HIGHWAY RESEARCH BOARD Bulletin 93

iSofI Density and Stabititg

National Academy of Sciences— National Research Council

II HIGHWAY RESEARCH BOARD Officers and Members of the Executive Committee 1954

OFFICERS

G. DONALD KENNEDY, Chairman K. B. WOODS, Vice Chairman

FRED BURGGRAF, Director ELMER M. WARD, Assistant Director

Executive Committee

FRANCIS V. DU PONT, Commissioner, Bureau of Public Roads

HAL H. HALE, Executive Secretary, American Association of State Highway Officials

Louis JORDAN, Executive Secretary, Division of Engineering and Industrial Re• search, National Research Council R. H. BALDOCK, State Highway Engineer, Oregon State Highway Commission PYKE JOHNSON, Consultant, Automotive Safety Foundation

G. DONALD KENNEDY, Executive Vice President, Portland Cement Association 0. L. KiPP, Assistant Commissioner and Chief Engineer, Minnesota Department of Highways

BURTON W. MARSH, Director, Safety and Traffic Engineering Department, Ameri• can Automobile Association C. H. SCHOLER, Head, Applied Mechanics Department, Kansas State College REX M. WHITTON, Chief Engineer, Missouri State Highway Department

K. B. WOODS, Director, Joint Highway Research Project, Purdue University

Editorial Staff

FRED BURGGRAF ELMER M. WARD W. J. MILLER 2101 Constitution Avenue Washington 25, D. C.

The opinions and conclusions expiessed in this publication are those of the authors and not necessaiily those of the Highway Research Board HIGHWAY RESEARCH BOARD BuUetin 93

Sof I Density and StahiUty

PRESENTED AT THE Thirty-Third Annual Meeting January 12-15, 1954

1954 Washington, D. C. Department of Soils

Frank R. Olmstead, Chairman; Chief, Soils Section, Bureau of Public Roads

COMMITTEE ON COMPACTION OF EMBANKMENTS, SUBGRADES AND BASES

L.D. Hicks, Chairman; Chief Soils Engineer North Carolina State Highway and Public Works Commission

W. F. Abercrombie, State Highway Department of Georgia Charles W. Allen, Research Engineer, Ohio Department of Highways W. H. Campen, Manager, Omaha Testing Laboratories Miles D. Catton, Assistant to the Vice President for Research and Development, Research and Development Laboratories, Portland Cement Association, Chicago C. A. Hogentogler, Jr., 6104 River Road, Washington 16, D. C. O.J. Porter, 415 Frelinghuysen Avenue, Newark, New Jersey Thomas B. Prlngle, Office, Chief of Engineers, Department of the Army L. C. Ritter, c/o Howard Jansen, Inc., 619 North Washington Street, Naperville, Illinoij James R. Schuyler, Assistant District Engineer, Soils and Subdrainage Section, New Jersey State Highway Department T. E. Shelburne, Director of Research, Virginia Department of Highways, Thorton Hall,! University of Virginia S. E. Sime, Senior Hi^way Engineer, Bureau of Public Roads W. T. Spencer, Soils Engineer, State Highway Commission of Indiana,

11 Contents

SELECTION OF DENSITIES FOR SUBGRADES AND FLEXIBLE-BASE MATERIALS Chester McDowell — - - 1 Appendix Preparation of Soil and Flexible Base Material 3 General Laboratory Compaction Test for Moisture- Density Relations for Soils 6 Use of Compaction Ratio for Selection of Densities for Subgrades and Flexible Bases 9 Discussion W.H. Campen - 14 Chester McDowell, Closure 14

RELATIONSHIP BETWEEN DENSITY AND STABILITY OF SUBGRADE SOILS H.B. Seed and Carl L. Monismith 16 Discussion Robert Horonjeff 30 W.H. Campen — 31 H. B. Seed and C. L. Monismith, Closure 32

EFFECT OF COMPACTION METHOD ON STABILITY AND SWELL PRESSURE OF SOILS H. B. Seed, Raymond Lundgren, and Clarence K. Chan 33

NEW METHOD FOR MEASURING IN-PLACE DENSITY OF SOILS AND GRANULAR MATERIALS Carl E. Minor and Herbert W. Humphres 49 Appendix A: Design and Operation Procedure 55 Appendix B: Results of Ejqperiments 58 Discussion Alfred W. Maner - 60

EFFECT OF REPEATED LOAD APPLICATION ON SOIL COMPACTION EFFICIENCY George F. Sowers and C. M. Kennedy III 61

111 Selection of Densities for Subgrades and Flexible-Base Materials

CHESTER MCDOWELL, senior Soils Engineer Texas Highway Department

# THE need for a realistic method of se- compaction with so little compactive ef• lectii^, from tests, densities which are fort, and the material is so loose after comparable to ideal field conditions has rolling that our field forces often do not led to the use of the term "compaction consider such compaction as satisfactory. ratio. " It expresses that portion of the Hence, there has been a disturbing trend possible densificatlon that it is desirable toward relying solely upon visual inspec• to accomplish in changing a material from tion in lieu of tests to control compaction. a loose to a very-dense state. It has In fact, in many instances the rigorous simplified matters materially by making use of specifications certainly would have it possible to select or predetermine produced compaction inferior to that ob• desirable roadbed densities for construc• tained by visual inspection. This trend will tion involving a wide range of soils and continue until better methods of testing base materials by means of unit-we^ht and and means of e}q>ressing the selection of moisture-density tests. densities are employed. Our experience with the compaction Donald M. Burmister indicated concern of soils involving all gradations from over these same matters in his excellent clays to granular base materials has in• article: "The Importance and Practical dicated the need for a means of expres• Use of Relative Density in Soil Mechanics," sion for significant states of grain struc• published in Volume 48, Proceedings tures which affect relative supporting ASTM, where he described the problem in values. Investigations of e^ansive prop• the following statements: erties of clays as related to strength and compaction, together with experience in the commonly used term, percent compaction, molding specimens consisting of large expressed as a simple percentage of the maxi• particles, often have shown the need for mum density only, is not significant but is such an expression. Data relative to this actually misleading, because it is based upon subject were presented at the annual meet• the obvious fallacy that zero percent compac• ing of the Highway Research Board in 1946. tion can represent a unit weight of zero for any soil, whereas there is always a definite physical Our procedure (see THD-83 in the appen• limit to the minimum density or loosest state. dix) for compaction of samples consisting ... A field control requirement of a certain of large, top-size aggregates in large percent compaction gives a false sense of a good molds has done much to help establish a degree of compaction, which does not in fact correlation between laboratory and field exist. To the average engineer or contractor 100 percent compaction represents perfection compaction. Additional investigations, and 90 percent compaction, according to common including in-place field densities, have standards of engineering practice, is considered shown that it is not practical to obtain to be very good indeed. ... It is evident that such an expression by use of percent com• field-compaction requirements and specifications paction or percent density and that the use based on percentage compaction are not good enough for the construction and treatment of of percent compaction as a means of field subgrades and of high-quality base courses for control may cause overcompaction of clays airport-runway pavements intended to support and undercompaction of granular soils. heavy airplane wheel loadings, particularly Occasionally, harsh mixtures of base under the vibrational effects. Such require• materials are encountered which cannot ments are inadequate because they cannot prop• erly fix the lower density limit considered per• possibly be compacted to a density as high missible and acceptable in earthwork construc• as that calculated for the total material tion, which will minimize objectionable consol• from density tests of the minus-y4-inch or idation of granular soils and objectionable lateral No. 4 material, specific gravity of the plastic displacements of clay-soils under such retained portion, etc. Rolling of most loadings. Only 100 percent compaction has any real practical significance. The use of the term, soils which are not of a harsh nature percent compaction, in specifications should produces the required percent density or therefore be discontinued. In attempting to work out a means of shown that the percent compaction desired, using relative density for analyzing this (DA/DD) 100, varies from 87 to 99 per• problem, a similar but different expression cent for all soils tested, and that the dry- was discovered which simplified matters: rodded unit weight for some of the soils, compaction ratio is defined as the ratio of when expressed as a percentage of DD , the difference between actual and loose varies from84 to 92percent;therefore, by densities to the difference between dense specifying QOpercent density, the resulting and loose densities, ejqpressed as a per• density would not be above the dry-rodded centage and is represented by the ex- unit weight in some cases. Hence, a more- rational approach was sought by use of presslon^^A " "^^00. It e^qjresses the compaction ratio which is expressed as follows: /DA - DL\ degree to which it Is desirable to compact CR in percent = \UD -DL/IOO- Figure soil materials on the basis of the compacti- C and Table A of THD-110 show that a good bility of the material. The experienced correlation exists between compaction technician often can select a density to ratio and DL for widely varying types of which it is desirable to compact a material soils. The relation may be expressed by by investigating the effects produced by a the equation CR = 58 + t:—^ . Then,actual variety of compactive efforts, as suggested 3.9 in THD-83 in the appendix. By reference Since to the procedure involving the use of com• density DA = ^ (DD - DL) + DL. paction ratio, THD-110, it is possible for CR can be expressed in terms of DL, it is the inexperienced technician to select obvious that we have eliminated one variable proper densities on the basis of two specific and it is possible to determine the desired tests, i. e., a unit-weight determination DA in terms of DL and DD. Figure 1 shows and a moisture-density test. The particular how simple it is to use this method. densities referred to above are obtained by the use of the following techniques: ,1-SSailjf "ti'Ttt 1. Loose density DL is obtained for clean sands and granular materials by running rodded unit weight tests (ASTM Designation C 29-42) and is e^^ressed in pounds per cubic foot dry. For other soils containing small amounts retained on the No. 40, DL is obtained by determining the dry unit weight per cubic foot of the soil shrinkage pat which has been molded at the liquid limit condition and corrected for any plus-No. -40 material which under field conditions will be floating in the soil. This loose structural condition perhaps is analogous to a formation freshly deposited by water. 2. Actual density DA is the drj' weight per cubic foot which experience has proven to be desirable and practical. Table B of THD-110 shows that a close approxima• tion of this density can be obtained in the laboratory by employing a particular com• pactive effort the intensity of which depends upon the character of the soil. D^ also may be obtained by calculation, as will be Figure 1. Chart showing relations between shown later. DL, DD and DA- 3. The term dense density "DD" is ob• tained from the peak of an optimum mois• CONCLUSIONS ture curve run at a high compactive effort of approximately 30 ft. -lb. per cu. in. It is believedthat desirable densities can ] From the data in Table A it may be be predetermined successfully by using a j compaction ratio ranging between 70 and tained by the application of normal amounts 90, depending upon the DL characteristics of rolling; however, no attempt is made to of the material. It should be emphasized present details of field control for such that the densities selected by the use of rolling in this paper. Nevertheless, a few simplified tests in conjunction with com• remarks on this subject seem appropriate paction ratio are comparable to those ob• at this time. In the first place, everyone tained by running extensive volume-change seems to need better equipment and tech• and strength tests. It is also important that niques for measuring in-place densities. moisture contents of high-volume-change We hope to overcome these difficulties soils be controlled during and following soon. In the second place, the condition construction in order to obtain the greatest of the soil as determined from tests and subsequent strength, small amounts of use of compaction ratio should be specified swell, and a minimum of dry weather in lieu of trying to accomplish the same shrinkage cracks. In order to do this It is end by specifying hours of rolling. It is necessary to select Ideal initial moisture- believed thatbettercontrol will be obtained density relations for construction in such a through testing, and once contractors are manner as to avoid volume change in aware of the moisture-density conditions amounts sufficient to cause excessive desired, they will find the best and most- movements and loss of soil stability. It is economical ways of obtaining them. The also necessary that such selected soil- amount of rolling required to obtain min• water systemabe protectedfrom excessive imum density requirements may not be evaporation so as to reduce destructive, adequate for finishing purposes; therefore, dry weather cracking. Methods of control the amounts of rolling hours for finishing of clays proposed in THO-110 offer many probably should be continued and handled possibilities and as evidence of their prac• as a separate item. tical use, it is pointed out that the methods proposed are based upon fieldperformance such as that of the Houston Expressway. ACKNOWLEDGEMENTS The clay subgrade soil on this project was The writer is indebted to many who sealed with asphalt membranes, and have contributed, encouraged or assisted moisture-density relations have remained in the development of compaction tests. A practically unchanged for over 5 years, al• few of these people or organizations have though dry weather cracks occurred ex• been mentioned in the text; however, the tensively in shoulders overlying slope writr r feels that the contributions of the membranes. mecoers of the Soils Section and other It is also concluded that density selected members of the Materials and Tests Di• by the proposed procedure is a desirable vision of the Texas Highway Department state for subgrade and base construction, have been a major factor in making this includiiig e^ansive clays, and can be ob• report possible.

Appendix

PREPARATION OF SOIL AND FLEXIBLE BASE MATERIAL TEXAS HIGHWAY DEPA:iTMENT THD-53 Revised 1953

Foreword AASHO Designation T 146-49 but differs from Standard Dry Preparation Methods AASHO T 87-49 and ASTMD 421-39 for This method describes In Part I the materials which contain particles larger preparation by washing of soil samples than the 40 mesh. Part II describes the for mechanical analysis and the de• preparation of materials for tests (com• termination of the soil characteristics. paction, triaxial, and stabilization) which This method is in close agreement with require laboratory compacted specimens. PART I: WET PREPARATION OF DIS• any particles larger than 40-mesh in size. TURBED SOIL SAMPLES FOR SOIL This can be done by visual inspection or by CONSTANTS AND HYDROMETER slaking a small representative portion in ANALYSIS TESTS water. If the material contains no par• ticles larger than 40 mesh in size, or if Prepared Soil Binder the amount of particles larger than 40 mesh is small and can be easily dis• The purpose of the "preparation" is to tinguished by eye, these particles should separate the material into sufficiently be removed by hand and steps 9 and 10 fol• small sizes so that all of the material will lowed in the preparation of the soil binder. be mixed thoroughly and uniformly wetted in the making of the various subsequent 3. For materials containing an appre• soil tests. That portion of the material ciable amount of aggregate, the air-dried passing the Standard U. S. 40-mesh sieve sample selected for the purpose of me• shall be known as "Soil Binder". It shall chanical analysis and physical tests should be separated from any larger particles be weighed and the weight recorded (un• without fracture of the individual grains. corrected) for hygroscopic moisture. The A representative sample shall be selected sample shall then be separated into two according to the method outlined in THD- portions by means of a No. 40 mesh sieve. 50. The material passing the No. 40 sieve shall be identified and set aside for later re• Apparatus combination with additional binder as de• scribed in steps 7 through 10. The apparatus consists of: (1) scale of 4. The material retained on the 40- 30-lb. capacity, sensitive to 1/100 lb; (2) mesh sieve shall be placed in a milk pan, set of standard screens and sieves for covered with clear water and allowed to screening small samples and another larger soak for a period of 2 to 24 hours. If the set, preferably with shaker, for screening sample is immersed for only a few hours, large samples; (3) 6-inch mortar; (4) stone care shall be taken to see that no lumps pestle; (5) rubber-covered pestle; (6) containing soil binder remain in the ag• scoop; (7) air-dryer with temperature gregate. The slaking time for base mate• range of 120 F. to 160 F.; (8) supply of rials can be determined as described in pans; and (9) small siphon tube. THD-54. 5. After slaking, the material shall be Size of Sample washed on a No. 40 sieve in the following manner: The empty sieve shall be set in For samples predominantly soil, ap• the bottom of a milk pan and the liquid from proximately one quart or three pounds will the wet sample poured into it. Enough be required. Larger samples should be additional water shall be added to bring the selected depending on the amount of ag• level of the water in the pan approximately gregate. For materials containing aggre• %-inch above the mesh of the sieve. A gate, the sample should be large enough to small portion of the soaked material shall produce one quart of soil binder (minus No. be placed in the water on the sieve and 40 mesh) or approximately 6 to 12 lb. total stirred by hand at the same time the sieve materiaL Larger samples are always is agitated up and down. If the material satisfactory. Such materials should have retained on the 40 sieve contains lumps that all aggregate crushed to the maximum size have not disintegrated, but which can be permitted in its contemplated use, and care crumbled or mashed between the thumb should be taken to prevent loss of the fine and fingersoas to pass the 40-mesh sieve, material produced by the crushing process. such lumps shall be broken and washed through the sieve into the pan. After all Preparation of Test Sample of the soil binder appears to have passed through the sieve, the sieve then shall be 1. Dry the soil sample in an air drier held above the pan and the retained mate• or oven of a temperature not to exceed 140 rial shall be washed by pouring a small F. Higher temperatures than this may amount of clean water over it and letting change the characteristics of the soil the water drain into the pan. The washed binder. material retained on the No. 40 sieve shall 2. Determine whether the material has be transferred to a clean pan. 6. The procedure of step 5 shall be re• PART n: PREPARATION OF SAMPLE peated until all of the soaked sample has FOR COMPACTION, TRIAXIAL AND been washed. STABILIZATION TESTS 7. The mate rial retained on the 40 sieve shall be dried and dry sieved on the No. 40. 1. Select approximately 200 pounds of The portion passing the 40 sieve in this material to prepare. Samples should be operation shall be added to the material obtained "crusher run" or as near the passing the 40 sieve obtained in step 3. condition In which they are to be used on The retained material shall be weighed and the road as possible. Caliche, rock and then set aside for use in the screen anal• similar materials obtained from test ysis of the coarse material. holes should be crushed in laboratory to pass 2" screen. Spread sample on clean 8. After all of the soaked material has floor to air dry. Do not slake or oven dry been washed, the pan containing the fine any of the material. soil and wash water shall be set aside and not disturbed until all of the soil particles 2. After the sample has air dried suf• have settled to the bottom of the pan and ficiently to handle, the necessary steps to the water above the soil is clear. As much be followed will be governed by the type of of the Clearwater as possible shall then be material encountered: (a) Clay and sandy decanted or siphoned off. In some in• clay subgrade materials having no appre• stances, the wash water will not become ciable amount of aggregate present shall clear in a reasonable length of time, in be crushed to pass %" screen. The sam• which case, the entire watercontents must ple is then separated into two portions by be evaporated. The soil remaining in the means of the No. 20 mesh sieve. This pan shall be dried at a temperature not to provides a material that can be handled exceed 140 F. without segregation; (b) clay subgrades containing aggregate should be prepared 9. The dried soil binder shall be pul• by breaking up all of the clay lumps to verized to pass the No. 40 mesh sieve by pass a %" screen without breaking up the suitable hand or mechanical methods, tak• aggregate. This may be done by means of ing care not to fracture the original grain a plastic mallet, rubber-covered tamp or sizes and then be combined with the mate• similar hand tool. The material shall rial passing No. 40 sieve obtained by the then be screened as described in (c). procedure described in steps 3 and 7. (c) The desired quantities of caliches, After all the soil binder has been pre• crushed rock, gravel and sand shall be pared to pass the No. 40 sieve, it should screened over the following suggested be weighed and thoroughly mixed to pro• sizes: 2",' 1%", 1", %", V*" and No. duce a uniform homogeneous mixture of all 10 mesh. particles. 3. Weight the various pans full of sizes 10. Add the weight obtained in steps 7 including the portion passing the No. 10 and 9 for the total weight of sample. This mesh and compute the percentages. These weight is used as a check against the percentages are not to be used as a true original weight and to calculate the percent screen analysis, but are to be used in re- of soil binder in the material. combining the sample for individual spec• imens. % soil Wt. passing No. 40 sieve ^ 4. Determine the hygroscopic moisture binder " Wt. of total sample ^ content of the total sample as follows: Weigh a, representative portion of minus No. The soil binder passing the No. 40 mesh 10 mesh material that has been thoroughly sieve obtained as described in paragraphs mixed. Oven dry this sample at 230 F. and 2 and 9 shall be used in performing the weigh. Compute the percent hygroscopic hydrometer analysis and for determination moisture based on total material. of the soil characteristics. Materials which contain aggregate prepared by this method % Hygro. _ Wet Wt. - Dry Wt. ^ (% minus usually have a higher percentage of soil Moisture Dry Wt. No. 10) binder (minus No. 40 mesh) and show more plasticity than those prepared by Standard In this determination, it is assumed that the Dry Methods ASTM D 421-39 and AASHO moisture in the plus 10-mesh material is T 87-49. negligible. GENERAL LABORATORY COMPACTION TEST FOR MOISTURE-DENSITY RELATIONS FOR SOILS TEXAS HIGHWAY DEPARTMENT THD-83

Scope larger mold, the use of a variable com- pactive effort, and the use of a fresh This method of test is intended for de• sample for each trial. termining the relationship between the moisture content of soils and resulting densities (oven-dry weight per cubic foot) Apparatus when the soil or flexible base material is compacted in the laboratory as specified (1) Compactive device (see Figure A) herein. This procedure differs from the with base plate to hold 6 inch I. D. molds Standard ASTM D 698-42T, AASHO T 99- equipped with 10-lb. ram and adjustable 49 and THD-84 methods in that the test height of fall. Striking face of 10-lb. ram- pertains to the use of the entire sample 40-deg. segment of 3-inch radius circle; consisting of large sizes, the use of a (2) compaction mold with removable collar — 6-inch inside diameter and 8- inch height; (3) measuring device, mi• crometer dial assembly (Fig. B) for de• termining height of specimen; (4) scale, rated 30 pound capacity, sensitive to 1/100 lb, ; (5) extra base plate to hold mold (Fig. C); (6) press, low capacity, to eject

