Surrounding rock deformation and stress evolution in pre-driven longwall recovery rooms at the end of mining stage
- 120 Downloads
Two case studies were conducted in the Shennan mining area of Shaanxi Province, China to evaluate the surrounding rock deformation and stress evolution in pre-driven longwall recovery rooms. These studies mainly monitored the surrounding rock deformation and coal pillar stress in the recovery rooms of the N1206 panel of 2−2 coal seam at Ningtiaota Coal Mine and the 15205 panel of 5−2 coal seam at Hongliulin Coal Mine. The monitoring results showed that the surrounding rock deformation of the main recovery room and the coal pillar stress in the N1206 and 15205 panels began to increase significantly when the face was 36 m and 42 m away from the terminal line, respectively. After the face entered the main recovery room, the maximum roof-to-floor convergence in the N1206 and 15205 panels was 348.03 mm and 771.24 mm, respectively, and the coal pillar stresses increased more than 5 MPa and 7 MPa, respectively. In addition, analysis of the periodic weighting data showed that the main roof break position of the N1206 and 15205 panels after the longwall face entered the main recovery room was − 3.8 m and − 8.2 m, respectively. This research shows that when the main roof breaks above the coal pillar, the surrounding rock deformation of the main recovery room and the coal pillar stress increase sharply. The last weighting is the key factor affecting the stability of the main recovery room and the coal pillar; main roof breaks at disadvantageous positions are the main cause of the support crushing accidents.
KeywordsPre-driven longwall recovery room Surrounding rock deformation Stress evolution Periodic weighting
Moving equipment is crucial in longwall coal mining practices, and the speed at which the equipment can be moved affects the mining efficiency. The moving method adopted by most coal mines in China is the pre-driven double recovery room. This room drives two roadways parallel to the longwall face along the terminal line. The longwall equipment is transported to the headgate or tailgate via the recovery room and is then moved to the main haulage roadway and the next panel. If the surrounding rock deformation of the recovery room is too large, the normal operation of all subsequent processes will be affected during the moving stage. Therefore, the surrounding rock stability of the recovery room is the key factor that affects how quickly the equipment can be moved.
A number of studies have been conducted in relation to pre-driven longwall recovery rooms. From the late 1980s to the 1990s, certain researchers studied the surrounding rock stability in recovery rooms and summarized the numerous cases of roof fall hazards and the effects of front abutment pressure on supports (Listak and Bauer 1989; Oyler et al. 1998). In the twentieth century, the support technology in recovery rooms improved with the maturity of high-strength support technology. Tadolini et al. (2003) and Tadolini and Barczak (2006, 2008) had designed the support system of pre-driven recovery rooms, which consists of bolts, cables, and standing supports, such as pumpable supports, timbers, and concrete cribs, and studied the behavior of rock mass and support response in recovery rooms. By evaluating the mechanics of support load transfer and ground response associated with the advancement of the longwall face into recovery rooms, additional design methods of support parameters have been proposed and advanced (Tadolini 2003; Barczak et al. 2007; Barczak and Tadolini 2008; Kang et al. 2016).
In previous studies, almost all coal mines have used concrete cribs to support the recovery room. In recent years, certain coal mines in China have used buttress hydraulic support instead of concrete cribs to reinforce the roof in recovery rooms. Compared with concrete cribs, buttress hydraulic supports require quick installation and are reusable. At present, the support technology of pre-driven recovery rooms in China is mature, but roof fall hazards still occur frequently. Between 2010 and 2013, numerous roof fall hazards and support-crushing accidents occurred in the Shennan mining area, Shaanxi Province, China. For example, the roof-to-floor convergence reached 1.2 m in the recovery room of N14201 longwall panel at the Zhangjiamao Coal Mine and resulted in most of the hydraulic supports in the recovery room being crushed, which seriously affected the production efficiency of the coal mines. Similar accidents have shown that the mechanism of large surrounding rock deformation in recovery rooms is still not well understood. Thus, researchers have focused on the mechanism of surrounding rock instability in the recovery room. Wichlacz et al. (2009) have analyzed the main parameters that impact the successful implementation of the recovery room and developed a program to evaluate longwall recovery operations. Lv (2014) has discussed the key factors that affect the stability of pre-driven recovery rooms and offered a mechanical model of the unmined coal pillar. Gu et al. (2015) has studied the load transfer mechanical mechanism of the unmined coal pillar and the coal pillar during the end of the mining stage based on theoretical analysis, derived load calculation formulas of the coal pillar, and proposed a method to determine the width of the coal pillar. Shu et al. (2018) has adopted numerical simulation and theoretical analysis to study the influence of buried depth on roof fall and rib sliding in recovery rooms. However, contrastive studies have not been conducted on the surrounding rock deformation and stress distribution in the recovery rooms of different coal seams, and few studies have been conducted on combining periodic weighting and surrounding rock deformation.
