A Practical Design Approach for Reducing The

A practical design approach for reducing theoccurrence of shallow slides in cohesionless tillhighway embankments

J.-M. Konrad and G. Grondin

Abstract: Shallow slides in cohesionless till occurred in several highway fills in the province of Quebec, Canada,following spring thaw and sometimes during rainy autumns. This study has shown that the strength parameters ofcohesionless tills can be related to the liquid limit for both drained and undrained conditions. These relationships canbe used as input in stability analyses to readily yield design values of stable slopes. Tills with liquid limits between15% and 25% are all very susceptible to liquefaction flow failures if loosely compacted and saturated. To reduce therisk of shallow slides in the cohesionless till embankments used for highways, a stabilizing coarse-grained fill placedatop of the till embankment appears to be an efficient technique. Design charts of minimum thickness of the stabilizingfill at the base of the till slope were developed using available stability programs and an empirically derived strengthof liquefied tills.

Key words: cohesionless, till, liquefaction, stability, slope, skin.

Résumé : Des instabilités superficielles des talus routiers en till pulvérulent ont été observées au Québec, Canada, lorsdu dégel printanier et quelques fois durant des automnes pluvieux. Cette recherche a démontré que les paramètres derésistance drainée et non drainée des tills pulvérulents pouvaient être reliés à la limite de liquidité. Ces relations peu-vent être utilizées pour établir l’inclinaison de pentes stables. Des tills ayant des limites de liquidité entre 15 % et25 % compactés dans un état lâche et saturés sont très susceptibles de se liquéfier. Afin de réduire le risque de couléede peau dans les talus routier en till, le tapis drainant placé par-dessus le remblai est une technique efficace. Des aba-ques de dimensionnement ont été développés pour déterminer la largeur minimale du tapis drainant à la base du talusen fonction de la résistance au cisaillement du till liquéfié.

Mots clés : pulvérulent, till, liquéfaction, stabilité, pente, peau.

Konrad and Grondin 206

Introduction

Shallow slides in till occurred in several highway fills inthe province of Quebec, Canada, following spring thaw anddisplayed typical flow slide features resulting from staticliquefaction (Konrad et al. 2000). Poor compaction controlnear the embankment slopes owing to the lack of confine-ment and to the fact that heavy compaction equipment usu-ally do not approach too close from the edge of the slope ismainly responsible for these shallow slides. Liquefactionfailures were also reported in Scandinavian tills, particularlyin Asele (Ekström and Olofsson 1985) where loose compac-tion was a result of winter placement.

This paper presents the results of a comprehensive studyaimed at the development of a design methodology to mini-mize the risk of shallow slides in till embankments. After a

brief review of the behaviour of saturated cohesionless soilssubjected to undrained loading, the paper presents the con-ceptual mechanisms involved in shallow slides using criticalstate soil mechanics principles.

The results of an extensive laboratory study are given forvarious tills from Quebec and were used to establish empiri-cal relationships between the strength parameters of the tilland index soil properties. These relationships, in turn, con-stitute key input parameters for the design of the slope angleof till highway embankments to minimize the risk of shallowsliding as the upper layers become progressively saturatedduring the infiltration of water.

Liquefaction in saturated cohesionless soils

As stated by Poulos (1988), any given soil element in situsubjected to a particular loading condition has two well-defined strengths, the peak strength, tp, and the steady statestrength, tF. The peak strength is the maximum shear stressin the soil element equal to one-half of the maximum devia-tor stress, qp. The steady state strength is defined as thestrength that exists after the shear loading has eliminated alleffects of the original structure of the element and after anyorientation of grains that might affect strength has occurred.At the steady state of deformation, soil is being sheared at

Can. Geotech. J. 42: 198–206 (2005) doi: 10.1139/T04-095 © 2005 NRC Canada

198

Received 4 November 2003. Accepted 3 September 2004.Published on the NRC Research Press Web site athttp://cgj.nrc.ca on 1 March 2005.

