Research ArticleExperiment Study of Lateral Unloading Stress Path and ExcessPore Water Pressure on Creep Behavior of Soft Soil

Wei Huang,1,2 Kejun Wen ,3 Dongsheng Li,1,2 Xiaojia Deng,4 Lin Li,5 Haifei Jiang,6

and Farshad Amini3

1School of Civil Engineering and Architecture, Chongqing University of Science and Technology, Chongqing 401331, China2Chongqing Key Laboratory of Energy Engineering Mechanics & Disaster Prevention and Mitigation, Chongqing 401331, China3Department of Civil and Environmental Engineering, Jackson State University, Jackson, MS 39217, USA4School of Civil Engineering, Chongqing University, Chongqing 400045, China5Department of Civil and Architectural Engineering, Tennessee State University, Nashville, TN 37209, USA6School of Civil Engineering, Chongqing Jiaotong University, Chongqing 400074, China

Correspondence should be addressed to Kejun Wen; kejun.wen@jsums.edu

Received 28 August 2018; Revised 23 November 2018; Accepted 29 November 2018; Published 14 January 2019

Academic Editor: Hossein Moayedi

Copyright © 2019 Wei Huang et al. +is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

+e unloading creep behavior of soft soil under lateral unloading stress path and excess pore water pressure is the core problemof time-dependent analysis of surrounding rock deformation under excavation of soft soil. +e soft soil in Shenzhen, China,was selected in this study. +e triaxial unloading creep tests of soft soil under different initial excess pore water pressures (0, 20,40, and 60 kPa) were conducted with the K0 consolidation and lateral unloading stress paths. +e results show that theunloading creep of soft soil was divided into three stages: attenuation creep, constant velocity creep, and accelerated creep. +eduration of creep failure is approximately 5 to 30mins. +e unloading creep behavior of soft soil is significantly affected by thedeviatoric stress and time. +e nonlinearity of unloading creep of soft soil is gradually enhanced with the increase of thedeviatoric stress and time.+e initial excess pore water pressure has an obvious weakening effect on the unloading creep of softsoil. Under the same deviatoric stress, the unloading creep of soft soil is more significant with the increase of initial excess porewater pressure. Under undrained conditions, the excess pore water pressure generally decreases during the lateral unloadingprocess and drops sharply at the moment of unloading creep damage.+e pore water pressure coefficients during the unloadingprocess were 0.73–1.16, 0.26–1.08, and 0.35–0.96, respectively, corresponding to the initial excess pore water pressures of 20, 40,and 60 kPa.

1. Introduction

Soft soil is a kind of regional special soil with significantrheological characteristics and widely distributed in China’sPearl River Delta, Yangtze River Delta, and other coastalareas.+e total area of the coastal zone is about 280,000 km2,which is the most economically developed region in China.With the rapid development of economic construction, theabovementioned areas have carried out large-scale devel-opment of soft soil, such as soft soil deep foundation pits,underground shopping malls, and subway [1, 2]. +e stressthat soil body experienced is mainly an unloading process inthe excavation of soft soil [3]. +e engineering practice

shows that the deformation, instability, and damage of thesurrounding rock of soft soil do not occur immediatelyduring the excavation or after the excavation, but they ex-perience a period of history, that is, the unloading creepdamage of soft soil occurs [4, 5]. Meanwhile, the high-levelgroundwater normally exists in soft soil areas, and thedisturbance or vibration of construction will lead to largeexcess pore water pressure. +e presence of pore waterpressure will weaken the microstructure of the soft soilparticles, and it can make the unloading creep damage of thesoft soil extremely strong and even cause the rheologicaldisasters. In 2003, Shanghai Metro Line 4 failed due to thefreezingmethod, which caused the soft soil around the shield

HindawiAdvances in Civil EngineeringVolume 2019, Article ID 9898031, 9 pageshttps://doi.org/10.1155/2019/9898031

tunnel to withstand high-pressure water and caused rheo-logical disasters [6]. erefore, the soft soil unloading me-chanics caused by lateral unloading stress path and excesspore water pressure has become an inevitable problem in thedesign and construction of excavation of soft soil.

