The preevaluation to the vulnerability of the surrounding rocks is proposed as one of the reliable indicators of the safety coefficient of gateway support and a foundation to optimize the support design parameters. In this study, taking the surrounding rocks, stress, geological environment, and service time into consideration, the safety coefficient is determined based on the vulnerability scores calculated by the vulnerability preevaluation model of the surrounding rock. Applying the safety coefficient to the instability evaluation of the composite rock-bolt bearing structure, the strength required to maintain the stability of the gateway is calculated, which further provides references and guidance to the optimization of the anchor support parameters. This method has been successfully adopted by the GuCheng coal mining project in N1303 tailgate to strengthen the anchor-bolt structure in the roof watering area especially the main inclined shaft. Applying more accurately calculated strength to the anchor-bolt structure can effectively avoid the issue of overcompensation, thus reducing the cost and increasing the driving speed. Furthermore, this method provides insights into optimizing design parameters of the gateway. This method provides a reliable basis for the optimization design of bolt support parameters in coal mine gateway.
Due to the influence of complex natural stress field in the surrounding rock, deep mining will face more intricate engineering rock mechanics problems than shallow mining [
As a key in bolt support of coal mine gateway, reasonably designed support parameters can maximize the advantages of bolt support and achieve the safety of gateway. Due to the complex underground geological environment, a safety coefficient is often included in the anchor design to ensure the safety construction. However, the safety coefficient is usually identified based on the empirical value. This approach is featured with randomness [
Generally speaking, the determination of the safety coefficient of the gateway needs to refer to the damage degree of the surrounding rock of the gateway, which can be expressed by the degree of vulnerability [
In this paper, the preevaluation to the vulnerability of the surrounding rocks is proposed as one of the reliable indicators of the safety coefficient of gateway support and a foundation to optimize the support design parameters. Taking the surrounding rocks, stress, geological environment, and service time into consideration, a vulnerability preevaluation model is established through the analytic hierarchy process method. Following the theory of composite rock-bolt bearing structure, the bolt support parameters are identified through the quantitative calculation. The proposed approach has been applied to the gateways with both major deformations and small deformations, proving to be effective in reducing overly estimated bolt support strength.
In order to maintain the stability of the surrounding rock stable, preloaded bolts are installed to reinforce the roof and two ribs of gateway. The preloaded bolts of proper dimensions and materials, together with the supporting accessories, form a bearing structure to the gateway surrounding rock, providing some strength and bearing capability. The geometric form of composite rock-bolt bearing structure is shown in Figures
Geometric form of composite rock-bolt bearing structure: (a) arch; (b) rectangle.
The strength of composite rock-bolt bearing structure can be calculated by [
If the gateway shape is arch,
If the gateway shape is arch,
The gateway strata weight is mainly borne by the deep surrounding rock, while the formed composite rock-bolt bearing structure mainly bears the weight of the loose strata of the gateway. According to the field damage conditions of the underground gateway and related research, when the external load exceeds the strength of the composite rock-bolt bearing structure, serious roof falling and excessive deformation will be observed in the gateway. Therefore, in order to ensure the stability of the composite rock-bolt bearing structure during the service life of the gateway, calculating the weight of the potential loose overlying rocks and determining the external load borne by the composite rock-bolt bearing structure become crucial. According to the relevant researches, the thickness
Following the method mentioned above, the evenly distributed load on the composite rock-bolt bearing structure is calculated, and its strength is tested. When the upper evenly distributed load is less than the strength of the composite rock-bolt bearing structure, the composite rock-bolt bearing structure is stable with limited displacement. Under the event that the upper evenly distributed load is greater than the load limit of the composite rock-bolt bearing structure and a low shrinkage of the bearing structure, the bearing structure tends to fail, resulting in the gateway roof collapse. On the other hand, a bearing structure with a high shrinkage rate can cause dramatic deformation and eventually lead to the gateway failure. Considering a reasonable safety coefficient, the instability evaluation of composite rock-bolt bearing structure should be based on
When
According to the analysis on the factors that affect the vulnerability of the gateway surrounding rocks, the indicators are selected to evaluate the vulnerability of the gateway surrounding rocks. Based on the Analytic Hierarchy Process (AHP) [
The preevaluation indexes of the surrounding rock vulnerability.
