Impact of wetting–drying cycles on hydro-mechanical behavior of an unsaturated compacted clay

Applied Clay Science 86 (2013) 38–46

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Research paper

Impact of wetting–drying cycles on hydro-mechanical behavior of an unsaturated compacted clay R. Chen a,⁎, C.W.W. Ng b,c a b c

Department of Civil and Environmental Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen University Town, Xili, Shenzhen, China Guangzhou HKUST Fok Ying Tung Research Institute, Guangzhou, China Department of Civil and Environmental Engineering, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

a r t i c l e

i n f o

Article history: Received 14 June 2012 Received in revised form 1 September 2013 Accepted 5 September 2013 Available online 31 October 2013 Keywords: Compacted clay Isotropic compression test Suction-controlled Irreversible swelling Hydraulic hysteresis Pre-consolidation stress

a b s t r a c t To further our understanding on the impact of wetting–drying cycles on the hydro-mechanical behavior of unsaturated soils, this paper presents experimental results from suction-controlled isotropic compression tests on an unsaturated compacted clay subjected to different wetting–drying histories. This clay exhibited complicated volumetric response to wetting–drying cycles such as irreversible swelling upon wetting, irreversible shrinkage upon subsequent drying and accumulated swelling after a wetting–drying–wetting cycle. The wetting-induced irreversible swelling contributes to a significant reduction in pre-consolidation stress. It was observed that a wetting–drying cycle leads to a smaller pre-consolidation stress and downward shifting of the post-yield compression curve at a given suction, whereas a wetting–drying–wetting cycle shows an opposite effect. These observations are attributed to both irreversible swelling and irreversible change in the degree of saturation resulting from wetting–drying cycles. It was found that irreversible swelling or an irreversible increase in degree of saturation makes the soil more susceptible to yield, exhibiting a softening effect. Regarding water phase, its response to isotropic compression is mostly related to the recent wetting–drying history rather than the overall wetting–drying history. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Most surficial soils are subjected to wetting–drying cycles due to alternative rainfall and evapo-transpiration. The wetting–drying cycles can significantly affect the hydro-mechanical behavior of unsaturated soils. A significant number of studies (e.g. Cui and Delage, 1996; Lloret et al., 2003; Zhan and Ng, 2006) have been conducted on the effect of moisture content on the hydro-mechanical behavior of unsaturated soils. Recently, the role of wetting–drying cycles in the hydromechanical behavior of unsaturated soils has received particular attention (Gens et al., 2006). It was found that the pre-consolidation stress at a given suction on the drying path is smaller than that on the wetting path (Alshihabi et al., 2002; Sharma, 1998; Sivakumar et al., 2006). This phenomenon is attributed to hydraulic hysteresis (i.e. the occurrence of irreversible changes in the degree of saturation during wetting–drying cycles) and can be well captured by some mechanical models, which consider the impact of degree of saturation on yield function (e.g. Gallipoli et al., 2003; Gens et al., 2006; Tamagnini, 2004; Wheeler et al., 2003). It should be noted that the previous work

⁎ Corresponding author at: Department of Civil and Environmental Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen University Town, Xili, Shenzhen, 518055, China. Tel.: +86 755 26617372; fax: +86 755 26033509. E-mail addresses: [email protected] (R. Chen), [email protected] (C.W.W. Ng). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.09.018

focused on the soils exhibiting collapsible deformation or reversible swelling upon wetting. Some soils such as expansive soils show significant irreversible bulk volume increase upon wetting. In other words, unrecovered volume changes are sustained along the subsequent drying paths (e.g. Chu and Mou, 1973; Likos and Lu, 2006). This behavior may be associated with irreversible changes in clay texture resulting from interlayer swelling upon wetting (Likos and Lu, 2006). The impact of wetting–drying cycles on the hydro-mechanical behavior of such soils is not yet fully understood. Expansive soils are widely distributed in the world as one type of problematic soil. On the other hand, compacted expansive soils are often used as impervious liners to contain toxic and radioactive wastes. Given the high transportation costs of imported materials, the use of local expansive soils as fill material in earth structures is sometimes unavoidable (Tripathy et al., 2009). Hence, studies on compacted expansive soils are both desirable and useful. The volumetric behavior of compacted expansive soils with wetting–drying cycles has been studied by many researchers using oedometers without suction control (e.g. Al-Homoud et al., 1995; Osipov et al., 1987). Only a few experimental programs have been conducted using suction controlled devices (Alonso et al., 2005; Chu and Mou, 1973; Cuisinier and Masrouri, 2005; Lloret et al., 2003; Nowamooz and Masrouri, 2008). It was found that substantial irreversible accumulation of swelling or shrinkage strain in an expansive soil may occur upon wetting–drying cycles. Whether irreversible swelling or shrinkage occurs is a function of compaction conditions and the

