Strata behavior and control strategy of backfilling collaborate with caving fully-mechanized mining

2.1 Technical principle and layout of BCCFM workface

BCCFM does not refer to mechanical superposition of traditional caving method and backfilling mining. In the same workface, the BCCFM workface integrates gangue backfilling section and caving section in the same workface, combining the backfilling method and the caving method to joint manage the strata roof. Through strict equipment matching, the backfilling section and the caving section share a set of equipment to complete coal mining. In the backfilling section, the surface accumulation gangue or downhole output gangue is directly used for local filling with the assist of a series of filling equipment, including filling hydraulic support, bottom-dump scraper conveyor, filling material loader, and waste scraper conveyer.

The BCCFM workface is equivalent to adding caving workface based on the conventional backfilling workface, and the caving section is mining synchronously when the backfilling section is filling. This method retains the filling capacity and improves the productivity and efficiency compared to the conventional backfill mining, so that the BCCFM method can meet the multiple technical requirements of high filling efficiency, excellent productivity, and being capable of disposing gangue waste. The layout of the BCCFM workface is shown in Figure 1.

Figure 1

Layout of BCCFM workface.

2.2 Technical characteristics of BCCFM

1. The length of the conventional backfilling workface is usually no more than 120 m considering the limited length of the bottom-dump scraper conveyor and filling efficiency. However, the total designed length of the BCCFM workface can exceed 200 m, which can meet the single-face capacity requirement. Moreover, the BCCFM workface has reduced the number of workface layouts and the frequency of workface removing and decreased the tunnel drivage ratio compared with the conventional backfilling workface.

2. Continuous mechanized filling is realized in the backfilling section, and the designed length of the backfilling section is more than 100 m, which can meet the demand of consuming a large amount of waste gangue.

3. In the BCCFM workface, filling process and mining process are carried out simultaneously without interfering with each other, so that improved filling efficiency can be achieved while satisfying the demand to filling efficiency and production capacity at the meantime.

But compared to the traditional caving method and the conventional backfilling mining, the weaknesses and limitations of BCCFM was that the required quantity of hydraulic supports and operating personnel was big. The initial investment of the BCCFM workface will be large if the coal mine has no idle hydraulic supports.

2.3 Mining and filling process

The BCCFM workface mining process includes coal mining technology and filling technology. BCCFM workface completed coal mining a under the joint assist of filling hydraulic support, transitional hydraulic support and traditional fully mechanized hydraulic support. Three different types of hydraulic supports share a set of shearer and bottom-dump scraper conveyor for mining and transporting coal. The coal mining process is the same as the traditional caving coal mining process [21].

The BCCFM workface fills only the goaf in the filling section, and the coal mining and the filling process are conducted in parallel. Unlike the conventional gangue filling surface, which is filled after the coal cutting is finished and all the hydraulic supports are moved, the BCCFM workface can be filled after the support is moved and straightened. The filling process is designed to be starting from the tail to the head, and the specific process flow is as follows: the dumping holes are numbered and grouped along the filling direction, and four dumping holes are divided as a group. First, open the first group of four dumping holes in the tail direction. When the first dumping hole meets the filling requirements (the stacking height reaches the bottom of the bottom-dump conveyor when the natural blanking occurs, and the first dumping hole in the second group continues to be rammed repeatedly when the filling is dense), open the first dumping hole in the second group. Fill the dumping holes of each group in this order until the filling section is completely filled. The specific filling process is shown in Figure 2.

Figure 2

Filling process of BCCFM workface.

3.1 Construction of numerical model

A numerical model was built based on Ji₁₅-31010 BCCFM workface of Pingdingshan No. 12 coal mine. The size of the model is 400 m × 300 m × 130 m (length × width × height), and the total length of the BCCFM workface is 220 m (L_c = 120 m; L_k = 120 m). To eliminate the influence of boundary on model excavation, the coal pillars with a length of 40 m are left at the four sides of the model. The horizontal displacement of the four sides of the model is constrained, while the bottom of the model is constrained on the horizontal and vertical displacements. The gravity acceleration of 9.8 m/s² is applied to the entire model, and a uniform load of 22.5 MPa is applied on the top boundary to replace the 900 m thick overburden weight. The model has a total of 4,72,500 units and 5,09,627 nodes, Numerical model is shown in Figure 3.

