Minin-induced water inrush from a confined aquifer due to subsided floor karst collapse column (SKCC) is a type of serious disaster in the underground coal extraction. Karst collapse column (KCC) developed in a confined aquifer occurs widely throughout northern China. A water inrush disaster from SKCC occurred in Taoyuan coal mine on February 3, 2013. In order to analyze the effect of the KCC influence zone’s (KCCIZ) width and the entry driving distance of the water inrush through the fractured channels of the SKCC, the stress, seepage, and impact dynamics coupling equations were used to model the seepage rule, and a numerical FLAC3D model was created to determine the plastic zones, the vertical displacement development of the rock mass surrounding the entry driving working face (EDWF), and the seepage vector and water inflow development of the seepage field. The hysteretic mechanism of water inrush due to SKCC in Taoyuan coal mine was investigated. The results indicate that a water inrush disaster will occur when the width of the KCCIZ exceeds 16 m under a driving, which leads to the aquifer connecting with the fractured zones of the entry floor. Hysteretic water inrush disasters are related to the stress release rate of the surrounding rocks under the entry driving. When the entry driving exceeds about 10 m from the water inrush point, the stress release rate reaches about 100%, and a water inrush disaster occurs.
Water inrush in coal mines has resulted in millions of fatalities in China. With the development of new technology, the level of coal production and construction has significantly improved [
Karst collapse column (KCC) is a special geological structure that is widely distributed in northern China. It is composed of fillings such as broken rocks and fine argillaceous or clay particles, and it causes the surrounding rock mass to settle and develop a large number of fissures [
Previous studies have provided strong theoretical support for the prevention and control of KCC water inrush disasters and have obtained significant engineering benefit. In the KCC water inrush in Taoyuan coal mine, the entire mine was flooded, but there was only one casualty. However, in the period of karst cavity formation and closed geological structure evolution, the ground stress and rock stratum of the KCC are destroyed and affected. Generally, the cavity of a KCC is broken, stress is released, and cracks appear around the cavity and in the roof rock, forming a KCC influence zone (KCCIZ) around the KCC (
In order to analyze the KCC water inrush in Taoyuan coal mine, based on research into the mining method and the KCC water inrush mechanism, a numerical model that considers the dynamic changes in the strain softening and rock mass permeability coefficient was established. FLAC
Taoyuan coal mine is located in Suzhou, Anhui Province, China, and working face 1035 is located in the third stage of the south wing of the south three mining area. The main coal seam is coal seam #10, which is 2.9 m thick and has an average burial depth of about 500 m. The SKCC water inrush disaster occurred in the entry driving working face (EDWF) of working face 1035. The EDWF was driven along the roof of coal seam #10, and explosion driving and bolt-mesh support were both employed. The cross-section is rectangular, with a width and height of 3.8 m and 2.8 m, respectively (
Strata | Thickness (m) | Volumetric weight ( |
Young’s elastic modulus (GPa) | Poisson’s ratio | Cohesion (MPa) | Friction angle ( |
Tensile strength (MPa) | Permeability coefficient ( |
Porosity |
---|---|---|---|---|---|---|---|---|---|
Siltstone interbedded with mudstone | 15 | 26.2 | 6.0 | 0.2 | 5.1 | 36 | 4.8 | 0.19 | |
Middle-fine sandstone | 20 | 26.0 | 5.8 | 0.23 | 4.8 | 38 | 4.6 | 0.20 | |
Coal | 2.9 | 14.3 | 1.8 | 0.35 | 2.0 | 32 | 0.8 | 0.28 | |
Mudstone | 4.0 | 24.1 | 3.0 | 0.3 | 2.5 | 30 | 1.0 | 0.22 | |
Siltstone interbedded with fine sandstone | 52.5 | 25.3 | 4.2 | 0.24 | 3.6 | 35 | 1.8 | 0.24 | |
Limestone | 135 | 24.9 | 5.0 | 0.28 | 4.0 | 36 | 2.8 | 0.32 |
On February 2, 2013, a water inrush accident occurred at EDWF 1035 in Taoyuan coal mine, which was accompanied by floor heave and increasing water inflow (
Data | Time | Water inflow (m3/h) | Character description | |
---|---|---|---|---|
2013-02-02 | 14:10 | 10 | Water coming out of the floor of EDWF 1035 | |
15:30 | 30 | Floor heaving occurred at 8–12 m in the floor of EDWF 1035 | ||
16:30 | 40 | / | ||
17:30 | 60 | / | ||
18:30 | 70 | / | ||
18:55 | 150–200 | / | ||
20:05 | 200 | The sump was half filled | ||
22:00 | 200 | The sump was full, and the head entry next to the EDWF began to accumulate water | ||
2013-02-03 | 00:20–06:10 | 29000 | Banging was heard in the EDWF, and the water level began to increase rapidly |
The water plugging drilling confirmed that the main water inrush channel was an SKCC (oval, long axis is 70 m, and short axis is 50 m), which was 20 m below the coal seam #10, and the water inrush point was located between the head roadway and the EDWF. There were two hydrology observation holes near the water inrush point, namely, the Ordovician limestone aquifer observation borehole (borehole #1) and the Permian limestone aquifer observation borehole (borehole #2), located at about 4 km and 1 km to the north of the water inrush point, respectively. After the disaster occurred, the water level in borehole #1 decreased by 58.12 m and that in borehole #2 only decreased by 16.5 m, which indicates that the main water inrush source was the Ordovician limestone water aquifer.
