In mining excavation, the retained entry with stiff coal pillar is situated in the strong mine ground pressure. Influenced by mining abutment stress and dynamic stress (the vibration signal) induced from the hard roof activation, the retained entry may be subjected to roof separation, supporting body failure, severe floor heave, and even roof collapse. Based on a 2D physical model, an experimental method with plane-stress conditions was used to simulate the mechanical behavior of the rock strata during mining. In this experiment, three monitoring systems were adopted to reveal the characteristics of the strong mine ground pressure in the stiff coal-pillar entry retaining. The results show that the hard roof undergoes bending down, fracture, and caving activation successively until it is able to support overlying loads. The abutment stress which is induced from the loading transfer in stiff coal pillar is larger than that in other rocks around the retained entry in amplification, and overlying loads above the worked-out area have a loading effect on the unworked-out area. When the hard roof is situated in the activation state, the dynamic stress is generated from the hard roof activation, which is verified by the great saltation of acoustic emission signals. The results of mining ground pressure in the physical model can clearly illustrate the mechanical behavior of the rock around the retained entry with stiff coal pillar.
As a fossil energy source, coal has provided enormous energy for human civilization in the past and in the future. Strip mining and underground mining are the main ways to maintain the sustainable development of coal production. Security maintenance of entries in underground mining requires solving many engineering and mechanical problems, which is a key technology to ensure the normal operation of the production system [
Layout of two-entry system in longwall panel.
Stage 1 is the in situ stress state of Tail entry 2, which is not affected by the hard roof activation above the adjacent gob; Stage 2 is the stress evolution state of Tail entry 2, which is affected by the hard roof activation above the adjacent gob; Stage 3 is the side abutment stress state of Tail entry 2, which is not affected by the hard roof activation above the adjacent gob; and Tail entry 2 should be retained to serve Panel 2.
Coal pillar width determines the stress condition around the retained entry during the hard roof activation above the gob in Panel 1 [
The method of cutting hard roof to achieve the pressure relief has been widely used in the world [
In this work, a 2D physical model with plane-stress conditions was established to simulate the mechanical behavior of the rock strata behind the working face during the mining process. In this physical model, three monitoring systems were used to reveal the characteristics of strong mine ground pressure in stiff coal-pillar entry retaining.
First Yangquan coal mine is located in the city of Yangquan, Shanxi Province, China. The two-entry system, which is employed in the longwall top coal caving operation, is approximately 2200 m long by 220 m wide in every panel as shown in Figure
Generalized stratigraphic column.
The physical experiment was conducted by a physical modeling system at the State Key Laboratory of Coal Resources and Mine Safety in China. As shown in Figure
Experimental scene with physical model and monitoring systems.
As shown in Figure
Materials used in the physical model.
Lithology | UCS of prototype (MPa) | UCS of model (kPa) | Sand (kg) | Calcium carbonate (kg) | Gypsum (kg) | Amounts (kg) | Water (L) |
---|---|---|---|---|---|---|---|
Mudstone group 1 | 35.27 | 144.06 | 70.31 | 7.03 | 7.03 | 84.38 | 9.38 |
Medium sandstone | 70.09 | 286.30 | 158.20 | 15.82 | 36.91 | 210.94 | 23.44 |
Mudstone group 2 | 35.27 | 144.06 | 585.94 | 58.59 | 58.59 | 703.13 | 78.13 |
Fine sandstone | 74.61 | 304.81 | 189.84 | 18.98 | 44.30 | 253.13 | 36.16 |
Mudstone group 3 | 35.27 | 144.06 | 386.72 | 38.67 | 38.67 | 464.06 | 51.56 |
Limestone | 71.83 | 293.42 | 142.38 | 14.24 | 33.22 | 189.84 | 27.12 |
Coal seam 15 | 24.83 | 101.41 | 79.98 | 5.71 | 5.71 | 91.41 | 10.16 |
Mudstone | 29.71 | 121.35 | 120.54 | 10.04 | 10.04 | 140.63 | 15.63 |
Evolution of the abutment stress, acoustic emission signals, and the rock strata displacement are determined as indexes of the mine ground pressure during the hard roof activation above the gob. UEILOGGER 3.0.0 data acquisition system, made by American UEL Company, was used to monitor the evolution of the abutment stress. The system consists of four parts, including the miniature pressure cell, UEILOGGER host, data transmission cable, and data processing software. The miniature pressure cell is capable of operating in the saturated aqueous medium. The measurement range of the miniature pressure cell is 0.02–1.5 MPa, the deviation is limited to 0.5% FS, and acquisition frequency is set at 1 Hz in this monitoring programme. The acoustic emission monitoring system (AEwin), made by American Physical Acoustics Corporation, was used to monitor acoustic emission signals. This system also consists of four parts, including the acoustic emission sensor, the acoustic emission host, the data transmission cable, and data processing software. The measurement range is 1 kHz–3 MHz vibration frequency, and maximum acquisition frequency reaches 40 MHz for the acoustic emission monitoring system. The resonant frequency, the sensitivity peak, and the effective acquisition frequency of the acoustic emission sensor are 40 kHz, 75 dB, and 15 kHz–70 kHz in this monitoring programme, respectively. In addition, TS3866 digital photogrammetry system was used to monitor the rock strata displacement.
Figure
Monitoring programme in the physical model.