Figure 3

Numerical model of BCCFM workface.

The Mohr–Coulomb model was adopted to simulate strata rock. Physical and mechanical parameters of coal seam were obtained according to the laboratory tests. The specific physical and mechanical parameters of the model are presented in Table 1.

3.2 Simulation method design of gangue filling material

To accurately simulate the compression characteristics of gangue filling material in the compaction process, YAS-5000 press machine was used in the laboratory to conduct compaction tests on gangue filling materials. The loading rate is 0.1 kN; the inner diameter of the steel tube is 250 mm, the height is 300 mm, and the wall thickness is 11.5 mm. The stress–strain curves are shown in Figure 4. Furthermore, Origin software toolbox of curve fitting was used to deal with the test data of gangue filling material. The fitting equation was obtained, as shown in equation (2). The correlation coefficient F is as high as 0.9966.

where ε is strain of gangue filling material and σ is axial stress, in unit of MPa. According to equation (2), the constitutive relationship of gangue filling materials is nonlinear under loading process, which indicates that gangues are not ideal elastic material. Therefore, it is impossible to choose fixed parameters to represent the stress–strain constitutive relationship in the compression process. When the gangue filling materials are compressed, the logarithmic nonlinear relationship between stress and strain is characterized by equation (3).

where A, B, and C are fitting parameters, A = 0.117, B = 2.45, and C = 1.12.

Figure 4

Compaction result of gangue filling material: (a) compaction equipment and (b) strain and stress curve.

Generally, compression modulus is used to represent the antideformation characteristics of granular materials [24]. By taking derivative of equation (3), the expression of compression modulus E_c of gangue filling materials can be obtained, as shown in equation (4):

The Mohr–Coulomb model was also used for simulation of the gangue filling material. The main parameters involved in the model include bulk modulus K, shear modulus G, internal friction angle θ, etc. The bulk modulus K, shear modulus G, and compression modulus E_c of gangue filling materials satisfy equations (5) and (6).

where μ is the Poisson’s ratio.

The nonlinear compression characteristic of gangue filling materials is programed by FISH language embedded in FLAC^3D software. According to the stress–strain relationship, the bulk modulus K and the shear modulus G are dynamically updated every 50 steps until the mining is finished.

3.3 Surround rock stress distribution of BCCFM workface

Taking the condition of φ = 85% and C = 120:100 as an example, this article analyzes the strata behavior of the BCCFM workface. When the BCCFM workface is advanced by 150 m, different profiles are made at advancing and tendency direction of workface, as shown in Figure 5. The stress (the stress is vertical stress, the same as below) distribution of the surrounding rock is analyzed. The distribution curve of the strata behavior is obtained after data processing, as shown in Figure 6.

1. The pressure distribution of the surrounding rock shows significant nonsymmetrical characteristics, with higher pressure in the caving section and lower pressure in the backfilling section, and there exists a high pressure transition area between the two sections.

2. Along the advancing direction of workface, the abutment pressure of backfilling section (A–A), caving section (B–B), and transition area (C–C) exhibits the pressure concentration, but the concentration degree and influence range are different. The peak value of the three sections is 0.49, 37.86, and 48.92 MPa, respectively, and the influence range is 17, 47 and 57 m, respectively. The degree of the abutment pressure can be ranked as follows: transition area > caving section > backfilling section. This indicates that the gangue filling body effectively bore the overburden loading and significantly reduced the influence range and peak value of abutment pressure in backfilling section.

3. Along the tendency direction of workface, there exists an obvious pressure concentration and rapid transition characteristics in the workface position (E–E) and goaf position (D–D). The peak values are 38.76 and 55.34 MPa, respectively. The influence range of high pressure in the transition area is about 5.53 m. However, there is no obvious pressure concentration phenomenon, but a slow transition characteristic in the front of workface position (F–F).

4. The peak value of lateral support pressure is 47.85 MPa in return airway, while that of intake airway is just 31.92 MPa. The peak value of caving side’s airway is 1.5 times of that of the backfilling side’s airway. The peak values all appear on the side of the protection coal pillar. The pressure of the surrounding rock in backfilling side is significantly lower than that of the caving side, and the difference should be emphasized in support design.

