1 Introduction

With the development of the technology and equipment of large mining height and fully mechanized top-coal caving, the organic combination of large mining height and fully mechanized top-coal caving has formed the unique mining method of 8–15 m coal seam mining, which has become an important technical approach to realize the safe, high yield and efficient mining of the extra-thick coal seam. But after coal mining height increasing, mining will cause some technical problems to be solved, which mainly includes: the determination of parameters of fully mechanized top-coal caving face with large mining height are broken, piece for roof caving working face of coal wall mechanism and prevention, the problem of working face coal recovery rate, the characteristics of roof breakage and the law of mine pressure appear research. A large number of scholars to solve these problems were studied. (Cheng et al.2019a, 2020; Kong et al. 2019; Yang et al.2016).

In recent years, Many scholars have deeply studied the roof structure of large mining and high mining, based on elastic mechanics and rock mechanics, presents a shallow buried coal seam roof breaking the theoretical calculation formula of Angle, and in the changes Mao auxiliary haulage roadway construction of coal mine working face 22,201 of 9 hole, identified by drilling into the roof of the breaking location, it is concluded that the working face of large mining height are broken roof breaking Angle, combining physical simulation experiment results are verified. (He et al. 2019) Based on the field measurement and analysis of support working resistance and overburden settlement at three different mining stages of the large mining height face 52307of Daliuta Mine, the author studied the law of ore pressure, change law of support working resistance and overburden subsidence characteristics during thick coal seam mining under the condition of thick sandstone roof. (Shen 2018) "lulu others for fully-mechanized caving the roof breakage structure and mine pressure appear in this research, in view of the thick and hard roof of full mechanized top-coal caving mining face of first weighting interval is difficult to determine the problem, in the engineering background of Dongzhouyao coal mine working face 8301, thick and hard roof of clamped beam model is established, using the energy method for distributed in the horizontal stress in basic roof, and then get the basic first step to pressure from the analytical formula, the numerical simulation of fracture characteristics and working face roof support working resistance is analyzed, and the working face different positions under different forward distance support working resistance monitoring was carried out. (Cheng et al. 2016) According to theoretical analysis, the range of direct roof and old roof in full-mechanized caving of extra-thick coal seam is obviously increased. From the perspective of whether the bracket deformation pressure roof strata can be divided into “no deformation of rock pressure” and “have rock deformation pressure”, and gives the sublevel caving support by the analytical method of load, according to the analysis of the fully mechanized top-coal caving face with large mining height thick coal mining thickness increase, the scope of “deformation pressure layers” will significantly increase, abnormally pressure which leads to the cause of the strong. (Yan 2009).

To sum up, many scholars have studied roof fracture and mining pressure development law under large mining height and roof fracture and mining pressure development during fully mechanized top-coal caving mining, and obtained excellent research results, which have laid a solid theoretical foundation for the safe advancement of large mining height and fully mechanized top-coal caving mining.(Cheng et al.2019b; Gou 2020; Zhang et al.2019) However, the law of overburden fracture and mine pressure development in fully mechanized top-coal caving face still lags behind the field practice. Therefore, in this paper, based on the background of mechanized top-coal caving face 101 with large mining height, the characteristics of roof breaking and the law of mine pressure developing in the mining process are studied through theoretical analysis and numerical simulation, and verified through an engineering example, providing a basis for roof control of fully mechanized top-coal caving face with large mining height.

2 Engineering Background

The working face 101was the first fully mechanized top-coal caving face with large mining height, the mine working face strike length of 350 m, the dip length of 120 m, mining coal seam of yan’an group 4 coal seam, the average thickness of coal seam is 13.4 m, mining height is 3.5 m, the average height of coal 5 m, mining put ratio of 1:1. 43, the dip angle of coal seam 2°–7°, the coal seam buried depth of 580–1195 m, as shown in Fig. 1 of 4 coal seam roof and floor strata histogram.

