| Transient electromagnetic method(TEM)is a time domain electromagnetic geophysical detection method.The application and theoretical research of geological survey,mineral exploration,groundwater search under semi-spatial conditions are quite mature,while the transient electromagnetic method under full space is relatively lagging.With the development of infrastructure construction,as a kind of underground engineering,there are more and more tunnel construction projects in China in recent years.However the adverse geological hazards which affect the construction of tunnel project and threaten the construction safety occur frequently.Transient electromagnetic method is sensitivity to low resistance,so it could identify the water-rich fault,fissure fracture zone,karst development section effectively.Altogether it is very important to study the theory and engineering application of tunnel transient electromagnetic method ahead of geological prediction under full space.Based on the advanced geological prediction project of Micangshan extra-long tunnel,the theoretical research,numerical simulation and engineering application of the combination of methods are used to analyze the general law of electromagnetic response of tunnel transient electromagnetic method.Also it's compared with other advanced geological prediction results to verify the feasibility and correctness of the advanced geological prediction of the transient electromagnetic method.The corresponding engineering treatment measures is proposed finally.The main research results are as follows:(1)The paper adopts the ANSYS finite element software to simulate the tunnel transient electromagnetic method and the three-dimensional model of the tunnel is established.The pure tunnel space and the unfavorable geological body with water are simulated calculation.The "water wave effect" of eddy current and electromagnetic induction intensity were proved by numerical simulation under the condition of full space.The sensitivity that transient electromagnetic method is sensitivity to low resistance provides a theoretical basis for transient electromagnetic method to be applied to advanced geological prediction.By changing the parameters such as azimuth,scale,thickness and water content of the unfavorable geological body in front of the tunnel face,the general law of the transient electromagnetic response under the condition of each parameter is obtained.The influences of metal body and karst low resistance upon the transient electromagnetic method response are studied to obtain the general law of electromagnetic response under the influence of the disturbance.(2)Transient electromagnetic instrument is carried out to detect the water bearing zone in Micangshan extra-long tunnel advanced geological prediction.The results show that the transient electromagnetic method has obvious detection effect on the water-rich fault,fissure fracture zone,karst development section.It also proves the effective avoidance distance of the metal space in the tunnel space should be greater than 30 m and the influence of the exploration result on the surrounding rock space in the back surface of the bracelet is limited.The transient electromagnetic method exhibits a higher detection accuracy compared with TSP and GPR in terms of surveying water.(3)The corresponding engineering treatment measures for the water-rich fault,fissure fracture zone,karst development section are proposed.For the water-rich fault and fracture zone,advanced pipe shed,advanced small pipe,ahead of the anchor as well as other support methods can be adopted.While for the karst development section,the principle of "blocking first,combine blocking with draining,local conditions and comprehensive management" is put forward.The efficiency and safety of tunnel construction could be improved greatly with these measures. |
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