SEEPAGE INDUCED CONSOLIDATION TEST FOR THE EVALUATION OF COMPRESSIBILITY CHARACTERISTICS OF SOFT CLAYS

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Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Invited Talk-12) SEEPAGE INDUCED CONSOLIDATION TEST FOR THE EVALUATION OF COMPRESSIBILITY CHARACTERISTICS OF SOFT CLAYS K.

1 Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Invited Talk-12) SEEPAGE INDUCED CONSOLIDATION TEST FOR THE EVALUATION OF COMPRESSIBILITY CHARACTERISTICS OF SOFT CLAYS K. Prakash, Professor, Dept. of Civil Engg., S.J. College of Engg., Mysore , ABSTRACT: The present day scenario of the activities of constructional industry is such that the constructions on soft clay deposits are becoming the necessity of the day. This calls for understanding the engineering behaviour of soft clay deposits. The major problems posed by such deposits include the difficulty one faces while taking undisturbed samples and carrying out laboratory testing on such soft soils. Even though the solution to overcome these difficulties was advanced during late 1970s in Japan by introducing the concept of seepage induced consolidation, the technical difficulties and instrumental sophistication involved had acted as barriers for the new methodology to become popular. In this context, a simplified version of seepage consolidation technique is discussed in this paper, which makes use of very simple instrumentation and analysis with no compromise with the accuracy. INTRODUCTION The term soft clay is normally used to represent a clay sediment with a high water content. These soft clays are characterised by their high compressibility and in many cases, by their sensitivity. The soils transported by water and deposited in water bodies such as lacustrine and marine sediments belong to this category. These sediments form nearly about 75% of the surfaces of the continental platforms and a considerably higher proportion of the ocean floors; about 5% of the terrestrial and sub-oceanic earths[1]. In India, marine clays occupy about 6400 km stretch of coastal belt from Rann of Cutch on the western side to Kolkata on the eastern shore line. This coastal belt is the source of well known marine clays of India namely, Cochin marine clay, Mangalore marine clay, Mumbai marine clay, Madras marine clay and the like and is the hub of many important constructional activities. In addition, the ever increasing land reclamation activities wherein the dredged geological finegrained materials as well as mine tailings at high water contents are pumped in to the low lying areas also contribute to the formation of soft clays. These soft clay deposits normally exhibit a meta stable structure by virtue of their high water content. Various problems encountered with these soft clay deposits, with marine clays in particular, such as very high void ratios, very high compressibility, poor shear strength, sensitive nature towards sample disturbance and drying [2, 3] make their sampling and testing in the laboratory in the conventional way very difficult or probably impossible. The ever increasing constructional activities involving the sites containing the soft clay deposits call for the engineering property characterisation of such soft clay sediments. In this context, this paper intends to discuss a very simple yet very useful laboratory technique of preparing and determining the compressibility characteristics of soft clay sediments. ARTIFICIALLY SEDIMENTED CLAYS In view of the very soft consistency of the sediments, which does not allow their satisfactory sampling, researchers have resorted to obtain the sediments in the laboratory by artificial means, simulating the field conditions. This involves the preparation of the slurry of the soil under consideration at very high water content in a specially designed sedimentation jars, which is then allowed to undergo sedimentation process. The major difficulty one faces with the process of getting the soft clay sediments in the laboratory is the grain size sorting, which prevents from obtaining homogeneous sediments [4, 5]. This grain size sorting tendency of the sedimenting soils depends upon many factors of which the initial water content of the soil-water slurry plays an important role. Sridharan and Prakash[6] studied the settling behaviour of fine-grained soils through a well planned and meticulously carried out jar tests and grain size analysis of soil sediments of different dominant clay mineralogy over their depths. Figs.1 through 6 represent some typical results obtained from such a study. Based on their observations, Sridharan and Prakash[6] have classified the settling of soils into three types based on the extent of grain size sorting. Homogeneous or uniform settling: If the initial water content of the soil-water slurry is below a limiting value, this type of settling takes place. This results in a sediment, which has identical grain size distribution over the entire depth of the sediment. Segregational settling: This type of setting takes place at high water contents for all soils. The resulting sediment has different, well sorted layers (with coarsest particles at the bottom most and finest particles at the top most layers). Transitional settling: This type of settling takes place when the initial water content of the soil-water slurry is slightly more than that required for homogeneous settling. This type of settling results in sediments which are neither well sorted nor uniform. The type of settling the soil particles undergo in a settling medium during sedimentation appears to be a function of soil type, initial water content of the soil-water slurry [6], ph of the settling medium [7] and also on pore water salinity. In view of these facts, it is necessary to take utmost care to select an appropriate initial water content of the soil-water slurry which produces a homogeneous sediment. 72

