Abstract: Because of complexity about property of unsaturated soil, getting correlative strength parameters needed high and extract equipment, the difficulty is big so that its application is limited. Based on an analysis of strength theory of unsaturated soil, CD test of clay about different combination of water content and dry density is carried out with normal three-axis test. According to the theory of effective strength of soil, the effective strength of clay about water content and dry density is discussed, thus functions of total strength indexes are established that the effect of water content(saturation) and dry density can be considered. Study shows that strength of clay has close connection with water content and dry density. Moreover, the relation between cohesion, internal friction angle and water content(saturation) is conic, the relation between cohesion and dry density is linear, and the mutual effect of the both can be expressed by multiplication. Our research can be used as a reference application of project.

Key words: lay, effective stress, strength indexes, water content, dry density