Fig.1
figure 1

Columnar diagram of coal roof and floor

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3 Roof Breaking Characteristics of Full Mechanized Top-Coal Caving Face with High Mining Height

For the fully mechanized top-coal caving face with large mining height, the movement effect of the basic roof will be weakened by the top-coal and become the secondary control object. Therefore, the direct roof and top coal are the strata to be controlled on the top coal caving face. However, when the thickness and strength of the basic roof are large and the space for the basic roof movement is large behind the direct roof caving, the coming pressure of the basic roof will be strong and dynamic pressure impact will occur. Under such conditions, the stope control design must consider the impact of the basic roof movement on the dynamic load of the stope support. In this paper, the fully mechanized top-coal caving face 101 with large mining height has small thickness and weak strength of the direct roof. The fine-grained sandstone above can be regarded as either the direct roof or the basic roof. When the fine-grained sandstone is broken, it is not only the first caving of the fine-grained sandstone in the direct roof, but also the first time to pressure of the basic roof.

3.1 Roof Structure of Fully Mechanized Top-Coal Caving Face with Large Mining Height

The average thickness of coal in the full mechanized top-coal caving face 101 with large mining height is 13.4 m. When caving top coal mining, the bottom coal of 2 m remains, the mining height is 3.5 m, the average caving coal thickness is 5.0 m, the remaining coal thickness of working face is 2.9 m, the mudstone of 3.6 m thick and the fine-grained sandstone of 9.9 m above it, a total of 16.4 m. Because the coal produced on the working face is thick and the average mining height is 8.5 m, the space of strata movement above is large, and the strength and bending deformation resistance of the coal seam and mudstone are poor, the coal seam of 2.9 m and the mudstone of 3.6 m above are irregular caving as the direct roof upper and lower strata. Due to the high thickness, strength and bending deformation resistance of the 9.9 m fine-grained sandstone above, it is easy to form the upper direct roof of the stope. The space of the rock strata movement after the direct roof collapse at the lower position will decrease, but there is still a large movable space. Therefore, the fine-grained sandstone collapse is generally in large blocks, and the large blocks squeeze each other, easily forming a semi-arch "rock-gangue" structure. As shown in Fig. 2:

Fig. 2
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Schematic diagram of "rock gangue" structure

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The "rock-gangue" structure refers to the semi-arch structure formed by extruding no caved rock and caved gangue. This structure form is the most common structure form in top coal caving stope, especially when the upper direct roof is sandstone with larger thickness and higher strength, the collapse of which is easy to form bulk gangue, while the mutual extrusion of bulk gangue is easy to form "rock-gangue" structure with the direct upper roof without caving.

As the roof structure of the working face is a "rock-gangue" structure, in this case, the basic roof is actually regarded as the direct roof. The position of arch structure will change during the process of coal drawing. In each coal drawing cycle, at the beginning of coal drawing, the top coal is released less, the space for roof movement is smaller, and the position of arch structure is lower, as shown in Fig. 3.

Fig. 3
figure 3

Schematic diagram of the "rock-gangue" structure of the fully mechanized top-coal caving face

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The lower arch structure will be damaged due to insufficient bending deformation capacity, making the arch structure move up. When all the top coal is discharged, the space allowing the roof to move reaches the maximum, and the position of the arch structure will develop to the maximum, as shown in Fig. 4.

