temporary stability of steep noncemented and lightly cemented soil slopes

7/23/2019 Temporary Stability of Steep Noncemented and Lightly Cemented Soil Slopes

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ARTICLE

Temporary stability of steep, noncemented and lightlycemented soil slopesPoul V. Lade and Jerry A. Yamamuro

Abstract:Many steep soil slopes are apparently stable beyond what is indicated by slope stability analysis. The mechanism of

slope stability in dilating soils is explained in detail, and the development of shear strength in such soils is demonstrated by

drained andundrained tests on dense sand. It is argued that appropriate shear strength parameters foranalysis of slope stability

in dilating materials describe the residual strength. It is explained how reliance on peak shear strength parameters is unsafe,

because the component of shear strength created by the additional effective confining pressure caused by development of

suction due to inhibited dilation can be exhausted by either access to water or by drying the soil. The fleeting phenomenon of

apparent additional shear strength causes super-stability of the slope. Exhaustion of the soils capacity to dilate results in

reduction of shear strength and instability of the steep slope. It is difficult to predict the time when the soils capacity to dilate

is exhausted and when the consequent decline in shear strength occurs. This is because this decline occurs with access to water.

This is demonstrated by triaxial compression tests on saturated and partly saturated, dilating specimens.

Key words: dilation, instability, noncemented soil, shear strength, slope stability, suction.

Rsum :Un grand nombre de talus de terre aforte pente sont apparemment stables au-delade ce quindiquent les analyses de

la stabilit des pentes. On explique en dtail le mcanisme de la stabilit des pentes dans un sol en dilatation et on dmontre le

dveloppement de la rsistance au cisaillementdun telsol pardes essaisdrains et nondrains mens surdu sable compact. On

fait valoir que les paramtres appropris de rsistance au cisaillement aux fins de lanalyse de la stabilit des pentes dans des

matriaux en dilatation dcrivent la rsistance rsiduelle. On explique comment il est risqu de se fier aux paramtres de

rsistance maximale au cisaillement, car llment de la rsistance au cisaillement cr par la pression de confinement effective

additionnelle, cause par le dveloppement de la succion due ala dilatation inhibe peut sattnuer par soit laccs aleau ou

le ressuyage du sol. Le phnomne passager de rsistance au cisaillement supplmentaire apparente cause la superstabilit de la

pente. Lpuisement de la capacit du sol adilater entrane une rduction de la rsistance au cisaillement et linstabilit de la

pente raide. Il est difficile de prvoir le moment o la capacit du sol adilater est puise et celui o la diminution consquente

de la rsistance au cisaillementse produit; la raisontantque cette diminution survient de pair avec laccs a leau. On dmontre

ceci par des essais de compression triaxiale sur des spcimens en dilatation saturs et partiellement saturs. [Traduit par la

Rdaction]

Mots-cls : dilatation, instabilit, sol non ciment, rsistance au cisaillement, stabilit des pentes, succion.

Introduction

In connection with studies of conditions for stability and insta-

bility of granular materials, it has become clear that dilating

sands can be very strong and perfectly stable as long as they are

maintained in undrained conditions. Such situations exist in the

field where steep slopes have been cut in dilating soil with low

permeability. Well-known examples exist in Hong Kong (Sweeney

and Robertson 1982), along the Pacific Coast Highway in Southern

California(Moran et al. 1959), at the Point Grey cliffs on the cam-

pus of The University of British Columbia, along the northwestcoast of Jutland and the northern coast of Zealand in Denmark,

and in many other places in the world. Relevant concepts relatingto this issue have been presented by Fredlund and his colleagues

(Fredlund 1987; Lim et al. 1996). In some locations, slopes have

been cut to produce level ground for highways and houses, or theocean has eroded the beach and cut away at the toe of the slopes

resultingin bluffs of different geometries, as exemplified in Fig. 1.

In Hong Kong, the slopes are cut in decomposed granite with

grain sizes in the sand and silt range. In Southern California, theslopes are cut in clayey sandstone and siltstone, and at the PointGrey cliffs and in Denmark the steep slopes consist of glacio-fluvial deposits such as sand, silt, and clay. The resulting steeperslopes are apparently stable with no signs of incipient failure, butmeasured friction angles are not sufficiently high to account fortheir stability under drained conditions. It has therefore beenthought that cohesion in the soil played an important role in thestability of such slopes.

