Residual shear strength of clay-structure interfaces

Transcription

1 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: 03 6 Residual shea stength of clay-stuctue intefaces Amma Rouaiguia Civil Engineeing depatment, faculty of Engineeing, Najan Univesity, P.O. Box 1988, Najan, Saudi Aabia ouaiguia@yahoo.com Tel: Abstact The inteface between stuctues and soils is a citical poblem in geotechnical engineeing. Undestanding the shea stength of soil-stuctue intefaces is impotant in detemining wall o shaft fiction fo etaining walls and piles, ancho ods, deep foundations, einfoced eath, and buied pipelines. This study deals with laboatoy tests on six diffeent clays, thee Algeian clays namely: Kaolin1, Kaolin2, and Kaolin3. Thee Bitish clays namely : Keupe mal, London clay, and Lias clay. The testing pocedues included modified diect shea tests. Each clay was sheaed alone, unde nomally consolidated dained conditions, and against both Sandstone ock and glass. These tests defined the minimum esidual stength obtained in each case and povided a basis fo a compaison with othe published eseach. It is demonstated that the esidual stength depends mainly on the inteface mateial and its oughness, the popeties of the soil, and the magnitude of the clay faction. The minimum value of esidual stength was obtained with the clay sheaed against glass. It is concluded that the shea-displacement behaviou of clay-stuctue inteface is simila to that of soil-onsoil. have occued in the field leading to the fomation of shea planes, knowledge of the esidual stength will be equied fo design puposes. The dained esidual shea stength of cohesive soils is a cucial paamete in evaluating the stability of pe-existing slip sufaces in new and existing slopes and design of emedial measues Stak & Eid [2]. The ultimate sheaing esistance at the inteface between soils and solid mateial is elevant to the stability of fiction piles, as shown schematically in figue1, etaining walls, ancho ods, eath einfocement, submaine pipelines, offshoe gavity stuctues and geomembanes. It is geneally diffeent fom the esidual stength of the soil itself, and depends on the inteface mateial and its oughness as well as on the popeties of the soil, the gain size distibution and shape of the soil paticles, the magnitude of the nomal stess and the ate of shea displacement Lemos & Vaughan [3]. Keywods: Residual shea stength, laboatoy tests, modified shea box appaatus, soil-stuctue intefaces. 1. Intoduction One of the most difficult poblems encounteed in geotechnical engineeing and constuction is how to ensue stability of the slopes of cuttings and etaining walls, and consequently how to assess the limiting stength of soil. The esidual shea stength is mobilized afte the application of lage shea displacements in dained conditions. Its pactical significance to slope stability was fist ecognized by Skempton [1], who poposed the concept of esidual stength to long-tem slope stability analysis. Theefoe, if pevious lage movements Figue 1. Influence of soil-pile fiction on pile capacity. Most of the eseach wok on soil-inteface shea behaviou epoted in the liteatue has been done on sands. Pevious studies dealing with the shea esistance of sand sliding on an inteface is dependent on the oughness of the contact suface with espect to the size of the sand, sand type, nomal stess, density of the sand and ate of displacement. Sand gains tend to slide on vey smooth sufaces, giving skin fiction angles as low

2 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: 03 7 as 10 ο Yoshimi and Kishida[4], Uesugi et al. [5]. Most of these investigations wee caied out with diffeent expeimental appaatus such as: diect shea tests have been used to study the behaviou of soil-stuctue intefaces. Seveal factos such as stuctual mateial, soil popeties, and suface oughness have been investigated to bette undestand thei effects on the inteface chaacteistics (Kulhaway and Peteson [6]). Yoshimi and Kishida [4] utilized a ing tosion appaatus fo inteface testing and obseved sand defomation by using x-ay photogaphy. Yin et al.[7] conducted a lage shea test to obseve the distibution of elative displacement along the inteface. Fost et al.[8] studied the evolution of the stuctue of sand adjacent to the geomembane, and found that it was diectly influenced by the suface oughness. Paikowsky et al.[9] developed a dual inteface testing appaatus that allowed measuement of fiction distibution along the inteface. Using nomalized oughness R n and oughness angle α, the inteface was categoized into thee zones: smooth, intemediate, and ough. As pointed out by Boulon [10], the fictional behaviou between sands and solid sufaces is contolled mainly by the complex phenomena that develop within a vey thin laye of soil (inteface) close to the contact aea. This laye of soil can be consideed as a zone of intense localisation of shea stains Cichy et al.