After coal is extracted from a working face in a steep coal seam (SCS), the immediate roof tends to cave in and refill the lower part of the goaf. Based on the geological conditions of a work area in a SCS and the characteristics of roof caving, this study proposed a formula for the width of the backfill in the goaf and analyzed the main factors influencing it. Based on the small-deflection theory for elastic thin plates, a working face model was created for the mechanical analysis of the main roof above a SCS before the roof fractures for the first time. Then, a roof deflection equation was derived for the estimation roof deformation under the action of both the load from overlying strata and the support provided by the backfill in the goaf. The theoretical analysis combined with the actual operational parameters at the Zuoqipian working face in #49 seam of Xintie Coal Mine shows that the maximum roof deflection is around 0.8 m and occurs at a location 39 m from the upper end of the working face. Fractures will first develop in the upper sections of the frontal and rear walls of the face and the middle of the upper suspended roof due to tension or shearing and ultimately form an
The roof of a working face consists of the immediate roof and the main roof. The immediate roof is normally composed of thin and soft rock strata. It is thus able to fall easily as mining progresses and thus has little influence on the mining operation. The main roof is usually made up of thick and hard rock strata. As the main roof has a long suspended section behind the face, its fracture has a great influence on the mining operation and is often accompanied by strong strata behavior, making it difficult to control the surrounding rock. Therefore, there is an emphasis on controlling the fall of the main roof during mining. To study strata behavior at a working face and develop roof control measures, it is necessary first to understand the stress-deformation-fracture characteristics of the main roof [
Some researchers have studied the stress-deformation-fracture characteristics of the main roof above a SCS. Kulakov [
Previous research on the stress-deformation-fracture characteristics of the roof above a SCS largely used a beam or plate model to represent the roof and assumed that the load on the roof had a uniform or trapezoidal distribution, which cannot reflect the actual mechanical behavior of the roof above a SCS. In situ monitoring shows that as the mining in a SCS progresses, the immediate roof can cave easily and the caved rock then refills the lower part of the goaf [
Stress distribution in a stope.
The supports are located at the coal wall of the working face and play a fixing role on the edge of the roof. This research is about the stress-deformation-fracture characteristics of the main roof in the goaf. However, according to the parameters (width 1.5 m, length 5.5 m and support resistance 2600 kN) of the supports in the Xintie Coal Mine, we can calculate the resistance pressure as 0.31 MPa, and this can be ignored compared with the weight of the roof overburden. So, this study did not consider the role of support resistance to the goaf roof.
During mining in a SCS, the immediate roof will collapse and refill the lower part of the goaf due to the large dip angle of the seam. Therefore, the roof caving height decreased roughly from the upper end to the lower end of the face. When the suspended section of the main roof above the goaf has a width smaller than the main roof’s ultimate caving interval, the main roof, backfill, and the rock block along the upper section of the face can form an arch-shaped structure in mechanical equilibrium. The backfill along the lower section of the face then exerts an influence on the force-deformation properties of the roof of the face.
Based on the lithology and mechanical properties of the overburden, the immediate roof beneath the main roof can be divided into strata very prone to caving and hysteretic caving strata. Strata very prone to caving collapse readily as face supports advance. The suspended section of such strata behind the working face is usually shorter than the width of a single support (the width of a single support is 1.5 m) [
Schematic of the backfill in the goaf.
Three-dimensional schematic
Profile along the dip direction
To facilitate subsequent quantitative analysis, the cavity created by roof caving along the middle-upper part of the working face was simplified to a space with trapezoidal cross section in the dip direction, as shown in Figure
Sketch of roof caving and the backfill in the goaf.
Sketch drawn
Geometrical relationship
The problem of determining the width of the backfill in the goaf can be converted to solving for the length of the segment DI or EF shown in Figure
Based on their geometrical relationship and equation (
After bulking, the caved rock from the region defined by ABCD in the cross section then occupies the space defined by DEFG in the cross section, according to the geometrical relationship shown in Figure
Variation in the width of the backfill with different operational parameters.
Figure
According to the theory of elastic thin plates, a plate can be treated as a thin plate if the ratio of its thickness to its minimum width is greater than 1/100 to 1/80 and less than 1/8 to 1/5 [
A mechanical analysis shows that the force exerted by overlying strata and the support provided by the backfill in the lower part of the goaf are the main forces that cause the main roof to deform. The force exerted by overlying strata consists of two components: a shear component parallel to the dip direction and a normal component. Due to the large dip angle, the shear component is greater than the normal component and thus nonnegligible. It is reasonable to infer that the deformation of the roof of the working face in a SCS is caused by the action of both the shear and normal loads.
The rectangular coordinate system shown in Figure
A working face model for the mechanical analysis of the main roof.
