Mining-induced fault reactivation associated with the main conveyor belt roadway and safety of the Barapukuria Coal Mine in Bangladesh: Constraints from BEM simulations

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Abstract

Fault reactivation during underground mining is a critical problem in coal mines worldwide. This paper investigates the mining-induced reactivation of faults associated with the main conveyor belt roadway (CBR) of the Barapukuria Coal Mine in Bangladesh. The stress characteristics and deformation around the faults were investigated by boundary element method (BEM) numerical modeling. The model consists of a simple geometry with two faults (Fb and Fb1) near the CBR and the surrounding rock strata. A Mohr-Coulomb failure criterion with bulk rock properties is applied to analyze the stability and safety around the fault zones, as well as for the entire mining operation. The simulation results illustrate that the mining-induced redistribution of stresses causes significant deformation within and around the two faults. The horizontal and vertical stresses influence the faults, and higher stresses are concentrated near the ends of the two faults. Higher vertical tensional stress is prominent at the upper end of fault Fb. High deviatoric stress values that concentrated at the ends of faults Fb and Fb1 indicate the tendency towards block failure around the fault zones. The deviatoric stress patterns imply that the reinforcement strength to support the roof of the roadway should be greater than 55 MPa along the fault core zone, and should be more than 20 MPa adjacent to the damage zone of the fault. Failure trajectories that extend towards the roof and left side of fault Fb indicate that mining-induced reactivation of faults is not sufficient to generate water inflow into the mine. However, if movement of strata occurs along the fault planes due to regional earthquakes, and if the faults intersect the overlying Lower Dupi Tila aquiclude, then liquefaction could occur along the fault zones and enhance water inflow into the mine. The study also reveals that the hydraulic gradient and the general direction of groundwater flow are almost at right angles with the trends of faults Fb and Fb1, which could act as barriers to groundwater flow into the mines.

Introduction

The reactivation of a fault in an area undergoing mining subsidence may result in the generation of a fault scarp along the ground surface, accompanied by compression and fissuring (Donnelly, 2006). These types of problems can often be expected when mining activities approach or breach a major geologic discontinuity, such as a fault (Crouch and Starfield, 1983). Thrusts, strike-slip faults, normal faults, and igneous intrusions (dikes and sills), which are referred to as ‘geologic discontinuities’, are associated with highly fractured rock strata and can cause problems in the safe operation of underground mines (Shepherd et al., 1981, Saghafi et al., 2008, Islam et al., 2009, Islam and Shinjo, 2009). The mining conditions in thick coal deposits in tectonically stressed masses are characterized by a number of features that are manifestations of mine pressure. The distribution of stress around a major fault zone that intersects the mine entry roadway is of considerable importance in determining the stability and safety of mining operations. The redistribution of longwall mining-induced stresses can cause important deformation inside and around the mine openings. Therefore, during mining excavation, special attention should be paid to the presence of major faults and the influence of other geologic discontinuities, especially joints, fractures/fissures, weak and flat laminated bedding planes, and small faults (Homand et al., 1997, Ozbek et al., 2003, Mark and Molinda, 2005, Sunwoo et al., 2006, Donnelly et al., 2008, Islam et al., 2009, Islam and Shinjo, 2009). Major roof collapses and instabilities in the main gate roadways can cause fatalities, injuries, and significant economic losses because these roads provide critical access to the longwall mining operations (Shen et al., 2008).

Barapukuria colliery (Fig. 1) is a highly tectonically deformed basin, and is influenced by a complex interaction of convergence-related tectonic processes between the continental Indian plate with the continental Eurasian plate (Islam and Hayashi, 2008, Islam et al., 2009). Numerous north-south-trending normal faults have been identified in this coal basin. Vertical displacements range from less than 1 m to about 3 m on small faults that lie within the mining levels between – 250 m and – 430 m. These small-scale faults are detected at the time of mining face inspections. Besides these faults, a total of 37 intra-basinal faults have been interpreted in seismic data, which have an estimated vertical resolution of about 10 m. The largest Eastern Boundary Fault (Fig. 2), which strikes NNW–SSE for at least 5 km, has controlled sedimentation within the basin. The fault plane dips 70–75° towards the east, and has an estimated vertical displacement of more than 200 m with a dominant dip-slip component. These faults correspond to the contemporary tectonic stress field of regional E–W extension along the basin axis that trends almost north-south. This extension is thought to account for the preferred orientation of roof failures in underground coal mines and jointing in roof sandstones. The second largest of these, fault Fb, strikes NW–SE in the south to N–S in the north, and has a displacement of up to 60 m (Wardell Armstrong, 1991, Bakr et al., 1996, Islam, 2005, Islam and Islam, 2005, Islam and Kamruzzaman, 2006, Islam and Hayashi, 2008, Islam et al., 2009). Fault Fb, which is further divided into Fb and Fb1, intersect the main conveyor roadway (CBR) of the Barapukuria mine (Fig. 2).

