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Innovative approach based on roof cutting by energy-gathering blasting for protecting roadways in coal mines
Tunnelling and Underground Space Technology 99 (2020) 103387
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Innovative approach based on roof cutting by energy-gathering blasting for protecting roadways in coal mines
T
⁎
Xingyu Zhanga,b,c, Jinzhu Hua,b, Haojie Xuea,b, Wenbin Maoa,b, Yubing Gaoa,b, , Jun Yanga,b, Manchao Hea a
State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining & Technology, Beijing 100083, China c Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiroadway layout Cutting roof Pressure relief Deep roadway protection
As underground mining progresses to deeper levels, roadway control has become a significant issue in the development of modern mines with characteristics of a high yield and high efficiency. To protect deep roadways from dynamic disasters, an innovative approach based on cutting the roof by energy-gathering blasting for protecting the roadway (CREGBPR) was studied. First, the key technique of energy-gathering blasting was introduced. CREGBPR was developed by using the blasting technique and to directionally cut the roof above the roadway following a specific design. Theoretical analysis, numerical simulation, and field tests were combined to analyze the roadway control mechanism of CREGBPR. Compared with the original approach, the CREGBPR approach uses the broken and expanded rock mass to support the fractured structure of the main roof, reducing the impact on the retained roadway. Furthermore, the improvement in the structure eliminates the roof overhanging the roadway coal pillar. The pillars can serve as integrated supporting bodies that bear the abutment stress. Meanwhile, the stress environment is improved significantly by the roof cutting effect; the stress evolution includes new areas in which the stress distributes more uniformly. As a result, these independent effects induced by CREGBPR are integrated to protect the roadway effectively. A field test was conducted to verify the effectiveness of CREGBPR. The roadway convergence and roof weighting were significantly reduced. The research results prove that the CREGBPR approach is feasible for engineering applications and can protect roadways in deep mines.
1. Introduction Tunneling projects are indispensable for coal mining. Numerous roadways are constructed underground every year worldwide. Tunnel roadways function as connecting channels for transportation, pedestrians and ventilation (Wang et al., 2020). With the development of the longwall mining method, the multiroadway layout has been widely adopted to achieve high productivity and efficiency (Zhang et al., 2015), and the length of a roadway can be thousands of meters in one longwall mining unit (Zheng et al., 2016). However, mining and thus roadway construction continue to extend deeper. Deep rock masses in mining engineering are associated with high in situ stress and largescale disturbance; the disturbance generated by mining activities leads to abnormal stress concentration and then causes sudden and unpredictable destruction of the roadways, which is manifested by a large
range of instability and collapse events (Ranjith et al., 2017; Feng et al., 2019). This issue seriously threatens mine safety and production efficiency (Fairhurst, 2017), so engineers must first ensure roadway safety. Worldwide, researchers have been conducting extensive research on this topic. The existing research concentrates on the mechanisms and protective countermeasures of large deformation and rock burst along deep roadways. Yang et al. (2018) presented a case study on the failure mechanism and support techniques of deep soft rock roadways; the improved support system effectively controlled the large deformation of the surrounding rock. Yang et al. (2015) proposed coupling support via a double-layer truss to control the deformation failure of deep roadways based on analyses of the composite failure mechanism. Xie et al. (2015) proposed a powerful anchor support system and anchor-cable adaption technology to resist surrounding rock deformation; in practice, these
⁎ Corresponding author at: State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing 100083, China. E-mail address: [email protected] (Y. Gao).
https://doi.org/10.1016/j.tust.2020.103387 Received 14 June 2019; Received in revised form 21 February 2020; Accepted 8 March 2020 Available online 20 March 2020 0886-7798/ © 2020 Elsevier Ltd. All rights reserved.
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measures reduced deep roadway convergence during a roadway’s service period. Likewise, other research was conducted to optimize and strengthen support systems against large deformation of deep roadways (Lu et al., 2011; Wang et al., 2018; Yu et al., 2015). However, regarding more complicated and severe rock burst disasters in roadways, it is not sufficient to rely on only support to ensure roadway safety (Dou et al., 2014a; Ning et al., 2012). Active measures of pressure relief are essential. Conventional approaches for pressure relief are water softening, hydraulic fracturing and deep-hole blasting. However, the complexity of geological conditions and ground stress increase for deeper roadways. Conventional approaches cannot meet the pressure relief requirements because of their disadvantages (Dou et al., 2014a). Therefore, with the development of monitoring technology and control theory (Alber et al., 2009; Cai et al., 2018; Dou et al., 2014b; Wu et al., 2018), some newly refined pressure relief techniques for controlling roadways were proposed. He et al. (2012) introduced directional hydraulic fracturing, which is characterized by cutting out an initial groove in the borehole and then injecting high-pressure liquid to break the rock to protect the head entry. Guo et al. (2016) studied the mechanism of rock burst of hard and thick upper strata and proposed a control technology for bed separation grouting. Zhang et al. (2019) focused on pressure relief drilling and studied its mechanism in different borehole layouts to refine this pressure relief technology for relieving the stress concentration in a coal mass and preventing rock bursts in underground coal mines. Therefore, the enhancement and optimization of roadway support systems seem to prevent large roadway deformation. However, depending on only support measures is not a long-term strategy. An effective and easily applicable pressure relief technique should be explored for protecting deep roadways. The fractured structure of a thick and hard roof and high stress concentrations are two major factors leading to roadway failure (Dou et al., 2014b; Gao et al., 2019; Wang et al., 2015; Zhao et al., 2017). High stress concentrations as the static load and movement of the fractured structure as the dynamic load act on the roadway surrounding rock. When roadways are under the coupling effect of static and dynamic loads, roadways may become unstable (Li et al., 2008). Therefore, from the perspective of relieving these two triggering factors, an innovative approach based on cutting the roof by energy-gathering blasting (CREGB) was studied to solve the roadway control problem.
Fig. 1. Model of the BEGD. (a) Overview. (b) Detailed view of the inner structure.
moves deeper into the rock via the crack and distributes the stress farther in the designed direction after the stress wave loading. When boreholes are arranged in a line at a certain interval and the charge quantity in the BEGD is properly designed, the rock roof can be cut along the energy-gathering direction, whereas the rock in the other directions remains intact owing to the protection from the BEGD. Thus, the CREGB technique is based on the effect of multiborehole blasting. As shown in Fig. 3, multiple holes are detonated simultaneously, and the rock mass is directionally cut due to the induced tension. We conducted a field test to observe the actual crack effect in a blasting engineering project. The loading rock mass was granite porphyry with locally developed joints, and its uniaxial compressive strength was 120–200 MPa. The boreholes drilled in the rock mass were 2 m deep with a diameter of 40 mm and a spacing of 500 mm; they were aligned in a line with a strike of 180°. The explosive loaded in the BEGD was #2 rock emulsion explosive. We adopted a nonelectric millisecond detonator to detonate the boreholes simultaneously. The field test demonstrated that a directional crack connected the boreholes, but no visible cracks were observed in the other directions (Fig. 3).
2. Technique and method 2.1. CREGB technique for rock Based on the high compressive strength and low tensile strength characteristics of rock, the bilateral energy-gathering tensile blasting technique was invented to achieve directional blasting in rock masses (He et al., 2017). The key to this technique is the application of a bilateral energy-gathering device (BEGD). Fig. 1(a) shows the general structure of a BEGD, which is designed in a tubular shape that allows the borehole to be loaded with explosives. On either side, a line of energy-gathering holes is arranged, as shown in Fig. 1(b). As shown in Fig. 2, conventional explosives are loaded into the BEGD in a predrilled borehole. According to the synthetic action hypothesis of the stress wave and explosive gases, when the loaded dynamite explodes in the device, the explosion energy is gathered to develop jet energy flow, which is unloaded from the energy-gathering holes. The directions of the jet flow are the planned crack directions. Initial cracks are first caused by the explosive shock wave and stress wave. Because rock is a brittle material, the ensuing local failure is easily caused by the stress concentration in already failed areas. Therefore, the stress wave loading in the initial cracking process is the crucial stage, which governs the nature of all subsequent crack extension, branching and coalescence behaviors. With the generation of such cracks, the outflow of detonation products, such as explosive gases,
2.2. Principle and design of CREGBPR In longwall mining, coal extraction causes movement of the overlying stratum. This movement often leads to destructive phenomena, such as the large deformation of roadways, coal pillar failure, and roof falling. Based on underground pressure theory, the roof stratum movement is divided into three stages, as shown in Fig. 4. Fig. 4(a) shows that the roof stratum is continuously distributed in layers before mining because of the sedimentary nature of the rocks. Above the coal seam, several thin rock layers form the immediate roof. As the mining face advances, the immediate roof caves into the gangue, as shown in Fig. 4(b). The main roof controls the overlying movement as the principal stratum. After the caving of the immediate roof, the main roof rotates and subsides to form a structure similar to a voussoir beam. This rotation-subsidence leads to mining-induced pressure, as shown in Fig. 4(c). In most cases, the gangue formed by the collapsed immediate roof cannot fill the gob, especially when mining thick coal seams; thus, a large space is left between the main roof and the uncompacted 2
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Fig. 2. The mechanical model of bilateral energy-gathering tensile blasting. (a) Bilateral energy-gathering jet flow. (b) The mechanical effect of the energy-gathering tensile action in the x–z plane.
immediate roof collapses. This failure continues until the roof contacts the caved material and a new equilibrium state is reached. In this process, the caved roof forms random irregular rock fragments, which causes the volume of caved material to become larger than that of the original intact strata. However, the increase in volume does not compensate for the void space created from the floor to the main roof. As a result, the main roof begins to deform, bend, crack, and rotate downwards. When the roof and caved material are in contact, a certain amount of cover pressure from the strata movement is applied to the caved material. The bulked material is compressed so that it becomes denser and stiffer. During the bulked material compression stage, it is not possible to compress the material back into its original volume because the pressure is not infinite. As seen from the above analysis, the process from the initial disturbed state to a new equilibrium can be divided into two stages: the caved material bulking stage and the bulked material compaction stage. Two key parameters are used to describe these stages separately: the initial bulking factor (bi ) and the residual bulking factor (br ). As seen in Fig. 6, for the original approach (Fig. 6(a) and (c)), bi and br are separately stated as follows:
Fig. 3. CREGB mechanism and field test effect.