J.-M. Konrad.1 Department of Civil Engineering, UniversitéLaval, Sainte-Foy, QC G1K 7P4, Canada.G. Grondin. Ministère des Transport, 930 chemin Sainte-Foy,Québec, G1S 4X9, Canada.

1Corresponding author (e-mail: [email protected]).

constant volume, constant effective stress, and constantstrain rate (Poulos 1981).

The peak strength and the peak strength envelope dependon the soil composition, the structure of the sample or fabricanisotropy, the void ratio and effective confining stress (of-ten referred to as state), rate of dilatancy and loading method(Poulos 1981, 1988; Bolton 1986). In contrast, Poulos(1981) states that the slope of the steady state strength enve-lope, φ′, is solely a function of the soil composition. Similarviews were developed by Bolton (1986), who states that thecritical state angle of shearing resistance of soil. which isshearing at constant volume. is principally a function ofmineralogy.

It was also well-established that in order for liquefactionflow failure to occur, the soil must be contractive when sub-jected to undrained shear and, moreover, the driving shearstress must be larger than the steady state strength of thesoil. A number of researchers (Vasquez-Herrera et al. 1988;Negussey et al. 1988; Konrad 1993) showed that triggeringof flow failure in monotonic and cyclic undrained testsoccurred when the effective stress path crossed the peakstrength envelope, which is a straight line passing throughthe origin and with a slope referred to as φpu′ . Work byDoanh et al. (1999) has shown that φpu′ was essentially afunction of soil fabric and stress anisotropy.

Conceptual failure mechanism for shallowslides

Till in dense statesThe fill for embankment highways is usually specified to

be compacted at least 95% of the maximum dry densityobtainable according to standard test methods. These specifi-cations provide adequate strength and compressibility char-acteristics in tills and often result in relatively densecompaction states associated with a dilative response duringundrained shearing, and thus generally, are not prone to de-velop liquefaction flow failures. This is well-illustrated inFig. 1, which is a 3D representation of the state and stressconditions in a soil element located in an infinite slope at adepth z. Initially, the soil is unsaturated, and the water tableis located at a depth greater than the soil element. Ratherthan using void ratio to describe the physical state of the soilelement, it is more practical to use water content in unsatu-rated and compacted soils since the latter can vary substan-tially, while void ratio essentially remains constant for thesedense tills.

For dense compaction states, the soil element in an infi-nite slope at depth z is represented by point A in Fig. 1: itswater content is equal to the water content at optimum Proc-tor, wopt, and its stress conditions can be approximated byeq. [1] assuming that the unit weight is constant:

[1] σ γ β τ σ βn a w nand′ = − − = ′[ ( )] cos tanz u u 2

where σn′ is the effective stress normal to the soil element, τis the shear stress, z is the depth, γ is the unit weight of thesoil, β is the slope angle, and (ua – uw) is the matric suction.

With increasing water content in the soil element as a re-sult of water infiltration, the suction decreases and the effec-tive stress path moves towards the stress space origin along a

line whose inclination is β. At point B, the soil element isfully saturated, its water content now becomes equal to wsat,and, since its state is below the critical state line (CSL), theresponse of the soil element during undrained loading isdilative, the soil element cannot thus liquefy.

If the water table progressively raises above the soil ele-ment, the effective stress path is a horizontal line passingthrough point B. Ultimately failure is reached at point Cwhen the water table is equal to:

[2] hw nB nC w= ′ − ′( )/ cosσ σ γ β2

At failure, slope movement is not dramatic since the soilbehaviour is dilative during undrained shear. Often, water ta-ble fluctuations induce creep movements of these dense tillslopes. This is not the case when the till is placed in a loosestate.

Till in loose statesHighways embankments are usually placed in lifts that are

successively compacted using heavy vibratory rollers toachieve the target density. However, this heavy compactionequipment does not approach too close from the edge of theembankment sloped face, which in turn, often results inpoorly compacted zones near the slope face. For thesepoorly compacted layers, undrained collapse can be trig-gered only if the soil in the vicinity of the potential failuresurface (usually at the interface between wel- compacted andpoorly compated layers) is at a compaction state above thecritical state and becomes saturated as a result of snow meltinfiltration or sustained rainy periods.

