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Downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 11/1 Stability of Natural Slopes in Quick Clays Stabilite des Pentes dans I'Argile Sensible G. AAS Civil Engineer, Norwegian G eotechnical Institute, O slo, Norway SYNOPSIS This paper attempts to explain the frequently occur ring flake- type quick clay landslides, characterize d by the ’arge volumes involved and by a sudden and very rapid fai lure along a nearly horizontal sliding surface. A conventiontional effective stress analysis based on a potential sliding surface and pore pressures p rior to failure is unable to express the risk of sliding. It seems pos sible, however, to determine a limit value of the s hear stress acting along such a sliding surface. This critical shear s tress implies that the clay will start yielding when subjected to any undrained load increment. Such critical shear stres s values have been determined by direct simple shea r tests, and are compared to the average shear stress along the slid ing surface calculated for a number of previous qui ck clay slides. INTRODUCTION vertical effective stress <Jn and a horizontal effective stress Ko*cr,j both principal stresses Statistically, at intervals of about four years, la rge quick clay slides involving clay masses of the orde r of millions of m^ occur in Norway. A nearly horizontal sli­ ding surface and a very gently inclined average slope are characteristic features of this type of earthslide. Ex­ Al ja\ perience (Aas, 1979) shows that the extent of the s lide area in the direction of sliding is of the order of 7- 15 t o times the corresponding difference in terrain level . t h t Quick clay slides frequently start without any warning and develop very rapidly embracing an area of sever al hundred thousands of m^ within a few minutes or les s. It seems to be a fact, however, that animals, like for inst­ ance horses, in some cases got nervous a few hours before the slide occurred. A possible explanation for this might be minor ground vibrations caused by stress redistr ibu­ tions in the quick clay. Quick clay slides seem to be, almost as a rule, initiated by a small increase in aver­ age shear stress caused by for instance intensive s tream Fig. 1. Stress conditions in clay element at potent i­ erosion, increased unit weight of top soil layer af ter al sliding surface before (A) and after (B) heavy rainfall, or earthworks for agricultural purp oses. applying a horizontal shear stress, The typical quick clay slide is frequently describe d in The applied horizontal force causes a shear stress Th the literature as a combination of a minor initial slide along the horizontal and vertical planes. When added to and a progressive failure process developing very r apidly the existing stresses, the applied stresses result in and spreading in all directions from the gate opened by both a change in magnitude and a rotation through an the first slide. As will be seen from examples pres ented angle v of the principal stresses. Denoting by tan ipm ob in this paper, another common failure mechanism imp lies a the maximum value of shear stress to effective normal clay volume sliding out as a huge flake. stress acting along the plane inclined ( 45*i/2ip'mob- v ) to the horizontal plane, and supposing that the str ess The paper attempts to explain these flake- type slid es change leads to an increase in pore pressure, Auw, one taking into account the quite peculiar strength proper­ may derive the following general expression for xH (Fig.l): ties of quick clays and the unique stress situation ex­ isting in the failure zone. x = cr ( D H o If one now assumes that t H is gradually increased under FAILURE MECHANISM IN QUICK CLAY fully drained conditions, yielding will take place along the critical plane when 'P'mob equals vp’ . The cor respon­ Fig. 1 shows a very idealized flake- type slide. A r igid ding stress conditions for the clay element are shown by block is displaced along a potential horizontal sli ding the circle of stress C in Fig. 2 a, and the value o f surface. Prior to being affected by the horizontal' force, xH,i»a 'S given by Eqn. 1 when inserting sinip'mob = a clay eTement on the failure surface was subjected to a sin tp' and Auw = 0. 