4 Variation of particle size distribution for the sediment of coarse kaolinite (w L = 48%), illustrating sediment homogeneity Fig.

5 Variation of particle size distribution for the sediment of brown soil - 1 (w L = 64.6%), illustrating sediment homogeneity Fig.

2 K. Prakash Fig. 1 Variation of particle size distribution for the sediment of coarse kaolinite (w L = 48%), illustrating particle segregation Fig. 4 Variation of particle size distribution for the sediment of coarse kaolinite (w L = 48%), illustrating sediment homogeneity Fig. 2 Variation of particle size distribution for the sediment of brown soil - 1 (w L = 64.6%), illustrating particle segregation Fig. 5 Variation of particle size distribution for the sediment of brown soil - 1 (w L = 64.6%), illustrating sediment homogeneity Fig. 3 Variation of particle size distribution for the sediment of red earth - 1 (w L = 38.6%), illustrating particle segregation Fig. 6 Variation of particle size distribution for the sediment of red earth - 1 (w L = 38.6%), illustrating sediment homogeneity 73

Seepage Induced Consolidation Test for the Evaluation of Compressibility Characteristics of Soft Clays COMPRESSIBILITY BEHAVIOUR OF ARTIFICIALLY SEDIMENTED SOIL DEPOSITES Geotechnical engineering

3 Seepage Induced Consolidation Test for the Evaluation of Compressibility Characteristics of Soft Clays COMPRESSIBILITY BEHAVIOUR OF ARTIFICIALLY SEDIMENTED SOIL DEPOSITES Geotechnical engineering literature has documented several techniques adopted by different researchers to study the compressibility behaviour of soft clay sediments in the laboratory. 1. Self weight consolidation: In this method, the soil-water slurry is prepared with a water content sufficiently high, yet maintaining the homogeneity. The slurry is poured into a settling column/tube/jar and is allowed to settle to form a sediment. This sediment is allowed to undergo consolidation under self weight. After reaching the equilibrium state, the compression curve (i.e., e log ' or w% - log ' ) over a very low effective stress range can be obtained through the measurement of water content variation over the depth of the sediment by slicing method [8, 9, 10] (Fig.7) or density profile over the depth of the soil slurry by measuring the density of the sediment at different depths by different techniques [11, 12, 13]. The self weight consolidation tests can also be performed in a centrifuge [14, 15]. Water content (w), % Fig. 7 Typical compression curves due to self weight consolidation {data source: (a) Imai [8]; (b) Scully et al, [10]} 2. Hydraulic consolidation test: This test was originally devised by Imai[16]. In this method, the soil sediment is prepared as in the case of self weight consolidation. After the self weight consolidation is over, the sediment is subjected to a seepage force of known magnitude due to which the soil sediment consolidates. Since the water content of normally consolidated soils decreases as the effective stress increases, the water content of the soil sediment varies with the depth z measured from the top of the sediment. Therefore, immediately after the applied seepage force on the sediment is removed, the flow rate v, distribution of pore water pressure u(z) and distribution of water content w(z) by the method of slicing are measured within a short time. From the water content and pore pressure distribution over the depth of the sediment, the compression curve in the whole range of consolidation stresses realized in the sediment can be obtained. Since the velocity of flow is constant throughout the sediment and the hydraulic gradient i is calculated from the pore water pressure distribution, the value of coefficient of permeability k can be obtained as a function of effective consolidation stress from which variation of coefficient of consolidation c v with effective consolidation stress can be obtained. Imai et al.[17] studied the applicability of the hydraulic consolidation test for very soft clayey soils. The hydraulic consolidation test as proposed by Imai[16], even though is a very useful technique to study the compressibility behaviour of soft sediments, requires sophisticated instrumentation for the test to be performed. Huerta et al, [18] have proposed a modified version of hydraulic consolidation test which involves the measurement of steady state flow rate as well as the void ratio at the base of the sediment at the end of the test and a new technique of mathematical analysis which involves the use of the compressibility and permeability constitutive relationships. Abu-Hejleh et al, [19] have proposed an alternate method of hydraulic consolidation test and analysis which involves the measurement of zero effective stress void ratio, sediment height and pore pressure difference across the sediment at steady state flow. However, the analysis requires special computer programmes. Even though Fox and Baxter [20] have presented a two-stage procedure for studying the compressibility behaviour of sediments using the hydraulic consolidation test, it has a limitation in that it can not be used for tests in which effective stresses at the top of the sediment is zero. In spite its usefulness, the hydraulic consolidation test formulated by Imai and the others requires sophisticated instrumentation and tools for the analysis as well as for the appropriate interpretation, which most of the geotechnical engineering laboratories may not afford to have. The simplified seepage consolidation test proposed by Sridharan and Prakash[21] overcomes all these difficulties posed by the earlier versions of hydraulic consolidation test. It does not require any sophistication in terms of instrumentation, conduction and analysis. Fig. 8 Experimental set up for simplified seepage consolidation test (Note: The Sketch is not to scale) 74