Failures of such slopes often occur following heavy rainfalls.These failures are usually preceded by creep movements in thedownwards directionin concert, butslightlyout of phase with the

heavy rainfalls. After cessation of the rainfall, the slopes mostoften slowlycome to a halt, i.e., thecreep eventually stops as theavailability of water is exhausted. However, once in a while one ormore slopes have apparently reached into the tertiary region ofcreep, and failure has followed.

Attempts at relieving pore-water pressures through drainagepipes have been unsuccessful, because most often no water came

Received 5 June 2014. Accepted 25 November 2014.

P.V. Lade.Department of Civil Engineering, The Catholic University of America, Washington, DC 20064, USA.J.A. Yamamuro.*School of Civil and Construction Engineering, Oregon State University, Corvallis, OR 97331, USA.

Corresponding author:Poul V. Lade (e-mail:[email protected]).

*Deceased.

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Can. Geotech. J.52: 13741384 (2015)dx.doi.org/10.1139/cgj-2013-0342 Published at www.nrcresearchpress.com/cgj on 13 February 2015.
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out from the drains, and efforts to measure pore-water pressureshave also been futile, because positive pressures could not be

measured.The explanation for the initial stability of the slopes, which

eventually fail, appears to be due to effects of cohesion in the drystate and due to the volume change behavior and the associatedstrength behavior as the dense soil transitions from undrained inthe short term to drained in the long-term condition. Experi-ments have been performedto demonstrate thebehavior of densesand under sustained stress conditions similar to those in a slope.It is shown that effective cohesion does not exist in saturated ornearly saturated soils, but is created by drying of the soil in theslopes, and it is demonstrated how the slope becomes unstable inthe long term if water is allowed access to the deeper soil layers.Itis finally shown that the long term stability is dependent on theresidual strength of the soil. Therefore, the true long-term stabil-ity of such slopes corresponds to a geometry in agreement with

the residual friction angle, which is similar to the angle of reposefor the soil.

Drained and undrained strengths of dilating sand

Fundamental understanding of the behavior of loose and densesand under drained and undrained conditions have been ad-dressed by, e.g., Casagrande (1936), Bishop (1966), Lee and Seed(1967),and Seed and Lee (1967).In critical state theory there is acomplimentary relationship between void ratio and confiningpressure. The higher the confining pressure and (or) the higherthe void ratio, the less dilative capacity a soil will have. Even aloose soil will dilate at low confining pressure. Further, the sandfabric was found byMulilis et al. (1977)to have important effectson the stressstrain and volume change behavior as well as theliquefaction potential of the sand deposits. The influence of sand

fabric is particularly pronounced in so-called locked sand, as ex-plained byDusseault and Morgenstern (1979).The fabric in lockedsands is impossible to reproduce in the laboratory, and intactsamples may be obtained only with difficulty, e.g., by takingblocksamples or by using a freezing technique. The interlocking geom-etry of the sand particles is the cause of the high shear strength,which includes an extraordinary large contribution from dilation,but still does not involve any true cementation. Nevertheless,granular soils with such naturally generated interlocking canstand with rather steep slopes, as explained in detail by Sitar(1983)and byCollins and Sitar (2009).

The behavior of dense sands that dilate during shear is illus-trated schematically in Fig.2 and with actualexperimental resultsas presented byLade and Yamamuro (1993).The diagrams in Fig. 2

show schematic results of drained and undrained triaxial com-pression tests on specimens of dense sand. The stress path for the

drained test is inclined at 3:1 (vertical:horizontal) on the Cam-bridgep=q diagram in Fig.2a, wherep= and q arethe mean normalstress and deviator stress, respectively. As the stress path crossesthe characteristic line (Luong 1980), the volumetric strain changesfrom contractive to dilative, as illustrated in Fig. 2c. The peakstrength is reached at the highest rate of dilation, and this isfollowed by softening towards the lower ultimate or residualstrength. Shear banding occurs in the softening regime in a tri-axial compression test when the hardening modulus reaches asufficiently low negative value (see e.g., Rice 1976; Peters et al.1988; Wang and Lade 2001; Lade 2003), as indicated in Fig. 2b.Under plane strain conditions, which are more likely to prevail insteep slopes, shear banding occurs in thehardeningregime result-ingin abrupt failure and more severe reduction in shear strength.

Figure 2also shows the results of an undrained test performed

on a dense specimen with the same initial confining pressure. Asthe effective stress path from the undrained test passes the phasetransformation line(Ishihara et al. 1975) (i.e., characteristic lineobtained from the drained tests(Luong 1980)), the pore pressuredevelopment changes from positive to negative. The negativepore pressure develops as a result of prevented drainage in theregionof dilation, and it results in higher effective confining pres-sure and therefore greater undrained than drained strength.

