INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING
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CONFINED COMPRESSION OF LOESS
COMPRESSION AVEC ETREINTE LATERALE DU LOESS C1KATHE J1ECCA BE3 BOKOBOrO PAC111MPEHMH
HARRISON KANT Ph. 0., Professor of Civil Engineerin g, the University of Iowa, Iowa City (USA!)
SYNOPSIS. Effective stress paths for a loessial soil subject to collapse during confined compression have been determined from the results of a testing program consisting of (1) confined compression tests on natural samples of loess with initial water contents ranging from air-dry to saturation, (2) negative pore-water pressure meas urements to -300 psi during these tests, and (3) Retests in which the lateral stress ratio was measured for one-dimensional strain. Before collapse, K was found to average 0.23, an extremely low value for a loose soil, whereas after collapse, Kq increased to 0.5?, which is consistent with values for other soils. From the stress path interpretation of the results, it is demonstrated that the collapse mechanism of loess in confined compres sion and during wetting Is a shear phenomenon and subject to analysis in terms of effective stresses.
INTRODUCTION pends also on the water content before wetting. Studies by Bally (1961) demonstrated large compressive strains This research has been undertaken to study and de upon wetting of air-dry loess but small strains for scribe, quantitatively, the mechanisms involved in the the same soil at an initial water content of 24%. collapse phenomenon in loess. Supporting data incor Similar observations have been made by others. porates two quantities which have not previously been measured in this context: the negative pore-water To provide the data for interpreting the behavior of pressure and the lateral stress ratio, both measured the soil, two special types of test6 were run: (1) during confined compression. confined compression tests in which the negative pore- water pressure was measured during the course of the Loess is a wind-deposited sediment transported from tests utilizing a specially constructed cell, and (2) the flood plains of glacial rivers. The natural, un a special series of tests in which the lateral stress disturbed loess is a loose, open-structured soil com ratio (the ratio of the horizontal to the vertical posed of silt particles separated by clay coatings or stress) was measured in a triaxial compression cell aggregates of clay particles (Larionov, 1965; Gibbs under a zero-lateral-strain condition (KQ-tests). and Holland, 1960). A typical midwestem loess has a clay content of 10% to 30%, with water contents from SOIL INDEX PROPERTIES 5% to 30% and densities from 70 pcf to 90 pcf (Sheeler, 1968). The significant properties in this study are The undisturbed soil samples were obtained from a road (1) the low natural density which permits the occur cut in a loess deposit on the Oakdale campus of the rence of large volume changes, (2) the bond strength University of Iowa, just west of Iowa City. Index provided by the clay coatings, and (3) the changes in properties for the Oakdale loess are listed in Table this bond strength that occur with changes in water 1. The natural dry density of the Oakdale loess is content. relatively high for loessial soils in Iowa. Densities in east-central Iowa from 80 pcf to 90 pcf have been The collapse of loess occurs when the stress between reported by Lyon, Handy, and Davidson (1954) and den particles exceeds the bond strength provided by the sities as low as 66 pcf have been measured. On the clay cdatings. This may be caused by an increase in basis of the properties in Table 1, the Oakdale loess stress due to an applied load, or by a decrease in is described as a relatively dense, silty loess of low strength due to swelling and softening of the clay plasticity. binder after wetting. The loss of strength on wetting has been recognized for some time (e.g. Holz and Gibbs, SAMPLING AND PREPARATION OF TEST SPECIMENS 1951). A recent description of this behavior has been given by V. G. Berenzantzev, et al. (1969). In this Hand-carved blocks of soil, 8 in. by 10 in. by 10 in. paper the subsidence deformation below a foundation is were removed from the test pit, wrapped in plastic, shown to occur in a zone where the shear strength has and transported directly to the laboratory. Upon ar decreased as a result of wetting to the level of the rival at the laboratory, the blocks were divided into existing shear stresses. The amount of settlement de smaller samples, sealed and stored in a moist room