This study mainly evaluated the surrounding rock deformation, coal pillar stress evolution, and periodic weighting in the pre-driven longwall recovery rooms of two different coal seams at the Ningtiaota Coal Mine (N1206 panel of 2−2 coal seam) and the Hongliulin Coal Mine (15205 panel of 5−2 coal seam). This study compared the surrounding rock deformation and coal pillar stress monitoring data of the recovery room in the two panels. Based on the monitoring data, the periodic weighting data were analyzed, and the main roof break positions of the two panels were determined. From the results of the study, the key factor affecting the surrounding rock deformation and coal pillar stress distribution in the recovery rooms were determined.
2 General conditions
2.1 Roof and coal seam conditions
Lithology and roof condition of 2−2 and 5−2 coal seams
Coal seam number
Coal seam average thickness (m)
Depth range (m)
Average thickness (m)
Average thickness (m)
2.2 Support and mining conditions
Pattern of supports in the main recovery room of 2−2 and 5−2 coal seams
Type of layout
Support pattern (main recovery room)
Coal pillar rib
Pre-driven double recovery room
Steel bolts, cables, and buttress hydraulic supports
Pre-driven double recovery room
Steel bolts, cables, and buttress hydraulic supports
Support parameters and mining conditions of the N1206 and 15205 panels
Buried depth (m)
Main recovery room
Mining height (m)
Chock-shield supports yield load (kN)
Buttress supports yield load (kN)
Coal pillar width (m)
3 Field monitoring instrument
Field monitoring contents include surrounding rock deformation, coal pillar stress, and canopy-to-base convergence of buttress hydraulic supports. All instruments were installed before the longwall face was 100–150 m away from the recovery room.
Displacement meters were used to measure roof-to-floor and coal pillar rib convergence.
Laser rangefinders were used to measure canopy-to-base convergence of the buttress hydraulic supports.
Borehole pressure cells were installed in the coal pillar rib to measure stress changes, and the installation depths were 1 m, 2 m, 3 m, 4 m, 6 m, and 8 m.
4 Monitoring results and analysis
4.1 Roof-to-floor convergence
Maximum roof-to-floor convergence of the main recovery room at each stage
Longwall distance (m)
Maximum convergence (mm)
Stage 1 (stable stage) The recovery room is far from the longwall face and is rarely affected by mining advance; the change of the roof-to-floor convergence is very small at this stage.
Stage 2 (increasing stage) When the longwall face is mined at a certain distance away from the terminal line, it enters into Stage 2. Began within Stage 2, the surrounding rock of the recovery room is gradually damaged, and the roof-to-floor convergence begins to increase significantly. The longwall face will gradually begin to reduce the mining height to the height of the main recovery room. The starting distances of the N1206 and 15205 panels to reduce the mining height were 15 m and 20 m, respectively.
Stage 3 (peak stage) From this stage, the longwall face is about to enter the main recovery room. The roof-to-floor convergence increases sharply, and the convergence rate reaches its peak value when the face enters the main recovery room.
Stage 4 (decreasing stage) At this stage, the longwall face has entered the main recovery room, but the movement of the equipment has not started yet. Mining stopped, and the roof-to-floor convergence rate decreased as the convergence deformation continued.
