Shucai Li

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.

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.

Characterization of shallow karst using electrical resistivity imaging in a limestone mining area

The Guangxi area is famous for its high degree of karstification due to a long period of groundwater erosion and the development of fracture networks. Karst collapse appeared during the mining process and caused many environmental problems. Applying electrical resistivity imaging (ERI) is the most cost-effective method to study the characteristic of the subsurface karst environment. In this area, more than 24xa0km of ERI profiles around a mining area is carried out to present the development of karst which will be used to evaluate the effects on the ground water and the surrounding environment. The area represents a classical limestone rock which is of high quality and is used for cement production. In this study, the ERI is used to determine the geometry of the karst range at depth. The results are in good agreement with drilling experiments. The ERI survey provided near-surface resistivity information, which is very useful for establishing the geometry and the position of potential karst.

Generating Irregular Models for 3D Spherical-Particle-Based Numerical Methods

The realistic representation of an irregular geological body is essential to the construction of a particle simulation model. A three-dimensional (3D) sphere generator for an irregular model (SGIM), which is based on the platform of Microsoft Foundation Classes (MFC) in VC

Study on the Mechanical Behavior of Excavation and Support Construction of Shallow Tunnel

In this paper, the mechanical behavior of excavation and support construction of Weishe tunnel, which is a section of the Yangwu expressway, is studied quantitatively using 3D finite difference numerical simulation method. A sequential excavation method is used and the results show that the vault settlement occurs mainly on the phase of upper bench excavation. The convergences of upper and lower sidewalls occur mainly on the phase of lower bench excavation. During the construction, the surrounding rock pressure in the vault and sidewall of the tunnel decrease. Axial force of anchor reaches the maximum value after the finish of second lining. However, the surrounding rock pressure and internal force of steel arch reach the maximum value after completing the upper bench excavation, and then become as smaller as half of the peak value during the lower bench excavation.

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.

Study of Advance Surrounding Rock Classification Based on TSP203 and Extenics

In order to conduct tunnel advance surrounding rock classification effectively, the authors put forward an advance surrounding rock classification method based on TSP 203 system and Extenics. The method extracts the useful information from the detection results of TSP 203, establishes the indicators system of advance surrounding rock classification and adopts extensible comprehensive evaluation method to conduct advance surrounding rock classification. In the progress of establishing surrounding rock advance classification index system, six rock classification parameters are adopted, which TSP203 could identify effectively. They are rock mass integrity coefficient, Poissons ratio, static Youngs modulus, the angle between main structural surface and the tunnel axis, discontinuous structural surface and the state of groundwater. Finally, the method is applied to practical engineering and the results show that the method has higher prediction accuracy in advance surrounding rock classification and provide a new idea for advance surrounding rock classification.

Development of Model Test System for Grouting Simulation in Flowing Water and Study of the Diffusion Form of Anti-Dispersion Grout

In order to simulate the diffusion form of anti-dispersion grout, the first visual large-proportion plane fissure test system for the physical model of fracture-grouting in flowing water is developed in China. The plane fissure test system is made up of plane fissure test-bed, monitoring system, water supply and drainage system, grouting system, control system and data acquisition system. The fiber Bragg grating (FBG) sensors for pressure, velocity and temperature monitoring are employed because of its small size and high sensitivity. The U-shaped diffusion form of anti-dispersion grout is found by the plane fissure grouting test in the flowing water.