The scores and evaluation level are listed below: Scores for the characteristics of the surrounding rocks The integrity of the surrounding rocks The integrity of the surrounding rocks can be influenced by the number of rock planes, shape, spacing, and roughness of the surrounding rock structure. The surrounding rock integrity is rated based on RQD for various integrity level and shown in Table The strength of the surrounding rocks The roof rock mass strength, side rock mass strength, and floor rock mass strength are important factors to evaluate the stability of the surrounding rock. The strength of the surrounding rock is indicated by the uniaxial compressive strength of the roof rock mass. The surrounding rock strength is rated and presented in Table Roof rock layer The reinforcement effect of the bolts varies depending on various rock layers, such as the rock roof, coal roof, and composite roof, making the rock layer an important factor to evaluate the roof stability. The impact of the rock layer characteristics on the stability evaluation is rated and presented in Table Gateway type The gateway type refers to the combination of various surrounding rocks of the gateway including the rock gateway, the coal gateway, and the semicoal gateway. The gateway type is rated and presented in Table The inclination angle of the surrounding rock The inclination angle of the surrounding rock is considered as one of the important factors to evaluate the vulnerability of the gateway surrounding rock, which is rated and presented in Table The stress The vertical stress The vertical stress is a fundamental factor to the deformation and failure of the gateway surrounding rock. The vertical stress is rated by the buried depth and presented in Table The horizontal stress The influence of horizontal stress on the surrounding rock can be represented by the maximum principle horizontal stress and the angle between the axis of the gateway and the orientation of the maximum principal horizontal stress, which are often considered as key factors to evaluate the stability of the gateway surrounding rocks. The maximum horizontal principal stress and the angle between the gateway axis and maximum principal horizontal orientation are rated and presented in Tables The construction disturbance During the service period of gateway, the surrounding rock vulnerability is affected by the disruptions generated by the excavation or mining [ The abutment pressure where where According to Equations ( The scores corresponding to various states of disturbance times are shown in Table Geological Structure Common geological structures in coal mine include folds, faults, and collapse columns. The influence of geological structure on the surrounding rock is evaluated based on the quantity and scale of geological structures within a radius of 50 m around the area to be assessed [ Section parameter The section parameter comprises the section type and section size, which exert significant impact on the stress distribution and stress concentration of the gateway surrounding rock. The scores for the section type and section size are shown in Tables Geological Environment Underground water Water accumulations in the boreholes degrade not only the strength of the anchor bolts, but also the strength of the surrounding rock, which jeopardizes the stability of the gateway surrounding rock. Besides, due to the softening property, the inhaled water tends to degrade the rock strength [ Underground temperature The temperature around the borehole can influence the resin anchor-hold [ Service time Inspection and maintenance Inspection and maintenance are among important methods to detect and solve problems in time. Manual monitoring means inspections conducted mainly through manpower. Machine monitoring indicates that the monitoring and recording are conducted by machines and auto systems, such as borehole stress meters, deep displacement meters, and data storage equipment. Smart monitoring indicates the smart data monitoring and analysis [ Service time With the increased service time, the gateway surrounding rock often develops the creep deformation, which escalates the internal damage, resulting in higher vulnerability of the gateway surrounding rock [
The scores of the surrounding rock integrity.
RQD (%) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of the surrounding rock uniaxial compression strength.
Strength of roof (MPa) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of roof characteristics.
Roof type | Rock roof | Coal roof | Composite roof |
---|---|---|---|
Scores | 3 | 6 | 9 |
The scores of gateway types.
Gateway type | Rock gateway | Semicoal gateway | Coal gateway |
---|---|---|---|
Scores | 3 | 6 | 9 |
The scores of the inclination angles.
The inclination angle (°) | ||||
---|---|---|---|---|
Scores | 2.5 | 5 | 7.5 | 10 |
The score of the buried depths.
Depth (m) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of maximum principal horizontal stresses.
Major horizontal principal stress (MPa) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of included angles between gateway axis and major horizontal principal stress.
Included angle (°) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
Stress redistribution on the solid coal along the gob [
The scores of width of protective pillar in the current coal seam.
Width of the protective coal pillar | IV | I | III | II | 0 m |
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of disturbance times from mining.
Disturbance times | 1 | 2 | 3 | Others | ||||
---|---|---|---|---|---|---|---|---|
Scores | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 10 |
“
The scores of geological structures.
Geological structure | None | 2 | 2 | Others | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Scores | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
The scores of section type.
