Computation of irreversible seismic displacements of rock wedges: an application to dam abutment safety assessment

The paper discusses a method to assess the seismic performance of rock wedges in contact with arch-dam abutments based on the Newmark displacement approach, conveniently modified to include all possible three-dimensional failure mechanisms and forces transmitted by the structure. Under rigid-block and no-rotations assumptions, the method allows to consider the three components of the seismic input, the progressive detachment of the block from the supporting discontinuity planes, the possible recover of the contact, and the temporary complete detachment of the block from the rock mass. In addition to gravitational and seismic inertial actions, static and dynamic forces exerted at the dam-wedge contact, including those related to the inertial interaction with the reservoir, are included in the analyses. A case study is presented, referred to the seismic verification of the Ridracoli arch-gravity dam (Italy), founded on a marly-arenaceous formation. In particular, the right abutment is thoroughly investigated against wedge sliding involving bedding planes of particularly poor resistance. The results in terms of maximum wedge displacements are compared with the corresponding factor of safety obtained with a limit equilibrium approach, indicating that even a wedge resulting unstable with limit equilibrium analyses shows relatively small displacements at the end of the seismic excitement.