Study On Impact Mechanism Of Deep Buried Karst To Tunnel Safety And The Treatment Technique

It is a world-wide technically difficult problem tunneling across Karst district. There are a series of problem with the design and construction of Karst tunnel. It is difficult to master the load distribution under high water pressure. The water-inrush and mud-inrush threat the safety of workers and equipments. Karst caves under bed and high water pressure is hidden danger to railway operation. Yi-Wan railway line is one of the main railway net in China, it is also one important part of the express route from Shanghai to Sichuan. The railway passes across Karst-develop district where Karst action generally lags behind Qingjiang valley cutting. The water level or the height of Karst cave and Karst tube is far below the drainage base level. So tunnels in this district are deep buried and under high water pressure. The invisible and enormous damage with deep buried Karst makes tunnel construction extremely difficult.Taking Yesanguan and Dazhiping Karst tunnels as examples, the forming condition, character of deep buried Karst caves and their impact on tunnel construction and railway operation are studied. The Principle and technique for dealing with karst is put up based on the study. The main contents and achievements are:(1) The forming condition, developing character and water-inrush mechanism of deep buried karst is studied by document analysis and engineering practice. The opinion is put up that the karst hydrodynamic zoning and tectonic position control the space distribution of water inrush. Based on this opinion, the karst water-inrush models are established. By ground surface karst investigation and tracer test, the karst developing character of Yesanguan tunnel is studied, and the geological factors causing water-inrush are analyzed.(2) By document investigation and engineering practice, the effect of karst type (including water pressure, the feature of cave filling, the hydro-connection with ground) on tunnel construction is studied. The possible process of water-inrush is analyzed. By this study, it is pointed out that karst caves with high water and complex filling are most dangerous to tunnel construction, and the broken mechanism of rock between tunnels and caves is analyzed. By theoretic and numeric method, the effect of position and scale of the karst cave on the tunnel is studied, it is put up that only the intact rock between tunnels and caves can play a role to stop water inrush. When rock depth between tunnels and rocks is bigger than the tunnel's diameter, the effect of cave on tunnel deformation is very small. Based on empirical analogy, theoretical analysis and numerical test, the safe rock depth between tunnel driving face and cave is put up combined with engineering practice, it is used in Yesanguan and Dazhiping tunnel construction.(3) The method for calculating original water pressure of karst tunnel is thoroughly discussed. The paper puts up the method to calculate water pressure on lining for karst tunnel, and analyzes the law of grouting on water pressure on lining and discharge, puts up the suitable grouting range. By three dimensional model test that laying the tunnel into true water and soil circumstance, the distribution rule of water pressure on lining during different drainage condition is studied, and the viewpoint is put up that tunnel lining bearing the load from seepage even under the condition of total drainage, and the traditional calculation method of no considering water pressure on lining during total drainage is not proper. By analyzing the situ monitoring data, the amount of precipitation, the water-inrush quantity and the water pressure character on lining of the typical section are mastered. In this paper, the ultimate data of water pressure of typical section is collected, it indicates that the water pressure is far below the design value after one year's operation, but still we must pay attention to the study of the relation between the amount of precipitation, the water-inrush quantity and the water pressure.(4) By the constant exploration of theory-practice-theory-practice, a systematic principle and technique to deal with karst disaster is refined. The suitable range of advance geology forecast during construction and the maximum design value of water pressure on lining are put up, and the achievements are used in practice. The paper puts up the principle of advanced geology forecast, the principle of guarding against water-inrush, the principle of cave treating, the principle of safe construction, the principle of protecting environment and the technique of treating karst caves. For different type of karst disaster, there is corresponding technique to deal with, it provides safeguard for tunnel construction and operation.(5) It is put up that it is with high risk to excavate inclined shaft in Karst district. It is illustrated that inclined shafts connect different aquifer, bring large amount of water inflow and cause high risk to construction, and it is uneconomical. So, inclined shafts should be abandoned in design and the pilot should be advocated in karst district or other aquifer, and it is with big advantage in adding excavation face, reducing water inflow, decreasing construction risk. For karst intruding into tunnel and with water pressure, the drainage conduit must be installed to maintain its flowing state. And the conduit should be connected with drainage tunnel permanently so as to safeguard the operation.(6) The construction difficulty of Yesanguan and Dazhiping tunnel is the most among the eight tunnels with high risk because of the complex geology. By thoroughly recording and analyzing the process of dealing with the two caves with high water pressure and complex filling, it is illustrated that it is not practical trying to seal it for this type of cave. A series of effective measure is put up to safeguard tunnels' construction. All this provides valuable material at first hand for similar project.