Study On Method Selection And Interpretation Threshold Of Advanced Geological Forecast In Tunnel

China attaches great importance to infrastructure construction,highway is the focus of infrastructure construction,tunnel construction is an important part of highway engineering.Especially in southwest China,the complicated terrain and geological conditions determine the necessity of tunnel construction.In the course of tunnel construction,it is inevitable to encounter unfavorable geologies,and the common unfavorable geologies includes broken rock mass,weak zone,fault,karst cave,etc.Unfavorable geologies will easily cause tunnel collapse,mud gushing,water gushing and other disasters.In order to find out the unfavorable geological conditions in the tunnel site area and effectively avoid the construction geological disasters,the advanced geological forecast technology was introduced into the tunnel engineering to find out the unfavorable geological conditions in a certain area in front of the tunnel face before the tunnel excavation.The geophysical exploration method is a common method of tunnel advanced geological forecast.The common geophysical exploration methods in tunnel are tunnel seismic prediction,ground penetrating radar,transient electromagnetic method,infrared water detection and land sonar method.Taking Tianchi Tunnel of Jiumian Expressway as the engineering support,the importance weight of tunnel seismic prediction,ground penetrating radar and transient electromagnetic method in targeted geophysical forecast scheme is calculated by analytic hierarchy process.This paper summarized the geological forecast data of tunnel seismic prediction in the Zhegushan Tunnel,Tongluoshan Tunnel and Dabashan Tunnel,and obtained the regularity of longitudinal wave velocity change value(?v_p)and longitudinal and shear wave velocity ratio change value(?v_p/v_s)of common unfavorable geologies.Based on the data of transient electromagnetic method in Tongluoshan tunnel and Mingyueshan tunnel,the regularity of apparent resistivity(?_T)of common unfavorable geologies was obtained.These regularities were applied to the interpretation of advanced geological forecast in tianchi tunnel of Jiumian expressway to improve the accuracy and efficiency of interpretation.The main research results of this paper are as follows:(1)The longitudinal wave velocity,shear wave velocity,longitudinal and shear wave velocity ratio are three important parameters in the interpretation of tunnel seismic prediction.They have thirteen models.The longitudinal wave velocity change value can reflect the change of rock integrity,the longitudinal and shear wave velocity ratio change value can reflect groundwater development.Through mathematical statistics,the corresponding longitudinal wave velocity change value and longitudinal and shear wave velocity ratio change value threshold ranges of three models most likely to reflect unfavorable geological conditions were obtained.This experience is applied to the advanced geological forecast of Tianchi tunnel.Firstly,the integrity of the rock mass and the water-rich condition are preliminarily determined according to the conventional interpretation,and then the interpretation is carried out quickly and accurately by looking up the table.This method has the effect of improving the accuracy and efficiency of tunnel seismic prediction.(2)In the interpretation of transient electromagnetic method,apparent resistivity is an important parameter to reflect the development of groundwater.The threshold range of apparent resistivity in common water-bearing joints and water-bearing karst regions was obtained by mathematical statistics.This experience is applied to the advanced geological forecast of Tianchi tunnel.Firstly,the water-rich condition of rock mass is preliminarily determined according to the conventional interpretation,and then the interpretation is carried out quickly and accurately by looking up the table.This method has the effect of improving the accuracy and efficiency of transient electromagnetic method.(3)In terms of the selection of advanced geological forecast method,this paper applies analytic hierarchy process to calculate the weight of importance in the geophysical forecast scheme,and establishes a set of advanced geophysical forecast system based on analytic hierarchy process.The hierarchical model includes all the factors that influence the advanced geological forecast scheme of Tianchi tunnel.The first forecast takes into account all the influencing factors and chooses a geophysical exploration method,when the unfavorable geologies are found in the first forecast,other geophysical exploration method for unfavorable geologies are selected by analytic hierarchy process for re-survey,two or more geophysical exploration methods can eliminate the multi-solution of forecast and improve the accuracy of forecast.