Residual Pore Pressures in Compacted Clay

Transcription

1 135 Residual Pore Pressures in Compacted Clay Pression de l eau interstitielle résiduelle dans l argile compactée by T. W illiam L am be, Professor and H ead, Soil Engineering M assachusetts Institute o f Technology Cam bridge, M ass. U.S.A. Summary This paper presents values of negative pore water pressure measured in laboratory compacted samples of clay. Tensions as large as 14 pounds per square inch below atmospheric pressure were recorded. The results obtained show that the pore pressures existing in clay following compaction are very sensitive to the following details of the compaction process : ( 1) molding water content ; (2) type and am ount of compaction ; (3) method of attaining the molding water content ; (4) tem perature ; and (5) confinement. The measured pore pressures are consistent with the compaction theory proposed by the author (1958), and supported and extended by Seed and Chan (1959). Residual pore pressure is considered an im portant and useful manifestation of soil structure. Sommaire Cet article présente des valeurs de la pression négative de l eau interstitielle sur des échantillons de laboratoire d argile compactée. Des pressions atteignant 14 livres par pouce carré au-dessous de la pression atm osphérique ont été enregistrées. Les résultats obtenus montrent que les pressions interstitielles existant dans l argile après compactage sont très sensibles aux éléments suivants de la méthode de compactage : (1) teneur en eau de moulage ; (2) mode et intensité de compactage ; (3) méthode d obtention de la teneur en eau de moulage ; (4) température ; (5) mode de confinement. Les pressions interstitielles mesurées sont en accord avec la théorie du compactage proposée par l auteur (1958), confirmée et développée par S e e d et C h a n (1959). La pression interstitielle résiduelle est considérée comme un im portant et utile élément de structure du sol. I. Introduction T he great influence o f com paction on the engineering properties o f clay is well know n. T he strength, perm eability, an d volum e change characteristics o f com pacted clay can depend very considerably on : ( 1) m olding w ater content, (2) am ount o f com paction, (3) type o f com paction, and (4) details o f com paction procedure, such as the m ethod used to get the soil to the m olding w ater content. In tw o papers published in the A.S.C.E. proceedings ( L a m b e, 1958) the au th o r proposed a m echanistic theory to account for the effects o f com paction on the behavior of clay. T he theory attem pted to explain the know n properties of clay in term s o f structure the arrangem ent o f soil particles and the electrical forces betw een adjacent particles. T he th eo ry also explained the effects o f com paction on clay structure. T est d ata w ere presented to help substantiate the theory. S e e d and C h a n (1959) presented a w ealth o f experim ental data w hich confirm ed and extended facets o f the structure concepts. The three papers show ed th at, in general : (1) Increasing the m olding w ater tended to result in a m ore parallel arrangem ent o f p a rtic le s; (2) Increasing the com pactive effort tended to result in a m ore parallel arrangem ent o f p a rtic le s; (3) K neading com paction tended to give a m ore parallel arrangem ent o f particles than did static com paction. The existence o f these tendencies was proved by actual m easurem ents o f particle arrangem ent. Since such m easurem ents are very difficult to m ake other m ethods o f determ ining structure were sought. O ne good indication o f the structure o f a soil sam ple was found to be the am ount of shrinkage the sam ple underw ent upon drying. T he theory suggested an o th er m anifestation o f soil structure w ould be th e pore w ater pressure existing in the com pacted clay p rior to the addition o f m ore w ater or o f shear stresses. Such pore pressures are herein term ed, residual pore pressures. T he rem aining portion o f this paper presents m easurem ents of residual pore pressures in tw o clays. T he effects o f m olding w ater, am ount and type o f com paction, other com paction variables and tem perature are show n and discussed. II. Soils used in tests Tw o soils were used in the investigation, nam ely : (1) A silty clay from V icksburg, M ississippi ( V icksburg L oess ), having a liquid lim it o f 34 per cent and a plastic lim it o f 26 per