Protective layer mining, as a dominating regional prevention measure, is generally adopted to prevent and control gas disasters in highly gassy or outburst mines of China. Interlayer distance is one of the most important factors that influences protection effect. However, how does interlayer distance affect the protection effect of steeply inclined upper protective layer mining is not understood fully. According to the engineering practice in Nantong mining district, a new method for similar material simulation experiment of steeply inclined upper protective layer mining is proposed, in which an orthogonal test of similar materials comprising of sand, cement (containing gypsum and fly ash), and water mixture is conducted to obtain relations between proportioning parameters and mechanical properties using a multiple regression method. And then the method is applied to study the protection effect of steeply inclined upper protective layer mining with varying interlayer distances. The results show the following. (1) The proportioning parameters of similar material have strong linear relations with its mechanical properties, and mechanical behaviors of such similar material denote that it can simulate most coal-rock lithologies in coal mine. (2) Both pressure-relief curves and swelling strain curves for protected layer present convex shapes; protection angles at lower excavation boundary are greater than those of upper excavation boundary; with the increase of interlayer distance, the pressure-relief curve evolves from pattern “
Currently, coal constitutes a major part of China’s energy consumption. As the depletion of shallow coal resources, coal working face would enter into deep mining area. Related studies show that the mining depth of China would increase to 1500 m in next twenty years [
Schematic diagram for protective layer mining.
As seen in Figure
Recently, based on theoretical analysis, numerical simulation, and field observation, scholars [
In brief, existing reports on protection effect of protective layer mining with varying interlayer distances mostly concentrated on slightly inclined or near-horizontal coal seams, lacking reports on steeply inclined coal seams, whose redistribution characteristics of stress and displacement would differ, thus affecting its protection effect [
The Nantong coal mine district is located in Chongqing city of China, and its geographical location is shown in Figure
Location map of Nantong coal mine district.
The schematic diagram for geological synthesis columnar is shown in Figure
Schematic diagram for geological synthesis columnar.
Currently, the average buried depth of C6 coal seam, overlying above the C4 coal seam, is 650 m. The C6 coal seam has lower risk of gas outburst, so it is chosen as the upper protective layer to eliminate gas outburst risk of C4 coal seam. A pitching oblique mining method [
Related physicomechanical parameters for each stratum.
Stratum number | Lithology | Volume weight (MN·m−3) | Elastic modulus (GPa) | Poisson’s ratio | Tensile strength (MPa) | Compressive strength (MPa) |
---|---|---|---|---|---|---|
1 | Limestone | 0.028 | 30.2 | 0.26 | 9.8 | 107.35 |
2 | Allitic shale | 0.024 | 23.3 | 0.30 | 6.6 | 91.00 |
3 | C6 coal seam | 0.014 | 0.91 | 0.35 | 0.4 | 3.98 |
4 | Calcareous shale | 0.025 | 26.4 | 0.23 | 8.7 | 103.35 |
5 | Silty shale | 0.024 | 23.1 | 0.28 | 9.8 | 107.35 |
6 | Limestone | 0.028 | 30.2 | 0.26 | 9.8 | 107.35 |
7 | Silty shale | 0.024 | 27.6 | 0.28 | 6.6 | 88.71 |
8 | Cherty limestone | 0.029 | 35.3 | 0.23 | 21.2 | 185.26 |
9 | Silty shale | 0.024 | 20.1 | 0.28 | 9.8 | 107.35 |
10 | C4 coal seam | 0.014 | 0.94 | 0.38 | 0.3 | 3.38 |
11 | Siltstone | 0.030 | 21.6 | 0.25 | 7.3 | 104.57 |
12 | Limestone | 0.028 | 30.2 | 0.26 | 9.8 | 107.35 |
As mentioned, the dip angle and buried depth of C6 coal seams do not vary largely, but the interlayer distance between two mineable coal seams ranges from 25 m to 70 m. To systematically and accurately study protection effect of steeply inclined upper protective layer mining with varying interlayer distances, the interlayer distances chosen to be studied should reflect conditions of close distance, remote distance, and super-remote distance. There are no acknowledged definitions of close distance, remote distance, and super-remote distance protective layers presently. But relevant studies [
In order to carry out the similar material simulation experiment, a rotatable physical similarity simulation bench, with the geometric dimensions of 2.0 m × 2.0 m × 0.3 m (length × height × width) and rotation angles ranging from 0° to 70° to manufacture coal-rock strata with various dip angles, is used, and other experimental devices consisting of pressure test devices, strain (or displacement) test devices, and loading devices (including levers and weights) are shown in Figure
Experimental devices (a) and setup (b) for similar material simulation.
As seen from Figure
The similar material simulation experiment is a reduced-scale approach according to similarity criteria [
To implement similar material simulation experiment, a suitable similar material is needed. The type and proportion parameters of similar material determine whether experimental results obtained can represent mechanical responses of prototype. Similar material simulation experiments have been widely employed, but how to obtain accurate and optimum proportioning parameters is urgently needed to be investigated in coal mining [
The three influencing factors (including sand-binder ratio, density and residual water content) are relaxed within specific regions, and each factor is divided into four levels; then Poisson’s ratio, compressive strength, and elastic modulus are determined. The orthogonal array
Levels of orthogonal design for similar materials.
