J. Cent. South Univ. Technol. (2008) 15(s1): 397-401

DOI: 10.1007/s11771-008-387-y

Effects of stress conditions on rheological properties of granular soil in large triaxial rheology laboratory tests

CHEN Xiao-bin(陈晓斌)^{1, 2}, ZHANG Jia-sheng(张家生)¹, LIU Bao-chen(刘宝琛)¹, TANG Meng-xiong(唐孟雄)²

(1. School of Architectural and Civil Engineering, Central South University, Changsha 410075, China;

2. Guangzhou Institute of Building Science, Guangzhou 510440, China)

Abstract: In order to study the rheological properties of red stone granular soil, a series of rheological experiments were executed on large tri-axial rheological apparatus. Under 100, 200 and 300 kPa confining stress conditions, the rheological tests were carried out. These experiment results showed that the stress conditions, especially the stress level were the critical influencing factors of the rheological deformation properties. Under the low stress level (S=0.1), the granular soil showed the elastic properties, and there was no obvious rheological deformation. Under the middle stress level (0.2＜S≤0.6), creep curves showed the linear viscoelastic rheological properties. However, under the high stress level (S＞0.8) creep curves showed the non-linear viscous plastic rheological properties. Especially, under the stress level of S=1.0, the accelerated rheological phase of creep curves occurred at early time with a trend of failure. The stress level had obvious effects on the final rheological deformation of the soil sample, and the final rheological deformation increments nonlinearly increased with stress level. The final rheological deformation increment and step was little under low stress level, while it became large under high stress level, which showed the nonlinearly rheological properties of the granular soil. The confining pressure also had direct effects on final rheological deformation, and the final rheological deformation linearly increased with confining pressure increments.

Key words: stress conditions; granular soil creep; large triaxial rheology test; redstone granular soil; final creep deformation

1 Introduction

The typical creep curves at constant principal stress differences in the triaxial test are shown in Fig.1. When the principal stress difference is low, the axial strain takes place in a short time and it tends to be constant with time. The failure does not occur and the soil shows the visco-elastic properties. When the principal stress difference increases, the rate of creep increases and the soil shows the visco-plastic properties. After the principal stress difference reaches a limit, the rate of creep is speeded up and the axial strain would increase to fail from creep. In the process, there are four parts as follows. (a) The instantaneous elastic strain OA is little for soil. (b) At the beginning of creep AB, the rate of creep decreases gradually. If unloading takes place, the instantaneous elastic strain is recovered at once and the initial strain because of creep is reinstated progressively. (c) The rate of creep BC is constant and the permanent strain remains after unloading. (d) The rate of creep CD increases in high speed until the failure takes place. It is evident that when the principal stress difference exceeds a limit, the rate of creep begins to increase quickly.

The rheological constitutive models are traditionally divided into linear model and nonlinear model^[1-2]. In general, there are two methods to set up a nonlinear rheological constitutive model^[3]. One is that the nonlinear rheological elements take the place of the traditional linear elements in the rheological model to set up a new model that can describe the visco-plastic rheological properties including the accelerating creep rate phase in the rheological procedure. The other is that the nonlinear rheological constitutive model is set up by the endochronic theory, fracture mechanic theory, etc.

Nowadays, granular soil is widely used as embankment fillings^[4], for its good engineering properties and broad distribution, especially redstone granular soil. However, the engineering practices show that many pavement damages were caused by granular soil embankments’ long-term rheological deformation^[5]. Unfortunately, in the road engineering domain, the research works about granular soils were insufficient, especially the rheological properties^[6-9]. With the quick construction steps of highway, it is necessary to study the rheological properties of the granular soil embankment fillings^[10]. In this work, a series of rheological experiments were conducted on the large tri-axial rheological apparatus (GST-1). Through analyzing the test data of the granular soil, the rheological properties of

Fig.1 Typical creep curve in triaxial rheological test

the redstone granular soil, especially, the effects of stress conditions on the final rheological deformation under different stress conditions were analyzed.

2 Experimental design

Three redstone granular soil samples were examined on the GST-1 to study the rheological properties. These redstone granular soil samples were taken from the k20+240 embankment in Huaixin highway as embankment fillings. The granule ometrica of test samples was reduced from the original granular soil (Fig.2).

The physical parameters of redstone granular soil samples in the laboratory tests are listed in Table 1.

The dry density of samples is 18.8 g/m³, and the

Fig.2 Test granulometrical curves of redstone granular soil

Table 1 Physical parameters of redstone granular soil sample

dimension is d 300 mm×600 mm. The stepping load path was adapted in the rheological tests, and the load was divided into 6 steps (Table 2). When the axial strain increment between the two load steps was less than 3% of the total axial strain, the next step load started. The testing plan is listed in Table 2.

Table 2 Redstone granular soil rheological test plan

3 Effects of stress conditions on creep curves

The creep curves under 50 kPa confining pressure are shown in Figs.3-8.

