Peng S., Zhang J., Engineering Geology for Underground Rocks

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9.4 Mining under bodies of water at shallow depths 255

Coal seam

Water-proof pillar

Bedrock

Fractured zone

Surface fissures

Fig. 9.32 c. A schematic diagram showing the water-proof pillar consisting of the

fractured zone, the protective layer and surface fissures without alluvium covered

on the bedrock.

Water-bearing sands

Clay

Coal seam

Water-proof pillar

Bedrock

Fractured zone

Fig. 9.32 d. A schematic diagram showing the water-proof pillar consisting of the

fractured zone, the protective layer and the weathered permeable zone in slightly

inclined strata.

256 9 Strata failure and mining under surface and ground water

The sand-proof pillar

The sand-proof pillar is designed for preventing sand in the alluvium from

flowing into mining workings through mining-induced caving. It allows

the fractured zone to penetrate partially into the aquifer. The function of

the sand-proof pillar is to make successful coal seam extraction without

sand inrush, but with a slight increase of groundwater discharge to the

mine. When the lowermost water-bearing layer in the alluvium belongs to

a weak aquifer, and the aquifer does not need to be protected as water re-

source, the sand-proof pillar can be applied (Zhang and Peng 2005). The

minimum height of the water-proof pillar must not be less than the maxi-

mum height of the caved zone and the height of a protective layer, as

shown in Fig. 9.33, i.e.

pscs

HHH t

(9.16)

where H

is the vertical height of the sand-proof pillar; H

is the maximum

vertical height of the caved zone, which can be obtained from the empiri-

cal formulae given in the previous section (Eqs. 9.6 - 9.8); H

is the height

of the protective layer (Table 9.5, Liu 1998).

Water-bearing sands

Clay

Coal seam

Sand-proof pillar

Bedrock

Caved zone

Fig. 9.33. The sand-proof pillar design.

9.4 Mining under bodies of water at shallow depths 257

Table 9.5. The height of the protective layer for the sand-proof pillar

Strata strength

(MPa)

e f

>40 4A 2A

20-40 3A 2A

20-10 2A 2A

<10 2A 2A

Note, A is the same as that in Table 9.4.

e – the thickness of the weak aquifer in lowermost alluvium is greater than the to-

tal thickness of the extraction; f – the total thickness of clay layers in the alluvium

is greater than the total thickness of extraction.

The caving-proof pillar

When the lowermost layer in the alluvium belongs to a thick layer of clay

or a drainable weak aquifer, the caving-proof pillar can be applied. The

minimum height of the caving-proof pillar is equal to the maximum height

of the caved zone, as shown in Fig. 9.34, i.e.

(9.17)

where H

is the vertical height of the caving-proof pillar.

Clay

Coal seam

Caving-proof pillar

Bedrock

Caved zone

Fig. 9.34. The caving-proof pillar design.

258 9 Strata failure and mining under surface and ground water

9.4.5 Technical measures for the shallow mining

Several case studies and in-situ experiments were conducted in shallow

coal mining areas in China, such as the Xingtai mine and the Dongpang

mine in Hebei Province, the Xuzhuang mine and the Longdong mine in Ji-

angsu Province, the Daliuta mine in Shaanxi Province, the Xing-

longzhuang mine and the Yangzhuang mine in Shangdong Province, and

the Zhuxianzhuang mine in Anhui Province, etc. Table 9.6 lists some

mines with successful reduction of outcrop coal pillar sizes without any

safety incidents.

Summarizing the experiences obtained from the case studies, the fol-

lowing mining techniques are of crucial importance for safe mining at

shallow depths:

(1). Design optimal coal pillar size according to the hydrogeological

conditions of the alluvium, mining parameters and the empirical formulae

given in the previous sections.

(2). Apply lift mining for thick seams. The lift mining offers the sub-

stantial benefit of reducing water and sand inflows by minimizing total

heights of the fractured and caved zones resulting from extraction. Reduc-

tion of the thickness of extraction within the first or second lift is ex-

tremely beneficial in minimizing the height of the failure zones.

(3). Adopt short-wall mining. Reducing mining width can decrease

permeability enhancement and reduce mining-induced fractured height

(Zhang 2005).