Lade and Yamamuro (1993)performed drained and undrainedtriaxial compression tests on dense, fine silica sand to study therelation between the capacity to dilate and the resulting un-drained strength obtained when this dilation is prevented. Allspecimens were initially consolidated at the same effective con-fining pressure of3

= 29.4 kPa (0.300 kg/cm2), and to avoid cavi-tation of the pore water in the undrained tests, which occurs

at approximately 98 kPa (1.0 kg/cm2

), back pressures up to6375 kPa (65 kg/cm2) were required. The specimens were initiallysheared under drained conditions, and at predetermined pointsalong thestressstrain relations thedrainagevalvewas closed andthe specimens were sheared under undrained conditions in theremaining portions of the tests.

It was clear that the capacity to dilate controls the entire behav-ior pattern of the granular material under undrained conditions.Thus, the strongest specimen was that with the lowest void ratioat the time undrained shearing was initiated. This specimen hadthe highest capacity for dilation, whereas the weakest specimenhad the highest void ratio and therefore the lowest remainingcapacity to dilate at the time the drainage valve was closed. Theweakest specimen has already exhausted a portion of its capacity

Fig. 1. Examples of temporarily stable, steep slopes cut in noncemented, dilating soil with low permeability. , friction angle.

Lade and Yamamuro 1375

Published by NRC Research Press


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to dilate before undrained shearing is initiated. The fact that therate of dilation decreases after peak failure under drained condi-tions was reflected in the lower rate of pore pressure reductionand in the consequent slower rise in stressstrain relation for theweakest specimen. Otherwise, the effect of having initiated theundrained portion of the test in the softening regime was notreflected in any way in the weakest specimen. Thus, the sand

experienced peak failure twice: once under drained conditionsfollowed by peak failure under undrained conditions. Thus, thestress path rides the failure line for most of the test. Thus, thestressstrain curves for these soils have peaks at very large strains,often not reaching a clear peak at all. All of the toughness dis-played by these curves is the result of increasing confining pres-sure caused by dilation. If the drainage valve were opened duringany part of the test, after the initial rise of the stress path, the soilwould fail. In other words, the dilation contained the failure untilits capacity was exhausted or the pore water caveated.

Each specimen reached its respective peak failure point underundrained conditions when the capacity to dilate had been ex-hausted. This capacity is, of course, diminished with the increas-ing effective confining pressure according to the concepts ofcritical state soil mechanics as advanced byCasagrande (1936) and

bySchofield and Wroth (1968): at the critical state or ultimatestate (as proposed byPoorooshasb 1989), there is no further ten-dency for volume change. The ultimate state is reached either bya change in void ratio of by a change in effective confining pres-sure or by a combination of the two changes, as exercised in thetests presented byLade and Yamamuro (1993).

Application to slope stability

Steep slopes cut in dense soil that tends to dilate during shearare stable as long as they remain undrained and retain their ca-pacity to dilate. Very high and steep slopes can be cut in suchmaterials. Maximum possible slope heights for given inclinationand a maximum pore suction or u = 98 kPa, which is the ap-proximate cavitation pressure for free water, may be calculated

on the basis of the charts developed byTaylor (1948). A suctionhigher than 98 kPa is possible in clays. The pore suction producesan apparent cohesion, which causes the slope to remain stable aslong as the suction is maintained. This is true whether the soil isoverconsolidated clay or dense sand.

Steep slopes of the type described here can occur on many scalesfrom very small to very large. The smaller slopes do not require the

high values of suction, and such smaller, steep slopes are seen, e.g.,along river banks consisting of sands and silts and clays. Figure 3shows photographs of three steep slopes at three different scalesfrom very small to very large.Figure 3ashows a cut, approximately10 cm high (note thescalefrom thepencil) made by erosion by a localstreamrunning over a moist sand beach. Figure 3b shows an approx-imately 3 m high slope in a river bank produced by erosion, andFig. 3cshows the failure of a 30 m high bluff indilating soilalongthePacific Coast Highway in Southern California. In each case the slopeis very steep, butstable as long as therequiredsuction is sustainedinthe soil mass. The required pore suction for stability of a slope isproportional with the slope heightH.

