OSIPOV, V. I., NIKOLAEVA, S. K. & SOKOLOV, V. N. (1984). G&technique 34, No. 2, 293-303

Microstructural changes associated with thixotropic phenomena in clay soils

V. I. OSIPOV,” S. K. NIKOLAEVA* and V. N. SOKOLOV*

Thixotropic phenomena in clay soils are accompanied by microstructural changes. The nature of these changes is unclear because of the short duration of the thixotropic processes and the difficulties involved in preparing a specimen at certain stages. However, these technological problems were overcome and SEM photographs were obtained of the thixotropic soil mic- rostructure during deformation of samples in a rotary viscometer with and without vibration. The new data obtained clarify the mechanism of thixotropic phenomena. Soil microstructure was not ruptured by vibration during the shear process. On the contrary, it became more homogeneous over the entire volume and at the same time this induced a decrease in strength in the system due to a reduction in cohesion at contacts. Disruption of some structural bonds is followed by their rapid restoration--the overall mi- crostructure remaining intact. As a result, in all the samples studied the shear zone disappears. In kaolinite clay and coarser dispersed soils (silty clay, water- saturated loess) the local areas of the structural framework (shear planes) become smoother and the orientation of the structural elements along the direc- tion of shear disappears. After the cessation of vibra- tion, the microstructure is rapidly restored to its initial state.

Les phCnom&nes thixotropiques dans les sols argilacts sont accompagnts de changements microstructurels. La nature de ces changements n’est pas clair, B cause de la courte durCe des phtnombnes thixotropiques et des difficult&s de la prkparation d’un Cchantillon g de certaines &tapes. Ces problkmes technologiques ont CtC rCsolus et des images SEM ont CtC obtenues de la microstructure du sol thixotropique pendant la d&formation des tchantillons dans un viscom&re rotatif avec et sans vibration. Les nouvelles donn6es obtenues clarifient le mecanisme des phtnomtnes thixotropiques. La microstructure du sol n’a pas 6tC rompue par la vibration pendant le cisaillement. Tout au contraire, elle est devenue plus homogkne sur le volume entier, tandis qu’en m&me temps ceci a cause une rCduction de la rCsistance du systkme due B une diminution de la cohCsion aux contacts. La rupture de quelques liens structurels est suivie de leur rCtablissement rapide, tandis que la microstructure entibre reste intacte. Comme rtsultat, la zone de cisaillement disparait dans tous les Cchantillons etudib. Dans l’argile kaolinite et les sols dispersts

Discussion on this Paper closes on 1 January 1085. For further details see inside back cover. * Moscow State University.

plus grossiers (argile limoneuse, loess saturC d’eau) les zones locales du cadre structure1 (plans de cisaille- ment) deviennent plus lisses, et l’orientation des Cl&men% structuraux le long de la direction de cisaille- ment disparait. Aprts que la vibration a cessC, la microstructure se rCtablit rapidement dans son &at initial.

INTRODUCTION

Thixotropic phenomena, exhibiting a gel-sol type of reversible isothermic transformation, are widespread in water saturated clay soils with coagulative types of structural bond, and occur upon complete remoulding (Avgustinik, 1940; Akkermann, 1948; Skempton & Northey, 1952; Gumensky, 1954; Mitchell, 1960, Gorkova, 1961; Van Olphen, 1977).

The phenomena lead to unfavourable conse- quences: landslide activation, loss in slope sta- bility and reduction in the bearing capacity of soil foundations. Therefore, a study of the na- ture of thixotropic phenomena and recognition of the characteristics of thixotropic soil be- haviour is of great theoretical and practical im- portance in soil mechanics (Trofimov & Mar- tynov, 1975).