Figure 2 and Table 4 show that the initial distance and the roof-to-floor convergence of the two panels at each stage were different. The initial distance affected by mining in each panel was 36 m (N1206 panel) and 42 m (15205 panel). The maximum roof-to-floor convergence in the main recovery room of each panel was 348.03 mm (N1206 panel) and 771.24 mm (15205 panel). Starting from Stage 3, the convergence data of Stations 2, 3, and 4 began to increase considerably compared with those of Stations 1 and 5. This increase was caused by the fact that the opposite ends of the main recovery room were close to the headgate and tailgate, and certain parts of the abutment pressure were carried by the unmined coal pillar.
Although the basic variation of the roof-to-floor convergence of the main recovery room in the two panels is consistent, the roof-to-floor convergence of the main recovery room in the 15205 panel was larger than that in the N1206 panel. The initial distance affected by mining in the 15205 panel was farther than that in the N1206 panel, which indicates that the higher the mining height, the more evident the influence of mining on the recovery room is.
4.2 Coal pillar rib convergence
Maximum coal pillar rib convergence of the main recovery room at each stage
Maximum convergence (mm)
4.3 Canopy-to-base convergence of buttress hydraulic supports
Canopy-to-base convergence of buttress hydraulic supports in the main recovery room
Average canopy-to-base convergence of supports
Mining rib (mm)
Coal pillar rib (mm)
Mining rib (mm)
Coal pillar rib (mm)
4.4 Vertical stress change in coal pillar
The large deformation of the surrounding rock of the main recovery room in the 15205 panel shows that the roof movement may be significant under certain circumstances after the longwall face enters the recovery room. Barczak et al. (2007) and Tadolini and Barczak (2008) have confirmed that the recovery room is significantly affected by the periodic weighting. Therefore, to determine the primary cause of the large deformation of surrounding rock in the recovery room, periodic weighting should be analyzed during the end of the mining stage.
6 Analysis of periodic weighting
Periodic pressure positions and steps of each panel before the longwall face enters the main recovery room
Number of periodic weighting
In Eq. (1), the last weighting step l is generally larger than the longwall distance s, thus, the main roof break position d will be negative, which means the break position is behind the terminal line. Correspondingly, Eq. (2) means the main roof break position is in front of the terminal line. The periodic weighting real-time observation system of the two panels shows that the periodic weighting occurs in the N1206 and 15205 panels after the face enters the main recovery room. Therefore, according to the data in Table 7 and Eq. (1), the main roof break position of each panel after the face enters the main recovery room is − 3.8 m (N1206 panel) and − 8.2 m (15205 panel). The calculation results show that the main roof break position in the N1206 panel is above the main recovery room, and the main roof breaks above the coal pillar in the 15205 panel.
An important reason for the large deformation of the surrounding rock in the recovery room of the 15205 panel is that the main roof breaks above the coal pillar after the longwall face enters the main recovery room. In this situation, the maintenance of the roof stability remains difficult even if the high-strength buttress hydraulic supports have been used in the 15205 panel, which has caused the damage of the coal pillars and the hydraulic support crushing accident. To avoid such cases of failure, the last weighting step should be adjusted artificially to reduce the roof pressure, such as caving the roof and yield mining.
The influence of the front abutment pressure on the recovery room could be divided into four stages. The initial distances of the increasing stage in the N1206 and 15205 panels were 36 m and 42 m, respectively. The initial distance affected by mining advanced with the increase of the mining height. When the longwall face was approximately 8–10 m away from the terminal line, the surrounding rock deformation of the main recovery room and the coal pillar stress began to increase sharply. The roof’s large deformation and the rib spalling failure mainly occurred before the face entered the main recovery room.
The maximum roof-to-floor convergences of the main recovery room in the N1206 and 15205 panels were 348.03 and 771.24 mm, respectively, and the coal pillar stress increased by more than 5 MPa and 7 MPa, respectively. The surrounding rock deformation in the middle of the main recovery rooms was much larger than that at the opposite ends, and the roof deformation form of the main recovery rooms was tilting and sinking. When designing the support parameters, the uneven deformation rule of the main recovery room should be considered. The strength and density of the supports in the middle of the main recovery room should be higher than that at the opposite ends.
