Mining Under Bodies of Water at Shallow Depths

To enhance recovery, it is often necessary to extract coal seams in very difficult geological and hydrogeological conditions and at shallow and deep depths (Peng et al. 2002b). However, in some cases coal production, envi ronment, and lives of miners have suffered serious threats from water inrushes, methane explosions and roof collapses. In a considerable number of coalfields in China the coal beds are covered by unconsolidated Cenozoic alluvium, such as those in the Yellow river and the Huai river alluvial plain areas.

The alluvium is comprised of mixed impermeable clay layers with water-bearing sands and gravels. Usually, the lowermost layer in the alluvium is permeable water-bearing sands mixed with gravels. Underlying the aquifer is the bedrock of coal beds. The water-bearing layers, particularly the lowermost aquifers, are potential threats to safe mining at shallow depth. In China, water inrush incidents happen frequently during mining at shallow depth. Statistical results show that water inrushes take place about 125 times annually, resulting in serious economic loss and personnel casualties. Figure 9.24 shows the schematic relationship between the alluvium, coal seam and surrounding strata. The size of the outcrop coal pillar is defined as the vertical distance between the lowermost layer in the alluviu and the uppermost part of the mining face (usually called mining tailgate).

If the mining face is very close to the alluvium aquifer, water inrush from the aquifer will be a serious threat. Therefore, the optimal design of the pillar size is of crucial importance for safe production and groundwater protection. If the size is too large, it will lead in loss of coal resources. Liu (1998) made statistical analyses of 20 coal mines with conservative designs of coal pillars in China. Total reserves in those mines were as high as 780 million tons, about 40 million tons for each mine. However, if the size is too small, then the water inrush will cause serious consequences, such as mine submersion, surface sinkhole collapse, and destruction of groundwater resources. There is a paradox between enhancing recovery and environmental protection, and the best solution should be safe mining with maximum coal recovery.

In the USA and other countries, groundwater, domestic water supply, and mining safety are also affected by shallow coal seam mining (Booth et al. 1998). For example, most alleged domestic water supply cases in 73 investigations were impacted by shallow underground mining in Virginia (Zipper et al. 1997). An extensive hydrological monitoring program was conducted at a longwall coal mine in West Virginia. In-situ experiments showed that when the monitoring wells were 36 m vertically away from the mining seam, the water wells went dry after mining and did not recover (Hasenfus et al. 1990, Liu and Elsworth 1997). Even though aquifers were not affected by mine drainage, they still exhibited changes in groundwater chemistry induced by mine subsidence (Booth and Bertsch 1999).

Incidents of mining at the shallow depth

Many incidents have happened in China during mining at shallow depths, causing serious loss of personnel casualties and in environment quality. The accidents have primarily been caused by the following aspects.

Small Outcrop pillar size

When the outcrop pillar size is not large enough to prevent mining induced fractures from reaching the lowermost aquifer in the alluvium, water, or even sand in the aquifer, can intrude into the mining space, causing excessive water drainage and destructive surface subsidence. For example, in the Daliuta coal mine of the Shenhua Corporation, a water inrush caused a serious impact on groundwater resource and farm land in 1995, as shown in Fig. 9.8.

Influence of faults

At shallow depth, when tunneling and mining expose faults, collapses of the coal seam roof with water and sand inrushes may occur. The strata near a fault have a much lower strength and are sometimes poorly consolidated. Furthermore, if the fault is permeable, water inrush occurs when the fault is unveiled. Many shallow mining incidents of water inrushes were caused by faults. Figure 9.25 shows a typical example of water and sand inrush from the alluvium into the mining face. The incident took place in the Lujiatuo coal mine of the Kailuan coalfield in Hebei Province. The thickness of the coal seam was 4 m. The hydraulic mining method was used in Face # 3371. The rocks in the roof were very fissured, water-bearing, and poorly consolidated. A faulted zone existed nearby. When the mining face was 15 m away from the fault, the roof collapsed, and rocks in the faulted zone rapidly fell into the mining space. Sand and mud with water from the alluvium intruded into the mining face. Consequently a cone-shaped collapse pit with a diameter of 14 m and depth of 4.2 m was formed on the surface.

Overburden strata failure induced by the shallow mining

Generally two failure zones that affect strata hydraulic conductivity are formed in the strata overlying the mined area: caved zones and fractured zones. On the surface, both subsidence troughs and tensile fissures are created due to shallow mining. Table 9.3 lists observed surface fissures induced by underground longwall mining in alluvial areas.

Figure 9.26 presents a typical observed section for strata failures at a deep depth. The profiles of the caved and fractured zones are broad in section with extended lobes over the headgate and tailgate. Maximum heights of caved and fractured zones are in the strata directly over the tailgate. Observed data show that the failure zones decrease as the strata become increasely softer. For shallow mining, because the strata are very close to the weathered rocks, the strata become much weaker.

Consequently the failure zones have lower maximum heights and more specific features than those in normal conditions (Liu 2000 personal communication). Figure 9.27 shows the in-situ measured failured zone of the shallow depth in the Yangzhuang coal mine, Feicheng coalfield, Shandong Province (Liu 1998). The heights of the caved and fractured zones in the upseam of the strata are much smaller, which is opposite from the deeper mining (Fig. 9.26). This is due to that the strata in the upseam in the shallow strata are much softer than those in the downseam (Fig. 9.28). The experimental results of the core samples obtained from the X-2 borehole in the Xuzhuang coal mine show that the uniaxial compressive strength of the rock sample (Vc) increases as the depth of cover (h) increases, which has the following relationship (Zhang and Peng 2005).

Design of outcrop coal pillars

There are three types of outcrop coal pillars, i.e., the water-proof pillar, the sand-proof pillar and the caving-proof pillar (Zhang and Peng 2005). Their functions, sizes and applicable conditions and response to hydrogeology vary. The design of the pillars in the shallow mining are based primarily on the hydrological characteristics, strata lithology and mining conditions.

The water-proof pillar

The water-proof pillar is used for preventing groundwater or surface water to flow into mining workings through the mining-induced fractured zone.

The water-proof pillar guarantees successful seam extraction with neither water inrush nor even excessive groundwater discharge to the mine. It requires that the fractured zone does not penetrate upwards into the overly ing aquifers. In this case, both water resources and mining safety can be achieved. When the alluvial aquifer, particularly the lowermost waterbearing layer, belongs to a strong aquifer (unit flowrate q > 1.0 l/m•s), or medium strong aquifer (unit flowrate q = 0.1 - 1.0 l/m•s), or any aquifer used as a water resource and needed to be protected, the water-proof pillar is applied. Normally, the minimum height of the water-proof pillar must not be less than the maximum height of the fractured zone and the height of a protective layer, as shown in Figs. 9.32a and 9.32b, i.e.

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