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Engineering Geology 168 (2014) 46–58

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Engineering Geology

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Influence of structure on shear characteristics of the unsaturatedloess in Lanzhou, China

Bao-Ping Wen ⁎, Ya-Jing YanSchool of Water Resources and Environment, China University of Geosciences (Beijing), No 29, Xueyuan Road, Haidian District, Beijing 100083, China

⁎ Corresponding author. Tel.: +86 10 82322081; f13910716003 (mobile).

E-mail address: [email protected] (B.-P. Wen).

0013-7952/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.enggeo.2013.10.023

a b s t r a c t
a r t i c l e i n f o
Article history:Received 14 February 2013Received in revised form 4 September 2013Accepted 26 October 2013Available online 1 November 2013

Keywords:Unsaturated loessShear characteristicsBreaking up of interparticle bondClayCalcium carbonate

Loess is typically a kind of soil with metastable structure, which makes loess sensitive to collapse and slidingupon wetting. Such structure has thus profound influence on loess's mechanical behavior. However, therehave been arguments about how the metastable structure influences on loess's mechanical behavior and whatthe mechanism behind is. In this study, the influence of structure on shear characteristics of the unsaturatedloess in Lanzhou city of Chinawas investigatedwith comparison of variation of shear strength and its parametersbetween six pairs of the undisturbed and remolded samples. It is found that the peak shear strength and strengthparameters of the loess significantly reduced once its structure was destroyed, while shear behavior of the loessshowed little change. Strength parameter c was much more sensitive to structure of the loess than ϕ. Shearstrength reduction of the loess due to change of structure should be largely attributed to breaking up of cemen-tation bonds between particles, evidenced by the difference in pore size distribution and microstructure of soilfabrics between the undisturbed and remolded samples. The bonds provided by clays and carbonates contributedthemost to structure of the loesswithminor by soluble salts, whilematric suction played little role in structure ofthe loess.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Loess is typically a kind of clayey silt with metastable structure andgenerally occurs in unsaturated state in the semi-arid climate regionsof the world. It has been widely recognized that loess's metastablestructure is characterized by its randomly open and loose particlepacking with high porosity and significant amount of macroscopicpores as a result of airfall deposition (e.g., Smalley and Vita-Finzi, 1968;Liu, 1985; Gao, 1988; Smalley et al., 1991). The nature of unsaturatedloess's open and loose packing structure has been found to be dominatedby varieties of weak, or short range bonds between skeleton particles,including matric suction (capillary force), clay, soluble salts and lesssoluble agents, such as carbonates, gymposium, and iron oxides (Holtzand Hillf, 1961; Matalucci et al., 1970; Rogers et al., 1994). Metastablestructure is thus fundamental to loess's mechanical behavior, whichdisplays that loess has a large loss of shear strength or great increasein compressibility with comparatively small changes in stress or defor-mation, particularly upon wetting, hence being highly susceptible tocollapse and sliding the world over (Holtz and Hillf, 1961; Matalucciet al., 1970; Derbyshire and Mellors, 1988; Derbyshire and Mellors,1988; Gao, 1988; Smalley et al., 1991; Rogers et al., 1994). Mechanicalproperties of loess upon wetting, specifically its hydroconsolidationand shear strength under different saturated states, have been

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extensively studied for many decades (e.g., Holtz and Hillf, 1961;Anayev and Volyanick, 1986; Milovic, 1988; Smalley et al., 1991;Dijkstra et al., 1994; Kruse Gerard et al., 2007). However, as to the influ-ences of the metastable structure, particularly relative roles of interpar-ticle bonds on loess's mechanical behavior and correspondingmechanisms behind, there seem considerable arguments. Some be-lieved that decrease in matric suction upon wetting should take majorresponsibility for reduction of loess's shear strength (e.g., Fredlund andRahardjo, 1993; Muñoz-Castelblanco et al., 2012). Others suggestedthat weakening of inter-particle bonds provided by clay and carbonatescontributes themost to loss of loess's strengthwhenwetted (e.g., Rogerset al., 1994; Dijkstra et al., 1995; Kruse Gerard et al., 2007). Handy(1973) attributed loess's collapsing behavior to interparticle's claybonds since such behavior occurred when loess had low clay mineralcontent. Exploring the effects of structure on loess's mechanical behav-ior hence still remains to be an active subject for research and engineer-ing practice (Jefferson et al., 2004; Kruse Gerard et al., 2007).