Studies have been conducted on di�erent stress paths forsoft soil since Lambe [7] proposed the concept of stress path.e early research on soft soil theory mainly focused on thestrength and characteristics of soft soil under the loadingstress path (e.g. AE loading stress path in Figure 1) [8]. Withthe deep understanding of the creep behaviors of soft soil,scholars have realized that the actual unloading stress path isneglected. e stresses and strains calculated by using pa-rameters and models obtained from conventional axialloading tests (AE loading stress path in Figure 1) are foundto be signi�cantly di�erent from the actual ones. However,the in�uence of the lateral unloading stress path on the creepbehaviors of soft soil is neglected, which results in a theo-retical di�erence between the theoretical calculation resultsand the actual stress and deformation. erefore, selectingthe stress path in accordance with the actual unloadingprocess to explore the unloading creep behaviors of soft soilhas attracted great attention from scholars and practicalengineering.

Zhou and Chen [9] found that the creep e�ect of soilunder lateral unloading conditions was signi�cantly higherthan that of axial loading through triaxial undrained tests onsoft soil in the Pearl River Delta. Negative pore waterpressure was generated when the soil sample was subjectedto undrained shear under lateral unloading conditions.However, their study did not further study the variation lawof negative pore water pressure. Fu et al. [10] conducted atriaxial lateral unloading creep test on soft soil in Shanghai,China. eir results showed that the creep of soft soil afterunloading was divided into three stages: attenuation creep,constant velocity creep, and accelerated creep. During thecreep process, the pore pressure coe�cient changed withtime. Zeng et al. [11] conducted a series of experiments onthe mechanical properties of soft soil in Guangzhou atdi�erent consolidation conditions. eir results showed thatthe lateral unloading would increase shear stress and de-crease volume stress, which could result in a dilatancy in thesoil. e coe�cient of pore water pressure was related to theconsolidation state, but they did not study the variation lawof pore water pressure coe�cient.

e weakening e�ect of excess pore water pressure onsoft soil in actual engineering is accompanied by the wholeprocess of unloading creep of soft soil, which is very im-portant to clarify the mechanism of unloading creep damageof soft soil. Li et al. [12] studied the changes of pore waterpressure of clay subgrade in the process of dynamic vibrationof construction and indicated that the dissipation time ofexcess pore water pressure in clay formation is 20–40 h. Zhuet al. [13] reported the pore water pressure caused byconstruction vibration in saturated soft soil area.ey foundthat the distribution range of excess pore water pressure isaround 43–64 kPa. Jian and Chang [14] analyzed the vari-ation of soft soil strain and pore water pressure during theprocess of principal stress rotation. ey found that shear

stress has a signi�cant impact on the accumulation of porewater pressure. Yan et al. [15] studied the variation of excesspore water pressure in saturated soft clay.e results showedthat in the triaxial tensile test, the excess pore water pressuregenerated by the change of the deviatoric stress is positive.Cai et al. [16] studied the e�ect of consolidation stress pathson the shear characteristics of overconsolidated clay. eyreported that the increase of the ratio of vertical and radialconsolidation stress (K) could lead to lower negative excesspore water pressures.

Previous studies mainly focused on the unloading me-chanical properties of soft soil under single lateral unloadingpath or the distribution of excess pore water pressure causedby construction vibration. e quantitative analysis of theunloading creep of soft soil under the coupling of lateralunloading stress path and excess pore water pressure issparse.erefore, this study considered the lateral unloadingstress path and the excess pore water pressure caused by theconstruction in soft soil area in Shenzhen, China. A series ofK0 consolidation triaxial undrained unloading creep testswere carried out to investigate the e�ects of di�erent initialexcess pore water pressures, di�erent consolidation con-�ning pressures, and lateral unloading on the unloadingcreep behaviors of soft soil. is research would providetheoretical guidance for the excavation of soft soil andfurther lay the foundation for the future numerical simu-lation analysis of soft soil unloading creep.

