A study of the mechanical properties of herbaceous roots and root-soil composite systems in the degraded alpine pasture artificially restored grassland
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摘要:
为研究人工恢复草本植物对青藏铁路沿线退化高寒草原土体抗剪强度增强作用，以青藏铁路沱沱河段人工草地建植区为研究区，通过对区内生长期为1 a的3种组合种植草本植物开展单根拉伸和根-土复合体直剪试验，评价了3种草本组合单根、根-土复合体力学强度特性。研究结果表明：组合类型I即扁穗冰草（Agropyron cristatum）+碱茅（Puccinellia distans）+冷地早熟禾（Poa crymophila），其3种草本平均根径（0.06～0.34 mm）、平均抗拉力（0.58～3.09 N）、平均抗拉强度（36.87~221.70 MPa）相对最大；3种草本组合其单根抗拉力与根径呈幂函数正相关关系，而抗拉强度与根径呈幂函数负相关关系；直剪试验结果表明，3种草本组合其根-土复合体黏聚力均随深度增加而降低，其中组合类型I其根-土复合体黏聚力相对最大为32.62 kPa，且较组合类型II、组合类型III根-土复合体黏聚力的增加幅度分别为15.3 %、57.7 %，表现出草本组合类型I具有相对更为显著的增强土体抗剪强度作用。研究结果可为青藏铁路沿线高寒草地退化、水土流失、浅层滑坡等灾害防治提供理论依据，同时对采用种植植被开展沿线生态恢复具有实际指导意义。
Abstract:To study the reinforcement to soil by herb roots along the Qinghai-Tibet railway, an artificially restored grassland along the Qinghai-Tibet railway is selected as test site. By conducting tensile tests to a single root and direct shear tests to the root-soil composite system with its growth period of one year, such mechanical indices as roots’ tensile strength and shear strength of root-soil composite system are quantitatively assessed. The results show that the values of the mean root diameter, tensile resistance and tensile strength for the combination I (Agropyron cristatum + Puccinellia distans + Poa crymophila) are greater than those for the other combinations. Moreover, a positive power function can be introduced to describe the relationship between the root diameter and its tensile resistance for the three species, while a negative power function is introduced to describe the relationship between the root diameter and its tensile strength. In addition, the cohesion of the root-soil composite system for the three species exhibits a decreasing trend with the increasing soil depth. Of the values for cohesion for the three species, the cohesion for the combination I is 32.62 kPa, greater than that of the other two combinations by 15.3 % and 57.7%, respectively, suggesting the excellent capacity in reinforcing soil. The results may provide a theoretical basis for the prevention and control of the alpine grassland degradation, soil erosion, shallow landslides and other geo-hazards and eco-hazards along the Qinghai-Tibet railway, and are of a practical guiding significance for ecological restoration by planting vegetation along the railway.
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表 1 试验区3种组合类型地表以下2种不同深度土体物理性质指标试验结果
Table 1. Experimental results of the physical property indexes of the soil mass at two different depths under the surface of three different combination planting types of herbaceous plants in the testing area
