Technical research on controlling major karst water hazards in China coalmines

China has the largest coal production in the world. The total annual output has maintained at 3 billion tons since 2014 and recorded at 3.68 billion tons in 2015, almost twice the total of US, India, and Australia, which are 900 million, 550 million, and 451 million tons respectively.

In 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 over 30 common types of water hazards.

China has a territory area of over 9.6 million km², in which limestone is present as much as 2 million km² or 20.83%. About 60% of the coal-bearing region in China, containing millions of tons of coal reservation, is affected by limestone karst water. Looking at the frequency and severity of water-related coalmine disasters in China, karst water has become one of the major water hazards threatening mining safety in China especially in North China and South China region. The frequent occurrence of mine flooding and human casualties caused by the large volumes of highly pressurized karst water is catastrophic to safe production in coalmines (Table 1) (Wu et al. 2013).

The China coal industry has accumulated rich achievements in karst water control and treatment, which will be of value and relevance to other countries facing similar threats from karst water in coalmines.

Taking North China coal-bearing region as an example, the strata development, from new to old, are Quaternary, Neogene (partially Paleogene), Permian, Carboniferous, and Ordovician. The Permian strata are the main coal seam, under the threat from karst aquifer in Carboniferous Taiyuan formation and Ordovician limestone.

If the actual water head pressure in karst aquifer H_re is smaller than the value of H_th in formula (1), then the roadway is safe. If H_re > H_th, water will break the bottom plate and burst. As a countermeasure, it is mandatory to drain water to lower water head pressure by △H so that H_re-△H < H_th.

In the formula:

• L_p – width of the safety water-resistant rock pillar (m);

• K – safety coefficient, ref. 2–5;

• M – height of coal roadway (m);

• H_re1 – actual water pressure at the contact point of coal roadway and top surface of karst aquifer (Mpa);

• k_p – average tensile strength of the rock strata in which the coal roadway passes through

In the formula:

• H^’_th – theoretical safety water pressure bearable to the bottom plate in the coal roadway (Mpa);

• k_p – average tensile strength of the bottom aquifuge in the coal roadway (Mpa);

• γ – average weight of the bottom aquifuge in the coal roadway (MN/m³);

• t – minimum distance from coal roadway boundary to top surface of karst aquifer (m);

• l – bottom width of coal roadway (m);

• α – inclination angle of rock strata (°).

If H^’_th > H_re2, the safety water-resistant rock pillar of width L_p is secure. If H^’_th < H_re2, the width of the safety water-resistant rock pillar is not secure and needs widening until H^’_th > H_re2.

When mining proceeds into coal seam in peril of karst aquifer, the main contributors to water burst are (Li et al. 1987; Gui et al. 1999; Huang & Li 2008):

• (1) Karst water head pressure on the bottom aquifuge (P_re). P_re is decided by the hydrogeological properties of the karst aquifer and can be directly measured by water level (pressure) observation port. When rock strata are in horizontal or near-horizontal position (Figure 1(b)), P_re is evenly distributed (and P_re = H_re). When rock strata are tilted (Figure 2(b)), P_re is unevenly distributed (it increases as it goes deeper), the water head pressure on the bottom plate at the mechanic roadway is higher than that at the airway by △P = H_red- H_reu = γ_wLsinα, in which γ_w represents the weight of water (MN/m³), L is the inclination length of the coal mining face (m), α the inclination angle of rock (coal) strata (°).

• (2) The effective thickness (h₂) and strength (k_p) of bottom aquifuge. The destruction caused by mining in the bottom plate (the depth as h₁) will decrease the effective aquifuge thickness to h₂ (Figure 1(b) and Figure 2(b)). Therefore, h₂ and k_p are two critical parameters to evaluate the water-resistance of the bottom aquifuge.

In the formulas, T_s is the water burst coefficient (Mpa/m), M₀ is the thickness of the bottom aquifuge in coal seam (m), h₁ is the destruction depth in bottom coal seam after mining (m), I is the critical indicator of water burst, σ₃ is the minimum horizontal main stress on the rock mass in the bottom aquifuge (Mpa), Z is the water-resistance coefficient (Mpa/m), P_fis the stress that fractures the bottom plate in the hydraulic fracturing tests (Mpa), R is the extension radius of the fissures caused by hydraulic fracturing (ref. 40–50 m).

It can be seen that, in formula (4), (5) and (6), the values of P_re, M₀ and R are known, the values of h₁, σ₃ and P_f have to be obtained through experiments.

If the water pressure on the bottom aquifer in the testing spot is P_re, formula (5) can be applied to get the value of I_i (=P_re/σ_3i, i = 1,2,…,n). Risks of water burst are higher at segments where I_i > 1, precautious measures are demanded (Murdoch & Slac 2002; Liu et al. 2007; Xie et al. 2009).

In the formula, is the weighted average thickness of water-resistance coefficient (Mpa/m).

When the tolerance of bottom aquifuge for water pressure (P_rea) is smaller than the karst water pressure exerted on the aquifuge (P_re), draining is required to reduce the water head pressure in karst aquifer to below P_rea. The preconditions for applying water draining and depressurizing are:

• (1) Within mining boundaries, if water replenishment to the karst aquifer is poor, it is feasible to lower water pressure to safety values through limited and controlled draining.

• (2) If the designed roadway must expose the karst aquifer, water must be drained to lower water pressure in the karst aquifer.

• (3) If consolidation grouting and revamp cannot enhance the water-resistance of the bottom aquifuge, which cannot undertake both karst aquifer pressure and mine pressure, water draining must be taken properly to lower water pressure in the karst aquifer.

