The maintenance of the support withdrawal space during the relocation at the end of fully mechanized mining faces has traditionally used square timber beam support, which not only has poor support strength but also involves high costs. Since 2002, No. 6 Mine has adopted bolt support technology for the relocation space and achieved good results.

 

 

　　The Wu 8-22190 working face is located in the Wu II dip mining area of the second level of No. 6 Mine. The coal seam has an average thickness of 2.2 m and a dip angle of 10°. The strike length is 1100 m, the dip length is 180 m, and the recoverable reserves are 594,000 tons. The immediate roof is siltstone with a thickness of 3.6–5.0 m; the main roof is fine sandstone with a thickness of 1.7–3.3 m; the first caving interval of the immediate roof is 10–15 m, the first weighting interval of the main roof is 15–28 m, and the periodic weighting interval of the main roof is 10–15 m.

 

　　The working face adopts full-seam mining at one time. The main equipment used includes: MGTY250/600-1.1D shearer, ZY5000-1.4/3.0 shield-type hydraulic supports (120 sets), SGZ-764/500 double-center-chain frame-type scraper conveyor, and SZZ-764/200 bridge-type stage loader.

 

　　2 Support Scheme Design

 

　　Since the support withdrawal space in the ending process of the fully mechanized mining face is equivalent to excavating a coal roadway, based on existing roadway support experience, we replaced the original timber shed support with bolt support in the support withdrawal space at the end of the fully mechanized mining face. The specific design is as follows:

 

　　2.1 Roof bolts.

 

　　Bolt length: L=L1+B+L2=100+1500+400=2000 (mm); the bolt length is extended to 2.2 m.

 

　　Where L1——exposed length of the bolt, taken as 100 mm;

 

　　L2——length of the bolt extending into the stable rock stratum, taken as 400 mm;

 

　　B——loosening range of the roof rock stratum, taken as 1500 mm based on measurement and calculation.

 

　　Bolt diameter: φ20 rebar bolts are selected. Bolt spacing and row spacing: after comprehensive analysis of various factors and based on experience, the bolt spacing and row spacing are first determined as 750×700 mm (see the figure below), and then the parameters are checked and calculated. The weight of roof rock supported by each bolt is Q=25×3.6×1.4×0.7÷5=17.64 (KN).

 

　　The safety factor K is:

 

　　K=Q/q=36/17.6=2.8 (meets the requirement),

 

　　Where Q——design anchoring force of the bolt, taken as 36 KN;

 

　　q——weight of loosened rock borne by each bolt;

 

　　K——safety factor.

 

　　2.2 The coal wall support in the support withdrawal space adopts split-set bolts. Since the service period of the support withdrawal space is relatively short, φ40 mm split-set bolts can be selected as the outer rib bolts, with a length of 1800 mm and spacing and row spacing of 800×800 mm. The anchoring force of each rib bolt is 15 KN, slightly greater than the coal rib side pressure of 14 KN.

 

　　2.3 Selection of anchor mesh. In the ending support process of the fully mechanized mining face, the metal mesh mainly serves to support the roof and prevent the broken roof from caving; the steel wire rope mainly supports the metal mesh, prevents the metal mesh from breaking and the roof from caving, and after the fully mechanized supports are withdrawn, roof props are installed to ensure an effective ventilation section. Therefore, in the ending support process of the fully mechanized mining face, the metal mesh and steel wire rope are still used in the original support form.

 

　　2.4 Support at the chamfered corner of the tail. The chamfered corner of the tail is a specially arranged turning passage for the withdrawal of supports in the fully mechanized mining face. Its support process still adopts bolt and anchor mesh support, and the bolt spacing and row spacing are the same as those of the basic support of the mining face. However, because the section here is large, for safety reasons, we erect a metal steel beam shed support of “one beam and two props” under the anchor mesh support.

 

 

　　3.1 Before the mining face stops mining, the supports on the mining face shall be straightened, and the mining height shall be moderate, reaching 2.8 m.

 

　　3.2 The support withdrawal space is cut by the shearer. The construction process sequence is: coal breaking → pushing the conveyor → temporary support → drilling bolt holes → installing bolts → laying (hanging) mesh → bolt quality inspection → moving the bolting machine → next cycle.

 

　　3.3 Two rows of holes are drilled for each support, respectively 0.38 m from the center line of the support; three holes are drilled in each row, respectively 0.2 m, 0.9 m, and 1.6 m from the end of the support beam (as shown in Figure a).

 

　　3.4 Bolt installation: bolt installation includes loading the anchoring agent, inserting the bolt, stirring the cartridge, and tightening the nut. The quality of the bolt hole, the quality of the bolt components, and the quality of the cartridge shall be ensured. At the same time, the stirring time of the cartridge shall be strictly controlled at 30 s ± 5 s, and the stirring process of the cartridge shall be continuous without interruption.

 

　　3.5 The roof control distance shall be strictly controlled to no more than 1200 mm, the roof bolts shall be installed within 2 hours, and temporary support shall be consistently used well.

 

 

　　4.1 A bolt support management group shall be established, with the production mine manager and the coal mining mine manager as the group leader, and the deputy chief engineer of coal mining as the deputy group leader. The coal mining district (section) chief and the team leader, party secretary, and technician of the construction unit shall be members of the leading group.

 

　　4.2 The coal mining district shall set up a bolt support management group, with 3–4 full-time personnel to supervise and manage bolt support work, and clarify the management responsibility system.

 

　　4.3 Personnel in the coal mining district and coal mining team must pass training and undergo on-site practice in the excavation unit until proficient before being allowed to work.

 

　　4.4 Supervision and inspection of bolt support quality shall be strengthened. If bolts are found to be ineffective or damaged, point props shall be promptly installed and bolts shall be re-drilled. If roof separation, overall subsidence, or insufficient bolt resistance near broken zones or faults occurs, shed support shall be promptly added.

 

　　4.5 The coal mining team shall establish a dedicated quality acceptance system, clarify reward and penalty rules for each process, and establish and improve various original data records. In case of major safety hazards, production shall be promptly stopped for rectification to ensure construction quality.

 

 

　　5.1 The application of bolt support technology in the ending process of fully mechanized mining faces is another innovation in the practical application of bolt support technology and has broad value for popularization and application. It not only simplifies the ending process of the working face, but also saves a large amount of material costs such as pit wood, metal mesh, and steel wire rope, reduces a large amount of heavy labor such as carrying pit wood, lowers workers' labor intensity, and improves work efficiency. Moreover, because two drilling machines are used for simultaneous operation, compared with the original ending process, the construction period is shortened by nearly two days.

 

　　5.2 The use of bolt support technology in the ending process of fully mechanized mining faces is safer than the original timber shed support process. During the ending period of the working face, the end distance of the support is small, and the roof is easy to control; during support withdrawal, the height is on average 300 mm higher than that with timber shed support, avoiding the frequent phenomenon of supports being crushed during withdrawal and ensuring safe production.

 

　　5.3 The use of bolt support technology in the ending process of fully mechanized mining faces, compared with the original timber shed support process, ensures an effective ventilation passage and section, avoids the frequent high temperature and gas overrun at the upper corner during support withdrawal, and improves the safety conditions and working environment during support withdrawal.