Abstract:Expansive soil is of potential for shrinking or swelling which can be caused by the change of water content. The shear strength of expansive soil is one of the key issues of soil mechanics and further studies are required for the need of analyses including evaluation of the stability of slopes,design of foundations and earth retaining structures. However,the strength behavior of expansive soil is rather complex especially compared to other types of soils. In this paper,the swelling pressure and strength of two typical expansive soils respectively collected from Heishan of Liaoning Province and Meishan of Nanjing are systemically investigated using the methods of conventional soil tests in order to understand its physical and mechanical properties and damage behavior. Meanwhile,the relationship between the swelling pressure and shear strength parameters such as effective cohesion and effective angle of internal friction and initial water content of unsaturated compacted expansive soil is established based on experimental test results. The swelling pressure and shear strength of expansive soils decrease sharply and the volume increases greatly with increasing water content. The ratio of the strength caused by matric suction to the swelling pressure is constant. Therefore a new formula of shear strength for unsaturated expansive soils is presented.