This study focused on large-scale roof-fall accidents occurred in large-section coal seam roadways of Bayangaole Coal Mine, Inner Mongolia, China, and investigated the occurrence mechanism of roof-fall and the related supporting control method in detail. Firstly, the fracture characteristics of the surrounding rocks on the roadway roof were measured using a stratum detector. The results showed that the roadway roof underwent the most severe failure with a maximum deformation of 3.53 m; the bedding separation and fracture zones were distributed at irregular intervals. Accordingly, the entire stratum was separated into several thin sublayers, significantly reducing the stability of roof. In addition, the roof medium grained sandstone of roadway is water-rich strata, and water aggravates the damage of roof. Next, the mechanism of the occurrence of roof-fall accidents in the roadway was elucidated in detail. The following three reasons are mainly attributed to the occurrence of roof-fall accidents: (i) effects of mining-induced stress and tectonic stress, (ii) existence of equipment cavern on the side of roadway, and (iii) unreasonable support parameters. On that basis, a new supporting design is proposed, including a more reasonable arrangement of anchor cables and bolts, bolts with full-length anchorage which are applicable in cracked and water-rich roadway, high-strength anchor cables, and crisscrossed steel bands. Moreover, high pretightening force was applied. Finally, a field test was performed, and the mining-induced roof displacement and stress on anchor cable (bolt) were monitored in the test section. The maximum roof displacements at the two monitoring sections were 143 mm and 204 mm, respectively, far smaller than the roadway’s allowable deformation. Moreover, the stress on roof anchor cables (bolts) was normal, and no anchorage-dragging and tensile failure phenomena were observed. The monitoring data indicated that the new supporting design was remarkable on the control of large-section coal seam roadway roof deformation.
With the improvement of modern comprehensive mechanization technology as well as the popularization and application of some high-yielding and high-efficiency coal mining techniques such as top-coal caving mining and full-seam mining, large-scale and fast-advancing intensive mining mode has now become an important development direction in China’s coal mining; moreover, the use of large-scale equipment in the work face, a significant increase in mining intensity and yield, and the enhancement of transportation capability in mines have set higher requirements for the section size of roadway [
Currently, a great deal of research has been performed on the mechanisms, prediction, and support control of roof-fall accidents in coal roadways. Some scholars systematically analyzed the causes of roof-fall accidents in mines and pointed out that roof-fall accidents in underground mines were mainly caused by many factors including geological conditions, crustal stress state, water, the arrangement in mines, and mine environment [
In previous studies, researchers analyzed the mechanisms of roof-fall accidents in coal roadways, investigated the related influencing factors from different perspectives, and obtained a lot of important results. However, based on a thorough knowledge of roadway roof-fall mechanism, more targeted supporting control measures should be further used for different types of roof-fall accidents to more effectively avoid the occurrence of accidents. He and Zhang [
This study focused on Bayangaole Mine located in Erdos, a city in Western China. First, the fracture characteristics of the surrounding roadway rocks were evaluated using the borehole peering technique; then, large-section coal seam roadway roof-fall mechanisms were elucidated in combination with a roof mechanical model. Based on the evaluation on original supports and parameters, a novel supporting scheme and related parameter setting are proposed. Finally, using this novel support system, field tests were conducted. According to the field monitoring results, this novel support system achieved favorable controlling performance on roof settling in a large-section coal seam roadway.
Bayangaole Coal Mine is located in the southern part of Erdos, the Nei Mongol autonomous region, China, and at the northeastern edge of Maowusu Desert. As shown in Figure
Geographic position and surface landform of Bayangaole coal mine.
Lithological characteristics of number 3-1 coal seam, roof, and floor in Bayangaole coal mine.
Numbers 311101 and 311102 working faces are the first two working faces in number 11 panel of Bayangaole coal mine. Figure
Illustration of the arrangement in numbers 311101 and 311102 working faces of Bayangaole coal mine.
Original supporting scheme in the roadway.
At about 8:00, April 23, 2015, when number 311102 working face was advanced to 210 m, the anchor cables in ventilation roadway were fractured, and roof stratum 51–62 m in front of the working face fell (i.e., the position filled with red grid lines shown in Figure
Pictures of rain-fall accidents in the air-return roadway of number 311102 working face.