Fig. 4
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The highest arch of "rock-gangue" in fully mechanized top-coal caving face structural position diagram

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3.2 Determine the thickness of Direct Roof and Basic Roof

According to the field practice, the direct roof thickness of the fully mechanized top-coal caving face with large mining height changes in different mining stages. From the safety point of view, the direct roof thickness increases to a basic stable value, about 2 times of the mining thickness in the normal promotion stage; the stable direct roof thickness can be divided into two parts, the upper direct roof and the lower direct roof according to the movement characteristics. The allowable rotation space of the lower direct roof after fracture is large, and the caving form is irregular collapse zone, while the upper direct roof rock stratum after fracture has small rotation space, and the collapse form is regular collapse zone. In the caving face, the coal is a dynamic process from the beginning of fracture to release, in this process, the thickness of the direct roof also changes with the amount of top coal released change. As shown in Fig. 5, considering the bending settlement characteristics of the rock stratum, the direct roof thickness of the caving coal stope can be predicted by the following formula: (Wang et al. 2016)

$$ m_{z} = \frac{{h + T - S_{A} - C}}{{K_{A} - 1}} $$
(1)
Fig. 5
figure 5

Calculation diagram of direct roof thickness

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In the formula, the mining height \(h\) is 3.50 m; the actual top coal thickness T is 5.00 m; the settlement value \(S_{A}\) is 0.25 h', in which h’ = h + ηT(80% of the top coal yield) can get SA is 1.875 m; for the residual coal thickness \(C\), It is concluded that \(C\) is 1.275 m by 85% recovery of actual coal thickness. The gangue swelling coefficient KA in the goaf is 1.30. The \(m_{z}\) is 17.80 m by substituting the relevant parameters into the formula. (Wang 2009).

4 Calculation of Support Strength and Caving Step of Working Face

Under the condition of fully mechanized top-coal caving face with large mining height, the direct roof of working face is caving of coal, mudstone and fine-grained sandstone, and their thickness, strength and flexural deformation resistance are different, so they fall in layers; That is, 2.90 m thick coal seam first collapsed, then 3.60 m thick mudstone collapsed, and finally 9.90 m thick fine-grained sandstone collapsed. Therefore, the initial caving steps of three strata should be determined separately.

4.1 The First Caving Step Distance of Coal seam

According to the theoretical prediction formula of the first caving step distance of direct roof, the first caving step distance of 2.90 m thick top coal seam is derived from the following formula:(Qian et al.2003; Wang 2009)

$$ c_{z01} = \sqrt {\frac{{2m_{z1} [\sigma_{zt1} ]}}{{\gamma_{z1} }}} $$
(2)

In the formula: cz01 is the first caving interval step distance of top coal, the unit is m; mz1 is the thickness of top coal, the thickness is 2.90 m; γz1 is the average gravity density of top coal, average gravity density is 2.45 × 104 N/m3; [σzt1] is the average tensile strength of top coal, average tensile strength is 0.89 MPa. The first caving step distance of 2.90 m thick coal seam is 14.52 m, which can be calculated by substitution parameters.

4.2 The first Caving Step Distance of Mudstone

According to the theoretical formula of the first caving step distance of direct roof, the first caving step distance of 3.60 m thick mudstone is derived from the following formula:

$$ c_{z02} = \sqrt {\frac{{2m_{z2} [\sigma_{zt2} ]}}{{\gamma_{z2} }}} $$
(3)

In the formula:\(c_{z02}\) is the first caving step distance of mudstone, the unit is m;\(m_{z2}\) is the thickness of mudstone, Its thickness is 3.60 m.; \(\gamma_{Z2}\) is the average gravity density of mudstone, average gravity density is 2.65 × 104 N/m3; \([\sigma_{zt2} ]\) is the average tensile strength of mudstone, average tensile strength is 2.13 MPa. The first caving step distance of 3.60 m thick mudstone is 24.06 m, which can be calculated by substitution parameters.

4.3 The first Caving Step Distance of Fine-Grained Sandstone

According to the theoretical prediction formula of the first caving step distance of direct roof, the first caving step of fine-grained sandstone with 9.90 m thickness is derived from the following formula:

$$ c_{z03} = \sqrt {\frac{{2m_{E} [\sigma_{zt3} ]}}{{\gamma_{E} }}} $$
(4)

In the formula: \(c_{z03}\) is the first caving step distance of fine-grained sandstone, the unit is m; \(m_{E}\) is the thickness of fine-grained sandstone, the thickness is 9.90 m; \(\gamma {}_{E}\) is the average gravity density of fine-grained sandstone, average gravity density is 2.45 × 104 N/m3; \([\sigma_{zt3} ]\) is the average tensile strength of fine-grained sandstone, average tensile strength is 3.12 MPa. The first caving step distance of 9.90 m thick fine-grained sandstone is 47.83 m, which can be calculated by substitution parameters.