R. Chen, C.W.W. Ng / Applied Clay Science 86 (2013) 38–46

subsequent variation of net stress and suction, rather than being determined solely by soil type (Alonso et al., 1995; Gehling et al., 1995; Nowamooz and Masrouri, 2009; Sharma, 1998). In dense compacted expansive soils, only reversible volume change response to wetting–drying cycle occurs (Cui et al., 2002). It seems that the irreversibility of volume change during wetting–drying cycles complicates the understanding on the impact of wetting–drying cycles on the mechanical behavior of unsaturated expansive soils. Regarding water-retention behavior, significant hydraulic hysteresis was observed for compacted expansive soils (Likos and Lu, 2003; Miao et al., 2002). Therefore, it is not fully understood whether hydraulic hysteresis alone or hydraulic hysteresis and irreversible volume change together are responsible for the impact of wetting–drying cycles on the hydromechanical behavior of unsaturated expansive soils. The effect of suction level on the shear strength characteristics of an expansive clay from China has been studied by Zhan and Ng (2006). It was found that the contribution of suction to the shear strength of the compacted expansive clay is more significant than that of a compacted kaolin (Wheeler and Sivakumar, 1995) at suctions less than 100 kPa. As a natural extension to the work presented by Zhan and Ng (2006), this study is aimed at investigating the impact of wetting–drying cycles on the hydro-mechanical behavior of the compacted expansive clay. The laboratory program in this study consists of three series of suctioncontrolled isotropic compression tests on soil samples subjected to three different wetting–drying histories, i.e. (i) wetting, (ii) a wetting– drying cycle and (iii) a wetting–drying–wetting cycle. Characteristics of changes in both volume and water content of the soil along wetting– drying cycles and isotropic compression were investigated. The effects of suction level and wetting–drying cycles on the isotropic compression behavior of the soil were revealed. 2. Material and methods 2.1. Testing material The tested soil was taken from an expansive clay slope at Zaoyang (ZY) in Hubei, China (Ng et al., 2003). This soil is classified as an expansive soil with medium expansion based on one-dimensional free swelling tests (Zhan, 2003). This soil is called as the ZY clay hereafter. The tested soil consists of 3% sand (0.063 mm b particle size b 20 mm), 58% silt (0.002mmb sizeb 0.063mm) and 39% clay (sizeb 0.002mm). The liquid and plastic limits are 51% and 20%, respectively, giving a plasticity index of 31%. This soil is classified as a silty clay with intermediate plasticity (BSI, 1990). The predominant minerals are montmorillonite (21%) and illite (16%), as determined from X-ray diffractometry. Each triaxial specimen, 38mm in diameter and 76mm in height, was prepared by static compaction in a split compaction mold. The mold consists of three pieces spaced at 120 degrees (on plan) to facilitate the ease of taking apart the mold from a compacted soil specimen after compaction. Each specimen was compacted at a gravimetric water content, w, of 18.5%. This compaction water content is on the dry side of the optimum water content, which was determined from the Standard Proctor compaction test (i.e., 20.5%). The compaction of soil specimen was conducted in six layers and each layer was compressed at a fixed displacement rate of 0.025 mm/s to reach the maximum compaction pressure of 800 kPa. This pressure was selected to achieve the average dry density of 1.56 Mg/m3 that was measured in the field. The compaction resulted in a void ratio, e, of 0.711 and a degree of saturation, Sr, of 69%. The initial matric suction of the compacted specimens was about 540 kPa as measured by a highcapacity tensiometer. 2.2. Testing equipment A computer-controlled triaxial stress-path testing system was set up for testing unsaturated soils (Fig. 1). This system is equipped with two

39

GDS digital hydraulic pressure/volume controllers to control the axial stress, σ1, and the pore water pressure, uw. The confining stress, σ3, and the pore air pressure, ua, are controlled by two automatic pneumatic controllers. Matric suction, s = (ua − uw), is then applied to a specimen by controlling pore water and air pressures based on the axis translation principle proposed by Hilf (1956). A 5 bar high airentry-value (HAEV) ceramic disk was used for this purpose. Volume of water flowing in/out of a specimen is measured using the GDS digital hydraulic pressure/volume controller (back pressure controller) with an accuracy of better than 0.1% measured value. A simple double-cell device for accurately measuring overall total volume change in an unsaturated soil specimen (Ng et al., 2002) was used within the triaxial apparatus. Based on detailed calibrations, the estimated accuracy of this measuring device is in the order of 32 mm3 (or 0.04% volumetric strain for the triaxial specimen used in this study). Further details on this device can be found in Ng et al. (2002). Although the HAEV ceramic disk is able to prevent free air from flowing into the water drainage system, air may diffuse through water into the ceramic disk and appear as air bubbles beneath the disk, thereby affecting the measurement of the volume of water inflow/ outflow. This problem becomes more prominent for long-lasting unsaturated soil testing due to low water coefficient of permeability of unsaturated soils. Following the technique proposed by Fredlund (1975), a diffused air volume indicator (DAVI) was developed and used to flush and measure the volume of diffused air underneath the ceramic disk daily. As shown in Fig. 2, the flushing operation starts by switching on the three-way valve and opening the shutoff valve to connect the flushing grooves underneath the ceramic disk and the flushing system. Under an applied pressure gradient, deaerated water is sent from one air–water interface cylinder through the flushing grooves beneath the ceramic disk and finally to the DAVI, which is kept at a slightly lower pressure than the air–water interface cylinder. By using a four-way valve, the flow direction for flushing diffused air underneath the ceramic disk can be reversed, so that any remaining air bubbles can be removed effectively. However, it is interesting to note that the amount of diffused air during testing was negligible, similar to the observations reported by Sivakumar (1993) and Sharma (1998). This may be related to the small contact area between pore air in soil voids and the ceramic disk in these studies, in which clays were used and maintained at a relatively high degree of saturation.

Pneumatic controllers ( 3, ua) Digital transducer interface

Hydraulic pressure/ volume controllers ( 1, uw) Total volume change measuring device (Ng et al., 2002) Bishop & Wesley triaxial cell Diffused air volume indicator Air-water interface cylinder Fig. 1. A computer-controlled triaxial system for testing unsaturated soils.

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R. Chen, C.W.W. Ng / Applied Clay Science 86 (2013) 38–46

Legend Pressure regulator Pressure gauge Shutoff valve Three-way valve Four-way valve

Air supply

DAVI

Air-water interface cylinder

Ceramic disc Flushing groove

Back pressure controller Fig. 2. Schematic diagram of flushing system for measuring the amount of diffused air.

