Shear characteristics and modified Coulomb model of alfalfa root-loess composite
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摘要: 植物根系可以显著提升土体的强度。为探究紫花苜蓿根系对黄土抗剪性能的增强作用，选取生长周期为60 d的紫花苜蓿根系−黄土复合体为研究对象，通过剪切试验和根系拉伸试验获取紫花苜蓿根系−黄土复合体抗剪强度和根系抗拉强度参数，采用电子放大镜观察紫花苜蓿根系结构与特征，利用经验公式建立根−土复合体库仑修正模型，并对比分析库仑修正模型、WWM模型及修正WWM模型的优缺点，对黄土地区植物根系增强土体强度进行评价。结果表明：紫花苜蓿根系对土体抗剪性能的增强效应显著；紫花苜蓿根系发达，根径在0.2~6.7 mm，随着根径的增大，单根极限抗拉力呈指数式增大，抗拉强度呈指数式降低；不同RAR(Root Area Ratio，即根系横截面积之和与土体横截面积的比值)下的根−土复合体抗剪强度相比素土样提升显著，RAR与黏聚力和内摩擦角之间具有显著的相关关系，其中RAR与黏聚力之间呈线性正相关关系，与内摩擦角之间呈高斯函数关系；根−土复合体库仑修正模型对黄土区草本植物根−土复合体的抗剪强度增量预测具有较好的合理性、有效性及适用性，修正WWM模型次之，WWM预测精度相对较差；而在木本植物根−土复合体的抗剪强度增量预测方面，根−土复合体库仑修正模型和修正WWM模型的预测精度要远高于WWM模型。研究结果对黄土地区植被护坡工程具有指导意义。Abstract: Plant roots can significantly improve the strength of soil. In order to explore the enhancement effect of alfalfa root on the shear strength of loess, study was conducted based on the root-soil composites of alfalfa with a growth period of 60 d herein. Specifically, the parameters on shear strength of the alfalfa root-soil composite and the tensile strength of alfalfa root were obtained through the direct shear tests and root tensile tests. By observing the structure and characteristics of alfalfa root with an electronic magnifier, a modified Coulomb model of root-soil composite was established with the empirical formula. Meanwhile, the advantages and disadvantages of the modified Coulomb model, the WWM model and the modified MMW model were comparatively analyzed. In addition, the plant root enhanced soil strength in the loess area was evaluated. The results show that the alfalfa root system has a significant enhancement effect on the shear resistance of soil. The alfalfa root system is well developed with the root diameter between 0.2-6.7 mm. With the increase of root diameter, the ultimate tensile resistance of a single root increases exponentially, but the tensile strength decreases in the same way. The shear strength of root-soil composite under different Root Area Ratio (RAR, which refers to the ratio of total cross-sectional area of root system to the cross-sectional area of soil) is significantly improved compared with the soil sample without roots, indicating that RAR is obviously correlated with the cohesion and internal friction angle. Definitely, RAR is in linear positive relation to the cohesion and in relation of Gaussian function to the internal friction angle. The modified Coulomb model of root-soil composite shows good reasonability, effectiveness and applicability to the increment prediction of tensile strength of root-soil composite of herbaceous plants in loess area, followed by the modified WWM model, and the WWM model has relatively poor accuracy of prediction. However, in predicting the shear strength increment of woody root-soil composites, the modified Coulomb model and the modified WWM model are much more accurate than the WWM model. The research results are of guiding significance to the vegetation slope protection projects in loess areas.
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Key words:
- root-loess composite /
- shear strength /
- root tensile /
- modified model /
- loess area
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图 7 根径与抗拉力和抗拉强度关系
Fig. 7 Relationship of root diameter with tensile resistance and tensile strength
图 10 抗剪强度参数与RAR拟合关系曲线
注：a、b、A、w均为高斯函数参数，其中a为高斯曲线峰值，b为峰值对应的横坐标，A、w为实数，R2为确定系数。
Fig. 10 Fitting curves of shear strength parameter with RAR
图 12 各模型获得的抗剪强度预测和实测数据对比
Fig. 12 Comparison of shear strength prediction obtained by each model with measured data
图 13 根系增强土体力学机理
注：Z为剪切区厚度；α为根系剪切变形角；Tr为根系抗拉力；x为根系在水平方向上的错动距离。
Fig. 13 Enhancement mechanism of soil strength by roots
表 1 试验用土的物理性质指标测试结果
Table 1 Test results on physical properties of test soil
土体
类型含水率/
%孔隙比
e液限
wL塑限
wP天然密度/
(g·cm−3)密度/
(g·cm−3)相对
密度粉质黏土 16.5 0.74 26.8 15.4 1.63 1.39 2.70 
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