:At present, almost all mountain tunnels are excavated and constructed utilizing the New Austrian Tunneling Method (NATM) in Japan. This tunneling method was advocated by Prof. Rabcewicz from Austria in 1964. In Japan, this method has been applied to tunnel construction since around 1978, after which there has been a subsequent decrease in the number of fatalities and casualties in tunnel construction. However, there is still a relatively higher incidence of labour accidents caused during tunnel construction than when compared to incidence rates in the construction industry in general. In tunnel construction, rock fall events at the cutting face, are particularly characteristic of the types of accident that occur.
In this document, we analyse labour accidents that possess the characteristics of being a rock fall event at a work site, which were collected by the Association of Nihon Tunnel Construction Sub-Contractors. We also propose accident prevention measures against rock fall events by considering the mechanisms of such events and the theory of NATM.
The contents of this document are summarized as follows:
- It is clear that many of the labour accidents due to rock fall events happen when workers set off explosive charges or install steel arch supports in the cutting face during tunnel construction using the mountain tunneling method, especially NATM. In addition, it was found that the dimensions of the fallen rock are on average 1.0m in length, 0.6m in width and 0.3m in thickness, and that these relatively small-scale and few rocks are the cause of deaths or injuries to workers.
- Prevention measures that were proposed against labour accidents due to rock fall events during tunnel construction include shotcreting the cutting face, bolting to the cutting face, removing rock masses in the cutting face, drilling the cutting face for drainage, measuring the displacement of the cutting face, sufficient lighting in order to observe the cutting face, and protecting workers directly using nets, mats and man cage guards and individual protection kits. These measures are shown in “Table-3” in this document.
- In the prevention measures against labour accidents due to rock fall events during tunnel construction, shotcreting the cutting face is very effective because it can control any weakness in the integrity of the mountain rock, protect against exposing mountain rock to the air, and makes it easier to observe new cracks and deformations in the rock face, etc. In addition, shotcreting the cutting face is very useful in terms of construction and economic aspects. It is, however, more important to use shotcreting in combination with other prevention measures because some labour accidents due to rock fall events occurred even after shotcreting had been implemented. The cause of such accidents is thought to be insufficient thickness in the shotcreting or a weakening in cohesive strength due to the upwelling of groundwater.
- Therefore, not only shotcreting alone, but also sufficient lighting in order to observe the cutting face, removing rock masses from the cutting face, and measuring the displacement of the cutting face, etc., should be used in combination to ensure that all prevention measures work synergistically. To this end it is important to ensure that these measures are implemented appropriately, paying due attention the “Items to Bear in Mind” detailed in Section 3.4 of this document, which, if complied with, are thought to be effective means of implementing prevention measures for accidents caused by rock fall events, which are a requirement under the stipulations of Article 384 of the Ordinance on Industrial Safety and Hygiene.
- In situations where the risk of rock falls remains high even after the above prevention measures have been applied, it is preferable for the contractor and the construction operator to engage in discussions to consider prevention measures from the design stage and other auxiliary measures, including forehand piling to a tunnel roof, and bore holing for drainage.