All tests in this study were conducted in a room with a temperature of 20 ± 1 °C to decrease the effect of daily temperature fluctuation on test results. 2.3. Test program and procedures The test program consists of three series of suction-controlled isotropic compression tests following different wetting–drying histories. In Test Series 1, the isotropic compression was applied after suction equalization (or wetting). Test Series 2 involved a wetting–drying cycle before carrying out the isotropic compression. In Test Series 3, a wetting–drying–wetting cycle was followed by the isotropic compression. Series 2 and Series 3 were used to investigate volumetric behavior and water-retention behavior of the ZY clay subjected to wetting–drying cycles and to study the impact of wetting–drying cycles on subsequent drained isotropic compression behavior with reference to Test Series 1. 2.3.1. Test Series 1 As shown in Fig. 3, Path OABC is a typical stress path in the (p: s) plane in Test Series 1. The initial state of each specimen after compaction is indicated by Point O. Each test included two stages, i.e., (i) suction

Series 1: OABC

Matric suction, s

O A

F

Series 2: OABDE Series 3: OAFGHIJK

I

equalization (Path OAB) and (ii) isotropic compression at a constant suction (Path BC). In the equalization stage, each specimen was wetted from the initial suction of 540 kPa to a lower suction (i.e., 0, 25, 50, 100 and 200kPa; see Table 1) at a net confining stress of 20 kPa, which was used to ensure good contact between the specimen and ceramic disk. It should be noted that equalization at zero suction did not imply forced saturation by means of flushing. The specimen could be still unsaturated even if the applied suction was equal to zero. The suction equalization process was terminated when the rate of water flow decreased to 30 mm3/day, which is equivalent to a 0.02% change in gravimetric water content per day. This rate is smaller than the one adopted by Sharma (1998) and Sivakumar et al. (2006), i.e. 0.04% change in gravimetric water content per day. It should be noted that such method does not necessarily assure suction equilibrium. A small change in water content may correspond to a significant change in suction if the soil were near residual saturation (Vanapalli et al., 1996). From water retention results presented later, it can be seen that the maximum applied suction of 400kPa corresponds to the transition stage where a small change in water content causes a small change in suction. As compared with changes in water content during wetting–drying cycles, the rate used for limiting change in water content at the end of suction equalization stage in this study is relatively small and it should assure high degree of suction equilibrium. The duration of suction equalization stage varied from 12 to 39days in a suction range from 0 to 200 kPa. After suction equalization, isotropic compression was applied by increasing the net confining stress from 20 to 350 kPa at a rate of 12 kPa/day. This rate was selected to limit excess pore-water pressure developed during isotropic compression. On reaching the target net confining stress, a 48-hour rest period was allowed to ensure full dissipation of excess pore-water pressure throughout the specimen. It was observed that the changes in volume and water content during the rest period were negligible. This indicates that excess pore-water pressure was also negligible and the used loading rate is acceptable. 2.3.2. Test Series 2 A typical stress path in the (p: s) plane in Test Series 2 is denoted as OABDE in Fig. 3. Each test included three stages, i.e., (i) wetting (Path OAB), (ii) drying (Path BD) and (iii) isotropic compression at a constant suction (Path DE). Stress paths involving a wetting path followed by a drying path are denoted as a wetting–drying cycle. A step-loading approach was adopted to change the suction during a wetting–drying cycle. At each step, the applied suction was kept constant until suction equilibrium. Specimens UW4 and UW5 were firstly wetted to a suction of 25 kPa and 50 kPa, respectively, under a net confining stress of 20 kPa (see Table 2). The two specimens were subsequently dried to a suction of 100 kPa and then were subjected to isotropic compression by increasing the net confining stress from 20 to 350 kPa at a suction of 100 kPa. 2.3.3. Test Series 3 A typical stress path in the (p: s) plane in Test Series 3 is denoted as OAFGHIJK in Fig. 3. Each test included four stages, i.e., (i) initial suction equalization (Path OAF), (ii) isotropic compression to a desired net

G J

K E

D

Table 1 Values of state parameters for Test Series 1. I.D.

H B

C Net mean stress, p

Fig. 3. Typical stress paths in the (p: s) plane for Test Series 1, 2 and 3.

UI16 UI12 UI13 UI14 UI15

Initial condition

After wetting

w (%)

v

Sr (%)

s p v (kPa) (kPa)

18.44 18.03 18.30 18.66 18.42

1.710 1.725 1.716 1.709 1.697

69.4 0 66.4 25 68.2 50 70.3 100 70.5 200

20

After isotropic compression

1.794 1.792 1.754 1.731 1.714

Sr (%)

s p v (kPa) (kPa)

93.5 0 76.8 25 76.1 50 72.7 100 70.9 200

350

Sr (%)

1.614 100.0 1.636 91.5 1.657 84.5 1.673 77.6 1.684 72.0

R. Chen, C.W.W. Ng / Applied Clay Science 86 (2013) 38–46

41

Table 2 Values of state parameters for Test Series 2. I.D.

