Search

This book focuses on the engineering properties of unsaturated widely graded soils and their influence on slope stability. This book characterizes the natural widely graded colluvial soils from macroscale, mesoscale, and microscale viewpoints, introduces the techniques for measuring hydro-mechanical properties for unsaturated widely graded colluvial soils, and clarifies the hydro-mechanical behavior and the failure mechanism of widely graded colluvial soils subjected to environmental loads. This book improves the understanding of rainfall-induced landslides on natural slopes. Researchers and engineers in the field of civil engineering can benefit from the book. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023.

The fabric, water retention curves (WRC) and shear strength of a fines-controlled soil, a coarse-controlled soil and a transitional soil were investigated through laboratory tests. The pore size distribution (PSD) and WRC of the fines-controlled soil show unimodal features dominated by intra-aggregate pores. During the desaturation process, the fines-controlled soil shrunk as water gradually drained from the intra-aggregate pores. The apparent peak and critical cohesions increased gradually with suction. For the coarse-controlled soil, its PSD and WRC exhibited bimodal features. Accordingly, the shear strength over the entire suction range also showed two distinct modes. The pore water drained from the inter-aggregate pores in the first mode while the intraaggregate pores remained saturated. The apparent critical cohesion reached a peak and then decreased with increasing suction. The apparent critical cohesion increased again as the clay aggregates became unsaturated in the second mode. However, the apparent peak cohesion kept increasing with suction. The transitional soil exhibited weak bimodal features in its PSD and WRC, and was strongly contractive when saturated as the clay aggregates tended to collapse into the large inter-aggregate pores.

Colluvial soils on natural terrains are usually coarse and their grain-size distributionsvary 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 fro