Research On The Stability Of Well Vegetated Slopes

Landslide, Debris flow and other geological disasters was widespread in the world, that the impact for the safety of people’s life and property can’t be ignored. The effect of gravity, hydrological condition, earthquake force, human activities, geologic forces and other factors that slope is changed constantly, so that the slope body original stress state is changed, leading to different forms of soil deformation and failure.This paper carries on the quantitative analysis for slope stability, combining the factors such as vegetation slope hydrology, rainfall infiltration, evaporation, interception, vegetation types, vegetation roots, topography&slope. Based on understanding and mastering of vegetation slope deformation and failure law of development, the effect mechanism and characteristics of vegetation to slope stability are studied, the penetration process and mechanism that rainfall on slope unsaturated zone are studied, the mechanism between the plant and its roots and slope are fully understand, the limit equilibrium state of slope under vegetation and water-rock, rock and soil interaction when vegetation slope in rainfall reaches a value (critical rainfall) are studied, the contribution and effect for vegetation to the slope stability are analyzed. Therefore, this paper focuses on the following:(1) The effect of vegetation roots and organic soil in unsaturated zone for the water permeability is studied, the water’s infiltration path and characteristics in the soil is analyzed. It’s interactions of the soil particles, granular structure with the vegetation root, worm’s secretions, and microbial in the unsaturated zone, some exhibit hydrophilic, while others appear to be hydrophobic. This will lead to the flow path is inconsistent in the soil, making hydrophilic aggregate together more water, and hydrophobic aggregates won’t have too much moisture penetration even the soil is dry, so that the water content is not consistent of the slope.(2) Applied scanning electron microscope (SEM) technology, analysis of soil aggregate structure and pore structure. From the graph of SEM:the soil structure near the surface of slope is very loose, containing a large number of litter, vegetation roots and organic aggregate structure, there are a lot of the fractures and pores that connectivity and variable shape, extending into different directions. Below the surface of3-8cm showed a large amount of aggregate structure, between the aggregate yielding some irregular, in different shapes of the pores space, some with trend of development, form a mesh structure channel, and visible detritus hollow bore of the root, between the lateralof root and soil have obvious gap, compared the surface soil, it’s structure to be densified. Below the surface8cm also have a large number of aggregate structures, around the aggregate structure emergence a large number of interconnected pores, and have some roots and large channel that the root rot formation, root hole and the soil have obvious gap, compared the upper layer of soil is dense.(3) Observation of the distribution form, scope, depth and effective interaction depth of vegetation roots in the soil. Under the action of water, the effect of vegetation roots to slope stability is analyzed. Under the condition of rainfall, the mechanism and characteristics of the vegetation and the rock-soil of slope, the interaction and characteristics of vegetation roots and water-rock, rock-soil are studied. Under certain rainfall conditions, if the rainfall infiltration reached a certain level, although the vegetation roots can be reinforcement the slope in a certain depth range of slope, the number of vegetation roots to the potential sliding surface of slope is less, the interaction between vegetation roots and soil has weakened, vegetation is a sliding force that made slope slid, the higher of the plant that it’s self-weight is the greater, which increased the slope body load, and transfer the wind load to slope, under the action of wind load and plant self-weight, vegetation exists only exacerbated the slope sliding, increase the slope instability.(4) Evapotranspiration and interception of the vegetation slope during rainfall are studied, monitoring and calculating the evaporation and interception of vegetation during the rainfall, the effect of vegetation for slope hydrological is analyzed, the basic law of rainfall water cycle is analyzed, and the effect of the runoff and infiltrate of the factor, such as pore water pressure, porosity, permeability coefficient and other factors of vegetation slope are analyzed. The effect of vegetation interception reduces rainfall intensity and effective rainfall that reach the surface, the hit chance and the splash erosion to slope is reduced greatly, that is important to the erosion control and protection of slope.(5) The rainfall infiltration process and mechanism, wetting front geometry and migration process in the unsaturated zone are studied. During the rainfall, rainfall infiltration and redistribution in unsaturated soil, the redistribution process depends on the soil moisture, water pressure, and unsaturated permeability. At the beginning of rainfall, soil with high penetration ability and all the rainfall infiltration into soil. With the process continues, the soil infiltration capacity decreased gradually, until the infiltration rate to reach a stable value. As the soil is saturated with the rainfall infiltration then promote wetting front, if rainfall intensity is higher than the soil hydraulic conductivity that lead to large amount of rainfall into the soil, underground water level will rise, the soil suction is reduced, increasing water content, the strength of the soil is reduced, increasing unit weight of soil, so that increasing the shear stress of slide surface. Along with rainfall time that rainfall infiltration is increasing gradually, the pore water pressure is rising gradually, the slope stability coefficient is decreasing gradually. In the long term infiltration, surface instability is due to the presence of the positive pore water pressure or negative pore pressure and suction to reduce. When the rain infiltration through the unsaturated zone, the surface of the slope near wetting front could lead to instability in the long-term rainfall, which is usually parallel to the slope. (6) To establish the corresponding slope hydrological stability model, Combining with field investigation and monitoring data, and the main features of the slope rock-soil mass, topography&geomorphology, rainfall intensity and duration, underground water level and other factors, the stability coefficient is less than1of the potential sliding slope unit number and the critical rainfall for the slope unit are calculated, Based on digital elevation model (DEM) could realize the visual analysis and prediction of slope stability. Slope instability in relation to rainfall, also in relation with the rainfall duration and the processes. In a situation that rainfall great but short duration, a part of the rainwater could not translate into groundwater flow, produce surface runoff, which on landslide contributed little, not necessarily a landslide. Another situation is that rainfall is small, but longer duration, rainfall intensity is lower than the soil infiltration capacity, rainwater can full infiltration into soil, within a certain period of time the total penetration is larger, may be produce landslide. Rainfall intensity and duration decided to rainfall infiltration quantity, which influence the slope stability. The calculation results show that the slope unit which located in the Chu-Meng highway beneath subgrade in the rainfall range of0-50mm/d has become a whole, the location may be integral sliding, While other unit of the slope is not connected, due to the mutual restraint effect between the soil of slope unit, does not have a large area of the slide, but local unit generates the surface spalling phenomenon. Potential landslide unit number with amount of rainfall assumes the trend of escalation, steep terrain landslide probability.(7) Considering the evapotranspiration, interception, and the effect of rainfall infiltration, under different rainfall conditions simulation the slope potential landslide unit by the rainfall threshold model, the simulation fits the real slope instability cases very well, the anastomosis rate with the actual landslide position more than80%.It is feasible that predictive simulation the required critical rainfall when the slope generating potential sliding and slope stability analysis by rainfall threshold model. Monitoring to the surface and underground displacement lag the duration of rainfall, with the rainfall duration increasing, the groundwater level of the monitoring borehole is rising, and the groundwater level rise time lags the rainfall time.Finally, summarizes the research results. Due to the slope that generates landslide and debris flow most is the vegetation slope, vegetation slope stability under rainfall condition is studied that has the certain practical significance, for the prediction, prevention landslide, debris flow and other natural disasters to provide the corresponding theoretical basis.