Engineering Geology 183 (2014) 31–38

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Engineering Geology

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Laboratory study on soil behavior in loess slope subjected to infiltration

Y.F. Zhou a,⁎,1, L.G. Tham b, W.M. Yan b, F.C. Dai c, L. Xu c

a Key Laboratory of Geotechnical Mechanics and Engineering of Ministry of Water Resources, Changjiang River Scientific Research Institute, Wuhan 430010, Chinab Department of Civil Engineering, The University of Hong Kong, Hong Kong, Chinac Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

⁎ Corresponding author. Tel.: +86 27 82927646.E-mail address: zhouyuefenghku@gmail.com (Y.F. Zho

1 Formerly Department of Civil Engineering, The UniveChina.

http://dx.doi.org/10.1016/j.enggeo.2014.09.0100013-7952/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 April 2014Received in revised form 23 September 2014Accepted 23 September 2014Available online 30 September 2014

Keywords:LandslideInfiltrationLoessUnsaturated soilStress pathConstant load

In recent years, Heifangtai Plateau in Gansu Province, China, has received significant attention as a loess region inwhich landslides frequently occur. This study aims to examine the soil behavior and failure mechanism(s) ofloess slopes subjected to water infiltration. High-quality samples were retrieved behind the failure plane of alandslide inHeifangtai. Three sets of stress path tests were conducted tomimic the process of loess slope failures;these tests include undrained compression on anisotropically consolidated saturated samples, drained shear bydecreasing mean effective stress at constant axial load on saturated samples, and wetting by decreasing matricsuction at constant axial load on unsaturated samples. Loess behavior was evaluated in terms of shear strength,deformation upon shearing, critical state, state parameter, and soil water characteristic curve. Threemajor obser-vationswere drawn from the test results. First, the anisotropically consolidated saturated loess exhibits a notice-able strain-softening behavior upon undrained compression. The peak shear strength ismobilized at less than 1%axial strain. The ultimate shear strength is only 20–30% of the peak value. Second,when subjected to shear by de-creasingmean effective stress at constant axial load, the saturated loess exhibits sudden failure when the confin-ing stress is low. At higher stresses, the soil exhibits progressive failure. Third, unsaturated loess deformsprogressively when subjected to a reduction in matric suction (i.e., wetting) at constant axial load. The failuremechanisms of the loess slopes are discussed based on the experimental findings.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

A loess is a geological formation mainly composed of silts that aretransported and laid down by the wind (Zhang et al., 1987). Commonlyreferred to as “problematic soil,” loess has two typical characteristics:structural feature andwater sensitivity (Lin, 2008). The structural featureof loess is influenced by the cementation, packing, and orientation of par-ticles in sediment composition. With regard to water sensitivity, loess, atpartially saturated state, has a high shear strength and stiffness, which issignificantly weakened when wet. These two features suggest a well-known loess bulk behavior, i.e., “collapsibility,” in which loess is proneto collapse during wetting. Covering an area of at least 6.31 × 105 km2

of China, loess predominantly exists in northern China, including Gansu,Ningxia, Shaanxi, Shanxi, Henan, and Xinjiang provinces (Derbyshireet al., 2000). The climate in the area is arid or semi-arid monsoonal. Formany decades, diversion irrigations have been carried out to support ag-ricultural activities in these areas. In recent years, numerous diversion ir-rigation projects have been planned and are being constructed. However,

u).rsity of Hong Kong, Hong Kong,

agricultural irrigations have induced many severe landslides at the edgesof the loess plateaus, which include Linxia, Heifangtai, Huining, Gaolan,Hexi, and Yuzhong in Gansu Province (Derbyshire et al., 2000; Wu andWang, 2006; Wen and He, 2012) as well as Jingyang (Jin, 2007; Jin andDai, 2008), Bailuyuan (Lei and Qu, 1991; Miao, 2003), and Hua country(Zhang et al., 2009) in Shaanxi Province. Among them, Heifangtai is fa-mous for frequent occurrences of loess landslides caused by irrigations.Heifangtai has a total area of 13.7 km2, andmore than 70 large-scale land-slides have been reported in this area since the 1960s (Xu et al., 2008).The increasing frequency of landslides inHeifangtai has urged researchersto fundamentally and systematically examine the trigger/failure mecha-nisms of loess slopes.

