Determining the stability of roll-back stretches in steep layers when unloading a coal rock massif

The object of this study is the condition of side rocks in a coal rock massif with preparatory workings. The task solved was to ensure the stability of preparatory workings to improve the safety of miners' activities. The bearing capacity of protective structures in the preparatory mining workings along the length of the excavation section in a coal mine was comparatively assessed. The operational condition of the roll-back stretch, when using a protective technique that involves coal pillars, is provided for at their relative deformation e<0.25 and relative volume change dV<0.1. At a distance of l>50 m behind the cleaning pit, at e³0.6–0.8, there is a decrease in the load-bearing capacity of coal pillars and the destruction of the roof. The loss of cross-sectional area of the roll-back stretch is 50 %. Under such conditions, the level of a roof collapse threat is approaching a critical state. When using rolling bundles made of wooden sleepers at e>0.21 and dV>0.16, conditions are created around the preparatory workings at which the integrity of the roof is ensured through the gradual sealing of protective structures. At a distance l>60 m, the increase in lateral rock displacements reaches the minimum values DU=10 mm, and the loss of the cross-sectional area of the roll-back stretch is 30 %. The kind of functional dependence between the length l (m) of the roll-back stretch with different protection techniques and the relative change in the volume dV of protective structures was experimentally determined. Such a dependence makes it possible to assess the bearing capacity of protective structures along the length of the excavation area in the zone of active influence of mountain pressure behind the cleaning pit. The operational condition of roll-back stretches when using coal pillars is ensured within the limits of their deformation resource, which limits the use of this protection technique. To ensure the stability of preparatory workings, it is advisable to use rolled bundles made of wooden sleepers. After their compaction, the movement of side rocks is limited in the worked-out space of the excavation site