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Fig. 1. State and stress conditions in a dense till slope.

The analysis of a failure by undrained collapse for thesimplified case of an infinite slope must therefore considerthe relationship between shear stress, q, mean principal ef-fective stress, p′, and void ratio. For the particular case ofskin flows in saturated till initially placed in unsaturatedconditions, it is convenient to consider the relationship be-tween shear stress, τ, normal effective stress, σn′ and watercontent, w, for a soil element parallel to the slope as indi-cated in Fig. 2. Two cases must be analysed: (i) the slope an-gle, β, is greater than the slope of the trigger surface, φpu′ ;and (ii) β < φpu′ .

Case 1: � �> pu′In Fig. 2, point I represents the state and stress conditions

of a soil element at the potential failure surface parallel tothe face of a till embankment with a slope β. Water infiltra-tion causes a progressive increase in the degree of saturation,Sr , and the representative point moves from I to i as the ef-fective stresses decrease since the suction decreases with in-creasing values of Sr . With a further increase in Sr , the soilelement reaches point j, which is located just above CSL,i.e., in the contractive domain. However, liquefaction flowfailure cannot occur since the soil element is not yet suffi-ciently close to fully saturated conditions. At point J, the soilelement is fully saturated. In the stress space (τ, σn′ ), theslope of line OJI is equal to β and in the state space (w, σn′ ),point J is located above CSL, which indicates that the condi-tions for liquefaction flow failure are fully met. Since β >φpu′ , liquefaction is triggered as soon as the soil elementbecomes fully saturated or near it. During undrained flow

failure, the water content remains constant, and the value ofthe induced pore pressure is given by the value of segmentJK. In the stress diagram, the strength of the liquefied till,τ(K), corresponds thus to the failure envelope for the effec-tive confining stress σn′ (K).

Case 2: � �< pu′In Fig. 3, point M represents the initial state and stress

conditions of a soil element at the potential failure surface.As the degree of saturation increases with infiltration, thestress and state conditions move progressively towards pointN. At point N, the soil element is fully saturated, but sinceβ < φpu′ , the effective stress path has not crossed the triggerline and liquefaction cannot occur. At point N, the water ta-ble is at the potential failure surface, i.e., at the interface be-tween the denser till and poorly compacted till. With furtherinfiltration, the water table rises progressively and the effec-tive stress path in the (τ, σn′ ) space is a horizontal line pass-ing through point N. When the effective stress path crossesthe trigger line, i.e., at point O, liquefaction is possible. Thepore pressure required to reach point O can be related to theheight of the water table above the potential failure surface,hw and β as:

[3] u h= γ βw w cos2

Once liquefaction is triggered, the state conditions of theliquefied till are given by point P in the state diagram (w,σn′ ), and the corresponding strength τ(P) is obtained from

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200 Can. Geotech. J. Vol. 42, 2005

Fig. 2. Poorly compacted till for the case β > φpu′ . Fig. 3. Poorly compacted till for the case β < φpu′ .

the failure envelope in the stress space (τ, σn′ ). Theundrained excess pore pressure during flow liquefaction isequal to the value of segment OP.

Properties of till studied

The analyses described above, however simplified, haveshown that undrained collapse in loosely compacted tills de-pends on φpu′ and the ultimate or critical state characteristicsof the saturated till. Both characteristics can be obtainedfrom triaxial laboratory tests.

Till was collected from 10 different sites from four geo-graphic regions in Quebec (Abitibi, Gaspésie, Beauce, andEstrie). Index properties were obtained for all soils andstrength characteristics from triaxial tests were obtained forfour different tills, where slope instabilities were observed.

