\)
HIGHWAY RESEARCH BOARD DIVISION OF ENGINEERING AND INDUSTRIAL RESEARCH NATIONAL RESEARCH COUNCIL
* * * Wa,·ti,ne Road Proble,ns * * * No. 11 COMPACTION OF SUBGRADES AND EMBANKMENTS
HIGHWAY RESEARCH BOARD 2101 Constitution Avenue, Washington 25, D. C. August, 1945 j
1 HIGHWAY RESEARCH BOARD * * * OFFICERS AND EXECUTIVE COMMITTEE Chairman, STANTON WALKER, Director of Engineering, National Sand and Gravel Asso ciation Vice-Chnfrmtm, R. L. MORRISON, Professor of Highway Engineering and Highway Trans port, Department of Civil Engineeriug, University of Michigan THOMAS H. MACDONALD, Commissioner, Public Roads Administration WILLIAM H. KENERSON, Executive Secretary, National Academy of Sciences and Na- tional Research Council HAL H . HALE, Executive Secretary, American Association of State Highway Officials PYKE JOHNSON, President, Automotive Safety Foundation F. C. LANG, E ngineer of Materials and Research, Minnesota Department of Highways, and Professor of Highway Engineering, University of Minnesota W.W. MACK, Chief Engineer, State Highway Department of Delaware BURTON W. MARSH, Director, Traffic Engineering and Safety Department, American Automobile Association CHARLES M. UPHAM, Engineer-Director, American Road Builders' Association J. S. WILLIAMSON, Chief Highway Commissioner, State Highway Department of South Carolina Director, RoY W. CRUM Assistant Director, FRED BURGGRAF
DEPARTMENT OF SOILS INVESTIGATIONS C. A. HOGENTOGLER, Chairman Committee on Compaction of Subgrades and Embankments L. D. Hicxs Chnirro:in ; Senior Materials and Testing Engineer, North Carolina State High- way n.na1 P ublic Works Commission , CB~LES W. A1,LEN, Acting 'hicf Engineer, Dure au of Tests, Ohio Department of Highways SBREVl'l CLA.RK , Testing Engineer, Virginia Department of Highways C. A. HoOENTOGLE n, Jn ., University of Maryland BEn·r MYERS , Materials nnd Construction Engineer, Iowa State Highway Commission CARL R. REID , Engineer of Materinls, Oklahom a State Highway Commi ssion S. E. S1m1, Materials Engineer, Public Itoada Administration W. T. SPENCER, Soils Engineer, Indiana State Highway Commission K. B. Wooos, Assistant Director, Joint Highway Research P roject, Purdue University 11' artime Boa,l P,•oble111,s
There are two major wartime l'Oad responsibilities; to keep the traffic essential to the war effort moving, and to carry the existing roads through the wai· period in as good condition as possible. Dischal'ge of these responsibilities entails consideration of many ne\V factors in view of the limitations on time, money, labor, equipment and use of critical materials imposed by the exigencies of the national situation. Obviously, changing emphasis from devising better and more economi cal methods to a program, within the wartime limitations of wartime traffic movement and conservation of the existing roads confronts highway engineers with many new problems and new aspects of old problems. The Highway Research Board believes that it can be helpful by aiding in disseminating in usable form the best available information on those phases of highway technology in which common practice has not become established or in which practice must be modified duifog the war. To this end a series of bulletins on WARTIME ROAD PROBLEMS will be prepal.'ed by qualified committees and pub lished by the Highway Research Board. Recommendations in this series of bulletins are based upon wartime restrictions and needs and are only intended for use as guides during the periods in which these conditions prevail. This program has been endorsed by the Executive Committee of the American Association of State Highway Officials. Suggestions for suitable subjects will be weJcomed.
The following report on compaction of subgrades and embankments is of immediate emel'gency importance. The recommendations, how ever, are general and need not be restricted to wartime practice as they are applicable whenever the described conditions prevail. COMPACTION OF SUBGRADES AND EMBANKMENTS
The subgrade is that portion of a road compacted. Compaction should not be way immediately below the surface, base considered a substitute for water-proof or sub-base. It may consist of the original ing admixtmes or other satisfactory materials, as in cut sections, or hauled in stabilization procedures. In arid regions materials, as in filled sections. or other localities where adverse moisture An embankment is that portion of a conditions are not present, compaction roadway which has been built above the obtained through control of the moisture original ground by depositing material content and compactive effort, may not obtained from cuts or borrow pits. The be economical or necessary. upper layer of an embankment is also the subgrade. Embankments and sub PHENOMENA OF COMPACTION grades must provide stability for the road. Moisture-Density Relations The stability of a subgrade or embank The density to which any given soil can ment is dependent upon the shearing be compacted depends upon its moisture resistance of the soils of which they are content and the amount of compactive composed. Shearing resistance consists effort expended upon it. A change in of two properties, internal friction and either moisture content or compactive cohesion. These combined properties of effort produces a change in density. This a given soil are affected by the amount of relationship is utilized to produce soil voids in the soil and the amount of water masses of high density with the ieast within these voids. A soil mass containing compactive effort. a high percentage of voids will become If a given soil in an air-dry condition is very unstable when exposed to high placed in a container and submitted to a moisture conditions. Conversely, a soil definite compactive effort, a certain density mass containing a low percentage of voids (usually measured in pounds per cubic will resist the entrance of water and will foot) will be obtained. If a small per in turn be much more stable than the soil centage of water is added and the soil is mass containing the higher percentage of again compacted with the same amount of voids. Any process which reduces the effort, a greater density is obtained. By amount of voids in a soil mass may be repeating this procedure, using the same called densification. compactive effort, but increasing the The most economical and feasible method moisture each time, a moisture content of improving the supporting power of will be found for which the density of the subgrades and embankments is densification soil is a maximum. The moisture in the by compaction. Under present war con soil at maximun density is called the ditions, when admixtures for stabilizing optimum moisture content for this com subgrades are at a premium, it is particu pactive effort. If the test is repeated, larly important that maximum supporting using a greater compactive effort, a higher power of the subgrade soil be obtained by maximum density will be obtained at a this means. lower optimum moisture content. On However, compaction cannot be con the other hand, if a smaller compactive sidered a cure-all for obtaining satisfactory effort is used, a lower maximum density subgrades and embankments. Rather, will be obtained at a higher optimum compaction should be considered as an moisture content. For .a given soil, economical means of improving most therefore, there are as many "maximum subgrade or embankment soils. Some densities" and "optimum moistures" as types of soil are expansive and may swell there are compactive efforts used. when subjected to adverse moisture and This phenomenon has been explained on temperature conditions even though well the basis of both the smface tension of 4 water and the effect of film moisture. The upper curve of Figure 1 shows the These explanations are of scientific interest penetration resistance offered by the but are not essential in the application of compacted soil mass at the various condi the principles of soil compaction and need tions of moisture and density. not be discussed here. The measure of density commonly used, The relations between density and as illustrated in Figure 1, is the weight of moisture content at a certain compactive a cubic foot of the compacted material effort are illustrated in Figure 1. The exclusive of the contained water. As lower curve shows a typical determination used in this bulletin the term "density" of optimum moisture and accompanying means the dry density as thus defined. density. Just above this curve is a com These curves reveal the advantage and plete saturation curve showing the per importance of thorough compaction. It centage of voids in the soil for the various will be noted that a maximum density densities and the moisture content if the of about 129 lb. per cu. ft. was obtained
