This article introduces the evaluation of mining conditions under pore aquifer and presents four control techniques on pore water, i.e. retaining safety coal (rock) pillars to resist water, sand, and collapse; draining bottom pore aquifer in Quaternary strata to lower underground water level; presplitting hard overlying rock by blasting to inhibit growths of water-conductive fissures; manually caving the roof strata over the steep sloping coal seam by extruding blast to prevent water-resistant coal pillars from caving. This article is to provide references for countries confronting similar problems and to facilitate international discussions on pore water control.
Looking at the distribution of known coal resources in the mainland China, the over 500,000 km2 of coal-bearing area is divided into six regions, namely North China Carboniferous-Permian, northeast and northwest Jurassic, South China late Permian, Tibet-West Yunnan Mesozoic, and Taiwan Paleogene (Fan 2003). The complicated hydrogeological conditions are inherent with as many as 30 common types of water hazards. With over 60 years of research and practice, certain types of water hazards, such as sandstone aquifers in coal seam, have been effectively contained by maturing technologies; while some other types, such as pore water, are frequent causes of water disasters in coalmines. On Dec. 19, 1977, a catastrophic disaster caused by pore water in the Cenozoic Quaternary loose strata in Meihe coalmine, Liaoyuan of Jilin Province, burst out 56,520 m3 of water and sand and killed 64 people (Wu et al. 2013a, 2013b). On May 31, 2001, Shanwan coalmine, in Shendong of northwest coal-bearing region, water-sand bursts from Quaternary pore aquifer silted about 400 m roadway, registering 4,700 m3 of sand and 50 ∼ 60 m3/h max water inrush. The mining area was closed for 20 days and suffered economic losses of over RMB 1.4 million (Gui et al. 2015).
Most of the coal seam in China are covered by thick and giant thick Cenozoic Quaternary loose strata. The north China coal-bearing area, in particular, has overlying Cenozoic Quaternary loose strata of as thick as 250 m to 500 m on top of the coal seam. Generally, loose strata breed multiple layers of pore aquifers, the bottom of which (hereinafter referred as the bottom aquifer) is in direct contact with coal seam. The bottom aquifer is the immediate water menace to coal mining and the main outlet for water-sand bursts (Zhang et al. 2013; Deng et al. 2014; Xu et al. 2015b). In order to prevent water burst from the bottom aquifer in Quaternary loose strata, safety coal (rock) pillars of 80 m are retained as precautions. But these pillars stock substantial tonnage of coal, which is estimated to be in tens of billions.
EVALUATION OF MINING CONDITIONS
Water disasters in coalmines border on two factors: water source and conduits. In the scenario of pore water accidents, the water source is in the Quaternary bottom aquifer and the conduits include pre-existing conduits (water-conductive faults, etc.) and man-made conduits (mining-caused rock caving and water-conductive fissures, etc.). The bottom aquifer supplies water, sand, and hydraulic pressure. Pre-existing fissures and mining fissures serve as carriers. Thus, the key to pore water control is to control water source and conduits (Duan 2013; Zhang 2015b; Zhao & Ren 2015).
Pre-mining evaluation shall include the followings to allow for effective pore water control plans.
Hydrogeological properties of Quaternary bottom aquifer, such as replenishment, run-off, drainage, water abundance, waterhead pressure, and physical composition, etc., as well as explicit opinion on the necessity of water recycling.
Lithological properties of weathering bedrock, thickness and degree of weathering.
Lithological properties of overlying rock over mining coal seam and its composition, etc.
Mining techniques and roof management.
(i) Retaining safety coal (rock) pillars. In the light of the evaluation results, safety coal (rock) pillars of three types, water-resistant, sand-resistant, and anti-collapse, can be retained to contain water-sand bursting into mining workface from Quaternary bottom aquifer and weathering bedrock.
(ii) Draining pore aquifer. Based on the analysis of Quaternary bottom aquifer, draining plan shall evaluate drainage effectiveness and adopt the proper type of safety coal (rock) pillar accordingly.
(iii) Improving mining techniques and roof management (such as fill mining, presplitting blast, etc.) to avoid water-sand bursting into mining workface from Quaternary bottom aquifer and weathering bedrock.
WATER CONTROL TECHNOLOGY
Retaining safety coal (rock) pillars
Water-resistant coal (rock) pillars. If the bottom aquifer of Quaternary loose strata has high water abundance and refills sufficiently or when there is a demand to recycle underground water, water-resistant coal (rock) pillars must be retained at a height by Hw ≥ Hf + S. Hf represents the maximum height of the water-flowing fractured zone. S represents the thickness of the protective strata (usually 2–4 times of mining thickness (MCI 2000; Shang et al. 2015), as shown in Figure 2(a).
Sand-resistant coal (rock) pillars. If the bottom aquifer of Quaternary strata has low water abundance and refills insufficiently or when there is no demand as to recycle underground water, sand-resistant coal (rock) pillars can be retained at a height by Hs ≥ Hc + S. Hc represents the maximum height of the caving zone as in Figure 2(b). Its role is to allow the water-flowing fractured zone to partially implicate bottom aquifer while keep caving zone away from it, so as to prevent the sand bursting from bottom aquifer into mining panel.
