Shanxi Puxian Huasheng Coal Industry Co., Ltd. is a resource integration mine after the merger and reorganization of coal mines in Shanxi Province in 2009. The mine field area is 8.9611 km2, with a production scale of 0.90 Mt/a. The main mineable coal seams are No. 2 and 3 of the Shanxi Formation and No. 9, 10, and 11 of the Taiyuan Formation; No. 2 coal seam has been extensively mined before integration (mostly by room and pillar, pillar, or room-and-pillar mining), and No. 3 coal seam has been mined in a small area; currently, No. 3 coal seam is being mined, with an average thickness of 2.2m and a coal seam structure of 0.5 (0.6) 1.1. The mining method is retreating longwall along the strike. The roof of No. 3 coal seam is mostly siltstone and mudstone. The lithology is gray-black, brittle, and well-cemented. The uniaxial compressive strength is 36.7-51.4 MPa; the uniaxial tensile strength is 1.34-1.98 MPa; the shear strength is 3.85-6.15 MPa. According to small mine investigations, the roof is moderately stable, easy to manage, and has good water resistance.

 

The floor of No. 3 coal seam is mostly siltstone and mudstone. The lithology is gray-black, blocky, and brittle. The uniaxial tensile strength is 1.34-1.98 MPa; the shear strength is 3.85-6.15 MPa. It is prone to mudification when encountering water, and under certain conditions (roof pressure), floor heave is likely to occur, but the water resistance is good, belonging to an unstable floor.

 

The No. 3 coal seam is currently being mined in the mine field. The water sources are mainly roof sandstone fissure water and goaf water from No. 2 coal seam. The strata generally dip to the southeast, with shallower burial in the southwest, and the No. 2 coal seam outcrops outside the mine field boundary. Thus, the roof sandstone fissure water has hydraulic connection with the bedrock weathering zone fissure water, and the roof fissure water is relatively large. Since the No. 2 coal seam in the construction area has been extensively mined, goafs have formed, and water has accumulated in the goafs. The interburden between No. 2 and No. 3 coal seams is 2-8 meters, so water prevention and roof management should be the focus.

 

 

The 3101 auxiliary transport roadway is 1700 meters long, with a trapezoidal cross-section, supported by 12# I-beam supports at 700mm spacing, with mesh on top and 4-6-4 side and roof support. Six months after the completion of the 3101 auxiliary transport roadway, the 3101 working face began extraction. The mining process is fully mechanized coal mining, full-seam extraction, and full caving roof management. When the extraction advanced about 150 meters, the mining-induced pressure in the 3101 auxiliary transport roadway became obvious, causing floor heave and support deformation, making the roadway unusable. In this situation, we started reinforcement support using a combined bolt + anchor cable scheme in the 3101 auxiliary transport roadway 100 meters ahead of the extraction advance.

 

Based on the actual construction conditions and after repeated demonstration, on the basis of the 3101 auxiliary transport roadway supported by 12# I-beam supports at 700mm spacing, with mesh on top and 4-6-4 side and roof support, a combined bolt + anchor cable support scheme was adopted. Reinforcement support started 100 meters ahead of the extraction advance. Although the construction cost increased somewhat, construction safety and quality are guaranteed. Considering the actual site conditions, this scheme was adopted.

 

(1) Roof support

 

1. Bolts

 

Bolt type and specifications: The bolt body is 20# left-handed ribbed steel bar without longitudinal ribs, made of 335# special steel for bolts, with a length of 2200mm and a tail thread of M22.

 

Anchoring method: Resin extended anchoring, using two resin cartridges, specification MSK2335. The drill hole diameter is 28mm, the anchoring length is 1300mm, and the design anchoring force is 105kN.

 

Bolt accessories: Arch-shaped high-strength bearing plate, steel grade not lower than Q235, with spherical washer and damping nylon washer. The bearing plate height shall be not less than 36mm, thickness not less than 8mm, and load-bearing capacity not less than 154kN.

 

Bolt layout: 5-3-5 arrangement, bolt spacing 800mm, row spacing 700mm.

 

Bolt angle: The middle bolts are installed perpendicular to the rock surface at 90°, and the side bolts are installed at an angle of 75°.

 

2. Anchor cables

 

When the interburden spacing is greater than 6.5m:

 

Anchor cable type and specifications: The anchor cable material is 17.8mm, 1x7 strand high-strength low-relaxation prestressed steel strand, length 6300mm, drill hole diameter 28mm, anchored with one MSK2335 and two MSZ2360 resin cartridges, anchoring length 1755mm; anchor cable bearing plate: 300mm×300×12mm high-strength adjustable spherical bearing plate and matching lockset, plate thickness not less than 12mm, plate height required to reach 60mm, load-bearing capacity not less than 350kN; anchor cable layout: spacing 1600mm, row spacing 1400mm; anchor cable angle: all anchor cables installed perpendicular to the rock surface; anchor cable pre-tension: anchor cables require over-tensioning to 200kN, and the pre-tension after loss shall not be less than 180kN.
 

When the interburden spacing is >3.5m and <6.5m:

 

Anchor cable type and specifications: The anchor cable material is 17.8mm, 1x7 strand high-strength low-relaxation prestressed steel strand, length 3500mm, drill hole diameter 28mm, anchored with one MSK2335 and two MSZ2360 resin cartridges, anchoring length 1755mm; anchor cable bearing plate: 300mm×300×12mm high-strength adjustable spherical bearing plate and matching lockset, plate thickness not less than 12mm, plate height required to reach 60mm, load-bearing capacity not less than 350kN; anchor cable layout: spacing 1600mm, row spacing 1400mm; anchor cable angle: all anchor cables installed perpendicular to the rock surface; anchor cable pre-tension: anchor cables require over-tensioning to 200kN, and the pre-tension after loss shall not be less than 180kN; when the interburden spacing is less than 3.5m: bolt support is used.

 

(2) Support in special sections

 

For structural sections such as broken roof, fault zones, and anticline/syncline peripheries, where the combined bolt + anchor cable support cannot be used, single hydraulic props are used for reinforcement support.

 

 

On the basis of the 3101 auxiliary transport roadway supported by 12# I-beam supports at 700mm spacing, with mesh on top and 4-6-4 side and roof support, after adopting the combined bolt + anchor cable support scheme for reinforcement, verification through mine pressure observation showed that the roof convergence was 0-1.3 cm, basically without major changes, and floor heave and support deformation no longer occurred in the roadway.

 

 

In underground coal mine roadways, especially in integrated mine construction, roadway support under overlying goafs is often encountered. When the roof is loose and broken, and its strength is insufficient to maintain its own stability, bolt support cannot be used, and only single hydraulic prop support can be adopted. Its role is to maintain the stability of the roadway surrounding rock, prevent roof fall accidents, and ensure construction safety. However, the effect of single hydraulic prop support is not ideal and needs further improvement.