Initial condition

UW4 UW5

After wetting

After drying

After isotropic compression

w (%)

v

Sr (%)

s (kPa)

p (kPa)

v

Sr (%)

s (kPa)

p (kPa)

v

Sr (%)

s (kPa)

p (kPa)

v

Sr (%)

17.90 18.15

1.712 1.725

67.1 66.8

25 100

20

1.777 1.762

78.0 75.4

100

20

1.769 1.758

75.6 74.6

100

350

1.648 1.664

83.4 79.7

Swelling of the ZY clay is observed in Fig. 4b and is likely attributed to the presence of montmorillonite. This active clay mineral has an expandable interlayer and its specific surface area is large enough to absorb a large amount of water (Bergaya et al., 2006). Generally, there are two levels of swelling, i.e. interlayer swelling at the microscopic scale and bulk volume increase at the macroscopic scale in clays containing montmorillonite (Gates, 2007; Likos and Lu, 2006). When subjected to wetting, water enters clay mineral interlayer spaces and forces clay mineral layers apart and thus causes interlayer swelling at the particle scale. The interlayer swelling is generally reversible, i.e., opposite to the observed behavior when it is subjected to drying. The interlayer swelling can result in bulk volume increase of soil. However, different from the interlayer swelling, the bulk volume increase is generally featured with irreversibility and hysteresis. The irreversibility of the wetting-induced swelling in the ZY clay is discussed later.

confining stress (Path FG), (iii) a wetting–drying–wetting cycle at a constant net confining stress (Path GHIJ) and (iv) isotropic compression at a constant suction (Path JK). Specimens UW2 and UW3 were firstly equalized at a suction of 200 kPa under a net confining stress of 20 kPa and then were subjected to isotropic compression by increasing the net confining stress from 20 to 70 kPa and 100 kPa, respectively (see Table 3). The two specimens were subsequently subjected to a wetting path followed by a drying path and then a re-wetting path at the net confining stresses of 70 kPa and 100 kPa, respectively. These test paths are referred as a wetting–drying–wetting cycle. Changes of suction along various wetting and drying paths are given in Table 4. The steploading approach mentioned above was adopted to change the suctions. After a wetting–drying–wetting cycle, UW2 and UW3 were isotropically compressed to a net confining stress of 350 kPa at the suction of 50 kPa. 3. Results and discussion

3.1.2. Wetting–drying cycles Fig. 5 shows the v ~ logs relationships during the wetting–drying cycles in the ZY clay at different net mean stresses and in the kaolin at the net mean stress of 50 kPa. In each ZY clay specimen, irreversible volumetric swelling occurred along the first wetting path, i.e. the swelling was not vanished via subsequent drying. This observation is in contrast to that in the kaolin, which exhibited reversible swelling. The irreversible swelling in the ZY clay is likely related to the irreversibility of bulk volume increase of montmorillonitic clays. Based on microscopic observations (Katti and Shanmugasundaram, 2001), clay aggregates containing montmorillonite are broken down into smaller sized ones upon wetting under external loading as they expand. Consequently, the soil texture of the ZY clay was altered upon wetting and could not be recovered in the subsequent drying. On the contrary, there could be only reversible swelling of clay aggregates occurring in the non-expansive kaolin upon wetting. As shown in Fig. 5a, specimen UW4 exhibits a larger irreversible swelling than specimen UW5 because UW4 was wetted to a lower suction. As a result, UW4 had a higher value of specific volume than UW5 at the same suction along the subsequent drying path. From Fig. 5b, it is found that the swelling behavior of the ZY clay upon wetting is stress-dependent. The magnitude and rate of volumetric swelling upon wetting decreased with an increase in an applied net mean stress. This means that confining stress inhibits wetting-induced swelling. Similar observations were reported by Alonso et al. (1995) and Sharma (1998). From the microscopic viewpoint, the efficiency of translating interlayer swelling to bulk volume increase is reduced with an increase in confining stress, likely due to that a larger confining stress results in a more stable soil texture. An irreversible shrinkage upon drying was observed in the ZY clay from Fig. 5b, suggesting yielding along the drying path. From the

3.1. Response to suction changes 3.1.1. Suction equalization Fig. 4 shows the specific water volume, vw, and the specific volume, v, equalized at different suctions in Test Series 1. vw is defined as the volume of water plus solids in a volume of soil containing unit volume of solid particles (Wheeler, 1991) and it is calculated by vw ¼ 1 þ wGs

ð1Þ

where Gs is the specific gravity. v is defined as the volume of void plus solids in a volume of soil containing unit volume of solid particles: v¼1þe

ð2Þ

Data from a compacted kaolin reported by Sivakumar et al. (2006) are included in the figure for comparison. The kaolin was wetted at a net mean stress of 50 kPa. As shown in Fig. 4a, there was an increase in vw in each ZY clay specimen because the applied suction was significantly lower than the initial value (i.e. 540kPa). When a specimen was equalized at a lower suction, it had a larger increase in vw. Generally, the ZY clay had a smaller increase in vw than the kaolin had at the applied same suction. A possible reason may be attributed to the similar volume expansion upon wetting in both soils (see Fig. 4b). The absorption of water in an unsaturated soil contributes both to total volume increase and to filling the voids previously occupied by air. By comparing with the kaolin, the initial specific volume of ZY clay was smaller and it had fewer soil voids. Consequently, the ZY clay needed a smaller amount of water to filling the voids when subjected to wetting. Table 3 Values of state parameters for Test Series 3. I.D.

UW2 UW3

Initial condition

After equalization

After 1st isotropic compression

After wetting

After drying

After re-wetting

After isotropic compression

w (%)

v

Sr (%)

s (kPa)

p (kPa)

v

Sr (%)

p (kPa)

v

Sr (%)

s (kPa)

v

Sr (%)

s (kPa)

v

Sr (%)

s (kPa)

v

Sr (%)

p (kPa)

v

Sr (%)

18.31 18.38

1.718 1.715

68.1 68.8

200

20

1.733 1.731

70.1 70.5

70 100

1.727 1.723

70.9 71.2

10

1.775 1.755

83.9 85.1

350

1.731 1.729

71.2 70.9

50

1.751 1.750

74.6 74.0

350

1.683 1.688

80.3 80.2

R. Chen, C.W.W. Ng / Applied Clay Science 86 (2013) 38–46

1.80

p (kPa)

Changes in s (kPa)

