Colluvial soils on natural terrains are usually coarse and their grain-size distributions vary with location. Shallow-seated failures are the main failure mode in colluvial soil deposits under rainfall infiltration and the corresponding confining pressures are typically low. So far the compression behaviour, shearing behaviour and instability behaviour of saturated and unsaturated coarse granular soils under low confining pressures have not been fully studied. The main objectives of this thesis are to characterize the microstructures of coarse and widely graded soils to provide a physical explanation of the soil behaviour, investigate the effect of coarse content on the soil-water characteristic curve (SWCC), compressibility and shearing behaviour of saturated and unsaturated co...[ Read more ]

Colluvial soils on natural terrains are usually coarse and their grain-size distributions vary with location. Shallow-seated failures are the main failure mode in colluvial soil deposits under rainfall infiltration and the corresponding confining pressures are typically low. So far the compression behaviour, shearing behaviour and instability behaviour of saturated and unsaturated coarse granular soils under low confining pressures have not been fully studied. The main objectives of this thesis are to characterize the microstructures of coarse and widely graded soils to provide a physical explanation of the soil behaviour, investigate the effect of coarse content on the soil-water characteristic curve (SWCC), compressibility and shearing behaviour of saturated and unsaturated coarse granular soils under very low confining pressures, develop a physical model for the bimodal SWCC and a general shear strength model for bimodal coarse soils, and study the critical state characteristics and instability of saturated and unsaturated coarse granular soils.

The microstructure, compressibility, SWCC, shearing behaviour and critical state characteristics of coarse granular soils are investigated in an integrated manner. As the coarse content increases, the soil microstructures change from fines-controlled to coarse-controlled. A transition is observed at a critical coarse content of approximately 70%. The skeletons of the fines-controlled and coarse-controlled soils are formed by clay aggregates and coarse particles, respectively. The high stiffness of the skeletons of the two soils results in the low compressibility and dilative behaviour of these soils. Clay aggregates and coarse particles together form the skeleton of the transition soil, which exhibits high compressibility and strong contractive behaviour as the saturated clay aggregates tend to collapse into the large inter-aggregate pores and contract. With increasing coarse content, the pore structure of soil changes from a unimodal intra-aggregate pore structure to a dual-porosity structure having both intra-aggregate pores and inter-aggregate pores. Accordingly, the SWCC changes from unimodal to bimodal. The effects of the pore water in the two pore series on the critical state parameters during the desaturation process are different. The results are useful for the development of constitutive models for unsaturated soils, especially bimodal soils.

A physical model is developed for the bimodal SWCC. The model is closely related to the dual porosity structure of the soil. The whole drainage process on the bimodal SWCC is divided into four stages based on the role of the pore water. In stage a, all the pores are filled with water. The bulk water drains from the inter-aggregate pores in stage b, in which the free water forms a continuous phase. The drainage of the meniscus water in the inter-aggregate pores follows in stage c, in which the water in the inter-aggregate pores becomes disconnected while the fines aggregates stay saturated. Further, the water stored in the intra-aggregate pores drains in stage d, and the clay aggregates become unsaturated.

The compressibility and shear strength of the soil with a bimodal SWCC also exhibit bimodal features. The overall shear strength behaviour over the entire suction range shows two modes, which is not reported in the literature. A shear strength model is proposed to calculate the shear strength of granular soils with a bimodal SWCC. The proposed equation can be degenerated to predict the shear strength of unimodal sand and Kaolin clay.

An equivalent mean effective stress taking into account the suction stress is proposed to describe the instability of an unsaturated soil subject to wetting. An instability line is defined for the unsaturated coarse grained soil. A relationship between the normalized slope of the instability line and state parameter is established for rapid assessment of the instability of both saturated and unsaturated coarse granular soils. The line separates the soil states into stable and unstable states in the stress space. If the stress conditions of a soil element reach this line, instability is to be initiated.