The Refined Application Research Of Tunnels’ Advanced Forecast In Karst Area

Karst is widely distributed in southwest China,tunnels construction often along with geological disasters such as water inrush and mud inrush.How to do a good job in advance prediction of tunnel and ensure the safety of tunnel construction are important research topics at present.Ground penetrating radar(GPR)has the advantages of lightness,high efficiency and high resolution,which can realize the accurate detection of short-distance targets in tunnel.Transient electromagnetic method(TEM)can distinguish conductive targets well.It also can realize the delineation of karst aquifer and qualitative analysis of geoelectrical structure in the middle-distance in front of the palm-face.Relying on the research content of the research project "Comprehensive Aeronautical Geophysical Data Processing Methods and Software Development"(No.2017YFC0602202)of the National Key R&D Program Project "Development of Aerospace Geophysical Technology Demonstration and Processing Interpretation Software Platform in Typical Coverage Areas" as well as "Sichuan Emei-hanyuan Highway Engineering Research Project"(LH-HT-45),based on the principles of above two geophysical exploration methods,this paper respectively discusses the radar wave propagation characteristics and transient electromagnetic field distribution characteristics in different targets.Then,above two methods are applied to Emei-hanyuan engineering application project with good complication results.In the research on GPR: Firstly,time domain finite difference forward modeling principle of GPR was introduced based on maxwell’s equations.Then,by using Gpr Max3.0 and Randa6.0software,the propagation process of radar wave were simulated in different geological targets,such as intact rock,karst cave,fault fracture zone,weak intercalations;meanwhile summarizing the propagation law and characteristics of radar signal.Finally,GPR was applied to the advanced detection of several tunnels with a lot of prediction achievements.In the research on TEM: Firstly,TEM responses of horizontal electric dipole,vertical magnetic dipole and square loop source in uniform full space are detailed derived.Through formula derivation and numerical calculation,it is found that uniform full space response of the central loop device is about 2.5 times that of uniform half space response.Then,the basic principle of three-dimensional finite element forward modeling is introduced and electromagnetic field distribution in vertical water-filled faults,water-filled caves and complex geological bodies in front of the tunnel face was simulated in this way.The results show that horizontal component of electric field in the symmetric model is divided into positive and negative parts and is symmetric about tunnel middle axis.Electric field lines are diffused outward from both sides of the symmetric model.The time derivative vertical component of magnetic induction intensity in the symmetric model is also symmetric about the tunnel middle axis and diffuses outwards from the center of model.By simulating the responses of caves with different resistivity,different buried depth and different scale,it is found that the lower resistivity,the shallower buried depth and the larger scale of geological targets,transient electromagnetic response is more obvious.A survey line was arranged on the tunnel face to observe the transient electromagnetic response of low resistivity cube.From the low resistivity anomaly on all-time apparent resistivity profile,it can be seen that using TEM to detecte low resistivity targets has obvious advantages.Finally,TEM is applied to the actual tunnel detection,meanwhile electrical structure in front of the tunnel face is given.According to geological conditions of excavated tunnel,verifying that applying TEM to the tunnel advance prediction is feasible.Combined with theory and practice,this paper has gained a lot of recognition on advanced tunnel detection.Among them,the distribution of abnormal targets field,the image interpretation marks of GPR and using TEM to detect water in tunnels can provide a little experience and reference for advanced prediction of other similar tunnels.