In most China coalmines, popular ways to drain water and depressurize are conducted on ground level, underground level, and dual-mode. For ground draining, big holes are drilled into the karst aquifer. Then use submerged pump to extract water from the karst aquifer. This method is reliable and simple to execute, commonly used for mines where coal seam is close to ground surface.

Underground draining (two scenarios: roadway draining and drill hole draining) is to either expose karst aquifer in the roadway or drill into the karst aquifer in roadway to lower water level. When in dual-mode, both ground and underground draining are utilized for mines where hydrogeological conditions are complicated.

When conducting underground draining, compatible equipment, including pumps, pipelines, water tanks, and power facilities, must be configured in place. Pumps include working pump, back-up pump, and maintenance pump. Working pump is required to ‘drain up normal mine water discharge of 24 h within 20 h’. Back-up pump capacity shall be no smaller than 70% of the working pump, while maintenance pump capacity no smaller than 25% of the working pump. The total capacity of back-up pump and maintenance pump shall ‘drain up the maximum mine water discharge of 24 h within 20 h’ (SACMS 2010).

No matter which draining method is chosen, implementation must take into considerations of prospecting facts. In addition, when draining high-pressured karst aquifer, drilling must be protected with back pressure and blow-out device. If water pressure in the drill holes surges or ‘resists’, drilling must pause immediately (without drawing out the drill pole) and resume after the site is technically conditioned (SACMS 2009) to avoid high-pressured water burst, which could evolve into fatal accidents.

If the karst aquifer has sufficient water replenishment with high water yield and pressure, draining does not suffice to reduce water pressure. Moreover, the massive drainage workload is not only costly, but also may cause secondary disasters such as karst collapse at ground level. In this case, the countermeasure is to consolidate and revamp the bottom aquifuge to enhance its water-resistant capability. ‘Consolidation’ is to grout into the pre-existing fissures and faults in the bottom aquifuge so as to improve rock mass strength and resistance against karst water pressure. ‘Revamp’ is to convert the limestone strata, located in the shallow part of the karst aquifer where water yield is low, into aquifuge so as to increase actual aquifuge thickness and its water-resistance against karst water. In China coalmines, the mainstream techniques are underground drilling for local consolidation grouting and revamp (Xu & Li 2014; Xu & Yang 2014; Xu et al. 2014), ground targeted branch drilling for holistic consolidation grouting and revamp through sequential strata (Li et al. 2013; Shi et al. 2013; Shi 2014).

• (1) Underground drilling for local consolidation grouting and revamp. As a first step, conduct geological prospecting to identify water yield anomaly in the bottom aquifuge and the shallow part in the karst aquifer. Then at coal mining face, drill in the airway, roadway, or open-off cuts slanting inwards the anomaly spots and grout. Lastly, observe and monitor grouting outcomes through drilling or prospecting (Hou 2013).

• (2) Ground targeted branch drilling for holistic consolidation grouting and revamp through sequential strata. As underground workspace is narrow, leaving limited room for bottom aquifuge revamp, only certain sections (where water yield is abnormal) in one coal mining face can be grouted at a time. In recent years, the ground targeted branch drilling for holistic consolidation grouting and revamp through sequential strata is widely adopted in many China coalmines to consolidate and revamp bottom aquifuge in an extensive area (such as multiple coal mining face in one mine).

Ordovician limestone strata are below Carboniferous Taiyuan formation and its thickness is over 500 m with developed karst fissures, high water pressure (5.0–7.5Mpa), and high water yield. If there are water-conductive faults or karst collapse column, it will strengthen the hydraulic relations among the Ordovician limestone, Carboniferous Taiyuan formation, and coal seam, thus threatening mining safety.

Bottom aquifuge consolidation grouting and revamp is shown as in Figure 5(b). At ground level, first to drill vertically into the ground (the parent hole) and then, at certain depth of the parent hole, to drill branch holes along L₃ limestone stratum and grout (the number of branch holes are determined by the dimensions of the bottom plate to be revamped and the grout spreading radius; drilling path is sophisticatedly controlled by the built-in measure and control system). L₃ limestone stratum, once grouted into aquifuge, will add to the original aquifuge thickness by △M = 23 m (i.e. the distance between L₁ top to L₃ bottom), lowering water burst coefficient by 27%. This technique has been applied successfully in Huaibei Zhuzhuang coalmine and Huaibei Hengyuan coalmine in the North China coal-bearing region, gaining effective and cost-saving results in water hazards control. As it ensures mining safety without draining underground water from karst aquifer, it will be the primary technique in China coalmines to treat high-pressured karst water hazard with high water yield.

Following conclusions can be drawn based on the above analysis:

• (1) The spatial relation between mining facility, including coal roadway and coal mining face, and karst aquifer is critical to the design and implementation of water hazards control and treatment. If faults do not exert an impact, the spatial relation can be generalized into two categories, the mining facility is either above or on the side of the karst aquifer.

• (2) Identification of water burst from karst aquifer can be realized through water coefficient, critical indicator of water burst, and water-resistance coefficient, etc. The key parameters relevant to the above-mentioned coefficients can be obtained onsite through the drill hole double-end leak stoppage device and the testing system of tectonic stress in hydraulic fracturing, such as destruction depth in bottom mining plate (h₁), the minimum main horizontal stress on the rock mass in the bottom aquifuge (σ₃), and the stress that fractures rock mass in the bottom aquifuge (P_f).

• (3) Technical solutions to contain karst water include retaining safety water-resistant rock pillar, water draining and depressurizing, and bottom aquifuge consolidation grouting and revamp, etc. Of all, the ground targeted branch drilling for holistic consolidation grouting and revamp through sequential strata is more effective and cost-saving. It will become the mainstream solution to contain high-pressured and abundant karst water hazard in China coalmines in the future.

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