Index propertiesThe grain size distribution for each till is presented in

Fig. 4 and details are given in Table 1. Till, defined as awell-graded soil, may be characterized by a wide range offines content, indicating that grain size distribution alone

may not be sufficient to classify adequately the till with re-spect to its liquefaction potential. Table 2 summarizes someof the index properties of the till studied, and highlights thetills used for triaxial testing. The liquid limit was determinedusing the fall cone test, according to the BNQ2501-092 stan-dard (CAN/BNQ 1986), on the fine fraction smaller than400 µm. The density of solid particles, ρs, was determinedaccording to the BNQ 2501-070 standard (CAN/BNQ 1986).The specific surface area, Ss, of the fraction passing throughthe No. 200 sieve (80 µm) was obtained using the methyleneblue technique.

Critical state characteristics from triaxial testsUnlike uniform sands, till has lower hydraulic conductivi-

ties, which leads to more difficulties in preparing a fully sat-urated sample for triaxial testing. In the field, the till nearthe embankment face is placed in horizontal layers at or nearoptimum water content, but is lightly compacted for the rea-sons discussed previously. The in situ mean effective stressranges between 15 and 20 kPa if the dense–loose interface islocated at a depth of about 1 m. Critical state characteristicsshould therefore be determined for these stress conditionsusing lightly compacted samples at different initial void ra-tios. These low stresses, in turn, require precise instrumenta-tion for adequate monitoring of pore pressure and deviatoricstress during monotonic loading.

The samples were prepared by the moist tamping methodto study representative field conditions. This, however, oftenleads to sudden collapse during saturation under water per-

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Fig. 4. Grain size distribution of till studied.

Localization d60 (mm) d50 (mm) d10 (mm) % <80 µm

Témiscaming 1 0.18 0.11 0.09 43Témiscaming 2 0.29 0.19 0.04 31Saint-René-de-Matane 0.065 0.022 0.00045 64St-Georges 1 0.16 0.075 0.0025 52St-Georges 2 0.07 0.04 0.0025 62St-Martin 1 0.0205 0.011 0.001 87St-Martin 2 0.031 0.015 0.0013 72Beauceville 0.11 0.068 0.0025 56Weedon 0.88 0.405 0.008 36Sherbrooke 1 0.09 0.062 0.0047 58Sherbrooke 2 0.18 0.12 0.0065 40

Table 1. Grain size characteristics of the till studied.

Sites wL (%) ρs (t/m3) % <80 µm Ss (m2/g)

Témiscaming 1 18 2.70 43 3.7Témiscaming 2 16 2.70 31 3.7Saint-René-de-

Matane33 2.74 64 36.8

St-Georges 1 19 2.71 52 8.9St-Georges 2 20.5 2.73 62 —St-Martin 1 30 2.75 87 18.5St-Martin 2 23 2.74 72 16.5Beauceville — 2.69 56 —Weedon 25 2.72 36 4.4Sherbrooke 1 20 2.72 58 —Sherbrooke 2 15.5 2.71 40 —

Table 2. Index properties of the till studied.

colation. During saturation with increasing backpressures,sufficient time must be allowed to reach a uniform watercontent distribution within the sample. Generally, 1 to2 days are required to obtain satisfactory conditions.

Once saturated, rate of shearing must be adapted to thesample’s swelling characteristics to ensure uniform porepressure distribution. At the end of testing, a water contentof the entire sample is determined and compared to the cal-culated changes in void ratio during the different steps of thetesting (saturation by percolation, backpressure saturation,consolidation).

Figure 5 presents typical CU test data obtained for tillfrom Weedon tested for different void ratio and stress condi-tions. The failure envelope in the Cambridge plot, i.e., p′ =( )/σ σ1′ + ′2 33 and q = σ1 – σ3, has a slope of 1.6, and a peakstrength envelope of 0.81. In a (τ, σn′ ) stress space, the fail-ure envelope is thus φ′ = 39.2°, and the peak strength enve-lope φpu′ = 20.9°. Table 3 presents the results from thetriaxial tests in terms of strength parameters, while Fig. 6summarizes the critical state characteristics of the till studiedin a conventional void ratio – mean effective stress plot.

