3.1 Rheological test results and analysis of coal rock containing gas under different confining pressures
The rheological tests of coal and rock containing gas under different confining pressures were carried out on three groups of specimens A-1～A-3, and the failure load and strength limit of specimens under different confining pressures were obtained. The relevant data are shown in Table.1.
Table 1
Rheological test results and analysis of coal rock containing gas under different confining pressures
|
Specimen number
|
Diameter /mm
|
Height /mm
|
Confining pressure /MPa
|
Failure load /kN
|
Strength limit /MPa
|
|
A-1-1
|
50.02
|
100.03
|
0
|
30.14
|
15.06
|
|
A-1-2
|
50.06
|
99.97
|
0
|
29.72
|
15.1
|
|
A-1-3
|
49.99
|
100.1
|
0
|
30.01
|
15.29
|
|
A-2-1
|
50.08
|
100.1
|
2.5
|
40.81
|
20.72
|
|
A-2-2
|
50.02
|
100.2
|
2.5
|
41.05
|
20.89
|
|
A-2-3
|
50.05
|
99.98
|
2.5
|
40.61
|
20.64
|
|
A-3-1
|
49.96
|
100.5
|
5
|
49.24
|
25.12
|
|
A-3-2
|
50.04
|
100.06
|
5
|
49.60
|
25.22
|
|
A-3-3
|
50.03
|
100.12
|
5
|
49.26
|
25.06
|
It can be seen from Table.1 that when the confining pressure is fixed, the test results of each group of coal and rock specimens are basically the same. Therefore, the test data of specimens A-1-2, A-2-1 and A-3-2 are selected as the reference data for subsequent tests. The full-range graded loading rheological curves of these three coal and rock specimens are shown in Figs.7–9.
It can be seen from the analysis in Fig.7 that in the whole loading process, the longitudinal deformation of the coal-rock specimen is about 32mm, and the transverse deformation is about 22mm. The longitudinal deformation of the coal-rock specimen is significantly different from the transverse deformation. From the point of view of strain under all levels of load, in the early rheological stage, when the first stage load (about 3MPa) is applied, there is a small deformation stage similar to the straight line. With the increase of time, the deformation of the specimen is also increasing, and finally tends to be stable. The longitudinal deformation of coal and rock under the first stage load is about 4.3mm, and the transverse deformation is about 2.6mm. When the second load (about 6MPa) is applied, it is obvious that the curve will show the characteristics of leap change when the load is just applied, and then the strain will tend to be stable with the increase of time. The longitudinal deformation of coal and rock under this first load is about 2.5mm, and the transverse deformation is about 2.7mm; The deformation of coal and rock under the third stage load (about 9MPa) is similar to that under the second stage load. The longitudinal deformation of coal and rock under this stage load is about 2.8mm, and the transverse deformation is about 2.3 mm. When applied to the fourth load (about 12MPa), the rheological curve has a trend of elevation, the longitudinal deformation of the specimen under the first load is about 7mm, the transverse deformation is about 5mm, the deformation is significantly higher than that under the second and third load levels. When applied to the fifth load, the deformation of coal rock specimen increases sharply until the specimen is destroyed. For the above changes, the author makes the following explanation: in the initial creep stage, the coal-rock specimen is in the range of elastic deformation, and the deformation increases linearly. Then, with the increase of time, the coal-rock specimen gradually enters the uniform creep stage from the initial creep stage, and the coal-rock specimen is mainly plastic deformation in this stage. Therefore, with the increase of time, the deformation tends to be flat. Due to the sudden increase of external load on the coal-rock specimen in the uniform creep stage, the deformation will suddenly increase, showing a sudden leap in the curve. However, since it does not exceed the plastic limit of the coal-rock specimen itself, the deformation will become stable over time. When the fourth stage load is applied, the applied load is more and more close to the plastic limit of the coal rock specimen itself, so the deformation will have a significant upward trend, but there is still no big change in the curve. When the fifth-order load is applied, the load has exceeded the plastic limit of the coal rock itself. The deformation of the coal rock specimen increases rapidly in a short period of time until it is destroyed. The curve shows an obvious acceleration increase. This stage is the accelerated creep stage.
