The shearing behaviour of clays

Applied Clay Science, 4 (1989) 125-141

125

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

The Shearing Behaviour of Clays MAX MULLER-VONM00S and T 0 R LOKEN

Tonmineralogisches Labor des Institutes fi2r Grundbau und Bodenmechanik, ETH-Zentrum, CH-8092 Ziirich (Switzerland) Norwegian Geotechnical Institute, Sognsveien 72, N-Oslo 8 (Norway) (Received November 28, 1988; accepted February 10, 1989)

ABSTRACT

Mfiller-Vonmoos, M. and Loken, T., 1989. The shearing behaviour of clays. Appl. Clay Sci., 4: 125-141. The shear behaviour of kaolinite, illite and montmorillonite depends primarily on the anisotropy of the clay particles and the negative charge of the layer surfaces. In contrast to the Atterberg liquid limit and the rheological behaviour of aqueous clay suspensions the charge at the edge of the clay particle seems to have little influence on the shear strength. The relatively high shear strength of kaolinite comes principally from the intergranular friction. The shear behaviour of illite depends strongly on the counter ions. The shear behaviour of montmorillonite is determined to a large extent by the force with which the layers are held together. It is assumed that the montmorillonite particles break up for the most part into their component layers during shearing.

INTRODUCTION

Silt and clay exhibit clear differences in their soil mechanical behaviour. Plasticity is the principal characteristic of clays that distinguishes them from silt. Clays also have a significantly lower shear strength than silty materials. This leads to the fact that many landslides are triggered in layers with high clay contents. The shearing behaviour of soils is of major concern for engineers, since the solution of stability problems in foundation engineering requires a knowledge of the shearing strength of the underlying soil. The shearing strength ~f of a soil is usually described by the Mohr-Coulomb failure condition: r f = c ' + a ' tan ~' where c' =effective cohesion, a' =effective normal stress, and ~' =effective friction angle. The effective cohesion can be disregarded, for all practical purposes, in the case of normally consolidated clays, i.e. clays that have not been subject to 0169-1317/89/$03.50

© 1989 Elsevier Science Publishers B.V.

126 ~hear ~;tress "

V"

!

]

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• (~) . I~ "

shear displacement

@

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h

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°

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Fig. 1. Full shearing curve for a drained shear test: 1 = maximum shear strength or peak strength rm; 2 = shear displacement at which maximum strength is reached; 3 = shear strength after failure, with indication of the residual strength ~r as minimum, constant shear strength; 4 = shear displacement at which residual strength is reached; 5=vertical deformation of the sample during shearing.

previous loads higher than the present state. The present article examines only normally consolidated clays, for which the effective friction angle ~' is sufficient to describe the shearing strength. Both drained and undrained tests can be run to determine the shearing strength of a normally consolidated clay sample. For undrained tests, however, the pore-water pressures must be measured in order to determine the shear parameters c' and ~'. This is not necessary for drained tests, since the porewater pressures in that case should have reverted to zero after consolidation, and the shearing should be run slowly enough to prevent any buildup of porewater pressures. This work focuses primarily on the results of drained shearing tests. The full shearing curve for a drained test is given in Fig. 1 with the values of importance for the description of the shearing behaviour. The drop from the maximum shear strength to the minimum constant shear strength, or residual shear strength, can vary widely for clays. The following relations hold between the values given in Fig. 1: ~m tan ~Pm-- G' tan ~'r -- ~r 0"

127

where ~" = effective friction angle at maximum shear strength, and ~ = effective friction angle at residual shear strength.