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TABLE 1 SOI L I NDEX PROPERTI ES in the p a rtia lly saturated specimens of loess w hile the s o il was undergoing compression under standard consolidation test loading. O a k d a le L o e s s The confined compression ce ll is shown in Fig. 1. T h e three main parts are the base, the cylinder, and th e Liquid Lim it 2 7 top. The base and top are machined from stainless P lastic Lim it 2 3 steel and the cylinder Is a section of 5-inch diame t e r P la sticity Index 4 aluminum pipe. A fine-grained porous stone w ith a S pecific G ravity 2 . 7 2 rated bubbling pressure of 225 psi is sealed into t h e Percentage Clay base. Two sm all-diam eter ports enter through the bo t L e s s t h a n 0 . 0 0 5 Dim 1 7 tom of the base to the lower surface of the stone. L e s s t h a n 0 . 0 0 2 mm 1 3 One port is fitte d w ith a valve and a supply of dea i r A c t i v i t y 3 0 . 5 0 ed water confined in a control cylinder capable of N atural Dry Density pcf forcing measured quantities of water through the st o n e R a n g e 90.4 to 92.5 into the so il. A 150-psi pressure transducer is mou n t A v e r a g e 9 1 ed at the other port. The ce ll top encloses the top o f N atural Void Ratio the cylinder. A 1/2-inch diameter loading ram is R a n g e 0.800 to 0.940 guided through the top by a ball-bushing and teflon A v e r a g e 0 . 8 6 1 seals provide a low -friction seal. The top is con N atural Water Content, % nected to the base by four bolts extending from the R a n g e 21.2 to 22.9 base to the top outside the cylinder. Accurate alig n A v e r a g e 2 2 ment of the loading ram is assured by the machined cylinder ends.
aA ctivity = p la sticity index/(% 0.002 ram clay - 5%) The so il specimen is carved in to a standard consoli d a (Seed, H.B. et a l, 1962) tion ring, 2.5 in. in diameter by 0.75 in. thick. T h e ring is located centrally in the hase by three lugs u n til needed for testing. and held in place by a co lla r. The load from the lo a d ing ram is transm itted to the so il by the loading c a p The confined compression testing program required and stone. The ring, co lla r, and loading cap are stru ctu ra lly undisturbed specimens w ith a range in standard consolidation c e ll components. water contents, nom inally 4%, 8%, 12%, 16%, 20%, 24 % , and 28%. Since the natural water content was about 22%, the specimens had to be wetted or dried to achieve the desired water contents before trim m ing t o test specimen size.
The a lteratio n of the water content was accomplishe d in the follow ing manner. To achieve a water content o f 28%, the sample as stored at its natural water cont e n t was unwrapped and wetted by spraying the surface w i t h a measured quantity of water. The sample was then r e wrapped and placed in a m oist chamber fo r several d a y s to perm it the dispersal of the water throughout the so il. This process was repeated u n til the water con tent of the sample, estim ated from the sample's wet weight and its orig in al weight and water content, w a s the desired 28%. The sample was then trimmed into t h e consolidation ring for testing. The water contents o f the specimen and the trim m ings were compared to che c k the uniform ity of water distribution in the sample a n d in a ll cases the differences were less than 1%. Water contents below the natural water content were achieved by perm itting the surface of the sample to air-dry for several hours, during which tim e the sa m ple was weighed periodically to determine the weigh t of water evaporated. The sample was then stored in the m oist chamber to perm it the rem aining so il mois ture to redistribute its e lf. This process was repea t ed u n til the desired water content was reached. It was found that air-drying the samples too rapidly p r o duced cracks which required discarding the sample. The m odified water contents were in general w ithin 2% of the desired nominal water content.