Thixotropic phenomena develop in structured systems and are accompanied by microstructural changes associated with thixotropic loss and re- storation of strength (Ovchinnikov, Kruglitsky & Mikhaylov, 1972; Rebinder, 1979; Uryev, 1980). Hence, it is impossible to understand fully the mechanism of thixotropy without a comparative analysis of the microstructural changes that take place in a system when it is deformed under static? and dynamic conditions. The microstructural changes in clay under shear in static conditions have been studied by a number of investigators (Popov, 1944; Morgenstern & Tchalenko, 1967; Vyalov, Pekarskaya & Maksimyak, 1970; McKeys & Yong, 1971; Tovey, 1971. However, the charac- teristics of these changes during deformation with applied vibration have received little atten- tion: this is because of the technical difficulties

*The term ‘static’ conditions is used in this Paper to characterize soil deformation without vibration.

293

294 OSIPOV, NIKOLAEVA AND SOKOLOV

Table 1. Characteristics of the soils studied

Description Granulometric composition: % Mineral composition Moisture of the clay fraction content at

0,055 O.Ol- 0.005- co.002 mm liquid 0.01 mm 0.005 mm 0.001 mm limit W,:

%

Clay 2 21 Clay 2 1 Silty clay 29 21

L.oess 16 9

30

31

4

47 97 19

11

Kaolinite Montmorillonite (100%) Illite (40%) mixed-layered (30%), chlorite (20%), kaolinite (7%), montmorillonite (3%) Illite (60%), mixed-layered (20%), kaolinite (lo%), chlorite (10%)

56 200

40

25

associated with the high sensitivity of thixotropic soils, and the complexity of the rapid fixation of the microstructure at various stages in the thixotropic transformation. The Authors have obtained new experimental data which provide a better understanding of thixotropic phenomena.

EXPERIMENTAL PROGRAMME Soils studied

Soil samples of variable dispersion and min- eral composition were used to study the charac- teristics of microstructural changes associated with thixotropic phenomena. Monomineral clays (kaolinite and montmorillonite) and polymineral silty clays and loess were used (Table 1). The experiments were performed on reconstituted paste samples prepared at moisture contents ranging from O.SW, to 2.2W,, where W, is the moisture content at the liquid limit. In this state the samples had a typical coagulation structure characteristic of the majority of modern and poorly lithified clay soils. The samples were thoroughly mixed with distilled water and the resulting paste was placed in a desiccator with a container of water for about 24 hours. This al- lowed the moisture in the paste to become uni- form without changing in value. Thereafter the paste was used in the experiments, with mois- ture content and temperature under constant control.

Shear tests. To characterize the thixotropic behaviour of the prepared samples, their resis- tance to shear was studied in static conditions and with applied vibration. Tests were per- formed during continuous shear deformation by employing a rotary viscometer, Reotest-2, with the cylinder measuring device mounted on a vibrostand (Fig. 1). To reduce the impact of

normal stresses and near-the-wall slip, hollow measuring cylinders with knurled surfaces were used in the experiments. The pastes were placed in the 3 mm wide ring gap of a coaxial cylinder system. The external cylinder of the viscometer was rigidly fixed on the electrodynamic vibro- stand, which served as a source of harmonic vertical vibration. The internal cylinder revolved at a constant velocity of 1 rev/min. The follow- ing vibration parameters were used in the exper- iments: frequency 20 Hz, displacement amp- litude 0.6 mm, acceleration 10 m/s’, duration 5 min. This set of conditions corresponds to the conditions for the majority of dynamic soil prob- lems of interest, e.g. those induced by earth- quakes or by traffic (Puchkov, 1974; Pavlenkov, 1978).

