Injury from mining

Japanese: 鉱害 - こうがい（英語表記）injury from mining

Damage caused to third parties by mines during the process of mining, ore dressing, smelting, etc. It can also be called pollution caused by mines. Article 109 of the Mining Act (Act No. 289 of 1950) defines it as "damage caused to others by excavating land for the purpose of mining minerals, discharging pit water or wastewater, piling up waste rock or tailings, or emitting mine smoke." Today, secondary mining pollution can also be seen from abandoned mines.

[Tadashi Nishida]

History of mining pollution

Mining pollution has a long history and is said to be the origin of pollution. Already in the Edo period, there are records of mineral poisoning in metal mines in Sado (Niigata), Besshi (Ehime), Ikuno (Hyogo), Toroku (Miyazaki), and other places. Full-scale mining pollution did not occur until the Meiji period, when modernization was promoted. There were successive incidents of mineral poisoning and smoke pollution in Ashio (Tochigi), Besshi, Hitachi (Ibaraki), Kosaka (Akita), and other places. Mine pollution caused by coal mining came next. Coal mining for about a century since the Meiji period caused significant damage in coal-producing areas such as Chikuho (Fukuoka) and Ube (Yamaguchi), and the damage is still evident today.

One of the most well-known mining disasters after World War II was the Itai-itai disease case, one of Japan's four major pollution lawsuits. Other problems include land subsidence caused by the extraction of water-soluble natural gas. Land subsidence is known in the Niigata gas field and the Minami-Kanto gas field, and became noticeable around 1955. It was caused by a drop in pressure in the gas layer due to excessive gas extraction. As a result of various regulatory measures such as limiting the amount of gas extracted and returning it underground, the subsidence has shown a tendency to slow down. Today, new problems include the occurrence of secondary mining disasters such as the leakage of underground water from abandoned metal mines, the occurrence of shallow subsidence in former coal-mining areas, and the occurrence of old cave water springs.

Coal mining pollution has been seen in foreign countries for a long time. Coal mining has been taking place in Germany and the UK since the 18th century, and the history of damage is long, as coal mining is often carried out in relatively small areas under farmland and industrial zones. Therefore, mining methods, surveying systems, and compensation systems for mining pollution prevention have been considered for a long time. For example, Germany has established a system of mining surveyors called Markscheiders who are independent of mining rights holders and landowners. They have been conducting precise surveys for a long time, and this is the basis for accurate prediction of mining pollution today. In the UK, the nationalization of coal mines (1946) led to the unification of mining laws, which has resulted in a reduction in mining pollution and smoother compensation for mining pollution.

[Tadashi Nishida]

Types of mine pollution and countermeasures

Mining damage caused by excavation of land

Land subsidence caused by coal and lignite mining is a major problem. Damage was also seen during the Meiji period, but it became especially severe during the Showa period. The damage was particularly severe in the Chikuho and Ube coalfields, where at their peak there were 246 mines that accounted for about 40% of the nation's coal production, and some areas saw cumulative subsidence of 7 to 8 meters. The damage was widespread, as stated in the opening part of a resolution on mining damage at the 7th Diet plenary session of the House of Representatives in 1950, which states, "Due to mining damage caused by coal mining, beautiful rice fields have turned into seas of mud, houses are in danger of collapsing day and night, transportation has been cut off, and ancestral graves have been submerged. Even people living on the roads find the terrible situation unbearable to look at."