Loess has been revealed to have some striking similarities with thequickclays, particularly its metastable structure featured by open fabricand weak bonds between particles (Lutenegger, 1981; Smalley et al.,1991; Assallay et al., 1997; Jefferson et al., 2004). For quickclay, theeffect of structure on itsmechanical behavior has long been investigatedby comparison ofmechanical strength of the undisturbed and remoldedsamples (e.g., Rosenqvist, 1953; Gasparre and Coop, 2008). Similarly,the influence of structure on loess's mechanical behavior was alsoexamined by many researchers (e.g., Matalucci et al., 1970; Lutenegger,1981; Rendell, 1988; Dijkstra et al., 1994; Assallay et al., 1997). Dijkstra

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47B.-P. Wen, Y.-J. Yan / Engineering Geology 168 (2014) 46–58

et al. (1994) found that the undisturbed loess samples from Lanzhou ofChina had greater effective cohesions and less internal friction anglescomparedwith those of their remolded, and they attributed these differ-ences to breaking up of cementation bonds between skeleton particlesand redistribution of these particles into denser packing, respectively.Assallay et al. (1997) observed that the undisturbedMalan loess samplesfrom Lanzhou of China showed less compression with increased stressthan their disturbed specimens, and they accounted this to breaking upof bonds of clay and carbonates in the loess. Hu et al. (2000) observedthat the difference in compressibility between the undisturbed andremolded loess near Xi'an of China was greater when the normal stresswas small. Jiang et al. (2011) concluded that the difference in shearstrength between the undisturbed loess samples from Shanxi of Chinaand their remolded was higher when they were tested under lowconfining pressure than under high confining pressure. Referring to thesensitivity generally used for evaluation of structural influence onthe mechanical behavior of quickclays, which is quantified as ratio ofthe undisturbed to fully remolded strength at the same water content,Smalley and Derbyshire (1991) mentioned that the sensitivity was 10or more for loess, while it is generally greater than 16 for quickclays.

China has themostwidespread loess in theworld, where loessman-tles about 631,000 km2 of itsmainland and equivalently to about 6.6% ofits total area. Of the loess in China, about 317,000 km2 is the loessplateau in northwest China, the greatest bulk accumulation of loess inthe world.With attempt to better understand the influence of structureon the mechanical behavior of the loess in China, particularly for thepurpose of getting insightful understanding on the mechanism of theloess landslides, shear characteristics of six loess samples collectedfrom the Late Pleistocene loess, namely the Malan loess, on six slopesin Lanzhou city of China, were investigated in this study. The influenceof structure on shear characteristics of the loess was estimated by com-paring shear strength between the undisturbed and remolded loessspecimens. The mechanisms behind were discussed in terms of matricsuction, interparticle bonds, pore size distribution and microstructuresof soil fabric. It is believed that the difference in shear characteristics

Fig. 1. Distribution of loess in China and locati

between the undisturbed and the remolded loess could not only reflectstructural influence on the loess, but also give implication on behaviorof loess landslides, as mentioned by Derbyshire (2001).

2. Study area and samples

Lanzhou, the capital city of Gansuprovince and located in thewest ofthe loess plateau in China (Figure 1), is world-widely well-known forthick loess terrain in terms of geology, where loess thickness generallyexceeds 30 to 50 m with the maximum record of up to 335 m (Liu,1985; Derbyshire and Mellors, 1988). All the three main stratigraphicunits of the loess in China, namely the Wucheng loess (most of theLower Pleistocene), the Lishi loess (upper part of the Lower Pleistoceneand the Middle Pleistocene) and the Malan loess (the Upper Pleisto-cene), occur in Lanzhou (Liu, 1985). Due to its terrain landform andloess soils' mechanical properties sensitive to wetting and disturbance,Lanzhou has suffered from loess landslides the most severely in Chinain addition to frequent hydro-collapse (Ding and Li, 2009). More than172 landslides have occurred in Lanzhou since 1950s, of which 78%were shallow and rapid loess landslides in the Malan loess (Ding andLi, 2009). Recent investigation reported that more than 179 loess slopesare undermarginal stable state, posing a great risk to public safety in thecity (Ding and Li, 2009). It appears that occurrence of loess landslides inthe city tends to be more frequent as a result of greater intensity ofirrational human activities, for instance, irrigation on loess terrace foragriculture and afforestation, cutting and filling slopes for construction,and discharging sewage on slopes in densely populated areas (Ding andLi, 2009). On May 16 of 2009, a rapid and shallow loess landslide atJiuzhou, which was induced by irrigation for afforestation, killed sixresidents and destroyed part of a building (Figure 2).With the intentionto better understand the rapid and shallow loess landslides in Lanzhoucity, this study focuses on shear characteristics of the Malan loess. Theundisturbed loess samples were collected from the Malan loess at sixloess slopes, namely Xujiashan, Xiguoyuan, Taoshuping, Hualinping,Wenchangge and Jiuzhou, respectively (Figure 1). All the samples

ons of the sampling sites in Lanzhou city.

