In mountain tunneling, construction works are in the proximity of tunnel faces where falling rocks and collapse of ground are often feared. It is however difficult to improve quickly such work environments e.g., by applying remote construction and complete mechanization. In reality, an exclusive watch guard stands in place and supervises instability of the tunnel face, when charging/connecting explosives and installing tunnel supports. When the guard judges that the continuation of tunneling work is dangerous, he gives warning to the workers, so they can immediately leave from the work area. A newly-developed face monitoring system applies a high-speed camera set beside the watch guard and the image recognition technology. It promptly arrests fall of pebbles and pieces of sprayed concrete, symptoms of possible tunnel face collapse, that could lead to disaster, gives warning within a mere 0.1 seconds after the detection of movements of falling objects, and gets the construction workers to leave immediately from the dangerous spot. Effectiveness of this system in securing safety by foretelling falling rocks was verified in a mountain tunneling site.

1 Introduction

In mountain tunneling, construction workers carry out various operations near the tunnel face, depending on the stage of construction. In particular, during preparations for blasting to excavate hard rock and during installation of steel arch supports (bent H-section steel beams) to support the surrounding rock after excavation, it is necessary for the construction workers to be close to the tunnel face where the risk of injury is high. Actual operations at the tunnel face are affected by rock failure or fragility of the ground, which can cause falling rocks or fragments of sprayed concrete to fall. Of the 47 persons that were killed or injured in the 44 falling debris accidents that occurred from the year 2000 to the year 2010, 6% were killed, and 42% were out of work for one month or longer, according to a technical document (National Institute of Occupational Safety and Health, Japan, 2012).

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