Landslides can be triggered by water infiltration through two differ-ent processes: (1) wetting of soil at a shallow depth because of short-term rainfall or irrigation (Brand, 1981; Anderson and Sitar, 1995;Sorbino and Nicotera, 2013) and (2) rising of the groundwater table indeep soil because of prolonged irrigation (Dai et al., 1999; Jin and Dai,2008). The increase in soil water content causes a reduction of matricsuction in unsaturated soil or an increase in positive pore water pres-sure in saturated soil, both of which lead to a reduction of shearstrength. The natural water content of the loess near the ground surfaceis low, generally lying below10% (Wang andHui, 2001). It is anticipatedthat a significant portion of shear strength of the unsaturated loess iscontributed by the matric suction, which keeps the slope temporarily

32 Y.F. Zhou et al. / Engineering Geology 183 (2014) 31–38

safe. The suction is destroyed during wetting, thereby reducing shearstrength.

Brand (1981) suggested that the field stress path for unsaturatedsoil subjected to rainfall-induced landslide can bemimicked in a triaxialtest by reducing the matric suction under constant axial stress. Thisconstant-stress condition can be imposed in two ways: (1) using aclosed-loop feedback controlling system tomaintain a constant deviatorstress or (2) applying constant axial load (e.g., dead weight) to thespecimen. Anderson and Sitar (1995) performed constant deviatorstress tests and stated that the mobilization of debris flow involvesboth drained initiation and triggering of undrained deformation of thecontracting soil. By analyzing the field anisotropic stress states, Ander-son and Sitar considered it appropriate to represent the field stresspath by constant deviator stress tests while ignoring the minor influ-ence of seepage stresses. For contractive saturated soils, continuous de-formation may trigger further development of excess pore waterpressure, leading to further soil collapse. Dai et al. (1999) performedsimilar tests on colluvial soils. The soils exhibited a dilative behavior,in which negative excess porewater pressurewas generated. Accordingto Dai et al., the surpluswater during an intense rainstorm can compen-sate for the reduction of pore water pressure as a result of dilation,leading to further deformation. Cascini et al. (2010) proposed a mathe-matical model for describing the failure and post-failure behaviors of ashallow landslide resulting from rainfall. The model was used to assessthe failure conditions and the potential of the failed soil mass to travela long distance. The initiation of an infiltration-induced landslide is gen-erally considered a drained process under constant total stress (Brand,1981; Chen et al., 2004; Cascini et al., 2010). After the onset of failure,fully or partially undrained shearing could develop in very loose mate-rials, and a flow slide could be triggered (Chu et al., 2003; Chen et al.,2004; Olivares and Damiano, 2007; Sorbino and Nicotera, 2013; Zhanget al., 2013).

In China, loess landslides are investigated mainly through depictionand statistics. Few studies have applied systemic stress path testing toexamine the soil behavior and failure mechanism(s) of loess slopes. Inparticular, the behavior of unsaturated loess under constant axial loadsubjected to a wetting path is seldom explored and therefore requiresfurther investigation (Xie, 1999). Three sets of stress path tests wereconducted in this study on retrieved loess samples to investigate the be-havior of saturated and unsaturated loess subjected to water pressureand matric suction, respectively. This study aims to provide insightinto the failure mechanisms of loess slopes resulting from irrigation.

2. Field investigation and sampling

This study is part of a comprehensive investigation on the interplayamong slope instability and cracks, sinkholes, and local collapseresulting from water infiltration in Heifangtai (Xu et al., 2011; Zhouet al., 2014). In recent decades, landslides in Heifangtai had demonstrat-ed three distinct features: (1) an increase in sliding frequency, (2) anincrease in sliding volume, and (3) an increase in the travel distanceof debris. The failures have brought about economic loss and societalimpact.

Fig. 1 depicts the No. 2 Qiaotou landslide in Heifangtai with a round-backed shape; the landslide had a travel distance of approximately

salt efflorescence

Fig. 1. No. 2 Qiaotou landslide in Heifangtai (photo taken on May 2010).

400 m. By using dry rotary drilling technique, a borehole was drilledat the test site to explore the ground profile. The test site's ground suc-cession consists of 42.5m of aeolianMalan loess and 4m of clay, follow-ed by a 4.5 m thick pebble stone layer, which in turn overlies the lowermudstone and sandstone strata. The permeability of the clay is relativelylow. Therefore, a perched groundwater table rises gradually in the loesslayer because of prolonged irrigation. Piezometer readings revealed thatthe current groundwater level in Heifangtai is approximately 26 mbelow ground.