Measurements of the rotary moment value on

I

Fig. 1. Diagram of the rotary viscometer, Reotest-2.1 internal cyfinder wtth kmuled surface; 2 external cylinder; 3 paste in the gap between the internal and external cylinders; 4 liqaid nitrogen container for sample freezing; 5 vibrostand; w angle speed; f fre- quency of the vibration

THIXOTROPIC PHENOMENA IN CLAY SOILS 295

i

0 Static Pynami4 Static I

Fig. 2. Shear resistance (7) against time (t) relation- ship for soil subjected to static and dynamic shear conditions: OA shear resistance of the ondisturbed stmctore under static conditions; AB shear resistance for the roptored stroctore (residual strength); CD shear resistance of the ruptured stroctore wlth vibra- tion; EF shear resistance of the thixotropically re- stored structure (after cessation of vibration); BC drastic drop in strength (with vibration) and DE strength restoration (after cessation of vibration). 0, 1, 2, 3 soil sample selection points for microstractoral StUdieS

the internal cylinder axis were taken at various known deformation velocities. Calculation of the shear resistance (T) was performed with the use of the following relation (Uryev, 1980):

M

r=zz

where M is the rotative moment, L is the height of the internal cylinder immersed in the clay paste and r is the radius of the internal cylinder.

A static shear test was begun and performed at a constant deformation velocity of 2.5 X 10m4 m/s, which corresponds to conditions of rapid shearing. After a constant shear resistance value (residual resistance) was set up the vi- brator was switched on. Five minutes later the vibrator was switched off and shearing deforma- tion continued in the absence of external vibra- tion. The shear resistance recorded is shown in Fig. 2 as a function of time for the static- dynamic-static test sequence. The figure sug- gests that the vibration disrupts the structural bonds, causing the soil strength to be signific- antly reduced.

To examine the microstructure at any stage of the test the microstructure had to be fixed as rapidly as possible while the test was in progress. This was done by pouring liquid nitrogen into the container surrounding the soil sample and the gap between the internal and external cylin- ders (see Fig. l), while simultaneously stopping the test. Because the sample thickness was only about 3 mm, freezing was essentially instantane- ous. It is considered that no change in the

microstructure was caused by this freezing pro- cess.

The apparatus was disassembled to remove the frozen soil sample. Portions of the sample were selected for moisture removal by sublima- tion in a vacuum chamber-the process is known as freeze-drying (Barden & Sides, 1977; Tovey, Frydman & Wong, 1973; Smart & Tovey, 1982).

In this technique very rapid freezing is used in an attempt to solidify the soil porewater instan- taneously without disruptive crystal growth. Subsequently, sublimation under vacuum re- moves the ice without disturbing the microstruc- ture of the soil. Details of the freeze-drying procedure are well described by Smart & Tovey (1982).

Three identical samples of each paste were prepared and tested. One was frozen after con- stant shearing resistance was achieved, corres- ponding to the condition at point 1 in Fig. 2. The second was frozen during vibration (point 2, Fig. 2), and the third was frozen at maximum strength after the cessation of vibration (point 3, Fig. 2). A fourth identical sample was prepared and then frozen without shearing. This sample represented the initial microstructure.

Microstructural studies of samples after freeze-drying were performed with the aid of the scanning electron microscope (CWIKSCAN- 106). The most interesting peculiarities of mic- rostructure were photographed at a number of magnifications (from Xl00 to x10 000). The images obtained were used for assessing the qualitative and quantitative characteristics of microstructural changes in the portions of inter- est on the shear curve. Quantitative microstruc- tural analysis was performed with the aid of the image analyser. This analysis provided informa- tion on the pore area, the distribution of pores and microaggregates by size and the orientation of the soil microstructural elements inside and outside the shear zone (Sergeyev, Grabowska- Olszewska, Osipov & Sokolov, 1980).

MICROSTRUCTURAL OBSERVATIONS Initial paste microstructure The specimens from the montmorillonite paste had a honeycomb microstructure (Fig. 3(a)), typ- ical of a fine-grained clay system with high water content (Sergeyev et al., 1980). This microstruc- ture is characterized by interaction between the montmorillonite microaggregates of the face-to- face and face-to-edge type, with the develop- ment of closed, predominantly isometrically shaped cells with diameters ranging from frac- tions of a micron to 2-3 km. (Fig. 3(a), A, C, D).