2. Materials and Methods

2.1. Soft Soil. e test soils are taken from a soft soilfoundation pit in Shenzhen. e soil is grayish black, smellyand contains a small amount of shells. In order to carry outsu�cient contrast test, the lateral unloading strength testunder di�erent consolidation con�ning pressures and initialexcess pore water pressures use disturbed soil samples. enatural density (ρ) and water content (w) of the soil sampleare determined by the statistical average of the undisturbedsoil. e above values are used as the expected values of the

II district

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k0A

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Figure 1: Schematic diagram of space excavation under soft soil.

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disturbed soil samples. +e cutting-ring density method isused to measure the natural density (ρ), the drying methodto measure the water content (w), the pycnometer method tomeasure the soil specific gravity (Gs), the GYS-2 photo-electric liquid-plastic limit instrument to measure plasticlimit, the TSZ automatic stress-controlled triaxial apparatustomeasure the undrained cohesion (c), and the friction angle(φ) and the variable head method to measure the perme-ability coefficient k. Other properties of soft soils are de-termined by Geotechnical Test Method Standard(GBT50123-1999). +e physicomechanical properties of softsoil are summarized in Table 1. +e size of standard soilsample is 80.0mm in height and 39.1mm in diameter.

2.2. SpecimenDesign. To carry out themechanical propertiesof soil unloading in accordance with engineering practice,the stress path involved in excavation must be firstly ana-lyzed. +e unloading path of space under soft soil (such as afoundation pit) can generally be simplified to threeunloading stress paths as shown in Figure 1. +e first is theAB stress path, which is the unloading path with only thelateral unloading of soil. +e second is the AC stress path,which is the unloading path with the constant lateral loadand the decrease of axial load.+e third is the AD stress path,which is the unloading path with decreasing of both axialand lateral loads and accompanied by the rotation of theprincipal stress axis.

+e AB unloading stress path involves analysis of lateraldeformation and excavation stability of supporting struc-ture. +erefore, this paper concentrated on the lateralunloading stress path (AB) in Zone I. According to study byZhu et al. [13] and the actual construction of soft soil inShenzhen, the excess pore water pressure of 17–64 kPa wasmonitored.+erefore, four different initial excess pore waterpressures are proposed, which are u0 � 0, 20, 40, and 60 kPa,respectively, to explore the unloading creep mechanicalproperties of soft soil under the coupling of excess porewater pressure and lateral unloading path.

+e TSZ automatic stress control triaxial instrument isused in this study. +e range of the axial pressure sensor is1 kN; the range of the displacement sensor is 50mm; theengineering range of the confining pressure is 1MPa; theresolution is 1 kPa; the volume range of the back-pressuresystem is 120mm3; the volumetric accuracy is 1mm3.

2.3. Experimental Procedure. +e back-pressure saturationsystem of TSZ automatic stress control triaxial apparatus isselected to saturate the soil sample. +e 110 kPa confiningpressure and 90 kPa back-pressure are applied, and thesaturation of the soil can reach 98%. +e three differentconsolidation confining pressures (σ3 �100, 200, and300 kPa) are selected in this study. +e unequal consoli-dation (K0 � 1− sinφ′ � 0.53) is used to restore the self-weight stress state of the soil. +erefore, the final axialpressures are 189, 377, and 566 kPa.

After the consolidation of the corresponding confiningpressureK0 is completed, the upper and lower drain valves ofthe triaxial apparatus are closed, and the initial excess pore

water pressures u0 � 0, 20, 40, and 60 kPa are applied to theinterior soil through the back-pressure system. +e lateralunloading creep test under undrained conditions is con-ducted in 6–7 stages. +e unloading rate is ∆q� 1 kPa/min.+e duration of each stage after unloading is around 2–4 d,and the detailed unloading process is shown in Table 2. +econfining pressure, axial pressure, axial deformation, andexcess pore water pressure are recorded during the tests untilaxial strain reaches 15%.

