BNGINEERING MONOGRAPHS No. 28
United States Department of the Interior BUREAU OF RECLAMATION
PETROGRAPHIC AND ENGINEERING PROPERTIES OF LOESS
by H. J. Gibbs and W. Y. Holland
November 1960 $1.00 ~
BUREAU OF RfCLAMA nON l' Cr~~ DENVER UBf!ARY If t (/l I'I'II~'IIIII~/II"~III'920251b3 tl J t, \r , ;;"" 1; "," I. I ,' ;' ~'", r '.'" United States Department of the Interior 'Jo' 0 I .'.~!1 .~ j FRED A. SEA TON, Secretary Bureau of Reclamation FLOYD E. DOMINY, COMMISSIONER
GRANT BLOODGOOD, Assistant Commissioner and Chief Engineer
Engineering Monograph
No. 28
PETROGRAPHIC AND ENGINEERING PROPERTIES OF LOESS
by H. J. Gibbs, Head, Special Investigations and Research Section Earth Laboratory Branch and W. Y. Holland, Head, Petrographic Laboratory Section, Chemical Engineering Laboratory Branch Commissioner's Office, Denver, Colorado
Technical Information Branch Denver Federal Center Denver, Colorado ENGINEERING MONOGRAPHS are published in lirnit~d ~@ition~for till! tl!chnicA.l !t!.ff of the Bureau of Reclamation and interested technical circles in Government and private agencies. Their purpose is to record devel- opments, innovations, and progress in the engineering and scientific techniques and practices that are employed in the planning, design, construction, and operation of Recla- mation structures and equipment. Copies may be obtained from the Bureau of Recla- mation' Denver Federal Center, Denver, Colorado, and Washington, D. C. CONTENTS
Page
INTRODUCTION...... 1
GENERAL DESCRIPTION OF LOESS...... 1
ORIGIN OF LOESS...... 4
DETAILED DESCRIPTION OF LOESS...... 6 Type of Matrix...... 7 Cementation...... 8 Particle Shape...... 8 Granular Components...... 9 Grain Coatings...... 9
OBSERVATION OF PHYSICAL PROPERTIES...... 9 Location of Engineering Studies...... 9 Gradation...... 11 Specific Gravity...... 11 Plasticity...... 11 Compaction...... 11 Permeability...... 11 Consolidation...... 14 Shear Resistance...... 14
RESEARCH FINDINGS FOR LOESS...... 20 '.' Results of Plate Load Tests on Loess...... 20 Results of Ponding Tests...... 20 Example of Prewetting...... 22 Behavior of Piles in Loess...... 22 Results of Pile Driving Studies...... 26 Results of Pile-load Tests...... 28 Summary - Study of Piles in Loess...... 28
DENSITY OF LOESS AS A CRITERION OF SETTLEMENT...... 32
CONCLUSIONS...... : ...... 35
LIST OF REFERENCES...... 37 LIST OF FIGURES
Number Page
1 Outline of major loess deposits in the United States. . 2
2 Photographs of typical characteristics of loess deposits...... 3
3 Photomicrographs of loess structure...... 5 4 Location of major loess deposits in the Missouri River Basin...... 10
5 Trends of gradation curves for loess...... 12
6 Trends of plasticity characteristics of loess ...... 13
7 Trends of permeability for loess ...... 15 8 Consolidation characteristics of loess...... 16 9 Generalized consolidation trends ...... 17 10 Shear tests of representative loessial soils...... 18 11 Shear and volume change characteristics of dry and wet loess ...... 19
12 Plate-load tests for a typical loess deposit for comparison of material at natural moisture and prewetted conditions...... 21 13 Records of ponding experiments at Ashton pile test areas ...... 23 14. Prewetting of loess foundation by ponding at Medicine Creek Dam. . . . . 24 15 Medicine Creek Dam foundation settlement installations...... 25 16 Profiles and plans of test sites...... 27 17 Gradation tests...... 27 18 Method of making pile load tests...... 27 19 Driving records...... 29
20 Comparison of pile load tests for friction piles in loess...... 29 21 Requirements for adequate supporting capacity of piles ...... 30 22 Requirements for practical placement of piles...... 31 23 Natural density versus liquid limit relationship for loessial soils. . . . . 34
24 An example of subsidence of low-density loess after saturation from canal operation...... 35
v INTRODUCTION
This monograph summarizes knowledge and canals in these areas, the Bureau re- of the physical and physical-chemical prop- quired information on the characteristic erties of loess gained from Bureau of Rec- properties of loess, particularly those prop- lamation experiments, investigations, and erties which would be affected by moisture observations. Because of the unstable prop- change s resulting from the operation of erties of loess which may cause settlement hydraulic structures. This monograph dis- of foundations of structures, such knowledge cusses the origin of loess and presents de- of the limitations of loess for engineering tailed descriptions of the pertinent charac- purposes is important not only to the geolo- teristics of loessial soils including matrix, gist and soil mechanics engineer but also cementation properties, and particle shape. to design and construction engineers who It discusses physical properties including are required to build structures on loessial permeability, compaction, consolidation soils. and shear resistance characteristics, and summarizes the results of loading tests of The Bureau of .Reclamation has parti- footings and piles. A criterion for deter- cular interest in the engineering properties mining the critical nature of loess in respect of loess because of its requirements for to the probability of settlement is given on large hydraulic structures in loessial re- the basis of observations and tests in the gions of the western United States. To de- Nebraska-Kansas area. sign and construct such structures as dams
GENERAL DESCRIPTION OF LOESS
Loess is an earth material which covers are also weakly resistant to moisture move- vast areas of the central part of several ments as displayed by the erosion patterns continents of the world. The major loessial of natural drainage streams, as shown in deposits in the United States are shown on Figure 2c. the map in Figure 1. The most extensive loessial area is in Nebraska, Kansas, Iowa, Olr studies lead us to support the theory Wisconsin, illinois, Tennessee, and Missis- that the finely divided rock-powder silt of sippi. Other loessial deposits occur in which loess is primarily composed was pro- southern Idaho and Washington. The deposits duced by interaction of sever~l geologi~al of major concern to the Bureau of Reclama- processes, but mainly by abrasIOn resultl?g tion are in Nebraska, Kansas, Idaho,'and from movements of continental glaciers. Washington. With retreat of the continental glaciers, the silt was deposited along flood plains of riv- ~cause the particle structure of natural ers. This silt was then transported, sorted, loess is often very loose and contains appre- and redeposited by wind action. Loess is ciable voids, problems related to large con- composed primarily of fine, angular grains solidation under certain moisture and load of silt, fine sand, and calcite admixed with combinations, poor stability, seepage, and relatively small amounts of clay and clay erosion frequently arise. On the other hand, coatings on the larger grains. Some organic loessiql soils which have been reworked by matter occurs in the form of roots and root water action are usually in a more dense hairs. The particle arrangement in the soil state and exhibit better engineering proper- mass resulting from the wind-blown method ties. Likewise, loessial soils, which are of deposition is frequently very loose. There- remolded and compacted as construction fore, loess is susceptible to high degrees materials, also exhibit better engineering of consolidation. However, the dry loess properties. Naturally dry loess exhibits which exists in the arid regions of the mid- considerable stability permitting it to stand western states has high strength owing to on steep cut slopes, as shown in Figure 2a, the binding characteristics of the clay and provided the natural moisture contents are is often capable of supporting relatively not increased. Natural drainage troughs large loads without excessive settlement. where higher moisture contents accumu- It is easily collapsed by heavy loading and late display slumping and subsidence, as wetting, and the bond between particles can shown in Figure 2b, which illustrates a cri- be reduced to practically no strength by tical condition that can occur in hydraulic wetting. structures that eventually saturate portions of their foundations. Wetted loessial soils Loessial soils have been analyzed in
1 ~.~?
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'f STEEP SLOPES CONSTRUCTED IN NATURALLY DRY LOESS SHOWING CONSIDERABLE STABILITY
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A NATURAL DRAINAGE CHANNEL WHERE HIGH MOISTURE CONTENTS ACCUMULATE, CAUSING SLUMPING AND SUBSIDENCE
TYPICAL EROSION PATTERN OF NATURAL DRAINAGE STREAMS IN LOESS (c) PHOTOGRAPHS OF TYPICAL CHARACTERISTICS OF LOESS DEPOSITS FIGURE 2 3 considerable detail by the Earth and Petro- which was cut from a similar undisturbed graphic Laboratories of the Bureau of Rec- sample of loess. This photomicrograph lamation.. The initial sununary of consolida- has a magnification of 30X, and shows the tion and related properties of loess observed relative size of the void spaces. The prep- by the Bureau of Reclamation* was reported aration of this thin section and other sec- by Holtz and Gibbs in a 1951 paper 1..1. tions was accomplished by impregnating a carefully cut specimen with a thermoplastic resin which acted as a binder during the Theundisturbed structure and the miner- grinding process. alogical characteristics of loessial soils have been observed. The internal structure of The majority of the silt grains in loess loess is shown in Figure 3. Figure 3a is a are coated with very thin films of clay. This photomicrograph of undisturbed loess soil clay is generally montmorillonite and forms taken with a magnification of 12X. This intergranular supports or braces within the photograph shows the loose arrangement structure. Calcite usually occurs in distinct of silt particles with typical numerous voids silt-sized grains throughout the loess in a and rootlike channels. The structure is finely dispersed state rather than as a ce- typical of natural wind-blown deposits of menting material. The thin clay coatings loess which have not been reworked. Figure and, to a lesser extent, the calcite, ap- 3b is a photomicrograph of a thin section parently bond the particles together. Upon *Numbers in superscript refer to publica- wetting, this bond is easily loosened caus- tions in the List of References. ing great loss in strength.