Shear strength of till from empirical relationshipsThe design of stable slopes in till requires as an input pa-

rameter the shear strength along the potential slip surface.The mobilization of shear can occur in drained or undrainedconditions. Triaxial testing is a common approach to deter-mine these strengths, but may not be practical nor economi-cal for preliminary studies to assess the various designoptions. The use of empirical relationships between thestrength parameters and more readily obtained index proper-ties of tills is an interesting and practical alternative.

Drained conditionsFor drained conditions, triaxial tests on saturated samples

have shown that all the till studied behave as cohesionlesssoils, i.e., with an effective cohesion equal to 0. The drainedstrength is thus simply related to the effective friction angle,φ′. For the till studied, the best correlation between φ′ and in-dex soil properties was obtained with the liquid limit of thefraction smaller than 400 µm, wL. As shown in Fig. 7, thisrelationship can be expressed as:

[4] φ′ = 66° – 1.4wL

with R2 = 0.95 and 15% < wL < 25%.

Undrained conditionsFor undrained conditions, two cases must be considered:

dilative and contractive behaviour during shear. For theproblem at hand, the important aspect is related to contrac-tive behaviour of poorly compacted tills near the slope facebecoming saturated with water infiltration. For flow slidingto occur, the stress path must cross the trigger envelope, φpu′ .Since the liquid limit appears to correlate well with the fric-tion angle, it should also be an acceptable indicator for thepeak undrained strength as indicated by Fig. 7, and the bestfit equation given below:

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Fig. 5. CU triaxial test results for Weedon till.

Till Mf φ (°) Mp φpu (°)

Témiscaming 1.65 40.3 0.6 15.8St-Martin 1.35 33.4 0.8 20.7Sherbrooke 1.5 36.9 0.63 16.6Weedon 1.6 39.2 0.81 20.9RTE 175 1.67 40.8 0.7 18.3

Table 3. Strength parameters for four tills.

Fig. 6. Critical state characteristics of tills studied.

Fig. 7. Empirical relationships for strength parameter of tills.

[5] φpu′ = 0.72wL + 3.4°

with R2 = 0.82 and 15% < wL < 25%.The undrained shear strength, Cu, mobilized along the slip

surface depends on the type of till and its void ratio. ForWeedon till, the test results shown in Figs. 5 and 6 can becombined to produce a complete state and stress diagram inan arithmetic scale as illustrated by Fig. 8. The undrainedshear strength of liquefied Weedon till placed at a void ratioof 0.54 (i.e., a water content in a saturated state of 20%)would be 8.4 kPa. Furthermore, the same diagram also givesthe minimum depth of the failure surface to ensure that thedriving shear stress is at least equal to the liquefied shearstrength. For instance, if the till embankment has a slope2H:1V, the vertical effective stress at the failure surface mustbe equal or greater than 17 kPa (i.e., a minimum depth ofabout 1.2 m considering that z = σ γ βn / cos2 ) and for a slope1.5H:1V, the stress at the failure surface must be greater than12.5 kPa, i.e., a minimum depth of about 0.85 m. Figure 8also illustrates that Weedon till compacted to 95% of itsProctor value, i.e., a void ratio of 0.32 (wsat = 12%) will dis-play a dilative response during undrained shear, and thusnever experience liquefaction flow failures.

For the case of shallow slides, it is convenient to developa relationship between Cu, wL, and wsat. Figure 8 can be usedto determine, for a given soil, wsat values for different valuesof undrained shear. Path 2 in Fig. 8 illustrates clearly that,for Weedon till, a shear strength value of 5 kPa is directlyassociated with a water content of a saturated sample of22.5%. As anticipated, for a given soil, the undrainedstrength increases with decreasing water content of the satu-rated till (i.e., decreasing void ratios) (see Table 4). Asshown above, the effect of soil type is well-reflected by itsliquid limit. Figure 9 indicates that, for a given water con-tent, the liquefied strength increases with increasing valuesof wL. The relationships presented in Fig. 9 can be used toreadily estimate the undrained shear strength of liquefied tillgiven its liquid limit and its in situ compaction state, usuallygiven in terms of dry unit weight, γd. Obviously, fully satu-rated conditions are a prerequisite for liquefaction, and wsatcan be calculated from γd as:

[6] wsatd s w

w= −⎛

⎝⎜

⎞

⎠⎟1 1

γ ρ γγ

A practical design approach

The empirical relationships established above can be usedto obtain the required input parameters in stability analysesto readily yield design values of stable slopes. Design chartsconsidering that the embankment face is approximated by aninfinite slope provide a rapid means to assess the factor of

safety as a function of various soil properties and environ-mental conditions.

Design chartsFor the sake of simplicity, it is considered that the slip

surface is approximately parallel to the ground surface (infi-nite slope procedure), and that the safety factor is given by:

[7] Fh h

h= − ′( ) cos tan

sin cosγ γ β φ

γ β βw w

2

in which h is the depth of the slip surface, γ is the soil unitweight, hw is the elevation of the water table above the slipsurface, β is the bedding of the slip surface, and φ′ is the fric-tion angle of the soil.

Figure 10 presents the relationship between slope angle,liquid limit and safety factor for drained conditions for afailure surface and a water table located a depth of 1 m.These charts can be used to determine the slope of dense tillembankments. For different conditions, eq. [7] readily givesthe value of the safety factor.

In the poorly compacted zone near the face, it can be ar-gued that the slope β should be less than φpu′ to prevent theonset of liquefaction. This, in turn, signifies that extremelyflat slopes must be constructed as shown on Fig. 10 (opensquares). For instance, for a till that has a liquid limit of15%, the minimum slope inclination should be 4H:1V, and

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Fig. 8. Characteristics of Weedon till in a (w, τ, σn′ ) space.

TillWater content %for Cu = 3 kPa

Water content %for Cu = 5 kPa



Témiscaming 17.3 16.9 16.5 16.0St-Martin 24.5 22.0 21.5 20.2Sherbrooke 21.8 18.6 17.5 17.0Weedon 24.5 22.5 21.4 20.0

Table 4. Water content values at given undrained strength of liquefied tills.

for wL = 25%, it becomes 2.5H:1V. These values are toolow to be of practical interest.

Figure 10 is also indicative of the liquefaction susceptibil-ity of the various tills if placed loosely, and which becomesaturated with time. The difference between the slope angledetermined for drained and undrained conditions is directlyrelated to the loss of strength during liquefaction flow fail-ure. It appears thus that tills with low liquid limit values aremore susceptible to develop significant flows than tills withhigher liquid limits. This is confirmed by the failure ob-served in RTE 175 till (Konrad and al. 2000) where liquefiedtill flowed over a distance of 60 m. The till at RTE 175 was

characterized by a liquid limit equal to 17%, thus with a sta-ble slope of 3.5H:1V to prevent liquefaction flow failure,while a slope of 1.35H:1V would display a safety factor of1.2 for drained conditions.

Design of stabilization fillIt is well-known that moist tills can be placed at relatively

loose states. The study presented above clearly indicates thatfor those loose conditions, most of the tills (15% < wL <25%) will be very susceptible to liquefaction when progres-sively saturated. To reduce the susceptibility of liquefaction,several remedial measures can be considered:• Improved compaction near the face of the slope (costly

and fairly difficult).• Construction of a wider crest with subsequent cut to final

design width (costly, time consuming, not always feasibleif right-away too narrow).

• Slope stabilization by vegetation (often skin failures occurin the first spring before vegetation roots have grown suf-ficiently deep).

• Geosynthetics (costly).• Control of drainage, i.e., directing surficial water to ar-

moured channels.• Coarse-grained stabilization fill.

This latter remedial measure is often used by the QuebecMinistry of Transportation. It has several advantages becauseit acts as a draining layer and reduces water infiltration, espe-cially at the level of the loose–dense interface parallel to thetill slope. It also acts as a mechanical stabilization since theshear strength mobilized in the coarse-grained fill is thedrained strength characterized by its effective friction angle.When placed immediately during construction, it is the mostcost-effective solution.