2BOO at the optimum moisture content of 8.5 t I\ per cent. The upper curve shows resist ;2,00 ance to penetration of about 2,350 lb. per \ ~2000 sq. in. at this moisture content and density. w ' At this density, as shown by the saturation ~ 1600 y-rYPICAL PEN ETRATION curve, the soil contains about 22 per cent ; RE;5ISTANCE C RVE 1 ~ 1200 ' voids. If the voids were entirely filled z with water instead of the optimum amount, Q BOO \ the soil would contain about 10.5 per ~ I wz 400 cent moisture, by weight. At this moisture lt' i "Ii"--- content the resistance to penetration for I I I 142 the same density would be reduced to ~ I 1 about 330 lb. per sq. in. : C i t IJB '- i :5 ,,~_qJ 6 u ! : When the soil was compacted at both i'--- ~~('}',) : ~ 134 " - and 11.5 per cent moisture the curves 'Z .;ct, I ;;: ~ ,~~~"',,,.,,,.,.,t,"' show that the same density ((122.5 lb. t I.JO i------""- .,:, ,"t;':~ ! per cu. ft.) and voids content (26 per ;;; / ~ :=s._.:- .- I ~ 126 cent) were obtained for each. At 6 per
~ _'::: , _MUt cent moisture the resistance to penetra a: 122 l::: -- 0 VJTYPI( AL CO MPAC K> N CU RVE ' ~- tion was relatively great but at 11.5 per 116 I 1 e g 10 11 12 13 14 1s cent, although the density was the same, PERCENT MOI STURE BY ORY WEIGHT OF SOJL the resistance became very small and if Fm. !.-Effects of Moisture upon Com the entire void space of 26 per cent should pacted Density. become filled with water, the resistance to penetration would be practically zero. voids are entirely filled with water. The The value of low porosity or void space data for this curve can be computed from in soils in embankments and subgrades the density of the soil and its specific is clearly shown. Porosity is a function specific gravity, thus: of the gradation and arrangement of the particles in a soil mass. The arrangement Percentage of voids = of the particles which is controlled by the 62.4 X Sp.G. - dry wt. per cu. ft. of soil ------X 100 degree of consolidation, depends upon the 62.4 X Sp.G. compactive effort, which produces its Percentage of Moisture for saturation (by greatest effect at the moisture content known as the "optimum". Poor soils dry weight of soil) cannot be made to function as good soils Wt. per cu. ft. of water (62.4) by high compaction alone, but their Dry wt. per cu. ft. of soil serviceability may be greatly improved. 1 Soils whose porosities, when compacted to - -- X 100 Sp. Gr. their maximum densities, are so great 5 that they will permit entrance of water in top of the specimen at the rate of ! in. detrimental amount are unsatisfactory of penetration per second. The penetra materials for subgrades and embankments. tion resistance in pounds per square inch is computed by dividing the total pressure Moisture-Density Tests by the area of the needle. Determination of the moisture-density The moisture-density and resistance relations of soil are comparatively simple, data are usually plotted graphically on involving only procedures for determining the same sheet as in Figure 1. unit weight and moisture content of samples subjected to a definite compactive SOIL CHARACTERISTICS effort. Several procedures, varying only The mineral particles of soil are grouped in detail have been used, each of which according to size into soil constituents as has been devised for applicability to follows: certain prevailing conditions. However, Coarse Material, stone or gravel, larger with due knowledge of the relationships than2mm. between density, optimum moisture and Sand ...... 0.05 mm. to 2.00 mm. compactive effort any of the methods Silt ...... 0.005 mm. to 0.05 mm. may be used to control subgrade Clay ...... smaller than 0.005 mm. and embankment construction. Obviously there are advantages in the general use Silt can be easily distinguished from of a common method, since in that way clay by its grittiness when bitten between experience in different places and with the teeth. Clay particles give no sensa different soils may be compared and cor tion of grittiness when bitten. It is the related. On that account the method clay particles that become cohesive when standarized by the American Association wet and produce tenacious mud. of State Highway Officials (T 99 - 38), which is the one most commonly used is Soil Classification recommended. The same procedure has A grouping of soils based on performance also been approved as a tentative method as subgrades was suggested by Hogentogler by the American Society for Testing Mate and Terzaghi in Public Roads, May 1929. rials. Hereafter it will be referred to as the A complete analysis of this grouping and "standard" method. It consists essen its application was published in Public tially in determining the weight per cubic Roads for June and July 1931. Desirable foot and percentage of moisture of samples changes in the system have been developed having moisture contents varying by through extensive use by highway en approximately 1 per cent, compacted in gineers. The revised classification based three equal layers in a cylindrical measure on these developments and descriptions of 4.0-in. diameter and 4.59 in. high, by of the tests of soil characteristics used 25 blows of a 2-in. diameter cylindrical in the classification were published in rammer weighing 5.5 lb. dropped from Public Roads, February 1942. 1.0 ft. above the soil. The optimum In this classification the soils are dis moisture content is that which gives the tinguished by particle size gradation and greatest weight per cubic foot. by the relationships of certain test con Methods used by the California Division stants determined by mixing soils with of Highways, the Kansas State Highway the necessary amounts of water to obtain Commission and the Corps of Engineers, definite consistencies. Particle size dis U. S. Army are described in references tribution and these "constants", liquid Nos. 5, 22, and 34. limit, plastic limit and plasticity index In connection with the test for moisture are more or less definitely related to density-compactive effort relations the service performance. Descriptions of these penetration resistance of the sample is · properties and the soils classes are given often determined by measuring the pres in the Appendix. sure required to force a blunt needle of Soils are also given a textural classifica known area a distance of 3 in. into the tion depending upon the amounts of the 6 "soil constituents". Limits of the con are not very compressible, they require stituents for each textural class are given very little compactive effort for consolida in Figure 2. tion, the vibration of the grading equip Soils are further classified as "cohesive" ment often being enough to produce high and "cohesionless". A cohesive soil is densities. one possessing plasticity or that property The members of the A-4 group are which causes the soil particles to cling fair soils for embankments if they are together. Soils containing clay in suffi handled in such a manner as to secure cient amourit belong in this category. 'high densities. These are soils in which the silt fraction predbminates and due to their texture and poor grading have high porosities (voids) even when compacted to the maximum density obtainable . . They vary widely in their textural compo 70 l-----,l--'-·'l.--t---l--t--1--1--i-1 sition, ranging from sandy loams to silts. (,' Hczavy Clay G 60 _,__...,...._, (See Fig. 2 on textural classification of soils.) The sandy loams may be compac ted to comparatively high densities and will have good stability through a wider range of densities than silt loams or silts. The silt loams and silts cannot be rolled to the densities necessary for good stability because of the excess of voids which results from poor grading and because of the lack of cohesive clay for binder material. Only fair stability can be obtained with them at best and then they Frn. 2.