Fig. 2. A shallow and rapid loess landslide occurred on May 16, 2009 at a residential areanear Jiuzhou Development District in Lanzhou.

48 B.-P. Wen, Y.-J. Yan / Engineering Geology 168 (2014) 46–58

were carefully collected from trial pits using customized thin-walltubes, which had a diameter of 120 cm and length of 25 cm. Depth ofthese trial pits ranged from 4.0 m to 4.6 m below the ground surface(Table 1). The collected samples, named after their locations, werewrapped carefully using plastic film and kept in wooded boxes. The

Table 1Basic physical properties of the loess samples.

Sample Sampling depth below ground surface (m) Dry density (kN/m3) Void ratio A

L

XJS 4.1 16.3 0.71 3XGY 4.2 14.7 0.89 2TSP 4.0 14.2 0.93 3HLP 4.3 14.1 0.94 3WCG 4.4 12.6 1.19 2JZ 4.6 12.1 1.24 2

Table 2Mineralogical compositions of the loess samples.

Sample Whole soil (%)

Quartz Feldspar Calcite Dolomite

XJS 32 16 12 3XGY 39 17 15 7TSP 41 19 16 3HLP 29 23 13 4WCG 40 15 14 5JZ 39 19 15 5

Note: I/S—interlayered illite/smectite, I—illite; K—kaolinite; C—chlorite.

Table 3Content of soluble salt and calcium carbonate and major ion compositions in the loess sample

Sample Major ions (mg/L)

K+ Na+ Ca2+ Mg2+ Cl− SO42−

XJS 2.6 192 191 16.4 181 632XGY 3.0 129 15.6 3.0 35 255.9TSP 4.61 191 98.4 14.5 209 387HLP 4.73 247 151.0 16.0 269 520WCG 4.71 234 171 18.7 246 638JZ 6.16 325 80 20.1 187 544.3

Note that the unit mg/100 g means total salts in mg in 100 g dry powder sample.

boxes were fully filled and covered with dry loess soils before beingcaped and transported to the laboratory.

Physical properties of the six loess sampleswere determined follow-ing the Chinese National Standards (CNS) GB/T50123-1999 (SAC et al.,1999). These loess samples had low dry densities ranging from 12.1 to16.3 kN/m3 with a void ratio of 1.24 to 0.71 (Table 1). Silts in the loesssamples ranged from 75 to 92% with clays of 3–22% and very minorsands (3–5%). Their liquid limits and plastic limits were in the range of25 to 29 and 15 to 19, respectively. Bulk and claymineralogy of the sam-ples were measured using X-ray diffractometer following the methodsdescribed by Moore and Reynolds (1997). Results of semi-quantitativeXRD analysis also using the method suggested by Moore and Reynolds(1997) show that in addition to primary minerals quartz (29–40%),clay minerals (20–35%) and feldspar (15–23%), these samples alsocontained significant amount of carbonate minerals of calcite (12–15%) and dolomite (3–7%), of which calcite was much more abundant(Table 2). In terms of clay mineralogy, all the samples had primarilyillite (53–60%) with some interlayered illite/smectite (11–16%), kaolin-ite (10–13%) and chlorite (15–23%). Ionic concentrations of solublesalts and calcium carbonate in the loess samples were also determinedin accordance with CNS GB/T50123-1999 (SAC et al., 1999). The resultsshowed that all the six loess samples had more than 0.3% soluble saltswith a maximum of up to 1.18%, and they had more than 13% calciumcarbonate with a maximum of up to 16.2% (Table 3). In the samplesXGY, TSP, HLP, WCG and JZ, Na2SO4 was the major soluble salt (about98%, 74%, 74%, 64% and 68%, respectively) and CaCl2 was the secondary,whereas in XJS sample, NaClwas themost abundant (about 86%) follow-ed by some Na2SO4. It is noted that calcium carbonate in all the samples

tterberg limits (%) Particle size distribution (mm,%)

iquid limit (%) Plastic limit (%) Clay (b0.002) Silt (0.002–0.075) Sand (N0.075)

1.8 18.5 22 75 38.4 18.0 14 83 30.0 18.9 22 75 30.5 18.2 17 79 47.9 18.4 12 84 46.1 17.3 3 92 5

Clay (%)

Hornblende Clay I/S I K C

2 35 11 53 13 231 21 16 57 10 172 20 14 60 11 153 28 15 57 11 17/ 26 15 58 10 171 21 15 55 12 18

s.