Stability calculations with SLOPE/W were conducted toestablish the minimum thickness for the granular pad toensure slope stability once the till has liquefied along theloose–dense interface parallel to the till slope. The minimumthickness is associated with the parameter L shown in

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Fig. 9. Undrained shear strength of liquefied till.

Fig. 10. Design charts for till slopes used for highway embank-ments.

Fig. 11. The conditions of the analysis are well-summarizedby the insert in Fig. 11. The geometry considered hereincomprises a till slope at an inclination of 2H:1V in whichthe loose-dense interface is located at a depth of 1 m. Theheight of the water table above this interface is considered tobe 0.3 m. The granular fill placed above the till slope is de-fined by its minimum length, L, at the base of the till slopewhose height is H.

The stability analyses were conducted for two potentialfailure surfaces, referred to as surface S1 and S2. S1 is par-allel to the till slope, coinciding with the actual loose–denseinterface with the interface between the stabilizing fill andthe till. S2 is considered as a more realistic failure surfacesince it assumes that it exits through the stabilizing fill nearthe base of the till slope. S2 corresponds more closely to ac-tual failure surfaces observed in the field for till embank-ment slopes.

The strength parameters in the stabilizing fill were takenas c′ = 0 and φ′ = 45°. The undrained strength of the lique-fied till was taken as 5 kPa. It is emphasized that the actualvalue will be related to the liquid limit and the actual densityin the loose zone as illustrated by the relationships presentedin Fig. 9.

The results of the stability analysis show that the mini-mum width, L, of the stabilizing fill is related to the slopeheight, H, and the geometry of the failure surface (Fig. 11).For the realistic S2 failure surface, a stabilizing fill is re-quired when the height of the till embankment is larger than4 m. For a height of 10 m, the width of the stabilizing fill isclose to 5 m, for a strength of the liquefied till equal to5 kPa.

The stability analyses also showed that the width of thestabilizing fill was a function of slope inclination of the tillembankment, the position of both the water table and theloose–dense interface. Light-weight dynamic penetrometertesting may prove to be a useful tool to obtain adequate dataon the position of the loose–dense interface near the slopeface. Density determinations in the loose zone are also of ut-most importance to estimate the undrained shear strength ofthe saturated till.

Conclusions

Flow slides in cohesionless soils can be related to an un-drained collapse phenomenon that depends upon many fac-tors such as the initial void ratio, the effective stress pathduring loading, the initial stress state, and obviously the typeof soil. Only soils that tend to decrease in volume duringshear, i.e., contractive behaviour, can suffer the loss of shear-ing resistance that results in a flow slide when the drivingshear stress is larger than the ultimate undrained strength ofthe saturated soil.

Loosely compacted tills may be extremely sensitive toundrained collapse depending on their critical state charac-teristics. These conditions exist near the face of highwayembankments owing to difficulties of adequate compactionnear the slope edge, especially with increasing slope heights.Once the loosely placed tills become saturated as a result ofinfiltration in the spring or fall, undrained collapse and liq-uefaction flow failures may be triggered.

This study has shown that the strength parameters ofcohesionless tills can be related to the liquid limit for bothdrained and undrained conditions. These relationships can beused to readily provide the necessary input parameters instability analyses and yield design values of stable slopes.

Tills with liquid limits between 15% and 25% are all verysusceptible to liquefaction flow failures if loosely compactedand saturated. To reduce the risk of shallow slides in the tillembankments used for highways, a stabilizing coarse-grained fill placed atop of the till embankment appears to bean efficient technique. Design charts of the minimum thick-ness of the stabilizing fill at the base of the till slope consid-ering the empirical relationships for the strength of liquefiedtills were developed using available slope stability programs.

Acknowledgements

This research was supported by the Quebec Ministry ofTransportation (MTQ). The technical assistance of FrançoisGilbert, Jean Côté, Olivier Turcotte, and Isabelle Leclerc isgratefully acknowledged.

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