| Collapse mound is a special phenomenon of water and soil loss in South China. The collapse mound can cause the water and soil loss and damage the land, which damage the national economy, the people’s lives and assets greatly. This test was held in two typical collapse mound areas and one collapse mound which just started to governance, from different indoor and outdoor tests, the basic physical properties, the shear strength and the resistance situation of different-layer soil in weathering crust the resistant spatial distribution and dynamic change of water in sidewall of collapse mound and the water characteristics at different soil layers of collapse mound plot. Linked up with analysis of force of collapse mound’s different layers, the conclusions can be drawn as follows.(1) The differences of different soil layers in granite collapse mound sidewall are obvious. From the research, it is found that soil layer of collapse is deep, texture of outer layer is viscosity-gravity, which thickness is more than450cm, and clay content is more than20%; the lower layer is relatively lighter, thickness is over200cm; sand content in two collapse mound areas are both more than70%; the water binding capacity in soil of upper layer is strong, which water content of capillary is around45%, while water content of capillary in lower layer is less than40%; the soil porosity in upper layer which is larger than15bar relative aperture is more than15%, while it is less than8%in the lower layer.(2) The happen of granite collapse mound depends on a special soil body configuration, which upper area is formed by viscous, tight and heavy layer with alleviated parent material, while the lower area is unconsolidated efflorescence from weathered granite. Differences of soil resistance and shear strength between various layers are obvious. When soil is under saturation state, viscosity-gravity surface layers’ cohesion reaches19200N/m2and internal friction angle is21.06°; While lower layer which texture is lighter, its cohesion is only6470N/m2and internal friction angle is24.29°. When soil is under humid state, surface layer’s cohesion is11363N/m2and internal friction angle is24.24°; lower layers’ cohesion is20716N/m2and its internal friction angle is24.29°. When soil is under dry state, surface layers’cohesion reaches68950N/m2and internal friction angle is49.03°; while lower layer is only2630N/m2and internal friction angle is29.34°. Viscosity-gravity surface layer in the normal condition can protect the lower layer, collapse mound, a special water and loss form, only happens when collapse limit is reached.(3) The limiting balance formula of soil collapse mound is proposed initially. According to interaction between various forces of soil and principle of balance, based on soil thickness, bond force, internal friction angle, bulk density, water content, formula is derived as follow:According to the tested soil characteristics of collapse mound area, soil collapse mound limit of weathered crust’s different layers under three circumstances are analyzed and distinguished by the formula, mechanism of why collapse mound happen is explained in certain level. When soil is under saturated condition, collapse mound would happen directly based on the theory. When soil is under humid condition, the limit depth of theoretical collapse mound in this experimental area’s layer B and C is25.64m; while when soil is under dry state the limit depth of theoretical collapse mound is0.67m.(4) Comment the existing managing measures in the collapse mound area. Collapse mound, which happens in granite area, is greatly related to the wetness of soil. In the managing process, decrease the level of water that infilter into collapse mound at a certain level, and avoid the erosion caused by surface runoff. Cutting slope can exposure C layer directly, which makes C layer be destroyed easily. The stability of C layer is a key factor for collapse mound to collapse or not. After cutting slope, C layer must be protected effectively. |
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