Separation water is a commonly-seen water hazard in China coalmines. This article, built on case studies of disasters caused by separation water, analyzes the key influencing factors in the formation of separation and the water hazard, as well as the features and causes of explosive, delayed explosive, and intermittent separation water burst. The article takes as an example of one accident caused by roof bed separation water burst in the 745 working face of Haizi Coalmine. The study has a particular interest in separation water burst accompanied by rock burst when mining under thick-hard igneous rock. The results are of reference to countries with similar mining conditions and researches on separation water burst and hazard control in coalmines.

INTRODUCTION

China is the largest coal producer, with an annual coal output around half of world production. Heavy coal mining breeds severe mining accidents and disasters, including water burst, gas explosion, coal spontaneous combustion, roof caving, of which water is the destructive of all (Gui et al. 2016). Mining practice in China has revealed over 30 types of water hazards (Gui & Lin 2016), of which roof bed separation water is one of the deadliest. It is the culprit for mass casualties and huge economic losses (Jing et al. 2006; Cheng & Wang 2008) and remains as the focus of researches among Chinese mining experts.

During mining, the natural stress balance inside the roof strata is disrupted and rebalanced over the goaf zone (Gui et al. 2015). As the roof strata are composed of different rocks, with varying thicknesses and distances to goaf zone, movements are not uniform. In particular, two contiguous strata, hard on top and weak below, may be displaced vertically and create a void called ‘separation’. If underground water accumulates in the separation void, it is called separation water (Cao 2013), which exerts hydrostatic pressure on the underlying strata. When the pressure builds up past a tipping point, the water may burst and endangers mining safety.

Lately, there have been extensive researches on mining-caused roof bed separation and separation water by Chinese mining experts. Liang & Sun (2002) utilized ‘composite beam’ analysis to discuss roof strata movement and proposed the equation to identify separations by quantitative criteria. Su et al. (2003), taking the rock strata above and below the separation as rock plates, built a mechanical model of rock plate deflection, which laid the theoretical foundation for quantitatively estimate the size of separation void. Zhang et al. (2001) studied potential emplacements of roof bed separation and proposed the calculation of maximum separation height. Zhang & Chen (1996) started with the forming conditions of separation water and found the distribution pattern of bed separations and separation water. Research results in this field, while enabling mining practitioners to understand the dynamic evolvement of bed separations and separation water, lays the theoretical foundation to identify and control separation water hazard (Wang et al. 2014).

Case history of separation water accidents from China coalmines have shown that the formation of separation water hazards, though closely related to geological properties and mining conditions, also displays certain patterns throughout the burst (Wang & Wang 2008; Qiao et al. 2011). It is paramount to paraphrase the patterns into practical uses.

DISTRIBUTION OF SEPARATION WATER HAZARDS

Coal reservation spreads over 500,000 km2 across Mainland China, partitioned into five coal-bearing regions, namely the South China, North China, Northeast, Northwest and Tibet-West Yunnan. Except for Tibet-West Yunnan, the rest of the four coal-bearing regions are all affected by separation water hazard to different extents. It is the most perilous in Guizhou Qianbei Coalmine, Chongqing Songzao Coalmine and, Nantong Coalmine of South China, Anhui Huaibei and Huainan Coalmine, Shandong Zaozhuang, and Xinwen Coalmine, Shanxi Datong Coalmine of North China, Liaoning Fuxin Coalmine, Fushun, and Laohutai Coalmine of Northeast, Ningxia Yuanyanghu Coalmine, Shaanxi Yonglong Coalmine, and Xinjiang Yili Coalmine of Northwest (Xu et al. 2013; Yang et al. 2014). Water burst even evolves into catastrophic calamities. For instance, on January 30, 2015, a water accident caused by separation water burst struck the 866-1 mining face of Huaibei Zhuxianzhuang Coalmine, Anhui of North China (Table 1), water discharge maximized at 7,200 m3/h. In addition to huge economic losses, 7 people died and 7 others seriously injured.

