The karst collapse pillar (KCP) is a common geological structure in the coal mines of northern China. KCPs contain many fractured coal rocks, which can easily migrate under the action of high-pressure water. The destruction or instability of the cementation structure between the rocks can directly induce coalmine water-inrush accidents. To study the seepage stability of cemented and fractured coal rock under triaxial pressures, a self-designed triaxial seepage testing system was used and the permeability
The karst collapse pillar (KCP) is a common geological phenomenon in coal mining. It is a special geological structure formed by sedimentation and cementation of fractured coal rock [
In recent years, research on KCP water inrush focused mainly on the formation cause, evolution mechanism, the KCP mechanical model, and seepage characteristics. Extensive statistical data indicate that KCP water inrush is caused by karst sedimentation in the carbonate rock distribution area [
The evolution of the KPC instability water-inrush system is shown in Figure
Illustration of KCP instability water-inrush system evolution.
In order to study the seepage stability of the cemented structure between the fractured coal and rock, a series of cemented samples were prepared to test the seepage stability of the structure. The sample preparation process was as follows: First, to produce the specimens, fractured coal rock with different particle sizes was screened, and four particle sizes were selected for gradation. The particle sizes were 0–2.5 mm, 2.5–5.0 mm, 5.0–7.5 mm, and 7.5–10 mm. According to the rock test requirements [
The mass ratio of the sample.
Particle sizes (mm) | Talbot index | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | |
0∼2.5 | 87.1 | 75.8 | 66.0 | 57.4 | 50.0 | 43.5 | 37.9 | 33.0 | 28.7 | 25.0 |
2.5∼5 | 6.2 | 11.3 | 15.3 | 18.4 | 20.7 | 22.5 | 23.7 | 24.4 | 24.9 | 25.0 |
5∼7.5 | 3.9 | 7.4 | 10.5 | 13.3 | 15.9 | 18.2 | 20.2 | 20.0 | 23.6 | 25.0 |
7.5∼10 | 2.8 | 5.5 | 8.2 | 10.9 | 13.4 | 15.8 | 18.2 | 22.6 | 22.8 | 25.0 |
Finally, applying the initial pressure of 2 kN and maintaining, each sample of
Cemented samples made at different values of
From the perspective of rock mechanics, the KCP belongs to a special geological structure, which is mainly due to the dissolution of underground high-pressure water. The fractured coal rock is dissolved in the long term and then cemented. The stability of the KCP depends on the integrity of the cemented structure. In order to further understand the seepage failure mechanism of the cemented structure in the KCP, a set of cemented coal-rock seepage test device was independently developed to carry out the triaxial seepage test on the cemented and fractured coal rock. The main structure of the device includes DDL600 electronic universal testing machine, three-axis percolation test cylinder, lateral pressure pump, and seepage pressure pump, with a computer, data acquisition device, voltage regulator, and so on. The three-axis seepage system of cemented coal rock is shown in Figure
Triaxial seepage test system for cemented fractured coal rock.
Considering the influence of the gradation structure on the seepage characteristics of the fractured coal-rock mass, the sample aggregate prepared in this paper includes four size ranges, and the distribution in the particle size range of each level obeys the Talbot theory [
Structural coal-rock specimens of four different grades were subjected to seepage tests under gradual loading by applying five-stage loading (1, 5, 10, 20, and 30 kN) in the axial direction. The specimens were tested at five levels of seepage pressure (0.5, 1.0, 1.5, 2.0, and 2.5 MPa) under each axial load (Figure
Schematic diagram of seepage pressure loading.
In order to ensure the reliability of the test data during the test, the percolation test under the same test conditions was repeated 3 times, and the average value was taken as the final test result. The specific test procedures mainly include (1) assembling the coal-rock three-axis seepage test system and measuring the initial height
In the triaxial seepage test, the schematic diagram of loading at each stage is shown in Figure
Schematic diagram of different stages of triaxial seepage.
Each group of coal samples with different cracks is set to 5 levels of lateral pressure, and each stage of lateral pressure is set to 4 levels of seepage pressure. The lateral pressure is selected from the range of 0.5–2.0 MPa. In the test, it is usually ensured that the lateral pressure is 0.2–0.5 MPa larger than the pore pressure [
Coal sample test parameters.