Fleeting effective cohesion in lightly cemented soils

It is evident that these steep slopes shown in Fig. 3 are suffi-

ciently stable, at least temporarily, and that structures can bebuilt along the bluffs, as they are in many places in SouthernCalifornia. The fact that effective cohesion is not present in thesoils in question is demonstrated by the experiment shown inFig. 4.The top two photographs show the effect of adding water toa dry lump of soil from the killer slide in Pacific Palisades. In thedry condition, this soil sample appears to possess considerablecohesion, but the soil lump completely collapses within 20 min oftheadditionof water.Because theshear stress insidethe soil lumpdue to self-weight is negligible, any small amount of cohesionwould prevent the collapse. However, the fact that it collapsesclearly indicates that effective cohesion is not present in the soil.Following this experiment the soil was allowed to dry out insidethe glass beaker. This caused the soil to form a new lump in the

Fig. 2. Schematic diagrams of (a) effective stress paths, (b) stressstrain and (c) volume change relations, and (d) pore pressure relations for soil

that tends to dilate under drained and undrained conditions.u, pore pressure change;1, axial strain;

v, volume change;

1and

3, major and minor

principal stresses,respectively;1, effective major principal stress.

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shape of the interior of the beaker, but slightly smaller due toshrinkage of the material. The newly formed soil lump is shownon the third photograph in Fig. 4. Because, in the absence ofwater, the new lump has similar apparent cohesion as the original

one, the experiment could be repeated with the same result,namely that effective cohesion is not present in the soil.

The apparent cohesion is reduced due to an adsorption pressurecaused by surface forces created by water and acting betweenparticles. These surface forces play an increasingly large role withdecreasing particle size. While surface effects are insignificant forgrains of sand, their significance increases tremendously for par-

Fig. 3. Photographs of temporarily stable slopes in noncemented,

dilating soil with three different heights: approximately (a) 10 cm

(4 in.), (b) 3 m (10 ft.), and (c) 30 m (100 ft.).

Fig. 4. (a) and (b) Collapse of dry soil lump from the killer slide in

Pacific Palisades, California, within 20 min of addition of water,

indicating absence of effective cohesion. (c) Drying of soil resulting

in new soil lump with cohesion, which is absent in the presence of

water.




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ticles in the powder and colloid range. A basic surface force is

created by the electrostatic chargeon particles. It causes electricalrepulsion between adjacent particles. Attractive forces, referred

to as Van der Waals forces are also present between particles. Theresulting force, attraction or repulsion, depends on the relative

magnitude of the two types of forces, and this in turn depends on

many factors, including, e.g., the pH of the surrounding liquid. Ifthe net force is repulsive, then the particles are repelled by the

electrostatic force, which increases with decreasing distance be-

tween the particle surfaces.The source of the electrostatic force is adsorbed ions that are

attached to the surface of the particles, as indicated inFig. 5.Theadsorbed ions create a counter-ion layer, which is attached to the

particle. Theinterface between the counter-ion layer and the bulk

liquid is called the slipping plane. The electrostatic charge pro-duces a difference in electrical potential, in millivolts, between a

particle surface, i.e., at the slipping plane between the surface of

the counter-ion layer and the surrounding liquid, as shown inFig. 5.This difference is called the zeta potential (-potential). The

magnitude of the zeta potential depends on the concentration ofions in the surrounding liquid.

As particles get closer together, the electrostatic repulsion be-

tween the counter-ion layers increases. For particles that are al-ready cemented together, thereby creating a bond of a certain

(tensile) strength, the electrostatic repulsion reduces the bond

strength, as indicated inFig. 6.Thus, the effect of the adsorptionpressure is to effectively reduce the (tensile) strength in the nor-

mal direction and therefore also the shear strength of the bondcreated by cementation.

While the effect of the adsorption pressure may be consideredsimilar to that of an increased pore-water pressure, the effect isnot quite the same. In the first place, there is no measurableincrease in pore pressure in the free water caused by the adsorp-tion pressure. Secondly, the effect of the adsorption pressure isacting directly between particles only. Thus, the adsorption pres-sure acts to flatten the particles perpendicular to the direction ofthe force between the particles; there is not an increased pressuresurrounding the particles, which would tend to reduce the vol-ume of the particles. Thus, the best way to characterize the effectof the adsorption pressure is through a reduction in the effectivecohesion created by the cementation. Dry soil has the higheststrength because there is no counter-ion layer present in the air

surrounding the particles and therefore no repulsive forces. Ad-dition of water immediately alters this situation as explainedabove.

