Studies On Distribution Characteristics And Erosion Mechanism Of The Collapsing Hill Of Yudu County

The collapsing hill is a special form of soil and water loss in southern hill side area of China. Yudu county is one of the most seriously soil and water loss part in jiangxi province.The areal water and soil loss is 843.82 km2 and the moderate water and soil loss dominates more than 64.01% of the total loss, especially comes from the collapsing hills. This research focused on studying the weathered soil of tuffaceous sands, granite and glutenite in collapse district in Yudu Jiangxi province by investigating the soil type of the collapsing hill, the type of the collapsing hill, joint number and directions, concave and convex condition of slope surface, altitude, slope, slope direction, then we combined the measured results of soil moisture content, bulk density, porosity, density, saturation, grain size distribution, soil water stability of soil aggregate, liquid plastic limit, organic matter content, soil disintegration test to study tentatively the mechanism of the collapsing hill in Yudu Jiangxi province. The results are as follows:(1)There were 4062 spots which had formed the collapsing hill, mostly of them were in exuberant growth period of erosion. The types of the collapsing hill were primarily strip, gourd ladle and hybrid. Most of the collapsing hill developed in red soil area which was developed from granite at altitude of 200 m, slope 10-30°. The collapsing hill was distributed in all 23 villages and towns of Yudu. The spatial distribution characteristic of the quantity and area of the collapsing hill was the north more than the south, the southeast more than the northwest.(2)Through the investigation of the collapsing hill developed in tuffaceous sands weathering crust in technology demonstration garden of Jinqiao village in Yudu county, we know that: the averaged joint density was 31 strips per one meter. The advantageous bearing of joint was NW- SE, NE – SW, the bearing of NS in the secondary position. The trend of joint was basically identical with the number decreasing direction of the collapsing hill distribution. The structural joints developed from tuffaceous sands were mainly high dip angle, distributed in 60 ~ 90 °. The joint developed in tuffaceous sandstone had the characteristics of intensive, complex of direction, depth of fragmentation and inheritance.The direction of joint developed from collapsing hill was agreement with that of collapsed hills.(3)The collapsing hills developed in granite and tuffaceous sandstone didn’t have the duanbi structure which collapsing hills developed in glutenite had. Through the comparison of joint, we found that the joint in glutenite are rather poorly developed and soil wall is complete. The joint in tuffaceous sandstone is rather developed, in granite is secondary and the joint in glutenite is rather poorly developed.(4)The bulk density of tuffaceous sands overall performed that upper soil < lower soil. The initial infiltration rate, the steady infiltration rate, average infiltration rate and infiltration volume of granite weathered soil were greater than tuffaceous sands weathered soil. The permeability rate of tuffaceous sands lower soil was faster than the upper soil.Soil permeability of granite was better in intrusive period. The infiltration amount of tuffaceous sands weathered soil had a power function relationship with time. The upper soil was y = 0.0197t0.9275. The lower soil was y = 0.0393t1.018. The soil infiltration characteristics of the area of the collapsing hill had a correlation with capillary porosity(x4),non-capillary porosity(x5), > 0.25 mm water stability aggregate content(x8), dispersion coefficient(x9), reunion degrees(x11).(5)The liquid plastic limit of tuffaceous sands was higher than that of granite and glutenite. The soil clay content of tuffaceous sands increased from the upper to the lower.The liquid limit, plastic index of tuffaceous sands increased with the increasing amount of soil clay content. The liquid plastic limit of granite was associated with soil clay content,the higher the content of clay soil, the bigger of the value of soil liquid plastic limit. The organic matter content of tuffaceous sands weathered soil had a binomial relation with both liquid limit and plastic limit with the correlations greater than 95%.(6)The tuffaceous sands has a high content of montmorillonite. The weathered soil of tuffaceous sands can collapse completely in dry conditions in 10 min. The disintegrating quantity of tuffaceous sands increases as the content of initial water decreases, which strongly verify that tuffaceous sands is easy to collapse, especially after experiencing the state alternation of dry and wet and having a weak water storage capacity.