NONLINEAR ANALYSIS OF PINE FLAT DAM INCLUDING BASE SLIDING AND SEPARATION M.N. Viladkar Indian Institute of Technology Roorkee, Roorkee - 247 667, India A.K.M.S. Al-Assady Formerly Research Scholar, Indian Institute of Technology Roorkee, Roorkee- 247 667, India SUMMARY Nonlinear earthquake response of the tallest, non-overflow monolith of Pine Flat Dam to Taft ground motion, scaled to 0.5g has been presented in this study so as to investigate the possibility of sliding and separation at the base of dam and to study its effect on the dynamic response of dam-foundation system. Proposed finite element based dynamic model has the ability to simulate radiation damping at the far field using a transient transmitting boundary. The interaction with both foundation and the reservoir has been considered. It has been found that both sliding and separation modes of the interface dominate the deformations as well as the principal stresses in the dam body. Moreover, sliding and rocking displacements have been found to be quite considerable, especially at the heel. Key words: Transient transmitting boundary, Dynamic soil- structure interaction, Pine flat dam, Non- linear Analysis, Sliding and separation. 1. INTRODUCTION Potential failure mode of a concrete gravity dam during an earthquake is extensive cracking and deformation in the region between the base of the dam and the foundation rock (base sliding displacement). Development of mathematical models for predicting the seismic response of concrete gravity dams, including interface zone deformation, is a complex problem due to interaction between the dam and compressible water and between the dam and the foundation rock (Chavez and Fenves 1995a, b). In the present study, nonlinear dynamic model, formulated directly in time domain (Al-Assady 2005), has been used to investigate the possibility of sliding and separation at the base of dam and to study its effect on the interactive dynamic response of Pine Flat Dam-foundation system. The model is based on the use of transient transmitting boundary to simulate a non-reflecting boundary at the far field (Al-Assady 2005). Nonlinearity of both concrete dam and the foundation material has been represented through proper constitutive modeling. Sliding and separation along the dam-foundation interface has also been considered through an interface element proposed by Al-Assady (2005). 2. EARLIER WORK El-Aidi (1988) and Hall et al. (1991) studied the nonlinear response of Pine Flat Dam including the opening of crack and its propagation along dam-foundation interface using smeared crack approach. The foundation was represented by frequency-independent impedance coefficients. Study indicated that although uplift over part of the base can occur, rocking of dam about the base is not that important and there is a permanent slip at the base due to sliding of dam. Leger and Katsouli (1989) studied the stability of concrete gravity dam in which sliding at the interface was represented by gap-friction elements. The results suggested that nonlinear deformation of dam-foundation rock interface reduces the seismic response of the dam. Chopra and Zhang (1991) studied the sliding response of a gravity dam founded on