3. Results and Discussion

3.1. Study of Strain-Time Curve. +e unloading strain-timecurves of soft soil under different initial excess pore waterpressure with 100 kPa consolidation confining pressure areshown in Figure 2. +e unloading strain-time curves of theconfining pressure of 200 kPa and 300 kPa are similar tothat of 100 kPa. +ey are not presented in this study. +edeformation obtained from the triaxial creep test under thelateral unloading stress path can be divided into in-stantaneous deformation and unloading creep. +eunloading creep of soft soil can be divided into threestages: attenuation creep, constant velocity creep, andfailure creep. It can be seen that attenuated creep andfailure creep occur mostly, and the constant creep happensonly once in A5 unloading of Figure 2(c). Fu et al. [10]studied the unloading process of soft soil in Shanghai,China. +ey pointed out that constant creep would notoccur during soft soil unloading creep, which is opposite tothe results in this study. +is could be due to that theloading time of each stage is too short in their study.Meanwhile, the unloading creep of soft soil is related todeviatoric stress (q � σ1−σ3). In the case of A1 unloading inFigures 2(a)–2(c), when the deviatoric stress is low, itexhibits attenuated creep, and the amount of creep de-formation is very small. In the case of A5 and A6 inFigure 2(a), when the deviatoric stress increases, the creepdeformation of the soft soil becomes greater, and the timerequired to reach the stable condition is longer, but thestage is still in the attenuated creep stage. In the case of A7in Figure 2(a), A6 in Figure 2(b), and A5 in Figure 2(d),when the deviatoric stress approaches the ultimate load,the creep deformation of the soft soil increases sharplywithin a few minutes until damage occurs, which is thefailure creep.

+e initial excess pore water pressure has an obviousweakening effect on the unloading creep of soft soil. Underthe same consolidation confining pressure, the unloadingcreep deformation of the soft soil produced by the samedeviatoric stress is greater while the initial excess porewater pressure increases. +e higher initial excess porewater pressure is more likely to cause unloading creepdamage. For example, in A4 unloading of Figures2(a)–2(d), the creep deformation generated at the initialexcess pore water pressures u0 � 0, 20, 40, and 60 kPa are0.33%, 2.41%, 5.14, and 9.74%, respectively. +erefore, theinitial excess pore water pressure in soft soil should bereduced as much as possible to prevent unloading rheo-logical damage in actual engineering.

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Under high initial excess pore water pressure, the softsoil can produce large creep deformation under low stress,which is beneficial to the discovery of unloading creepdamage of soft soil in practical engineering. However, it isnot easy to find unloading creep damage of soft soil underthe low initial excess pore water pressure. For example, inFigures 2(a) and 2(b), the deformation of the soft soil afterthe first three stages of unloading is very small. With theaccumulation of lateral unloading, the unloading creep ofthe soft soil gradually increases and eventually occursunloading creep damage. +e unloading creep damage ofsoft soil is more concealed and sudden under low initialexcess pore water pressure. +erefore, during the actualunloading excavation process, the soft surrounding rockshould be supported in time to prevent excessive lateraldeformation and the soft soil unloading damage.

3.2. Study of Deviatoric Stress-Strain Isochronal Curve. It isincapable of studying the nonlinear creep properties of softsoil through strain-time curve. Zou et al. [17] and Dob et al.[18] reported that the creep of soft soil is nonlinear, and thelinear creep only occurs at very lower stress. +e lateralunloading creep deviatoric stress-strain curves of soft soilunder different initial excess pore water pressures at con-solidation confining pressure 100 kPa are shown in Figure 3.When the deviatoric stress is low, the curve can be ap-proximately defined as a straight line, and the creep be-haviors of soft soil exhibit linear viscoelastic properties.When the deviatoric stress is relatively high, the curvegradually changes from a straight line to a curve. +eunloading creep characteristic exhibits strong nonlinearviscoplasticity, and the bending origin of the curve corre-sponds to the yield stress (σs) of the soft soil unloading creep.Wang et al. [19] reported a similar result in studying thecreep behaviors of loess. +eir study also indicated that theyield stress of loess is significantly higher than that of soft soilat confining pressure of 100 kPa.