Soluble salts in total (mg/100 g) Calcium carbonate (%)

HCO3−

31.2 1038.5 13.918.05 384.2 14.425.3 774.8 16.229.7 1031.2 15.434.2 1122.2 13.620.11 1182.9 15.7

image of Fig.�2

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XGY( Sr=20%)


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HLP( Sr=20%)


WCGSr=20%


JZ( Sr=20%)

Fig. 3.Representative curves of shear stress versus shear displacement for the loess samples. (Note that the solid symbols denote for the undisturbed samples, and the open symbols for theremolded.)


determined using chemical methods described in CNS GB/T50123-1999(SAC et al., 1999) was not consistent with calcite content measuredusing XRD. It is believed that this should be due to incomparable natureof the two methods. Nevertheless, both of the results indicate thatcarbonates, particularly calcium carbonate, were abundant in all thesamples.

3. Methods and sample preparation

3.1. Methods

The shear strength of unsaturated soils can be measured using asuction-controlled triaxial equipment or direct shear apparatus (Ganet al., 1988; Fredlund and Rahardjo, 1993; Hu et al., 2000; Maleki andBayat, 2012). However, these tests are very costly, time-consumingand often impractical to obtain. Instead, determining the unsaturatedshear strength using combined methods of suction measurement and

direct shear test is a widely accepted in practice. Several studies havedemonstrated that the shear strength of unsaturated soils determinedusing conventional direct test and its correlation with matric suctionmeasured using pressure plate test are comparable with those ob-tained using sophisticated modified direct shear test (Oloo andFredlund, 1996; Vanapalli and Fredlund, 2000; Vanapalli and Lane,2002). In this study, due to equipment availability, shear strength andmatric suction of the unsaturated Malan loess were measured usingthe conventional direct shear test and the pressure plate test, respec-tively. RSI-SHEARTRAC-II (by Geocomp Corporation) was employed toconduct direct shear tests, which is a fully automated direct shear appa-ratus and has a circular shear box with a diameter of 10 cm and heightof 2.5 cm. 1500F1 and 1600 pressure plate extractors (by the Soil Mois-ture Equipment Corporation) were used for suction measurement,which have capacities of 15 bar and 5 bar respectively and sample re-tainers of 50.8 mm in diameter and 10.2 mm in height. Microstructureand pore size distribution of the loess soils were analyzed using a

image of Fig.�3

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Fig. 4. Relationship between shear strength and saturation degree of the loess samples. (Note that Undis. and Rem. denote the undisturbed and remolded samples, respectively.)


scanning electron microscope (SEM) of Zeiss EVO 50 and a mercury in-trusion porosimetry (MIP) of Quantachrome, respectively.

3.2. Sample preparation

Asmentioned earlier, structural influence on shear characteristics ofthe loess soils in this study was investigated by comparing the differ-ence in shear characteristics between the undisturbed and remoldedloess samples. The undisturbed specimens for direct shear test and pres-sure plate test were trimmed very carefully using the samples takenfrom the field. The remolded specimens with the same bulk dry densi-ties as their undisturbedwere prepared byhandpulverizing and slightlytamping the air dried loess powder. Before being remolded, the air driedloess powder was thoroughly crumbed to such an extent that all visibleaggregates were destroyed. For each sample, six pairs of groups of theundisturbed and remolded specimens were prepared for direct shear

tests, whereas nine pairs of groups of the identical specimens wereprepared for pressure plate test.

For direct shear test, the six groups of specimens were air dried orwetted by adding distilledwater onto the specimens to saturation degreeof 20%, 40%, 50%, 60%, 70% and 80%, respectively. For each group of thesamples, triplicate specimens were prepared. The dried or wetted speci-mens were then kept in airtight containers for at least 24 hours be-fore being sheared. Such length of time could be sufficient to ensureair and water in the specimens in the state of equilibrium, as unsaturat-ed loess in Lanzhou has hydraulic conductivities generally ranging from1.8 × 10−3 cm/min and 1.0 cm/min when its saturation degree isbetween 20% and 80% (Yao et al., 2012). To prevent significant changein matric suction (water content) and volume of the specimens duringshearing, direct shear testwas doneunder unconsolidated andundrainedconditions, as suggested by Vanapalli and Lane (2002). Such shearingcondition was also believed to be analogous to that of shear failure in

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Fig. 5. Representative peak shear strength envelopes of the samples. (Note that the solidsymbols denote for undisturbed samples, and the open symbols for remolded.)

Table 4Shear strength parameters of theundisturbedand remolded loess specimenswith differentsaturation degree.