CONFINED COMPRESSION TESTS
Special equipment was designed and constructed to p e r F i g . 1 Conf i ned Compr essi on C el l m it the measurement of'negative pore-water pressure s
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During operation the stone in the base and the port 6 the measured pressure below the stone tends to become below the stone were saturated with water. The pres negative. As noted above, the negative pressure was sure in this water was measured by a 150- pei pressure compensated by increasing the cell pressure. transducer mounted directly on the base. The confined compression cell was mounted in a consolidation test Equilibrium was reached when there was no change in machine having dead- weight loading at a lever ratio of cell pressure and water pressure below the stone for 10 to 1. The base of the cell was bolted to the test a period of ten minutes or more. At this time the machine and the desired loads were applied to the t op initial negative pore- water pressure was computed as of the ram through the lever system. The compression the difference between the cell pressure and the pr es of the soil specimen during loading was measured with sure below the stone if any. a dial indicator reading 0.0001 in. During consolidation a loading sequence of 1/2, 1, 2, The measurement of the negative pore- water pressures 4, 8, 16, and 32 tsf was used for soils at or wetter has been made using a technique known as the axis than a water content of 16 percent. The 1/2 tsf load translation technique (Olson and Langfelder, 1965). was omitted for soils with water contents less than 16 This technique works in the following manner, if a percent. For each load the compression, the cell pr es partly saturated specimen of soil is placed on the sure, and the water pressure below the stone were r e fine ceramic porous stone shown in Fig. 1, the capi l corded at time intervals of 1/2, 1, 2, 4, 8, 15, 30 lary suction in the soil will tend to draw the water and 60 minutes. At the end of sixty minutes the next out of the porous stone. With valve A closed and the load was applied. voids in and below the stone completely saturated, the water below the stone Is in a closed system. Thus t he In a special test series, the water content of the water pressure below the stone will become equal to soil was Increased while the soil was under constant the pore- water pressure in the soil with a negligible load; during this process the compression of the spec flow. imen and changes in pore- water pressure were measured. The increase in water content was accomplished through However, if the pore- water pressure is below - 10 to the use of the back pressure control cylinder shown in -15 psi, cavitation will occur and the system below Fig. 1. The control cylinder was constructed to dis the stone will no longer behave as a closed system. place 0.1 cc of water per rotation. This condition is avoided in the axis translation meth od by placing the soil and the porous stone in a cl osed RESULTS OF CONFINED COMPRESSION TESTS chamber. A positive gas pressure applied in the cel l ^ cancels the negative pore- water pressure in the pore Six specimens have been selected to illustrate the £ fluid. The negative pore- water pressure in the soil, trends in the data. The test results for these speci therefore« is equal to the measured water pressure be mens are presented in Fig. 2 a four- variable plot of low tiic stone minus the gas pressure in the cell. log stress, void ratio, water content, and negative pore- water pressure. The specimens had initial water In order to determine the initial negative pore- wat er contents ranging from 4.6% to 28.3% and approximately pressure, an undisturbed soil sample at the desired the same Initial void ratio. moisture content was carved into the consolidation ring. The soil and the consolidation ring were weighed The void ratio- log stress plots in Fig. 2 (a) show and the trimmings used to check the nominal moistur e that the wetter specimens were more compressible than content. The cell was prepared by removing the top and the drier specimens which is, of course, to be expect cylinder and wiping the excess water off the porous ed. The stress level at which the curve steepens for stone after having closed valve A. A small negative the driest specimen is nearly ten times that for the water pressure developed immediately in the water below wettest specimen. the stone as a result of evaporation and the format ion of menisci on the surface of the stone. This pressure The void ratio- water content plots in Fig. 2 (b) show was monitored on a transducer indicator. Next the sam lines for 50% and 100% saturation and the change in ple was placed on the stone and the collar placed degree of saturation of the sample during the course around the consolidation ring and secured with three of the tests may be observed. The water content re nuts. A circle of filter paper was placed on top of mained constant during the compression of the dry the sample and the loading cap and stone were put into specimens. For these specimens the degree of satura position. Following these steps the cell was assembled tion increased as a result of the reduced volume of and the top secured to the base by means of the bol ts voids but the specimens remained partially saturated. extending from the top to the base. The above work in On the other hand, the wettest two specimens became general took less than two minutes to complete. Wit h 100% saturated during compression and the further r e the cell assembled it was then possible to apply ni tro duction in voids caused water to be expelled from t he gen pressure in the cell. The applied cell pressure specimen and a consequent decrease in water content . tends to cancel the measured negative water pressur e and the cell pressure was continually adjusted to main The measured initial pore- water pressures are plott ed tain a pressure of zero in the water below the stone. against initial water content in Fig. 3. The curve drawn through the data points has the equation Finally, a seating load of 1/32 tsf was applied to the ,,2.6 6oil specimen and an additional load was applied to the uw (psi) = - (^) (Eq. 1) top of the loading ram to compensate for the cell pres sure acting upwards on the bottom of the ram. These loads were generally in place six minutes after the This equation is limited to the pressure range between 6tart of the tests. 3 psi and 300 psi. Twenty of the twenty- five measured pressures are within 33% of the value given by this As the pressure i n the soi l and i n the stone equal i zes, equation.
