Given the increase in mining depth and intensity, tunnel failure as a result of rock burst has become an important issue in the field of mining engineering in China. Based on the Composite Rock-Bolt Bearing Structure, which is formed due to the interaction of the bolts driven into the surrounding rock, this paper analyzes a rock burst prevention mechanism, establishes a mechanical model in burst-prone ground, deduces the strength calculation formula of the Composite Rock-Bolt Bearing Structure in burst-prone ground, and confirms the rock burst prevention criterion of the Composite Rock-Bolt Bearing Structure. According to the rock burst prevention criterion, the amount of the influence on rock burst prevention ability from the surrounding rock parameters and bolt support parameters is discussed.
In China, coal reserves have proven to amount to as much as 5059.2 billion tons, constituting nearly 11.1% of the total reserves worldwide [
Based on the Composite Rock-Bolt Bearing Structure, which is formed due to the interaction of the bolt and surrounding rock [
The shock stress caused by the coal mining activities is a critical factor of tunnel failure in the burst-prone ground. The shock stress propagated from the shock center and the initial rock stress has a superposition. Once the stacked stress is more than the ultimate strength of the surrounding rock, the equilibrium state of the surrounding rock would be lost. As a result, the rock fails instantaneously due to the repeated tensile and compression by the stress wave [
To maintain the tunnel stability, the tunnel’s roof and sides are supported by bolts with some pretightening force. After the installation of multiple bolts with reasonable bolt length and density, the Composite Rock-Bolt Bearing Structure with some strength and deformability is formed due to the interaction of the bolts and the surrounding rock [
Given the activities involved typically with mining, the force of the shock center induces a stress wave, which propagates to the tunnel. Firstly, the stress wave propagates in the intact rock mass and then passes to the Composite Rock-Bolt Bearing Structure. Once the shock stress is more than the strength of the Composite Rock-Bolt Bearing Structure, rock burst would occur in the tunnels. The mechanical model for burst-prone ground control is given in Figure
Mechanical model for burst-prone ground control.
Mechanical model of surrounding rock stability
Mechanical model of Composite Bolt-Rock Bearing Structure
(1) Homogeneous broken rock circles around the tunnel are formed after the tunnel excavation and the Composite Rock-Bolt Bearing Structure is in the broken area [
(2) The rock material follows the Mohr-Coulomb yield criterion under shock stress as shown in the following formula [
(3) The bolt support is intensive and the working resistance distributes on the tunnel surface evenly. The overlying stress distributes on the external surface of the Composite Rock-Bolt Bearing Structure evenly.
(4) The tunnel cross section forms a circle and the surrounding rock is an isotropic homogeneous plane strain model without any creeping and viscosity behavior.
(5) The stress wave can be regarded as having normal incidences and is well-distributed when it propagates to the Composite Rock-Bolt Bearing Structure.
A mechanical analysis was based on half of the Composite Rock-Bolt Bearing Structure as shown in Figure
The conical compression zone around the bolt head can be approximately described as [
The thickness of the Composite Rock-Bolt Bearing Structure is equal to the bolt length minus the thickness of the conical compression zone:
The Composite Rock-Bolt Bearing Structure is under the action of the vertical force, the homogeneous stress, and the support strength. In the horizontal direction, external force of the Composite Rock-Bolt Bearing Structure can achieve self-balancing. Therefore, to achieve an external balancing force, a vertical balancing force is needed.
Bolt support strength can be described as
In order to fully use the bolt’s working resistance, the bolt should be yielding but not to the extent of causing tensile failure. Therefore, the bolt’s working resistance can be described as
It is assumed that surrounding rock is in a limit state under external stress. Based on the Mohr-Coulomb yield criterion, the tunnel surface stress can be described as
The vertical force of the Composite Rock-Bolt Bearing Structure can be described as
A mechanism analysis is developed on an arc block of the tunnel surface:
Similarly, a mechanism analysis is developed on an arc block of the Composite Rock-Bolt Bearing Structure which is shown in Figure
Overlying homogeneous stress distribution of Composite Rock-Bolt Bearing Structure.
In the vertical direction, the static equilibrium equation can be described as
After submitting formulas (
Therefore, the overlying stress on the Composite Rock-Bolt Bearing Structure can be described as
After submitting formulas (
Based on above analysis, the stress of the Composite Rock-Bolt Bearing Structure can be described as
It is assumed that the energy-dampening index of the stress wave propagation in the medium is
The incident strength of the stress wave of the external surface of the Composite Rock-Bolt Bearing Structure can be described as
Therefore, the external surface stress of the Composite Rock-Bolt Bearing Structure is described as
When
Hence, the rock burst prevention criterion of the Composite Rock-Bolt Bearing Structure can be described as
It may be indicated by this study and demonstrated by formula (
After the shock center parameters are concerned then the rock burst prevention ability of the Composite Rock-Bolt Bearing Structure depends on the surrounding rock parameters (cohesion and internal friction angle) and the bolt support parameters (bolt length, interval, space, and diameters). The authors of this paper chose only one parameter to investigate the relationships among the
Calculation parameters.
|
2.0 |
|
2.4 |
|
9 |
|
20 |
|
400 |
|
0.5 |
|
30 |
|
1 |
|
500 |
|
100 |
|
1.5 |
After submitting formulas (
It is clear from formula ( there is a negative linear correlation between the there is a negative linear correlation between the there is a negative linear correlation between the when the tunnel radius is increased from 1.4 m to 2.6 m, the relationship between the when the tunnel section bolt numbers are increased from 5 to 12 and the bolt interval is equal to the bolt space that increased from 0.57 m to 1.2 6 m, the relationship between the when bolt length is increased from 1.6 m to 2.8 m, the relationship between the
Relationship between
Relationship between
Relationship between
The Composite Rock-Bolt Bearing Structure with some strength and deformability is formed due to the interaction of the bolt and the surrounding rock and has certain abilities to prevent rock burst in the tunnel. Based on a circular tunnel, the mechanical model for burst-prone ground control is established and the strength and the rock burst prevention criterion of the Composite Rock-Bolt Bearing Structure are obtained. Based on the rock burst prevention criterion of the Composite Rock-Bolt Bearing Structure, the influence degree on the rock burst prevention ability from the surrounding rock parameters and the bolt support parameters is discussed. Rock burst occurs more easily in a large section tunnel; however, the increasing bolt support density, length, and diameter can enhance the rock burst prevention ability of Composite Rock-Bolt Bearing Structure.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This paper is supported by “Natural Science Foundation of Jiangsu Province, China” (Grant no. BK20130189), and “Priority Academic Program Development of Jiangsu Higher Education Institutions,” funded by “Open Projects of State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM12X05).”