砂质黏性紫色土粒组缺失对降低持水特性的影响

Effects of grain group absence of sandy clayey purple soil on the reduction of water-holding characteristics

紫色土结构松散、具高生产性,但具有较弱的抗侵蚀能力,侵蚀后表现出土壤干密度减小、细颗粒流失严重等现象,影响其持水特性。为探讨紫色土降雨侵蚀后土壤持水特性的变化,该研究选取砂质黏性紫色土在降雨条件下最易变化的3组粒组(>0.250~0.500、>0.075~0.250、0~0.075 mm),开展广义吸力范围内粒组缺失的土-水特征曲线(soil-water characteristic curve,SWCC)试验,分析粒组缺失对砂质黏性紫色土持水特性的影响。结果表明:1)粒组缺失会降低土体持水特性,在边界效应区和过渡区最为明显,残余区影响较小。缺失粒组质量分数与进气值和残余含水率呈反比;2)使用粒径参数(限制粒径、平均粒径、中值粒径、有效粒径)表征土体级配状况,建立土体级配曲线与SWCC特征值的关系,发现除有效粒径,其余参数均与进气值和残余值呈线性反比关系;3)使用Fredlund-Xing模型对不同方法所得数据进行拟合,发现使用单一试验方法所得数据结果拟合优度较高,但SWCC明显偏离其余试验数据点。提出获得准确的紫色土全吸力SWCC仅需确定吸力最大值,并引入3种评价指标进行验证,拟合结果表明,此方法可有效缩短SWCC试验周期,为SWCC在三峡库区紫色土地区的工程运用提供了可靠的思路。

Purple soil is widely distributed in southwestern China and is one of the important natural resources.Because of the loose parent rock and low degree of differentiation,purple soil is characterized by a loose structure that is easily eroded and high soil fertility.Soil erosion is very serious in the purple soil region,which is one of the main sources of sediment in the upper reaches of the Yangtze River,and has a certain impact on the agricultural production and engineering development in the purple soil region.After erosion,purple soil shows the characteristics of reduced dry density of soil and serious loss of fine particles,which affects its water-holding characteristics.In order to explore the changes of soil water-holding properties of purple soil after rainfall erosion,this study was carried out to analyze the effect of missing grain groups on the water-holding properties of sandy clayey purple soil under rainfall conditions by selecting three test methods,namely,axial translation technique,filter paper method,and saturated salt saturated saline solution vapor equilibrium technique for the three grain groups which are the most susceptible to changes in the rainfall conditions,and carrying out the soil-water characteristic curve(SWCC)test with the absence of grain groups within the full suction range to analyze the effect of the missing grain groups on the water-holding properties of sandy clayey purple soil.The results show that:1)the missing grain group reduces the waterholding characteristics of sandy clayey purple soil,which is most obvious in the boundary effect zone and transition zone,and has less effect on the residual zone.The mass of the missing grain group is inversely proportional to the air intake value and the residual water content.2)Using the grain size parameters(constrained diameter、average diameter、median diameter、effective diameter)to characterize the soil grading condition,the relationship between the soil grading curves and the characteristic values of SWCC was established.It was found that except for effective diameter,the other parameters were linearly inversely related to the air intake value and residual water content;3)The data obtained from different methods were fitted using the Fredlund-Xing model,and it was found that the results of the data obtained from a single test method fitted better,but the SWCC significantly deviated from the test data points in the boundary effect zone and the transition zone.Root mean square error(RMSE)、percent bias(PB)and mean absolute percent relative error(MAPRE)were introduced as evaluation indexes,and it was found that the suction range was the main factor affecting the fitting effect of SWCC,and within a certain suction range,the larger the range of data points involved,the better the model fitting effect was,and after a certain suction value was reached,the suction range had very little effect on the model fitting effect.It is proposed that to obtain the accurate SWCC of purple soil quickly,only the maximum suction value needs to be determined,and the number of test points with high suction is reduced,which can effectively shorten the SWCC test period.The results lay a foundation for the subsequent study of the role of soil erosion and water-holding characteristics of purple soil,and also provide reliable ideas for the prevention of soil erosion and the application of soil-water characteristic curves in the Three Gorges Reservoir area.