In the case of the Barapukuria rift basin, mining-induced deformation of strata occurred inside and around the gate roads (Islam et al., 2009). Throughout the development stage (2001–2003), it was observed that where the CBR passed through faults Fb and Fb1, the roadway was affected by a large wedge fall along a slickensided fault surface. The CBR has been driven through sandstone, siltstone, and interbedded shales and clays. Continuous collapse of clay bands and slickensided material through the fault planes created major hazards and instability in the roofs of the roadways (Islam, 2005, Islam and Islam, 2005, Islam and Kamruzzaman, 2006).

Knowledge of the mining-induced reactivation of faults, stress propagations, displacements, and deformation characteristics, as well as the failure behavior of rock strata in and around the two major faults that overlie the main CBR, remains limited. No interpretation of the mining-induced stress distribution with respect to the reactivation of the two major faults was realized, although this interpretation is essential in forecasting of mine operations. The main objectives of this paper are to present:

  • an interpretation of the mining-induced stress distributions to take into account the reactivation of the two major faults associated with the CBR,

  • an understanding of the displacements and rock failure trajectories around the two faults,

  • a prediction of rock failure loading conditions around the mine roadway, and to then apply that understanding to the development of appropriate reinforcement strategies, and

  • an assessment of safety regarding earthquakes induced hazards due to the geotectonic location of the mine.

Section snippets

Geology of the main conveyor belt roadway

The principal constraints on the design of the Barapukuria mine relate to the presence of the ‘massive’ overlying Gondwana sandstones and the unconsolidated and water-bearing Dupi Tila Formation. The Dupi Tila Formation, and the Gondwana sandstone that is in hydraulic continuity with it, represents a major potential hazard to the mine from water inflow (Wardell Armstrong, 1991, Islam et al., 2009). The entry of water into mine workings is governed by a combination of complex factors related to

Numerical modeling

We used Boundary Element Method (BEM) numerical modeling, which emphasizes the role of stresses that lead to the failure of the rock surrounding the two major faults. The term ‘boundary element’ is used to indicate the method whereby the external surface of a domain is divided into a series of elements over which the functions under consideration can vary in different ways, in much the same manner as in finite elements. In terms of mining engineering, the boundary of the underground excavation

Model results

Results of the simulation are illustrated in Fig. 5, Fig. 6, Fig. 7. The modeling results are presented in terms of seven rock mechanical parameters as follows:

Discussion

The measurements of the various parameters of the stress system in the present study may be the key to successful prediction of fault reactivation and safety in the Barapukuria coal mine. As the first underground coal mine in Bangladesh, Barapukuria is also an experimental mine where the evaluation of different geo-environmental hazards is essential. In our previous two research papers, we focused on water inflow hazards due to multi-slice longwall mining (Islam et al., 2009) and gas outburst

Conclusions

In this paper, BEM numerical modeling was used to simulate mining-induced fault reactivation associated with a key roadway of the Barapukuria Coal Mine. Previous studies have indicated that reactivation of faults could lead to widespread damage to an underground mine. Disputed causes of fault reactivation (Donnelly, 2006) in underground coal mining have resulted in large-scale roof collapse and damage to mining equipment, fatal accidents, hindering of production, and economic losses.

Acknowledgements

The Ministry of Education, Culture, Sports, Science and Technology (Monbukagakusho) of Japan is acknowledged for its financial support of M. R. Islam. He thanks Dr. Ozgen Karacan, the editor, for help with illustrations and the International Journal of Coal Geology reviewers for helpful comments.

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