ORIGIN OF LOESS
Certain conditions must be present for water silt issuing from glaciers after the the development of loessial deposits. These silt had dried upon outwash plains, and re- conditions are: deposited this silt in regions bordering the glaciers. Other sources probably contri- (1) A source of intermixed silt and clay buted to the formation of loess but their adequate enough to account for the known contribution was small. deposits In the Missouri River Basin and Upper (2) A period of strong winds prevailing Mississippi River Valley the source of the from one direction wind-blown silts is generally agreed upon to be of glacio-fluvial origin. At various (3) A place of deposition interglacial periods and at the close of the Pleistocene ice epochs, large areas along (4) Arid to semi -arid conditions at least the flood plains were left devoid of plant following the issuance of silt from the gla- cover and exposed to strong winds. The ciers and at all times following deposition flood plains of these rivers were composed mainly of sand, silt, and clay derived from The origin of loess has been a subject of the abrading action of the glaciers. The much debate for many years. It is now gen- silt and clay particles with intermixed fine erally agreed that loess was wind transported sand were sorted from the coarser sand, and deposited, and that the sediments were transported, resorted, and redeposited by of glacio-fluvial origin. The finely divided wind, the coarser sized particles being de- rock powder of which loess is composed was posited close to the source and the finer produced by the interaction of several geo- particles deposited at greater distances logical processes, but mainly by abrasion from the source. Smith 3/ in his investi- resulting from movement of continental or gation of the Illinois loess, and Swineford large mountain glaciers. With the retreat and Frye~..I in their investigation of Kansas of the glaciers, the suspended load of the loess showed that the grain size of loess glaciers was deposited in outwash plains decreases logarithmically away from the and subjected to erosion both by wind and source. Swineford and Frye traced the by water. The material carried off by the source of most of the Kansas loess by parti- streams was later deposited as silt along cle size distribution to the Republican and the flood plains of rivers and then redepos- Platte Rivers. These rivers carried much ited by wind action. As noted by Hobbs 2/, silt which had been derived by the abrading anticyclonic winds blowing from the retreat- action of glaciers in the Rocky Mountain ing glaciers could have acted upon the melt- region.
4 (A) PHOTOMICROGRAPH SHOWINGTHE TYPICAL LOOSE (8) PHOTOMICROGRAPH OF A THIN SECTION OF LOESS OPEN STRUCTURE OF SILTY LOESS SOILS. THE TAKEN TO SHOW THE MICROSTRUCTUREOF AM AREA MICROBOTRYOIDAL TYPE OF STRUCTURE WHICH OCCURS SIMILAR TO THAT SHOWN IN FIGURE 4. NOTE THE FREQUENTLY THROUGHOUT THE LOESS IS ALSO SHOWN EXTREME POROSITY(LIGHT AREAS) OCCURRING IN IN THE LOWER CENTRAL PORTIONOF THE PICTURE. MAGNIFICATION 12X SUCH AREAS. MAGNIFICATION 30X PHOTOMICROGRAPHSOF LOESS STRUCTURE FIGURE 3 Vegetation, as pointed out in some the- was and continues to be a source of inter- ories, may have contributed to holding the mixed silt and some clay but the wind-blown soil together after its deposition. Fine material is deposited either in the Atlantic clayey silt materials, however, are not Ocean, or as in the Arabian Desert, the generally disturbed by wind and are picked wind-blown material is deposited into the up only if there is intermixed silt or sand. Red Sea or the Persian Gulf. Loess then could have been deposited with- out the sustaining benefit cover of grasses, Intermixed silt and clay may be picked and the advent of grasses after the deposi- up by the wind from its source only when tion of the wind-blown silt would provide an dry climatic conditions exist following their excellent growing material for larger plants. deposition from either a fluvial or glacio- The development of approximately vertical fluvial environment. Winds are unable to root systems might give the loess its verti- pick up intermixed wet silt and clay. Cli- cal cleavage and would contribute in forming matic conditions, favorable to deposition, in loess . its vertical porosity and permea- therefore, do not necessarily have to be bility. arid or semi-arid but may be Arctic. De- posits of loess have been noted in the Ma- Wind-blown, intermixed silt and clay tanuska Valley, Alaska 5/, and Greenland 6/. that eventually became loess must have been Photographs of silt risfrig from the dry flood deposited directly to the ground surface from plains of the Tanana River, Alaska, il- aerial transportation. Silts transported by lustrate that the material must be dry be- wind, but deposited under subaqueous con- fore it can be transported. After deposi- ditions do not have the typicalloessial fabric. tion and the development of a cohesive fabric, Adequate sources for loess may exist, but the climatic conditions must remain arid if no suitable terrestrial areas for deposi- or semi-arid for loess to retain its open tion exist, no deposits of loess will form. fabric. For example, the Sahara Desert probably
DETAILED DESCRIPTION OF LOESS
Loess is a quartzose, somewhat feld- Many materials transported by either spathic clastic sediment composed of a UBi- water or glaciers alone or a combination form]y sorted mixture of silt, fine sand, and thereof yield silts which resemble loess in clay particles arranged in an open, cohesive gra.in size, distribution, and mineralogical fabric frequently resulting in a natural dry composition. In the majority of water-laid density of 70-90 pounds per cubic foot. De- sediments, however, the constituent grains posits may reach considerable thickness. are fairly closely packed after the deposit has been subjected to several cycles of alter- Materials which are not cohesive and nate wetting and drying and burial. It is which are composed predominately of either possible that freshly deposited water-laid granular silt, and fine sand particles, or silts may develop an open fabric similar clayey silt and fine sand size aggregations, to loess and may be cohesive to some ex- are not considered as loess. Dust deposits tent. However. with subsequent wetting which may be composed of particles finer and the deposition of another layer of silt than or equal to predominant grain sizes the open fabric is lost and consequently the found in loess are not considered as loess. unit weight or density of the water-laid silt The dust deposits may be potentially capable increases over the freshly deposited silt. of forming loess, but until the coarse wind- blown material develops cohesion, the term loess cannot be applied. Similarly, clayey silt and fine sand size aggregations may ac- Since wind-blown silt, which is deposited quire cohesive properties and an open fabric under subaqueous conditions, is not consid- similar to loess, but these materials cannot ered as loess and will have a different fab- be classified as loess if the wind-blown clay ric from the wind-deposited silt, it is not consolidates and the mineralogicalcomposi- impossible to have wind-blown silts de- tion is different from loess. Deposits which posited under lacustrine or playa conditions consist of cohesionless silts, fine sands, or to be mixed with or graded into loess. Small clayey silt and fine sand size aggregations lakes most likely existed after Pleistocene transported and deposited by wind should be glaciation and gradually became filled with referred to as aeolian or wind-deposited wind-blown silt, fluvial silts, or sands and silts or fine sands or clays. could have been covered over with loess.
6 Silts transported and deposited by wind there is an increase in moisture content of vary considerably in their dynamic physical the loess body and the moisture content may properties depending upon whether the silt become so large that soil flow results. was deposited under arid, semi-arid, or humid conditions. The wind-blown silts From an engineering standpoint, loess deposited directly to the ground surface is a potentially unstable clastic sediment. under arid and semi-arid conditions develop Loess is in equilibrium with its environ- an open cohesive fabric and retain this fabric ment, but as the moisture conditions change as long as the climatic conditions do not or the overburden load becomes too great, change. Local variations in the density or loess tends to consolidate and assume a closeness of packing in the massive loess more stable arrangement of its grains. probably occur, as local heavy rains would Consolidated loess assumes properties collapse some of the open fabric. Loess similar to those of any other water-laid deposited under semi -arid conditions would silt having a similar granular and miner- probably develop a protective rather than alogical composition. impervious surface layer, as the effect of rainfall over a long period of time would tend The properties of loess, whether con- to be distributed over a large area rather sidered from a physical or physical-chemical than being localized as in an arid region. viewpoint, are dependent upon two factors, namely, fabric and mineralogical composi- Wind-blown silt deposited under.humid tion. If two loessial materials with the same conditions or under arid conditions and later fabric and mineralogical composition are subjected to humid conditions develops a subjected to physical tests, they should and close packed fabric by a collapse of inter- do act similarly. However, if anyone of the granular supports or a gradual bringing to- factors constituting fabric changes or the gether of the granular components by wetting composition of the clayey fraction changes, and drying. The densities of these wind- the properties are changed materially. A blown silts are frequently more than 100 change in the granular components, such as pounds per cubic foot. Samples of material substituting feldspar for quartz, does not obtained from Natchez and Vicksburg, Mis- affect the physical properties materially, sissippi,. have densities ranging from 102 to but with the introduction of volcanic glass 112 pounds per cubic foot. with its shard-like form as a granular com- ponent, the properties of the loess would The upper few feet of loess frequently certainly change. show a collapsed or a denser fabric than the underlying loess. This collapse in fabric may be produced by a variety of factors such Type of Matrix as the activities of man, a progressive change to a more humid climate with in- The type of matrix in loessial materials creased weathering, or by sheet erosion. is commonly clayey and less commonly cal- The upper few feet of loess form top-soil. careous. Calcite is present in many loessial The primary or open fabric of the top-soil materials as grains and crystals but may be is destroyed by man during plowing and farm- secondary. In the open fabric types of loes- ing operations and possibly irrigation. These sial materials the larger silt grains are con- operations tend to disperse the clay fraction nected by what might be called inter granular more evenly throughout the granular compo- supports which are composed mainly of a nents and produce a soil in which the granu- montmorillonite-type clay mineral and pos- lar components tend to be in contact. If sibly intermixed. with small amounts of illite. loess is subjected to humid climatic condi- tions, it gradually consolidates and the whole In other types of loess, the clayey com- deposit attains a dense fabric. However, the ponents are distributed interstitially but with upper few feet in this case are subjected to the silt grains not in contact. Voids may be weathering and sheet erosion, and the com- distributed throughout the matrix but are ponents begin to alter. Also the clayey com- usually not distinct. This type of fabric ponents are more evenly distributed through- seems to be the more common type. The out the granular components. density of loess with this type of fabric is generally less than or equal to the loess with The upper few feet of loess or the whole intergranuJar supports. In a loess from Ken- massive loess deposit may be affected by newick, Washington, the granular compo- slope wash. If loess is deposited on a slope, nents are so fine grained that the arrange- gravitational forces act upon the body and ment of the grains in respect to the matrix the mass slowly moves down the slope. In is difficult to see. The silt grains are evi- this process the open fabric is destroyed dently dispersed throughout the clay, which and a closely packed fabric results. Gravi- is mainly illite, and a petrographic thin sec- tational forces become more active when tion reveals only a continuous fabric.
7 In wind-blown silts which have been de- nate would tend to concentrate in these pores posited under subaqueous conditions or humid with formation of concretions. With re- conditions the matrix remains clayey, but peated dissolution and precipitation the car- in the se materials the clay evidently is bonates would tend to develop better crystal- slightly more compacted and the grains are line forms as are observed in many loessial in contact which gives stability to the mass. masses.
