| Inner Mongolia has a complex climate and various geomorphological types,severe soil erosion and frequent geological disasters.In recent years,efforts have been made to restore the damaged ecological environment through plant measures.Plant roots interweave and entwine in the soil to form root-soil complex,which forms reinforcement and anchoring to the soil,so as to strengthen the resistance of the soil to the external environment,become the main performance of plant soil-fixing ability.In this thesis,Medicago sativa roots were taken as the research object,and the theory and method of soil mechanics and material mechanics were adopted,in-situ limit single root tensile test,direct shear test of root-soil complex,in-situ self-healing test of root-soil complex damaged by shear,direct shear friction test of root-soil complex and pull-out friction test of single root were carried out,the ultimate tensile properties of Medicago sativa roots,the mechanical properties of self-healing after shear damage,the shear properties of root-soil complex and the frictional resistance of root-soil interface were studied.To study the internal factors,including root diameter,number of buried roots,and external factors,which affect the mechanical properties of Medicago sativa roots,such as soil moisture,shear rate,soil type,vertical load and loading rate.By analyzing these factors,the mechanical characteristics of Medicago sativa root system were revealed.Research shows:(1)The distribution characteristics of root morphology and quantity of the standard plant showed that the diameter classes of Medicago sativa representative roots were0-0.5mm,0.5-1mm and 1-1.5mm.It is mainly distributed in the range of small path level and exists in the form of fine roots.(2)The ultimate tensile test of single root in situ showed that the ultimate tensile strength of single root of Medicago sativa showed a positive power function correlation with the increase of root diameter in the range of 0 ～ 4mm.The ultimate tensile strength and elastic modulus showed a negative power function correlation with the increase of root diameter,and the ultimate strain showed a polynomial function relationship that first increased and then decreased with the increase of root diameter.The mean values of ultimate tensile strength,ultimate tensile strength,ultimate strain and elastic modulus were 25.22 N,48.90 Mpa,14.89% and 5.91 Mpa,respectively,in the range of representative root diameter class 0 ～ 1.5mm.(3)The direct shear test of alfalfa root-soil complex showed that the root-soil complex of alfalfa was better than that of normal soil under the condition of 8.60%natural water content and 1.50 g/cm3 soil dry density,the increase rate of shear strength is 46%,the increase rate of cohesion is 68%,and the increase rate of internal friction angle is 11%.The results showed that the existence of root system increased the shear strength of soil.With the increase of soil moisture content,the shear strength and cohesion of Medicago sativa root-soil complex increased at first and then decreased,but the internal friction angle did not change obviously.The study on the influencing factors of root-soil complex shows that under the vertical load of 12.5 KPa,the soil water content is 16.5%,the number of roots is 10,and the diameter of roots is 1.5-2mm,the effectiveness of plant roots in improving shear strength of soil is the best.The shear strength at the shear rate of 0.8 mm/min is less than that at the shear rate of 0.1 mm/min.(4)The results of in-situ callus self-healing test showed that the shear strength and residual shear strength of the in-situ root-soil complex were higher than that of plain soil but lower than that of parallel control group.When the root system was damaged by the shear forces of grade Ⅰ(35% of average ultimate shear),grade Ⅱ(55% of average ultimate shear)and grade Ⅲ(75% of average ultimate shear),the shear strength of root system after 3 months of repair was 10.58,6.38,5.38 times of that of plain soil.Decreased by 14.75%,48.64% and 57.00%,respectively;After 5 months of repair,the shear strength was 11.57,7.71 and 6.38 times of that of plain soil,which decreased by12.36%,41.64% and 51.63%,respectively,compared with parallel control group.The residual shear strength after 3 months of restoration was 9.83,6.12,4.93 times of that of plain soil,decreased by 5.04%,40.86%,52.41% compared with the control group,respectively.After 5 months of repair,the residual shear strength was 9.18,6.07 and 5.32 times of that of plain soil,which decreased by 10.78%,41.02% and 48.29% compared with parallel control group,respectively.For the same degree of damage,the longer the self-repair time,the higher the survival rate;for the same degree of self-repair time,the heavier the damage degree,the lower the survival rate.(5)The direct shear friction test of root-soil complex showed that the cohesion of Medicago sativa root-soil complex was 7.82 KPA and the internal friction angle was33.91 ° at the natural moisture content of 8.50% and the soil dry density of 1.50 g/cm3,the coefficient of friction is 0.62.With the increase of soil water content,the cohesion of root-soil complex increased first and then decreased,and the internal friction angle and coefficient of root-soil complex decreased.(6)The indoor drag test of single pull shows that: The pull force of Medicago sativa was linearly positively correlated with the increase of root diameter,while the pull shear strength of Medicago sativa was negatively correlated with the increase of root diameter.Under the representative root diameter class of Medicago sativa(1-1.5mm),the average pull force of Medicago sativa was 23 N and the average pull shear strength of Medicago sativa was 97.65 Kpa.With the increase of soil moisture content,the single pull strength of Medicago sativa first increased and then decreased,and the peak value appeared when the moisture content was 10.86%,and the value of Medicago sativa was 97.65 Kpa.The peak shear strength of single pull strength of Medicago sativa in loess was 3.39 times that in sandy soil... |
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