Large deformation analyses and control measures of retaining entry along the backfilling working face with thick hard roof in deep mining

To control the large deformation of the gob-side retained entry along the backfilling working face in deep mining,taking the retained entry of a “mining+dressing+backfilling” mine in Shandong,China,as the study site,the theoretical analysis,numerical simulation,and field measurements were conducted.The influencing factors of the large deformation of retained entry along the backfilling working face were analyzed,and a mechanical model for the structure of gob-side retained entry along backfilling working face under the thick hard roof was established.The relationship between the backfilling percentage of working face and the subsidence of the thick hard roof of the retained entry was given based on the equivalent mining height theory and related control countermeasures were proposed accordingly:a priority should be given to strengthening the gob backfilling on the side of the entry retaining,increasing the strength of the gangue lining wall,and optimizing the supporting parameters of the gangue lining wall.FLAC3D,a simulation software,was adopted to calculate and analyze the factors:backfilling percentage of gob,length of the strengthen backfilling working face,the strength and the pressure relief supporting length of gangue lining wall,which influence on the deformation of the retained entry.The retained entry areas with different engineering conditions were selected and the entries displacement were observed.The results show that in the section of retained entry well backfilled,the adoption of reasonable pressure relief length of gangue wall supporting can effectively control the large deformation of the surrounding rock of the retained entry.Also,the research shows that where the backfilling percentage is enough high,the thick and hard roof as a whole is turned to the side of the gob and the measured roof subsidence is similar to the calculated result of the mechanical model described.The backfilling percentage is the main controlling factor for the large deformation of retaining entry,with its increase,the mining-induced stress appears gently in the stope and the subsidence of the roof in the retained entry is reduced.The measures to strengthen the backfilling properly at the working face entry retaining side can effectively control the subsidence of the roof above the retained entry in the case of backfilling materials were insufficient.Increasing the strength of the gangue wall and selecting the reasonable parameters of the pressure relief supporting can effectively control the lateral large deformation of the gangue lining wall.In the future,there are two prospects need to be addressed:① to develop high-strength,low-cost,easy-to-construct concrete materials for roadside support structures and ② to investigate the problem of gangue logistics and equipment matching of the working face under the condition of one-sided reinforced backfilling.