饱和砂土剪切吸水流滑的物理规律和数学描述

Flowslides in dilative sand slopes or foundations with static driving stress were found in past earthquakes and model tests. It has confirmed that strain hardening or cyclic mobility happened in saturated dilative sands during an application of monotonic or cyclic shearing in undrained conditions, so flowslides cannot develop in dilative sands in undrained conditions. One of the main reasons why reduction or loss of the shear strength happened, which lead to the failure of dilative sand slopes or foundations, is attributed to redistribution of water content or void redistribution during the shear process. Existing methods of evaluation based on the undrained behavior cannot always provide a realistic prediction to post-liquefaction deformation and strength behavior of saturated sands in practical natural drained conditions. Although this point of view was widely accepted, until now no important advances was made in the area of void redistribution in practical boundary value problems and stress-strain behavior of saturated sand in the condition of water absorption..The presented research project arises from practical needs to seek more rational theory and methodology for evaluating the stability and large liquefaction-induced deformation of slopes of foundations composed of saturated dilative sand strata. Main contents of the study are: 1) a series of cyclic triaxial tests on saturated sands in the condition of different water absorption rates are designed to reproduce the stress-strain behavior of saturated sand depending on water absorption; 2) to reveal the mechanism regarding the different deformation behavior of saturated sand depending on water absorption; 3) to develop a cyclic constitutive model for prediction of deformation behavior of saturated under different water absorption conditions 4) to implement the present model into a fully coupled finite element code to develop a numerical method for evaluating the practical problems.

震害调查和室内模型试验表明：即使是剪胀性砂土构成的边坡或地基（具有驱动剪应力）也可能产生流滑破坏，这种破坏与超静孔隙水压力的扩散和转移导致的局部砂土在剪切过程中吸水变松有关。现有的基于不排水条件的边坡稳定性评价方法不能解释中密到密实的剪胀性砂土边坡出现流滑破坏的现象。该问题虽然已得到岩土工程领域的广泛共识，成为土工抗震领域研究的新热点，但是目前还未取得重要突破。.本项目拟通过控制剪切吸水率的三轴试验来再现饱和砂土在剪切吸水条件下的力学响应，探究饱和砂土在剪切吸水条件下的应力应变特征和强度变化的规律性，揭示剪胀性饱和砂土在自然排水条件下流滑的物理机制，建立能够描述饱和砂土剪切吸水条件下应力应变行为的本构模型，提出一套能够用于自然排水条件下流滑相关边值问题的稳定性评价方法。

在地震作用下，由中密至密实的饱和砂土构成的成层饱和砂土倾斜地基或边坡会发生一种特殊的延迟滑动变形破坏现象，即破坏不是发生在惯性力作用的地震过程中，而是发生在地震结束后的某一时段。现有的基于不排水条件的边坡稳定性评价方法不能解释中密到密实的剪胀性砂土边坡出现流滑破坏的现象，更不能模拟边坡破坏的延迟时间效应。. 针对上述问题，本项目主要在以下方面取得了进展：（1）设计开发了一套控制剪切吸水速率的三轴试验系统和大剪切变形作用下土的渗透性测试系统，以模拟边坡中土单元在地震后超静孔隙水压力扩散转移过程中的典型应力路径；（2）系统进行了标准砂在流滑应力路径下的剪切试验，观察得到了土单元在流滑过程中的吸水率与剪应变的相互影响规律；（3）从边值问题中孔隙水压力扩散转移过程以及土单元的应力应变响应的角度，揭示了剪胀性砂土边坡地震后延迟变形的内在机理；（4）在揭示的机理的基础上，基于状态剪胀理论，建立了剪切吸水条件下饱和砂土的本构模型和数值算法；（5）给出了由吸水体变预测流滑剪应变的实用图表，建立了预测饱和砂土边坡地震后流滑变形的简化计算方法和复杂边界条件下的有限元计算方法；（6）利用发展的计算方法分析了土的物理性质、具体边值条件对边坡流滑变形的影响规律。本项目的研究成果可用于剪胀性砂土边坡的地震安全性评价和加固设计。. 另外，本项研制的设备还可用于复杂应力路径下的土的变形特性测试，在本项目中相关设备和理论也应用于击实膨胀土的胀缩变形以及路基在交通荷载下的变形计算，取得了一些研究成果。