Abstract: With the soft clay samples of the 4th soil layer in Shanghai prepared by isotropic compression and K0 compression respectively, some mercury intrusion porosimetry (MIP) tests were carried out to investigate the microstructural characteristics of the volumetric deformation of clay under different compression stresses. By analyzing and comparing the pore size distribution curves (PSDC) obtained from MIP tests, some conclusions are drawn as follows. The pores in tested clay can be divided into macropore, mesopore, fine pore and micropore; where the fine pores account most on the volumetric deformation of clay; under the isotropic compression state, the soil particles move and get closer to each other with the development of compression, which make the main peak according to fine pores moves towards left (become smaller) with the pore shape unchanged in PSDC. While under the K0 compression state, the soil particles rotate and the pores become flat, which presents that the peak value of fine pores in PSDC drops down evidently.

Key words: volumetric deformation, compression stresses, pore size distribution, microstructure