Figure A. Figure B. specimens from mold; (7) drying oven, the height of the specimen in inches. ir^i ^ u area in sq. in. x 1 in. cubic controlled to 230 F. ; (8) pans, wide and Vol.permch= ^ ^^^^ shallow for mixing materials; (9) a supply of small tools, hammer, plastic mallet, Vol. of specimen = volume per lin. in. x level and others; and (10) circular porous height of specimen in stones slightly less than 6 in. in diameter inches and 2 in. high. Preparation of Sample Calibration of Mold Prepare the material according to the The method of calibrating a 6-inch procedure given in THD-53, Part IL mold is different from that employed for regular Proctor and similar molds. In the Compactive Effort case of the 6 inch mold, the total volume of the mold would be of no use since the Compactive effort is defined as the total

Figure C. mold is neither filled nor trimmed off. energy, expressed in ft. -lb. , delivered by Instead, a volume of per inch of height is the compaction ram to each cubic inch of employed. The diameter of the mold is specimen. As proposed in this procedure, measured (by means of a micrometer the modified hammer (weight 10-lb. fall caliper and a dial micrometer) at the ends 1. 5 ft.) is used to compact layers 2 inches and several intermediate points to obtain thick and 6 inches in diameter; 25 blows an average diameter. The average di• per layer will represent a compactive effort ameter is used to calculate a mean cross of 6. 63 ft. lb. per cu. in. (25 x 10 lb. x sectional area of the mold. The area thus 1. 5 ft. ) divided by (3 in. x 3 in. x 3.1416 computed is then used to compute the vol• x 2 in.). ume in cubic feet for one inch of he^ht. In selecting a compactive effort for Thereafter, the volume for any height can running triaxial tests, THD-80, it is im• be found by multiplying the above volume by portant to choose the one that will produce T«jr«9 MfhnM7 Daftrtmmt TaD-8j 4. Clean, cohesionless sands are ex• Form ceptions and require special treatment. Specimens representing field conditions MOISTURE AND DENSITY WORK SHEET Sample No Count;/. Projec-t No can be con:q)act$d by use of the proposed equipment if the hammer is converted to Mold NO val\ime of Mold.. Cu Fi/Ia ]^ghi-ofMold. have a twin striking face and weighing 10 Mam Dry •V lb. It is also necessary to use a live rub• ber pad y2-inch thick to convert some of the impact into vibrations. The only other modification in technique is that the use of approximately 100 blows per 1-inch layer to produce the desired densities is indicated. 5. For selection of compactive efforts, for soils stabilized with cement, asphalt and lime, see THD No. 96, 98 and 82. 6. For control of compaction by rol• lers, see THD-110. Test Procedure L Calculation of Mix for Individual Spec• imens. 1. Estimate the weight of air-dry material that will, when wetted and com• pacted, almost fill the 6" x 8" mold. 2. Using the weight estimated in (1), Figure D. compute the accumulative weights of the the degree of compaction required by the various sizes of the material based on the conditions of the proposed use. The usual percentages obtained in the preparation of compactive efforts shall be as follows: the large sample. 1. For flexible base materials and 3. Weigh up the sample as calculated select soils with little or no tendency to• in (2), keeping the minus No. 10 or No. 20 ward shrinkage or swell, use 50 hammer mesh material separate. blows per layer (13.26 ft. lb. per cu. in.) There are special cases where this IL Mixing of Specimen: degree of laboratory compaction will not 1. Place the a^regate portion (plus produce densities and strengths which Na 10 mesh material), weighed out in are as high as those produced with rol• lers. In cases where this is strongly evident and the strength of the material when tested at 50 blows per layer com• paction method is lower than appears reasonable, it is suggested that 75 blows per layer be used. In such cases field density should be controlled by testing so that it will be as great or greater than obtained by use of 75 blows per layer. 2. For moderately active soils use 25 blows per layer (6.63 ft. lb. per cu. in. ). Most moderately active soils will have a PI between 20 and 35 if practically all i particle sizes pass the No. 40 sieve. Most granular, select materials with Pi's be• tween 20 and 35 should be molded in ac• cordance with step 1. 3. For very plastic clays, exhibiting high volume change, use 15 to 20 hammer blows per layer (4 or 5 ft. lb. per cu. in.). Figure E. Moisture-density curve. 9

step 1, in the mixing pan and add a known the right height, it can be easily corrected amount of water (a percentage based on the by weighing out air-dry material = dry weight of the total material) pounds of water to be added equals pounds of water amount used „ . J„„I„„J desired minus hygroscopic. Mix thorough• height obtained ^ ^^"^^^ ^^^^^^^ ly with a trowel, breaking up the soft 5. Place a porous stone on bottom of slakeable lumps of material. Do not crush mold and extrude specimen from mold, any material that would not ordinarily taking care not to lose any of the materiaL slake. 6. Extrude specimen and if triaxial 2. Add the minus No. 10 mesh mate• tests are not being run, break up material rial to the wet aggregate and mix until by hand and dry at 230 F. uniform. Try not to lose any of the mate• 7. Vary the amount of molding water, rial. Cover mixture to prevent loss of and repeat above operations in order to moisture. obtain several points for the moisture- density curve. nL Molding of Specimens IV. Moisture Density Curve 1. The ten pound ram of the compac• The dry density of each specimen is tive machine should be adjusted to drop 18 calculated as follows: inches. 1. Volume of specimen = height of 2. The material is now molded in four specimen x vol. per linear inch (see equally weighed layers at the number of "Calibration of Mold"). blows per layer as specified under "Com• pactive Effort." In case of harsh aggre• 2. Wet weight of specimen = wet wt; of gate or material with low amount of fines, spec, and mold minus weight of mold. some of the fines out of the first layer , ,„ . wet wt. of specimen should be placed on the bottom of the mold 3. wet density = ^^^^ specimen to insure a smooth bottom on the specimen. ii J wet density of specimen 3. After the last layer is compacted, 4. Dry densitymolding moistuTj- the specimen is placed on the extra base IDD plate and finished with the various hand tools, such as putty knife, plastic mallet, 5. Zero air voids density = heavy lead hammer and circular plate with a plane surface. The surface of the speci• specific gravity x 62. 5 men is checked by means of the small 1 + sp. gr. X % moisture carpenter level to see if it is plane and ^IDD level with respect to the top of the mold. 6. Plot dry density and zero air voids Do not trim the top of the specimen. density against molding moisture (see 4. Remove the mold from the small Figure E). t-Where the specific gravity base plate, weigh and measure height of is not known, the value of 2. 65 maybe used specimen. Record test data on form shown as an average specific gravity for the in Figure D. If the specimen is not of majority of materials.

USE OF COMPACTION RATIO FOR SELECTION OF DENSITIES FOR SUBGRADES AND FLEXIBLE BASES TEXAS HIGHWAY DEPARTMENT THD-110

Foreword ture contents are calculated on the basis* of oven dry weight of soil (dried to con• Throughout this discussion the term stant weight at 105 C.). "density" or "dry density" of any given The fact that the same compactive ef• soil mass, regardless of its moisture fort produces different densities for the content, will refer to the calculated weight same soil when compacted at various of dry soil per cubic foot. The "wet den• moisture contents has been discovered sity" will include the weight of soil plus independently by several sources, but Mr. water in pounds per cubic foot. All mois• R. R. Proctor worked out the first widely 10

Compaction Ratio (CR) From LL PI SL SR "L °A iiT Fig. 3 Soil Triaxial Lab Binder Strength Lab. No. Material Location Tests Calculated** % Class 49-1-R Crushed Servtex 126 141 139.5* 89.9 90.2 21 7 13 2.0 32 1 stone New Braunfels, Texas so-eg-R Gravel Hybla Valley, 122 138 136. 5* 90.4 89.5 22 7 15 1.8 21 3.3 Va. 53-1-R Light Lubbock, 85 101.5 98* 78.8 79.8 63 22 45 1.2 13 3.2 Caliche Texas 3g-7-MR Sand Austin Co. 107* 127 124* 85.0 85 4 18 3 15 1. 8 97 3.5 Clay Texas 51-51-R Clean Canadian 103 112 110.5 83.4 84.3 21 0 21 1. 65 100 Fine River Sand Sand Roberts Co. Texas 51-112-R Silt Idaho 77. 5* 100 95* 79.5 77.9 39 13 23 1. 56 99 3.5 WASHO Test Section 39-11-MR Black Manor, 56* 106 92 72.0 72.4 74 45 10 2 0 100 5.0 Gumbo Texas Clay

* Materials compacted to densities equal to those obtained from normal amounts of rolling by employing a laboratory compactive effort of 13. 26 ft. lb. per cu. in. See THD-83. The Canadian River sand wascompactedby theuseof a specialtechniquetoobUin vibration from drop hammers as outlined in THO-83. The black clay 39-11-MR was found to have best strength when compacted to this density by use of 6. 62 fL lb. /cu. in. and subjected to capillary wetting. This density was also found to exist underneath old pavements on this type of soil (see Table B) **CH =/DAJDL\=^''A-DL^ , 100

+ DL calculated from the formula DL = 1 + LL^- SL values were obtained from rodded unit weight tests —IDD— recognized test for determining the opti• earth dams, and the compaction obtained in mum moisture content of the soil and the Proctor test mold was presumably checking it in the field. His tests were equivalent to that obtained by the sheep's made in connection with the building of foot rollers used on these first jobs. These tests are based on the fact that for each soil there is but one moisture content, termed the optimum moisture content, at which the maximum density is produced by a specific degree of compac• tion. Our triaxial test studies relative to thickness and quality of flexible bases have thoroughly convinced us of the need for a method of density selection which can be applied successfully to widely vary• ing types of soils. In order to do this it is recommended that the method described herein which is based upon "Compaction Ratio" "be used. In its simplest terms compaction ratio is defined as the ratio of the difference between actual and loose densities to the difference between the dense and loose densities and is expressed in percentage as follows:

/°A-PL\ CR 100 PES-Cll Lgure F. Chart for calculating Dp from Where: LL and SL CR = Compaction ratio in percent 11

DL = Loose Density (Dry Unit Weight in densities (DQ - DjJ. This method is pre• Lbs. per Cu. Ft) ferred in lieu of the usual method of spec• D^ = Actual Density to be obtained (Dry ifying an arbitrary percentage of some Unit Weight in Lbs. per Cu. Ft.) maximum density. The method of specify• DD = Densest condition the material can ing percent density is not sound because be compacted to by using 30 ft. lbs. / it is a percentage of a variable density cu. in.compactive effort in the lab• which has an important influence upon oratory. See THD-Procedure 83. strength. As an example, assume that a base material can be compacted in the The terms "CompactionRatio" and "Rel• laboratory to a maximum density DQ and ative Density" should not be confused be• 90 % density is required. The density so cause CR must be multiplied by the ratio specified can usually be obtained by lightly of DD/DA to obtain "Percent Relative Den• rodding the dry material into a one-haU sity". The term "Relative Density" is cu. ft. bucket. It is obvious that when such based upon a concept of filling voids with a material is at such a low density that its solids and is used frequently in soil me• strength is only a fraction of what It should chanics and its use for control of com• be. If the same minimum density of 90% of paction of subgrades and base materials DD for some high volume change soils is was proposed in an interesting article by used, a condition of overcompaction could Professor Donald M. Burmister and pub• be attained. Therefore, various soil mate• lished in Volume 48 of the Proceedings of rials should be compacted to different the ASTM. percentages of their maximum densities. "Compaction Ratio" e:q)resses the de• The desirable percent density to use can be gree to which it is desirable to compact predetermined from strength and volume soil materials on the basis of the com- change tests but they are too time con• pactibility of the material; namely, the suming to be used during construction. By use of the proposed method the compaction difference between the dense and loose

TABLE B Density lb. / cu ft. Dry Description Lab Highway Field Lab. Compactive Effort Before Rolling No County Number of Material No. Ft lb. / cu. in Tests 63 13. 26 19.89 PI % S. B. 49-4-R Lampasas 190 Caliche and 126 8 1 127. 0* 7 41* Limestone with Soft Sandstone R-1 Williamson 79 Clay-Gravel 128.9 5 128. 2* 8 28 thru Plus Waste R-10 Lime L-1 Llano 29 Granite Gravel 128 0 10 127. 6* 11 25 thru Plus Waste , L-5 Lime 48-29-R Bexar(San Crushed Rock 129 7 48 128 0* 128 9 12 27 Antionio Ex• Subbase from pressways) Rabke Pit UGI 1083 (S) 48-40-R Crushed Rock 137 1 34 133. S* 134.5 8 29 Base from McDonough Pit 4B-21-R Leon Creek 142 4 32 141 9* 142 7 13 31 Gravel-Sand Admix i^ubbaae + 39-11-MR Travis 20 Black Clay 92. 0»» 98.5 102.2 45 98 (Gumbo) 3 92 0*"* 47-471E Williamson U.S Black Clay 85. 0** a 85.0* 37 97 81 47-136-E Bastrop St. Sand Clay lis. 8 IS 111. S 114 9* 6 98 Hwy. 20 „ „ 47-143-E 118.9 14 117.7 121.4* 14 95 47-1S8-E 117.1 3 112.7 116 5* 14 96 49-14-R Crushed Stone 126.0 4 125. 0+ 6 2« * Lab. specimens and road samples contamed from 48 to 50 percent soil binder after compaction, showing a similar breakdown of particle sizes for field and laboratory operations ** Samples from old road subgrade after clay had consolidated or swelled so as to come to its final density. **» standard AASHO density = 85 lb /cu. ft. Values obtamed m same manner D^ values were obtained in Table A. 12

ratio and the minimum desirable density (a) Determine the dry unit weight can be predetermined from simple unit Dp of the soil at the LL condition from the weight and compaction tests -which, will be formula e^lained under procedure. The pro• SR X 62. 5 cedure for laboratory compaction (THD- Dp = 83) used in determining provides for . , LL - SL compacting coarsely graded soils and base materials without having to split the Where: sample over such screen sizes as in. or SR = Shrinkage Ratio No. 4, etc. There will be instances \diere SL = Shrinkage Umit it will not be practical to use the proposed LL = Liquid Umit method. Some of these are as follows: J Lds. 1. Where soils are considerably wetter 4- than optimum and it is not practical to il-T —1-I—r— 120 1 1 1 1 1 71 */ aerate them. --4-I 1 1 1 1 n/T .t'zr. 1 1 1 1 1/i ; • • .~f TT~ i 111 1111 TT" kjLl—!—1— 1 1 -! 1 • -1— - Oh 4—1— -<-,-M- -H- -t-+^Lifi O• - "TI ,i i 1 1 Oi pit / ' -|-h4 1- 1 1 . 1 \i dp;-ol Viipa 1 '/\ 1 xrrr Pdfmu —+f -f ••- I -t- tn TSr cc snierj • /- 1 / J_ 1 -LIZ ' 1 1 TL' I fj 1 • T T tt, ii/< Tt*/ rl" J I L-i -t */ / 1 ! l-i, + r- t 4-4-1 '- nr . - - ri- 1 <-t r r 1 i1t -'1i r • . 1 1 ft 1 r -ja I. + i I ' L 1 I +- 4-UU- i- T 1^ rf / ' 1 ! ! ' T ^ -1- H- -t- ' t"'t ~r •"-M - -hi-.- -L 111 ~ H——1~ + 1111 -U1- 1 t 1 -M- "7 < ^« 1' -t- -\ 7- t r xt -H- -1— - ;/1 4-4— -4-4—l—t— i-t • 01- ' 7 aw M£,TIB. IIf ! 1 ! U T _XJ_ 1 1 —t

Figure H. Chart for calculating CR from Figure G. Chart for calculating Di from Dp. ^ Op may also be determined graphically by use of Figure A. 2. Where hig^ fills (above 15 ft.) are (b) Obtain DL by correcting Dp being constructed out of high volume change for aggregate portion. soils. In this case higher densities and less moisture than proposed should be Dt IQQ used because the surcharge weight of the " J^pags#4p ^ % Ret, on # 40 fill restricts excessivie swellage and less Dp 62. 5 (bulk gravity of settlement will be obtained. portion ret. on# 40) In cases where bulk gravities of plus Procedure #40 portion varies between 2.3 and 2. 5, Figure B may be used. Bulk specific 1. Secure and prepare samples in ac• gravity is determined by THD-74. cordance with THD-53, Part II. B. For materials containing pre• 2. Determine loose density DL by one dominant amounts of plus #40 material and of the following methods: clean fine sands passing the #40 mesh A. For all soil materials contain• sieve. ing small amounts of plus No. 40 mesh (a) Run rodded unit weight on total material except clean sands. air-dry material in accordance with ASTM 13

C 29-42. Briefly, this procedure is as or greater than DA- Hig^ volume change follows: Rod material in three layers at clay soils (PI above 35 and containing little 25 blows per layer, in a standard one- or no plus #40) should not be compacted to a half cu. ft. unit weight bucket with the density in excess of DA and can be sat• blunt end of a % in. diameter steel rod 24 isfactory ¥rtien compacted to a density of as low as seven pounds/cu. ft. below DA' 6. Estimate optimum moisture con• tent for rolling as follows: (a) Plot the optimum moisture vs. maximum density obtained instep 4 of each of these soils on a sheet similar to Figure D. The "absolute specific gravity" of each soil can bedeterminedby test (THD-73) and the correct zero air voids density curve drawn or an assumed Sp. Gr. of 2. 65 can be used with satisfactory accuracy. " The zero air voids density curve can be plotted from the following equation:

Zero Air Sp. Gr. X I Voids Density lM%molsture^gp )

(b) Draw a line of equal air voids passing through DQ and parallel to the Figure I. Chart for estimating compaction zero air voids line. See dashed line in moisture content. Figure D. (c) This line intersects DA at a de• in. long. The material is struck off with sirable moisture content for compaction. the rod and its unit weight in lbs. per cu. This moisture content may need to be ft. calculated. This is OL- varied for various rollers and the number 3. Enter DL on Figure C to determine of passes they make and thicknesses of lifts CR or calculate by the formula being rolled. High volume change clay DL soils should be compacted when containing CR=58+3— at least as much moisture as determined in this step but should not contain more A comparison of CR values obtained by this than 5 percent above this minimum. In method to those obtained from tests are order to materially reduce heaving and shown in Table A. dry weather cracking, the compaction 4. Run compaction test for moisture- moisture for such soils should always be density relations in accordance with THD- permanently preserved by whatever means 83 using 112 blows per layer. Maximum is economically feasible. This can us• density at optimum moisture is OQ. ually be accomplished by placing bitumi• 5. Calculate DA, the actual density nous membranes or granular materials on desired, by the following formula: top of the final layer of any series of layers placed during one fairly continuous series of operations. DA DL) + DL The usefulness of the above formula Concluding Remarks was checked by: (1) selecting a laboratory compactive effort which produces densities It probably will e^edite matters to comparable to normal amounts of rolling select a few typical samples representative in the field as shown in Table B; (2) using of each of the large deposits or general this laboratory compaction procedure to types of materials to be encountered, and determine D^. values shown in Table B. run the necessary tests to determine DD These data indicate that all soils materials and DL for each material prior to the except swelling clays could and should be start of construction. Additional tests for compacted to a minimum density equal to determination of DQ and DL can and should 14

be made during construction especially (2) increase Inflation pressures and pos• when materials are encountered which sibly the weight of the roller; (3) vary the have not been tested. The Inspector who moisture content; (4) reduce the thickness will control this part of ihe project should of lift. Throughout the progress of the familiarize himself with the "looks" and job, in place density tests should be made "feel" of each of these soils at their opti• Immediately after rolling and the moisture mum moisture content in order that he content and density compared to those can do part of the inspection for moisture required. control in the field by observation. So This procedure calls for an amount of long as the specific gravity and soil con• rolling which will compact a material to a stants for a given material do not v^y desirable state for subgrade and base con• appreciably, DD and DL will remain fairly struction. For purposes of densification constant for wide variations in gradation. control, the condition of the soil mate• At the start of the project the effectiveness rials as proposed herein should be spec• of the number of passes of a given roller ified in lieu of trying to accomplish the used on a given depth of layer containing same end by paying for hours of rolling. optimum moisture content (step 6) should This is because better control will be be checked by making in place density obtained through testing and once con• tests (see THD-85). If these densities tractors are told what moisture-density meet the requirements for DA, discussed conditions are desired, they will find the in step 5, this rolling procedure can be best and most economical ways of ob• used for compacting the remainder of this taining them. The amount of rolling re• material. If this rolling procedure pro• quired to obtain minimum density re• duces densities lower than DA, any one of quirements may not be adequate for fin• a number of changes will increase the den• ishing purposes, therefore, the amounts sities from rolling. Some of these are as of rolling hours for finishing probably follows: (1) increase the number of passes; should be handled as a separate item.

