Abstract:It is very difficult to resolve or measure matric suction in engineering practice,because the matric suction is not constant especially under undrained condition. The degree of saturation is relatively easy to be measured,so the shear strength formula about saturation degree instead of suction has valuable significance in practical engineering. A new formula based on strength indexes of saturated soil is proposed,which can be used in whole engineering practice range including unsaturated soil and saturated soil. More intrinsical characteristics of engineering can be reflected. Proper parameters for Yunnan red clay in the above-mentioned formula are determined based on the results of soil-water characteristics and constant water content triaxial tests. Finally,the undrained shear strength formula is utilized to analyze the bearing capacity of foundation;and the conclusion that the bearing capacity increases with the reduction of degree of saturation linearly is drawn.