Level number | Sand-binder ratio | Density (kg·m−3) | Water content (%) |
---|---|---|---|
1 | 6 | 1550 | 5 |
2 | 8 | 1600 | 3 |
3 | 10 | 1650 | 2 |
4 | 12 | 1700 | 1 |
Sixteen combinations and mechanical parameters of similar materials.
Experimental number | Sand-binder ratio | Density (kg·m−3) | Residual water content (%) | Compressive strength (MPa) | Elastic modulus (MPa) | Poisson’s ratio |
---|---|---|---|---|---|---|
1 | 6 | 1550 | 5 | 0.442 | 91.23 | 0.251 |
2 | 6 | 1600 | 3 | 0.504 | 81.67 | 0.254 |
3 | 6 | 1650 | 2 | 0.821 | 217.83 | 0.213 |
4 | 6 | 1700 | 1 | 1.202 | 261.84 | 0.243 |
5 | 8 | 1550 | 3 | 0.270 | 48.75 | 0.235 |
6 | 8 | 1600 | 5 | 0.416 | 74.36 | 0.296 |
7 | 8 | 1650 | 1 | 0.572 | 96.42 | 0.194 |
8 | 8 | 1700 | 2 | 0.731 | 166.22 | 0.284 |
9 | 10 | 1550 | 2 | 0.315 | 59.22 | 0.190 |
10 | 10 | 1600 | 1 | 0.333 | 65.51 | 0.186 |
11 | 10 | 1650 | 5 | 0.181 | 27.36 | 0.311 |
12 | 10 | 1700 | 3 | 0.354 | 53.88 | 0.316 |
13 | 12 | 1550 | 1 | 0.193 | 47.02 | 0.149 |
14 | 12 | 1600 | 2 | 0.206 | 38.50 | 0.202 |
15 | 12 | 1650 | 3 | 0.239 | 40.60 | 0.286 |
16 | 12 | 1700 | 5 | 0.267 | 43.84 | 0.343 |
It is worth noting that mechanical parameters (including Poisson’s ratio, compressive strength, and elastic modulus) are obtained by uniaxial compression tests of similar material specimens with a diameter and a height of 50 mm and 100 mm in the laboratory, as shown in Figure
Mechanical testing of similar material specimens.
As can be seen from Table
Utilizing the multiple regression method [
According to equations (
Finally, proportioning parameters for each coal-rock stratum in the similar material simulation experiment can be derived by solving equations (
According to the physicomechanical properties of each stratum in Table
Proportioning parameters and mechanical parameters of similar materials for each stratum.
Stratum number | Lithology | Density (kg·m−3) | Sand-binder ratio | Elastic modulus (MPa) | Compressive strength (MPa) |
---|---|---|---|---|---|
Sand : cement | |||||
1 | Limestone | 1.656 | 6 : 1 | 178.61 | 0.63 |
2 | Allitic shale | 1.637 | 6 : 1 | 162.31 | 0.63 |
3 | C6 coal seam | 1.513 | 9 : 1 | 10.19 | 0.04 |
4 | Composite medium stratum | 1.551 | 6.5 : 1 | 140.80 | 0.70 |
5 | C4 coal seam | 1.513 | 9 : 1 | 10.09 | 0.04 |
6 | Siltstone | 1.539 | 7 : 1 | 110.81 | 0.54 |
7 | Limestone | 1.656 | 6 : 1 | 178.61 | 0.63 |
The coal-rock strata of on-site engineering are simulated by similar materials including sand, cement (containing gypsum and fly ash), and water mixtures. To ensure proportioning ratios and mechanical parameters for each stratum are satisfied, similar material for each stratum or layer is manufactured using a method of layered filling and compaction [
Similar material model.
The main purposes of this experiment are to study pressure-relief characteristics and deformations within underlying C4 coal seam (protected layer) induced by C6 coal seam (protective layer) mining. Hence, internal pressures and deformations (or strains) within the protected layer are needed to be monitored during excavation. The internal pressures are monitored using ASMD3-16 resistance strain gauge and BX-1 pressure sensors, while the displacements (or strains) are monitored using a digital image correlation method [
Pressure monitoring devices. (a) Pressure sensor. (b) Resistance strain gauge.