Figure 5

Profile of BCCFM workface.

Figure 6

The stress distribution of surround rock: (a) advancing direction and (b) tendency direction.

3.4 Main influencing factors

The influencing factors (φ and C) were analyzed through numerical simulation. Three-dimensional pressure distribution of surrounding rock under different mining conditions is shown in Figure 7.

Figure 7

Pressure distribution under different mining conditions: (a) φ = 85% (C = 120:100), (b) φ = 70% (C = 120:100), (c) φ = 55% (C = 120:100), (d) C = 50:170 (φ = 85%), (e) C = 100:120 (φ = 85%), and (f) C = 150:70 (φ = 85%).

To compare the effects of different influencing factors on strata behavior in the BCCFM workface, the pressure distributions shown in Figure 7 are further analyzed and the results are presented in Table 2.

4.1 Main influencing factors

There is a significant difference between the backfilling section and the caving section of BCCFM workface, and there exists a strong mine pressure area between the two sections. The mine pressure control of transition area is the core of the BCCFM workface. It is necessary to master the strong mine pressure law and its influencing characteristics. In this article, the analysis focuses on the effects of influence range of high pressure and the peak value of strong mine pressure in the transition area. The influence range of high pressure refers to the area where the overburden pressure value exceeds the average pressure value of the filling section by 1.3 times.

The strata behavior in the transition area affected by φ is shown in Figure 8.

Figure 8

Impact characteristics of φ: (a) peak value, (b) influence range, and (c) ratio of pressure.

Figure 7 shows that

1. When φ was 85%, 70%, and 55%, the maximum stress concentration factor in the transition area was 1.57, 1.48, and 1.19, respectively. It indicates that the pressure concentration degree decreased with the decrease of φ, and the decreasing amplitude became increasingly large.

2. When φ was 85%, 70%, and 55%, the influence range of high pressure was 5.53, 5.21, and 4.94 m, respectively. The influence range of high pressure in the transition area was all less than 6 m, without obvious change. This indicates that the factor φ had no significant effect on the influence range of high pressure in the transition area.

3. The strata behavior in the transition area was significantly higher than that in the backfilling section. With φ decreasing from 85% to 55%, amplitude decreases by as high as 43%.

The strata behavior in the transition area affected by C is shown in Figure 8. Figure 9 shows that

1. As the C gradually changed from 70:150 (<1) to 150:70 (>1), the maximum stress concentration factors in the transition area were 1.32, 1.61, 1.57, and 1.36, respectively. The maximum stress concentration factor first increased and then decreased. It indicates that the factor C had a significant effect on pressure concentration degree in the transition area.

2. As the C gradually changed from 70:150 to 150:70, the influence range of high pressure was 5.42, 5.53, 5.63, and 5.71 m, respectively, The influence range of high pressure in the transition area was less than 6 m and exhibited no significant change. It indicates that the factor C had no significant effect on the influence range of high pressure in the transition area.

3. With the C gradually changes from 70:150 to 150:70, the ratio of pressure between the transition area and the backfilling section increases at first and then decreases.

Figure 9

Control characteristics of C: (a) peak value, (b) influence range, and (c) ratio of pressure.

In conclusion, the pressure concentration in the transition area was significantly affected by factors φ and C. While factors φ and C had little influence on the influence range of high pressure, and the influence range was less than 6 m. Through analysis, it was found that change of factors φ and C only transformed the distribution of the pressure but did not change the fracture characteristics of overburden in the transition area. No matter how the mining conditions changes, the overburden damage in the transition area inevitably will occur.

4.2 Support structures in BCCFM workface

There are three types of hydraulic supports in the BCCFM workface, namely, filling hydraulic support, transition hydraulic support, and traditional hydraulic support. The basic frame of the filling hydraulic support and transition hydraulic support is the same. The support structures and working states are shown in Figure 10.

Figure 10

Support structures and working states.