Discussion W.H. CAMPEN, Manager, Omaha Test• varied to suit different soils. Also the ing Laboratories — McDowell's paper in• compactive effort of the standard methods volves a consideration of the principles can be varied as has been done by the U. S. pertaining to the densification of soils. Engineers. He claims that the general practice of In a general way McDowell indicates specifying percentage of maximum lab• that certain densities are required, in the oratory density in the compaction of field, for different soils in order to ob• soils is not a satisfactory criterion for tain satisfactory performance. Why not two principle reasons: (1) because in• then determine these densities and use adequate strength is developed in co- them as a basis of preparing specifications. hesionless mixtures and (2) because un• stable conditions are developed in highly CHESTER McDOWLL, Closure - In plastic mixtures. Due to these objec• order to carry out Campen's suggestion tionable features, he has worked out a of using densities as a basis of preparing method whereby desirable densities may specifications, it would be necessary to be determined and specified for various develop a tabulation showing the densities soils. These densities are not directly required for all variations of subgrade soil related to maximum laboratory values and subbase and base materials to be en• and are so selected as to produce max• countered. Such an approach becomes imum strength consistent with durability. impractical when various formations are I certainly agree that the same de• encountered, especially when a contractor gree of compaction does not give desir• may use from any of a number of possible able results with all soils. Furthermore, sources of flexible base materials existing no one can criticize his method of ob• within a 50-mile radius of a given project. taining desired results. However, the Campen may believe that experience with same results may be obtained by use of various compaction equipment has estab• the standard methods. For instance, the lished the values of densities necessary percentage of relative density may be for good construction. However, the 15 a.uthor feels that it is doubtful if we know DL/3. 9, in which DD and DL are de• enough about compaction to be able to termined by tests in the laboratory and DA specify densities in pounds per cubic foot is obtained from in-place density tests, as necessary for all soil materials en- a specification appears to offer the most pountered on any one given project. It ideal method of securing good compaction. ^uld seem desirable to be able to have ia the case of clay soils it is desirable i method whereby the intent and purpose to consider 58 + DL/3. 9 as a maximum and }f the specifications can be checked on values of 5 to 10 lower should be accept• the job every time necessary and especially able. In the case of all other soils, es• when the nature of the material being pecially granular base materials, 58 + compacted changes unexpectedly. The 9 should be considered a minimum ise of compaction ratio equal to 58 plus requirement for good compaction. Relationship Between Density and Stability of Subgrade Soils H. B. SEED, Assistant Professor of Civil Engineering, and CARL L. MONISMITH, Instructor in Civil Engineering, University of California

This paper discusses some of the limitations of the concepts that the stability of a compacted soil increases with an increase in density and that every effort should be made to attain the highest practical density in field compaction. Com• prehensive test data on the relationship between density, stability, water con• tent, and degree of saturation for two soils compacted by kneading action are presented, together with a description of the Triaxial Institute kneading com• pactor. In the tests, stability is measured by triaxial-compression tests and by the Hveem Stabilometer. The significance of the criterion of stability adopted with respect to the relationship between density and stability is discussed. The relationship between the attainable stabilities and those determined by specifica• tions based on the standard Proctor and modified AASHO compaction tests is demonstrated. Data are also presented on the density-versus-stability relationships of soils compacted by impact and static procedures, and the effect of compaction method on the density-versus-stability relationship is shown. The conflicting conclu• sions which may be reached on the basis of tests on samples prepared by static and impact methods or by static and kneading methods are indicated.

# SINCE the principles of soil compaction and maximum density of a soil, even for were first described by R. R. Proctor (t) the same compaction method, are not some 20 years ago, the use and interpreta• necessarily attained at the same time; this tion of soil-compaction tests have changed evidence has been obtained in both lab• relatively little. Thou^ new compaction oratory and field tests. Nevertheless, procedures have been developed, tests are many engineers seem to believe that the still conducted to determine the optimum higher the density to which a soil is com• water content at which a given compactive pacted, the greater will be its strength and effort will produce the maximum density stability and that every effort should always of the soil. This water content and den• be made to obtain the highest possible den• sity are then used as criteria for field sity. It would seem pertinent, therefore, compaction of earth fills and pavements. to discuss some of the limitations of this In adopting these criteria, it is im• concept as indicated by the results of recent plicitly assumed that density is a meas• laboratory tests. Such a discussion is ure of the desirable characteristics of particularly desirable now that pavements the compacted soil, such as strength or are being designed for conditions other stability, compressibility, and some• than complete saturation (8); an under• times permeability. A recent article (2) standing of the relationship between density states: "This optimum condition, pro• and stability would seem to be essential ducing a maximum density for the given for the intelligent design of such pave• compaction method, is generally the ments. strongest and most permanently stable condition for the soil resulting from the particular compaction procedure." Prop• LABORATORY COMPACTION TESTS erly interpreted, this statement is no The object of any laboratory compac• doubt true, but its validity will depend on tion test is to reproduce in the laboratory the conditions to which the compacted soil the compaction effects produced by equip• is ejcposed. There is considerable evi• ment in the field. A variety of procedures dence (3, 4, 5, 6, 7) that maximum strength are in use at the present time. In most

16 17 tests, the soil is compacted by dropping a falling weight on to the surface of the soil, a process referred to as "impact compac• Ceatiling tion. " In some cases, soil is compacted Spring (K' aOOOItpaiial by subjecting it to a static load, which is built up slowly to some predetermined value and then released, a process re• Tem-Tura Cam S/tlti ferred to as "static compaction." The fact that these two methods of compaction result in density - water -content curves having different forms and that they pro• aeiatrm b/ FraMr vol duce soil specimens having quite different stress-strain characteristics has long Figure 2. Compactor mechanical tamping been recognized (3). system. specimens are used for design purposes, in recent years considerable effort has been devoted to the development of lab• I oratory procedures which will satisfactori• ly duplicate the effects of field compaction. These efforts have resulted in a com• paction method which more closely sim• ulates the effect of sheepsfoot or rubber- tired rollers. The action of this equip• ment is to build up the pressure on a small area of soil to a definite value, maintain this pressure for a small ele• ment of time, and then gradually reduce the pressure; this method of load ap• plication has been termed a "kneading action. " It has been shown that the com• paction of soil specimens by this method offers good possibilities for the satisfactory preparation of laboratory samples having properties sufficiently close to those of the soil compacted in the field. A laboratory kneading compactor has been developed at Northwestern University (2) and, on behalf of the Triaxial Institute, a model originally designed and used by the California Division of Highways has been modified and developed at the Uni-

Figure 1. Triaxial Institute Kneading Compactor. Extensive studies of soils compacted in the field by sheepsfoot and rubber-tired rollers have also shown that the stress- strain characteristics of those soils are different from those prepared in the lab• oratory by either static or impact com• Figure 3. Compactor hydropneumatic con• paction. Since laboratory - prepared trol system. 18

versity of California. A miniature knead• on the test specimen by the tamper, a ing compaction device has also been de• combination hydraulic-pneumatic control veloped at Harvard University (9). In the system is used. A schematic drawing of tests reported in this paper, all samples this ^stem is shown in Figure 3. Air compacted by kneading action were pre• from a high-pressure line passes through pared by the Triaxial Institute Kneading a pressure regulator, which can be set a^ Compactor. any predetermined value, into the upper portion of the oil reservoir. This reser• TRIAXIAL INSTITUTE KNEADING voir is situated in the pipe column which also serves as a member of the com• COMPACTOR pactor frame. A feeder valve controls This equipment, a view of which is the flow of oil into the cylinder containing shown in Figure 1, may be used for com• the piston, which is attached to the lower pacting soil or asphalt samples. It is a link of the press. This feeder valve is mechanical device for applying a series of used to adjust the height of the tamper in tamps of fleeting pressures by means of the mold prior to the start of the compact• a tamping foot to a sample contained in a ing procedure. cylindrical mold. The shape of the tamp• The maximum load on the tamping foot ing foot is approximately that of a seg• remains constant throughout the compac• ment of a circle having the same diam• tion procedure since, as soon as the piston eter as the forming mold; its area is ap• exerts more pressure on the oil than proximately a fourth that of the cross- exists in the compressed air in the tank, sectional area of the mold. oil is squeezed out from under the piston

1 , 1 ^ Dilution of ReGordinir - 1— — — •

: 0.2 0 sec . 1 1 tt .4 0 sec - L J 1 r - i ).2 0 sec . 1

i.'i • i/ r \ [ 3 >0 38: / \ ! \\ \ 4

i: !, • 1 On e (3y c le 2. 00 se c. 1 • 1 •ir 1 iijl 1 1 i'i " 1 1 'li' ill Illjil.' ,i i 1 ml •!l Figure 4. Typical oscillogram showing load vs. time relationship for the Triaxial Institute Kneading Compactor.

A schematic drawing of the mechanical throu^ a one-way check valve back into tamping system, which employs a toggle- the oil reservoir and a pop-off valve, press principle, is shown in Figure 2. which is set at the predetermined pressure, Power for the operation of the toggle- allows excess air to escape. To ensure press mechanism is provided by an elec• that the. full pressure will always be ap• tric motor through a speed reduction gear, plied to the sample, a bypass valve is flywheel, and connecting rod. The action kept open a certain amount during the is such that in any one tamp the pressure entire process. The pressure of the com• is gradually built up and then allowed to pressed air and the setting of the bypass dwell on the sample for a fraction of a valve govern the pressure exerted by the second before being released. The com• tamper on the sample. paction rate is 30 tamps per minute. Before the compactor is used, it must The overall dimensions were selected first be calibrated by measuring the load to provide ample space for compacting exerted by the tamper foot for different specimens up to 6 inches in diameter and air pressures and settings of the bypass 12 inches in height. valve. This is done by inserting a dyna• In order to control the pressure exerted mometer above the tamper foot and obtain- 19

ing oscillograms of the relationships of vertical load on the specimen is applied load versus time. A typical oscillogram at a constant rate of strain (0.05 inches obtained in this way during compaction of per minute) while the pressure is allowed a silty clay is shown in Figure 4. It will to build up in the liquid cell which en• be seen that the pressure-versus-time circles and confines the specimen lat• curves consist of three distinct parts: erally. The lateral pressure, PH, trans• first, the application, second the dwell, mitted through the specimen when the and third, the release. vertical pressure, Py, is 160 psi. is recorded. An indication of the surface roughness of the specimen is then obtained by determining the displacement, D, of the specimen, and the resistance value or stabilometer Rvalue is computed from the PitiM for Bpptjuag /Bad to spoetmoH formula R = 100 - 100

1 +1 •FloolUo Blophroim D , lltolt ooOor omoll The R values determined in this way loitlol prOMtoro have been correlated with the service be• Atlootoblo havior of soils under pavements and have 510,0 been found to be a satisfactory measure of relative stabilities. Tootloo maetiiito A third test which may be used to measure the relative stability of a soil is Matt- Spjcmoa gtvOH lottrol ouppert by flontblt Old* molt mtlietl the triaxial compression test, performed froHsmilM tmaontal pnoourt to litmd Motiiltudo of under constant lateral pressure. Either protsun may bo rood on gaago the ultimate strength of a test specimen or Figure 5. Hveem Stabilometer. the modulus of deformation at a particular strain maybe used as an index of stability. STABILITY In the tests reported in this paper, all three of the above methods were used. The stability of a soil may be broadly The methods are used, however, only defined as the load or pressure which it to compare relative stabilities of samples, can support without excessive deforma• and no attempt is made to compare the tion. Exactly what constitutes excessive values obtained by the various methods. deformation is a matter of opinion. Be• cause of the difficulty of measuring stability in the field a variety of laboratory methods RELATIONSHIP BETWEEN DENSITY for measuring stability have been developed AND STABILITY OF SOILS COMPACTED and correlated with the performance of soils underlying pavements. Thus these BY KNEADING ACTION test methods provide fairly reliable means Since kneading compaction best sim• for measuring the relative stabilities of ulates the effects of compaction equip• soils. ment, it was considered desirable to de• Probably the most widely used index of termine the relationship between density the stability of a subgrade soil is the and stability for samples compacted by California Bearing Ratio which is based on this method. A comprehensive investiga• the resistance to penetration of the soil tion was made using a silty-clay soil by a plunger having a base area of 2 sq. (liquid limit = 37, plastic limit = 23) from in. Vicksburg, Mississippi. A series of In California, the index of stability tests were made to establish the relation• used for design is the resistance value ships between density and water content as measured by a Hveem Stabilometer and between strength or modulus of de• test (10). This is a closed-system, tri- formation and water content for the soil axial-compression test, as shown in compacted at each of six different com• Figure 5, using a specimen 4 inches in pactive efforts. diameter and about 2% inches high. The For tests at any one compactive effort, 20

the soil was first oven dried and samples Tamps were then mixed at about six different Lojftra ! La/er

590 psi water contents. Each sample was placed SSOpt in a sealed contamer and allowed to con• 175 PSI dition for one day prior to compaction. 125 psi The samples were then compacted in 6- inch-diameter molds to form specimens ^/i inches high. The compacted samples were used to determine the relationship of dry density versus water content in the usual manner. After the weight and vol-

Tbaps per -— Foot La/tfs Lofer Pressor* J 25 590 psi s - 3 iS 350psi S es S85PSI «: lie s 5 J* 175 psi 5 £5 US PSI I'"I 3 BS 125 psi 3 . IS 1 SOati

• •

Wattr CeiHtnt — ptreenl

Figure 6. Relationship between water con• tent, dry density, and modulus of deforma- ti.on at 1% strain. \ I - \ S/«| I r

390 PH X 105 nil 115 fiv 1 I im wain Cenlanl- pwretm

Figure 8. Relationship between water con• tent, dry density, and stress at 10 per• cent strain.

ume of a compacted sample had been meas• ured and a sample had been taken for water-content determination, a specimen was cut from the sample having a diam• eter of 1..4 inches and a height of about 4 inches. s no This specimen was subjected to a tri- axial-compression test of the uncon• solidated, undrained type, using a con• fining pressure of 1 kg. per sq. cm. Load was applied at the rate of 8 kg. per min. until failure, and the stress-versus- strain relationship for the specimen was Wttltr Conmt- ptrctnl determined. In this way the water con• Figure 7. Relationship between water con• tent, density, strength, and deforma• tent, dry density, and stress at 20 per• tion characteristics of each compacted cent strain. sample were obtained. 21

The results of these tests are shown in modulus of deformation from 190 to in Figures 6, 7, and 8. Figure 6 shows 560 kg. per sq. cm., but a further in• the density-versus-water-content curves crease from 114 to 120 lb. per cu. ft. and the modulus of deformation at 1 per• caused a reduction in modulus of deforma• cent strain versus water content for each tion from 560 to 175 kg. per sq. cm. The compactive effort. It will be seen that for effect of increased density may thus be each compactive effort the modulus of to increase or reduce the modulus of de• deformation is low on the wet side of opti• formation depending on the range of den• mum but that it begins to increase as the sities concerned. optimum water content is approached and, Conditions under which an increase in for the range of water contents used in density may cause a reduction in strength these tests, continues to increase with or deformation index are associated with decreasing water content on the dry side the characteristic overlapping of the of optimum even though the density of the curves, in Figure 6, showing the rela• samples is decreasing. tionship of this index to the water content

loe I04 we fOB 110 us 114 im iie leo at Off Omltf 'tbptfeuft Figure 9. Relationship between dry density Figure 10. Relationship between dry density and modulus of deformation at 1% strain and stress at 20% strain for constant for constant water contents. water contents.

The significance of these results is of the soil. best seen from the relationship of modulus The effect of dry density and water con• of deformation, density and water con• tent on the strength of the compacted soil tent as shown m Figure 9. These curves, IS shown in Figures 7 and 8. On the dry showing modulus of deformation versus dry side of the optimum water content for density at a series of constant water con• any given compactive effort, specimens tents, were interpolated from the results failed by shearing along a well-defined in Figure 6. It will be seen that the ef• plane. At about the optimum water con• fect of increased density on the modulus tent and at higher water contents, speci• of deformation of the soil depends on both mens bulged considerably but continued to the water content and the range of den• support increasing loads. For such sities considered. At a water content of specimens, failure is considered to occur 13 percent, an increase in dry density when the deformation becomes excessive; from 100 to 110 lb. per cu. ft. caused an it is often defined as the stress required increase in modulus of deformation from to cause say 10 or 20 percent strain of the 190 to 470 kg. per sq. cm.; at a water test specimen. content of 17 percent, however, the same Figures 7 and 10 show the variation of increase in density caused a reduction in strength with dry density and water con• modulus of deformation from 120 to 50 tent when failure is defined as the stress kg. per sq. cm. Thus, a given increase required to cause 20 percent strain. As in density may increase or decrease the for the modulus of deformation data, the modulus of deformation, depending on the curves in Figure 10 have been obtained by water content of the soil. interpolation from the data in Figure 7. Again, at a water content of 13 per• It is readily seen that within the range of cent, an increase in dry density from 100 densities investigated there is a marked to 114 lb. per cu. ft. caused an increase difference between the effect of increasing 22

content. At a water content of 14 percent, an increase in density from 100 to 110 lb. per cu. ft. caused an increase in strength of about 50 percent, while at a water content of 17 percent, the same in• crease in density had practically no effect on the strength. The test results when failure is de• fined as the stress required to cause 10 m'trx percent strain are shown in Figures 8 and 11. For these results, as for the modulus of deformation at 1 percent strain data, Figure 11. Relationship between dry den• there is some overlapping of the curves of sity and stress at 10% strain for constant strength versus water content for constant water contents. compactive efforts, indicating that an in• density on strength and on modulus of crease in dry density may cause a reduc• deformation. Except for the high water tion in strength. In this case there is content of 18 percent, an increase in dry more variation in form of the curves of density in all cases caused an increase strength versus density at constant water in strength; this type of density-stability contents than for the curves in Figure 10. For the range of densities and water con• tents investigated, the strength increases J,ES£m. 1 1 1 I • Sptcimen with >?' ¥alii» '225 consistently with density for water con• — Sp»etmtn w th 7i ¥itue ' 3 tents below 13 percent, but for water con• tents above 14 percent, the strength may I increase or decrease with increasing density depending on the range of densities considered. It will be seen from these results that the effect of density and water content on the stability of the soil depends on the criterion used to define stability. If the ultimate strength or the stress at 20 per• cent strain is used as a criterion of stabil• ity, then it may in general be true that stability increases with density. If the modulus of deformation at 1 percent strain or the strength as defined by the stress required to cause 10 percent strain is used as a criterion of stability, then for partially saturated soils, stability may

40 eo ao MO 110 140 ISO ISO \ Vrtieal Prtssiirt, - psi Figure 12. Typical results of stabilometer tests on specimens of silty clay with high and low stabilities. relationship is associated with cases 30 40 so SB 70 where there is no over lapping of the curves StaHlonMtr V valM of stability versus water content for constant compactive efforts. The mag• Figure 13. Relationship between stabil• nitude of the effect of increasing density, ometer R value and axial strain at which R | however, decreased with increasing water value was determined. 23 increase or decrease with increasing reduction with increase in density; at a density. In tests which are customarily relative compaction of 100 percent based used as criteria of stability, the strain on the standard Proctor test, the modulus of the specimen at which the measurement of deformation and the strength are higher is taken is not always clearly defined. than can be attained at higher values of For example, in the California Bearing relative compaction. Ratio test, the strain of the soil sub• If compaction is controlled by the mod• jected to load at the time the measure• ified AASHO test, the soil might be com• ment is made is probably of the order of pacted at a water content of 14 percent to a 5 percent. In the Hveem Stabilometer relative compaction of 90, 95 or 100 per• test, the strain of the specimen at th6 cent depending on the specifications,that is, time the stability is determined varies to a density between 105 and 117per cu. ft. with the water content and stability of the It will be seen from Figure 9 that at a specimen. water content of 14 percent the maximum Typical results of stabildmeter tests on value of the modulus of deformation is specimens of the silty clay having high and attained at a density of 113 lb. per cu. ft., low stabilities are shown in Figure 12. It that is, a relative compaction of 97 per• will be seen that for the specimen having cent. However, a further increase in a resistance value of 92. 5 the strain at relative compaction to 100 percent causes which this value was determined was 0.9 the modulus of deformation to be reduced percent, while for the specimen having a to only 33 percent of its maximum value. resistance value of 22. 5 the strain was In the case of strength (for 10 percent 3.7 percent. For the Vicksburg silty strain), the strength increases with den• clay, the variation of the strain at which sity up to a relative compaction of 99 per• the resistance value was determined for cent with only a slight reduction occurring specimens with resistance values ranging if the relative compaction is further in• from 7 to 92. 5 is shown in Figure 13; the creased to 100 percent. strains range from 0.9 to 4.2 percent. These results illustrate the deleterious Thus in both the CBR and the Hveem Sta• effects of overcompacting a soil as well as bilometer tests, the stability of the soil is those of using too high a water content. At determined at strains appreciably less than the optimum water content of the standard 10 percent; on the basis of the compression- Proctor compaction test (18 percent), the test data, it would be expected that an in• maximum modulus of deformation at 1 per• crease in density would not necessarily cent strain was only 95 kg. per sq. cm., lead to an increase in stability, therefore, while at the optimum water content of the for soils compacted by kneading action. modified AASHO compaction tests (14 per• In order to obtain some idea of the con• cent), the maximum modulus of deforma• dition to which this soil might be com• tion was 385 kg. per sq. cm. Again, at a pacted in the field, standard Proctor and water content 1 percent above the modi• modified AASHO tests were conducted to fied AASHO optimum, the maximum sta• determine the optimum water contents and bility for a relative compaction of 95 per• maximum dry densities given by these cent was only about 65 percent of that at• compaction procedures. The tests gave tained at the same relative compaction at the following results: the optimum water content. The im• portance of careful water content control Maximum in obtaining the maximum stability of Optimum Dry partially saturated soils cannot be over• Compaction Test Water Content Density emphasized. Standard Proctor IF"^ 105 pcf. Examination of the data in Figures 6, 7, Modified AASHO 14 % 117 pcf. and 8 shows that for each compaction On the basis of the standard Proctor curve there is a marked change in stability test, this soil might be compacted at a at about the optimum water content. At water content of 18 percent to a relative water contents above the optimum the sta• compaction of 100 percent, that is to a dry bility is low, while at water contents below density of 105 lb. per cu. ft. At this the optimum the stability is relatively high. water content, both the modulus of defor• These results would seem to indicate that mation at 1 percent strain and the strength the lower values of stability are associated of the compacted specimens, show a slight with higher degrees of saturation, rather 24

higher wiU be the stability.

