抗震液化的总应力合成分析方法

Total Stress Synthesized Analysis Method for Liquefaction under Seismic Loadings

基于总应力动力分析法,运用二维显式有限差分程序FLAC对某大坝在地震荷载作用下的动力响应进行模拟分析。编制了分析大坝液化的数值模型的分析模块并与FLAC接口。分别考虑了水平、竖向地震荷载以及两个方向的耦合和不同水位深度对大坝动力特性的影响,得到了大坝在地震荷载作用下液化区域和位移矢量的分布态势。

There are two concerns for assessing liquefaction response: (1) Will liquefaction be triggered in significant zones? And if so, (2) what displacement will occur? Liquefaction can cause a large drop of strength, and damaging displacement may result if the liquefaction zones are of sufficient extent. These displacements can be predicted by two approaches: (1) empirical equations based on past experiences; and (2) mechanics-based methods. A mechanics-based approach is presented for assessing post-liquefaction displacements of slopes. The approach is derived from total stress procedures in current use, with two major advantages; (1) the triggering and post-liquefaction responses have been synthesized into one analysis, and (2) the modeling of the post-liquefaction clement behavior is greatly improved. Mechanics-based methods estimate the field response by approximating soil behavior using numerical models. The models capture the fundamental physics of dynamic soil response, although their success is limited by inherent simplifications. The accepted state of practice is often a three-phase total stress procedure :( Ⅰ )Triggering evaluation: Zones of liquefaction are predicted by comparing estimates of applied loading or cyclic shear stress to the anticipated resistance to liquefaction. The resistance to triggering is typically based on in-situ tests, such as the standard penetration test. (Ⅱ ) Flow slide evaluation; If significant zones of liquefaction are predicted, the potential for a flow slide is evaluated using limit equilibrium techniques. The susceptibil ity to large deformations is found by assigning residual or post-liquefaction strengths to the liquefied zones and appropriate undrained strengths to the material. If the safety factor is less than or near 1, the structure is not stable. ( Ⅲ) Displacement evaluation: large but limited displacement may occur even if the structure has sufficient strength to maintain overall stability. These limited displacements accumulate during an earthquake and are a direct result of the continuing inertia loading. Techniques for estimating th ese displacements are often based on Newmark method and model the displacing soil as a rigid block translating on a plane. The triggering of liquefaction is assessed by continuously evaluating predicted shear stress. When sufficient stress cycles have accumulated to trigger liquefaction at a location, then the element properties at that location are modified to reflect liquefaction behavior. Additional element liquefies as the shaking continues and the displacements increase with the duration of shaking. The dynamic responses on a dam under seismic loadings are simulated and analyzed by using the code FLAC which is two-dimensional explicit program according to this method. The code of the analysis model which is used to analyse liquefaction of a dam is programmed and interfaced with FLAC code. The dynamic characteristic of the dam caused by vertical, horizontal and two-dimension coupled seismic loadings and different depths of water are considered. The distributing states of displacement vector and liquefaction zones of the dam are gained.