Samples were obtained behind the failure plane of the landslide.First, loess blocksmeasuring 300mmon each sidewere cut tominimizethe disturbance on the samples. Before being mounted into a samplingcontainer made of sheet iron, the blocks were trimmed into cylindricalsamples with 150 mm diameter and 220 mm height. The small gapsbetween the sample, the mold, and the end caps were filled by loesspowder. The samples were then sealed by waterproof membrane andadhesive tape in the field. The samples were placed inside boxes filledwith sawdust before being sent to the laboratory to reduce vibrationduring transportation. Prior to storage, the samples were sealed withmolten wax immediately upon arrival at the laboratory.

3. Stress path tests

3.1. Test program

Table 1 summarizes the basic physical properties of the studiedloess, which were determined in accordance with BS1377 (BritishStandard Institution, 1990). Fig. 2 shows the particle size distributionof the loess, which mainly consisted of silt-sized particles with low tomedium in situ density.

A series of three tests was conducted in this study; these testsincluded (1) undrained compression of anisotropically consolidatedsaturated samples (ACU), (2) drained shear on saturated samples by de-creasing themean effective stress p′ (by increasing pore water pressureuw) at constant axial load (SCL), and (3) wetting (by decreasing matricsuction) unsaturated samples at constant axial load (UCL). The purposeof the ACU tests is to study the potential of static liquefaction of the sat-urated loess aswell as to deduce its critical state strength and associatedparameters. The SCL tests aim to investigate the initiation of a slide insaturated loess as a result of the rising of the groundwater table. TheUCL tests aim to examine the behavior of unsaturated loess subjectedto wetting at a constant axial load, whichmimics near groundwater in-filtration. In ACU tests, a constant rate of axial deformation was appliedto shear the saturated loess samples. In SCL and UCL tests, the axial loadwas provided and maintained by placing ingots on the specimens atstages with the aid of a hanger. To initialize deformation and failure,the pore water pressure (SCL tests) and matric suction (UCL tests)were increased and decreased, respectively. A UCL test takes approxi-mately 4 months to be completed; thus, only two tests were conductedbecause of time constraints.

Shallow landslides are commonly classified as those having slidingmass depth smaller than 2–3 m, whereas deep-seated landslides havegreater depth slides (Laprade et al., 2000; Baum et al., 2005). Tomimic a low stress level (e.g., at 3 m depth), the specimens in ACU1and SCL1 were first isotropically consolidated to p′ = 50 kPa prior tothe conventional drained shearing (at constant cell pressure) to achievea deviator stress q=50kPa. The specimens became anisotropically con-solidated, and the stress state corresponded to p′= 66 kPa with an ini-tial stress ratio K = 0.5. The K-value was adopted to mimic the in situearth pressure, which was estimated by using the well-accepted Jáky'sformula (Jáky, 1944) under an assumed internal friction angle of 30°.This friction angle was assumed prior to any testing. The material's fric-tion angle (32.3°) was later found to be slightly greater than the as-sumed value. However, a small difference in K-value was consideredto have an insignificant effect on the soil behavior in the studied testsand the interpretations of results. Higher stress levels with the same

Table 1Physical properties of Heifangtai loess.

Size content In-situ density (Mg/m3) Dry density (Mg/m3) Specific gravity Liquid limit (%) Plastic limit (%) Saturated permeability (10−6 m/s)

Sand (%) Silt (%) Clay (%)

8 72 20 1.45–1.55 1.38 2.7 26.5 18.3 4.0–8.4

33Y.F. Zhou et al. / Engineering Geology 183 (2014) 31–38

stress ratio were adopted in the remaining ACU and SCL tests to mimicloess at greater depths. After the isotropic consolidation in theUCL tests,the specimen was consolidated anisotropically to the target stress ratiounder drained condition by incrementally increasing the axial load(Table 2). All of the results were interpreted according to BS1377(1990), with the considerations the constraint of membrane and areacorrections. The results are summarized in Table 2.

Table 2Testing program and result summary of the tests.

3.2. Test procedures

Prior to each test, the top lid of a sealed sample was opened in thelaboratory. A thin-wall tube (inner diameter: 76 mm; area ratio: 1:15)was used for the coring. Confined by a split mold, the top and bottomof the sample were then trimmed to minimize soil disturbance andconsequently obtain a specimen measuring 76 mm in diameter and152 mm in height.