gangue. Because of the compressibility of the uncompacted gangue and the open space, the main roof exhibits a large deformation, which is likely to cause high mining-induced pressure. To control the disastrous movement of the main roof, an innovative approach using CREGB for protecting the roadway (CREGBPR) was presented. Fig. 5(a) shows that the CREGB technique is applied at the inclined top of the belt roadway. By cutting the roof at a certain height, the roof rock on both sides is separated along the cutting line. When the coal seam is unmined, a weak plane remains along the roadway strike direction in the roof. As the mining face advances, the roof close to the gob falls (forming gangue) due to the combined effect of the mining pressure and roof cutting. Thus, the rock to the roof cutting height is effectively used to compensate for the space. In this way, the broken and expanded rock mass provides the bulking force that resists later roof movement, as shown in Fig. 5(b). Consequently, compared with the original approach, CREGBPR resists the dangerous movement of the main roof. The deflection and rotation of the main roof are still small when the system enters a new equilibrium state, as shown in Fig. 5(c). On the other hand, the main roof body in motion is smaller with CREGBPR, as shown via the difference in Figs. 4 and 5.
bi =
h¯1a hI
(1)
br =
h¯1b hI
(2)
where h¯1a and h¯1b are the average heights of the gangue in the bulking and compaction stages of the original approach, respectively, and hI is the depositing height of the immediate roof in the strata. From the bulking stage to the compaction stage, the amount of compression is determined from the following equation:
C1 = h¯1a − h¯1b = hI (bi − br )
(3)
To obtain the subsidence of the lowest main roof, the unfilled space is expressed via
Δ1 = hI + m − h¯1a = m − hI (bi − 1)
(4)
where Δ1 is the gap height in the original approach and m is the mining height. According to Eqs. (3) and (4), the subsidence of the lowest main roof S1 is derived from the following equation:
S1 = Δ1 + C1 = m − hI (br − 1)
(5)
From Eqs. (4) and (5), specific to a given mining condition, the gap height Δ1 is determined by the initial bulking factor bi , and the final subsidence of the main roof is determined by only the residual factor br . In the CREGBPR approach, as shown in Fig. 6, the roof is cut at a certain depth in advance. After coal extraction, the upper roof within the cutting range caves under the mining pressure. If the roof cutting
3. Mechanism and effect of CREGBPR 3.1. Effect on roof structure The overburden strata are disturbed as the face advances and the 3
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Fig. 4. Schematic diagram of roof stratum movement with the original method. (a) Unmined. (b) Initial state after mining. (c) Final state after mining.
where h¯2a and h¯2b are the average heights of the gangue in the bulking and compaction stages by CREGBPR, respectively, and hc is the roof cutting length. θ is the angle from the roof cutting line to the normal direction, and hc cos θ is the roof cutting height and expressed as follows:
height is high enough and involves the main roof, the main roof will act as the immediate roof, caving instantly. Thus, bi and br are separately stated as
bi =
h¯2a hc cos θ
(6)
hc cos θ = hI + hcm h¯2b br = hc cos θ
(8)
where hcm is the main roof height within the roof cutting range. According to Eqs. (6), (7) and (8), the amount of compression of the
(7)
Fig. 5. Schematic diagram of roof stratum movement with the CREGBPR approach. (a) Unmined. (b) Initial state after mining. (c) Final state after mining. 4
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Fig. 6. Geological calculation model. (a) Bulking stage of the original approach. (b) Bulking stage of the CREGBPR approach. (c) Compaction stage of the original approach. (d) Compaction stage of the CREGBPR approach.
Fig. 7. Stress distribution comparison between the original approach and the CREGBPR approach.
In this equation, hc and θ are the variables. When the roof cutting height hc cos θ satisfies
gangue is as follows:
C2 = h¯2a − h¯2b = (bi − br ) hc cos θ
(9)
m bi − 1
The other component of the subsidence of the lowest main roof (i.e., the gap height Δ2 ) is calculated by the following equation:
hc cos θ ⩾
Δ2 = hc cos θ + m − h¯2a = m − hc cos θ (bi − 1)
the caved material can fill the gob. Therefore, the uncompacted gangue directly provides the bulking force to resist the upper roof movement in
(10) 5
(11)
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previously determined; on the other hand, the transferred stress is shifted to the deep gob and increases the stress there so that it approaches the initial stress (Campoli et al., 1993). The effect on the stress transfer of CREGBPR is obtained by comparison with the results of the original approach. The stress field for the CREGBPR approach is also divided into five zones. However, many notable changes arise due to the roof cutting, such as the zone length, the stress level, and the degree of concentration of the abutment stress. These changes are due to two substantial effects. One effect is that the roof structure over the pillar is optimized, as stated in Section 3.1, and the coal pillar is exempt from the influence of the overhanging roof and ultimately remains intact, without failing in compression after mining. The designed roadway protection pillar can serve as an integral supporting body against the abutment stress, wherein the stress is equally distributed. Meanwhile, the bulking body volume in the gob rim is sufficient to fill the gob to support the upper roof. These effects increase the stress level of the gob destress zone, but the stress there is still lower than the initial stress due to the mined-out effect; the stress transferred from zone V decreases, and the stress level in zone Ⅳ decreases accordingly. Therefore, the cover pressure applied on the roadway and coal pillar not only distributes more uniformly but also remains at a lower level. The other substantial effect is the cut-off on the stress transfer path in the low roof position, as shown in Fig. 7. The stress transferred from the gob is significantly restricted. During the stress redistribution, the released stress from the gob must seek a longer path for the stress transfer. There is an excellent possibility that this path change will lead to a shift in the position of the highest stress. To verify the correctness of the theoretical analysis and further investigate this topic, a 3D finite-difference fundamental model was constructed via FLAC3D to simulate the original approach and the CREGBPR approach, as shown in Fig. 8. The model size was 328 m × 80 m × 100 m, and its establishment was based on the field test in the Hongqinghe coalmine, as described in Section 4. In this model, the cross-sectional dimensions of the roadways were 6 m × 4 m, and the height of the coal seam was 6 m; the length of the coal pillars between the roadways was 30 m; and the burial depth of the coal seam was set to 700 m. The model boundaries except the upper boundary were fixed, and the upper boundary was assigned a vertical stress boundary condition with a value of 16.2 MPa; the horizontal in situ stress coefficients were all set to 1.2. The rock mass adopted the general Mohr-Coulomb constitutive model, and the coal mass adopted the strain-softening constitutive model to observe the yielding features during its postfailure stage (Hoek and Brown, 1997; Li et al., 2015). According to the geological experimental report of the coalmine and the research of Mohammad et al. (1997) and Cai et al. (2013), the input mechanical parameters were first scaled and then calibrated in reference to the in situ measurement of the roadway displacement; the parameters are listed in Table 1. The postfailure parameters of the coal mass were additionally calibrated by comparing the numerical simulation and laboratory test results of conventional triaxial compression tests. The fitting result is shown in Fig. 9, and the final scaled parameters for the postfailure of the coal mass are listed in Table 2. Based on the CREGBPR mechanism, the boreholes at a certain diameter would be connected by the directional continuous crack and thus form a roof cutting zone that separated the rock mass on both sides. Therefore, a roof cutting zone was generated with a 0.01 m width in the model during the simulation of the CREGBPR approach, and the built-in material model of “null” was used in FLAC3D to realize this roof cutting. Using the FLAC3D software, the model stress environment and block states were solved, as shown in Fig. 10. Fig. 10(a) shows the stress distribution of the original approach. While undergoing coal extraction, the system forms a large destress zone. The highest abutment stress is concentrated in the coal pillar due to the effect of stress transfer, and its value reaches 56.8 MPa, far beyond the in situ stress and the coal mass strength. Considering the plastic zone shown in Fig. 10(c), the part of the coal pillar adjacent to
the bulking stage. Eq. (11) potentially provides a design guide for CREGBPR. The subsidence S2 can be derived from
S2 = Δ2 + C2 = m − (br − 1) hc cos θ
(12)
From Eqs. (9), (10), and (12), the compression amount of the gangue C2 , the gap height Δ2 and the subsidence of the lowest main roof S2 are related to the roof cutting height hc cos θ in the CREGBPR approach. Notably, only the gap can be eliminated according to Eq. (11) because space-time relations exist among the roof caving, caved material expansion and expanded material compaction, and the compaction stage always exists. Accordingly, both the amount of compression of the gangue C2 and the subsidence of the lowest main roof S2 exist as well. Although the support by broken and expanded materials cannot eliminate the deformation, the CREGBPR approach is effective in reducing the fracture impact of the upper roof. Furthermore, the overhanging roof above the roadway pillar is eliminated, as shown by the comparisons in Fig. 6. The hanging arch consists mainly of the immediate roof (Zone I1) and part of the main roof (Zone M1), which is changed into gangue via the roof cutting. This elimination protects the integrity of the coal pillar and has a positive effect on roadway stability. 3.2. Effect on stress evolution and distribution Not only does the CREGBPR approach improve the fractural roof structure, but this approach can also optimize the evolution and distribution of mining-induced stress. Mining activity and roadway excavation cause underground stress redistribution. The induced abutment stress is applied to the adjacent solids and is 4–5 times the initial value (Wang et al., 2018). This high abnormal stress is due to the coupling effect of mining disturbances. As shown in Fig. 7, the abutment stress evolution and distribution for the original approach and the CREGBPR approach are combined for comparison and analysis. For the original approach, according to the stress distribution and magnitude, the stress field is divided into five zones: I: the initial stress zone, II: the excavation overstress zone, III: the excavation destress zone, Ⅳ: the coupling overstress zone, and Ⅴ: the gob-pillar destress zone. The horizontal red dashed line above the coal seam represents the initial stress level. Zone I is far from the disturbed areas such as the mining face and the excavated roadway. Thus, the stress in zone I is distributed uniformly and equals the in situ stress. From zone I to the left, a zone of higher stress occurs close to the retained roadway due to the influence of the excavation and is defined as the excavation overstress zone II. Zone III includes the area around the roadway and a small area of the coal mass adjacent to the roadway; here, the stress is lower than the initial stress because of the pressure relief effect of excavation. As a result, the released stress transfers to the adjacent coal wall, forming the abutment stress in zones II and IV. The stress in zone IV, except for the abutment stress from the roadway excavation, is affected by the mining activity, which is the principal formation factor of the high abutment stress. The coupling effect of the abutment stress arises in the roadway pillar. Notably, only part of the pillar, away from the gob, bears extraordinarily high stress. The other part, close to the gob, is compressed to failure by the overhanging roof. According to the elasticplastic theory, the part that fails cannot play the role of the protection pillar and bear a high stress, so a destress zone clearly exists in the pillar. When the high coupling stress acts on only a small intact part of the pillar close to the retained roadway, it causes the mass accumulation of elastic energy. The energy is easily activated and released by mining activity, which is unfavorable for roadway protection. Then, from zone Ⅳ to the left, there is a destress zone due to the pressure relief due to coal extraction and the effect of the partly yielded pillar. The destress area consists of the gob area and the pillar area close to the gob, so this zone is defined as the gob-pillar destress zone Ⅴ. Similarly, the stress released from zone Ⅴ is transferred to the adjacent zones. On the one hand, this transfer exacerbates the roadway pillar stress, as 6
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Fig. 8. 3D numerical calculation model.