[11], and the suounding soil can be thought of as a estaining elastic medium. Xue et al.[12] have studied the sandstone-concete joints by using a lage diect shea machine (with sample size up to 600 mm in length) unde a ange of constant nomal stiffness and initial nomal stess conditions. They found that significant wea of the sandstone suface occus duing shea displacement, and this wea has a significant affect on the behaviou of the joints. Bandis et al. [13] showed that the chaacteistics of joint sufaces, especially the hadness and oughness of such sufaces, have a majo influence on the shea stength of joints. A numbe of studies have shown that the suface topogaphy is impotant in the behaviou of soilstuctue intefaces [14], [15]. The soil-inteface sheaing esistance is nomally slightly less than the stength of the soil alone, and tends to decease with deceasing suface oughness [16]. It is woth noticing that most of the pevious investigations have studied the inteface shea between sands and solid sufaces. Howeve, the amount of data on the inteface shea between clays and solid sufaces is significantly smalle. 2. Expeimental Pogam and Testing Pocedues This study has been caied out by using a modified sheabox appaatus, designed and built in the laboatoy, which allows the test specimen to be sheaed continuously to displacements lage enough to establish esidual conditions. The movement of the box and the sheaing foce developed ae ecoded automatically fom tansduces by using data logge systems. The wok compaes the shea stength of soil intefaces (Sandstone ock and glass) with the same soil when sheaed alone. 2.1 Desciption of the clays used London clay The London clay used in this investigation is obtained fom a site in Essex (U.K). Block samples wee taken fom the base of a tench at depths of between 2 and 3 metes, and consisted of a bown, fim clay. The soil classification and popeties of London clay ae given in Table 1 and Table 2. The clay minealogy of London clay is dominated by Smectite, Illite, Mica, and Chloite (see Table 3) Lias clay The block samples of blue-gey Lias clay is taken fom a limestone quay nea Southam in southen Wawickshie (U.K). The blue gey deposit is vey had, and exteme diffulty has been encounteed in pepaing satisfactoy undistubed samples fo testing due to the tendency fo the mateial to open along bedding planes. It consists mainly of clays and shales with occasional bands of limestone and ionstone. Block samples ae taken fom depths of between 12 and 15 metes below gound level. The soil classification and popeties of this Lias clay ae given in table 1 and Table 2. The clay minealogy consists of illite, Mica, kaolinite and Chloite, the most dominant being Illite and Mica (see table 3) Keupe mal The Keupe Mal is a heavily ove-consolidated deposit beaking along joints and fissue-planes with a stacky factue. The keupe mal used in this study is oiginally obtained in powdeed fom a

3 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: 03 8 local Supplie. The soil classification and popeties ae given in Table 1 and Table2. The clay minealogy compositions using X-ay diffaction shows that the keupe mal used in this study has Chloite as the dominant mineal with taces of Illite and Mica (see Table 3) Kaolin1, kaolin2, and kaolin3 These thee kaolin samples ae obtained fom an aea which is situated in the east of Algeia, Noth Afica. These mateials ae used geneally fo the manufactue of pottey and ae emoved fom a kaolin quay. Kaolin1 and kaolin2 ae simila, both being white in colou, soft and smooth. Howeve Kaolin3 is between blue and black in colou, is had and it looks as though. The soil classification and popeties ae given in Table 1 and Table2. The clay minealogy of all the Algeian clays ae dominated by kaolinite with few taces of Illite and Mica (see table 3). The standad soil classification tests ae caied out in accodance with B.S. The pipette method being used to detemine the paticle size distibution. Figue 2 and figue 3 show the paticle size distibution fo Algeian and Bitish clays. Table 1. Popeties of Clays used in this study AC1 AC2 AC3 LonC LiaC KM using X-ay diffaction Minealogy Composition using in dominant ode Kaolinite (odeed layes) ; Illite/Mica (vey little) Kaolinite (bette odeed layes) ; Illite/Mica (vey little) Kaolinite (pooly odeed layes) Illite/Mica (moe in compaison to Kaolin 1 and kaolin 2) Smectite, Illite/Mica, and Chloite Illite/Mica, Kaolinite, and Chloite Chloite and Illite/Mica LL (%) PL (%) PI (%) A c AC AC Clay F. M. C. F. M. C.. Silt Sand Figue. 2. Paticle size distibution fo Algeian clays. AC LonC LiaC KM Table 2. Gain size distibution (Accoding to Bitish Standad) Sand (%) Coase Silt (%) Medium Silt (%) Fine Silt (%) Clay (%) AC AC AC LonC LiaC KM Table 3. Clay Minealogy Composition

