Under unstable roof conditions in thin coal seam blasting mining faces, normal support methods cannot meet the roof support requirements. The application of slotted pipe rock bolts in thin coal seam blasting mining faces to control unstable roof effectively solves the support of fractured roof zones.

 

 

① The 1702 working face is the second extraction face of the No. 17 coal seam in the No. 11 mining area, located in the southwest of the No. 11 mining area. The face is adjacent to the No. 11 mining area track incline to the east, the Nantun and Beisu mine field boundary to the south, the western fault of the No. 1 mining area and the No. 13 mining area to the west, and the 1704 working face (not prepared) to the north. The surface elevation is +40.51～+41.8m, higher in the east and lower in the west. The working face elevation is -385.3～-409.3m.

 

② The main roof of the 1702 working face is mudstone, tightly cemented, brittle, with horizontal bedding, locally siltstone, with a Protodyakonov hardness of 5～6. The immediate roof is No. 11 limestone, grayish white, dense and brittle, locally absent and changed to sandy mudstone, with a Protodyakonov hardness of 10～11. The No. 17 coal seam thickness is 0.90～1.20m, averaging 1.06m.

 

③ The 1702 working face is generally a monocline structure, higher in the southeast and lower in the northwest. The coal seam strike is nearly east-west, dipping north. The coal seam dip angle is 2～8°, averaging 3°.

 

 

The 1702 working face uses ZZ2400/06/12B hydraulic supports, single hydraulic props, and plank beams in combined action to support the roof. In the hydraulic support support area, every three sections form a group, with two supports and one hydraulic single prop support interval. Both the maximum and minimum roof control distances adopt the "23" support method, i.e., two single props behind the scraper conveyor, and a group of three props in a triangular pattern at the roof cutting row. In the hydraulic single prop support interval, a plank beam is set up in combined action to support the roof, forming one beam with two props.

 

When the working face roof is locally absent and changed to sandy mudstone with severe roof fracturing, normal support cannot meet the support requirements. In order to effectively prevent roof caving and ensure safe production, slotted pipe rock bolts can be driven into the coal wall in advance to maintain the roof.

 

① Specifications of slotted pipe rock bolts: The rock bolt is Φ34×2000mm.

 

② Working principle of slotted pipe rock bolts: The slotted pipe rock bolt is installed into a pre-drilled hole. When the support is advanced, the forepoling beam of the support holds the exposed part of the rock bolt, forming advance support to maintain the roof.

 

 

4.1 Construction scope: areas with relatively fractured unstable roof in the working face.

 

4.2 Construction plan: To ensure the integrity of the roof, slotted pipe rock bolts are driven into the coal wall for advance roof maintenance. Wet drilling with an electric coal drill is adopted, with a drill bit specification of ?34mm, and a 1.8m auger drill pipe.

 

4.3 Technical requirements

 

4.3.1 A row of rock bolts is driven into the coal wall at a height of 1.0m～1.1m from the floor, with a bolt spacing of 0.15±0.05m, and the exposed length of the bolt is 200～300mm. According to the site coal wall conditions, additional bolts are installed at fractured coal wall locations, and the bolts are driven horizontally into the coal (rock) seam. When the drill hole opening has rib spalling causing the bolt to loosen, small wooden wedges should be used to wedge the hole opening so that the bolt and the coal (rock) seam are fully stressed. 

 

4.3.2 After drilling in the coal (rock) seam of the working face is completed, the coal (rock) powder in the drill hole is cleaned out, and the slotted pipe rock bolt is inserted. The plank beam is placed parallel to the coal wall on the forepoling beam of the support to hold the exposed part of the rock bolt. The forepoling beam should be in flat and solid contact with the roof. Then, under the combined support of the rock bolt, support, and metal mesh, the adjacent support is moved forward to ensure that the roof between adjacent supports does not cave during forward movement.

