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European Journal of Environmental and Civil Engineering

ISSN: 1964-8189 (Print) 2116-7214 (Online) Journal homepage: http://www.tandfonline.com/loi/tece20

Assessment of the effects of initial water contentson the pore pressure response of reconstitutedclays

Xia Bian, Ling-Ling Zeng, Jian-Wen Ding & Sen Qian

To cite this article: Xia Bian, Ling-Ling Zeng, Jian-Wen Ding & Sen Qian (2016):Assessment of the effects of initial water contents on the pore pressure response ofreconstituted clays, European Journal of Environmental and Civil Engineering, DOI:10.1080/19648189.2015.1131202

To link to this article: http://dx.doi.org/10.1080/19648189.2015.1131202

Published online: 12 Jan 2016.

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Assessment of the effects of initial water contents on the porepressure response of reconstituted clays

Xia Biana* , Ling-Ling Zengb, Jian-Wen Dingc and Sen Qianc

aGeotechnical Research Institute, College of Civil and Transportation Engineering, HohaiUniversity, Nanjing, P.R. China; bInstitute of Geotechnical Engineering, College of CivilEngineering, Fuzhou University, Fuzhou, P.R. China; cInstitute of Geotechnical Engineering,School of Transportation, Southeast University, Nanjing, P.R. China

(Received 17 May 2015; accepted 9 December 2015)

Isotropically consolidated undrained triaxial compression shear tests were performedon three reconstituted clays to investigate the effects of initial water contents on thepore pressure behaviour. The test results show that during undrained shearing theexcess pore pressure normalised by isotropic consolidation pressure has a linear rela-tionship with the effective stress ratio under a given initial water content, and theslope of the linear relationship changes with initial water contents for a given recon-stituted clay. The pore pressure coefficient at failure also varies with initial watercontents, ranging from .5 to .96 for the reconstituted clays investigated. By compar-ing the pore pressure response induced by shear stress, the mechanism of decrease inundrained strength with an increase in initial water content is identified from highershear-induced pore pressure. Finally, a simple equation is also proposed to link theslope of linear relationship between normalized pore pressure and effective stressratio with undrained strength and effective friction angle.

Keywords: clays; initial water content; excess pore pressure; effective stress ratio;pore pressure coefficient at failure

1. Introduction

During undrained analyses, excess pore pressure plays an important role in determiningthe stability of embankments on soft clay, assessing the soil pressure on undergroundtunnel lining and other geotechnical engineering practice (Georgopoulos, 2010; Law &Bozozuk, 1979; Leroueil, Tavenas, Trak, Rochelle, & Roy, 1978; Mira, Pastor, Li, &Liu, 2004; Terzaghi, Peck, & Mesri, 1996; Yi, Rowe, & Lee, 1993). It is common prac-tice to study pore pressure characteristic of soils using triaixal tests (e.g. Aris,Benahmed, & Bonelli, 2012; Atkinson & Little, 1988; Jrad, Sukumaran, & Daouadji,2012; Parry, 1960; Sheahan, Ladd, & Germaine, 1996). It has been well documentedthat the pore pressure response is correlated to undrained strength and stress–strainbehaviour of clays (e.g. Bruhn, 1972; Mantaras, Odebrecht, & Schnaid, 2014; Mayne &Swanson, 1981; Muir Wood, 1991). Previous studies showed that the undrained strengthof clays is dependent on the effective stress at failure for reconstituted clays; thus, thefundament mechanism of change in strength with K0, OCR or stress path is the variationof effective stress path of clays, which is correlated to the pore pressure change during

*Corresponding author. Email: xia.bian@hhu.edu.cn

© 2016 Taylor & Francis

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undrained test (e.g. Georgopoulos, 2010; Koutsoftas, 1981; Leonards et al., 1984; Mayne& Swanson, 1981). Hence, pore pressure coefficient is commonly introduced to link porepressure generation with changes in total stress or effective stress (e.g. Henkel, 1960;Skempton, 1954). Numerous researchers have reported that the pore pressure coefficientduring undrained shearing depends on several important features such as: loading rate(e.g. Katti, Tang, & Yazdani, 2003; Richardson & Whitman, 1963; Sheahan et al., 1996)and stress path (e.g. Henkel & Sowa, 1963; Law & Holtz, 1978; Prashant & Penumadu,2005; Muir Wood, 1991), as well as overconsolidation ratio (e.g. Balasubramaniam,Handali, & Wood, 1992; Eigenbrod & Burak, 1991; Graham & Li, 1985; Kamei, Ogawa,& Tanaka, 1987; Khera & Krizek, 1968; Mayne & Stewart, 1988).

