Study On Construction Risk Assessment And Key Parameters Of Karst Tunnel

Tunneling in western China, especially in southwest areas, inevitably encountersKarst circumstance. According to high demand of safety and Eco-environmentalconservation, there appear various technology challenges of tunneling in Karst area.Therefore, the risk assessment and early warning, prevention suggestion and the keyparameters of minimum safety thickness of karst caves and tunnels between stratabecome the most significant problem to be solved.Based on the Chang’Anzhai tunnel in Wanli highway project, this paper uses theresearch methods of literature survey, theoretical analysis, numerical analysis, etc. Itscontents are as follows: using the analytic hierarchy process (ahp) and fuzzycomprehensive evaluation theory to evaluate flood risk underwater-enriched karsttunnels and to get the flood risk level of water-enriched karst tunnels; according to thelevel of risk assessment, determining their corresponding early warning level andmaking a different level of early warning pre-control countermeasures; using ANSYS tobuild a Karst tunnel model and then analyzing the minimum safety thickness of karstcaves and tunnels strata; summarizing the disciplines based on numerical analysis of theminimum thickness according to different karst cave span and depth-span ratio.we arrive at the following six conclusions.①The water disasters of water-enriched Karst tunnel are mainly affected by theKarst development degree and water storage structure, and the water inrush risk can bedivided into four grades, among which the highest risk is level Ⅰ and the lowest risklevel Ⅳ.②Corresponding to the damage level of flood risk assessment, the warning levelabout water inrush and burst mud in water-enriched karst tunnels can be divided intofour levels, and be expressed in four colors, respectively: Red (I level, the most serious),Orange (Ⅱ level, serious), Yellow (Ⅲ level, the heavier) and Blue (Ⅳ level, general).③For the prevention of water-enriched karst tunnel, first of all, we must attachimportance to advance geological forecast, monitoring and measurement work,preparation of enough emergency equipment for water, such as pump, and formulationof emergency preplan. When encountering the contact zone between Karst and nonKarst, pipe roof, small pipe and pre-grouting are adopted in preliminary reinforcement. ④For the cave spans, no matter which side the cave is located in the tunnel, thegreater the cave span, the larger the minimum safe thickness between the cave and thetunnel rock.⑤For the cave depth-span ratio, no matter which side the cave is located in thetunnel, When the cave in horizontal or vertical direction, the minimum safe thicknessbetween the cave and the tunnel rock increase gradually. When the cave depth-spanratio tend to1.0, the minimum safe thickness between the cave and the tunnel rockreduce gradually. When the cave depth-span ratio equal to1.0. the minimum safethickness between the cave and the tunnel rock is relatively smaller.⑥Caves located in the lateral part of the tunnel exert greatest influence onminimum safe thickness, and next the caves located in the bottom.