INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING
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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 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. During the shearing phase the pressure and increased >p'mobmay be derived from Eqn. 1. vertical stress is continuously adjusted to keep the This is achieved by replacing xH with ( ih - Axh ), simp^ volume of the specimen constant, thus ensuring zero ex with(sin SHEAR STRENGTH OF QUICK CLAY The solid curves show the relationship between recorded As indicated above, the fundamental difference in stress- values of xH and vertical stress, a i during the tests. strain- strength behaviour between a quick clay and a clay The maximum value of xH which equals the undrained shear 334 11/1 Fig. 3. Results from undrained, active tri axial tests on specimens reconsoli dated to in- situ stresses strength of the clay, suq, is of the order of 0.18 - 0.22 times oi , and appears at a time when the reduction of vertical stress is 30- 45%. In Fig. 4 is shown the theoretical value of xH when ass uming Oy = constant = oj, and that the shear stress is increased under drained conditions until yielding t akes place along the critical plane. A constant horizont al stress in the specimen equal to 0.5ai was also assumed. It can be seen from all diagrams that, if yielding is assumed to be defined by = vp’cr , the corresponding value of t h =x D^cr becomes approximately equal to or slightly greater than surj. On the other hand, if y ielding requires that > the critical value of is considerably greater than sU[). Comparable direct simple shear tests on quick clay have clearly demonstrated (Bjerrum and Landva, 1966) tha t the value of th in a drained test at which strains start to increase rapidly, corresponds very well to the valu e of SuD from a conventional constant volume (undrained) test. A comparison between such results from drained and con stant volume tests is given in Fig. 5. The experien ce given above thus seems to verify that the undrained shear strength of quick clay determined also in direct si mple shear tests is defined entirely by ip’ and not by t he ultimate friction angle, >p'. cr Returning to the idealized slope instability in Fig . 1, one finds that a shear stress equal to sud acting along the horizontal plane in reality implies a safety fa ctor equal to unity, irrespective of whether t H is appli ed under drained or undrained conditions. In either ca se any sudden additional increase in shear stress would re sult in Fig. 4. Results from direct simple shear tests on immediate failure in a quick clay. quick clay specimens reconsolidated to in- situ stresses 335 11/1 HORIZONTAL SHEAR STRAIN % Fig. 7. Critical long term stress conditions in normally consolidated, quick clay. The dotted Fig. 5. Results from a drained (A) and a conven curve indicates the waytp will vary with tional constant volume (B) direct simple effective normal stress when a small stress shear test on quick clay. The specimens were increment brings the clay to complete failure cut out from a block sample taken just below the sliding surface in a slide at Skjelstad- Fig. 8 shows a corresponding slide in a somewhat overcon marken, Stjordal 14.September 1962 solidated, quick clay, unloaded by stream erosion. Here all elements along the plane sliding surface have e xperi enced almost the same stress history as the specimen in a conventional direct simple shear test, namely a rel ease APPLI CABI LI TY OF LABORATORY TEST DATA IN SLOPE STABILITY in vertical stress and an increase in horizontal shear ANALYSES stress. So, the critical value of t;-| is equal to sud de termined in a direct simple shear test consolidated to For simplicity, the assumptions made above specifie d that the original effective overburden. As the ratio Oh/ o y the sliding surface is horizontal and that a horizo ntal might well be quite near to unity under these circum shear stress is applied without changing either the ver stances, the critical value of T g (Eqn. 4) will not tical or the horizontal effective stresses. These c ondi differ very much from su0 . tions are seldom entirely satisfied for actual quic k clay slopes. The general expression for the critical long term value of shear stress acting along a plane sloping at an angle B is derived from Fig. 6 : (3 ) T Pc = CT cos2 (3 or expressed as a function of t h c r : X Pcr = t Hcr cos2 P4°v d-K') S'n2P (4) -f HQpK' -a; Fig. 8 . Critical long term stress conditions in slightly overconsolidated quick clay. The | T H cr dotted curve indicates how Tg varied with effective normal stress during the process of vertical unloading K -crv A frequently occurring type of flake slide which compri ses an eroded slope in front of a nearly horizontal plane Fig. 6 . Stress conditions in quick clay element is shown in Fig. 9. In this case the stress conditi ons de at first yielding along critical plane scribed previously for a normally consolidated and an overconsolidated clay, are quite representative for the Here oi and K1 • aj denote vertical and horizontal effec parts of the sliding surface located under the near ly tive stresses, respectively. On the basis of Eqn. 4 and horizontal area and the slope, respectively. Howeve r, near the reported experience from direct simple shear te sts, the toe of the slope the clay might at a certain ti me have one may evaluate the applicability of the data from such been locally overstressed without causing overall s lope in tests in stability analysis, as discussed below. stability. Consequently, the available shear resist ance in this zone might have passed through its