While stronger cementation may be present in the large blockslying near the foot of the slope inFig. 3c,the cementation is notlikely to be evenly distributed in the soil and the cohesion willtherefore be reduced by intruding water in zones with light ce-mentation. This creates weaker soil with negligible cohesion,which allows failure to occur.

Fig. 5. Particle with zeta potential created by electrostatic forces along surface.

Fig. 6. Particles with cementation bonding and repulsive forces.




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Experimental study of mechanics of deformation

and failure

To study the soil behavior along the stress path followed by atypical soil element located near the future failure surface insidethe soil mass of a steep slope, as exemplified in Fig. 1, two series oftriaxial compression tests were performed on dense, fine silicasand. The sand tested, the specimen preparation procedure, thesequence of events leading to the stress paths followed in the

tests, andthe results of the twotest seriesare describedas follows.

Sand testedTo avoid experimental problems due to excessive effects of

membrane penetration caused by changing effective confiningpressures(Lade and Hernandez 1977;Martin et al. 1978;Kramerand Sivaneswaran 1989), the tests were performed on a fine sand.Membrane penetration into pores of granular soils is small tonegligible when the average diameter of soil grains is smallerthan 0.10.2 mm(Frydman et al. 1973).

Fine silica sand composed of angular particles consistingmainlyof quartz was used for thestudy.The characteristics of thissand are summarized as follows: (i) mean diameter, 0.18 mm;(ii) coefficient of uniformity, 2.0; (iii) specific gravity of grains,2.66; (iv) maximum void ratio, 0.85; and (v) minimum void ratio,

0.55. Tests were performed on dense specimens with void ratio of0.58, corresponding to a relative density of 90%.

Specimen preparationAll triaxial tests in the experimental program were performed

on cylindrical specimens with diameter, D, equal to 7.1 cm(2.80 in.) and with height,H, equal to 19.0 cm (7.5 in.) correspond-ing toH/D = 2.68. The tall specimens were employed in the tests toallow anyinstabilities, e.g., development of shear planes or bands,to occur freely and uninterrupted by the end plates. In addition,lubricated end plates were used in all tests to avoid developmentof significant shear stresses at the cap and base.

The specimens were prepared by dry pluviation of sand andsaturated using the CO

2method(Lade and Duncan 1973). In addi-

tion, sufficiently high back pressures (most often 392 kPa) wereapplied to thespecimens to ensurea high degreeof saturation and

to avoid cavitation in the undrained tests. B-values of 0.97 or bet-ter were measured in most tests. Full saturation was produced tobe able to study the mechanism of strength mobilization in thesand during the imposed stress path. In the field such back pres-sures would not be present and the limiting pore-water suctionwould be 98 kPa or lower.

The triaxial compression tests were performed in a testing ma-chine that could apply loads under load control and deformationcontrol. To simulate the field loading condition and to allow de-velopment of shear bands and subsequent instability, the speci-mens were tested under load control. The vertical load, theconfining pressure, the pore-water pressure, the vertical deforma-tion, and the volume changes were measured during each test.Corrections were applied to the measured vertical load for upliftforces on the piston. A frictionless bushing of the type described

byChan (1975)was employed for the piston, thus avoiding correc-tion for piston friction. Corrections to linear as well as volumetricstrains were found to be negligible.

Sequence of events and stress paths followedTwo series of tests were performed. In one series the specimens

were fully saturated, and partly saturated specimens were testedin theother series to study theeffect of degreeof saturation on thesoil behavior.

All specimens were initially consolidated to an effective confin-ingpressureof 29.4 kPa. Each specimen was then loadedverticallyunder load control and undrained conditions to simulate therapid cutting of thesteepslopein thefield,as exemplified in Fig.1.Because the dense sand tends to dilate under the relative low,

initial confining pressure, negative pore-water pressures are gen-erated, as also explained in connection with Fig. 2. The corre-sponding soil behavior is indicated by the stressstrain, volumechange and pore pressure relations, and the effective stress pathshown inFig. 7.The initial field condition after cutting the slopehas now been established at point B in Fig. 7:the externally ap-plied total confining pressure shown in Fig. 7band the deviatorstress (and therefore the shear stress) shown inFig. 7a are held

constant, thus simulating theinitial state of stress at a point alongthe potential failure surface in the steep slope, as indicated by thesoil elements inFig. 1. In the particular test shown in Fig. 7, theeffective confining pressure at point B is 294 kPa, and the state ofstress at point B corresponds to the condition inside and near thebottom of a very high slope.