Table 1

Accidents caused by separation water in China coalmines

Region Coalmines Date Max. water discharge/m3 h−1 Spot of water burst Roof strata Direct trigger 
North-west Jiaoping Yuhua Coalmine, Shaanxi Mar. 31, 2012 600 1,408 working face sandstone, conglomerate, mudstone water-conductive fissure zone channeling separation water 
May 26, 2012 500 
Yonglong Cuimu Coalmine, Shaanxi Jun. 18, 2013 500 21,302 working face sandstone, mudstone water-conductive fissure zone channeling separation water 
Yuanyanghu Hongliu Coalmine, Ningxia Nov. 3, 2009 31,800 1,121 working face sandstone, mudstone water-conductive fissure zone channeling separation water 
Nov. 21, 2009 5,670 
Mar. 3, 2010 55,700 
Mar. 25, 2010 121,880 
North China Huaibei Zhuxianzhuang Coalmine, Anhui Jan. 30, 2015 7,200 866-1 working face mudstone, sandstone water-conductive fissure zone channeling separation water 
Huaibei Yangliu Coalmine, Anhui Jul. 17, 2011 7,845.6 working face igneous rock, sandstone igneous rock fractures and causes rock burst 
Chongzhou Jining No. 2 Coalmine, Shandong Oct. 6, 2007 490 11,306 working face limestone, mudstone hydraulic pressure and load of separation water 
Xinwen Huafeng Coalmine, Shandong Sep. 2005 720 1,409FMTC working face alternating strata of thick gravel, claystone, sandstone rock burst 
Huaibei Haizi Coalmine, Anhui May 21, 2005 3,887 745 working face igneous rock, siltstone, medium sandstone, mudstone igneous rock fractures and causes rock burst 
Huainan Xinji No. 1 Coalmine, Anhui Jan. 30, 2003 400 1,307FMTC working face gneiss, limestone, fine sandstone gneiss fractures and causes rock burst 
Huainan Xinji No. 2 Coalmine, Anhui Dec. 28, 2001 85 1,113,104 working face gneiss, limestone, mudstone gneiss fractures and causes rock burst 
South China Songzao Datong No. 1 Coalmine, Chongqing Dec. 2, 2008 650 S1821 working face limestone, mudstone roof strata falls 
Nantong Nantong No. 1 Coalmine, Chongqing Aug. 28, 1994 963 6,404 working face limestone, sandstone, shale shear crack cuts through separation zone 
Nantong Yutianbao Coalmine, Chongqing 1985 1,500 2,403 working face limestone, sandstone, shale shear crack cuts through separation zone 
Nantong Nantong No. 2 Coalmine, Chongqing Jun. 2, 1966 442 5,404 working face limestone, sandstone, shale shear crack cuts through separation zone 
Region Coalmines Date Max. water discharge/m3 h−1 Spot of water burst Roof strata Direct trigger 
North-west Jiaoping Yuhua Coalmine, Shaanxi Mar. 31, 2012 600 1,408 working face sandstone, conglomerate, mudstone water-conductive fissure zone channeling separation water 
May 26, 2012 500 
Yonglong Cuimu Coalmine, Shaanxi Jun. 18, 2013 500 21,302 working face sandstone, mudstone water-conductive fissure zone channeling separation water 
Yuanyanghu Hongliu Coalmine, Ningxia Nov. 3, 2009 31,800 1,121 working face sandstone, mudstone water-conductive fissure zone channeling separation water 
Nov. 21, 2009 5,670 
Mar. 3, 2010 55,700 
Mar. 25, 2010 121,880 
North China Huaibei Zhuxianzhuang Coalmine, Anhui Jan. 30, 2015 7,200 866-1 working face mudstone, sandstone water-conductive fissure zone channeling separation water 
Huaibei Yangliu Coalmine, Anhui Jul. 17, 2011 7,845.6 working face igneous rock, sandstone igneous rock fractures and causes rock burst 
Chongzhou Jining No. 2 Coalmine, Shandong Oct. 6, 2007 490 11,306 working face limestone, mudstone hydraulic pressure and load of separation water 
Xinwen Huafeng Coalmine, Shandong Sep. 2005 720 1,409FMTC working face alternating strata of thick gravel, claystone, sandstone rock burst 
Huaibei Haizi Coalmine, Anhui May 21, 2005 3,887 745 working face igneous rock, siltstone, medium sandstone, mudstone igneous rock fractures and causes rock burst 
Huainan Xinji No. 1 Coalmine, Anhui Jan. 30, 2003 400 1,307FMTC working face gneiss, limestone, fine sandstone gneiss fractures and causes rock burst 
Huainan Xinji No. 2 Coalmine, Anhui Dec. 28, 2001 85 1,113,104 working face gneiss, limestone, mudstone gneiss fractures and causes rock burst 
South China Songzao Datong No. 1 Coalmine, Chongqing Dec. 2, 2008 650 S1821 working face limestone, mudstone roof strata falls 
Nantong Nantong No. 1 Coalmine, Chongqing Aug. 28, 1994 963 6,404 working face limestone, sandstone, shale shear crack cuts through separation zone 
Nantong Yutianbao Coalmine, Chongqing 1985 1,500 2,403 working face limestone, sandstone, shale shear crack cuts through separation zone 
Nantong Nantong No. 2 Coalmine, Chongqing Jun. 2, 1966 442 5,404 working face limestone, sandstone, shale shear crack cuts through separation zone 