4 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: 03 9 Clay F. M. C. F. M. C. Silt Sand Figue3. Paticle size distibution fo Bitish clays. Figue 4. Plasticity chat showing esults of Attebeg limits on Algeian and Bitish clays As shown in figue 4, the Algeian clays fall into the MH (high-plasticity silt) goup. Howeve, Lias clay and Keupe mal fall into CL (low-plasticity clay) goup, and London clay falls into CH (highplasticity clay) goup. 3. Desciption of the intefaces used Two intefaces ae used: glass inteface and sandstone ock inteface of 100 x 100 x 5 mm dimension each wee pepaed by cutting a piece fom both sandstone ock and glass into the equied size by using a special cutte as shown in figue 5. deposited by moving wate o by wind. Some of the sandstone ae so homogeneous and soft that they ae capable of eceiving most elaboate caving and filigee wok. The colo of the ock is lagely detemined by the cementing mateial - ion oxides poduces ed o eddish-bown sandstone, and the othe mateials poduce sandstone in white, gayish o yellowish sandstone. The chemical constitution of sandstone is the same as that of sand, the ock is thus composed essentially of quatz. The natual cementing mateial that binds the sand togethe as ock is usually composed of silica, calcium cabonate, o ion oxide. Chemically sandstone is vey esistant Mono-Minealic ock, with silica as the pincipal. The pecentage of each constituents is as follows : SiO % ; Ion (Fe2O3) 1.5%-1.6% Alumina (Al2O3) 1.4 to 1.5% ; Soda (Na2O) and Potash (Ko) 1.0% to 1.2% Lime (CaO) 0.8% to 0.9% ; Magnesia (MgO) 0.2 to 0.25% ; Loss On Ignition (LOI) 1.0% to 1.2% Sandstone physical popeties The physical popeties of sandstone includes following : Colo: The colou vaies fom ed, geen, yellow, gay and white. The vaiation is esult of the binding mateial and its pecentage constituent. Wate Absoption: The capacity of wate absoption is not moe than 1.0% Hadness: Lies between 6 to 7 on Mohs Scale Density 2.32 to 2.42 Kg/m 3 Poosity: The poosity vaies fom low to vey low. Compessive Stength: Vaies fom 365 to 460 Kg/m 2 4. Modified shea box test Figue 5. Intefaces used in this study Sandstone is a sedimentay ock composed of small gains cemented by siliceous, felspathic, o calcaeous cementing mateial. The duability of ock depends on the cementing mateial. Sandstone is often fomed in layes and has vaied applications as building stones. This coase-gained sedimentay ock is fomed by the consolidation and aggegation of sand and held togethe by a natual cement, such as silica. It is an extemely had and tough mateial and consists of consolidated masses of sand The need fo paticula laboatoy test techniques, such as those fo achieving lage stains necessay to measue the esidual shea stength, has become appaent. In ode to save technician time and educe the difficulties of intepetation by obtaining consistent test esults which is a vital step. The eseach wok has been caied out by using a modified shea box appaatus, designed and built in the laboatoy, which allows the test specimen to be sheaed continuously to displacements lage enough to establish esidual conditions. The nomal stesses wee applied by using an electo-pneumatic convete contols the pessue of the ai supply, the

5 Inteenational Joun nal of Civil & Envionmental E l Engineeing IJCEE-IJENS I V 10 No: 033 Vol: ooutput pessuue being popotional to t the diectt c cuent suppplied. The co onvete givees a pessuee o output of kpa. Fig gue 6 showss the electo-p pneumatic convete wh hich contols the appliedd n nomal stessses. F Figue 6. Shows the electo-pneeumatic convetee whichh contols the app plied nomal steesses. IIn this appaatus, the geaas have emooved and thee m moto has been b eplaceed by a stteppe moto c contolled diiectly fom the t data loggge by fixingg t numbe of the o pulses necessay to divee the moto too t equied distance the d (see figue 7). 10 The ate of sheaa fo the moodified shea box was fixedd to mm/min, a value that was w found by peliminay p t tests to ensue dained conditions thouughout the tesst. 