UW2 UW3

70 100

Wetting: 200 → 100 → 50 → 25 → 10; Drying: 10 → 25 → 50 → 100 → 200 → 350; Re-wetting: 350 → 200 → 100 → 50.

microscopic viewpoint, this indicates that the pore contraction of montmorillonitic clay leads to an irreversible denser soil texture. Similar to the wetting-induced swelling, the irreversible shrinkage is also stress-dependent. Its magnitude also decreased with an increase in the applied net mean stress, suggesting that the increasing values of net mean stress enhance the resistance to shrinkage caused by drying in the ZY clay. A volume increase was observed along the rewetting path in specimens UW2 and UW3 (Fig. 5b). Furthermore, an accumulated swelling occurred after a wetting–drying–wetting cycle. The magnitude of accumulated swelling was reduced by an increase in the applied net mean stress. The accumulated swelling due to wetting–drying cycles may have an impact on subsequent isotropic compression behavior. Fig. 6 shows the Sr ~ logs relationships during the wetting–drying cycles in specimens UW2 and UW3. The starting points in this figure denote the commencement of the wetting–drying cycles, i.e. the state at the suction of 200 kPa. It is found that the Sr ~ logs curves are somewhat stress-dependent. Degrees of saturation in specimens at different net mean stresses were almost the same along a wetting path whereas the degree of saturation at a given suction was larger for a higher net mean stress along a drying path. A marked hysteresis loop was observed in each Sr ~ logs curve. The size of the hysteresis loop seemed to increase with an increase in the applied net mean stress.

(a)

1.75

2.05

1.95 UI16

1.85

1.65 UI12

Initial (average)

UI13

1.55 ZY clay (Series 1)

UI15

Kaolin (Sivakumar et al., 2006)

1.45 1

1.75

UI14

10

Specific water volume,vw (kaolin)

Specific water volume, vw (ZY)

1.85

1.65 1000

100

(a)

2.05

1.78

2.03

1.76

2.01

1.74

1.99 UW4 (20 kPa)

1.72

Starting

1.97

UW5 (20 kPa) Kaolin (Sivakumar et al., 2006)

1.95 1000

1.70 1

10

100

Suction, s (kPa) 1.80

Specific volume, v (ZY)

I.D.

Specific volume, v (ZY)

Table 4 Testing conditions for a wetting–drying–wetting cycle in Test Series 3.

Specific volume, v (kaolin)

42

(b)

1.78 1.76 1.74 UW2 (70 kPa)

1.72

Starting

UW3 (100 kPa)

1.70

1

10

100

1000

Suction, s (kPa) Fig. 5. Variations in the specific volume in relation to applied suction during wetting– drying cycles at the net mean stresses of (a) 20 kPa and (b) 70 kPa and 100 kPa.

3.2. Response to isotropic compression 3.2.1. Effect of suction level Fig. 7 shows variations in v, vw and Sr during isotropic compression following suction equalization in Test Series 1 in relation to the net mean stress, p. In each specimen except for specimen UI15, the yielding is identified by obvious changes in the slope of the v ~ logp curve (see Fig. 7a). Since the compression curves are relatively smooth and it is nearly impossible to define a clear break corresponding to the yield stress, the pre-consolidation stress (or yield stress), py, is thus estimated by approximating a compression curve as two linear segments and extending them to meet at a point (Cui and Delage, 1996; Roscoe and Burland, 1968). While such a definition of pre-consolidation may result in a large estimate error, it is observed that pre-consolidation stress increases with suction. This demonstrates the contributions of suction

Suction, s (kPa)

1.80 UI16

2.00

UI12

1.75

1.95

UI13 UI14 UI15

1.70 ZY clay (Series 1)

1.65 1

10

1.90 Initial (average)

Kaolin (Sivakumar et al., 2006)

100

90

2.05

1.85 1000

Suction,s (kPa) Fig. 4. Equalized (a) specific water volume and (b) specific volume at different applied suctions.

Degree of saturation, Sr (%)

(b)

Specific volume, v (kaolin)

Specific volume, v (ZY)

1.85

Suction paths: wetting drying rewetting

85

80

75 UW2 (70 kPa)

70

Starting

UW3 (100 kPa)

65

1

10

100

1000

Suction, s (kPa) Fig. 6. Variations in the degree of saturation in relation to applied suction during wetting–drying cycles at different net mean stresses.

R. Chen, C.W.W. Ng / Applied Clay Science 86 (2013) 38–46

1.80

1.75

1.70 UI16 (s = 0 kPa) UI12 (s = 25 kPa)

1.65

UI13 (s = 50 kPa) UI14 (s = 100 kPa) UI15 (s = 200 kPa)

1.60 10

100

1000

Net mean stress, p (kPa)

Specific water volume, vw

1.80

UI16 (s = 0 kPa)

(b)

1.75

UI12 (s = 25 kPa) UI13 (s = 50 kPa) UI14 (s = 100 kPa)

1.70

UI15 (s = 200 kPa)

1.65 1.60 1.55 1.50 1.45 10

100

1000

Net mean stress, p (kPa)

Degree of saturation, Sr (%)

100

(c) UI16 (s = 0 kPa)

90

UI12 (s = 25 kPa) UI13 (s = 50 kPa) UI14 (s = 100 kPa)