In the ACU and SCL tests, the saturation of the specimen comprisedtwo steps. First, the specimen was flushed with carbon dioxide forapproximately half an hour and then circulated with de-aired waterfor an hour at an initial confining pressure of 15 kPa. Second, the cellpressure and the back pressure were increased alternately, with an in-crement of 25 kPa. A B-value of 0.98 can be easily achieved with aback pressure 250 kPa. Thereafter, the specimen was consolidatedisotropically, and the consolidation is terminated when 95% of the ex-cess pore water pressure is dissipated. The specimen was then shearedunder drained condition to the designed stress ratio (K = 0.5). In theACU tests, drained compression using a shearing rate of 0.05 mm/minwas adopted to shear the specimens and bring them to an anisotropicstress state prior to undrained compression. During this process, thepore water pressures at the top and bottom of the specimen were mea-sured. The difference was always smaller than 1 kPa, implying a suffi-ciently slow shear rate to maintain the drained condition. When thetarget anisotropic stress state was reached, the drainage valve wasclosed, followed by an undrained compression shearing. In the SCLtests, both the cell pressure and the axial load were kept constantafter anisotropic consolidation. The anisotropically loaded specimenwas then sheared by increasing the pore water pressure at a rate of 4kPa/h, whereas the axial load was maintained. Junaideen et al. (2010)investigated the rationale for choosing the rate of pore pressure incre-ment for soil with permeability of 10−5–10−6 m/s, and stated that therate used in the tests minorly affect the soil behavior. In this study,

0

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60

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0.001 0.01 0.1 1 10

Particle size (mm)

Perc

enta

ge p

assi

ng (

%)

Test 1Test 2

Fig. 2. Particle size distribution of the loess.

pore pressure was measured at the bottom and top of the specimen toconfirm the uniformity of stress distribution.

Loess collapsibility was traditionally investigated by submerging theground surface of loess underwater in thefield or by fully saturating thespecimen in the laboratory (Ye et al., 2004). However, the progressivewetting of loess seemed more realistic. The behavior of loess followinga wetting path is critical in examining the mechanism of infiltration-induced landslides. On basis of axis translation technique (Hilf, 1956),the high air-entry ceramic disk (HAECD) was adopted for the majorityof the unsaturated soil tests. The air pressure and the water pressureare controlled at both sides of the specimen, and the difference betweenwhich is regarded as the “controlled matric suction.” In view of the lowcoefficient of permeability of the unsaturated soil and HAECD, the vari-ation inwater content in soil is laggedwhen compared to the controlledmatric suction. Several UCL tests were performed by continuously re-ducing the controlled matric suction to simulate slope failure resultingfromwetting (e.g., Jin, 2007). However, suchmethod is not appropriatefor depicting soil deformation and failure because this approach ignoresthe time lag between the actual matric suction in the soil and the con-trolled matric suction. Therefore, the UCL tests should be performedby achieving an equalization of water content and matric suction in astepwise manner to accurately investigate soil deformation and failure.

The UCL tests were performed at a controlled temperature of20 ± 1 °C to minimize thermal influence on the measurement of cellvolume. Calibration was first conducted to estimate the volume changeof the system, including the effects of cell pressure, water absorption,and load ram movement (Zhou, 2012). The HAECD attached on thebase pedestal of the triaxial specimen was saturated before each test.Under an initial cell pressure of 15 kPa, axis translation technique wasutilized to apply initial suction to the specimen. The specimenwas load-ed under drained condition by increasing dead weights (axial load) instages until the target stress ratio was achieved. Thereafter, the speci-mens were subjected to wetting by reducing the matric suction,which is achieved by increasing the back pressure while maintaining aconstant cell pressure and pore air pressure. The back pressure was in-creased by 5–25 kPa in each step, which was maintained for the equal-ization of matric suction for 5–10 days because of the low coefficient ofpermeability of the unsaturated soil and HAECD.

Test Afterisotropicconsolidation

After anisotropic compressionbut prior to shear

Critical/final statewhen test isterminated

p' or p(kPa)

v p' or p(kPa)

q/p' orq/p

σ'1/σ'3 orσ1/σ3

v p' or p(kPa)

q/p' orq/p

v

ACU1 50 1.946 65 0.76 2.02 1.937 8 1.35 1.937ACU2 100 1.958 132 0.75 2.00 1.948 11 1.35 1.948ACU3 199 1.885 261 0.76 2.02 1.866 35 1.36 1.866SCL1 49 1.971 66 0.76 2.02 1.943 13 0.80 1.943SCL2 101 1.912 134 0.76 2.02 1.876 58 1.30 1.848SCL3 200 1.910 269 0.74 1.98 1.861 114 1.30 1.839UCL1 30 2.002 60 1.50 4.00 1.992 55 1.38 1.932UCL2 50 1.988 93 1.39 3.59 1.982 90 1.33 1.926

Note: p',σ'1,σ'3 are themean effective stress, the effectivemajor principal stress, the effec-tive minor principal stress, respectively. p, σ1, σ3 are the net mean stress, the net majorprincipal stress, the net minor principal stress, respectively. q is the deviator stress; v isthe specific volume.