Sample Strengthparameter

Undisturbed/remolded


20 40 50 60 70 80

XJS c (kPa) Undisturbed 111 82 77 71 59 43Remolded 56 34 43 32 32 27

ϕ (°) Undisturbed 34.8 34.2 31.6 31.3 27.5 22.8Remolded 34.3 33.7 31.2 30.5 21.9 20.1

XGY c (kPa) Undisturbed 81 52 42 48 27 15Remolded 60 35 40 30 22 13

ϕ (°) Undisturbed 34 19.9 18.8 18.0 16.1 13.3Remolded 33.5 19.2 18.7 17.8 14.3 11.7

HLP c (kPa) Undisturbed 77 30 27 31 15 9Remolded 50 22 26 18 13 7


TSP c (kPa) Undisturbed 76 37 32 37 21 10Remolded 52 24 28 24 13 8


WCG c (kPa) Undisturbed 37 28 32 18 9 5Remolded 17 22 15 10 6 4


JZ c (kPa) Undisturbed 37 26 30 12 5 1Remolded 11 19 10 5 4 0



shallow and rapid loess landslides in Lanzhou city. Shear rate appliedin this study was 14 mm/min. Considering shallow occurrence of theloess soils (4.0–4.6 m) (Table 1), three low level normal stresses25 kPa, 50 kPa and 75 kPa were applied during direct shear test, which

was thought to approximately represent the loess samples' overburdenin the field.

Matric suction measurement by pressure plate test was carried outin accordance with ASTM D3152-72 (ASTM, 2000). For each pair ofthe sample (i.e., the undisturbed and remolded), nine specimens wereexamined under pressure of 7 kPa, 10 kPa, 50 kPa, 100 kPa, 200 kPa,300 kPa, 400 kPa, 500 kPa and 650 kPa, respectively.

Small block specimens of the air dried undisturbed andremolded loess specimens were carefully prepared for microstruc-ture observation with scanning electron microscopy (SEM) andpore size distribution measurement using mercury intrusionporosimetry (MIP).

4. Results

4.1. Shear behavior of the unsaturated loess soils

The representative results of the direct shear test of both the undis-turbed and remolded loess samples are shown in Fig. 3. It is seen thatshear stresses of all the undisturbed samples were much greater thantheir remolded, illustrating that shear strength of the loess soils had asignificant reduction after being remolded. On the other hand, for thesamples with dry densities greater than 12.6 kN/m3 (i.e., samplesXGY, XJS, TSP and HLP), including both the undisturbed and remoldedspecimens, shear stress reached a peak value at relatively small dis-placement and thereafter displayed a gradual reduction, whereas forthosewith lesser dry densities (i.e., samplesWCGand JZ), shear stress in-creased gradually to an ultimate value without a prior peak (Figure 3).This indicates that the loess soils are of the shear behavior either stainsoftening or strain hardening largely depending on their densities, butthe nature of their shear behavior had little change after beingremolded. Additionally, it seems that for the undisturbed specimenswith dry densities greater than 12.6 kN/m3, shear displacement neededto reach the peak shear stresses were smaller than that for theirremolded, implying that the loess soils' strain softening behavior wasweakened after being remolded.

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Fig. 6. Relationship between shear strength parameters and saturation degree of the samples. (Note that Undis. and Rem. denote the undisturbed and remolded samples, respectively.)


4.2. Variation in shear strength of the unsaturated loess soils and theirstrength parameters

When the peak or ultimate shear stresses are defined as peak shearstrength (hereafter shear strength for short) for the loess samples, itcould be seen from Fig. 4 that shear strength variations of the undis-turbed loess samples exhibited undulatingdecrease trendswith increasein saturation degree, during which a slight trough and crest occurredaround saturation degrees of 40% and 60% respectively for the sampleswith dry densities greater than 12.6 kN/m3 (i.e., XJS, XGY, HLP andTSP), and 40% and 50% respectively for those with lesser densities(i.e., JZ and WCG). Shear strength of the remolded specimens with drydensities greater than 12.6 kN/m3 varied with similar trends to theirundisturbed, but the troughs and crests occurred at less saturationdegrees (Figure 4). Shear strength variations of the remolded speci-mens with dry densities less than 12.6 kN/m3 displayed no troughs in

the range of saturation degree in this study (Figure 4). It is believedthat their troughs would occur when saturation degree is less than 20%.

