We have developed a new technology for surveying natural ground ahead of a tunnel face. The technique, called “DRi-Scope,” visualizes the state of ground forward of a tunnel face using an industrial endoscope. The natural ground at the tip of a bit is observed directly by inserting an industrial endoscope in the water supply conduit of a rod on a hydraulic drill jumbo.

It became possible to evaluate geological conditions ahead of the tunnel face using images obtained by DRi-Scope. Combined with drilling logging systems, this can produce highly accurate surveys while minimizing the impact on the tunneling process.

Since the rod substitutes for a casing, DRi-Scope surveys can be conducted in all types of ground conditions, including highly collapsible ground that was formerly difficult to observe. Maximum survey range is around 30 m ahead of the tunnel face, similar to the general range of drilling logging systems, and in combination with such systems it is possible to complete a survey in about two to three hours.

This paper provides an outline of the technology and describes its features. It then introduces application examples for visualization of natural ground ahead of tunnel faces.

1. Introduction

For enhanced safety and economical tunnel excavation, it is important to survey geological conditions ahead of tunnel faces with high accuracy. To this end, advance boring and drilling logging systems have been used, but advance boring is time-consuming, and drilling logging systems present problems because they make presumptions from indirect information.

DRi-Scope survey technology uses an industrial endoscope to directly visualize ground conditions ahead of a tunnel face via a hydraulic drill jumbo, which is usually deployed for tunnel excavation.

With DRi-Scope, a portion of endoscope (about 98mm) is inserted into the water supply conduit (about φ10mm) of a rod during cutoff and addition of the rod at the desired boring depth (Fig. 1.1).

It then protrudes from the water supply aperture in front of the bit, enabling direct observation of the bottom and wall of a borehole. The observed picture can be displayed on the monitor as a live image and can be recorded as a video or still image.

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