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The mining spatial structure of isolated island face in extra-thick fully mechanized top-coal caving mining is unique, which leads to a complex mining stress distribution and serious safety hazards. In this study, combined with a specific engineering example, the mining stress distribution characteristics of isolated island face are expounded, and a bearing structural mechanical model of the continuous beam of overlying strata is established using elastic–plastic mechanics theory. The mechanical equations of the mining stress distribution and failure depth of coal–rock mass are then obtained. Comparison of theoretical calculation results with numerical simulation and field measurement results shows basically consistent stress distribution characteristics. The derived mechanical equations can provide an estimation method for the analysis of mining dynamics on isolated island face in extra-thick fully mechanized top-coal caving mining. The following conclusions are acquired. The coal–rock mass should bear not only the lateral mining superposition influence but also the advance mining influence in front of the coal wall, so the isolated island face is in the complex environment of multiple mining stress superposition. In the mining process, the maximum advance mining stress concentration factor is 4.0–6.0 and is located at the upper and lower ends of the isolated island face. The lateral mining failure depth of the coal wall of the isolated island face increases by 2.0–5.0 m under the influence of advance mining. Therefore, compared with the nonisolated island face, the mining pressure appearance is intense. The mining influence in the range of 20–30 m of the upper and lower ends is intense, and the mining stress in this area is characterized by “cone distribution.” This zone is an important hidden danger area with coal–rock mass mining instability on isolated island face, which requires special attention to avoid mining disasters. According to the analysis of the influencing mining factors and laws of isolated island face, it is concluded that the longer the isolated island face size is, the closer the goaf size on both sides of the isolated island face is, the smaller the coal seam buried depth is, the better the mechanical conditions of coal and rock medium are, and the smaller the mining height of coal seam is, the more favorable the safe mining of isolated island face is.

The safe mining of isolated island face is of great significance to the safety production and economic benefit of coal mines. In the process of isolated island face formation, the overlying strata are affected by the lateral mining superposition influence of bilateral goafs. Consequently, the bearing pressure of coal–rock mass is more serious than that of nonisland working face, and the pressure step distance of isolated island face is smaller than that of nonisolated island face. Although failure occurs in coal–rock mass at the lateral edge of isolated island face due to mining, the coal–rock mass of the isolated island face and the overlying strata of the fracture zone in goafs still maintain continuous and complete structural characteristics and have good continuous bearing conditions and mining force transmission [

The bearing pressure of an isolated island face comes from its own and the rock load carried by the overlying strata slabs or beams on both sides. Therefore, the mining influence of isolated island face is more complex and severe than that of nonisolated island face. The safe mining of islands under the influence of strong mining has been concerned for a long time, and the research has made great progress. For instance, Jiang et al., Shi et al., Cheng et al., and Hou et al. classified the overlying stratum space structure of isolated island face stopes into “O,” “S,” and “C” types [

However, a mechanical analysis of complex mining effects, such as mining failure depth, the complex characteristics of mining stress distribution, and the intensity of the mining stress concentration of the coal–rock mass in isolated island face in fully mechanized top-coal caving mining, should be conducted. The safety production of isolated island face in fully mechanized top-coal caving mining urgently needs relevant theoretical research as technical guidance. The current study bases on the analysis of the bearing capacity characteristics of coal–rock mass of isolated island face in Tangjiahui Coal Mine and the analysis of the force-bearing characteristics of the coal–rock mass through numerical simulation. A mechanical model analysis of mining bearing in the isolated island face is performed. The mining load–force transfer mechanism of the coal–rock mass is studied. This study provides an estimation method for the calculation of mining stress distribution of the isolated island face in fully mechanized top-coal caving mining and plays a certain role in engineering guidance for the safe production of isolated island face.

The 61102 isolated island face in fully mechanized top-coal caving mining in Tangjiahui Coal Mine has the following characteristics: the average thickness of the main mining 6# coal seam is 18.3 m, the average dip angle of the coal seam is 2°, the buried depth is 520 m, and the plane layout is shown in Figure

Space arrangement and rock occurrence conditions of the isolated coal face in fully mechanized caving mining.