Combined with the tunneling condition in roadway and the mining conditions in working face, a YTJ20 stratum detector as shown in Figure
(a) Picture of guide rod and (b) picture of YTJ20 stratum detector.
Arrangement of drilling holes on two monitoring sections and related drilling parameter settings. (a) Monitoring section 1 and (b) monitoring section 2.
Figures
Detection results of different drilling holes on roof and two sides of monitoring section 1. (a) Observations in drilling hole 1, (b) observations in drilling hole 2, (c) observations in drilling hole 3, and (d) observations in drilling hole 4.
Detection results of different drilling holes on roof and two sides of monitoring section 2. (a) Observation in drilling hole 5, (b) observation in drilling hole 6, and (c) observation in drilling hole 7.
Figures
Detection results of roadway roof separation and fracture distribution. (a) Monitoring section 1 and (b) monitoring section 2.
Based on the composite beam theory of anchor blot support, the anchoring force of the bolt increases the contact stress among various rock strata within the anchorage range to avoid the occurrence of stratum separation. Additionally, under the action of anchoring force, the shearing strength between any two rock strata also increases, inhibiting the horizontal diastrophism between any two strata and forming a thick combined rock beam via anchoring within the anchorage range and finally significantly improving the stability of roadway roof [
Next, the roof-fall mechanism in the roadway was analyzed by taking the ventilation roadway in number 311102 working face as an example. Based on the theory of mechanics of materials, the roof in the ventilation roadway of number 311102 working face was simplified as a simply supported beam, and the mechanical model of rock and beam in the roof was established, as shown in Figure
Mechanical model of the upper coal roof.
According to the theory in mechanics of materials, the maximum bending deformation and maximum tensile stress exist at the center of a simply supported beam. When the applied maximum tensile stress on the roof rock beam reached the stratum’s tensile strength, the rock beam snapped. The limit span when a simply supported beam with a unit width fractured can be calculated as follows:
The actual load on the rock stratum includes not only the gravity of rock stratum but also the load induced under stratum. Based on the principle of composite beam, considering the effect of overlying
As shown in Figure
As shown in Figure
According to (
According to the abovementioned drilling TV detection results, several obvious separation failures appeared in the roof at a depth below 3.53 m, separating the rock stratum 2.25 m below the sandy mudstone from the above stratum, and an obvious stratification was observed. In contrast, as shown in Figure
According to the above analysis, the top coal seam around the cavern in roadway and the sandy mudstone layer at a thickness of 2.25 m satisfied the condition of fracture failure and were likely to fracture. When the top coal seam was fractured, the load on the stratum was bore by the anchor cable for suspension. Therefore, the load on each anchor cable when the fracture occurred in the top coal seam can be estimated using the following formula:
In the original supporting scheme, steel strand anchor cables with a nominal diameter of 17.8 mm were used, and the parameters are listed in Table
Cable parameters in original support.
Properties | Values |
---|---|
Diameter (mm) | 17.8 |
Length (mm) | 7300 |
Elastic modulus (GPa) | 237 |
Tensile strength of steel strand (MPa) | 1860 |
Maximum tensile load (kN) | 353 |
Elongation (%) | 4 |
Pre-tension (kN) | 0 |
Based on the above analyses, roof-fall accidents in the main belt conveyor roadway of number 311102 working face and ventilation roadway of number 311102 working face can be attributed to the following three aspects: Stress condition is a key factor in the occurrence of roof-fall accident in the roadway. Under the combined effect of mining-induced stress and tectonic stress, the surrounding rocks in the roadway exhibited severe deformation and destruction, the roof was broken, and the stratum was separated. These are the preconditions for the occurrence of roof-fall accident in the roadway. Because of the existence of equipment caverns, the actual span of the roof stratum increased significantly, also providing favorable conditions for the occurrence of fracture and failure of roof stratum in the roadway. The original supporting scheme is unreasonable, where the anchor bolts in the roadway roof were short and the fractures developed in the surrounding rocks within the anchorage range, thus leading to an insufficient anchoring force. Therefore, the appearance of separation of coal seam and stratum in roadway roof could be hardly suppressed, and the role of the anchor bolt support as the composite beam could be hardly fully played. In addition, the array pitch between anchor cables was too high, and the used anchor cables were poor in specifications and models. Therefore, an insufficient limit tensile load of the anchor cable could hardly support the gravity load of the fractured stratum and easily induced tensile failure.