USE OF LINE OF OPTIMUMS FOR PRE• DICTING STABILITY CHANGES It has been suggested (7) that the den• sity and water-content conditions at which further increase in density causes a re• duction in stability might be predicted from the position of the line of optimums de• 106 IDS termined by compaction tests. The line Dry D*»utr~ lb pvcutt of optimums is the smooth curve passing Figure 14a. Relationship between dry den• through the peaks of a series of compac• sity and modulus of deformation at 1% strain tion curves and is usually approximately for constant d/egrees of saturation. parallel to the zero air-voids curve. Con• than particular water contents, and that sideration of the fact that the density the degree of saturation may be a more- versus stability relationship is dependent important factor in determining stability on the definition of stability will show that than the water content. Curves of mod• ulus of deformation at 1 percent strain 1 versus density for various constant values s V of the degree of saturation are shown in s i" Figure 14a; these curves have been ob• i V tained by interpolation from the data in < < Figure 6. For a given degree of satura• tion, in all cases the modulus of deforma• y tion increases with density. The same is true of strength as may be seen from I, Figures 14b and 14c. It is interesting to compare the curves in Figure 14a with those showing the relationship between den• sity and modulus of deformation at con• Figure 14c. Relationship between dry den• stant values of water content in Figure 9. sity and stress at 10% strain for constant While the stability of the soil at a given degrees of saturation. water content may increase or decrease any relationship between the line of opti• with an increase in density, it maybe con• mums and the configuration of the curves cluded from Figure 14 that, within the of density versus stability will also de• range of densities and water contents in• pend on the definition of stability, that is, vestigated, for a given degree of satura• on the magnitude of the strain used in the tion, the stability will increase with an determination of stability. It is somewhat increase in density and, for a given density, fortuitous, therefore, if the position of the the lower the degree of saturation the line of optimums, when plotted on the curves of the density versus stability, happens to coincide with the peak points of these curves. ;T This may be seen from the results in Figures 6 to 11. The line of optimums « i/ determined by the compaction curves I (Figures 6, 7, and 8) has been plotted on the curves of density versus modulus of t 4 deformation in Figure 9 and on the curves of density versus strength in Figures 10 and 11. It will be seen that the peak points of the curves of density versus modulus of lOo loe 104 /oe loa no iis lit na ito lit Brf Onutf - /» f»r eti ft deformation occur at densities lower than Figure 14b. Relationship between dry den• those determined by the line of optimums; sity and stress at 20% strain for constant that if strength is determined on the basis degrees of saturation. of 10 percent strain, the line of optimums 25

100 100 a ao

so I I 40

I SO •

104 lOa 112 116 120 124 I2B Dry Density - lb per cu ft ^ >S4

Tomps per Foot 1'^ Laytn Layers Pressure • 5 25 550 116 o 5 25 350 •— 5 25 150 IIZ V

9/ • \ loe

104^ 10 12 14 16 16 20 Water Content-percent

Figure 15. Water content, density, and stability relationships for sandy clay for kneading compaction. corresponds fairly closely with the peak isfactory. A similar approximate corre• points of the density - versus - strength lation should not necessarily be expected, curves; and that if strength is determined however, if other methods of stability de• on the basis of 20 percent strain, the termination are used. peak points of the curves of density versus strength occur, if at all, at densities considerably higjier than those determined EFFECT OF COBflPACTION METHOD ON }y the line of optimums. THE STABILITY VERSUS DENSITY Thus it would appear that if stability RELATIONSHIP s defined as the stress required to cause In order to determine whether the > to 10 percent strain in the triaxial-com- general form of the relationship of sta• >ression tests, the line cf optimums will bility versus density established for the >e a good indicator of the density and Vicksburg sllty clay is applicable to other vater content conditions at which further types of soil and to determine whether the ncrease in density will cause a reduction form of the relationship is affected by the n stability; for other amounts of strain, method of compaction, two series of tests he method would be unsatisfactory. It is were made on a sandy clay from Antioch, nteresting to note that the magnitude of California. he strain for which the line of optimums All samples of soil were mixed and con• ^ves a satisfactory indication of the peak ditioned in the same manner as that used oints on the density versus stability for the sllty clay soil previouslj de• urves is of the same order of magnitude scribed. s that used in the CBR test. It was for In the first series of tests, samples 4 tability determinations made by this test inches in diameter and 4?-/2 inches in height liat the use of the line of optimums for were prepared using the kneading com• idicating peak points of stability was pactor. Each sample was compacted in uggested and found to be relatively sat• five layers using 25 tamps per layer. 26

Samples were prepared using tamping in metal molds with plungers top and bot• pressures of 150, 350, and 550 psi. After tom and the pressure was increased to th3 the density of the compacted soil had been desired value at a loading rate of 600 lb. determined, the upper inches of the per min. After compaction the resistance sample was trimmed off and used for value of each specimen was determined by water content determination while the lower a Hveem Stabilometer test. The results 2y4 inches was subjected to a Hveem are shown in Figure 16. Stabilometer test and a measure of its In this case there is no overlapping of relative stability obtained by determina• the curves for stability versus water con• tion of the resistance value. The results tent for constant compactive efforts, and of these tests are shown in Figure 15i the curves of stability versus density for It will be seen that there is the character• constant water contents show a consistent istic overlapping of the curves of stability increase in stability with increasing den• versus water content for the various com- sity. Thus for the same soil, the general pactive efforts and that the general form form of the relationship of density versus of the curves of stability versus density stability for specimens prepared by static at constant water contents is similar to compaction is quite different from that for that obtained for the silty clay soil. specimens prepared by kneading com• In the second series of tests, saipples paction. Conclusions with regard to the 4 inches in diameter and about 2V4 inches density-stability relationship drawn from high were prepared by subjecting the soil tests on statically compacted soils may to static pressures of 250, 500, and 1,000 therefore be entirely erroneous when ap• psi. These pressures were chosen to give plied to soils compacted by kneading a range of densities approximately the action. same as that obtained inthekneading-com- In the majority of pavement - design paction tests. The samples were compacted procedures, the samples on which stability

100 too

BO W'12% r w=l3X .

0\— \ 104 lOB 112 116 120 124 I2BI

124 Dry Density -lb per cu ft

^ izo 1000psiLEGEND Static Pressure Mm SOOpsi Static Pressure ^- 250psi Static Pressure J) § 112

Q lOB

104 10 12 14 16 IB 20 Water Content-percent

Figure 16. Water content, density, and stability relationships for sandy clay for static compaction. 27

rrsres AS UOLOCO, NO SOMim

fee.

If V \

(a) MOISTURE - DENSITY

MY KNSITT MLB KR CU FT

CBR VS DENSITY WATCH cmrcNT w KR CEHT mr WCMMT FOR EQUAL MOISTURE CONTENTS (bl CBR VS MOISTURE FROM LABORATORY TESTS

Figure 17. Water content, density, and stability relationships for sandy clay for impact compaction (courtesy CR. Foster). tests are made are prepared by impact shown in Figure 17. The general form compaction. The question is thus raised of the curves is similar to that obtained as to how closely this method of compac• for the specimens prepared by kneading tion will reproduce the properties of soils compaction. Although samples prepared compacted in the field. At the present by impact compaction show some diver• time only limited information is available gence from the properties of field-com• on this aspect of soil compaction, due to the pacted soils (3), they have the same gen• difficulty of obtaining data for field-com• eral characteristics as samples pre• pacted soils. However, considerable data pared by kneading compaction and show the same loss of stability, in spite of in• creased density, at the higher degrees of saturation.

DISCUSSION OF RESULTS The primary purpose of this investi• gation was to determine the limitations of the widespread belief that soil stability increases with increasing density. The results- have shown that for soils com• Dry Density — lb percu ft pacted by impact methods or kneading ac• tion, the validity of this concept depends Figure 18. Relationship between density on the criterion of stability adopted; when and stability for saturated samples of larger strains are permissible in defin• clayey sand for kneading compaction. ing a stable soil mass, the concept may be has been obtained by the Corps of Engi• quite true. But for low strains and strains neers on the density-versus-stability re• of the order used in the CBR and Hveem Sta• lationship of soil samples prepared by im• bilometer tests, an increase in density at a pact compaction. Typical results obtained given water content may cause a decrease by the Corps of Engineers in tests of this in stability depending on the range of den• type, in which the California Bearing Ratio sities and the water content of the soil in• is used as an index of soil stability, are volved. For samples of a soil compacted 28

by static forces, however, an increase in such soils may develop extremely high density was always associated with an stabilities if properly protected. However, increase in stability as measured by a the desirable water content and density for Hveem Stabilometer. this purpose would have no particular re• While it is believed that kneading com• lationship to the optimum water content and paction most-closely duplicates the action maximum density determined by a stand• of rollers in the field and that compacted ard conyiaction test. subgrades may become less stable if com• Finally, it would seem desirable to pacted beyond a certain limit, this result mention briefly the difficulty of determin• does not necessarily mean that every effort ing accurately the stability of specimens should not be made to obtain the highest having water contents varying along their possible density. The tests reported in lengths. The significance of this effect this investigation show that for a given was clearly demonstrated in the triaxial degree of saturation, stability always in• compression tests on the silty clay, when creases with increasing density. This on one occasion, the porous base plate of result is apparently equally true for all the specimen was accidentally wetted and degrees of saturation, including the con• after placement of the test specimen, the dition of complete saturation. Typical lower Vio inch or so of the 4-inch-tall results of the relationship between density specimen absorbed this moisture. In• and stability for saturated soil samples stead of the anticipated modulus for 1 are shown in Figure 18. Thus, if a sub- percent strain of about 800 kg. per sq. grade is likely to become saturated at any cm., the test data showed the specimen time in the life of the pavement which it to have a modulus of about 520 kg. per supports, the higher the density to which sq. cm. This low value could not be ac• it can be compacted without subsequent counted for until the water in the bottom swelling, the higher will be the subgrade of the specimen was discovered during the stability for which the pavement can be dismantling of the apparatus. Assuming a designed. Since the large majority of modulus of 40 kg. per sq. cm. for the pavements are designed for the saturated bottom Vio inch of the specimen, it may condition, compaction to the highest prac• readily be shown that although the re• tical density is justified. maining 3. 9 inches of the specimen had a It is only for partially saturated soils modulus of about 800 kg. per sq. cm., that increased density may have a dele• the overall modulus of deformation would terious effect on stability. It is necessary, appear to be about 520 kg. per sq. cm. therefore, to take this effect into con• Such results emphasize the need for care• sideration when the most-economical ful testing techniques and procedures and pavements are to be designed for areas in the obvious difficulty of interpreting the which it is reasonably certain that the stability of nonuniform test specimens. supporting soil will not become saturated. The importance of the relationship be• tween density and stability under those CONCLUSIONS conditions has been recognized for some The main conclusions presented in years by the Corps of Engineers and has the preceding pages may be summarized been incorporated into their pavement- as follows: design method (8). 1. The relationship between density and The density-versus-stability relation• stability of soil depends on the criterion ship is also important in the design of base used to define stability: the greater the courses protected by asphaltic membranes, permissible strain before a sample is such as those used in the construction of considered unstable, the greater is the the fill sections of the Houston Freeway possibility that an increase in density will and in other areas (LL). By this means, cause an increase in stability. changes in water content of a soil may be 2. For samples of two soils, a silty prevented and soils which would have low clay and a sandy clay, compacted by stability -when saturated may be safely used kneading action, an increase in density at for base-course construction. By select• a given water content caused an increase ing the water content at compaction and or a decrease in stability (for strains less the desirable density in accordance with than 10 percent) depending on the water the relationship of density versus stability, content and the range of densities in- 29 volved; however at a constant degree of the higher the density of the subgrade the saturation, an increase in density always greater will be its stability. However for caused an increase in stability. partially saturated subgrades, the de• 3. The relationship between the line sirable density for maximum stability will of optimums determined by compaction depend on the water content of the subgrade tests and the density and water content and too high a density may have a dele• conditions at which further increase in terious effect on stability. density will cause a reduction in stability 7. In designing and constructing pave• will depend on the method used to evaluate ments resting on partially saturated sub- stability. grades, the selection of the desirable 4. Samples prepared by impact com• density of the subgrade should be based paction have the same general character• on the anticipated maximum water con• istics as samples compacted by kneading tent of the subgrade and the density versus action. stability relationship for the soil. 5. Samples of a sandy clay compacted by static pressure always showed an in• crease in stability, as measured by a ACKNOWLEDGMENT Hveem Stabilometer, for an increase in The assistance of F. N. Finn and C. K. density at constant water content, even Chan in performing some of the tests de• though samples compacted by kneading scribed in this paper and of G. Dierking, action showed no consistent relationship. •who prepared the figures, is gratefully 6. For saturated subgrade conditions. acknowledged.

References 1. Proctor, R.R., "Fundamental Prin• Engineering Tests on Steel, Pierced Type, ciples of Soil Compaction," Engineering MB and Aluminum, Pierced Type M9, News-Record. August31, September?, 21, Technical Memorandum 3-324. Vicksburg, 28, 1933. Mississippi: May 1951. 2. McRae, J. L. and Rutledge, P. C., 7. Foster, C.R., Reduction in Soil "Laboratory Kneading of Soil to Simulate Strength with Increase in Density. Pro- Field Compaction," Proceedings, High• ceedings Separate No. 225^ New York: way Research Board, Vol. 31, 1952, pp. American Society of Civil Engineers, 593-600. July 1953. 3. U.S. Waterways Eiqperiment Sta• 8. U.S. Engineer Dept. Airfield Pave• tion. • Soil Compaction Investigation, Tech• ment Design; Flexible Pavements, Engi• nical 13emorancEm~3^^27Tl 1-5. Vicks- neering Manual for Military Construction, burg, Mississippi: April 1949-June 1950. Partxn, Chapter 2, AppendixB. July 1951. 4. U.S. Waterways Ejqperiment Sta• 9. Wilson, S. D., "Small Soil Compac• tion. Investigation of the Design and Con• tion ^paratus Duplicates Field Results trol of Asphalt Paving Mixtures, Tech• Closely," Engineering News-Record, Vol. nical Memorandum 3-254, 3 V. Vicks- 145, No. 18: November, 1950. burg, Mississippi: May 1948. 10. Hveem, F. N. and Carmany, R. M., 5. U.S. Waterways E^eriment Sta• "The Factors Underlying the Rational tion. Investigation of Effects of Traffic Design of Pavements," Proceedings, High• with High-pressure Tires on Asphalt way Research Board, Vol. 28, 1948, pp. Pavements, Technical Memorandum 3- 101-136. 312. Vicksburg, Mississippi: May 1950. 11. "Asphalt Membranes — Their Serv• 6. U. S. Waterways Experiment Sta• ice Record on Gulf Freeway Fills, " Roads tion. Airplane Landing Mat Investigation, and Streets, April, 1953. 30

Discussion ROBERT HORONJEFF, Lecturer and Re• fining stability as the authors have de• search Engineer, Institute of Transporta• fined it. It would be interesting to com• tion and Traffic Ei^neering, University of pare the relationships for 1 percent strain California — Seed and Monismith have with the corresponding relationships with presented an excellent paper on the rela• 5 percent stram. tionships between density, water content, The authors have shown conclusively and stability. Of particular interest to the that generally it is most advantageous from -writer are the relationships between var• a strength standpoint to compact earth ious degrees of saturation on the modulus fills on the dry side of optimum regard• of deformation of a soil as shown in Fig• less of the compactive effort. ures 14a, 14b, and 14c. As the axial Most pavement-design procedures re• strain is increased the modulus of defor• quire that the thickness of the pavement mation is affected less by the degree of structure be predicated on a saturated saturation. For example, for a density of subgrade. Seed and Monismith have 110 lb. per cu. ft. an increase in degree stated that their relationships apply to of saturation from 60 to 85 percent results partially saturated soils; also that the in a reduction in the modulus of deformation Corps of Engineers has given considerable in the amount of 550 kg. per sq. cm. when attention to the relationship of density, the strain is one percent, whereas when water content and stability. The writer the strain is 20 percent the reduction in has taken some data developed by the modulus is only 26 kg. per sq. cm. Corps of Engineers to show the importance It is unfortunate that the authors were of the moulding water content. Figure A not able to include the relationships shown shows the relationships of density and in their paper for a strain of 5 percent. It water content to strength for a silty clay. is the writer's opinion that 5 percent strain The chart on the right-hand side of the represents an upper limit of strain for de• figure shows the relationship between

SAMPLE LESEHO NOTES; SILTY CLAV OL (DSURCHAROESOAKINS AND *PENETRATION lOLB . Rl - II OSS BLOWS PER LAYER (C) ALL SPECIMENS SOAKED LL-S2 a esBLOWS PER LAYER TOP AND BOTTOM FOR 4 DAYS A 10 BLOWS PER LAYER (S)ALL SPECIMENS COMPACTED • WATER CONTENT TOP INCH IN B LAYERS,lOLS HAMMER IB INCH DROP IN CM MOLD. FISURE BESIDE CURVE IS MOULDING WATER CONTENT

' 0 AIR VOIDS S«C.74

olOS

S 9S n 8 10 IE 14 IB IS BO 100 110 ISO WATER CONTENT IN PERCENT DRY WT. MOLDED DRY DENSITY IN LBS PER O.FT MOLDINB WATER CONTENT VS DENSITY BCBJt MOLDED DENSITY VS CBJI. RECOMMENDED PROCEDURE FOR PERFORMINB LABORATORY CB.R TESTS FOR DESISN. Figure A. 31