Note that recent experimental studies showed that initial water contents significantlyaffect the compressibility (e.g. Cerato & Lutenegger, 2004; Hong, Yin, & Cui, 2010;Hong, Zeng, Cui, & Cai, 2012) as well as undrained strength (Hong, Bian, Cui, Gao, &Zeng, 2013) of reconstituted clays. However, the change in pore pressure response withrespect to initial water content was not investigated in these literatures. Hence, a quanti-tative study on the variation of pore pressure due to initial water contents is required tocharacterise the change of mechanical behaviour of reconstituted clays. Moreoverwhether the pore pressure coefficient is affected by initial water content also need to befurther identified.

The objective of this study is to quantitatively assess the effects of initial water con-tents on pore pressure response during undrained triaxial test for reconstituted clays. Aseries of isotropically consolidated undrained triaxial compression tests were carried out.Specifically, the impacts of initial water contents on pore pressure generation duringundrained shearing and pore pressure coefficient at failure are investigated. This studyalso presents a discussion on the variation of undrained strength with initial watercontents via the knowledge of change in shear-induced pore pressure.

2. Materials and test programme

The basic physical properties of the investigated clays are shown in Table 1. Note thatHong et al. (2013) analysed the effects of initial water content on undrained strengthcharacteristic with Wenzhou clay, Kemen clay and Lianyungang clay. In this study, theexperimental data of the pore pressure response from the tests on 10 reconstitutedspecimens of the three clays in Table 2 are analysed for investigating the pore pressurebehaviour.

Triaxial tests were performed on slurry-consolidated specimens with initial watercontent w0 ranging from 1.0 to 2.0 times corresponding liquid limits. The slurry is putin a large-diameter consolidometer cell with the area of 630 cm2 and the height of

Table 1. Basic physical properties of testing clays.

SoilsDensity of soil particles

(Mg/m3)Liquid limit

(%)Plastic limit

(%)

Clay(<.002 mm)

(%)

Silt(.002–.06 mm)

(%)

Sand(.06–2 mm)

(%)

Wenzhouclay

2.72 78.9 34.1 39 58 3

Kemen clay 2.67 61.0 30.0 26 65 9Lianyungang

clay2.71 73.7 33.0 28 59 3

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15 cm. Then step loading was adopted to preconsolidate the specimens. The preconsoli-dation pressures were of 20 kPa for Wenzhou clay and Kemen clay and of 40 kPa forLianyungang clay. After preconsolidation, triaxial specimens with 80 mm in height and39.1 mm in diameter were trimmed from the preconsolidated cakes. Finally, the speci-mens were placed in the triaxial apparatus for consolidated undrained test. More detailedinformation of sample preparation can be found in Hong et al. (2013).

A back pressure of 100 or 150 kPa was applied in all the tests to ensure the com-plete saturation of the specimens. Skempton’s B-value of .98 was obtained for all thespecimens. The isotropic consolidation stresses prior to shear for Wenzhou clays andKemen clays were 25 kPa; 50 kPa; 100 kPa; 200 kPa. For Lianyungang clays, they were50 kPa; 75 kPa; 100 kPa; 150 kPa; 200 kPa. All the applied confining stresses are largerthan the preconsolidation pressure. It has been reported that the mechanical behaviourof the reconstituted–reconsolidated samples is the same as that of the reconstitutedsamples for a certain clay when the applied consolidation stress is larger than thepreconsolidation stress (e.g. Hong et al., 2013; Sridharan & Prakash, 1996).