4 K. Prakash Simplified Seepage Consolidation Test The experimental set up in simplified seepage consolidation test is very simple (Fig.8). It consists of a graduated glass cylinder of 50 mm internal diameter with an outlet at its bottom. The graduations are such that, any change in the sediment height up to 0.5 mm accuracy can be measured. The filter provided at the bottom of the cylinder is made of filer paper and coarse sand. The glass cylinder is connected to the down stream reservoir through a flexible tube. Studies of Sridharan and Prakash[22] have shown that there are three characteristics water contents for any fine-grained soil-water system namely, free swell limit (w FS ), settling limit (w SL ) and shrinkage limit (w S ). The settling limit water content is the maximum initial water content of the soil slurry for which the final water content of the sediment formed is also the same. If the soil slurry has settling limit water content as its initial water content, then homogeneous sediment will result. The given soil of known fixed dry weight is mixed with water to form a slurry and then, its value is increased to the required value that results in homogeneous settling. For most of the natural soils in fresh water, it has been observed that an initial water content in the range 1.3 w L 1.6 w L would result in homogeneous sediments, where w L is the liquid limit of the soil. The soil slurry is thoroughly mixed, transferred carefully in to the test cylinder and allowed to settle to form the required soft sediment. Sufficient time is allowed to reach the equilibrium. During this entire process, the water surface levels in both upstream and down stream reservoirs are maintained equal. Once the equilibrium is reached, the top surface of the bottom reservoir is lowered to a level as determined from the consideration of the stress increment ration adopted slowly and gradually. Due to this hydraulic head difference created between the upstream and downstream reservoir levels, water flows in the downward direction through the sediment, resulting in the seepage consolidation of the sediment. During this seepage consolidation process, the upstream and downstream reservoir levels are kept overflowing / maintained constant. Once the steady state is reached, the sediment height and consequently, the compression occurred are noted. Then, variable head permeability test is conducted by allowing the upstream reservoir level to reduce, keeping the downstream reservoir level overflowing. In this way, a set of values of average velocity of flow and average hydraulic gradient can be recorded, which can be used to get the value of coefficient of permeability. Once the permeability measurements are taken, the top surface level of the down stream reservoir is lowered to get a predetermined effective stress as guided by the adopted stress increment ratio and the entire procedure outlined above is repeated. With this simplified seepage consolidation test procedure, the consolidation parameters can be obtained in a very low effective stress range of 0.01 kpa 10 kpa. In the original version of hydraulic consolidation test proposed by Imai, the nature of the seepage force varies with the depth (i.e., zero at the inlet end of the flow and maximum at the exit end of the flow). Because of this, different cross sectional elements of the sediment experience seepage force of varying magnitude. Thus, the top most layers will have higher void ratio, which decreases with depth. Hence, e log ' relationship for the sediment can be obtained over its depth. However, in the simplified seepage consolidation technique, instead of considering varying void ratio with depth, the entire soil sediment (of smaller thickness) is represented by its average void ratio. Let w, sat and sub be the unit weight of water, saturated unit weight and submerged unit weight of soil mass respectively. Let d ' be the change in effective stress and du be the change in pore water pressure due to the seepage of water from the top of the sediment of thickness dz downwards. Then, d = sat dz du (1) The change in the hydraulic head between the top and bottom surfaces of the sediment is given by Substitution of eq. (2) in eq. (1) results in eq. (3). Seepage force is the force exerted by the flowing water per unit volume of soil and is given by eq. (4). Hence, eq. (3) can be rewritten as eq. (5). Eq. (5) indicates that the seepage force and submerged unit weight of the soil cause an effective stress gradient. In other words, the seepage force gets converted into effective consolidation stress. For the case of steady flow, the effective stress at the base of the sediment can be shown to be equal to where h is the head causing the flow. (2) (3) (4) (5) = sub L + (6) 75