-Right Augle Soil Texture Chart for must be compacted with smooth rollers Classifying Soils. in layers not to exceed about 6 in. in loose thickness, using a very narrow mois A cohesionless soil does not possess plastic ture range. They possess relatively low ity and is quite unstable unless confined. stabilities at any moisture content and Sands belong in this group. The term are quite unstable at higher moistu:re "granular" is sometimes used to designate contents, a condition which is very likely soils that have a preponderance of material to occur during wet seasons due to high retained on the No. 200 sieve, usually 65 capillarity of this type of soil. The per cent or more. A "granular" soil silty clay loams of this group are somewhat may be "cohesive" or "cohesionless", better since they are better graded and depending upon its plasticity. possess some cohesion. They may be compacted effectively with tamping Soils for Embankments (sheepsfoot) rollers. Some soils are not suitable for embank A-5 soils are similar to those of the ment construction-others can be made A-4 group except that they contain mica satisfactory only by special handling. or diatoms which cause high capillarity Soils of the A-1, A-2, and A-3 groups and elasticity. They are generally more (see P. R. A. classification in Appendix) poorly graded and have high porosities are satisfactory for embankments and, even when compacted to maximum den with the exception of the A-3 soils, can sities. Their elastic properties interfere be compacted to high densities by rolling. with consolidation and may be so pre Soils of the A-3 groups are cohesionless dominant as to cause instability at low sands and cannot be compacted with a moisture contents. Like the A-4 soils, 10-ton roller but may be consolidated to they vary in texture and grading, which high densities by heavy duty track type variation is reflected in their capability tractors, ponding or jetting. Since they of compaction to comparatively high 7 densities. The cohesive variety of A-5 compaction on the job necessary for soils are the best in the group in that the satisfactory performance. This is ex elastic properties are overcome by cohe pressed as the mi1tlmum percentage of sion; however, if allowed to absorb suffi the dL-y weight determin ed by the st-Ondo.rd cient water, they may become ela tic as compaction test, that should be attained well as unstable due to a decrease in the when the mnteriaJ is compacted in the cohesive force caused by the higher mois embanlanent. ture content. Erosion of slopes and The weights per cubic foot and per washouts are quite common with the · centages of voids in Table 1 are based on non-plastic and feebly plastic varieties soil having a specific gravity of 2.65. For of this type of soil. soils of different specific gravities cor The A-6 and A-7 groups are predomi rections should be made to the weights nantly clay soils, the chief difference being per cubic foot in the table for each range that the A-7 soils are elastic when wet in voids in direct proportions to the and have higher porosities due to poorer specific gravities. For instance for a grading. For maximttm stability, both soil of 2.55 specific gravity the weights should be compacted to their maximum per cubic foot for the voids range of possible densities. In this condition, 39-45 per cent would be:
TABLE 1.- EMBANKMENT SOIL COMPACTION REQUIREMENTS
CONDITION 1 Co~'Dl'l'lON 2 Fills 10 ft. or less high and not subject to extensive floods Fill~ higher than 10 ft. or subject to long periods of flooding
Minimwn Field Minimum Field Maximum Laboratory Compaction Maximum Laboratory Compnction Dry Weights,a 'Rcquircmc11t5{ Voids, % Dry Weights ,a Requirements, Voids,% lb. per cu. ft. perccnto11e o lb. per cu. ft. perccntngc of dry wcighl tlry weight 89 . 0 and less b 46 plus 94 . 9 and less • 42 plus 90.0- 99 .9 95 39- 45 95 . 0- 99 .9 100.0 39-42 100.0-109.9 95 33-39 100. 0-109. 9 100.0 33-39 110.0-119.9 90 27-33 110.0-119.9 95.0 27-33 120.0-129.9 90 21-27 120 .0-129.9 90.0 21-27 130 .0 and more 90 21 minus 130 . 0 and more 00.0 21 minus
a Mnximum lnborntory dry weight la obln!nod b)• tho et.11nd11rd compaction test, see Appendix. b Soils hn.ving m11Jsoil type being figure of 95 per cent. Other specifications used and will give him an optimum mois require that a procedure be followed which ture for that soil approximating that requires moisture control and stipulates determined by laboratory test. Figure 3 the compactive effort, while still others shows a set of typical moisture-density specify both density and procedure. The curves for the state of Ohio. Similar following procedure is recommended when sets of curves may be made for any locality rigid control of density is desired. It and . used as guides in selecting the requires that the soil be compacted by· a optimum moisture percentage to be used. stipulated compactive effort at optimum moisture content as determined by the Moisture Control standard test. Prior to the grading operations, a Control of the moisture in the soil profile survey should be made as described being used in embankment construction in "Standard Methods of Surveying and consists in determining the moisture Sampling Soils for Highway Subgrades", content of the soil and correcting it to ASTM Designation D420-42T or AASHO the optimum by the addition of water, Designation T-86-42, and samples taken if below the optimum and by aeration, representing all of the various soils to be if above. The water may be added to encountered. Tests should be performed the layer after it is placed or to the soil on the samples for mechanical analysis, in the cut or pit before excavation. ·The physical characteristics, and moisture latter method is more desirable as the density relations and these data furnished moisture will be mixed with the soil by the field inspector for use during grading the grading operations before it is placed, operations. With these data as a guide, thus eliminating the necessity of other it will be possible to select the most mixing operations. Reduction of mois satisfactory materials for embankments ture is accomplished by evaporation, and subgrade. assisted by harrowing and plowing. The optimum moisture of the soil may Fortunately, the moisture content of be taken from these laboratory test data many soils beneath the ground surface, or may be determined directly in the but above the water table, is practically field if there is any doubt that the soil at the optimum and needs little or no under consideration is covered by labora correction. tory test data. The percentage of moisture in the soil In cases where a soil type is encountered may be determined by drying 100 or on which the moisture-density relationship 200 g. of a representative sample and has not been determined by previous substituting the wet and dry weights of tests or where a contractor wants to start the soil in the following formula: in a new borrow pit before sampling, temporary maximum densities for immedi Percentage of Moisture ate usage may be determined by compact Wt. of Wet Soil - Wt. of Dry Soil ing a sample of the soil in the field using . X 100 the standard test procedure but limiting Wt. of Dry Soil 9 In drying the sample care should be curves and the corresponding moisture exercised to avoid "burning" as some content selected. In case of a discrepancy, soils contain chemically combined water the moisture content corresponding to
2000 C,: IIJ OO - ~ 1r,o'o - ~/100 s: C:, !ZOO <)- ") 1000 ~ 'l "°° «)() ~"' 4Dq -
~ ~()() () HAK:oRr ,.,,, CURVE WT OPT. · NO LfJ./'fR MOIST. ,.,z ~u.rr. PCT. I 1!1.Z ,oo 90. 7 t6.6 '"" - 'J 9l.l 26.0 /91, 4 94. S 24.8 s 97.S 23.!J /!16 9 9. S n .z •7 /Oll zas IJJt / 0,1.4 /9. 7 •9 /06.9 18.4 /JJZ /0 109. 4 17. Z II 111.7 IS. 9 I,. / 30 ll /14.2 /,f.8 <:, . 117.Z 11.6 ~ fll! //9. J IJ. O I t; !2lS l l .O ~ lt6 IU..7 //.4 ~ 126.tJ /Q6 v /24 fl8. 9 9. 3 /31.0 9.0 ~m 'l "' Jt () - "'~ 1/tJ 'l"' //6 /1,f /IZ
l + 6 tJ 10 /Z / I / 6 /8 20 22 U l 6 llJ 30 3 2 ,J,f. , 36 18 /'10!.5TU/ft: -P£lfC£NT OF ORY 1¥£/GH T Frn. 3.-Typical Moisture-Density Curves. Prepared by Ohio State Highway Testing Laboratory from Tests on 6619 Ohio Soil samples.
