Study On The Rock-Soil Characteristcs Of Sheat Strength And Distintegration Of Granite Collapse Mound In Southestern Hubei

Collapse mound is a special and great dangerous soil and water erosion phenomenon in southern China, which is taken place by an interaction of water power and gravity.It’s the most important cause of deterioration of ecological environment and can brought great damage to the national economy and the safety of people’s lives and property.Researchers conducted a lot of research about the collapse mechanism and erosion control measures, and made certain findings,But the erosion mechanism of collapse mound is not completely clear,although there were a lot of studies about the relationship of shear strength,disintegration and water content,still less applied to the analysis of the mechanism of collapse mound.The reason of the soil unstability is largely due to the decline of soil shear strength and the increase of disintegration,in this process,changes in soil moisture plays a big role.Meanwhile,from the field observation,found that collapse mound often occur in sunny collapse,and the Wet and dry cycle effect of sunny collapse is greater than the shady.So presumably wet and dry cycle effects will promotethe occurrence of collapse mound.In this experiment two activity type collapse mounds were selected in Tongcheng county, Hubei province with field investigation and laboratory experiments. The measuring items were the basic geotechnical characteristics of collapse mound, the relationship of soil shear strength and water content, the effect of initial water content on the disintegration about the collapse mound, the destruction of alternation of wetting and drying on the stability of the soi.The main conclusions are as following:1.The basic geotechnical characteristics are slight difference between BG1(collapse mound number1) and BG2(collapse mound number2),The basic geotechnical properties are quite different in the layers of a soil profile of collapse mound a. The value of bulk density in soil profile of collapse mound is:red soil layer> transition soil layer> sandy layer> toplayer> debris layer. Total porosity and non-capillary porosity in debris layer is the maximum of the layers, red soil capillary porosity is the largest of the layers. The variation of clay content is:red soil layer>toplayer transition soil layer> sandy layer> debris layer..The changes of soil moisture boundaries of each layer are great similar to the clay content, the highest of which is in red soil layer, other soil layer decreases from top to bottom. by measuring the indicators of soil chemistry of collapse mound, the pH value is increasing with the increasing of soil depth, and the overall are acidic; Changes of organic matter consistent with pH the red soil layer’s cation exchange capacity and free iron content was the highest.2. Soil shear strength of each layer is great impact by water content, totally speaking soil shear strength tended to decrease with the increase of water content,, but the margin of the decline of different soil layers are not the same. By the experiments of soil shear strength:firstly the shear strength of surface soil was increased with the water content increased from3.12%to14.21%, cohesive force increase from59.62KPa to96.51KPa in BG1, when the water content continues to increase until close to saturation, cohesion start decreased rapidly to3.56KPa, while the water content changed from2.45%to11.26%the cohesive force increase from45.39KPa to78.69KPa in BG2. When the water content more than11.26%, the cohesion began to decline, when the water content close to saturation, cohesion just is5.36KPa. The angle of internal friction is showing a declining trend with the water content increased, The intensity of cohesion attenuation by the water content is very large in red soil layer, when the water content change from5%to35.53%, the cohesion may have been dropped from122.52KPa to3.65KPa in BG1, a slight increase when the water content continues to increase near to the saturation. There is a similar trend in BG2, the cohesion decline from137.15KP to5.39KPa as water content changed from7.82%to saturation,. The tendency of Clastic’s shear strength decreases gently with water content changed in transition soil layer, the including friction angle reduced with water content decreased in sand layer and debris layer.3.The disintegration disparities are quite different in each layer of collapse mound. The disintegration intensity was significantly higher in sand layer and debris layer than the surface soil layer and red soil layer, but the disintegration most affected by the initial moisture content in transition soil layer. the amount of the disintegration is only8.89%at34.06%of soil water content in BG1, the amount of the disintegration when the water content reduced to6%. There is a similar situation in BG2as in BG1, but the amount of disintegration can reach to100%when the water content reduced to9%, when the water come in to the dry soil, the amount of disintegration can reach100%in a short time in the red soil layer in BG2. While the top soil layer and the red soil layer in the BG1, with the reduction of the initial moisture content, the amount of disintegration will increase, but overall, the disintegration of the weak. So, from the perspective of disintegration characteristics, the disintegration strength of the sand layer and debris layer are largest,most affected by the initial moisture content of the transition layer.Thus, as long as water enter the body collapsed gully subsoil,and the erosion strength will increase greatly.4.Alternation of wetting and drying have a big impact on the overall stability of the collapse mound. soils produce fissures with alternation of wetting and drying, especially in the first two times of alternate of wetting and drying. the developing of fractures is more broad and deep. and form the main fissures. the small cracks are gradually produced on these major cracks after others loop. the soil deformed mainly in the vertical direction with alternation of wetting and drying in debris layer,. Soils become looser with more times of alternation of wetting and drying After four times of alternation of wetting and drying, the soil fissures ratio occurred in layers of callapse mound is:the red soil layer> transition layer> top soil layer> sand layer> debris layer.. The fissures ratio can reach9.78%in red soil layer. Since the development of fissures the soil shear strength will continue decay when increasing the times of alternation of wetting and drying and the disintegration become more easy.