Damage caused by coal mining can be classified into five elements: subsidence, inclination, curvature, horizontal movement, and distortion of the ground surface (the five elements of mining damage). These elements, either individually or in combination, cause various types of damage to surface properties such as fields, houses, railways, bridges, roads, rivers, wells, and water and sewerage systems. When a horizontal coal seam is mined, the ground surface moves within an area determined by the limit angle (the angle between the line connecting the mining edge and the subsidence edge of the ground surface and the horizontal plane). The limit angle varies by region, but in Japan it is around 55 degrees. Subsidence is greatest immediately above the center of the mine. Horizontal movement of the ground surface is small in the center of the mine and greatest immediately above the mining edge. However, the direction of movement differs between the left and right sides of the mine. The amount of subsidence is proportional to the thickness of the mined coal seam (called mountain height or coal depth), but it can be reduced by filling in the mined area and by using mining methods that leave coal pillars for safety. Also, by appropriately adjusting the mining sequence, speed, and area (harmonious mining method), the inclination or tensile strain in specific areas on the ground surface is eliminated, minimizing damage. Today, a theory of subsidence that is tailored to the characteristics of Japan's ground has been established, and it is possible to predict and calculate subsidence.

After the closure of a coal mine, shallow subsidence becomes a new problem. This phenomenon occurs when the ground surface instantly collapses into a tunnel or a cavity (old cavern) remaining at a shallow depth underground due to a decrease in ground strength, the load of structures, fluctuations in groundwater level, etc. This phenomenon is also called "a sinkhole." It is strongly related to rainfall and occurs frequently during the rainy season and heavy rain. In the Chikuho region, a former coal mining area, 50 to 60 cases occur every year. Earthquakes can also trigger the occurrence of shallow subsidence. During the Miyagi Prefecture offshore earthquake (magnitude 7.4) in June 1978 (Showa 53), more than 200 shallow subsidences occurred in the lignite mining areas of the Tohoku region (Miyagi, Kitakami, Mogami, Soma, etc.), although there are usually 15 to 20 shallow subsidences every year. Shallow subsidences are generally less than 30 meters deep in old caverns, and occur frequently near the coal seam outcrop line. Most sinkholes are about 3 meters in diameter and 2 meters deep. Countermeasures for constructing heavy structures in former coal-mining areas include (1) the filling method, (2) the soil stripping method, (3) the pile driving method, and (4) the beam method, and the method is selected taking into consideration the existence of the old cavern, the type of structure, and economic efficiency.

The restoration of damage caused by coal mining is carried out based on the Temporary Coal Mine Damage Rehabilitation Law (Law No. 295 of 1952), also known as the Temporary Mining Law. The Temporary Mining Law was originally a 10-year law, but because of the enormous amount of accumulated damage caused by mining, it has been extended twice. Restoration is carried out under a no-fault liability system, meaning that the matter is left to the discretion of the parties involved, the tortfeasor and the victim, but the national and local governments also shoulder some of the costs of restoration. The burden on farmland is particularly heavy, with mining rights holders only shouldering 15% of the costs. In cases where the mining rights holder is insolvent or whereabouts are unknown, restoration is carried out by the national and local governments. By 1981, the national government had spent 400 billion yen on restoration work for mining damage, but farmland and houses still remain that have not been restored.

[Tadashi Nishida]

Mine pollution caused by mine wastewater

The Ashio Copper Mine Pollution Incident and the Itai-itai Disease Incident are well-known examples of this type of mining pollution. Mine wastewater from the Ashio Copper Mine and the Kamioka Mine polluted the Watarase River and the Jinzu River, respectively, causing damage to residents and agricultural crops in the coastal areas. Mine wastewater includes mine water, open-cut mining wastewater, ore dressing wastewater, and seepage water from waste rock or tailings piles. In many cases, it contains harmful substances such as free acids, heavy metal ions, and fine mineral particles. Even after a mine has stopped operating, mine water may leak from the mine mouth and cause damage. According to documents from the Ministry of International Trade and Industry (now the Ministry of Economy, Trade and Industry), at least 600 of the approximately 5,700 closed and abandoned mines are at risk of mine water leaking, and measures to prevent mine pollution are needed. Methods for doing so include (1) the neutralization precipitation method and (2) the tunnel blocking method. (1) is the most reliable method, in which the free acids in the mine water are neutralized with hydrated lime or calcium carbonate and then precipitated and removed, but it requires the construction of a treatment facility, treatment costs, and a disposal site for the neutralized sediment. In the case of the former Matsuo Mine (Iwate), construction costs were estimated at about 6.5 billion yen, and treatment costs were estimated at about 500 million yen per year, and it is said that there is only enough storage space for the neutralized sediment to last for 20 years. (2) is a method in which the mine tunnel is sealed with a concrete plug (dam) to prevent the outflow of mine water, and does not require maintenance after construction, but is unsuitable when the bedrock around the tunnel is weak or there are many blockages. Other methods include the underground reduction method, in which the mine water is returned underground, and the method of covering the ore body so that groundwater does not come into contact with it.

