Seismic Stability Of Rock Masses Available to Purchase

Pour les calculs de la charge sismique agissant sur les massifs rocheux on propose d'utiliser la methode dynamique spectrale. On utilise le modèle "corps elastique sur la fondation elastique" dont l'execution s'effectue à l'aide de la methode des elements finis. L'analyse de l'influence des divers facteurs sur la valeur de la charge sismique, est presentee. La mise en pratique de la methode proposee pour l'estimation de la resistance aux seismes du talus de la centrale hydroelectrique de pompage Dniestrovskaya, est decrite.

Zur Berechnung der seismischen Belastung des Felsmassivs wird die Ausnutzung der dynamischen spektralen Methodik vorgeschlagen. Es wird das Modell "elastischer Körper auf dem nachgiebigen Untergrund" benutzt, das mit der FEM realisiert wird. Es wird der Einfluß verschiedener Faktoren auf die Größe der seismischen Belastung analysiert. Die Anwendung der empfohlenen Methodik zur Einschatzung der Erdbebensicherheit der Böschung des PSW Dnestrowskaja wird beschrieben.

While analyzing the stability, the rock masses as a rule are assumed to be rigid disks interacting with the stiff space through friction and cohesion. Therefore the static theory of seismic resistance is applied to determine seismic loading on them. In accordance with the theory: 1) horizontal seismic force is assumed to be equal to the product of the rock mass weight and the seismic coefficient for this region; 2) vertical component of the seismic force is considered to be small and is neglected in the stability analysis. The paper views the estimation of seismic loading using the dynamic spectral method that is widely applied for calculations of seismic stability of industrial, civil and hydrotechnical structures. This method takes into account dynamic properties of the rock mass, i.e. frequencies and natural modes. The method of finite elements is used as well. The result obtained in the course of studies are in a good agreement with the results of H.Ito and T.Watanabe (1985).

The basic provisions for the design model are as follows: 1) the rock mass is thought to be separated by more or less distinctly defined sliding plane from the surrounding medium considered as the rock mass foundation; 2) the rock mass is thought to be elastic located on the elastic foundation; 3) the rock mass is believed to have the weight, but the foundation is thought to be inertialess (Fig. 1).

Two-dimensional problem is being solved. The rock mass together with the part of foundation are approximated by the finite element mesh (Fig. 2).

The rock slope is an infinite half-plane. Therefore the application of the method of finite elements requires some restriction on the dimension of the area to be analysed. Three areas are considered to analyse the influence of the foundation width of the study area on the seismic loading (Fig. 3). "The usual area" is characterized by the foundation dimension, generally accepted in the statistic analyses using FEM. Table 1 shows average values of Biηik for three study areas in the rock mass. The area of "usual" dimensions was considered under three boundary conditions to analyse the influence of boundary conditions along the outlines of the foundation on the seismic loading value (Fig. 4) (Table 2). The analysis of the influence of the relationship between the modulus of deformation of the rock mass proper and the foundation was made. The dimensions of the analysed area are "usual", the boundary condition - overall embedment around the whole outline. The rock slopes with the ratio of modulus of deformation E found/E rock mass = 1.0;1.25; 2.5; 5.0; 10.0; 20.0 were considered. Basing on the results of calculations the diagram of (Biηik) average in the horizontal and vertical directions against the foundation stiffness is plotted (Fig. 7). Figures 5, 6, 7 show: 1. Seismic loads along the rock mass height are extremely nonuniform. Maximum accelerations occur near the surface approximately in the middle part of the slope. 2. In the slope more or less uniform heightwise horizontal seismic forces calculated by dynamic spectral methods exceed that determined by the static theory of seismic stability almost by 20%. 3. Vertical seismic forces are rather high and are comparable with the horizontal ones.

The slope at the Dnestrovskaya pump storage station is composed of disintegrated argillite, marls, limestones alternating with clay interlayers.