140 lowed to increase to 16 percent the CBR 0 55 Blow. per layer would be reduced materially. Consider• • S5 Blow.1 per layer t^lO Blow s per layer ing a single wheel load of 50, 000 lb. and a tire pressure of 100 psi., an increase 130 in water content from 11 to 16 percent ax Density means a reduction in CBR from 11 to 2. —• This reduction would require an increase 120 in pavement thickness of over 20 inches. I Silty S ind Figure B shows a similar relationship LLS for a sandy clay. It will be noted that for PI 5 a specified density of 125. 5 lb. per cu. ft. no 10 15 an increase in water content from 8 to 10 Water Content - percent percent reduces the CBR from 50 to 9. The effect of moulding water content BO on the stability of a soil cannot be over• 1 1 1 emphasized. The moulding water content 70 95X MaxDensity^ 1 , affects not only the stability immediately Moil t iASH 0 upon completion of construction of a pave• 60 •igur 95 r*pre. ent '1 ment but also the stability when the sub- ng wi Iter eontents m 50 n .[so 1 « grade soil reaches a nearly saturated (0 condition. 40 1 o 30 1 \ W.H. CAMPEN, Manager, Omaha Test• eo •> ing Laboratories — This paper is not only 10 ) interesting but it also deals with a funda• r mental aspect of soil stabilization. In 105 110 115 120 125 130 fact, the principal purpose of the authors is Dry Density- lb per cu ft to show that excess densification of soils is not only possible but that it may result Figure a in loss of strength. soaked CBR and density for various mould• Before expressing my views on the ing water contents. The chart is similar main subject of the paper, it should be to the relationships presented by the authors mentioned that the data in the paper sub• in Figures 9, 10, and 11, except that the stantiates two well-established properties strength is e^ressed in terms of soaked of soil: (1) density of compacted soils in• CBR. The chart supports the authors creases with the compactive effort and (2) findings that the greater strengths occur strength of compacted soils increases as on the dry side of the optimum water the moisture content decreases with all content. compactive efforts. The writer recognizes that soaking a The data in the paper also show that at soil sample for 4 days does not necessarily a given moisture content the strength in• mean it is saturated, not even in the top creases with density up to a point where inch or so. Nevertheless there are many the compactive effort produces excessive soils which are close to full saturation kneading or manipulation. This compactive within the depths influenced by the CBR effort is somewhat greater than the one test piston after a 4-day soaking period. required to produce optimum moisture Thus one can see that the moudling water equal to the given moisture content. At content has a profound effect on the strength this point an increase in density takes place of a soil in a saturated or nearly saturated but the strength decreases, if strength is condition. From a practical standpoint measured at strains similar to those ob• the importance of controlling the water tained in the CBR test. content in the field is evident. Again re• The increase in density must be due to ferring to Figure A for a specified density the esqpulsion of air or gases. The re• of 114 lb. per cu. ft., a range in water duction in stability is no doubt due to an contents between 11 and 13 percent does increase in the lubricatii^ action of the not affect the CBR values very much. clay portion of the soil. The fact that However, if the water content were al• compaction by static force does not re- 32

veal this behavior substantiates this as• a point where the asphaltic cement be• sumption. comes a lubricant. On the other hand, it While it should be recognized that in has already been pointed out that soils are the laboratory it is possible to reduce not likely to be densified or activated by strength by increasing density, it must be traffic, and even if they are, the loss of pointed out the deleterious effect does not strength is comparatively small. have much practical significance. For In concluding the comments, I wish to instance, the data in Figure 17, obtained emphasize the fact that water is the prin• by the modified AASHO method of compac• cipal foe of stability, and since the amount tion and the CBR method of measuring of water can be limited by density, as much strength, shows that the soil used has a density as possible, within practical maximum density of 119 lb. and an opti• limits, should be developed in the com• mum moisture of 12 percent. This soil paction of soils. might be compacted in the field to a rel• ative density of 95. 5 percent or 114 lb. H.B. SEED and C. L. MONISMITH, Clo• per cu. ft. At this density the soil has an sure — The authors express their appre• optimum moisture of 14.75 percent and a ciation to Horonjeff and Campen for their CBR of 27 percent. At a density of 116 interesting discussions. In closing, it Ibo at the same moisture content the soil would seem desirable to comment on has a CBR of 25 percent. If the soil were Campen's observations that the deleter• compacted to 99. 5 percent or 117. 5 lb. ious effect of too hi^ a density does not per cu. ft., the corresponding optimum have much practical significance and that moisture would be 13 percent. At this it is doubtful if the same thing could hap• water content and density the soil has a pen in the field. Campen reaches these CBR of 79 percent. At a density of 119 conclusions by using data from the curves lb. and the same water content it has a for samples prepared by impact com• CBR value of 77 percent. Thus it will be paction. The stabilities of samples pre• noted that the loss of strength due to over pared by this method are not so greatly densification is comparatively small. affected by changes in density as are Even though it can be shown in the lab• those of samples prepared by kneading oratory that a loss of strength can ac• compaction. (Seed, H.B., Lundgren, R., company an increase density, it is doubtful Chan, C. K., "The Effect of Compaction if the same thing can happen in the field. Method on the Stability and Swell Pres• It may be that compaction by sheepsfoot sure Characteristics of Soils," Proceed• rollers can produce this result, but it is ings of the Highway Research Board, 1954). unlikely that traffic can do so. The rollers The optimum water contents for samples might do it if compaction were done at compacted to a given density by field equip• constant moisture content by a series of ment are somewhat greater than those de• rollers capable of applying pressure in termined by impact methods, and an anal• ascending order. On the other hand, traf• ysis based on compaction and stability data fic could hardly do it for the reason that for samples prepared by kneading com• it caimot expel air or apply a kneading ac• paction would show that density changes of tion. These two reactions can hardly take the order considered by Campen can have place, since the subgrade soils are usually a substantial effect on stability values. covered with a considerable thickness of That this effect can be of practical sig• superimposed layers. nificance and that the same effect can occur In connection with this discussion, it in the field is illustrated by a recent paper would seem appropriate to mention loss of (Foster, C. R., "Reduction in Soil Strei^h stability in bituminous mixtures, because with Increase in Density," Proceedings it is often confused with similar occurrence Separate No. 228, New York: American in soils. Traffic can and often does reduce Society of Civil Engineers, July 1953). the stability of bituminous mixtures to a Results obtained by the Corps of Engi• large degree. That is because the traffic neers and presented in this paper show acts directly on the mixtures, which are that a large change in stability occurred usually used as wearing surfaces, and thus in the field as a result

H. B. SEED, Assistant Professor of Civil Engineering, Institute of Transportation and Traffic Engineering, University of California, RAYMOND LUNDGREN, Partner, Woodward, Clyde and Associates, Oakland, California, and CLARENCE K. CHAN, Graduate Research Engineer, Institute of Transportation and Traffic Engineering, University of California

Test data are presented for two soils, a silty clay and a sandy cl^, comparing the stabilities at various water contents and densities of partially saturated sam• ples compacted by Impact methods, static pressure, and kneading action; the stabilities of samples prepared by these three methods and soaked to near sat• uration at constant density are also compared. A possible explanation for the different effects of the compaction methods is suggested and some of the diffi• culties of preparing saturated samples in the laboratory are discussed. Data are alsopresented comparing the deformation characteristics Intrlaxlal- compression tests of silty-clay specimens prepared by impact, static, and kneading compaction in the laboratory with those of the same soil compacted by sheepsfoot and rubber-tired rollers in the field. A comparison is also made of the stability and swell pressures developed at various densities for samples of a sandy clay compacted by kneading and static methods and subsequently saturated by exudation of moisture under static load; the great difference in test results obtained is illustrated.

# THE object of a laboratory compaction having the same water content and density test is to reproduce in the laboratory the but prepared by impact and static compac• compaction effects produced by equipment tion have different stress-strain character• in the field. Not only should laboratory- istics (1). Recent investigations have compacted samples exhibit the same re• showntHistobe true also for samples pre• lationship of density versus water content pared by kneadii^ and static compaction (2). as the soil compacted in the field, but since It becomes important, therefore, in order the laboratory samples are used for design to satisfactorily design a pavement on the purposes, they should also possess the basis of laboratory tests, to know which same deformation characteristics under method of compaction reproduces most load. At the present time, three main closely the effects of field equipment and methods of compaction are in use for the the magnitude of the differences instability preparation of samples in the laboratory. of a sample resulting from the use of dif• In the majority of tests, the soil is com• ferent compaction methods. pacted by dropping aweight onto the surface Relatively little information is available of the soil, a process referred to as Impact on the extent to which the stabilities of lab• compaction. In some cases the soil is com• oratory compacted samples compare with pacted by subjecting it to a static load which those of samples compacted in the field. is built up slowly to some predetermined Some tests conducted by the Corps of Engi• value and then released, aprocess referred neers have shown that samples of a silty to as static compaction. In other methods, clay taken from the field have different the soil is compacted by repeatedly ap• stress-strain characteristics from those plying a predetermined pressure to small prepared in the laboratory by impact and areas of the soil, maintaining the pressure static compaction; additional data are pre• for a small element of time and then grad• sented in this p^er to compare these re• ually reducing the pressure, a process sults with those for the same soil compacted termed kneading compaction. by kneading action. However, the main It has long been recognized that samples purpose of the investigations described is

33 34

to illustrate the extent to which different density, water content, and stability. For methods of laboratory compaction affect each soil and for each method of covapac- the stability of soils and to draw tentative tion, series of tests were made to establish conclusions with regard to the qualitative the relationships between dry density and nature of these effects. water content and between stability and In the pavement-design procedure used water content at each of three different by the California Division of Highways, the compactive efforts. For each soil, the expansion or swell pressure developed by compactive efforts were selected to give a soil is used to determine the desirable results over ^proximately the same rai^e pavement thickness. The effect of com- of densities and water contents for each of p action method on the swell-p ressur e char• the three methods of compaction; this range acteristics of soils, as measured by the was ^proximately between the densities California design procedure, is therefore obtained in the standard Proctor and the also important and has been included within modified AASHO compaction tests. the scope of the investigation. For tests at any one compactive effort. lOO

98 lOZ 106 no 114 IIB Dry Density — lb per cu ft

Tamps Foot Layers per Layer Pressure 5 25 400psi o• 5 25 ISOpsi

•— 3 25 40pst

9 II 13 15 17 19 21 Water Content- percent

Figure 1. Water content, density, and" stability relationships of silty clay for kneading compaction. EFFECT OF COMPACTION METHOD the soil was first oven-dried and samples ON THE STABILITY OF PARTIALLY were then mixed at about six different water SATURATED SOILS contents. Each sample was placed in a sealed container and allowed to condition Comprehensive series of tests were for one day prior to compaction. Speci• made on two soils, a silty clay from Vicks• mens 4 inches in diameter and A% inches burg, Mississippi, and a sandy clay from in height were then prepared using the se• Pittsburg, California, to determine the lected method of compaction and compactive effect of impact, static, and kneadii^ com• effort. After the density of the compacted paction on the relationship between dry soil had been determined, the upper 2% 35 inches of the specimen was trimmed off and the two soils are reproduced in Figures 4 usedfor water-content determination while and 8. Figure 4a compares the stabilities the lower inches was tested in a Hveem at various water contents and densities, Stabilometer and a measure of its stability for specimens of the silty clay compacted

100

I—°— %so 5 Jk 60

B — \ \40\ \ \ '15% W'I7X

I > 98 102 106 no 114 IIB 122 Dry Density - lb per cu ft v" s

LEGEND • ^ Blows Weight Drop Layers per Layer Hammer in inches B— 5 25 lOlb IB • 3 25 lOlb 12 4> °— 3 25 5 5 lb 12 • 3 25 5 Sib 8 I

II 13 15 17 19 21 Water Content— percent Figure 2. Water content, density, and stability relationships of silty clay for impact compaction. Obtained by determination of the resistance by kneading and impact methods. It will or R value (^). The resistance value is be seen that, in general, the curves for used as an index of stability in the California these two methods of compaction are simi• method and has been correlated with the lar in form but that kneading compaction required thickness of pavement for various produces slightly higher stabilities at the types of loading conditions. lower densities and impact compaction The results of these tests on the silty results in somewhat higher stabilities at clay, for kneading, impact and static com- the higher densities. The higher densities pactlonprocedured respectively, are shown on the curves in this type of plot are asso• in Figures 1, 2, and 3. Similar results for ciated with the higher degrees of saturation; the sandy clay are shown in F^ures 5, 6, thus at higher degrees of saturation impact and 7. On the left of each of these figures compactionproducesthe higher stabilities, the test data is presented and on the while at lower degrees of saturation, knead• right is shown the relationship between ing compactionproduces higher stabilities. density and stability at various constant Comparison of the densities at which impact values of the water content; this latter re• compaction begins to give higher stabilities lationship was in all cases interpolated with the position of the line of optimums from the test results shown on the left of for the compaction curves in Figures 1 and the figure. 2 will show that it is at water contents just For purposes of comparison, the re• below and above the optimum water content lationship of density versus stability for for the particular compactive effort being I00\

5 eo

so 5! I 40

S 20

M 98 102 106 no 114 lie 122 Dry Density — lb per cu ft

LEGEND

900psi Static Pressure 200psi Static Pressure 90psi Static Pressure

9 II 13 15 17 19 21 Water Content - percent

Figure 3. Water content, density, and stability relationships of silty clay for static compaction. used that impact compaction causes higher these two methods. stability than kneading compaction; at water It is interesting to note that both kneading contents well below optimum, kneading and impact compaction result in samples compaction results in the higher stabilities. which, at lower degrees of saturation, show However, at no stage is there any great an increase in stability with increase in difference between the results obtained by density at a given water content but at the

LEGEND LESEND Kneading Compaction Kneading Comppchon Impact Compaction Static Compaction

wist

wist

wist •~»-.l7tI »>/* » wist irx 94 SB lOe 106 no 114 lis lli 101 loe no 114 Dry Density - lb per cu ft Dry Density - lb per cu ft

Figure 4a. Effect of kneading and impact Figure 4b. Effect of kneading and static compaction on density versus stability compaction on density versus stability relationship at constant water contents relationship at constant water contents for for silty clay. silty clay. 37

100 water Content. '-H*

"fist

lie 114 lie lie no izz Dry Density - lb per cu ft

S lie Tamps Foot Layers per Layer pressure • 5 25 AOOpsi o 5 25 SOOpsi • 9 25 200psi

10 12 14 16 Water Content- percent Figure 5. Water content, density, and stability relationships of sandy clay for kneading compaction. higher degrees of saturation show a re• pronounced for specimens prepared by duction in stability with increase in density kneading compaction than for those pre• at constant water content. This reduction pared by impact compaction. in stability with Increase in density Is more The stabilities at various water contents

100, waterContent,

W'l3t

w=l4%

wiex

no 112 114 116 lie 120 122 Dry Density - lb per cu ft

LEGEND Blows Weight Drop per Layer Hammer in inches 25 lOlb 18 25 lOlb IS 25 lOlb 12

10 12 14 Water Content - percent Figure 6. Water content, density, and stability relationships of sandy clay for impact compaction. 38

and densities of samples of silty clay pre• is increased at the higher degrees of sat• pared by kneading and static compaction uration. This difference results in a con• are compared in Figure 4b. The most- siderable discrepancy between the stabil• important difference in the results for these ities of samples prepared by static and compaction methods is that samples pre• kneading methods. For example, atawater pared by static compaction always show an content of 15 percent and a density of 116 increase in stability with an increase in lb. per cu. ft. the resistance value of a density at any water content, while for sample prepared by kneading compaction is samples prepared by kneading compaction only 22, while that for a sample prepared the stability is reduced if, at water contents by static compaction is 56, an increase of greater than about 12 percent, the density approximately 150 percent. In terms of

wmer'CmtSiSjES.

'•

w=l5X eo WI6X

112 114 lie IIS 120 122 Dry Density-lb per cu ft

LEGEND

IZOOpsi Static Pressure 114 600psi Static Pressure <0 300psi Static Pressure

I 110

10 12 14 16 20 Wafer Content -percent

Figure 7. Water content, density, and stability relationships of sandy clay for static compaction.

mater C m'llX — wl4X — •"•——• ^ ^'I3X WISX- - 1'I3X wISX \ IT' 14% wISX \'I4X W'I4X I •lex H •ISX ex 1 LEGEND Kneading compaction Kneading compaction Static compaction 114 lie lie I20 122 110 112 114 116 IIB 120 122 124 Dry Density — lb per cu ft Dry Density - lb per cu ft Figure 8a. Effect of kneading and impact Figure Bb. Effect of kneading and static compaction on density versus stability re• compaction on density versus stability re• lationship at constant water contents for lationship at constant water contents for sandy clay. sandy clay. 39 pavement thickness for a typical flexible impact compaction at the higher degrees pavement designed by the California pro• of saturation. For example, at a water cedure for a highway with heavy traffic, a content of 15 percent and a dry density of resistance value of 22 would indicate a 118, the resistance value of a sample com• required thickness of pavement and base pacted by static pressure was 75, com• of about 18 inches, while a resistance value pared with a resistance value of 27 for a of 56 would indicate a required thickness sample prepared by kneading compaction. of only about 9 inches. If kneading com• From the results presented in Figures paction most satisfactorily reproduces the 4 and 8 for the two soils investigated, the effects of field compaction, the dangers of following general conclusions may be designing a pavement for a partially sat• drawn: urated subgrade condition on the basis of 1. At a given density and water content, tests on samples prepared by static com• samples compacted by static pressure have paction are immediately evident. higher stabilities than samples prepared by At lower degrees of saturation, the kneading or impact compaction; this is stabilities of samples of the silty clay pre• particularly true at higher degrees of sat• pared by static and kneading compaction uration when there is a large difference ^pear to be almost identical. Thus, at between the stabilities of samples prepared equal water contents and densities, the by static and impact or static and kneading stabilities of samples prepared by static compaction. compaction were always equal to or greater 2. At any given density and water con• than those of samples prepared by kneading tent, samples prepared by impact and compaction. Comparison of the curves in kneading compaction have similar sta• Figures 4 a and 4b show this to be true also bilities, with kneading compaction resulting for static and impact compaction. in somewhat higher stabilities for samples The stabilities of samples of sandy clay compacted on the dry side of the optimum prepared by kneading and impact com• water content and impact compaction pro• paction are compared in Figure 8a. As for ducing higher stabilities for samples com• the silty clay, kneading compaction gives pacted at water contents above the optimum slightly higher stabilities at lower degrees for the particular compactive effort being of saturation, and impact compaction gives used. slightly higher stabilities at the higher 3. For samples prepared by impact or degrees of saturation; for this soil it is kneading compaction, an increase in density approximately for samples compacted on may cause an increase or decrease in sta• the dry side of the optimum water content bility depending on the water content and for the particular compactive effort being the range of densities involved; for samples used that kneading compaction gives the prepared by static compaction, an increase higher stabilities and for samples on the in density always results in an increase in wet side of optimum that impact compaction stability. gives the higher stabilities. However, both methods of compaction again show that at higher degrees of saturation, an increase in EFFECT OF TAMPING PRESSURE ON density may lead to a decrease in stability. STABILITY OF PARTIALLY SATURATED SOILS Comparison of the stabilities resulting from kneading and static compaction of the In the tests on samples of silty clay sandy clay in Figure 8b, shows that, for prepared by kneading compaction, the dif• any given water content and density condi• ferent compactive efforts were obtainedby tion within the range investigated, static varying the tamping pressure from 40 to compaction gives the higher stability. 400 psi. It is of interest to determine how, Furthermore, in contrast to the results for for any given water content, the stability kneading and impact compaction, the sam• of a sample will vary with the tamping pres• ples prepared by static compaction show a sure used to compact it. Such results may consistent increase in stability with in• be interpolated from the data in Figure 1, crease in density, even at higher degrees and the variation of the resistance values of saturation. As a consequence of this, of samples with the tamping pressure used there are again large differences in sta• in the compaction tests, for a series of bility between samples prepared by static constant water contents, are shown in and kneading compaction or by static and Figure 9. The relationships of water con- 40

122 too OOpsi Kn toding Pn ssure Note Data taken from Fig. I content w'lIZ IIB /iHodifi ed / ' AASh0 /

114 fiSOpsi Kneading I frt •ssure •\ \ I'" /stand ird ^ / f AAS 10 . 14% Note D tto token 40psi Ki leading from Fii IS 1 and 2 Pres wre 102 10 12 14 16 It 20 too ZOO 300 400 500 Water Content- percent Kneading Pressure-psi Figure 9. Density versus water content and stability versus kneading-pressure relationships for silty clay. tent versus density resulting from these lower stability than would be obtained for tamping pressures are shown on the left of any tamping pressure between 40 and 400 Figure 9, together with the curves obtained psi. It is interesting to note that, for com• by the standard A A SHO and modified A ASHO paction at this water content, the maximum compaction tests for comparison. stability would be obtained by using a tamp• It will be seen that an increase in tamping ing pressure of about 175 psi. which, ac• pressure may lead to an increase or de• cording to the positions of the compaction crease in stability of the compacted soil curves shown in Figure 9, would produce depending on the water content at which the a relative compaction of about 95 percent. soil IS compacted. The optimum water However, an increase in tamping pressure content as determined by the modified from 175 to 400 psL , would reduce the AASHO compaction test for this soil is about resulting stability of the soil by over 50 13 percent. A tamping pressure of 400 psi. percent. results in densities comparable to those For compaction at the optimum water obtained by the modified AASHO test; yet, content as determined by the standard if this pressure is used at a water content AASHO compaction test, the stability de• of 13 percent, the compacted soil has a creases as the tamping pressure increases

100 odified A, \SHO Water >rfl2X X A

1SI \*? 119/3OOP fPressure f Kneadi ng X Pressure

> /200pi ^Kneading 'ressure W'I4X 112 "I5X ' / w=l6%

108 Xstaiidard X AASHO Note Dal 3 taken Nate. Data taken from Fig 5and6 from Fig 5 104 12 14 16 18 20 200 300 400 Water Content - percent Kneading Pressure-psi

Figure 10. Density versus water content and stability versus kneading-pressure relationships for sandy clay. 41

compaction may have on the resulting sta• bility of a partially saturated soil are

o-ioa] clearly evidentfromthe curves in Figure 9. Kubber-tired For this silty clay at water contents more than 1 percent below the optimum for the modified AASHO compaction test, an in• i 104 o Rubber-tired Sheepsfoot crease in tamping pressure at least up to ill. • Sheepsfoot • f —~~—^ 400 psL had a beneficial effect on stability; ^sneepsroor at water contents more than 1 percent above 17 IB 19 this optimum, an increase intampingpres- Water Content —percent sure above 40 psL had a deleterious effect Figure 11. Water content versus density on stability. Near the optimum water con• relationship for field-compacted samples of tent for the modified AASHO test, the most- silty clay. desirable tamping pressure decreased as