 

4.3.3 Blasting requirements: At locations with relatively fractured unstable roof, the method of drilling dense holes and firing small shots is adopted for blasting. The charge quantity should be appropriate and not too large to reduce blasting damage to the roof.

 

4.4 Safety technical measures

 

4.4.1 During construction, the "knocking on the roof and asking about the roof" system before and during work shall be implemented. This work should be carried out by two experienced veteran workers, one to sound the roof and one to observe the roof and the retreat route. The long-handled tool for roof sounding shall be a wooden pole or pointed hammer not less than 2.0 meters. The person sounding the roof should stand in a safe location and use the long-handled tool to sound the roof to prevent coal gangue from sliding down the pole and injuring people; the observer should stand to the side and behind the person sounding the roof and ensure the retreat route is unobstructed. Roof sounding should start from a location with intact support, proceed from outside to inside, first the roof then the coal wall, in sequence, and no other personnel are allowed to enter the roof sounding area. When large broken gangue blocks with separation are encountered on the roof, temporary support should first be set up, and after ensuring safety, the roof should be slowly sounded down along cracks and bedding; hard digging or forced excavation is not allowed.

 

4.4.2 Before entering the face for operations, the roof and coal wall must be maintained first, the "knocking on the roof and asking about the roof" system must be implemented, loose gangue and dangerous rocks must be removed with long-handled tools, the retreat route must be cleared, and a dedicated person must observe the roof. Working under an unsupported roof is strictly prohibited.

 

4.4.3 When drilling holes for rock bolts, personnel must stand firmly and cooperate well, with a dedicated person watching the roof and monitoring safety. If safety hazards are found, work must be stopped immediately for handling to ensure construction safety.

 

4.4.4 The electric coal drill must be in good condition, and all protections of the comprehensive protection device for the electric coal drill must be complete and reliable. After using the electric coal drill, the power must be cut off and locked.

 

4.4.5 When using the electric coal drill, the following matters should be noted:

 

① Wearing gloves is prohibited when drilling with the electric coal drill. Work clothing cuffs must be fastened tightly, collars and jacket corners must be tied, and buttons must be fastened to avoid being entangled by the drill pipe.

 

② Use the electric coal drill with uniform advancement; do not apply excessive force. Strong pushing or hard pressing is not allowed to prevent broken drill rods from injuring people, and overload operation is prohibited.

 

③ When drilling, avoid blast holes, and the driller must check for misfires and residual explosives.

 

④ Drilling is performed by two people. Drill at the pre-marked positions, with one person guiding the drill rod and one person holding the drill. First jog the power handle, then gradually press the handle with uniform force. At regular intervals, the drill pipe should be pulled back and forth to clean out the coal (rock) powder in the drill hole.

 

⑤ Drilling and charging are strictly prohibited to be carried out simultaneously. Starting the scraper conveyor is strictly prohibited during drilling and charging.

 

4.4.6 When entering the face for operations, the current support and adjacent supports must not be operated. The main liquid inlet valves of the current support and adjacent supports must be closed to prevent misoperation or collision of materials with the operating handles. The machine and conveyor must be stopped, and power must be cut off and locked. The system of "whoever cuts off power shall restore power" shall be implemented to ensure operation safety.

 

4.4.7 Ensure that hydraulic supports do not leak or channel fluid, do not automatically descend, are raised tightly to the initial support force, and are in flat and solid contact with the roof.

 

4.4.8 During blasting at the working face, the "four-three" blasting system shall be strictly implemented. Before and after blasting, water shall be sprayed within a 20m range to reduce dust. After blasting, roof sounding shall be carried out promptly to remove loose gangue and dangerous rocks.

 

 

The application of slotted pipe rock bolts under unstable roof conditions in the thin coal seam blasting mining face of Nantun Coal Mine is successful. It has played a good support role in controlling the unstable roof, eliminated roof caving accidents during the period of passing through the unstable roof, and has certain guiding significance for the control of unstable roof in thin coal seam blasting mining faces in the future.