| With the continuous investment and construction of some world-class major projects such as "Sichuan Tibet Railway","Sichuan Tibet expressway" and "Central Yunnan water diversion project",the focus of tunnel engineering in China has gradually shifted to the western mountainous areas and areas with extremely complex geological environment characterized by high ground stress,high ground temperature,high water pressure,strong karst and multiple active faults.The accidents caused by water inrush are about 45% in the major geological disasters of tunnels,which are easy to cause heavy casualties and adverse social impact,and are the most challenging problems in the process of tunnel construction.However,for the advanced prediction of tunnel water abundance,the traditional prediction methods mainly focus on the indirect prediction of the geological interface,and the direct detection research on the characteristics of water abundance is still insufficient.The research results of transient electromagnetic three-dimensional finite element method in complex tunnel environment are less.It is of great theoretical and practical significance to carry out the research on transient electromagnetic method(TEM)response characteristics and advanced identification method of tunnel water rich structure..Based on this,this paper takes the tunnel water rich structure as the research object,the transient electromagnetic method as the detection method,and makes full use of the modern engineering geology,geophysics,statistical induction,numerical simulation,comparative research and other methods,from the geological and geophysical model and characteristics of water rich structure,the geological and geophysical characteristics of the tunnel water rich structure,the geological and geophysical characteristics of the tunnel water rich structure,the geological and geophysical characteristics of the tunnel water rich structure,the geological and Based on the indepth study of the three-dimensional forward algorithm of transient electromagnetic finite element and field response,as well as the advanced identification and application of water rich structure based on transient electromagnetic characteristics,the geological and geophysical fusion analysis framework for the advanced identification of tunnel water inrush is preliminarily constructed.The main achievements and understandings are as follows:(1)The tunnel water rich structure can be divided into three typical types: water rich karst,water rich fault and water rich fracture zone.Based on the case statistics,it is known that the water filled karst structures account for 51%,mainly in the form of water rich structures such as karst cavity,pipeline and underground river;Water rich faults account for 28% of the total,and most of them are water rich water conducting faults and water resisting faults,which form major or even extremely serious disasters of water and mud inrush;Water rich fracture fracture zone accounts for 21% of the total,mainly including fold fracture fracture zone and lithologic junction fracture fracture zone.The difference of electrical and dielectric properties of water rich karst,water rich fault and water rich fracture zone in direct response to water rich property is the physical property basis for the detection and application of transient electromagnetic method..(2)In the tunnel environment TEM time domain vector finite element threedimensional forward modeling method research.In view of the limitation of small space and multi angle detection of tunnel,the first-order tetrahedron mesh method is used to simulate the changes of complex geological conditions more accurately;In the numerical method,the time domain is used to solve the problem directly,which avoids the reduction of the accuracy of solving in the frequency domain and then in the time domain.Firstly,the governing equations of time-domain electromagnetic field are derived by using time-domain Maxwell equations and boundary conditions;Secondly,based on the Galerkin weighted residual method,the corresponding weak form equations of the vector finite element governing equations are derived,and the firstorder tetrahedral element is used to derive the interpolation functions at the nodes and the vector basis functions at the edges for element analysis.The unstructured mesh generation method with local refinement is used to encrypt the source area and abnormal area of tunnel environment field.The loop source is divided into multiple current elements in series to overcome the singularity around the source,and the transient electromagnetic field response calculation of tunnel is realized.Finally,the uniform full space model and Newman model are used to verify the correctness and accuracy of the algorithm.The calculation results in this paper are in good agreement,and the relative error is controlled within 2.5% and 2%.(3)The transient electromagnetic response characteristics and laws of typical water rich structures in tunnel are obtained.For the characteristics of water rich fault and uniform transient electromagnetic field,the vertical component of magnetic field is backward with time,the positive part of vertical magnetic field is outward with time,the central extremum is gradually smaller,the maximum value is always in the central position,and the contour line is thinning from dense to sparse until the final Hz intensity is weaker and weaker.Transient electromagnetic detection has directionality,and it detects along the detection direction of the detection coil axis to obtain the strongest induction field strength.From the curves of different positions,the response curve of the measuring point facing the face is the strongest,and the response of the side measuring point gradually weakens with the increase of distance.The simulation results show that the measurement angle is 0 ° The results show that the transient electromagnetic field has the best coupling with low resistivity geological body,and its response value is the largest.The contour of TEM response on tunnel axis section can basically describe the shape,location and boundary of water rich structure.The induced electromotive force shows a strong response in the water rich structure and a maximum,corresponding to the low resistivity of the water rich structure,and a relatively weak response and a minimum outside the water rich structure.When the rock fracture is rich in water,the response of induced electromotive force will close in a small range.For the water rich fracture zone,the isoline presents multiple induced electromotive force trap areas,and the single trap area is small,showing obvious differences in different angles of induced electromotive force profiles;The closed area of induced electromotive force is different in shape,similar to the development trend of solution and fracture.The main source of transient electromagnetic field affected by metal equipment in roadway is construction trolley.The closer the trolley is to the detection coil,the stronger the interference is.The farther the trolley is,the weaker the interference is.When the trolley is more than 30 m away from the coil,the influence of the trolley on the transient electromagnetic field of the tunnel can be ignored.(4)The fuzzy neural network identification method of water rich structure is established,which is mainly based on transient electromagnetic water rich index and geological characteristics direct water rich index.Under the analytical framework of geological and geophysical multi-source information constraints,the transient electromagnetic field response characteristics of water rich structures obtained by numerical simulation are combined with the image recognition characteristics of typical cases.The forward modeling is used to assist the design of transient electromagnetic field detection parameters,and the forward modeling is applied to the advanced recognition process of water rich structures.Finally,the identification features of TEM apparent resistivity image,statistical threshold of apparent resistivity,ground water of tunnel face,survey and design data and the main indicators of TEM direct water identification are selected to establish the advanced identification method of water rich structure based on fuzzy neural network model.(5)The magnetic characteristics of transient electromagnetic response,auxiliary parameter design of forward modeling and multi-source advanced identification method are applied to the advanced identification of water rich karst fissures in Tianchi tunnel.At the same time,the fuzzy neural network increases the direct water rich index of Geology and TEM,which improves the prediction accuracy.In addition,the transient electromagnetic method is successfully applied to the detection of water rich karst cave in Xinhua tunnel and water rich fault in xinyuexi tunnel.The example shows that the detection accuracy of TEM for water rich structure is high. |
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