5. In nteface testts An extensive poogam of tessts was condducted to exam mine the peakk and esidual stength, andd to study the shea-displace s ement behaviiou of six claays when sheaaed alone annd against two t plane inntefaces. Evey clay is shheaed againsst sandstone ock and againnst glass. Thhe sheabox is assembleed in the testinng machine, with the saandstone occk in the bottoom half and thhe clay placed in the top half h of the box. A poous sttone is placedd above the specimen s and below the saandstone ockk and gentle pessue on thhe uppe pooous stone foced the speciimen into the coect c positioon fo testingg. The loading head is then assembled. The nom mal stess is applied thouughout the tesst by using thhe mico-comppute and the sample is shheaed with ate of shea fixed to mm/miin fo both fowad fo and backwad b sheaa cycles. Fo the tests off clay sheaedd against glasss, the poceddue of testingg is the same, except that smooth platee glass insteadd of sandstonne ock is locatted in the boottom half of the box. Afte A five compplete cycles of shea, thee dive is diismantled and the suounding wate in i the wate bath is dainned. The nom mal stess is emoved autom matically thouughout the compute, the sheaabox is disseembled and excess e moisttue emovedd pio to studyying the sheaa suface. A pimay puposse of this studdy is to investtigate the esiddual stength of the intefaaces and thei elative esiddual stengthh compaed to the clay sheaed alone. Fo this eason tests aee caied out using the samee ate of sheea and the same s nomal stesses. Thesse tests have an applicatioon in the studdy of the intecalation betw ween had oock layes, whee w the pobblems of zonees of weakneess in ock masses m is signiificant. Such a poblem caan occu in naatual and atifi ficial slopes,, open cutts and exccavations, founndations andd abutmentts of dam ms, and undeegound opennings. 6. Su uface ougghness F Figue 7. Diecct connection bettween the moto and the dive unit. The impotance of o the degeee of suface oughness at innteface behavviou and soiil-stuctue innteaction espoonse, have been b noted by b many inveestigatos (e.g., [17], [18], [15].

6 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: The suface oughness can be measued with a Taylo-Hobson Talysuf stylus pofilomete. A schematic of the pofilomete is shown in figue 8. As the figue illustates, the pofilomete comes equipped with a diamondd stylus tip that tavesess the suface to be chaacteized. Tansduces in the pofilomete help ecod the vetical and hoizontal coodinates of the styluss tip. Theefoe, the aw suface pofile data consists of a seies of discetee coodinate pais (x, z) that can be post-pocessed and analyzed to obtain the suface topogaphy chaacteization of the mateial investigated [15]. The suface oughness values pesented by the aithmetic aveage oughness (Ra) of some inteface mateial ae summaized in table 4. Table 4. Suface oughness values of some inteface mateial. Inteface mateial type Aithmetic aveage oughness (Ra) ; (μm) Glass (vey smooth) Concete 28.0 Steel 5.7 Sand blasted with coase 7.0 sand Sandstone ock 10.0 Note: 1 (μm) = mmm 7. Discussion of Test esults Figue 8. Stylus pofilomete sketch [21]. Figue 9. Gaphical epesentation of oughnesss paametes Rt, Sm, and Ra. Suface topogaphy was quantified by theee commonly used oughness paametes: the maximum peak-to-valley height (Rt), the aveagee mean line spacing (Sm), and the aithmetic aveagee oughness (Ra). Sm is twice the mean distance between locations at which the pofiles coss the centeline dawn though the centoid of the pofile. Ra is the aithmetic aveage value of the pofile depatue fom the mean line along the pofile length. These thee oughness paametes ae illustated in figue 9. Moe detailed discussion of these paametes is given in Intenational Oganization fo Standadization (ISO) Standadd 4287 (ISO 1997)[19]. All the soils behaviou is discussed in tems of effective stess. The esults of tests pefomed thus give moe undestanding to the poblems of stuctues whichh ae subjected to lage elative displacements, and the effect of othe phenomena such as the stength of shea zones on the sides of diven piles. Tomlinson [20], in his significant contibution to the undestanding of the behaviou of piles in clay ove the yeas, ageed that it was impotant to focus on the behaviou of a thin laye of soil adjacent to the pile. An attempt has also been made to obtain a moe complete undestanding of the diffeence which existss between the clays sheaed alone and the clay-stuctue intefaces. The esults of all these investigations ae discussed below. 7.1 Discussion on the esults of Intefacee tests The pupose of this section is to epot the tests in ode to investigate the shea stength of intefaces. The stess-displacement cuves (kaolin1 on kaolin1) fo clay sheaed alone ae shown in Figue 10(a) fo fowad cycles and figue 10(b) fo backwad cycles.