300 OSIPOV, NIKOLAEVA AND SOKOLOV

velopment of shear planes-local disruptions of the structural framework. Outside the shear zone the orientation of the structural elements is absent and the clay microstructure is similar to that of the kaolinite initial sample (Fig. 4(a)). Evaluation of the kaolinite paste pore area dis- closed that its value within the shear zone com- prises 68%, and beyond the shear zone 58%.

Marked orientation of the structural elements is also characteristic of coarse-grained systems- the silty clay and loess pastes. Referring to Fig. S(c), at contacts the grains and clayey substance become orientated along the shear direction, although a marked shear plane is absent here. Increasing porosity in the shear zone is observed in coarse-grained systems, as for montmorillo- nite and kaolinite pastes.

Microstructural changes under dynamic shear Applying vibration during the shearing pro-

cess leads to considerable microstructural changes in the samples studied. The vibration of the montmorillonite paste makes for a more regular honeycomb microstructure (Fig. 4(c)) in comparison with that of the initial structure (Fig. 4(a)), with cells as a rule not more than 1.5- 2.5 urn in diameter. The decrease in size of the microaggregates is observed as a result of their disruption by vibration. It is important to note that with applied vibration the shear zone disap- pears.

A similar effect is observed with kaolinite paste; the vibrational impact leads to elimina- tion of the orientation of structural elements along the shear direction, and the shear zone also disappears. In this case the microstructure becomes more homogeneous through the entire specimen (Fig. 5(c)). The quantitative pore area estimation of the photographs produced by the image analyser (Sergeyev et al., 1980) shows that the pore area in a kaolinite paste actually does not change when it is removed from the internal to the external viscometer cylinder, and comprises on average 62%.

The gross changes in the microstructure of the silty clay and loess pastes that take place with applied vibration result in a more homogeneous and regular honeycomb microstructure. At the same time, oscillation of the coarse sandy and silty grains in these soils disturbs their structure, making it less cohesive. SEM photographs taken after the impact of vibration on soils appear to show that the vibration brings about disruption of the clay bridges (Fig. 5(d)).

Microstructural changes upon cessation of vibration

The cessation of vibration (during shear de-

formation) is followed by a process of rapid reversal which restores the microstructure: mic- roaggregate size increases, with the formation of larger and less homogeneous pores. Thus, for instance, in montmorillonite paste the size of the pores approaches 2-3 pm. Pores regain their initial size, and the total pore area comprises 53%. The SEM photographs (Figs 3(a), (d); 4(a), (d); 5(a), (e)) show the similarity of the micro- structure before and after the cessation of vibra- tion, The clay bridges are restored between the sandy and silty grains in silty clay and loess pastes (Fig. 5(f)).

DISCUSSION Microstructural investigations of thixotropic

processes in clay soils have shown soil strength under static and dynamic shear to be dependent on microstructural changes.

Under static shear the soils studied react im- mediately to shear stress by displaying minor displacements. At this stage reversible elastic deformations dominate, as a result of the partial tilting of particles and microaggregates in the direction of the shear force without displace- ment relative to one another (Tshukin & Rebin- der, 1971). Load removal at this stage of defor- mation leads to elastic restoration with time of the initial state of particle arrangement. As the maximum strength of a test sample r,, (Fig. 1) is approached and deformation increases, the elastic deformation gradually changes to visco- plastic in the narrow shear zone, leading to rebuilding of the soil microstructure in this zone. In the case of the fine-grained systems such as the montmorillonite paste, the rebuilding is manifested through partial microaggregate dis- ruption resulting in a more regular honeycomb microstructure in the shear zone. In the case of coarser systems formed by isometric mic- roaggregates of a plate type (e.g. kaolinite paste), reorientation of the microaggregates also occurs along the direction of shear stress. Both processes are followed by an increase in soil porosity in the shear zone, the development of negative pore pressure and an increase in mois- ture content (Skempton, 1974; Smolin, 1977; Meschan, 1978). A number of large sized pores elongated to the shear zone are formed besides (Fig. 6, part B). The development of similar pores (canals) in the shear zone is evidently connected with intensive moisture migration to the deformed zone under the impact of negative pore pressure. This originates as a result of decreasing density and increasing porosity in the soil in the shear zone. Microstructural rebuilding and the development of a shear zone bring about a reduction in growth for the strength of