Section type | Circular | Arch | Straight wall semicircle arch | Trapezoid | Rectangle |
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of section size.
Section size (m2) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of water pouring amount.
Water pouring amount (ml/min) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of underground temperatures.
Ground temperature (°) | |||||
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
The scores of service time.
Inspection and maintenance | None | Manual monitoring | Machine monitoring | Smart monitoring |
---|---|---|---|---|
Scores | 10 | 7.5 | 5 | 2.5 |
The scores of service time.
Service time (y) | <5 | 5∼15 | 15∼30 | 30∼45 | >45 |
---|---|---|---|---|---|
Scores | 2 | 4 | 6 | 8 | 10 |
In the judgement matrix,
Factors
1–9 scaling method.
The ratio of factors | Quantized values | The ratio of factors | Quantized values |
---|---|---|---|
Equally important | 1 | Highly important | 7 |
Somewhat important | 3 | Extremely important | 9 |
Important stronger | 5 | Intermediate value between two adjacent judgments | 2, 4, 6, 8 |
Thirdly, both maximum eigenvalues
Fourthly, a judgement matrix consistency test was conducted. Since both subjective and approximate evaluations are included during the development of the judgement matrix, deviations are possible. Thus, a consistency test is required. The CR ratio can be calculated by
Value of RI.
Matrix order | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
---|---|---|---|---|---|---|---|---|---|---|---|
RI | 0 | 0 | 0.58 | 0.90 | 1.12 | 1.24 | 1.32 | 1.41 | 1.45 | 1.49 | 1.51 |
Table
Both CI and CR must be below 0.1 [
Lastly, the weight of each indictor was calculated. After the consistency test, the weight values were determined by following the steps listed in Figure
Steps to calculate weight values of indicators.
The highest eigenvalue of judgement matrix is
The weight matrix of the standard layer is
The weights of the evaluation indexes.
Layer-1 | Weight | Layer-2 | Weight | Layer-3 | Weight |
---|---|---|---|---|---|
Characteristics of the surrounding rock | 0.31 | Surrounding rock integrity | 0.35 | ||
Strength of the surrounding rock | 0.35 | Roof | 0.35 | ||
Rib | 0.4 | ||||
Floor | 0.25 | ||||
Roof rock types | 0.1 | ||||
Gateway type | 0.1 | ||||
Dip angle of the surrounding rock | 0.1 | ||||
Stress condition | 0.4 | Section parameter | 0.1 | Section type | 0.5 |
Section size | 0.5 | ||||
Construction disturbance | 0.35 | Degree | 0.5 | ||
Time | 0.5 | ||||
Geological structure | 0.3 | ||||
Buried depth | 0.1 | ||||
Major horizontal principal stress | 0.15 | Value | 0.4 | ||
Angle | 0.6 | ||||
Geological environment | 0.1 | Underground water | 0.5 | ||
Underground temperature | 0.5 | ||||
Service condition | 0.19 | Inspection and maintenance | 0.2 | ||
Service time | 0.8 |
The value of a gateway surrounding rock’s vulnerability evaluation can be calculated according to the weight listed in Table
After calculating the vulnerability score, the vulnerability of surrounding rock can be rated into five levels. The corresponding safety coefficients are exhibited in Table
Classification standard for vulnerability of surrounding rock and safety coefficient.
Score | |||||
---|---|---|---|---|---|
Vulnerability levels | I | II | III | IV | V |
Safety coefficient | 3.3 | 3.3∼4 | 4∼4.7 | 4.7∼5.4 | >5.4 |
In order to examine the feasibility and effects of increasing the density of the bolt support, Gucheng coal mine of Luan Group, located in Shanxi Province, China, was selected. No water flooded into the tunnel when the main inclined shaft penetrated the soil surface and weathered bedrock section. However, after the main shaft entered into the bedrock section and reached certain depth, water was detected and even burst in the anchor boreholes. The anchor cable pull-out tests revealed that the anchor force dropped by nearly 32% and the anchor-hold was decreased by about 15%, which indicates that the water flooding and burst severely degraded the support for the main inclined shaft, resulting in a 100 mm deposition of the gateway roof with local cracks. This change significantly jeopardizes the 65-year designed gateway surrounding rock support.