If nothing further occurs to the soil specimen, it will remainstableunderthe load controlas will thespecimenin thefield.Thisis because any decrease in pore suction will reduce the effectiveconfining pressure, and this will cause shear deformation and atendency for dilation to occur, which in turn will generate addi-tional pore suction and therefore increase the effective confiningpressure by virtue of its relative impermeability. Thus, as long asthe tendency for dilation is not exhausted, the soil will generatesufficient effective confining pressure to remain stable.

The effect of rain water seeping down from the ground surfaceor entering laterally through horizontal soil layers and penetrat-ing into the soil element is simulated in the triaxial compressiontest by opening the drainage valve slowly and letting measuredamounts of water be sucked into the specimen through the drain-ageline attached at the base (Fig.7c), while theresultingchange inpore-water pressure is monitored from the drainage line attachedto the specimen cap (Fig. 7b). While the total confining pressureand the vertical load (under load control) are maintained con-stant, thewater imbibed by thespecimentends to reduce theporesuction, which in turn decreases the effective confining pressureand causes the specimen to shear and dilate. The deformation ofthe specimen will continue until sufficient pore suction in thedilating soil has been generated to maintain sufficient effectiveconfining pressure to carry the imposed shear stress. Thus, as longas the capacity to dilate has not been exhausted, the soil element

will remain in equilibrium and the slope will remain stable, evenas water enters into the slope.

Tests on fully saturated soilThis sustained state of equilibrium is demonstrated in the test

shown inFig. 7.While the triaxial specimen is able to sustain theimposed load, it undergoes axial strain, i.e., it appears to creep,and it dilates in response to the water sucked in. The deviatorstress (and therefore the shear stress) in the laboratory specimenactually decreases slightly due to the increasing cross-sectionalarea, because the experiment is performed with constant verticalload rather than with constant deviator stress. In the field theshear stress is generated by the weight of the sliding mass, and itis essentially constant at a given point along the potential failuresurface.

As the soil imbibes water, it moves towards failure, and in fact,it moves beyond peak failure as defined in terms of the maximumeffectivestress ratio, as indicated in Figs. 7a and 7d.This fact is notidentifiable from the variation in shear stress, but from the initialreduction in effective confining pressure followed by increasingeffective confining pressure, as shown inFig. 7b.Thus, stability ismaintained well beyond the point at which the failure surface isencountered, because the soil in the triaxial compression testretains its capacity to dilate well into the post-peak failure regionwhere softening occurs. Such stability in the softening regime wasdiscussed in detail byLade and Yamamuro (1993).

Collapse of the initially dense specimen occurred when shearbanding developed in the load controlled test. This happened asthe capacity of the sand to dilate was nearly exhausted and the




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rate of dilation had decreased to a small value, as seenin Fig.7c. Intriaxial compression tests, this occurs in the softening regimeafter a critical hardening modulus has been reached (Rudnickiand Rice 1975;Rice 1976;Peters et al. 1988;Lade 2003). Thus, themaximum strength that can be expected to be present in the soilat the time of collapse is the ultimate or residual strength. Anycontribution to the shear strength beyond this value is based onthe volume dilation of the soil, but this contribution can be ex-hausted under constant shear stress as demonstrated earlier. Con-sequently, any shear strength beyond the residual shear strengthis illusive and cannot be counted on to contribute to the stabilityof slopes that are based on pore suction.

The residual strength of a soil is based entirely on the basicfriction between soil grains and a small contribution due to rear-ranging of grains at constant volume. Thus, the residual friction

angle is equal to the critical state or the ultimate friction angle,and it is theoretically equal to the angle of repose for the soil.Consequently, the ultimate, long term geometry of a slope in soilthat initially derives part of its strength from pore suction is onethat is in agreement with the angle of repose for the soil.

Several additional experiments were performed in this series tostudy the behavior over a range of initial confining pressures.While all speci mens were first consolidate d to a confiningpressure of 29.4 kPa, they were subsequently sheared under un-drained conditions until the desired effective confining pressurehad been achieved, and this was followed by suction of water intothe specimens, as explained above. All experiments showed verysimilar soil behavior, as indicated in Fig. 8. The experimentalresults shown inFig. 7are also presented inFig. 8for reference.

In one experiment inFig. 8,test No. 6, the access to water wasstopped twice for 60 min to observe the soil behavior during thestoppage. One stoppage was imposed at an axial strain of 3%, i.e.,in the hardening regime before the effective stress failure point,and the other stoppageoccurred at anaxial strain of 6%, i.e., inthesoftening regime after the effective stress failure point. The axialstrains during test No. 6 are shown plotted versus time inFigs. 9aand9b, and they show that the soil responds by slowly coming toa stop in the axial straining. During this period, the pore-waterpressure continues to change slightly, as seen inFig. 8cand thismaybe thereasonfor the continuedaxial straining. This responsewith time resembles creep that slowly comes to a stop as access toadditional water is cut off.

