减饱和松砂静态液化的水-气两相流耦合分析

Coupling analysis of water-air two-phase flow in static liquefaction of desaturated loose sand

减饱和法是一种通过减小饱和砂土地基中的饱和度,提高地基抗液化强度的新方法。基于水-气两相流反应与土体骨架变形的耦合模拟方法,建立了单调加载条件下减饱和砂土静态液化的数值分析模型。开展了减饱和松砂的三轴不排水试验数值模拟研究,与室内试验结果对比,发现两相流模型能够准确描述减饱和砂土加载过程中的应力-应变关系、应力路径及孔隙水压力增长规律,验证了两相流模拟方法的正确性。数值分析结果还表明,加载过程中减饱和松砂中的饱和度会增加,直至达到一个稳定值,当围压一定时,减饱和松砂加载结束时的饱和度与初始饱和度呈线性关系;且砂土中气体会在荷载作用下被压缩,使得减饱和砂土在不排水条件下发生剪缩。计算发现,当砂土饱和度从100%减小到94.5%时,孔隙水压力系数B值会减小约80%,最大孔隙压力值会降低40%~50%,不排水剪切强度提高2.0~2.5倍,残余强度会提升10倍以上,由此可知,此为减饱和法抗液化的主要机制,而基质吸力不是减饱和法提高砂土抗剪强度的主要原因。

Desaturation is a new method to improve the liquefaction resistance of foundation by decreasing the saturation degree of saturated sand. Based on the coupling simulation method of water-gas two-phase flow reaction and soil skeleton deformation, a two dimensional numerical model is established to simulate the static liquefaction behavior under monotonic loading conditions. Also numerical simulation of undrained triaxial tests are carried out. By comparing the numerical results with the laboratory test results, it is found that the two-phase flow model shows great accuracy in describing the stress-strain relationship, stress path and pore water pressure growth in the static liquefaction process, which verifies the effectiveness of the two-phase flow simulation method. In addition, the saturation degree of desaturated loose sand during loading process increases until a stable value is reached. While the confining pressure is constant, the saturation degree at the end of loading linearly increases with the initial saturation. At the same time, the air in the sand is compressed under loading, which enables the desaturated sand to develop shear shrinkage with undrained condition. When the initial saturation degree of loose sand is reduced from 100% to 94.5%, pore water pressure coefficient B will decrease by 80%, the greatest pore pressure will decrease by 40%-50%, the undrained shear strength will increase by 2.0-2.5 times, and the residual strength can improve by more than 10 times. Therefore, this is the main mechanism of desaturation method to improve the liquefaction resistance, and the matric suction isn＇t the main reason for desaturation method to improve the sand shear strength.