Abstract

Evaluation of the distribution of fractures in a rock mass ahead of a tunnel face is required for safe and reasonable tunnel excavation. To evaluate not only the position of fractures, but also their inclination before tunnel excavation, a directional borehole radar may be a suitable technique. A directional borehole radar can determine the inclination of fractures with a single pilot boring because it can estimate the arrival direction of the waves reflected by fractures, which are measured by a dipole array antenna with multiple elements. In this study, we first confirmed experimentally that glass fiber reinforced plastic (GFRP) tubes with steel joint couplers can be used as borehole casings for measurements with radar. Then, a directional borehole radar was applied to the survey of fractures in granite in a horizontal borehole drilled in the sidewall of a tunnel in order to confirm its applicability for evaluating the three-dimensional distribution of fractures. The results show that the strike and dip of each reflector estimated by the directional borehole radar measurements approximately correspond to the fractures observed with a borehole scanner. Therefore, it was concluded that a survey with a directional borehole radar in a horizontal borehole can evaluate the three-dimensional distribution of fractures surrounding the borehole.

1.
Introduction

Because the collapse of a tunnel face has occurred after the unexpected appearance of faults or fractures, evaluation of the distribution of fractures in the rock mass ahead of a tunnel face is required for safe and reasonable tunnel excavation. A survey of a pilot boring has been widely applied to evaluate the geological conditions ahead of a tunnel face (Fig. 1). Although the position where a fracture intersects with a pilot boring can be detected by the survey, it is difficult to evaluate its inclination with this method. As shown in Fig. 1, if a fracture intersects a tunnel at a low angle, it will appear on the tunnel face at a different position than that detected by the pilot boring. This suggests that it is important to evaluate not only the position of fractures, but also their inclination, before tunnel excavation. On the other hand, although the inclination of fractures may be evaluated by a survey with two or more pilot borings, the cost becomes high. Therefore, a geological technique capable of evaluating the inclination of fractures in a single pilot boring is needed.

A directional borehole radar may be a suitable technique for evaluating the inclination of fractures because it can estimate the arrival direction of the reflected waves, which are measured by a dipole array antenna with multiple elements. This study first examined the application of a directional borehole radar to investigate within a single pilot boring. Next, a directional borehole radar was used to survey fractures in granite in a horizontal borehole drilled in the sidewall of a tunnel in order to confirm the applicability of the directional borehole radar for evaluating the three-dimensional distributions of fractures.

This content is only available via PDF.
You can access this article if you purchase or spend a download.