c e n t; (2) A pure kaolinite from B ath, South C arolina, having a liquid lim it o f 55 per cent, and a plastic lim it o f 33 per cent. These are essentially the sam e silty clay and kaolinite used in the Seed and C han (1959) te sts; i.e., they are from the sam e sources, but different batches. III. Pore water pressure measurements P ore w ater pressure m easurem ents in soil-w ater system s w ere first m ade m any years ago. In a review o f the developm ents in p ore pressure m easurem ent technique B i s h o p (1960) described tests by O sborne R e y n o l s in approxim ately R e y n o l d s m easured w ith a m ercury m anom eter a negative pore pressure o f 13 lb. per sq. in. in a sam ple o f saturated sand subjected to a shear stress. T a y l o r (1943) describes tests by L. R e n d u l i c in approxim ately 1937 in w hich pore pressures were m easured w ithin a sam ple o f clay during shear. R e n d u l i c used a core o f sand-m ica as his sensing elem ent. M ore recent pioneering w ork in pore pressure m easurem ents was done by the B ureau of R eclam ation (at the ends o f triaxial specim ens) and by T a y l o r at M.I.T. (w ithin triaxial specim ens by m eans o f probes). In another paper ( W h i t m a n, R i c h a r d s o n and H e a l y, 1961) to this 207

2 C onference is described a system used to m easure a negative pore pressure o f 20 lb. per sq. in. T his system em ployed an electrical pressure transducer connected to an oscilloscope. T he system used to obtain the data presented in Figs. 1-6 consisted essentially o f a sensing elem ent connected to a m easuring elem ent. Tw o sensing elem ents w ere used, nam ely : a porous ceram ic disk (G rade 0-6 m anufactured by the Selas C orporation, having an air entrance pressure o f 60 lb. per sq. in.) ; and a 332 in. diam. probe o f stainless steel tube containing a 200 mesh screen, tipped w ith clay and then fired (after H ilf, 1956). Tw o m easuring elem ents were used : an electrical pressure transducer (D ynisco PT 25, m an u factured by the D ynam ic Instrum ent Co.) connected to an oscilloscope ( H e w l e t t - P a c k a r d 130 A ); and a null section connected to a pressure and vacuum source. The transducer-oscilloscope m easuring elem ent is m uch m ore convenient than the null section. The transducer gives an instantaneous pressure reading and requires essentially no fluid flow to actuate - the entire diaphragm deflection, corresponding to a pressure range o f 100 psi, is only inches. As show n by the data in Fig. 1 good agreem ent was ob tained w ith th e various arrangem ents o f sensing and m easurem ent elem ents. T he great m ajo rity o f the test data reported Fig. 1 Pore pressures measured with various setups. Pressions interstitielles mesurées avec différents appareillages. herein (in particular, the data in Figs. 3-6) were obtained using the H ilf-type probe and a null section. Several probes (necessitated by breakage) were used. A ir entrance pressures for the probes varied from 6 to 10 lb. per sq. in. The arrangem ent o f elem ents is sim ilar to those show n in the literature ( B i s h o p, 1960; H i l f, 1956 ; L a m b e, 1948 ; T a y l o r, 1944, etc.). All pore pressure m easurem ents were m ade on clay sam ples com pacted in the H arv ard M iniature m old (2.816 inches high by inches in diam eter) using one o f the three follow ing m ethods : (1)3 layers, 25 tam ps per layer w ith 40 lb. spring (approxim ately equal to stan d ard A A SH O effort). (2) 5 layers, 25 tam ps per layer w ith 40 lb. spring. (3) Static load necessary to give the desired density. IV. Test results T he results o f the pore pressure m easurem ents are show n in Figs. 1 through 6 ; the results show th e follow ing : A. Measurement Technique Fig. 1 presents p ore pressures m easured in V icksburg Loess by various experim entors using several different types o f equipm ent. T he close agreem ent o f the data indicates th at hum an factors and equipm ent setups had relatively m inor influence on the accuracy o f the m easurem ents. It was not, how ever, an objective o f this investigation to evaluate various techniques-in fact, all o f the data in Figs. 3 through 6 were obtained by the sam e person using the sam e technique. As noted in the preceding section, the transducer-oscilloscope (or X -Y recorder) system is m uch faster and m ore convenient th an a hand-balanced null system. B. Molding Water Content All the d