5 Seepage Induced Consolidation Test for the Evaluation of Compressibility Characteristics of Soft Clays Figs.9 13 present the e log ' curves of some typical soils subjected to both simplified seepage consolidation test and the conventional one-dimensional oedometer consolidation tests at overlapping effective stress ranges, which confirm the validity of the simplified seepage consolidation technique. The study of log m v log ', e log k and log c v log ' relationships at the over lapping effective stress ranges obtained from simplified seepage consolidation tests and the conventional one-dimensional oedometer consolidation tests by Sridharan and Prakash[21] has further strengthened this validity. Simplified seepage consolidation technique has been successfully used to study the compressibility & permeability behaviour of both homogeneous and segregated soft soil sediments[23], to define the limiting compression curves[24] and to study the permeability of soft soil layered sediments[25]. methodology have been validated with those from the conventional oedometer consolidation tests at overlapping low effective stress ranges. Fig. 11 Composite e-log curve for red earth - 1 Fig. 9 Composite e-log curve for coarse kaolinite Fig. 12 Composite e-log curve for black cotton soil - 2 Fig. 10 Composite e-log curve for brown soil - 1 CONCLUSIONS Soft clays are becoming the part and parcel of the near shore and offshore constructional activities and also of most of the land reclamation projects involving soil deposits of high water contents. Most of the problems encountered in getting reasonable estimates of the consolidation parameters of such soft clay deposits can be overcome by the simplified seepage consolidation technique. The simplified seepage consolidation technique makes use of very simple instrumentation and procedure. The results from this Fig. 13 Composite e-log curve for Hesaraghatta soil ACKNOWLEDGMENTS The author gratefully acknowledges the guidance he received from Prof. A. Sridharan, INSA Honorary Scientist, Formerly Professor in Civil Engineering, Indian Institute of Science, in preparing this paper. 76