In the lower part of the goaf behind the working face, the caved rock backfill experiences compressive deformation partly as a result of the movement of overburden. Coordinated interaction exists between the overlying strata and backfill. As the amount of downward deflection of overburden increases, the backfill tends to be compacted to a greater degree, have a higher bearing capacity, and thus be able to provide a greater force to support the roof.
Mechanical tests on backfill samples have shown that the behavior of the backfill in triaxial compression can be divided into four stages: elastic deformation, yielding, plastic deformation, and plastic failure, and the stress in the backfill was asymmetric along the width of the backfill [
Let
According to the theory of elastic thin plates, a thin plate’s deformation is negligible when it is subjected to only a load parallel to the plate’s plane, but the action of normal and shear loads combined can cause considerable deformation [
The deformation of the backfill in equation (
Previous mechanical analyses indicate that roof deformation caused by a normal load is much greater than that caused by a shear load [
The mechanical model shown in Figure
Then, we have
The roof deflection caused by the shear load can be obtained using the solution for the deflections caused by the normal loads:
High solution accuracy is not required for problems in mining engineering. When
The mechanical characteristics of the roof above a SCS indicate that the distribution of forces on the roof is symmetrical about the bimedian of the rectangular roof perpendicular to the strike direction and is asymmetrical in the dip direction. Then, substituting
The Xintie Coal Mine analyzed in this section is operated by the Qitaihe branch of Heilongjiang LongMay Mining Holding Group Co. Ltd. More than 50% of its coal seams are steep seams, which have 95.57 million tons of coal reserves. The main seam being worked is the steeply dipping #49 seam, where coal is extracted from the Zuoqipian working face in the 5th district of the 1st mining level. It is 1.6 m thick on average (Figure
Composite stratigraphic column.
The stratigraphic column shown in Figure
The first weighting interval of the main roof was measured at 57 m during actual mining. The coal seam dips at 55° on average and is 372 m deep at the upper end of the face and 460 m deep at the lower end of the face. The main roof has a thickness of 6.5 m, Poisson’s ratio
Substituting the abovementioned parameter values into equation (
Stress distribution in the roof in different directions.
As Figure
According to various failure criteria, the fracture of a material should first take place at the location of maximum principal stress. The relationships between principal stress, normal stress, and shear stress are governed by the following equations [
These equations can be used to predict the distribution of maximum principal stress and maximum shear stress.
Figures
Distribution of maximum principal stress.
Distribution of maximum shear stress.
Normally, the compressive strength of a rock is significantly greater than its tensile strength and is also greater than its shear strength. This, combined with the characteristics of stress distribution in the roof, suggests that the upper sections of the frontal wall and rear wall and the middle of the upper roof section may first fracture due to tension or shearing. The fractures in the three locations will interconnect to form an
Pattern of roof fractures.
Three-dimensional distribution of roof deflection.
Figure
Data on support pressure measured during mining can provide qualitative information about roof deformation. To validate the deformation distribution obtained by theoretical calculation, 5 observation stations (Figure
Distribution of observation stations along the dip direction.
Distribution of support pressure along the dip direction.
The measurements of support pressure suggest that the supports along the middle section of the working face experienced the highest pressure, followed by those along the upper section. The supports along the lower section of the working face were subject to the lowest pressure, which was roughly equal to the pressure at the pump station. A greater amount of roof deformation was associated with a higher pressure on roof support. This consistency indirectly confirms the accuracy of the pattern of roof deformation determined by theoretical analysis.
The immediate roof of a working face in a SCS can be divided into strata very prone to caving and hysteretic caving strata, based on the lithology and ability of immediate roof strata to cave. A formula for calculating the width of the backfill in the lower part of the goaf behind the working face was derived. The main factors affecting the backfill width were analyzed. A mechanical model of a working face above a SCS was constructed based on the theory of elastic thin plates. A roof deflection equation was obtained for estimating the roof deformation under the load from overlying strata combined with the support provided by the backfill in the goaf. A case study of the Zuoqipian working face in the #49 seam of the Xintie Coal Mine shows that the lower section of the roof experiences relatively low levels of stress while the upper section of the roof exhibits high levels of stress. The upper sections of the frontal and rear walls of the working face and the middle of the upper section of the roof show relatively high levels of tensile stress and shear stress. The fractures in the roof may form an According to the theoretical analysis combined with the actual operational parameters of the Zuoqipian working face, the maximum roof deflection is around 0.8 m and occurs at a location 39 m from the upper end of the working face. The roof deformation obtained by theoretical analysis and the measured support pressure follow a similar distribution pattern, thus confirming the accuracy of the theoretical results.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support from the National Key R&D Program of China (2018YFC0604701) and the Independent Research Project of the State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM18X003).