4.4 The support strength of working face

According to the previous analysis, under the condition of caving mining, the roof structure is "rock gangue" structure, and the support surrounding rock relationship of "rock gangue" structure is shown in Fig. 6.

Fig. 6
figure 6

The support-surrounding rock relationship of "rock-gangue" structure

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Theoretically, it is predicted that the relationship between surrounding rock and support under the "rock gangue" structure is as follows:( Qian et al.2003; Wang 2009;Yu 2005)

$$ P_{T} = T\gamma_{t} + m^{\prime}_{z} \gamma_{z} + P_{c} $$
(5)

In the formula:\(P_{T}\) is the reasonable support strength of support design, the unit is Pa; \(T\) is the thickness of top coal, the thickness is 7.90 m;\(\gamma_{{\text{t}}}\) is the average gravity density of top coal, average gravity density is 2.45 × 104 N/m3;\(m^{\prime}_{z}\) is the thickness of the direct roof under arch structure, from the perspective of safety, the limit value is 13.5 m;\(\gamma_{z}\) is the average gravity density of the direct roof under arch structure, average gravity density is 2.67 × 104 N/m3,\(P_{c}\) is the contact stress which is produced in and near the contact area when two contact objects squeeze each other, according to a large number of measured data, the dynamic load coefficient of contact stress is generally 1.05–1.80, although the direct roof thickness is large and the ore pressure is not obvious, in order to ensure safety,\(P_{c} = 0.5(T\gamma_{t} + m^{\prime}_{z} \gamma_{z} )\). Substituting the relevant parameters into the formula,\(P_{T}\) is 1.13 MPa.

According to the above calculation, in the special case of failure and instability of "rock-gangue" structure, the reasonable support strength PT of the support shall be no less than 1.13 MPa to meet the requirements of roof support.

5 Numerical Simulation

Overlying strata movement law is an important research content of strata control technology, and it is also the main component of key mining technology of fully mechanized top coal caving. In this paper, the geological conditions and mining technology of fully mechanized top-coal caving face 101 with large mining height are studied. Using Mohr coulomb model in the state of the software to establish numerical model, Mohr coulomb model based on the destruction of the shear yield to define the material, and the yield stress is only related to the maximum and minimum principal stress, apply to the mining engineering problems such as underground excavation, so the simulation scheme of material constitutive relation for the Mohr coulomb model.

5.1 Model Building

In the model, the coal seam strike is the X-axis and the Y-axis along the vertical direction of lead is the Y-axis. The calculated model length is 200 m and the height is 100 m, of which the coal seam thickness is 13.4 m, the roof thickness is 66.6 m and the floor thickness is 20 m. The mining depth of the simulated working face is 800 m and the mining height of the simulator is 3.5 m. The vertical stress of 20 MPa was applied. In order to reduce the boundary influence, 40 m transition zones were reserved on both sides of the boundary of the model. The left and right boundary of the model limited the horizontal displacement and the lower boundary limited the vertical displacement. The bottom is also a fixed boundary, the velocity is zero, and the gravitational acceleration is 9.8 m /s2. The relationship between the initial model and the lithology is shown in Fig. 7.

Fig. 7
figure 7

Initial model and the lithology

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According to the rock mechanics test results provided by field geological survey and related research, considering the scale effect of rock, the rock mechanics parameters used in the simulation calculation are shown in Table 1.

Table 1 Mechanical parameters of rock mass
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5.2 Roof Breaking Characteristics

Under different advancing distances, the characteristics of roof caving distribution in the direction of working face under vertical stress are shown in Fig. 8.

