Microstructure and shear strength of widely graded soils during desaturation

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 intra-aggregate 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.