基于单相流的减饱和砂土流固耦合改进算法

减饱和法是一种通过减小饱和砂土地基中水的饱和度来处置可液化砂土地基的方法。基于单相流-固耦合理论，将减饱和砂中水-气两相流体等效为单相流体，提出一种可以考虑加载过程中孔隙流体体积模量变化的减饱和砂土静态液化的单相流改进算法，用其进行单调荷载作用下三轴不排水压缩试验数值模拟研究，分析了不同饱和度条件下的减饱和砂土的不排水反应并与饱和砂土三轴不排水试验结果进行对比。研究结果表明，单相流改进算法能够很好地反映减饱和法的抗液化特性。此外，对比不同数值分析方法模拟结果，得出以下结论：采用单相流简化算法分析减饱和砂土的不排水反应时，因为不考虑加载过程中的孔隙流体体积模量变化，所以初始的流体体积模量取值会影响减饱和砂土的强度，初始围压为100 kPa、饱和度为96%的减饱和砂土在单调荷载作用下，气体体积模量取值从100 kPa增加至200 kPa时，减饱和砂试样的峰值偏应力会减小约30%，孔隙压力会增加约40%；通过对比同等条件下的单相流改进算法和两相流算法的应力-应变关系曲线以及饱和度和体积应变变化曲线，两者结果相近，误差在5%以内。综上所述，单相流改进算法是一种较为合理而简洁地模拟减饱和砂土静态液化的计算方法。

A modified method for mechanical-hydraulic coupled simulation based on a single-phase fluid for desaturated sand soil

Desaturation is a method to improve the liquefaction resistance of ground by decreasing the degree of saturation of saturated sandy soils. A simplified single-phase fluid numerical method considering water-air flow in desaturated sand as a saturated pore fluid, a modified single-phase fluid numerical method is proposed to investigate the static liquefaction behavior considering the change of pore fluid bulk modulus. The numerical simulations of the undrained triaxial test are carried out in this study. By comparing the numerical results with laboratory tests, it indicates that the modified method can predict static liquefaction performance of saturated sand accurately. Moreover, some conclusions are obtained from numerical results of different methods, namely, simplified single-phase fluid method, modified single-phase fluid method, and two-phase flow method. First, the initial value of pore fluid bulk modulus will have a significant impact on the prediction of the undrained behavior of desaturated sand in simplified single-phase fluid numerical method due to the constant pore fluid bulk modulus. When the initial value of pore fluid bulk modulus increases from 100 kPa to 200 kPa, the peak deviator stress can decrease by 30% and the pore pressure can increase by 40%. Second, the stress-strain response and volumetric strain curve of modified method agree with that of two-phase flow method and the error is less than 5%. Overall, the modified numerical method is a simple and reasonable method for describing the static liquefaction of desaturated sand.