The undrained shear tests reported here were all monotonic loading tests with porepressure measurement at the base of the specimen. For these tests, a strain rate of .09%per minute was adopted and the tests were stopped when axial strain reached about20%. It should be mentioned that the failure point was defined as the peak point ofdeviator stress q before or when the axial strain reaches 15%.

3. Test results

3.1. Pore pressure response during shearing

Figure 1 shows the excess pore pressure (Δu) vs. axial strain (ɛa) curves for the threereconstituted clays investigated, in (a), (b) and (c) respectively. It is found that for agiven clay with a given initial water content, the pore pressure curve of the specimenconsolidated at a higher isotropic consolidation pressure lies above that consolidated atlower isotropic consolidation pressure. The graph also shows that for a given clay at agiven consolidation pressure, the pore pressure curve of a sample with a higher initialwater content lies above that with a lower initial water content, and the differenceincreases with the increase in isotropic consolidation pressure. This behaviour indicates

Table 2. Programme of consolidated undrained triaxial tests.

SoilsInitial water

content w0 (%)Liquid limitwL (%) w0/wL Preconsolidation pressure (kPa)

Wenzhou clay 100.1 78.9 1.3 20Wenzhou clay 118.5 78.9 1.5 20Wenzhou clay 135.5 78.9 1.7 20Wenzhou clay 156.4 78.9 2.0 20

Kemen clay 63.0 61.1 1.0 20Kemen clay 70.4 61.1 1.2 20Kemen clay 83.4 61.1 1.4 20Kemen clay 98.0 61.1 1.6 20Kemen clay 113.9 61.1 1.9 20

Lianyungang clay 86.5 73.7 1.2 40Lianyungang clay 126.6 73.7 1.7 40

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that the pore pressure response of reconstituted clays at a higher initial water content ismore sensitive and greater than that at lower ones.

Balasubramaniam et al. (1992) proposed a method to quantify the pore pressurechange during undrained shearing for reconstituted clays. In which, the normalized porepressure, defined as the ratio of the excess pore pressure Δu to isotropic consolidationpressure p00, had a linear relationship with effective stress ratio (η = q/p′). The experi-mental data in Figure 1 were replotted in terms of the normalized pore pressure (Du=p00)against effective stress ratio (η), as shown in Figure 2. It is evident that there emerges agood linear relationship between Du=p00 and η curve at a given initial water content,with correlation coefficient r of .97 for all reconstituted clays investigated, as reportedby Bian, Hong, and Zeng (2012). Hence, the relationship between Du=p00 and η can beexpressed as follow:

Du=p00 ¼ Cg (1)

where C represents the slope of Du=p00 : g linear relationship.Note that for a given clay, the normalized pore pressure line of a specimen with a

higher initial water content lies above that at lower initial water content. Hence, theslope C of Du=p00 : g line is not a soil constant, but varies with initial water content.Figure 3 shows the relationships between C and initial water content w0 for the threereconstituted clays investigated. For a given reconstituted clay, the value of C has anincreasing tendency with the increase in w0. Meanwhile under the same initial watercontent the value of C increases with the decrease in liquid limit. It should be men-tioned that the influence of initial water contents on C seems less important for the twosets experiment of Lianyungang clay. However, the overall trends of C increasing withw0 are still validated for the tests on 10 reconstituted specimens of the three clays.

3.2. Pore pressure coefficient at failure Af

Pore pressure coefficient A defined by Skempton (1954) is often used to link porepressure generation with the change in total stress,

Du ¼ B½Drr þ AðDra � DrrÞ� (2)

where A and B are the pore pressure coefficient. The value of B is unity for saturatedclays and the value of A is known to change with the strain. However, the pore pressure

Table 3. Slop of C for different clays.

