The tertiary soft rock (or tertiary hard soil) is half-diagenetic rock (soil) and its properties are different from hard rock and soil. A lot of engineering practices show that the stability of thick tertiary soft rock slopes is affected seriously by underground water and rainfall. A high open-pit slope which is made up of super-thick tertiary soft rock is taken as a case, firstly the variation laws of the physical and mechanical parameters of the tertiary soft rock in this slope with different water content had been analyzed through drill hole sampling and laboratory testing. Then a fluid-solid coupling model had been established based on the limit equilibrium theory, finite element method and saturated-unsaturated seepage theory, to study the effect of seepage field caused by underground water and rainfall on the stability of the slope. The experimental and numerical results show that the cohesion of the tertiary soft rock is rather sensitive to water content but the internal friction angle is insensitive; the slope will be unstable in the present level of underground water, and only the depth of waterline is dropped for 100m can the slope be safe; both transient saturated zone and pore water pressure increase with longer duration of rainfall, but the heaviest rainfall can only lead to the shallow slope sliding within the thickness of 20 m and local rainfall has less impact on the whole slope stability.
The stability of slopes greatly affects the economic benefits and production safety of open-pit mines in the process of strip mining, so studies on open-pit slopes have always been the hot subjects. There have many research achievements on the issues of hard rock slopes, soft rock slopes and soil slopes in the field of open-pit mines, hydroelectric dams, highway and railway slopes etc (Naghadehi et al. 2013, Dine et al. 2011, Huang 2007). However, the tertiary soft rock (or called the tertiary hard soil) is a kind of half-diagenetic soft rock (soil) which is in the status from soil to rock, and at present there aren't targeted rules and enough research results for determining the bearing capacity of this kind of rock by existing different standards (Zhang & Qu 2000a). A lot of engineering practices show that the tertiary soft rock (soil) is a kind of regional special soil which is highly vulnerable to environmental changes (including the natural environment and engineering environment) and has significant drying shrinkage and water absorption expansion characteristics (Zhang & Qu 2000b, Feda et al. 1995).
