Seismic water-crack interaction in gravity dams: Experimental study and numerical simulations.

摘要

Oscillating crack wall opening and closing modes, with the crack mouth in contact with pressurized reservoir water, are going to modify the prevailing pre-seismic uplift pressure magnitude and spatial distributions in cracked concrete dams. Uplift pressures acting along cracks and joints creates external forces that favour crack propagation in the dam body and affects the dynamic stability of cracked concrete components. Seismic uplift pressure variations during earthquakes remains a major source of uncertainty in design and safety assessment of concrete dams. Review of dam safety guidelines indicates that assumptions regarding uplift pressure intensity in a crack during an earthquake may vary from full reservoir pressure to zero pressure. The state-of-the-practice illustrates the lack of knowledge in defining transient seismic uplift pressures in existing or new cracks in concrete dams. The objective of this study is thus to develop a rational seismic crack-water hydro-mechanical model to predict dynamic uplift pressure variations during earthquakes.; An experimental study to characterize the transient seismic uplift pressure response of small concrete specimens with either newly induced or existing 0.4 m long seismic cracks was first performed. Cyclic crack wall frequency motions varying from 2 to 10 Hz were applied, with different magnitudes of crack mouth water pressures. The existing crack test results show that water pressure decreases in crack opening mode and increases in closing mode. For the first time the cavitation phenomenon is observed along the cracks in opening mode with 10 Hz excitation frequency.; A dynamic water-crack interaction model is then formulated to reproduce the experimental results, and extrapolate the computed uplift pressure variations to cracks of arbitrary lengths likely to develop in actual dams. The transient pressure gradients in a crack of specified length is modelled as a function of the crack mouth opening displacement (CMOD(t)) and crack mouth pressure (Pcrm(t)) time histories assuming: (i) 1D flow along the crack, (ii) continuity condition with an incompressible fluid of a constant viscosity, (iii) pressure-flow relations governed by the crack hydraulic conductivity using variations of the so called "cubic law" accounting for the crack roughness, laminar or turbulent flow conditions according to Reynold's number, and cavitation, (iv) impervious crack walls, and (v) residual crack aperture during cyclic motions (zero or larger). (Abstract shortened by UMI.)