+e unloading creep behaviors of soft soil are not onlyrelated to the deviatoric stress, but also closely related totime.+e tangent slope at any point of the curve is defined asthe viscoelastic modulus of the creep curve (Evel(t)). It can beseen from Figure 3 that at any deviatoric stress level, theEvel(t) of soft soil creep decreases with time, especially at ahigher level of deviatoric stress. It also indicates that thenonlinearity of soft soil unloading creep increases with time.

+e initial excess pore water pressure also has a sig-nificant weakening effect on the yield stress (σs) of soft soilunloading creep. Table 3 shows the relationship between theinitial excess pore water pressure and the yield stress (σs). Itis found that the soft soil yield stress (σs) decreases linearlywith the increase of the initial excess pore water pressure.Meanwhile, the slope of the fitting relationship tended toincrease with the confining pressure, indicating that theeffect of excess pore water pressure on the yield stress isgreater at a high confining pressure.

3.3. Change of Excess Pore Water Pressure. +e pore waterpressure-time curve under different pore water pressureswith 100 kPa consolidation confining pressure is shown inFigure 4. From the whole process of unloading creep behaviorof soft soil, the excess pore water pressure generally shows adownward trend with the intensification of lateral unloadingof soft soil. +is is mainly due to the reduction of the lateralrestraint of the soil and the dilatation of the soil. Especiallywhen the creep failure of the soil sample is about to occur, thehigh stress can immediately cause excessive plastic sheardeformation of the soil, and the excess pore water pressurewill drop sharply. According to the principle of effective stressof Terzaghi, while the excess pore water pressure dropsdramatically, the effective stress will suddenly increase, whichmay cause the unloading creep failure of the soil. +erefore,the excess pore water pressure should be closely monitored inactual engineering to prevent the creep failure of soil. Figure 5showed the pore water pressure-time curve at initial excess

Table 2: +e test unloading schemes.

Unloading processσ3c � 100 kPa σ3c � 200 kPa σ3c � 300 kPa

σ3 (kPa) σ1 (kPa) σ3 (kPa) σ1 (kPa) σ3 (kPa) σ1 (kPa)K0 consolidation 100 189 200 377 300 566A1 90 189 180 377 270 566A2 80 189 160 377 240 566A3 70 189 140 377 210 566A4 60 189 120 377 180 566A5 50 189 100 377 150 566A6 40 189 80 377 120 566A7 30 189 — — — —

Table 1: Physicomechanical properties of soft soil.

Soil samplesPhysical properties Shear properties

ρ (g/cm3) w (%) e Sr (%) Gs wL (%) wP (%) Ip IL c (kPa) φ (°)

Undisturbed sample∗ 1.80 39.2 1.105 96.0 2.73 41.5 25.2 16.3 0.88 18.1 26.4Disturbed sample 1.82 39.6 1.094 98.8 19.9 28∗+e average value of 30 undisturbed samples.

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pore water pressure (40 kPa). At the initial stage of each stageunloading, all excess pore water pressures suddenly drop.When they drop to the lowest value, the excess pore waterpressure gradually rises and remains stable. e excess porewater pressure drops slightly in the A1–A3 level but dropssharply in the A4 and A5 levels. is is consistent with theresults of creep deformation of soft soil in Figure 2(c).erefore, when the soft soil unloading creep increases, thevolumetric volume expansion of the soil will increase,resulting in an increase in excess pore water pressure.

e above changes in the excess pore water pressure arerelated to the unloading creep mechanism of soft soil. eunloading creep of soft soil actually is the process of damageand self-healing e�ect of the grain structure inside the soil.After the lateral unloading occurs, the force balance betweenthe soil particles is broken, and the soil particles try to adjustthe relative position to reach a new balance. When the self-healing e�ect of the soil is greater than the damage e�ect, thesoil will enter the attenuated creep stage, the deformationtends to a stable value, and the excess water pressure will rise

