| During the construction of highway,excavation of soil and stone produces a large number of embankment and cutting slopes,which leads to serious problems such as surface exposure,soil erosion and ecological environment damage.In recent years,the construction of green flexible protection system for embankment slopes has developed rapidly because of the improvement of environmental protection awareness.Due to the complexity of root-soil interaction,the design calculation of green flexible protection system mainly stays on empirical model and qualitative analysis level,and the theoretical knowledge lags far behind the development of engineering construction.In this paper,the effect of root content on soil shear strength parameters was quantitatively analyzed through tensile and direct shear tests,and the existing theoretical calculation model was modified.A simple calculation model for the root-permeated soil shear strength increment was also proposed.In addition,the effects of root reinforcement,anchoring and water uptake on slope stability were analyzed by finite element method.The main research contents are as follows:(1)Single root tensile tests were carried out on four types of root samples,namely,Solanum nigrum Linn.,Chenopodium album Linn.,Sonchus oleraceus Linn.,Cynodon dactylon Linn.The stress-strain curve and tensile strength of single root were obtained and compared.The highest tensile strength is Chenopodium album Linn.,followed by Cynodon dactylon Linn.,Solanum nigrum Linn.and Sonchus oleraceus Linn.The tensile strength and diameter of plant roots are negatively correlated by power function,namely,the larger the diameter of a single root,the smaller the tensile strength.(2)Direct shear tests were carried out on undistributed root-permeated soil under three different water contents.The effects of root content on shear strength and strength parameters were quantitatively analyzed.The experimental results were compared with the calculated values of Wu model,and the theoretical model correction was proposed.In addition,the spatial distribution of plant roots was measured,and the simplified calculation method of root-permeated soil shear strength increment with depth was derived.The results show that cohesion of composite soil increases linearly with cross-sectional area ratio of root system,while the friction angle remain constant.When designing the engineering highway slope,the K value in the Wu model should be corrected to 0.2.Based on the normalized formula describing root spatial distribution,the root-permeated soil shear strength increment at any depth can be deduced by the simplified calculation formula derived in this paper.(3)Considering the variation of plant root distribution with depth,a piecewise variation function of additional root cohesion increment was defined.And the influence of root type(bare,herb,shrub,tree)and slope geometry(height,slope)on slope stability were analyzed respectively.The result indicate that tree has the largest strengthening effect on the slope because of the larger value of additional cohesion and influenced depth.The safety factor of vegetation slope decreases with slope height in power function,while that decreases linearly with slope gradient.(4)Considering the root structure and spatial distribution,threedimensional finite element models of root-soil interaction were established,and slope stability analyses were carried out.The results show that slope can be effectively strengthened when plant root passes through the potential slip surface,but there is no obvious reinforcement effect after a certain value.When the angle between the root system and slope is between 80° and 120°,the vegetation slope has the best strengthening effect.The root system located in the lower part of the slope was more effective than that in the upper part,and the effect of staggered distribution was more obvious.(5)By defining the shape functions of four different roots and adding them into the seepage equation of unsaturated soil slope,the influence of root uptake time,shape and depth of root on the distribution of pore water pressure were investigated.The safety factor of slope stability was also calculated quantitatively.The results show that the envelope shape of pore water pressure and the safety coefficient ratio of four plant roots different morphological are consistent with the distribution of shape function.In the case of the same root content,the greater root depth,the greater influenced range of root water uptake,while the corresponding shallow horizontal root content,the less improvement effect on soil shear strength and smaller increment of slope stability. |
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