and other substances which can be driven the wet weight value rather than to the off at high temperatures and thus cause an penetration resistance should be used. error in the determination. With experience, it 'is possible to esti Another method of determining the mate the proper moisture content for moisture content of soil in the field is to maximum density by inspection. When compact the soil, as it is being used in supplemented by field density determina the embankment, in the mold and deter tions, this method is known to produce mine its wet weight per cubic foot and very satisfactory results. penetration resistance. These values The maximum allowable variation in are compared with the laboratory test moisture content of the embankment wet weight and penetration r ist.auce material from the optimum determined 10 by test should be governed by the shape particles that will not pass a No. 4 sieve of the moisture-density curve. Curves should be excluded from the test. This with flat peaks indicate rather wide may be done by separating them by ranges in moisture content at which accept hand from the soil as it is removed from able densities may be secured while the hole and placing them in the hole as curves with sharp peaks do not permit the oil or sand is introduced for the volume such a wide range in moisture content. determination. When this is not feasible, Many organizations allow a tolerance as when using the rubber balloon, the of 2 per cent moisture, above or below volume of the larger particles may deter the optimum; however, if this tolerance mined by the water-displacement method is not supported by curve data, it should and subtracted from the volume of the not be used. hole. , The standard density for the embank Compaction Control ment material is the maximum density Compaction control in embankment shown on the moisture-density curve for construction consists in determining the that particular soil. When great accuracy degree of compaction of the materials is desired, it is necessary to test the material as they are placed and increasing the taken from the hole in the embankment density, if deficient, by increased com density determination; however, the maxi pactive effort. The degree of compaction mum density shown on the laboratory is determined by comparing the dry moisture-density curve is sufficiently embankment density with the maximum accurate on most work, if the curve is dry density obtained for the soil in a representative of the soil. compaction test and expressing the The degree of compaction is the ratio of the embankment density to the standard relationship as a percen~e of that obtained by the compaction test. density and is expressed as a percentage The embankment dry density may be of the standard density, thus determined by measuring the weight per Degree of Compaction, percent cubic foot of a sample of the embankment Embankment Density in place (wet density), determining the ------'- X 100 moisture content in percentage of the Standard Density dry weight of the sample and calculating the dry density by the following formula: Compaction requirements are generally more rigid for subgrades than for embank Dry Density (wt. per cu. ft.) ments due to the fact that the subgrade is in contact with the road surface, base, Wet Density (Wt. per cu. ft.) X 100 or sub-base and the live loads applied to 100 + percentage of moisture the road are transmitted more directly to it. Also, the subgrade is nearer the The wet weight per cubic foot of a sample surface of the ground and is more exposed of the embankment may be determined to the prevailing climatic conditions. A by weighing the soil taken from a hole high degree of compaction increases its about 6 in. deep and 4 in. in diameter stability and reduces the voids thereby and dividing by the volume of the hole. 1 tending to limit the amount of moisture The volume of the hole may be determined it can absorb. by filling it with a measured amount of A subgrade is that portion of the road sand or oil of known weight per cubic on which the pavement structure rests foot or it may be measured directly by and may be over an embankment or introducing a rubber balloon into the through a cut section. The density of hole and filling it with water from a cali undisturbed soil is usually less than that brated vessel under pressure. Rock of the same soil when compacted by the t Jlor dotailod dcecripUon of tbus l<.'lli aeo Wnrlime standard compaction test, hence that noiut Problotn8 No. 6,' Gmnulnr Stsbilizo. 11. should be compacted if necessary to bring 11 it up to the required density. To do stress due to the weight of the embank this, it is necessary to plow up and pul ment. The higher the embankment and verize the subgrade, bring it to the correct the less stable the soil, the flatter the moisture content to the desired depth slope required. This slope, the angle (usually 6 to 12 in.) and compact it to tbe slope makes with the hm•izontal which the required densi ty. It is usually is known as the angle of repose in soil specified that a ubgrnde be compacted mechanics, varies with the character of to fro.m 95 to 100 per cent of 'the density the soil, its moisture content, and the obtained by the standard compaction pressure applied. It has been common test. Some specifications require 100 per practice to use a slope of 1! to 1 (lf cent and it is reported that this high horizon tal to 1 vertical) on all embank degree of compaction is secured without ments regardless of the soil, height of much difficulty. embankment, and condition of service. This practice has caused many embank EMBANKMENT DESIGN AND ment failures in the form of slides. The CONSTRUCTION proper slope can be determined by soil Certain features of the design and mechanics but the process i laborious construction of subgrades and embank and is not within the scope of this bulletin. ments should be mentioned in connection Individual cases, such as exceptionally with compaction. high embankments, that justify the refine The first consideration in embankment ment should be designed by the principles design should be the foundation on which of soil mechanics after sufficient investiga it is to rest. Embankments placed on tion and testing has been done to obtain soft foundations will subside under their the data needed in the solution of the own weight, causing failure of the embank problem. The heights of embankments ment itself or a rough riding surface on constructed for highways, however, do the road. The soft condition of the not usually require this refinement and foundation soils is generally due to poor expense. The following slopes are sug drainage as embankments are in most gested as the minima to be used on ade instances used across low areas. Where quate foundations: possible, these areas should be drained, Slope Soil and Condition but if this is impracticable, the condition 1! to 1 All sand fills whether inun must be accepted and proper allowance dated or not. made for subsidence. The use of sandy Fills of cohesive soils less materials in the lower portion of the fill than 5 ft. high not subject will greatly accelerate subsidence as the to inundation. water in the foundation soil will be squeezed 2 to 1 Fills of cohesive soils more than 5 ft. but less than 50 out through this porous layer. Muck ft. high and not inundated. should be removed by excavation or Fills whose heights exceed 50 blasting as this type of material behaves ft. should be investigated as a viscous fluid instead of a plastic and designed by a rational solid. Fills placed over muck layers have method based on soil me- been known to require years for final chanics. subsidence. 3 to 1 All fills of cohesive soils sub Another important feature of embank ject to total or partial in ment design is the angle of the side slope undation not exceeding 50 ft. in height. which should vai·y with the height of the embankment, the character of the soil, These recommendations are for average and possible subjection of the embankment soils as they occur in embankment con to inundation due to impounded water. struction. Embankments containing a The function of the side slopes of an high percentage of rock present a different embankment is similar to that of a retain problem which is quite complicated and ing wall, that is, to resist the lateral difficult to analyze. It is couceded that thrust of the soil which is under vertical the slopes of embankments composed of 12 this type of material may be steeper than as a necessity in good embankment con for soil which does not contain a pre struction. Fills should be constructed ponderance of large rock. in layers or lifts of uniform depth and of The preparation of the embiinkment uniform moisture content. After a lift foundation consists of clearing, grubbing, of loose soil has been placed it should be and removal of root mat and other obj ec leveled with a blade grader or bulldozer tionable material. The first layer of fill to insure unifo rm depth and eliminate material acts as a leveling course, filling local depressions. Obviously any appre in stump holes, etc. When water is ciable differences in the thickness of a present in the foundation or the foundation lift or any significant differential in the is in a swamp, granular material is the moisture content of the soil in a lift will most suitable for this course as it permits produce variable densities since the roller easy flow of water squeezed from the or compacting unit applies a constant undersoil due to the weight of the embank amount of cornpactive energy to the ment. surface and the amount of work to be The embankment should be constructed done or the amount of energy required in layers not exceeding 8 in. thick loose for uniform compaction would vary with measttrement, and soil should contain depths and moisture contents. Care must the moisture necesssu-y to produce maxi be taken to insure uniform depths and mum density (optimum moisture for the moisture contents of each lift if uniform soil). Compaction an be secured by densities are to be obtained. In order rolli ng with sui table tamping ( heepsfoot) to guard against delays, the grade should rollers, smooth rollers, or pneumatic have sufficient crown at all times to tired rollers. However, mooth rollers or