紫色土坡耕地埂坎土壤抗剪性能对含水率的响应

Response of soil shear strength to soil water content in purple soil slope cropland bunds

选取三峡库区典型紫色土坡耕地埂坎进行试验,通过室内三轴试验研究不同含水率(质量分数6%、11%、16%、21%、26%和31%)对埂坎土壤抗剪强度指标的影响,以深化紫色土坡耕地埂坎力学性质研究。结果表明:1)试验含水率范围内,紫色土坡耕地埂坎土壤黏聚力受含水率影响显著(P<0.05),且随着含水率增加呈现出先增大后减小的趋势,明显的峰值出现在含水率质量分数11%左右,黏聚力为85.52 k Pa;2)埂坎土壤内摩擦角随含水率增加而减小,呈非线性衰减,符合一阶指数衰减规律。高含水率时,衰减缓慢;3)紫色土坡耕地埂坎抗剪强度受含水率变化影响显著(P<0.05),埂坎土壤极限主应力差随含水率和围压的变化明显且具有规律性。相同围压下,埂坎土壤极限主应力差随含水率增大而迅速减小,即土体的抗剪强度降低。相同含水率下,极限主应力差随围压增大而增大,低含水率时增加明显,高含水率时增加缓慢。当含水率质量分数达到26%左右,埂坎土壤抗剪强度趋于低值;4)紫色土埂坎土壤的应力-应变曲线随含水率递增依次呈现应变软化型、硬化型和弱硬化型。研究结果可为三峡库区高标准基本农田等工程的埂坎建设提供依据和技术支撑。

Soil bunds play a key role in controlling soil erosion on sloping farmlands, however, the performance and mechanism of purple soil bunds against water erosion is little reported. The objective of the present study was to analyze the soil shear strength features of purple soil bunds under different soil water content scenarios. Samples were selected from 3 types of purple soil bunds in Zhongxian County in the Three Gorges area. An unconsolidated-undrained（UU） triaxial compression test was carried out to study the soil shear strength in terms of the cohesion, internal friction angle and principal stress difference. Considering the actual water content in soil bunds and other soil physical properties, we set 6 levels of water content（from 6% to 31% with an interval of 5%） to prepare soil specimens for the test. The results showed that： 1） Within the testing water content from 6% to 31%, the soil cohesion had significant differences among different water content（P0.05）. It was increasing and then decreasing with the increment of the water content, which could be fitted by a quadratic curve. The maximum of the soil cohesion 85.52 k Pa for purple bund soil occurred at the soil mass fraction of water content of 11%. When the mass fraction of water content reached 26%, the decreasing rate of soil cohesion was faster, resulting in a soil shear strength attenuation; 2） The soil water content had significant effects on the internal friction angle of bund soil（P0.05）. It decreased from 33.19° to 1.36° with the increasing soil water content, showing a first-order exponential decay. However, it decreased slowly at a high water content of 26%; 3） Significant differences（P0.05） occurred in the soil limit principal stress difference of the purple soil bund under different soil water content. It changed obviously with the varying soil water contents and confining pressures. Under the same confining pressure, the limit soil principal stress difference decreased rapidly with increasing soil water content, indicating the reduction of soil shear strength. Under the same water content, the limit principal stress difference increased with the increment of the confining pressure. It increased relatively fast at low water contents, while slowly at high water contents. When the mass fraction of water content was 11%-25%, the soil shear strength was relatively larger than the other water content conditions. While the soil shear strength tended to be a minimum when the mass fraction of water content was about 26%; 4） The stress-strain curves of purple soil bunds affected by different water content could be categorized into 3 types, that is, softening type, hardening type and weak hardening type. In the condition of small confining pressure and low water content, a maximum appeared in the stress-strain curve, showing a softening type. When the water content and confining pressure increased, the stress-strain curve gradually changed to a hardening type. However, the stress-strain curves gradually transformed into a weak hardening type under a condition of high water content. The results can provide support for bund construction in ecological projects, for example, high standard basic farmland construction, in the Three Gorges Reservoir area.