Fig. 5 illustrates the representative peak shear strength envelopes ofthe samples. Table 4 lists the strength parameters of these samplesobtained from their strength envelopes, again showing that the shearstrength parameters of the loess samples had a reduction after beingremolded. It should be mentioned that these strength parameterswere total stress parameters because total normal stress was used toobtain strength envelopes. With increase in saturation degree, varia-tions of the cohesions (c) of both the undisturbed and remoldedsamples exhibited consistent decrease trends with those of the shearstrength, while variations in their internal friction angles (ϕ) alsoshowed decrease trends, but without notable troughs and crests(Figure 6). In terms of reduction degree at the same saturation degreefor each sample after being remolded, the reduction in c was greaterthan that in ϕ, latter of which had little reduction when the loess's dry

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1.4

1.6

25kPa 50kPa 75kPa

St f

or s

hear

str

engt

h


HLP

0 20 40 60 80 1001.0

1.2

1.4

1.6

25kPa 50kPa 75kPa

St f

or s

hear

str

engt

h


WCG

0 20 40 60 80 1001.0

1.2

1.4

1.6

1.8

2.0

25kPa 50kPa 75kPa

St f

or s

hear

str

engt

h


JZ

Fig. 7. Variation of sensitivity (St) for shear strength of the samples with saturation degree.


density was lesser than 12.6 kN/m3. It is noted that the differencesequence in c between the undisturbed and remolded loess was assame as that observed by Dijkstra et al. (1994), but that such sequencefor ϕwas different from that by Dijkstra et al. (1994). This may bemainlybecause of the difference in normal stress applied during shearingbetween this study and that by Dijkstra et al. The normal stressesapplied by Dijkstra et al. (1994) ranged from 25 kPa to 350 kPa, whichwere generally much higher than those applied in this study.

4.3. Influence of structure on shear characteristics of the unsaturated loess

Apparently, the significant difference in shear characteristics betweeneach pair of the undisturbed and remolded specimens with same bulkdry density and at the same saturation degree should be attributed totheir difference in structure due to remolding. Referring to sensitivity(St) used for quantifying structural influence on themechanical behaviorof quickclays, this parameter is also employed in this study to estimate

structural effect on shear characteristics of the unsaturated loess. Differ-ently, two kinds of sensitivities are used for shear strength and forstrength parameters at the same saturation degree, respectively, whichare defined as ratios of the undisturbed to remolded shear strengthand strength parameters (c and ϕ), respectively.

Fig. 7 shows that sensitivity (St) for shear strength of the loess soilsranged from 1.03 to 2.02, and that the St varied with both normal stressand saturation degree. It seems that for all the samples in this study thegreater the normal stress, the lesser the St was, indicating that the influ-ence of structure on shear strength of the loess soils decreases whennormal stress increases. Variation of the St with saturation degree wasundulated in the range of saturation degree from 20% to 80% with oneor two peaks and troughs, suggesting that there is a certain saturationdegree at which structure had the strongest or weakest influence onshear strength of the loess soils. For the samples with dry densitiesgreater than 12.6 kN/m3, it seems that structure had the strongest andweakest influence on their shear strength at saturation degrees around

0 20 40 60 80 1001.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4 XJS

c φ

φ φ

φ φ

φ


0 20 40 60 80 1001.0

1.2

1.4

1.6XGY

c


0 20 40 60 80 1001.0

1.2

1.4

1.6TSP

c


0 20 40 60 80 1001.0

1.2

1.4

1.6HLP

c


0 20 40 60 80 1001.0

1.2

1.4

1.6

1.8

2.0

2.2 WCG

c


0 20 40 60 80 100

1.2

1.6

2.0

2.4

2.8

3.2

3.6 JZ

c


St f

ot s

tren

gth

para

met

ers

St f

ot s

tren

gth

para

met

ers

St f

ot s

tren

gth

para

met

ers

St f

ot s

tren

gth

para

met

ers

St f

ot s

tren

gth

para

met

ers

St f

ot s

tren

gth

para

met

ers

Fig. 8. Variation of sensitivity (St) for strength parameters of the samples with saturation degree.


60% and 50% respectively, while for the samplewith lesser dry densities,such saturation degrees were about 50% and 40% respectively.

On the other hand, the St for strength parameter c (cohesion) of eachsamplewasmuch greater than that for itsϕ (internal friction angle), andthat the St variation for c of each sample with saturation degree wasabout consistent with that for its shear strength (Figures 7 and 8). Thissuggests that c of the loess soils is much sensitive to structure than itsϕ, and that c should take major responsibility for shear strength reduc-tion of the loess soil after being remolded. Prominently, the St for ϕ ofall the samples was very low when their saturation degree was lessthan 60%, and the drier the soil, the lesser the Stwas (Figure 8). It appearsthat for the samples with dry densities greater than 12.6 kN/m3, the Stfor ϕ reached the greatest value at a saturation degree around 60% or70%, whereas for the samples with lesser dry densities, the St for ϕshowed an increase trend when the samples' saturation degree wasgreater than 60%.