The same problems as those in metal mines are occurring in former coal mining areas. The mine water (old cave water) springs up due to the rise in the mine water level after the mine was closed. There are more than 50 springs in the Chikuho region alone, causing wet fields, red water, and river pollution. The old cave water generally springs up in areas where mining is done at a depth of less than 50 meters, and is mostly in lowlands and valleys. Because the mine entrance, mining site, and mountainous areas are at a higher altitude than the springs, the old cave water is pressurized and springs up to the surface through mining sites, faults, and fracture zones. The water quality is acidic with a pH of 3 to 5 and contains a large amount of sulfate ions, and varies widely from high concentrations that cause red precipitates of iron oxide at the springs to low concentrations that are almost the same quality as river water. To prevent old cave water from springing up, a method called suppression water is used, in which the mine water level is lowered by pumping.

[Tadashi Nishida]

Mine pollution caused by mine smoke

This mining pollution, also known as smoke pollution, is often caused by smoke emitted from dry smelters of non-ferrous metals. Mine smoke often contains harmful substances such as smoke dust and sulfurous acid gas. Smoke dust adsorbs sulfurous acid gas to its surface, preventing its dilution and diffusion in the air, which exacerbates the smoke pollution. However, with the development of the Cotterell dust collector, which deposits smoke dust electrically, the damage caused by smoke dust has decreased. Sulfurous acid gas has a specific gravity of 2.264 compared to air, and it is said that at a concentration of 500 ppm, a person will suffocate with the first breath. It also has a significant harmful effect on plants, and during the Meiji period, when smoke pollution was common, the forests around smelters were devastated. The Besshi Copper Mine smoke pollution incident is one such example. The smelter of Sumitomo Mining's Besshi Copper Mine was originally located in Niihama (Ehime Prefecture), but because the sulfur dioxide gas in the smoke caused great damage to crops and other things, the smelter was relocated to Shisakajima, about 20 kilometers from the mainland of Shikoku, in 1904 (Meiji 37) in an effort to prevent smoke damage, but the smoke damage did not stop even after the relocation, causing serious damage to 96 towns and villages on the mainland of Shikoku. In an effort to prevent smoke damage, low, thick chimneys called sulfur smoke dilution devices were adopted, but this was ineffective. Research into the treatment of sulfurous acid gas in mine fumes was also actively carried out, and in 1929 (Showa 4) the Petersen sulfuric acid manufacturing plant was introduced, which resulted in over 70% of the smelting sulfur being converted into sulfuric acid, and the concentration of sulfurous acid gas in the mine fumes, which was 1% in 1916 (Taisho 5), fell to 0.53% in 1931 and 0.19% in 1935. This was followed by a switch to a neutralization method using ammonia, and the smoke pollution problems that had continued since the Meiji era were resolved for the time being. Even today, mine fumes are basically treated by removing smoke dust using the Cotterell plant and converting sulfurous acid gas into sulfuric acid, and as treatment technology has advanced significantly, there is almost no smoke pollution like before.

