Research On Stability Of Collapsing Gully Wall In Granite Region Of South China

The collapsing gully erosion is the main important and specific soil erosion type in the hilly granite region of tropical and subtropical South China. Due to granite weathering crust have deep soil thickness with coarse texture and loose structure, meanwhile accompanied by a large number of joints and fissures and with unstable mechanical properties. The differences of geotechnical characteristics in each soil layer are significant. Therefore Collapsing gullies are widely distributed in this region, and large-scale erosion cause serious soil and water loss and serious damage to farmland and water conservancy facilities, etc. Carry out the researches of geotechnical properties and its relationship with stability of collapsing gully wall will have important significance for the further understanding the instability machinism and the control of collapsing gully.In this paper, four collapse gullies were selected from the four counties Tongcheng County(TC), Gan County(GX), Anxi County(AX) and Wuhua County(WH) in the hilly granitic region, taking the four soil layers(surface soil layer, red soil layer, sandy soil layer and detritus layer) of the collapse gully wall as the study objective. According to the thickness of each soil layer, 1-3 sampling points were set in each soil layer, and 6 soil sub-layers were collected in the TC collapsing gully(no detritus layer), 8 soil sub-layers were collected in the other three collapsing gully. Based on field survey and data collection and lab soil analysis, the environmental characteristics of the collapsing gully erosion regions are counted. We analyzed the soil physical-chemical properties, soil particle size distribution, soil disintegration, soil infiltration and shear strength under different water content. The material basis for the development of collapsing gully, the soil disintegration mechanism and the influence of infiltration characteristics on the stability of the collapsing gully wall was obtained. Under different soil water content, the decay mechanism of the shear strength and collapsing critical height and stability safety coefficient of the different soil layers was also studied. The main results are listed as following:(1) The lower the latitude, the numbers and erosion areas of the collapsing gullies are increasing, and the average area and density of the collapsing gullies is also increasing. The lower the latitude, the proportion of the mixed type collapsing gullies show an increasing trend and the numbers of the active collapsing gullies increase dramatically. On the whole, the lower the latitude, there are more collapsing gullies formed. Meanwhile the lower the latitude, the vegetation in the collapsing gully erosion area become sparser, and Pinus massoniana is dominated in the tree layer and understory vegetation is the Dicranopteris linearis in the four collapsing gully erosion regions.(2) The coarse soil particles are higher in the sandy soil layer and detritus layer than those in the surface soil layer and red soil layer, but the finer soil particles show the opposite trend. With the declining of weathering degree(from surface layer to detritus layer), contents of each size fraction in the four collapse gullies show the same changing trend, especially the sand content and clay content. In addition, with the increasing of soil depth(from surface layer to detritus layer), soil fractal dimension(D) of different weathering profiles in the TC, GX and WH collapse gullies decreased, from 2.851 to 2.723, 2.880 to 2.732 and 2.845 to 2.717, respectively. But in the AX, the average of fractal dimension value of the red soil layer was the highest(2.91), and then declined to 2.67(detritus layer). The result of linear regression analyses and second degree polynomial regression show that fractal dimension have a remarkable negative correlation with the content of 2.0-1.0mm, 1.0-0.25 mm, 0.25-0.05 mm and 1.0-0.05mm(P<0.01), a strong positive correlation with clay content(p<0.01).(3) The sandy soil layer and detritus layer prone to disintegrate, but the surface soil layer and red soil layer have higher disintegration index(Kc) value than that in the sandy soil layer and detritus layer, so it is not easy to show disintegration. Soil is easier to disintegrate under the air-dried condition than the soil under the natural state condition. There is a remarkable positive correlation between the disintegration index(Kc) value and the content of <0.002mm(p<0.01), but negative correlation was found between the Kc value with the content of other particle size. The soil disintegration mechanism under the different soil water content and the difference of soil particle size distribution was studied. Under the lower water content, the main controlling factors to soil disintegration mechanism have been regarded as effective void ratio inside the soil. But with difference of soil particle size distribution, the content of the cementing material like clay and organic matter control the soil disintegration process, the less cementing material, disintegration rate is higher.(4) With the increasing depth, the total porosity of the different soil layers of the four collapsing gullies is reduced after the first increase, but the capillary porosity and non-capillary porosity variation with depth show opposite trend each other. Non-capillary porosity is lowest in the sub-layer of the red soil layer and sandy soil layer. The cumulative infiltration and infiltration rate of the surface soil layer are significantly higher than those of the other three soil layers, and red soil layer is higher than sandy soil layer and detritus layer. The result show that in the middle layer of the collapsing gully wall(cross position of the red soil layer and sandy soil layer), the cumulative infiltration and infiltration rate are lower than in the other layers, this indicate that a relatively impermeable layer locate in the collapsing gully wall. Horton model is the best model to simulate the soil infiltration process, followed by Kostiakov model, while soil infiltration process does not comply with the Philip model.(5) In the WH collapsing gully, the soil cohesive force(c) of the surface soil layer is higher than that in the other soil layer, but in the TC, GX and AX collapsing gully, the soil cohesive force(c) of the red soil layer is higher than that in the surface soil layer and strongly higher than that in the sandy soil layer and detritus layer. On the whole, with the increasing the soil water content, soil cohesive force(c) and soil internal frictional angle(φ) of the surface soil layer and the several sub-layers of the red soil layer decrease, but the c and φ of the other layers increase firstly and then decline. In the air-dried stage, the soil cohesive force(c) and soil internal frictional angle(φ) show relatively close relationship with coarser and finer particles, and the correlation coefficient with <0.002 mm particle content is higher than those with the other size particle content. In the higher water content condition, there was a significant correlation between soil cohesive force(c) with the content of the most of particles, but the correlation coefficient to the soil internal frictional angle with the each sizes particles is not very high.(6) The decay mechanism of the shear strength of the surface soil layer and red soil layer is as follow: in the air-drying stage, crack evolution is the dominant factor affected shear strength declining, while in the humidifying stage, the dissipation of the strengthening cohesion and adsorption cohesion between soil particles is the main factor for shear strength decline. The decay mechanism of the shear strength of the sandy soil layer and detritus layer is as follow: in the air-drying stage and natural state, matric suction was the main factor controlling the shear strength. While in the humidifying stage, the dissipation of the strengthening cohesion and adsorption cohesion between soil particles is the main factor controlling the shear strength.(7) Assumed collapsing gully wall as upright slope, the collapsing critical height formulas was used to calculate the collapsing critical height under different moisture content, the results show that surface soil layer will not collapse in each moisture conditions, and the red soil layer will collapse when the moisture near saturation, but the sandy soil layer and detritus layer are prone to collapse with slightly humidifying based on the natural condition. In this paper, the calculation formula of the stability safety coefficient(K) of different soil layers in the two cases(cracks presence and no cracks presence) is presented. The collapsing critical height and stability safety coefficient(K) show relatively close relationship with the soil water content, and it decrease with the soil water content increase. Results also show that cracks presence can reduce the stability of the collapsing gully wall and largely increase the risk of collapsing.