Figure 9 shows that the filling hydraulic support is used in the backfilling section. The bottom-dump conveyor is hung at the bottom of the rear top beam to transfer the filling gangue material. The punning apparatus is installed on the base of the support to pre-compress the gangue material and improve the density of the filling body. Traditional hydraulic support is arranged in the caving section. The basic frame of the transition hydraulic support is the same as that of the filling hydraulic support, and the only difference is that the punning apparatus is replaced by the baffle plat, which prevents goaf gangues and caved rocks from flooding into the support operation space and thus avoids occurrence of equipment damage and casualties.

4.3 Control strategy of strong mining pressure in transition area

The backfilling section and the transition area have the same type of hydraulic support, while the strength of strata behavior in the transition area is higher than that of the backfilling section. When designing the support resistance, one must consider the situation that one support may have obvious different load bearings. Under these circumstances, this article proposes to properly improve the support strength of the transition hydraulic support within the influence range of transition area. Therefore, the high pressure concentration degree and the influence range of strong mining pressure should be calculated according to parameters φ and C. Moreover, according to the pressure concentration coefficient, the support strength of the transition hydraulic support should be properly increased so as to control the strong mining pressure in the transition area.

5.1 Control strategy of strong mining pressure

The total length of the Ji₁₅-31010 BCCFM workface is 218 m, in which the length of backfilling section is 120 m and the length of the caving section is 98 m. The average thickness of Ji₁₅ coal seam is 3.2 m. The aim of backfilling is to consume the waste gangue, so the filling of waste gangue is realized under the force of gravity. It has been calculated that the filling ratio is about 55%. The type of the traditional hydraulic support of the caving section is ZY6800/20/40, and the support strength is 1.0 MPa.

In the backfilling section, the type of the filling hydraulic support is ZC5200/20/40. According to the equivalent mining height theory [12], the support strength of the filling hydraulic support is 0.225–0.45 MPa. Finally, for the sake of security, the support strength is set to 0.84 MPa. The type of the hydraulic support in the transition area is as same as that of the filling hydraulic support. It can be obtained by numerical simulation that the pressure in the transition area is 1.3 times of that in the backfilling section. Through calculation, it can be known that theoretical strength of the transition hydraulic support should be 0.508–0.585 MPa. Considering the low filling ratio and the weak pressure concentration degree of the transition area, which is consistent with that of the filling hydraulic support, the support strength of the transition hydraulic support is also set to 0.84 MPa.

According to the simulation calculation, the influence range of strong mine pressure of the transition area is 5.53 m, so four transitional hydraulic supports (the width of support is 1.5 m) are arranged in Ji₁₅-31010 BCCFM workface. However, it needs to be emphasized that the support strength of the transition hydraulic support must be properly increased to ensure the safety when the filling ratio is high and the pressure concentration degree in the transition area is strong.

5.2 Measurement and analysis of mine pressure

In consideration of different types of supports in the BCCFM workface, 17 support resistance monitors are arranged along the Ji₁₅-31010 workface. Among them, one-channel resistance monitors (1#–7#) are arranged in traditional hydraulic support, two-channel resistance monitors (8#–17#) are installed in transition hydraulic support and filling hydraulic support. The specific monitoring scheme is shown in Figure 11.

Figure 11

Monitoring scheme.

The measured results of working resistance are shown in Figure 12.

Figure 12

Pressure distribution of support: (a) advance direction of workface and (b) tendency direction workface.

Figure 12 shows that:

1. After Ji₁₅-31010 workface advanced to 87 m, the working state of supports was stable and the safety valves of hydraulic support remained unopened. But the working resistance of supports in different sections is different.

2. Through monitoring date analysis of No. 44, 69, and 107 hydraulic supports (respectively represent caving section, transition area, and backfilling section), it can be known that the support resistance varied greatly in the caving section and the transition area and obvious periodic pressure phenomenon was observed, which indicates that the periodic break of overburden occurred. In contrast, the support resistance of filling hydraulic supports was relatively stable and the backfilling section exhibited no obvious periodic pressure phenomenon, indicating that the gangue filling material effectively reduced the intensity of pressure of the backfill section.

3. Along the tendency of workface, the average support resistance of supports in the backfilling section, transition area, and caving section was 25.9, 32.7, and 27.6 MPa, respectively. The average support resistance of the transition hydraulic support was 1.26 times of that of the filling hydraulic support. The field measurement result was consistent with numerical simulation, which verifies the reliability of control strategy of strong mining pressure in the transition area.