80

Opposite to the changes in v and vw, an increase in Sr was observed in each specimen during the isotropic compression (Fig. 7c). In the early stages, the increase in Sr was relatively small. As the applied net mean stress increased, an obvious change in the slope of the Sr ~ logp curve was observed, corresponding to yielding observed in Fig. 7a. Fig. 8 shows the relationship between the identified preconsolidation stress and suction for the kaolin and the ZY clay. Similarly, the kaolin was subjected to isotropic compression following initial suction equalization. In the ZY clay, the pre-consolidation stress decreased from 285 to 80 kPa as the suction decreased from 100 to 0 kPa. The reduction in the pre-consolidation stress was significantly larger than the decrease in suction value. On the contrary, the reduction in the pre-consolidation stress in the kaolin was much smaller than the decrease in suction value. It should be noted that both clays swelled in the previous suction equalization stage (see Fig. 4b). The significant reduction in the pre-consolidation stress in the ZY clay is likely related to both wetting-induced irreversible swelling and remarkable interlayer swelling. As a result, the ZY clay became more susceptible to yielding upon external loading due to changes in clay texture upon wetting (Gens and Alonso, 1992; Likos and Lu, 2006; Likos and Wayllace, 2010). The clay texture is the arrangement of the individual clay particles into quasi-crystals and quasi-crystals into larger aggregates. Each of these scales has pores associated with them, and loading results to collapse of pores. Therefore, the irreversible swelling also produced a reduction of the size of the yield locus. The lower the applied suction during the equalization stage (along a wetting path), the larger the magnitude of the irreversible swelling, and hence the greater the reduction in the size of the yield locus. Therefore, as shown in Fig. 8, different loadingcollapse (LC) yield curves in the (p: s) plane may be drawn through yield points identified from isotropic compression at different suctions in the ZY clay. The yield points in the kaolin identified from isotropic compression may be on the same LC yield curve since there was no irreversible swelling associated with suction decrease in this clay.

UI15 (s = 200 kPa)

70

60 10

100

1000

Net mean stress, p (kPa) Fig. 7. Effect of suction level on variations in (a) specific volume; (b) specific water volume and (c) degree of saturation during isotropic compression in Test Series 1.

to stabilizing soil skeletons. The gradient of the pre-yield isotropic compression curve seemed to be only marginally dependent on the applied suction. Regarding post-yield behavior, only specimens loaded at suctions of 50 kPa or below travel along the normal compression line far enough for this to be well defined. In the suction range from 0 to 50 kPa, the slopes of the normal compression lines are fairly similar, suggesting negligible effect of suction on post-yield stiffness. In company with the decrease in v, a decrease in vw was observed in each specimen during isotropic compression, as shown in Fig. 7b. Except for specimen UI16 compressed at zero suction, the magnitude of the decrease in vw for each specimen was substantially smaller than that in v. In other words, the change of external loading exerts a more significant effect on the soil skeleton than on water phase in an unsaturated specimen. This is because the external loading is applied directly to unsaturated soil skeleton rather than the water itself. For specimen UI16, the decrease in vw with net mean stress was also small until its degree of saturation reached 100% (see Fig. 7c). Subsequently, the soil water in UI16 was forced out of the soil voids due to its small compressibility as net mean stress was further increased. As a result, the magnitude of decrease in vw in UI16 became as large as that in v.

3.2.2. Effect of a wetting–drying cycle In Test Series 2, specimens UW4 and UW5 were subjected to isotropic compression after a wetting–drying cycle. Fig. 9a shows the variations in v during the isotropic compression in specimens UW4 and UW5. Results for specimen UI14 are included for comparison. The three specimens were isotropically compressed at the suction of 100 kPa. Yielding was identified from each v ~ logp curve. The estimated values of pre-consolidation stress for UI14, UW4 and UW5 are 285 kPa, 148 kPa and 203 kPa, respectively. A wetting– drying cycle results in smaller pre-consolidation stress in UW4 and UW5 by 48% and 29%, respectively. The significant reduction in preconsolidation stress is also likely attributed to the irreversible swelling resulting from the wetting–drying cycle (see Fig. 5a). The irreversible swelling produced a more open texture and developed larger voids

300 ZY clay (Series 1)

250

Suction, s (kPa)

Specific volume, v

(a)

43

Kaolin (Sivakumar et al., 2006) Postulated yield curve

200 150

UI14

100 50

UI13

UI12 UI16

0 0

50

100

150

200

250

300

Pre-consolidation stress, p y (kPa) Fig. 8. Effect of suction level on pre-consolidation stress (Test Series 1).

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(a)

1.75

UI14 (after wetting)

1.65

UW5 (after a wetting-drying cycle)

1.60 10

100

1000

Net mean stress, p (kPa) 1.60

(b)

1.55

1.50

UI14 (after wetting)

1.45

UW4 (after a wetting-drying cycle) UW5 (after a wetting-drying cycle)

1.40 10

100

1000

Net mean stress, p (kPa) Fig. 9. Effect of a wetting–drying cycle on (a) volume changes and (b) response of water phase during isotropic compression.

(Gens and Alonso, 1992). As the void ratio of the soil increases, the resistance of the soil to external loading is reduced, i.e. the soil is softened. A larger irreversible swelling generally results in a larger reduction in the pre-consolidation stress. As expected, the preconsolidation stress of UW4 is lower than that of UW5 because UW4 has a larger irreversible swelling (see Fig. 5a). It should be noted that hydraulic hysteresis is likely another reason (Sivakumar et al., 2006; Wheeler et al., 2003). An irreversible increase in the degree of saturation occurred in UW4 and UW5 after a wetting–drying cycle as a consequence of hydraulic hysteresis. This resulted in fewer soil-meniscus water contacts and then reduced the stabilizing effects provided by water menisci. Consequently, the pre-consolidation stress decreased. The observed relation between the irreversible increase in the degree of saturation and the reduction in pre-consolidation stress is consistent with the constitutive model proposed by Wheeler et al. (2003). The post-yield isotropic compression curves of UW4 and UW5 lie below that of UI14 in the (v: logp) plane. This indicates that the effect of a wetting–drying cycle on isotropic compression cannot be fully erased by yielding. In other words, the position of the post-yield isotropic compression curve is not only related to suction level but also influenced by wetting–drying cycles. In correspondence with Fig. 9a, b shows the variations in vw during the isotropic compression in specimens UI14, UW4 and UW5. The decrease in vw during isotropic compression after a wetting–drying cycle (UW4 and UW5) is larger than that after wetting (UI14). It is believed that some voids air-filled at a given value of suction during wetting become water-filled at the same suction after wetting to a lower suction followed by drying (Wheeler et al., 2003). Therefore, UW4 and UW5 may have a larger number of water-filled voids than UI14. During isotropic compression, voids (both air-filled and waterfilled) were compressed due to an increase in the net mean stress (see Fig. 9a). Soil water was forced out of the soil specimen when the water-filled voids were compressed. As a result, more water was drained out from UW4 and UW5 than from UI14, since UW4 and UW5 had more water-filled voids. The observation implies that the

