Research On Occurrence Conditions And Disaster-causing Mode Of Water Inrush During Mountain Tunnel Construction

Mountain tunnel is an important engineering project in modern transportation construction.Due to the complex geological conditions in mountainous areas,the development of karst,and the enrichment of groundwater;during the construction of tunnels,groundwater and surface water can easily enter the tunnel,causing water inrush,mud inrush,collapse,etc.It is extremely difficult and even endangers the safety of personnel’s life.Therefore,it is very necessary to study the disasters caused by water inrush in mountain tunnels.In this paper,by collecting and arranging typical cases of tunnel water inrush projects,summarizing and analyzing the occurrence conditions of tunnel water inrush,combined with the structure classification of the anti-outburst layer,the water inrush disaster mode is divided into two categories,and the typical engineering cases are selected for theoretical analysis.The software simulates and analyzes the process of water inrush disasters,and analyzes the influence of different factors on water inrush disasters.The research results mainly include:(1)Collect and organize 22 typical engineering cases of tunnel water inrush,and summarize the conditions for tunnel water inrush: geological conditions,hydrogeological conditions,and construction conditions.It is concluded that there are two types of rock mass structures in the outburst prevention layer: the water-resisting rock mass with complete rock mass structure,and the filling structure composed of fillers.The type of water inrush disaster mode occurs when the water-resisting rock mass is gradually weakened due to the impact of tunnel excavation.Theoretical analysis of typical cases is carried out,and the influencing factors are obtained: thickness of water-resisting rock mass,elastic modulus,and water pressure.(2)Seepage instability type water inrush disaster mode,the occurrence condition is affected by groundwater,and the characteristic is that the occurrence time of water inrush lags behind.Theoretical analysis of its typical cases shows that the influencing factors are: water pressure of the aquifer on the filling medium,effective volume of the filling structure for water resistance,and support force of the tunnel.(2)For the gradual destruction of rock mass caused by water inrush,the COMSOL numerical simulation software was used to establish a corresponding step-by-step excavation model to simulate and analyze the water inrush disaster.In the common surrounding rock area,the seepage and displacement changes caused by excavation are very small.When the excavation enters the water-resistant rock mass,the seepage and displacement of the surrounding rock gradually increase.When the excavation exposes the water-rich fault zone,the stress on the face of the tunnel is concentrated at the vault,and the plastic zone of the surrounding rock increases and penetrates deep into the tunnel vault.,the risk of flood disasters increases.Analyzing the influence of different factors,the grade of the surrounding rock increases,the water pressure in the fault zone increases,and the risk of water inrush due to damage to the water-resisting rock mass increases.(3)For seepage instability-type water inrush disasters,it is divided into tunnel excavation disturbance stage and hydraulic seepage instability stage.In the excavation disturbance stage,tunnel excavation causes the porosity and permeability of the fault zone to increase.The Richards equation of the COMSOL software was used to simulate and analyze the process of the groundwater seepage fault zone in the aquifer.The time ranged from 0d to 50 d.The entire fault zone infiltrated from an unsaturated state to a saturated state,and the seepage velocity and water inflow near the arch circle increased by about25 times.,the water pressure near the vault increases with time.The greater the hydraulic pressure of the aquifer above the fault zone and the greater the width of the fault zone,the faster the time for groundwater to infiltrate the filling medium will be.(4)According to the influence of tunnel support force on different water inrushes,ANSYS software load structure method is used to establish a model,simulate the increase of water pressure in the whole section to analyze the internal force change of the lining structure,and the maximum positive bending moment appears at the span shoulder when there is no water pressure,the water pressure increases,the force deformation of the lining structure increases,and the maximum positive bending moment appears at the arch foot when the water pressure increases to 1MPa.The grade of surrounding rock deteriorates,and the force deformation of the lining structure increases.Water inrush at different positions of instability causes inward deformation of the lining structure.The increased thickness of the lining structure can effectively resist the influence of groundwater pressure.