34 Y.F. Zhou et al. / Engineering Geology 183 (2014) 31–38

4. Test results

4.1. ACU tests

Fig. 3 summarizes the results of the ACU tests. Three levels of initialstress, which mimicked soil at different depths, were investigated.When subjected to undrained compression, the anisotropically consoli-dated specimens required only approximately 1% axial strain to attainthe peak strength (Figure 3(a)). The deviator stress exhibited a remark-able reduction with further compression. In addition, the excess porewater pressure initially increased noticeably when the axial strain wasbelow 5% and then increased in a much slower rate (Figure 3(b)). It im-plied a strong contractive tendency at the beginning of compressiveshearing when the shear strength first rose to a peak and remarkablydecreased subsequently. Both the deviator stress and the excess porewater pressure of ACU1 and ACU2 achieved steady values at an axialstrain of approximately 20%. However, aminor change in deviator stressand porewater pressure occurred beyond20% in ACU3. The critical stateof the soil was estimated by the final state of each specimen.Fig. 3(c) shows the stress path of each specimen. The effective confiningstress of ACU1 was the smallest, indicating that only a small increase in

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Axial strain εa (%)

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ACU2

ACU3

ACU1

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Exc

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-wat

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ress

ureΔ

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a)

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b

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ress

q (

kPa)

Peak states

Critical states

ACU2

Critical state line

ACU1

ACU3

c

a

Fig. 3. Results of the ACU tests.

excess pore water pressure can substantially reduce the shear strengthof the soil and lead to a flow type of sudden failure. In the three tests, atremendous drop in shear strength can be observed when the stresspath approached the critical state line (will be discussed in Section 5).

4.2. SCL tests

In the SCL tests, the deformation of a soil specimen was initiatedby reducing the mean effective stress, p′, by increasing the pore waterpressure, uw. Fig. 4(a, b) shows the evolution of the axial and volumetricstrains, respectively, with decreasing p′. Fig. 4(c, d) shows the develop-ment of the volumetric strain and the deviator stress with axial strain.Fig. 4(e) shows the stress path. In the figures, yielding (indicated bythe triangles) was determined as the state where a variation of stiffnessoccurred in the v–lnp′ plane (Zhou, 2012). As shown in Fig. 4(a), thechange in axial strain was negligible at the beginning of each test. In re-sponse to the decrease in p′, specimen SCL1 showed a sudden increasein axial strain when p′ was reduced to just below 50 kPa. Shortly afterthe axial strain responded to the decrease in p′, the axial strain of thespecimen increased to 30% in 2 s only. The soil structure was destroyedquickly, leading to a rapid collapse of the soil skeleton when subjectedto the imposed stress path (decreasing p′ at constant axial load). Thecollapse then led to a rapid increase in excess pore water pressure,which cannot be fully drained and thus triggered a sudden flow-typefailure in an “undrained” manner. SCL2 and SCL3, with higher initialconfining stresses (134 and 269 kPa, respectively), showed a similar re-sponse. However, the increase in axial strainwasmuch slower. Progres-sively accelerated axial strain in SCL2 and SCL3 became obviouswhen p′dropped to below105 and 190kPa. The induced excess porewater pres-sure should dissipate effectively, and the soil only showed progressivedeformation in a drainedmanner. The SCL tests were performed at con-stant axial load; thus, the deviator stress q decreased with an increasein axial strain and a corresponding increase in cross-sectional area(Figure 4(d)). In Fig. 4(b), a small volumetric increase (a negative volu-metric strain denotes swelling) can be identified before the yield points(triangles), suggesting the elastic swelling of the soil. Beyond the yieldpoint, the rate of swelling slowed down. At this stage, the soil contrac-tion (because of the tendency of soil collapse) was anticipated tobalance part of the unloading swelling. As mentioned previously,specimen SCL1 collapsed in 2 s only, in which the pore water did nothave sufficient time to drain. Therefore, the instrument was unable torecord any volumetric strain in SCL1. After the test, the total volumetricstrain in SCL1 was found to be negligible. By contrast, SCL2 and SCL3gradually deformed after the yield point, and the volumetric contractionwasmeasured by a volumetric measuring device. After yielding, the ac-celerated axial and volumetric strainswere developed in the three tests.A sudden or gradual collapse of the soil structure occurred at the yield-ing stress state. In this process, themobilized strength dropped togetherwith volumetric collapse and disturbance on the soil skeleton or contactbetween soil particles. Thereafter, a state of constant volumetric strainand mobilized stress ratio can be identified, albeit with a continuouslyincreasing axial strain (Figure 4(c, e)). The tests were then terminated,and the final states were assumed to be the critical state of the speci-mens. The accumulated volumetric strains in SCL2 and SCL3 were 1.5%and 1.2%, respectively, which suggests an in-situ spatial difference inloess samples. The instability and critical state line in the p′–q planewas defined accordingly (Figure 4(e)). The stability line was definedas the peak state line and the lower bound of the region of potential in-stability (Zhou, 2012). The distinct reduction in the deviator stress wascaused by the increase in cross-sectional area during the loading whilethe axial load was maintained.

