Bimodal Shear-Strength Behavior of Unsaturated Coarse-Grained Soils: Fid-Bau Portal

Bimodal Shear-Strength Behavior of Unsaturated Coarse-Grained Soils

Zhao, H. F. / Zhang, L. M. / Fredlund, D. G.

Abstract

The microstructure, compressibility, and shear strength of an unsaturated widely graded coarse soil were investigated through a series of laboratory tests. The results show that coarse soil has a dual-porosity structure that has both intra- and interaggregate pores. The soil-water characteristic curve (SWCC) shows bimodal features. Accordingly, the shear strength and compressibility over the entire suction range also show two distinct modes. The pore water drains from the interaggregate pores in the first mode while the intraaggregate pores remain saturated. The shear-strength behavior during this drainage period resembles that of uniform coarse sand. The apparent cohesion reaches a peak and then decreases with increasing suction, whereas the compressibility decreases first and then increases. The clay aggregates become unsaturated in the second mode as the pore water drains from the intraaggregate pores. The compressibility decreases and the apparent cohesion increases with suction as in a typical fine-grained soil. A shear-strength model is proposed for granular soils with a bimodal SWCC. The proposed equation can also be degenerated to predict the shear strength of a wide range of soils.