The rock mass presents the mechanical characteristics of low strength and poor cementation under the later geological structure because of the late diagenesis and low maturity of the coal measure strata, which are mostly argillaceous and calcareous cementation. Problems, such as in mine pressure control, support bearing stability, and surrounding rock control of the mining roadway, occur in the mining process of the working face. Therefore, the mining stress distribution and failure in the isolated island face should be studied urgently.

In accordance with the research on the behavior of mining pressure in isolated islands [

As the “bearing and transmitting” structure of the gob on both sides and its overlying strata, the island coal face bears “part” load of the overlying strata on both sides, and the bearing load is higher than that in the nonisolated island face

The characteristics of the spatial bearing structure in the isolated island face lead to the lateral mining superposition influence of bilateral goafs, resulting in intensified mining stress peak value, mining failure, and deep bearing stress distribution. The effects of many factors make the bearing state and force failure of coal–rock mass present complex and strong mining pressure in the isolated island face

Under the stress environment affected by the lateral mining superposition influence of bilateral goafs, the isolated island face mining results in a serious mining influence, overlying stratum pressure, fracture and rotation of the overlying stratum block, and a relatively “positive” mechanical response

The mining intensity, overlying stratum failure, pressure appearance characteristics, and surrounding rock stability of mining roadway of the isolated island face differ from those of a nonisolated island face. Hence, the characteristics of bearing capacity of coal–rock mass under the mining influence of the isolated island face should be analyzed

On the basis of field investigation and analysis, numerical simulation analysis is used to reveal the distribution characteristics of lateral mining superposition influence and the advance mining pressure in the isolated island face. The objective is to provide a strong reference for the mechanical model analysis of mining stress transfer in coal–rock mass regarding mining influence.

This study regards the 61102 isolated island face in fully mechanized top-coal caving mining in Tangjiahui Coal Mine as the object. Numerical simulation is performed, and the model is shown in Figure

The medium is a Mohr–Coulomb model. In accordance with the mechanical test of geological drilling and coring rock in Tangjiahui Coal Mine, the mechanical parameters of coal–rock are obtained through numerical simulation, as shown in Table

The boundary stress and displacement conditions are set and the initial stress field balance calculation is completed

The 61101 and 61103 goafs are excavated step by step, and the simulated mining step distance is 18.0 m. On the basis of the double-yield model used in references [

After the simulation calculation balance of the 61101 and 61103 goaf compaction, the 61102 isolated island face is excavated step by step. The simulated mining step distance is 12.0 m, and the mining area is at 250 m in

After balance calculation, the results of 100–246 m in

Numerical calculation model.

Mechanical parameters of coal–rock.

Medium name | Shear modulus (GPa) | Bulk modulus (GPa) | Cohesion (MPa) | Friction angle (°) | Tensile strength (MPa) |
---|---|---|---|---|---|

Medium fine sandstone | 1.62 | 1.71 | 2.6 | 35 | 0.62 |

Bedrock | 1.04 | 1.14 | 3.3 | 33 | 0.82 |

Sandy mudstone | 0.91 | 0.86 | 3.3 | 37.5 | 2.25 |

Coal seam | 0.67 | 0.50 | 1.3 | 28 | 0.54 |

Fine sandstone | 1.04 | 1.14 | 3.3 | 33 | 0.82 |

Coarse sandstone | 1.54 | 1.68 | 3.3 | 35 | 0.40 |

Mudstone | 2.04 | 2.15 | 1.6 | 37.4 | 0.41 |

Mechanical parameters of the goaf medium in the double-yield model.

Medium name | Shear modulus (GPa) | Bulk modulus (GPa) | Friction angle (°) | Dilatancy angle (°) |
---|---|---|---|---|

Goaf medium | 7.78 | 5.83 | 12 | 8 |

Stress–strain relationship of the goaf medium in the double-yield model.