INFLUENCE OF THE GRAIN SIZE ON THE RESIDUAL STRENGTH OF QUARTZ AND FELDSPAR

Quartz and feldspar are minerals typically encountered in the silt fraction. Their presence in the clay fraction, on the other hand, is insignificant compared to that of clay minerals. A question that poses itself is how much of the change in the soil mechanical behaviour between the silt and the clay fractions depends solely on the decrease in grain size. The results of the investigation on the influence of grain size on the residual strength of quartz and feldspar shown in Table I provide a quite interesting answer to that question. The tests were carried out by Kenney (1967) in a direct-shear apparatus, with a normal stress of 0.1 N mm -2. The results clearly show that the grain size has a minimal effect on the shear behaviour of quartz and feldspar. The high residual strength of the silt samples (in the 60-2 pm range ) remained unchanged after a reduction of the grain size into the clay fraction ( < 2/~m). Even flyash with a very small grainsize (0.50.01/~m) showed a high residual strength. Like quartz, for which grinding to very small fractions does not induce plasticity, flyash does not exhibit a claylike shear behaviour despite its being extremely fine-grained. The peculiar soil mechanical behaviour of clays depends primarily on the layer structure of the clay minerals, i.e. on the anisometry of the clay particles and on their surface charge characteristics. TABLE I Residual shear strength of quartz and feldspar, according to Kenney, 1967 (direct shear test, normal stress 0.1 N mm -2) Sample

Quartz broken Quartz broken Feldspar broken Feldspar broken SiO2-flyash

Grain size

Residual shear strength

(~m)

Cr°

60-2 <2 60-2 <2 0.5-0.01

34.6 35.0 34.6 34.6 29.3

128 SHEARING BEHAVIOUR OF KAOLINITE, ILLITE, AND MONTMORILLONITE

Examination of the maximum shear strength with the triaxial apparatus The maximum shear strengths of kaolinite, illite ( < 2 #m fraction of Fithian illite ), and montmorillonite (Wyoming bentonite ) have been examined by O1son ( 1962, 1963, 1974 ), Olson and Hardin ( 1963 ), and Mesri and Olson ( 1970 ) in the triaxial apparatus. The samples were poured into plastic tubes as a clay slurry, consolidated isotropically at 0.035 and 0.14 N mm-2, and then built in to the triaxial cell. For determination of the peak strength, the samples were subjected to compression up to failure at constant rates of deformation, both under drained and undrained conditions. The deformation rates were low enough to ensure equilibrium of pore pressures. Tests focused on the sodium and calcium forms of the samples, with varying concentrations of' NaC1 and CaCE in the pore water. The maximum shear strength of kaolinite in the sodium and calcium forms corresponded to a friction angle of about 25 °. The change from a positive charge at the edges of the clay particles to a negative one, through an increase of the pH from 5 to 9 (see Lagaly, 1989, fig. 3) had an effect on the consolidation behaviour (the water content at the transition to a dispersed form at pH 9 decreased), but not on the shear strength. It was therefore assumed that the shearing behaviour of kaolinite depends primarily on intergranular friction. Thus, it would be influenced by the size and shape of the kaolinite particles, on their packing density, and on their physical properties, such as elasticity, ultimate strength, and friction coefficient. Friction angles between 23 ° and 26 ° were found for calcium illite. An increase of the CaC12 concentration in the pore water from 0.001-n to 1-n did not have any measurable effect on the m a x i m u m shear strength. The friction angle for the sodium illite, on the other hand, was clearly lower and showed a high dependence on the NaC1 concentration. The friction angle was measured as 16 ° in an 0.007-n NaC1 solution, and as 21 ° for an 0.1-n solution. This led to the conclusion that the maximum shear strength of illite depends mostly on intergranular friction for the calcium form of the mineral, whereas it rests mainly on the repulsion of the double layers at the surface of the particles for the sodium form. The maximum shear strength for calcium montmorillonite corresponded to a friction angle of 10-15 °, and was independent of the concentration of CaC12 in the pore water (0.001 to 0.l-n). The friction angle for the sodium form was only 4 °. No dependence of the shear strength on the electrolyte concentration could be observed here either. The friction angle values were so low, however, that measuring accuracy was not enough to reach a reliable conclusion. The exceptionally low shear strength of the sodium montmorillonite is especially to be noted. It is assumed that the montmorillonite particles break up into

129 layers along the shearing plane; there, they are forced into a face-to-face arrangement, and they repel one another by their double layers. An aggregated structure and a stronger bonding of the layers was assumed for the calcium montmorillonite, on the other hand.

