Abstract: In order to find a thorough stress-strain characteristic in clay, a series of consolidation undrained plane strain compression experiments were made. Local lateral non-homogeneous deformation in the samples were recorded. Through analysis for the experimental results, it is found that characteristic values of localized deformation began to form are very close to suddenly increased turning point of the pore water pressure. But the suddenly increased turning point of the pore water pressure is its yield point in clay soil. It is shown that the deviator stress of shear band began to form is very close to its yield point in clay soil. Exceeding this point, the localized deformation speeds up. The discovery will conduce to a thorough study on strength theory, deformation and stability analysis in soil.

Key words: clay, localization of deformation, deviator stress, pore water pressure, yield point