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F ig . 2 Confined Compr ession Test IfesuLTS
For a given water content, the negative pore-water K -T E S T S o pressure is independent of the degree of saturation except when Sr exceeds 85% or 90%. This is shown in A series of tests designated as KQ-te sts was run to Fig. 4 where the pore-water pressure is plotted supplement the confined compression tests described against degree of saturation for representative tes t s . above. These tests were run in a standard tria x ia l For each test, the water content is constant, but t h e c e ll on specimens having diam eters of 1.5 in . and degree of saturation increases as a result of the r e heights of 3 in. Lateral strains were detected by duction in voids during compression. The approximat e means of a specially designed la te ra l-stra in indica t o r . ly horizontal curves show that the pore-water press u r e The test was set up in the same manner as a standar d does not change during this process. The independen c e tria x ia l compression test w ith a zero ce ll pressure . of pore-water pressure from degree of saturation ma y The axial stress was then increased and the ce ll pr e s be explained as follow s. The clay content is d istri b sure was increased as necessary to m aintain zero la t uted as coatings on larger particles or as aggregat e s eral strain. A ll Specimens were undisturbed, and at of clay particles. The water in the soil is dispers e d th e ir natural water contents. in the clay voids and on the surface of the larger particles. The clay voids do not change in volume The results are summarized in Table 2. The ra tio of w ith changes in voids of the whole so il and thus th e the tota l lateral stress to the tota l axial stress i s shapes of the m enisci, and the pore-water pressures , defined here a6 KQ, the la te ra l stress ra tio at res t . are independent of the to ta l volume of voids. When Before the collapse of the s o il structure, which oc the degree of saturation exceeds 90%, however, the curred at axial strains of 1% to 4%, the average va l u e volume of a ir is at a point where a further reducti o n o f K q was 0.23 (K .). A fter collapse, Kft increased to in the voids causes the larger pores to become satu 0.54 (Ko f). rated and the pore water pressure Increases. At a d e gree of saturation of 100%, the water fills the voi d s and further compression is accompanied by the expul s i o n of water from the so il. For this saturated state, t h e pore-water pressure w ill be zero when consolidation i s c o m p l e t e .
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TABLE 2 SUMMARY OF K -TESTS o
1. Test No. 1 2 3 H - l H - 2 H - 3 A v e r a g e
IN IT IA L MEASUREMENTS
2. Water content % 2 2 . 6 2 1 . 5 2 2 . 9 2 2 . 6 2 2 . 7 2 3 . 0
3. Void ratio 0 . 8 8 3 0 . 8 6 2 0.851 0.871 0.848 0 . 8 3 3 —
4. Degree of saturation % 6 9 . 3 6 7 . 5 7 3 . 1 7 0 . 2 7 2 . 6 7 4 . 8 —
LATERAL STRESS RATIO, K ------o
5. Before collapse, 0 . 2 5 0 . 2 2 0 . 1 5 0.23 0.33 0.17 0.23
6. A fter collapse, KQ^ 0 . 5 2 0 . 5 6 0 . 5 4 0 . 5 4 0 . 5 6 0 . 5 0 0 . 5 4
Fig. 4 Pore- Water Pressure vs Degree of Saturation During Confined Compr ession
ANALYSIS OF MECHANICAL BEHAVIOR
W ater Content, % During confined compression, the ve rtica l stress crv is increased and the radial or horizontal stress cr^ Fig. 3 Relation Between Initial Pore- increases su fficie n tly to m aintain the condition of zero la teral strain. The stress path method of repr e Water Pressure Arc Water Content senting the successive states of stress (Lambe, 196 4 ) forms the basis for the analysis of the behavior of
119 4/ 19 the so il in the follow ing discussion. In this metho d , The general character of effective stress paths for t h e s t ate of stress is represented by a stress point the confined compression of loess is shown in Fig. 6 . on a p - q diagram in which p = (ov + O h)/2 and q = Four particular test conditions are considered in (av - a^)/2. Successive stress points are connected this figure: (a) loading without wetting, (b) wetti n g to form the stress path for a loading. The stress before collapse, (c) w etting after collapse and path may be drawn using either to ta l stresses or (d) collapse due to w etting. The firs t condition effective stresses. The la tte r is used here and thu s corresponds to the basic test series and the last the effective stresses in the loess during confined three to conditions which existed during the specia l compression are required. t e s t s .
