非饱和土双应力变量广义土水特征曲线模型验证

利用基于轴平移技术的Geo-Expert高级型应力相关土-水特征曲线压力板仪研究不同覆土压力(0、40、100、200 k Pa)对南阳膨胀土土水特征曲线(soil-water characteristic curve,SWCC)的影响;并对提出的考虑土体变形及多孔隙分布形态的双应力变量广义SWCC表征方程进行如下试验验证:1)不同覆土压力下微多孔隙分布形态的南阳膨胀土侧限固结试验及脱湿试验SWCC验证;2)零净法向应力状态双孔隙尺度硅藻土双峰SWCC试验验证;3)不同净围压状态下单孔隙尺度韩国残积土SWCC试验验证;4)多应力路径下法国Bapaume黄土,不同初始干密度下日本Edosaki砂土在脱湿-吸湿过程SWCC试验验证;并比较分析新方程与van Genuchten方程及Fredlund等方程的差异性。结果表明:1)覆土压力会显著改变膨胀土结构及孔隙通道,进而改变SWCC边界效应区、过渡区形态;也改变了双重孔隙尺度土壤的进气值;第1个波峰SWCC进气值均在1 k Pa左右;相比于零覆土压力试样,40、100、200 k Pa覆土压力试样第2个波峰SWCC进气值分别高4.74、17.58、67.23 k Pa;2)未考虑净法向应力影响的单应力状态多峰SWCC、考虑侧限双应力状态多峰SWCC、各向同性净法向应力单峰SWCC、不同脱湿-吸湿路径SWCC及不同初始干密度的SWCC试验拟合曲线均表明,双应力广义SWCC具有包容复杂影响因素的能力;3)新方程能够利用至少3个土-水数据即可拟合出具有较高的精度的整条SWCC。研究为定量描述土壤的持水、渗透及强度特性提供参考。

Verification of generalized soil-water characteristic curve model considering two stress state variables for unsaturated soils

Abstract: This study investigated the influence of geostatic stress on soil structural evolution, water retention characteristics, permeability properties as well as the air-entry value and validated the reliability of the proposed generalized soil-water characteristic curve model (generalized SWCC). Nanyang expansive clay (depth of 0-10 m) was collected for determination of SWCC by a Geo-Expert high-type pressure plate apparatus for stress-dependent soil-water characteristic curve (SDSWCC-H) based on axis translation technique. A series SWCCs for Nanyang expansive clay were obtained under the vertical depth of 0, 2, 5, 10 m (the geostatic stress was 0, 40, 100, 200 kPa respectively). The coefficient of uniformity and the coefficient of curvature of the expansive clay were 60.44 and 9.08, respectively. With the obtained data and literature data, the generalized SWCC considering two stress state variables and multi probability pore-size density function (M-PDF) was validated against a wide range of experiments under the following different conditions: 1) the confined compression tests and drying-SWCCs for slightly double-series porosity Nanyang expansive clay under different geostatic stress; 2) Double-series porosity diatomaceous earth mixtures SWCCs with zero net normal stress state; 3) One porosity Korean residual soil SWCCs at different net confining stress; and 4) Drying-wetting path of France Bapaume loess under different stress path, and Japan Edosaki sandy soils under different initial dry density. Performances of the proposed equation, van Genuchten equation and Fredlund&Xing equation were compared. The results showed that the void ratio of the expansive clay at the matric suction of 0.1 kPa was 0.688, 0.678, 0.627 and 0.576 under the geostatic stress of 0, 40, 100, 200 kPa, respectively. The void ratio decreased with increasing matric suction and increasing geostatic stress. During the drying process, the water content of the expansive clay at the geostatic stress of 40, 100 and 200 kPa was lower than that at the geostatic stress of 0 kPa. It suggested that the net normal stress had great influences on the water content-SWCC curves. The influence of the net normal stress on water content in boundary effect zone and transition zone was different. In the first boundary effect zone, the water content almost equaled to the initial water content; In the zone at matric suction of 1-10 kPa, water content decreased rapidly with increasing matric suction; In the zone with matric stuction of 10-100 kPa, the water content change was smaller; In the zone with matric suction of 100-450 kPa, the change of water content tended to the same decreasing trend under the different geostatic stress. The air entry of the SWCC was about 1 kPa at the first peak and 25.25-92.48 kPa at the second peak under the different geostatic stress. For the Korean residual soil, the generalized SWCC could fit well with the R2>0.99 and root mean square error (RMSE) smaller than 1.5%. Based on 3 data points, the SWCC could be well fitted with the R2 of 0.997 and RMSE of 0.55%, indicating the feasibility of using 3 data points to fit SWCC curves. For the diatomaceous earth mixtures, the generalized SWCC model showed high fitting accuracy of R2 higher than 0.97 and RMSE smaller than 2.9%. Under different net confining stress, the generalized SWCC model well fitted the SWCC in the expansive soil with R2 higher than 0.99 and RMSE smaller than 0.28%. Similarly, the generalized SWCC model had high accuracy in fitting drying path and wetting path of Japan Edosaki sandy soil under different dry density and France Bapaume loess. Compared with the van Genuchten and Fredlund et al''s equations, the generalized SWCC model had higher accuracy (R2=0.98 and RMSE<1.3%) in France Bapaume loess and Korean residual soil at the net confining stress of 0 kPa. This study validated the feasibility and reliability of the proposed generalized two stress bimodal SWCC model in simulating SWCC under different conditions and in different soils. The model would be helpful in quantitatively describing soil water-retention characteristics, permeability, strength and soil erosion mechanism.