Enlong Liu

A Fractional Creep Constitutive Model for Frozen Soil in Consideration of the Strengthening and Weakening Effects

The triaxial creep tests of frozen silty clay mixed with sands were performed under different pressures, and the test results demonstrated that, under the low confining pressure, when the shear stress is lower than the long-term strength, the test specimen exhibits an attenuation creep because the strengthening effect is greater than the weakening effect. When the shear stress is higher than the long-term strength, the test specimen exhibits a nonattenuation creep due to the level of the strengthening and weakening effects change in different stages. As the confining pressure increases, the test specimens only exhibit an attenuation creep because of the enhancing strengthening effect. Both the hardening parameter and the damage variable were introduced to describe the strengthening and weakening effects, respectively, and a new creep constitutive model for frozen soil considering these effects was put forward based on the theory of elastoviscoplastic and the fractional derivative. Finally, the model parameters were analyzed and their determination method was also provided to reveal the trend of parameters according to the triaxial test results. The calculated results of the constitutive model show that the proposed model can describe the whole creep process of frozen soil well.

Deformation Mechanisms of Crushable Blocky Materials Upon Lateral Unloading for a Biaxial Stress State

The geological materials encountered in engineering are mainly particulate materials, which are composed of distinct particles that interact with one another and the associated voids among them. Upon loading or unloading, the particles may slide, rotate, crush, or break. The deformation and breakage mechanisms of particulate materials are sufficiently complicated that many experimental studies have attempted to characterize these processes (see, e.g., Rowe 1962; Drescher 1976; Chappell 1979; Oda et al. 1980; Calvetti et al. 1997; Lanier 2001; Wolf et al. 2003; Delenne et al. 2004). Furthermore, several researchers have employed theoretical and numerical methods to study the deformation mechanisms of particulate materials (McDowell et al. 1996; Anandarajah 2004; Chang and Hicher 2005). The aforementioned experimental and theoretical studies focused primarily on the microscopic deformation mechanisms that occur in stiff materials, such as sliding and rolling or the rotation of particles. Upon loading, however, the particles of an element of soil are crushed or broken (Hardin 1985; Nakata et al. 2001; Bartake and Singh 2007). This breaking, in turn, profoundly affects the strength and stress–strain behavior of the soil elements. Although a number of theoretical methods have been employed to study the crushing mechanism of granular materials, the simulation results need to be confirmed using experimental data. When formulating the constitutive model of particulate materials, we should consider the influence of microstructure deformation mechanisms. For this approach, we performed biaxial tests on crushable blocky materials at the meso scale and investigated the breakage properties of crushable blocky materials with a monotonic stress path (Liu 2011). In this study, we tracked the mesoscopical breakage process of crushable blocky materials for a lateral unloading stress path. Biaxial compressive tests involving the unloading of lateral stress at three different stress states were performed on an assembly of crushable blocky materials. Finally, we comprehensively investigated the deformation mechanisms and breakage processes of particulate materials for a lateral unloading stress path.

Hydro-mechanical analysis of rainfall-induced fines migration process within unsaturated soils

Seepage-induced fines migration under rainfall infiltration is a main cause leading to shallow failures in loose colluvial slopes. To describe the full process of fines migration within unsaturated soils during rainfall infiltration and the associated hydro-mechanical behaviors, a seepage-erosion-deformation coupled formulation is proposed in this paper. The governing equations proposed are implemented into a finite element code and used to investigate the influences of skeleton deformation on the rainfall infiltration process through unsaturated soil columns. The numerical results were presented in detail for a better understanding of the rainfall-induced fines migration process within unsaturated soils. Further, the obtained results are integrated into an infinite slope model for slope stability analysis. The results show that, the skeleton deformation will affect the rainfall infiltration rate and hence the timing of slope failures; meanwhile their influences are more evident if the fines deposition process is taken into account. Moreover, the slope stability could be reduced gradually due to the soil strength loss along with loss of fine particles. Therefore, particular attentions should be paid to analyzing the stability of soil slopes susceptible to internal erosion.

A Binary-Medium Constitutive Model for Artificially Structured Soils Based on the Disturbed State Concept and Homogenization Theory

AbstractTriaxial compression tests were carried out on artificially structured soil samples at confining pressures of 25, 37.5, 50, 100, 200, and 400 kPa. A binary-medium constitutive model for art...

Investigation on the Nonlinear Strength Properties and Damage Statistical Constitutive Model for Frozen Sandy Soils

There are many flaws, such as fissures, cavities, and inclusions, in geomaterials, which make their mechanical properties with great randomness and uncertainty. Upon loading, the soil structure gradually losses the bearing capacity due to the transformation from microdefects to macroscopic breakage bands. Based upon the experimental data of frozen sandy soils, a new nonlinear strength equation between the first and third principal stresses was proposed, and then the nonlinear strength properties for frozen sandy soils in σ-τ plane were analyzed. In addition, by assuming that the microstrength of frozen sandy soil obeys the Weibull distribution function, a statistical damage constitutive model was established based upon the framework of continuum damage mechanics (CDM), with few parameters and a high accuracy. Compared with experimental data, the new model can well grasp the nonlinear strength properties and simulate the stress-strain relationships under different confining pressures for frozen sandy soils.

Analysis on velocity distribution and displacement along the profile of a slope using both empirical and analytical methods

Assessing the slope deformation is significant for landslide prediction. Many researchers have studied the slope displacement based on field data from the inclinometer in combination with complicated numerical analysis. They found that there was a shear zone above the slip surface, and they usually focused on the distribution of velocity and displacement within the shear zone. In this paper, two simple methods are proposed to analyze the distribution of displacement and velocity along the whole profile of a slope from the slip surface to the slope surface during slow movement. In the empirical method, the slope soil above the shear zone is assumed as a rigid body. Dual or triple piecewise fitting functions are empirically proposed for the distribution of velocity along the profile of a slope. In the analytical method, the slope soil is not assumed as a rigid body but as a deformable material. Continuous functions of the velocity and displacement along the profile of a slope are directly obtained by solving the Newtons equation of motion associated with the Bingham model. Using the two proposed methods respectively, the displacement and velocity along the slope profiles of three slopes are determined. A reasonable agreement between the measured data and the calculated results of the two proposed methods has been reached. In comparison with the empirical method, the analytical method would be more beneficial for slope deformation analysis in slope engineering, because the parameters are material constants in the analytical solution independent of time t, and the nonlinear viscosity of the soil can be considered.

Micro-scale Modelling of the Breakage Mechanism of Structured Soils

The two dimensional discrete element method (DEM) was employed to develop a numerical simulation model for simulating the stress-strain relationship and the breakage mechanism of structured soils at micro-scale in the plane strain state. Considering the distribution of micro-pore and bonding between soil particles, the user-defined language was used to validate the model of structured soils and the boundary conditions. Comparing with the SEM photograph of the artificial structured soils, the validity of model was proved. The influences of particle properties and test conditions on the macroscopic mechanical properties of structured soils were investigated in microscopic level. The results revealed that the breakage mechanism of structured soils could be simulated well in micro-scale based on discrete element method. From the simulated results, the axial peak strength dramatically increases with the growth of friction coefficient and bonding strength between particles respectively; the axial peak strength and residual shear strength increase with the growth of confining pressure and the volumetric strain is contracting at first, and then dilating.