THIXOTROPIC PHENOMENA IN CLAY SOILS 301

the system during continuous deformation, with the value r reaching its maximum and then beginning to decline. A transfer over maximum (the structural strength limit) is explained by the formation of a shear zone with weakened struc- tural bonds between the particles and the mic- roaggregages, a result of the increased moisture content and partial reorientation of the struc- tural elements along the shearing force direc- tion. However, some investigators are inclined to attribute this to breaking up of the spatial structural framework into separate blocks (Ab- duragimova, Rebinder & Serb-Serbina, 1955; Uryev, 1980).

The portion AB of the graph (Fig. 2) shows progressive development of the shear zone, ac- companied by a further increase in porosity and orientation of the structural elements, enhancing soil moisture content within the zone and reduc- ing strength. The progress of deformation leads to completion of the shear zone development and stabilization of the shear stress, expressed by the appearance of a horizontal portion on the deformation graph (residual strength T,,,.). Mic- rostructural studies of samples at this stage of deformation show the existence of a clearly defined shear zone, whose width at the plane shear of plastic clays changes from a few mic- rons to a few millimetres. The experiments prove that the width of the soil shear zone grows while the microaggregate size decreases and the moisture content of the soil increases.

The impact of vibration under dynamic shear leads to a drastic reduction in the shear resis- tance of soils (portion BC on the shear curve, Fig. 2), depending on the vibration parameters, and remaining permanent up until the cessation of impact (portion CD on the shear curve, Fig. 2). Some investigators explain this phenomenon by the complete structural disruption and loss of structural cohesion (Trofimov & Martynov, 1975), others consider that vibration facilitates particle orientation in the direction of a shear displacement, thus causing a reduction in the strength of a system (Lishtvan, Bityukov & Terentyev, 1977).

The microstructural studies carried out sug- gest that the joint action of shear force and vibration results, not in rupture, but, on the contrary, in the development of a more homogeneous microstructure over the entire sample volume. A dynamic state is induced in a soil by the impact of a vibration field as a result of the forced oscillation of particles and their microaggregates. The structural elements are displaced, structural cohesion weakens and par- tial disturbance of microaggregates and an in- crease in soil dispersion takes place. The relative

mobility of the structural elements grows and the relaxation processes are accelerated. A more homogeneous microstructure appears along the entire gap of the viscometer, and the shear zone disappears, probably because of reorientation of the structural elements and their interaction within the most energetically advantageous areas.

The orientation of the structural elements along the direction of shear force disappears because the vibration displacement gradient ex- ceeds the velocity gradient of shear deformation.

Cessation of vibration leads to rapid restora- tion of the soil structure. This is shown by the fast growth of the shear strength of soils some seconds after vibration has ceased, and by the occurrence of a second maximum on the shear curve (portion DE of the shear curve, Fig. 2). A drastic increase in soil strength upon cessation of vibration is caused by the mutual fixation of particles and the creation of coagulation con- tacts between them. As a result, over the vol- ume a structural framework is formed in the specimen, characterized by the uniform distribu- tion of the water in the entire specimen. This process is attended by an increase in the size of microaggregates and the formation of a less homogeneous porosity. The soil microstructure formed is similar to the initial microstructure.

Rapid restoration of the soil strength at rest is followed by a further strengthening which con- tinues for a long period of time. This is because, in a soil after vibration, the establishment of thermodynamic equilibrium is related to the microstructural changes, which are not accom- plished instantaneously, but proceed for a cer- tain period of time until the thixotropic restora- tion of the system is complete.

