Optimization study on reaming parameters of under-reaming and back-filling technology for bolt support

In coal mine roadways with weak surrounding rock, after resin bolt support is applied, the low shear strength at the surrounding rock-anchoring agent interface tends to cause slip failure of resin bolts, which further leads to large roof deformation and even roof collapse, endangering the safe production of coal mines. To effectively improve the shear strength between the surrounding rock and the anchoring agent, a new support technology, namely under-reaming and back-filling support, is proposed. Different from the shear-slip failure mode at the surrounding rock-anchoring agent interface in conventional anchoring, after under-reaming and back-filling anchoring, the weak surrounding rock is replaced by a back-filling body at the same position. With the increase of drawing force, shear failure occurs at the bolt-back-filling body interface, and the bolt is pulled out directly. Through the combination of theoretical analysis, numerical simulation and laboratory tests, the key reaming parameter combinations of this support technology are comprehensively analyzed. The results show that the reaming angle and the length of the reaming section are two key parameters affecting the support effect of under-reaming and back-filling support. The ultimate bearing strength of the under-reaming and back-filling anchorage system requires the synergistic control of reaming angle and reaming section length. Among the four reaming parameter combinations with good support effect, when the reaming angle is 20° and the reaming section length is 100 mm, the bolt stress distribution in the anchorage model is uniform, the surrounding rock stress is stable, and the support effect is optimal. The ultimate pull-out force of the 20°-100 mm under-reaming and back-filling anchorage system is about 1.7 times that of the conventional anchorage system. The optimized reaming parameters of the under-reaming and back-filling support technology is conducive to the performance of bolts in roadways with weak surrounding rock.