SoilsSampletype wL w0 w0/wL M C Su/p00

Strain rate(%/ min) Author

Bangkok clay Undisturbed 116 124 1.07 1.17 .53 .36 .075 Balasubramaniam et al. (1992)M30 clay Reconstituted 55 70 1.27 1.58 .47 .5 .07 Nakase and Kamei (1983)Boom clay Reconstituted 64 80 1.25 1.15 .42 .43 – Bouazza, Van Impe, and Haegeman (1996)Grundite clay Reconstituted 54.5 100 1.83 .99 .51 .39 .035 Khera and Krizek (1968)Pure clay Reconstituted 42 20 .48 .96 .45 .41 .013 Shafiee, Tavakoli, and Jafari (2008)Kaolin clay Reconstituted 65 155 2.38 1.2 .57 .3 .05 Prashant and Penumadu (2005)Fat clay Reconstituted 62 125 2.02 1.02 .54 .36 1 Richardson and Whitman (1963)Fat clay Reconstituted 62 125 2.02 1.04 .60 .31 .002 Richardson and Whitman (1963)

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0

20

40

60

80

100

120

140

160

Exc

ess

por

e p

ress

ure

u /

kP

a

Axial strain a / %

Initial water content = 100.1%Initial water content = 118.5%Initial water content = 135.5%Initial water content = 156.4%

200kPa

100kPa

50kPa

25kPa

Consolidation pressure

0

20

40

60

80

100

120

140

160

180

Exc

ess

por

e p

ress

ure

u / k

Pa

Axial strain a / %

Initial water content = 63.0%Initial water content = 70.4%Initial water content = 83.4%Initial water content = 98.0%Initial water content = 113.9%

Consolidation pressure

200kPa

100kPa

50kPa

25kPa

0

20

40

60

80

100

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140

0 5 10 15 20 25

0 5 10 15 20 25

0 5 10 15 20

Exc

ess

por

e p

ress

ure

u / k

Pa

Axial strain a / %

Initial water content = 86.5%Initial water content = 126.6%

200kPa

150kPa

75kPa

50kPa

Consolidation pressure

100kPa

(a) Wenzhou clay

(b) Kemen clay

(c) Lianyungang clay

Figure 1. Relationships of excess pore pressure against axial strain.

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0.0

0.1

0.2

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0.7 Initial water content = 100.1%Initial water content = 118.5%Initial water content = 135.5%Initial water content = 156.4%

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 Initial water content = 63.0%Initial water content = 70.4%Initial water content = 83.4%Initial water content = 98.0%Initial water content = 113.9%

0.0

0.1

0.2

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0.4

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0.7

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Initial water content = 86.5%Initial water content = 126.6%

(a) Wenzhou clay

(b) Kemen clay

(c) Lianyungang clay

Figure 2. (Δu/p00:η) relationship at different initial water contents.

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coefficient at failure Af was often investigated as a representative of particular clays. Forconventional triaxial compression test, pore pressure coefficient at failure Af is expressedas

Af ¼ Dufqf � q0

(3)

where Δuf is excess pore pressure at failure, q0 and qf is the deviator stress at beginningand failure, respectively.

Figure 4 depicts the variation of Af with respect to initial water content. It is foundthat the value of Af increases with an increase in initial water content for a givenreconstituted clay. Under the same initial water content, Af decreases in the order ofincreasing liquid limit. This is consistent with the change law of parameter C. Withinthe initial water content range investigated, the value of Af scatters from .5 to .96.

Af and effective friction angle is related to the undrained strength ratio, expressed as(e.g. Khera & Krizek, 1968; Mayne & Stewart, 1988)

Su=p00 ¼

sin/0

1þ ð2Af � 1Þ sin/0 (4)

where Su is undrained strength, expressed as the peak value of (r0a-r0r)/2 at failure, r0a

and r0r is effective axial stress and effective radial stress respectively; φ′ is effective fric-tion angle, yields the Mohr–Coulomb failure envelope. The experimental data ofundrained strength and effective stress parameters are obtained from Hong et al. (2013).

Then the pore pressure coefficient at failure Af becomes:

Af ¼ 1

2Su=p00� 1

2 sin/0 þ1

2(5)

Figure 5 shows the comparison of Af between the predicted values using Equation (5)and the values determined by Equation (3) with experimental data for reconstituted

0.2

0.3

0.4

0.5

0.6

0.7

0.8

40 60 80 100 120 140 160

C

Initial water content w0 / %

Wenzhou clayKemen clayLianyungang clay

Figure 3. Parameter C vary with initial water content.