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A5: σ3 = 50kPa, σ1 = 189kPaA6: σ3 = 40kPa, σ1 = 189kPaA7: σ3 = 30kPa, σ1 = 189kPa

(a)

A1: σ3 = 90kPa, σ1 = 189kPaA2: σ3 = 80kPa, σ1 = 189kPaA3: σ3 = 70kPa, σ1 = 189kPaA4: σ3 = 60kPa, σ1 = 189kPaA5: σ3 = 50kPa, σ1 = 189kPaA6: σ3 = 40kPa, σ1 = 189kPa

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Figure 2: e unloading strain-time curves of soft soil under di�erent initial excess pore water pressures (consolidation con�ning pressure100 kPa). (a) Excess pore water pressure u0� 0 kPa. (b) Excess pore water pressure u0� 20 kPa. (c) Excess pore water pressure u0� 40 kPa.(d) Excess pore water pressure u0� 60 kPa.

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slightly. is process is particularly evident in the A1–A3 levelwhere the initial excess pore water pressure is 40 and 60 kPa.When the damage e�ect is greater than the self-healing e�ect,the new balance can not be achieved by the adjustment of thesoil particles. e plastic deformation caused by shearing willdirectly appear as a dilatancy phenomenon, resulting in asharp decrease in the excess pore water pressure as shown inthe A5 level of Figure 5.

Skempton [20] analyzed unloading problems of clay soiland de�ned the excess pore water pressure as

Δu � B[Δσ3 + A(Δσ1 −Δσ3)]. For saturated soft clay, B isequal to 1. erefore, the equation can be rede�ned asΔu � Δσ3 + A(Δσ1 −Δσ3), where A is the pore pressurecoe�cient, σ1 is the axial pressure, σ3is the con�ningpressure, and u is the excess pore water pressure. e re-lationship between pore water pressure coe�cient and timeis shown in Figure 6. It can be seen that the highest porewater pressure coe�cient is found at excess pore waterpressure (20 kPa) and the lowest at excess pore waterpressure (60 kPa). Moreover, the pore water pressure

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Figure 3: Unloading creep deviatoric stress-strain curves of soft soil under di�erent initial excess pore water pressure (consolidationcon�ning pressure 100 kPa). (a) Initial excess pore water pressure u0� 0 kPa. (b) Initial excess pore water pressure u0� 20 kPa. (c) Initialexcess pore water pressure u0� 40 kPa. (d) Initial excess pore water pressure u0� 60 kPa.

Table 3: e �tting relationship between creep yield stress and initial pore water pressure.

Consolidation con�ning pressure (kPa) e relationship between σs and u R2

100 σs�−0.325u+ 38.5 0.9486200 σs�−0.415u+ 65.7 0.9901300 σs�−0.460u+ 68.8 0.9832

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coe�cients at initial excess pore water pressure 20, 40, and60 kPa are approximate 0.73–1.16, 0.26–1.08, and −0.35–0.96, respectively. Fu et al. [10] reported the pore waterpressure coe�cients to be around 1.0–1.3. e coe�cientsfound in this study are generally lower than the reportedones. is could be due to that the initial excess pore waterpressure is not applied in their study.

3.4. Inuence of Consolidation Con�ning Pressure on Creep.e creep deformation of soft soil under lateral unloadingis not only related to axial stress and initial excess porewater pressure, but also closely related to the consolida-tion con�ning pressure of soft soil. e comparison of

axial strain at di�erent con�ning pressures under the sameaxial unloading stress is summarized in Table 4. It can beseen that the higher excess pore water pressure can yieldgreater maximum strain at all consolidation con�ningpressures. Meanwhile, the maximum strain increases withthe decrease of consolidation con�ning pressure under thesame axial unloading stress with all initial pore waterpressures. e greatest strain value happened at con�ningpressure (100 kPa) is approximately two orders of mag-nitude higher than that at con�ning pressure (300 kPa),indicating that the con�ning pressure has a great in�uenceon the unloading and creeping behavior of soft soil.However, the higher con�ning pressure can contribute toconsolidating the soft soil samples. erefore, softfoundation treatment methods such as preloading andvacuum preloading can be implemented in advance toincrease the degree of consolidation of soft soil duringexcavation of soft soil.