Based on the survey and the field investigation to the boreholes and the drainage holes, two new aquifers were identified between gateway mileage points 580 m and 740 m. During the gateway construction, cracks were generated around the anchor cable when the cable went through the aquifer and the middle-layer surrounding rock, which allowed the water to flow out from the anchor cable boreholes ad anchor bolt boreholes. Due to the tight schedule and the lacking of the emergency response plan, the project supervisor decided to replace the anchor cable with the anchor support. Therefore, the vulnerability evaluation should be conducted to the gateway surrounding rock and develop optimization to the anchor bolt support parameters.
The evaluation parameters of the main inclined shaft are shown in Table
Evaluation parameters and scores of the main inclined shaft.
Indictors | Value | Score | Indictors | Value | Score |
---|---|---|---|---|---|
Surrounding rock integrity (%) | 52 | 6 | Mining disturbance time | 1 | 2 |
Roof strength (MPa) | 25.48 | 8 | Geological structure | 2S | 4 |
Rib strength (MPa) | 25.48 | 8 | Buried depth (m) | 178 | 4 |
Floor strength (MPa) | 25.48 | 8 | Major horizontal principal stress (MPa) | 3.2 | 2 |
Roof rock type | Rock | 3 | Angle between the axis of the gateway and the major horizontal principal stress (°) | 42° | 6 |
Gateway type | Rock | 3 | Underground water (ml/min) | 600 | 8 |
Dip angle of the surrounding rock (°) | 15 | 5 | Underground temperature (°C) | 17.3 | 2 |
Section type | Straight wall semicircle arch | 6 | Inspection and maintenance | Manual | 7.5 |
Section size (m2) | 24.7 | 10 | Service time | 65 | 10 |
Pillar width (m) | ∞ | 2 | |||
Vulnerability score | 5.67 | ||||
Vulnerability preevaluation level | IV | ||||
Safety coefficient | 5.2 |
The main inclined shaft is jointly supported by anchor cable and a 150mm thick shotcrete. From the perspective of conservative estimation, without taking the bearing load of 150 mm thick shotcrete into account, the detailed parameters of the main inclined shaft are shown in Table
Detailed parameters of the main inclined shaft.
Parameter | Value | Dimension | |
---|---|---|---|
Parameters of the surrounding rock | Cohesion | 0.76 | MPa |
Internal friction angle | 17 | (o) | |
Section size | Width | 6400 | mm |
Height | 4400 | mm | |
Bolt support parameter | Length | 2400 | mm |
Strength | 335 | MPa | |
Diameter | 22 | mm | |
Column spacing | 800 | mm | |
Row spacing | 900 | mm |
According to Table
Based on the information listed in Table
Based on the existing bolt types, reducing the distance among anchor bolts was recommended to increase the strength of the composite rock-bolt bearing structure. When the distance among anchor bolts was 824 mm, the strength of the composite rock-bolt bearing structure reached 1.55 MPa. Considering the safety construction, 800 mm was selected for the distance among anchor bolts, which provided 1.567 MPa as the composite rock-bolt bearing structure. The optimized supporting parameters can be viewed in Figure
New support parameter of inclined shaft.
After the optimization, a deformation observation station is set at gateway mileage 594 m. As the deformation curve and the image of the completed optimization demonstrated in Figure
The deformation curve and the image of the completed optimization.
More specifically, water has a great impact on the safety of underground engineering, especially the long-term use of underground engineering. In order to further reduce the influence of water on the stability of surrounding rock, the grouting reinforcement measure was applied in the main inclined shaft. Parameters and effect of grouting reinforcement are shown in Figure
Parameters and effect of grouting reinforcement.
N1303 working face is located at Gucheng No. 1 mining zone, which mainly mines Coal seams #3 with a 6 m thickness through a fully mechanized caving technology. The tunnel for the conveyer in N1303 was excavated 600 m ahead the tailgate N1303. The deformation of conveyer tunnel is well controlled during excavation with low excavation speed (only 6.4 m/d), resulting in a high construction cost. Gateway layout of N1303 working face is shown in Figure
Gateway layout of N1303 working face.
In order to increase excavation speed and reduce the cost of the supporting system, the vulnerability preevaluation method was adopted to identify the safety coefficient of the tailgate N1303 and develop an optimization plan.
The evaluation parameters of tailgate N1303 are shown in Table
The evaluation parameters and scores of tailgate N1303.