Strain | Stress (MPa) | Strain | Stress (MPa) | Strain | Stress (MPa) |
---|---|---|---|---|---|

0.01 | 0.003 | 0.19 | 0.103 | 0.37 | 0.452 |

0.04 | 0.015 | 0.22 | 0.131 | 0.40 | 0.620 |

0.07 | 0.028 | 0.25 | 0.166 | 0.43 | 0.910 |

0.1 | 0.042 | 0.28 | 0.210 | 0.46 | 1.533 |

0.13 | 0.059 | 0.31 | 0.267 | 0.49 | 3.842 |

0.16 | 0.079 | 0.34 | 0.343 | 0.50 | 7.122 |

As shown in Figure

Distribution characteristics of lateral mining stress in the isolated island face.

As shown in Figure

Distribution characteristics of advance mining stress in the isolated island face.

In the analysis of mining dynamic problems, the investigation of the distribution of mining stress and the calculation of mining failure depth require determining the mining stress concentration coefficient (or the peak value of mining stress). However, most determination methods adopt an artificial assumption of mining stress concentration coefficient (the empirical value is 2.0 and 2.5 mostly) or numerical simulation of mining stress peak value. The artificial assumption lacks of a strong basis. Many factors, such as coal seam occurrence conditions, original rock stress environment, and isolated island face layout, affect the mining stress state of isolated island face. This condition leads to the artificial assumption that the mining stress concentration coefficient results in a substantial deviation. Although numerical simulation analysis has evident advantages in revealing the mechanical problems of complex engineering, many problems occur in numerical simulation analysis. Problems include the time-consuming modeling and calculation, the accuracy influence of zone size, the heavy simulation workload of influencing rules and factors, and the high requirements for the numerical simulation operation and analysis of researchers. However, the field measurement lags behind the actual engineering activities due to the constraints of deep mining space conditions, monitoring equipment level, and time conditions. Therefore, mechanical model analysis of the mining influence characteristics of the isolated island face is conducted to realize mechanical theoretical analysis of the mining influence of the isolated island face. This analysis provides a timely and effective calculation method for the mechanical estimation of mining stress concentration and failure depth, which can be used for collaborative analysis with numerical simulation and field measurement.

As shown in Figure

Distribution characteristics of mining stress in the coal–rock mass in the isolated island face in fully mechanized top-coal caving mining.

On the basis of the theory of mine pressure [

The characteristics of load transfer of the overlying strata in the isolated island face and stress transfer of the surrounding rock in the goaf are similar to those in “the stress structure of slab or beam.” From the previous research on the bearing structure of overlying strata in an isolated island [

Continuous beam structure model of the overlying strata in the isolated island face.

The mechanical model is simplified to facilitate its solution and meet the engineering applicability. The following basic assumptions are made.

The coal–rock mass in the isolated island face in fully mechanized top-coal caving mining is an ideal elastic–plastic medium, and the medium failure meets the Mohr–Coulomb strength criterion

Before the formation of goafs on both sides of the mining area, all coal–rock masses are in the original rock stress environment

The peak value of mining stress concentration is located at the boundary of the elastic and failure areas of coal–rock media

The unstable bearing effect of goafs is disregarded

On the basis of the bearing structure model of the continuous beam in the overlying strata of the isolated island face in fully mechanized top-coal caving mining, the internal supporting point of the continuous beam model is considered to be located in the middle of the coal–rock mass in the isolated island face. The lateral mining stress carried by the coal–rock mass in the isolated island face is symmetrically distributed in the middle supporting point

The end supporting point of the continuous beam model is considered to be located at the lateral mining failure boundary of the coal–rock mass on both sides of goafs

In accordance with the abovementioned basic assumptions, as shown in Figure

The statically indeterminate problem of the continuous beam is solved in accordance with structural mechanics [^{4};

In the middle of the isolated island face of the beam structure model, with

The deformation coordination condition indicates that at the middle fulcrum position (

The results are as follows:

In view of the lateral force state of the coal–rock mass in the isolated island face in fully mechanized top-coal caving mining, the lateral mining results in the mining stress concentration at the edge of the lateral coal–rock mass. This phenomenon leads to the failure of coal–rock mass in the lateral edge and forms the failure area in the edge of the coal–rock mass in the isolated island face. From the literature [

Stress analysis model of the edge coal–rock mass.

