Gob-water is a serious hazard in Chinese coalmines. Such inrushes have caused more than 80% of the coalmine water accidents recorded in recent years. This study is based on reviews of past gob-water hazard cases in China. Six inrush patterns are suggested, caused by gobs exposed by tunneling, channeled by faults, or fissure or caving zone channeling, damage to or collapse of water-resistant coal pillars and multi-factor interactions. The results are of relevance in all coal-producing countries wishing to contain and treat gob-water hazards.
Gob is the term used to describe mined-out areas, including abandoned tunnels and mines remaining after mineral extraction. Any groundwater that accumulates in gobs is called gob-water. Gob-water is a serious threat to safety in subsequent mining and tunneling – e.g., in coal mining. An analysis of inrushes in British collieries between 1851 and 1970 (Vutukuri & Singh 1995) identified the source and number of fatalities in each case. They show that gob-water is the most significant source of accidental inundation. The total of 176 cases from gob-water is well in excess of all the other inrush sources put together – 9 accidents caused by surface water, 8 by shallow groundwater in unconsolidated deposits, 2 by coal measure strata water, and 13 accidents arising from shaft sinking, failure of underground dams, leakage from boreholes, etc.
Modern coalmining in China started in the early 20th century. In coalmines operated by foreign companies – e.g., Tangshan Kaiping Coalmine in Hebei (British, 1900) and Zibo Beida Coalmine in Shandong (French, 1904), the prevalent mining technologies were generally primitive. Since the founding of the People's Republic of China in 1949, substantial support has been initiated for coalmining, starting mainly with small mines (annual output under 1 million tonnes) and shallow excavations (less than 200 m deep). The construction of large coalmines started in the 1960s and excavations went deeper. Annual Chinese coal output quickly rose to 32.4 million tonnes in the early 1950s and 1 billion tonnes by the late 1980s, and exceeded 3 billion tonnes in 2009 (Gui et al. 2016a).
As more and more small-sized coalmines were deserted in China, and shallow coalmining intensified, gob-water became a common threat, resulting in numerous water-inrushes (Li et al. 2014). On April 6, 1990, a gob-water related accident occurred in Leshan Dongfeng Coalmine, Sichuan Province, killing 57 people. On August 7, 2005, another inrush in Meizhou Daxing Coalmine, Guangdong Province, cost 121 lives. In light of the reality, research on the mechanisms and control of gob-water hazards are high priorities in China (Yang et al. 2007; Yang & Song 2012). Yang et al. (2013) assessed the role of gob water in mining underlying coal seams by analyzing the post-mining heights of caving and fractured zones, as well as the depths of coal seam floor destroyed. Cui & Ning (2007) studied an abandoned mine in the Pittsburgh coal field, and pointed to the influence of the hydrogeological conditions on the stability of water-resistant coal pillars and the threat to adjoining mines.
Recently, new geophysical prospecting techniques, such as borehole electromagnetic surveys (Su et al. 2016) and transient electromagnetic surveys (Xue et al. 2013; Li et al. 2015; Chang et al. 2016) have been used widely in prospecting gob-water. Long-distance directional drilling is also used extensively for the exploration and drainage of gob-water (Yao et al. 2013). Grouting is frequently applied to fill gob areas (Bai et al. 2014; Liu et al. 2015), etc. All of these technologies have produced favorable results in controlling gob-water hazards.
Gob-water hazard management must start with a thorough understanding of the formation mechanisms. Based on analysis of water-inrush characteristics and the gob-water disasters recorded in recent years, this paper includes suggestions about some gob-water inrush patterns arising, for instance, from gobs exposed by tunneling, water conducting fault channeling gobs, etc.
OVERVIEW OF GOB-WATER INRUSH IN CHINESE COALMINES
Taking a historical view of Chinese coalmine gob-water disasters, the degree of damage by any inrush is closely related to when the gob was formed. Gob-water is categorized chronologically in Chinese coalmines. Usually, that in gobs formed before the 1960s is known as ancient gob-water, while modern gob-water occurs in those formed subsequently. Until the 1960s, mining technologies in China were rudimentary, and gobs from then were small due to low production capacity and shallow mining depths. During decades of compaction, the gob space shrank to hold relatively small volumes of water, which, if the gob was breached, would only delay mining or lower efficiency slightly, rather than causing a severe accident or human casualties. From the 1960s onward, coalmining expanded to large or super-large scales, creating much larger gobs containing huge amounts of water. As a result, a burst can inundate a mine and take lives. On May 18, 2006, a gob-water inrush in Datong Xinjing Coalmine, Shanxi Province, yielded 422,000 m3 of gob-water and killed 56 miners (Wu et al. 2013).
The most characteristic feature of a gob-water inrush is that the flow-rate attenuates rapidly. The time to attain maximum flow-rate is very short and attenuation is quick – typically a few minutes. At 23:40 on November 24, 1996, gob-water struck at Fengfeng Sun Zhuang Coalmine, Hebei Province. The inflow increased to its maximum of 2,400 m3/h at 00:30 on November 25. In those 50 minutes, the entire mine was flooded (with direct economic losses of RMB 224 million or about $ 27 million), after which the inflow rate fell rapidly to about 30 or 40 m3/h. In the Jiangjiawan Coalmine incident, it only took 6 minutes for the inflow rate to maximize at 56,956 m3/h, after which it decreased quickly to 40 or 50 m3/h. Over the next four days, water from the gob area flowed in at 10 m3/h, which was the normal rate in the sandstone aquifer.