Examination of the residual strength with the direct shear apparatus The residual strength of drained samples of kaolinite, illite ( < 2 pm fraction of a quick clay), and montmorillonite ( < 2 pm and <0.1 ,um fractions of a bentonite) have been examined by Kenney (1966, 1967) in a direct shear apparatus with a rigid frame. The samples were prepared with a water content slightly over the liquid limit, consolidated, and sheared at a rate of 0.14-1.0 mm h -1, with a normal stress of 0.02-0.8 N mm -2. The kaolinite was little examined. Its residual strength was remarkably low, corresponding to a friction angle of 15.1 °. The ionic form of the kaolinite was not given, and the sample contained up to 5% illite and/or illite-montmorillonite mixed layers. The residual strength of the illite corresponded to friction angles of 23.8 ° for the calcium form, 21.3 ° for the potassium form, and 16.2 ° for the sodium form. An increase of the electrolyte concentration in the porewater to 30 g NaC1 or KC1 per litre led to an increase of the friction angle from 16.2 ° to 23.3 ° and from 21.3 ° to 24.7 °, respectively. Raising the calcium content of the pore water to 15 g CaC12 per litre, however, had little effect on the shear behaviour; the friction angle increased only to 25.2 °, from 23.8 °. The residual strength of calcium montmorillonite < 0.1 pm corresponded to a friction angle of 9.7 °. The friction angle in a saturated CaSO4 solution was measured as 10.2 °. Raising the electrolyte concentration in the calcium form therefore had no effect on the shear strength. The friction angle for < 0.1 pm and < 2 pm sodium montmorillonite was only 4.0 °. The angle increased to 10.2 ° and 8.5 ° for an electrolyte concentration of 30 g NaCl per litre. Kenney also examined 15 other soils with high clay contents and known mineralogical composition, grain-size distribution, plasticity, and ion-exchange capacity. He found no close relation between the residual strength and the plasticity or the grain-size distribution. The important factors were the quantity and type of clay minerals, their ionic form, and the electrolyte concentration of the pore water.

Examination of the residual strength with the ring shear apparatus Large shear displacements are needed to reach the residual strength in clays. The residual strength can therefore only be measured by reversals of direction in a direct shear apparatus such as that used by Kenney (1967), in which the maximum displacement is only 2.5 mm. The ring shear apparatus presents the

1'30

advantage of' allowing unlimited displacements in the shearing direction. The following data (M~iller-Vonmoos et al., 1985 ) on the shear behaviour of' kaolinite, illite, and montmorillonite were measured in a G E O N O R ring shear apparatus (Fig. 2; Bishop et al., 1971 ~. The samples were smoothed into the apparatus at a water content slightly above the liquid limit and progressively consolidated to a normal stress of 0.1 N mm -=' In the process, the water was pressed out through filter stones, at the top and bottom of the sample. Average thickness of the samples was 19 mm. The samples were also tested in the oedometer, to check the consolidation behaviour and the permeability. The upper confining rings were lifted slightly (0.02-0.05 mm) before begin of' the shearing, in order to leave a gap between them and the lower confining rings. The sample was sheared by rotation of' the bottom half' relative to the top one. Shearing was conducted at a rate of 0.1 to 1.0 mm h -1 up to the maximum strength, and at 1.0 mm h - 1 after the peak strength was reached. Raising the shearing rate has a negligible effect on the residual strength measured (Bucher,

frame ring shear apparatus

rotating table

normal loading system

torque measuring system

upper confining ring l i f t i n g system

soil sample

Fig. 2. Schematic section of the GEONOR ring shear apparatus.