7 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: Figue 10(a). Stess-displacement elationship fo Ac1/AC1 Using Modfied Sheabox at mm/min. Figue 11(a). Stess-displacement elationship fo Ac1/Rock Using Modfied Sheabox at mm/min. Figue 10(b). Stess-displacement elationship fo Ac1/AC1 Using Modfied Sheabox at mm/min The stess-displacement cuves fo the inteface (kaolin1 on sandstone Rock) ae shown in Figue 11(a) fo fowad cycles and figue 11(b) fo backwad cycles. Figue 11(b). Stess-displacement elationship fo Ac1/Rock Using Modfied Sheabox at mm/min. The stess-displacement cuves fo the inteface (kaolin1 on Glass) ae shown in Figue 12(a) fo fowad cycles and figue 12(b) fo backwad cycles.

8 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: clay sheaed against glass although the oughness is not the same. This can be explained by the paticles have not been displaced a sufficient distances elative to thei initial contact positions. Lupini et al..[21] pesented an extensive study on the esidual stength of cohesive soils as measued in the ing shea appaatus. They found that esidual stength measued at slow dained displacement ates esulted fom thee types of sheaing mechanism. Figue 12(a). Stess-displacement elationship fo Ac1/Glass Using Modfied Sheabox at mm/min. Figue 12(b). Stess-displacement elationship fo Ac1/Glass Using Modfied Sheabox at mm/min. It is woth noticing that the cuves fo inteface tests (Figues 11 and Figue 12) ae simila except that the dop in stength fo soil sheaed against eithe sandstone ock o glass occus quickly afte the peak stength is eached. This is explained by the plane suface facilitating the eoientation of clay paticles and the destuction of the bond between paticles duing sheaing being aligned in the shea zone quickly. The peak shea stength is eached at about 0.1 mm fo both clay sheaed against sandstone ock and In the fist mechanism lage stain involves otation of the otund paticles, as in ganula soils, and paticle oientation has a negligible effect. This mode of defomation is temed tubulent shea. If a high popotion of clay paticles is pesent, a continuous oientation shea suface can fom between any otund paticles. This mode of defomation is temed sliding shea. At intemediate popotions of clay paticles, oiented shea sufaces can patly fom, but ae continuously disupted by the otund paticles. This mode of shea is temed tansitional shea. Tubulent esidual shea has thus been defined as the state of esidual shea at constant volume fo which no paticle oientation occus. In this case soils that shea at esidual conditions exhibit typically high esidual stengths with φ in excess of 25 degees. The esults epoted heein indicate that all the esidual shea stengths ae unde 25 degees, which in tun indicates that the samples exhibit eithe sliding shea mode o tansitional shea mode. It can thus be stated that paticle oientation is involved in all of the shea mechanisms and this will lead to a esidual state being eached at lage displacements. The sliding shea mode is chaacteised by a shea suface that is fomed by stongly oiented clay paticles and usually has a low esidual fiction angle (typically in the ange fom 5 to 12 degees). The highest esidual stength angle, φ fo Kaolin3 is typically 20.8 degees, when sheaed in the modified sheabox. An explanation of this high value could be attibuted to the minealogy of Kaolin3, which is entiely dominated by kaolinites. This was shown by Lupini et al.[21] who tested soils of diffeent minealogies, and found that Montmoillonite soils had the lowest esidual fiction angle, and Illite o kaolinite soils the highest. With egad to this the esults indicated that all the thee Algeian clays contained pedominantly

9 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: kaolinite, and the esidual fiction angles ae 11.8, 17.6, and 20.8 degees fo Kaolin1, Kaolin2, and Kaolin3 espectively. Despite the fist clay having a slightly lowe value in compaison with the othe two clays, such a elationship is not always tue. The Lias clay with Illite constituting the dominant mineal, gave φ =12.1 degees. Fom which it can be seen in fact that thee is only a elatively small diffeence, between this value and the value of Kaolin1. In this study, Kaolinitic soils gave esults anging between 11.8 and 20.8 degees. Despite keupe mal not being dominated by eithe Illite o by kaolinite, it gave a high esidual stength angle of 20.4 degees. As fa as the clay faction is concened, keupe mal is mainly dominated by silt paticles which ae not platelets, as epoted by Mitchell [22] elated esidual stength to paticle shape. They found that low esidual fiction