| With the economic development and large-scale urban construction, geological disaster is becoming increasingly frequent; this seriously influences the process of urbanization. Karst collapse in Wuhan City began in the 1930s, and was mainly due to the rain and local groundwater pumping. After 1990, karst collapse occurred frequently, due to the increase of constructions and groundwater pumping. Karst collapse was controlled in the years 2011 due to the decrease of groundwater pumping, and with an effective monitoring of ground deformation. Despite this, due to the engineering drainage, karst collapse still occurred sometimes, the people’s lives and property were seriously affected by karst collapses. Therefore, we selected a typical karst collapse area-Qingling Town in this study, Based on the study and analysis of the Research area’s geography, engineering geology and hydrogeological conditions, combined with the karst cave history of this area, the characteristics and mechanism of karst collapse were studied. The mathematical model for describing karst collapse is discussed, a water-soil-rock model has been established. The model is used to predict the quantitative evaluation of karst collapse under different affecting factors. The main conclusions are as follows:1. The distribution of dissoluble rock was investigated and analyzed based on the dissolution experimental data. Ruanjia alley, Sifa School, and Fenghuo village were under the most extensive bedrock surface corrosion degree. The results showed under the control of geological structure, dissolution rate in boreholes was 73% in pressure shear fault zone of Qingling village. The limestone cave encounter rate was 22.6%. the extensive development of dissoluble rock in vertical zone located in-5-10 m with a dissolution rate of 15.7%.2. A statistical analysis of the filling characteristics of karst cave was employed. The fillings in karst cave around Ruanjia alley-Lujia street-Sifa school-Qiaomu village-Yujia village and Maotangang village were filled with sand, clay and gravel in the plan. The vertical zone between 20m and 40m reached a filling rate of 67.39%.3. The relationship between development of karst and chemical composition of minerals in soluble rock with the formation time was analyzed. CaO content and the specific dissolution ratio in the Triassic were lower than that in Permian with a specific dissolution ratio ranging from 0.93 to 1.17. Permian strata was Not widely distributed while the underlying bedrock of corrosion phenomenon was relatively common with highest density 3 per km2.3. The relationship between development of karst and chemical composition of minerals in soluble rock with the formation time was analyzed. CaO content and the specific dissolution ratio in the Triassic were lower than that in Permian with a specific dissolution ratio ranging from 0.93 to 1.17. Permian strata was Not widely distributed while the underlying bedrock of corrosion phenomenon was relatively common with highest density 3 per km2.4.We analyzed and summarized the spatial and temporal distribution of karst collapse:① collapse area all located in the southern strip of limestone, limestone caves or fissures;② in the topography, the collapse of all points are confined to the first terrace on both sides of the Yangtze Riverside on the ground, where the groundwater and the Yangtze River with the seasonal changes in water conservancy closely alternate contact;③ distributed along the railway as well as the location of major construction projects.④ on the geological structure, many collapse point occurred in Qingling Temple pressure-shear fault and Wutong-Tangxun Lake fault zone within 500m range.⑤ collapse point generally occurs in the Quaternary limestone Looking at the results from the current statistics. Collapse occurred mainly in the 6-8 month period and 12-2 during the month.5. We studied influencing factors of karst collapse:① The topography influence, terrace deposits in the presence of the dual structure, the upper for cohesive, fine sand and the lower part of the gravel layer directly overlying limestone, within dynamic role of groundwater in the area can easily be hidden eclipse. ② The influence of the upper soil structure on the ground collapse lithology, cohesion higher, stronger shear strength, resistance to collapse the higher capacity. The maximum thickness of the sand layer in the Fenghuo village is 36m, with maximum density collapse. It is easy to collapse under the effect of the osmotic pressure of the groundwater. ③The influence of the geological structure, the collapse pressure points along the Qingling Temple twist of fault distribution are all in 500m range. The mechanism can be explained that the faults accelerate the groundwater flow causing a much more extensive latent erosion of groundwater.④ Underground the karst cave development and degree of soil, soil disturbance on ground subsidence, Village disturbance flames with a multi-profile way interconnected, multiple sand disturbance with mutual influence, forming dense ground subsidence pits more than one place at the point of collapse. Disturbed belts consisting of one or more profiles with inter-permeation make soil accelerate to migrate with the groundwater flow. The inter-permeation disturbed belts provides a better environment for migration. In addition, rainfall increase vertical seepage pressure as well as pumping. Furthermore, the rainfall also increases the weight of stress in the soil. Therefore this type of collapse can be explained as penetration collapse, gravitational collapse.7. Based on the research on spatial distribution of Karst geological conditions, the characteristics of Karst collapse development and genetic mechanism analysis, a coupled model integrated with groundwater flow and stress-strain was constructed in subsidence area of study area. In addition, the Coulomb-Mohr principle and Griffith principle were applied in the present study.8. The groundwater table and the value of stress-strain of soil were calculated using GMS software and a Fottran program form the January of 2009 to July of 2013. The verification procedure indicated a good precision for the model.9. The water level of Yangtze River, construction drainage and rainfall were predicated using the above model in Fenghuo village of Qinglin town. It could provide scientific basis for prevention of Karst collapse.The main innovations in the present study:(1)Proposed coupled model of groundwater flow and stress-strain model for modeling of karst collapse and mechanism in Qingling Town.(2)A coupling model is proposed which contains dynamics of groundwater flow and mechanics model. In this model, karst breakdown problem in Qingling Town which under change of water level of Yangtze river and construction drainage around is simulated and the problem under rainfall condition is also analyzed. |
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