the belt roadway fails and cannot bear the high stress, so the high abutment stress exists in the intact coal mass close to the failure zone. These results correspond to the work of Mark (1990), who used field measurements from an instrumented pillar to show that the initial abutment loads were usually observed as stress increases near the ribs of the pillar; as the abutment load increased, the stress peaks shifted to the core of the pillar, while the stress meter measurements near the rib often indicated fully transferred loads. Moreover, considering the possible influence of the overhanging roof, as was analyzed previously, the pillar rib may be subject to more severe damage, which would lead to a further shift in the stress. Some research has concluded and verified that with the conversion between the elastic strain energy and the plastic strain work, the mass stores elastic energy in the coal pillar, and the strain localization in the rock and coal masses present a great threat to the retained roadway stability and may even cause rock burst and coal bumps (Wang, 2006; Liu et al., 2010; Wang and Kaunda, 2019; Weng et al., 2017), especially in deep mining conditions and under dynamic mining disturbance (Vacek, 2001; Whyatt, 2008). Therefore, this stress evolution and distribution of the original approach (Fig. 10(a)) is not favorable for the stability of the retained roadway and cannot adapt to the challenges of deep mining. CREGBPR blocks the stress transfer in the lower roof from the gob to the coal pillar side. As shown in Fig. 10(b), the stress transfer path is altered into an arc that passes the outside end of the roof cutting line. The transfer distance is stretched. The path end that the highest stress lies in is shifted to above the coal pillar, changing from a position in the coal to a position in the rock. Moreover, the highest stress value is 49.4 MPa, a 13.0% decrease
Fig. 9. Calibration result of coal specimen by conventional triaxial compression test.
compared to that determined with the original approach, and the rock strength is greater than that of the coal. As a result, roof cutting not only transfers the highest abutment stress into the stronger rock but also reduces the abutment stress magnitude. From the comparison of the plastic zones between these two approaches, as shown in Fig. 10(c) and (d), the failure area of the roadway surroundings with the original approach is obviously larger than that with the CREGBPR approach. With the CREGBPR approach, the damage depth on the pillar side of the
Table 1 Numerical simulation model properties. Strata
Bulk modulus (GPa)
Shear modulus (GPa)
Density (kg/m3)
Friction angle (°)
Cohesion (MPa)
Tension (MPa)
Overlying strata Fine sandstone Pelitic siltstone (roof) Coal seam Pelitic siltstone (floor)
3.68 2.97 2.15 1.62 2.02
2.42 1.86 1.04 0.98 1.02
2500 2400 2300 1500 2300
38 36 34 28 33
7.4 5.9 4.2 3.8 4.0
5.0 4.2 3.6 3.0 3.5
7
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shown in Fig. 12, the immediate roof is composed of soft pelitic siltstone with an average thickness of 8.18 m; the upper roof is primarily hard sandstone, and the lowest main roof is fine sandstone with an average thickness of 10.65 m; the immediate floor is soft pelitic siltstone with an average thickness of 17.35 m. The zone of 12.0 m roofcutting depth in the test zone was selected for the following analysis. Detailed support conditions are also shown in Fig. 12. To understand the displacement pattern of the surrounding rock of the retained roadway and verify the pressure relief effect of CREGBPR, the test utilized displacement sensors that were installed in the material roadway and stress sensors that were installed on the working face support. The detailed layout of the sensors is shown in Fig. 13. Three displacement measuring points were arranged with a spacing of 30 m to monitor each approach. Sixteen hydraulic supports among the 145 supports were equipped with stress sensors. The displacement sensors were used to monitor the roof-to-floor and rib-to-rib convergence; the stress sensors were used to monitor the pressure variation in the supports to understand the dynamic change in the roof structure.
Table 2 Variation in postfailure properties with plastic strain. Coal mass
Plastic strain
Friction angle (°)
Cohesion (MPa)
0 0.01 0.03 1
28 24 23 23
3.8 1.6 0.8 0.8
material roadway decreases from 7.57 m to 4.50 m, and the damage height of the roadway roof decreases from 5.37 m to 3.18 m. Thus, the roadway stability is improved. In addition, CREGBPR maintains the overall stability of the coal pillar and makes it a uniformly loaded area, which reduces the possibility of rock burst and coal bump events. The results of the numerical simulation analysis correspond to the results of the previous theoretical analysis and can be used to further explore the stress transfer process in the system.
4.2. Analysis of the pressure relief effect 4. Field test The roadway deformation can be directly used to evaluate the pressure relief effect. Fig. 14 shows the measured convergence curves of the material roadway as the working face advanced away from the displacement measuring points for both approaches. First, Fig. 14(a) shows that the vertical convergence gradually increased as the working face advanced. From the trends of the six curves, when the face reached approximately 130 m, the roof-to-floor displacement increased less compared to that of the previous deformation. The ends of the curves show that the roof-to-floor convergence of the CREGBPR approach became stable. The measured maximum convergence stabilized at approximately 218 mm. However, the convergence of the original approach showed a small increase at the end; the measured maximum convergence reached 798 mm when the face advanced 162.3 m. As shown in Fig. 14(b), the horizontal convergence pattern of the material roadway was similar to the vertical convergence pattern. The measured maximum convergence of the original approach reached 650 mm with
4.1. Test overview The Hongqinghe coalmine, which is located in Ordos City, Inner Mongolia Province, North China, was selected for the case test. Hongqinghe is a modern mine with a high yield and high efficiency, and its annual approved production capacity is 1.5 × 109 kg. The mined coal seam is the No. 3−1, and its average thickness is 6.2 m with an average inclination of 3°. The average burial depth is 731.5 m. The experimental site was in the 3−1101 working face (Fig. 11), which employed fully mechanized mining technology to mine the overall height simultaneously. The incline and strike lengths of the face were 245.7 m and 3212.7 m, respectively. The field test began at 2811 m from the open-off cut. The CREGB technique was used in the 3−1101 belt roadway, as shown in Fig. 11. The drill data, especially the roof condition, near the belt roadway were analyzed in the test zone. As
Fig. 10. Stress distribution, evolution and plastic zone. (a) Stress state of the original approach. (b) Stress state of the CREGBPR approach. (c) Plastic zone of the original approach. (d) Plastic zone of the CREGBPR approach. 8
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Fig. 11. Layout of the test site.
Fig. 12. Section of roadway conditions and the CREGBPR design.
Fig. 13. Layout of displacement and stress sensors.
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Fig. 14. Deformation curves of the material roadway. (a) Roof-to-floor convergence. (b) Rib-to-rib convergence.
pressure clearly decreased after adopting CREGBPR; a larger lowpressure area (0–3800 kN) and no obvious high-pressure area (11,000–15,000 kN) existed in zone 4 compared to those in zone 3. The average pressure decreased from 6850.90 kN in zone 3 to 5572.00 kN in zone 4, which is an 18.7% drop. In the vertical comparison, the pressure levels were comparable between zones 1 and 3 in the original approach, and their average pressures had only a 212.24 kN difference. When the working face entered the range of the CREGBPR approach, the pressure on the roof cutting side (zone 4) was 2372.90 kN less than that on the opposite side; the pressure decreased by 29.9% under the influence of roof cutting. In addition, the pressure level increased from zone 1 to zone 2, while the pressure in zone 4 was still lesser than that in zone 3. The above comparative analyses illustrate the pressure relief effect of CREGBPR on the roof cutting side, causing the mine pressure to become more moderate; the results also verify the roadway convergence and explain the support effect of the collapsed gangue formed by roof cutting. CREGBPR was used to cut the roof directionally via the energygathering blasting technique. The roof structure was optimized during the mining process, and the stress evolution and distribution in the system were improved. Fig. 16 compares the effects before and after adoption of CREGBPR. The convergence of the retained roadway
an advanced distance of 162.3 m. The curve trends showed that the convergence would increase continually as the face advanced. In contrast, the measured maximum convergence of the CREGBPR approach nearly stabilized at 236 mm when the working face reached 150.1 m. Additionally, the convergence of the original approach increased more rapidly at the beginning of the mining. For the CREGBPR approach, when the distance was in the range of 20–40 m, the convergence began to increase rapidly. Besides, the displacement curves of the different sensors corresponding to the CREGBPR approach were closer to each other than those corresponding to the original approach. These phenomena may suggest that the CREGBPR approach influenced the roof movement and then changed the deformation process, reducing the sagging rotation of the fractured roof compared to that with the original approach. To clarify the changes in the roof movement, the pressure data extracted from the supports were further processed and analyzed. As shown in Fig. 15, a plot of the support pressure was generated to present the pressure history before and after the adoption of the CREGBPR approach. We selected and divided the roadway side area of the working face into four zones for comparison to illustrate the pressure relief effect. Table 3 gives the average support pressure of the four zones. In the horizontal comparison between zones 3 and 4, the mining 10
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Fig. 15. Support pressure monitoring results.
divided into two stages: the gangue bulking and gangue compaction stages. The upper roof deformation and the minimum roof cutting height were calculated by two control parameters: the initial and residual bulking factors. The CREGBPR approach eliminates the overhanging roof above the pillar and the roof deformation in the bulking stage, and reduces the impact in the compaction stage, thereby protecting the integrity of the coal pillar and the roadway stability. In addition, CREGBPR cuts off the original stress transfer path in the lower roof and thus alters the stress transfer path, which optimizes the stress evolution and distribution of the roadway surroundings. The failure area of the retained roadway surrounding rock for the original approach was smaller with the CREGBPR approach than that with the original approach, and the potential threat of rock burst and coal bump was reduced. Finally, a field test was performed. The monitoring results and the field application showed that the roadway convergence was reduced and the pressure relief effect was significant. Therefore, the feasibility and effectiveness of the CREGBPR approach were illustrated.
