Rock fall

The term "rockfall" generally refers to the falling of the bedrock of the ceiling (usually called the roof or crown) or the collapse of the side walls (called "dobera" in mining jargon) in a mine. The scale of occurrence varies from small to large, ranging from a few small rocks falling to a collapse in which a long section of the mine or a wide area of ​​the mining site is buried by falling rocks.

Although some of the causes of rockfalls are human-caused, they are more likely to occur in areas where the rock is weak, where tunnels in the rock have many cracks, or where there are faults and folds in the rock, and in areas of geological disturbance. In other words, natural conditions are the major factor. For this reason, not only have predictions been established, but also complete prevention measures have not yet been established. To completely prevent them, it would be possible to solidify the entire tunnel with concrete, but this would make the cost of mining ore and coal extremely high and economically unviable. Civil engineering projects also build underground structures such as tunnels and underground power plants, but these are intended to be used permanently and are constructed solidly with concrete. However, the principle for tunnels in mines and coal mines is to dig them as cheaply as possible and not spend too much on repairs. Moreover, when the mining area controlled by the tunnel is finished, it is necessary to remove the facilities as quickly as possible and abandon the tunnel. Therefore, the tunnel only needs to be maintained at the lowest possible cost even during its usage period. This is why it is so difficult to completely prevent cave-ins. Generally speaking, the ratio of victims and fatalities caused by cave-ins to the total number of victims and fatalities in coal mines is between one third and one quarter. This ratio is true not only for coal mines but also for other mines. Meanwhile, the number of cave-ins in both coal mines and mines has been decreasing year by year. This is partly due to a decrease in the number of coal mines and mines, but also when looking at the number of accidents per million workers and per million tons of coal produced, the trend is downward. In other words, this is the result of improved safety awareness, the introduction of shield frames that completely cover the ceiling of the coal mining face (the part being excavated), and the widespread use of steel frames for tunnel support. It is also believed that the reduction in cave-ins has been due to the requirement to use a device called a "first support" that temporarily supports the ceiling before the installation of the main frame at the end of the tunnel in order to minimize the time when the ceiling is bare and to help prevent cave-ins. However, the most important measure against rock falls is to use good support with the utmost care. Specifically, this comes down to two points: carefully enclosing the gaps between the tunnels and supporting the roof of the coal mine with as much strength as possible.

[Toshiro Isobe]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

坑内で天井の岩盤部分（通常この部分を天盤または冠(かむり)という）が落下したり、側壁部（鉱山用語で土平(どべら)という）が倒壊してくる現象を総称して落盤ということが多い。規模は大小さまざまであり、小石が少数個落ちてくる落石から、崩落といって坑道の長い区間あるいは採掘場が広範囲に落下岩石のため埋没する現象まで含まれる。

　落盤には一部に人為的要因もあるが、岩盤がもろく弱い箇所とか、亀裂(きれつ)の多い岩盤内の坑道あるいは断層、褶曲(しゅうきょく)が存在する地質的擾乱(じょうらん)地帯におこりやすい。つまり、自然条件が大きな要因となっている。そのため、予知のみならず、完全な防止対策もまだ確立されていない。完全防止のためには、坑内をすべてコンクリートなどで固めればよいかもしれないが、それでは鉱石や石炭の採掘費は甚だしく高額になり経済的に引き合わなくなる。土木事業でもトンネル、地下発電所などの地下構造物をつくるが、これらのものは永久的使用が目的で、コンクリートで堅固に構築される。しかし、鉱山、炭鉱の坑道はできるだけ安く掘り、あまり修理費をかけないのが原則である。しかもその坑道の支配する採掘区域が終掘すれば、できるだけ速やかに施設を撤去して坑道を放棄することが必要条件である。そのため坑道は使用期間中さえ最少の費用で維持されればよい。落盤の完全防止のきわめてむずかしい理由はこれである。一般的にみて、落盤による罹災(りさい)者数、死亡者数の比率は、炭鉱における全羅災者数、全死亡者数の全災害数の3分の1から4分の1の間にある。このような比率は炭鉱ばかりでなく、鉱山でも変わりがない。一方、炭鉱、鉱山とも落盤の回数は年度を追って減少している。それは炭鉱、鉱山数の減少もあるが、稼働延べ100万人、および出炭100万トン当りの災害回数をみても減少傾向にある。つまり、保安意識の向上、採炭切羽(きりは)（掘っている箇所）の天井を全面的に覆うシールド枠の導入、坑道支保に鋼枠が全面的に使用されてきた結果でもある。また坑道掘進先には裸天井(はだかてんじょう)の時間をできるだけ短縮して落盤防止に役だてるため、「先受け」と称する、本枠の取り付け前に一時天井を支持する器具を使用することを義務づけたことなども落盤減少につながっていると考えられる。しかし、最大の落盤対策は、細心の注意でよい支保を行うことである。具体的には坑道の枠間(わくま)をていねいに囲うこと、採炭場の天井はできる限り強い力で支えるという二点に帰する。

［磯部俊郎］

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