These experiment results show that the stress condition, especially the stress level is the critical influencing factor of the rheological deformation properties. There are different rheological properties of

Fig.3 Creep curve of stress conditions (σ₃=50 kPa, S=0.1)

Fig.4 Creep curve of stress conditions (σ₃=50 kPa, S=0.2)

Fig.5 Creep curve of stress conditions (σ₃=50 kPa, S=0.4)

Fig.6 Creep curve of stress conditions (σ₃=50 kPa, S=0.6)

Fig.7 Creep curve of stress conditions (σ₃=50 kPa, S=0.8)

granular soil under different stress levels. Under the low stress level (S=0.1), the granular soil shows the elastic properties, and there is no obvious rheological deformation. The axial strain takes place in a short time and it tends to be constant with time (Fig.3). Under the middle stress level (0.2＜S≤0.6), creep curves show the linear viscoelastic rheological properties (Figs.4-6). The

Fig.8 Creep curve of stress conditions (σ₃=50 kPa, S=1.0)

rate of creep decreases gradually. The linear viscoelastic rheological model (Burgers) could describe the linear rheological properties well. However, under the high stress level (S=0.8), creep curves show the non-linear viscous plastic rheological properties (Fig.7). The rate of creep increases in high speed until the failure takes place. Especially, under the stress level (S=1.0), the accelerated rheological phase of creep curves occurs at early time with a trend of failure (Fig.8). The Burgers rheological model can describe the linear rheological properties including the attenuation creep phase and steady state creep phase well, but cannot describe the non-linear rheological properties, especially, the accelerated creep phase. New rheological models should be developed to describe them.

The tests results show that the stress conditions are the main factors that affect the creep properties of redstone granular soil embankment fillings. Different rheological properties of the creep curves appear under different principal stress levels.

4 Effects of stress conditions on equal time strain—stress curves

In order to study the nonlinear viscous elastoplastic rheological property of the redstone granular soil, the test data of S1 soil sample were selected as the analysis emphases. The nonlinear viscous elastoplastic stress—strain relationship could not be described directly in the creep curves. So, in order to expediently study the nonlinear viscous elastoplastic relationship, the equal time stress—strain curves were deduced from the creep curves of S1 sample under different stress conditions (Fig.9). The total creep strain of the redstone granular soil consists of the viscoelastic strain part which is linear and the viscoplastic strain part which is nonlinear. Usually, the strain under the low stress condition is taken as the linear viscoelastic strain part which can be expended to the high stress condition to get the viscoelastic strain part under the high stress condition^[11]. And the total strain was subtracted by the linear viscoelastic strain part to get the nonlinear viscoplastic strain part in the equal time curves^[12].

Fig.9 Equal time stress vs strain curves of of S1 soil sample:  (a) Total stress—strain curves of equal time; (b) Viscous plastic strain curve of equal time

The curves in Fig.9(a) were the equal time total stress and strain curves of redstone granular soil S1 sample, and the curves in the Fig.9(b) were the equal time viscoplastic stress and strain part curves. From the characteristics of the equal time stress—strain curves, we knew that the stress—strain curves were linear under the low stress condition, which stand for the linear visoelastic properties of the redstone granular soil. When the stress reached the yield limit, the stress—strain curves were obviously nonlinear, which stand for the nonlinear viscoplastic properties of the redstone granular soil. The equal time viscoplastic stress and strain part curves in Fig.9(b) showed the viscoplastic stress and strain nonlinear relationship of the redstone granular soil, and with the time going on, the nonlinear viscoplastic properties become more obvious. The linear viscoelastic rheological model cannot describe nonlinear viscoplastic stress and strain relationship anymore.

5 Effects of stress conditions on final rheolo- gical deformation

According to the creep tests results, the final rhecological deformations of the redstone granular soil under different stress conditions are listed in Table 3.

The stress level and final rheological deformation relationship curves are drawn in Fig.10 according to the data in Table 3.

Table 3 Effect of stress conditions on final rheological strain of redstone granular soil

Fig.10 Effects of stress level on final rehological strain of red- stone granular

Fig.10 showed that the stress level had obvious effects on the final rheological deformation of the soil sample. The final rheological deformation increments nonlinearly increased with stress level increment. And the final rheological deformation increment step was little under low stress level, while the final rheological deformation increment step became large under high stress level, which showed nonlinearly rheological properties of the granular soil.

The different confining pressures and final rheological deformation relationship curves are also drawn in Fig.11 according to the data in Table 3.

Fig.11 showed that the confining pressure also had direct effects on final rheological deformation, and the final rheological deformation linearly increased with confining pressure increments in the laboratory test.

Fig.11 Effect of confining stress on final rehological strain of red stone granular

The test results showed that the stress conditions including the stress level and confining pressure were the main factors which affected the creep properties of redstone granular soil embankment fillings.

6 Conclusions

1) The stress conditions including the stress level and confining pressure were the main factors which affected the creep properties of redstone granular soil embankment fillings. There were different rheological properties of the creep curves under different stress levels.

2) The stress level had obvious effects on the final rheological deformation of the soil sample. The final rheological deformation increments nonlinearly increased with stress level increment. And the final rheological deformation increment step was little under low stress level, while it became larger under high stress level, which showed the nonlinearly rheological properties of the granular soil.

3) The confining pressure also had direct effects on final rheological deformation, and the final rheological deformation linearly increased with confining pressure increments.

Acknowledgements

The Huaixing Highway Construction CO., LTD is acknowledged for supplying the redstone granular soil samples. The authors will also like to acknowledge Dr. JIANG Jian-guo for his help in redstone granular soil creep tests.

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(Edited by YANG Bing)

Foundation item: Project(200413) supported by Communication Science and Technology Fund of Hunan Province, China

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: CHEN Xiao-bin, PhD; Tel: +86-731-2656563; E-mail: cxb528@163.com