Table 3 Average mine pressure in different zones. Position Zone Average pressure (kN)
No-roof-cutting side
Roof cutting side
1
2
3
4
7063.14
7944.90
6850.90
5572.00
decreased significantly, which ensured roadway safety and improved productivity. Thus, the CREGBPR approach is an effective way to achieve roadway pressure relief and mining safety. 5. Conclusions In this study, an innovative approach for protecting the retained roadway was proposed based on cutting the roof via energy-gathering blasting. Compared to the original approach, the advantages of the CREGBPR approach are reflected in the optimizations of the roof structure and the stress evolution and distribution, which control the mechanism of pressure relief via CREGBPR. A geological calculation model was established to further explore the effect on the roof structure. The change in the roof structure was
CRediT authorship contribution statement Xingyu
Zhang:
Conceptualization,
Fig. 16. Overall effect of the retained roadway before and after the CREGBPR application. 11
Methodology,
Funding
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acquisition, Software, Writing - original draft, Formal analysis. Jinzhu Hu: Investigation, Visualization, Resources, Writing - review & editing. Haojie Xue: Investigation, Visualization. Wenbin Mao: Software, Investigation, Visualization. Yubing Gao: Conceptualization, Methodology, Funding acquisition, Supervision, Resources, Writing review & editing. Jun Yang: Supervision. Manchao He: Conceptualization, Methodology, Funding acquisition.
design: a case study. Rock Mech. Rock Eng. 48 (1), 305–318. Li, X., Zhou, Z., Lok, T.S., Hong, L., Yin, T., 2008. Innovative testing technique of rock subjected to coupled static and dynamic loads. Int. J. Rock Mech. Min. 45 (5), 739–748. Liu, S.X., Liu, C.W., Cao, L., 2010. Post-peak softening of porosity coal and its influence to rock burst occurred in work face. J. China Coal Soc. 35 (12), 1990–1996. Lu, X.L., Liu, Q.S., Su, P.F., 2011. Deformation mechanism and support measures for stress-induced cracked rock mass of deep coal mine roadway. Geomech. Geotech.: From Micro to Macro Two Set 845–853. Mark, C., 1990. Pillar Design Methods for Longwall Mining. U.S. Dept. of the Interior, Bureau of Mines, Washington, DC. Mohammad, N., Reddish, D.J., Stace, L.R., 1997. The relation between in situ and laboratory rock properties used in numerical modelling. Int. J. Rock Mech. Min. 34 (2), 289–297. Ning, J.G., Liu, X.S., Tan, Y.L., 2012. Mechanism of rock-burst prevention by synergizing pressure relief and reinforcement of surrounding rocks with zonal disintegration in deep coal roadway. Disaster Adv. 5, 1237–1242. Ranjith, P.G., Zhao, J., Ju, M.H., De Silva, R.V.S., Rathnaweera, T.D., Bandara, A.K.M.S., 2017. Opportunities and challenges in deep mining: a brief review. Eng. -Proc. 3, 546–551. Vacek, J., 2001. Stress distribution in a coal seam before and after bump initiation. Acta Polytechnica 41, 4–5. Wang, F., Kaunda, R., 2019. Assessment of rockburst hazard by quantifying the consequence with plastic strain work and released energy in numerical models. Int. J. Min. Sci. Technol. 29 (1), 93–97. Wang, G.F., Gong, S.Y., Li, Z.L., Dou, L.M., Cai, W., Mao, Y., 2015. Evolution of stress concentration and energy release before rock bursts: two case studies from Xingan coal mine, Hegang, China. Rock Mech. Rock Eng. 49, 3393–3401. Wang, P., Zhao, J., Feng, G., Wang, Z., 2018. Interaction between vertical stress distribution within the goaf and surrounding rock mass in longwall panel systems. J. S. Afr. I. Min. Metall. 118, 745–756. Wang, X.B., 2006. Numerical simulation of deformation and failure for two bodies model composed of rock and coal. Rock Soil Mech. 27 (7), 1066–1070. Wang, Q., He, M.C., Yang, J., Gao, H.K., Jiang, B., Yu, H.C., 2018. Study of a no-pillar mining technique with automatically formed gob-side entry retaining for longwall mining in coal mines. Int. J. Rock Mech. Min. 110, 1–8. Wang, Y.J., He, M.C., Yang, J., Wang, Q., Liu, J.N., Tian, X.C., Gao, Y.B., 2020. Case study on pressure-relief mining technology without advance tunneling and coal pillars in longwall mining. Tunn. Undergr. Space Technol. 97, 103236. Weng, L., Huang, L., Taheri, A., Li, X., 2017. Rockburst characteristics and numerical simulation based on a strain energy density index: a case study of a roadway in Linglong gold mine. China. Tunn. Undergr. Space Technol. 69, 223–232. Whyatt, J., 2008. Dynamic failure in deep coal: recent trends and a path forward. In: Proceedings 27th International Conference on Ground Control in Mining, pp. 37–45. Wu, Q.S., Jiang, J.Q., Wu, Q.L., Xue, Y.C., Kong, P., Gong, B., 2018. Study on the fracture of hard and thick sandstone and the distribution characteristics of microseismic activity. Geotech. Geol. Eng. 36, 3357–3373. Xie, S.R., Li, E.P., Li, S.J., Wang, J.G., He, C.C., Yang, Y.F., 2015. Surrounding rock control mechanism of deep coal roadways and its application. Int. J. Min. Sci. Technol. 25, 429–434. Yang, J., Wang, D., Shi, H.Y., Xu, H.C., 2015. Deformation failure and countermeasures of deep tertiary extremely soft rock roadway in Liuhai coal mine. Int. J. Min. Sci. Technol. 25, 231–236. Yang, X.J., Wang, E.Y., Wang, Y.J., Gao, Y.B., Wang, P., 2018. A study of the large deformation mechanism and control techniques for deep soft rock roadways. Sustainability 10 (4), 1100. Yu, W.J., Wang, W.J., Chen, X.Y., Du, S.H., 2015. Field investigations of high stress soft surrounding rocks and deformation control. J. Rock Mech. Geotech. 7, 421–433. Zhang, S., Li, Y., Shen, B., Sun, X., Gao, L., 2019. Effective evaluation of pressure relief drilling for reducing rock bursts and its application in underground coal mines. Int. J. Rock Mech. Min. 114, 7–16. Zhang, Z., Bai, J., Chen, Y., Yan, S., 2015. An innovative approach for gob-side entry retaining in highly gassy fully-mechanized longwall top-coal caving. Int. J. Rock Mech. Min. 80, 1–11. Zhao, T., Liu, C.Y., Yetilmezsoy, K., Zhang, B.S., Zhang, S., 2017. Fractural structure of thick hard roof stratum using long beam theory and numerical modeling. Environ. Earth Sci. 76, 751. Zheng, Y.L., Zhang, Q.B., Zhao, J., 2016. Challenges and opportunities of using tunnel boring machines in mining. Tunn. Undergr. Space Technol. 57, 287–299.
Acknowledgement This work was supported by the State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing (No. SKLGDUEK1928), the State Key Research Development Program of China (No. 2016YFC0600900), a program of the China Scholarship Council (No. 201806430070) and the China Postdoctoral Science Foundation (No. 2019M650896), which are gratefully acknowledged. Declaration of Competing Interest The authors declare that they have no conflict of interest. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2020.103387. References Alber, M., Fritschen, R., Bischoff, M., Meier, T., 2009. Rock mechanical investigations of seismic events in a deep longwall coal mine. Int. J. Rock Mech. Min. 46, 408–420. Cai, M., He, M., Liu, D., 2013. Rock Mechanics and Engineering, second ed. Science Press, Beijing. Cai, W., Dou, L.M., Zhang, M., Cao, W.Z., Shi, J.Q., Feng, L.F., 2018. A fuzzy comprehensive evaluation methodology for rock burst forecasting using microseismic monitoring. Tunn. Undergr. Sp. Tech. 80, 232–245. Campoli, A.A., Barton, T.M., Dyke, F., Gauna, M., 1993. Gob and gate road reaction to longwall mining in bump-prone strata. RI: Bureau of Mines 48, 9445. Dou, L.M., Mu, Z.L., Li, Z.L., Cao, A.Y., Gong, S.Y., 2014a. Research progress of monitoring, forecasting, and prevention of rockburst in underground coal mining in China. Int. J. Coal Sci. Technol. 1, 278–288. Dou, L.M., He, X.Q., He, H., He, J., Fan, J., 2014b. Spatial structure evolution of overlying strata and inducing mechanism of rockburst in coal mine. T. Nonferr. Metal. Soc. 24, 1255–1261. Fairhurst, C., 2017. Some challenges of deep mining. Eng. -Proc. 3, 527–537. Feng, G., Kang, Y., Sun, Z.D., Wang, X.C., Hu, Y.Q., 2019. Effects of supercritical CO2 adsorption on the mechanical characteristics and failure mechanisms of shale. Energy 173, 870–882. Gao, Y.B., Wang, Y.J., Yang, J., Zhang, X.Y., He, M.C., 2019. Meso-and macroeffects of roof split blasting on the stability of gateroad surroundings in an innovative nonpillar mining method. Tunn. Undergr. Sp. Technol. 90, 99–118. Guo, W.J., Li, Y.Y., Yin, D.W., Zhang, S.C., Sun, X.Z., 2016. Mechanisms of rock burst in hard and thick upper strata and rock-burst controlling technology. Arab. J. Geosci. 9, 561. He, H., Dou, L., Fan, J., Du, T., Sun, X., 2012. Deep-hole directional fracturing of thick hard roof for rockburst prevention. Tunn. Undergr. Sp. Technol. 32, 34–43. He, M.C., Zhang, X.H., Zhao, S., 2017. Directional destress with tension blasting in coal mines. ISRM Eur. Rock Mech. Symp. Eurock 2017 (191), 89–97. Hoek, E., Brown, E.T., 1997. Practical estimates of rock mass strength. Int. J. Rock Mech. Min. 34 (8), 1165–1186. Li, W., Bai, J., Peng, S., Wang, X., Xu, Y., 2015. Numerical modeling for yield pillar
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Innovative approach based on roof cutting by energy-gathering blasting for protecting roadways in coal mines
T
⁎
Xingyu Zhanga,b,c, Jinzhu Hua,b, Haojie Xuea,b, Wenbin Maoa,b, Yubing Gaoa,b, , Jun Yanga,b, Manchao Hea a
State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining & Technology, Beijing 100083, China c Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiroadway layout Cutting roof Pressure relief Deep roadway protection
As underground mining progresses to deeper levels, roadway control has become a significant issue in the development of modern mines with characteristics of a high yield and high efficiency. To protect deep roadways from dynamic disasters, an innovative approach based on cutting the roof by energy-gathering blasting for protecting the roadway (CREGBPR) was studied. First, the key technique of energy-gathering blasting was introduced. CREGBPR was developed by using the blasting technique and to directionally cut the roof above the roadway following a specific design. Theoretical analysis, numerical simulation, and field tests were combined to analyze the roadway control mechanism of CREGBPR. Compared with the original approach, the CREGBPR approach uses the broken and expanded rock mass to support the fractured structure of the main roof, reducing the impact on the retained roadway. Furthermore, the improvement in the structure eliminates the roof overhanging the roadway coal pillar. The pillars can serve as integrated supporting bodies that bear the abutment stress. Meanwhile, the stress environment is improved significantly by the roof cutting effect; the stress evolution includes new areas in which the stress distributes more uniformly. As a result, these independent effects induced by CREGBPR are integrated to protect the roadway effectively. A field test was conducted to verify the effectiveness of CREGBPR. The roadway convergence and roof weighting were significantly reduced. The research results prove that the CREGBPR approach is feasible for engineering applications and can protect roadways in deep mines.