Calcite may occur asa secondary matrix Particle Shape material in loess owing to leaching from above or by the evaporation of capillary The shape of the individual granular water from ground water below. However, components of loess ranges from irregular zones containing calcite are generally not to platy, fibrous, or prismatic. The de- of great thickness under semi-arid condi- gree of rounding ranges from subround to tions. angular. Quartz and the feldspars are com- monly irregular in shape, whereas the mica Cementation minerals are platy or shred-like. Horn- blende, zircon, pyroxenes, and apatite are Many investigators have stated that the commonly in prismatic form and sillimanite primary cementing material in loess is cal- in fibroUs form. cite and/or dolomite, mainly because the sample effervesces if hydrochloric acid is Calcite is present as irregularly shaped added. In samples of loess from Kansas, grains, rhombohedral crystals, and as Nebraska, Iowa, Washington, and Israel patches composed of very fine-grained parti- which were analyzed by the Bureau of Rec- cles. Dolomite is usually present as rhom- lamation laboratories, calcite and dolomite bohedral crystals. were found by microscopic investigation to occur as anhedral detrital silt-size grains Acidic volcanic g]ass in the shape of vol- and as euhedral crystals, whereas the clay canic shards is often observed in loess from minerals occurred as the main cementing the western United States. These shards materials. In samples of loess from Natchez have a variety of shapes and are fresh or . and Vicksburg, Mississippi, calcite and moderate]y altered to montmorillonite. Dia- dolomite also occur as crystals and grains. toms and globigerina tend to Occur rarely Calcite and dolomite may help slightly to bind in loess. In some loessial materials, very the soil particles together, but the major small tubes composed of opal are present. cementing materials appear to be the clay The tubes are thin walled and have a very minerals. irregular surface. These tubes are thought to have formed around the fine root hairs of If the main cementing minerals were plants. carbonates, the loessial materials should not slake or soften appreciably in water. All The clay minerals, montmorillonite of the samples of loess examined slake read- and illite, occur as very fine platy crystals ily in water, either immediately or within which are commonly cemented into aggre- one or two minutes. Further evidence that gl;ltes between the granular components. the carbonates are not the cementing mate- Montmorillonite commonly occurs as thin rial in loess is shown in consolidation and hulls around the grains. Illite, however, triaxial shear tests. If the carbonates were has a tendency to occur as individual shred- the main cementing material, the test speci- like crystals. Platy crystals of kaolinite mens should not show (as they do) immediate and octohedral crystals of cristobalite in consolidation when water is added to the soil loess deposits in Kansas have been observed having low natural moisture content. by Swineford and Frye 4/ with the electron microscope. - Zones of loess may contain carbonates as the principal cementing material where the carbonates have been leached from the Organic matter, which is visible under upper loessial material and redeposited at the petrographic microscope, ranges from lower depths. The introduction of carbo- fine capillary root hairs to plant stems. nates may be accomplished also by capil- These plant stems may be partially decom- lary action. If the ground water table is posed or totally carbonized. sufficiently close to the loessial body, water carrying Ca++ in solution as the bicarbonate may be drawn up into the loess, and with a The majority of the granular components change in pressure or temperature,- calcium are subangular in shape, the grains becom- carbonate would be precipitated in the pore ing more angular as the grain size decreases spaces. In the large pore spaces. the evapo- and mare rounded as the grain size increase s ration should be greater and calcium carbo- towards the fine sand size.
8 Granular Components Grain Coatings
Most of the ~ranular components of The granular components, as deter- loess from Kansas, Nebraska, and Iowa mined by X-ray diffraction and petrographic are coated with a very thin hull or shell of analysis of the loessial materials from a montmorillonite type of clay mineral. Sherman Dam site and Trenton Dam site, These clay hulls are firmly attached to the Nebraska, range in value as follows: grains and cannot be removed even after long dispersion in a dispersion machine and subsequent treatment with 1: 1 hydro- chloric acid. Swineford and Frye 4/ also Quartz 25 to 37 percent have observed these clay coatings~on the silt grains inloessialmaterialfromKansas. Feldspar, including orthoclase (which In some instances, the clay coatings. is predominant), completely envelop the grains, whereas in plagioclase, and a few instances the grains are only partially micro cline 10 to 22 percent enclosed. In petrographic thin sections tabular or prismatic grains which have a Acidic volcanic glass 5 to 10 percent complete clay coating have been observed in which the coatings on opposite sides of Biotite and musco- the grain extinguish together as the micro- vite 4 to 6 percent scope stage is rotated.
Green and brown It has been observed that in the maJority hornblende and of instances water-laid silts that are derived chalcedony O. 5 to 1 percent from the decomposition of pre-existing rocks do not carry clay hulls, whereas the grains. in wind-blown silts commonly do. Clay hulls may be formed on the grains of some present soils because of the downward concentration Minor accessories include clinozoi- of colloidal clay. These clay hulls probably site, epidote, chlorite, zoisite, sillimanite, aid in bonding the granular components to- augite, sphene, garnet, rutile, zircon, gether and in bonding the grains to the clay apatite, magnetite, tourmaline, hematite, in the matrix. The presence of these clay and a glauconite-like mineral. Small amounts hulls would also aid in retaining a certain of gypsum, 0.1 percent, are occasionally minimum moisture content, and the pres- associated with loess. Similar minera- ence of moisture in these hulls would exert logical compositions of loess from various a surface tension effect thereby binding the parts of Kansas ha;ve been reported by Swine- grains more tightly together with evapora- ford and Frye 4/ except that the samples tion or rise in temperature. Hence, mois- examined from-the various dam sites are ture films or moisture associated with the lower in quartz and higher in clay content clay rims and matrix are important in the than other samples from Kansas examined. binding action of loess.
OBSERVATIONS OF PHYSICAL PROPERTIES
Location of Engineering Studies map bounded by the heavy line shows the location of the large loessial area which is Numerous B.1reau of Reclamation struc- of major concern to the Bureau of Reclama- tures are proposed, are under construction, tion and from which most of the test data or have been constructed on the Missouri for this monograph were obtained. River Basin Project. Investigation for sev- eral structures, as well as special research The physical properties of loess, when studies, have permitted the gathering of test ,summarized for loesses from locations data from several locations throughout the spread over wide areas in Nebraska and major loessial regions of Nebraska and Kansas, indicate basic characteristics Kansas. These particular locations are which are useful for engineering purposes. shown on the map of the Missouri River Based on numerous tests of soil samples Basin area in Figure 4. The portion of this from these areas, it was found that the basic
9 "~,,~,' , BUREAU OF R£CLAi'vIAliON .... 0 EXPLANATION . Dam sites investigated . Canals investigated
LOCATION OF MAJOR LOESS DEPOSITS IN THE MISSOURI RIVER BASIN FIGURE 4 properties of the loess are quite similar loess in western Iowa w~ very similar to and, predominantly, the loess has silty the predominating silty loess of the Ne- characteristics. However, some of the braska-Kansas area; loess farther east had loesses are more clayey and others are increased plasticity and cation exchange somewhat sandy. Other investigators, capacity,-and was thus more clayey in Davidson and Sheeler 7/, have shown that characterlstics. Some clayey loess was the changes in properties of loess are re- also found in the Bureau of Reclamation lated to the distance of the deposits from inve?tigahons~ -;Although systematic trav- their probable source. The coarser or more ersesof.:locations were not studied, it is sandy loess is deposited nearest to the probabIE(that'distances of deposits from source and the predominance of the clayey sour-ce.s'tnay also be an explanation for and finer loess increases farther from the these ~Hte,r;ences, . in properties. source. Although this trend is apparent, it is believed that properties of the loess, Compacti
The specific gravities of these samples Permeability were found to range between narrow limits of 2.57 and 2.69; the average was 2.65. The permeability of natural loess is principally related to the density when it Pla sti city is considered that basic properties of gra- dation and plasticity, as previously shown, The plasticity characteristics were de- are relatively uniform. However, there termined by consistency (Atterberg limits) are other features which influence this tests on many samples from 11 locations. trencl to some degree. Natural loess is The results are plotted in Figure 6 for the known to have rootlike voids which would purpose of showing the areas on a graph in cause considerable variations in permea- which values are concentrated; a standard bility; however, to some extent these root- classification chart was used. It is evident like voids are also reflected in the low den- that the major concentration of points occurs s ity. In very dense and reworked or for plasticity indices from 5 to 12 combined recompacted loess, these natural rootlike with liquid limits from 25 to 35 percent. voids are not present, and permeability When these data were studied in conjunction values are lower. Although loess is con- with the gradation curves, it was observed sidered relatiyely uniform in basis proper- , that this group is generally characteristic ties, the loessial soils which fall in the of the material which was previously identi- more clayey or more sandy groups should fied as silty loess. Points of higher plas- be expected to have somewhat lower or ticity indices and liquid limits represented higher permeability values, respectively. more clayey loess, and points of lower plasticity indices and liquid limits repre- In Figure 7a, all of the permeability sented the sandy-loess soils. test results are for undisturbed natural loess with tests made in the vertical direc- Davidson and Sheeler 7/, in their 'tion using the one-dimensional consolida- studies of loess in western IOwa, showed tion apparatus. By enclosing the concentra- that the loess becomes more clayey when tion of points with lines a trend is shown. the distance of the wind-blown deposits is In reviewing the gradation and plasticity of farther from their probable sources. The each sample it was found that t~ "E~ 'tecllniCe. l\ecla:tne.tiO-o. of .e.C-O 11 ~uree.u coloI -pen