131

1975). The shearing resistance is measured with a torque measuring system, in which the shear stress is transmitted through a torque arm bar to two proving rings. The shear stress, the normal stress and the vertical deformations of the sample were measured during the test. Poorly crystallized CMS-KGa-2 Georgia kaolinite (Source Clay Minerals Repository, University of Missouri) was used for the examination of the shear behaviour of kaolinite (Fig. 3). The sample contained 95% kaolinite, with a specific surface of 23 m 2 g-1 and a cation exchange capacity of 3.8 meq. per 100 g. The maximum and residual strengths of the calcium and sodium forms of the kaolinite corresponded to friction-angle values of ~ = 25 ° and ~'r = 23 °. The drop in the shear strength from failure to the residual strength was small. {0" and ~'r decreased to 21 ° and 19.5 ° after the induction of a negative charge at the particle edges (by addition of 0.5 g Nat P2 07" 10H20 to 100 g oven-dried sodium kaolinite). The opposite could have been expected, since the water content decreased from 57.6% to 31.6% (i.e. the bulk density of the kaolinite was significantly raised). The shear displacement necessary to reach maximum and residual strengths was reduced by the addition of the phosphate. Slip

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shear displacement (logarithmic}

Fig. 3. Development of the shear strength of kaolinite as a function of the shear displacement (linear and logarithmic ) and settlement of the sample during the shear test.

I:ti!

surfaces could be distinguished at the level of the gap between the confining rings. The surfaces adhered strongly, however, and could not be separated. A sample from the Massif Central (France) with an illite content of 75~;i and 25c4 orthoclase was examined for the investigation of the shearing behaviour of illite (Fig. 4). The sample had a specific surface of 105 m ~ g ~ and a cation exchange capacity of 26.5 meq. per 100 g. According to Gabis (1963) this illite was formed from montmorillonite through the uptake of potassium. Maximum and residual strengths corresponding to friction angles of ~01,~= 28.5 and ~i-= 24 ~ were measured for the calcium fbrm. A change into the sodium form brought the friction angles down to 15.5 ~ and 13.5 ~. Despite a decrease of the water content from 65.5% to 55.3%, with an accompanying increase of the bulk density, a sharp decrease in the shear strength was observed after the change into the sodium form. The residual strength remained unchanged after addition of the phosphate, although a further decrease of the water content to 46.9% occurred. Thus, the change in charge at the edges had no effect on the shear behaviour of illite. The slip surfaces also adhered together to a great extent, and only small areas could be separated. Two samples were examined for the investigation of the shearing behaviour C '4Q Nc~* No4P207

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Fig. 4. D e v e l o p m e n t of the shear strength of illiteas a function of the shear displacement (linear and logarithmic ) and settlement of the sample during the shear test.

133

of montmorillonite: the relatively highly charged CMS-SAz-1 Arizona montmorillonite, with an interlayer charge of 0.346 per formula unit and a montmorillonite content of 90%, and the CMS-SWy-1 Wyoming montmorillonite with an interlayer charge of 0.270 per formula unit and a montmorillonite content of 85%. The shearing curves for the Arizona montmorillonite in its sodium, calcium, aluminium and thorium forms are shown in Fig. 5. The types of ions in the "aluminium" and "thorium" forms could not be determined exactly. The samples were treated with the acid nitrate solutions of the added elements for the change into the ionic form. In the "aluminium" form, the incorporated ions could be A1 (OH)2+ or a combination of A13+ and H~O +. In the "thorium" form, the most likely interlayer cations are Th(OH)22+. The friction angles corresponding to the maximum and residual strengths were measured as follows: 36 ° and 14 ° for the thorium form; 33.5 ° and 12 ° for the aluminium form; 29.5 ° and 7.5 ° for the calcium form; and 15 ° and 4.0 ° for the sodium form. Especially to be noticed are the large drop in shear strength from the maximum to the residual strength and the large shear displacement reCa