4.3. UCL tests

Fig. 5 shows the variations of the axial and volumetric strains withdecreasing matric suction at constant axial load. For specimens UCL1

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trai

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(%

)

SCL2 SCL3SCL1

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a (%

)

c

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Axial strain εa (%)

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(kP

a)

d

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Dev

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ress

q (

kPa)

Yielding

Critical states

SCL2

Critical state line

SCL1

SCL3

e

Instability line

Fig. 4. Results of the SCL tests.

35Y.F. Zhou et al. / Engineering Geology 183 (2014) 31–38

and UCL2, matric suction reduced from 90 kPa and 140 kPa, respective-ly, to close to zero. Before matric suction reached approximately 50 kPa,both the axial and the volumetric strains were negligibly small. A slightincrease in both strains can then be observed when matric suction re-duced from 50 kPa to 30 kPa.With a further reduction in matric suction(below 30 kPa), the strains then increased noticeably, which suggeststhe onset of loess instability. This phenomenon was similar to theloess behavior in the SCL tests with decreasing p′, which suggests apotential slope instability that was induced by continuous wetting.Based on the micro-scale studies on loess, Gao (1980) and Yang(1988) reported a contractive behavior of loess, which was because ofsaturation, using the scanning electron microscope (SEM) technique.The saturation was induced by the destruction of soil cementation,and the subsequent rearrangement of the soil particles. In the UCLtests, the stress condition at 30–50 kPa matric suction showed acceler-ated axial and volumetric strains, which suggests sufficient weakeningof soil cementation because of wetting. The rearrangement of particlescould induce filling of pore space by particles and volumetric collapse.Nevertheless, no sudden failure of a specimen was observed duringmatric suction equalization period of 5–10 days in each step of the

UCL tests. The soil was progressively deformed. At a lower stress level,specimen UCL1 shows a smaller volumetric strain than UCL2. The finalvolumetric strains in the UCL tests (1.9% in UCL1 and 2.7% in UCL2,respectively) are generally greater than those in the SCL tests (themaximum value of 1.5%) despite the stress level of the former onesbeing much lower (Table 2). Hence, the unsaturated loess would expe-rience a stronger structural collapse in the wetting process. Moreover,no evidence suggests that the UCL specimens have reached the criticalstate.

5. Discussion

5.1. Saturated loess

5.1.1. Potential of flow slidesTo evaluate the static liquefaction potential of soil, Poulos et al.

(1985) presented three factors, namely, (1) ratio of critical-statestrength to peak-state strength, (2) axial strain required to reach thepeak undrained strength, and (3) rate of change of post-peak undrainedstrength. Despite thewide controversy that comeswith the term “static

0

3

6

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15

Matric suction (ua-uw) (kPa)

Axi

al s

trai

n ε a

(%

)

UCL2

UCL1

0

1

2

3

4

5

0 30 60 90 120 150

0 30 60 90 120 150

Matric suction (ua-uw) (kPa)

Vol

umet

ric

stra

in ε

v (%

)

UCL2

UCL1

b

a

Fig. 5. Results of the UCL tests.

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Mean effective stress p' (kPa)

Dev

iato

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ress

q (

kPa)

CSL: q = 1.317p’

1.7

1.8

1.9

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2.1

0 50 100 150 200 250 300

1 10 100 1000

Mean effective stress p' (kPa)Sp

ecif

ic v

olum

ev =

1+e

initial state (ACU tests) initial state (SDL tests)

critical state (ACU tests) critical state (SDL tests)

b

CSL: v = -0.050lnp’+2.064

ACU1

SCL3

a

Fig. 6. Critical state lines of the studied loess.