Fig. 8
figure 8

Roof breaking characteristics under different advancement distances

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When working face advancing to 10 m, with the coal produced after the performance at this time as an end top-coal caving of direct roof, with fully mechanized working face continue to push forward, there was a direct roof in the working face advance to 20 m in the first caving coal seam part, direct roof in coal seam section initial caving step distance between 10 and 20 m, with the theoretical calculation of caving step distance consistent; When the working face is pushed to 30 m, the first caving of mudstone occurs, and the caving step distance is between 20 and 30 m, which is basically consistent with the caving step distance of mudstone calculated theoretically. Working face advancing to continue, because of the direct roof in fine-grained sandstone thickness, high strength, resistance to bending deformation ability, at the top of the vertical stress direct roof in the fine-grained sandstone section deformation is not easy to show, with the increase of advance distance, when the working face advancing to 40 m, at this time is fine-grained sandstone in central goaf downward bending deformation, rock cracks, on both ends of the working face of coal wall caving of direct roof in coal and mudstone in inverted trapezium, working face advancing to 50 m, the fine-grained sandstone portion of direct roof appeared first caving, this means that fine-grained sandstone first caving step distance is between 40 and 50 m, this is basically consistent with theoretical calculation.

5.3 Plastic Zone and Stress Distribution of Model

When the working face is advancing 20 m, the top coal caving is few, the stress is mainly concentrated in the coal seam and the upper coal seam of the goaf. Because of the large thickness, high strength and strong bending deformation ability, the two ends of the coal seam are in the stage of stress concentrated development, which shows plastic failure. When the working face is advancing to 40 m, the plastic area is dispersed and developed gradually to the fine-grained sandstone part of the direct roof, and the development degree is obviously different, which indicates that the fine-grained sandstone part of the direct roof is about to collapse; As the working face advanced to 60 m, the deformation and failure zone is mainly concentrated in the middle of the goaf, while the roof subsidence increases sharply, the collapse area is large, the stress changes violently, the direct roof overburden is greatly affected, and a series of motions such as direct roof complete deformation-displacement-damage-collapse occur, resulting in the direct roof collapse appears. At this time, the stress at both ends of the direct roof and coal seam is rapidly weakened, but the stress concentration zone is developing to the deep to reach the new stress equilibrium state.

Figure 9 shows the plastic region changes and stress environment changes of different lengths of working face.

Fig. 9
figure 9

Plastic zone and stress of the model

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According to the above analysis, plastic deformation and local failure occur in the overlying strata where the stress concentration and energy accumulation occur, and the coal and rock mass accumulate more energy under the action of mining stress, which leads to roof collapse, resulting in crushing expansion and deformation, which makes the rock stratum and coal wall move in a wide range of space. For fully mechanized top-coal caving mining large mining height, because of the coal seam thickness, the direct roof collapse, forming "rock- gangue" structure, in the mining process, the direct roof part of the top coal, mudstone, fine-grained sandstone in turn collapse phenomenon, due to the large thickness and high strength of fine-grained sandstone, the caving of fine-grained sandstone can be regarded as the primary pressure of the basic roof, resulting in severe ore pressure and a sharp increase in roof subsidence.

5.4 Field Observation

In order to deeply study the roof breaking characteristics and the law of ore pressure development in the fully mechanized top-coal caving face 101 with large mining height, the field measurement and ore pressure control technology were carried out in combination with the specific mining engineering technical conditions of the fully mechanized top-coal caving face 101 with large mining height. The characteristics of roof breaking and the law of ore pressure development in full mechanized caving with large mining height can be objectively reflected in the change of working resistance of hydraulic support column, and the relation between working resistance of hydraulic support and working face advancing distance reflects the process of ore pressure development caused by roof movement.

