Study On The Reduction Coefficient Method Of Assessing The Tunnel External Water Pressure

Since the 21st century,with the rapid development of national economy and the continuous improvement of comprehensive national strength,the state has put forward the strategic plan of "ten vertical and ten horizontal high-speed railway network" and "five vertical and seven horizontal motorway network",railway and highway undertakings have made rapid progress.However,China is a mountainous country,the mountainous area accounts for about two-thirds of the territorial area,so a large number of deep-buried cross-mountain tunnel emerged as the times require,especially in the southwest region.During the construction of cross-mountain tunnel,it is unavoidable to cross the strata with complicated geological conditions,even through high pressure and water-rich strata,serious water gushing and high water pressure problems will occur in the tunnel,which poses a great threat to the safe construction and normal operation of the tunnel.At present,the reduction coefficient method is often used to calculate the tunnel external water pressure,which is mostly empirical or semi-empirical values,and slightly deficient in quantitative calculation.Therefore,this paper on the basis of comprehensive consideration in the influence of surrounding rock permeability and lining parameters on the tunnel external water pressure,according to the theory of groundwater seepage mechanics,deeply analyzing the head loss caused by tunnel excavation and groundwater seepage,and deriving the analytical formula of the reduction coefficient in uniform hydrogeological structure and gradual hydrogeological structure.Then,aiming at physical simulation test of the external water pressure for tunnel crossing uniform hydrogeological structure,quantitatively analysing the test corrected formula of the reduction coefficient,verificating and optimizating the test corrected formula in combination with the analytical formula.Finally,taking the Xiema tunnel in Zhongliang mountain of Chongqing as an example,verificating the reliability of the external water pressure calculated by the test corrected formula and the analytical formula of the reduction coefficient,in order to provide a reference for predicting and evaluating the tunnel external water pressure,the conclusions as follows:(1)According to the permeability and spatial combination characteristics of surrounding rock media,the hydrogeological structure of tunnel crossing can be divided into uniform structure and gradual structure.For the gradual hydrogeological structure,a negative exponential model is proposed to modify the permeability coefficient of rock mass in tunnel depth kZ,according to the average values of the permeability coefficient of surface rock mass k0 and the permeability coefficient of rock mass in tunnel depth kZ,to optimize the permeability coefficient of rock mass?(2)Based on the theory of groundwater seepage mechanics and the well flow formula of groundwater moving towards the confined well,the loss coefficient of the tunnel external water pressure head caused by tunnel excavation is deduced.Then,by using the formula of velocity head in hydraulics,the loss coefficient of tunnel external water pressure head caused by groundwater seepage is deduced.According to the law of conservation of energy,the reduction coefficient of tunnel external water pressure? is the product of the loss coefficient of tunnel excavation ?1 and the loss coefficient of groundwater seepage ?2.When tunnel crosses through the uniform hydrogeological,the analytical formula of the reduction coefficient is obtained as follows:without grouting circle,?=(?);with grouting circe,?=(?).The reduction coefficient of tunnel crossing the gradient hydrogeological structure is basically the same as that of the uniform hydrogeological structure,it is only necessary to repair the permeability coefficient of rock mass kr with the equivalent permeability coefficient.(3)Through the physical simulation test of the external water pressure for tunnel crossing through uniform hydrogeological structure,it is concluded that,with the decrease of the lining permeability coefficient kl,and the increases of the permeability coefficient of surrounding rock kr,the reduction coefficient of the tunnel external water pressure ? increases continuously,and fit the test formula of the reduction coefficient.The reliability of physical simulation test is higher by combining the analytical formula of the reduction coefficient,and it is more appropriate to use the negative exponential function relationship to fit the reduction coefficient,importing the ratio of influence radiusr2 to lining radius r1 to optimize the test formula,the test corrected formula of the reduction coefficient is ?=(?).(4)Taking the Xiema tunnel in the north section of Zhongliang mountain in Chongqing as an example,the test corrected formula of the reduction coefficient,the analytical formula,and other empirical formulas to calculate the tunnel external water pressure,and through the comprehensive comparative analysis of the field measured tunnel external water pressure,it is concluded that the test corrected formula method of the reduction coefficient and the analytical formula method proposed in this paper are more accurate and reliable for calculating the tunnel external water pressure.