Sample | M-1 | M-2 | M-3 | M-4 | M-5 | M-6 | M-7 | M-8 | M-9 | M-10 | |
---|---|---|---|---|---|---|---|---|---|---|---|
Talbot | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | |
51.2 | 49.3 | 51.8 | 49.8 | 52.0 | 52.1 | 51.9 | 51.2 | 50.9 | 50.3 | ||
2.043 | 1.086 | 1.042 | 1.069 | 2.951 | 3.113 | 3.447 | 2.694 | 3.662 | 2.017 | ||
Critical value | −15.0 | −14.9 | −13.1 | −13.4 | −15.1 | −15.3 | −14.1 | −13.2 | −13.8 | −15.4 |
Effective stress is the fundamental factor leading to particle deformation of fractured coal rock. The principle is to simplify the macroscopic three-dimensional force of coal rock, which is equivalent to the effective stress of coal-rock matrix, so as to simplify the complex stress conditions. Therefore, the effective stress can better reflect the stress of the cemented and fractured coal rock under the triaxial stress. The effective stress reflects the stress of the cemented fractured coal rock under triaxial stress. According to the effective stress principle of Terzaghi [
The effective stress in the test is described by the mean effective stress:
When
The relationship between effective stress and permeability is shown in Figure
Relationship between effective stress and permeability of samples. (a)
The results can be explained as follows: when the lateral pressure is constant, the permeability of the fractured coal rock will decrease with increasing axial pressure. At this stage,
For porous media, pore channels refer to microchannels that penetrate each other in porous media and are also the main parameters for fluid transport properties of porous media [
When liquid seeps through the rock sample,
The distribution of
Distribution of
A large number of experiments have shown that water-inrush or coal and gas outburst accidents caused by seepage loss in mining projects are often caused by sudden changes in the seepage parameters. Therefore, a dynamic method should be used to study the non-Darcy seepage differential dynamic system of fractured coal rock [
For an incompressible fluid and ignoring the acceleration
The first and second formulas of (
We define the variable
Then, (
Among them, the traveling wave method is an important way to solve this kind of nonlinear wave equation. Zakharov equation [
Substituting (
The stability criterion is then
When
When
When
So, the dynamics of the system depend on the initial conditions; the stability conditions of the non-Darcy seepage differential dynamic system are as follows:
If
If
In this test, the skeleton structure is cemented and fractured coal rock, and seepage stability was tested under triaxial pressure. The boundary conditions of the equation are
Then,
Thus, the system instability conditions with the permeability, hydrodynamics, and pressure boundary conditions are obtained:
Here,
DTE22 hydraulic oil was used as the permeate in the tests, with a density of
Taking the logarithm on both sides of (
Similarly, using the above calculation method, the threshold value of the seepage loss for
The seepage states of stability of fractured coal. (a)
For the stability analysis in Figure
The instability curve of the test is
At this point, the threshold of instability is
The analysis results can be explained as follows: (1) In cemented fractured coal rock with different grading structures, when there is a high content of small particles or the rock is finely fractured, blockage of the seepage channel is likely to cause the seepage state to change. (2) Under external loading, the pressure increases and damage to the skeletal structure can lead to the collapse of the pore channels, resulting in a sudden change in gas permeability, which ultimately leads to a stable runoff.
Symbol comment list.
Symbol | Physical meaning and interpretation | Symbol | Physical meaning and interpretation |
---|---|---|---|
Density and initial density, kg/m3 | New functions defined, | ||
Porosity and initial porosity, % | Representation function and its various derivatives | ||
Comprehensive compression factor | New variables defined, | ||
Acceleration coefficient | Permeability and initial permeability, m2 | ||
Seepage pressure, MPa | Non-Darcy factor and initial factor, m−1 | ||
Time, s | Facial strength, | ||
Seepage velocity, m/s | Integral constant | ||
Seepage displacement, m | Constant | ||
Dynamic viscosity, Pa·s | Height, m |
To study the evolution mechanism of KCP-induced mine water-inrush events, a triaxial seepage test of fractured coal rock with different grading structures was carried out. The important factors affecting the permeability of fractured coal rock were obtained, and the formulation for estimating the seepage flow was developed. Two criteria were found for instability assessment. The following conclusions were drawn: The effective stress is a key factor in determining the permeability of fractured coal rock. The permeability of coal with different grades of rock will decrease exponentially as the effective stress increases. When the effective stress is less than 2.05 MPa, the skeleton structure of the coal rock is in the rapid compaction stage which increases the pressure in the skeleton structure; then, the skeleton pressure damage enters the structural readjustment stage. The change in the order of magnitude of the non-Darcy flow factor The value of the seepage parameter
The data used to support the findings of this study have not been made available because of confidentiality reasons.
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 under Grant nos. 51774234 (Study on the Mechanism of Water-Gas Coupling Fracture Expansion and Ultrasonic Characteristics of Coal Rock in Drilling Holes), 51874234 (Study on Mechanism and Parameter Optimization of Carbon Dioxide Deep Hole Pre-Cracking Blasting), and 51604214.