[Tadashi Nishida]

Mine damage caused by the accumulation of waste rock and tailings

This type of mining damage is often caused by the collapse or runoff of deposits. Spill rock includes gangue (worthless ore) and waste tailings from flotation, while slag is waste material known as noro in iron smelting and kane in nonferrous smelting. It can be disposed of in several ways, including (1) filling in mining sites, (2) using it for coastal reclamation, (3) using it as civil engineering material, and (4) disposing of it in a dumping site. Option (4) is the most common, where a retaining structure (dam) is built in a valley to prevent runoff, and the spilling of the spoil rock and slag is disposed of there. When spoil rock and slag absorb water, their fluidity generally increases, so drainage facilities such as culverts are also necessary. Accidents are more common during heavy rains, and a typical example of a collapse is the collapse of the dam at the Nakazawa dumping site of the Osarizawa Mine (Akita) in November 1936. Nearly 500 people were killed or injured, and over 300 houses were damaged. There are few examples of collapses due to earthquakes, but in January 1978, the Mochikoshi Mine Hozukizawa Sedimentary Site (Shizuoka) collapsed due to an earthquake (magnitude 7.4) near Izu Oshima. The collapse is thought to have been caused by liquefaction of the sediments by the earthquake.

Coal mining also produces large amounts of waste rock called botah or zuri , which is usually piled up outside the mine (called botah or zuriyama), causing problems such as collapse and landslides due to weathering. Slag heaps and zuriyama were once symbols of coal-producing areas, but they also pose the risk of spontaneous combustion. The best solution would be to demolish the slag heaps, but this would require an enormous amount of money. There is currently an idea to use the demolished slag to fill in the coastline, and to use the reclaimed land and the former slag heap sites as public land.

[Tadashi Nishida]

"Mine Reader" by Yoshikazu Hagiwara and Hiroki Hayashi (1963, Gijutsu Shoin)" ▽ "Chronology of Pollution, Industrial Accidents, and Occupational Diseases" by Nobuko Iijima (1977, Pollution Control Technology Association)"

[References] | Ashio Copper Mine Pollution Incident | Itai-itai Disease | Pollution

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

鉱山が採掘、選鉱、製錬などの過程で第三者に与える害。鉱山に起因した公害ともいうべきものである。鉱業法（昭和25年法律289号）第109条には「鉱物の掘採のための土地の掘さく、坑水若(も)しくは廃水の放流、捨石若しくは鉱さいのたい積又は鉱煙の排出によって他人に与えた……損害」と定義されている。今日では休廃止した鉱山による二次的な鉱害もみられる。

［西田　正］

鉱害の歴史

鉱害は公害の原点ともいわれるようにその歴史は古い。すでに江戸時代に佐渡（新潟）、別子(べっし)（愛媛）、生野(いくの)（兵庫）、土呂久(とろく)（宮崎）など、金属鉱山における鉱毒の記録がある。本格的な鉱害の発生は、近代化が推進された明治時代に入ってからである。足尾（栃木）、別子、日立（茨城）、小坂(こさか)（秋田）などで鉱毒、煙害事件が続出した。石炭採掘による鉱害が次に続く。明治以来約1世紀にわたる石炭採掘のため、筑豊(ちくほう)（福岡）、宇部（山口）などの産炭地では著しい被害を被り、被害はまだ残存している。

　第二次世界大戦後の鉱害では、日本の四大公害裁判の一つであるイタイイタイ病事件が著名で、ほかに水溶性天然ガス採取に起因した地盤沈下問題がある。新潟ガス田、南関東ガス田における地盤沈下が知られ、1955年（昭和30）ごろから顕著となった。過剰なガス採取に伴うガス層内の圧力低下が原因である。ガス採取量の制限、地下への還元など種々の規制措置の結果、沈下は鈍化傾向を示している。今日では休廃止金属鉱山の坑内水の流出、旧産炭地における浅所(せんしょ)陥没の発生、古洞水(ふるとうすい)の湧水(ゆうすい)など二次的な鉱害の発生が新たな問題となっている。

