The Affect Of Different Scales Of Environmental Factor On The Collapse Distribution And Developmental Processes

The collapse is the most harmful soil erosion type in South China, which is a special type of soil erosion with the combined action of hydraulic and gravity effect on a hilly land, with main distribution in tropical, subtropical granite in red soil region. The formation of collapse is subject to many natural conditions. In the granite weathering crust development area, with vegetation destruction, surface erosion aggravated, and the hollows formed by sheet flow quickly evolve into the gully. With the deepening and expansion of gully, it may incise into a certain depth to a steep. The runoff on slope transformed to the waterfall flow in their steep. The waterfall flows severely destruct the soil underneath, forming sputtering pit in the sand layer quickly and gradually developing into a niche. Soil on niches saturate with water, under the gravity effect it will collapse to form the embryonic form of dilapidated granite. With the influence of surface runoff and waterfall flow new niches come into being, and collapse may take place once again. Such repeated occurrences contribute to the formation of collapse.This study, taking collapsed gully in Hubei province as the macroscopic scale, Yang Long small watershed gully in Tong Cheng county of Hubei Province as mediumscopic scale, and two typical collapse mounds in Wu Li and Cheng Feng in Tong Cheng county of Hubei province as a microscopic scale, conducts three layers of research on the distribution and formation mechanism of collapsed. The conclusion can be achieved as following:1. Collapse distribution in Hubei province has close relation with geological conditions, topography, climate conditions and land use types (human factors). They mainly locate among the red clay developed with maternal granite, in regions with elevation of 100～500m, slope of 10 to 30 DEG in hilly areas, annual rainfall of 1500mm, and annual average temperature of 16.5 DEG. The collapse are in the form of arc, gourd shape, strip, claw and mixed form, in which 85% of the collapse mound is active collapsing type.2. There are 19 collapses distributed in Yang Long small watershed in Tong Cheng County of Hubei province. The collapse distribution is closely related to landform, grade and slope of the watershed; land use types and regional geological structure and joints. There are mainly six types of land uses in Yang Long small watershed:dam, residence, dry land, forest land, paddy field and pond, etc. There are 10 collapses evenly located on both sides upstream,4 collapses on just the west side in the middle reach of the river, and 5 collapses on both sides downstream in the watershed. And the collapses are mainly located near the paddy field or near ponds,The collapse follow a regular pattern in the vertical spatial distribution:collapses in Tong Cheng are mainly located in low hilly area with from 98m to 235m, and no development of collapse has been found in elevation lower than 98m and higher than that of 255m.3. The typical collapses in Wu Li and Cheng Feng in Tong Cheng County of Hubei province are increasing as for the collapse area and erosion quantity of collapse in the time sequence, but because of installing experimental facilities there, the collapse development is slowed down. Therefore, appropriate engineering measures are beneficial for treatment of collapsing hill.4. Through rock-soil anti-shear strength weakening law analysis in different wet and dry effect in the collapse zone, conclusions can be drawn as the following:(1) For drying stage, the impact degree of anti-shear strength follows in descending order as:plasticity index> free iron oxide> clay content less than 0.002mm> water soluble salt> dry density; in the moistening stage, the impact degree of anti-shear strength follows in descending order as:dry density> water soluble salt> clay content less than 0.002mm> plasticity index> free iron oxide.(2) before the natural moisture content, fissure is the control factor of anti-shear strength for red clay; and after natural water content, matrix suction is the main factor influencing anti-shear strength for red clay.