Extraction of a protective coal seam (PVCS)-below or above a coal seam to be mined with the potential of coal and gas outburst risk-plays an important role not only in decreasing the stress field in the surrounding rock mass but also in increasing the gas desorption capacity and gas flow permeability in the protected coal seam (PTCS). The PVCS is mined to guarantee the safe mining of the PTCS. This study has numerically evaluated the stress redistribution effects using FLAC3D model for a longwall face in Shanxi Province. The effects of mining depth, mining height and inter-burden rock mass properties were evaluated using the stress relief angle and stress relief coefficient. Vertical stress distribution, stress relief angle and stress relief coefficient in the PTCS were analyzed as the face advanced in the PVCS. The results showed that the stress relief achieved in different locations of the PTCS varied as the face advanced. Sensitivity analyses on the pertinent variables indicate that the stress relief in the PTCS is affected most by the mining depth followed by the inter-burden lithology and the mining height. Furthermore, the elastic moduli of different layers within the inter-burden rock mass are more important than their uniaxial compressive strength (UCS) and Poisson’s ratio. These observations can guide gas drainage borehole design to minimize the accidents of coal and gas outbursts.
Over 50% of the coal mines in China mine coal seams with high gas content. The definition of high gas content coal mine is very complex but the commonly used indicators are the absolute gas emission (AGE) and the relative gas emission (RGE). When the AGE is greater than or equal to 40 m3/min, or the RGE is greater than or equal to 10 m3/t, the mine can be regarded as having high gas content [
Most coal seams in China are characterized by low gas pressure, low gas saturation, low permeability due to their high degree of metamorphism. This has resulted in low success in gas drainage before coal mining [
The mine is located in NW China around Gaoping City of Shanxi Province (
Group | Coal seam number | Thickness (m) | Layer space (m) | Minable or not | Rock mass properties | Thickness uniformity | |||
---|---|---|---|---|---|---|---|---|---|
Min.–Max. Average | Min.–Max. Average | Roof | Floor | Structure | Parting/m | Uniformity | |||
Shanxi | 2# | 0–3.020.71 | 9.40–25.6920.68 | Portion | MudstoneSandy-mudstoneSiltstone | MudstoneSandy-mudstone | Simple | 0–2 | No |
3# | 4.56–6.835.70 | 30.53–41.0735.13 | Minable | Sandy-mudstoneSiltstone | MudstoneLimestoneFine-grained sandstone | Simple | 0–3 | Yes | |
Taiyuan | 8# | 0–3.051.22 | Portion | MudstoneSandy-mudstone | MudstoneSandy-mudstone | Simple | 0–2 | No | |
15# | 2.20–6.414.18 | 38.92–60.7950.32 | Minable | MudstoneSandy-limestone | Mudstone | Simple-Complex | 0–5 | Yes |
Coal seam | Coal face | Mining thickness/m | Gas content/(m3/t) | Original gas pressure/MPa | Coal stiffness | Outburst risk |
---|---|---|---|---|---|---|
#3 | 4306 protected longwall face | 5.23 | 3.5 |
0.38 |
0.44 |
Yes |
#8 | 84306 protective longwall face | 1–3 | 2 |
None | 0.51 |
No |
It is desired to have a good understanding of stress redistribution around a longwall face, and associated fracture development, and gas flow behavior during the mining process and then effectively apply the PVCS concept in the field. A few studies on these topics have been conducted under the condition of extracting single seam and multiple seams [
Rock mass permeability is influenced by the two important factors- rock mass fracturing and stress redistribution. Stress relief and mining-induced fractures can increase the permeability of coal and rock mass. Therefore, identification of the stress relief zones in the upper PTCS above a lower PVCS is an important objective of this paper. The stress relief area in PTCS depends on several parameters such as mining depth, panel width, face advance rate, gob loading behavior and mining height of the PTCS, the thickness and inter-burden strata properties between the PVCS and PTCS. All these factors should be considered to analyze the effect of mining the PVCS. However, in this study three main factors- mining depth, mining height of the PVCS and inter-burden strata properties were chosen since their effects on the stress relief of PTCS during the excavation of PVCS are not well documented [
The concept of gas drainage before mining and PVCS mining has been used extensively to eliminate coal and gas outburst risk [
where
The pressure relief area is controlled by geology, gas pressure, deformations associated with mining activity, and the vertical stress in PTCS [
where
Selection of the PVCS should meet two conditions: (1) Its excavation should not destroy the ability to mine PTCS (which means the PTCS should be located outside of the caved zone of the PVCS); and (2) It should achieve the best pressure relief effect. Upon excavation of the lower PVCS, the overlying strata may be divided into three relatively distinct zones based on fracture development: the caved zone, the fractured zone, and the continuous deformation zone [
Here
Mining height of 8# coal seam/m | Location of 3# coal seam | Stress relief effect of 3# coal seam |
---|---|---|
1 | Above fractured zone | Poor |
2 | Upper border of fractured zone | General |
3 | In fractured zone | Excellent |
Coal and rock mass have naturally-existing geological discontinuities (joints, cracks, and faults). Gas is stored in the pores and fractures mainly in an adsorbed state. The mining of the PVCS increases coal deformations and redistributes stress around adjacent coal and rock mass with stress relief or concentration. The permeability of coal and rock mass in some areas may increase through additional bedding plane fractures and cracks. The gas in both the PTCS and PVCS may desorb and flow into high-permeability regions and the newly formed fracture network [
Li et al. [
where
The gas migration rate is related to coal permeability, which is directly determined by the opening and closure of fractures. Therefore, the relationship between stress and permeability can be studied indirectly by studying the relationship between changes in stress, cracks formation and deformation, and rock mass permeability.
