Huaifeng Sun

Three-dimensional Modeling of Transient Electromagnetic Responses of Water-bearing Structures in Front of a Tunnel Face

We present a finite-difference time-domain (FDTD) approach for the simulation of threedimensional (3-D) transient electromagnetic diffusion phenomena for the detection of waterbearing structures in front of a tunnel face. The unconditionally-stable du Fort-Frankel difference discrete method is used and an additional fictitious displacement current is introduced into the diffusion equations to form explicit difference equations. We establish a new excitation loop source which considers Maxwell’s equations in source media to overcome the limitations of the precondition that the near-surface resistivity of the model is uniform in the well-known 3-D FDTD algorithm demonstrated by Wang and Hohmann in 1993. The algorithm has the ability to simulate any type of transmitting current waveforms and arbitrarily complicated earth structures. A trapezoidal wave is used to simulate a step-off source. The fictitious permittivity is allowed to vary during the computation to ensure the stability and optimize an efficient time step. Homogeneous full-space models with different resistivities are simulated and compared with the analytical solutions to demonstrate the algorithm. Transient electromagnetic (TEM) responses of a tunnel with and without a water-filled vertical fault in front of the tunnel face are simulated and compared. 3-D models with water-filled fault and karst caves in front of a tunnel face are simulated with different parameters considered.

Multi-component and multi-array TEM detection in karst tunnels

Emerging applications of transient electromagnetic methods (TEM) in tunnelling require higher resolution on the distributions and shapes of low resistivity bodies, such as karst water and karst pipes, using multi-component and multi-array receivers. However, there are no apparent resistivity definitions for both vertical and horizontal components with offsets inside the loop. Although the raw field can show the differences of the earth electric structure, it is not straightforward. Apparent resistivity is very convenient and easy for engineers. We have developed a method for multi-component and multi-array TEM which can be applied in tunnelling and defined the expressions of apparent resistivity. This method takes advantage of the difference in resolution among components. A homogeneous half-space model and four typical three-layered models are used to test the effectiveness of the new definition. A field case history is carried out and analysed to demonstrate the viability of this technique. The results suggest that it is feasible to use the technique in tunnelling, especially for identifying the spatial distribution of karst water and karst pipes.

Application of the comprehensive forecast system for water-bearing structures in a karst tunnel: a case study

While tunneling in karst terrains, engineers may encounter unpredictable well-developed karst conduits, which frequently lead to water inrush accidents. Geological processes significantly affect the varieties and characteristics of water-bearing structures. Therefore, a comprehensive system for water-bearing structure prediction is first put forward, and then the geological and hydrogeological engineering conditions of the Yuelongmen tunnel in Southwest China are analyzed. To accurately predict the geometric characteristics of water-bearing structures and their spatial relationship with the tunnel face, the transient electromagnetic method (TEM) and ground-penetrating radar (GPR) were comprehensively applied. Then, the induced polarization method (IP) was utilized separately to detect the three-dimensional position and spatial distribution pattern of the water-rich area. According to the comprehensive forecast conclusion, targeted boreholes were drilled, which were selected to verify the water-bearing structure in the survey area. The drilling and detection results matched. Furthermore, the curtain grouting method was adopted for the treatment of the water-rich area. By establishing a comprehensive prediction technology system with the principle of “from qualitative analysis to quantitative identification, from structure locating to the water-bearing structure discrimination, as well as from far and near,” this comprehensive prediction system was successfully put into practice in the karst tunnel in Sichuan; it can play a guiding role in similar projects.

Practice of TEM tunnel prediction in Tsingtao subsea tunnel

Summary Introduce the TEM tunnel prediction and its application in Tsingtao subsea tunnel. Give an introduction about the apparent longitudinal quadratic differential conductance imaging method. It has the characteristic of more sensitivity to geological surfaces. Finally, give a field test case of TEM prediction in Tsingtao subsea tunnel. The result forecast two water bearing structure ahead of the tunnel face successfully.

Application of Ground Penetrating Radar to Rock Failure Analysis in High Risk Tunnels

The authors found a type of sidewall failure during the construction of a tunnel with high geostress. And relative strong rockburst usually occurred in the areas nearby during the excavation. This paper shows this type of failure called Crisp Fissure Failure and some characteristics of the failure. Reasons of the failure are also discussed. Further more, the authors studied the fracture distribution under this type of failure by Ground Penetrating Radar. Suggestions of support in a high risk tunnel are put forward based on the research of the Crisp Fissure Failure.

Finite Element Analysis of Grab Crane’s Metal Construction

This paper aims at presenting a theoretical basis as well as a simulation data for both strength check of grab crane and amendment of dynamic load. It introduces the methods and results of conducting a finite element analysis on the metal construction of grab crane which was accomplished by using ANSYS Workbench. Firstly, a simplified model of the grab crane’s metal construction was established. In accordance to the different working conditions of the grab crane, static analysis, modal analysis and transient analysis were performed. The results of simulation showed that the main beam’s maximum deformation was 1.8mm and the maximum equivalent stress was 78.367MPa, the design of grab crane’s metal construction was feasible.

Technology of transient electromagnetic synthetic aperture method in tunnel prediction

Summary As an electromagnetic induction method, TEM (transient electromagnetic method) is sensitive to low resistivity water-filled fracture zone, and has made some achievements in the advanced prediction. However, as the geological and hydro geological conditions encountered in the excavation of buried long tunnel become more and more complicated, it is difficult to forecast the water hazard problems in the construction process of complex geological conditions, so new interpretation methods that fit to such conditions is in urgent need. In this paper, the principle of transient electromagnetic synthetic aperture method is introduced. Firstly, we get longitudinal conductivities with equivalent conductive plane method. Then, by borrowing ideas from synthetic aperture radar, correlation superposition can be used in the processing of differential conductance signals. Lastly, draw profiles with synthesized signals and give a joint interpretation combined with apparent resistivity sections. The results of imaging and interpretation for measured data have shown that this method is sensitive to geological disasters in front of the tunnel face, and compared with previous methods, the resolution has improved greatly.