4. Conclusion

e undrained triaxial unloading creep tests of soft soil inShenzhen, China, were conducted in this study. e e�ect ofdi�erent initial excess pore water pressures and the lateralunloading stress path was also studied. e following can beconcluded:

(1) e unloading creep curve of soft soil under thelateral unloading stress path is closely related to thedeviatoric stress. With the increase of the deviatoricstress, the creep can be divided into attenuated creep,constant velocity creep, and failure creep stages.Meanwhile, the creep deformation is also related tothe consolidation con�ning pressure. With the re-duction of the con�ning pressure, the soft soil ismore prone to have unloading creep.

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Figure 4: e pore water pressure-time curve at di�erent porewater pressures.

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A1: σ3 = 90kPa, σ1 = 189kPaA2: σ3 = 80kPa, σ1 = 189kPaA3: σ3 = 70kPa, σ1 = 189kPaA4: σ3 = 60kPa, σ1 = 189kPaA5: σ3 = 50kPa, σ1 = 189kPa

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Figure 5: e pore water pressure-time curve at initial excess porewater pressure (40 kPa).

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Figure 6: e relationship between pore water pressure coe�cientand time.

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(2) +e initial excess pore water pressure has an obviousweakening effect on the unloading creep of soft soil.Under the same deviatoric stress, the unloadingcreep deformation of soft soil becomes larger withthe increase of initial excess pore water pressure.

(3) +e unloading creep behaviors of soft soil are relatedto both deviatoric stress and time: when the devia-toric stress is lower than the yield stress, thedeviatoric stress-strain curve exhibits linear visco-elastic properties. When the deviatoric stress ishigher than the yield stress, it exhibits strong non-linear viscoplastic properties. Meanwhile, the non-linearity of unloading creep is gradually enhancedwith the increase of time.

(4) Under undrained conditions, the excess pore waterpressure drops slightly in the early stage but dropssharply in the failure creep stage. Also, the pore waterpressure coefficient decreases with the increase of theinitial excess pore water pressure.

Data Availability

All the data supporting the conclusions of this study arepresented in the tables of the manuscript. All data areavailable upon request from the corresponding author(kejun.wen@jsums.edu).

Conflicts of Interest

+e authors declare that they have no conflicts of interest.

Acknowledgments

+is paper is based on the work supported by the Scientificand Technological Research Program of Chongqing Mu-nicipal Education Commission (Nos. KJ1713327 andKJ1600532), China Scholarship Council Program (No.201708505131), Chongqing University of Science andTechnology Research Program (No. ck2017zkyb013), Schoolof Civil Engineering and Architecture (Grant No. JG201703)from Chongqing University of Science and Technology, andChongqing Research Program of Basic Research andFrontier Technology by Chongqing Science and TechnologyCommission (No. cstc2015jcyjA90012).

References

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Table 4: +e comparison of axial strain at different confining pressures under the same axial unloading stress.

Excess pore waterpressure (kPa)

Axial unloading stress (40 kPa)

εA4/εB2

Axial unloading stress (60 kPa)

εB3/εC2σ3c � 100 kPa σ3c � 200 kPa σ3c � 200 kPa σ3c � 300 kPaA4 maximumstrain (εA4)

B2 maximumstrain (εB2)

B3 maximumstrain (εB3)

C2 maximumstrain (εC2)

0 1.03 0.19 5.42 0.57 0.26 2.1920 3.45 0.27 12.78 0.71 0.30 2.3740 6.78 0.32 21.19 0.92 0.32 2.8860 14.94 0.38 39.31 1.13 0.36 3.14

8 Advances in Civil Engineering

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