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Fig. 6. Developmentof the shear strength of kaolinite, illite, Arizona montmorillonite,and Wyoming montmorillonite in the calcium form as a function of the shear displacement (linear and logarithmic). quired to reach the residual strength. The shear-strength values and the displacement to reach residual strength decreased with decreasing valence of the counter ions. A very small friction angle of 4 ° was measured for the residual strength of the sodium form. Maximum shear strength of the Wyoming calcium montmorillonite was only half that of the Arizona calcium montmorillonite, with a friction angle of 14 °; the residual strength, however, was only 1.5 ° lower, with a value of 6 ° (Fig. 6). In contrast with the kaolinite and the illite, the slip surfaces for the montmorillonite were clearly formed and could easily be separated. SUMMARIZINGCONCLUSIONS ON THE SHEARING BEHAVIOUROF CLAYS The kaolinite has the largest grain size among the examined minerals. The kaolinite particles have the smallest anisometry, i.e. the smallest diameterthickness ratio. The particles thus have a relatively large edge surface. The kaolinite also has a distinct distribution of the charge, with one highly charged and one uncharged basal plane. The same, high shear strength for both the sodium and the calcium forms, and the similar profile of the shear curves leads to the conclusion that the shear behaviour is determined principally by the friction between the relatively large, plate-shaped particles. The kaolinite particles only partially align themselves face-to-face during shearing, and a rough

135 shear surface results. Furthermore, the change from a positive to a negative charge at the edge of the particles, through an increase of the pH from 5 to 9 or through addition of phosphate, has little influence on the shear behaviour. The same observations about the influence of the ionic form (sodium or calcium) of the tested clay and of the addition of phosphate, and about the profile of the shear curve were made by Sonderegger (1985) on samples of CMS-KGa1 Georgia kaolinite and the kaolinite China Clay (St. Austin, Cornwall, G.B. ) in the ring shear apparatus. Except for the value obtained by Kenney, all friction-angle values for the sodium and calcium forms of kaolinite mentioned in this paper and given by Sonderegger (1985) lie between 29 ° and 21 ° for the maximum shear strength and between 26 ° and 18 ° for the residual shear strength. The illites examined, and especially the illite from the Massif Central, were fine-grained, and the particles had a high level of anisometry. Both basal planes are highly charged. The charge is very effective because the charge centres are in the tetrahedral sheets on the outer sides of the layers, and thus close to the surface of the basal planes. In the calcium form, the double layers of neighbouring particles are destroyed by the formation of a central layer of calcium ions, and the particles take on a preferred band-like configuration (see Lagaly, 1989, fig. 5b). This explains the relatively high maximum strength values of 23 ° to 28.5 ° measured in the triaxial and ring-shear apparatuses and residualstrength values of about 24 ° measured in the direct-shear and ring-shear apparatuses. Since the calcium ions place themselves in the middle plane between neighbouring illite particles, rather than forming diffuse double layers, changes of the calcium ion concentration in the pore-water solution cannot have a significant effect on the shear behaviour of calcium illite. On the other hand, for the sodium form, the shear behaviour is determined by the strong repulsion of the double layers on the faces of the particles. The maximum shear strength corresponded to a friction angle of about 16 °, and residual strength was given by angles between 13.5 ° and 16 °. An increase of the electrolyte concentration in the pore water causes a compression of the double layer and a consequent decrease of the repulsion between the particles. This explains the increase in shear strength with higher NaC1 concentrations, which was also observed by Sonderegger (1985) in the tests on illites from the Massif Central. It can be assumed that the highly anisotropic illite particles take a face-to-face arrangement in the shear plane. This would also be pointed out by the fact that the change of the charge at the edges through the addition of phosphate to the sodium illite has no effect on the shear behaviour, although the plasticity index decreased from 47% to 25% (Table II). In contrast to what the liquid-limit values indicate, the negative charge at the particle edges did not have an effect for shearing across the face-to-face arrangement under a normal stress of 0.1 N mm -2. Sonderegger (1985) made similar observations with sodium illite

t:~6 TABLE II Liquid limit, plastic limit, plasticity and friction angles 9~ and ~0'rof kaolinite, illite and montmorillonite in different ion forms, according to M~iller-Vonmoos et al. (1985) Material