1. Introduction Tunneling projects are indispensable for coal mining. Numerous roadways are constructed underground every year worldwide. Tunnel roadways function as connecting channels for transportation, pedestrians and ventilation (Wang et al., 2020). With the development of the longwall mining method, the multiroadway layout has been widely adopted to achieve high productivity and efficiency (Zhang et al., 2015), and the length of a roadway can be thousands of meters in one longwall mining unit (Zheng et al., 2016). However, mining and thus roadway construction continue to extend deeper. Deep rock masses in mining engineering are associated with high in situ stress and largescale disturbance; the disturbance generated by mining activities leads to abnormal stress concentration and then causes sudden and unpredictable destruction of the roadways, which is manifested by a large
range of instability and collapse events (Ranjith et al., 2017; Feng et al., 2019). This issue seriously threatens mine safety and production efficiency (Fairhurst, 2017), so engineers must first ensure roadway safety. Worldwide, researchers have been conducting extensive research on this topic. The existing research concentrates on the mechanisms and protective countermeasures of large deformation and rock burst along deep roadways. Yang et al. (2018) presented a case study on the failure mechanism and support techniques of deep soft rock roadways; the improved support system effectively controlled the large deformation of the surrounding rock. Yang et al. (2015) proposed coupling support via a double-layer truss to control the deformation failure of deep roadways based on analyses of the composite failure mechanism. Xie et al. (2015) proposed a powerful anchor support system and anchor-cable adaption technology to resist surrounding rock deformation; in practice, these
⁎ Corresponding author at: State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing 100083, China. E-mail address: [email protected] (Y. Gao).
https://doi.org/10.1016/j.tust.2020.103387 Received 14 June 2019; Received in revised form 21 February 2020; Accepted 8 March 2020 Available online 20 March 2020 0886-7798/ © 2020 Elsevier Ltd. All rights reserved.
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measures reduced deep roadway convergence during a roadway’s service period. Likewise, other research was conducted to optimize and strengthen support systems against large deformation of deep roadways (Lu et al., 2011; Wang et al., 2018; Yu et al., 2015). However, regarding more complicated and severe rock burst disasters in roadways, it is not sufficient to rely on only support to ensure roadway safety (Dou et al., 2014a; Ning et al., 2012). Active measures of pressure relief are essential. Conventional approaches for pressure relief are water softening, hydraulic fracturing and deep-hole blasting. However, the complexity of geological conditions and ground stress increase for deeper roadways. Conventional approaches cannot meet the pressure relief requirements because of their disadvantages (Dou et al., 2014a). Therefore, with the development of monitoring technology and control theory (Alber et al., 2009; Cai et al., 2018; Dou et al., 2014b; Wu et al., 2018), some newly refined pressure relief techniques for controlling roadways were proposed. He et al. (2012) introduced directional hydraulic fracturing, which is characterized by cutting out an initial groove in the borehole and then injecting high-pressure liquid to break the rock to protect the head entry. Guo et al. (2016) studied the mechanism of rock burst of hard and thick upper strata and proposed a control technology for bed separation grouting. Zhang et al. (2019) focused on pressure relief drilling and studied its mechanism in different borehole layouts to refine this pressure relief technology for relieving the stress concentration in a coal mass and preventing rock bursts in underground coal mines. Therefore, the enhancement and optimization of roadway support systems seem to prevent large roadway deformation. However, depending on only support measures is not a long-term strategy. An effective and easily applicable pressure relief technique should be explored for protecting deep roadways. The fractured structure of a thick and hard roof and high stress concentrations are two major factors leading to roadway failure (Dou et al., 2014b; Gao et al., 2019; Wang et al., 2015; Zhao et al., 2017). High stress concentrations as the static load and movement of the fractured structure as the dynamic load act on the roadway surrounding rock. When roadways are under the coupling effect of static and dynamic loads, roadways may become unstable (Li et al., 2008). Therefore, from the perspective of relieving these two triggering factors, an innovative approach based on cutting the roof by energy-gathering blasting (CREGB) was studied to solve the roadway control problem.
Fig. 1. Model of the BEGD. (a) Overview. (b) Detailed view of the inner structure.
moves deeper into the rock via the crack and distributes the stress farther in the designed direction after the stress wave loading. When boreholes are arranged in a line at a certain interval and the charge quantity in the BEGD is properly designed, the rock roof can be cut along the energy-gathering direction, whereas the rock in the other directions remains intact owing to the protection from the BEGD. Thus, the CREGB technique is based on the effect of multiborehole blasting. As shown in Fig. 3, multiple holes are detonated simultaneously, and the rock mass is directionally cut due to the induced tension. We conducted a field test to observe the actual crack effect in a blasting engineering project. The loading rock mass was granite porphyry with locally developed joints, and its uniaxial compressive strength was 120–200 MPa. The boreholes drilled in the rock mass were 2 m deep with a diameter of 40 mm and a spacing of 500 mm; they were aligned in a line with a strike of 180°. The explosive loaded in the BEGD was #2 rock emulsion explosive. We adopted a nonelectric millisecond detonator to detonate the boreholes simultaneously. The field test demonstrated that a directional crack connected the boreholes, but no visible cracks were observed in the other directions (Fig. 3).
2. Technique and method 2.1. CREGB technique for rock Based on the high compressive strength and low tensile strength characteristics of rock, the bilateral energy-gathering tensile blasting technique was invented to achieve directional blasting in rock masses (He et al., 2017). The key to this technique is the application of a bilateral energy-gathering device (BEGD). Fig. 1(a) shows the general structure of a BEGD, which is designed in a tubular shape that allows the borehole to be loaded with explosives. On either side, a line of energy-gathering holes is arranged, as shown in Fig. 1(b). As shown in Fig. 2, conventional explosives are loaded into the BEGD in a predrilled borehole. According to the synthetic action hypothesis of the stress wave and explosive gases, when the loaded dynamite explodes in the device, the explosion energy is gathered to develop jet energy flow, which is unloaded from the energy-gathering holes. The directions of the jet flow are the planned crack directions. Initial cracks are first caused by the explosive shock wave and stress wave. Because rock is a brittle material, the ensuing local failure is easily caused by the stress concentration in already failed areas. Therefore, the stress wave loading in the initial cracking process is the crucial stage, which governs the nature of all subsequent crack extension, branching and coalescence behaviors. With the generation of such cracks, the outflow of detonation products, such as explosive gases,
2.2. Principle and design of CREGBPR In longwall mining, coal extraction causes movement of the overlying stratum. This movement often leads to destructive phenomena, such as the large deformation of roadways, coal pillar failure, and roof falling. Based on underground pressure theory, the roof stratum movement is divided into three stages, as shown in Fig. 4. Fig. 4(a) shows that the roof stratum is continuously distributed in layers before mining because of the sedimentary nature of the rocks. Above the coal seam, several thin rock layers form the immediate roof. As the mining face advances, the immediate roof caves into the gangue, as shown in Fig. 4(b). The main roof controls the overlying movement as the principal stratum. After the caving of the immediate roof, the main roof rotates and subsides to form a structure similar to a voussoir beam. This rotation-subsidence leads to mining-induced pressure, as shown in Fig. 4(c). In most cases, the gangue formed by the collapsed immediate roof cannot fill the gob, especially when mining thick coal seams; thus, a large space is left between the main roof and the uncompacted 2
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Fig. 2. The mechanical model of bilateral energy-gathering tensile blasting. (a) Bilateral energy-gathering jet flow. (b) The mechanical effect of the energy-gathering tensile action in the x–z plane.
immediate roof collapses. This failure continues until the roof contacts the caved material and a new equilibrium state is reached. In this process, the caved roof forms random irregular rock fragments, which causes the volume of caved material to become larger than that of the original intact strata. However, the increase in volume does not compensate for the void space created from the floor to the main roof. As a result, the main roof begins to deform, bend, crack, and rotate downwards. When the roof and caved material are in contact, a certain amount of cover pressure from the strata movement is applied to the caved material. The bulked material is compressed so that it becomes denser and stiffer. During the bulked material compression stage, it is not possible to compress the material back into its original volume because the pressure is not infinite. As seen from the above analysis, the process from the initial disturbed state to a new equilibrium can be divided into two stages: the caved material bulking stage and the bulked material compaction stage. Two key parameters are used to describe these stages separately: the initial bulking factor (bi ) and the residual bulking factor (br ). As seen in Fig. 6, for the original approach (Fig. 6(a) and (c)), bi and br are separately stated as follows:
Fig. 3. CREGB mechanism and field test effect.