angles ae associated with platy paticles, and that subangula and needle-shaped paticles gave high esidual fiction angles. Regading these findings, and that because silt contains many ounded paticles, the mechanism of failue involved paticle olling and tanslation, athe than diect sliding, this being pevented by intelocking of the paticles. It is theefoe, possible that duing shea, the continuous oiented planes ae inteupted by the silt paticles, such that the silt paticles gave high esidual fiction angles. In contast, London clay has the lowest esidual stength angle of φ =8.2 degees, fo which a possible explanation could be elated to the high clay faction which is 55%. The low esidual fiction angle is associated theefoe, with good oiented bands of high clay faction, Pefeentially oientated. This poduced a low esidual stength duing shea. As mentioned ealie, concening the low fiction angles (typically in the ange fom 5 to 12 degees) it seems that only kaolin1, London clay, and lias clay values lie in this stage. The dop in stength post-peak is found to occu quicke with the glass inteface than with the ock inteface. The elatively quick dop in stength with the glass inteface can be explained by the smoothness of the suface paallel to the plane and hence to each othe. Evidence that the fomation of oientation domains begins at elatively small stains was achieved by Goldstein et al.[23]. Thee is also such evidence fo the pesence of continuous bands of almost pefectly oientated paticles in clays subjected to lage stains, both in the laboatoy Astbuy[24] and in the field Skempton[1]. It is clea fom this study that the glass acts solely as an inteface fo the eoientation of clay paticles and that the smoothe the suface, the moe apid the eduction in stength and the lowe the measued esidual angle. Anothe pupose fo using the glass inteface is to find a elationship between the esults fom the modified sheabox and the Bomhead ing shea, with the aim of poducing compaable values so that the commonly available standad sheabox can be used instead of the much ae ing shea fo esidual stength testing. Kanji and Wolle [25]tested soils against had polished ock. They found that the peak shea stength τ max was lowe and occued at small displacement, and also that thee is a apid dop in stength afte the peak stength had passed. They explained this dop by stating that the had polished sufaces encouage the development of esidual stength at small shea displacement. In this study, it was found that the clay sheaed against glass gave the lowest values of peak and esidual stength, fo all six clays, compaed with ock. The diffeence between the stengths fo both sandstone ock and glass inteface tests given below in Table 5. Table 5. The diffeence between stengths of sandstone ock and glass inteface tests. Clay Δ φ = - φ (Clay-Rock) φ (Clay-Glass) Δ φ P = φ P (Clay-Rock) - φ P (Clay-Glass) (Degees) (Degees) AC AC AC LonC LiaC KM The tests have thus yielded the following anges. 1.6 ο φ (Clay-Rock) ο φ (Clay-Rock) - P φ (Clay-Glass) 4.8 ο φ (Clay-Glass) 5.1 ο P Since the development of sheas in clay is accompanied by paticle oientation, the diffeence between the two intefaces could theefoe be attibuted to the fact that the smooth aea of glass pemits the clay paticles to be moe stongly oientated in the diection of movement than the sandstone ock inteface. It is woth noticing fom

10 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: these diffeences that the smooth inteface, against which the paticles have attained thei maximum degee of oientation, must possess the minimum possible esistance to shea, which is defined as the esidual stength of the clay. Fom this examination it is easonable to suppose that the inteface leads to the eady destuction of the cohesion and that thee is a stong oientation of clay paticles paallel to the suface of shea. Futhemoe, clays tested against a smooth inteface (ock o glass) show lowe stength values. This eduction in stength could be explained lagely by the oientation of paticles along the shea zone, due to the smoothness of the plane suface of the intefaces. The stength values obtained by the Bomhead ing shea fo clay-on-clay ae found to lie in between the stength values of clays tested against the smooth plane suface (ock o glass), with the lowest values obtained with the clay sheaed against glass. This coelation shows definitely the geat ole played by the eoientation of the paticles duing shea. The absolute esidual stength values as a pecentage of the esidual Bomhead ing shea values ae given below in table6. Table 6. Residual stengths measued in the MSB as a pecentage