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Void fraction distribution in overburden disturbed by longwall mining of coal

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Abstract

Voids in overburden disturbed by coal mining provide channels for transfer of heat, methane, groundwater, oxygen, exhaust gas and fire-fighting materials in the underground environment. Based on expressions of strata and ground subsidence, a group of void fraction distribution models are proposed, including porosity in the caved zone, the fracture ratio in the bed separation zone and the fissure ratio in the ground subsidence zone. Combined with a case study of the Antaibao coal mine, China, theoretical calculation and numerical simulation indicate that porosity distribution in the caved zone is U-type and the transverse void fractions in the bed separation zone and ground subsidence zone have M-type distributions. Additionally, the longitudinal void fraction distribution changes from ∩-type to M-type along the strike and dip of the coalbed after the complete subsidence of ground. The distribution of fractures in the disturbed overburden of the goaf presents a “fractured arch”, and fracture density gradually decreases from arch foot to crown. Fire-prone zones, propagating paths of coalbed fires, areas prone to water influx and outlets of boreholes used for fire-fighting all usually appear around the perimeter of disturbed overburden. The transverse void fractions distribution in the bed separation zone changes from ∩-type to M-type with an increase in the size of the goaf. It is suggested that the inlets of gob vent boreholes should be arranged near the center of the bed separation zone in the early stages of longwall mining, and around the perimeter in middle to late stages of mining.

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Abbreviations

W xi :

Subsidence of the ith key stratum (KS) in section y = 0

W yi :

Subsidence of the ith KS in section x = x 0

W 0i :

Maximum subsidence amount of the ith KS

W i :

Subsidence amount of the ith KS in the x–y plane

W 1 :

Subsidence amount of the first KS

l x :

Strike length of the goaf

l y :

Dip width of the goaf

l i :

Broken length of the ith KS

v(t):

Speed of mining advance

M :

Thickness of the coal seam

Σh i :

Distance between the ith KS and the coal seam

Kp i :

Bulking factor of the rock mass between the ith KS and the coal seam

h i :

Thickness of the ith KS

σ ti :

Tensile strength of the ith KS

q :

Load on the ith KS

r(z):

Radius of the influence zone on the ground caused by the subsidence of a micro-unit in the main key stratum (MKS)

β :

Influence angle

W e :

Ground surface subsidence caused by the subsidence of a micro-unit in the MKS

W g :

Ground surface subsidence

W m :

MKS subsidence

H :

Distance between the ground surface and the MKS

V :

Spatial scale of MKS subsidence

φ c :

Void fraction in the caved zone

h d :

Thickness of the immediate roof

\(\varphi_{{{\text{s}}(i,i + 1)}}^{\text{T}}\) :

Transverse fracture ratio between the ith and (i + 1)th KS

\(\varphi_{{{\text{s}}(i)}}^{\text{L}}\) :

Longitudinal fracture ratio of the ith KS

\(\varphi_{\text{g}}^{\text{T}}\) :

Transverse fissure ratio in the topsoil

\(\varphi_{\text{g}}^{\text{L}}\) :

Longitudinal fissure ratio on the ground

V v :

Volume of void

V r :

Volume of rock mass

ΔS :

Areal increment of the stratum after subsidence

S :

Original area of the stratum

ζ (or x), η (or y), z :

Three axes of a Cartesian coordinate system

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Acknowledgments

The project was sponsored by the Joint Funds of the National Natural Science Foundation of China and Shenhua Group Corporation Limited (No. 51134020), the National Natural Science Foundation of China (No. 11472311), and the Fundamental Research Funds for the Central Universities of Central South University (No. 2015zzts083). The authors would like to thank Antaibao Opencast Coal Mine of ChinaCoal Pingshuo Group Corporation Limited, which gives a lot of support in fire district survey and data collection. Thanks also to Jian Zhou and Wenzhuo Cao for language check.

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Authors and Affiliations

  1. School of Resources and Safety Engineering, Central South University, Changsha, 410083, China

    Shaofeng Wang & Xibing Li

  2. School of Safety Engineering, China University of Mining and Technology, Xuzhou, 221116, China

    Deming Wang

  3. State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology, Xuzhou, 221008, China

    Shaofeng Wang & Deming Wang

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  1. Shaofeng Wang
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Correspondence to Shaofeng Wang.

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Wang, S., Li, X. & Wang, D. Void fraction distribution in overburden disturbed by longwall mining of coal. Environ Earth Sci 75, 151 (2016). https://doi.org/10.1007/s12665-015-4958-6

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