CONCLUSION The studies have given a more complete view

of the microstructural changes that take place in thixotropic soils during deformation in a rotary viscometer with and without vibration, thus pro- viding a deeper insight into the mechanism of thixotropic phenomena. The data obtained indi- cate that without vibration clay deformation oc- curs over a limited volume-in the shear zone- and is attended by marked changes in the soil microstructure in this zone: i.e. a decrease in the size of the microaggregates and the diameter of the predominant pores, a decrease in the density of the system and an increase in moisture con- tent. These microstructural changes are directly related to the value of the maximum and minimum shear strengths of the soils. It is im- portant to note that, under shear test, for the system composed of small-sized particles (gener-

302 OSIPOV, NIKOLAEVA AND SOKOLOV

ally less than a micron, i.e. montmorillonite) the deformation in the shear zone is volumetric in character, and does not lead to disruption of structural continuity (shear planes). It is obvious that the mechanism of deformation in such sys- tems is relaxational or thixotropic, i.e. the microstructure-forming cells are ruptured at shear, and at the same time are instantaneously restored, without forming a linear defect in the structure. In coarser dispersed clayey soils (ka- olinite), in addition to the microstructural changes described above, in a shear zone the microaggregates become orientated along the direction of shear stress and local shear planes develop, along which the orientation of struc- tural elements is particularly marked. For these soils during shear therefore both volumetric de- formation and local disruption of the structural framework occur along developing shear planes.

Observations indicate that vibration applied during the shearing process does not disrupt the microstructure; on the contrary, it becomes more homogeneous over the entire volume, as a result of a reduction in cohesion at contacts. The loosening of the structural bonds between parti- cles and microaggregates during vibration prom- otes disruption of large microaggregates and more uniform stress distribution along the entire gap of the viscometer. It also accelerates the relaxation processes and facilitates the mutual reorientation of particles and microaggregates. In these systems disruption of some structural bonds is followed by rapid restoration, with the overall microstructure remaining intact. This re- sults in the disappearance of the shear zone in all the samples studied. In kaolinite paste and coarser dispersed soils (silty clay, loess) the local areas of the structural framework (shear planes) become smooth and the orientation of the struc- tural elements along the direction of shear dis- appears. The deformation spreads over the vol- ume compared with the situation during shear before vibration is applied.

The cessation of vibration is followed by rapid fixation of the existing spatial structural framework at the expense of strengthening of the coagulation contacts due to speed, causing a drastic increase in soil strength some seconds after switch-off. Concurrently with strengthening of the system, a number of processes lead to an enlargement of the microaggregates and a re- duction in porosity. As a result, the clay micro- structure after thixotropic soil restoration shows a similarity to its initial state before the shear tests. A subsequent increase in thixotropic soil strength with time is evidently related to the gradual establishment of thermodynamic equilibrium and complete restoration of the ini- tial microstructure.

REFERENCES

Abduragimova, L. A., Rebinder, I’. A. & Serb- ina, N. N. (1955). The elastic-viscous properties of thixotropic structures in aquatic suspensions of bentonite clays. Colloid J., 17, No. 3, 184-195 (in Russian).

Akkerman, E. (1958). The thixotropy and fluidity of fine-grained soils. Problems of Engineering Geol- ogy, No. 1, 10-29. Moscow (in Russian).

Avgustinik, A. 1. (1940). The thixotropic strengthen- ing of clays. Transactions of the Leningrad Tech- nological Institute of Vsekomprosuet, No. 3, 51-70 (in Russian).

Barden, L. & Sides, C. (1971). Sample disturbance in the investigation of clay structure. Geotechnique 21, No. 3, 211-222.

Gorkova, I. M. (1961). The quicksand and thixotropy of dispersed sedimentary soils. Colloid .I. 23, No. 1, 12-19 (in Russian).

Gumensky, B. M. (1954). Soil thixotropy phenomena and the vibration technique. Colloid J. 16, No. 6, 421424 (in Russian).

Lishtvan N. I., Bityukov N. N. & Terentyev A. A. (1977). A study of the vibrorheological properties of peat systems in fluid consistency. Colloid J. 39, No. 4, 786-789.

