| Author: | Zhan, Zhiqi |
| Title: | Thermo-hydro-mechanical coupled material point method and its applications in landslide analysis |
| Advisors: | Zhou, Chao (CEE) Yin, Jianhua (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | 223 pages : color illustrations |
| Language: | English |
| Abstract: | Rainfall-induced slope failures in unsaturated soils represent a significant geohazard, particularly in regions like Hong Kong. This issue is further complicated by climate change and frictional heating-induced pore water pressure. Understanding the initiation mechanism and post-failure motion of landslides is crucial for risk mitigation. Traditional mesh-based methods, like the Finite Element Method (FEM), cannot easily simulate large deformation problems due to mesh distortion. As a result, several mesh-free methods, including the Material Point Method (MPM), have been developed. However, existing MPM formulations for unsaturated soil generally have several major limitations. First of all, they cannot analyse the interactions between soil and free water, such as the interaction between unsaturated soil slope and surface ponding and runoff. Secondly, modelling the hydro-mechanical behaviour of unsaturated soil is highly simplified, such as the assumption of perfect plasticity (e.g., not able to simulate contractive behaviour in loose fill slope) and the disregard for the density-dependency of hydraulic behaviour. Thirdly, the effects of thermal effects on unsaturated soil slope failure have not been explored. To address these challenges, this study develops a two-phase, two-point MPM formulation incorporating thermo-hydro-mechanical (T-H-M) coupling, applicable to both saturated and unsaturated soils. Separate sets of material points represent the solid and liquid phases, allowing for the modelling of free water, dry soil, and saturated and unsaturated soils in a unified approach. A method was proposed to simulate rainfall boundary conditions while considering infiltration, ponding, and surface runoff. The T-H-M coupling effects are accounted by a void ratio and temperature-dependent SWRC, permeability function, and the impact of porosity and saturation on thermal conductivity. The mechanical behaviour includes unsaturated effects, thermal strain, and strain hardening/softening, suitable for large deformation problems involving soil volume changes, such as loose fill landslides. Experimental, numerical, and analytical results validate the MPM code, demonstrating its effectiveness in capturing H-M and T-H-M coupled large deformation problems. Sand column collapse tests were conducted to further validate the code and assess the impact of water content and density on collapse behaviour. The results show that the effects of water content and density on the duration of different failure stages, sand column collapse modes, as well as the profile and runout distance, are very significant. With the increase of Bishop's stress and shear strength, the sand column exhibits a partial collapse mode, the thin layer flow (third stage) is not obvious, resulting in a decrease in both the runout distance and the degree of collapse. The MPM approach is applied to simulate the 1976 Sau Mau Ping landslide, a typical loose-fill slope failure in Hong Kong. This simulation spans the entire process from rainfall infiltration to the final deposition of the sliding mass. Based on the stress path of the points on the sliding surface and the evaluation of the sliding volume, the post-stage movement of a landslide, which includes the process from sliding to flowing, is reclassified into three phases, each associated with a specific slope failure mode. Furthermore, parametric studies are conducted to investigate the influence of dilation angle, strain softening rate, and void ratio-dependent SWRC on slope failure. Additionally, several parametric studies were conducted to investigate the influence of several factors on the initiation and post-failure motion of landslides, including (i) the contraction/dilation during shearing and its influence on the hydraulic behaviour; (ii) frictional heating induced excess pore water pressure; and (iii) surface ponding and surface-subsurface conjugated water flow. The computed results suggest that if the effects of contraction/dilation on hydraulic behaviour are not considered, the post-failure displacement is overestimated for loose soil slope and underestimated for dense soil slope. With a consideration of frictional heating, the landslide displacement is larger. Finally, as expected, the post-failure displacement is underestimated if the surface ponding is not considered. |
| Rights: | All rights reserved |
| Access: | open access |
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