Liquid limit I%)

Plastic limit ~c,~)

Plasticity index (%)

~P',, "

(P'r :

Kaolinite Ca-form Na-form Na-form + Na4 P2 O7

74 69

31 31

43 38

25 25

23 23

33

24

9

21

19.5

93 76

32 29

61 47

28.5 15.5

24 13.5

54

29

25

18

13.5

137 158 190 431

85 56 50 48

52 102 140 383

36 33.5 29.5 15

14 12 7.5 4

264

26

238

14

6

IUite Ca-form Na-form Na-form + Na4 P2 07

MontmoriUonite, Arizona Th-form Al-form Ca-form Na-form

Montmorillonite Wyoming Ca-form

after an addition of phosphate, for a 2-n NaC1 concentration in the pore water and with potassium illite after an addition of K4P2OT. In contrast to kaolinite and illite, in which the layers are strongly held together, the montmorillonite layers are held so loosely that water can penetrate between them. Montmorillonite is subject to innercrystalline swelling, a process in which the interlayer distance can increase to 1 nm (1 n m = 10 -8 mm, for references on innercrystalline swelling, see Madsen and Mtiller-Vonmoos, 1989). The montmorillonite layers are held more strongly together with increasing layer charge and with increasing valence and ionic radius of the interlayer cations. In the calcium form, for example, they will be held more strongly in the highly charged Arizona montmoriltonite than in the Wyoming montmorillonite, which has a lower charge. This also explains the fact that the interlayer distance stays limited to 1 nm for free swelling of the calcium, aluminium, and thorium forms in deionized water, whereas the swelling of sodium montmorillonite has no such limit. Thus an interlayer distance of 3 nm was measured for the sodium montmorillonite at the end of the shear test, while it

137

did not exceed 1 nm for the other samples. The weak force holding the montmorillonite layers together is critical in determining the shear behaviour of this clay. During shearing the montmorillonite breaks down into individual layers and layer packets. Innercrystalline water is freed in the process, and the very thin (about 1 nm thick) layers arrange themselves into a smooth shear surface. These slip surfaces could be separated only for the montmorillonite samples tested. The clear rise to the maximum shear strength and the strong drop to residual strength are best explained by the overcoming of the force holding the layers together and their configuration in a shear plane. Thus the force between the layers in the Arizona montmorillonite increased with an increasing valence of the interlayer cations, i.e. going from sodium to thorium. As a result, the angles corresponding to the maximum and residual-shear strengths went up, and the shear displacement required to reach residual strength became larger (Fig. 5). At an interlayer distance of 3 nm, for the sodium form, the attraction between the layers is very small, and the friction angle at residual strength is correspondingly reduced to 4.0 °. The friction angle was still low for the calcium montmorillonite, with a value of 7.5 °. The influence of the layer charge on the shear behaviour of montmorillonite can be seen by comparing the shear curves for the calcium forms of Arizona and Wyoming montmorillonites (Fig. 6). Both samples had approximately the same montmorillonite content. The Arizona montmorillonite, however, had a higher charge than the Wyoming montmorillonite, with an interlayer charge of 0.346 per formula unit, as opposed to 0.270 for the latter. As a result, the maximum strength friction angle for the former, with a value of 29.5 °, was more than twice as high as for the latter, with a value of 14 °. The difference in residual strength was not as pronounced, with values of 7.5 ° and 6 °, respectively - - a difference of only 1.5 °. Sonderegger ( 1985 ) changed Arizona montmorillonite into its potassium form and measured angles of 29 ° and 16 ° for the maximum and the residual shear strengths. The drop to the residual strength was smaller than for the calcium form. The interlayer distance for montmorillonite in the potassium form, however, was at the end of the shear test only 0.43 nm, as opposed to 1 nm for the calcium form. The interlayer distance was therefore probably already closed in part through the ion exchange and the fixation of the potassium. After 100 cycles of exposure to potassium, with drying of the samples at 105 ° C after every cycle to guarantee a better fixation of the potassium, the interlayer distance was reduced to 0.16 nm, i.e. the interlayer space was almost completely closed up. The friction angle values for the maximum and the residual shear strength of this sample were 32 ° and 30 °, i.e. the drop in strength became very small after an extensive loss of the innercrystalline swelling potential. In the calcium, aluminium and thorium forms, the innercrystalline swelling is limited to an interlayer distance of 1 nm. The cation concentration then corresponds to a 2 to 3-n saline solution. A decrease in the interlayer distance and a consequent increase of the shear strength of the montmorillonite could