The whole test involves six steps: (1) Preparation of experimental tools, such as the high stiff loading frame, physical materials, mixing barrel with electric power, electronic scale, three monitoring systems, and other essential tools. (2) Model and compact the eight physical rock strata one by one, and separate every rock strata with certain mica powder. (3) Apply the vertical load 0.056 MPa through 20 loading rams in the top frame to simulate the overburden loads, fix the normal displacement in the floor boundary, two-side boundaries with the frame, and keep the free state for the front and back boundary of the model after two months of the model completion. (4) Conduct the excavation of the retained entry. From the view of mechanics, the additional stress around the retained entry generally comes from the activation of the hard roof structure near the retained entry, while the collapsed hard roof structure in the gob center is independent of the additional stress around the retained entry. (5) Perform the longwall successively which retreats from the panel center to panel boundary to simulate the activation effect for the hard roof of the retained entry. In each stage, 50 mm-long coal is excavated by using a mini shovel. Then wait 20 minutes before the next excavation. During the excavation, three monitoring systems should be operated in a normal state for recording until the test procedure ends. (6) Apply additional vertical loading 200 Pa per second through the 20 loading rams, so as to simulate the abutment stress induced from the retreating of Panel 2. According to the existing monitoring data in the field, the additional vertical stress in the rock in the front of the working face increased by 17.29 MPa in 24 hours. So the increasing rate can be calculated approximately as the 200 Pa per second.
As the pictures shown in Figure
Roof activation characteristics. (a) Roof bending. (b) Roof fracture. (c) Roof caving. (d) Cantilever construction. (e) Cantilever construction fracture. (f) Cantilever construction caving. (g) Cantilever construction. (h) Cantilever construction fracture. (i) Upper roof condition. (j) Suspended upper roof. (k) Upper roof fracture. (l) Upper roof fracture and caving.
Along the vertical direction, roof bends down in the lower roof near the gob initially, and then the upper roof begins to bend down gradually. The lower roof is larger than the upper roof in vertical displacement distinctly. As the increasing distance from the gob center, the roof vertical displacement decreases gradually as the measuring data shown in Figure
Vertical displacement of the roofs above the gob. (a) Panel 1 retreating 500 mm. (b) Panel 1 retreating 650 mm. (c) Panel 1 retreating 1150 mm. (d) Panel 1 retreating 1200 mm. (e) Panel 1 retreating 1600 mm. (f) Applying additional abutment stress.
At first, additional abutment stress increases slowly, then decreases sharply, and presents stabilization finally in P1, P2, and P3 as shown in Figures
Additional abutment stress evolution during the hard roof activation above the gob. (a) Measuring point of P1. (b) Measuring point of P2. (c) Measuring point of P3. (d) Measuring point of P4. (e) Measuring point of P5. (f) Measuring point of P6.
Distinct acoustic emission phenomenon occurs during the hard roof activation above the gob as measuring point S4 in Figure
Acoustic emission signals during the activation of the hard roof above the gob. (a) Ring count in measuring point S4. (b) Energies in measuring point S4. (c) Amplitude in measuring point S4.
In mining excavation, stiff coal-pillar entry retaining is under the condition of strong mine ground pressure, which is validated by the physical method. The rock around the retained entry experiences not only the evolution of the side abutment stress and the front abutment stress [
Deformation characteristics of the 19.5 m stiff coal-pillar entry retaining [
The suspended roof encounters bending, fracture, and caving activation when the suspended area is large enough. During the activation process, overlying loads above the worked-out area have a loading effect on the unworked-out area. Besides, the generating dynamic stress wave to the nearby retained entry, which makes the rock around the retained entry under the synergy effect of abutment stress and dynamic stress. The results of mining ground pressure in the physical model of plane stress can clearly illustrate the mechanical behavior of the rock around the retained entry with stiff coal pillar under the hard roof [
However, the physical model can only simulate the mechanical behavior of the rock strata behind the working face during the mining process when it is under the plane-stress condition. When there are several hard roofs near the mining coal seam, this experimental method is a great option for predicting the mine ground pressure of the stiff coal-pillar entry retaining, while it is inappropriate for predicting the deformation behavior of the retained entry when the dimension of the entry is just 31 mm in width and 25 mm in height. In addition, when monitoring the dynamic stress induced from the hard roof activation, the monitoring system of high-frequency pressure cell is more convinced than vibration signals with the acoustic emission.
In order to reveal the mine ground pressure of stiff coal-pillar entry retaining and verify whether this entry can be retained to serve the next panel influenced by the hard roof activation, a 2D physical model with plane-stress conditions was established to simulate the mechanical behavior of hard roofs behind the working face. The results of the experimental method are concluded as follows. The hard roof closed to the gob undergoes bending down, fracture, and caving activation successively unless its upper hard roof is strong enough to support overlying loads. The cantilever structure, which is supported by the stiff coal pillar above the gob, faces the potential fracture activation above the stiff coal pillar under the front abutment stress induced from the retreating of the next panel. Under the synergistic effects of the side varying abutment stress and dynamic stress, the entry cannot be retained to serve next panel successfully even though it is protected with a stiff coal pillar. Overlying loads above the worked-out area mainly have a loading effect in the stiff coal pillar. Besides, the dynamic stress induced from the hard roof activation can wave to the underneath retained entry. The experimental method can be used to analyze the mechanical behavior of the rock strata during the mining process. Since the geological and engineering conditions are different, the procedure is still necessary for other cases.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported by the National Natural Science Foundation of China (contract nos. 51804099, 51774111, and 51704098), the Key Scientific Research Project Fund of Colleges and Universities of Henan Province (19A440011 and 19A130001), the Natural Science Foundation of Henan Polytechnic University (B2018-4 and B2018-65), and the Regional Collaborative Innovation Project of the Xinjiang Uygur Autonomous Region (2017E0292).