Influences of morphology and hierarchy of roots on mechanical characteristics of root-soil composites
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摘要: 植物根系的结构特征包括形态和层次结构,对根土复合体力学特性影响显著。通过开展黑麦草生长参数测试及直剪试验,获得了不同时期根土复合体的力学特征;采用自主研发的MechRoot程序建立了符合黑麦草根系结构特征的根土复合体模型,研究黑麦草不同形态和层次结构根系在直剪过程中的轴力水平以及占比情况,阐明了根系形态与层次结构对根土复合体力学特性影响及固土作用机理。研究表明:植物根系能明显提高土体的强度,根土复合体的抗剪强度增加主要是由黏聚力增加引起,增幅最大达4.99 kPa;随根系形态复杂程度增加,剪切过程中根系能够调动更大范围土体抵抗剪切变形,根系周围的剪切带和塑性区分布范围不断增加,并向根系周围集中,根土复合体的抗剪强度提高;根土复合体剪切过程中,各层次根系发挥作用逐渐变化,一级根、二级根、三级根的轴力水平分别为3.87,1.50,0.15 N,随根系生长发展,二级根、三级根参与到根系固土作用的比例不断增大,最大贡献分别为43.69%,13.80%。Abstract: The structural characteristics of plant roots, including morphology and topological structure, have significant effects on the mechanical characteristics of root-soil composites. The mechanical behaviors of the root-soil composites at different time are obtained by carrying out the Lolium perenne root system morphology parameter tests and direct shear tests. By using the self-developed MechRoot program, a numerical model for the root-soil composites with more realistic root structural characteristics of Lolium perenne is established, and the axial force levels and proportions of roots with different shapes and topological structures in the process of direct shear are studied, which illustrates the influences of morphology and topological structures of roots on the mechanical characteristics of root-soil composites and the mechanism of soil consolidation. The results show that the plant roots can strengthen soil significantly, and the increase of shear strength of root-soil composites is mainly caused by the increase of cohesion, with the maximum increase of 4.99 kPa. During the shearing process, with the increase of the morphological complexity of roots, the more range of soil can participate in the resistance of shear by the roots, the shear zone and plastic zone around the roots increase and are concentrated around the roots, and the shear strength of the root-soil composites increases. At the same time, the effects of the roots at several levels change gradually. The axial forces of the primary roots, secondary roots and fibrous roots are 3.87, 1.50 and 0.15 N. With the growth of the roots, the proportion of the secondary roots and fibrous roots participating in soil fixation of the roots increases continuously, with the maximum contributions being 43.69% and 13.80%, respectively.
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表 1 试验土体基本物理性质指标
Table 1 Basic physical properties of test soils
土粒相对质量密度Gs /(g·cm-3) 孔隙比 液限/% 塑限/% 渗透系数k/(cm·s-1) 2.58 1.63 0.82 48 23 2.5×10-6 表 2 不同层次根系的生长数量指标
Table 2 Growth indices of root system
根系指标 时间/d 数量/条 长度/mm 直径/mm 一级根 7 3 98.160 — 10 3 175.320 0.28 14 3 244.060 0.52 21 3 278.100 0.65 28 3 285.980 0.68 二级根 7 11 7.640 — 10 21 15.260 0.16 14 33 17.520 0.36 21 36 17.600 0.47 28 42 17.700 0.50 三级根 7 0 — — 10 45 1.660 0.08 14 91 4.700 0.14 21 134 5.350 0.16 28 168 5.400 0.16 注:表中根系直径为距离根顶100 mm处根系的平均值;二级根和三级根的长度为距离根顶80~120 mm范围内的平均值。 表 3 根系的力学参数指标
Table 3 Mechanical parameters of roots
根径/mm 抗拉力/N 弹性模量/MPa 0.21 0.50 73.88 0.32 1.43 55.72 0.46 2.74 52.82 0.56 3.52 40.21 0.67 5.88 28.79 0.79 7.56 31.11 0.84 9.12 25.41 0.91 11.87 23.98 1.02 15.64 20.59 表 4 室内直剪试验结果
Table 4 Results of direct shear test
生长时间/d c/kPa φ/(°) 0 13.60 25.5 7 13.82 25.3 10 15.32 25.0 14 17.26 24.2 21 18.21 24.8 28 18.59 24.5 表 5 黑麦草的关键生长参数
Table 5 Key growth parameters of Ryegrass
生长参数 s/(mm·d-1) lb/mm la/mm ln/mm n /(rad) c/(mm·d-1) 0th
(种子)30 0 0 0 3 1 1st
(一级根)60 32.2 42 15.3 15 0.218 2nd
(二级根)10 3.2 3.3 2.8 10 0.146 3rd
(三级根)2.0 3.3 2.1 — — — 0.0925 表 6 不同生长时间的根系模型生物量
Table 6 Growth parameters of root model
不同生长时间/d 单元数 根系总长/mm 根系总面积/mm2 根系总体积/mm3 10 589 1445.236 1089.63 116.59 14 1143 2441.007 2090.78 252.34 28 2172 4456.762 3777.45 452.01 表 7 土体参数
Table 7 Soil parameters
天然重度/(kN·m-3) 内摩擦角/(°) 黏聚力/kPa 杨氏模量/Pa 泊松比 14.9 25.5 13.6 2.72×106m2 0.22 表 8 剪切面上各层次根系轴力水平统计表
Table 8 Statistics of axial force of roots at different levels on shear plane
不同生长时间/d 一级根数量/条 一级根平均轴力/N 二级根数量/条 二级根平均轴力/N 三级根数量/条 三级根平均轴力/N 10 3 2.46 3 0.60 6 0.002 14 3 3.87 6 0.78 17 0.100 28 3 3.47 6 1.50 18 0.150 表 9 不同形态根系的根土复合体抗剪强度统计表
Table 9 Statistics of shear strength of root-soil composites with different root forms
生长时间/d 根土复合体抗剪强度 试验结果/kPa 模拟结果/kPa 误差/% 10 15.58 15.84 1.60 14 17.19 18.94 10.18 28 17.98 20.46 13.93 -
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