循环渗流-应力耦合作用下土体渗流特性及力学响应机理试验研究

A great number of embankment dams have been built in China. Most of them are under very complex stress conditions and subjected to frequent changes of water level. Such changes not only induce soil internal erosion but also cause failures and accidents of embankment dams. However, the deformation and mechanical responses of soils subjected to internal erosion under coupled effects of cyclic hydraulic gradients and stress conditions are still unknown and urgently require further study. In this research project, extensive internal erosion tests will be conducted by a modified triaxial system to investigate changes of soil permeability, deformation and strength induced by internal erosion under cyclic hydraulic gradients with various stress states. More importantly, evolution of the microstructure of the post-eroded soil will also be measured to provide more insight in the mechanism about the hydro-mechanical responses of the eroded soil. Several series of seepage tests considering different complex stresses, frequency and magnitude of cyclic hydraulic gradients are carried out to find relations of soil permeability, critical hydraulic gradient, void ratio, volumetric deformation with particle loss, as well as to study stress-strain relations, peak and critical strength, critical state parameters of the eroded soils. After that, the pore size distribution, particle rearrangement and fabric structure of the eroded soil are also examined. After the comprehensive experimental investigation, an empirical relation between soil permeability, critical hydraulic gradient, void ratio and the applied stress ratio, magnitude and frequency of the cyclic hydraulic gradient will be proposed. The outcome of this research will advance our understanding of the fundamental behavior of internal erosion in soils and provide scientific evidence for seepage control and safety assessment of reservoir dams, dikes and slope engineering.

我国已建土石坝数量庞大,服役期应力环境复杂,水位升降频繁,由于渗透破坏导致的溃坝事件时有发生。但循环渗流-应力耦合作用下土体渗透变形特性及其力学响应机理尚不明确，迫切需要深入研究。本项目将通过大量的渗流-应力耦合三轴试验及土体微观结构测试分析，研究复杂应力环境中，循环水力梯度作用下土体的渗透特性、变形特性、强度特性，提出土体渗透系数、临界坡降、孔隙率、体积形变及渗透变形土体应力-应变、峰值及临界强度、临界状态参数等随循环水力梯度渗流-应力耦合环境的变化规律，以及土体颗粒运移、孔隙演化等土体微观结构随循环渗流场变化的空间演化规律，揭示循环渗流-应力耦合作用下土体渗透变形特性及其力学响应机理，构建土体渗透系数、临界坡降、孔隙率等与应力比、循环水力梯度、频次的经验公式。研究成果将丰富岩土渗流理论，为水库大坝、江河堤防、边坡工程等防渗控制与安全评估提供科学依据。

土体渗透侵蚀是引起边坡、土质堤防、土石坝等破坏的主要原因。受全球气候变化影响，极端降雨、风暴潮、山洪等灾害频发，导致水库和江河水位的快速变化，引起土质堤坝孔隙水压力的快速改变。相较于稳定水力载荷而言，这种复杂循环水力环境会对土体的渗流特性及其力学响应行为产生深刻影响。但截至目前，国内外针对循环渗流-应力耦合作用下土体渗流特性及其力学响应机理的研究几乎还未开展。本项目应用自主研发的渗流-应力多场耦合三轴试验系统，通过对粗粒土开展大量循环水力载荷-应力耦合试验，探究了不同循环幅度（Δi）及平均循环水力梯度（iavg）对土体渗透侵蚀发生-发展全过程的影响，以及土体应力-应变、强度、临界状态指标等力学行为的响应机制及参数变化规律；同时，采用离散元软件PFC5.0对3种典型内部结构土体进行微观结构数值模拟，结合其宏观渗流特性，揭示了不同土体微观结构对单调/循环渗流的影响机制。该项目揭示了复杂应力环境中，循环水力梯度作用下土体的渗透特性、变形特性、强度特性，丰富了岩土渗流理论，为江河堤坝与岩土工程防渗控制与安全评价提供了重要科学依据。