| Rely on major projects such as the Shanghai-Kunming high-speed railway and systematic summary large amount of geological disasters in case of karst water bursting during tunnel construction, this paper studies the types, characteristics, influencing and controlling factors, disaster mode of karst water bursting disaster. With three-dimensional seismic tomography, ground penetrating radar and three-dimensional laser scanning test, we analyzed the identification and estimation methods of karst disaster risk sources such as karst reservoir structure positioning and cave wall thickness. Based on the typical disasters which come from data analysis and experiment data, a conceptual calculation model can be established. By using numerical analysis software to do finite element simulation, we analyze different cross-section shape, thus we can get the deformation characteristics and rules of rock in tunnel face considering the environmental conditions like different karst water pressure, different cave scale and different cave wall thickness. By means of numerical analysis method, we studied the risk assessment method of karst water bursting disaster. By analyzing the critical warning indicators, monitoring and early warning information system can be studied on the basis of risk assessment.The main results are as follows:(1) Most of the tunnels in study area are across the carbonate strata like limestone and dolomite or contact zone of soluble and non-soluble rock, which means in study area, karst strongly develops. The surface karst forms are including karst depression, sinkhole, grike, fluid bowl and clint. The underground karst forms are mainly underground water, caverns, corroded fissure and karst pipeline. The karst forms are developed in the height of1200-1500m.The karst negative terrain development density in tunnel site are respectively3per square kilometers in Dadushan Tunnel,4per square kilometers in Gangwu Tunnel and4in Doumo Tunnel. In Dadushan Tunnel from D1K855+300-D1K858+918, there exists15karst negative terrains in fault zone which are large scale and highly affective to the tunnel.(2) Water inflow and pressure are important factors determining the extent of karst water-related disasters. The study found that most of the karst water inrush disaster occurs at high pressure, water-rich cavity or fracture zone, and low water pressure, water-rich cavity also can be formed disasters, this paper carried on the classification respectively from the karst gushing water level, gushing water dynamic, storage structure and composition of protrusion of karst water bursting, disaster types and basic pattern. Combined with the geological condition and mechanism which lead to karst water bursting in Dadushan Tunnel and others in Shanghai-Kunming passenger line, macro hazard zones are regionalized in Dadushan Tunnel, Gangwu Tunnel and Doumo Tunnel. It provides fundamental information for the later construction safety and early warning.(3) Through field test, with three-dimensional seismic tomography to remotely recognize Poor water storage structure, with ground penetrating radar to recognize in short distance, meanwhile combined with advanced borehole and three-dimensional laser scanning test, risk sources like the thickness of aquifuge and water inflow can be located and estimated. The research shows that in seismic tomography maps, a complete and hard rock stratum with higher wave velocity is in yellow, and fractured aquifer with lower wave velocity is in blue. Where in the map are spotted yellow and blue means aqueous fractured zone. If karst fissures are fully developed, we can see blue zones connected in large scale. Karst cavern fully developed shows stereoscopic blue zone in the map. The typical response of aquifer is the velocity ratio of longitudinal waveand transverse wave. In the GPR maps, the essential signs of water-rich zone are increase of permittivity and decrease of electromagnetic wave velocity. Simultaneously there exists strong reflection and diffraction, scattering, wave disturbance, discontinuous phase axis and drastic changes from high to low frequencies.(4) Numerical simulation shows that after the tunnel excavation, maximum deformation of karst cavern wall in front of tunnel face occurs in the geometric center of tunnel face. When the size of karst cavern in front of tunnel face is smaller than the diameter of the tunnel, the deformation would be small after excavation. But high water pressure also causes greater deformation in that situation. When the size of karst cavern in front of tunnel face is similar to or greater than the diameter of the tunnel, the deformation would be great in the effect of water pressure.(5) Under the karst water pressure, tunnel face rock deformations are mainly occurred in the range from0to0.1m ahead of excavated surface. While near cave rock thickness keeps constant, with the water pressure increases, the deformation gradually increases. When the karst water pressure is greater than2.0MPa, the increasing trend of maximum deformation is clear. Under the karst water pressure, the tunnel face rock deforms great when the thickness of karst cavern wall is less than1.0m. When the thickness is greater than2.0m, the deformation increases little with the water pressure increases.(6) Through the analysis of controlling and influencing factors of karst water bursting disaster, a risk assessment index system in tunnel construction period can be established which includes the degree of karst development, geological structure, topography, surface water and groundwater systems and buried depth of tunnel, etc., adding up to6level1indexes and16level2indexes. Using fuzzy mathematics method, we can quantify the risk indexes and then establish karst water bursting risk model with the weight of indexes and fuzzy comprehensive evaluation method. Combined with Dadushan Tunnel, Gangwu Tunnel,Doumo Tunnel and other important sections karst water bursting risk assessment case, fuzzy evaluation method proves to be applicable.(7) On the foundation of study result about karst water bursting risk assessment, take water pressure, water inflow and deformation of surrounding rock as early warning indexes, we raise a four color early warning level for karst water bursting and classify the early warning indexes. Tunnel Geological Prediction and real-time monitoring data can help to do a dynamic risk assessment for karst water bursting. With the stacking and coupling analysis between monitoring data of early warning indexes and conclusion of risk assessment, using GIS and AutoCAD software as second development platform, karst water bursting disaster during tunnel construction monitoring and early warning system and its application can be analyzed. |
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