36 Y.F. Zhou et al. / Engineering Geology 183 (2014) 31–38

liquefaction,” this evaluation approach is adopted herein to analyze thepossibility of flow slide of a slope. The test results showed the following:(1) the ratio of critical-state strength to peak-state strength is approxi-mately 20%; (2) the axial strain at the peak state lies below 1%; and(3) the curve of shear strength at the post-peak state is concave againstaxial strain, which suggests a rapid decrease in shear strength. The re-sults indicate that flow failure can be triggered in the Heifangtai loess.Here, the term “flow” refers to the geological motion type of fluid orsemi-fluidmaterial (Hungr et al., 2001), because the pore water insteadof the soil skeleton carried the majority of total stress.

0

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0.1 1 10 100 1000

Matric suction (ua-uw) (kPa)

Vol

umet

ric

wat

er c

onte

nt (

%)

UCL1

UCL2

Fig. 7. Soil-water characteristic curves of the studied loess during wetting path.

5.1.2. Critical state lineThe critical state depicts an ultimate condition of perfect plasticity, in

which plastic shearing could indefinitely continue without changes inthe volume or effective stresses (Muir Wood, 1990). The critical statesobtained from the ACU tests and the SCL tests are plotted in Fig. 6. Thepoints converge to a unique straight line in the p′–q plane but showslight scattering in the ν–lnp′ plane, as also shown inmany other exper-imental characterizations of geomaterials perhaps owing to the uncer-tainty of density evaluation at the critical state (Wang and Yan, 2006;Yan and Ma, 2010; Yan and Li, 2012). The parameters of the criticalstate line areM= 1.317, Γ= 2.064, and λ= 0.05, where M is the crit-ical stress ratio, Γ is the critical state specific volume of the soil at p′=1kPa, and λ is the gradient of CSL in the v–lnp′ plane.

The state parameter describes the initial state of soil in terms of thedifference in void ratio between the initial state and the critical state lineat the samemean effective stress in the v–lnp′ plane (Been et al., 1991).As shown in Fig. 6(b), the initial state of saturated soil in this studyalways lies above the critical state line, which suggests a positive stateparameter and a loose soil at the corresponding stress level. In accor-dance with that shown in Figs. 4 and 5, the soil behaves as a highly con-tractive soil. To clearly demonstrate the soil behavior in the v–lnp′plane,two tests (ACU1 and SDL3) are shown in Fig. 6 representatively. InACU1, the path continuously moves to the left until the critical state is

reached. In SDL3, a great and dominant contraction occurs after slightswelling prior to reaching the critical state.

5.2. Unsaturated loess

5.2.1. The soil water characteristic curve (SWCC)The soil water characteristic curves (i.e., SWCCs) of loess can be de-

duced from the wetting paths of the UCL tests (Figure 7). The SWCC ofUCL2 locates slightly lower than that of UCL1 because of its higher stresslevel and lower initial porosity. An air-entry value of about 1 kPa to3 kPa is expected for the studied loess and concurs with the range ofair-entry value of siltymaterials from theprobabilistic analysis conduct-ed by Chiu et al. (2012a,b). The features of the SWCC can be used to pre-dict the variation of soil strengthwithmatric suction (Zhao et al., 2013).Ng and Pang (2000) introduced stress-dependent SWCC to clarify theeffects of net normal stress and stress path on thewater retention capa-bility of an unsaturated soil. Essentially, the stress level not only causes achange in the soil density (or void ratio), but also results in the variation

37Y.F. Zhou et al. / Engineering Geology 183 (2014) 31–38

of pore size distribution. The SWCC will be correspondingly affected bythe stress state.

5.3. Mechanism of loess landslides

Whilewater infiltration is believed to be themain cause of triggeringloess landslides, infiltration has different roles, which depend on thematerial's degree of saturation. Irrigation is expected to cause an in-crease in the shallow ground soil water content in short-term and alsoa rise of the ground water table (often in deeper ground) in long-term. These phenomena are sometimes referred to as the formation ofa wetting band and a perched water table.

Irrigation wets the initially unsaturated shallow ground loess andyields progressive deformation (as revealed in the UCL tests). Despitethe lack of detailed local records of shallow landslides because of the rel-atively low economic loss andminor scale, a landslide frequently occursin irrigation, and inevitably leads to a great loss of agricultural land.Together with the critical state lines at the saturated state and at92–100 kPa matric suction, the stress path in UCL2 is re-plotted inFig. 8 (Zhou, 2012). As stated by Zhao and Zhang (2014), the stressstate can be expressed using a single stress state variable expression(Bishop, 1959; Lu and Likos, 2006) to consider the combined actionsof total normal stress and matric suction in the wetting test. Duringwater infiltration, the shear strength is reduced with the downwardmovement of the critical state line from CSL′ to CSL. Meanwhile, the ef-fective stress decreases at a roughly constant deviator stress, which isaccompanied by the destruction of the loess structure and the develop-ment of soil deformation. When the degree of saturation becomes high(formation of wetting band and perched water table in soil at lowerstress ratio, such as SCL1), any further infiltration causes an increase inpore water pressure. As shown in SCL1 (Figure 8, representing soil fail-ure in 3 m depth), a sudden collapse could then be triggered and resultin slope failure.

