Abstract: Compacted soil, which widely exists in many sorts of engineering projects, is commonly unsaturated soil; and its physico- mechanical characteristics are influenced by soil moisture. Because of the difficulty in matric suction measurement, it is hard to apply the unsaturated soil strength theory to compacted soil projects. This paper aims to research how much influence on shear strength of compacted soil when soil moisture varies, and tries to find an easy and useful way in determining the shear strength of unsaturated compacted soils. Research works start from the theory of unsaturated soil, and the mechanism, with which soil moisture affects shear strength of compacted soil, is analyzed. Triaxial shear test results of five sorts of unsaturated compacted soils are rearranged and analyzed; and strength parameters of direct shear test are collected and arranged from data of some filled and compacted soil projects. Analysis shows that matric suction contributes a considerable portion of shear strength in the compacted soils; and the influence of soil moisture on shear strength of compacted soil is obvious and can not be neglected. A fine linear relationship between logarithmic saturation and cohesion of compacted soil is found in project data; and a simple method for calculating shear strength of compacted soil, which can take saturation degree influence into account, is presented

Key words: compacted soils, shear strength, unsaturated soils, matric suction, water content