Multi-Field Information Comprehensive Analysis of Tunnel Karst Cave Prediction Shao-shuai Shi1, Shu-cai Li1, Li-ping Li*1, 2, Cong Liu1, Zong-qing Zhou1, Qing-han Wang1 1. Geotechnical and Structural Engineering Research Center，Shandong University, Jinan, Shandong 250061, China 2. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Jiangsu, Xuzhou 221008, China

 

ABSTRACT Water inrush and burst mud can be caused by the unknown Karst caves. In this paper, the prediction principle of Ground Penetrating Radar (GPR), Transient Electromagnetic Method (TEM) and High Density Resistivity Method (HDR) are compared. The multi-field information comprehensive analysis of tunnel Karst cave prediction is introduced. The application in Xiakou tunnel of Yiba highway shows that this comprehensive method can detect the position and scale of Karst cave accurately, and supply some useful guides for tunnel construction to avoid the water inrush and burst mud.

KEYWORDS:

Tunnel, Karst cave, Multi-field information, GPR, TEM, HDR,

Application

INTRODUCTION With the construction rapidly develops in China, more tunnels are constructed in western Karst areas with complicated geological conditions [1-3]. In this area, Karst cave is one of the most common accidents in the tunnel construction [4-5]. As shown in Figure 1, Water inrush and burst mud can be caused by the unknown Karst caves.

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Figure 1: The pictures of water inrush and burst mud The single method has been used to detect the unfavorable geology . However, every method has different response for the Karst cave. These methods have a drawback of one-sided. The GPR[6-9] and TEM belong to electromagnetic wave method [10-13], and the HDR belongs to electrical method [14-16]. The electromagnetic response of Karst cave is different from electrical method. When the electromagnetic or electrical single encounter Karst cave, the particular precursory information should be received by instrument. In order to achieve the purpose of accurate prediction, the comprehensive analysis method of multi- field information is applied in Xiakou tunnel of Yiba highway. The prediction result shows that this comprehensive method can detect the position and scale of Karst cave accurately, and supply some useful guides for tunnel construction to avoid the water inrush and burst mud.

METHODS Ground Penetrating Radar（GPR） GPR working principle is that the transmitting antenna directionally transmits the high frequency short pulse electromagnetic waves, the electromagnetic waves will be reflected or refracted when they encounter formation or unfavorable geology body with different electrical property. [6-9] The reflected wave energy is determined by reflection coefficient R. For the tunnel project, the unfavorable geology body and surrounding rock all have high impedance, so the reflection coefficient R and refraction coefficient T can be represented below: R

1   2 1   2

(1)

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T

2 1

1   2

(2)

where 1 and  2 are relative dielectric constant of media before and after the interface.

Figure 2: The principle of GPR

Transient Electromagnetic Method (TEM) The excitation field sources of Transient Electromagnetic Method mainly have types. One type is the magnetic source of loops form (or current-carrying coil), and another is the current source of grounding electrode forms. Below uniform transient electromagnetic response of the earth as an example, to discuss transient electromagnetic field inspired by loops form magnetic dipole source, thus states the principal theory of plumbing using Transient Electromagnetic Method. [10-11] When homogeneous isotropic surface of the earth where electrical conductivity is σ and magnetic permeability is μ is laid by rectangular transmitter loop of which area is S and being supplied with the step pulse current in the return wire. I t  0 I (t )   0 t  0

(3)

Before the current is disconnected ( t  0 ), transmitting current establishes a stable magnetic field in the earth and space around the loops, as shown in Figure 3.

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t