McKeys E. & Yong R. N. (1971). Three techniques for fabric viewing as applied to shear distortion of a clay. Clays Clay Miner. 19, 289-293.

Meschan, S. R. (1978). The initial and prolonged strength of clays, Chap. 5, 171-177. Moscow: Nedra.

Mitchell, J. K. (1960). Fundamental aspects of thixo- tropy. .I. Soil Mech. Fdns Div. Am. Sot. Civ. Engrs 86, SM 3, 19-52.

Morgenstern, N. R. & Tchalenko, J. S. (1967). Mic- roscopic structures in kaolin subjected to direct shear. Gtotechnique 17, No. 4, 309-328.

Ovchinnikov, P. F., Kruglitsky, N. N. & Mikhaylov, N. M. (1972). Thixotropic system rheology, pp 7-8. Kiev: Naukova Dumka (in Russian).

Pavlenkov, V. A. (1978). Experimental studies of the seismic oscillations of the earth bed in the conditions on the Baikal-Amur main-line route. Geological and seismic conditions in the Baikal- Amur main-line area, pp 172-180. Novosibirsk (in Russian).

Popov, I. V. (1944). The criptostructure of clays at deformation. Trans. USSR Acad. Sci. Moscow 45, No. 4, 174-176 (in Russian).

Puchkov, S. V. (1974). The regularities of soil oscilla- fions during earthquakes, chap. 5, pp 65-99. Mos- cow: Nauka (in Russian).

Rebinder, P. A. (1979). On the rheology of thixotropi- tally structured dispersed systems. In Surficial phenomena in dispersed systems: the physico- chemical mechanics (selected papers) pp 104-111 Moscow: Nauka (in Russian).

Sergeyev, Y. M., Grabowska-Olszewska, B., Osipov, V. I., & Sokolov V. N. (1980). The classification of microstructures of clay soils. J. Microscopy 20, No. 3, 237-260.

Skempton, A. V. (1967). The prolonged stability of clay slopes, Problems of Engineering Geology, No. 4, 142-176. Moscow (in Russian).

THIXOTROPIC PHENOMENA IN CLAY SOILS 303

Skempton, A. W. & Northey, R. D. (1952). The sensitivity of clays. Gtotechnique 3, No. 1, 30- 53.

Smart, P. & Tovey, N. K. (1982). Electron microscopy of soils and sediments: techniques, pp 51-55. Ox- ford: Clarendon Press.

Smolin, Yu. P. (1977). A study of the pore pressure in soil at shear moment. Trans. Novosibirsk Inst. Railway Engrs, No. 180, 89-91.

Tovey, N. K. (1971). A selection of scanning electron micrographs of clays. CUED/C-soil/TRSa, pp 2-4. University of Cambridge. Department of En- gineering,

Tovey, N., Frydman, S. & Wong, K. Y. (1973). A study of swelling clay in the scanning electron microscope. F’roc. 3rd lnt. Con?. Expansive Soils, Haifa. pp 45-54.

Trofimov, V. T. & Martynov, A. P. (1975). On the

quicksands and thixotropic properties of silty soils in the seasonally thawing layer of the Yamal penin- sula. Natural Conditions of Western Siberia, No. 5, 256-263. Tjumen (in Russian).

Tshukin, E. D. & Rebinder, P. A. (1971). On the mechanism of the elastic after-effect in structurized bentonite suspensions at minor concentration. Col- loid J. 33, No. 3, 450-458 (in Russian).

Uryev, N. B. (1980). Highly concentrated fine-grained systems, chap. 5, pp 173-190 Moscow: Khimia (in Russian).

Van Olphen, H. (1977). An introduction to clay colloid chemhy (2nd edn), pp 138-139. New York.

Yvalov, S. S., Pekarskaya, N. K. & Maksimyak, R. V. (1970). On the physical essence of the deformation and disruption processes in clay soils. Fountains and Soil Mechanics, No. 1, 7-9. Moscow (in Rus- sian).