ata (Figs. 1-6) show th a t the higher the m olding w ater content, th e less negative is th e residual pore pressure in the com pacted sam ple. C. Amount o f Compaction T he data on the effects o f am ount o f com paction are scant. T hey (Fig. 2) suggest a higher residual p ore pressure from higher com pactive effort. D. M ethod o f Compaction F o r the sam e m olding w ater content and the sam e com pacted density, kneading com paction gives higher residual pore pressures th an does static com paction (Figs. 2 and 3). E. Effect o f Moisture Attainment The pore pressure curve on th e right in Fig. 4 was obtained on soil brought to the desired m olding w ater content, equilibrated, and then com pacted. T he curve on th e left was obtained on soil m oistened, equilibrated, dried back approxim ately 2 per cent o f m oisture content, equilibrated and th en com pacted. T he d ata show th a t sam ples w hich have once been w etter th an the m olding w ater content have higher residual pore pressures. F. Effect o f Temperature T he data in Figs. 5 and 6 show: (1) Sam ples com pacted cool have higher residual pore pressures th an those com pacted w a rm ; (2) C ooling the sam ple and m old causes an increase in pore pressure ; heating the sam ple and m old causes a decrease in pore p re ssu re ; (3) C ooling an unconfined sam ple causes a decrease in pore pressure. G. Effect o f Confinement R em oving a sam ple o f kaolinite from the m old in w hich it was com pacted causes a reduction in pore pressure (Fig. 6). N o change in pore pressure occurred in the silty clay upon rem oval from the mold. V. Discussion of test results Three concepts will be used in discussing the test re s u lts; these a re : (1) water deficiency, (2) prestressing and (3) stress equilibrium. These concepts will be considered before discussing the data presented in Figs F o r several reasons (hydration o f clay surface, hydration of exchangeable ions, etc.) a clay particle has the ability to attract w ater. T hus a soil elem ent has a certain capacity 208

3 a m = ratio m ineral-m ineral contact area to to tal a r e a ; p a = air pressure w here contact is air-m in eral; u = pore w ater pressure ; aw = ratio o f area o f w ater-m ineral contact to to tal a r e a ; R = electrical repulsive pressure betw een p a rtic le s; A = electrical attractive pressure betw een particles. i ' 1 Vicksburg Loess a. AStatic compaction + 3 layer, kneading comp. r -4Ī 1 a Kaolinite + Static compaction O 3 lager, kneading comp. f+ O 4+ + k ^ A A 3>. X % o * V Nk 100 V ; N- 9 8 It H! i ZZ 14 M olding wafer content % 96 ' x 0 N Fig. 2 Pore pressures for different types and amounts of compaction. Pressions interstitielles pour différents modes et intensités de compactage. n X N v ' î H ; + + o f w ater w hich it can im b ib e ; this capacity depends on the com position o f the soil in the elem ent, on th e structure of the elem ent and the external pressures acting on the elem ent. If the existing w ater content o f a sam ple is less th an its capacity, a w ater deficiency exists. In other w ords, the w ater deficiency is the th irst the sam ple has. T he com paction o f a clay sam ple can be considered a prestressing o f the soil. T he com paction process rearranges soil particles and forces them into positions closer than equilibrium for the condition o f no external loads. U pon rem oval o f the com pacting force the soil skeleton tends to expand to an equilibrium volum e. If sufficient w ater is in the soil and a w ater deficiency exists, the tendency o f the soil sam ple to expand is opposed by negative pore pressures. T hus for m olding w ater contents reasonably n ear optim um, a com pacted clay sam ple has a com pressive prestress w hich is held by a negative pressure in the pore w ater. In an o th er pap er (L a m b e, 1 960) the au th o r proposed the follow ing equation to express an equilibrium o f norm al stress in soil, w here,... ( 1) ct = com bined stress betw een adjacent p a rtic le s; a = contact stress w here the particles are in m ineralm ineral c o n ta c t; ZZ 2 4 Zb ZB Molding water content % Fig. 3 Effect of compaction method on residual pore pressures. Effet des différentes méthodes de compactage sur les pressions interstitielles résiduelles. If anything is done to a soil elem ent to change any term in Eq. 1, som e other term (s) m ust changed to re-establish pressure equilibrium. F o r exam ple, if an electrolyte