gangue. Because of the compressibility of the uncompacted gangue and the open space, the main roof exhibits a large deformation, which is likely to cause high mining-induced pressure. To control the disastrous movement of the main roof, an innovative approach using CREGB for protecting the roadway (CREGBPR) was presented. Fig. 5(a) shows that the CREGB technique is applied at the inclined top of the belt roadway. By cutting the roof at a certain height, the roof rock on both sides is separated along the cutting line. When the coal seam is unmined, a weak plane remains along the roadway strike direction in the roof. As the mining face advances, the roof close to the gob falls (forming gangue) due to the combined effect of the mining pressure and roof cutting. Thus, the rock to the roof cutting height is effectively used to compensate for the space. In this way, the broken and expanded rock mass provides the bulking force that resists later roof movement, as shown in Fig. 5(b). Consequently, compared with the original approach, CREGBPR resists the dangerous movement of the main roof. The deflection and rotation of the main roof are still small when the system enters a new equilibrium state, as shown in Fig. 5(c). On the other hand, the main roof body in motion is smaller with CREGBPR, as shown via the difference in Figs. 4 and 5.
bi =
h¯1a hI
(1)
br =
h¯1b hI
(2)
where h¯1a and h¯1b are the average heights of the gangue in the bulking and compaction stages of the original approach, respectively, and hI is the depositing height of the immediate roof in the strata. From the bulking stage to the compaction stage, the amount of compression is determined from the following equation:
C1 = h¯1a − h¯1b = hI (bi − br )
(3)
To obtain the subsidence of the lowest main roof, the unfilled space is expressed via
Δ1 = hI + m − h¯1a = m − hI (bi − 1)
(4)
where Δ1 is the gap height in the original approach and m is the mining height. According to Eqs. (3) and (4), the subsidence of the lowest main roof S1 is derived from the following equation:
S1 = Δ1 + C1 = m − hI (br − 1)
(5)
From Eqs. (4) and (5), specific to a given mining condition, the gap height Δ1 is determined by the initial bulking factor bi , and the final subsidence of the main roof is determined by only the residual factor br . In the CREGBPR approach, as shown in Fig. 6, the roof is cut at a certain depth in advance. After coal extraction, the upper roof within the cutting range caves under the mining pressure. If the roof cutting
3. Mechanism and effect of CREGBPR 3.1. Effect on roof structure The overburden strata are disturbed as the face advances and the 3
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Fig. 4. Schematic diagram of roof stratum movement with the original method. (a) Unmined. (b) Initial state after mining. (c) Final state after mining.
where h¯2a and h¯2b are the average heights of the gangue in the bulking and compaction stages by CREGBPR, respectively, and hc is the roof cutting length. θ is the angle from the roof cutting line to the normal direction, and hc cos θ is the roof cutting height and expressed as follows:
height is high enough and involves the main roof, the main roof will act as the immediate roof, caving instantly. Thus, bi and br are separately stated as
bi =
h¯2a hc cos θ
(6)
hc cos θ = hI + hcm h¯2b br = hc cos θ
(8)
where hcm is the main roof height within the roof cutting range. According to Eqs. (6), (7) and (8), the amount of compression of the
(7)
Fig. 5. Schematic diagram of roof stratum movement with the CREGBPR approach. (a) Unmined. (b) Initial state after mining. (c) Final state after mining. 4
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Fig. 6. Geological calculation model. (a) Bulking stage of the original approach. (b) Bulking stage of the CREGBPR approach. (c) Compaction stage of the original approach. (d) Compaction stage of the CREGBPR approach.
Fig. 7. Stress distribution comparison between the original approach and the CREGBPR approach.
In this equation, hc and θ are the variables. When the roof cutting height hc cos θ satisfies
gangue is as follows:
C2 = h¯2a − h¯2b = (bi − br ) hc cos θ
(9)
m bi − 1
The other component of the subsidence of the lowest main roof (i.e., the gap height Δ2 ) is calculated by the following equation:
hc cos θ ⩾
Δ2 = hc cos θ + m − h¯2a = m − hc cos θ (bi − 1)
the caved material can fill the gob. Therefore, the uncompacted gangue directly provides the bulking force to resist the upper roof movement in
(10) 5
(11)
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previously determined; on the other hand, the transferred stress is shifted to the deep gob and increases the stress there so that it approaches the initial stress (Campoli et al., 1993). The effect on the stress transfer of CREGBPR is obtained by comparison with the results of the original approach. The stress field for the CREGBPR approach is also divided into five zones. However, many notable changes arise due to the roof cutting, such as the zone length, the stress level, and the degree of concentration of the abutment stress. These changes are due to two substantial effects. One effect is that the roof structure over the pillar is optimized, as stated in Section 3.1, and the coal pillar is exempt from the influence of the overhanging roof and ultimately remains intact, without failing in compression after mining. The designed roadway protection pillar can serve as an integral supporting body against the abutment stress, wherein the stress is equally distributed. Meanwhile, the bulking body volume in the gob rim is sufficient to fill the gob to support the upper roof. These effects increase the stress level of the gob destress zone, but the stress there is still lower than the initial stress due to the mined-out effect; the stress transferred from zone V decreases, and the stress level in zone Ⅳ decreases accordingly. Therefore, the cover pressure applied on the roadway and coal pillar not only distributes more uniformly but also remains at a lower level. The other substantial effect is the cut-off on the stress transfer path in the low roof position, as shown in Fig. 7. The stress transferred from the gob is significantly restricted. During the stress redistribution, the released stress from the gob must seek a longer path for the stress transfer. There is an excellent possibility that this path change will lead to a shift in the position of the highest stress. To verify the correctness of the theoretical analysis and further investigate this topic, a 3D finite-difference fundamental model was constructed via FLAC3D to simulate the original approach and the CREGBPR approach, as shown in Fig. 8. The model size was 328 m × 80 m × 100 m, and its establishment was based on the field test in the Hongqinghe coalmine, as described in Section 4. In this model, the cross-sectional dimensions of the roadways were 6 m × 4 m, and the height of the coal seam was 6 m; the length of the coal pillars between the roadways was 30 m; and the burial depth of the coal seam was set to 700 m. The model boundaries except the upper boundary were fixed, and the upper boundary was assigned a vertical stress boundary condition with a value of 16.2 MPa; the horizontal in situ stress coefficients were all set to 1.2. The rock mass adopted the general Mohr-Coulomb constitutive model, and the coal mass adopted the strain-softening constitutive model to observe the yielding features during its postfailure stage (Hoek and Brown, 1997; Li et al., 2015). According to the geological experimental report of the coalmine and the research of Mohammad et al. (1997) and Cai et al. (2013), the input mechanical parameters were first scaled and then calibrated in reference to the in situ measurement of the roadway displacement; the parameters are listed in Table 1. The postfailure parameters of the coal mass were additionally calibrated by comparing the numerical simulation and laboratory test results of conventional triaxial compression tests. The fitting result is shown in Fig. 9, and the final scaled parameters for the postfailure of the coal mass are listed in Table 2. Based on the CREGBPR mechanism, the boreholes at a certain diameter would be connected by the directional continuous crack and thus form a roof cutting zone that separated the rock mass on both sides. Therefore, a roof cutting zone was generated with a 0.01 m width in the model during the simulation of the CREGBPR approach, and the built-in material model of “null” was used in FLAC3D to realize this roof cutting. Using the FLAC3D software, the model stress environment and block states were solved, as shown in Fig. 10. Fig. 10(a) shows the stress distribution of the original approach. While undergoing coal extraction, the system forms a large destress zone. The highest abutment stress is concentrated in the coal pillar due to the effect of stress transfer, and its value reaches 56.8 MPa, far beyond the in situ stress and the coal mass strength. Considering the plastic zone shown in Fig. 10(c), the part of the coal pillar adjacent to
the bulking stage. Eq. (11) potentially provides a design guide for CREGBPR. The subsidence S2 can be derived from
S2 = Δ2 + C2 = m − (br − 1) hc cos θ
(12)
From Eqs. (9), (10), and (12), the compression amount of the gangue C2 , the gap height Δ2 and the subsidence of the lowest main roof S2 are related to the roof cutting height hc cos θ in the CREGBPR approach. Notably, only the gap can be eliminated according to Eq. (11) because space-time relations exist among the roof caving, caved material expansion and expanded material compaction, and the compaction stage always exists. Accordingly, both the amount of compression of the gangue C2 and the subsidence of the lowest main roof S2 exist as well. Although the support by broken and expanded materials cannot eliminate the deformation, the CREGBPR approach is effective in reducing the fracture impact of the upper roof. Furthermore, the overhanging roof above the roadway pillar is eliminated, as shown by the comparisons in Fig. 6. The hanging arch consists mainly of the immediate roof (Zone I1) and part of the main roof (Zone M1), which is changed into gangue via the roof cutting. This elimination protects the integrity of the coal pillar and has a positive effect on roadway stability. 3.2. Effect on stress evolution and distribution Not only does the CREGBPR approach improve the fractural roof structure, but this approach can also optimize the evolution and distribution of mining-induced stress. Mining activity and roadway excavation cause underground stress redistribution. The induced abutment stress is applied to the adjacent solids and is 4–5 times the initial value (Wang et al., 2018). This high abnormal stress is due to the coupling effect of mining disturbances. As shown in Fig. 7, the abutment stress evolution and distribution for the original approach and the CREGBPR approach are combined for comparison and analysis. For the original approach, according to the stress distribution and magnitude, the stress field is divided into five zones: I: the initial stress zone, II: the excavation overstress zone, III: the excavation destress zone, Ⅳ: the coupling overstress zone, and Ⅴ: the gob-pillar destress zone. The horizontal red dashed line above the coal seam represents the initial stress level. Zone I is far from the disturbed areas such as the mining face and the excavated roadway. Thus, the stress in zone I is distributed uniformly and equals the in situ stress. From zone I to the left, a zone of higher stress occurs close to the retained roadway due to the influence of the excavation and is defined as the excavation overstress zone II. Zone III includes the area around the roadway and a small area of the coal mass adjacent to the roadway; here, the stress is lower than the initial stress because of the pressure relief effect of excavation. As a result, the released stress transfers to the adjacent coal wall, forming the abutment stress in zones II and IV. The stress in zone IV, except for the abutment stress from the roadway excavation, is affected by the mining activity, which is the principal formation factor of the high abutment stress. The coupling effect of the abutment stress arises in the roadway pillar. Notably, only part of the pillar, away from the gob, bears extraordinarily high stress. The other part, close to the gob, is compressed to failure by the overhanging roof. According to the elasticplastic theory, the part that fails cannot play the role of the protection pillar and bear a high stress, so a destress zone clearly exists in the pillar. When the high coupling stress acts on only a small intact part of the pillar close to the retained roadway, it causes the mass accumulation of elastic energy. The energy is easily activated and released by mining activity, which is unfavorable for roadway protection. Then, from zone Ⅳ to the left, there is a destress zone due to the pressure relief due to coal extraction and the effect of the partly yielded pillar. The destress area consists of the gob area and the pillar area close to the gob, so this zone is defined as the gob-pillar destress zone Ⅴ. Similarly, the stress released from zone Ⅴ is transferred to the adjacent zones. On the one hand, this transfer exacerbates the roadway pillar stress, as 6
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Fig. 8. 3D numerical calculation model.