of Bomhead ing shea values. Clay AC1-AC1 % AC1- Rock % Clay-Glass % AC AC AC LonC LiaC KM It is clea fom table 6 that the ange fo clay-clay is highe than the two esidual inteface tests, which is between 121% - 139%, wheeas fo ock and glass, the esults lie between 106% - 114% and 85% - 95% espectively. disaggegate upon sheaing especially at low nomal stess values. Accoding to the plasticity chat, the Bitish clays classified as CL-CH goups. The nonlineaity of the esidual failue envelope fo London clay ageed with the findings of Stak and Eid [26] that the nonlineaity of the esidual failue envelope is significant fo cohesive soils with a liquid limit between 60% and 220% and a clay faction geate than 50% whee the London clay (liquid limit is 87% and clay faction is 55%) fulfil the condition suggested by stak and Eid [26]. Howeve, Keupe mal shows nonlinea esidual failue envelope although have liquid limit less than 60% and clay faction of 39% and is classifies as CL goup. It is woth noticing that the Algeian clays showed a linea failue envelope and they can be fitted into the linea egession with R 2 equal to although these clays ae classified as MH goup. London clay and Keupe mal can be fitted into the linea egession with R 2 equal to 0.97 and 0.98 espectively. Residual shea stess (kpa) Effective nomal stess (kpa) Figue 13. Dained esidual shea envelope fo all clays (by using modified sheabox tests) AC1 AC2 7.2 Residual shea envelope Figue 13 pesents the dained esidual stength failue envelope fo the six clays tested duing this study. It can be seen that the dained esidual stength envelope is nonlinea. This nonlineaity is moe significant fo Bitish clays especially fo effective nomal stess between 100 kpa and 200 kpa. This nonlinea behaviou may be caused by the pesence of weak patly weatheed paticles that

11 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: Residual shea stength (kpa) Ac1 /SS R AC 2/S SR 4. Conclusions The following conclusions have been dawn fom this study : The shea failue envelope fo the studied Clay-stuctue intefaces obeys the Moh Coulomb failue citeion. It was found that the mode of shea mechanism in geneal fo clays/stuctues intefaces was changed by the stuctue suface oughness. Effective nomal stess (kpa) Figue 14. Dained esidual shea envelope fo clay against sandstone ock inteface. Residual shea stength (kpa) Effective nomal stess (kpa) Figue 15. Dained esidual shea envelope fo clay against sandstone ock inteface. AC 1/G L AC 2/G L Figue 14 and figue 15 pesent the dained esidual failue envelopes fo the six clays sheaed against the sandstone ock and against glass espectively. It is woth noticing that the dained esidual failue envelopes fo clays sheaed against intefaces(sandstone ock and glass) gave less nonlineaity compaed to clay-on-clay. Theefoe, it may be concluded that the nonlineaity of the esidual failue envelope fo soil-on-soil is moe geate and significant than the clays sheaed against intefaces (sandstone ock and glass). The degee of the nonlineaity of the dained esidual shea envelope is minimized especially fo the clayglass inteface which is clealy obseved at figue 15. It seems that the degee of nonlineaity is stongly dependent on the suface oughness. Fo the studied intefaces, the shea stength of the clay-on-clay is geate than that of the clay-stuctue inteface fo simila stess conditions. Results that ae pesented in this study will enhance the undestanding of the shea behaviou of clay-stuctue inteface that is cucially impotant in stability analysis of the inteface between soils and solid mateials in geneal. Fom the wok epoted on the inteface tests, it is possible to suggest that if a soil shows a tansitional esidual mode of behaviou which involves a combination of both tubulent and sliding shea and is sheaed against a smooth had inteface (e.g. glass used in this study), the esidual conditions can be alteed to a sliding shea mode involving a low esidual shea stength in compaison to the soil sheaed alone. This is demonstated by values given by kaolin1. The inteface test is a best method to obtain the esidual stength of clay soils, constituting a simple, apid and economical method. This fact is due to easie clay paticle oientation at the vicinity of the contact with the intefaces. Although this study is pefomed on a limited ange of inteface oughness. It is believed that the inteface oughness has a geat influence on clay-stuctue shea behaviou. Futhe eseach is needed to povide a bette undestanding of the