3.2.3. Effect of a wetting–drying–wetting cycle In Test Series 3, specimens UW2 and UW3 experienced a wetting– drying–wetting cycle before isotropic compression. Fig. 10a shows the variations in v during the isotropic compression in specimens UW2 and UW3. Results of specimen UI13 are included for comparison. The three specimens were isotropically compressed at the same suction of 50 kPa. After a wetting–drying–wetting cycle, accumulated swelling (see Fig. 5b) and an accumulated decrease in the degree of saturation (see Fig. 6) occurred in UW2 and UW3. As shown in Fig. 10a, yielding is identified from each v ~ logp curve. The estimated values of pre-consolidation stress for specimens UI13, UW2 and UW3 are 180kPa, 205kPa and 220kPa, respectively. It was expected that a reduction in pre-consolidation stress should be observed in UW2 and UW3 due to the accumulated swelling. The results, however, show that a wetting–drying–wetting cycle leads to larger pre-consolidation stress in UW2 and UW3 by 14% and 22%, respectively. Hydraulic hysteresis may contribute to the unexpected observation. Specimens UW2 and UW3 had lower degrees of saturation than UI13 had, then they may have had a larger number of voids affected by water menisci. Consequently, there are stronger stabilizing effects provided by water menisci in UW2 and UW3, leading to an increase in the resistance to external loading. The stabilizing effect of water menisci is dominant over the softening effect caused by soil swelling. The existence of a stabilizing effect of water menisci is also consistent with the model proposed by Gallipoli et al. (2003), who postulated the existence of a “bonding” effect associated to capillary menisci from which they developed a full mechanical model of soil behavior. The stronger stabilizing effects provided by water menisci also cause the post-yield isotropic compression curves of UW2 and UW3 to lie above that of UI13 in the (v: lnp) plane.

1.80

(a) Specific volume, v

1.70

UW4 (after a wetting-drying cycle)

Specific water volume, vw

response of water phase to isotropic compression is also affected by a wetting–drying cycle.

1.75

1.70

UI13 (after wetting)

1.65

UW2 (after a wetting-drying-wetting cycle) UW3 (after a wetting-drying-wetting cycle)

1.60

10

100

1000

Net mean stress, p (kPa) 1.60

Specific water volume, vw

Specific volume, v

1.80

(b)

1.55

1.50 UI13 (after wetting)

1.45

UW2 (after a wetting-drying-wetting cycle) UW3 (after a wetting-drying-wetting cycle)

1.40 10

100

1000

Net mean stress, p (kPa) Fig. 10. Effect of a wetting–drying–wetting cycle on (a) volume changes (b) response of water phase during isotropic compression.

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Regarding water phase, Fig. 10b shows the variations in vw during the isotropic compression in specimens UI13, UW2 and UW3. As compared with a wetting–drying cycle (see Fig. 9b), the effect of a wetting–drying–wetting cycle on change in water content is negligible. This indicates that the response of water phase to isotropic compression is mostly related to the recent wetting–drying history, i.e. wetting or drying just prior to isotropic compression, rather than the overall wetting–drying history. 4. Conclusions In this study, suction-controlled isotropic compression tests were carried out on an unsaturated silty clay subjected to different wetting– drying histories. The following conclusions are reached based on the experimental study. 1. Due to the presence of montmorillonite, the ZY clay showed complicated volume change behavior in addition to hydraulic hysteresis during wetting–drying cycles, e.g. irreversible swelling upon wetting, irreversible shrinkage upon subsequent drying and accumulated swelling after a wetting–drying–wetting cycle. Both volume change behavior and hydraulic hysteresis are stressdependent. 2. In contrast to the non-expansive compacted kaolin, the reduction in pre-consolidation stress along a wetting path in the ZY clay is much larger than the decrease in suction value. The significant reduction in pre-consolidation stress is attributed to the wetting-induced irreversible swelling as well as the reduction of stabilizing effect of meniscus water due to water absorption. 3. Pre-consolidation stress is affected by wetting–drying cycles in addition to suction level due to both irreversible swelling and change in the degree of saturation resulting from wetting–drying cycles. It was found that irreversible swelling or an increase in the degree of saturation leads to a decrease in pre-consolidation stress. 4. The position of the post-yield isotropic compression curve is also influenced by wetting–drying cycles. This experimental evidence is very important for further development of most existing models that assume that the post-yield compression curve is only related to suction level. 5. The response of water phase to isotropic compression is mostly related to the recent wetting–drying history, i.e. wetting or drying just prior to isotropic compression, rather than the overall wetting– drying history. The decrease in water content during isotropic compression following drying is larger than that following wetting due to having a larger number of water-filled voids.