the belt roadway fails and cannot bear the high stress, so the high abutment stress exists in the intact coal mass close to the failure zone. These results correspond to the work of Mark (1990), who used field measurements from an instrumented pillar to show that the initial abutment loads were usually observed as stress increases near the ribs of the pillar; as the abutment load increased, the stress peaks shifted to the core of the pillar, while the stress meter measurements near the rib often indicated fully transferred loads. Moreover, considering the possible influence of the overhanging roof, as was analyzed previously, the pillar rib may be subject to more severe damage, which would lead to a further shift in the stress. Some research has concluded and verified that with the conversion between the elastic strain energy and the plastic strain work, the mass stores elastic energy in the coal pillar, and the strain localization in the rock and coal masses present a great threat to the retained roadway stability and may even cause rock burst and coal bumps (Wang, 2006; Liu et al., 2010; Wang and Kaunda, 2019; Weng et al., 2017), especially in deep mining conditions and under dynamic mining disturbance (Vacek, 2001; Whyatt, 2008). Therefore, this stress evolution and distribution of the original approach (Fig. 10(a)) is not favorable for the stability of the retained roadway and cannot adapt to the challenges of deep mining. CREGBPR blocks the stress transfer in the lower roof from the gob to the coal pillar side. As shown in Fig. 10(b), the stress transfer path is altered into an arc that passes the outside end of the roof cutting line. The transfer distance is stretched. The path end that the highest stress lies in is shifted to above the coal pillar, changing from a position in the coal to a position in the rock. Moreover, the highest stress value is 49.4 MPa, a 13.0% decrease
Fig. 9. Calibration result of coal specimen by conventional triaxial compression test.
compared to that determined with the original approach, and the rock strength is greater than that of the coal. As a result, roof cutting not only transfers the highest abutment stress into the stronger rock but also reduces the abutment stress magnitude. From the comparison of the plastic zones between these two approaches, as shown in Fig. 10(c) and (d), the failure area of the roadway surroundings with the original approach is obviously larger than that with the CREGBPR approach. With the CREGBPR approach, the damage depth on the pillar side of the
Table 1 Numerical simulation model properties. Strata
Bulk modulus (GPa)
Shear modulus (GPa)
Density (kg/m3)
Friction angle (°)
Cohesion (MPa)
Tension (MPa)
Overlying strata Fine sandstone Pelitic siltstone (roof) Coal seam Pelitic siltstone (floor)
3.68 2.97 2.15 1.62 2.02
2.42 1.86 1.04 0.98 1.02
2500 2400 2300 1500 2300
38 36 34 28 33
7.4 5.9 4.2 3.8 4.0
5.0 4.2 3.6 3.0 3.5
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shown in Fig. 12, the immediate roof is composed of soft pelitic siltstone with an average thickness of 8.18 m; the upper roof is primarily hard sandstone, and the lowest main roof is fine sandstone with an average thickness of 10.65 m; the immediate floor is soft pelitic siltstone with an average thickness of 17.35 m. The zone of 12.0 m roofcutting depth in the test zone was selected for the following analysis. Detailed support conditions are also shown in Fig. 12. To understand the displacement pattern of the surrounding rock of the retained roadway and verify the pressure relief effect of CREGBPR, the test utilized displacement sensors that were installed in the material roadway and stress sensors that were installed on the working face support. The detailed layout of the sensors is shown in Fig. 13. Three displacement measuring points were arranged with a spacing of 30 m to monitor each approach. Sixteen hydraulic supports among the 145 supports were equipped with stress sensors. The displacement sensors were used to monitor the roof-to-floor and rib-to-rib convergence; the stress sensors were used to monitor the pressure variation in the supports to understand the dynamic change in the roof structure.
Table 2 Variation in postfailure properties with plastic strain. Coal mass
Plastic strain
Friction angle (°)
Cohesion (MPa)
0 0.01 0.03 1
28 24 23 23
3.8 1.6 0.8 0.8
material roadway decreases from 7.57 m to 4.50 m, and the damage height of the roadway roof decreases from 5.37 m to 3.18 m. Thus, the roadway stability is improved. In addition, CREGBPR maintains the overall stability of the coal pillar and makes it a uniformly loaded area, which reduces the possibility of rock burst and coal bump events. The results of the numerical simulation analysis correspond to the results of the previous theoretical analysis and can be used to further explore the stress transfer process in the system.
4.2. Analysis of the pressure relief effect 4. Field test The roadway deformation can be directly used to evaluate the pressure relief effect. Fig. 14 shows the measured convergence curves of the material roadway as the working face advanced away from the displacement measuring points for both approaches. First, Fig. 14(a) shows that the vertical convergence gradually increased as the working face advanced. From the trends of the six curves, when the face reached approximately 130 m, the roof-to-floor displacement increased less compared to that of the previous deformation. The ends of the curves show that the roof-to-floor convergence of the CREGBPR approach became stable. The measured maximum convergence stabilized at approximately 218 mm. However, the convergence of the original approach showed a small increase at the end; the measured maximum convergence reached 798 mm when the face advanced 162.3 m. As shown in Fig. 14(b), the horizontal convergence pattern of the material roadway was similar to the vertical convergence pattern. The measured maximum convergence of the original approach reached 650 mm with
4.1. Test overview The Hongqinghe coalmine, which is located in Ordos City, Inner Mongolia Province, North China, was selected for the case test. Hongqinghe is a modern mine with a high yield and high efficiency, and its annual approved production capacity is 1.5 × 109 kg. The mined coal seam is the No. 3−1, and its average thickness is 6.2 m with an average inclination of 3°. The average burial depth is 731.5 m. The experimental site was in the 3−1101 working face (Fig. 11), which employed fully mechanized mining technology to mine the overall height simultaneously. The incline and strike lengths of the face were 245.7 m and 3212.7 m, respectively. The field test began at 2811 m from the open-off cut. The CREGB technique was used in the 3−1101 belt roadway, as shown in Fig. 11. The drill data, especially the roof condition, near the belt roadway were analyzed in the test zone. As
Fig. 10. Stress distribution, evolution and plastic zone. (a) Stress state of the original approach. (b) Stress state of the CREGBPR approach. (c) Plastic zone of the original approach. (d) Plastic zone of the CREGBPR approach. 8
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Fig. 11. Layout of the test site.
Fig. 12. Section of roadway conditions and the CREGBPR design.
Fig. 13. Layout of displacement and stress sensors.
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Fig. 14. Deformation curves of the material roadway. (a) Roof-to-floor convergence. (b) Rib-to-rib convergence.
pressure clearly decreased after adopting CREGBPR; a larger lowpressure area (0–3800 kN) and no obvious high-pressure area (11,000–15,000 kN) existed in zone 4 compared to those in zone 3. The average pressure decreased from 6850.90 kN in zone 3 to 5572.00 kN in zone 4, which is an 18.7% drop. In the vertical comparison, the pressure levels were comparable between zones 1 and 3 in the original approach, and their average pressures had only a 212.24 kN difference. When the working face entered the range of the CREGBPR approach, the pressure on the roof cutting side (zone 4) was 2372.90 kN less than that on the opposite side; the pressure decreased by 29.9% under the influence of roof cutting. In addition, the pressure level increased from zone 1 to zone 2, while the pressure in zone 4 was still lesser than that in zone 3. The above comparative analyses illustrate the pressure relief effect of CREGBPR on the roof cutting side, causing the mine pressure to become more moderate; the results also verify the roadway convergence and explain the support effect of the collapsed gangue formed by roof cutting. CREGBPR was used to cut the roof directionally via the energygathering blasting technique. The roof structure was optimized during the mining process, and the stress evolution and distribution in the system were improved. Fig. 16 compares the effects before and after adoption of CREGBPR. The convergence of the retained roadway
an advanced distance of 162.3 m. The curve trends showed that the convergence would increase continually as the face advanced. In contrast, the measured maximum convergence of the CREGBPR approach nearly stabilized at 236 mm when the working face reached 150.1 m. Additionally, the convergence of the original approach increased more rapidly at the beginning of the mining. For the CREGBPR approach, when the distance was in the range of 20–40 m, the convergence began to increase rapidly. Besides, the displacement curves of the different sensors corresponding to the CREGBPR approach were closer to each other than those corresponding to the original approach. These phenomena may suggest that the CREGBPR approach influenced the roof movement and then changed the deformation process, reducing the sagging rotation of the fractured roof compared to that with the original approach. To clarify the changes in the roof movement, the pressure data extracted from the supports were further processed and analyzed. As shown in Fig. 15, a plot of the support pressure was generated to present the pressure history before and after the adoption of the CREGBPR approach. We selected and divided the roadway side area of the working face into four zones for comparison to illustrate the pressure relief effect. Table 3 gives the average support pressure of the four zones. In the horizontal comparison between zones 3 and 4, the mining 10
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Fig. 15. Support pressure monitoring results.
divided into two stages: the gangue bulking and gangue compaction stages. The upper roof deformation and the minimum roof cutting height were calculated by two control parameters: the initial and residual bulking factors. The CREGBPR approach eliminates the overhanging roof above the pillar and the roof deformation in the bulking stage, and reduces the impact in the compaction stage, thereby protecting the integrity of the coal pillar and the roadway stability. In addition, CREGBPR cuts off the original stress transfer path in the lower roof and thus alters the stress transfer path, which optimizes the stress evolution and distribution of the roadway surroundings. The failure area of the retained roadway surrounding rock for the original approach was smaller with the CREGBPR approach than that with the original approach, and the potential threat of rock burst and coal bump was reduced. Finally, a field test was performed. The monitoring results and the field application showed that the roadway convergence was reduced and the pressure relief effect was significant. Therefore, the feasibility and effectiveness of the CREGBPR approach were illustrated.