(4). Detect faults, coal pillar size and aquifers prior to active mining

reaching the area in question. The form and magnitude of the potential

hazard can be diagnosed, and appropriate remedial measures need to be

taken.

(5). Grout permeable faults or leave certain coal seams intact for

maintaining distance between the faults and the mining face.

Table 9.6. Outcrop coal pillar reduction in some coal mines in China

Coal mine Thickness

of allu-

vium (m)

Original

pillar

height

(m)

Reserves

in the

pillar

(Mt)

Height

Reduc-

tion of

pillar (m)

Pillar type

for reduc-

tion

Xingtai 80-290 60 28 35-45 Sand-proof

Longdong 154-213 45 21 27 Sand-proof

Xinglongzhuang 180-200 80 73 20 Water-roof

Baishan 114-117 40 14.2 20 Sand-proof

References 259

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10 Water inrush and mining above confined

aquifers

10.1 Introduction

In many coal mines the limestone confined aquifers underlie coal seams.

During coal extraction from these mines, water inrushes occur frequently

with disastrous consequences. The most serious one of possible water dis-

asters affecting the safe operation of coal mines in China is water inrushes

from the Ordovician limestone under the Permo-Carboniferous coal seams

in Northern China (Zhang and Shen 2004). The Ordovician limestone is a

confined karst aquifer containing an abundant supply of water and a very

high water pressure. Furthermore, the strata between coal seams and the

aquifer are relatively thin varying in thickness from 30 to 60 m. Due to

these characteristics of the aquifer, plus mining-induced strata failure and

inherent geological structures (such as water-conducting faults, fractures)

high pressure groundwater can break through seam floors and burst into

mining workings. Therefore, water inrushes from the aquifer occur fre-

quently, and coal mines often suffer from serious water disasters during

coal extractions (Peng 1997, 1999). Water inrush incidents have shown

that the maximum water inflow in a coal mine has reached as much as

2,053 m

/min, which submerged the mine in a very short time.

The total coal reserves threatened by bodies of water are estimated at

25 billion tons. For example in Northern China, the yearly coal production

from the Permo-Carboniferous coal-bearing formations is more than 200

million tons. However, the coal extraction has been threatened by frequent

water inrushes from the Ordovician aquifer. In this region, the lower level

seams which have more than half of the total coal reserves are much more

difficult to mine due to this threat of water inrushes (Table 10.1).

262 10 Water inrush and mining above confined aquifers

Table 10.1. Coal reserves threatened by the Ordovician aquifer in China’s coal

mines

Coalfield Total reserves

(100 million tons)

Reserves threatened

(100 million tons)

Ratio of threat-

ened to total (%)

Jiaozhuo 5.65 4.95 87.6

Fengfeng, Han-

dan, Xingtai

70 35 50

Hancheng 12.7 7.83 61.7

Chenghe 3.26 2.2 67.5

Feicheng 4 2.5 63

Huozhou 1.37 0.81 59.1

Total 96.98 53.29 55

For example, from 1950 to 1990 a total of 222 serious water inrush

incidents took place in China causing collieries to be submerged by water

intrusions from the confined karst aquifers. More recently, total water in-

rushes at nationalized key coal mines occur about 125 times annually re-

sulting in an economic loss of about 180 million US dollars. In addition,

local coal mines run by provinces, counties, and private business have a

larger annual economic loss induced by water inrushes.

