No abstract

A significant number of landslides occur in cold regions because of freezing and thawing cycles. The instability of thawing slopes can cause serious damage to transportation infrastructure and property, and even loss of human life. This type of landslide is difficult to analyze by the traditional limit-equilibrium methods, however, because of the complicated multi-physics processes involved. This paper describes a holistic microstructure-based random finite element model (RFEM) to simulate the stability of a thawing slope. The RFEM model is developed to simulate the bulk behaviors of frozen and unfrozen soils based on the behaviors of individual phases. The phase coded image of a frozen silty clay is first custom built and then converted into a finite element model. The mechanical behaviors of individual phases of the frozen soil are calibrated by uniaxial compressive test. The triaxial test is then simulated by RFEM to obtain the shear strength parameters of frozen and unfrozen soils. Coupled thermal-mechanical REFM models are developed to simulate the effects of temperature on the displacement field and stress field in the slope. From the results, the local factor of safety field can be determined. The development of local factor of safety and potential failure surface associated with the thawing process over a typical year are simulated by this new model. The variations in the stability of thawing slopes predicted by this model are consistent with field observations as well as the global-wise slope stability analysis.

Hydraulic conductivity and soil-water retention are two critical soil properties describing the fluid flow in unsaturated soils. Existing experimental procedures tend to be time consuming and labor intensive. This paper describes a heuristic approach that combines a limited number of experimental measurements with a computational model with random finite element to significantly accelerate the process. A microstructure-based model is established to describe unsaturated soils with distribution of phases based on their respective volumetric contents. The model is converted into a finite element model, in which the intrinsic hydraulic properties of each phase (soil particle, water, and air) are applied based on the microscopic structures. The bulk hydraulic properties are then determined based on discharge rate using Darcy’s law. The intrinsic permeability of each phase of soil is first calibrated from soil measured under dry and saturated conditions, which is then used to predict the hydraulic conductivities at different extents of saturation. The results match the experimental data closely. Mualem’s equation is applied to fit the pore size parameter based on the hydraulic conductivity. From these, the soil-water characteristic curve is predicted from van Genuchten’s equation. The simulation results are compared with the experimental results from documented studies, and excellent agreements were observed. Overall, this study provides a new modeling-based approach to predict the hydraulic conductivity function and soil-water characteristic curve of unsaturated soils based on measurement at complete dry or completely saturated conditions. An efficient way to measure these critical unsaturated soil properties will be of benefit in introducing unsaturated soil mechanics into engineering practice.

Frost heave can cause serious damage to civil infrastructure. For example, interactions of soil and water pipes under frozen conditions have been found to significantly accelerate pipe fracture. Frost heave may cause the retaining walls along highways to crack and even fail in cold climates. This paper describes a holistic model to simulate the temperature, stress, and deformation in frozen soil and implement a model to simulate frost heave and stress on water pipelines. The frozen soil behaviors are based on a microstructure-based random finite element model, which holistically describes the mechanical behaviors of soils subjected to freezing conditions. The new model is able to simulate bulk behaviors by considering the microstructure of soils. The soil is phase coded and therefore the simulation model only needs the corresponding parameters of individual phases. This significantly simplifies obtaining the necessary parameters for the model. The capability of the model in simulating the temperature distribution and volume change are first validated with laboratory scale experiments. Coupled thermal-mechanical processes are introduced to describe the soil responses subjected to sub-zero temperature on the ground surface. This subsequently changes the interaction modes between ground and water pipes and leads to increase of stresses on the water pipes. The effects of cracks along a water pipe further cause stress concentration, which jeopardizes the pipe’s performance and leads to failure. The combined effects of freezing ground and traffic load are further evaluated with the model.

No abstract