Study On Seepage Failure Mechanism And Evolution Process Of Water Inrush Induced By Filled Latent Karst Cave In Karst Tunnels

With the rapid development of economy and the gradual improvement of infrastructure construction,China’s tunnel engineering has been highly developed,and the karst geological environment faced by tunnel construction is complex and changeable.In the process of tunnel construction,the geological disaster of water inrush is often caused by the filling latent karst cave,which poses a serious threat to the construction personnel and equipment.By means of literature review,theoretical analysis and laboratory tests,this paper conducts an in-depth study on the mechanism of infiltration failure and the evolution process of water inrush in the buried dissolved cavity of filling type.The main research work and the progress made are as follows.(1)The karst geological conditions of latent karst caves are analyzed from three aspects: negative topography,geological structure and spatial distribution of soluble rocks.The development and expansion of latent karst caves and the characteristics of filling substance are summarized.The excitation conditions of water inrush in concealed karst caves were analyzed from three aspects: energy storage,water-resistant structure stability and artificial disturbance.According to the characteristics of karst cave filling substance,water inrush frequency and water-resistant structure,water inrush hazard was divided into eight different types.The research results provide a theoretical basis for the research on the seepage failure mechanism and the evolution process of water inrush in the filling substance of concealed karst caves.(2)The seepage failure characteristics of filling substance were studied from the perspective of particle migration and loss,and the process of filling substance’s seepage failure was divided into three stages: piping failure stage,water-inrush(sand)channel expansion stage and water-burst(sand)stage.Based on the relative theory of river dynamics,in view of the piping failure stage to establish a model of multilayer spiral capillary,consider the grain size distribution and fine particle content determine the level of the capillary tube model,further analysis of the influence of seepage force,based on the stress analysis was carried out on the movable particles,movable particles through the analysis of the moment balance critical starting velocity.A circular section channel model was established for the expansion stage of the water(sand)inrush channel.The critical starting velocity of the channel sidewall particles was determined according to the torque balance by three-dimensional force analysis of the sidewall particles.With the change of permeability as the bridge between the two stages,the water(sand)burst stage is the result after the channel is extended to a certain extent.The seepage failure theory of filling substance was verified by a practical engineering case.(3)The simulation test device of “evolution process of water inrush in latent karst cave and failure characteristics of filling substance” was developed.The device consists of four parts,including deposition storage device,water inrush switch control device,flume flow device and recovery device.Based on the observation of the experimental phenomena,the water inrush process was divided into four types according to the changes of flow rate,flow rate and spray distance,including seepage-inrush-burst water(sand),inrush-burst water(sand),water burst(sand),seepage-burst water(sand).The development and evolution of the flow state in the process of water inrush is the external manifestation of the seepage instability of the filling substance,which indirectly explains the rationality of the theoretical analysis of the seepage failure of the filling substance.(4)The effects of the height of overburden head,the height of substance deposition,the type of substance and the size distribution of substance particles on the duration of water inrush were analyzed by laboratory tests.The results show that the particle size distribution and the type of substance have significant influence on the water(sand)seepage stage: the relation between fine sand particle content and medium sand particle content and duration can be expressed by polynomial function,the duration of sediment type is larger than that of gravel type.The influence of substance deposition height and substance type on water(sand)inrush stage is obvious: the substance deposition height of gravel type and sediment type is positively correlated with the duration of water inrush(sand),the duration of sediment type is smaller than that of gravel type.The influence of particle size distribution and height of substance deposition on water(sand)burst stage is obvious: the relation between fine sand particle content and duration can be expressed by polynomial function,the relationship between the deposition height of gravel type and the duration was first increased and then decreased.(5)The effects of the height of overlying head,the height of substance deposition,the type of substance and the size distribution of particles on the failure zone and the failure pattern of filling substance of latent karst caves were analyzed by laboratory tests.The results show that length of failure zone along the direction of water inrush: The failure length of sediment type is almost unchanged at different overlying head heights,the deposition height of gravel type is positively correlated with the failure length,the failure length of gravel type is larger than that of sediment type,the relation between the content of coarse and gravel grains and the failure length can be expressed by polynomial function.Failure zone length in vertical water inrush direction: the height of the overlying head of the sediment substance is positively correlated with the failure zone length,the deposition height of sediment type is positively correlated with the failure zone length,the failure zone length of the gravel type is always near the maximum boundary,the relation between gravel content and failure zone length can be expressed by polynomial function.By analyzing the failure morphological characteristics of filling substance,the failure pattern can be abstractly classified into two types: semi-cylindrical and semi-conical.The corresponding volume formula is used to calculate its spatial capacity.