For p a rtia lly saturated so ils, the expression for In the case of loading without w etting, Fig. 6(a), effective stress cf is (Bishop, 1960): the effective stress path for loading prior to collapse is AC. Its slope is determined by the valu e of K ^. It should be noted that the total and ° = [a ~ ua " x(ua " Uw3 ^Eq‘ ^ effective stress paths are parallel since the pore- where o is the to ta l stress, ua is the pore-air pre s wate: pressure does not change sig n ifica n tly (Fig. 4 ) . sure, uw is the pore-water pressure, and x is an At j
The x vs Sr relations for this so il, based on the c o n fined compression tests, are shown in Fig. 5. These results were deduced from an effective stress fa ilu r e envelope, the to ta l stress at collapse as determine d by measured Kc -values (Table2), and the measured pore-water pressures. In spite of the scatter for Sr > 70%, it is clear from the remaining portion of the curve that x is approxim ately equal to unity wh e n Sr exceeds 70%. E ffective stresses used to construc t the stress paths in the follow ing diagrams were calculated using Eq. 2 w ith x~values from Fig. 5.
F i g . 5 x vs SR f or Oakdal e Lo e s s Fig. 6 Effective Stress Paths for Confined Compr ession of Loess
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20 60 30 100 120 m J5, psi
Fig, 7 Effective Stress Path for Confined Compr ession Test
p, psi
Fig. 8 Effective Stress Path for Confined Compr ession Test With Wetting i n av causes slippage between particles. As a re su lt, slopes downward from the Kf-line at a slope of 45 the so il compresses and new stru ctu ra l configuratio n s degrees if the pore pressure remains constant durin g develop. Collapse may be considered to have con c o lla p s e . cluded when the particles lock in new stable positi o n s at which point K0 = KQf, point D. The transition The stress path fo r a representative confined compr e s f r o m 0^ = Koi av to = K0f av occurs with A = sion test is shown in Fig. 7. The stress path follo w s Aav; that is , the stress path C D is horizontal. the idealized stress path in Fig. 6 (a) and the cor This conclusion is based on the measurements obtain e d responding points are shown by the same le tte r desi g from the KQ-tests. The path for loading after nations. At each data point the vertical strain is collapse is DF and its slope is determined by the given. Because of the re la tively wide spacing of th e value of K -. data points, the collapse stress at point C is not o f precisely known. The portion of the stress path dur If, during the course of the test, the loess is wet ing the collapse of the structure is shown by a das h e d ted, the stress paths are changed in the manner line and drawn so that point D is a data point and shown in Figs. 6 (b), (c), and (d). W etting under a the line CD is horizontal. Point C is not on the Kf - constant av reduces the negative pore pressure and line as it is in Fig. 6 (a), because the collapse thus p decreases w hile q remains constant. The stre s s stress is not precisely known and because the K f-li n e path during w etting is horizontal (BB* and EE'). If , represents an average strength. However, it is evi during w etting, the stress path intersects the K f- dent from these figures that (1) there is a dispro lin e , as for path BC* in Fig. 6 (d), collapse occur s portionately high increase in strain as the path ex as a result of wetting. In this case, av remains tends from C to D (since the load doubles, doubling constant during collapse but increases because the strain is proportionate), indicating a structur a l
K q increases from KQ^ to The stress path C'D1 collapse, and (2) point C, where this collapse in itia te s, is near the Kf- line. 121 4/ 19
Thus the test data substantiates the idealized BEREZANTZEV, V. G., A. A. MUSTAFAYEV, N. N. SIDOROV, concept for the effective stress path, Fig. 6(a). I . V. KOVALYOV, and S. K. ALIEV, (1969), "On t he The stress path for a test in which the sample was Strength of Some Soils," Proceedings, 7th Inter wetted is shown in Fig. 8. The path follows the pat h national Conference on Soil Mechanics and Foundation in Fig. 6 (d) which describes collapse due to wetting. Engineering, Mexico, Vol. 1, pp. 11- 19. The strain increased to over 8% (path C'D') due to wetting alone with no increase in stress. This test , BISHOP, A. W., (1960), "The Principle of Effective and other special tests having similar results, agree St ress," Publication No. 32, Norwegian Geotechnical with and support the concepts illustrated by the Institute, Oslo, pp. 1- 5. rtress paths in Fig. 6 (b), (c), and (d). BISHOP, A. W., and G. E. BLIGHT, (1963), "Some Aspects CONCLUSIONS of Effective Stress in Saturated and Partly Saturat ed Soils," Geotechnique, Vol. 13, No. 3, September, pp. (1) The initial pore- water pressure was related to 177- 197. the .water content through the empirical equation GIBBS, H. J. and W. Y. HOLLAND, (1960), "Pet rographic and Engineering Properties of Loess," Engineering uw (Psi > = - (£| ) 2-6 (Eq. 1) Monograph No. 28, U.S. Bureau of Reclamation, Denver,
Colorado. fsso During compression, no significant change in the pore water pressure occurred except when the degree of HOLTZ, W. G., and H. J. GIBBS, (1951), "Consolidati on saturation Sr exceeded 85% to 90%. Thus, with this and Related Properties of Loessial Soils," Symposium exception, the pore- water pressure is independent of on Consolidation Testing of Soils - 1951, American the degree of saturation, and Eq. 1 is applicable t o Society for Testing Materials, Special Technical Pub more than the initial conditions. lication No. 126, pp. 9- 26.