CLAY (PLASTIC) TO SAN D GRAVEL COBBLE S SILT (NON PLASTIC) FI NE MEDIUM FINE I I COARSE I COARSE Gradation data was obtained on 148 For all samples tested samples from projects in the 76% were in the silty (oess zone Missouri River Basin area 18 % were in the clayey loess zone The curves generally take the direction 6% were in the sandy loess zone shown by the boundary lines TRENDS OF GRADATION CURVES FOR LOESS FIGURE5 30 CH
CLAYEY LOESS" ';..- ..... 20 e. X UI / 0 / e. Z I . . >- I~ !:: CL \ e.+ . 0 \ I/)l- SI L TY LOESS-, , * eX ", A .~ ...... J - Q. }:' ~ \ MH end OH 10 -/ lie. / ty. 0 J CL / e.y ~ye.e.~ f I ~n 1 ...... ~ ,/ CL-ML--- - ,f'8,:~~ */I . e.L ~ '0-.-:'-
SANDY LOESS / /1 ML- end OL
0 cO 0 0 10 2~0E 30 40 * 50 60 LIQUID LIMIT
Consistency Ii m its data were obtained for samples from t he following structures:
8 Trenton Dam and Railroad Relocation + Cushing Dam A Bonney Do m 0 Milburn Dam * Davis Creek Do m . .Amherst Dam e. Ashton pi Ie test area . Courtland Canol 0 Rockvi lie Dom . Cambridge Conal Y Medicine Creek Dam
TRENDS OF PLASTICITY CHARACTERISTICS OF LOESS FIGURE S falling within these lines were generally which may develop In structures placed for the silty loess. Points falling above on wetted loess, on dry loess, or on dry these lines and toward values of higher loess which is wetted after construction. permeability were for sandy loess, and points falling below these lines and toward Using the data of the actual tests in values of lower permeability were for clayey Figure 8 as guides for establishing consol- loess and possibly reworked natural loess. idation trends, the curves of Figure 9 were drawn. These curves demonstrate distinct Using these trends in Figure 7a as a contrast in settlement between loess of wet standard for comparison, the results of and dry conditions for loads of 10 to 100 recompacted and reworked loess are shown psi, which is the general range of loading in Figure 7b. Most of these tests were for most foundations. It is evident that the made on specimens which were recompacted most pronounced settlements occur for for the purpose of embankment investiga- loess at densities less than 80 pounds per tions. Therefore, it will be noted that most. cubic foot, and relatively smaller settle- of the specimens have a density range from ments occur for loess at densities more 102 to 112 pounds per cubic foot. For these than 90 pounds per cubic foot. tests, the permeability range is rather low, from 0.01 to 0.5 foot per year. Such values Shear Resistance correspond to values observed for dense natural loess. Compacted samples having The variations in shear resistance may lower densities were not available. be attributed to the same general factors which cause variations in consolidation. Consolidation There is evidence of definite influences of initial density, initial moisture content, Only general trends are evident in the and clayeyness of the loess. settlement characteristics shown in Figure 8, because considerable variations occur In Figure 10, the solid lines represent in density and moisture. Slight variations the shearing resistance curves (Mohr en- in clay content and differences in moisture velopes of triaxial shear tests) of natural un- conditions influence the bonding character- disturbed loess and the dashed curves repre- istics from particle to particle. sent loesses which were prewetted. Although the tests include both sealed and drained The prominent effect on the amount of specimens, the results have been adjusted settlement is found to be initial density. for pore pressure, and the shear resistance Under a loading of 100 psi and at wetted curves represent strengths for effective conditions, natural loess, which varied in (grain-to-grain) stresses. initial density from 73 to 92 pounds per cubic foot, consolidated to densities from The curves are generally of similar 95 to 103 pounds per cubic foot, or a~ much slope indicating similar frictional resist- as 30 percent or as little as 5 percent, as ance. This would be expected because shown in Figure 8a. However, as shown loess has such similar basic physical prop- in Figure 8b, under relatively dry natural erties. However, there is considerable conditions, loess consolidated little until difference in the position of the resistance loads are in excess of 100 psi. This indi- curves; This is caused by ttle variations cates that loading alone does not break down in cohesion which are relate1d to the clay the particle-to-particle bonding character- content, moisture content, and density. istics until relatively large loadings are The several curves which are bracketed reached, whereas increased moisture appre- and located high on the graph are those ciably affects settlement. showing high cohesive characteristics. It may be noted in the table in the figure that Figure 8c is another generalized picture, these soils are either clayey, of very high published previously by Holtz and Gibbs density, or of very low moisture content, 1/, of the consolidation characteristics or are soils having combinations of these Plotted to linear scales. The trends for proper tie s. loess which is thoroughly wetted (shaded area) show that it consolidates quite easily The curves bracketed at the lower levels under small loads. Three examples of on the graphs are those of typical silty loess consolidation tests -show the contrast of generally of relatively low density. The low settlement under dry conditions and majority of the curves are contained in this high settlement under wet conditions. When grouping. This lower group of curves con- water is added during the test the curves tains test results for two test methods; the drop to values near those obtained by com- solid lines represent tests on loess at natural panion prewetted tests. These curves (dry) moisture content, and the dashed curves demonstrate settlement characteristics represent tests on natural loess which has
14 t--- II111 110 - .. UNDISTURBED LOESS
. y ~II< i ... 105 , I'- y / * y-'---:> c . 4' ... * ' , /"oj '"' c ::> 100 . .y b"'~'" 0 - -, .~/c,f)~A 0 0: " a. ~~~'"'' I"'. y '" I "I ai 95 y ...J .. y "I' y b6- / ....>- ~~,c.c,a c,C? ,, q, / """y -.J y ~ 90 ",°q,'t:,/ # ~/ 0 9..' ' '-" '" t)~~t) I Cj "- >- 0: <.'l.// 0/ ~yy 1"\ 0 , 0 " 85 .. . y ,.Y[' }j 0 '\ {'f ,. "" ~~~y, 80
"-;yyy
I 75 0.1 1.0 10 100 1000 PERMEABILITY - FT. PER YR. (0)
* ~-- *" I II1111 110 ~- 6 *y .. REMOLDED LOESS " y" " .f-" ...... ' y !*" t O.y 105 * """"' . , I. " , ,," ,.: * / 09.. ... ,v ::>100 -..: b'" 0 ' "< CO",'" 0:
a. """""' ...... '" "I'\. iii 95 ...J I * ,
....>- ().c::.~c,c, I ' q,c,c,,,oq, / , ~ 90 ",°'l.'b/ 0 ~/ "\ '" '" 9..' ,o""q,...oI >- c..; // "\ 0: <. 0 85 "
80 ""
'\.
75 0.1 1.0 10 100 1000 PERMEABILITY(b)-FT. PER YR.
EXPLANATION . Trenton Dam and Railroad Relocation 0 Milburn Dam .. Bonny Dam . Amherst Dam Davis Creek Dam . Court land Conal * Ashton pile test area 0 Cambridge Conal "c Rockville Dam Enders Dam y * Medicine Creek Dam N Erickson Dam + Cushi ng Dam
TRENDS OF PERMEABILITY FOR LOESS FIGURE 7
15 1.3 I.J
1-- .7< ~75 1.2 1.2 I ~1'-- 14Z-524~ I 1\ I \ " 80 14Z-52b, 85 I r I - --- I a ~i\ 1.0 - -t+- -- I ... ., ' ... ., - - - u U> - ---- 0 ~U> 0 .. -- W ;:: "- - --- >- -...: ~ I-.. ~~>-I- '" -~~~f --- - ,\ 1- '" -j- -- \ : .9I---in ~r---: .9 -in " - - 0 z " Z '"ci. - -- ' \ (5 '" --- If4: \ 0 70 ., I I I U> 1.3 N NOTE ... The 5hoded area represent5 the consolidation 0 characteristics shown by samples which >- were te5ted in a pre wetted condition (wetting I-;; 75 1.2 by ponding). The results of all tests with .. -Low denSity loess from initial densities varying from 74 to 90 lb. .... : Ashton Pile Test Area '"u'" per cu. ft. fell within this area and the ...... ,;Iesteddryj"--_ok I >. curves hod the general directional pattern of the area. g;8 80 -. ~-Tested wet": >< , '" , -' " """1-..... '": 0 1.0 Med. density loess > ;:: -- . from Trenton Dam '"« 85 « " z -ed~)I... .. '" .-Tested dry', .:Test z eO.9 -- 0 0 ;;Woteroddedt---__- 0 > zt-f --- ~ 90 , I .. - -- ~-Woter added ~ o.a :;' /'/ /7. ;:; 95 , , u m, , ::; 0.7 . .. CTesteddry-, CWoteradded ~ roo I-Testedwet-- // I->- --, in 0.6 Highdensity loess ~ 105 0 I from Trenton Dam 110 I 0.50 20 40 60 ao 100 120 140 160 reo 200 220 240 260 lOAD-PSI (c) SUMMARY OF CONSOLIDATION CHARACTERISTICS OF DRY AND WETTED lOESS OF VARYING INITIAL DENSITIES CONSOLIDATION CHARACTERISTICS OF LOESS FIGURE-8 16 , 1.3 75 1.2 in 110 .5 0.1 10 100 1000 LOAD- P.S.L Wetted Loess (above 20%moisture content) - Dry Loess (below 10% moisture content) GENERALIZED CONSOLIDATION TRENDS / FIGURE 9 been wetted prior to testing. It can be seen for the undisturbed loess at natural mois- that the low density loess which has been ture conditions, and the dashed curves are thoroughly wetted is of very low shear re- for the same material which has been wetted sistance. Under low normal stress, this prior to testing. The shear curves in Figure loess shows a flat shear resistance curve, lla show distinct differences in shear re- and on some occasions it has virtually no sistance between wet and dry conditions. shear strength. Under higher normal stress, The larger circles for dry loess indicate this strength increases because frictional greater shearing resistance under applied resistance was developed after consolidation pressures. The higher level of the envelope took place. line indicates pronounced cohesion. The smaller circles for prewetted loess indicate As a further explanation of the differ- lower shear resistance for applied stresses ence between wet and dry conditions, the similar to those used in the dry tests. In curves in Figure 11 are shown. These the wet tests, appreciable pore pressure curves are for a typical silty loess of rel- developed causing lower effective normal atively low density. The solid curves are stress values. 17 LAB. TYPE DENSITY NDIST. TEST SAMPLE OF AT TINE AT TINE REMARKS /1 NO. CONDITIONS LOESS OF TEST OF TE ST -120-202 /' 120-202 NATURAL CLAYEY 100.2 10.1 ~I /' 150 12F-44 SILTY 94.1 8.4 I I // 120-200 CLAYEY 84.9 8.6 I I 14!