12 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: behaviou of the clay-stuctue inteface fo a wide ange of suface oughness. Abbeviations MSB : Modified Shea Box AC1 : Kaolin 1 AC2 : Kaolin 2 AC3 : Kaolin 3 LonC : London clay KM : Keupe Mal LiaC : Lias Clay SSR : sandstone ock GL : Glass CL : Low-plasticity clay CH : High-plasticity clay ML : Low-plasticity silt MH : High-plasticity silt OH : High-plasticity oganic soil LL : Liquid Limit PL : Plastic Limit PI : Plasticity Index A C : Activity A C = (PI)/CF F. : Fine M. : Medium C. : Coase SSR : Sandstone Rock GL : Glass Acknowledgements The authos wish to expess thei gatitude to the technical staff of the Civil Engineeing Laboatoy at Loughboough Univesity, who always esponded pomptly and in a vey pofessional way. Refeences [1] A.W. Skempton,. Long-tem stability of clays slopes. Géotechnique, London, England, 14(2), pp , [2] T.D. Stak, and H.T. Eid. Dained esidual stength of cohesive soils. Jounal of Geotechnical Engineeing, Vol. 120, No. 5, pp ,1994. [3] L.J. Lemos, and P.R. Vaughan. Clay-Inteface shea esistance. Géotechnique 50, No.1, pp.55-64, [4] Y. Yoshimi, And T. Kishida. A ing tosion appaatus fo evaluating fiction between soil and metal sufaces. Geotech. Test. J. ASTM 4, No. 4, , (1981). [5] M. Uesugi, H. Kishida, and Y. Tsubakihaa. Fiction between dy sand and concete unde monotonic and epeated loading. Soils Found., 30, No.1, , [6] F.H. Kulhaway, and M.S. Peteson. Behaviou of sand and concete intefaces. Poc., 6 th pan Ameican Conf. on soil mechanics and foundation engineeing, Bazil, No.2, ,1979. [7] Yin, Zong-Ze, Zhu, Hong, and Xu, Guo-Hua. A study of defomation in the inteface between soil and concete. Comput. Geotech., 17,75-92, [8] J.D. Fost, S.W. Lee, and P.E. Cagill The evolution of sand stuctue adjacent to geomembanes. Poc., Geosynthetics 99, Industial Fabics Association Intenational, Boston Balkema, Rottedam, The Nethelands, ,1999. [9] S.G. Paikowsky, C.M. Playe, and P.J. Connos. A dual inteface appaatus fo testing unesticted fiction of soil along solid sufaces. Geotech. Test. J., 18(2), ,1995. [10] M. Boulon. Contibution a la mécanique des intefaces sols-stuctue. Application au fottement latéal des pieux. Diplôme dhabilitation a diige des echeches, Univesité Joseph Fouie, Genoble, Fance,1988. [11] W.Cichy, M. Boulon, and J. Desues. Edude expéimentale stééophotogammétie des intefaces sols-foundations a la boite de cisaillement diect. 4th colloque fanco-polonais de mécanique des sols appliquée. Genoble, pp ,1987. [12] F.Gu. Xue, Julian, P. Seidel, and Chis, M. Habefield. Diect shea test of sandstone-concete joints. Int. J. Geomech. Vol. 3, Issue 1, pp , [13] S.C. Bandis, A.C. Lumsden, and N.R. Baton. Fundamentals of ock joint defomation. Int. J. Rock Mech. Mining Sci. Geomech., 20(6) : , [14] J.E. Dove, and J.C. Haping. Geometic and spatial paametes fo analysis of geomembane/soil inteface behaviou. Poc., Geosynthetics 99, Industial Fabics Association Intenational, Boston Balkema, Rottedam, The Nethelands, ,1999. [15] J. E. Dove, and J.B. Jaett. Behavio of dilative sand inteface in a geotibology

13 Intenational Jounal of Civil & Envionmental Engineeing IJCEE-IJENS Vol: 10 No: famewok.. J. Geotech. Geoenvion. Eng., 128(1), 25-37, [16] L. M. Kaft, W.R. Cox, and E. A. Vene. Pile load tests: cyclic loads and vaying load ates. ASCE 107, No. GT1, [17] J. Han. An expeimental and analytical study of the behavio of fibe-einfoced polyme piles and pile-sand inteactions. PhD dissetation, Geogia Tech, Atlanta, GA,1997. [18] J.E. Dove, and J.D. Fost. Peak inteface fiction behavio of smooth geomembane-paticle intefaces. Jounal of Geotechnical Engineeing, ASCE, 125(7), pp ,1999. [19] ISO. Geometical poduct specification (GPS)- suface textue: pofile method-tems, definitions, and suface textue paametes. ISO Standad 4287, Intenational Oganization fo Standadization, Geneva, Switzeland,. (1997). [20] M.J.Tomlinson. Some effects of pile on skin fiction. Poc. ICE Conf. Behaviou of piles, London, pp ,1971. [21], J.F, Lupini, A.E. Skinne, and P.R. Vaughan. The dained esidual stength of the cohesive soils. Geotechnique, 31(2), pp ,1981. [22] J. K. Mitchell. Fundamental of soil behaviou. 2 nd Ed., Wiley, New Yok,1993. [23] M.N. Goldstein, V.A. Misumsky, and L.S. Lapidus. The theoy of pobability and statistics in elation to the heology of soils. Poc., 5 th Int. Conf. Soil mechanics, Pais, 1 : pp , [24] N.F. Astbuy. Science in the ceamic industy. Poc. Roy. Soc. A., pp. 258 : pp ,1960. [25] M.A. Kanji, and C.M.Wolle. Residual stength new testing and micostuctue. Poc., 9 th I.C.S.M.F.E., Tokyo, Vol. 1, pp ,1977. [26] T.D. Stak, and H.T. Eid. Dained esidual stength of cohesive soils. Jounal of Geotechnical Engineeing, Vol. 120, No. 5, pp ,1994.