Microstructural evolution of loess soils from the Loess Plateau of China

A series of laboratory tests were conducted on intact specimens of two loess soils to characterize their collapsibility, shear strength, microstructure and mineralogy. In addition, microstructural observations were performed on the specimens after various mechanical tests (odometer-collapse tests and triaxial tests). The micrographs were processed using MATLAB program and the morphology properties of soil pores (including area, major axis length, eccentricity and orientation) were obtained. The micrographs and variations in distributions of the pore morphology properties were used to interpret the microstructural evolution of loess soils due to loading and wetting. Results of the study highlight that collapsible loess soils have an open structure, where clay coated silts and clay-silt aggregates functioning as fundamental units are connected to each other with a few cementations. Upon loading and wetting, disintegration of clay aggregates (cementations), breakdown of carbonate cementations and other bonding agents initiate the failure of soil structure. This is followed by particle movement and rearrangement, transforming the initial open structure into a closer one. The microstructural evolution is dependent on the stress level and stress path. With the increase in stress level, large-sized inter aggregate pores transform into small-sized infra-aggregate pores; the pores are flatter and still randomly orientated. Fissures with the size and connectivity related to the stress level may develop in the specimen under a triaxial stress state, depending on the stress path.