The effect of bermuda grass root morphology on the displacement of slope

: During recent years, with the drastic development of highway, lots of slopes are produced which cause the increase of geological disasters including soil erosion on the slope and shallow sliding surface, it makes significant impact on the ecological environment. Adopting ecological protection technique of plant slope protection not only can prevent geological disasters, but also benefit the protection of ecological environment. In the paper, bermuda grass, as one kind of ordinary slope protection plants, was chosen as specimen, and the root-soil composite was regarded as organic system which consists of soil and root system; the effect of root morphology on displacement of slope surface under the function of rain erosion and slope runoff erosion was studied and analyzed by using nonlinearity finite calculation method. The result indicates Bermuda grass root system can defense the rain erosion of slope surface soil, restrain the soil displacement and prevent water and soil erosion of slope .


Introduction
Traditional slope protection generally use geotextiles protection measures such as bolt shotcrete or dry pitching, but these measures totally confine the growth environment of plant while protecting the slope [1][2][3], this makes the original plant on the slope unable torecover in short period of time and destroy the ecological environment [4][5][6][7], besides, geotechnical structure always produce weathering and shedding over time which induce the decrease of strength, so the performance of protection becomes worse and worse.
The ecological protection technique of plant slope protection not only can benefit the slope stability, but also can be good to the ecological environment preservation [8][9][10], so it is applied to gradually replace the traditional technique to protect slope and conduct greening [11][12][13]. In the paper, the bermuda grass root system was used as slope protection plant, the engineering simulation finite element software, ABAQUS, was applied to conduct digital analysis, this software can be used to work out problems range from comparatively easy linear analysis to complicated nonlinear analysis, meanwhile , it has abundant unit database that can simulate any geometrical shape so as to tackle the issues about strain and displacement of slope. The effect of root morphology on displacement of slope surface under the function of rain erosion and slope runoff erosion was studied and analyzed by using ABAQUS, this makes it feasible to quantitively analyze soil solidity of root system and provide the effective proof for the plant choice of ecological slope protection [14][15][16][17][18]].

2
Numerical model size and parameters

slope model and bounding condition
The model was simplified to be convenient for calculation and the slope soil was assumed to be same and isotropic, the model of slope is showed in Fig.1 and

model of root system
The bermuda grass root system was simplified in numerical calculation, the main root and fibrous root are treated in equal diameter, the model of root system is ideal elastic model of flexible pole element which can only sustain tensile force but the press force. The diameters of main root and fibrous are 5mm, their length was considered as 200mm, 300mm, 400mm, the respective model is showed in Fig.3.

model of root-soil composite
The root-soil composite was regarded as an integrate in the calculation without considering the sliding between soil and root. The root system and soil were set as materials with different parameters. It was assumed that the soil and root system connected with each other directly and the displacement coordination was realized automatically. In the model, the root system was adopted as liner structure because the root system is regarded as pole element. The interface between soil and root system was tackled with Embedded region in the bounding constraints order in ABAQUS, the root system was set as Embedded Region embedded into surrounding soil.

Result and analysis
In the condition that the diameter of rain point is 5mm, the flow speed of rain point is 9.11m/s, the flow depth of surface water is 100mm, the effect of different arrangement density, different length, and different amount of fibrous root of root system on the horizontal and vertical and total displacement of slope surface was investigated. The finite element displacement filed cloud map without implanting root system is showed in    In order to reflect the effect of arrangement density of root system on displacement of slope surface soil better, the differences in displacement between slope without root system and slope which was implanted with bermuda grass root system with 7 strains of fibrous root whose arrangement density of 10 strains per meter and 15 strains per meter were compared and analyzed. The variation curves of horizontal, vertical and total displacement were drawn out., showed in Fig.12, Fig.13, Fig.14.  After fitting and analyzing the data above, it was indicated that it was more accurate to regress the relation between fibrous root length of bermuda grass root system and difference in vertical displacement of slope surface by using linear fitting method. The fitting formulas are as follows:

Conclusion
By simulating the coaction between soil and bermuda grass root system with the finite element numerical calculation software, the effect of root system on the stability of soil were described quantitatively, the study result shows: (2) The length of bermuda grass root is short, mainly in the range of 39cm,the soil solidity performance of root system is performed within the sphere of influence of root system, but the amount of bermuda grass fibrous root id large and the elastic modulus of root system is much bigger than that of soil, so with the increase of amount of fibrous root and plant density, the cohesive force of soil increase, forming the reinforcement effect to soil.
Especially in the condition of raining, the exist of (3) Because of spatial distribution and the complication of distribution pattern of plant root system, in this paper, the root morphology was simplified in the process of establishing model of root system, the actual root morphology is more complicate than that of simulation model. The problem of how to consider the actual distribution morphology of plant root system and real spatial features of slope soil is worth of being studied further.