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.