• As xl mpoeted 122 • After sookint

Am —

\ » I 114 110 to 12 16 IB 20 20 40 Water Content-percent Stabilometer '/lvalue Figure 12a. Density versus water content and density versus sta• bility of samples soaked at constant density for static compaction. from 40 to 150 psi., but beyond this point, the water content increased. Unfortunately, further increase in tamping pressure has the tamping pressures used in the labora• no effect on stability; presumably the ulti• tory tests have not been correlated with mate bearing capacity of the soil is reached those producing similar degrees of com• at a pressure of about 150 psi. and tamping paction in the field, but the nature of the pressures exceeding this value cause shear effects produced by field equq>ment will failure and no change in condition of the be similar to those obtained in the labor• soil. atory. The significant effects which the water The relationships between stability and content and the kneading pressure used for tamping pressure at various constant values TABLE 1- SUMMARY OF TEST RESULTS

Water Content - percent 15.4 15.7 17 7 19.0 20. 0 Dry Density - lb per cu ft 103.7 101 7 106.8 101.0 102 0

Modulus of Deformation at 1% Strain

Field Compaction - sheepsfoot roller 320 130 120 Field Compaction - rubber-tired roUe: 400 180 Lab. Compaction - Impact - Corps of .ng. 330 330 120 120 45 Lab. Compaction - Impact - Univ. of Calif. 290 250 155 120 90 Lab. Compaction - Kneading - Univ. of Calif, 350 300 195 130 90 Lab Compaction - Static - Corps of Eng. 330 320 225 160 - Percent Difference between Moduli of Deformation for Field and Laboratory Compaction

Laboratory Impact Compaction -22.5 -9.5 -23.5 -7.5 -44.0 Laboratory Kneading Compaction -12.5 -6.5 +8.5 0 -25.0 Laboratory Static Compaction -17.5 0 +25.0 +23.0 - 42

of water content for the sandy clay are shown of silty clay compacted by sheepsfoot and in Figure 10. The general form of these rubber-tired rollers in the field were com• curves is similar to that for the silty clay, pared with those of samples, at the same though the effect of tamping pressure on densities and water contents, prepared by stability is not so great. Furthermore, at static and impact compaction in the labor• the optimum water content for the modified atory. Through the courtesy of the Water• AASHO compaction test, an increase in ways Ejqjeriment Station at Vicksburg in tamping pressure above 200 psi. caused a supplying some of the soil used in the tests, slight reduction in stability. The effect of this comparison has been extended to in• water content at compaction, for any given clude samples prepared by kneading com• tamping pressure again has an important paction. effect on stability. For a tamping pressure In the field the soil was compacted in of 300 psi., compaction at the optimum 6-inch lifts by six passes of either a sheeps• water content for the standard AASHO test foot roller with a foot pressure of 500 psL (17 percent) would produce a sample with or a rubber-tired roller with a wheel load a resistance value of only 21, compared of 20,000 lb. Undisturbed samples were with a value of about 80 for compaction at cut from a depth of 12 to 21 inches below the optimum water content for the modified the top of the compacted soil and trimmed AASHO test (12 percent). to 1.4-lnch-diameter specimens for test-

124 124 • Aseoi tpaeted • After tooliittg

2 ~ 120 ^- St I lie IIS 1 112 I 112

XIkP * lOB 108^ 10 12 14 IS le 20 O 20 40 60 80 Water Content — percent Stabilotneter "W Value

Figure 12b. Density versus water content and density versus sta• bility of samples soaked at constant density for impact compaction. These results emphasize the importance ing. The water contents and densitiesof of careful control of construction condi• five samples taken from the field are shown tions and the possible dangers of the arbi• in Figure 11, and their moduli of deforma• trary selection of these conditions on the tion at 1 percent strain, as measured by basis of standard compaction tests, when triaxial-compression tests using a constant placing soils for maximum stability under lateral pressure of 1 kg. per sq. cm., are conditions where the water content is un• presented in Table 1. likely to change appreciably from that at In the tests conducted by the Corps of which the soil is compacted. Engineers, the greatest discrepancy be• tween the stress-deformation character• istics of samples prepared in the labora• COMPARISON OF STABILITIES PRO• tory by impact and static compaction was DUCED BY FIELD AND LABORATORY found to occur at low strains. In this in• COMPACTION PROCEDURES vestigation, the modulus of deformation at In a previous investigation conducted by 1 percent strain was therefore selected as the Corps of Engineers (1^), the strength the basis for comparison of the effects of and deformation characteristics, as meas• different compaction methods. ured by triaxial-compression tests of the In Table 1 are summarized the moduli unconsolidated, undrainedtype, onsanqiles of deformation at 1 percent strain of sam- 43

124 124 O As ct impacted • After soaHing \<5 IZO \ X \^ 5> I 116

112 ^

loa 10 12 14 16 IB 20 20 40 60 BO Water Content-percent Stabilometer'fTValue Figure 12c. Density versus water-content and density versus stability of samples soaked at constant density for kneading compaction. pies at the same water contents and den• field compacted samples, are summarized sities as those taken from the field and at the foot of Table 1. For samples pre• tested in the same manner but prepared in pared by impact compaction, the average the laboratory by impact, kneading, and of the values obtained by the Corps of Engi• static compaction. In order to ensure that neers and the University of California have the testing procedure used for the samples been used. It will be seen that, in general, prepared by kneading compaction was the the moduli of deformation of the field com• same as that used by the Corps of Engineers, pacted samples agree more closely with test data was al3o obtainedf or samples pre• those of samples prepared in the labora• pared by impact compaction. It will be seen tory by kneading compaction than with those that there is reasonably good agreement be• of samples prepared by impact or static tween the results of tests on samples pre• compaction. pared by impact compaction conducted in The relative values of the moduli ob• the different laboratories. tained by the various laboratory compac• To facilitate comparison of the test re• tion methods seem to be in general agree• sults, the differences of the moduli of de• ment with the results obtained in the tests formation of laboratory compacted samples previously described. The moduli for from those of the field compacted samples, kneading compaction are consistently e^qpressed in percent of the moduli of the slightly higher than the moduli for impact

124 124

8 120

I 116

I 112

lOB 10 12 14 16 IB 20 20 40 60 ao Water Content-percent stabilometer 'R'Value Figure 12d. Density versus water-content and density versus sta• bility of samples prepared by kneading compaction and saturated by exudation. 44

f " of specimens 4 inches in diameter and 4% 1Soakeil at com tant de isity / Saturot ion by t 'xudatii inches high were prepared in metal molds by impact compaction at water contents both / / / above and below the optimum. The density / was determined and the upper 2^4 inches / / of each specimen was then trimmed off and Kneadi ng Comp action-^ f usedfor water-content determination while 1 116 r the remainder of the specimen was main• / I / tained at constant density by confining it between two rigid porous plates and sub• ~Jmpaa Compoi "tion 1 / jected to a water pressure of about 10 psi. / atic Cot npaction I 1 atone of its ends. The water pressure was /— 1 maintained until free water was seen to ac• 1 1 i cumulate at the opposite end of the speci• 1 ~M men, atwhich stage the specimen was con• ' // sidered to be saturated; the time required for this to occur was about 7 or 8 weeks. ZO 30 40 SO 60 When all of the specimens in the series Stabilometer 'R" Value were saturated in this way, the water pres• Figure 13. Density versus stability of sure was removed, the stabilities of the samples soaked at constant density and specimens were measured by Hveem Sta• samples saturated by exudation.' bilometer tests, andthe densities and water contents were determined. Similar series compaction; and although the moduli for of tests were performed on samples pre• static compaction are approximately equal pared by kneading and static compaction, to those for kneading compaction at the the compactive efforts being selected to lower degrees of saturation, they are give samples in the three series with ap• appreciably higher than those for kneading proximately similar density ranges. compaction at the higher degrees of satu• ration. The results of these tests are shown in In general it may be concluded from Figures 12a, 12b, and 12c. At the left of these tests that static compaction appears each figure is shown the density and water to give the highest modulus values; impact content of the sample as it was prepared compaction, the lowest values; and field and kneading compaction give intermediate values. While the results are by no means conclusive, they appear to substantiate previous indications that laboratory knead• ing compaction offers the greatest possi• bility for satisfactory duplication of field compaction effects. Proving

EFFECT OF COMPACTION METHOD ON THE STABILITY OF SAMPLES SATURATED BY SOAKING In the large majority of cases, pavement designs are based on the assumption that the compacted subgrade will become sat• Adjustable urated at some stage in the life of the pave• Base ment which it supports. Thus, the design thickness is usually determined by the sta• bility of a sample compacted in the labor• atory and subsequently saturated by soaking at approximately constant density. The effect of compaction method on the stability of samples treated in this manner was investigated by three series of tests on a sandy clay. In the first series, a number Figure 14. Expansion pressure device. 45 and after saturation, while on the right is so shown the relationship between the resist• • ance value and the dry density of the sat• 40 urated specimens. It will be seen that only :o^ Stan e Camp in isolated samples was a condition ap• 30 • proaching complete saturation achieved, < though the degree of saturation usually 30 • Ik exceeded 95 percent. In general the average •S - Kiiea ding Co npactit 1 degree of saturation for samples prepared 10 by impact compaction was slightly lower than that for samples prepared by static or 0 kneading compaction. /// 113 IIS 117 119 121 123 125 Dry Density- lb per cu ft The relationships of density versus sta• bility after saturation for samples prepared by impact, static, and kneading compaction are compared in Figure 13. At equal den• 0 • / sities, samples prepared by static com• Sta, le Comf action • paction had the highest stabilities, and samples prepared by kneading compaction I 0 i had the lowest stabilities. However, the stabilities for samples prepared by impact and kneading compaction did not differ greatly, and this difference might have been • due to the slightly higher average degree of

/ft eading tompaex

109 III 113 IIS 117 119 121 123 I2S Dry Density — lb per cu ft Sfafie Compoctton Figure 16. Density versus stability and density versus swell pressure of samples of sandy clay prepared by static and kneading compaction and saturated by exudation. saturation for the samples prepared by kneading compaction.Sthe average degrees of saturation for samples prepared by im• pact and kneading compaction had been the 109 III 113 IIS 117 same, the samples prepared by impact Dry Density - lb per cu ft compaction might have had the lower sta• bilities. The stabilities, as measured by the resistance values of samples prepared y by static compaction, were from 10 to 25 11 percent greater than those for samples pre• I / 1 Slat e Compi CtlOli — pared by kneading compaction. These re• / 1 sults are in general agreement with the 1 effects of compaction method on the stabil• ities of partially saturated soils. In the Califomiadesign procedure, sam• ples are prepared by kneading compaction ^^^^^ Compot //Off—^ at water contents on the wet side of optimum

107 109 III 113 IIS 117 and are then saturated by applying static Dry Density - lb per cu ft pressure until moisture is exuded. It was considered of interest to compare the sta• Figure 15. Density versus stability and bilities of samples prepared by this method density versus swell pressure of samples with those obtained by the method of sat• of silty clay prepared by static and uration previously described. The results kneading compaction and saturated by exudation. of tests conducted in accordance with the 46

California design procedure are presented of the pressure, the sample was allowed to i in Figure 12d, and the density-versus- stand for % hour with the ends of the mold stability relationship is shown in Figure 13. covered. A perforated metal disc with a Itwillbeseen that, for equal densities, the vertical stem was then placed on top of the stabilities of these samples are consider• sample, and the mold was fitted in the swell ably lower than those prepared by kneading pressure device. In this position the lower compaction and saturated by soaking. While end of the sample rested on the adjustable this difference may be due partly to the in• base of the device, and the base was ad• fluence of the molding water contents on the justed until the stem of the perforated plate stabilities of samples even after saturation, on top of the sample was just tight against it may also be due to an increase in strength the proving bar. Water was then poured on due to electrochemical action during the top of the sample, and the subsequent de• 8-week period of soaking the samples in flection of the proving bar over a period of metal molds. If this should be the case, several days was measured by a dial gauge. the influence of this effect during long The proving bars are relatively stiff, a periods of soaking is clearly of considerable pressure of 1 psL exerted by the sample importance and indicates the need for causing a deflection of only 0.003 inch; special provisions for the saturation of thus, only a slight expansion of the soil is samples after compaction. permitted during the swell-pressure meas• urements. After the maximum swell pres• sure developed by each sample had been EFFECT OF COMPACTION METHOD ON measured, the water was poured off the THE STABILITY AND SWELL PRES• samples, the dimensions and weight of SURES OF SAMPLES SATURATED BY each sample were determined, and the EXUDATION stability was measured by a Hveem Stabil- ometer test. Finally the water contents of Since the stabilities of samples saturated the samples were determined. by long periods of soaking were consider• ably higher than those of samples saturated The entire process was repeated for by exudation and were apparently affected samples at the same water contents, but by the test procedure, a further investiga• compacted by static pressure, implied at tion was made to compare the stabilities of the rate of 600 lb. per min., until moisture samples of two soils, a silty clay and a exuded from the base of the mold. The sandy clay, prepared by static and kneading swell pressures and stabilities of these compaction and then saturated by exudation samples were determined as before. ] of moisture under static load. Samples The results of these tests on the silty prepared In this way by kneading compac• clay are shown in Figure 15. It will be { tion are used for pavement design by the seen that for a density of about 107 lb. per California Division of Highways, the sam• cu. ft. , which is the maximum density as ples being used to determine the stability determined by the standard AASHO com- | of the soil and, also, the swell pressure paction test for this soil, the resistance i exerted by the soil when it is confined in a values and swell pressures of samples pre• mold and immersed in water; swell pres• pared by static and kneading compaction sures are measured in the standard device are about the same, but at higher densities shown in Figure 14. samples prepared by static compaction have The test procedure for each soil was the higher resistance values and swell pres• essentially that used by Calif ornia and may sures. At a dry density of 117 lb. per cu. be outlined as follows: ft., which is the maximum density as de• Four or five samples at different water termined by the modified AASHO compac• contents on the wet side of optimum were tion test for this soil, the resistance value prepared in 4-inch-diameter metal molds of a sample prepared by static compaction by means of the Triaxial Institute Kneading is about 200 percent greater than that of a Compactor, using 125 tamps and a tamping sample prepared by kneading compaction; pressure of 350 psi. Each sample was then and the swell pressure of a sample pre• subjected to a static pressure applied at the pared by static compaction is about 800 per• rate of 600 lb. per min. until moisture was cent greater than that of a sample prepared seen to be exuding from the base of the by kneading compaction. mold; at this stage the sample height was The results of the tests on the sandy between T^U and 2% inches. After release clay are shown in Figure 16. As for the 47

silty clay, at equal densities, samples to accomplish this by forcing water through prepared by static compaction have higher the samples. Even after a number of weeks resistance values and swell pressures than the samples were not completely saturated samples prepared by kneading compaction. and the properties of the soil seemed to be For the sandy clay, the maximum density affected in some way, possibly by electro• in the standard AASHO compaction test was chemical action resulting from the use of 110 lb. per cu. ft., and the maximum den• metal molds. This effect mi^ht have been sity in the modified AASHO compaction test eliminated by the use of plastic equipment, was 123 lb. per cu. ft. At a density of yet even if this were done, a waiting period 110 lb. per cu. ft., the resistance value of of perhaps two months before a saturated a sample prepared by static compaction is sample could be obtained would be an un• about 30 percent greater than that of a sam• desirable situation for a testing laboratory ple prepared by kneading compaction, and processing a large number of samples. the swell pressure is about 400 percent Yet another method of saturation might greater; at a density of 123 lb. percu. ft,, be to soak a sample without necessarily the resistance value of a sample prepared preventing expansion. However, this pro• by static compaction is also about 30 percent cess would also take considerable time and greater than that of a sample prepared by would probably lead to a nonuniform den• kneading compaction, while the swell pres• sity and water content in the sample and sure is probably about 100 percent greater. to a higher degree of saturation at the ends If, as the previous results would indi• than in the center section. cate, kneadmg compaction duplicates more The preparation of completely saturated closely the effects of field compaction samples of clayey soils in the laboratory equipment than static compaction, the er• which will have the same properties as roneous values for desirable pavement similar samples saturated in the field pre• thickness which would be obtained if designs sents a number of problems, and it would were based on the results of tests on sam• seem that no simple method of accomplish• ples prepared by static compaction are ing this has yet been developed. It may immediately apparent. well be that the methods of saturation pres• ently being used in various test procedures are entirely adequate for all practical pur• PREPARATION OF SATURATED poses yet this cannot be ascertained until SAMPLES reliable test results for fully saturated In the tests described above it has been soils can be obtained. It is regretted that shown that the compaction of a soil to a no simple solution can be offered in this saturated condition by the application of paper, but it is hoped that this brief dis• static pressure causes the stability and cussion may stimulate further attention to swell pressure of the soil to be higher than this problem. those for a similar sample prepared by kneading compaction and then saturated by the application of static pressure. If static CONCLUSIONS pressure causes this difference in meas• In methods of pavement design based on ured properties, the question is raised as the results of tests performed on samples to the extent to which it affects the proper• prepared in the laboratory, it is desirable ties of the samples prepared by kneading that the test samples should have the same compaction. It would seem likely that the properties as those of the soil compacted stabilities and swell pressures of samples in the field. The method of compacting a prepared by kneading compaction and then soil sample in the laboratory has a signi• saturated by the application of pressure ficant effect on the resulting properties of until moisture is exuded would be some• the soil. The main conclusions, with regard what higher than for samples of the same to the effect of compaction method on soil density, prepared by kneading compaction properties, resulting from the investiga• and then saturated without the application tions described in this paper may be sum• of static pressure. marized as follows: However, the complete saturation of 1. For the two soils investigated, a samples without the application of pressure silty clay and a sandy clay, samples com• is not an easy task. In one of the test series pacted to equal densities and water contents previously described an attempt was made by kneading and impact methods do not show 48 any large difference in stability as meas• rubber-tired rollers are in better agree• ured by a Hveem Stabilometer test. At ment with those of samples at equal den• lower degrees of saturation, samples pre• sities and water contents prepared in the pared by kneading compaction have higher laboratory by kneading compaction than stabilities and at higher degrees of satura• with those of similar samples prepared by tion, samples prepared by impact compac• impact and static compaction. tion have higher stabilities. 4. Samples of the two soils investigated 2. At lower degrees of saturation, sam• prepared by static compaction and saturat• ples of the two soils investigated prepared ed by soaking at constant density have con• by static compaction have somewhat higher siderably higher stabilities than samples stabilities, in stabilometer tests, than of equal densities prepared by kneading and samples compacted by kneading and impact impact compaction and saturated by soaking methods to the same densities and water at constant densitv. contents; at higher degrees of saturation, samples prepared by static compaction have 5. For the two soils investigated, sam• much-higher stabilities than similar sam• ples compacted by static pressure until ples prepared by impact and kneading com• moisture is exuded have much-higher sta• paction. bilities and swell pressures, as measured 3. The moduli of deformation at 1 per• by the test procedure of the California Di• cent strain, as measured in triaxial-com- vision of Highways, than samples of the pression tests, for samples of a silty clay same density prepared by kneading com• compacted in the field by sheepsfoot and paction and then saturated by exudation.

References

1. U. S. Waterways Experiment Sta• Stability of Subgrade Soils. Paper prepared tion, Compaction Studies on Silty Clay; for presentation at the annual meeting of the Soil Compaction Investigation, Report No. Highway Research Board, January 11-15, 2, Technical Memorandum No. 3-271: 2. 1954, Washington, D. C., Berkeley, Vicksburg, Mississippi: July 1949. 49 pp. California. : University of California, 2. Seed, H. B. and Monismith, C. L. Institute of Transportation and Traffic Some Relationships Between Densitv and Engineering, 1953. 12 pp. 13 Figs. New Method for Measuring In-Place Density of Soils and Granular Materials

CARL E. MINOR, Materials and Research Engineer, and HERBERT W. HUMPHRES, Senior Materials Engineer; Washington State Highway Commission

Conventional methods of measuring the in-place density of soils and granular materials are time-consuming and oftentimes subject to considerable error. Presently used methods, such as measuring volumes by use of oil or calibrated sand, have one or more of the following shortcomings: (1) suitable only for ap• plication to a small range of hole sizes, (2) subject to considerable error when surface of ground is, rough, (3) accuracy affected by vibration or temperature, (4) cannot readily check densities of successive lifts by merely extending depth of same hole, and (5) require lengthy calculations and correction factors for final answer. In an attempt to overcome these deficiences, the Washington Densometer has been developed. The basic principle of the instrument is the same as that used in some other methods, namely that of inflating a rubber balloon with fluid until it fills the excavated hole and measuring the volume of the hole by measuring the amount of fluid so required. However, to the authors' knowledge, the apparatus is unique in the method of application of this principle. The device uses a closed system with a cylinder and piston to activate inflation and deflation of the balloon. This permits extremely rapid operation, plus the direct reading of volumes in cubic feet from the calibrated piston rod. By the inclusion of known-volume rings, rapid and accurate measurements of holes varying from 0.000 cu. ft. up to 0. 500 cu. ft. can be made.

