EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING BETWEEN COHESIVE SOILS AND SOLID MATERIALS AT LARGE DISPLACEMENT

Transcription

1 ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING) VOL. 7, NO. 1 (2006) PAGES EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING BETWEEN COHESIVE SOILS AND SOLID MATERIALS AT LARGE DISPLACEMENT F. Hammoud a, A. Boumekik b a Department of Civil Engineering, University of Batna, Batna, Algeria b Department of Civil Engineering, Mentouri University, Constantine, Algeria ABSTRACT The assessment of piles lateral friction involves the determination of the interface shearing resistance at large displacement. Consequently, the shearing resistance mobilised at large displacements, at the interface between cohesive soils and steel and concrete of varying roughness has been studied in the ring shear apparatus. The latter is known to offer the best mean of studying shearing resistance when soils undergo large displacements. The focus of this work is to highlight the solid material surface roughness effect. Results obtained from soil-soil and soil-interface shearing tests are presented. They indicate that the surface roughness and the average diameter of particles, which are combined into the influence of relative roughness R, have a significant effect on the interfacial shear strength at a given normal stress level. With respect to R, three shearing modes are likely to take place: shearing at the interface, shearing within the soil and an intermediate mode involving these two types of shearing. For the second mode, the shear strength at the interface soil-rough solid materials (steel and concrete) is likely to be greater than the shear strength of the soil, particularly for soils whose major mineral component is montmorillonite. Keywords: cohesive soil, concrete, drained shear, friction, laboratory test, pile, residual strength, steel 1. INTRODUCTION Friction between soil and construction materials is of significant concern in soil-structure interaction problems including retaining structures, deep foundation, earth reinforcement and so on. It is one of the primary mechanisms through which load is transferred. Therefore, it is necessary to know the friction parameters between soil and construction materials in order to assess the stability of such structures. In applications where deformations at the interface are small, as in the case of retaining walls, the peak value of interfacial friction is appropriate. Whereas, in the case of piles, the assessment of lateral friction involves the -address of the corresponding author: hammoud_farid@yahoo.fr

2 64 F. Hammoud and A. Boumekik determination of the interface shearing resistance for large displacements under drained conditions (Bond and Jardine, 1991). In these conditions, the shear strength falls below the peak value and eventually reaches a residual value at a relatively large displacement. According to Lupini et al. (1981), three shearing modes are likely to take place as far as residual behaviour is concerned: a turbulent mode in soils with a high proportion of rotund particles or with platy particles of high interparticle friction, in which preferred orientation of platy particles does not occur, a sliding mode in which a low strength shear surface of strongly oriented low friction platy particles form, and a transitional mode involving both turbulent and sliding shear. More research work have been conducted to study shearing at the interface between sand and solid interfaces than for clays, which resulted in less studies regarding the mechanisms involved in the shearing at the interface in clays. Furthermore, only few studies regarding large displacement have been carried out. Shear zones at or close to residual strength are likely to be present in the vicinity of displacement piles as a result of driving and subsequent loading. Although studies regarding interface behaviour of piles have been conducted mainly by means of in-situ large scale piles loading tests, their results cannot be generalized to predict the behaviour of other piles. On the contrary, laboratory shear tests at the interface are very useful to study the fundamental behaviour of lateral friction around piles because they have well defined boundary limit conditions and only small soil samples are needed to conduct interface tests. The purpose of this study is to indicate a realistic view of interface shear mechanism between cohesive soils and solid surface over a wide shear deformation region from prepeak to post-peak until residual conditions are established. In addition, the appropriate operative angle of interface friction occurring will be clarified based on results from interface ring shear tests in which shearing of soil against steel and concrete interfaces at large displacements was performed, with particular emphasis to the effect of surface roughness. 2. TESTING EQUIPMENT It is now well established in the literature that the ring shear apparatus offer the best mean of studying shearing resistance when soils undergo large displacements. It is also recommended for interface studies. Kishida and Uesugi (1987), after reviewing different apparatuses used in the study of shear strength at the soil solid material, concluded that the ring shear apparatus is the ideal machine for this kind of investigation because of its unlimited interface. This machine has been designed to solve certain problems encountered with the reversal shear box. In the latter, the sample is placed in a Casagrande type box, consolidated under a vertical pressure and sheared alternatively in opposite directions around its initial position. Failure takes place along the plane of the separation between upper and lower boxes. The main disadvantage of this test concerns the variation of shearing area which lead to a non uniform shear stress distribution. Moreover changing the direction of shearing lead each time to a secondary peak. Several studies regarding soil structure interaction involving large displacement have been conducted for different materials using