138

only be achieved with higher salt concentrations in the pore water. Such considerations are without practical meaning for the construction engineer, however. The montmorillonite in the sodium form is somewhat more sensitive to the electrolyte content of the pore water, because the innercrystalline swelling is carried on as osmotic swelling, which leads to larger interlayer distances. The sodium ions located on the middle plane between the layers move to the layer surfaces and there form electric double layers. The electrostatic attraction thus becomes a repulsion through the double layers. Mtiller-Vonmoos et al. (1985) measured an interlayer distance of 3 nm for the Arizona sodium montmorillonite with a water content, after the tests, of 245%. The sodium ion concentration in the interlayer space was less than 1 - n ; this would substantiate the slight increase of the residual strength for an increase of the NaC1 concentration of 30 g l - 1 observed by Kenney ( 1967 ). Organic molecules, such as alkylammonium ions can have a great influence on the shear behaviour of montmorillonite. Such molecules, which are found in the seepage water of hazardous waste dumps, are selectively taken up in the interlayer space and make the surfaces hydrophobic. As a result, the montmorillonite adopts the behaviour of a silty material. Grain-size distribution and plasticity are properties examined for the clas• sample ~'r

number

tan ~P'r

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139 Quartz % I00

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60

Clay minerals - - ~ - - Kaolinite - - o - - Na-Montmorillonite

Fig. 8. Residualshear strength of quartz, 60-2 pxa, with increasingkaolinite and sodiummontmorillonite,accordingto Kenney (1967). Direct shear tests. sification of fine-grained soils. It is therefore sensible to examine the possibility of a relation between the shear behaviour and the grain-size distribution or the plasticity. No relation between the two properties and the residual strength could be found by Kenney (1967). The nature of the exchangeable cations was not given, however, so that it is not known whether the soils were in the sodium or in the calcium form. Bucher (1975) pointed out a decrease of the residual strength with increasing plasticity in his examinations of Swiss soils {Fig. 7 ). The scatter was quite large, however, and it was not altogether sensible to determine the residual shear strength based on average plasticity values. On the other hand, the minimal value of the residual shear strength for Swiss soils could be given with a reasonable certainty after determination of the plasticity. The liquid limit, the plastic limit, the plasticity index, and the friction angles (0" and ~'r for kaolinite, illite and montmorillonite for different ionic forms are given according to Miiller-Vonmoos et al. (1985) in Table II. While the residual strength for the clay minerals without innercrystalline swelling, and especially illite, increases for higher plasticity indexes, a decrease can be noted for montmorillonite. The predominant exchangeable ions in our soils are magnesium and, most of all, calcium; the sodium content is usually low. In the