-478 SILTY 90.2 7.4 ~ExtremelY clayey SILTY 84.2 4.5 120-183 I Loess and Loess / ./ I 12F-143 SILTY 103.5 18.0 of high density I and low mois- Extremely clayey Loess and Loess of / I high density and low moisture contenty /~/ 14Z-526 CLAYEY 83.1 13.3 ture content 8.2 120-181 SILTY 79.1 ~I 6.9 I \, 14Z-527 SILTY 89.5 I // /~20-200 I /~ 14Z-476 SILTY 83.8 8.6 I m I CL 13S-106 SILTY 89.0 6.0 I V '\ ) 12F-44,>-///13S-la6,././ I en 100 13S-111 SILTY 81.7 6.5 I m I W f\\ d~./7 13S-109 SILTY 83.8 7.3 I a:: /./~F-143 I I- m 13S-112 SILTY 81.3 9.5 I /l,/\I ~ ././', 135-111 I a:: 14Z-527-,L .Y ./~ 14Z-524 SILTY 82.0 10.6 I ~ ~ ./~ ./' .//./ 120-191 CLAYEY 78.6 20.3 Typical si Ity w r :J: I Loess of low m ./V I /-?' I'Y:;:~0-180V ./ / I density ./ V /' "j~;'/ /"/ - I ..... I 14Z-476- PRE- I CO 13S-105 WETTEO SILTY 81.1 29.3 ~:-109 AND135-112 I /V~ ~~~;: SILTY 82.5 30.4 I 13S-111 I 14Z-476 SILTY 81.6 34.8 I 141:~~~::~~~~~3S-111 I 50 14Z-521 SILTY 78.2 34.9 I /...;~ 135-1015 I 14Z-526T__, [.7~V I~~ SILTY 78.0 33.9 I ~V/ '~.;..;, I 14Z-524 I 13S-108 SILTY 78.4 28.9 I f14Z-524 V~'~ ~;~./ '-Typical silty Loess of lowdensity \. //' EXPLANATION V \ Typicalsilty Loess at '- .:;:::' r~ \~ /:;:;r... '-120-191 natural moisture content - Tests at natural moisture condition ~ ' wetted cond ition /~~ ~".. '~Typical silty Loess of --- Tests at ~ ~- highly wetted condition E::;}i<".,4Z-476,521 & 524 -~~~---t3S-108 I I 00 50 100 150 EFFECTIVE NORMAL STRESS-PSI SHEAR TESTS OF REPRESENTATIVE LOESSIAL SOILS FIGURE 10 INITIAL CONDITIONS NATURAL PONDED DRY DENSITY (pcfL 82.02 78.02 MOISTURE CONTENT (%L- - 10.57_------__33.96 DEGREE OF SA TU RATION (%) - - -- 27.50------__80.23 SPECI FIC GRAVITY 2.660 FAILURE (MAX. OJ/ 80 I I .- , Sealed Tests -- ~'- ~', 1.1 ~./ V Q) 80 r--.o ~ 60 1 ~IF 1 0 u \ (J) u i= _I n t'-.... (J) ~~~V SHEAR AND VOLUME CHANGE CHARACTERISTICS OF DRY AND WET LOESS FIGU RE II Figure lIb shows the consoliaation is related to the greater consolidation which which occurred in the shear specimens. occurs in these specimens. It is interesting The dashed curves showed that wet loess to note that wetting has caused expansion in consolidated under much lower stress con- the loe ss since the initial density of the ditions than the dry loess. The rise in wetter loess is considerably less than the shearing resistance under higher stresses initial density of the dry loess. RESEARCH FINDINGS FOR LOESS Several research studies, both in the The important observation is that the laboratory and in the field, were conducted wetted material for footings of sizes 5 by to clarify specific problems related to the 5 feet or larger showed increased settle- use of loess as a foundation and construction ment under loads as low as 0.8 ton per material. square foot. For safety reasons, this ma- terial would probably only permit loadings Results of Plate Load Tests on Loess of about 500 pounds per square foot. In contrast, the dry (natural moisture content) To demonstrate the settlement and bear- material did not show excessive settlement ing capacity of spread footings on loess, a under loadings as high as 5 tons per square series of plate load tests were conducted in foot. a typical low density loess near Ashton, Nebraska 1/. - " T he computed values of settlement The settlement characteristics of this checked the actual measured settlements soil are shown by the laboratory consolida- fairly well for the large size plate. As the tion curves marked 14Z-521 and 524 in plates become smaller, computed estimates Figure 8a. The field tests involved the load- become less accurate. It is of interest to ing of square plates of three sizes: (1) 1 by note that the curve computed from consolida- 1 foot, (2) 3 by 3 feet, and (3) 5 by 5 feet. tion tests and the plate load test curve (for Each plate was placed at a depth of 5 feet example, the 3- by 3-foot plate) have similar in holes equal to the plate size so that the shapes for the beginning part, indicating that full effect of surcharge existed. consolidation is the primary cause of settle- ment of the plate in that range of loading. The results of the tests are shown in For greater loadings, the plate load-test Figure 12. Settlements are shown in rela- curve shows greater settlement and crosses tion to the pressure. The results of each the computed curve, indicating progressive loading test are indicated for two sets of settlement and greater effects of shear plates: for the loess at natural moisture failure. content, and for loess which was prewetted. In addition, the theoretically computed settlements, obtained from the consolida- tion tests in Figure 8, are shown for the Results of Ponding Tests in Loess 3- by 3-foot and the 5- by 5-foot plates. (Computing settlement for the 1- by 1-foot Ponding of loess may in some cases be plate was not practical because of its small desirable to develop the weakest condition size. ) in the loess prior to construction. This is likely to be most important in hydraulic The results show that this loess at a structures which will have their foundations natural moisture content of about 10 to 12 thoroughly wetted by normal operation. Such percent is capable of supporting consider- ponding would permit settlements to take able load, about 5 tons per square foot, place before or during construction. If the without serious settlement. When this foundations are of low density and suscepti- loess was prewetted by ponding, consider- ble to large settlement, ponding would be able settlement occurred after loading. very desirable in preparing them for pile The settlements of the larger plates were driving. This will be discussed further greater because the distributions of pres- under pile driving and pile-load testing in sure included greater depths of compressi- loess. Also, ponding of foundations for , ble loess than the smaller plates. Thus, earth embankments may be desirable so these tests clearly show the error that could that settlement can take place as the em- arise if the results of field load tests on bankments are constructed and thus reduce small plates were used directly to estimate the possibility of the foundation slumping settlements of large footings. as saturation by the reservoir occurs. 20 0 ~It:.: r"-==.~..:.i- -~ "-'=':~ - -::::--~F-- :.-=;'Il-:..-:::I--"- -- ,-, ~ ~ . 1" 1,\ .: ::::, "-IIt'- ""' / .-:; . ~ :'" "r~ -- -~ ,/ ?"-- -~":"'-7~::: :~~=.:::; ::~~- .02 " ,,/ --- - "" '\', / " ? " l--r-::::- --.. ,/ -""""",,--r \ -..... "" " \ \ >- \ .16 ~ \ \ 2 3 4 5 LOAD TONS PER SQ. FT. PLATE - LOAD TESTS FOR A TYPICAL LOESS DEPOSIT FOR COMPARISON OF MATERIALATNATURAL MOISTURE AND PREWETTED CONDITIONS FIGURE 12 Figure 13 shows the results of surface the settlement curves become flat, indicat- ponding in typical loess of low density near ing that settlements virtually ceased. This Ashton, Nebraska. The two ponding experi- example illustrates the use of prewetting to ments are identified in the figure as Site 1, permit a large part of the settlement to take where the loess was about 40 feet deep place during construction. underlain by a medium to coarse sand, and Site 2, where the loess was in excess of 60 Behavior of Piles in Loess feet deep. Piles are sometimes used as a solution In Site 2, the soil was wetted to a depth to the problem of large settlements of rigid of 60 to 70 feet. In Site 1, the loess was irrigation structures such as powerplants, wetted to the top of the sand stratum at about pumping plants, gate structures, bridges, 40 feet. The wetting was accomplished by and appurtenant canal structures. In at- continuously ponding 1 foot of water over tempting to visualize the suitability of piles, the areas for a period of 6 to 9 weeks. Mter these questions were asked: 2 weeks of ponding, it was decided that open drill holes smuld be made to expedite wet- 1. How must piles be placed since the ting so that it could be accomplished in the loess is quite firm and sometimes hard allotted time. These drill holes were made when dry? in the western one-half of Site 2 and around the edge of Site 1. The locations of mois- 2. What happens to the pile-bearing ture observation holes are shown in the plan strength when the loess becomes thor- views in Figure 13, and the depths of mois- oughly wetted? ture penetration for various numbers of days of wetting are shown in the profiles. On 3. Can the natural structure of loess Sections AA and BB, it may be seen that the support friction piles or must there always open drill holes definitely assisted wetting. be firm bearing? The wetting of Site 1 was somewhat slower than Site 2 because of greater densities. 4. What kind of piles are desirable? After ponding was stopped, Test Holes No. DHP-23 and -25 in Site 2, and DHP-26 in 5. Will end bearing piles become over- Site 1 showed that wetting was accomplished loaded as consolidation of surrounding to the proposed depths. Forty days of pond- loess takes place? ing were required for Site 2 and 63 days for Site 1. A research study, near Ashton, Ne- braska, of full-scale pile tests was con- Example of Prewetting ducted to answer these questions. The test sites contain loess which is representative The foundation of Medicine Creek Dam, of that found at many proposed irrigation Nebraska, was thoroughly wetted by pond- projects. Site 1 contained end-bearing piles ing and sprinkling before embankment con- resting in firm sand underlying shallow loess. struction (described by Clevenger 8 n. Site 2 contained friction piles in loess greater Figure 14 is a photograph of the dikes -and than 60 feet deep. The loess at both sites the ponds. Moisture content of the loess was at a medium range of density, 80 to 90 was raised from about 12 to 28 percent in pounds per cubic foot. The study included from 25 days to 2 months during which driving 44 piles and performing 28 pile-load time 33 million gallons of water were used. tests. Two general types of piles were Settlement measuring points were established studied: nondisplacement (steel-H) piles throughout these ponded areas. No signifi- and displacement (timber and concrete) piles. cant settlement occurred from saturation Each test site contained an area in which alone. Base plates for measuring the foun- piles were placed in loess at a natural mois- dation settlement caused by fill construction ture content of about 11 percent and an area were installed on top of the loess stratum which was prewetted by surface ponding to a in four locations, designated BP-l through depth greater than the proposed pile length. BP-4 in the plan shown in Figure 15. The natural moisture area of each site Fill construction on the loess began contained the following types of piles and about mid 1949 and was completed in late placement methods: 1949. Base plate settlement measurements were made periodically from beginning of Group (1)--3 timber piles placed by fill construction until July 1954. Three of driving only or preboring the plates had settled from 0.8 to 1. 