(2) For stresses below the collapse stress, the lat LAMBE, T. W., (1964), "Methods of Estimating Settle eral stress ratio for confined compression K0 was ment," Journal of the Soil Mechanics and Foundations found to be small in comparison with values commonly Divi s i on, ASCE, Vol . 90, No. SM5, pp. 4^- 67. measured in other soils. The average measured value for the loess tested was 0.23; in contrast, sands LARIONOV, A. K., (1965), "Structural Characteristics have K0- values from 0.4 to 0.6 for dense to loose of Loess Soils for Evaluating Their Constructional densities respectively. After collapse, however, K0 Properties," Proceedings, 6th International Conference was found to be 0.54 which compares favorably with on Soil Mechanics and Foundation Engineering, Montr eal, values for other soils. Thus the structural change Vol. 1, pp. 64- 68. during collapse has a marked effect on the soil properties. LYON, C. A., R. L. HANDY, and D. T. DAVIDSON, (1954), "Property Variations in the Wisconsin Loess of East - (3) The interpretation of the test results permitted Central Iowa," Iowa Academy of Science Proceedings, important qualitative conclusions. The collapse mech Vol. 61, pp. 291- 312. (Reprinted in Bulletin No. 20, anism of the soil in confined compression is a shear Iowa H.i®h«av Research Board, December 1960, pp. 44- 64). phenomenon; that is, the collapse stress is determined by the shear strength expressed as a function of OLSON, R. E., and L. J. Langfelder, (1965), "Pore effective stresses. The behavior of the soil before, Water Pressures in Unsaturated Soils," Journal of t he during, and after collapse can be illustrated by use Soil Mechanics and Foundations Division, ASCE, Vol. 91, of effective stress path diagrams (Fig. 6). No. SM4, pp. 127- 150.
ACKNOWLEDGEMENT SEED, H. B., R. J . WOODWARD, and R. LUNDGREN, (1962), "Prediction of Swelling Potential for Compacted This work was supported by the Iowa State Highway Com Clays," Journal of the Soil Mechanics and Foundations mission and the National Science Foundation, Grant Divi s i on, ASCE, Vol . 88, No. SM3, pp. 53- 87. No. GK- 15112. The author was assisted by Mr. Altaf ur Rahman and Mr. Bharat Mathur, Graduate Research SHEELER, J. B., (1968), "Summarization and Comparison Assistants. of Engineering Properties of Loess in the United St at es," Highway Research Record No. 212, Highway Re REFERENCES search Board, pp. 1- 9.
AITCHISON, G. D., (1961), "Relationships of Moistur e SKEMPTON, A. W., (1961), "Effect ive Stress in Soils , Stress and Effective Stress Functions in Unsaturated Concrete and Rocks," Pore Pressure and Suction in Soils," Pore Pressure and Suction in Soils, Butter- Soils, Butterworths, London, pp. 4- 16. worths, London, pp. 47- 52. YOSHIMI, Y., and J . 0. OSTERBERG, (1963), "Compr ession BALLY, R. J ., (1961), "Data on the Homogeneity and of Partially Saturated Cohesive Soils," Journal of the Compressibility of Loesses Treated with the Help of Soil Mechanics and Foundations Division, ASCE, Vol. Statistical Methods," Studii de Geotechnica, Fundat ii 89, No. SM4, pp. 1- 24. si Constructii Hidrotechnice, Vol. 3, I.S.C.H., Bucharest.
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