Many instability events of bank slopes due to the construction of reservoir have occurred. The material strength deduction and the hydraulic effect are two important provocative factors. The slope deformation and instability manner are linked with the geometry of the potentially unstable rock-mass, the mechanics of materials and the water level fluctuation. The slope stability evolution is controlled by the geometry of the unstable rock-mass, the hydrogeological condition and the shear resistance of the sliding surface.
The deformation and the slope failure provoked by the water reservoir is a common problem in the hydraulic project. But there was not enough attention until the Vaiont event in 1963 caused 2600 of death. In 1961, still in the construction of Zhaxi dam (Hunan, China), the reservoir water fluctuation induced a rockslide that is very near the dam. This sliding caused 60 of death. During the filling and first fluctuation of the Gepatsch reservoir (Austria) between 1965 and 1969, more than ten meters of displacement was measured in the slope just before the dam. In 1992, the reservoir filling of Geheyan dam (Hubei, China) resulted in some important deformation and several old landslides. Most of this deformation became stable after 2–3 years, but there is one landslide (Maoping) whose deformation increases till today. In the reservoir region of the Three-Gorges dam, about 1300 old landslides or unstable slopes have been found. Most of the landslides will be partly submerged. The stability evolution of these landslides will be a great study topic. The deduction of material strength and the hydraulic effect are thought two main provocation factors of slope failure (Fujita, 1977; Zaruba and Mencl, 1982: Nakamura, 1985: Y.J.Cai, 2000). R.Cojean et al (1990) made a quantitative study of stability evolution relative to the slope structure. But our knowledge is still not enough to predict the future's situation of a slope in the reservoir. We don't know exactly how evolve the material strength following the submersion, how evolve the actual slope stability following the water level fluctuation.
Three states of saturation exist for the geological material: dry, non-saturated and saturated. The strength of rock changes following the water content. For the clay rock, the deduction of compressive strength reaches 50–60% but only 10–20% for the sandstone. The deduction of cohesion is more important than the interne friction angle. The first one reaches often to 20–30%, the later only 10–20% (Fig. 1). For the fractured rockmass, the stress concentration and the fissure extension are another important effect that promotes the mobilization of a new failure. We construct a slope with 70m of height and take a modelling of the stress evolution with the following parameters: compressive module 10GPa, shear module 9GPa, specific weight 25kN/m3, cohesion 2MPa and friction angle 45°. Four groups of fissure with 0.5m thickness and 10m in length are set at different positions. Each group is composed of 3–4 fissures who are near but don't intersect each other.