0 foot, if necessary and the fourth settled a total of 2 feet by 1954, as shown in Figure 15. After the Group (2)--3 steel H-piles placed by reservoir had been full for about 3 years, driving only 22 T ,., N 0 rf) - v It) U) , '"N, N, -!2 I" I" T T I I I I I T '" ...::-,8 000 0'" a 0 a 0 0'" 0'" , , 0 00 :i ::t!!; :!ii G.S. Elev. 2219 2 := ::!!: 2 2 G.S. Elev. 2219 "- "- ...." MO-I. Elev, 2216.8 Q Q =0-131 '" '" Elev. 2216.8 '" '" '" MQ-2 2210 14 Days 21 Do s ODH-P-25 2200 20 Doys 6 Days ODH-P-23 9 Days t 2190 \.iLOEPTHS OF WETTING ~ 2180 OF WETTING MO-4 MO-S .0-3.MO-5. . 2170 .MO-12 ~~ ~ ":' .MO-II. .MO-IQ --1 A""'-- A 2160 MO-15. 10' 0 40 Doys SECTION A-A SECTION B- B t..:> '" MO-9 ..., ~ 0 0':' NOTE ...::JB "--'",I These profiles should only be considered G.S. Elev. 2170 5~ approximate as they are based on observations SITE NO.2 '" Elev. 2167 '" ot scattered time intervals. The observations PLAN OF PREWETTED AREAS were obtained from rapidly mode drill holes. 10, 5 0 10 20 ~O 40, 2160 Penetration holes, DH-P-23,25 and 26. mode " SCALEOF FEET 17 00 S after ponding was stopped, ore the best proof of wetting. 2150 t I'll N . 1,,/Ij,' i ~ MO-14 , 2140 0DH-P-26 DEPTHS OF WETTING MO-7 !. 2130 7r MO-B C~L- C I 40' (a 63 Days SECTION C-C RECORDS OF PONDING EXPERIMENTS MO-19 f AT ASHTON PILE TEST AREAS SITE NO. I FIGURE 13 PREWETTING OF LOESS FOUNDATION BY PONDING AT MEDICINECREEK DAM FIGURE 14 ..y PONDED AREA , '-- RIGHT ABUTMENT CUTOFF TRENCH 200 0 200 400 600 Note: BP = Settlement measuring I I I I I plates on the foundation. SCALE OF FEET 0.00 0.20 0.40 a:: DAM CREST_, I- 0.60 ~2420 a:: "' ~ en ~ "' "'a:: BP4 I- 0.80 0::2400 Z 0 .... "'.. 0 BP3 Z ..J 2.00. '- -BPI 1950 1951 1952 1953 1954 MEDICINE CREEK DAM FOUNDATION SETTLEMENT INSTALLATIONS FIGURE 15 25 Group (3)--3 steel-shell, cast-in-place could easily be interpreted and maintained concrete piles placed by constant for comparing the driving of one driving only or preboring if pile to another. The 5, OOO-pound driving necessary hammer was dropped 36 inches and was op- erated at 55 to 60 blows per minute. Group (4)--3 timber piles placed in jetted holes The preboring of dry holes was done by dri ving an open end pipe into the ground for The prewetted area of each site con- about 4 feet, pulling it out, and blowing the tained only displacement piles placed by soil plug from the pipe with steam. The driving onJy. At the friction pile site (Site 2), process was repeated until the desired depth in addition to the single separated piles, a was reached. cluster of nine piles was driven to determine the consolidation effect of group pile driving. The jetted holes were made by ejecting a straight stream of water at 100- to 120-psi Representative piles of each type and pressure from a 1-l/2-inch-diameter jet placement method were load tested. Piles pipe into the ground using a long vertical pipe which were placed in the natural moisture guided by the leads of the pile driver. The areas were load tested before and after wet- jet pipe was worked up and down with short ting the areas. strokes and followed the hole down as it was made. The finished hole was somewhat ir- One of the purposes for testing end- regular in shape, varying from 7 to 12 inches bearing piles was to study downward drag in diameter. on the piles resulting from the upper loess settling when wetted. However, this part lDad tests were made by stacking stand- of the study did not materialize because the ard railroad rails on a loading frame above loess at this site did not settle as a result the pile to be tested. A hydraulic jack be- of wetting alone. tween the loading frame and the pile per- mitted the application of the load in controlled Soil profiles and plans of the two test amounts. The method of load testing is sites are smwn in Figure 16. In Site I, Fig- shown in Figure 18. ure 16a, the loess was from 35 to 40 feet deep. The Grand Island sand formation, Results of Pile Driving Studies was considered the bearing material, was ,at about 45-foot depth. Between the loess and The principal results of driving studies the Grand Island sand was a transition zone of are presented here by means of three graphs, sand with silt. The loess varied in density Figures 19a, b, and c. The8e graphs dis- from 84 pounds per cubic foot near the sur- play the record of a representative pile of face to 90 pounds per cubic foot at lower each of the general conditions. depths. The Grand Island sand was quite dense, having a density of 112 pounds per Figure 19a represents the driving rec- cubic foot. ords of the displacement piles placed by var- ious methods in deep loess at Site 2, the In Site 2, Figure 16b, the loess was friction pile site. The solid line curve is of considerably greater depth than the pro- the driving record of a timber pile placed posed pile length of 55 feet. The loess from by driving onJy in thoroughly prewetted loess. 0 to 40 feet was of relatively low density, 80 Such a pile drove rather easiJy and gradually to 85 pounds per cubic foot. This was under- developed resistance of about 36 blows per lain by a somewhat clayey, old soil horizon, foot. It was apparent that the denser and 1 to 2 feet in thickness. Below this old soil more sandy loess below the old soil horizon horizon, to 62-foot depth, the loess was caused resistance to increase more rapidly. slightly more sandy and denser, being at 85 The Engineering News formula indicated a to 95 pounds per cubic foot. safe load of 35 tons for this pile. A similar pile placed in dry loess, shown by a dash- The loess in this study was described dot line, indicates a distinct contrast in as "silty loess" and the physical properties driving resistance. It was only possible to were similar, in respect to gradation, plas- drive this pile 25 feet before refusal resist- ticity, and mineral contents, to other loess ance was reached. The safe load of this pile in the Kansas-Nebraska area. Typical gra- after the soil is wetted should be that indi- dation curves are shown in Figure 17. cated by driving a pile in prewetted loess to a similar depth, and, in this case, by the The equipment used for pile driving was Engineering News formula, would be about a single-acting steam hammer which was 9 tons. A safe load computed from the driv- particularly suitable in a research study of ing record in the dry loess was 90 tons, this kind because the energy of the blows which shows that driving in dry loess, which 26 "r Plles in Not. in Plies I" Not Plies in Plies Morsi Are0 Prewetfed Area -Moisl Area Prewetfed- Area + C-sur1oc.e 1 20 ~PEORIANLOESS !SILT. LOESS I- ,-OLD SOIL HORIZON ;LOVELAND -LOESS 'SILT, SOME SAND t G ;GRAND ISLAND SAND FORMATION NAl MOIST ARE?, ,PREWETTED 8 OGALLALA kcemented FORMATION Silly Sond 'OGALLALA ' *. ,Piles In preweitedoreo Jeited..~+O O Ct;'.~~~~~~~~~ AREA Timber------,' ' I// Steel-H-----' \ , .-Tim be, Timber Steel shell (Preboredl' \ '\~lmberCluster (PrebaredJ-- Jetfed' '-Steel Shell (Preboredl (0) PROFILE OF SlTE NO. I (b) PROFILE OF SlTE NO. 2 (END-BEARING SITE, (FRICTION PILES) PROFILES AND PLANS OF TEST SITES FIGURE 16 SIEVE SIZES .005.009.019.037.074 DIAMETER OF !ARTICLES IN rnm. I GRADATION TESTS FIGURE 17 METHOD OF MAKING PILE LOAD TESTS FIGURE 18 would be subsequently wetted, would give sults of load tests on various representa- erroneous indications of strength. tive piles in deep loess at Site 2, the friction- pile site. Figure 20a is a load test for a It was evident that preexcavation was displacement pile, driven in prewetted loess necessary to place a displacement pile in to a depth of 55 feet. This pile was tested dry loess to the proposed depth. Further, in two series of loadings. The second series it was found that to place a pile in a dry pre- was carried to complete failure at 170 tons. bored hole, the hole had to be practically However, the curve indicates beginning fail- as large as the pile. A representative pile ure at 120 tons. This curve is used as a placed in a dry prebored hole, shown in measure of comparison for the other piles. Figure 19a by a dash-double dot line, dropped Figure 20b shows the test of a displacement in the hole to 35 feet without resistance and pile in a jetted hole. The pile was tested rapidly picked up high resistance from there before wetting the area and showed similar to the proposed depth of 55 feet. A pile in strength to that of the pile in the prewetted a jetted hole, shown by the dashed line, had area. However,' after the area was wetted, gradually increasing resistance from a depth greater movements may be seen and failure of 12 feet. occurred at a somewhat lower load. Of the three placement methods which Figure 20c shows the test of a displace- permitted the piles to be placed to the pro- ment pile in a prebored hole. Although the posed depth of 55 feet, the pile placed in the strength of this pile was fair under dry con- prewetted loess by driving only was con- ditions with an indication of beginning failure sidered to give highest strength because at 60 tons, the pile was considerably weaker maximum consolidation of surrounding soil when the area was wetted. The test indi- was obtained. Piles placed in jetted holes cates great movement under very light loads, were considered to be the next be st as the particularly when comfBI'ed to the pile driven piles drove easily and some consolidation in the prewetted area. Figure 20d shows a of adjacent wetted loess was obtained. A test for a pile placed by hard .driving to only pile in a dry prebored hole was considered a shallow depth in dry loess. This pile to give the least strength because practically showed high strength with only slight move- no consolidation was obtained. Pile tests ment under dry conditions, but after wetting shown later support these opinions. the area, complete failure was obtained under 60 tons. In summary, these tests show In Figure 19b, a similar comparison that great loss of strength can be anticipated is made for piles placed in Site 1, the end- if friction piles are driven in dry loess or bearing site, where the loess was relatively placed in prebored holes in dry loess which shallow. In general, the driving resistance is subsequently wetted. in the loess was similar to that of Figure 19a. The impor:tant consideration is that all piles Load tests in Site 1, the end-bearing which reached the firm Grand Island sand site, were all of high strength, indicating stratum increased in resistance almost im- that firm bearing on the sand was entirely mediately regardless of whether the pile was satisfactory for all placement methods. Non- in a jetted hole, a prebored hole, or in pre- displacement steel H-piles in this area were wetted material. Therefore, it appears that found to be satisfactory, when embedded 5 this sand will adequately support piles placed feet into the bearing sand. Since it was pos- by any of these methods, provided they are sible to drive steel H-piles in dry loess