141~

calcium fbrm only the residual strength of montmorillonite is low, as can be seen in Fig. 6. The plasticity index of montmorillonite is furthermore very high, compared to kaolinite and illite. It can therefore be assumed that the decrease in residual strength with increasing plasticity index, as observed by Bucher ( 1975 ), is tied to an increasing content of montmorillonite or mixed-layer clays with innercrystalline swelling layers. Thus, as Kenney showed (1967), very small additions of montmorillonite to quartz can lead to a drastic drop in the residual strength of the latter (Fig. 8). In conclusion it can be said that the shear behaviour of the examined clay minerals depends primarily on the anisotropy of the clay particles and the negative charge of the layer surfaces. On the other hand, in contrast to the Atterberg liquid limit and the rheological behaviour of aqueous clay suspensions, the charge at the edges of the clay particles seems to have little influence on the shear strength. For shearing with a normal stress of 0.1 N mm-2, the clay particles in the consolidated samples take on a face-to-face configuration in the shear plane; a change of the charge at the edges therefore has no significant influence. The relative high shear strength of kaolinite comes principally from the intergranular friction between the relatively large, plate-shaped particles. On the other hand, the shear behaviour of illite, with its thin and highly charged particles, depends strongly on the counter ions. The shear strength decreases noticeably, through the repulsion of the double layers, with the change from the calcium into the sodium form. The shear behaviour of montmorillonite is determined to a large extent by the force with which the layers are held together; thus, the shear strength goes up with increasing layer charge and valence of the interlayer cations. It is assumed that the montmorillonite particles break up for the most part into their component layers during shearing. Since the predominant ions found in clays from Central Europe are calcium and magnesium, low residual strengths can be expected in soils with montmorillonite and mixed-layer clays with swelling layers, and, very generally stated, the shear strength decreases with increasing plasticity.

REFERENCES Bishop, A.W., Green, G.E., Garga, V.K., Andresen, A. and Brown, J.D., 1971. A new ring shear apparatus and its application to the measurement of residual strength. Gdotechnique, 21: 273328. Bucher, F., 1975. Die Restscherfestigkeit nattirlicher BSden, ihre EinflussgrSssen und Beziehungen als Ergebnis experimenteller Untersuchungen. Mitt. Inst. Grundbau Bodenmech., ETHZiirich, 103. Gabis, V., 1963. Etude mindralogique et g~ochimique de la s6rie s~limentaire oligoc~ne du Velay. Bull. Soc. Fr. MinSral. Cristallogr., 86: 315-354. Kenney, T.C., 1966. Residual strength of fine-grained minerals and mineral mixtures. Norwegian Geotechnical Inst., 68: 53-58.

141 Kenney, T.C., 1967. The influence of mineral composition on the residual strength of natural soils. Proc. Geotech. Conf. Oslo, 1: 123-129. Lagaly, G., 1989. Principles of flow of kaolin and bentonite dispersions. Appl. Clay Sci., 4: 105123. Madsen, F.T. and Mtiller-Vonmoos, M., 1989. The swelling behaviour of clay. Appl. Clay Sci., 4: Mesri, G. and Olson, R.E., 1970. Shear strength of montmorillonite. Gdotechnique, 20: 261-270. Miiller-Vonmoos, M., Honold, P. and Kahr, G., 1985. Das Scherverhalten reiner Tone. Mitt. Inst. Grundbau Bodenmech., ETH-Ztirich, 128. Olson, R.E., 1962. The shear strength properties of calcium illite. Gdotechnique, 12: 23-43. Olson, R.E., 1963. Shear strength properties of a sodium illite. J. Soil Mech. Found. Div., Sml, 89: 183-208. Olson, R.E., 1974. Shearing strengths of kaolinite, illite and montmorillonite. J. Geotech. Eng. Div., GTll: 1215-1229. Olson, R.E. and Hardin, J., 1963. Shearing properties of remolded sodium illite. Proc. 2nd Panam. Conf. Soil Mech. Found. Eng., 2/1: 203-218. Sonderegger, U., 1985. Das Scherverhalten von Kaolinit, Illit und Montmorillonit. Mitt. Inst. Grundbau Bodenmech., ETH-Ztirich, 129.