Calculation of Residual Surface Subsidence Above Abandoned Longwall Coal Mining

Ximin Cui,Bing Zhang,Yuling Zhao,Guorui Wang,Chunyi Li

doi:10.3390/su12041528

Abstract

Exhausted or abandoned underground longwall mining may lead to long-term residual subsidence on surface land, which can cause some problems when the mined-out land is used for construction, land reclamation and ecological reconstruction. Thus, it is important to assess the stability and suitability of the land with a consideration of residual surface subsidence. Assuming a linear monotonic decrease in the annual residual surface subsidence, the limit of the sum of the annual residual subsidence factor, and continuity between surface subsidence in the last year of the weakening period and the residual surface subsidence in the first year, we establish a model to calculate the duration of residual subsidence and the annual residual surface subsidence factor caused by abandoned longwall coal mining. The duration of residual surface subsidence increases with the increase in mining thickness as well as the factor of extreme residual subsidence. The proposed method can quantitatively calculate the annual residual subsidence, the accumulative residual subsidence, and the potential future accumulative residual subsidence. This approach can be used to reasonably evaluate the stability and suitability of old mining subsidence areas and will be beneficial for the design of mining subsidence land reclamation and ecological reconstruction.

Highlights

During underground longwall mining, the equilibrium conditions in the overburden strata are disturbed
The extreme residual surface subsidence factor qm can be expressed as qm = n(1 − q) where q is the surface subsidence factor, which ranges from 0.65 to 0.90 in China; n is the coefficient related to the compaction of caved or broken rock, 0 < n ≤ 1
As shown in Expression (7), the duration of residual surface subsidence is predominantly related to the mining thickness and the factor of extreme residual subsidence

Summary

Introduction

The equilibrium conditions in the overburden strata are disturbed. The initial bulking factor is typically greater than one because the volume of broken roof rocks is larger than that of the original intact strata. It varies with the shape of the fragments, their size, and size distribution, as well as the configuration of the caved rock fragments. It is impossible to compress the material back into its original intact volume; the compaction of such material to its original volume may be possible under infinite pressure [36] Based on this assumption, the extreme residual surface subsidence factor qm can be expressed as qm = n(1 − q).

Determination of Annual Residual Surface Subsidence Factor

Duration of Residual Surface Subsidence

Discussion

Findings

Field Study