Dense and mudium dense sands dilate under drained conditions and develop negative excess pore water pressure when sheared under undrained conditions. There exists a phase transformation line for dense and medium dense sand which characters the state from contractive to dilative. In the proposed model

the state parameter which makes the phase transformation line as reference line is introduced into the stress-dilatancy equation and the plastic hardening modulus expression to develop a new constitutive model for dilatant sand

9 model parameters which are convenient to be calibrated are involved. The numerical simulation is in good agreement with the data of triaxial compression tests.