Using the original supporting scheme, many severe roof-fall accidents occurred in the large-section mining roadway. By analyzing the occurrence mechanism of roof-fall accidents in the roadway, it was found that the original support design and related parameter setting were unreasonable, lacking some targeted control measures according to the roadway roof’s deformation failure characteristics. To avoid the recurrence of roof-fall accident in the roadway, a more reasonable supporting scheme should be proposed to better control the roof deformation and instability in the roadway. Based on the abovementioned analysis results on the occurrence mechanisms of roof-fall accidents in large-section mining roadway and many successful experiences of roadway support in China’s mines, the following points should be considered while designing a novel roadway support scheme: The support should be rapidly installed after the roadway excavation [ The anchor cable’s supporting performance should be guaranteed. In the support system of roadway surrounding rocks, anchor cables were always used for overhanging. The failure of anchor cables in support is a key factor that leads to the occurrence of roof-fall accidents. First, the anchor cables should be long enough and fixed into a complete and hard stratum so that they cannot be dragged. Second, the anchor cable’s model should match well with the length, and the cable’s limit tensile load should exceed the gravity of the stratum within the supporting range to avoid the occurrence of tensile failure. The anchor bolt should be grouted in full length. After the excavation in the large-section roadway, under the combined action of mining stress and tectonic stress, the surrounding rocks were severely destroyed, and both fracture and separation zones were developed in the stratum within the anchoring range. The traditional end-anchorage method should anchor the end of the bolt into the full stratum to fully play the bolt’s supporting and reinforcement function; in cases when the roof was water-rich and severely destroyed, the end-anchorage method could hardly accurately anchor to the full region in the stratum, easily causing the invalidation of anchorage and thus triggering roadway instability. In these cases, the bolt should be installed using full-length anchorage so that the invalidation of anchorage can be avoided and the supporting performance can be effectively enhanced. Additionally, full-length anchorage further increases the anchor bolt’s shearing resistance capability; in the region with more severe separation, this anchorage mode effectively prevents the relative lateral deformation among the roof sublayers [ The pre-tension in cable supporting should be increased. According to the field monitoring results, after the excavation in the large-section roadway, the roof stratum was separated into multiple thin sublayers. These thin sublayers were more easily bent, fractured, and even fell than a complete stratum. During the installation, a great pretightening force should be first applied on the cable. Research showed that great pretightening force could increase frictional resistance along bedding planes and prevent the separation failure in the stratum [
With these considerations in mind, a new supporting scheme is proposed in this study for overcoming roof-fall accidents in large-section roadways. Figure
Based on the actual condition of both roof and floor strata in number 3-1 coal seam,
Cable parameters in new support.
Properties | Values |
---|---|
Diameter (mm) | 21.8 |
Length (mm) | 9000 |
Elastic modulus (GPa) | 237 |
Tensile strength of steel strand (MPa) | 1860 |
Maximum tensile load (kN) | 607 |
Elongation (%) | 7 |
Pre-tension (kN) | 200 |
Compared to the original support in roadway, the designed new support system did not change the side support but only modified the top support. As shown in Figure
New supporting design in the roadway.
The installation effect of the anchor cable determines the suppression performance of roof-fall accidents in large-section roadways. Table
To validate the effectiveness of the new designed support, a field test was performed in the ventilation way of number 311102 working face of Bayangaole coal mine. A test section at a length of 100 m in the roadway was selected 150 m in front of the working face along the advancing direction, and a new support was installed according to the new design scheme. Two monitoring sections (sections I and II) with a spacing of 20 m were set in the test section for tests. An equipment cavern was arranged on the side of the roadway in section II, whose size was the same as that of the cavern at roof-fall accident. Figure
Monitoring design.
During the installation of measuring instruments on two monitoring sections, the section in the roadway was already excavated, and the initial deformation of the roadway was already released. Thus, the measuring instruments can only measure the roof deformation and anchor cable (bolt) change in the roadway during the period when subjected to mining in the face. After the installation, the measuring instruments started monitoring until the working face passed the monitoring section. The monitoring lasted approximately 70 days, and the working face was advanced at a velocity of 2.8 m/day.