5. Discussion

Although the remolded samples had the same mineralogy, particlesize distribution and bulk dry density (or overall void ratio) as their un-disturbed, bonds at skeleton particle contacts in these samples providedby the cementing agents may be completely or partially lost when theirundisturbed loess was thoroughly remolded. Reduction in the loesssoils' shear strength and their strength parameters compared withtheir undisturbed should thus be resulted from breaking up of inter-particle bonds, or change of the loess soil'smetastable structure. Asmen-tioned earlier, there are varieties of interparticle bonds in loess, includingmatric suction (capillary force), clay, soluble salts and less soluble agents,such as carbonates, gymposium, and iron oxides (Holtz and Hillf,1961; Smalley and Vita-Finzi, 1968; Matalucci et al., 1970; Rogerset al., 1994). Shear performance of unsaturated loess is therefore closelyrelated to breaking up of interparticle bonds when it is remolded.

30 40 50 60 70 80 90 100

100

200

300

400

500

600

700a

Rem-XJS Rem-XGY Rem-TSP Undis-XJS Undis-XGY Undis-TSP

Mat

ric

suct

ion

(kPa

)


10 20 30 40 50 60 70 80 90 100

100

200

300

400

500

600

700

Rem-HLP Rem-WCG Rem-JZ Undis-HLP Undis-WCG Undis-JZ

Mat

ric

suct

ion

(kPa

)


b

Fig. 9. Relationship betweenmatric suction and saturation degree of of the samples. (Notethat Undis. and Rem. denote the undisturbed and remolded samples, respectively.)

30 40 50 60 70 80 90 1000.5

0.6

0.7

0.8

0.9

1.0


Rat

io o

f m

atri

c su

ctio

n

XJS XGY TSP HLP WCG JZ

Fig. 10. Variation of the ratio of matric suction in the undisturbed samples to that in theremolded with saturation degree. (Note that Undis. and Rem. denote the undisturbedand remolded samples, respectively.)


Results of matric suction measurement using pressure plate testconfirm that matric suction did occur in both the undisturbed andremolded loess soils, and that matric suction decreased with increasein saturation degree (Figure 9), as general unsaturated soils (Fredlundand Rahardjo, 1993). This evidences that matric suction gave some con-tribution to reduction in shear strength of both the undisturbed andremolded loess soils and their strength parameters. Comparing matricsuction in the undisturbed samples and that in their remolded, it isseen that although they had very similar variation patterns with satura-tion degree,matric suction in all the remolded sampleswas greater thanthat in their undisturbed at the same saturation degree (Figure 9). Thisindicates that matric suction may give more contribution to shearstrength of the remolded loess than to that of the undisturbed loess,i.e. the loess with metastable structure. Fig. 10 displays that ratio ofmatric suction in the undisturbed samples to the remolded varied dif-ferently from what the St for both their shear strength and strengthparameters exhibited (Figures 7 and 8). This suggests that matric suc-tion may not provide a notable contribution to loess soil's metastablestructure, or in otherwords, that structure of unsaturated loess is largelystabilized by cementation bonds between particles other than matricsuction.

Occurrence of considerable clays, calcium carbonate and solublesalts in all the samples (Tables 1 and 3) indicates that they are primarycementing agents of the loess soils, most likely in forms of both coatingand bridges or plus micro- and phanero-crystals (Kruse Gerard et al.,2007). Once destroyed during fully remolding, cementation bonds pro-vided by these cementing agents at skeleton particle contacts whichformed during airfall deposition of the loess soils cannot be replicated,as discussed by several researchers (e.g., Dijkstra et al., 1994; KruseGerard et al., 2007). Thus reduction in shear strength of the loess soilsafter being remolded should be attributed to breaking up of these ce-mentation bonds. Loss of cementation bonds in the remolded loesssoils explained why cohesions of all the remolded samples were muchlesser than those of their undisturbed (Figure 6), and why cohesionsof all the loess soils were sensitive to their structure (Figure 8).Considering the weakening of the cementation by soluble salts mayremarkably occur when moisture in the soils increases, for the undis-turbed and remolded loess soils at the same saturation degree thecementation by soluble salts may have much lesser contribution to thereduction in their shear strength than those by clays and calciumcarbonate, also possibly magnesium carbonate. In other words, thecementation bonds provided by clays and carbonates may be key con-trols ofmetastable structure of the unsaturated soils. On the other hand,the difference in St for shear strength and strength parameters of sam-ples XGY, TSP and HLP, which had about same dry densities, but differ-ent amounts of clays, calciumcarbonate and soluble salts, demonstratedthat overall effect of the bonds provided by these cementation agentson the loess's shear characteristics may be rather comprehensive, ora complex combination (Figures 7 and 8, Tables 1 and 3). The St forshear strength and c of sample HLP was the greatest among the threesamples,while it had lesser clays than TSP andgreater calciumcarbonatethan XGY. Similar results were also observed in samples WCG and JZwhichhad about samedry densities too (Figures 7 and8, Tables 1 and 3).