Comparing Fig.7 to Fig.9, it can be found that the variation characteristics of coal and rock under different confining pressures have the above variation characteristics, but the final deformation of each specimen under different confining pressures is obviously different : the final longitudinal deformation of coal and rock specimen under confining pressure 0MPa is about 32mm, and the transverse deformation is about 21mm; The final longitudinal deformation of coal-rock specimen under confining pressure of 2.5MPa is about 27mm, and the lateral deformation is about 15mm. The final longitudinal deformation of coal-rock specimen under confining pressure of 5MPa is about 22mm, and the lateral deformation is about 12mm. From the above changes, it can be concluded that with the increase of confining pressure, the total deformation of coal rock specimens will be reduced. For this change, the author believes that when the axial compression is applied to the specimen, due to the existence of confining pressure, the internal pores of coal rock specimens will be gradually compressed, the gas between pores will be gradually reduced, and the pore pressure will be reduced. According to the principle of effective stress:
σ' = σ - μ
σ' — Effective stress
σ — Total stress
μ — Pore pressure
It can be seen that when the total stress (σ) is constant, with the increase of confining pressure, the internal pores of the coal-rock specimen are gradually compacted, and the gas in the internal free state is gradually reduced. Since the pore pressure is mainly generated by the free gas, the pore pressure (μ) is gradually reduced, and the effective stress (σ') in the specimen is gradually increased. The bearing capacity of the coal-rock specimen will gradually increase, and the deformation ability to resist the external load will naturally increase, showing that the deformation of the coal rock decreases with the increase of confining pressure.
3.2 Test results and analysis of gas - bearing coal rock under step loading
The above analysis shows that the longitudinal deformation is more obvious than the transverse deformation, so the following analysis adopts longitudinal deformation. Table 2 is the loading deformation of specimen A-3-2 under confining pressure 5MPa. Fig.10 is the longitudinal deformation of coal rock under confining pressure 5MPa.
Table 2
The deformation of specimen A-3-2 under different levels of confining pressure 5MPa
|
Load grade /MPa
|
Longitudinal initial strain
|
Longitudinal end strain
|
Amount of deformation /mm
|
|
5
|
0
|
0.023
|
2.3
|
|
10
|
0.044
|
0.048
|
0.4
|
|
15
|
0.064
|
0.071
|
0.7
|
|
20
|
0.08
|
0.11
|
3
|
|
25
|
0.13
|
0.22
|
9
|
Combined with Fig.10 and table 2, it is obvious that under the confining pressure of 5MPa, the coal rock under the axial compression of 5MPa, 10MPa, 15MPa and 20MPa is in the uniform creep stage for most of the time. Under the axial compression of 5MPa, the coal rock has entered the uniform creep stage from the initial creep stage. Under the axial compression of 10MPa and 15MPa, the coal rock is in the early and middle stage of uniform creep stage, the deformation of the specimen is not obvious, the total deformation is only1.1mm. When the axial pressure is loaded to 20MPa, it is temporarily called that the coal rock is in the late stage of uniform creep. The deformation of coal rock during this period is more obvious than that of the early and middle stages of uniform creep. The deformation is 3mm. The rheological curve has a tendency to rise with time, but the deformation rate is still relatively flat. When the axial pressure was loaded to 25MPa, the coal rock entered the accelerated creep stage. During this period, the deformation rate of the specimen increased, and the specimen was quickly destroyed. The rheological curve was significantly increased, and the deformation reached 9mm during this period, which was significantly greater than that in the later stage of uniform creep.
From the above analysis, it can be concluded that the deformation of coal rock specimens in the late stage of uniform creep has a significant upward trend, and the strain is significantly higher than that in the early and middle stages of uniform creep, but the total deformation remains unchanged. The deformation of coal rock in the accelerated creep stage is significantly higher than that in the later stage of uniform creep, and the deformation is significantly increased. In this regard, we can regard the deformation of coal and rock in the late stage of uniform creep as a strain threshold. When it is lower than this threshold, coal and rock will not undergo large deformation, and once it is higher than this threshold, coal and rock will undergo obvious deformation and damage quickly. The vicinity of this threshold is called the strength limit neighborhood of gas-bearing coal rock.
