Experimental study on the hydro mechanical behaviour of compacted loess-role of soil microstructure

Loess is a loosely cemented silty soil, transported and deposited by wind, and formed in semi- arid continental climates. Loess in the Chinese Loess Plateau is famous all over the world for its continuity and high development. Loess is treated as problematic soil due to its collapsibility upon wetting. The predominance of loess deposits in northern China, together with the rapid expansion of urban areas, has led to the widespread use of loess in several infrastructure projects, mainly as a filling material. The long-term settlement, differential settlement and stability of loess fill slopes are the most urgent problems for those loess fill projects. However, these problems are highly correlated to the behaviour of unsaturated compacted loess under complex environmental actions. Therefore, study the hydro-mechanical behaviour of compacted loess is the primary task that is required to solve those engineering problems. In addition, soil behaviour is highly related to microstructure. Investigating soil microstructure change upon hydraulic coupling loading will help to reveal the mechanism of soil behaviour. Based on this, a comprehensive laboratory study, including one-dimensional and isotropic compression tests under constant water content conditions, one-dimensional compression tests under suction control conditions and gas permeability tests were conducted on compacted loess with modified testing equipment. These tests were combined with qualitative (Scanning Electron Microscopy, SEM) and quantitative (Mercury Intrusion Porosimetry, MIP) microstructure observation techniques to give additional insights into compacted loess behaviour, including compression, collapse and gas permeability. The main innovative works in this study are as follows: A series of one-dimensional and isotropic compression tests under constant water content conditions were carried out on compacted loess with modified one-dimensional oedometer and stress-path triaxial apparatus. Effects of the initial microstructure, stress level and loading history on the compressibility, collapse and stiffness of compacted loess were studied. The evolution of microstructure upon compression and wetting was investigated via MIP test and SEM analysis. The preliminary mechanism of compression, collapse and creep of unsaturated loess were revealed from both macro- and micro perspectives. Using the axis translation technique, a series of suction control tests were conducted to investigate the compressibility and wetting collapse of compacted loess. The compression index and secondary compression coefficient were analysed. Yielding of compacted loess under wetting and cyclic wetting-drying were investigated. The expansion of the loading- collapse curve and hardening of compacted loess under hydraulic coupling loading were further studied, based on the existing constitutive framework. A series of numerical analyses were conducted with SEEP/W to simulate the moisture distribution in a case deep loess fill under different wetting conditions (rainfall and rise of underground water level). Settlement caused by wetting in a case loess fill was analysed using the layerwise summation method by combining numerical analysis results with the one-dimensional wetting test results. These results would be helpful for evaluating the wetting settlement in similar projects. With the modified stress-path triaxial apparatus, the gas permeability of compacted loess under a wide range of as-compacted states, on wetting/drying paths and a constant stress ratio loading path were studied. Microstructure of compacted loess was investigated with MIP test and SEM analysis to understand gas permeability changes under different loading paths. The new approach for estimating the pore structure parameter PSP, as well as the gas permeability model k_eff = C_s.PSP (C_s is the pore shape parameter) proposed by Nguyen et al. (2020) has been verified on compacted loess.