Abstract：Research on softening regularities of mechanical properties of saturated soft rocks and their application is one of the important theoretical and practical problems in geotechnical fields. The mechanical regularities of some typical kinds of soft rocks such as red siltstone，red mudstone，and black carbonaceous mudstone，which are widely distributed in South China，are discussed by designing a series of experiments of interaction between soft rocks and water，and their mechanical parameters are correspondingly tested. By sampling according to a time sequences including natural state and saturated states in different time (1 month，3 monthes，6 monthes，and 12 monthes) of soft rocks，the uniaxial compressive strength，cleave tensile strength，shearing strength，and their varying regularities along with time developing are analyzed. The results show that the mechanical properties mentioned above probably conform to an exponent curve after different saturated time on which the point in 6 month is the critical time for mechanical properties changing from gradually declining to stable state. On the basis of experimental data，by selecting a main section，the BIII2 slope stability in Water Supply Reconstruction Project from Dongjiang to Shenzhen，Guangdong China，is analyzed by nonlinear strength reduction finite element method (FEM). The calculated safety factor is 0.83，which coincides very well with the actual state of the slope. It means that the test results have an important significance for application to engineering and can provide a scientific reference for designing and construction as well as long term safety monitoring and analysis for other similar engineering problems in South China.