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clays at different initial water contents. The predicted values agree well with theexperimental data.

The relationship between pore pressure coefficient at failure Af and undrainedstrength Su or effective friction angle /0 is depicted in Figure 6. A decreasing trend ofSu and /0 is observed with an increase in Af. This behaviour indicates that the decreaseof undrained strength and effective friction angle with respect to initial water content ismainly due to the higher pore pressure response at higher initial water content. Themechanism of variation of undrained strength with initial water content will bediscussed in later sections.

3.3. Effective stress paths

As a result of pore pressure responses, the effective stress paths are also significantlyaffected by initial water contents, as shown in Figure 7 (e.g. Hong et al., 2013).

Figure 7 shows that under a given initial water content the shape of effective stresspath at different confined consolidation pressure are almost the same (Muir Wood,1990). More importantly for a given clay, the effective stress path at less initial watercontent lies on the right of that at higher initial water content, and becomes more nearlyvertical. Graham, Crooks, and Lau (1988) suggested undrained strength and effectivestress path were able to characterise yield behaviour or state boundary surface for nor-mally consolidated clays. For a given clay, both undrained strength and effective stresspath varies with initial water content due to pore pressure response. Hence, it may bededuced that the state boundary surface will not be fixed for a given clay, which varieswith the initial water content. Then the yield characteristic of reconstituted clays is not asoil constant but also depends on initial water content.

4. Discussion

4.1. Link parameter C with undrained strength and effective stress parameter

For isotropic normally consolidated specimen at failure, it assumes ηf = M, thenEquation (1) becomes,

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

40 60 80 100 120 140 160

Af

Initial water content w0 / %

Wenzhou clayKemen clayLianyuangang clay

Figure 4. Af vary with initial water content.

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Duf =p00 ¼ Cgf ¼ CM (6)

The relationship between M and effective friction angle /0 is expressed as

M ¼ 6 sin/0

3� sin/0 (7)

Combining with the definition of pore pressure coefficient at failure (see Equation (3)),

Af ¼ CMp00qf

¼ CM

2Su=p00¼ C

2Su=p00

6 sinu0

3� sinu0 (8)

0.0

0.4

0.8

1.2

1.6

0.0 0.4 0.8 1.2 1.6

calc

ulat

edA

f

measured Af

1

1

Figure 5. Comparison of measured Af with calculated Af using Equation (5).

0.1

0.2

0.3

0.4

0.5

0.6

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Wenzhou clay

Kemen clay

Lianyuangang clay

Wenzhou clay

Kemen clay

Lianyuangang clay

Su / p'0

Figure 6. Relationship between Af and undrained strength Su or effective friction angle /0.

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200

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Dev

iato

r str

ess

q / k

Pa

Effective mean stress p' / kPa

Initial water content = 100.1%Initial water content = 118.5%Initial water content = 135.5%Initial water content = 156.4%

upuqu fromchang in q

u fromchang in p

0

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280

0 50 100 150 200 250 300

0 50 100 150 200 250

Dev

iato

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ess

q / k

Pa

Effective mean stress p' / kPa

Initial water content = 63.0%Initial water content = 70.4%Initial water content = 83.4%Initial water content = 98.0%Initial water content = 113.9%

0

20

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100

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0 50 100 150 200 250

Dev

iato

r str

ess

q / k

Pa

Effective mean stress p' / kPa

Initial water content = 86.5%Initial water content = 126.6%

(a) Wenzhou clay

(b) Kemen clay

(c) Lianyungang clay

Figure 7. Effective stress paths.

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where Equation (8) is an indicative of the relationship between parameter C and porepressure coefficient at failure Af.

Comparison of Equation (5) with Equation (8), the parameter C is given by

Af ¼ C

2Su=p00

6 sin/0

3� sin/0 ¼1

2Su=p00� 1

2 sin/0 þ1

2(9)

C ¼ 3� sin/0

6 sin/0 ð1� Su=p00sin/0 þ Su=p

00Þ (10)

Equation (10) suggests that the parameter C is related to undrained strength andeffective friction angle, similar to the pore pressure coefficient at failure.

