Shear Strength Characteristics Of Bund Soil On Purple Sloping Croplands

Soil bunds play a key role in controlling soil erosion on sloping farmlands,however,the performance and mechanism of purple soil bund against water erosion has less been reported.The objective of the study was to analyze the soil shear strength features in purple soil bunds under different soil water content or dry density scenarios.Thirty six groups of samples were selected from three types of purple soil bunds in Zhongxian County in the Three Gorges area,China.An unconsolidated undrained(UU)triaxial compression test was carried out to study the soil shear strength in terms of cohesion(c),internal friction angle(?)and principal stress difference(?1-?3).Test results indicated that:(1)At a certain dry density condition,the soil cohesion is increasing and then decreasing with the increment of the water content which could be fitted by a quadratic curve.A turning point appears at a coincident water content of near 11% in all dry density conditions.The internal friction angle of bunds soil decreases with the increasing soil water content,showing a first-order exponential decay at a certain dry density condition.When the moisture content reaches about 26%,the internal friction angle decreases slowly.The maximum principal soil stress difference changes with water content regularly,in detail,it decreases rapidly when the water content increases under a same confining pressure.(2)At a certain moisture,the soil cohesion increases when the dry density increases.Generally,the interaction between the dry density and the water content has a small influence on the cohesion;The soil friction angle increases when the dry density increases.Generally,the effect of water content on soil friction angle is much higher than that of dry density and the interaction between water content and dry density had little effect on the soil internal friction angle.However,under the same water content,the difference of the maximum principal stress difference of soil increases with the increase of dry density,but the growth is slow.With the change of soil moisture content,the variation of the maximum principal stress difference of soil is larger than that of dry density,and the interaction between them is not obvious.(3)In addition,the bunds soil stress-strain curve is more or less affected by the dry density and water content.With the increase of water content,and it presents a strain softening,hardening and weak hardening type sequentially.However,and it presents a hardening,weak hardening and strain softening type sequentially with increasing dry density,it often appears a hardening type as confining pressure increasing.(4)The effect of reinforced material on the cohesion of reinforced soil is greater than that of internal friction angle.Three kinds of test materials showed that the cohesion of soil increased with the increase of the content of reinforcement,and then decreased.However,there was no obvious change law of soil internal friction angle.The shear strength of soil can be improved by different reinforcement materials compared with pure soil.But all kinds of reinforced materials to enhance the level of soil shear strength is different,which wheat husk reinforcement effect is best,followed by bamboo,the last is rice straw.Three kinds of reinforced soil show that the difference of the maximum principal stress difference of the soil increases first and then decreases with the increase of the reinforcement content.The optimal reinforcement content,respectively when wheat husk content is 0.8%,bamboo is 0.5%,rice straw is 0.3%.