紫色土埂坎典型草本根系摩阻特性对土壤含水率的响应

为进一步探索紫色土埂坎草本根-土界面摩阻特性与土壤含水率的关系，以紫色土埂坎常见稗草、马唐和牛筋草3种草本根系为研究对象，设置不同土壤含水率（5%、10%、15%、20%和25%），通过直剪和拉拔摩阻试验测定草本根-土复合体的摩阻特性指标（黏聚力、摩擦系数、最大抗拔力和抗拔强度），分析了土壤含水率对不同草本根-土界面摩阻特性的影响。结果表明：1）当含水率为15%和20% 时，根-土界面黏聚力达到较小值，而摩擦系数、最大抗拔力和抗拔强度达到较大值。2）3种草本根-土界面拉拔摩阻特性差异显著（P<0.01），牛筋草根-土界面平均最大抗拔力和抗拔强度分别是马唐的1.18和1.30倍，是稗草的1.14和1.10倍。3） 草本根-土界面间抗剪强度和垂直荷载的关系服从莫尔-库伦准则。当含水率为20%和25%时，根-土界面间抗拔力达到较大值。4）在相同垂直荷载和土壤含水率条件下，牛筋草根-土界面抗剪强度显著高于马唐和稗草（P<0.05）。由此可知，牛筋草根系能增强紫色土埂坎稳定性，其根系对埂坎的加固作用约在土壤含水率为15%时效果最佳。研究结果可为三峡库区紫色土埂坎固埂护坡草本植物的筛选提供参考。

Response of root-soil friction characteristics of three common grasses to soil water content in purple soil bunds

This study aimed toelucidate the reinforcement effect of grass roots on purple soil bunds. Three grasses commonly found growing on purple soil bunds （Echinochloa crusgalli， Digitaria sanguinalis and Eleusine indica） were studied. Soil is a material that cannot withstand tension but can withstand pressure， while the root system within the soil can withstand tension to some extent. When external force is applied， the contact interface between the root system causes roots and soil to dislocate each other and generate frictional resistance and aspects of this root-soil interaction were measured for roots artificially packed into specially constructed test apparatuses. Root systems were harvested from the field and manually packed into the test apparatuses. Parameters measured included： resistance to shear force under vertical load （5 pressures ranging from 25 to 150 kPa）， maximum force （N） to pull packed roots out of the test apparatuses， and tensile strength （MPa） of the roots in the test conditions. Tests were conducted under five soil water contents （5%， 10%， 15%， 20% and 25%） and the root-soil friction characteristics of the three grasses were examined by direct-shear force tests and ‘pull-out’ （extraction） force tests. In addition， the mechanisms of interaction between grass roots and soil during the friction test process were analyzed. It was found that： 1） There was a significant difference between the three grasses in extraction friction force （P<0.01）. The maximum pulling force of E. indica was on average 1.18 times greater than that of D. sanguinalis which in turn was 1.14 times higher than that of E. crusgalli. Similarly， the tensile strength of E. indica was on average 1.30 times higher than that of D. sanguinalis which in turn was 1.10 times higher than that of E. crusgalli. 2） The root-soil friction characteristics of the three tested grasses showed significant differences across the five different soil water contents tested （P<0.05）. At a soil water content of 15%-20%， the cohesion of the tested grass root-soil interface was relatively lower than at the other soil water contents， while the friction coefficient tended to be a maximised. In addition， the maximum extraction force and tensile strength of root-soil interface occurred at a soil water content of 15%. 3） The constitutive relationship between the shear strength at the root-soil interface and vertical load can be described by a hyperbolic model that satisfies the Mohr-Coulomb criterion. The shear strength of the root-soil interface reached a maximum at a soil water content of 20%-25%. 4） Under the same vertical load and soil water content， the shear strength of the goosegrassroot-soil interface was significantly higher than that of D. sanguinalis and E. crusgalli （P<0.05）. It can thus be concluded that among the three tested grasses herbs， E. indica had the most significant effect on enhancing soil shear strength， followed by D. sanguinalis and E. crusgalli. These results provide scientific data to assist with selection of grass species for soil-fixation to protect purple soil bunds.