This mine adopts the working resistance monitoring subsystem KJ216 of fully mechanized top-coal mining support to monitor the working resistance of the hydraulic support online. The support is a group of four-column supporting and shielding hydraulic support composed of the front column and a group of rear columns. Each mining support digital pressure gauge can simultaneously collect the working resistance of the two channels, namely the front and rear columns. 4 groups of hydraulic supports were selected for online monitoring of working resistance, and the mean values of the columns and front of the supports were used for analysis. The measured results of working resistance of hydraulic supports with the advancing distance of the working face were shown in Fig. 10. The working resistance peak of the hydraulic support and the corresponding propulsion distance are shown in Table 2.

Fig. 10
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Hydraulic support working resistance measured results

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Table 2 The working resistance peak of the hydraulic support
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It can be seen from Fig. 10 and Table 2 that the working resistance of the hydraulic support on the working face increases significantly for the first time when the working face advances to 26.0 m. Combined with the theoretical prediction of the initial caving step distance of the mining face 101, it can be judged that the place is the initial caving step distance of the coal seam of the direct roof. The monitoring results of the working resistance of the support show that the working resistance of the support increases, and the goaf roof collapses in a large area and gradually fills the goaf, accompanied by frequent coal cannons and other macroscopic failure phenomena. The above theoretical analysis shows that the average initial caving step of the mudstone part in the direct roof is 24.06 m, which is not far from the field measured results, and it can be judged that the initial caving step of the mudstone part in the direct roof is 26.0 m. The change of working resistance of working face bracket also reflects the pressure of basic roof. After the initial collapse of the direct roof, when the working face advances to 46.4 m, the overall working resistance of the support appears the second obvious peak value. It is preliminarily judged that at this time, the initial pressure of the basic roof will occur. The above theoretical analysis shows that the theoretical estimated initial pressure step of 9.9 m fine-grained sandstone is 47.83 m. To sum up, it can be judged that when the working face advances to 46.7 m, the initial pressure of the basic roof and the initial pressure step distance are 46.7 m, that is, the initial caving step distance of fine-grained sandstone in the "rock-gangue" structure.

6 Conclusions

  1. (1)

    According to the specific mining conditions of the fully mechanized top-coal caving face 101 with large mining height, theoretical analysis is carried out on the roof fracture of the working face. The bulk fine-grained sandstone above the working face is extruded to form "rock—gangue" structure, through the theoretical calculation, the working face in the direct roof caving of the first caving step distance of 14.52 m, mudstone first caving step distance of 24.06 m, fine-grained sandstone of the first caving step distance of 47.83 m, in the special case of failure and instability of "rock-gangue" structure, the reasonable support strength PT of the support shall be no less than 1.13 MPa to meet the requirements of roof support.

  2. (2)

    Through numerical simulation of coal seam excavation process, it is obtained that the initial caving distance of the top coal seam of the working face is about 10 m–20 m, the initial caving step distance of mudstone is 20 m–30 m, and the initial caving step distance of fine-grained sandstone is 40 m–50 m, which is basically consistent with theoretical calculation. When the working face advances to 40 m, the plastic region develops in dispersion and gradually develops to the fine-grained sandstone part in the direct roof. It is also affected by the nature of the rock layer and the development degree is obviously different. When the working face advancing to 60 m, a series of motions such as direct roof complete deformation-displacement-damage-collapse occur, all caused by direct roof caving, the stress decrease rapidly on both ends of the direct roof and coal seam, the stress concentration area to deep development, in order to achieve a new stress equilibrium, the direct roof caving zone and plastic area of the upper strata as the main.

  3. (3)

    Through the analysis of the working resistance of the hydraulic support on site, it is obtained that the initial caving step distance of the direct roof is 26.0 m, that is, the caving step of the mudstone in the direct roof, and the initial fracture step distance of the basic roof is 46.4 m, that is, the initial caving step of the fine-grained sandstone in the rock-gangue structure, which is consistent with the theoretical analysis and the numerical simulation.