and snugly held in the upper loess for adequate not displacement piles, a nondisplacement lateral support. The loadings are discussed pile was considered the only type which was below. practical to place in dry loess by driving only. Figure 19c shows the comparison of dif- ferent types of piles, all of which were placed Summary--Study of Piles in Loess by driving only. The timber pile in pre- wetted material drove easiest. In dry mate- 1. From this overall study, opinions of rail, the nondisplacement steel H-pile drove general requirements for piles and pile easiest; however, the effect of greater clay- driving in loess have been summarized in eyness in the old soil horizon was parti- Figures 21 and 22 on the basis of moisture cularly noticeable. Displacement piles, both content and density of the natural loess. timber and steel-shell piles, drove with much difficulty in dry loess and reached re- 2. Friction piles in deep loess had con- fusal values at relatively shallow depths. siderable difference in load-carrying capa- city, depending on their placement method. Results of Pile-load Tests a. For displacement piles in loess Figure 20a, b, c, and d show the re- which will be wetted in the future, pre- 28 .01 .02 .02 f- lUf- .03 W.03 IU , Beginning of "' ....':- f-"- 50 failure --- Z ~ .04 .04 ::E "':E ,, 60 0 20 80 \00 \20 180 200 "'-' -'"' ' , f- f- .05 , BLOWS PER FOOT .05 ~ 60 0 20 80 100 120 140 160 180 200 .02 BLOWS PER FOOT (b) Driving record of displacement piles in Site I} the f- f- end- bearing site ~.03 ~.03 "', \ "', f- f- ' 0 Z Z ~.04 ~.04 \ \ 10 \ \ f-"'-' \ f-"'-' \ ~.05 , ~.05 20 .08 .080 150 150 175 600 140 25 75 100 175 0 25 50 75 100 125 160 180 200 LOAD - TONS LOAD-TONS (c) Pile placed in dry pre bored hole. (d) Pile driven in dry natural material. (c) Comparison of piles of different ty pes placed by driVing only in deep loess COMPARISON OF PILE LOAD TESTS FOR FRICTION PILES IN LOESS ,.. DRIVING RECORDS FIGURE20 FIGURE19 60 (I) Firm end -bearing is required for piles. (2) Piles are generally required for most rigid structures. r: 70 (3) If end -bearing is not available, 20 to 40 feet of end- "-:> embedment in loess of densities above 80 pcf is u required. CI: ... n. u) 80 m -'I Friction piles of adequate embedment are permissible. )0- l- (30 to 60 feet total embedment is generally required) II) Z ~ 90 CI:-' ::>'" I- (I) Ordinary structure need not have piles. '"Z (2) Critical structures may use short friction piles, 100 generally requiring less than 30 feet of total embedment. 110 0 10 15 NATURAL MOISTURE CAS E 1. When higher moistu re contents are anticipated. (As in the case of hydraulic structures) 60 (a) f2) p. Is ft) COl}dl ./:f' 1":' . I": e lIes ref/fl' 1/"0 1'101} /"ICf; IId,o el}d, I 01} '10 I&/" o/"e I"ed (.10 II} /'eef eO"liJ 00Sf Uel}e" 1&,,/).40"1 70 ib PI It r: Oess Is Ile~ "u /'ef/". 60 ,..Ies ot c: oie0/ I} "IUld 0111'" "- I"ed) eef u:> Of' ;'def/,,:;dl!IOI} COI}0/~Q'0e:: .0VO/~:/""c:::I"'ed (I) 01'01 e 'J>.. 101} 'b CI: e "< ./lIS /"e Ie 8S. ... n. t';?) O/"dl/}O e00e 00e(j o/" f/"I,,~ 201'0 u) 80 C/"I!' Co d0el} 0el}f d ICOI"1' IIdl!; fls m PI;. Sf"" 01} O"e /; es. / Sf/" Cf"/,, 7>.. gel} Pe/" -'I (Uel} e I} " 110 0 20 MOISTURE CONTENT- PERCENT CASE 2. When natural moisture contents are not ex pected to increase. REQUIREMENTS FOR ADEQUATE SUPPORTING CAPACITY OF PILES FIGURE 2/ 30 30 '11\\\\011\ Q\~teO o\\\~ 25 \)e of\~I\\~ \\\~~ \)~ Q\Ot\\\~ ?\\eS . IIWI \\\ It '011\"'1'0. t- .\" fe \\~ Z \)es \ "'eoflV o\\\~. W " {f\\\ 0 . U ,Of \0 I II: IIseO QlIeS of\~\\\~ oe~e\OQe W 20 \)e \)~ IS. 0.. 11\0 \\\e\\\ \\~ f\\\~ '11\\\ e\\\\\~ to\ Q\\eS. OISQ\~t\ It~ e\\O-\)eOe\\e\fO\IO\\ t- 0\" Q Z - W '11?f:I~Q\~~e~fe:\\\~f'll~~~0\)\ ~f':~\e\\:o~o'/.\\\\\)\\\ t- , Z ':;,\ee\- Q\ote \\\~~ fO\)~\)\ \ot\\\~ 0 ,~'\ \)e QI'\eS Q Q U 15 \\\o~ \\\e\\'1 IIseo, \\\ IS feS\) \\'0 , W \sQ\~te O\\\~ \\\'00 II: ,ee \. \)e'0\ oeQ ::> ,:'1 D\, o~\~I\\~O tltO \~~o \oQeo. (/)l- ,tlt1 \)se 0 'o~ 0\\ oe~e \)e:'o \)e \~\\\~ . \'0 0 0 o~ O~I\\~ \\~~e ::;; '0\\011\ \)~ 10 e\'I\\\~ Q\\e\ Q\~teo ., e\\O-\)e \\\~~ \\\ \) I \\ -'00: ,11?~e.'II \ote~e \\\o~ . e'l\eO \\\e\\Oeo, II: OISQ QI\es \)e \ ~eto\\\ ::> '\ \\\~~ t- ':;,\ee\ -'0 tltO,ee'l'''I\es \\0'( ~~\\\~. oO: ,~'\ \0 ~ \'0 \)e Z :.0 \\\e\\\ "'O~I\\~ e\\0 - \~te Q~ev. I~\\\ D\sQ \\ M~ 'o~, ,:'1 II~ IIse ,tlt1(:>.\\;~ss\\)\e 0 60 70 80 90 100 DRY DENSITY- LBS. PER CU. FT. CASE 1. When higher moisture contents are anticipated. (As in the case of hydraulic structures) 30 . ,,\Ote\\\;~\\tll\'I~ olS~ \\0\\- ,\\\011'( "I' 25 O~ 'III \\\ Qloc.e0 . \oc.e\\\e \)e , ,\t\)I\~ DIsQ \\\o~ \~, I- ,,1\eS 0\\ ~eo,'I 01' Z ~ M\~I\\"." 11\ ~ 0 W '0\\ u '\)~ '11\\\\0 e\fO\1 II: Q\oc.e0 Qe\\ W 20 \e \e 0.. '\)e oeo.lIO oeo.lI~ , o~ ~ ~ '\)o~eo ~1\eS ,of o~\Oeo Q~e I- 0\\~~, '\)\e Qf \\\e Z ':;,\ee\- '0\~\\\~ oes\~O eO eQ\\\ 0' 1\\ w 11'0 0 Q\\e I- ,11 o~ '\)e \\eS, '\)e '(\\e . o\)\e z '\)~ o~ ,'\)\~ 0 '\)e ~e'l\\\\~ ~e\\\e\\'1 Qo~ QOSSIO\)e'jo\\ oesl\\ \\0\ 8 15 \\\ \\\0'0\ ,~1 o\sQ\O o\)'(~\\\e \\\e 'III W '\)of\\\~ IS e loess ('e II: Qf~~o\\O\\ s'If\)c.\1I ' ::> D~~ '11011\0~1~'II\\e\\\~: (/)l- 1:'1'Qe\\e e\\\\\~ '( Ic.o'\)\e 0 \\o\e, Q~e'll QfOC.'I' fO\\O\\ 0' ::;; \\ \\0~ oQe 10 (:>.\\\\O\)~ \)\~ 0\ ,tlt1 QfO\)O \\of\\\ -'00: I VJ II: ''0 \eO "'~ ::> 'IIe\ I- 00: z 0 60 70 80 90 100 DRY DENSITY- LBS. PER cu. FT, CASE 2. When natural moisture contents are not expected to increase, REQUIREMENTS FOR PRACTICAL PLACEMENT OF PILES FIGURE22 31 wetting the soil is recommended since 4. End-bearing piles resting on firm- highest strength will be obtained be- bearing material such as dense sand were cause of maximum consolidation of sur- found to be satisfactory regardless of the rounding material. placement method, provided they are snugly held for adequate lateral support. b. Placing displacement piles in jetted holes may be possible in certain 5. Both types of displacement piles, instances since holes smaller in diam- timber and steel shell, had similar driving eter than the piles may be made and some and loading characteristics. The strength wetting of the adjacent loess is obtained limitations of the pile itself would mainly from the jet water. govern which pile would be desirable. c. Placing displacement piles in 6. For piles placed in loess and assum- pre bored holes in loess at low natural ing material underlying a pile foundation is moisture content is not recommended competent, the following rules are recom- since virtually no consolidation of sur- mended: rounding material is obtained on driv- ing, and in this case, after the founda- tion is wetted, a weak bond between the a. When loess is at densities below soil and the pile can be anticipated. In 80 pa.m.ds per cubic foot, piles must have addition, considerable weakening of the firm end-bearing or a substantial amoo.nt loess surrounding the pile can be ex- of embedment of the ends in dense mate- pected. rial of permanently high shearing resist- ance. d. The use of displacement piles placed by driving alone in loess at low b. When loess is at densities from natural moisture content is not recom- 80 to 90 pounds per cubic foot, friction mended, since driving is extremely dif- piles will generally be satisfactorily sup- ficult and adequate penetration cannot ported only by adequate embedment in the be obtained. Also, the load-bearing loess, based on wetted conditions. capacity after wetting the soil is low. c. When loessisatdensitie'sgreater 3. Nondisplacement steel H-piles can than 90 pounds per cubic foot, moder- be driven in loess at low natural moisture ately loaded structures may be sup- content, but adequate embedment in per- ported without piles, and if piles are manently firm material is necessary since found necessary for critical structures, shearing resistance in loose wetted loess relatively short lengths may be used. is quite low. DENSITY OF LOESS AS A CRITERION OF SETTLEMENT In the construction and operation of lb- Limiting values of density for various reau of Reclamation dams, reservoirs, and types of soil, which indicate the suscepti- canals, the effects of saturation on the set- bility of a soil to subsidence on saturation, tlement of loess is of particular importance. can be defined using the following basic A simple criterion for judging the suscepti- principles: bility of loess to settlement is of value in predicting problem areas and the require- ment for foundation treatment. Approxi- (1) The liquid limit represents the mois- mate limits of density have been used for the ture content, based on a standard labora- loess of the Nebraska-Kansas area, since tory test, at which the soil is in a weak or other basic properties are generally simi- approaching liquid condition. lar. Extremely low-density loesses ap- parently subside under their own weight (2) If the natural density of the soil is when they are subjected to saturation. Mod- low enough that the porosity is more than erately low-density loess settles appreciably that required for the liquid limit moisture when subjected to loadings and saturation. content to saturate the soil, it is logical These critical coriditions of loess frequently that the soil could develop this weakened occur because of low natural moisture con- condition and could be susceptible to sub- tents and arid and semiarid climatic condi- sidence as added moisture approached this tions. amount. 