In accordance with the basic assumption (5), the lateral mining stress carried by the coal–rock mass in the isolated island face is symmetrically distributed with the central supporting point. Therefore, microelement is considered for the coal–rock mass in the isolated island face. Stress analysis is conducted with the microelement body as the research object. The lateral edge coal–rock mass boundary is regarded as the origin, and the

The tangential stress caused by the dislocation of the upper and lower boundaries of the lateral margin coal–rock mass is expressed as follows:

Equations (

Let

Equation (

In the position (

Therefore, Equations (

On the basis of the literature [_{1} and _{1} are the undetermined coefficients.

In the position (

From the stress transfer of continuous medium, we can determine

The three boundary conditions in Equations (

Therefore, the distribution equation of the lateral mining stress in the elastic area of the isolated island face is obtained as follows:

On the basis of the bearing structure model of the continuous beam of the overlying strata in the isolated island face in fully mechanized top-coal caving mining and the basic assumption (5), the lateral mining force carried by the coal–rock mass in the isolated island face is symmetrically distributed at the supporting point of the middle isolated island face. The load transfer balance equation of the lateral mining superposition influence is established by integrating

Equations (

The only unknown solution of the equation is the lateral mining failure depth

Under the lateral mining superposition influence, mechanical analysis of mining stress state in front of coal wall in the mining process of the isolated island face is performed. The stress analysis of advance mining is consistent with the dynamic analysis principle of lateral mining. The stress analysis of advance mining is based on the back mining boundary of isolated island face as the origin, the direction of coal wall mining is

In the elastic area of the coal–rock mass in front of the coal wall of the isolated island face, the influence of mining far away from the coal wall along the mining direction gradually converges to the lateral mining stress state. That is, the vertical stress distribution of the coal–rock mass in front of the coal wall of the isolated island face is

Then, the mining vertical stress equation of the elastic area in front of the coal wall of the isolated island face is set as follows:

At the advance mining failure boundary (

If the boundary conditions in Equations (

In accordance with masonry beam theory and rock beam transmission theory in the literature [

Cantilever structure characteristics of the coal wall roof in the isolated island face in fully mechanized top-coal caving mining.

The cantilever structure length of coal wall roof affects the advance mining stress distribution in front of the coal wall and leads to the appearance of periodic pressure characteristics. Therefore, in the mechanical model analysis, the cantilever structure length is regarded as the periodic pressing step distance

Equations (

Accordingly, the advance mining failure depth

The advance mining depth in front of the coal wall increases because of the advance mining influence of the isolated island face. Consequently, the advance mining stress distribution changes.

Therefore, the advance mining stress and failure of coal–rock mass should be recalculated in combination with the mining influence in front of the mining coal wall of the isolated island face. When

The 61102 isolated island face in fully mechanized top-coal caving mining is regarded as the engineering research object. The initial conditions of the example analysis are as follows:

Characteristics of the distribution of lateral mining stress in the isolated coal face.

Characteristics of the distribution of advance mining stress in the isolated coal face.

As shown in Figure

In Figure

Theoretical calculation and analysis show that the stress concentration coefficient of lateral mining superposition influence of the isolated island face is 2.6, and the lateral deep bearing stress is 1.67 times of the original rock stress. During the mining process of the isolated island face, the advance mining stress concentration coefficient of the upper and lower ends is as high as 5.0. The lateral mining failure depth increases by 2.0–5.0 m under the advance mining influence in the isolated island face. The mining influence in the range of 20–30 m of the upper and lower ends is intense, and the mining stress in this area is characterized by “cone distribution.” Therefore, this area is the key bearing structure of mining stress concentration in the isolated island face. The advance mining stress concentration of the isolated island face should be determined, and the support strength of coal–rock and the working resistance of the isolated island face support should be appropriately improved to avoid the instability of the bearing structure of coal–rock.

