Abstract:A series of experiments were performed in rock mechanics rigidity servo-testing system (MTS815.02) and rock odometer to determine the swelling behavior of red sandstone taken from a foundation pit of Hongshanyao project (HSYP) in Nanjing. The experimental data show that the curve of swelling strain-water content is logarithmic. Because of variation of hydroscopicity ratio,the mechanical properties of swelling rock,such as the elastic module,Poisson’s ratio,and yield limit and etc. will change. As a result of hydroscopicity ratio effect,the swelling stress,plastic flow,and yielding criteria which varies with the hydroscopicity ratio,are coupled together. Based on the experimental results,the elasto-plastic constitutive model with swelling and strain softening for swelling rock is proposed .