| A large number of gently inclined red stratum distribute in eastern area of Sichuan basin,whose inclination angles mostly ranges among 10°~20°. According to the traditional view of mechanics and engineering geology,it is hard to break out large-scale landslides. However,in recent years,large area and batches of landslides happened as common because of the extreme rainfall conditions,such as thousands of red stratum destroyed by the extreme rainfall on September 16 th ~18,including some landslides whose volume reach to one million to ten million cubic meters,causing a lot of property losses. There is no unified and clear conclusions on the cause of rock landslides in 10°~20°red stratum slope. The current achievements in the research believe that the driven forces come from hydrostatic-pressure in the cracks distributing in the trailing edge of slope and uplift pressure on the sliding surface. But through the field investigation of multiple rock landslides in Nanjiang county, it is discovered that landslides lack of the conditions producing hydrostatic-pressure in the trailing edge of slope. The typical landslide stability calculation result by the mudstone and natural slip zone soil shear strength shows that landslides can’t be destablized by hydrostatic-pressure and uplift pressure. Therefore, it is necessary to study the formation mechanism of the gently inclined landslide in red stratum region and to build landslides’ early warning model and the criterion to provide technical support for the landslides’ prevention and mitigation in red stratum region in eastern Sichuan province.Depending on the geological survey project of “Major Geological Hazard Survey and Warning in the Southwest of China” and 973, using field investigation, laboratory tests and other methods, combining with regional geological conditions, this thesis focuses on the in-depth study of gently inclined red stratum landslides’ formation conditions,rock and soil characteristics, formation mechanism and warning. The following achievements have been achieved in this thesis:(1)The distribution discipline of landslides was found out and the formation conditions were summarized from the aspects such as landform, stratum lithology, geological structure and so on. The results of landslides distribution by ArcGis show that landslides follow a planar distribution, landslides density reaches to 0.22/km2 and mainly with medium and large scale. As to topography and geomorphology, landslides are more likely to happen on 400~900m elevation and 10°~30° slopes, espcially 10°~20° slopes account for the vast majority. The highest contribution directions of slope include S,SE and E, free face combinations include type I and III. In addition, the old rift trough、steep cliffs and open cracks provide good catchment and infiltration conditions. The highest contribution strata to landslides’ area and volume is J3p2. The wide distribution of stized intercalations provides the material basis for rock landslides and the cataclastic rockmass produced by geological structure and weathering provides the structure condition. Besides, rainfall is the main external factors triggering landslides.(2)The physical and mechanical properties of slip(slip soil)in the red stratum landslides distributed in Nanjiang county are obtained. The tests on mineral composition、particle size distribution、liquid-limit、plastic-limit and swelling show that the clay mineral content is about 50% and fine soil content is more than 60%. The stized intercalation is low liquid-limit clay with weak expansibility. According to the water-saturated softening experiments, the microcosmic and mechnical properties variation laws are aquired. The results show that microcosmic structure get worse as time goes on,.Both cohesion and internal friction angle variation conform to an exponent curve, maximum attenuation amplitude reaches 41% and 81% respectively. The direct shear softening test of remodeling stized intercalation reveals that cohesion and internal friction angle respectively decrease to 1.28 kpa and 1.24 degree. With different depth of mudstone and different stages of siltized intercalation in typical landslide as the object of study, according to the indoor experiments, the variation laws of mineral composition, the content of chemical elements and microcosmic structure during the process of mudstone to siltized intercalation were found out. Mudstone hydration experiment suggest that the process of mudstone to siltized intercalation was slow and weak. The PH variation of solution conformed to an exponent curve and experienced the process from weak acid to weak base. Electrical conductivity flutuated and tended to stable state after 20 days. The concentration variation of Ca2+,Na+,Mg2+,Al3+,Si4+,Cl- presented an exponent curve.(3)The formation mechanism of rock landslides in the red stratum of Nanjiang county is revealed. Based on the geological prototype, The red bed slopes in Nanjiang county are divided into three categories: homogeneous slope,steep cliff slope and old rift trough slope. Three geological models and rainfall infiltration models are built too. There are two failure modes of rock landslides including creep-sliding mode and translational-sliding mode. Creep-sliding failure mode applies to homogeneous slope and is caused by sliding soil’s progressive softening effect from time and space after rainfall accumulates on the sliding surface. Translational-sliding failure mode applies to steep cliff slope and old rift trough slope, and is caused by hydrostatic-pressure in the cracks distributing in the trailing edge of slope,uplift pressure on the sliding surface and sliding soil’s softening effect. The start-up criterion is verified through the typical landslide’s stability calculation. The calculation results show that slope reach imit equilibrium state when softening sliding soil’s area accounts for 87% of the total area.( 4) The regional rock landslides’ early warning criterion and single rock landslide’s warning model in the red stratum of Nanjiang county are built. Based on the relationship between the number, area,and volume of landslides, and accumulative rainfall, 3 levels of landslide’s warning in the condition of single rainfall and accumulation precipitation are built. The single rainfall threshold is 0~80mm, 80~230mm and over 230 mm respectively, and the accumulation precipitation threshold is 0~350mm, 350mm~500mm and over 500 mm. Creep-sliding and translational-sliding landslide’s warning model in different rainfall level are built based on the limit equilibrium method. |
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