3 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING ring shear apparatus (Tika-Vassilikos, 1991 and 1999; Lemos and Vaughan, 2000; etc). Several ring shear devices have been developed during the last thirty years. For the time being, the two most used machines and which are available commercially are those described by Bishop et al. (1971) and Bromhead (1979). The former known as the IC/NGI apparatus has been developed conjointly by the Norwegian Geotechnical Institute, Norway and Imperial College, England. The latter, which is known as the Bromhead apparatus, has been designed and evaluated at Kingston Polytechnic in England. The fundamental difference between tests carried out in both machines lies in the mode of shearing of the two devices. In the IC/NGI apparatus failure occurs within the body, at the mid-height of the specimen away from the loading platens while in the Bromhead apparatus it occurs adjacent to the top loading platen. Although having different principles, tests carried out on virtually identical samples in both machines showed that there is a very good agreement between the results obtained (Hutchinson et al. 1980). Likewise La Gatta (1970) demonstrated that the residual strength was the same whether shear surface formed in the centre of the specimen or near a porous disc. This would suggest that the above machines are capable of accurate residual strength measurements. The Bromhead ring shear apparatus (Figure 1) is based on the original design described by Bromhead (1979). The apparatus is manufactured by Wykeham-Farrance Engineering Limited in Slough, United Kingdom. A complete description of the apparatus, its design and principles of operation are given by Bromhead (1979). The ring shear specimen is annular with an inside diameter of 7 cm and an outside diameter of 10 cm. Drainage is provided by two bronze porous stones secured to the bottom of the specimen container and to the top loading platen. The specimen, which is 5 mm deep, is confined between pairs of inner and outer confining rings. It is loaded normally through a top loading platen by a dead load lever system. Rotation is imparted to the specimen container through a variable ratio gearbox, and torque transferred to the specimen is measured by two matched proving rings acting on a torque arm fixed to the top loading platen. The sample assembly is surrounded by a Perspex water bath to prevent the sample from drying out during testing. It will be noted that linear displacement transducers have been added to supplement the dial gages so that data can be recorded automatically during prolonged tests. The Bromhead ring shear apparatus was modified to permit interface strength testing. This was achieved by substituting the upper porous stone by a steel or concrete annulus (interface). Digital dial gages and a microcomputer data acquisition system were used to measure vertical displacement and shear stress during the test. Average horizontal displacements values and shear displacement rates reported in the present paper were calculated using a diameter of 85 mm, which is the average diameter of the annular specimen. For the geometry of the Bromhead apparatus ring shear samples, the shear displacement of elements adjacent to the inner circumference would be 70% of the shear displacements of elements adjacent to the outer circumference of the annular sample. This would lead to a slight underestimation of peak strength values. However, assessment of large strain or residual shear strength parameters would not be affected by the presence of such strain gradients, which are inherent to all ring shear devices.

4 66 F. Hammoud and A. Boumekik Figure 1. View of the Bromhead ring shear apparatus 3. TEST PROCEDURE AND SPECIMEN Because of the sample container dimensions, relatively small soil quantity was needed for conducting a test in the Bromhead apparatus. Since both initial soil structure (Bishop et al. 1971) and water content (Voight, 1973) were found to have no significant effect on residual strength, a sample in a remoulded state has been used. Test specimen were prepared by kneading evenly soil paste into the annular space in the Bromhead ring shear device. The sample was produced by mixing dry soil with deionised water at the liquid limit. After ensuring a thorough mixing, it was then left to dry until it reached a water content equivalent to 1.5 times the plasticity limit approximately. Consolidation was then initiated at a normal stress of 70kN/m 2. Once consolidation was complete shearing took place at a rate of shearing of mm/min, which should ensure full dissipation of excess pore water pressure. Table 1. Index properties of soils studied Properties B K X K X Liquid limit w L (%) Plastic limit w P (%) Plasticity index I P (%) Clay fraction (% < 2µm) Average diameter D av (µm)

