Influence of the Elastic Dilatation of Mining-Induced Unloading Rock Mass on the Development of Bed Separation

Weibing Zhu,Jingmin Xu,Shengchao Yu

doi:10.3390/en11040785

Abstract

Understanding how mining-induced strata movement, fractures, bed separation, and ground subsidence evolve is an area of great importance for the underground coal mining industry, particularly for disaster control and sustainable mining. Based on the rules of mining-induced strata movement and stress evolution, accumulative dilatation of mining-induced unloading rock mass is first proposed in this paper. Triaxial unloading tests and theoretical calculation were used to investigate the influence of elastic dilatation of mining-induced unloading rock mass on the development of bed separation in the context of district No. 102 where a layer of super-thick igneous sill exists in the Haizi colliery. It is shown that the elastic dilatation coefficient of mining-induced unloading hard rocks and coal were 0.9~1.0‰ and 2.63‰ respectively under the axial load of 16 MPa, which increased to 1.30~1.59‰ and 4.88‰ when the axial load was 32 MPa. After successively excavating working faces No. 1022 and No. 1024, the elastic dilatation of unloading rock mass was 157.9 mm, which represented approximately 6.3% of the mining height, indicating the elastic dilatation of mining-induced unloading rock mass has a moderate influence on the development of bed separation. Drill hole detection results after grouting, showed that only 0.33 m of the total grouting filling thickness (1.67 m) was located in the fracture zone and bending zone, which verified the result from previous drill hole detection that only small bed separation developed beneath the igneous sill. Therefore, it was concluded that the influences of elastic dilatation of mining-induced unloading rock mass and bulking of caved rock mass jointly contributed to the small bed separation space beneath the igneous sill. Since the accurate calculation of the unloading dilatation of rock mass is the fundamental basis for quantitative calculation of bed separation and surface subsidence, this paper is expected to be a meaningful beginning point and could provide a useful reference for future, related research.

Highlights

Coal seam excavation induces the movement of overlying strata, which can be divided into three zones; the caving zone, fracture zone and bending zone from the immediate roof upwards according to the degree of failure of the overburden [1,2,3,4]
Since the rock unloading dilatation coefficient in the axial direction had the closest relationship with the development of bed separation, only the axial test results are shown in this paper
The test results with an axial load of 16 MPa and 32 MPa are shown in Figures 9 and 10, respectively

Summary

Introduction

Coal seam excavation induces the movement of overlying strata, which can be divided into three zones; the caving zone, fracture zone and bending zone from the immediate roof upwards according to the degree of failure of the overburden [1,2,3,4]. Coal mining is an unloading process of the rock mass from an original stress state, which inevitably results in bed separation due to the out-of-step movement of the unloading strata [11]. Xuan et al [17] applied the technology, isolated grout injection into dynamic bed separation in the overburden under mining conditions, to control the surface subsidence and ensure the safety of the buildings in the village at the surface. Xing et al [18] proposed the mechanism of water-inrush and its preventative measures based on the water-inrush accidents resulting from water accumulation at the bed separation zone in the mining overburden. Under the circumstance of dewatering and danger of flooding resulting from mining and mining-induced subsidence, a prediction for the optimization of drive-in filters was applied to ensure mining safety [19]

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