Indictors | Value | Score | Indictors | Value | Score |
---|---|---|---|---|---|
Surrounding rock integrity (%) | 83.5 | 2 | Mining disturbance time | 1 | 5 |
Roof strength (MPa) | 116.65 | 2 | Geological structure | 1 | 2 |
Rib strength (MPa) | 15.33 | 10 | Buried depth (m) | 490 | 6 |
Floor strength (MPa) | 21.32 | 8 | Major horizontal principal stress (MPa) | 13.8 | 4 |
Roof rock type | Coal | 6 | Angle between the axis of the gateway and the major horizontal principal stress (°) | 51 | 6 |
Gateway type | Coal | 9 | Underground water (ml/min) | 25 | 2 |
Dip angle of the surrounding rock (°) | 5 | 2.5 | Underground temperature (°C) | 18.9 | 2 |
Section type | Rectangle | 10 | Inspection and maintenance | Manual | 7.5 |
Section size (m2) | 18 | 6 | Service time (y) | 3 | 2 |
Pillar width (m) | ∞ | 2 | |||
Vulnerability score | 3.88 | ||||
Vulnerability preevaluation level | II | ||||
Safety coefficient | 3.95 |
The detailed parameters of tailgate N1303 are shown in Table
Relevant parameters of tailgate N1303.
Parameters | Value | Dimension | |
---|---|---|---|
Parameters of surrounding rock | Cohesion | 0.4 | MPa |
Internal friction angle | 21 | (o) | |
Section size | Width | 5000 | mm |
Height | 3600 | mm | |
Bolt support parameter | Length | 2400 | mm |
Strength | 335 | MPa | |
Diameter | 22 | mm | |
Column spacing | 800 | mm | |
Row spacing | 800 | mm |
According to Table
Based on Table
New support parameter of tailgate N1303.
After the optimization, a deformation observation station A is set at mileage 50 m to measure the deformation during excavation period (as shown in Figure
Deformation of gateway N1303 after support parameters optimization.
The observation for continuous 40 days suggests that the tailgate N1303 roof deposition is 55 mm, and the heave at the tailgate bottom is measured at 6 mm. In addition, the deformation between two ribs is limited to 77 mm. The deformation rate grows lower on the tenth day after the excavation, indicating that the deformation of the tailgate N1303 has been effectively controlled.
As demonstrated in Figure
The N1303 conveyer tunnel and tailgate N1303 are compared in Table
The Comparison of N1303 conveyer tunnel and tailgate N1303.
Technical and economic indicators | Support parameter of N1303 conveyer tunnel | Optimization parameter in tailgate N1303 | Difference | |
---|---|---|---|---|
Total amount of deformation (mm) | Roof | 193 | 272 | +40.9% |
Floor | 53 | 102 | +92.4% | |
Two ribs | 241 | 326 | +35.2% | |
Average stress during excavation(kN) | Bolt | 63.4 | 98.6 | +55.5% |
Anchor | 115.6 | 235.8 | 104% | |
Excavation speed (m/d) | 6.4 | 9 | +40.6% | |
Cost (%) | 100 | 74.6 | −25.6% |
As the data shows in Table
In this study, taking the surrounding rock, stress conditions, geological environment, and service time into consideration, a vulnerability preevaluation model is established by adopting the analytic hierarchy process.
A vulnerability preevaluation method to the surrounding rock is proposed as the basis to evaluate the gateway support safety coefficient and optimize the design of the support system. The safety coefficient is obtained based on the vulnerability scores of the surrounding rock through the vulnerability preevaluation model. Applying the safety coefficient factor to the composite rock-bolt bearing structure for the instability evaluation, a required strength of the composite rock-bolt bearing structure can be identified quantitively with the optimization of the anchor bolt support.
This method has been successfully adopted to optimize the anchor bolt support in the gateway water pool area of the main inclined shaft located in the Gucheng Coal Mine. The application suggests that this method can effectively maintain the stability of the surrounding rock and lower the support strength of the anchor bolt in N1303 tailgate, resulting in lower cost and increased excavation speed. This method provides reliable basis for the gateway bolt support optimization in coal mining industry.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by Self-Financing Project of Scientific Research and Development Plan of Langfang Science and Technology Bureau (Grant no. 2020013046), Scientific Research Found of Key Laboratory of Building Collapse Mechanism and Disaster Prevention, China Earthquake Administration (Grant no. FZ201207), and the Fundamental Research Funds for the Central Universities (Grant nos. 2020013046 and ZY20215156).