The borehole stress meter is arranged in the coal body of the 61102 isolated island face. The layout of measuring points is shown in Figure

Measuring point location for advance mining stress in the isolated island face.

Contrast of the distribution of advance abutment pressure in the 61102 isolated island face in fully mechanized top-coal caving mining.

The measured results show that the peak value of the advance mining stress in the coal body of the 61102 isolated island is nearly 36 MPa and 14 m away from the coal wall. The supporting load at 25 m away from the mining coal wall is nearly 20 MPa, and the stress distribution tends to be stable. The theoretical calculation results show that the peak value of the advance mining stress is 53.6 MPa, and it is close to 17.5 m from the mining coal wall. The supporting load outside at 40 m tends to stabilize at 30 MPa. The numerical simulation shows that the peak value of the advance mining stress is 52.6 MPa, and it is within 15-17 m from the mining coal wall. The supporting load outside at 40 m tends to stabilize at 35 MPa. Comparative analysis and verification imply that the theoretical calculation is basically consistent with the distribution trend of advance mining stress in the numerical simulation and field measurement. The mechanical equation of the mining stress distribution and failure depth of the isolated island face is derived. It provides a timely and effective calculation method for the estimation of mining stress distribution and failure depth of the isolated island face and plays a certain engineering guiding role in the safe production decision making of the isolated island face in fully mechanized top-coal caving mining.

Taking the 61102 isolated island face condition in Tangjiahui Coal Mine as the engineering background, the influencing factors and laws of mining stress transfer on isolated island face are analyzed.

For mining conditions, as shown in Figure

Influence characteristics of various factors on mining stress of isolated island face.

Different mining heights of coal seams (

Different isolated island widths (

Different goaf widths (

Different goaf widths (

Different depths of coal seam (

Different cohesions of coal seam media (

Different internal friction angles of coal seam medium (

Different friction coefficients on coal-rock upper and lower boundary (

The coal–rock mass in an isolated island face in fully mechanized top-coal caving mining is forced to be the “bearing and stress-transferring” structure of the overlying strata of bilateral goafs. The coal–rock mass should bear not only the lateral mining superposition influence but also the advance mining influence in front of the coal wall; therefore, the isolated island face is in the complex environment of multiple mining stress superpositions

Compared with the nonisolated island face, the mining pressure appearance is intense. In the mining process, the maximum advance mining stress concentration factor is 4.0–6.0 and is located at the upper and lower ends of the isolated island face. The lateral mining failure depth of the coal wall of the isolated island face increases by 2.0–5.0 m under the influence of advance mining

A serious mining influence area is formed under the superimposition of the lateral and advance mining influences of the isolated island face. The mining influence in the range of 20–30 m of the upper and lower ends is intense, and the mining stress in this area is characterized by “cone distribution.” This zone is an important hidden danger area with coal–rock mass mining instability on isolated island face

According to the mining influencing factors and laws of isolated island face, the longer the isolated island face size is, the closer the goaf size on both sides of the isolated island face is, the smaller the coal seam buried depth is, the better the mechanical conditions of coal and rock medium are, and the smaller the mining height of coal seam is, the more favorable the safe mining of isolated island face is

The mechanical model of the bearing structure of an overburden continuous beam on an island face is established. The mechanical equation of mining stress distribution and coal–rock mass mining failure is obtained using elastic–plastic mechanics theory and limit equilibrium method. It provides an estimation method for the mining dynamic analysis of isolated island face in extra-thick fully mechanized top-coal caving mining

The known data in this paper come from practical engineering case data, which are reliable and available.

The authors declare that there is no conflict of interest regarding the publication of this article.

This study was supported by the NSFC Project (51574007, 51674007), the Inner Mongolia Natural Science Foundation Project (2019MS05055), and the Open Research Fund Project of Key Laboratory of Safety and High-Efficiency Coal Mining of Ministry of Education (JYBSYS2019208).