The main coalfields threatened by the Ordovician aquifer are Jiaoz-

huo in Henan Province, Fengfeng, Handan and Xingtai in Hebei Province,

Zibo and Feicheng in Shandong Province, and Hancheng and Chenghe in

Shaanxi Province. If the problems of safe mining over the aquifer cannot

be solved efficiently, some coal mines in the mentioned coalfields will be

faced with gradual reduction of production or even abandonment of the

mines. In general, there are two different ways to solve the problems of

mining over confined aquifers. One is to drain the aquifer before mining

operation, the other is to mine without drainage. Geological investigation

and mining practice have unveiled that there are many environmental prob-

lems induced by water drainage from limestone aquifers, such as the Or-

dovician limestone aquifer in Northern China, and the Maokuo limestone

aquifer in Southern China. Fissures in these karstified limestones are well

developed and interconnected within the aquifer such that when water is

drained from one particular region, the drainage has an extensive influence

on the whole aquifer. For example, a dewatering in the Ordovician lime-

stone aquifer was conducted in Wangfen colliery, Hebei Province. The

pumping flowrate was 96 m

/min. However, the drawdown in the central

well was only 2.8 m, and the radius of the cone of depression extended to

10 km which caused the loss of many drinking water wells. This resulted

in a shortage of water supply for 100,000 people. Therefore water drainage

is infeasible. The only solution for coal extraction over the limestone aqui-

10.2 Underlying strata failure due to mining 263

fers is to mine with technical measures and without drainage. In order to

do so, it is of vital importance to study strata failure characteristics and hy-

draulic conductivity changes due to mining and thereafter find a way to

predict and prevent water inrushes. Various research has been conducted in

this area (e.g. Zhang 1989, Gao and Li 1992, Zheng et al. 2000, Peng and

Wang 2001, Peng and Meng 2002, Li and Guan 2002, Wang et al. 2002, Li

and Gao 2003, Yang et al. 2003, Zhang 2005, Yin and Zhang 2005). How-

ever, the mechanism of water inrushes is still not well understood.

10.2 Underlying strata failure due to mining

10.2.1 Determination of the water-conducting failure zone in the

seam floor

Coal extraction causes strata deformation and failure which may en-

hance hydraulic conductivity in the surrounding strata. Therefore, it is de-

sirable to accurately determine pre- and post-mining hydraulic conductivi-

ties in the overburden and underlying strata of the coal seam. To measure

the hydraulic conductivity in the underlying strata, boreholes are drilled

pre-mining in underground roadways for observation. In each borehole,

water injection and a number of well logging techniques (such as electric

resistivity, sonic log, acoustic emission, and hole televiewer, etc.) are used

to determine rock strength, borehole fissure, and changes in hydraulic con-

ductivity. Figure 10.1 gives a schematic diagram of a water injection in-

strument (Zhang and Zhu 1994). The key technique during measurements

is to control the injection pressure. The pressure should not be high enough

to create new fractures in the strata, since the experiment is to determine

the changes in hydraulic conductivity induced by mining. Therefore, the

injection pressure should not exceed the least principal stress of the sur-

rounding strata.

Figure 10.2 shows the observing borehole locations and layout in Xing-

tai coal mine. In-situ stresses in this area were:

= 7.4 MPa,

= 4.5 MPa.

The roadway in Fig. 10.2 was located 36 m below the mining face and four

boreholes were drilled at different angles. The water injection instrument

described in Fig. 10.1 was applied to measure the flowrate of water injec-

tion pre- and post-mining, using an injection pressure of 0.35 - 0.5 MPa.

The water injection along each borehole was conducted by pumping water

into the instrument, then into the borehole. The measurements were taken

in each hole at different sections throughout the borehole and at different

264 10 Water inrush and mining above confined aquifers

times.

1 – packer

2 – observed area

3 – borehole

4 – hose

5 – flow meter

6 – pressure meter

7 – pump

8 - reservoir

Fig. 10.1. The instrumentation for water injection observation in a borehole.

Headgate

Coal seam

Tailgate

36 m

Face 7802

Hole 1

Hole 4

Observing gate

25º

Hole 2Hole 3

Depth of mining 320 m

Fig. 10.2. Observing borehole layout for water injection measurements in Xingtai

coal mine, Hebei Province.

Figure 10.3 presents the measured pre-mining and post-mining

flowrate of water injection in Hole 1 (refer to Fig. 10.2). Note that in this

context pre-mining corresponds to a state before the mining face passes the

borehole, and post-mining means after the mining face passes the borehole.

Before the mining face passed Hole 1 (in pre-mining the mining face was

32 m away from the borehole), the injection rate was zero from 53 to 68 m

in the inclined borehole. This means that the strata in this area were im-

permeable. However, when the mining face passed the borehole, the injec-

tion rate (refer to Fig. 10.3 for post-mining at 19 and 63 m) increased dra-