干湿交替下苜蓿根系对黄土抗剪力学性能影响

Effects of Alfalfa Roots on the Shear Properties of Loess Under Alternate Drying-Wetting Conditions

[目的]探究干湿交替下植物根系对黄土抗剪性能的影响,为常年处于干湿交替状态下的黄土区生态建设和工程建设提供理论基础。[方法]以黄土区典型草本植物苜蓿为研究对象,采用室内直剪试验,研究黄土抗剪特性与干湿交替作用、根系尺寸(根长、根径)、土体容重、含水率等影响因素的关系,并基于Wu模型评估了根系对土体抗剪特性的增强作用。[结果]经历干湿交替后的根土复合体表面变得粗糙、松散、易脱落,并出现不均匀裂隙,而且其抗剪性能指标均呈降低趋势。干湿交替下,根系尺寸显著影响根土复合体抗剪特性,根长越长,根径越大,其抗剪强度越大。在一定范围内,黏聚力、内摩擦角与土体容重呈正相关关系。不同容重的根土复合体,经历干湿交替作用后,其黏聚力、内摩擦角均降低。相同干湿交替作用下,黏聚力随着含水率的增加呈先增加再降低的趋势;相同含水率水平下,进行干湿交替土体抗剪强度指标均弱于原状土体。Wu模型的局限性导致模型估算根系附加黏聚力结果与实际值相差较大,可以采用根系抗拔强度替换根系抗拉强度,优化Wu模型对根系附加黏聚力的估算结果。[结论]干湿交替下,植物根系尺寸、土壤容重与土壤含水率都对黄土抗剪性能有一定影响。

[Objective]The aim of this study is to explore the influence of plant roots on shear properties of loess under alternate drying-wetting conditions and provide a theoretical basis for the ecological construction and engineering construction in the loess region under the state of alternate drying-wetting.[Methods]Direct shear test in laboratory was conducted on loess samples with/without alfalfa roots to study the relationship between shear properties of loess and influencing factors including alternate drying-wetting,root size(root length,root diameter),soil bulk density and water content,and to evaluate the reinforcement of roots on shear resistance of loess based on the Wu model.[Results]The surface of root-soil composite became rough,loose and easy to fall off after alternate drying-wetting,and uneven cracks appeared.The shear indexes of the composite decreased after alternate drying-wetting.Root size significantly affected the shear behavior of root-soil composite under alternate drying-wetting conditions.Longer and larger roots resulted in greater soil shear strength.Soil cohesion and internal friction angle were positively correlated with soil bulk density in a certain range,and they decreased after alternate drying-wetting.Under the same alternate drying-wetting condition,the cohesion first increased and then decreased with soil water content.At the same level of soil water content,shear strength was weaker in soil under alternate drying-wetting than in soil without alternate drying-wetting.The limitations of the Wu model led to the relatively wide difference between the results by the model and the values by the shear tests.The estimation results of root cohesion by Wu model could be optimized by replacing root tensile strength with root pullout strength.[Conclusion]Under alternate drying-wetting conditions,the size of plant roots,soil bulk density,and soil moisture content all have a certain impact on the shear resistance of loess.