　外国では石炭採掘による鉱害が古くからみられる。ドイツ、イギリスなどではすでに18世紀以来、石炭採掘がなされ、しかも比較的狭い地域で農地や工業地帯などの下が多く採掘されているため被害の歴史も古い。したがって、鉱害防止のための採掘方法、測量制度、鉱害賠償制度なども早くから検討されている。たとえばドイツでは、マルクシャイダーMarkscheiderの名称で鉱業権者および土地所有者から独立している鉱山調査測量技師の制度が確立されている。古くから精密な測量を継続して行っており、これが今日の正確な鉱害予測の基礎となっている。イギリスでは、炭鉱の国有化（1946）によって採掘法が統一されたため、鉱害の減少、鉱害賠償の円滑化などの成果を得ている。

［西田　正］

鉱害の種類と対策

土地の掘削による鉱害

石炭、亜炭の採掘に伴う地盤沈下が大きな問題である。被害は明治時代にもみられたが、昭和に入ってとくに激烈を極めた。筑豊炭田、宇部炭田などの被害が著しく、最盛期には246鉱山で全国出炭量の約40％を占めた筑豊炭田では、累計沈下量が7～8メートルに及んだ所もあった。「石炭採掘による鉱害のため、美田変じて泥海と化し、住宅は日夜倒壊の危険に脅かされ、交通機関は杜絶(とぜつ)し、祖先の墳墓は水底に没する等惨憺(さんたん)たるその実情は、路傍の人もなお、正視するに忍びないものがある」（1950年、第7回国会衆議院本会議での鉱害に関する決議の冒頭部分）ともあるように、被害は広範囲に及んだ。

　石炭採掘による被害現象は、地表面の沈下、傾斜、湾曲、水平移動、およびひずみの五つの要素（鉱害五要素）に分類される。これらの要素が単独あるいは複合して、田畑、家屋、鉄道、橋梁(きょうりょう)、道路、河川、井戸、上下水道などの地表物件に諸種の被害を与える。水平な炭層が採掘された場合、限界角（採掘端と地表面の沈下端を結ぶ線が水平面となす角）で規定される範囲の地表面が移動する。限界角は地域により異なるが、日本では55度前後である。沈下は採掘中央部の直上付近がもっとも大きくなる。地表面の水平移動は採掘中央部では小さく、採掘端の直上付近が最大となる。ただし、採掘の左右では移動の方向が異なる。沈下量は、採掘された炭層の厚さ（山丈(やまたけ)または炭丈(すみたけ)という）に比例するが、採掘跡の充填(じゅうてん)、保安のための炭柱を残す採掘方法などにより沈下量を減少させられる。また、採掘の順序、速さ、範囲などを適当に調整して採掘（調和採掘法）を行うと、地表上の特定な地域の傾斜あるいは引張りひずみがなくなり被害が最小となる。今日では日本の地盤特性に応じた沈下理論も確立され、沈下予測計算も可能となっている。

　炭鉱閉山後は浅所陥没が新たな問題となる。地盤強度の低下、構造物などの荷重、地下水位の変動などが原因となり、地表面が地下浅所に残存している坑道あるいは採掘による空洞（古洞）へ瞬時に陥没する現象であり、つぼ抜けともいわれる。降雨との関連性がきわめて強く、雨期、豪雨時に頻発する。旧産炭地の筑豊地区では毎年50～60件発生している。地震も浅所陥没発生の誘因となる。1978年（昭和53）6月の宮城県沖地震（マグニチュード7.4）のとき、東北地方の亜炭採掘地域（宮城、北上、最上(もがみ)、相馬(そうま)など）では、通常毎年15～20件の浅所陥没が、一挙に200件以上発生している。浅所陥没は一般に古洞深度が30メートル以浅で、しかも炭層露頭線近くに多発している。陥没孔の大きさは直径3メートル、深さ2メートル程度のものが多い。旧産炭地に重量構造物を構築する場合の対策工法には、(1)充填工法、(2)剥土(はくど)工法、(3)杭(くい)打ち工法、(4)梁(はり)工法などがあるが、古洞の賦存状況、構造物の種類、経済性などを考慮して選択される。