5 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING Percentage passing (%) B K XK X Particle sizes (mm) Figure 2. Grading curves The soils used herein include two artificial high plasticity clays: kaolin (K) and bentonite (B), Xeuilley silt (X) and a mixture of 75% Xeuilley Silt and 25% kaolin (XK). The main mineral constituent of kaolin and bentonite are kaolinite and calcium montmorillonite, respectively. Xeuilley silt is a very plastic clayey silt cut off from Xeuilley (20 km in the south-east of Nancy, France). The mineral analysis shows that this silt contains quartz (60%), montmorillonite (20%), feldspath (11%), kaolinite (4 to 5%) and mica (4 to 5%). Their index properties are presented in Table 1 and Figure 2 shows the grading curves. 4. SURFACE ROUGHNESS CHARACTERIZATION In this study four different stainless steel interfaces and two micro concrete interfaces with different degrees of roughness have been used. Stainless steel interfaces were used to avoid roughness changes during testing due to rust. In order to design homogenous interfaces, a combination of two factors has to be taken into consideration, namely surface roughness (i.e. height of asperity) on the one hand and texture model (distribution of asperities) on the other hand. The roughness of each specimen was finished to a specified roughness. The smooth one was obtained by plane rectification while the rough ones were obtained by milling. End milling is the most common metal removal operation encountered. The rough interfaces were obtained with squared teeth knurls allowing a well formed and precise milling. The depth of impression can be adjusted according to the pressures applied to the knurls to obtain different roughness. Figure 3 illustrates the four steel rings used in this investigation.

6 68 F. Hammoud and A. Boumekik Figure 3. Steel rings Because of the small volume of concrete required and the exaggeration of interface surface roughness which would have occurred if big aggregates had been used, micro concrete was used. A high workability concrete is required for placement in the mould. Consequently, concrete interfaces were obtained by mixing ordinary Portland cement, graded fine aggregates and water. The concrete rings were prepared in a Perspex mould which can be dismantled. The rings were reinforced with 1 mm diameter wire owing to the fact that they had a thickness of only 6 mm. After the concrete was placed it was tamped with rod to ensure maximum density. The concrete rings were allowed to cure in water at normal temperature for several days prior to use in ring shear tests. In order to obtain a smooth ring, the latter was polished in a polishing turret using a fine abrasive. The rough interface was obtained using sandpaper. The concrete rings obtained are shown in Figure 4. Figure 4. Concrete rings Many investigators have shown that surface roughness plays a major role in interface behaviour (Uesugi and Kishida, 1986; Paikowski et al. 1995; etc.). To conduct a meaningful examination of the fundamental mechanisms controlling interface shear it is first necessary

7 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING to characterize the materials involved. For particulate media such as soil, characterization can be performed through a series of standard laboratory index tests. For a solid material interface, a quantitative evaluation of the surface is required. Various methods of surface roughness quantification have been proposed in studies of interface shear. The roughness R max (L a =2.5mm) is defined as the relative height between the highest peak and the lowest trough along a surface profile over an analysis length (L a ) of 2.5mm. The latter is a parameter of surface roughness only and does not involve particle size. In the present study a method of roughness description which is standard in tribology was adopted, the centre line average, R a. In discrete form, it is given by the following equation: 1 R (1) n a = y i n i= 1 where: n: number of evenly spaced sampling points y: amplitude of the current sampling point about the centreline The method proposed by Subba Rao et al. (1998) which uses a relative roughness R was also adopted. It consists to normalise R a with respect to the average diameter of the soil particles D av. The latter can be obtained from the grain size distribution curve of the soil studied, using an arithmetical scale for the particles sizes. R is defined as: R a R = (2) Dav The average diameters of the soils investigated are given in Table 1. The relative roughness R, decreases with an increase of D av. The decrease in relative roughness reflects the fact that large particles will move more smoothly than small ones when they slide across a surface with the same roughness. In geotechnical applications, roughness of a surface must be defined in terms of the predominant length scale of soil grains at the interface. Hence, measurements of the roughness were made by means of laser profilometer. Typical measured surface roughness profiles of steel and concrete rings are shown in Figure 5 and Figure 6, respectively. Since the distribution of asperities along the surfaces for the different specimens is not uniform, the average value of roughness parameters computed in 10 different locations was taken into consideration. The average values of R max, R a and R obtained are given in Tables 2 and 3. Table 2. Values of R a and R max of used rings Roughness arameters SI1 SI2 SI3 SI4 CI1 CI2 R a (µm)

