Experiment and modeling of soil-water characteristic curve of unsaturated residual soil

土-水特征曲线是指基质吸力与含水率或饱和度的关系曲线。土-水特征及其相关数学模型可用于非饱和土性质如渗透系数和抗剪强度模型的建立。通常,土-水特征曲线仅有脱湿曲线。笔者详细讨论了初始含水率、干密度、竖向应力及干湿循环对脱湿曲线的影响。采用滤纸法测试了厦门地区残积土的土-水特征曲线(SWCC)及滞回环。结果表明,初始含水率对SWCC影响较小,最优含水率干侧试样SWCC的进气值小,滞回环小;湿侧试样的进气值大,滞回环大;最优含水率试样的滞回环居中。初始干密度对SWCC有显著影响,低干密度试样SWCC进气值小,脱湿速率快;高干密度试样SWCC进气值高,脱湿速率低。滞回环的大小随着初始干密度的减小而增大。不同竖向应力下SWCC变化较大。随着竖向应力的增加,进气值增大,脱湿速率减小,滞回环减小并趋于稳定。第1次干湿循环对SWCC影响最大,随着循环次数的增加,进气值减小,滞回环减小并趋于稳定。由于残积砂质黏性土SWCC的过渡区和残余区不易区分,残余含水率难以确定,因此,提出5种剔除残余含水率参数的修正SWCC模型,计算分析得出,修正gArdnEr模型最适合厦门地区残积土的SWCC建模。The soil-water characteristic curve(SWCC) is defined as the relationship between the suction and the water content or degree of saturation.The soil-water characteristics and related mathematical models can be used to model the unsaturated soil properties such as the permeability and the shear strength.Conventionally, only the drying soil-water characteristic curve of soil is determined.In this work, the influences of initial water content, initial dry density, the vertical stress and drying and wetting cycles are examined and discussed in detail.The SWCCs and hysteresis loops of residual soils in Xiamen were obtained by the filter paper method.The results show that the initial water content has negligible effect on SWCCs.The specimen at the wet of optimum moisture content(OMC) has the higher air entry value and the larger hysteresis loop than that of the soil specimen at the dry of OMC.The specimen with OMC lies between them.While, the initial dry density has considerable influence on SWCCs.The specimen at lower initial dry density exhibits smaller air entry value and higher rate of desorption than that of the specimen at higher initial dry density.The size of hysteresis loop decreases as the initial dry density increases.The specimens subjected to different vertical stresses have different SWCCs.The higher the applied load on the specimen, the higher the air entry value, the higher the rate of desorption, the smaller the size of hysteresis loop.In the drying and wetting process, the size of hysteresis loop has been substantially affected by the first drying and wetting cycle.With the increasing of drying and wetting cycle, the air entry value and the size of hysteresis hoop decrease.Since the features of soil-water characteristic of sandy clay are not well defined especially for the residual state of saturation, which desaturates continuously without a distinct break point, the residual suction or residual water content, one of the key parameters in the mathematical models of SWCC, is not well defined.As a result five improved models without the residual water content are proposed to describe the SWCCs of residual soil.The improved Gardner model has been found to be the most suitable model for all SWCCs of the residual sandy clay in Xiamen.国家自然科学基金项目(No.51209180

VARIATION of THE SHEAR STRENGTH of XIAMEN UNSATURATED RESIDUAL SOILS WITH WATER CONTENT

采用普通应变控制式直剪仪对非饱和残积砂质黏性土和黏土进行慢剪强度试验,试验结果表明,砂质黏性土的黏聚力和内摩擦角均对含水量变化敏感,黏聚力随含水量的变化呈二次抛物线关系,内摩擦角随含水量增大呈幂函数减小;含水量对黏土强度的影响主要是降低了黏聚力,对内摩擦角影响很小。黏聚力随含水量的增加呈指数减小。针对上述变化规律,分别建立实用的强度经验公式,避免了吸力量测和计算困难。对提出的强度公式进行试验验证,计算预测强度并与实测强度相比较,结果表明不同竖向应力条件下,高竖向应力下强度的预测精度更高。2类残积土的强度公式预测精度均满足工程要求,可为工程实际应用提供参考。Shear strength of unsaturated sandy and clay residual soils were conducted by consolidated-drained shearing method using the conventional strain controlled direct shear apparatus.Results show that the shear strength of two kinds of residual soils decreases with increase of water content.The initial water content has effect on both cohesion and frictional angle of sandy clayey soils.The relationships between cohesion,frictional angle and water content of sandy clayey soils are quadratic and power respectively.While the initial water content has negligible effect on friction angle of clay.The considerable influence of increasing water content on shear strength of residual clay is decreasing the cohesion.The variation of cohesion with water content can be described by an exponential relationship.On the basis of variation of shear strength indexes with water content,two new strength formulas of unsaturated sandy clayey and clay were established without suction component and were demonstrated by further tests.Comparisons of the calculated and measured shear strength show that the accuracy of the prediction is higher under high vertical stress.Two strength formulas can provide the good estimation of shear strength of unsaturated residual soils in Xiamen for engineering application.福建省自然科学基金资助项目(2014J01208