## 1 Introduction  
Pre-driven recovery rooms (PRR) allow for the installation of the necessary supports and reinforcement before the influence of the front abutment pressures, which provides a chance to improve the efficiency of moving mining machinery from the completed panel to the next working face to be extracted [1]. The longwall face advancing into the PRR is a critical step in the process of coal extraction, since it must ensure the roof stability of the PRR during this step so as to provide sufficient space for the retrieval of equipment such as shearers, scraper conveyors and shields [2]. However, catastrophic results such as casualty incidents and equipment damage often occur in some recovery work owing to improper supporting measures, or a failure in limiting the continuous subsidence of the roof strata. At the end of last century, the National Institute for Occupational Safety and Health (NIOSH) in America constructed a comprehensive worldwide database containing 130 case histories and found that roof failures of the PRR can be divided into two types. One type is a roof collapse generated by the weight of nearby friable roof as the fender (referring to the remaining coal body to be mined between the longwall face and the PRR) narrows. The width of fender pillar gets smaller as the mining machinery gets closer to the PRR. The abutment force inevitably transfers to the support system positioned in the PRR and the outby abutment pillar system. Heavy roof reinforcement was adopted to significantly reduce the incidence of such failure. Another type is an overburden weighting failure where the main roof fails to bridge the face area and the PRR, meanwhile damagingthe rocks above the immediate roof. Strengthening of the roof structure was evidently not effective in eliminating weighting failures [3; 4]. Over the past several years, in Chinese underground longwall mining practices, many coal mines in the Shendong mining district have encountered overburden weighting failures at the final stage of coal extraction, even though double-row chock hydraulic supports with extremely high strength have been employed in the PRR to minimize roof subsidence [5; 6]. During the mining machinery entering the PRR, although the internal support is essential to establish an immediate roof beam that helps support the main roof, movement above the anchor layer is common, especially when the fracture region of the main roof is nearby the PRR in the last weighting [7; 8; 9]. The main force source of hydraulic support damage is the impact force generated by the unstable main roof rock block, and the strain energy stored in the broken rock block during structural instability is converted into kinetic energy of the rock block falling. This leads to non-static contact between the unstable rock block and the immediate roof below it, and the resulting contact collision phenomenon can produce an impact force much greater than the static self-weight of the main roof rock block in a short time. The impact force is transmitted directly to the face support through the immediate roof so that the support needs to bear the impact load it generates [10; 11]. In order to lessen unfavorable impacts of the overburden weighting on the PRR, it is common to stop advancing the longwall face temporarily while the remaining fender is about 5 m wide, and wait for the signs of weighting on the work face. These signs usually include the increasing resistance of the shields and the aggravation of coal wall spalling. Then the face continues to progress until it enters the PRR. During the longwall stop period, the immediate roof behind the supports can fully collapse and the main roof strata experience a certain amount of rotation. Hence, to some extent yield mining is able to alter the periodic weighting position and reduce the duration of the weighting. Nevertheless, a notable drawback of yield mining is that it is complicated and difficult to determine a reasonable longwall stop position.