Abstract: The interlayer cracking and collapse of the roof rock mass is one of the main diseases that cause the structural instability of the roof of flat-top caves in sandstone caves. Investigation on the flat roof caves in Northern Grottoes shows that lithology, tensile strength between rock layers, thickness of stripped body, opening width and connectivity rate between stripped body and bedrock are the main factors affecting the stability of cracking body. In order to effectively improve the safety reserve of the flat roof and enhance the overall stability of the roof, according to on-site investigation and quantitative calculation and analysis, the roof cracked rock mass is divided into four levels: low risk, medium risk, high risk and extremely high risk. According to the force characteristics and deformation failure mode, the roof cracked rock mass is simplified into two main failure modes: “cantilever fracture failure” and “falling failure”. Through the on-site grouting bonding test of the North Grottoes, it is found that the area coverage of the filling slurry is generally from 40% to 60%, and the bonding and reinforcement of the cracked rock with a thickness of no more than 11.4 cm can achieve ideal results. Considering of 8 degree for earthquake fortification intensity, the effective bonding thickness is not more than 10.89 cm.

Key words: North Grottoes, flat roof caves, stability, risk level, bonding