Effect of Seismic Uplift Pressure on the Behavior of Concrete Gravity Dams with a Penetrated Crack

Proper consideration of the uplift pressure at the base of a concrete gravity dam is of great importance in practical engineering, since it is crucial to the safety of the dam, specifically for a cracked dam under seismic conditions. However, constant uplift pressure, which is suitable for the static case only, was adopted in almost all the seismic analyses of cracked concrete gravity dams. To adequately estimate the seismic behavior of cracked concrete gravity dams, a seismic uplift pressure model is proposed for a penetrated crack. In this model, the amount and the distribution of the uplift pressure along the assumed rigid crack walls are determined by the earthquake acceleration, the water heads, the aperture of the crack, and the opening/closing velocity. Application of the model to a typical concrete gravity dam with a penetrated crack at the base reveals that the seismic behavior of the dam is markedly affected by the seismic uplift pressure. In general, the residual downstream sliding is considerably enlarged compared to that of constant uplift pressure. Computations show that the seismic uplift pressure can be several times higher than the constant one, increasing the dynamic instability of the cracked dam. It is also revealed that the dynamic water flow plays the role of a wedge while the upper mouth of the crack is closing. When the dam rocks back to upstream, the uplift pressure increases until it is so high that the pivot at the toe is raised up and the whole dam loses its contact. Then the resultant uplift pressure remains constant until the dam is inclined to the upstream. During this period of time, the cracked dam is normally drifting towards the downstream due to the hydro pressure.