The values of C calculated by Equation (10) are compared with the measured onesas shown in Figure 8. The pore pressure data of other reconstituted clays compiled fromthe available literature (see Table 3) are also plotted in Figure 8. It seems that Equation(10) can simulate the parameter C of reconstituted clays with various initial water con-tents and different strain rates reasonably well.

Balasubramaniam et al. (1992) proposed an equation to link parameter C and effec-tive friction angle, expressed as C ¼ 3� sin/0ð Þ2= 12 sin/0ð3� 2 sin/0Þ½ � 12 sin/0½ð3� 2 sin/0Þ�. The comparison between the results of Equation (10) and Balasubrama-niam’s equation is plotted in Figure 9. It is interesting to note that the predicted varia-tion of parameter C with effective friction angle is quite different from these twoequations. Balasubramaniam’s equation predicts a decrease tendency of parameter Cwith effective friction angle. However, Equation (10) illustrates that for a givenundrained strength ratio, parameter C is not monotonic function with effective frictionangle. Under a given effective friction angle, the parameter C varies with undrainedstrength ratio ranging a wide spectrum. Hence, undrained strength is more sensitive tothe variation of pore pressure response with respect to initial water content or strain rate.This behaviour implies that any empirical or theoretical method of undrained strength

0.2

0.4

0.6

0.8

1.0

0.2 0.4 0.6 0.8 1.0

calc

ulat

edC

measured C

This study(0.09%/min)Bangkok clay(0.075%/min)Pure clay(0.013%/min)M30 clay(0.07%/min)Grundite clay(0.035%/min)Kaolin clay(0.05%/min)Fat clay(1%/min)Fat clay(0.002/min)Boom clay

1

1

Figure 8. Comparison of measured C with calculated C using Equation (10).

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prediction at different initial water contents requires the knowledge of pore pressureresponse at failure.

4.2. The mechanism of variation of undrained strength with initial water content

During an undrained shear test on a saturated soil specimen, the excess pore pressure isgenerated due in part to the increase in mean total stress which the test operator hascomplete control over, and in part due to the plasticity of the clays or shear deformation(Henkel, 1960; Muir Wood, 1990). As suggested by Sheahan et al. (1996), the strengthincrease during conventional triaxial test is partly due to an increase in mean total stress,which also causes a corresponding increase in pore pressure. Hence, the response ofundrained strength to strain rate or stress path is mainly related to the shear-inducedpore pressure Δuq (Prashant & Penumadu, 2005; Sheahan et al., 1996). The shear-in-duced pore pressure Δuq is adopted to assess the mechanism of variation of undrainedstrength with initial water content. As shown in Figure 7, the change in pore pressuredue to the change in mean total stress is represent by Δup, and due to applied shear isrepresent by Δuq. Figure 10 shows the evolution of normalized shear-induced pore pres-sure Duq=p00 with axial strain. Due to the limit of external axial deformation measure-ment, there are no reliable data at ɛa below .01%, then the value at ɛa = .01% is set aszero points in this study.

It can be seen from Figure 10 that negative shear-induced pore pressure is generatedup to about ɛa = 1%. This behaviour may be resulted from the response lag of base porepressure devices. The similar responses have also been observed by Sheahan et al.(1996). Sheahan et al. (1996) and Kimura and Saitoh (1983) reported that when theaxial strain region is larger than 1%, the difference of pore pressure measurementsbetween midhight and base is less significant during undrained shear tests. The variationof Duq=p00 with axial strain in the axial strain region of main interest (ɛa > 1%) show a

0.2

0.3

0.4

0.5

0.6

0.7

0.8

20 25 30 35 40

0.3

0.4

0.5

C=(3-sin 2/[12sin (3-2sin )]Balasubramaniam et al. (1992)

Equation (10)

Figure 9. Comparison of the variation of C with effective friction angle between Equation (10)and equation proposed by Balasubramaniam et al. (1992).