组合种植类型 平均密度/（g·cm−3） 平均含水率/% 上层 下层 上层 下层 组合类型I 2.03±0.06 2.06±0.06 16.70±1.98 15.99±2.69 组合I素土 2.00±0.10 2.05±0.23 18.30±1.12 16.45±5.66 组合类型II 1.99±0.03 2.00±0.06 17.16±2.45 15.39±3.22 组合II素土 1.96±0.09 2.04±0.09 16.99±2.15 17.41±0.04 组合类型III 2.01±0.06 2.05±0.06 18.06±1.98 16.87±2.69 组合III素土 1.99±0.00 2.00±0.05 16.99±0.88 15.35±0.49 注：①组合I素土、组合II素土、组合III素土分别指的是同一位置不同植物组合的不含根系素土；②组合类型I、组合类型II、组合类型III分别指的是相同位置由不同植物组合构成的根-土复合体。 
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表 2 试验区3种组合类型草本植物根-土复合体试样根系特征统计结果
Table 2. Statistics of the root characteristics of the root-soil complex samples of three combined planting types of herbaceous plants in the testing area
组合
种植类型位置 平均干重
/g平均根数
/根平均根径
/mm平均根面积比
（RAR）/%组合类型I 上层 0.146 60 0.12 0.026 下层 0.040 26 0.10 0.008 组合类型II 上层 0.213 57 0.20 0.064 下层 0.052 17 0.19 0.017 组合类型III 上层 0.355 68 0.19 0.069 下层 0.081 24 0.18 0.021 注：①数据均为每组根-土复合体试样中4个环刀试样的统计结果；
②RAR指的是根面积比即根-土复合体剪切面上根系横截面面积之和与土体横截面面积的比值，其计算方法为RAR=Ar / As，Ar为根-土复合体试样横截面上所有根系面积之和/mm2，As为根-土复合体试样的横截面面积/mm2[27]。
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表 3 试验区3种组合类型草本植物根-土复合体试样根系与力学强度之间拟合函数关系式
Table 3. Fitting function relationship between the root and mechanical strength of the root-soil composite samples of three combined planting types of herbaceous plants in the testing area
组合种植类型 草本植物名称 单根抗拉力 单根抗拉强度 回归方程式 R2 回归方程式 R2 组合类型I 扁穗冰草 y=11.03x1.3726 0.9215 y=14.072x−0.626 0.7088 碱茅 y=4.4347x0.7457 0.7236 y=5.6482x−1.254 0.881 0 冷地早熟禾 y=9.2589x0.9996 0.7028 y=11.789x−1 0.7031 组合类型II 碱茅 y=4.2647x0.8769 0.7379 y=4.094x−1.177 0.8434 垂穗披碱草 y=17.366x1.4068 0.9303 y=22.111x−0.593 0.7037 星星草 y=6.8425x1.287 0.9072 y=8.7114x−0.713 0.750 0 组合类型III 垂穂披碱草 y=2.5499x0.6092 0.7014 y=3.2906x−1.385 0.921 0 冷地早熟禾 y=8.0364x1.2622 0.8802 y=9.9354x−0.753 0.7229 中华羊茅 y=3.0709x0.9027 0.7383 y=3.7758x−1.117 0.8193
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表 4 试验区3种组合类型中6种草本单根拉伸试验结果
Table 4. Tensile test results of the single root of 6 herbs of 3 different herb combination planting types in the testing area
组合
种植类型植物名称 平均根径
/mm平均抗拉力
/N平均抗拉强度
/MPa组合类型I 扁穗冰草 0.34±0.24 3.09±3.20 40.68±27.23 碱茅 0.28±0.12 1.70±0.63 36.87±21.32 冷地早熟禾 0.06±0.02 0.58±0.21 221.70±100.80 组合类型II 碱茅 0.15±0.06 0.83±0.33 63.22±49.26 垂穗披碱草 0.10±0.04 0.73±0.47 96.19±31.43 星星草 0.16±0.06 0.68±0.34 35.16±11.96 组合类型III 垂穗披碱草 0.15±0.07 0.79±0.27 69.67±49.48 冷地早熟禾 0.12±0.05 0.60±0.35 56.65±25.25 中华羊茅 0.13±0.06 0.50±0.27 50.60±35.47 注：表中单根拉伸试验采用的样本数量均为30株。
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表 5 试验区3种组合类型草本植物根-土复合体直剪试验结果
Table 5. Direct shear test results of the root-soil complex of three combined planting types of herbaceous plants in the testing area
组合种植类型 平均黏聚力c值/kPa 平均内摩擦角φ值/(°) 上层 下层 上层 下层 组合类型I 32.62±11.74 18.61±11.62 19.78±10.86 23.81±8.01 组合I素土 19.81±4.20 22.85±23.48 20.74±19.39 26.73±7.51 组合类型II 28.28±9.61 27.61±20.57 29.96±3.60 23.93±11.84 组合II素土 14.94±12.16 21.42±14.29 32.88±4.77 19.15±13.89 组合类型III 20.69±11.74 7.36±11.62 31.55±10.86 24.87±8.01 组合III素土 14.37±9.62 14.94±4.10 27.02±4.24 24.68±1.00
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