Figures
Monitoring results of roof displacement and stress on anchor cable (bolt) at section I. (a) Monitoring results of roof displacement, (b) displacements of roof stratum at different depths, and (c) monitoring results of stress on anchor cable (bolt).
Monitoring results of roof displacement and stress on anchor cable (bolt) at section II. (a) Monitoring results of roof displacement, (b) displacements of roof stratum at different depths, and (c) monitoring results of stress on anchor cable (bolt).
As the displacement of the shallow stratum (within a range of 0–4 m above the roof) increased gradually, stresses on the bolt at both sections I and II increased, indicating that a bolt with full-length anchorage can effectively suppress the displacement of a shallow stratum on the roof. The stress on roof anchor cable also increased with the increase in roof displacement and reached the maximum when the working face moved to the measuring point. The maximum stresses on roof anchor cables on sections I and II were 39.3 MPa and 52.5 MPa, respectively. According to the forced area of the stressmeter, the forces on the anchor cables were 275 kN and 365 kN, respectively, far smaller than the anchor cable’s tensile load limit. This indicates that the used anchor cable was reasonable.
Section II was located in the roadway’s equipment cavern with a large span, also close to DF14 section. Therefore, the maximum roof displacement on section II was higher than that on section I; moreover, as shown in Figure
Additionally, as shown in Figures
This study focused on a coal seam in a large-section roadway of Bayangaole coal mine, Inner Mongolia, China, and investigated the occurrence mechanism of mining-induced roof-fall accidents and related supporting methods. The long-wall mining method was used in numbers 311101 and 311102 working faces for coal extraction. During the mining, severe roof-fall accidents occurred in these two working faces, which have caused severe impacts on the life safety of miners and production activities.
The fracture characteristics of the surrounding rocks in the ventilation roadway of number 311102 working face were evaluated using a stratum detector. The results show that the surrounding rocks of roadway exhibited a discontinuous failure; fracture and crushed zones appeared in the drilling holes alternately and irregularly. The maximum fracture ranges of the roadway roof and side were 3.53 m and 1.36 m, respectively; apparently, the destruction on roadway roof far exceeded that on two sides of the roadway in both range and degree. The interval distribution of separation layers and crushed zones in roadway roof divided the entire roof stratum into multiple thin sublayers, thus significantly reducing the stability of roof. This is also as an inducing factor for roof-fall accidents in the roadway.
Next, according to the engineering geological conditions of the working face, the occurrence mechanism of a roof-fall accident was elucidated in detail. It can be concluded that mining-induced stress and tectonic stress are the key factors that led to roof-fall accidents in the roadway; these are also the preconditions for the occurrence of accidents. The equipment cavern on the side of the roadway increased the span of roof stratum and thus provided advantageous conditions for the occurrence of roof-fall accidents in the roadway. In the original supporting scheme, some parameters were set as inappropriate values, the anchor bolts on roadway roof were too short, and the end-anchorage mode could not provide an effective anchorage effect. Additionally, the array pitch between anchor cables was too high, and the used poor cables could not sustain the self-weight load of the fractured stratum and easily snapped. On that basis, a novel supporting scheme was proposed: the anchor cables and bolts were more reasonably arranged, and the bolts were anchored in a full-length way. Additionally, high-strength anchor cables were used, a high pretightening force was applied on the anchor cable, and the steel bands were arranged in a crisscross pattern.
Finally, a test section with a length of 100 m in the ventilation roadway of number 311102 working face was selected to validate the performance of the new supporting scheme. Two monitoring sections were set in this test section to measure the mining-induced roof displacement and stress on anchor cable (bolt). During the mining, the maximum roof displacements at the two monitoring sections were 143 mm and 204 mm, respectively, far smaller than the allowable deformations. The stresses on anchor cable and bolt were normal, and no anchorage-dragging and tensile failure phenomena were observed. Further, the roadway roof in the test section remained intact during the mining. According to the field monitoring results, by using the proposed new supporting scheme, the roof deformation in large-section roadways can be effectively suppressed.
The authors declare that all data supporting this study are available within the article.
The authors declare no conflicts of interest.
This work was partially funded by the State Key Research Development Program of China (2016YFC0801403), the Shandong Provincial Natural Science Foundation, China (ZR2018MEE009), and the Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2017RCJJ012).