Two experiments inFig. 8,test Nos. 3 and 4, were performed asdrained tests once the desired effective confining pressures were

reached under initial undrained conditions. The effective confin-ing pressure in the drained portion of test No. 3 was 32.4 kPa, andit was 297 kPa in test No. 4. These two tests therefore show con-ventional, curved stressstrain curves, as indicated inFig. 8a.

Tests on partly saturated soilThe second series of experiments was performed to study the

effect of the soil being partly saturated. The specimens were pre-pared as fully saturated specimens as explained above. To simu-late the effect of part saturation, a predetermined volume of air atatmospheric pressure was allowed to be part of the specimen byoutfitting the water-filled drainage line with a side branch inwhich the air was contained behind an on off valve. The volumeof air was measured at atmospheric pressure and then exposed to

Fig. 7. Measured (a) stressstrain, (b) volume change, and (c) pore pressure relations; and (d) effective stress path from load controlled test on

fully saturated fine silica sand undergoing events simulating cutting of steep slope and subsequent failure by addition of water.




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the same back pressure as applied to the pore water in the speci-men before opening the on-off valve to connect the pore waterwith the air in the side branch. The volume of air was monitoredduring application of back pressure to ascertain that it compliedwith Boyles law for compression of gas and that the measure-ments were correct. While the effect of the compressible air inconnection with the pore water will buffer the pore pressure de-velopment during shearing, thus providing the effect of partialsaturation, the effect of having the pore air distributed in bubbleswith surface tension in the soil is not simulated. This effect isdescribed by Kelvins law, but it is negligible for the purpose ofthis study. The degree of saturation is represented as the volumeof water at atmospheric pressure in the specimen divided by thetotal volumeconsisting of thevolume of airin theside branch andthe volume of water in the specimen.

Experiments similar to those performed on the fully saturatedspecimens were conducted on the partly saturated specimens.Only the stressstrain and pore pressure relations are shown inFig. 10. As the degreeof saturation reduces, the buffer effect of theincreasing amounts of compressible air causes the suction to re-duce, as shown in Fig. 10b, and this in turn produces reducingeffective confining pressures, and the stressstrain curves, showninFig. 10a, consequently become less steep in the undrained por-tions of the tests. Once access to water is provided at the pointwhere the effective confining pressure has reached 294 kPa, thespecimens with relatively high degrees of saturation show similarbehavior as the fully saturated specimens. This behavior appearsas creep in the axial direction as the water is sucked in by thedilating sand. The two specimens with degrees of saturation of

68% and 51% did not even reach effective confining pressures of294 kPa due to the buffer effect of the compressible air. These

specimens simply indicated conventional stressstrain behavior

as the water was imbibed and the pore suction was reduced withdecreasingdegree of saturation.The undraineddrained test fromFig. 8 performedwith an effectiveconfiningpressureof 32.4 kPaisalso shown in Fig. 10. In the context of the tests shown in thisdiagram, this test corresponds to a degree of saturation of 0%, i.e.,a completely dry sand specimen would produce the stressstrainrelation indicated by this test. However, in the presence of siltand clay, the specimen would develop some cohesion as the soilreached a degree of saturation of 0%, and the strength wouldconsequently be higher than that indicated by the clean sand.

Mechanism of slope instability

The mechanics of stability of slopes is relatively easy to compre-hend, and it serves to explain the general behavior of stability andinstability of dilating soils as also observed in other stability prob-lems. To cause movement and subsequent overall slope instabil-ity, water has to percolate to deeper layers, because they hold upthe upper layers. While shallow slides may occur, deeper slopefailures are also observed, and for these to occur the deeper layersare required to deform thereby allowing the upper layers to de-form as well. This requires the water to penetrate into the deeperlayers before any substantial deformation can occur. It is not suf-ficient to have water penetrateinto the upper layers only, becausethat does not result in any deformations in the underlying layersandconsequentlydoes not allow significant shearing in theupper

Fig. 8. Results of six tests on fully saturated fine silica sand performed over a range of effective stresses and following stress paths simulating

cutting of steep slope and subsequent failure by addition of water: measured (a) stressstrain, (b) volume change, and (c) pore pressure

relations; and (d) effective stress paths from load controlled test. Test No. 7 shown inFig. 7.




