| While tunneling in karst terrains,engineers may encounter some specific large-scale geological disasters such as water inrush and mud gushing affected by complicated geological condition.With the features of high incidence,unpredictability and great damage,water inrush seriously restricts the development of underground engineering construction in karst terrain.Accompany with immeasurable losses,water inrush usually causes tunnel to be flooded,mechanical equipment to be damaged and construction to be forced to interrupt.In addition,it also can cause the irreversible consequences like water and soil erosion and unbalance of water resource,even casualties.As karst terrain is distributed widely in our country,the underground space resources have broad development prospects.The issue of water inrush in tunnel,a typical geological hazard problem in karst terrains,has been a significant leading edge and hot topic which remains to be further studied urgently.This paper starts with the detail investigation of several representative water inrush events occurred during the construction of typical domestic karst railway tunnels.By using these methods of project case investigation,numerical calculation,theoretical analysis and numerical model tests,the mode and catastrophic mechanism of water inrush in karst tunnel are analyzed comprehensively.Based on different theories or mechanical response of construction,the prediction method of safety thickness of against-inrush layer is also studied.The research achievements which are as follows have a certain theoretical significance and engineering application value.(1)Based on the statistical analysis according to massive amount of data about water inrush in karst tunnel,the karst development characteristics are analyzed from the necessary conditions of karst development,and the types of water bearing structures in different tectonic zones and karst development belt are obtained.The main line of water inrush formation consists of water inrush energy storage,energy release condition and energy outburst catalytic.As a consequence,the catastrophe characteristics of water inrush can be studied step by step.The influencing factors of occurrence conditions of water inrush are also illustrated based on this main line,and the control factor set of water inrush is formed.(2)A number of representative events about water inrush in karst tunnel are selected to investigate and survey the disaster-causing process and mechanism,and generalized modes of each water inrush events are constructed.After some comparison and analysis,the modes of water inrush in karst tunnel are divided into two kinds: mode of againstinrush rock mass failure and mode of filling material failure.The disaster-causing key of the former is against-inrush rock mass,while the other is filling medium in water-bearing structure.On the basis of spatial location relationship of karst tunnel and water-bearing structure,the mode of against-inrush rock mass failure can be subdivided into intersecting type and separating type,and the other is classified as upper type,lateral type,under type,perforative type.The classification mentioned above can provide a foundation to the study of water inrush mechanism and the thickness of against-inrush layer.(3)The catastrophic mechanism of water inrush mode of against-inrush rock mass failure is the destabilization failure of against-inrush rock mass.In the first place,the weakening effect of karst water on against-inrush rock mass is analyzed from the view of physics and mechanics,respectively.In the second place,according to the quality of against-inrush rock mass,the failure mechanisms of against-inrush rock mass with different failure modes are studied by using the catastrophe theory,rock fracture mechanics and key block theory,respectively.In addition,it is pointed out that the failure mode of against-inrush rock mass in the process of water inrush is non-deterministic.Eventually,for the non-deterministic property of rock mass failure,the failure process of against-inrush rock mass is analyzed by utilizing numerical software which can consider the seepage-damage coupling mechanism,and the formation process of water inrush channel in rock mass is reproduced.Moreover,the failure mechanism of the againstinrush rock mass is revealed base on the evolution laws of several mechanics and seepage fields in the tunnel excavation process.(4)The catastrophic mechanism of water inrush mode of filling material failure is the destabilization failure of filling material.Based on the investigation and analysis of a load of engineering projects about water inrush,the classification of filling material in waterbearing structure is put forward.Then the difference on permeability of filling material is analyzed.In accordance with the difference mentioned above,the instability mechanisms of filling material affected by penetration and slippage are analyzed and verified by the engineering cases,respectively.(5)Based on the analysis of the influencing factors of the against-inrush layer stability,it is proposed that the quality of the against-inrush layer and karst water pressure are the most important factors which lead to the unstable failure of the against-inrush layer.According to the elastic theory,under the condition that the water-bearing karst cavity is located on the side of,above or under the tunnel,the against-inrush layer is simplified into the beam model fixed at both ends,while the against-inrush layer with water-bearing karst cavity developed in front of the tunnel face is simplified into the circular plate model fixed around the plate,thus the forecasting method of the safety thickness of againstinrush layer is established based on the flexure,shear or extension strength standard of beam and plate.Besides,the against-inrush layer located above or under the tunnel is simplified into the beam model fixed at both ends,while the against-inrush layer existing in front of or on the side of the tunnel face is simplified into the circular plate model fixed around the plate,thus the formula of the safety thickness of against-inrush layer is established on the basis of the cusp catastrophic model.(6)In order to form different calculation conditions,the common-level surrounding rock and karst water pressure are chosen,graded,intersected and combined.In the light of these conditions,the numerical model tests of karst tunnels where the intersecting type and separating type water inrushes may occur are built.Considering the coupling effect of fluid-solid,three-dimensional fast Lagrangian method is adopted to study the mechanics response during the process of tunnel construction under the conditions of step-by-step excavation and support.In different composition conditions of surrounding rock and karst water pressure levels,the evolution rules about displacement field of the model and against-inrush layer during the process of excavation are put forward and analyzed.Accordingly,the safety thickness values of against-inrush layer under all the calculation composition conditions are obtained.Through the regression analysis of the results,the safety thickness formulas of against-inrush layer in the intersecting type and separating type tunnels under different surrounding rock conditions are established,respectively.(7)Four practical karst tunnels where there exists occurrence probabilities of intersecting type and separating type water inrushes are selected as the engineering cases to test the feasibility and rationality of the safety thickness forecasting methods for against-inrush layer mentioned above.It is found that the forecasting methods based on the strength theory and the catastrophe theory can be applied in some working conditions,but they all still have the limitations.Nevertheless,the forecasting method based on the construction mechanics response is more simple with perfect engineering performance,and strong applicability and operability.It can be accepted more easily by the field engineers,and provides a powerful reference tool for the practical engineering. |
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