Permeability Evaluation Method Of Tunnel Water-bearing Structure Based On Modified Marshall-madden Model

Chinese basic engineering construction is showing a vigorous development trend.A number of railways,highways,water conservancy and hydropower tunnels are under construction or are about to enter the construction schedule,and there are a large number of deep and long tunnels.During the construction of deep and long tunnels,disasters such as water bursts and water inrush are often encountered,which brings serious harm to the safety of life and property and engineering construction.The water permeability of the aquifer has a direct relationship with the magnitude of the disaster.Therefore,the evaluation of the permeability of the water-bearing structure in front of the tunnel face is of great significance for early warning and prevention of the construction period.However,traditional permeability evaluation methods such as hydrogeological tests often have characteristics such as hysteresis and one-sidedness.Geophysical methods have advantages in overcoming the above shortcomings.Among them,the induced polarization method has the advantage of being sensitive to structural parameters of water-bearing structures.Tectonic advance prediction,hydrogeological survey and other aspects have been widely used,and the use of induced polarization method to evaluate the permeability of water-bearing structures is an emerging direction in the world.However,at present,the relationship between induced polarization observation data and formation permeability model has some problems such as strong empirical dependence and difficulty in obtaining structural parameters of strata,and it is difficult to meet the urgent needs of permeability evaluation of tunnel water-bearing structures.In view of the difficulty of evaluating the permeability of water-bearing structures in tunnels,this paper uses theoretical analysis and laboratory tests to improve the Marshall-Madden capillary model,and establishes a relationship between the induced polarization observation data and the structural parameters of the capillary model,revealing the positive correlation between relaxation time and permeability.A method for inversion of the capillary model structure parameters based on the induced polarization observation data is proposed,combined with the capillary model permeability calculation method,a permeability prediction model for porous geotechnical media is established,and finally the permeability of the aquifer in front of the tunnel face is formed.The evaluation method,relying on the Gaoligongshan Tunnel Project and Baoshan Tunnel Project,carried out project verification and achieved good results.The main research work and results of this article are as follows:(1)Induced polarization response characteristics based on the improved Marshall-Madden model.The characteristics of the induced polarization effect model based on the capillary model are studied and analyzed.The Marshall-Madden induced polarization effect model that characterizes the pore structure parameters of the geotechnical medium is improved.A large number of forward simulations were carried out based on the improved model.The response characteristics of the induced polarization observation data to the capillary structure parameters were systematically studied,and the relationship between the induced polarization observation data such as relaxation time and the capillary model structure parameters was revealed.It provides a theoretical basis for the evaluation of permeability.(2)Estimation method of equivalent permeability of geotechnical porous media based on inversion of capillary structure parameters.Based on the induced polarization observation data,genetic algorithm and least square method are used to obtain the structural parameters of the capillary model,and the model parameter inversion is performed based on the synthetic data to verify the effectiveness of the inversion algorithm.Combined with the composite mapping relationship of induced polarization observation data-capillary model-permeability,a method for estimating equivalent permeability of porous media based on the induced polarization method was established,and a corresponding estimation program was independently developed to realize porous geotechnical media permeability prediction.(3)Model test verification and model modification of water-bearing sand layer.Eight kinds of quartz sands were used to simulate the water-bearing sand layer under different working conditions.The induced polarization test and Darcy seepage test were carried out on the water-bearing formations with different pore structures.The prediction results of the induced polarization method for the permeability estimating proposed in this paper are compared with those of Darcy seepage test.The measured results of the seepage test verify the effectiveness of the induced polarization permeability estimation method,and modify the parameters of the permeability prediction model.(4)Evaluation method of permeability of tunnel water-bearing structure based on the induced polarization relaxation time.Through the systematic simulation of the induced polarizationt tunnel advance detection,the positive correlation between the position of the maximum value of the induced polarization relaxation time and the position of the water-bearing structure is revealed.Combining the model of the relationship between the induced polarization relaxation time and permeability,the qualitative prediction of the location and permeability of water-bearing structures ahead of tunnel face is realized.The research results have been applied to engineering practices such as Gaoligongshan Tunnel and Baoshan Tunnel,which have verified the effectiveness of the induced polarization evaluation method of tunnel permeability.