6 K. Prakash REFERENCES 1. Winterkorn, H.F. and Fang, H.Y.(1986), Soils technology and engineering properties of soils in Foundation Engineering Handbook, Edited by H.F. Winterkorn and H.Y. Fang, Galgotia Book Source, New Delhi. 2. Jose, B.T., Sridharan, A. and Abraham, B.M. (1988), Physical properties of Cochin marine clay, Indian Geotechnical Journal, 18(3), Rao, S.M., Sridharan, A. and Chandrakaran, S. (1989), Influence of drying on the liquid limit behaviour of a marine clay, Geotechnique, 39(4), Bjerrum, L. and Rosenqvist, I.Th. (1956), Some experiments with artificially sedimented clays, Geotechnique, 6, Katagiri, M. and Imai, G1994), A new in-laboratory method to make homogeneous clayey samples and their mechanical properties, Soils and Foundations, 34(2), Sridharan, A. and Prakash, K.(1997), Settling behaviour of fine grained soils, Indian Geotechnical Journal, 27(3), Badiger, R.M., Preetham, M., Bhat Nikhil, S. and Haresh Kumar, S. (2007), Effect of ph on physical properties of fine-grained soils, B.E. Thesis, Department of Civil Engineering, Sri Jayachamarajendra College of Engineering, Mysore. 8. Imai, G.(1981), Experimental studies on sedimentation mechanism and sediment formation of clay materials, Soils and Foundations, 21(1), Umehara, Y. and Zen, K.(1982), Consolidation characteristics of dredged marine bottom sediments with high water content, Soils and Foundations, 22(2), Scully, R.W., Schiffman. R.L., Olsen, H.W. and Ko, H.Y.(1984), Validation of consolidation properties of phosphatic clay at very high void ratios, Proceedings of Symposium on Sedimentation Consolidation Models: Predictions and Validation, ASCE, R.N. Yong and F.C. Townsend (Eds), New York, Been, K. and Sills, G.C.(1981), Self weight consolidation of soft soils: An experimental and theoretical study, Geotechnique, 31(4), Tan, T.S., Yong, K.Y., Leong, E.C. and Lee, S.L. (1990), Sedimentation of clayey slurry, Journal of Geotechnical Engineering, ASCE, 116(6), Li, H. and William, D.J.(1995), Sedimentation and selfweight consolidation behaviour of coal mine tailings, Proceedings of the International Symposium on Compression and Consolidation of Clayey Soils, H. Yoshikuni and O. Kusakabe (Eds), Hiroshima, Japan, Mikasa, M. and Takada, N.(1984), Self weight consolidation of very soft clay by centrifuge, Proceedings of Symposium on Sedimentation Consolidation Models: Predictions and Validation,ASCE, R.N.Yong and F.C.Townsend (Eds), New York, Takada, N. and Mikasa, M.(1986), Determination of consolidation parameters by self weight consolidation test in centrifuge, Consolidation of Soils: Testing and Evaluation, ASTM STP 892, R.N.Yong and F.C. Townsend (Eds),, Imai, G.(1979), Development of a new consolidation test procedure using seepage force, Soils and Foundations, 19(3), Imai, G., Yano, K.and Aoki, S.(1984), Applicability of hydraulic consolidation test for very soft clayey soils, Soils and Foundations, 24(2), Huerta, A., Kriegsmann, G.A. and Krizek, R.J. (1988), Permeability and compressibility of slurries from seepage-induced consolidation, Journal of Geotechnical Engineering, ASCE, 114(5), Abu-Hejleh, A.N., Znidarcic, D. and Barnes, B.L.(1996), Consolidation characteristics of phosphatic clays, Journal of Geotechnical Engineering, ASCE, 122(4), Fox, P.J. and Baxter, C.D.P. (1997), Consolidation properties of soil slurries from hydraulic consolidation test, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123(8), Sridharan, A. and Prakash, K.(1999), Simplified seepage consolidation test for soft sediments, Geotechnical Testing Journal, ASTM, 22(3), Sridharan, A. and Prakash, K.(1998), Characteristic water contents of fine grained soil-water system, Geotechnique, 48(3), Sridharan, A. and Prakash, K.(2001a), Consolidation and permeability behavior of segregated and homogeneous sediments, Geotechnical Testing Journal, ASTM, 24(1), Sridharan, A. and Prakash, K.(2001b), Limiting compression curves, Geotechnical Testing Journal, ASTM, 24(3), Sridharan, A. and Prakash, K.(2002), Permeability of two-layer soils, Geotechnical Testing Journal, ASTM, 25(4),

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