土石坝-覆盖层-基岩体系动力相互作用研究

Study on the dynamic interaction of earth dam-overburden-bedrock system

在我国西部强震区拟建的土石坝多坐落于覆盖层上,覆盖层土体具有动力非线性和饱和特性,土石坝-覆盖层-基岩体系动力相互作用复杂。为研究土石坝-覆盖层-基岩体系动力相互作用特性,分别采用地震动一致输入方法(联合地震惯性力和约束边界)和非线性波动输入方法(联合自由场非线性动力响应和非线性人工边界),对覆盖层上土石坝进行有限元动力反应分析,考虑了覆盖层厚度、土体动力特性、地震强度及地震波频谱特性的影响效应,着重讨论了大坝的加速度反应峰值。结果表明:在覆盖层土体饱和特性的影响下,一致输入方法不能合理模拟大坝体系动力的相互作用,所获得的坝体竖向加速度反应误差显著。非线性波动输入方法理论上更为严密,其所获得的坝顶竖向加速度峰值较一致输入方法降低明显,降低率为20%～62%,且当输入地震动以高频成分为主时一致输入方法的误差较大。对于坝顶水平向加速度峰值,非线性波动输入方法较一致输入方法的降低率为5%～32%。当土体材料阻尼效应明显时一致输入方法的误差较小,当输入地震动以低频成分为主时其误差较大。一致输入方法会明显低估覆盖层上土石坝的极限抗震能力。当土石坝可能遭受竖向分量较大的近场地震时,有必要采用非线性波动输入方法评价大坝抗震性能。

Many earth dams are planned to be built on overburden in the intensive earthquake area of West China.The earth dam-overburden-bedrock dynamic interaction is extremely complex because of the behavior of overburden soils such as dynamic nonlinearity and saturated characteristics.To study on the dynamic interaction behavior of dam-overburden-bedrock system,two different seismic input methods,the uniform excitation method (combined seismic inertia force and constrained boundary)and the nonlinear wave input method (combined nonlinear dynamic response of free field and nonlinear artificial boundary),were employed to conduct dynamic response analyses of earth dams built on overburden.The effects of series parameters,such as thickness and dynamic properties of overburden,strength and spectral behavior of seismic wave,were detailed studied.The research indicates that there is an outstanding error in vertical acceleration with uniform excitation method induced by the saturated behavior of overburden soils.The nonlinear wave input method is based on a more precise theoretical framework.The maximum vertical acceleration at dam crest obtained with this method is 20%-62%lower than that with uniform excitation method,and there is a larger error when the primary component of the input motion is high frequency.The maximum horizontal acceleration at dam crest with nonlinear wave input method is 5%-32%lower than that with uniform excitation method,and there is a smaller error when the effect of soil damping is obvious,there is a larger error when the primary component of the input motion has low frequency.The limit aseismic capacity of dams will be underestimated greatly with uniform excitation method.It is necessary to employed nonlinear wave input method to conduct seismic performance evaluation of earth dam built on overburden located at near-fault seismic zone where the vertical earthquake is prominent.