Nomenclature e void ratio, the ratio of the volume of voids to the volume of solid particles Gs specific gravity, the ratio of the density of solid particles to the density of water at a temperature of 4 °C under atmospheric pressure conditions p net mean stress, the difference between total mean stress and pore air pressure py pre-consolidation stress, the net mean stress at yielding s suction, the difference between pore air pressure and pore water pressure Sr degree of saturation, the ratio of the volume of water to the volume of voids ua pore air pressure uw pore water pressure v specific volume, the volume of voids plus solids in a volume of soil containing unit volume of solid particles vw specific water volume, the volume of water plus solids in a volume of soil containing unit volume of solid particles

w σ1 σ3

45

water content, the ratio of the mass of water to the mass of solid particles axial stress confining stress

Acknowledgments The authors would like to acknowledge the financial support from research grants HKUST09/CRF//09 provided by the Research Grants Council (RGC) of HKSAR, National Basic Research Program of China (973 Program) No. 2012CB719805 supported by the Ministry of Science and Technology of China, and No. 50909030 provided by the National Natural Science Foundation of China. References Al-Homoud, A.S., Basma, A.A., Husein Malkawi, A.I., Al-bashabsheh, M.A., 1995. Cyclic swelling behavior of clays. J. Geotech. Eng. 121, 562–565. Alonso, E.E., Lloret, A., Gens, A., Yang, D.Q., 1995. Experimental behaviour of highly expansive double-structure clay. Proceedings of 1st International Conference on Unsaturated Soils, Balkema, Rotterdam, The Netherlands, pp. 11–16. Alonso, E.E., Romero, E., Hoffmann, C., García-Escudero, E., 2005. Expansive bentonite– sand mixtures in cyclic controlled-suction drying and wetting. Eng. Geol. 81, 213–226. Alshihabi, O., Shahrour, I., Mieussens, C., 2002. Experimental study of the influence of suction and drying/wetting cycles on the compressibility of a compacted soil. Proceedings of 1st International Conference on Unsaturated Soils, Balkema, Rotterdam, The Netherlands, pp. 541–545. Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), 2006. Handbook of Clay Science, 1st ed. Elsevier Science, Amsterdam. BSI, 1990. BS1377: Methods of Test for Soils for Civil Engineering Purposes. British Standards Institution. Chu, T.Y., Mou, C.H., 1973. Volume change characteristic of expansive soils determined by controlled suction tests. Proceedings of the 3rd International Conference on Expansive Soils, Haifa, pp. 177–185. Cui, Y.J., Delage, P., 1996. Yielding and plastic behaviour of an unsaturated compacted silt. Geotechnique 46 (2), 291–311. Cui, Y.J., Yahia-Aissa, M., Delage, P., 2002. A model for the volume change behavior of heavily compacted swelling clays. Eng. Geol. 64, 233–250. Cuisinier, O., Masrouri, F., 2005. Hydromechanical behavior of a compacted swelling soil over a wide suction range. Eng. Geol. 81, 204–212. Fredlund, D.G., 1975. A diffused air volume indicator for unsaturated soils. Can. Geotech. J. 12, 533–539. Gallipoli, D., Gens, A., Sharma, R., Vaunat, J., 2003. An elasto-plastic model for unsaturated soil including the effect of saturation degree on mechanical behaviour. Geotechnique 53 (1), 123–135. Gates, W.P., 2007. Geosynthetic clay liner technology: understanding bentonite. Proceedings of National Landfill and Transfer Station Conference, Melbourne, pp. 1–18. Gehling, W.Y.Y., Alonso, E.E., Gens, A., 1995. Stress-path testing of expansive compacted soils. Proceedings of the 1st International Conference on Unsaturated Soils, Paris, vol. 1, pp. 77–82. Gens, A., Alonso, E.E., 1992. A framework for the behaviour of unsaturated expansive clays. Can. Geotech. J. 29, 1013–1032. Gens, A., Sánchez, M., Sheng, D., 2006. On constitutive modelling of unsaturated soils. Acta Geotech. 1, 137–147. Hilf, J.W., 1956. An investigation of pore water pressure in compacted cohesive soils. U.S. Bureau of Reclamation Technical Memorandum 654. Katti, D.R., Shanmugasundaram, V., 2001. Influence of swelling on the microstructure of expansive clays. Can. Geotech. J. 38 (1), 175–183. Likos, W.J., Lu, N., 2003. Automated humidity system for measuring total suction characteristics of clay. Geotech. Test. J. 26 (2), 1–12. Likos, W.J., Lu, N., 2006. Pore-scale analysis of bulk volume change from crystalline swelling in Na+- and Ca2+-smectite. Clay Clay Miner. 54 (4), 515–528. Likos, W.J., Wayllace, A., 2010. Porosity evolution of free and confined bentonites during interlayer hydration. Clay Clay Miner. 58 (3), 399–414. Lloret, A., Villar, M.V., Sanchez, M., Gens, A., Pintado, X., Alonso, E.E., 2003. Mechanical behaviour of heavily compacted bentonite under high suction changes. Geotechnique 53 (1), 27–40. Miao, L.C., Liu, S.Y., Lai, Y.M., 2002. Research of soil–water characteristics and shear strength features of Nanyang expansive soil. Eng. Geol. 65, 261–267. Ng, C.W.W., Zhan, L.T., Cui, Y.J., 2002. A new simple system for measuring volume changes in unsaturated soils. Can. Geotech. J. 39 (2), 757–764. Ng, C.W.W., Zhan, L.T., Bao, C.G., Fredlund, D.G., Gong, B.W., 2003. Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration. Geotechnique 50 (2), 143–157. Nowamooz, H., Masrouri, F., 2008. Hydromechanical behaviour of an expansive bentonite–silt mixture in cyclic suction-controlled drying and wetting tests. Eng. Geol. 101, 154–164.

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