8 70 F. Hammoud and A. Boumekik R max (µm) Table 3. Values of R Interfaces B K XK X SI SI SI SI CI CI Verticl distance (mm) Horizontal distance (mm) Figure 5. Typical profile of steel interface Verticl distance (mm) Horizontal distance (mm)

9 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING Figure 6. Typical profile of concrete interface 5. RESULTS AND DISCUSSION The friction shearing between the soil and the interface is often represented by the friction coefficient τ/σ n. Where τ and σ n are the shearing and normal effective stresses acting at the soil solid material interface. Two types of friction coefficients are of interest: the peak friction coefficient (τ p /σ n) and the residual friction coefficient (τ r /σ n). Figure 7 and Figure 8 show friction coefficient-tangential displacement curves obtained for K samples sheared against steel and concrete, respectively. The results obtained are summarised in Tables 4 to 7. In these tables φ p and φ r are the peak and the residual angles of internal friction obtained for soil-soil shearing. For interface shearing δ p and δ r refer to the peak and the residual angles of interface friction respectively. p and r are the displacements necessary to attain pic and residual conditions. It is important to emphasize that the present study concerns mainly residual shear strength rather than peak shear strength. τ / σ ' n XOI XSI4 XSI3 XSI2 XSI Tangential displacement (mm) Figure 7. Typical friction coefficient-tangential displacement curves of Xeuilley silt-steel interfaces XOI XCI2 XCI1 τ / σ ' n Tangential displacement (mm)

10 72 F. Hammoud and A. Boumekik Figure 8. Typical friction coefficient-tangential displacement curves of Xeuilley silt-concrete interfaces The standard ring shear tests (i.e. BOI, KOI, XKOI and XOI), where soil soil shearing takes place, have shown that all materials exhibit a brittle behaviour. Each test showed first a well defined peak in the friction coefficient tangential displacement. Bentonite and kaolin have peak angles of internal friction (φ p ) of 17.8 and 19.3 respectively; whereas X and XK have almost the same value (i.e and 26.9 respectively). The development of peak strength is derived primarily from the destruction breakdown of the bonds in the soil structure. This develops relatively high strengths over a small displacement. The peak strength obtained for these tests is a remoulded one because the specimen used is disturbed. As a result the value of φ p measured in the ring shear apparatus is expected to be lower than that obtained in the triaxial apparatus. Table 4. Summary of results on B soil TEST τ p /σ n φ p or δ p degrees p mm δ p /φ p τ r /σ n φ r or δ r degrees r mm δ r /φ r BOI BSI BSI BSI BSI BCI BCI TEST τ p /σ n Table 5. Summary of results on K soil φ p or δ p degrees p mm δ p /φ p τ r /σ n φ r or δ r degrees r mm KOI δ r /φ r KSI KSI KSI KSI KCI

11 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING KCI Table 6. Summary of results on XK soil TEST τ p /σ n φ p or δ r (deg) p (mm) δ p /φ p τ r /σ n φ r or δ r (deg) r (mm) δ r /φ r XKIO XKSI XKSI XKSI XKSI XKCI XKCI For large displacements, mean values of residual angle of internal friction equal to 4.3 and 14 were obtained for samples of bentonite and kaolin, respectively. The latter contain large proportions of clay particles (87% and 76% respectively). Since both soils are formed predominantly by clay particles of platy shape, brittleness is due only to particle orientation. The energy required to accomplish the full parallel arrangement accounts for the major component of residual strength. Thus the residual shearing mode which governs the behaviour at large displacement is the sliding mode as defined by Lupini et al. (1981). In this mode, the clay particles adopt a parallel face to face structure with little interlocking oriented in the direction of shearing, as a result less resistance is offered to sliding. Lupini et al. suggested that soils exhibiting such a mode of behaviour will have values of φ r ranging from 5 to 20. Furthermore, Skempton (1985) showed that when the percentage of clay mineral of a soil exceeds 50% it will exhibit the sliding mode. The shearing resistance is highly dependant on mineralogy and is mainly governed by the sliding friction of the clay minerals, the residual angles of friction being approximately 15 for kaolinite and 5 for montmorillonite. XK sample, which contains about 34% of platy clay particles and 66% of silt particles, is likely to exhibit the transitional mode with a value of φ r =23.1. The residual behaviour is attributed to rolling and translation of silt particles and to sliding of clay particles. Skempton (1985) suggested that the transitional mode is likely to occur in soils where the clay fraction is ranging from 25% to 50%. Regarding Xeuilley silt, since granular particles (i.e.78 %) predominate, interference between them prevents the formation of a shear zone of oriented clay. As a result, relatively high values of residual strength are obtained, with values of φ r >20, typically (Skempton, 1985). φ r obtained for this study is