provide adequate surface drainage in pneumatic tired rollers are satisfactory case of rain. Borrow pits should also only when the embankment layers do be kept smooth, and sloped to insure uot exceed 4 to in. in loose thickness, uniform moisture conditions in case of depending upon the character of the soil. rain. It has been reported that, with certain highly compressible soils, the use of COMPACTING EQUIPMENT smooth and pneumatic tired rollers has Equ ipment for compacting soil in produced excellent results with layers embankment construction consists of much thicker than 4 in.; however, unless rollers and tampers. Tamping equipment investigation proves this to be true for is used in areas too small for, or inaccessible the particular soil under consideration, to rolling equipment, such as around it is not wise to allow layers thicker than structures. It consists of hand tampers, about 6 in. when these types of rollers pneumatic tampers and vibrator tampers. are used. Tamping or sheepsfoot rollers Rollers belong to three general types; have the advantage of penetrating through smooth rollers, pneumatic tired rollers, the loose soil layer and compacting the and t;ampin or sh epsfoot rollers. soil from the bottom up. Of course it is Generall y, specifiCJ1.tions for smooth necessary to use a greater number of roll ers require tlm.t they weigh not less pa ·es with tamping rollers- usually about than 10 tons and be capable of exerting a two po. ses pel' inclt th ickness of soil pressure of 325 lb. per in. width of tread. layer, as compiwecl with about 3 0 1· 4 The requirement for pneumatic tired pnsses for the entire lay01· with the ·mooth rollers is that· they exert a pressure of 225 and J>neumatic tired typ . The oil lb. per in. wid th of trend. These two layer will be compacted uniform ly from types of rollers a:re more desirable for the bottom up, when the tamping roil el' compacting subgrades than tiimping rollers is used until the tamping feet do not as they prod uce a smooth surface . They penetrate the soil layer but "walk out". ore rathe1· cumbei-some nncl function best To obtain good compaction it is where the surface to be rolled is not rough neceswry to utilize certain elementary or steep; however, they are used exten principles that have been long recognized sively in many localities. Also certain 13 soils, such as cohesionless silty soils, may 3. The tamping feet shall have a be compacted to higher densities with bearing area of 5 to 12 sq. in. (This these types of rollers than with the tamping varies with the type of soil. See Table type. 2.) The sheepsfoot or tamping roller is the most common type used for compacting CURRENT PRACTICE embankments. It is capable of com In 1942 the Committee obtained in pacting layers of soil, up to 8 or 9 in. formation from the highway departments thick to a high degree of consolidation for of the 48 States and District of Columbia the full depth of layer, and can be used on their experiences with embankment over rough, steep surfaces. This type stability with special emphasis on methods of roller is so designed that its weight of compaction and results achieved. A may M increased materially by filling digest of these data are shown in Tables the drum with water or sand. The 3 and 4. For convenience of comparison, tamping feet are obtainable in several the various States are arranged in groups sizes, ranging from 5 to 12 sq. in. in area. more or less indicative of similar soil These two controllable features make and climatic conditions. It is the purpose this type of roller suitable for compaction of the data in Table 3 to disclose whether of practically all soils. Table 2· gives or not experience in highway construction the areas of tamping feet and ground had indicated need for controlled com pressures recommended for the three paction. Table 4 furnishes information main types of soils. It is necessary to on the influence of controlled compaction consider these features in order to secure on construction cost. TABLE 2 EMBANKMENT PERFORMANCE Arcn o! Trunp Ground Pres Type of Soil ins Feet, sure, lbs. per As shown in Table 3 embankment sq. in. sq. in. settlements and failures were reported by
Sandy Soils ...... 10 to 12 60 to 100 practically all of the States. Settlements Silty Clay Soils ...... 7 to 9 100 to 200 within the body of the fill, however, were Clay Soils ...... •• .. ... 6 to 6 200 to 400 mentioned only in connection with what efficient performance of tamping rollers. are now practically obsolete methods of A roller most suitable for the compaction construction such as end dumping and of clays will not "walk out" of a layer placing the soil in thick layers without of sandy soil, and a roller most suitable for provision for rolling or control. the compaction of a sandy soil will not Since end dump methods have been produce the required density in clay soil. superseded, earth fills have been built In this connection, it might be mentioned in layers, not exceeding 12 in. thick, com that very sandy soils can be satisfactorily pacted either by rollers or by hauling consolidated with track type tractors. equipment, or both. In most cases fills Specifications for sheepsfoot or tamping constructed in this manner are allowed rollers generally stipulate the following to season during one winter and spring requirements; before pavement is placed on them. 1. The weight shall be such that the Settlement under these conditions has, minimum ground pressure on the tamper in general, not been detrimental, especially feet will be not less than 50 to 200 lb. in the granular types of soils. per sq. in. (this varies with the type Several States indicated that difficulties of soil. See the recommendations in with heavy clays and silts have been Table 2.) largely overcome by adopting moisture 2. The tamper feet shall be not less and density control in accordance with than 6 in. nor more than 8 in. long and the method or some variation of the method 2 shall be located in staggered rows so described by Proctor in 1933. No detri spaced as to obtain a contact of about mental settlements have been observed on two tamping feet per square foot of • "Fundnmontnl Principles of Soil Compaction", R. R . P root.or, Engincl!l"in g News-Record, Aug. 31, Sept. 7, tamped area. 21, Md 20, 1033 . 14 pavements placed less than a week after to do so and are tlciW revising or planning completion of embankments constructed to revise their specifications to incorporate under specifications requiring moisture some form of control measures. Further and density control. more, many States not having a definite TABLE 3.-EMBANKMENT PERFORMANCE
Have Failures Would Proper Is Definite Compnction Ocrully State or Settlement Eliminnla Generally Contributor Occurred? Trouble? Specified?
North -East Mnlno ...... Yes Yes No L. D . Barrows New HRlllpsluro...... Yea Yea No D. H. Dickinson Vermont...... Yea Yea No Mossnohusotta ...... Yes Yea No R. W. Coburn C Cru:olina ...... Yes Partly No C. R. MC)Millan Oeorgin ...... Yes Partly No W. E'. Abercrombie Florida ...... Yes Partly No C. B. Cooko North-Control ...... Minnosotll ...... Yes Partly No J. l ' , E llison Yea No No Dort lyoni t?:o~i{: ::: ::: ::::: :: ::: ::: :: Yes Yes Yea F. V. Ro11gel Norlh 011kotn...... Yes Yes No Keith Ooyd South Dakota ...... Yes Yes No R. K. Morn>II obrnsk1, ...... Yes Partly Yes M. B. Jooea KllllSII!! ...... Yes Yes Yes R . D .irinney South-Contml ArkansM ...... Louisiana ...... Yes Partlyb No• H. L. Lehman Okhlhomn ...... , .. Yea Yes No Cnrl R. Reid 'l'oxoo ...... Mountain Montann ...... Yes Partly No Ray Kuhns ldnbo ...... Yea Partly Yes C. C. Hallvik Wyoming ...... Yes Partly No I.E. Russell Utllh ...... Yes Yes Yea Levi Muir• Colorado ...... Yes Yes Yea 0. T. Reedy Novndn ...... Yes Partly Yes F. H. Morrison Now M.o.xi<:0 ...... Arh:onn ...... Pnolflo WMhinglon ...... Yes Partly No Bailey Trompcr Yes No No R. H . Bnldo<:k g~,~~~::::: :::: : :::: :: ::: :: Yes Yes Yes T. E. Stanton • u.... A.A.S.Er.O. Sr>ccifionUons. b With prcscn~ control-11ctUomont reduced but groat& per cent due to subgrade. • Dofi.rulo dor.u,iLy fu rnished when result of pit is obtninod. It seems highly significant that the density requirement in their specifications majority of the States which do not do make laboratory and field tests to exercise moisture and density control use as guides to determine when satis over compaction, other than visual inspec factory compaction is obtained. tion, mention that it would be desirable In practically all cases, fill compaction 15 requirements are the same whether the pacting with several passes of a tamping surfacing is to be a rigid or flexible base roller followed by use of a 3-wheel IO-ton type. roller until the layer is satisfactorily Settlement adjacent to structures is compacted. Under certain soil conditions common to all States and it is admitted the moisture content is controlled within that it is a result of inadequate compaction 1 or 2 per cent of the optimum and com in spite of the fact that hand tamping paction to a predetermined density is in thin layers is specified for places inac required. In some cases the rolling is cessible to other equipment. Apparently done almost exclusively by tamping rollers the reason for unsatisfactory work at using the 3-wheel IO-ton roller at the structures is the difficulty of obtaining end of the day's work to compact the the necessary compaction economically loose top material or to protect the surface and rapidly with hand tools and even from saturation by storms. When the with mechanical tampers. Consequently, fill is of coarse granular material, the most engineers are satisfied to get the 3-wheel roller is used to a greater extent. best result they can with the tools avail New Hampshire reports the use of able. Development of an improved water or binder to stabilize loose sandy mechanical devise for this purpose is soils and sometimes jetting of fills for needed. rigid pavements. Otherwise, most States In contrast to settlement occurring in this area indicate that their difficulties within the body of the fill, failures such are confined to clays and silts. as lateral flow and slipouts causing destruc Massachusetts reports that 95 per cent tion of road and pavement are rare. Use of maximum A. A. S. H. 