is leached from a dispersed soil sam ple (thereby increasing R), the sam ple will expand (reducing R-A) or the confining pressure a needed to m aintain the volum e will increase. C apillarity plays an im portant role in soils w hen the pore w ater has an interface w ith a i r ; such a condition exists in u n saturated soils and in unconfined sam ples. Because o f its w ater deficiency, the soil sam ple tries to suck in w ater, m uch like a person draw ing a drink through a straw. Since w ater wets a m ineral surface and since a surface tension can exist at the air-w ater interface, capillary m enisci can develop. These m enisci tend to resist the soil s suction, ju st as an obstruction can stop flow into o u r straw. In both cases the fluid is subjected to a tension, relative to atm ospheric pressure. In very sm all pores, as exist in fine-grained soils, the pore fluid tension can far exceed absolute zero pressure. T hus capillarity can help m obilize negative pore pressures by resisting soil suction. It should n o t be considered the 209

4 1 1 K a o lin it e 1 O S o il m o is t e n e d, e q u ilib r a t e d, d r ie d b o c k, e q u ilib r a t e d, co m pa ted. j + S o il m o is t e n e d, e q u ilib r a t e d, c o m p a t! J r + V,+ + As noted in the preceding subsections, the flocculated structure has a higher capacity for prestressing. Thus increased compactive effort and kneading compactive action tend to destroy the prestressing, thereby resulting in higher pore water pressures. C. M ethod o f M oisture Attainm ent W etting a soil sample above the molding water content tends to give the sample a dispersed structure, some of which is retained on drying the sample. Thus overwetting the sample tends to give higher residual pore pressures. D. Effect o f Temperature There is considerable experimental evidence that cooling a soil element tends to increase R -A in Eq. 1. Increasing R permits the soil to be compacted in a more dispersed structure. A reduction in tem perature acts as a reduction in salt or an increase in molding water content. While the theoretical reasons for this tem perature effect are not clearly established, the experimental facts (heat tends to flocculate a soil-water suspension; cooling an element of soil in contact with water causes an expansion) are convincing. The three effects of tem perature shown in Figs. 5 and 6 can thus be explained : (1) The colder the soil at time of com paction, the more dispersed the structure therefore, the higher the residual pore pressures. (2) Cooling a sample mold decreases the size of the mold and thus increases tr in Eq. 1. An increase a is accompanied by an increase in the pore pressure u. This increase in a tends to occur before (and tends to outweight) the R-A increase. Fig. 4 Effect of method of obtaining molding water content on residual pore pressures. Effet de la méthode d intégration de l eau de moulage sur les pressions interstitielles résiduelles. fundamental cause of, or an essential ingredient to, negative pore pressures. The truth of this statement is indicated by the fact that W h itm a n, et al (1961) measured negative pore pressures below absolute zero pressure in a soil sample which was saturated and had no surface exposed to air ; i.e., no capillarity existed. As illustrated by the W h itm an tests, the soil suction of pore water can be resisted by seepage forces. The concepts described in the preceding paragraphs will now be used to explain the trends shown in Figs A. M olding Water Content The higher the molding water content, the less is the water deficiency. Increasing molding water tends to give a more dispersed structure (as discussed in detail in Lambe, 1958) : the dispersed structure (particles tending toward a parallel array) is more compressible (in the low pressure range). The low water deficiency means a low tendency to generate negative pore pressures. The higher skeleton compressibility means a lower capacity for prestress. These two factors combine to give higher pore pressures, i.e., smaller tensions. That is to say, the smaller the water deficiency the lower the pore tensions ; and the more nearly parallel the particles, the lower the pore tensions. B. Amount and M ethod o f Compaction The greater the compactive effort, the more nearly dispersed the structure ; kneading com paction gives a more dispersed structure than does static compaction. Kneeding introduces large shear strains between particles and, in effect, remolds the clay during the com paction process. These tendencies are discussed a t length in the Lambe (1958) and Seed and Chan (1959) papers. The larger shrinkage upon drying for dispersed than for flocculated structures is illustrated a t the tops of Figs. 2 and 3. o Ç sr 5-4 & ~5 to C -8 c. Q. u 2 to -12 u =9 c -16 L. Q> K o o h n i te S t a t ic c o m p a c t io n S o il e q u ilib r a t e d a t 4 0 ' c o m p a c t e d a t 40 F. S o il e q u ilib r a t e d a t 80 c o m p a c t e d a t 8 0 F., t h e n m o ld c o o le d to a pprox. 