3.3 Experimental results and analysis of rheological disturbance effect of gas-bearing coal rock under impact disturbance
Fig.11 shows the stress-strain curves of coal and rock containing gas under different confining pressures and the same impact disturbance (impact weight M=1kg, impact height h=10cm ). It can be seen that with the increase of stress, the strain growth gradually bends downward, and then tends to be stable, and the three lines gradually change from dense to sparse, but the three curves do not have a large span in the whole process, which indicates that although the rheological state of coal and rock containing gas under the impact disturbance is affected by the confining pressure, the change of confining pressure does not have a great influence on the deformation of coal and rock. Fig.12 shows the rheological curve of gas-bearing coal rock at the late stage of uniform creep stage. It is obvious that the strain increases with time from the trend of the curve. The author believes that the coal rock specimen enters the strength limit neighborhood described in 2.2 in this period. Once the coal rock enters the strength limit neighborhood, it will soon enter the accelerated creep stage and change greatly until the specimen is destroyed. The deformation difference of # three curves is about 1 mm, indicating that confining pressure has an impact on the rheology of gas-bearing coal rock, but the impact is not large.
Fig.13 is the stress-strain curves of gas-bearing coal and rock obtained by different impact disturbance under confining pressure 5MPa. It can be seen that the three curves also gradually move from dense to sparse, but compared with the rheological curves of gas-bearing coal and rock under the same confining pressure, the span of the three curves is larger. When the same deformation occurs, the greater the impact disturbance, the smaller the required stress. At the same stress level, the greater the impact disturbance, the greater the deformation. This shows that the impact disturbance has a great influence on the rheology of coal and rock containing gas, and the greater the disturbance amplitude is, the greater the influence on the rheology of coal and rock containing gas is. The rheological curves of gas-bearing coal and rock in the early and late stage of uniform creep stage are given in Fig.14 and Fig.15 respectively. It can be found that under axial compression of 10MPa, the deformation trend of the three curves in the early stage of uniform creep stage is basically synchronous. Under the action of impact disturbance, the strain of the three curves increases, but the strain rate decreases gradually. At this time, the coal and rock has not entered the neighborhood of strength limit, and the impact disturbance has little effect on the deformation of coal and rock. However, when the axial pressure is 25MPa, the coal rock enters into the late stage of uniform creep stage and enters into the neighborhood of the strength limit. The two coal rock specimens subjected to large impact disturbance have entered into the neighborhood of the strength limit in advance and have been damaged, and the strain reaches the maximum and remains near the deformation of the damage. Therefore, there are two flat linear characteristics on the curve. The coal rock specimens subjected to small impact disturbance are in the neighborhood of the strength limit, and the strain has been accelerating, showing a concave feature on the curve. In addition, the deformation gap of the three coal rock specimens is about0.4mm under the axial compression of10MPa, and the deformation gap of the coal rock specimens is about8mm under the axial compression of25MPa. It can be seen that the impact disturbance on the coal rock containing gas in the strength limit neighborhood is far greater than the influence outside the strength limit neighborhood.
For the above experimental results, the author believes that at the initial stage of loading, under the combined action of external load and gas, the internal gas and coal rock are in a stable solid-gas equilibrium state. Due to the adsorption and free state of gas in coal rock, the gas in the free state generates pore pressure on the surface of coal rock fracture. At this time, the effective stress in coal rock is small. When the external impact disturbance is applied, the coal rock skeleton, gas adsorption force and pore pressure jointly resist the external impact load, and the resistance is strong. Therefore, the deformation of coal rock does not change greatly. At this time, the coal rock has not entered the strength limit neighborhood described in 2.2. When loading to 25MPa, the effective stress inside the coal rock increases, and the pore pressure no longer resists deformation, but accelerates the crack propagation inside the coal rock. At the same time, due to the increase of external load, the pores inside the coal rock are gradually compressed, and the adsorbed gas is gradually reduced. At this time, only the coal rock skeleton and a small part of the adsorbed gas resist deformation, and the coal rock skeleton, the adsorbed gas and the free gas are in a very unstable fragile equilibrium state. When the impact disturbance is applied to it, the impact disturbance will accelerate the expansion of the internal cracks in the coal rock and produce a large number of new cracks in a short time. At the same time, the unstable fragile equilibrium state is broken instantly. The coal rock specimen reaches the yield limit in a short time and is destroyed. At this time, the coal rock specimen has entered the strength limit neighborhood described in 2.2. At the same time, it can be seen from the experimental results obtained in Fig.15 that the larger the external impact disturbance is, the faster the propagation speed of internal cracks in coal rock will be, and the time required for failure time will be reduced.