Anti-collapse coal (rock) pillars. If the bottom aquifer of Quaternary loose strata has poor water abundance with static reserve (i.e. without extra water supply), anti-collapse coal (rock) pillars can be retained at a height by Hcf. This will allow the water-flowing fractured zone to implicate bottom aquifer and the caving zone to approach it as well (Figure 2(c)). Its height Hcf is commensurate to the maximum height Hc of the caving zone.
Draining quaternary bottom aquifer
Case on pre-mining draining. The workface No. 20601 of Daliuta coalmine, Shendong of northwest coal-bearing region, had a dimension of 220 m in width and 2,800 m in length. The bedrock over the 2−2 coal seam had a thickness of 35–45 m. Hydrogeological analysis showed that the coal seam was in close vicinity of the Quaternary pore aquifer which was rated with medium to high water abundance. Water inrush, if were to happen, was estimated at 400 ∼ 600 m3/h. The geological and hydrogeological conditions were prone to inducing water-sand accidents (Li 2004; Du et al. 2013). Countering the natural perils, proactively deployed 2 drainage holes in workface, 8 drainage holes in roadway, 3 observation holes and 1 forced draining hole. Before mining, manual draining was conducted on the pore aquifer for 74 days, resulting in an average water level decrease of 6.3 m and a max of 9.4 m. Total water discharge was as high as 210,000 m3. On Aug. 13, 1995, after test mining on the workface, the first manual caving incurred water inrush of 28 m3/h and maximized at 50.18 m3/h on the Aug. 18. No further water-sand inrush accident.
Case on concurrent mining and draining. In the north China coal-bearing region, the No. 810 mining panel, in Luling coalmine of Huaibei, had a coal reserve of over 5 million tons under the threat of Quaternary pore water. Hydrogeological prospecting concluded that the bottom aquifer was predominated with static water and could be drained up. To this end, a number of drilling sites were arranged along the ventilation roadway, mechanical roadway, and open-off cut (Figure 3). Each site was organized with 3 draining holes (Figure 4) to extract water from Quaternary pore aquifer during mining (Tu et al. 2004; Chen et al. 2014; Gui et al. 2015).
Technical parameters as follows:
Draining site: L × W × H = 4 m × 3 m × 3 m, at an interval spacing of 50 m and supported by U-sized steel structure;
Draining hole diameter: φ75 mm
Elevation of draining hole: 50–60°
End hole locations (Figure 4): the end holes corresponding to the three draining holes in each site are located in the following fashion: 1# inside the workface, 2# outside the workface, 3# right above the roadway.
Drilling depth: 20–40 m as end holes reach mid-way inside the bottom aquifer.
The No. 810 mining panel, taking example after No. 8101 workface (Figure 3), set up 18 drilling sites and 54 draining holes. In 3-month time, total water discharge amounted to 3,000 m3, leaving bottom aquifer almost drained up. In later mining operation, there were occasional sand bursts locally but didn't escalate into major water-sand accidents.
Inhibiting growths of water-conductive fissures by presplitting blast
When mining under pore aquifers, pore water would not evolve into a water disaster if the water-resistant coal (rock) pillars are high enough that the water-flowing fractured zone cannot reach the aquifer. But there have been exceptions. Qidong coalmine in Huaibei of Anhui Province of north china coal-accumulation zone went into production in Nov. 2001 with a test mining on the first mining panel 3222 on Nov. 5. As mining proceeded to 42 m, the pore water in the bottom aquifer of Quaternary loose strata rushed out and continuously grew up to 1,520 m3/h within one day. The entire coalmine was flooded and cost hundreds of millions of economic losses. For panel 3222, the water-resistant coal (rock) pillars were designed for a height of 65 m, whereas the actual height (Hw') was 70 m as verified afterwards by drilling results.
Manual caving by extruding blast
To proactively control water-resistant coal pillar caving, manual caving by extruding blast (Shen & Wen 2000) is applied in some coalmines and has harvested favourable results in practice. This technique is to blast the overhead rock and sever or shatter the rock to fill the goaf space, so as to prevent water-resistant coal pillars caving.
Based on above analysis, conclusions can be drawn as follows:
In pore water accidents, the inrush water-sand and hydraulic pressure are sourced from pore aquifer through pre-existing and mining fissures as the inrush conduits. Therefore, the key to tackling pore water hazard is to control the water source and inrush conduits.
With accurate evaluation of mining condition, main techniques to contain pore water hazard include:
(i) Retaining safety coal (rock) pillars of verifiable heights to separate Quaternary bottom aquifer from mining workface. The pillars can resist water and sand and prevent collapse.
(ii) Draining Quaternary bottom aquifer before mining or during mining to lower waterhead level and decrease hydraulic pressure.
(iii) Presplitting hard overlying rock by blasting to inhibit growths of water-conductive fissures.
(iv) For mining steep sloping coal seam, manual caving by extruding blast to prevent coal pillar caving and water-sand inrush accidents.
This work was supported by the National Natural Science Foundation of China (41373095) and Anhui Science and Technology Breakthrough Project (1501zc04048).