where
The degree of fracture closure is expressed as follows:
The permeability of a single crack is expressed as:
In the above equation,
Permeability can then be expressed in a dimensionless form:
where
Numerical models were developed to simulate rock mass behavior during the mining of the lower PVCS at the study mine [
The generalized Hoek–Brown criterion (
where
where s and a are constants for the rock mass, and we have the following relationships:
where
Lithology | GSI | s | a | |||||
---|---|---|---|---|---|---|---|---|
#3 Coal seam | 0.33 | 6 | 75 | 11 | 4.504 | 0.0622 | 0.501 | 2938.86 |
Fine-grained sandstone | 0.19 | 90 | 90 | 16 | 11.195 | 0.3292 | 0.500 | 6566.53 |
Medium-grained sandstone | 0.20 | 73 | 88 | 15 | 9.772 | 0.2636 | 0.500 | 5020.52 |
Mudstone | 0.28 | 16 | 80 | 12 | 5.874 | 0.1084 | 0.501 | 3433.36 |
Sandstone | 0.24 | 40 | 86 | 13 | 7.885 | 0.2111 | 0.500 | 4203.05 |
Sandy-mudstone | 0.26 | 35 | 85 | 13 | 7.084 | 0.1512 | 0.500 | 4095.07 |
Limestone | 0.19 | 75 | 90 | 10 | 6.997 | 0.3292 | 0.500 | 9682.03 |
# 8 Coal seam | 0.29 | 6.4 | 75 | 11 | 4.504 | 0.0622 | 0.201 | 3020.50 |
The estimation of the rock mass strength is:
Two panels of the PVCS (#8 coal seam) are simulated in the FLAC3D model, but only 84306 longwall face is excavated in this study. As shown in
Vertical stress redistribution and pressure relief coefficient in the PTCS are shown in
1) For 30 m face advance in the PVCS, the minimum vertical stress in the PTCS is located about 18 m behind the coal face. The stress distribution curve in the PTCS is approximately V-shaped, and the maximum pressure relief area is located around the middle of the gob. The vertical stress is about 10.5 MPa, and the pressure relief coefficient is about 0.19. The vertical stress increased ahead of the longwall face and behind in the set up room areas (
2) For 50 m face advance in the PVCS (
3) For 100 m face advance in the PVCS, the vertical stress contour in the PTCS is shown in
In summary, the degree and extent of pressure relief in the PTCS increases gradually with the face advance in the PVCS. With collapse of the immediate roof and main roof behind the face, the overburden stress is transferred ahead of the face and on to the caved gob. With the increase in roof deformation and its vertical movement, the broken rock mass in the gob is gradually compacted, which gradually increases the supporting effect for the main roof, thus reducing to some degree the stress relief of the PVCS in the middle and rear of the gob. Assuming the central axis of the gob as the center line, stress relief around the front of the face and back in the gob area are asymmetrical. Most of pressure relief is concentrated ahead of the face.
The above analyses show that the stress relief degree and extent in the PTCS changes with different face advance distance of the PVCS. The stress relief zone in PTCS for 100 m face advance of PVCS (cutting plane Z-
For 100m face advance of PVCS, the vertical stress in the PTCS (
When the distance from the coal pillar is 130 m which is close to the central axis (X-
When the distance from the coal pillar is 30 m (blue lines in
The maximum pressure relief coefficient in the PTCS is similar for different distances away from the coal pillar. However, as the monitoring lines get farther and farther from the coal pillar, the location of the maximum pressure relief coefficient gets closer to the coal face. Therefore, the PTCS goes through several stages along the direction of face advance: Compression, stress relief, and expansion, increased stress relief and expansion, stabilized stress relief and expansion, decrease in stress relief and expansion, compression, and stable stress relief and expansion.