Many signs foreshadow gob-water inrush (Gui et al. 2016b). As examples: (1) before an inrush, small seeps and flows often appear on roadway walls, but flows increase quite quickly to tens of cubic meters per hour, before the wall explodes abruptly. (2) The water in the flows is dark and smelly. It also contains large amounts of suspended matter of complex composition – e.g., pulverized coal, bits of wood, shreds of canvas air duct (widely used in small mines before the 1980s for ventilation – shown in Figure 2(b) by the red arrow). (3) Gobs are usually filled with loosely-compacted broken rock, etc. When mining under gobs, the combined weight of the water and rock is born by the underlying tunnel rock and scaffolding. As pressure grows from above, abnormal signs appear, such as squeaks from the scaffolding, and peeling and cracks on the tunnel wall. (4) Gob-water, under pressure, may oppose prospecting drill pipes, in which case, the water spouts. (5) Within the coal seam, gases (including CH4, CO, H2S, etc.) accumulate and concentrate in the closed gob space. As mining approaches the gob area, the gases escape to the working face. (6) Gob-water deep underground is usually isolated from the surface environment and confined in an anoxic, reductive environment for long periods. In some cases, the water would be low in pH, and strongly acidic and corrosive – often because of the oxidization and then dissolution of sulfide minerals. For example, the pH of the gob-water in some coalmines in Zibo, Shandong Province, is between 1 and 2, a level that is detrimental to underground facilities (Pan 2014).
GOB-WATER INRUSH PATTERNS
Gob-water is present in many coalmines in China and exists in complex conditions. Generally, there is no evidence indicating the position or coverage of ancient gob-water. Because of this, mining without precautions could expose ancient gob-water, with catastrophic consequences. Immersion in gob-water over long periods tends to erode and weaken rocks, adding to the likelihood of an inrush.
Modern gob- water can be located and mapped, but such efforts are often based on records of low accuracy. By analyzing Chinese gob-water disaster records, six patterns of gob-water inrush have been determined.
I: Water-inrush from gobs exposed by tunneling
Tunneling frequently exposes gob-waters in Chinese coalmines. This is particularly true of ancient gob-water, whose location and volume are difficult to evaluate due to lack of data.
II: Water-inrush along faults connected to gobs
III: Water-inrush via fissure or caving zones
The strata overlying mine workings are classified into three zones, i.e. the caving, fissure, and bending zones. If the caving and/or fissure zones extend as far as the gob-water overhead, drainage will occur leading to an accident.
However, if the caving zone channels the gob-water into coal seam 7#, the flow rate would be higher through the gaps between rocks in the caving zone, maximizing flow velocity and volume, and generating high water pressure and strong kinetic energy, and thus causing extensive destruction. On November 3, 1983, a gob-water burst of 4,800 m3 via the caving zone in 1,106 working panel in Jingyuan Honghui No.1 Coalmine, Gansu Province, killed 4, damaged 48 items of equipment and flooded 2,400 m of tunnel. On April 6, 1990, in the Beiyi working face of Jingmen Yaohejiaoyu Coalmine, Hubei Province, a gob-water inrush caused by caving zone channeling reached tens of thousands of cubic meters per hour, flooding the mine and killing 5 people.
IV: Water-inrush when water-resistant coal pillars are damaged
K – safety coefficient, usually between about 2 and 5
M – thickness of coal seam or mining height (m)
P – head pressure of gob-water (MPa)
Kp – tensile strength of coal (MPa)
On December 2, 2005, illegal mining of water-resistant coal pillars caused a sudden collapse during excavation in 08# tunnel in Luoyang Sigou coalmine, Henan Province. Some 110,000 m3 of gob-water from the neighboring Qiaobei coalmine killed 42 people and inflicted direct economic losses of RMB 9.726 million (about $ 1.17 million).
V: Water-inrush because of waterproof coal pillar collapse
In Meizhou Daxing Coalmine, Guangdong Province – see above – the seam inclination is between about 55 and 75°. On August 7, 2005, a gob-water inrush occurred when a waterproof coal pillar caved during work on the −290 m crosscut. During the first 17 minutes, some 231,000 m3 of water entered the mine, with a maximum flow rate of 816,000 m3/h. In the next 30 minutes, the mine was flooded to +245 m and a further 19,000 m3 accumulated at up to 37,980 m3/h. In total, 250,000 m3 of gob-water were discharged and the direct economic losses amounted to RMB 47.25 million or about $ 5.70 million (Zhao et al. 2006; Wu et al. 2013).
VI: Water-inrush arising from multiple factors
A number of conclusions can be drawn from the above:
The characteristics of gob-water are that it is highly destructive and corrosive, and enters at high speed, but that the inrush flow rate attenuates quickly.
Gob-water inrushes are foreshadowed. For example, the initial flow is dark and smelly, containing lots of suspended matter and high concentrations of gas. The increase in pressure in the mine causes scaffolding to squeak and tunnel walls to peel.
There are six characteristic patterns of gob-water inrush in Chinese coalmines. These comprise gobs being exposed by tunneling, channeled by water-conducting faults, or by fissure or caving zones, damage to water-resistant coal pillars, and waterproof coal pillars collapsing, and, finally, multiple factors in combination.
This article is funded by National Natural Science Foundation of China (41373095), University Natural Science Research Project of Anhui Province (KJ2017A445) and Anhui Science and Technology Breakthrough Project (1501zc04048).