Anisotropy and Volume-contraction of Soil due to Axial Unloading in CD Test

When axial load increases and then decreases in CD test, volume of some Kinds of soil is notexpanding but contracting.This unusual phenomenon,caused by the soil anisotropy. S0il anisotropyoriginates from both depeitional and induced anisotropy, and the latter can only come into being underanisotropic consolidation stress. The induced anisotropy of the soil sample is more obvious when theprincipal stress ratio（σ₁/σ₃）is higher. Experiments showed there exists soil volume-contraction due toaxial unloading in CD test as σ₁/σ₃ exceeds about 2.6.