maximum val ue, Fig. 7 shows a realistic type of slide in normally con sun, and as a result of yielding ended up at a cons ider solidated, quick clay. The terrain has a fairly con stant, ably smaller value, o^- tamp' as indicated in Fig. 9. gentle slope, and the sliding surface is more or le ss running parallell to the terrain surface. In this c ase From the three examples given above it may be concl uded elements along the sliding surface will have a stat e of that the necessary conditions for a quick clay slop e to stress quite similar to the specimen in a direct si mple be prone to a flake- type failure, are as follows: shear test when a horizontal shear stress xHis incr eased a) The shear strength has to be fully mobilized in the under drained conditions. The critical value of t H = active and passive zones at the head and toe of the tDifj, = suo (Fig. 4), and the corresponding value of slide, respectively. « suB • cos 2p»^j( 1-KJ) )si n 26 (Eqn. 4). A critical stability condition requires that the shear stress acting b) The average shear stress acting along a potentia l, along the sliding surface equals this critical valu e of plane sliding surface inclined a small angle 6 to the Tg at the same time as the shear strength is fully mobi horizontal has to be approximately equal to the un lized in the active and passive zones at the top and the drained shear strength measured by conventional dir ect bottom of the slide. These zones by the way may not ne simple shear tests on specimens reconsolidated to cessarily consist of quick clay. the maximum effective vertical stress experienced by 336 11/1 the clay. This undrained strength may underestimate the actual shear resistance along the sliding surfa ce X P C r / ( T o in normally consolidated clays when the angle 0>1O°. ( F rom la r g e in situ SuDA o Ibast ad.t rogst ad I (Average value from Agricultural directsimple shear eartt, works tests) J I 0 18 I Fig. 9. Critical long term stress conditions for overstressed quick clay element (A) below the toe of the slope. The dotted curve in dicates a possible variation of Tg in re lation to effective normal stress as a re sult of the substantial unloading of the clay Even if the average shear stress along the plane sl iding surface for a flake slide is slightly lower than the cri tical value pointed out above, a slide could be ini tiated by an increase in shear stress for reasons as menti oned in the.introduction, or due to some tendency of pro gress ive faiiure. RISSA.SOR-TRONOELAG RECALCULATION OF ACTUAL FLAKE- TYPE SLIDES Stability calculations were carried out for 5 Norwe gian quick clay slides, all of which have been submitted to a comprehensive investigation including a determinati on of the position of the sliding surface. Moreover, in a ll these cases there exist no doubt that the whole sli de or a major part of it went out very rapidly and appare ntly monolithically. As seen from Fig. 10, a 11 e five s lides are flake- formed, and a major part of the sliding s ur Fig. 10. Comparison between calculated average shea r face was plane and sloped very gently. stress acting along plane sliding surface and measured strength from shear tests for 5 The investigations at four of the sites included di rect Norwegian quick clay slides sinpleshear tests on quick clay similar to the soil in the sliding zone reconsolidated to a vertical stres s stress along the plane sliding surface was on avera ge equal to the effective overburden. At one site (Fur re) O.140o which checks quite well with the measured va lue, large scale in- situ shear box tests were run (Hutchinson x p [r= 0.16oJ, . and Rolfsen, 1962). In these latter tests which wer e run very slowly and thus under drained conditions, the shear The slide_at_B|stad_5.December_1974 destroyed an ar ea of resistance was measured along a plane just below and 80T000'ir|2 (i ’mTTTT"in3]"nn"the”courie of a minute or less parallel with the actual sliding surface. (Gregersen and Loken, 1979). The slide most probabl y was initiated by earth works for agricultural purposes which The measured values of shear strength, normalized with may have led to overstressing of the quick clay nea r the respect to , are shown by the numbers to the left i n toe of the slope. Fig. 10. These strength values are compared with the values of calculated shear stress, Tg, acting along the In this case the soil within the active and passive zones plane sliding surface at the moment just before fai lure. consisted mainly of quick clay. Thus, failure was here assumed to take place at a stress level correspondi ng to The slide.at .Furre, Namdalen1_14iAgril_1959 embraced an a mobilization of the undrained shear strength meas ured aria'of T80.000 m^ [2.5 - 3 miTT. m3] of which 30 - 50% in active and passive triaxial tests, respectively. Cal slid out like a monolithic flake along an only 0.1 m thick culated average shear stress acting along the plane sli quick clay layer embedded in silt (Hutchinson, 1961). ding surface was O.16o'0 , i.e. slightly less than the Some few weeks before the slide, strong erosion in the recorded value from direct simple shear tests, sud = river caused a certain stress increase in the quick clay, 0.18O- . and the slide possibly started with a local initial in stability near the river. According to eye- witnesse s the The slide_at_SemJ_Heddal 17..Agril_1974 was initiate d by whole sliding process took about half a minute. pTacTng'a minor earth fTTT causing i TO m displacement of a flat area of 6.000 m^ (50.000 uw) in the course o f 2- 3 The stability analysis was based on an assumption o f fully mi nut es. mobilized friction angle in the silt and sand masses within