12 74 F. Hammoud and A. Boumekik equal to The mode of residual behaviour is either the transitional or the turbulent mode. However the brittleness observed is atypical of soils showing the latter pattern of behaviour. Therefore, the mode of shearing could be the transitional one. Table 7. Summary of results on X soil TEST τ p /σ n φ p or δ p degree p mm δ p /φ p τ r /σ n φ r or δ r degree r mm δ r /φ r XIO XSI XSI XSI XSI XCI XCI Although shearing mechanism is influenced by roughness, the interface shear strength at large displacement is still governed by the sliding mode for K and B. As far as interface shearing tests between kaolin and steel or concrete are concerned, τ r /σ n values were found to vary between and and the corresponding residual angles of interface friction (δ r ) between 12.2 and 13.2, with δ r /φ r equals approximately to 0.9, for the rough interface. The interface values are therefore lower than the soil soil value which is The same trend have been found for XK which seems to have as upper boundary limit the friction coefficient of the soil. As expected the smoothest interface (SI1) gave a value considerably lower (i.e for K and for XK) and for XK a sliding mode of shearing takes place at large displacement, instead of the transitional one which would take place if soil-soil shearing was initiated. Regarding X and B soils, δ r /φ r is likely to pass beyond unity, especially for the latter. For bentonite, values between about 1.3 and 1.5 were obtained for rough steel and concrete interfaces. On the other hand, for Xeuilley silt, ratios of about 1.1 were obtained for SI3 and SI4. It is noted that similar results, with about 25% increase of interface friction angle compared to internal friction angle, were also reported by Tsubakihara & Kishida (1993) regarding tests conducted by means of direct shear box on a reconstituted marine clay sheared to a displacement of about 10 mm especially for interfaces for which R max is greater than 10µm (L a =0.2mm). It is also worth noting that for both soils mentioned above, the major mineral component is montmorillonite and thus such result is likely to be attributed to mineralogy and roughness degree.

13 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING Both steel and concrete rings asperities embed into the samples tested resulting in failure planes formed just outside the embedded face of the solid material thus, a soil-soil failure surface resulted. It is hypothesized that as a result of montmorillonite particles being very small, the clay matrix adjacent to the interface through a more or less thick zone is likely to be dragged when large deformations take place, resulting in interface friction higher than internal values. For Xeuilley silt because montmorillonite percentage is less important (20%), there is only a slight increase (about 10%). The variation of τ r /σ n with R a, which is given in Figure 9, shows that the friction coefficient increases with R a. The increase tendency is more or less the same for K and B. However for X and XK, it is different. The plot also shows that the correlation between the interface friction coefficient at large displacement and R a is almost unique for both clays studied, regardless of the kind of interface material (steel or concrete). Nevertheless, this was not the case as far as X and XK are concerned. It is also evident that X and XK have higher shear strengths, presumably because of particle dimensions. This is illustrated in Figure 10 which shows a marked influence of D av on τ r /σ n. It is noted that the friction coefficient variation range decreases with the average diameter. Thus indicating that for very small diameters, roughness has a small influence and the bigger the particle diameters the more important are the friction coefficients are also analysed in Figure 11 which shows the variation of δ r /φ r with the relative roughness R which combines the influence of both R a and D av. It is shown that the residual friction coefficient increases with increasing relative roughness. It seems that there are two distinctive relationships when the values of R are greater than about 0.3, one corresponding to X and B and which is likely to go beyond one, and the second one corresponding to XK and K for which the upper limit is unity. It is worth noting that each relationship is related to a major mineral component: montmorillonite for the first one and kaolinite for the second one. This distinction is clearer for R values above unity. For the curve corresponding to K and XK, δ r is not influenced by roughness when D av is equivalent to R a (i.e. for R=1), whereas regarding the second curve, corresponding to B and X, the influence limit corresponds to R=3 approximately, in other words when R a 3D av.

