A microstructural perspective on soil collapse

Soil collapse is a phenomenon triggered by wetting a loaded soil, the structure of which contains large pores. This type of soil response has been studied since the 1970s and models that can predict its occurrence and magnitude were proposed from the 1990s. In particular, the concept of loading collapse (LC) curves has been developed in the framework of unsaturated soil mechanics and it has been validated using low-reactivity soils. Several publications have highlighted that the microstructure of expansive soils evolves significantly during swelling, a phenomenon that may affect the location of the LC curve. With that perspective, some studies mention a need to shift the LC curves for experimental data and model predictions to agree. This paper brings new insight into the significance of microstructure for the collapse of a reactive soil. A series of tests was conducted to characterise the microstructure of soil specimens compacted under different initial conditions. Then, three pairs of wetting tests under constant load and under controlled suction were conducted in order to identify the zone of onset of collapse. It was found that the conventional LC curves cannot adequately predict the occurrence of collapse. In contrast, analysing the swelling response in the light of microstructural characterisation led to the conclusion that, for Maryland clay, occurrence of collapse can be tracked by the evolution of the void ratio associated to the macro porosity during wetting.