40 F M o ld i n g N o t e r c o n t e n t % Fig. 5 Effect of tem perature on residual pore pressures. Effet de tem pérature sur les pressions interstitielles résiduelles. y S o il e q u ilib r a t e d a t 40 F., c o m p a c t e d a t 40 F., t h e n m o ld h e a te d to 80 F. S o il e q u ilib r a t e d a t 8 0 F a n d c o m p a c t e d a t 8 0 F

5 Fig. 6 Effects of confinement and temperature on residual pore pressures. Effets de confinement et de la température sur les pressions interstitielles résiduelles. (3) Cooling an unconfined sample causes an increase in R-A which in turn causes a decrease in u. The effects of temperature are not as simple as the above suggests. The influence of temperature on surface tension at air-water interfaces, pressure in air bubbles, and air-inwater solubility must also be considered. They are, however, generally of less importance, especially in plastic clays, than the effects discussed in the preceding paragraphs. E. Effect of Confinement Removing a compacted sample from its mold results in a reduction of cr in Eq. 1 ; this reduction in turn causes a reduction in the pore pressure. (This stress change is similar to that which occurs during clay sampling below the water table. See, for example, Hvorslev, 1948.) I. Acknowledgements T he tests described herein w ere perform ed by three o f th e au th o r s research students, nam ely : O liver H. G ilbert, J r.; R aul S o lo rz a n o ; and D a rio Fernandez. Special credit is due M r. F ern an d ez w ho ran m ost o f the tests and review ed a draft o f this paper. Bibliography [1] B i s h o p, Alan W., (1960). The M easurement of Pore Pressure in the Triaxial Test. London Conference on Pore Pressures. [2] C o l e m a n, J. D., (April, J959). An Investigation of the Pressure M embrane Method for Measuring the Suction Properties of Soil. Road Research Laboratory, Research Note No. RN3464JDC. [3] C r o n e y, D., C o l e m a n, J. D. a n d B r id g e, P. M., (1952). The Suction of Moisture Held in Soil and Other Porous Materials. Road Research Technical, in Paper 24, London. [4] F e r n a n d e z, Dario (June, 1960). Pore Pressures in Compacted Kaolinite, C. E. Thesis, M.I.T. [5] G il b e r t, O liver H., Jr. (Sept. 1959). The Influence of Negative Pore W ater Pressures on the Strength of Compacted Clays, 5. M. Thesis, M.I.T. [6] H il f, Jack W. (Oct., 1956). An Investigation of Pore- W ater Pressure in Compacted Cohesive Soils, Tech. Memorandum 654, Bureau of Reclamation, Denver, Colorado. [7] H v o r s l e y, M. J u u l ( N o v. 1949). Subsurface Exploration and Sampling of Soils for Civil Engineering Purposes, Waterways Experiment Station, Vicksburg, Mississipi. [8] L a m b e, T. W. (1948). The Measurement of Pore Water Pressures in Cohesionless Soils. Proc. of Second International Conference on Soil Mechanics and Foundation Engineering, Rotterdam. [9] (M ay 1958). The Structure of Compacted Clay and The Engineering Behavior of Compacted Clay, Journal o f Soil Mechanics and Foundation Div., Proc. of A.S.C.E. [10] (June, 1960). A Mechanistic Picture of Shear Strength in Clays. A.S.C.E. Research Conference on Shear Strength o f Cohesive Soils, Colorado. [11] S e e d, H. B. and C h a n, C. K. (Oct. 1959). Structure and Strength Characteristics of Compacted Clays, Journal o f Soil Mechanics and Foundations Division, Proc. of A.S.C.E. [12] S o l o r z a n o, R. (June, 1959). Investigation of the Influence of Compaction Method on the Pore Pressures of Compacted Clays, S. M. Thesis. [13] T a y l o r, D.W., Shear Research. Ninth Report (1943) ; Tenth Report (1944) ; for U.S. Engineer Department, M.I.T., Soil Mechanics Laboratory. [14] W h it m a n, R. V., R ic h a r d s o n, A. M., a n d H e a l y, K. A. (1961). Time-Lags in Pore Pressure Measurements. Proc. 5th International Conference o f Soil Mechanics and Foundations Engineering, Paris. 211