The locations in the C4 protected layer, which correspond to upper and lower boundaries of the C6 protective layer excavation region, generally have minimum swelling deformation. Thus, the variations of pressure and displacement nearby these positions are keys for determining protection angle and protection region. Taking such into account, pressure sensors with an interval are embedded in the C4 protected layer to record the pressures, whose embedding locations correspond to the excavation region of the C6 protective layer. In addition, as image coordinates and similar material model coordinates need to be corresponded when the digital image correlation method is used to compute displacement and strain after the C6 protective layer excavation, some grid points (i.e., control reference points) are also arranged on similar model surface [
The data acquisition system of pressure and displacement should be calibrated and reset before excavation. When the maintenance time of similar material model reaches (20–30 days in general), and the similar material strength satisfies the designed requirements, the C6 coal seam is excavated with a length of 700 mm to simulate C6 coal seam mining, which can be shown in Figure
The schematic diagram for relevant parameters in upper protective layer mining is illustrated in Figure
Schematic diagram for relevant parameters in upper protective layer mining.
After the C6 protective layer has been excavated, pressure-relief curves for C4 protected layers with varying interlayer distances are obtained through analysis of monitoring results of pressure sensors, as shown in Figure
Pressure-relief curves for C4 protected layers with varying interlayer distances.
In Figure
As aforementioned, if the pressure-relief value is greater than zero, it indicates that the ground pressure is less than the initial pressure, thus the distance between the two intersection points of pressure-relief curve and horizontal coordinate is the length of pressure-relief region (Figure
Pressure-relief region and offset distance with varying interlayer distances.
Interlayer distance (mm) | Length of pressure-relief region (mm) | Offset distance (mm) |
---|---|---|
250 | 669 | −50 |
450 | 652 | −47 |
650 | 504 | −46 |
The pressure-relief ratio
Maximum pressure-relief value and pressure-relief ratio with varying interlayer distances.
Interlayer distance (mm) | Maximum pressure-relief value (kPa) | Maximum pressure-relief ratio (%) |
---|---|---|
250 | 94.80 | 50.8 |
450 | 68.99 | 37 |
650 | 43.89 | 23.5 |
The pressure concentration coefficient
Pressure concentration coefficient with varying interlayer distances.
Interlayer distance (mm) | Pressure concentration coefficient at descending location | Pressure concentration coefficient at ascending location |
---|---|---|
250 | 1.218 | 1.202 |
650 | 1.046 | 1.080 |
In brief, as the interlayer distance increases, the pressure distribution pattern in the protected layer evolves from “U” to “V”; the pressure concentration coefficient in the protected layer decreases, but the influencing range of pressure concentration increases; the maximum pressure-relief ratio decreases, which are 50.8%, 37%, and 23.5% in the close, remote, and super-remote distance protected layers, respectively; and the length of pressure-relief region decreases, which are 669 mm, 652 mm, and 504 mm in the close, remote, and super-remote distance protected layers, respectively. According to the above results, a large pressure-relief region will develop in the underlying protected layer and lead to a significant increase in coal permeability in Nantong coal mine district [
After the excavation of the C6 protective layer, according to the digital image correlation method [
Swelling strain curves for the C4 protected layer with varying interlayer distances.
In current Provisions [
Schematic diagram for calculation of protection angles.
Protection angles at excavation boundary of C6 protective layers are calculated using equation (
Protection angle at the excavation boundary.
Interlayer distance (mm) | Protection angle at the lower excavation boundary, |
Protection angle at the upper excavation boundary, | ||
---|---|---|---|---|
Present results | Results in Provisions | Present results | Results in Provisions | |
250 | 78 | 80 | 65 | 70 |
450 | 75 | 80 | 71 | 70 |
650 | 78 | 80 | 65 | 70 |
Related studies show that the underlying floor be divided into floor heave-induced fissure zone, floor heave-induced deformation zone, and original floor zone from top to bottom [
Relationship between length of protection region and interlayer distance.
In brief, when mining steeply inclined upper protective layer with varying interlayer distances in Nantong coal mine district, both the protection region and protection angle determined based on the deformation protection criterion are less than the empirical values based on the dip angle in Provisions, denoting that the method proposed in this study is safer than that in Provisions.
According to an engineering practice of Nantong coal mine district, a method for the similar material simulation experiment of steeply inclined upper protective layer mining is proposed, and then it is adopted to investigate the protection effect with varying distances. The obtained conclusions are as follows: The new similar material, which comprises of sand, cement (containing gypsum and fly ash), and water mixture, can simulate various coal-rock lithologies in coal mine. Strong linear relations exist between mechanical parameters and proportioning parameters of similar material. The effects of the sand-binder ratio on compressive strength and elastic modulus are more remarkable than the effect of the density and residual water content; both the compressive strength and elastic modulus of similar materials increase as the density increases and decrease as the residual water content or sand-binder ratio increases; the effect of residual water content on Poisson’s ratio is more remarkable than the effect of density, and sand-binder ratio has little effect on Poisson’s ratio of similar materials. Both pressure-relief curves and swelling strain curves for the protected layer present convex shapes. With the increase of interlayer distance, the pressure-relief curve evolves from pattern “
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (Grant no. 51604111); the Scientific Research Fund of Hunan Provincial Education Department (Grant no. 16C0654); and the Doctoral Scientific Research Foundation of Hunan University of Science and Technology (Grant no. E51502).