0. standard of an unsatisfactory fill material placed density is easily obtained with granular at a high moisture content with inadequate soils compacted by hauling equipment compaction will cause such a result. All and power rollers without moisture and other failures of this kind have occurred density control. However, settlements on unstable foundations consisting of and failures have occurred in fills of silts muck and peat and in landslide areas and and clays constructed in this manner. have no relation to method or degree of On the other hand, no trouble was compaction within the fill. experienced with these same soils com The more important details of fill pacted in accordance with moisture con construction practice in the various States tents and densities as previously deter are presented in the following summary. mined by laboratory tests. Michigan varies the methods of con North East struction and degrees of control according to the type of soil as follows: Practically all of the northeastern 1. Light clays and loams are spread group of States require that fill material in 12-in. layers and compacted by be placed in layers not thicker than 12 in. tamping rollers. and rolled with a 3-wheel IO-ton roller 2. Sandy and gravelly soils are spread operating until each layer is satisfactorily in 10-in. layers and compacted with compacted as determined by visual in heavy tread or pneumatic tired equip spection. Only Vermont depends on ment or by inundation in special cases. distribution of movement of hauling 3. Very sandy or gravelly soil having equipment to obtain compaction. a fairly uniform size range is deposited On specific projects, Wisconsin pre and spread in layers about 3 ft. thick scribes sheepsfoot rollers and compaction and compacted by hauling equipment, equal to at least 95 per cent of the maxi saturation, or vibration. mum determined on the same soil in the 4. Heavy clays, loams and silts are laboratory by the A. A. S. H. 0. standard deposited in layers 8 to 10 in. thick and test. compacted by a sheepsfoot roller with One New York district specifies spread rigid control of moisture content and ing the material in 8-in. layers and com- density. 16 Of practical interest is the requirement layer more than 1-in. However, when in the New York specifications that at the soil is so sandy that it is not practica least one double drum tamping roller be ble to compact it with rollers, the material available for each 125 cu. yd. of excavation is deposited in 6-in. layers, saturated with moved per hour. water and rolled with a 10-ton tractor until the entire surface of each layer is Middle East covered at least once by the tractor Half of the middle eastern group of treads. States specify definite density varying from 90 to 100 per cent of the maximum North Central determined by standard laboratory pro Nebraska and Kansas specify that fills cedure. Thickness of layers ranges from be placed in 6-in. layers at optimum 6 to 12 in. Some of these States specify moisture content compacted to 90 per a particular type of roller while others cent of maximum density as determined leave the choice to the contractor. in the laboratory. No particular type The States which do not require density of roller is specified. Missouri requires control rely on visual inspection to deter the same control but demands a density mine satisfactory compaction. Most of of 95 per cent of maximum with a sheeps these States specify a 3-wheel 10-ton foot roller. roller and layers not more than 12 in. Standard practice in North Dakota thick. Pennsylvania calls for 8-in. layers. requires 10-in. layers compacted by a Virginia requires a moisture content sheepsfoot roller until the feet "walk that will permit thorough compaction out". South Dakota also specifies "walk without adherence of soil to the roller. ing out" by the sheepsfoot roller but On fill widening projects 6-in. layers requires that the soil be placed in 6-in. and compaction with a sheepsfoot roller layers. A minimum of 90 per cent of are specified. maximum density is required by North Maryland mentions excellent results Dakota under certain conditions. obtained by the use of a 16-ft. I-beam Iowa specifications require that all dragged behind a bulldozer to obliterate embankments be carried up in horizontal tracking and rutting and thus obtain layers 6-in. thick with each layer leveled better distribution of the hauling equip by a blade glader or bulldozer. Com ment on wide roadway sections. paction is obtained by distributing the Those States specifying density control hauling over the entire surface of each permit variations in the degree of com layer and rolling at least twice with a paction depending on the type of soil. power roller having a weight of at least Greater compaction is required on plastic 275 lb. per in. of width of rear rollers. clays and silts than on granular soils. In Minnesota the fills are compacted with sheepsfoot rollers in layers not South East exceeding 12 in. thick up to within a foot With the exception of Florida, all of of the surface where maximum thickness the southeastern States require that fills of layer is reduced to 6 in. be constructed in 6-in. layers compacted with sheepsfoot rollers. South Central In lieu of a definite density requirement, Louisiana, while not specifying a definite North Carolina specifies at least one trip density, instructs its field men to make with the roller for each inch of thickness: density tests to determine if the fills have South Carolina depends on visual inspec been satisfactorily compacted. Compac tion; and Georgia calls for maximum tion is obtained by means of hauling compaction in the opinion of the engineer equipment and sheepsfoot rollers on layers but never less than five trips of the roller. from 6 to 8 in. thick. Florida specifies 8-in. layers compacted Oklahoma requires all fills to be placed with a sheepsfoot roller until the roller in 6-in. layers at a moisture content near feet do not penetrate the surface of the the optimum compacted with sheepsfoot 17 rollers to the satisfaction of the engineer. a definite density on all jobs but had On sandy soils of such a nature that required close control on particular jobs, compaction with sheepsfoot rollers is not a unit price increase of from 3.5 to 6 cents possible a pneumatic tired roller is specified. per cubic yard was reported. Of the States reporting that a definite density Mountain was specified, about half indicated that Idaho, Utah, Colorado and Nevada construction costs were increased, about require fills to be constructed in layers not one-third said no change had occurred and over 8 in. thick and specify a definite the rest had no information. The esti- density ranging from 90 to 100 per cent TABLE 4.-RELATIVE COST OF CONTROLLED of maximum. Of these four States, only COMPACTION Utah specifies a sl1eepsfoot roller. Ut.ah De- Increase, requires one roller for each 100 cu. yd. of State No cents pe:r change crease cubic yard excavation per hour. ---- Wyoming l'equires 5-in. Lifts, watering North-E8Bt Maine ...... X and rolling with a sheepsfoot roller. Connecticut .... ,, ...... X New York ...... 4- 6 Although no definite density is specified, 3.5 the grade inspectors use laboratory com ~t~~~: i~ :::: ::: ::: :: :: " paction test data to determine, with the Middle-East aid of field density tests when satisfactory Illinois ...... X Indiana ...... X compaction is obtained . Ohio ...... X Pennsylvania . .. , ...... X Montana depends on the distribution of K.en~u.cky ...... 3-6 movement of hauling equipment to obtain V1 rg1n1a ...... ,, • 5 Maryland ...... X satisfactory compaction on fills placed in South-E 8Bt 8-in. layers. Rollers are required only Alabama ...... 2 when fill s are constructed by side casting North Carolina ...... X X or where tractor drawn equipment is ~1:i~~:~ :::: :: ::: :: ::: X not used. North-Central Minnesota ...... X Pacific X ~~ao;;~(·.: :::::::: :: : :: 5 North Dakota...... X On the Pacific Coast, Washington South Dakota ...... X specifies that fills be constructed in layers NebraBka ...... 1 Kansas ...... X not exceeding 8 in. thick, watered as South-Centml required, and rolled until satisfactory Louisiana X compaction is secured as determined by Oklahoma ...... 3. 5---4 visual inspection. Oregon uses laboratory Mountain Montana ...... X compaction tests and field density test.s Idal10 ...... X as a guide to the amount of rolling neces Wyoming ...... 3.52 Utnh ...... 2.5---4 sary but does not include any definite Colortu.lo ...... X density requirement in its specifications. Novndn ...... X Control of both moisture content and Pacific Oregon . . . .•.. ••.... •. •• 3 density in accordance with predetermined values is specified on all jobs in California. Tots.I ...... 14 2 16 Each layer, varying from 4 to 6 in. in thickness, must meet these requirements mated increase varied from less than 1 before the next layer is placed. cent to 6 cents per cu. yd. In most cases where water and rolling were paid for as COST OF CONTROLLED separate items, it was reported that this COMPACTION represents the amount of increased cost. Definite information on the effect of Several of the States noted that there controlled compaction on construction had been a general tendency toward costs was furnished by only a few States. decreased costs due to increased yardage The data reported are given in Table 4. on most jobs and to the use of bigger In all cases where a State did not specify units of grading equipment which had 18 more than offset any increase due to In Nebraska some contractors have special compaction procedures. remarked that if moisture control had not In many insto.nces, comments showed been specified, they would have added that the engi neers prepu.ring the replies water at their own expense. to the questionno.ire fel t that tile increased Oklahoma states that, while costs have cost was more than offset by subsequent gone up on an average of 3} cents per cu. savings in maintenance. Iowa experience indicates that on jobs yd., many settlements have unquestionably where moisture-density control is 1·equired, been eliminated and that over a period the cost will vary greatly depending on of years, it is believed an actual saving the size of the job. will accrue.