According to previous research results [
where
The distance between the coal seam floor and the Ordovician limestone roof was 195 m, and there was almost no threat of water inrush damage without the influence of the faulted geological structure. However, the SKCC linking the Ordovician limestone aquifer was about 20 m below the coal seam floor, and the pore water pressure in the SKCC was about 4.3 MPa, which greatly increased the water inrush damage threat of the Ordovician limestone aquifer. However, in the process of a SKCC water inrush disaster, there must be a seepage channel between the SKCC and the coal seam floor, which may be composed of a floor failure zone and a KCCIZ (
Therefore, the water inrush mechanism in the “2.3” Taoyuan coal mine disaster was as follows. As the driving of the EDWF, the rock mass behind the EDWF was continuously unloaded under high stress, generating a large number of mining-induced fractures. When these new fractures connected with the seepage fissure channels in the KCCIZ, the high-pressure water in the Ordovician limestone aquifer lead to water inrush through the SKCC, the KCCIZ, and the mining-induced fractures in turn. At the same time, during the water inrush, the water inrush channels were continuously widened under the action of hydraulic erosion, which further intensified the water inrush, resulting in well flooding (
Considering that the rock mass around the entry will yield after driving, which decreases the bearing capacity, a three-line strain softening model was adopted to simulate the relationship between the stress and strain of the surrounding rock (
Strata | Volumetric weight ( |
Young’s elastic modulus (GPa) | Poisson’s ratio | Cohesion (MPa) | Friction angle ( |
Tensile strength (MPa) | Permeability coefficient ( |
Porosity |
---|---|---|---|---|---|---|---|---|
KCCIZ | 21.3 | 6.0 | 0.28 | 1.08 | 35 | 0.54 | 0.32 | |
SKCC | 20.0 | 1.12 | 0.32 | 0.6 | 30 | 0.2 | 0.35 |
After strain softening occurs in the rock, the rock will be damaged by the development of fine fractures. The rock damage model based on the strain softening and residual strength modification is as follows:
where
In the process of rock mass strain softening, permeability is correlated with the damage evolution trend of the rock mass. The relationship between the damage factor
where
There is a power function relationship between the porosity and the permeability of the rock mass [
where
In FLAC
The peak pressure of the shock wave on the rock surrounding the borehole after the explosion of a single-borehole cylindrical explosive cartridge is [
where
When multiple boreholes are blasted simultaneously, the peak stress at a certain point in the surrounding rock is [
where
If the action time of the blast stress shock wave is taken into account, the shock wave pressure at the calculated point at different explosion action times can be expressed by the following equation [
where
The EDWF of working face 1035 was driven by explosion using coal mine allowable ammonium nitrate #2 explosives.
When the “2.3” water inrush disaster occurred, the KCCIZ under the coal floor was seriously damaged, so its width was difficult to verify. In order to reproduce the process of the SKCC water inrush disaster, a key cross section (cross section A–A in
Initial balance
First, the water inrush model of the entry was established. Then, the seepage mode was closed (set flow to off), the mechanical parameters were applied using the parameters in
Explosion driving simulation
As the explosion driving was a short-term dynamic behavior, the seepage mode was closed and the dynamic mode was turned on before the explosion pressure was applied. The pressure-time relationship shown in
where
Add support and write fish language
After the explosion driving simulation, the rock mass surrounding the entry was supported using the methods shown in
Water inrush simulation
In order to analyze the effect of the width of the KCCIZ on the SKCC water inrush in Taoyuan coal mine, we conducted a total of six sets of different numerical simulation calculations using KCCIZ widths of 0, 5, 10, 15, 16, and 18 m.
The SKCC water inrush in Taoyuan mine occurred after the EDWF passed the water inrush point of 12 m, which indicates that the SKCC water inrush was a hysteretic disaster. Hysteresis was an important characteristic of the SKCC or KCC water inrush, which occurred in the Fangezhuang SKCC water inrush, the Zhangji SKCC water inrush, and the Luotuoshan SKCC water inrush [
The stress release rate
where
It can be seen from the figure that (1) when f was less than or equal to 80% (i.e.,
In this paper, the water inrush disaster from SKCC in Taoyuan mine was studied, and the hysteretic mechanism of the water inrush was discussed using the method of coupling the stress, displacement, and impact dynamics. The following conclusions were drawn from the results of this study.
A multi-field coupling calculation method was established to simulate water inrush from the SKCC under entry driving, and the water inrush process in Taoyuan coal mine was simulated.
The influence of the width of the SCCIZ on the water inrush disaster was analyzed, and it was determined that a water inrush disaster will occur only when the width exceeds 16 m.
The water inrush from SKCC in Taoyuan coal mine had hysteresis, which was related to the stress release rate of the surrounding rock mass under entry driving. When the entry driving distance from the water inrush point exceeded about 10 m, the stress release rate reached about 100%, and a water inrush disaster occurred.