MIP measurement and SEM observation revealed that pore size dis-tribution and the nature of skeleton particles' contacts of the remoldedsamples were quite different from their undisturbed (Figures 11 and12), confirming that the loess soils' metastable structure was signifi-cantly destroyed as a result of breaking up of interparticle bonds follow-ing remolding. It is seen that all the remolded samples had much lesspores with a diameter greater than 10 μm and more pores with adiameter lesser than 10 μm than their undisturbed (Figure 11). Thisevidences that a collapse of the pores larger than 10 μm occurredin the undisturbed samples, or a denser particle packing after beingremolded as a result of breaking up of cementation bonds. Becausematric suction (capillary force) in unsaturated soil is inversely propor-tional to diameter of pores (Terzaghi et al., 1996), the difference in pore

1E-3 0.01 0.1 1 10 100 10000

1

2

3

4

5

Vol

ume

perc

enta

ge (

%)

Pore diameter (µm)

1E-3 0.01 0.1 1 10 100 10000

2

4

6

8

10

12

14

16

Vol

ume

perc

enta

ge (

%)

Pore diameter (µm)

1E-3 0.01 0.1 1 10 100 10000

2

4

6

8

10

12

14

16

Vol

ume

perc

enta

ge (

%)

Pore diameter (µm)

1E-3 0.01 0.1 1 10 100 10000

2

4

6

8

10

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16

Vol

ume

perc

enta

ge (

%)

1E-3 0.01 0.1 1 10 100 10000

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2

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Vol

ume

perc

enta

ge (

%)

Vol

ume

perc

enta

ge (

%)

1E-3 0.01 0.1 1 10 100 10000

2

4

6

8

10

12

Pore diameter (μm)

Pore diameter (μm)

Pore diameter (μm)

Remolded Undisturbed

JZ

RemoldedUndisturbed

TSP


XGY


WCG


HLP


XJS

Fig. 11. Pore size distribution curves of the samples measured using MIP.


size distribution between the undisturbed and remolded samples inter-prets why matric suction in the remolded samples was greater at thesame saturation degree. SEM photographs exemplifies that more abun-dance of larger pores in all the undisturbed samples, while pores withsmaller sizes were extensive in their remolded (Figure 12). In termsof interparticle contacts, it is clear that that the edge to edge contactsbetween particleswere prevailing in all the undisturbed samples, where-as the face to face contacts became overwhelming in their remolded,again indicating the occurrence of a denser particle packing once loess'original structure was destroyed (Figure 12).

6. Conclusions

By comparison shear characteristics between the unsaturated andremolded loess in Lanzhou of China and their pore size distributionand microstructure, it is found that structure had profound influence

on shear characteristics of the loess. Such influence could be concludedas the following.

1) Once the loess's metastable structure is destroyed, its shear strengthand strength parameters would significantly reduce, whereas thenature of shear behavior of the loess seems to have little change.Strength parameter c of the loess ismuchmore sensitive to structurethan its ϕ.

2) Themechanism for the reduction in shear strength of the unsaturatedloess and its strength parameterswas due to breaking up of cementa-tion bonds between particles. For the loess in this study, the cemen-tation bonds were largely provided by clays and calcium carbonate,and also possibly magnesium carbonate, and minor by soluble salts,little bymatric suction. Change of pore distribution, interparticle con-tacts and occurrence of a denser particle packing evidenced breakingup of interparticle bonds.

A2

B2B1

C1

A1

C2

D1 D2

Fig. 12. SEM photographs of the samples. A1, B1, C1, D1, E1, and F1 are the photographs of the undisturbed samples XJS, XGY, TSP, HLP,WCG and JZ, respectively; A2, B2, C2, D2, E2, and F2are the photographs of the remolded samples XJS, XGY, TSP, HLP, WCG and JZ, respectively.



3) Reduction extent of shear strength of the unsaturated soil and itsstrength parameters also related to overburden pressure and drydensity (overall void ratio) apart from breaking up of interparticlebonds.

Acknowledgments

This researchwas funded by theNational Natural Science Foundationof China (No. 40872182). The authors are thankful to Mrs Zhi-Heng Li,Cheng Zhao, Yong-Jun Zhang, and Rui-Dong Li from the Gansu Instituteof Geo-Environmental Monitoring for their assistance during field work.Ms Lei He, Mrs. Shu Jiang and Jing-Fang Yan also joined the field work.

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