It can be inferred from Table 1 that thick-hard rock strata (such as conglomerate, limestone, and igneous rock, etc.) in the roof strata are the preconditions for the formation of separations. Therefore, these thick-hard rock strata are called ‘key stratum’ (Qian et al. 1996; Xu et al. 2004). Once the key stratum fractures, the water confined in the separation will is lashed out, sometimes accompanied with rock burst, and threatening mining safety and personal safety.

FEATURES OF SEPARATION WATER HAZARD

Separation water bursts in three patterns, namely explosive, delayed explosive, and intermittent, with differing features and formation mechanisms.

Features of explosive separation water burst

When the thick-hard rock strata (i.e. the key stratum) above the separation are such suspended past a critical balance, the strata fracture abruptly and exert huge impact on separation water, which then is forced through the cushion zone between the separation and fissure zone (Figure 1), in a flash, into the working face. Such water burst is characterized as:
  • (1) Initial water flow is large and forceful. Water flow maximizes at outbreak, rampant and ferocious. Physical ramifications range from production halt to equipment destruction and human casualties. On May 21, 2005, the 745 working face of Huaibei Haizi Coalmine, Anhui Province of North China, was struck by a separation water burst (Figure 1), in which water discharge maximized at 3,887 m3/h. The instantaneous burst came without warning. Evacuation was impossible, killing 5 miners in the accident (Li 2006; Zhu et al. 2009).

  • (2) Water flow quickly dies down. Explosive separation water burst, though massive at beginning, soon attenuates (Cheng & Wang 2008; Ren et al. 2008). In the same accident cited above, 18 minutes after the water burst at 746 working face of Haizi Coalmine, water flow decreased from 3,887 m3/h to 905 m3/h, by a drop of 165.67 m3·h−1/min. Then to 139 m3/h 210 minutes later (3.5 hours). The average decrease in the 210 minutes was 17.85 m3·h−1/min (Figure 2).

Figure 1

Spatial location of separation zone.

Figure 1

Spatial location of separation zone.

Figure 2

Time curve of water discharge in the separation water burst at 745 working face, Haizi Coalmine.

Figure 2

Time curve of water discharge in the separation water burst at 745 working face, Haizi Coalmine.

Features of delayed explosive separation water burst

Delayed explosive separation water burst is featured by mild water flow at beginning, peaked after a while and followed by rapid decrease. On Aug. 28, 1994, the 6,404 working face of Nantong Nantong No. 1 Coalmine, Chongqing of South China, experienced a roof separation water burst (Xie 1997). Initial water flow was 200 m3/h, then maximized at 963 m3/h and dropped to 10 m3/h in 10 hours (an average decrease of 95.3 m3·h−1/h); later to 1.4 m3/h in 48 hours, by a decrease of 99.9% (Figure 3).
Figure 3

Time curve of water discharge in the separation water burst at 6,404 working face of Nantong No. 1 Coalmine.

Figure 3

Time curve of water discharge in the separation water burst at 6,404 working face of Nantong No. 1 Coalmine.

Among the many influencing factors to the delayed maximum water flow, the energy created by key stratum fracture and resistance of the cushion layer are the most critical. Generally, the key stratum, from bending to fracture, undergoes a dynamic evolution. If the key stratum has large thickness and strengths, so is its flexural rigidity. In the formation of separations, the key stratum flexes, accumulating flexural elastic energy, and compresses on the underneath separation water, which transmits the pressure down to the cushion layer and cracks it open, letting out the separation water (Figure 4(a)). When it happens, the cushion layer is still an entirety containing a small number of water conduits and water flow is comparatively small. As the key stratum flexes further, as soon as it surpasses its elastic limit, the stratum fractures abruptly and exerts enormous energy onto the cushion layer, causing it to collapse drastically (Figure 4(b)) and separation water rushing out to the maximum through many more conduits.
Figure 4

Key stratum flex and fracture.

Figure 4

Key stratum flex and fracture.