the passive and active zones. Calculated she ar Also in this case the active and passive pressures were 22 -01 7129 337 11/1 calculated on the basis of the active and passive un ance, corresponding to a sinOmob equal to 0.42 - 0.46 drained shear strength values of the quick clay. Ca lcu (i.e. 80 - 85% of sin*p' ) along the most critical p lane. lated average shear stress along the sliding surfac e was 0.18oi compared to the value from direct simple she ar Together the two above mentioned relations define a cri tests, 0.21oj tical shear stress level for horizontal or slightly in clined planes which cannot in practice be exceeded with Jhe slide_at_Bekkelaget 7^0ctober_1953 embraced an area out immidiately resulting in failure. For Norwegian quick of’T6 7000” rri2”( T007050’ m3j ' whTch~Tn"T5i s than half a mi clays this critical shear stress seems to be of the order nute moved 15- 20 m (Eide and Bjerrum, 1954) . Here too, of 0.15 - 0.20 times the vertical consolidation pre ssure, sonE filling operations had been going on just before the oi , and may most realistically be determined by di rect slide took place, and may well have been responsibl e for simple shear tests on specimens reconsolidated to a verti local yielding, leading to some stress increase ove r the cal stress equal to . entire slide area. The average shear stress along the plane sliding su rface was calculated to be 0.1 8 0 q , whilst the measured value ACKNOWLEDGEMENTS from recent shear tests was 0.22CTq • As shown in F ig.10, however, if one assumes a somewhat reduced length o f the The author wishes to express his gratitude to Direc tor sliding body, the calculated stress in this case agree Kaare Hoeg, for stimulating discussions and for re with the recorded strength, thus indicating a progr essive viewing the manuscript. type failure. Ihe_slide at Rl§sa_29iAgril_1978 embraced an area o f 330.660 m’ [5- 6 mill. nP). The slide started with a REFERENCES series of successive minor slides covering 6- 8% of the total area. About 40 min. after the initial slide t ook Aas, G. (1979). Kvikkleireskred.Norske sivilingeni0 rers place, the first one of a series of large flake- type forening. Skredfare og arealplanlegging; vurdering slides occurred, and the whole sliding process was com av faregrad og sikringstiltak; kurs. Lofthus 1979. pleted about 5 minutes later. Bjerrum, L. & Landva, A. (1966). Direct simple- shea r tests on a Norwegian quick clay. The profile shown in Fig. 10 is representative for t he G§otechnique, (16), 1, 1- 20. Also publ. in: Norw. first flake- slide in Rissa, the extent of which is quite Geotechn. Inst., Publ. 70. well documented by eye- witnesses (Gregersen, 1981). This area was almost unaffected by the preceding minor s lides, Bjerrum, L. & Kenney, T.C. (1968). Effect of struct ure on and the stability calculations were carried out as for an the shear behaviour of normally consolidated quick independent flake- type slide. As seen from Fig. 10, the clays. Proc. Geotechn. Conf. Oslo 1967 on Shear calculated average shear stress acting along the pl ane Strength Prop, of Natural Soils and Rocks, (2), sliding surface in this case is 0.21Oq and is in ex 19- 27. cellent agreement with the measured undrained stren gth. Eide, 0. & Bjerrum, L. (1954). The slide at Bekkela get. Proc. European Conf. on Stability of Earth Slopes, A few examples given above seem to clearly indicate that (2), 1- 15, Stockholm 1954. Also publ. in: it is a characteristic feature of the flake- type sl ides Geotechnique, (5), 1955, 1, 88- 100; and in: Norw. that the shear stress mobilized along the sliding s urface Geotechn. Inst., Publ. 10. just prior to failure, is approximately equal to the un drained shear strength determined by conventional d irect Gregersen, 0. & Loken, T. (1979). The quick- clay sl ide at simple shear tests. Theoretically, the critical she ar Baastad, Norway, 1974. stress along an inclined plane should be somewhat h igher Engineering Geology, (14), 2/3, 183- 196. Also publ. than SyQ. However, in practice this must be compens ated in: Norw. Geotechn. Inst ., Publ. 128. for by various factors like for instance the tendency for Gregersen, 0. (1981). The quick clay landslide in Rissa, progressive failure and rate effects. Norway; the sliding process and discussion of failure modes. To be publ. in: Proc. 10th ICSMFE, Stockholm 1981. CONCLUSION Hutchinson, J.N. (1961). A landslide on a thin laye r of quick clay at Furre, Central Norway. A recalculation of flake- type slides in quick clay has Geotechnique, (11), 2, 69- 94. Also publ. in: Norw. shown that the shear stress acting along the gently slop Geotechn. Inst., Publ. 44. ing sliding surface at failure is extremely low in re Hutchinson, J.N. & Rolfsen, E.N. (1962). Large scal e lation to the effective overburden, corresponding t o an field shear box tests on quick clay. apparent angle of internal friction of only about 10 de Geologie und Bauwesen, (28), 1, 31- 42. Also publ. grees mobilized along the sliding plane. To underst and in: Norw. Geotechn. Inst., Publ. 51. the failure mechanism in quick clay slides, one has to be aware of the following relations: The geometrically defined sliding surface does not repre sent the most critical plane where yielding in the clay elements within the failure zone first occurs. As soon as yielding takes place, however, pore pressures devel op very rapidly causing a critical condition also for other planes, for instance the one coinciding with the fi nal sliding surface. It seems to be characteristic for quick clays that a structural collapse, as indicated above, starts at a rather low degree of mobilization of frictional res ist 338