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-0.1

0.0

0.1

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Nor

mal

ized

she

ar-i

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ed p

ore

pre

ssur

e u q

/ p'

0

Axial strain a / %

Initial water content = 100.1%Initial water content = 118.5%Initial water content = 135.5%Initial water content = 156.4%

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.01 0.1 1 10

0.01 0.1 1 10

Nor

mal

ized

she

ar-i

nduc

ed p

ore

pre

ssur

e u q

/ p'

0

Axial strain a / %

Initial water content = 63.0%Initial water content = 70.4%Initial water content = 83.4%Initial water content = 98.0%Initial water content = 113.9%

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.01 0.1 1 10

Nor

mal

ized

shea

r-in

duc

ed p

ore

pre

ssur

e u q

/ p'

0

Axial strain a / %

Initial water content = 86.5%

Initial water content = 126.6%

(a) Wenzhou clay

(b) Kemen clay

(c) Lianyungang clay

Figure 10. Normalized shear-induced pore pressure vs. strain.

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clear increasing trend with an increase in initial water contents. Figure 11 shows therelationship of normalized undrained strength and shear-induced pore pressure at peakwith initial water contents. It can be observed that a decrease in Su is accompanied by acorresponding increase in the shear-induced pore pressure at failure. This behaviouremphasised that the primary mechanism of strength decrease with respect to initial watercontent is the increase of shear-induced pore pressure with increasing in initial watercontent.

5. Conclusions

A series of isotropically consolidated undrained triaxial compression tests wereperformed to study the influence of initial water content on pore pressure response. Themain conclusions are summarised as follows:

(1) For a given clay consolidated under a given consolidation pressure, the porepressure–axial strain curve at a higher initial water content lies above that at alower initial water content, and the difference increases with the increase in iso-tropic consolidation pressure. Pore pressure coefficient at failure Af increaseswith the increase in initial water content for a given clay.

(2) The normalized pore pressure (Du=p00) has a good linear relationship with theeffective stress ratio (η) for reconstituted clay at a given initial water content, andthe slope of the linear relationship C shows an increase tendency with the increasein initial water content. The relationship of parameter C with undrained strengthand effective friction angle is proposed as C ¼ 3�sin/0

6 sin/0 ð1� Su=p00sin/0 þ Su=p00Þ.

(3) The variation of shear-induced pore pressure Δuq at higher initial water con-tent lies above that at lower initial water content, indicating that the primarymechanism of strength decrease with respect to initial water content is resultedfrom higher shear-induced pore pressure with the increase in initial watercontent.

0

0.1

0.2

0.3

0.4

0.5

0.6

0

0.1

0.2

0.3

0.4

0.5

0.6

40 60 80 100 120 140 160

S u/ p

' 0

u q/ p

' 0

Initial water content w0 / %

Wenzhou clayKemen clayLianyuangang clayWenzhou clayKemen clayLianyuangang clay

Su / p'0

uq / p'0

Figure 11. Relationship of normalized undrained strength and shear-induced pore pressure withinitial water contents.

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Notation

Af pore pressure coefficient at failureC the slope of linear relationship between normalized pore pressure and effective

stress ratioM critical state stress ratioq deviator stress, equal to (r0a − r0r)p′ mean effective stress, equal to (r0a + 2r0r)/3p00 isotropic consolidation pressureq0, qf deviator stress at beginning and failure, respectivelyPI plasticity indexΔu excess pore pressurew0 initial water contentwL liquid limitη effective stress ratio, equal to (=q/p′)φ′ effective friction angler0a effective axial stressr0r effective radial stressɛa axial strain

Disclosure statementNo potential conflict of interest was reported by the authors.

FundingThis work was supported by the National Natural Science Foundation of China [41372309],[41502263], [51378118]; National Key Basic Research Program of China (973 Program)[2015CB057803]; Natural Science Foundation of Jiangsu Province of Jiangsu Province[BK20150819].

ORCID

Xia Bian http://orcid.org/0000-0002-4126-2117

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