Features of intermittent separation water burst

Intermittent separation water burst is related to the periodic culmination of mining pressure. When the roof strata pressure down and fracture the key stratum, separation water bursts and drains before the separation begins to close up. This process repeats when the mining pressure culminates in the next cycle. For instance, on March 31, 2012, the 1,408 working face of Jiaoping Yuhua Coalmine, Shaanxi of Northwest (Table 1), experienced a separation water burst with flow of 600 m3/h. Two months later (on May 26, 2012), another burst (flow of 500 m3/h) attacked the working face when the roof strata pressurized again. Understanding the pattern, the Coalmine took preventive measures by draining the separation water before the culmination cycle returns. These measures effectively prevented concentrated separation water burst at apex of mining pressure and also ensured operation safety on the working face.

Water burst accompanied by rock burst

As presented above, separation water burst is usually caused by key stratum fracture. If the fracture energy is adequately huge, it will also bring about rock burst.

  • (1) The flexural elastic energy of the key stratum. Mining causes the thick-hard rock strata (the key stratum) to flex and accumulate elastic energy. When the key stratum first fractures, the elastic energy within transforms into kinetic energy instantly. The kinetic energy, on one hand, is exerted on the separation water and breaks through the cushion layer (between the separation water and fissure zone) and into the coal mining face (Figure 1), compromising mining safety. On the other, the kinetic energy causes a great surge in mining pressure, which may trigger pernicious rock burst disaster.

Theoretically, the flexural elastic energy U (Dou et al. 2004), which is confined in the key stratum when it flexes and deforms before the first fracture, is: 
formula
1
 
formula
2

In the formula:

  • q — the load on the key stratum and overlying strata/Mpa;

  • L0 — length of first fracture of the key stratum (distance to the open cut)/m

In Formula (2), which is derived based on the beam theory:

  • E — elastic modulus of the key stratum/Mpa;

  • H0 — thickness of the key stratum/m;

  • Rt — tensile strength of the key stratum/Mpa.

Substituting Formula (2) into Formula (1): 
formula
3

It is thus evident that the flexural elastic energy (U) of the key stratum is in positive proportion to the squared thickness of key stratum (H0) and to the 2.5 power of its tensile strength (Rt). In other words, the thicker the key stratum, the strong it is, then the more flexural elastic energy within. When it first fractures, the flexural elastic energy will be released and spread out in the form of seismic waves.

From the spot of first fracture, within a distance of dl, energy loss is denominated as dU, which is: 
formula
4
When the elastic energy, generated by the first fracture of the key stratum, reaches the roadway or the working face, the remnant energy Ul is: 
formula
5

In the formula:

  • U0 — initial energy when l = 0, i.e. the epicenter energy released by the first fracture of the key stratum/J, substituted into Formula (3)

  • λ — energy attenuation coefficient, correlated to the types of roadway and working face, as well as the magnitude of epicenter energy (Konopko 1994), usually λ = 0.012 ∼ 0.039.

Obviously, the larger the epicenter energy (U0), the larger the energy that will reach the roadway or the working face (Ul), raising the possibility of rock burst.
  • (2) An engineering case taken from Huaibei Haizi Coalmine in Anhui Province, targeted at the formation mechanism of rock burst when mining under thick-hard igneous rock.

    • a) Lithology of roof strata. The roof strata on top of the 745 working face of Haizi Coalmine are shown in Figure 5 and Table 2. Drilling hole R455# reveals that the igneous rock stratum has a thickness of 87.6 m, tensile strength (Rt) of 10.91 ∼ 16.94 Mpa, and RQD of 91 to 95%. It is remarkably strong and shows noticeable features of an integral entirety. The lower stratum is as thick as 61.2 m, composing of ductile medium-grained sandstone, siltstone, mudstone, etc.

    • b) Separation and separation water. There are 3 distinct separations (T1, T2, T3) under the mining in the 7# seam. T1 is between Stratum ② medium-grained sandstone and Stratum ③ siltstone, with a height of more than 1.2 m and unit water discharge qT1 = 0.32 L/(s.m). T2 locates between Stratum ③ siltstone and Stratum ④ medium-grained sandstone, with a height of more than 1.5 m and unit water discharge qT2 = 0.056 L/(s.m). T3 emplaces between Stratum ⑤ siltstone and Stratum ⑥ igneous rock, with a height of over 3 m and unit water discharge qT3 = 0.16 L/(s.m). Mining height of 7# seam is 2.5 m, which gives out the Hf, maximum height of water-conductive fissure zone, of 40 m. As a result, the thickness of the cushion layer between T1 and fissure zone, ho = (H + H) -Hf = (23.13 + 18.07) −40 = 1.2 m. consequently, T1 is the closest separation, of the three, to the working face and contains the largest body of water.