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soil layers. The associated volume changes will occur in the upperlayers, only if shearing occurs in the lower layers. Thus, smallrainfalls do not have any real consequence for the stability of theslopes. The water must penetrate into the deeper layers where itreduces the effect of cementation and causes them to shear andexpand, thus exhausting the capacity for further dilation. Eventu-ally, when all capacity to dilate has been exhausted, the residualstrength will be reached, and this is the highest strength that cansafely be assumed for the long term stability of the slope.

The capacity to dilate during shearing of the soil is not restoredduring dry periods in which the soil stops shearing, i.e., the ap-parent creep of the slope comes to an end due to lack of access to

additional water. The exhaustion of the capacity to dilate is a

cumulative process that eventually leads to failure if the slope is

steeper than the angle of repose, which is equal to the residual

friction angle.

It is difficult to tell to which degree the capacity to dilate has

been exhausted in a given slope, but a fairly large amount of water

is required to percolateinto theground and reach deeper layers to

finally cause the slope to fail. Because it takes some time for the

water to reach the deeper layers, there is often a delay between

the heavy rainfall and the occurrence of slope failures of the types

discussed here. Thus, while the heavy rain occurs in the winter in

Fig. 9. Apparent creep of fine silica sand as the water imbibed into the specimen distributes into equilibrium and associated deformation

comes to a halt with time at (a) 3% axial strain (before effective stress peak failure) and (b) 6% axial strain (after effective stress peak failure).

Apparent creep tests performed at stop 1 and stop 2 in test No. 6, as shown inFig. 8a.




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Southern California, the large slope failures typically occur in thelate summer and fall of the same year.

Considering the mechanism of slope failure explained above, itis difficult to predict whether a given slope will reach the point ofinstability or remain stable until additional rainfalls occur in fu-

ture years to further exhaust the capacity to dilate. Besides, theshear strength required to maintain stability may be decliningbeyond the present shear stress as water is imbibed, thus reducingthe factor of safety below unity. Evaluating the time at which thefactor of safety reaches unity is difficult.

The fact that these slopes are stable beyond what is indicated bythe residual strength envelope, which is the effective strengthenvelope at large strains, may be referred to as super-stability.This phenomenon is attributable to the soils capacity to dilateand its capacity to regain some cohesion as the soil dries in thelong term, i.e., thestability of theslopesurpasseswhat is expectedon the basis of frictional strength alone. This phenomenon ofsuper-stability cannot be sustained if the slopebecomes saturated.There is very little indication of the decline in its stability until the

slope fails due to the exhaustion of the soils capacity to dilate,

thus indicating the fleeting phenomenon of stability beyond fric-

tional effects only.

Conclusions

Steep slopes in noncemented and lightly cemented soils thatdilate are stable only as long as they remain dryand their capacity

to dilate is not exhausted. The mechanism of slope stability in

dilating soil is presented, and it is shown that the contribution to

the soil shear strength and to the stability of a steep slope pro-

duced by inhibiteddilationis illusive andwill disappearas thesoil

imbibes water. This temporary component of shear strength is

therefore likely to expire as heavy rainfalls saturate the slope,

reduce the cementation, and exhaust the capacity to dilate. The

slope may remain temporarily stable at developed shear stresses

higher than the drained strength and lower than the undrained

strength of a dilating soil. The component of shear strength from

developed suction caused by the tendency for dilation during

Fig. 10. Measured (a) stressstrain and (b) pore pressure relations from load controlled test on partly saturated fine silica sand undergoing

events simulating cutting of steep slope and subsequent failure by addition of water. S, saturation.




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shear can be exhausted as water enters the slope and exhausts thecapacity to dilate and to imbibe additional water. Thus, a point intime will arrive at which further dilation cannot occur and it isshown that thelong-term shear strength availableto maintain thestability of the slope is the residual shear strength. The contribu-tion due to dilation is not permanent as are the basic frictionbetween grains and the contribution from energy required forrearrangement of grains at constant volume. Due to the nature of

the illusive dilation component, it is difficult to predict when thesoil in a given slope has exhausted its capacity to dilate, and theonly safe strength for prediction of slope stability in dilating soilsis the residual shear strength. Thus, the maximum safe inclina-tion of a slope in a dilating material is represented by the residualfriction angle, which theoretically is equal to the angle of reposefor the soil.

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temporary stability of steep noncemented and lightly cemented soil slopes

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