Research On Formation Mechanism Of Tension Crack In Gentle Terrane Slope Of Red Bed Region At Nanjiang Sichuan

In the rainstorm on September 9th, 2011, large-scale red-bed rock landslide was induced in the Territory of Nanjiang County, which resulting in severe property damage and casualties. The red-bed rock landslide in Sichuan, according to different dip angle of terrine, can be divided two types as near-horizontal terrane landslide and low-angle-dip terrane landslide and they are collectively known as gentle terrane landslide. Predecessors have conducted wide research about gentle terrane landslide. They think that the gentle terrane landslide is jointly induced by hydrostatic pressure of trailing-edge crack, uplift pressure of bottom slip surface and sliding soil softening after the rainfall infiltrates into dominant structure plane. However, currently, there is still no in-depth research about the preferential migration passage of rainfall infiltration---the tension crack of middle and back gentle terrane slop. As for this problem, the author dares to assume that the formulation of the sand inside the slop body and the tensile fracture in mudstone is mainly caused by the transformation of rock mass structure by ground's tectonic movement and non-compatible deformation between sandstone and mudstone in sand shale interbed.This thesis takes “The Survey of Geological Disaster in Nanjiang of Ba River Basin in East Sichuan”, the geological survey project of the Institute of Prospection Technology under China Geological Survey Bureau, as the support. The gentle stone slope of soft-hard-alternation-bed structure of typical red bed region in Nanjiang County in Sichuan Province is selected as the research object. Based on the research of landform, stratum lithology and structural geology features of red bed region in Nanjiang County, through statistical analysis of developmental state of cracks in middle and back edge of slope, maximum orientation of major principal stress and tectonic stress history experienced by research area are proved through investigation; in order to further prove the difference of rheological property between sandstone and mudstone, direct shear rheology experiment of stone is conducted and long-term strength, modulus of creep and coefficient of viscosity of sandstone and mudstone are obtained, which provides theoretical basis for physical simulation experiment and numerical simulation experiment. Taking red-bed gentle rock slop as the typical prototype, based on correspondence theorem of physical simulation experiment, physical simulation experiment is launched to prove the non-compatible deformation existed in sandstone and mudstone terranes and the formulation process of middle and back tensile cracks is observed; using 3DEC discrete element method, the deformation and destruction process of gentle slope of thick mudstone inside the upper supper-thick sandstone is simulated and the results of physical simulation experiment are proved, thus, the holing process of middle and back tensile cracks is revealed. According to the results of the above field investigation, laboratory test and numerical simulation, the following conclusions and acknowledgements are obtained:(1) The dominant joint planes of middle and back gentle terrane slope in red-bed region in Nanjiang County and tectonic stress history is identified. The direction of maximum principal stress of early Yanshan tectonic movement in Nanjiang County is EW and the direction of maximum principal stress of mid Himalaya tectonic movement is NNW-SSE, which lead to formed in the Yanshan tectonic movement joints(direction is NEE) to the closed state in compression in the Himalaya movement period.(2) The brittle feature of sandstone and the plasticity and glutinousness features of silty mudstone are proved through direct shear creep test. Under long-term shearing stress, the long-term strength, and shear modulus of sandstone and silty mudstone all drop sharply and the decrease of cohesive force of sandstone is more obvious.The viscous coefficient of mudstone is far smalle than that of sandstone in the same vertical normal stress conditions, which indicates mudstone rheological properties far stronger than the sandstone.(3) Through physical simulation experiments and numerical simulation, the formation mechanism of tension cracks in middle and back edge of the gentle slope is revealed. In the interbeddings of sandstone and mudstone, due to the difference of Poisson's ratio, long-term strength and coefficient of viscosity, the non-compatible deformation happens on the front slope at first and then, such non-compatible deformation extends from the front slope to the back slope, eventually lead to the principal tensile crack formation, development and through in middle and back edge of the sandstone terrane.(4) According to the physical model test and numerical simulation, the forming process of tension cracks on the middle and back slope of gentle terrane is revealed. There are following four processes:(1) The formation of strike jiont in compression on the middle and back slope;(2) The opening of strike jiont in compression, the formation and development of tensile crack;(3) The expansion of shear breaking plane on the surface of terrane of soft and hard rock;(4) The tensile crack open completely.