Rock mass properties used in numerical modelling.

The re-mined face using top coal caving system is the most applicable method for recovering the remaining reserves of a previous partially-mined thick coal seam. However, this mining method may encounter the problems of low recovery and unpredicted geological conditions. A numerical model is developed using PFC2D for studying the movement of top coal mass and development of coal-rock mass interface at a longwall top coal caving re-mined face. The re-mined face advances in the lower seam below the upper solid coal pillar, previous entries and gob. A theoretical analysis according to the unsteady flow model is developed to calculate the proper time duration of caving operation. The results showed that the top coal to be recovered through the caving window before the initiation of caving operation is a partial spheroid-shaped geometry. The boundary between the coal and rock mass then develops to a funnel-shaped coal-roof interface as the caving operation continues. The top coal recovery is 98.1%, 77.1% and 70.5% for caving operations below the solid coal, entries and gob area in the upper seam, respectively. The proper timing of caving and interval of caving operation is important for obtaining a high coal recovery. Good agreement is achieved between the proposed model and the improved Boundary-Release model for short of B-R model. The study in this work may provide reference for the safety and efficiency of the extraction of the longwall top coal caving re-mined face.

采用放顶煤开采法的复采工作面，是回收前期部分开采厚煤层剩余储量的最优适用方法。然而该开采方法可能面临采出率偏低、地质条件不可预知等问题。研究人员基于二维颗粒流程序（PFC2D）构建数值模型，用以探究长壁放顶煤复采工作面的顶煤运移规律与煤岩界面演化特征。该复采工作面布置于上层实体煤柱、原有巷道及采空区下方的下层煤层中并向前推进。研究基于非恒定流模型开展理论分析，用以确定放顶煤作业的合理持续时长。结果表明：放顶煤作业启动前，可通过放煤窗口回收的顶煤呈现局部椭球形态；随着放顶煤作业推进，煤岩界面逐渐演化呈漏斗状顶煤-顶板界面。分别在上方煤层的实体煤柱下方、原有巷道下方及采空区下方开展放顶煤作业时，顶煤采出率依次为98.1%、77.1%与70.5%。确定合理的放顶时机与放煤作业间隔，对实现高采出率至关重要。本研究构建的模型与改进型边界释放模型（Boundary-Release Model，简称B-R模型）的计算结果吻合度良好。本研究可为长壁放顶煤复采工作面的安全高效开采提供理论参考。