Risk Evaluation of Water Inrush in Karst Tunnel Based on Karst Evolution Model

The occurrence of water burst and water inrush in karst tunnel mainly depends on the development degree of karst cave and the structure type of karst system,which is closely related to the evolution of karst system.The evolution of karst system begin with the dissolution widening of cracks in rock mass by aggressive water.This study introduces the establishment of the karst evolution simulation model coupled with seepage model and dissolution widening model based on the regional hydrogeological conditions.The evolution simulation model is used to reconstruct the changes of water-bearing medium during the evolution process of Jigongling karst system.The result shows that,with the karst system's evolution,the fissures are widened progressively by dissolution with obvious differential dissolution and the heterogeneity of karst aquifer becomes more and more obvious.However,karst development extent and heterogeneity level become milder with the increase of depth.The average widening rate of fissures in the shallow system is almost 1500 times higher than that in the deep system.At the portal elevation of Jigongling tunnel,the karst development extent is mild,with the smaller dissolution widening fractures.The tunnel surrounding rocks are still a fractured rock mass with the permeability coefficient 0.51 m/d.The water inflow of 127.9m³/d when water burst accident happened is considered to be quite a small amount to exert effects on the tunnel construction.