333 11/1 of low sensitivity, lies in the different ways the two types of clay react to a sudden change in applied s tress. This is clearly demonstrated in Fig. 3 which gives data from undrained, active (compression) triaxial tests on specimens from 3 quick clay deposits and 2 clays of low sensitivity. All specimens were recondolidated to i n- situ stresses and then loaded to failure by increasing t he vertical stress. The lower diagram in Fig. 3 indicates that for all clays there exist a nearly unique relationship between changes in (a, - 03 )/( 0 | *Oj ) and subsequent changes in pore pressures uw as long as the shear stress/effective normal stress ratio does not exceed a certain value. If one assumes no cohesion intercept for these normally co nsoli­ dated clays, ( 0.-03 )/(aj'*Oj) equals sir"Pjnob and one Fig. 2. a) Stress conditions in clay element at gets from the diagram &uw/Oo'Asinvp1 - o.75 first yielding (C) and at drained failure along horizontal plane (D) At a certain value of sin*p' ob = sin, which varies b) Corresponding stress conditions if quick between 0.42 and 0.46 for tne three quick clays con sider clay ed, the rati0 Auw/cJ- Asinip1 suddenly increases drastically to values of 2.5 - 6 . The plastic clay of low sensitivi­ Due to the fact that sliding along the critical pla ne ty does not show any tendency for changing its rate of in the clay element, is not kinematically possible under pore pressure increase, while there is a very moder ate the rigid block (Fig. 1), yielding along this plane will increase in Auw/oj,- Asinip' for the lean clay of l ow sen­ not lead to a sliding failure. Thus, a further incr ease sitivity at about same value of 'P^ob as for t(le q uick in is possible because a simultaneously induced in­ clays. crease in horizontal stress provide for the stabili ty of the clay element. If one again assumes drained cond itions, The upper diagram in Fig. 3 shows how the maximum s hear the circle of stress moves outwards in Fig. 2 a, at all stress in the specimens, normalized with respect to tTg , times touching the failure envelope until the horizontal 1/2 (0 ,- o3 ) _/ Oo varies with sin'Pjncib . As a consequence plane becomes the critical sliding one. Then xH = CTo t an ' > of the regular increases in pore pressures, all cla ys be­ which may correspond to twice the value of xHat sta rt of have the same way for vP(|1ob<>Pcr corresponding to yielding. This value represents the maximum possibl e A [^- (o, - o3) ] / Og. Asinvp'~ 0.75 • For the quick clays,in shear resistance that can be mobilized along a hori zon­ contrast to the other ones, the dramatic increases in tal plane for a long term condition in normally con soli­ pore pressures thereafter, lead to a rapid decrease in dated clay. shear resistance in spite of the fact that mobilize d friction angle is continually increasing towards it s This paper attempts to illustrate that the correspo nding ultimate value corresponding to ( O,1 /a j ) max shear strength along a horizontal plane that governs the long term stability in quick clay, is considerably lower The most important information to be drawn from Fig . 3 than the theoretical value given above. This is par tly is that the values of Auw/Oq- Asinip1 at mobilizati on of due to the fact that yielding in quick clay starts at a Vtr in the quick clays, exceed by far the critical mobilized friction angle ip' lower than ip' . The main value figured out previously. Consequently, in- situ reason is, however, that ip'cr represents a critica l shear elements of these clays, carrying shear stresses co rre­ stress/effective normal stress ratio (Bjerrum and Kenney, sponding to vr>'ob = >p’Cr along any plane, will yi eld at 1968) at which a further sudden increase in shear s tress once if subjected to a sudden shear stress incremen t, is impossible as the increased value of 'P'mob''S more although 'Pjnot) is substantially less than ip' . than compensated for by the simultaneous decrease i n effective stresses due to rise in pore pressures. This is The stress- strain conditions in elements along a pl ane, visualized in Fig. 2 b, which shows that the circle of horizontal or gently inclined slip surface in- situ are stress moves towards the origin without Th being changed, best simulated in the laboratory by direct simple s hear until failure occurs along the horizontal plane when tests (Bjerrum and Landva, 1966). A specimen mounted in a confining membrane, is reconsolidated to in- situ (°o -A u w ) = TH f/fan«Pj stresses and then subjected to a gradually increasi ng The critical value of the ratio between increase in pore horizontal shear stress.
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