# THE need for a fast, accurate means of the data serving primarily as a record to determining the field density of in-place serve as a guide in future work. Often, the granular base course and surfacing mate• end result is a considerable waste of rials, as well as of subgrade soils, has rolling time for compaction equipment, or become critical in recent years in the high• in some cases, the need to disrupt the way-construction field. Widespread ap• progress of the job because of the nec• plication of moisture-density specifications essity of reprocessing a completed section. to insure uniformity of roadbed construc• Other phases of highway work have also tion requires that field methods be suf• been curtailed by the madequacies of ficiently rapid that control data can be presently accepted standard field density made available to the engineer and the test methods. In the field of research, contractor during the compaction process this department has begun a long-range and without causing delay to the progress program of investigation of base-course of the job. and surfacing materials. A considerable This is particularly critical in projects number of pavement failures can be ex• utilizing equipment-train methods of con• plained only in terms of failure of the struction, such as soil-cement or cement- granular surfacing materials and base stabilized-base projects, where each phase courses. To determine what correction of operation is integrated in time and steps are required, it is necessary first order with other operations. The rapid to determine whether the deficiencies are progress of such projects does not al• in the materials themselves, or a result low sufficient time for determining field of the manner of placement or alteration densities during the compaction phase of physical characteristic due to traffic. using presently accepted methods. This Extensive field studies will be required to has resulted in actually running "control" find the correct answers, and one phase tests after a section of road is completed; of these field investigations involves de-

49 50

Head (cylinder assembly lot shown)

:—Balloon

' •'emplote "7777777 7777mTTTfm, 7777777777777. Bolloon being lower• Condition of balloon During inflation. Completely inflated. ed into rings for in• with apparatus clamp Note how balloon Record initial reoding itial reading. Approx• ed in place prior to fills from bottom up. (IR). Deflate balloon imately I titer of fluid inflation. and remove Densom- in balloon. eter from rings.

(Remove rings, dig hole, and leave out ring or combination of rings whose volume most nearly equals the estimated hole volume. Sum of volumes of rings left out= ring constant.

577777

Balloon being lowered Condition of balloon During inflation, fill• Hole ana head com• into hole. Rings not on prior to inflation. Fluid ing fronn bottom all• pletely filled. Record template. in balloon causes bottom ows balloon to fill final reading (FR). of hole to be filled with• voids displacing air up• out trapping air. wardly and out vents.

D. Calculate volume: (FR + ring constant) - IR = volume of hole.

Note: The sketches above show a hole volume of approximotely 0.2 cu. ft. and all rings were omitted for the final reading. For smaller holes,one or more of the rings would be used during the final reading. The purpose of the rings is to cause the balloon to be inflated to approximately the some degree for both the initial and final readings. Figure 1. Sequence illustrating characteristics of densometer during operation. termination of field densities both during is calibrated; (3) do not provide for initial the construction period and after various surface reading, which Introduces appre• periods of traffic. The large volume of ciable error when surface of ground is such data required dictates the need of rough; (4) accuracy seriously affected by field equipment that is both accurate and outside influences such as vibration, hu• rapid. Presently accepted methods are midity, temperature; (5) will not permit not suitable for such studies for the rea• checking densities of successive lifts by sons stated below, and this is one of the merely extending the depth of the same hole; primary reasons the Washington Densom• (6) require frequent recalibrationto insure eter was developed. accuracy; (7) require a level surface for successful operation; (8) not suitable for use m clean granular materials; (9) dan• STUDY OF EXISTING METHODS ger exists of trapping air in bottom of In 1952 this department reviewed the holes dug in fine-grained soils; and (10) several existing methods generally ac• require lengthy calculations and correc• tion factors for final answer, which in• cepted by various agencies for determining creases the possibilities for errors. field densities, and found that each method possesses one or more of the following shortcomings: (1) required too much time DISCUSSION OF for operation, exclusive of time required WASHINGTON DENSOMETER to dig hole; (2) suitable only for applica• tion to a small range of hole sizes close With these deficiencies in mind, de• to the size of hole for which the apparatus velopment of the Washington Densometer 51

was started. Successive improvements method of measuring hole volumes: in design resulted in a device that has 1. Operation time is sufficiently short none of the siiortcomings listed above. that "wet densities" can be determined The schematic drawing shown in Figure in from 10 to 20 minutes depending OB the A illustrates the basic features of the ap• time required to dig the hole. Of the paratus. The principle utilized is the total time consumed, only about 3 min• same as that used in some other methods; utes are required for setting up the den- namely, that of inflating a rubber balloon someter and making the necessary read• with fluid until it fills the excavated hole ings. and measuring the amount of fluid so re• 2. An initial reading that will account quired to determine the volume of the for variations in roughness of the original hole. However, to the writers' knowledge, ground surface at the hole site is taken, the apparatus is unique in the method of which increases the accuracy of the de• application of this principle. The use of a termination. closed system with a cylinder and piston 3. Volumes of holes from 0.000 cu. activating the inflation and deflation of the ft. to 0. 250 cu. ft. can be measured with balloon permits extremely rapid opera• equal accuracy. (Actually, this volume tion and the direct reading of volumes in can be doubled by recharging the cylinder cubic feet from the calibrated piston rod. during the operation). The large capacity The inclusion of known-volume rings per• of the device makes it applicable lor de• mits the use of a large-size balloon for termining densities in coarse, granular both small and large holes without loss materials as well as in fine-grained soils. of accuracy. The purpose and use of 4. The balloon membrane is very thin, these rings are illustrated in Figure 1. and the size of the balloon is such that it In summary, the following features fills the excavated hole without being and advantages are found inthedensometer stretched appreciably. These features.

Figure 2. Gjmplete fi^ld kit for Washington Densometer; (left) including tools and equipment for field maintenance and operation; (right) loaded for transport in trunk of passenger car. 52 coupled with the fact that the balloon is one man can carry it with no difficulty lowered into the hole in such a manner that (see Fig. 2). the hole is filled from the bottom up, in• sure that air is not trapped and that prac• RESULTS OF EXPERIMENTS tically all pits and voids in the hole walls are filled. To verify the accuracy of the densometer

Figure 3. Lowering balloon into rings and template (left) and tak• ing initial readings (right). 5. Successive determinations on any and to determine its adaptability to actual number of thin layers can be made in the field conditions, a series of both labora• same hole. Holes up to 18 inches in depth tory and field tests were performed. A have been tested; however, the anticipated complete review of the tests including all maximum depth of hole is about 22 inches. pertinent data is appended to this paper. 6. Results obtained are of equal or greater accuracy than those obtainable by LABORATORY EXPERIMENTS presently accepted standard field methods. 7. Once calibrated, no further cali• 1. A series of readings was made on bration is necessary. Little field mainte• the same hole site by two different op• nance is required, except for periodic erators. It was found that the apparatus bleeding off air and occasional replacement exhibits a definite stopping point when the of the balloon. These procedures have piston is depressed; that an operator will been simplified to a point where very little repeat his determinations accurate to less operational time is lost. than 0. 0002 cu. ft.; and that two different 8. Readings are made directly in cubic operators can "repeat each other's deter• feet, which greatly simplified calculations. minations with equal accuracy. 9. The apparatus is of such size that 2. A container whose volume was ac• it can be carried in the trunkof apas- curately determined by precise water-fil• senger car, and is sufficiently light that ling methods was checked with thedensom- 53 eter. The volume determined agreed comparative results were obtained. The exactly with the true volume on two trials, densometer consistently gave slightly and varied 0.0001 cu. ft. on one trial. Dif• larger hole volumes for the same hole, ferent operators were used for the third and the difference increased as the rough• trial. This accuracy greatly exceeds that ness of the hole surface increased. This required for field density tests. indicates that the densometer more-com-

Figure 4. Balloon, flexible suction tube and tapered sleeve, carrying base, head, lock ring, wrench. Upper right shows balloon slipped over suction tube and sleeve and being placed in head. Lower left shows head clamped to carrying base and lock ring being tightened to seal balloon in place. Lower right shows cyl• inder in position on head and being tightened with special spanner wrenches.

3. A series of parallel tests was run pletely fills the voids than does the sand. with the sand-volume method and the Densometer. It was found that if extreme FIELD EXPERIMENTS care was used in calibrating and operating the sand-volume apparatus, and if an 1. The densometer was used for field initial ground-surface reading was made control of compaction of cement-treated with the sand-volume apparatus, close and granular base materials on five dif- 54

P 8H No. Cont. Ho. 4^30 section ^nr^arLe./ /Jn/i y^/Ve^

Inspector A^u^^Jir^ - (SaJAeU-

Test Hole No. 7 Station 72.3 + OO Reference to ^ Reference to Grade r..T£ DENSITT BETBRMIKATIOn Can No. / Densometer Final Beading "A" 0 17Jk Ring Constant "C" O.ITOO Sum of Final Reading plus "C" g 3i.i2. Densometer Initial Reading "B" n.l933 Vol. of Hole-Cu.Ft.(A+C-B; Vt. Wet Soil from Hole + Can Wt. of Can J.19 Wt. Wet Soil - lbs. lg.3g Wet Density - Ibs./ou.ft. Dry Density - Ibs./cu.ft. )3¥.-i Max.Denslty - Ibs./cu.ft. I3(, 1 5b of Max.Denalty-lbs./eu.ft. MOISTURE DEOERMINATION Wt. of DaDQ) Soil + Tare Wt. of Dry Soil + Tate Wt. of Moisture nJ} Wt. of Tare 9.IS- Wt. of Dry Soil 2..10 % Moisture i.Z

Wet Density = Wt. of Wet Soli Remarks; //o/^t £ '''of-e.-e^ J> / n r-ci^^i/'- Vol. of Hole

Dry Density = Wet Density i+i Moisture 100

^ Max.Denslty=Dry Density x 100 Max. Density it Moisture = Wt. of Moisture x 100 Wt. of Dry Soil

1 Gram = 0.0022 lbs.

1 lb. = lt53.6 grams Figure 5. Suggested form with sample data for field density test report. ferent projects. On one project parallel ometer while completing one with the determinations were made with the sand- sand. The trials included the time re• volume method. On this project results quired to clean the hole and refill it with of the two methods agreed closely (10.0004 fresh cement-aggregate mix and the time cu. ft. for holes approx. 0.10 cu. ft. in required to calculate results. volume) except occasionally when con• An alcohol-burning method of drying struction equipment working nearby vi• the samples was correlated with oven- brated the ground while running the sand- drying, and was used on the grade to de• volume determination. Differences then termine moisture content in conjunction increased to a maximum of 0. 0015 cu. ft. with the densometer. With two men and Intermittent checks made on the other using this procedure, it required approx• projects gave similar results. Speed imately 10 to 15 minutes to determine the trials using the two methods showed that wet density, and an additional 5 minutes it was possible to complete two deter• to determine the dry density. At no time minations of wet density using the dens• was it necessary to delay the operations of 55 the contractors for density information. the sand-volume method gave good results It was consistently possible to check at least and verified the accuracy of the densometer. one density, and sometimes two or three densities in each 400-foot to 800-foot section of roadway before the rollers were needed on the next section. It was found SUMMARY possible to complete a wet-density deter• In actual field use, the Washington mination for each pass of a roller when Densometer has proven itself to be con• the sections were over 600 feet long without siderably faster, equally or more ac• interfering with the roller operation. curate, and extremely more versatile On one of the above projects, it was than other existingacceptedtypes of equip• necessary for one man to work alone tem• ment designed for measuring in-place porarily. Using the densometer and the hole volumes in soils. alcohol-burning method of drying, he was The accuracy, versatility, and speed able to make six complete dry density de• attainable with this equipment have greatly terminations in 3 hours. These tests were increased the practical degree of actual made in an 8-inch lift of 1 inch of clean control possible on controlled-compaction crushed gravel. On several occasions on projects. The range of material types this project, tests were made on succes• for which control can be considered prac• sive 4-inch layers in the same hole with tical has been broadened considerably, and excellent results. the effectiveness of various types of rol• 2. The densometer has been used to lers on thin or thick lifts of soil can now check the densities of crushed stone sur• be readily investigated. facing, top course. Three densities were Maintenance requirements of the equip• taken at each of several sites at different ment during use has proven to be prac• offsets from centerline. The alcohol- tically negligible. The balloon mortality burning method was used for determining rate is lower than anticipated, with re• moisture content. Moves at about 3, 000 placements being required only after the feet were made between test sites. It was equipment has been stored unused for possible to make up to 30 complete de• considerable time. terminations per 8-hour day under these Reception by field inspectors, resident conditions. Results were very consistent engineers, and contractors has been and logical. Occasional comparisons with favorable.

Appendix A

DESIGN AND OPERATION PROCEDURE

The schematic sketches and section- or by turning the rod with a wrench, and plan drawings (Figs. A-D) show the phys• will then work freely and smoothly. ical arrangement and dimensions of the The balloonused was selected primarily apparatus. It is designed to handle hole for its size, which allows digging holes of volumes up to approximately ^a. CU. ft. about 9 inches in diameter, a desirable without recharging the cylinder, or up to size for granular materials. It has a approximately % cu. ft. or more with re• thin wall, and a high stretch ratio, which charging. Tests to date have been con• allows it to conform to small irregularities fined to cu. ft. and less, so no state• in the hole surface. Being a standard ment of satisfactory performance at vol• weather balloon, it is commercially avail• umes above that can be made at this time. able at low cost. The piston is designed to move freely Soluble oil, the type used by machinists in the tube, and yet give a sufficiently to make cooling water, is added to the tight seal to allow the fluid to be sucked up• water used in the apparatus to avoid rust ward into the tube without loss of seal. The and corrosion, and to lubricate the cylinder leather seals were selected after trying walls. Other fluids, such as hydraulic several materials and have worked very brake fluid, could be used, but the water- well. They tend to freeze temporarily oil solution is ine:q>ensive and satisfactory. on setting unused for long periods, but The detachable handle is calibrated can be freed by light tapping on the handle. in divisions of 0.001 cu. ft., and readings 56

nsTON ASSEMBLY -cap a Rod Guld*

BKedsr vom nog

-Cylinder HB-Tdpcrtd SIttv* lof l/2«1 i.r«idi

I /-TrOvstStop

B-BI«dv Note The exact spoong " ViMPliK lor the groduolions is cdcokited from Iht moon insAdalMolthcpir- twultf section of lulhng used tor the c>hnder

HOII section

Coliholld Ringi

Figure A. Schematic assembly sketch. Figure C. accurate to 0.0002 cu. ft. are made by apparatus between readings during op• interpolation between divisions. With the eration (see Fig. 2). handle removed, the Densometer will fit The calibrated rings are machined as easily into the back seat or trunk of a car. closely as possible to volumes of 0.0500 The carrying base is clamped to the head cu. ft., and 0.1000 cu. ft. ,the actual vol• to protect the balloon during transporta• umes, both singly and in various combina• tion, and also serves as a stand for the tions, being established by careful check with water or other precise methods. Once CYLINDER ASSEMBLY SPECIAL WRENCH Spomr Tjpd- 2 riq. ^A» vtnt- drdi a top for 1/8 n p calibrated, these volumes are established

1*1, a as constants for the apparatus and so marked on the respective rings. Change of the ring volumes due to normal tempera-

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Bolhion not shoMi hi drraifeig ire a top for 3/4 in nippio Figure D. Head assembly,side view of half Figure B. section. 57

ture fluctuations are not of sufficient mag• 9. Inflate balloon in carrying case nitude to Introduce appreciable error in several times by opening main valve and final results. However, if the apparatus operating the piston to insure removal of is to be used in areas where extreme all air from balloon. After last run, close temperature changes are common, it would main valve and exert a strong upward pull be good practice to establish a volume- on piston to de-air the fluid in the cylinder. temperature calibration curve for the de• 10. Remove bleeder vent plug and vice. It has been established that tem• presspiston down until fluid raises to top of perature fluctuations of less than 30 F. vent. Replace plug. The apparatus is now I have no noticeable effect on final results. de-aired and ready for use. (This step Further investigative work of temperature should be repeated whenever Densometer , fluctuations is contemplated. has not been In use for some time). Although the above assembly operation OPERATING PROCEDURE appears quite involved, it is quite simple in practice, and can be completed in ap• A. Assembly and filling with fluid. proximately 10 minutes. (Necessary occasionally when balloon is replaced). B. Field Density Determination: 1. Remove head from cylinder at 1. After selecting site, smooth joint below main valve and remove tapered ground sufficiently so that template sets balloon sleeve and flexible suction tube solidly. Do not attempt to level surface (Fig. 4). that will lie within center area of tem• 2. Slip neck of balloon over tube and plate. Place rings on template. tapered balloon sleeve until flare of balloon 2. Place Densometer on template and neckis flush with bottom,of tapered sleeve. rings, and clamp in position (Fig. 3). With 3. Trim off neck of balloon at or two men standing on template for rigidity, subtly below groove near top of balloon open main valve and apply gentle pressure sleeve. Lower deflated balloon through to calibrated rod. Sufficient pressure opening in head until the tapered sleeve is should be used to cause piston to lower at firmly seated. a constant, medium rate. When the bal• , 4. With head clamped to carrying loon fills the rings and head, a definite base, place tapered sleeve lock ring in shock will be felt. Maintaining slight position and tighten, forcing tapered sleeve pressure on handle, note reading on rod into final seated position. at top of cylinder cap. Repeat three times ' 5. Place head gasket in head coupling by lifting and depressing piston about 10 I and connect the upper cylinder to the head. inches to ascertain that no air is trapped 6. With densometer clamped to and apparatus is functioning properly. carrying base, and piston removed, open This will be shown by the consistency of main valve and fill with fluid until about 6 the readings. (No trouble has been ex- inches of fluid raises in cylinder. Rock periencedf rom this source to date). Record gently to facilitate passage of air bubbles reading as "Initial Reading". Note: One from balloon and head upward into cylinder. man can operate Densometer if heavy bag of dirt is used to balance load on 7. Insert piston, bleeder vent open, template. and push down until fluid raises in vent hole. Close piston bleeder vent with 3. Evacuate fluid from balloon by calibrated rod and raise piston, sucking lifting piston, close main valve, remove fluid from balloon into cylinder. Leaving Densometer, and set on carrying base. approximately 1, 500 cc. of fluid in bal• 4. Remove calibrated rings from loon, close main valve. template, and, being careful not to move 8. Remove piston. (If piston is too template, dig hole as for other methods. low m cylinder to allow removal by open• Save all material in a sealed can to avoid ing piston vent, open drain valve at bot• loss of moisture. The excavated hole should tom of cylinder while pulling up piston). be as smooth as possible, avoiding small Adjust fluid level to within about 3 inches radius holes or extreme undercutting of of top and replace piston. Cover top of the template. piston with enough fluid to cover the leather 5. Upon completion of hole, leave seals, install bleeder vent plug and out whatever number of calibrated rings cylinder cap. •are estimated to most nearly equal the 58 volume of the hole, and clamp Densom• to template. Open main valve and allow eter to template. (Care should be taken fluid in cylinder to pass into balloon until to lower balloon into hole in such a position piston is just short of extreme lower that it will not be twisted when the Den• position. Close main valve and record someter is seated.) Repeat Step 2, and reading as F. R, No. 1. record reading as "Final Reading". Re• 6. Open drain valve to allow air into peat Step 3. cylinder, and raise piston to extreme top 6. Add volumes of rings left out in position. Close drain valve, remove Step 5, and record as "Ring Constant". handle, bleeder vent plug, and cylinder Determine volume of hole by following cap, and remove piston from cylinder. formula: 7. Fill cylinder with additional fluid, replace piston, de-air and reassemble (Final reading + Ring Constant) - Initial cap, plug and handle. Record reading as Reading = Vol. of hole in cu. ft. I.R. No, 2. 7. Weigh wet soil taken from hole 8. Open main valve and complete and calculate wet density: filling of holes as in step B-2 above. Re• cord final reading as F. R. No. 2. 9. Fluid is evacuated as in step B-3 above, except that excess fluid is eliminated throu^ the drain valve into a 8. Determine moisture content by container. accepted means and calculate dry density. 10. Calculate hole volume as follows: C. Determination of volume of holes (See Fig. 5). larger than 0.230 cu. ft. (F.R. No. 1 - I.R. No. 1) + (F.R. No. 2 - For such holes, it is necessary to re• I. R. No. 2) + 0.2000 = Vol. in cu. ft. charge the piston to furnish sufficient fluid to fill the hole. The following procedure Note: Minor modification of the apparatus may be used: to include a filling tube connected to the 1. See B-1 above. bottom of the cylinder and rising along the 2. See B-2 above. side will permit recharging the cylinder 3. See B-3 above. without removing the piston. Details of 4. See B-4 above. this modification are nowbeing worked out 5. Upon completion of hole, leave and will be published as an addendum to out all of the rings and clamp Densometer this report when completely tested. Appendix B

RESULTS OF EXPERIMENTS

1. Check for consistency of readings. Bled air before start of second series.

Initial readings taken with all three Series No. 2 (Ring No. 2 removed for final rings in place. Final reading taken after reading) removal of specified ring. All readings I. R. 0.1970 0.1969 0.1969 0.1970 taken at same position on smooth, flat F.R. 0.1475 0.1475 0.1474 0.1474 concrete slab. Vol. 0.0495 0.0494 0. 0495 0.0496 cq. ft.