　石炭採掘による鉱害の復旧は、臨鉱法とよばれる臨時石炭鉱害復旧法（昭和27年法律295号）に基づいて行われている。臨鉱法は当初10年の時限法であったが、累積している鉱害が膨大であったため、これまで二度延長されている。復旧は無過失賠償責任制度、すなわち加害者・被害者当事者間の解決に任された形であるが、国および地方自治体も一部負担して復旧している。とくに農地についてはその負担割合が大きく、鉱業権者は15％の負担でしかない。鉱業権者が無資力または所在不明の場合は国および地方自治体で復旧を行っている。1981年までに国が投じた鉱害復旧費は4000億円にも達するが、未復旧の農地、家屋などはまだ残存している。

［西田　正］

鉱廃水による鉱害

足尾銅山鉱毒事件、イタイイタイ病事件などがこの種の鉱害では著名である。足尾銅山および神岡鉱山の鉱廃水がそれぞれ渡良瀬(わたらせ)川、神通(じんづう)川を汚染し、沿岸流域の住民、農作物などに被害を与えたものである。鉱廃水とは、坑内水、露天掘り廃水、選鉱廃水、捨石または廃滓堆積(はいさいたいせき)場からの浸出水などである。多くの場合、遊離酸、重金属イオン、微細な鉱物粒子などの有害物質を含有する。また、鉱山が操業停止後も、坑口から坑内水を流出し被害を及ぼす場合もある。通商産業省（現経済産業省）の資料によると、約5700の休廃止鉱山のうち少なくとも約600鉱山は坑内水流出の危険性があり、鉱害防止対策が必要とされている。その方法には(1)中和沈殿法、(2)坑道閉塞(へいそく)法などがある。(1)は坑内水中の遊離酸を消石灰、炭酸カルシウムなどで中和し沈殿除去する、もっとも確実な方法ではあるが、処理施設建設費、処理費、さらに中和沈殿物の廃棄場所が必要である。旧松尾鉱山（岩手）の場合、建設費として約65億円、処理費として毎年約5億円が必要とされ、中和沈殿物の堆積場は20年分しかないといわれている。(2)は坑道をコンクリート製のプラグ（ダム）で密閉し坑内水の流出を防止しようとする方法で、施行後の維持管理は不要であるが、坑道周辺の岩盤が軟弱な場合、閉塞箇所が多い場合などは不適である。ほかに、坑内水を地下に戻す地下還元法、地下水が鉱体と接触しないように鉱体を被覆する方法などがある。

　旧産炭地でも金属鉱山の場合と同様な問題が生じている。炭鉱閉山後の坑内水位上昇による坑内水（古洞水）の湧水である。筑豊地区だけでも湧水箇所は50か所以上もあり、田畑の湿潤化、赤水、河川の汚染などを生じている。古洞水の湧水は一般に50メートル以浅の採掘がある地域で、しかも低地、谷部がほとんどである。湧水箇所よりも標高の高い坑口、採掘跡、山地などのため、古洞水が被圧され、採掘跡、断層、破砕帯などを経由して地表に湧水する。水質は、pH3～5の酸性で硫酸イオンを多量に含み、湧水箇所に酸化鉄の赤い沈殿物を生ずる高濃度のものから、河川水とほぼ同水質の低濃度のものまで多種多様である。古洞水の湧水防止には、揚水により坑内水位を低下させる抑(おさ)え水とよばれる方法がとられている。