According to the stress path testing on saturated specimens (SCL2and SCL3), an increase in pore water pressure could lead to a progres-sive deformation when the stress level is high (e.g., SCL2 in Figure 8).Such a progressive deformationmay be used to shed light on the occur-rence of deep-seated slope failure with a rising groundwater level. Awell-known case is the Jiaojia landslide group in Heifangtai, which in-cluded at least 16 landslides (e.g., the No. 2 Qiaotou landslide inFigure 1) that occurred adjacently in the recent 30 years. The landslidegroup shows a number of common features: first, all of the landslidesoccurred in the loess layer with the thickness of 7–30m (i.e., a field em-bodiment of stress levels in SCL2 and SCL3); second, cracks are com-monly found at crests of loess landslides; third, spring lines andsaturated soil expose at back-scars of slopes; and fourth, the existenceof the reactivity of the landslides. Field monitoring shows that thegroundwater level gradually rises because of large-scale irrigation,with a rising rate of 0.3 m/year in the recent years (Jia et al., 2013).

Fig. 8. Comparison of stress paths in SDL1, SDL2 and UDL2.

The continuous increase in pore water pressure is responsible for theinitiation of the deep-seated landslide as an environmental factor. Initiallocal failure does not always induce a flow failure. However, once localfailure is initiated, subsequent stress transfer could lead to a rapid soilloading compared with a slow excess pore pressure dissipation underin situ partially drained or even undrained conditions. The strain-softeningbehavior of loess in the ACU tests suggests that the loess struc-ture is potentially collapsible, thus providing an internal factor for thetransformation from initial local failure to flow failure. A chain processof failure possibly results in a landslide if the condition of topographyon the plateau is satisfied (Figure 1).

Remedial measures based on the analyses of this paper are sug-gested to prevent frequent occurrence of landslides in loess regions.These remedial measures are as follows: (1) the flooding irrigationmust be improved into amore efficient approach to reduce intense infil-tration on ground surface (i.e., preventing the development of the fieldstress path for unsaturated soil in Figure 8); and (2) a vertical drainagesystem penetrated to the pebble stone layer (refers to ground succes-sion introduced in Section 2) can be considered to reduce groundwaterlevel in Heifangtai. A reverse or rightward movement of the stress pathfor saturated soil from the critical state strength envelope in Fig. 8 candecrease slope instability.

6. Conclusions

In this study, three sets of stress path tests were performed to exam-ine the mechanism of infiltration-induced loess landslides. The follow-ing major conclusions were drawn from results of the stress path tests:

(1) Contraction or tendency of contraction is the main volumetricbehavior of soil following different stress paths, which could beattributed to the destruction of the loess structure. A flow typefailure could be triggered on the basis of Poulos' liquefactionevaluation approach.

(2) Under constant axial load condition, the saturated loess showedtwo modes of failure with increasing pore water pressure. At alower stress level (e.g., mimicking a shallow depth), the initiallydrained deformation could evolve into an undrained suddenfailure. On the contrary, progressive failure is observed at ahigher stress level.

(3) Under constant load condition, the unsaturated loess shows aprogressive deformation with decreases in matric suction. Nosudden failure is observed in the UDL tests.

(4) In saturated loess at low stress, a rapid increase in pore waterpressure at constant axial load is believed to be themain trigger-ing mechanism of loess landslides.

(5) Remediation measures, which include the improvement of theflooding irrigation method and the construction of a verticaldrainage system penetrated to the pebble stone layer, can beconsidered in Heifangtai.

Acknowledgment

The authors acknowledge the financial support of the ResearchGrants Council of the Hong Kong Special Administrative Region (ProjectNos. HKU7140/08E; HKUST9/CRF/09; HKUST6/CRF/12R). The authorsappreciate the assistance provided by Mr. H. Min of the Institute ofRock and Soil Mechanics, Chinese Academy of Sciences and Mr. T. C.Chan of the Department of Civil Engineering, the University of HongKong.

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