14 76 F. Hammoud and A. Boumekik τ r / σ'n BSI KSI XKSI XSI BCI KCI XKCI XCI R a (µm) Figure 9. Effect of roughness τ r / σ'n soil-si4 soil-si3 soil-si2 soil-si1 soil-ci2 soil-ci D av (µm) Figure 10. Relationship between τ r /σ n and D av From these results are analysed, three modes of interface shearing, at large displacement, are postulated: - a mode in which sliding at the interface takes place for smooth interfaces. In this mode, the friction coefficient is clearly smaller than that obtained in the case of a soil-soil shearing. Sliding is confined to the interface and only particles close to the interface surface are involved in the interaction between the soil and the interface. As a result a

15 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING thin strength shear surface of oriented platy particles develops which gives a major difference between soil-soil and interface resistance for very smooth surfaces. Sliding occurs along a solid material-soil contact surface as long as R is smaller than a critical value equal to about a mode in which shearing takes place within the soil if the value of R is greater than 1 (for K and XK) or 3 (for B and X). The effect of roughness is then practically negligible. δ r /φ r is upper-bounded by unity for kaolin and XK whereas for bentonite and Xeuilley silt samples it may be greater than one. In this mode, shearing at the interface resulted in the development of a horizontal thin slip surface within the body of sample. Thus indicating that if deformation is continued long enough, a failure plane is formed and may divide the soil into two parts which may slide, one over the over, along the slickensided planes. In this mode more particles in the sample (not limited to particles close to the interface surface) tend to be involved in the shearing process. - an intermediate mode for interfaces of intermediate roughness in the range or 3. In this mode there is a clear increase of interface friction with roughness, as indicated in Figure 11. It is suggested that a thin layer of soil is likely to stick to the solid material in places causing both soil soil as well as soil solid materials shearing to take place simultaneously. δ'r / φ'r 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 BSI 0,01 0, R KSI XKSI XSI BCI KCI Figure 11. Variation of δ r /φ r with R The modes of interface shearing suggested above agree quite well with the results obtained by Tsubikihara et al. (1993) who tested cohesive soils against mild steel interfaces, using a direct simple shear type of test apparatus, for displacements of 15 mm and concluded that the frictional behaviour can be classified into three failure modes : full sliding at the interface; shear failure within the soil; and mixed behaviour where interface sliding and shear deformation of the soil specimen proceed simultaneously. For the shearing

16 78 F. Hammoud and A. Boumekik at the interface soil - steel with values of R max (L a = 0.2 mm) of 20 and 30µm, interface sliding and shear deformation proceed simultaneously. 6. CONCLUSIONS Friction at the interface between cohesive soils in one hand and steel or concrete of different roughness on the other hand, under consolidated drained condition has been studied using a torsional shear type of test apparatus. The following conclusions may be drawn: The Bromhead ring shear apparatus can be modified to measure the shear strength at the interface between cohesive soils and solid materials. Steel and concrete interfaces have been solidly fixed in order to conduct interface tests. The main advantage of a ring shear test is that large shearing displacements can be applied in one direction until the establishment of residual conditions. Kaolin and bentonite tested exhibit the sliding shearing mode and have low residual strengths for both interface and soil-soil ring shear tests whereas X and XK soils gave higher values. It is possible to suggest that if a transitional residual mode of behaviour, which involves a combination of both turbulent and sliding modes shear, is sheared against a smooth interface (SI1, SI2 and CI1): the residual conditions can be altered to a sliding shear mode involving a low residual shear strength in comparison to the soil sheared alone. This is demonstrated by values given by X and XK soils. The soils studied showed different stress-tangential displacement behaviour with respect to roughness and average diameter of particles. The relative roughness combines between the effect of roughness and mean diameter of particles. More or less good correlations are obtained between R and the residual friction coefficient with respect to the major mineral component (i.e. montmorillonite or kaolinite), delimiting three modes of shearing at the interface. A good correlation is also obtained between R a and τ r /σ n. The latter shows different trends of increase of τ r /σ n with roughness. Thus indicating that the shearing resistance at the interface depends on the interface roughness, as well as on the properties of soils. The constitutive material does not seem to have a significant effect for kaolin and bentonite. The results obtained showed that the characteristics of soil-concrete and soil -steel shearing are similar for kaolin and bentonite. Interface shearing regarding X and XK, seems to be affected by interface material. For the residual shear strength at the interface between cohesive soils and construction materials, there existed limiting values of relative roughness, which may depend on mineral composition. When the solid material surface was smoother than the lower value, a sliding shearing mode at the interface took place. When the relative roughness exceeded the upper value, however, a shearing mode within the soil occurred instead of interface sliding. The maximum interface shearing resistance was upper-bounded by the shear strength of the soil, for kaolin and the mixture Xeuilley silt kaolin. For bentonite and Xeuilley silt, however, interface shear strength did not agree with upper limit of the maximum shearing resistance, which was obtained when shear failure occurred within soil specimen. This was thought to be due to the presence of montmorillonite as major mineral component. Finally, when the relative roughness was situated between the two limits mentioned above an intermediate mode involving sliding and internal shearing was likely to take place.

