Comprehensive Evaluation Method Of Submarine (Underwater) Tunnel Permeability Basedon Induced Polarization And Seismic Wave Joint Detection

With the continuous improvement of comprehensive national strength and the rapid development of the national economy in China,infrastructure projects have ushered in a peak period of construction.Cross-regional interconnection and blue ocean development have put forward an urgent need for the construction of submarine(underwater)tunnels.At present,the construction speed of submarine tunnel in China is the fastest in the world,but at the same time,its construction safety is also facing severe challenges.Complex geological conditions such as faults and strong weathering zones,as well as the extreme occurrence environment of infinite water strong recharge and high water pressure load all have the possibility to induce large-scale water inrush disasters.Therefore,it is a key prerequisite to carry out theoretical research on the prevention of water inrush disaster to ensure the safe and efficient construction of submarine(underwater)tunnels.Water-bearing structure is the main geological inducement of water inrush disaster in submarine tunnel,and the location,water volume and permeability distribution of disaster-causing water body are important factors that determine the occurrence of water inrush disaster and evaluate the magnitude of water inrush disaster.Seismic wave method and induced polarization method have become the mainstream methods in the ahead prospecting of adverse geology in tunnel due to their advantages in obtaining structural information and water-bearing information of adverse geological bodies respectively.However,there are the following problems when applied to the permeability evaluation and water inflow prediction of submarine tunnels:(1)The existing porosity prediction methods are difficult to be widely used in the face of complex geological conditions and porous surrounding rocks of submarine tunnels,resulting in limited application scope and low accuracy of permeability prediction models based on porosity;(2)The single permeability prediction method is greatly influenced by geological environment,and it is difficult to accurately evaluate the permeability in front of the palm face.There is no comprehensive permeability evaluation method suitable for complex adverse geological conditions of submarine tunnels and high salinity seawater environment.Aiming at the above problems,this paper proposes a solution of combining induced polarization and seismic wave method to evaluate the permeability of submarine(underwater)tunnel by means of theoretical analysis,physical experiments,inversion imaging and field test.The relationship model between longitudinal wave velocity and porosity and permeability,and the relationship model between relaxation time and permeability are studied respectively,then a comprehensive permeability prediction model based on relaxation time and longitudinal wave velocity is established.A permeability distribution evaluation method based on comprehensive permeability prediction model is proposed.which realizes accurate evaluation of tunnel permeability,and the tunnel water inflow is predicted by combining empirical formulas.Finally,the effectiveness of the method proposed in this paper is verified by field tests.The main research work and achievements of this paper are as follows:(1)Prediction model of porosity and permeability based on wave velocity.Aiming at the problems that it is difficult to obtain porosity directly and accurately in the complex geological environment of submarine tunnel,permeability-porosity-longitudinal wave velocity test is carried out based on the typical lithologic rock samples in the sea area.And the relationships between porosity and longitudinal wave velocity,permeability and porosity,permeability and longitudinal wave velocity are studied systematically.The porosity prediction model based on wave velocity is established,and the permeability prediction model based on wave velocity is established by taking porosity as the medium,which lays the physical model foundation for evaluation of surrounding rock permeability by seismic waves.(2)Permeability prediction model based on relaxation time.Aiming at the problem that the complex pore structure of submarine tunnel and high salinity of seawater are not taken into account in the existing permeability prediction model,the induced polarization tests with different salinity are carried out based on the typical lithologic rock samples in the sea area.And the relationship between permeability,salinity and relaxation time is discussed.A permeability prediction model based on induced polarization relaxation time is established,which lays a physical model foundation for evaluation of surrounding rock permeability by induced polarization.(3)Comprehensive evaluation of permeability and estimation of water inflow.Aiming at the problem that it is difficult to accurately predict permeability with a single physical parameter,a comprehensive permeability prediction model based on relaxation time and longitudinal wave velocity is established by taking the relationship between induced polarization parameters,elastic wave parameters and pore structure as a bridge,which improves the accuracy of permeability prediction.An imaging method of tunnel permeability distribution based on relaxation time distribution and wave velocity distribution is put forward.Then the comprehensive permeability evaluation method suitable for submarine(underwater)tunnel is formed.Based on the results of permeability prediction,combined with the empirical formula of water inflow,the water inflow within the detection range in front of the tunnel is estimated.Based on the above research,the field test is carried out in the near-underwater tunnel section of Xianglushan Tunnel of Yunnan Central Water Diversion Project and Shantou Bay Submarine Tunnel in Guangdong Province.The test results accurately characterize the three-dimensional permeability distribution in front of the face of the tunnel,and the predicted water inflow is basically consistent with the actual excavation water inflow,which verify the feasibility and effectiveness of the comprehensive permeability evaluation and water inflow prediction method of submarine(underwater)tunnel based on induced polarization and seismic wave joint detection.