The main roof break position after the face entered the main recovery room of the two panels was determined based on the periodic weighting data. The main roof break positions of the N1206 and 15205 panels were 3.8 m and 8.2 m behind the terminal line, respectively. The main roof’s last weighting is the key factor affecting the stability of the main recovery room and the coal pillar. When the main roof broke above the coal pillar, the surrounding rock deformation of the main recovery room and the coal pillar stress increased sharply and caused the hydraulic support crushing accident.
Support for this work was provided by the National Natural Science Foundation of China (No. 51679199), Key Laboratory for Science and Technology Co-ordination and Innovation Projects of Shaanxi Province (No. 2014SZS15-Z01) and is thankfully acknowledged by the authors.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Barczak TM, Tadolini SC (2008) Pumpable roof supports: an evolution in longwall roof support technology. Trans Soc Min Met Explor 324:19–31Google Scholar
- Barczak TM, Tadolini SC, Zhang P (2007) Evaluation of support and ground response as longwall face advances into and widens pre-driven recovery room. In: Proceedings of the 26th international conference ground control min. Morgantown, WV, pp 160–172Google Scholar
- Gu SC, Wang BN, Huang RB, Miao YP (2015) Method for determining coal pillar load and width in retracement channel at ending stage of fully-mechanized face. J China Univ Min Technol 6:990–995 (in Chinese) Google Scholar
- Listak J, Bauer E (1989) Front abutment effects on supplemental supports in pre-driven longwall equipment recovery rooms. In: The 30th U.S. symposium on rock mechanics, Morgantown, WV, 19–22 Jun, pp 693–703Google Scholar
- Lv HW (2014) The mechanism of stability of pre-driven rooms and the practical techniques. J China Coal Soc S1:50–56 (in Chinese) Google Scholar
- Oyler D, Frith D, Dolinar DR, Mark C (1998) International experience with longwall mining into pre-driven rooms. In: Proceedings 17th international conference on ground in mining, Morgantown, WV, 4–6 Aug, pp 44–53Google Scholar
- Shu CX, Jiang FX, Han YW, Li D et al (2018) Long-distance multi-crosscut rapid-retracement technique in deep heavy fully mechanized face. J Min Saf Eng 3:473–480 (in Chinese) Google Scholar
- Tadolini, SC (2003) Ground control support considerations for pre-driven longwall recovery rooms. Ph.D. Dissertation submitted to College of Engineering and Minerals Resources, West Virginia University, Morgantown, WV, p 163Google Scholar
- Tadolini SC, Barczak TM (2006) Design parameters of roof support systems for pre-driven longwall recovery rooms. In: SME annual meeting and exhibit, pp 1–13Google Scholar
- Tadolini SC, Barczak TM (2008) Rock mass behavior and support response in a longwall panel pre-driven recovery room. In: The 6th international symposium on ground support in mining and civil engineering construction, pp 167–182Google Scholar
- Tadolini SC, Barczak TM, Zhang Y (2003) The effect of standing support stiffness on primary and secondary bolting systems. In: Proceedings of the 22nd international conference on ground control min, Morgantown, WV, pp 300–307Google Scholar
- Wang XZ, Ju JF, Xu JL (2012a) Theory and applicable of yield mining at ending stage of fully-mechanized face in shallow seam at Shendong mine area. J Min Saf Eng 3:151–156 (in Chinese) Google Scholar
- Wang XZ, Xu JL, Zhu WB, Ju JF (2012b) Influence of high mining speed on periodic weighting during fully-mechanized mining in a shallow seam. J China Univ Min Technol 3:349–354 (in Chinese) Google Scholar
- Wichlacz D, Britten T, Beamish B (2009) Development of a pre-driven recovery evaluation program for longwall operations. In: Coal 2009: coal operators’ conference. University of Wollongong & the Australasian Institute of Mining and Metallurgy, 12–13 Feb, pp 23–36Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.