    • c) Bursting liability of igneous rock. Measured by the bursting liability indices of the igneous rock (Stratum ⑥) (Li et al. 2008), DT (duration of dynamic fracture) is 165.96 ms, WET (elastic strain energy index) is 6.81, KE (bursting energy index) is 8.59, UwQ (bending energy index) is over 100 kJ. According to the China Classification Standards of Bursting Liability of Rock (MT/T866-2000), the igneous rock in discussion has a strong bursting liability.

    • d) Rock burst as a geological disaster. Substituting parameters from Table 2 (Li 2011) and into Formula (3) and obtain the maximum epicenter energy at first fracture of the key stratum (igneous rock), U0 = 4.613 MJ, equivalent to 2.3 times of the energy released in an earthquake of Magnitude 1 (2 MJ). The impact of such magnitude of energy will destroy the whole thin cushion layer (as thin as 1.2 m), forcing T1 separation water into the working face at a maximum speed of 3,887 m3/h. More gravely, 400 m3 of waste rock exploded out creating a mud slide, cutting all roadways in its way, flushing out a ditch of 1.2 m deep, and killing 5 people.

Figure 5

Structure of roof strata and locations of separations at 745 working face, Haizi Coalmine.

Figure 5

Structure of roof strata and locations of separations at 745 working face, Haizi Coalmine.

Table 2

Physico-mechanical indices of the roof strata at the 745 working face, Haizi Coalmine

Stratum Thickness/m Lithology Density γ/t.m−3 Compr-essive strength Rc/Mpa Tensile strength Rt/Mpa Visco-Sity C/Mpa Internal friction angle φ/° Elastic modulus E/Gpa Poisson's Ratio μ 
⑦ 243.53 loose bed 2.000       
⑥ 87.60 igneous rock 2.767 144.21 10.91 12.63 43.33 28.64 0.17 
⑤ 6.50 siltstone 2.622 31.06 3.63 5.70 36.32 2.72 0.23 
④ 2.20 medium-grained fine sandstone 2.653 71.35 5.10 9.91 39.64 9.46 0.18 
③ 11.30 siltstone 2.626 31.06 3.63 5.70 36.32 2.72 0.23 
② 18.07 medium-grained fine sandstone 2.643 71.35 5.10 9.91 39.64 9.46 0.18 
① 23.13 mudstone 2.578 18.08 1.16 4.42 36.54 1.94 0.26 
 2.52 7# coal seam        
Stratum Thickness/m Lithology Density γ/t.m−3 Compr-essive strength Rc/Mpa Tensile strength Rt/Mpa Visco-Sity C/Mpa Internal friction angle φ/° Elastic modulus E/Gpa Poisson's Ratio μ 
⑦ 243.53 loose bed 2.000       
⑥ 87.60 igneous rock 2.767 144.21 10.91 12.63 43.33 28.64 0.17 
⑤ 6.50 siltstone 2.622 31.06 3.63 5.70 36.32 2.72 0.23 
④ 2.20 medium-grained fine sandstone 2.653 71.35 5.10 9.91 39.64 9.46 0.18 
③ 11.30 siltstone 2.626 31.06 3.63 5.70 36.32 2.72 0.23 
② 18.07 medium-grained fine sandstone 2.643 71.35 5.10 9.91 39.64 9.46 0.18 
① 23.13 mudstone 2.578 18.08 1.16 4.42 36.54 1.94 0.26 
 2.52 7# coal seam        

CONCLUSIONS

Following conclusions are drawn from the above study:

  • (1) In the five coal-bearing regions of Mainland China, except for the Tibet-West Yunnan region, the other four regions (South China, North China, Northeast, Northwest) all are inflicted with separation water hazards to different extents.

  • (2) Separation water hazard in coalmines is dependent on many mining factors. Of all, the presence of thick-hard key stratum in the roof strata is the precondition to the formation of separations. Key stratum, once fractures, will transform separation water into a minacious hazard or event cause catastrophic water disaster.

  • (3) There are three types of separation water burst: explosive, delayed explosive, and intermittent. Explosive separation water burst reaches the maximum water flow at the beginning. Delayed explosive separation water burst is mild at first and continues to grow into the maximum. Intermittent separation water burst is related to the periodic pressurizing of the roof strata on top of the working face.

  • (4) Both the explosive and delayed explosive separation water burst display three notable features: ① When the water flow reaches the maximum, it is aggressive and destructive. ② After it reaches the maximum, the water flow quickly dampens. ③ Separation water burst may be accompanied by rock burst, threatening facility integrity and human safety.

ACKNOWLEDGEMENTS

This article is funded by Anhui Science and Technology Breakthrough Project (1501zc04048) and National Natural Science Foundation of China (41373095).

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