Study On Unloading Seepage Characteristics And Treatment Technology Of Tunnel Surrounding Rock Excavation

The seepage characteristics of surrounding rock in water-rich tunnel depend on fracture network of rock mass.Due to the redistribution of stress under excavation and unloading,the fracture will be closed,developed,germinated or developed,which will cause the change of the seepage field.The change of the seepage field will react on the fracture network,showing a coupling situation of interaction and interaction.Therefore,it is necessary to understand the mechanism of excavation unloading seepage from multiple perspectives,so as to effectively prevent geological disasters such as water inrush and mud inrush caused by groundwater seepage in the process of excavation unloading.Based on this,this paper takes a water diversion tunnel project in Hainan Province as the background,studies the seepage characteristics of tunnel surrounding rock excavation unloading by theoretical research,numerical simulation and monitoring verification,and puts forward reasonable and applicable comprehensive treatment technology.The main research contents and conclusions are as follows:(1)This paper summarizes the calculation formulas of permeability coefficient and seepage flow of water rich fractured rock mass under test conditions or based on normal deformation of fracture,and points out that the essence of different seepage flow calculation formulas is the change of various parameters.Based on the unloading rock mechanics and bandis hyperbolic model,the calculation formulas of seepage stress coupling permeability coefficient and seepage flow under the unloading tension and pressure stress of single fracture rock mass are given.Combined with a water diversion tunnel project in Hainan,the variation law of permeability coefficient and seepage flow under the unloading stress path and the fracture action mechanism are analyzed.The research shows that with the increase of unloading rock mass,the permeability coefficient and seepage flow are increased The change of permeability coefficient will be affected by fracture water pressure and unloading amount.The change of permeability coefficient corresponds to the five unloading stages of rock mass.When unloading causes rock mass to enter the plastic deformation stage,the sensitivity of fracture deformation to the change of unloading stress increment is strong,the permeability coefficient begins to increase significantly,and the permeability of rock mass increases.(2)The numerical model is established from three aspects of different groundwater level,different excavation step distance and different thickness of overburden layer,and the change of permeability coefficient is used to simulate the excavation and unloading process.Through the control variable analysis,the change laws of pore water pressure,node seepage flow,hydraulic gradient,seepage velocity and surrounding rock deformation during the excavation and unloading process are studied.The research shows that the higher the groundwater level,the larger the excavation cycle distance and the thickness of the overlying soil layer,the more violent the variation of the seepage movement parameters in the range of three times the tunnel diameter is,and the seepage point is easy to form at the key nodes of the tunnel vault,arch foot and so on,and the high-pressure water rich area will be formed after the tunnel face.At this time,the tunnel face needs to be set with appropriate outburst prevention safety thickness to deal with the high-pressure behind the tunnel face Water.(3)The monitoring data of the excavation section with the same or similar hydrogeological conditions in the construction site are compared with the numerical simulation results,and the vertical and horizontal displacement laws of the surrounding rock are analyzed to prove the reliability and Simulation of the model.The results show that the excavation unloading seepage deformation of the surrounding rock of the tunnel is greatly affected by different factors.(4)Based on the results of theoretical and Numerical Simulation Research on the seepage characteristics of excavation unloading,and based on the existing grouting and water plugging technology,a series of comprehensive measures are put forward for treatment,such as verifying the underground water state in front of the face,verifying the advance exploratory hole,determining the safe thickness of the face against outburst,arranging the relief hole and the pipe valve drainage of the relief hole,surface grouting and surface backfilling,and advancing the full section curtain grouting Seepage and water inrush in the excavation of surrounding rock in the water rich section.