17 EXPERIMENTAL STUDY OF THE BEHAVIOUR OF INTERFACIAL SHEARING Acknowledgements: The work described in the present paper was carried out at LAEGO, National High School of Geology at Nancy (France). The author gratefully acknowledges the facilities provided and expresses his thanks to the laboratory technicians and staff for their helpful suggestions. REFERENCES 1. Bishop, A.W., Green, G.E., Garga, V.K., Andersen, A. & Brown, J.D. A new ring shear apparatus and its implication to the measurement of residual strength, Géotechnique, No. 4, 21(1971) Bond, A.J. and Jardine, R.J. Effects of installing displacement piles in a high OCR clay, Géotechnique, No. 3, 41(1991) Bromhead, E.N. A simple ring shear apparatus, Ground Engineering, No. 5, 12(1979) Hutchinson. H.G., Bromhead. E.N., and Lupini. J.F. Additional Observations on the Folkeston Warren landslides, Quarterly Journal of Engineering Geology, 13(1980) Kishida, and Uesugi, M. Tests of interface between sand and steel in the simple shear apparatus, Géotechnique, No. 1, 37(1987) La Gatta, D.P. Residual strength of clays and clay shales by rotation shear tests, Harvard Soils Mechanics series, No. 86, Cambridge, Massachussets, University of Harvard (USA), Lemos, L.J.L. and Vaughan, P.R. Clay-interface shear resistance, Géotechnique, No. 1, 50(2000), Lupini. J. F., Skinner, A. E. and Vaughan. P. R. The drained residual strength of cohesive soils, Géotechnique, No. 2, 31(1981), Paikowsky, S.G., Player, C.M., and Connors, P.J. A Dual interface apparatus for testing unrestricted friction of soil along solid surfaces, Geotechnical Testing Journal, No. 2, 18(1995) Skempton, A.W. Residual strength of clays in landslides, folded strata and the laboratory, Géotechnique, No. 1, 35(1985) Subba Rao, K.S., Allam, M. M. and Robinson, R.G., Interfacial friction between sands and solid surface interfaces, Geotechnical Engineering, Proceedings of the Institution of Civil Engineers, 131(1998) Tika-Vassilikos, T.E. Clay-on-steel ring shear tests and their implications for displacement piles, Geotechnical Testing Journal, No. 4, 14(1991) Tika, T.E. Ring shear tests on a carbonate sandy silt. Geotechnical Testing Journal, No. 4, 22(1999) Tsubakihara, Y. and Kishida, H. Frictional behaviour between normally consolidated clay and steel by two direct shear type apparatuses, Soils and Foundations, No. 2, 33(1993) Uesugi, M. and Kishida, H. Influence factors of friction between steel and dry sands, Soils and Foundations, No. 2, 26(1986)

18 80 F. Hammoud and A. Boumekik 16. Voight, B., Correlation between Atterberg plasticity limits and residual shear strength of natural soils, Géotechnique, No. 2, 23(1973)