32 (3) On the other hand, if the porosity of ures 13 and 12, respectively, indicated that the natural soil is less than that required loessial soils having densities between 80 for the liquid limit moisture content, such and 90 pcf were weak and suceptible to ap- as for a dense condition, it is logical that preciable settlement under loading, but they the soil could become saturated and still did not subside by wetting alone. It should be in a plastic state and thus not subside be remembered, however, that this ponding unless loadings are great enough to com- may not have fully developed saturation press it. (It may even expand if it con- that would occur in extended canal and res- tains expanding-type clay minerals. ) ervoir operation. Such prewetting suffi- ciently weakens the soil to permit settle- (4) Denisov, 9/, an investigator and ment to take place as relatively small struc- author of publications on loessial soils turalloads are applied, as shown in Figure in Soviet Russia, has used an equation 12. Pile driving for pile foundations can which may be applicable in correlating easily be accomplished under such conditions. density and liquid limit: It has further been observed that loess with densities below 80 pcf is extremely suscep- Porosity at liquid limit tible to settlement upon saturation. Kct = Porosity at natural density When the ratio, Kd, is less than Subsidence has been observed on some 1, the soil has possibilities of sub- canals in loessial soils. Figure 24 is a pho- siding on saturation. tograph of some subsidence that has occurred at a canal lateral on the Columbia Basin Based on the above principles, a chart Project in Washington. The density and liq- or graph can be drawn with dry density ver- uid-limit were determined on undisturbed sus liquid limit moisture content, as shown samples in nonsubsided soil next to the sub- in Figure 23. The curves on this graph, as sidence that has occurred. These data are shown in the figure, represents the condition plotted in Figure 23 by x-points. Also shown at which the porosity for a particular dry in Figure 23 are data from Meeker Canal in dmsity equals the porosity at the liquid limit. Nebraska (asterisk points) where similar For practical purposes the curves are con- subsidence has occurred. The data fall well sidered as a single line, because the position above the dividing line; three points were of this line varies only slightly according to near the value of 80 pcf density, and all the specific gravity of the soil. Points above points were below 90 pcf. These soils are this line represent conditions easily suscep- relatively low in liquid limit, having values tible to subsidence upon wetting, and the from 22.0 to 27.5 percent which cause a further the points are above the line, the density of 80 pcf, as shown in Figure 23 to more susceptible the soil is to subsidence. be critical in respect to the average loessial Soil with points below the line are not sus- soils. ceptible to subsidence unless loadings are applied to compress the soil. It has been observed that the natural loesses of the Nebraska-Kansas area fre- In Figure 23, liquid limit data from Fig- quently have moisture contents below 10 per- ure 6 are plotted for samples having natural cent. Pile tests and plate-load tests have density determinations. These data show shown that such loess resist very large loads that most natural density values of loessial without appreciable settlement. Ponding soils fall above the dividing line. Also, as tests in the field and wetting of consolidation mentioned previously in regard to Figure 6, test specimens in the laboratory indicated most liquid limit values of loessial soils fall that when moisture contents were raised to between 25 and 35 percent. above 20 percent, appreciable settlements could easily take place under loading. How- On the basis of revious observations ever, as can be seen in Figure 23 about 35 by Holtz and Gibbs 1 r., without regard to the percent moisture content or greater is needed density-liquid limit relationship, it was con- to saturate low density loess. Therefore, cluded that the probability of extreme settle- increased moisture content is an important ments of loessial soils was not great for den- characteristic for permitting settlement to sities above 90 pcf. In viewing the plot in take place, but density is the primary feature Figure 23 for values of liquid limit between which governs the susceptibility to settle- 25 and 35 percent, this conclusion is con- ment. The critical ranges of density values sidered reasonable. Later experiments in are governed by the type of soil which is in- ponding and field plate-load testing, Fig- dicated by the liquid limit. 33 LIQUID LlMIT-CIJo 10 20 30 40 50 60 70 90 60 IL. 0 Q.. 70 I >- I- (/)- X Z w 80 a Q:>- a 90 0 -I 110 r ( \ 120 , \ "Specific gravity = 2.70 i " . r- Lines of 100 "10 saturation "--Specific gravi t y = 2.60 ) . --- Trenton Dam and Railroad Relocation 11---Ashton, Nebraska Pile Test Area 0 ---Amherst Dam . ---Courtland Conal ~ ---Cambridge Conal '* -- -Meeker Conal x -- -La tera I PE41.2 1 Col urn bia Basin Project NATURAL DENSITY VS. LIQUID LIMIT RELATIONSHIP FOR LO E S S IA L SO I L S FIGURE 23 34 AN EXAMPLE OF SUBSIDENCE OF LOW-DENSITY SOIL AFTER SATURATIOY\I FROM CANAL OPERATION FIGURE 24 CONCLUSIONS Certain features which govern the soil to settlement on saturation. The higher the mechanics and foundation properties of loess plotted points are above the lines shown the have been established in this monograph. more susceptible the soils are to subsidence on saturation. The critical values of density The basic properties of loess in the will vary according to the plasticity of the Nebraska-Kansas area are similar. This soil. However, silty loess is similar in its similarity has been noted in other locations. plasticity characteristics and the liquid limit However, some trends of changing charac- values range from 25 to 35 percent. Because teristics have been observed in relation to of this similarity in basic properties along the distances of deposits from their probable with consolidation and strength studies, it source and in relation to climatic environ- is concluded that initial (in place) density is ment s which prevail. The loess of the Ne- a good criterion for indicating the proba- braska-Kansas area is predominately silty bility of settlement for structures. The fol- (75 percent of all samples tested). There lowing are recommendations of logical bound- are some instances of more sandy loess and, ary values for density of silty loess. More on the other hand, some clayey loess. From sandy material will be somewhat more criti- a general soil mechanics viewpoint, loess cal. appears to have rather uniform character- istics of gradation and plasticity. a. Silty loess having densities of Figure 23 is a chart on which natural less than 80 pounds per cubic foot is density and liquid limit values can be plotted considered loose and highly suscep- to aid in evaluating the susceptibility of a soil tible to settlement upon wetting. b. Silty loess having densities d. Moisture contents above 20 per- from 80 to 90 pounds per cubic foot cent are considered somewhat wet to is moderately loose to medium dense moist, and will generally permit full and is moderately susceptible to settle- consolidation to occur under load. ment. particularly for critical or heavily loaded structures. Extreme saturation e. Experience has shown that mois- may result in subsidence. ture contents of near 25 to 28 percent can easily be obtained by surface pond- c. Silty loess having densities of ing, and about 35 percent moisture (de- above 90 pounds per cubic foot is some- pending on density) is required for com- what dense and may be capable of sup- plete saturation. porting ordinary structures without serious settlement. Settlement will be of normal characteristics with loading. Certain shear characteristics have been established: d. A more general criterion has been used for earth dams in which 85 a. Dry loess at natural moisture pounds per cubic foot is used as the contents of less than 10 percent gen- division between low and high density erally has inherent shear strengths loess or the division where special foun- resembling cohesion which permit nat- dation treatment is required for lower ural loess to stand on very steep slopes densities. for considerable heights, although it may be of low density. This cohesive Because loess is predominately a silty strength is apparently due to the small soil and has at least a small amount of clay, amount of clay which gives the loess a there is pronounced adjustment in particle- particle-to-particle bond. This co- to-particle strength as a result of adjust- hesive strength has been observed to ment in moisture content. In the Nebraska- be as much as 15 psi. Generally, it is Kansas area, natural moisture contents are about 5 to 10 psi for the Nebraska-Kan- frequently below 10 percent, and the natural sas loess. The slope of the shear re- loess is frequently capable of supporting very sistance curve (tan ~) is about 0.60 to large loads. However, when moisture con- 0.65. Such values of cohesion and tan 0 tents are raised due to wetting, as in foun- permit loess to stand on steep (or nearly dations of hydraulic structures, the mini- vertical) slopes 50 to 80 feet high. mum shear-strength conditions and full set- tlement with load should be expected. The b. Wetted loess is greatly reduced following are recommendations of boundary in strength. Experience shows that co- values for moisture content: hesion is generally reduced to less than 1 psi when the loess is wetted and even a. Moisture contents below 10per- for initially dense loess, it becomes less cent are considered very dry and maxi- than 4 psi. An im}X>rtant finding in these num dry strength and high resistance to studies is that high pore pressure de- settlement should be expected. velops in wetted loess as consolidation takes place, and therefore the develop- b. Moisture contents from 10 to 15 ment of the additional shear resistance percent are considered somewhat dry, resulting from friction may be restricted giving moderately high strength. until drainage takes place. Drainage maybe sufficiently slow to cause lateral c. Moisture contents from 15 to 20 movement of embankment foundations percent are approaching moist conditions. during normal construction periods. Acknowledgment Much of the petrographic and geologic narrative was written from original notes and work compiled by members of the Petro- graphic.Laboratory Section, in particular the late M. E. King. 36 LIST OF REFERENCES 1. Holtz, W. G., and Gibbs, H. J., "Con- 6. Hobbs, W. H., "The Origin of the Loess solidation and Related Properties of Lo- Associated with Continental Glaciation essial Soils, " Special TeChnical Publica- Based upon Studie s in Greenland, " Inter- tion No. 126, American Societyfor Test- national Geographic Congress, Paris ing Materials, 1951. 1931, Compte rendu, 1933, tome 2, fasc. 1, pages 408-411. 2. Hobbs, W. H., "TheGlacialAnticyc1ones and the European Continental Glacier, " 7. Davidson, D. T., and Sheeler, J. B., American Journal of Science, Volume "Cation Exchange Capacity of Loess 241, No.5, May 1943, pages 333-336. and Its Relation to Engineering Prop- erties, " Special Publication No. 142, 3. Smith, G. D. (1942), "Illinois Loess-- American Society for Testing Materials, Variations in its Properties and Dis- 1953. tribution, " University of Illinois Agri- cultural Experiment Station Bulletin 8. Clevenger, W. A., "Experiences with 490. Loess as Foundation Material, " Trans- actions American Society of Civil Engi- 4. Swineford, Ada, and Frye, J. C., "A neers' Volume 123, 1958, page 151. Mechanical Analysis of Wind-blown Dust Compared with Analyses of Loess, " 9. Denisov, N. J., "Settlement Properties Symposium on Loess, 1944, American of Loessial Soils, " a book published by Journal of Science, Volume 243, No.5, the Government Publishing Office -- May 1945, pages 249-255. Soviet Science, Moscow, 1946 (Trans- lated and abstracted by K. P. Karpoff 5. Tuck, Ralph, "The Loe ss of the Mata- and H. J. Gibbs, Bureau of Reclama- nuska Valley, Alaska, " Journal of Ge- tion, Denver, Colorado, June 1951, ology, Volume 46, No.4, May-June page 14). 1938. 37 GPO 842939