Note: Volumes indicated are slightly smaller than true volume of ring re• moved because balloon is not inflated to same degree for both readings. Series No. 3 (After setting in sun 1 hr., temp. 86 F.; Ring No. 2 re• moved for final reading) Series No. 1 (Ring No. 1 removed for final reading) I.R. 0.1954 0.1953 0.1953 0.1953 0.1458 0.1458 0.1458 0.1458 I. R. 0.1862 0.1860 0.1862 0.1861 F.R. 0.0495 0. 0495 0.0495 cu. ft. F.R. 0.1366 0.1364 0.1365 0.1365 Vol. 0.0496 Vol. 0. ()496 0. 0496 0. 0497 0.0496 CU. ft. Bled air before start of series 4. 59

Series No. 4 (Ring No. 3 removed for final bottom joint was used, and its volume reading) determined by the water method. Initial readings were taken on a smooth, flat I. R. 0.2233 0. 2233 0. 2233 0. 2233 concrete surface with the densometer, F.R. 0.1238 0.1238 0.1238 0.1238 and then the template was set on the con• VOL 0.0995 0.0995 0.0995 0.0995 cu. ft. tainer, rings No. 2 and 3 removed, and the final reading made after filling the Series No. 5 (Rings No. 1 and 2 removed container. Cylinder was bled between each for final reading) trial to give different set of readings. Es• LR. 0. 2244 0. 2244 0.2244 0. 2244 tablished volume of container = 0.1675 cu. F. R. 0.1254 0.1254 0.1254 0.1254 ft. VoL 0. 0990 0.0990 0. 0990 0. 0990 cu. ft, Triall Trial 2 Trials F.R. 0. 2127 0.2175 0. 2230 CONCLUSION: Apparatus exhibits a very Ring Constant 0.1500 0.1500 0.1500 definite stopping point which will repeat Sum 0.3627 0.3675 0.3730 itself consistently. Variations of 0.0002 I. R. 0.1952 0. 2001 .2055 cu. ft. are possible in making individual Vol. 0.1675 0.1674 6.1675 readings due to human variations in inter• CONCLUSION: The device will accurately polating the scale. When initially filled determine the volume of a hole. Operators with water, air should be bled frequently, were changed for each of the above trials, as a small amount of air will separate and part of the fluid was bled out of the each time fluid is pulled into piston. This cylinder to avoid repetition of readings. effect becomes practically negligible after The trials show that excellent results are several runs. Normal temperature changes obtainable independent of the operators. do not appreciably affect the accuracy of the apparatus. 4. Comparison to sand cone method 2. Calibration of rings: Rings were (California large sand-volume apparatus) calibrated by a careful, exact water- in actual field conditions: In these tests, volume method. One ring was sealed to it was found imperative to use extreme a rubber base, leveled, and filled with care in making the sand cone determina• water, using a glass plate to establish when tions to get consistent results. The cone full. The other rings were then added, was calibrated by the water method, and the joints being sealed with tape, and their the sand was calibrated frequently during volumes determined successively, the the tests. To eliminate error due to un• procedure was then reversed to establish even ground surface, initial readings were the volume of the bottom ring. Three made, using a template and a thin rubber checks were made, and temperature cor• membrane (dental dam) so the sand could rections were included. The following volumes were determined: be recovered and weights were determined accurately to 0.01 lb. Parallel deter• Ring No. 1 = 0. 0498 cu. ft. minations were made on the same hole Ring No. 2 = 0.0500 cu. ft. with the sand cone and the Densometer. Ring No. 3 = 0. 0998 cu. ft. The following results are typical of this series of tests: By volume of rings in pairs, it was Vol. (SandCo'ne^ established that each joint increased the 2 3 4 volume 0.0002 cu. ft., and: I ,0906 1120 0.0878 0.1057 cu ft, Rings No. 1 and 2 = 0.1000 cu. ft. Vol. (DensometerT Rings No. 2 and 3 = 0.1500 cu. ft. 12 3 4 Rings No. 1 and 3 = 0.1498 cu. ft. .0907 0.1124 0.0880 0.1058 cu. ft Rings No. 1, 2, and 3 = 0. 2000 cu. ft. Note: The Densometer consistently gave For practical purposes, the constants slightly large hole volumes for the same used were established as 0. 0500 cu. ft. hole, and the difference increased as the for each small ring, and 0.1000 cu. ft. roughness of the hole surface increasjes. for the large ring. This indicates that the Densometer fills the small pits and irregularities more 3. Check of Known Volume: A cylin• completely than does the sand. drical metal container with a rounded Occasionally it was not possible to avoid 60 having construction equipment working content, and it was possible for a two-man nearby. On these occasions, the volume crew to make as many as 30 complete dry determined by the sand cone was con• density determinations in an 8-hour day. sistently larger than that determined by Three tests were made at each location, the Densometer. The degree of error and moves of about 1, 000 feet were made caused by vibration of the sand is indicated between locations. by the following typical results: On one project, density determinations were made in an eight-inch lift of clean 1 2 3 crushed gravel base course material. One Vol. man working alone was able to complete 0.1192 0.1289 0.1692 cu.ft. six dry density determinations in three (Sand Cone) hours. Vol. 0.1180 0.1277 0.1674 cu. ft. 6. Measuring Large Holes: Two holes were dug 17 Inches and 18 inches in depth (Densometer) 0. 0012 0.0012 0. 0018 cu. ft. respectively. The volume of each was Error checked with the Densometer and by the 5. Time trials: Actual operation of sand-volume method using the large Cali• the Densometer including set-up time, fornia sand-volume apparatus. Careful but excluding hole digging time is about 3 calibration of the sand apparatus was minutes. Under actual field conditions, made for each determination. Initial where it was necessary to clean up the readings were taken with both types of hole and re-fill with compacted soil, ap• equipment. proximately two complete determinations Hole #1 Hole #2 could be made with the Densometer while (17" deep) (18" deep) making one with sand-volume apparatus. Densometer 0.4853 cu. ft. 0. 5081 cu. ft. Including average hole digging time, Sand Cone 0.4830 cu. ft. 0. 5060 cu. ft. wet density determinations require from Difference 0.0023 cu. ft. 0. 00^1 cu. ft. 10 minutes to 15 minutes under actual field conditions. The density of 2-Inch to 4- Results show good agreement and dif• inch layers of crushed stone surfacing ference would affect a normal density was determined on several projects during less than 1 lb. /cu. ft. The slightly larger construction. The alcohol-burning meth• volume shown by the Densometer indicates od was used to determine the moisture more complete filling of the voids. Discussion ALFRED W. MANER, Highway Research had occasion, in Virginia, to determine Engineer, Virginia Council of Highway the relative densities of two sections of .Investigation and Research — The authors waterbound macadam compacted by two are to be congratulated for developing a different methods. Taking a cue from piece of equipment that permits rapid and North Carolina we used a heavy, 15-lnch- accurate determination of field densities. diameter ring as a guide in digging the With the Washington Densometer they hole, a 100-lb. bag of Ottawa sand for seem to have overcome to a great extent measuring the volume, large buckets and the deficiencies of the more -common platform scales for weighing, in addition methods of measuring field densities. to other necessary equipment. We would Of prime Importance is the reduction have welcomed a device like the densom• of time for operation. Undoubtedly we eter at that time. shall someday have equipment that will In using the densometer for determin• measure moisture and density in a very ing the density of different layers in the few minutes without disturbing the soil or same hole, it seems that extreme care other material, but to have a device now must be exercised In order not to change that is fast enough to permit control during the volume of the hole in the upper layers the rolling operation is a big step forward. when digging in the lower layers. It would It is particularly interesting to learn be easy to change the volume of the hole that the volume of relatively large holes after the determination has been made and can be determined with the Washington thus give erroneous results in succeeding Densometer. During the past year we layers. Effect of Repeated Load Application on Soil Compaction Efficiency

GEORGE F. SOWERS, Professor of Civil Engineering, and C. M. KENNEDY m, Graduate Student; Georgia Institute of Technology

Soil compaction is essentially a process of consolidation. Our knowledge of that process leads to the conclusion that the greatest amount of soil consolidation is produced by the fewest cycles of application of load. This has been demonstrated by recent research at the Georgia Institute of Technology. Different methods of compaction such as tamping, hammering, and squeezing were employed, each exerting the same amount of work. The fewer the appllqations of pressure nec- cessary to exert the same amount of work, the greater the density obtained.

•IN the past, soil engineers have empha• ear, however, for the rate of Increase in sized the need for controlling and evaluating density decreases with increasing work. compaction and have largely left the prob• For example, research by the Waterways lem of obtaining it to the contractors and Experiment Station (1) indicates that a lin• equipment manufacturers. The few studies ear relationship exists between dry density which have been made, such as those by the and the logarithm of the number of hammer U. S. Waterways Experiment Station (1^) and blows in a laboratory test or the logarithm the Road Research Laboratory in England of the number of passes of a sheepsfoot (2) have done much to point out how better roller. compaction can be obtained easily. Much Little has been said about the effect of research remains to be done, however, on the way in which the compactive effort is the basic factors which control the effec• applied to the soil. In fact, some investi• tiveness of compaction. It is the authors' gators have Implied that soil density will purpose to discuss just one aspect of the always be about the same for a given effort problem, the effect of repeated load appli• in foot-pounds per cubic foot regardless of cations on compaction efficiency, and to the manner in which the effort is exerted. point out how this affects both laboratory Others have pointed out that considerable and field compaction results. differences in compaction effectiveness do exist. For example, moisture-density curves developed by the Standard AASHO COMPACTIVE EFFORT AND procedure (Standard Proctor Test) do not COMPACTION necessarily have the same shape as the The amount of work exerted in compact• moisture density curves developed by actual ing a soil is the compactive effort. It may rolling in the field. The difference has been be described by the number of blows of a attributed to the fact that the standard lab• certain weight hammer falling a fixed dis• oratory test involves tamping or dynamic tance, the number of applications of a cer• compaction while the compaction of a tain pressure to the soil surface, or by the sheepsfoot roller isproducedby an increas• number of passes of a roller of known weight ing static pressure. Some laboratory pro• and pressure over a specified lift of fill. cedures have been developed to simulate the Technically it is most accurately ejqpressed action of the sheepsfoot. by the work in foot-pounds or inch-pounds Research in the Soil Mechanics Labor• applied to each cubic foot of soil. atory, at Georgia Institute of Technology (3) A number of field and laboratory studies has shown considerable difference between of compaction utilizing different amounts of the densities produced by different compac• work applied have come to the same conclu• tion methods even though all utilized the sion: that soil density increases as the same total compactive effort. For example, compactive effort Increases for any given identical soil samples werecompactedusing soil condition. The relationship is not lin• first, the Standard AASHO method,and sec-

61 62

methods produced substantially the same densities while at moisture contents equal to or below the optimum, the densities were > quite different. I A number of factors were considered which might be the cause of the difference. DENSITY These included: (1) velocity of hammer at point of impact; (2) momentum of the ham• mer; (3) hammer weight; (4) ratio of the diameter of the compacting device to the thickness of the soil layer; and (5) percent• age of the total energy exerted during each tamp or application of pressure. The results of these tests indicate that the velocity of the hammer or tamper as it strikes the soil has no discernible influence "I 10 100 PERCENTAGE OF TOTAL ENERGY EXPENDED on the effectiveness of compaction. Neither IN EACH TAMP OF THE COMPACTION DEVICE do the momentum or the weight of the ham• mer. These results contradict the conclu• Figure 1. Relationship between the com- pacted density of the soil and the per• sion reached by some investigators that the centage of total energy exerted by each difference between the standard laboratory tamp of the compaction device (typical tests and field results lies in the fact that example). the laboratory tests are essentially dynamic ond, single ^plications of static pressure (with high velocity of impact) while thefield to each of three layers of soil in a standard work is essentially static (with low velocity compaction mold. The second method was of impact). adjusted by trial and error to utilize the The ratio of the hammer or tamper diam• same amount of total effort as the first. eter to the soil layer thickness was found The density obtained by the first procedure to be an important factor. Research is con- was 107 pcf. and by the second method was 119 pcf. This indicates that the second method is more effective in utilizing the work done in producing compaction.

EXPERIMENTAL STUDY OF FACTORS AFFECTING COMPACTION EFFECTIVENESS A program of study was undertaken at Georgia Tech to determine the factors which affect compaction effectiveness (3). The DEFLECTION work was carried outwith a single soil type IN IN (low plasticity clay, A-6), a limited number of moisture contents, and a single compac• tive effort of 34,000 foot-pounds per cubic foot. The latter was selected as it is be• tween the standard and the modified AASHO efforts and is representative of modern compaction requirements. A number of different devices were used. These included hammers weighing 5. 5, 10, and 25 lb. with heights of fall ranging from 3 to 18 inches, a low velocity punching tam• per to simulate the action of a sheepsfoot 50 100 roller foot, and apiston for applying slow, PRESSURE IN PSI static pressure to the soil. The results of 64 tests indicate that for moisture contents Figure 2. Pressure-deflection curve for above the standard Proctor optimum all three load applications. 63 tinuing on this point at Georgia Tech, but ratio of the amount of compaction to the the tentative conclusion is that the soil den• work done in producing it. Figure 3 shows sity increases with the square of this ratio, that it becomes less with each successive until the ratio equals approximately 1. pressure application until it probably even• The most-important factor was found to tually becomes zero. In other words, the be the percentage of the total energy which first application of pressure produces by was applied in each tamp, blow, or ^pli• far the most compaction for the amount of cation of energy to the soil. The greatest work required, and is therefore the most density in every case was produced when efficient. all the energy was utilized in a single ^pli• cation; the greater number of applications 1.2 required to apply the same amount of en• loj ergy, the smaller will be the resulting den• AMOUNT OF 8 sity. A t}rpical curve from these tests is COMPACTION - IN IN " given in Figure 1. 4

.2 CONSOLIDATION AND COMPACTION 0 The cause of this reduction in compac• 40 WORK DONE PER tion effectiveness as the number of appli• COMPACTION 30 IN-LB PER SO 90 cations of the compaction energy increases IN OFSURFACr must lie in the mechanics of the compaction 10 process. Additional research was there• fore directed toward that end. Compaction of cohesive soils is essen• tially consolidation with limited lateral sup• RATIO OF THE AMOUNT OF port. This is produced by pressure regard• COMPACTION TO THE WORK less of whether the soil is tamped, ham• DONE mered, or just loaded statically. When in• creasing pressure is ^plied to a soil it de• forms or compacts. Vfhen the pressure is released, the soil swells but not to its orig• inal volume. If the same pressure is re• applied, additional consolidation will take I 2 3 NUMBER OF TIMES PRESSURE IS place, but not nearly as much as during the APPLIED first application. The rebound will be pro• portionally greater. Each successive ^- Figure 3. Ihe amount of compaction, the plicationof pressure will produce less and work done and the compaction ratio (ratio less consolidation and proportionally more of compaction to work) as functions of the number of pressure applications. and more rebound until the two are equal and no further consolidation occurs. The The cause can be inferred from our pressure-deflection curves for three suc• knowledge of soil structure. The compac• cessive applications of a 140-psi. pressure tion of the soil under pressure is the to a 3-sq. -in. compaction foot acting on a result of elastic deflection of the soil struc• 2-inch soil layer are shown in Figure 2. ture and plastic movement of the soil grains The amount of work exerted in each cycle into a more dense arrangement. The elastic of pressure application and release can be deflection absorbs work in the form of strain found by integrating the pressure deflec• energy while the plastic deformation ab• tion curve. From Figure 2 it can be seen sorbs work and transforms it to heat that the amount of work exerted is greatest When the pressure Is removed, the elastic during the first pressure application and part of the deflection is largely recovered. much less during each successive applica- The strain energy is dissipated in the vis• tic n. Figure 3 shows how both the amount cous resistance of the soil to swelling and of compaction and the amount of work de• in some plastic deformation and re-ar• crease sharply with successive load appli• rangement During a second load appli• cations. cation the deflection is largely elastic with only a small amount of plastic deformation The effectiveness of the compaction can because the grains were already readjusted be expressed by the compaction ratio—the 64

durii^ the first loading. The only reason For greatest efficiency these should have additional compaction is produced is that tire spaclngs and arrangements so that all some readjustment took place during swell• parts of the surface c an be covered just once ing. After many load ^plications the without appreciable overly. deflection is entirely elastic and no addi• Specifications should be written to re• tional grain adjustment takes place. Work quire just one complete surface coverage. is still exerted, however, to overcome the If sufficient density is not obtained in a sin• viscous resistance to deformation although gle coverage (and the soil moisture is cor• no additional compaction results from it. rect) the equipment is inadequate, and addi• tional rolling with the same equipment would be largely a waste of time and money. This SUMMARY OF CONCLUSIONS is particularly important when the owner The results of this research lead to must pay for all passes of the roller above three conclusions which have important a certain minimum. implications in field and laboratory prac• tice: (1) Each successive application of The difference between the moisture- the same pressure to the soil results in density curves developed in the laboratory less and less work done per application. by the customary 25 blows of a hammer on (2) Each successive application of pressure layers of soil in a 4-inch-diameter mold results in less compaction per unit of work. and moisture-density curves developed by (3) For a given amount of work, the greatest rolling can be easily explained by these compaction results when the work is exerted studies. In the laboratory test the large in a single application. number of blows results in an average of six ^plications of pressure to the soil. In contrast, even 10 passes of a sheepsfoot APPLICATIONS TO FIELD AND roller (a large number) may not even pro• LABORATORY COMPACTION duce one complete coverage of the surface. When the action of modern compaction Furthermore the constant pressure exerted equipment is evaluated with respect to the by the sheepsfoot roller means a decreasing above conclusions, some devices are seen amount of work for each pass of the roller to be inefficient. The sheepsfoot roller, while the laboratory compaction hammer for example, requires theoretically from exerts an equal amount of energy each time. 10 to 15 passes to secure complete coverage The remedy would be to use a much heavier of an area. Actually with random rolling hammer for laboratory compaction and many parts of an area are rolled two or fewer blows, say six per layer. On this three times and other parts not at all. A basis the author believes that a laboratory roller designed with more and larger feet compaction test can be developed which will would produce less overlapping and better retain the simplicity of the present standard compaction for the amount of work done. Proctor and yet be similar in its results to The same applies to rubber-tired rollers. field experience.

References 1. Corps of Engineers, "Soil Compac• ment of Scientific and Industrial Research, tion Investigations, Reports 1-4, "Techni• Road Research Laboratory, London, 1950. cal Memo 3-271, u. S. Waterways Exper• 3. C. M. Kennedy, A Laboratory In• iment Station, 1949. vestigation of the Efficiency of Different 2. F. H. P. Williams and D. J. Mc• Methods of Soil Compaction, M. S. Thesis, Lean, "The Compaction Soil," Road Re• School of Civil Engineering, Georgia Insti• search Technical P^er No. 17, Depart• tute of Technology, 1953.

flflB:Jf-255 HE NATIONAL ACADEMY OF SCIENCES—NATIONAL RESEARCH COUN• CIL is a private, nonprofit organization of scientists, dedicated to the Tfurtherance of science and to its use for the general welfare. The ACADEMY itself wras established in 1863 under a congressional charter signed by President Lincoln. Empowered to provide for all activities ap• propriate to academies of science, it was also required by its charter to act as an adviser to the federal government in scientific matters. This provision accounts for the close ties that have always existed between the ACADEMY and the government, although the ACADEMY is not a govern• mental agency.

The NATIONAL RESEARCH COUNCIL was established by the ACADEMY in 1916, at the request of President Wilson, to enable scientists generally to associate their efforts with those of the limited membership of the ACADEMY in service to the nation, to society, and to science at home and abroad. Members of the NATIONAL RESEARCH COUNCIL i-eceive their appointments from the president of the ACADEMY. They inc! ide representa• tives nominated by the major scientific and technical societies, repre• sentatives of the federal government designated by the President of the United States, and a number of members at large. In addition, several thousand scientists and engineers take part in the activities of the re• search council through membership on its various boards and committees. Receiving funds from both public and private sources, by contribution, grant, or contract, the ACADEMY and its RESEARCH COUNCIL thus work to stimulate research and its applications, to survey the broad possibilities of science, to promote effective utilization of the scientific and technical resources of the country, to serve the government, and to further the general interests of science.

The HIGHWAY RESEARCH BOARD was organized November 11, 1920, as an agency of the Division of Engineering and Industrial Research, one of the eight functional divisions of the NATIONAL RESEARCH COUNCIL. The BOARD is a cooperative organization of the highway technologists of America operating under the auspices of the ACADEMY-COUNCIL and with the support of the several highway departments, the Bureau of Public Roads, and many other organizations interested in the development of highway transportation. The purposes of the BOARD are to encourage research and to provide a national clearinghouse and correlation service for research activities and information on highway administration and technology.