Determination of caved and water-conducting fractured zones of “two soft and one hard” unstable coal seam


The height of the development of a caved zone (CZ) and a water-conducting fractured zone (WCFZ), hereinafter “CZ and WCFZ”, in mining of soft coal with a soft mine floor, hard mine roof and different thicknesses of the coal seam, hereinafter “two soft and one hard” unstable coal seam, is of interest in the long wall coal mining. An observation borehole was drilled above a coal seam with an average thickness of 4.0 m. By using borehole imagery technique and comparing the borehole images at different distances from the working face, the height of “CZ and WCFZ” at the working face is calculated under the condition of insufficient mining. Based on field observations, a 3D numerical model is developed under the condition of full mining, the height of CZ is calculated to be 14.4 m and the height of WCFZ calculated to be 67.0 m. The numerical model also shows changes in the vertical stresses in the overburden strata above the roof of the coal seams and it is related to the development of the “CZ and WCFZ”. At the same time, based on the key strata theory and multiple linear regression, the height of WCFZ in the working face of this case study is predicted to be 67.0 m and 64.8 m respectively, which is close to the numerical simulation results. At the same time, 30 groups of measured data are collected to analyze the main factors affecting the WCFZ, and the influence of different lithology characteristics of overburden strata on the WCFZ is mainly discussed. The results show that the height of WCFZ is hard–hard > hard–soft > soft–hard. The results provide important practical guidelines for the prevention and control of roof water hazards in coal mine thus improving the safety of mining.

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Caved zone

WCFZ Hli :

Water-conducting fractured zone

k p :

Fragmentation coefficient

hi, hn :


r i :

Bulk density

E i :

Elastic modulus

q1(X)|n :

Load of the nth hard rock stratum on the first hard rock stratum

l n :

Limited span

σn :

Tensile strength

q n :

Load of the nth hard rock stratum

n :

Total number of rock layers

L n :

Broken distance

H n :

Distance of the hard rock stratum at the nth stratum from the upper surface of coal seam

\(\gamma\) :

Average broken angle of the rock stratum

Δs :

Maximum subsidence of broken block in main key stratum

M :

Thicknesses of the mined seam

h :

Distance between main key stratum and top of coal seam

k :

Opening of crack in main key stratum

s :

Thickness of the main key stratum

L :

Broken block length

β :

Opening angle between the broken block and the key stratum

α :

Change in angle before and after the broken block

l zn :

Periodic weighting step

L x :

Width of the working face


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This research is supported by the Fundamental Research Funds for the Central Universities (Grant No. 2017CXNL03) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).The authors are grateful to the anonymous reviewers for their helpful comments on the manuscript.

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Correspondence to Shuyun Zhu.

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Chen, Y., Zhu, S. Determination of caved and water-conducting fractured zones of “two soft and one hard” unstable coal seam. Acta Geod Geophys (2020).

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  • Water-conducting fractured zone height
  • Borehole imaging
  • Numerical simulation
  • Key stratum
  • Lithology characteristics of overburden strata