Entry driven along goaf-side (EDG), which is to develop the entry of the next longwall panel along the goaf-side and isolate the entry from goaf with a small-width yield pillar, has been widely employed in the past few decades in China. The width of such a yield pillar has a crucial effect in EDG layout in terms of ground control, isolation effect and recourse recovery rate. On the basis of a case study, this paper presents a methodology of evaluation, design and optimization of EDG and the yield pillar by considering results from numerical simulation and field practice. In order to analyze the ground stability rigorously, the numerical study begins with the simulation of goaf-side stress and the ground condition. Four global models with identical conditions except the width of the yield pillar are built and the effect of the pillar width on ground stability has been investigated with comparison in aspects of stress distribution, failure propagation, and displacement evolution in the entire service life of entry. On the basis of simulation results, the isolation effect of the pillar acquired from field practice is also taken into consideration. The suggested optimal yield pillar design is validated from a field test in the same mine. Thus the presented methodology provides references and can be utilized for evaluation, design and optimization of EDG and yield pillars under similar geological and geotechnical circumstances.
The stability of roadways is a long-standing issue in underground coal mines, especially for entries that serve and ensure the safe production of longwall panels. The ground stability and failure mechanisms of entries vary depending on stress, geological and geotechnical conditions.
Entry driven along goaf-side (EDG), which is the development of an entry of the next longwall panel along the goaf-side and the isolation of the entry from the goaf with a small-width yield pillar, has been widely employed in China over the past several decades (Li et al. 2015; Wang et al. 2015, Zhang et al 2017). A yield pillar, which is designed to deform progressively during its service life, can transfer its load to adjacent abutments and control the mining-induced stress distribution around the entries (Peng 2008). Hence, it contributes to preventing coal bumps and excessive ground deformation by employing yield pillars, and it has been successfully applied in many coal mines of China and USA (Peng 2008; Li 2015; Chen et al. 2014). For instance, Carr et al. (1985) employed a yield-abutment-yield pillar layout to a four-entry longwall system in the Blue Creek seam in Alabama to control its severe floor deformation. The application of yield pillars in Utah coal fields has achieved a notable effect on preventing coal bumps (Peperakis, 1958; Agapito et al., 1988).
