直剪试验中土体含水率对根系失效方式的影响

doi:10.16843/j.sswc.2021.02.006

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摘要植物根系在土体剪切过程中的受力状态和失效方式随含水率的变化会影响根系固土能力，而根增抗剪强度的模型计算时往往忽略该方面的考虑，使估计值远高于实验值。为准确估计非饱和状态下根系的固土能力，以香根草根-土复合体为对象，设计以土壤含水率和根系密度为变量的大盒直剪试验及单根拉拔试验，观察含根土体的抗剪强度和根系失效方式，基于此对根增抗剪强度进行估计。结果表明：1）在含水率相同的情况下，根增抗剪强度与根系密度呈正相关；根系密度一定的情况下，根增抗剪强度与含水率呈负相关。2）直径在0.2~2.2 mm单根样品的抗拉力和抗拉强度区间分别为3~19 N和5~29 MPa，根系的抗拉强度T_r和抗拉力t随直径D增大分别呈负幂函数和正幂函数（T_r=11.514D^-0.885，t=9.576 3D^1.095）；根系的拔出力p随直径变化也服从正幂函数（p=αD^β，α>0，β>0），且含水率越高，α值越小。3）ω₁%、ω₂%、ω₃%、ω₄% 4个含水率梯度下Wu模型根增抗剪强度值分别为试验值的1.834、1.864、1.889、1.873倍，而考虑含水率-失效方式计算的根增抗剪强度值分别为试验值的1.337、1.028、0.788、0.481倍。因此：1）不饱和状态下，根系固土能力的大小主要与剪切过程中的拔出根数量比有关，直剪试验过程中含水率越高，根系的拔出根数量比越大，能充分发挥抗拉强度的根系数量越少，根增抗剪强度越小，根系固土的能力越弱；2）根系的抗拉强度、抗拉拔力与直径均呈幂函数关系，且随着含水率升高，抗拔力的拟合衰减系数增大；3）考虑含水率及根系失效方式后，所得根增抗剪强度的估计值比Wu模型更接近实验值。

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关键词 ：根系固土, 抗剪强度, 根系失效方式, 含水率

Abstract：[Background] The stress state and failure mode of plant roots in the shearing process will affect the root's ability of reinforcing soil,which is usually neglected in the classical models when quantifying the shear strength increment provided by roots and results in the overestimation of root reinforcement. [Methods] Aiming to explore the resistance behaviors and failure model of roots in the unsaturated soil under shearing condition, the large scale direct shear test for Vetiver root-soil composite, root tensile and pullout test were conducted based on the various soil moisture content and root quantity gradient. And the fractured or pulled out root ratio of each treatment shearing sample were observed simultaneously. Based on which the shear strength increment by roots are calculated by a revised model considering effect of soil moisture and root failure. [Results] 1) Under the condition of the same moisture content, the increased root shear strength was positively correlated with the root density. Under the condition of a certain root density, the increased root shear strength had a negative correlation with the moisture content. 2) The tensile force T_r and strength t with diameter 0.2-2.2 mm was 3-19 N and 5-29 MPa, and can be described as a negative power function and a positive power function with the increase of diameter D respectively (T_r=11.514D^-0.885,t=9.5763D^1.095). Root pullout force p also followed the positive power function with diameter (p=αD^β,α>0,β>0) and higher the moisture content, smaller the α value. 3) Among the four water moisture contents, Wu model values was 1.834, 1.864, 1.889, and 1.873 times of test values respectively, however the model value was 1.337, 1.028, 0.788, and 0.481 times of test values respectively when taking the root failure mode into account. [Conclusions] 1) Under unsaturated conditions, the reinforcing soil capacity of the root system is mainly related to the ratio of the number of roots extracted during the shearing process. The higher the water content during the direct shear test, the lower the extraction strength of the root system, and the root extraction. The larger the ratio of number of the extract roots, the smaller the amount of roots that can fully contribute to the tensile strength, and thus the weaker the root system's ability to stabilize the soil. 2) The root tensile strength, pull-out force and diameter are all in a power-exponential function relationship, and as the moisture content increases, the fitting attenuation coefficient of the pull-out force increases. 3) After taking moisture content and root failure mode into account, the estimated value of the obtained root shear strength is closer to the experimental value than that by the Wu model.

Key words： root reinforcement shear strength root failure mode moisture content

收稿日期: 2020-06-08

PACS:

TU411.7

基金资助:国家自然科学基金项目"降雨条件下植物根系动态固土护坡效应研究"（31971726）

通讯作者: 王云琦(1979-),女,教授,博士生导师。主要研究方向:水土保持工程。E-mail:wangyunqi@bjfu.edu.cn E-mail: wangyunqi@bjfu.edu.cn

作者简介: 王余靖(1995-),男,硕士研究生。主要研究方向:水土保持。E-mail:wangyujing@bjfu.edu.cn

引用本文:

王余靖, 周利军, 王云琦, 李洪飞, 李通, 王鑫皓. 直剪试验中土体含水率对根系失效方式的影响[J]. 中国水土保持科学, 2021, 19(2): 43-51.
WANG Yujing, ZHOU Lijun, WANG Yunqi, LI Hongfei, LI Tong, WANG Xinhao. Effect of soil moisture content on root failure mode in direct shear test. SSWC, 2021, 19(2): 43-51.

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http://journal12.magtechjournal.com/Jweb_stbc/CN/10.16843/j.sswc.2021.02.006 或 http://journal12.magtechjournal.com/Jweb_stbc/CN/Y2021/V19/I2/43

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