［西田　正］

鉱煙による鉱害

この鉱害は煙害ともいわれ、非鉄金属の乾式製錬所から排出される鉱煙による被害が多い。鉱煙は多くの場合、煙塵(えんじん)、亜硫酸ガスなどの有害物質を含有する。煙塵は表面に亜硫酸ガスを吸着するため、空気中での希釈、拡散を妨げ煙害を助長する。しかし、電気的に煙塵を沈積させるコットレル除塵装置の開発の結果、煙塵による害は減少している。亜硫酸ガスは空気に対する比重が2.264であり、500ppmの濃度では人は最初の一息で窒息するといわれている。植物に対する有害作用も著しく、煙害が多発した明治時代には製錬所周辺の山林は荒廃した。別子銅山煙害事件もその一つである。住友鉱業別子銅山の製錬所は当初、新居浜(にいはま)（愛媛）にあったが、排煙中の亜硫酸ガスが農作物などに多大の被害を与えたため、1904年（明治37）四国本土から約20キロメートル離れた四阪島(しさかじま)に製錬所を移転して煙害防止に努めたが、移転後も煙害はやまず、四国本土96町村に重大な被害を及ぼした。煙害防止のため硫煙(りゅうえん)希釈装置と称する低く太い煙突を採用したりしたが、効果は得られなかった。鉱煙中の亜硫酸ガス処理についての研究も積極的に行われ、1929年（昭和4）ペテルセン硫酸製造装置が導入された結果、製錬硫黄(いおう)量の70％以上が硫酸に転化され、1916年（大正5）には1％もあった鉱煙中の亜硫酸ガス濃度は、1931年に0.53％、1935年には0.19％まで減少した。その後、アンモニアを用いた中和法に切り替え、明治時代から続いた煙害事件はいちおう解決した。今日でも鉱煙処理は基本的にはコットレル装置による煙塵除去、亜硫酸ガスの硫酸転化であり、処理技術も大幅に進歩したため、従来のような煙害の発生はほとんどない。

［西田　正］

捨石や鉱滓の堆積による鉱害

この種の鉱害では堆積物の崩壊または流出による被害が多い。捨石は脈石（無価値の鉱石）、浮遊選鉱の廃滓など、鉱滓とは、鉄製錬ではのろ、非鉄製錬ではからみなどといわれている廃棄物である。その処理には、(1)採掘跡の充填に使用、(2)海岸埋立てに使用、(3)土木材料として利用、(4)堆積場に廃棄などがある。(4)が一般的で、谷間に流出防止用の土止め施設（ダム）を築造して、捨石、鉱滓が廃棄される。捨石、鉱滓は含水すると総じて流動性が増加するため、暗渠(あんきょ)などの排水施設も必要である。事故は豪雨時などに多く、代表的な崩壊事例に、1936年11月、尾去沢(おさりざわ)鉱山中沢堆積場（秋田）のダム決壊がある。500人近くの死傷者があり、300戸以上の家屋が被害を被った。地震による崩壊例は少ないが、1978年1月には、伊豆大島近海地震（マグニチュード7.4）による持越(もちこし)鉱山ほうずき沢堆積場（静岡）の崩壊が記録されている。地震による堆積物の液状化が崩壊の原因と考えられている。

　石炭採掘の場合も、ぼたまたはずりとよばれる捨石を多量に産出し、普通は坑外に山積み（ぼた山またはずり山という）されるため、風化に伴う崩壊、地すべりなどの問題を抱えている。ぼた山またはずり山はかつては産炭地の象徴であったが、自然発火などの危険性も有する。対策は、ぼた山の取り崩しが最良であるが、膨大な費用が必要である。現在、取り崩したぼたを用いて海岸の埋立てを行い、埋立地およびぼた山の跡地を公共用地として利用しようとする考えもある。

［西田　正］

『萩原義一・林裕貴著『鉱山読本』第3巻第20号（1963・技術書院）』▽『飯島伸子著『公害、労災、職業病年表』（1977・公害対策技術同友会）』

[参照項目] | 足尾銅山鉱毒事件 | イタイイタイ病 | 公害

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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