Stress relief angle ( |
Distance to coal pillar (m) | 130 | 120 | 110 | 100 | 90 | 80 | 70 | 60 | 50 | 40 | 30 | 20 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Location | Coordinate | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 |
Open-off cut side | 49.7 | 50.8 | 52.6 | 53.2 | 53.3 | 53.7 | 54.8 | 55.6 | 57.3 | 59.1 | 67.0 | 66.9 | |
Coal face side | 68.2 | 69.4 | 71.1 | 72.5 | 73.8 | 75.1 | 75.2 | 76.2 | 78.0 | 79.5 | 80.8 | 76.2 |
In order to analyze the vertical stress changes along the strike of the PTCS for different positions in the gob during the mining of the PVCS, four cross-section planes were analyzed perpendicular to the Y-axis for different positions behind the face in the gob during the PVCS mining: 100 m (
For 30 m face advance, the stress relief extent and the coefficient are almost the same. The maximum values of the stress relief coefficient for 1 and 3 m mining heights are 0.12 and 0.14. The location center of the relieved area is about 17 m behind the working face.
For 50 m face advance, the above coefficients are 0.21 and 0.28. Similar data for 70 and 120 m face advances vary 0.23 to 0.24. The above results show that the influence of the mining height on the extent and the degree of stress relief in the PTCS vary only in the initial stage of mining.
Additional analyses were performed to quantify the influence of the mining height on the stress relief coefficients and stress relief angles along the strike direction (
Stress relief angle ( |
Variables | ||||||||
---|---|---|---|---|---|---|---|---|---|
Face advance (m) | 30 | 50 | 70 | 120 | |||||
Mining height | 3 | 1 | 3 | 1 | 3 | 1 | 3 | 1 | |
Open-off cut | 82.0 | 74.5 | 74.5 | 71.6 | 64.7 | 63.5 | 41.3 | 41.3 | |
Coal face side | 82.0 | 77.5 | 80.6 | 76.0 | 77.5 | 74.5 | 76.0 | 68.1 |
The rock mass properties within the interburden layers affect the stress relief in the PTCS. Rock mass properties of interest include elastic moduli, cohesion, and bulk density. The general consensus is that the disturbance and damage within the overlying strata will increase with the decrease in strength of the inter-burden layers, and that should increase the extent and degree of stress relief. To quantify these effects, three interburden lithologies with engineering properties in
Nine numerical models were run based on the three-level and four-factor orthogonal experiment design without a consideration of interaction among those three factors (Table L9, 34) [
Model No. | Mining height/m | Mining depth/m | Rock mass properties | Stress relief coefficient |
---|---|---|---|---|
Model 1 | 1 | 400 | Sandy mudstone | 0.20 |
Model 2 | 1 | 600 | Fine-grained sandstone | 0.21 |
Model 3 | 1 | 800 | Limestone | 0.21 |
Model 4 | 2 | 400 | Fine-grained sandstone | 0.18 |
Model 5 | 2 | 600 | Limestone | 0.18 |
Model 6 | 2 | 800 | Sandy mudstone | 0.28 |
Model 7 | 3 | 400 | Limestone | 0.15 |
Model 8 | 3 | 600 | Sandy mudstone | 0.25 |
Model 9 | 3 | 800 | Fine-grained sandstone | 0.26 |
The F value reflects the degree of influence of each factor on the experimental results. It is obtained by the ratio of the squared sum of the mean deviations of the influencing factors to the sum of the squared deviations of the error [
Influencing factor (variables) | Sum of squares of deviations | Degree of freedom | F-Value | Critical value of F |
---|---|---|---|---|
Mining height | 0.000 | 2 | 0.000 | 5.140 |
Mining depth | 0.008 | 2 | 1.714 | 5.140 |
Rock mass properties | 0.006 | 2 | 1.286 | 5.140 |
Error | 0.01 | 6 |
Mining a PVCS has been widely used to reduce the coal and gas outburst risk where multiple coal seams are present near. This paper has numerically analyzed for a case study mine the progressive effects of this practice on the extent (stress relief angle) and the degree of stress relief as the face in a lower PVCS seam is advanced. The relative importance of mining depth, inter-burden lithology between the two seams, and mining height on the above variables are also analyzed.
(1) With face advance in the PVCS, each point in the protected seam (PTCS) undergoes thru varying stress redistribution in different stages of mining that can be beneficially used from gas drainage point of view. The 3-D shapes of these zones outlined in the results section can aid gas drainage boreholes design.
(2) Both the extent of stress relief and the coefficient of stress relief in the PTCS vary ahead of and behind the face in the mined-out areas. These are affected by the mining face position in relation to caving behind the face in the PVCS.
(3) For this case study, the stress relief angle behind the face in the mined-out area is smaller than ahead of the face. The pressure relief angle along the gob side and the face side increases first and then decreases with face advance to an equilibrium value. The maximum relief angle value is
(4) Mining height affects the stress relief angle during the early stages of mining on the gob side. Ahead of the face, the stress relief angle increases with the mining height.
(5) The pressure relief in the PTCS is affected most by the mining depth, followed by the inter-burden lithology engineering properties and the mining height. The elastic modulus of the inter-burden layers has the most effect on the stress relief in the PTCS among considered engineering properties.
The authors would also like to express special thanks to the editor and anonymous reviewers for their professional and constructive suggestion.