ABSTRACT

A major factor to select a suitable place as foundation for high concrete arch dams is the existence of rock abutments with high degrees of strength and stiffness. However, discontinuities are found almost in any rock masses and thus dam abutments often consist of joints, bedding planes and faults. The existence of discontinuities in rock mass and the influence of some characteristics associated with the joints such as aperture, filling, roughness, normal and shear stiffness, dip and dip direction, spacing and geometric situation relative to dam body cause jointed rock mass behaviour to be complicated. Furthermore, hydraulic and mechanical interaction causes more complicated behaviour in rock mass. Accordingly, disregarding of joint effects may lead to an unsafe design. Malpaset dam failure in 1959 demonstrated that hydromechanical effects in abutments might lead to instability of abutment and the arch dam. Today, most of dam designers still use conventional methods paying no attention to hydromechanical process. In this paper, the authors present the response of a typical concrete arch dam abutment under hydraulic and mechanical interaction using UDEC software. In this study, the values of sliding and opening and the resulting water flow along discontinuities were utilized as criterions for estimation of dam abutment safety. The results show that these indexes are good stable or unstable condition indicators.We observe that the mechanical parameters influence arch concrete dams abutment hydraulic behaviour such as maximum flow rate. Moreover, the initial aperture of joints has the noticeable effects on abutment response.

1 INTRODUCTION

The joints have an important role in the discontinuum media response. The safety and failure mechanism are indicated by separation and sliding values and rotation along joints. The existence of large joint displacement inside abutments and foundation and hydromechanical interaction can be the main reasons of failure in arch dams. Accordingly, disregarding of joint effects problem with them such as hydromechanical interaction may lead to an unsafe design. Malpaset dam failure in 1959 demonstrated that hydromechanical effects in abutments may lead to instability of abutment and also the arch dam. In this paper the stability of a typical concrete arch dam abutment under hydraulic and mechanical interaction was studied using distinct element methods and finally, three failure indices, sliding, opening and the flow rate along discontinuities, used as criterions for the estimation of dam abutment safety.

2 DISTINCT ELEMENT METHOD

The essential feature of the distinct element method is its ability to model the arbitrary motion of each block with respect to any other. Blocks may be rigid or deformable. The integration of the motion law provides the new block position and therefore, the joint displacement increments (or velocities). Blocks are assumed to interact at discrete points referred to as "contacts". A force-displacement relation describing joint behaviour at contacts is then used to obtain forces that are applied to the blocks at the next timestep.

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