Table 3 Average mine pressure in different zones. Position Zone Average pressure (kN)
No-roof-cutting side
Roof cutting side
1
2
3
4
7063.14
7944.90
6850.90
5572.00
decreased significantly, which ensured roadway safety and improved productivity. Thus, the CREGBPR approach is an effective way to achieve roadway pressure relief and mining safety. 5. Conclusions In this study, an innovative approach for protecting the retained roadway was proposed based on cutting the roof via energy-gathering blasting. Compared to the original approach, the advantages of the CREGBPR approach are reflected in the optimizations of the roof structure and the stress evolution and distribution, which control the mechanism of pressure relief via CREGBPR. A geological calculation model was established to further explore the effect on the roof structure. The change in the roof structure was
CRediT authorship contribution statement Xingyu
Zhang:
Conceptualization,
Fig. 16. Overall effect of the retained roadway before and after the CREGBPR application. 11
Methodology,
Funding
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acquisition, Software, Writing - original draft, Formal analysis. Jinzhu Hu: Investigation, Visualization, Resources, Writing - review & editing. Haojie Xue: Investigation, Visualization. Wenbin Mao: Software, Investigation, Visualization. Yubing Gao: Conceptualization, Methodology, Funding acquisition, Supervision, Resources, Writing review & editing. Jun Yang: Supervision. Manchao He: Conceptualization, Methodology, Funding acquisition.
design: a case study. Rock Mech. Rock Eng. 48 (1), 305–318. Li, X., Zhou, Z., Lok, T.S., Hong, L., Yin, T., 2008. Innovative testing technique of rock subjected to coupled static and dynamic loads. Int. J. Rock Mech. Min. 45 (5), 739–748. Liu, S.X., Liu, C.W., Cao, L., 2010. Post-peak softening of porosity coal and its influence to rock burst occurred in work face. J. China Coal Soc. 35 (12), 1990–1996. Lu, X.L., Liu, Q.S., Su, P.F., 2011. Deformation mechanism and support measures for stress-induced cracked rock mass of deep coal mine roadway. Geomech. Geotech.: From Micro to Macro Two Set 845–853. Mark, C., 1990. Pillar Design Methods for Longwall Mining. U.S. Dept. of the Interior, Bureau of Mines, Washington, DC. Mohammad, N., Reddish, D.J., Stace, L.R., 1997. The relation between in situ and laboratory rock properties used in numerical modelling. Int. J. Rock Mech. Min. 34 (2), 289–297. Ning, J.G., Liu, X.S., Tan, Y.L., 2012. Mechanism of rock-burst prevention by synergizing pressure relief and reinforcement of surrounding rocks with zonal disintegration in deep coal roadway. Disaster Adv. 5, 1237–1242. Ranjith, P.G., Zhao, J., Ju, M.H., De Silva, R.V.S., Rathnaweera, T.D., Bandara, A.K.M.S., 2017. Opportunities and challenges in deep mining: a brief review. Eng. -Proc. 3, 546–551. Vacek, J., 2001. Stress distribution in a coal seam before and after bump initiation. Acta Polytechnica 41, 4–5. Wang, F., Kaunda, R., 2019. Assessment of rockburst hazard by quantifying the consequence with plastic strain work and released energy in numerical models. Int. J. Min. Sci. Technol. 29 (1), 93–97. Wang, G.F., Gong, S.Y., Li, Z.L., Dou, L.M., Cai, W., Mao, Y., 2015. Evolution of stress concentration and energy release before rock bursts: two case studies from Xingan coal mine, Hegang, China. Rock Mech. Rock Eng. 49, 3393–3401. Wang, P., Zhao, J., Feng, G., Wang, Z., 2018. Interaction between vertical stress distribution within the goaf and surrounding rock mass in longwall panel systems. J. S. Afr. I. Min. Metall. 118, 745–756. Wang, X.B., 2006. Numerical simulation of deformation and failure for two bodies model composed of rock and coal. Rock Soil Mech. 27 (7), 1066–1070. Wang, Q., He, M.C., Yang, J., Gao, H.K., Jiang, B., Yu, H.C., 2018. Study of a no-pillar mining technique with automatically formed gob-side entry retaining for longwall mining in coal mines. Int. J. Rock Mech. Min. 110, 1–8. Wang, Y.J., He, M.C., Yang, J., Wang, Q., Liu, J.N., Tian, X.C., Gao, Y.B., 2020. Case study on pressure-relief mining technology without advance tunneling and coal pillars in longwall mining. Tunn. Undergr. Space Technol. 97, 103236. Weng, L., Huang, L., Taheri, A., Li, X., 2017. Rockburst characteristics and numerical simulation based on a strain energy density index: a case study of a roadway in Linglong gold mine. China. Tunn. Undergr. Space Technol. 69, 223–232. Whyatt, J., 2008. Dynamic failure in deep coal: recent trends and a path forward. In: Proceedings 27th International Conference on Ground Control in Mining, pp. 37–45. Wu, Q.S., Jiang, J.Q., Wu, Q.L., Xue, Y.C., Kong, P., Gong, B., 2018. Study on the fracture of hard and thick sandstone and the distribution characteristics of microseismic activity. Geotech. Geol. Eng. 36, 3357–3373. Xie, S.R., Li, E.P., Li, S.J., Wang, J.G., He, C.C., Yang, Y.F., 2015. Surrounding rock control mechanism of deep coal roadways and its application. Int. J. Min. Sci. Technol. 25, 429–434. Yang, J., Wang, D., Shi, H.Y., Xu, H.C., 2015. Deformation failure and countermeasures of deep tertiary extremely soft rock roadway in Liuhai coal mine. Int. J. Min. Sci. Technol. 25, 231–236. Yang, X.J., Wang, E.Y., Wang, Y.J., Gao, Y.B., Wang, P., 2018. A study of the large deformation mechanism and control techniques for deep soft rock roadways. Sustainability 10 (4), 1100. Yu, W.J., Wang, W.J., Chen, X.Y., Du, S.H., 2015. Field investigations of high stress soft surrounding rocks and deformation control. J. Rock Mech. Geotech. 7, 421–433. Zhang, S., Li, Y., Shen, B., Sun, X., Gao, L., 2019. Effective evaluation of pressure relief drilling for reducing rock bursts and its application in underground coal mines. Int. J. Rock Mech. Min. 114, 7–16. Zhang, Z., Bai, J., Chen, Y., Yan, S., 2015. An innovative approach for gob-side entry retaining in highly gassy fully-mechanized longwall top-coal caving. Int. J. Rock Mech. Min. 80, 1–11. Zhao, T., Liu, C.Y., Yetilmezsoy, K., Zhang, B.S., Zhang, S., 2017. Fractural structure of thick hard roof stratum using long beam theory and numerical modeling. Environ. Earth Sci. 76, 751. Zheng, Y.L., Zhang, Q.B., Zhao, J., 2016. Challenges and opportunities of using tunnel boring machines in mining. Tunn. Undergr. Space Technol. 57, 287–299.
Acknowledgement This work was supported by the State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing (No. SKLGDUEK1928), the State Key Research Development Program of China (No. 2016YFC0600900), a program of the China Scholarship Council (No. 201806430070) and the China Postdoctoral Science Foundation (No. 2019M650896), which are gratefully acknowledged. Declaration of Competing Interest The authors declare that they have no conflict of interest. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2020.103387. References Alber, M., Fritschen, R., Bischoff, M., Meier, T., 2009. Rock mechanical investigations of seismic events in a deep longwall coal mine. Int. J. Rock Mech. Min. 46, 408–420. Cai, M., He, M., Liu, D., 2013. Rock Mechanics and Engineering, second ed. Science Press, Beijing. Cai, W., Dou, L.M., Zhang, M., Cao, W.Z., Shi, J.Q., Feng, L.F., 2018. A fuzzy comprehensive evaluation methodology for rock burst forecasting using microseismic monitoring. Tunn. Undergr. Sp. Tech. 80, 232–245. Campoli, A.A., Barton, T.M., Dyke, F., Gauna, M., 1993. Gob and gate road reaction to longwall mining in bump-prone strata. RI: Bureau of Mines 48, 9445. Dou, L.M., Mu, Z.L., Li, Z.L., Cao, A.Y., Gong, S.Y., 2014a. Research progress of monitoring, forecasting, and prevention of rockburst in underground coal mining in China. Int. J. Coal Sci. Technol. 1, 278–288. Dou, L.M., He, X.Q., He, H., He, J., Fan, J., 2014b. Spatial structure evolution of overlying strata and inducing mechanism of rockburst in coal mine. T. Nonferr. Metal. Soc. 24, 1255–1261. Fairhurst, C., 2017. Some challenges of deep mining. Eng. -Proc. 3, 527–537. Feng, G., Kang, Y., Sun, Z.D., Wang, X.C., Hu, Y.Q., 2019. Effects of supercritical CO2 adsorption on the mechanical characteristics and failure mechanisms of shale. Energy 173, 870–882. Gao, Y.B., Wang, Y.J., Yang, J., Zhang, X.Y., He, M.C., 2019. Meso-and macroeffects of roof split blasting on the stability of gateroad surroundings in an innovative nonpillar mining method. Tunn. Undergr. Sp. Technol. 90, 99–118. Guo, W.J., Li, Y.Y., Yin, D.W., Zhang, S.C., Sun, X.Z., 2016. Mechanisms of rock burst in hard and thick upper strata and rock-burst controlling technology. Arab. J. Geosci. 9, 561. He, H., Dou, L., Fan, J., Du, T., Sun, X., 2012. Deep-hole directional fracturing of thick hard roof for rockburst prevention. Tunn. Undergr. Sp. Technol. 32, 34–43. He, M.C., Zhang, X.H., Zhao, S., 2017. Directional destress with tension blasting in coal mines. ISRM Eur. Rock Mech. Symp. Eurock 2017 (191), 89–97. Hoek, E., Brown, E.T., 1997. Practical estimates of rock mass strength. Int. J. Rock Mech. Min. 34 (8), 1165–1186. Li, W., Bai, J., Peng, S., Wang, X., Xu, Y., 2015. Numerical modeling for yield pillar
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