Analysis on the influence of backfilling of abandoned open-piton underground roadways stability

In order to ensure stability of the shallow underground mine roadway underneath it during and after the backfilling of the abandoned open pit in the open-pit combined mining area in the high-cold and high-altitude area, the main influence factors of roadways stability are analyzed, including: backfill body load, construction vehicle load, water body load and the hydraulic connection of surface to the roadways. By analyzing vertical distance between the vertical and transverse sections of the mining roadway and the bottom of the mining pit, a section of the return air inclined road is determined as the key evaluation object, delineating the scope of evaluation. The analysis suggests that in cold seasons, the existence of the frozen soil layer acts as a water barrier. Although there is a seasonal freezing and thawing effect in warm seasons, the hydraulic connection between the ground and underground is weak and will not be strengthened by the backfill of the pit. The elastic mechanics theory is used to calculate the load of the backfill, the load of the construction vehicle and its dynamic load, and the maximum additional stress caused by the water load after the pit is impounded. The value of the maximum additional stress with the depth is calculated. The depth at which the maximum additional stress reaches 10% of the self-weight stress at this point is the additional stress impact depth. By comparing the sum of the impact depth of the maximum additional stress and the ultimate bearing overburden height of the roadway with the actual overburden thickness, it can be concluded that the return air inclined roadway in No. 1 section can maintain the overall stability, but the thickness of the local safety overlying rock is less than 1 m; after considering the water load of the mining pit, the roadway within a range of 60 m above the starting point of the return air inclined roadway will be in an unstable state as a whole. Based on the results, a roadway protection method that combines the technological measures to reduce the disturbance of the construction load and the underground grouting anchor cable to strengthen the support and monitoring is put forward.