APPENDIX CLASSIFICATION OF SOILS TESTS AND THEIR SIG· to the water content at the plastic limit. NIFICANCE3 Above the liquid limit the materials are For use in soil cla.ssification, particle considered to be in a liquid state. Mate grading is measured by sieve analyses, rials such as coarse sand that do not have and the binding properties of the f:ine spaces of capillary size are not plastic soil fraction by the tes~ for liquid limit, although they may hold water in the plastic limit and plasticity index. The film phase. binding property tests are made upon The liquid limit test measures the that portion of a soil sample passing a maximum amount of water that the No. 40 sieve. material will hold in the film and capillary conditions. A high liquid limit therefore Sieve Analysis indicates that the material will hold large amounts of film and capillary water This test measures the relative amounts due to the pl'esellce of f:ine particles with of various sizes of particles in the material. Lbearing capacity divided into two parts, namely, the plastic at high densities and function satisfactorily and friable types. The friable type when used as bases for relatively thin usually has a plasticity index ranging wearing courses. from NP to less than 3 and can be used as Grading: The soil mortar, that fraction base course material for bituminous sur passing the No. 10 sieve, should be graded faces where a moisture content sufficient as follows: Clay, 5 to 10 per cent; silt, to insure stability can be maintained or 10 to 20 per cent; total sand, 70 to 85 where the material is completely confined. per cent; coarse sand, 45 to 60 per cent. This type is also suitable for use as a Constants: The liquid limit is usually blanketing material for very plastic sub greater than 14 and less than 35; the grades over which concrete pavement is plaRticity index ranges from 4 to 9. to be placed. The plasticity index of The characteristics of this group of the plastic type ranges from 3 to 15. When soils are such that the test constants fall the plasticity index of this type of soil into a rather narrow band inasmuch as exceeds 6, it is not suitable for use as a • From "Classification of Soils and Control Procedures base for light bituminous surfacing and Used in Construction of Embankments". Harold Allen, Public Roads Feb., 1942. may cause warping of concrete pavements 20 if large fluctuations of moisture content do not produce severe pavement warping are likely to occur. even though compacted in the dry state. Soils of this group (plastic or friable) The silty loams and silts cannot be may be considered as stable if well com rolled to high densities because of the pacted and they are satisfactory for the excess of voids which results from inferior construction of embankments or the grading and because of a lack of binder blanketing of the plastic or silty soils. material. They are relatively unstable They can be drained and may have at all moisture contents but especially sufficient plasticity to cause detrimental at the higher moisture contents when they volume changes. Bituminous materials, have very low stability (low bearing portland cement, and other admixtures capacity). Silts and some silty loams can be mixed with soils of this group with are difficult to roll because best rolling comparative ease. results may be obtained only through a Group A-3.-The soils of this group very narrow range of moisture. Uniform are composed entirely of coarse materials compaction can be obtained on these such as sand and gravel; they lack stability soils by the use of smooth-faced rollers, under wheel loads except when damp; provided the soil is neither too wet nor are only slightly affected by moisture too dry. If the moisture content is too conditions; have no volume change. They high or too low, "bridging" will occur cannot be compacted by rolling but in with heayy smooth-faced roller (soils will most instances may be settled by disking bulge up ahead and behind the roller) and ponding. They drain rapidly and, resulting in nonuniform compaction. when adequately confined, make suitable The clay loams of this group are some subgrades for all types of pavement. what better graded than are the silts Grading: The fraction passing the No. and can be rolled to higher densities. 200 sieve is less than 10 per cent. On heayy clay loams tamping rollers have Constants: Soils of this group have proved more effective than rollers of the no plasticity. smooth-faced type. The clay loams are A-3 soils are of common occurrence. quite stable at the lower moisture contents Many of them can be stabilized success and higher densities but under these fully with bituminous materials. conditions are likely to show detrimental Group A-4.-This group consists pre volume change if the moisture content is dominantly of silt soils containing only increased. moderate to small amounts of coarse Grading: The sand content is less than material and only small amounts of sticky 55 per cent. colloidal clay. When fairly dry or damp, Constants: The liquid limit of soils in A-4 soils present a firm riding surface this group varies from 20 for sandy loams which rebounds but little upon removal of to 40 for clay loams. The plasticity load. When water is absorbed rapidly, index varies from O for coarse silts with no they may expn.».d detrimentally or lose binder to 15 for clay loams. Group A-4 stability even in the ab ence of manipula soils are likely to be highly expansive. tion. They a.re subject to frost heave. The wide range of soils in this group The soils of this group vary widely in extends from those which border the A-2 textural compostion and range from the group to those which approach the lower sandy loams to silt and clay loams. A limits of the A-5, A-6, and A-7 groups. comparison of the grain-size analysis The borderline soils are often designated curves indicates wide variation in grading as A-4-2, A-4-5, A-4-6 and A-4-7, indicating within the group. that they approach the latter group in The sandy loams can be rolled to com characteristics, grading, and values of parnti vely high densities with either test constants. tamping or mooth-faced rollers and have Since the soils in this group are subject good stability through a wider range of to frost heave, they should be covered densities than do the silts and silty loams. with granular materials in areas where They have only small volume change and extremely low temperatures prevail and 21 conditions conducive to frost heave exist. compacted to maximum practical density; The thickness of cover required to prevent are compressible and rebound very little heaving varies from 18 to 48 inches. upon removal of load; are very expansive When wet, these soils may become and productive of severe warping in elastic and show considerable rebound concrete slabs if placed sufficiently dry to upon removal of load. allow water to be absorbed in large quanti The most plastic types in the group will ties. expand with increases in moisture in Grading: The sand content is less than sufficient degree to cause warping at the 55 per cent. joints in concrete slabs if the soils are Constants: The liquid limit exceeds 35, placed at moisture contents lower than the plasticity index is greater than 18. the optimum. Bituminous surfaces re The high plasticity indexes of the soils quire substantial base courses when placed of this group indicate the very cohesive on subgrades consisting of any of the nature of the binder material (clay and varieties of this group. colloids) at the lower moisture contents. Group A-5.