A Study on the Shear Strength Characteristic of Unsaturated Red Clay

The red clay in Chenzhou, Hunan province is mostly in unsaturated state.
Simply applying the mechanical properties that derived from classic saturated
soil mechanics often leads to slope failures in this region. In order to study
the shear strength characteristic of unsaturated red clay in Chenzhou and to
explore a shear strength equation that can be easily applied in engineering
practice, a series of triaxial tests of saturated and unsaturated red clay samples
were performed using the regular triaxial testing apparatus. The testing results show that the peak strength of red clay drops slightly before the moisture content of 30% but decreases sharply after that. The friction angle of red
clay under unsaturated state is basically equal to the effective friction angle
under saturated state, while the cohesion of unsaturated red clay is far much
bigger than that of saturated one, which indicates that the matric suction
makes a great contribution to the cohesion. By fitting the testing results with
appropriate curves, the relationships between total strength parameters total c
and ϕtotal with moisture content were obtained. The total total c increases
logarithmically before the moisture content of 35% then decreases linearly,
while ϕtotal decreases cubically with increasing moisture content.