-This group is similar to The cohesion decreases as the moisture the A-4 group except that it includes content increases. Therefore, since group very poorly graded soils which contain A-6 soils do not possess much internal materials such as mica and diatoms which friction, they have low stabilities at the are productive of elastic properties and higher moisture contents. Consequently, very low stability. Soils of this group they are suitable for use in fills and as are likely to be elastic and to rebound subgrades only when they can be placed upon removal of load even when dry. and maintained at a relatively low moisture Elastic properties of these soils interfere content. with the proper compaction of flexible-type In the field, group A-6 soils are character base courses during construction and with ized by the presence of shrinkage cracks the retention of good bond afterward. on all surfaces exposed to drying. Grading: The sand content is less than Water moves very slowly through soils 55 per cent (exceptions occur). of the A-6 group even when under a very Constants: The liquid limit is usually considerable head. Thus, these soils will greater than 35. The plasticity index take up water very slowly unless manipu usually ranges from O to 20 but in some lated, and, conversely, once they become cases may be as high as 60. wet, they will dry out very slowly. The The soils in this group are not suitable flow of gravitational water through them for use as subgrades for thin stabilized is negligible and, consequently, ordinary base courses or bituminous surfaces. They drainage installations are of little value. are subject to frost heave and should be It should be emphasized that while the covered with granular materials when rate of flow of water through group A-6 they are encountered in subgrades in areas soils is very slow, the capillary pressure where extreme freezing conditions prevail. which causes moisture to .move from the They are usually difficult to compact due wetter to the drier portions is very great to their tendency to rebound upon removal and large forces can be developed for of load. It has also been observed that that reason. pavements laid over subgrade soils of Soils of the A-6 group are confined this group crack excessively. within closer limits in their general char Group A-6.-This group is composed of acteristics than are those of either the predominately clay soils with moderate A-4 or A-7 group. Borderline soils are to negligible amounts of coarse material. often designated as A-6-4 or A-6-7 soils. In the stiff or soft plastic state they absorb Soils in this group are not suitable for water only when manipulated. They use as subgrades under thin flexible base can be compacted to relatively high courses or bituminous surfaces because of densities by the use of heavy rollers and the large volume changes that are caused can best be compacted with tamping by moisture fluctuations and the loss of rollers; have good bearing capacity when bearing power upon the entrance of mois- 22 ture. When concrete slabs are placed constitutes a wide range of soils varying over these soils, the subgrades should be in characteristics from those bordering blanketed with nonexpansive materials on the A-4 and A-5 groups of silts and or should be compacted to high densities loams and the A-6 group of clays to those at carefully controlled moisture contents. approaching the lower limits of the A-8 Areas immediately adjacent to the slab group, which contain excessive organic which are exposed to drying should be material. Such borderline soils are often protected by covering with nonexpansive designated as A-7-4, A-7-5, A-7-6, and material, such as A-1 or friable A-2 soil, A-7-8 indicating similarities to the latter or other insulating material to prevent groups. loss of moisture by evaporation from the Since the soils in this group are even subgrade and subsequent warping of the more expansive than those of the A-6 pavement due to reentrance of moisture. group, the same precautions in their use Soils of this group occurring in subgrades should be observed. Due to their elasticity for macadam or similar porous base courses and tendency to rebound, they should should be covered with an impervious, be compacted with great care when used nonexpansive material similar to soil of as subgrades for concrete slabs, and should the A-1 or A-2 groups. not be subjected to excessive loading Group A-7.-Soils of this group are immediately ahead of paving, if early similar to those of the A-6 group except cracking of the slab due to the force that at certain moisture contents they are exerted by the rebounding soils is to be elastic and deform quickly under load avoided. and rebound appreciably upon removal In areas where low temperatures pre of load. This characteristic results from vail, the soils in this group should be an inferior grading (steep grain-size curve regarded with suspicion because some of through the silt fraction); from extraneous them are subject to frost heave. material such as organic matter, mica Group A-8.-The soils in this group are fla.kes, lime carbonate; from a vaiiation in composed of very soft peat and muck. grain shapes or from a combination of They contain excessive quantities of any two or mol'e of tbe.se causes. Alter organic matter and moisture. They are nat-e wetting and drying of the A-7 soils obviously unsuitable for use in subgrades under field conditions leads to rapid and or embankments. detrimental volume changes. Grading: The grading is not significant. The soils of this group are more diffioult Constants: The liquid limit ranges from to compact by rolling than are those of 35 to 400, the plasticity index from O to the A-6 ,group. Heavy tamping rollers 60 and is usually less than 25. have been found most effective for rolling The tendency to contain capillary A-7 soils. Soils in this group have good moisture in large amounts far above the bearing capacities when compacted to water table makes these soils unsatis high densities but are subject to excessive factory for use as foundation soils for volume change unless properly compacted embankments. Their use in any type of at a moisture content sufficiently high to construction should be avoided whenever insure minimum air voids. These soils possible. have produced more severe warping of concrete slabs than have soils of other REFERENCES groups. Grading: The sand content is less than 1. Allen, Harold, "Compaction of Em bankments, Subgrade Treatments and 55 per cent. Soil Stabilization", Proceedings, Constants: The liquid limit for soils Highway Research Board, Vol. 18, of this group exceeds 35 and the plasticity P art. 1, p. 39 , 193 . index is greater than 12. 2. Casagrande, A., ar.\d Stanton, T. E., The ma.jor dillerence between soils of "RoaDams Fill Construction", Proceedings, High and Emb1mkmcnts, ProceMlinos, way Research Board, Vol. 17, p. 503, An1criMn Society of ivil E11gineers, 1937. Vol. 66, No. 8, Part!, p.1401, October, 17. Peckworth, Howard F., "Field Control 194.0. of Compact Earth Fills", Civil En 4. Gilboy, Glennon, "Mechanics of Hy gineering, Vol. 9, o. 1J., p. 221, April, d1·aulic-Fill Dams", J 01tmal Boston, 1939. Discussioo: Abdun-Nur, E. A., Society of Civil Engincel'S, Vol. 21, No. 6, p. 372, June 1939, and Peck o. 3, p. 185, July, 193 11-. worth, H.F., No. 9, p. 563, Septem 5. Hamilton, L. W., Preece, E. F., Stan ber, 1939. ton, T. E., John on, A.. W., Woods, 18. Proctor, R. R., "Fundamental Princi IC. B., and Casagmndc, L., "Compac ples of Soil Compaction", Engineering tion of Earth Embankments", Pro News-Record, Vol. 111, Nos. 9, 10, 12, ceeding:1, Highway RcsQa.rch Boal'd, and 13, Aug. 'ept., 1933. Vol. 18, Part 2, p.142, December, J93 . 19. Sours, H. G., and Shepard, C. H., 6. Hogen.togler, C. A., "Developments in "Embankment Compaction Control as Soil SLa.bi lization", ProcceDam", 13. Lyman, A. K. B., "Compaction of Soils Proceedings, International Conference by Explosives", Civil Engineering, on Soil Mechanics and Foundation Vol. 10, No. 4, p. 205, April, 1940. Engineering, Vol. 1, p. 266, 1936. 14. Mullis, Ira B., "Principles Applying to 27. Woods, IC D., "Soil Mechanics in High Highway Roadbeds" (with discus way Construction" Civil E1igineering, sion). Transactions, American So Vol. 7, ro. 1, p. 5, January, 1937. ciety of Civil Engineers, Vol. 104, p. 28. Woods, IC. B., and Litehiser, R. R ., 1381, 1939. " oil Mechanics Applied to Ilighway 15. Nikirk, F. A., "Report of Committee on Engineeri11g in Ohio", hio tutc Soil Compaction, Report of Sub-Com University Eugineer.ing Experiment mittee No. 2", Proceedings, American , tation Bulletin, rro. O!l , 1938. 24 29. Woods, K. B., "Application of Soil Materials for Franklin Falls Dam", Mechanics in Construction of Earth Corps of Engineers, U.S. Army, Bos Embankments", American Associa ton, Massachusetts, April, 1938. tion of State Highway Officials Con 32. "Compacting Highway Fills with Crane vention Group Meetings, p . 136, and Ball", Public Works, Vol. 68, No. December, 1938. 8, p. 12, August, 1937. 30. Woods, K . B ., "Engineering and Con 33. Taylor, Donald W., "Stability of Earth struction Control of Embankments Slopes". Journal, Boston Society of and Bases", Proceedings, American Civil Engineers, Vol. 24, No. 3, p. 197, Road Builders Association, 1941, p. July, 1937. 103. 34. "Engineering Field Manual," Chapter 31. "Compaction Tests and Critical Den XX, Office of Chief of Engineers, War sity Investigation of Cohesionless Department.
25