Bidirectional Tensile Plastic Net for Mining Roadway Support

General situation of coal roadway support development

Roadway support technology is one of the important contents of coal mining. With the popularization and application of bolt support technology, the superiority of bolt support is shown. Having the advantages of being stable and reliable, low cost and fast construction speed, bolt support has been widely used in coal mine and has become the main direction of the development of roadway support technology. The support form of bolt support technique in coal mine can be divided into two types: bolting net belt support and bolting net shotcrete support.

The form of bolting net belt support:

• Roof supporting: anchor bolt + M-type steel belt + metal diamond-shaped net.
• Side supporting: anchor bolt + M-type steel belt + bidirectional tensile plastic net (or metal diamond-shaped net).

The form of bolting net shotcrete support:

• anchor bolt + once shotcreting support of metal warp and weft net.
• anchor bolt + twice shotcreting support of metal warp and weft net.

In order to make the bolt support play more supporting effect and ensure the safety of the roadway use, the development trend of bolt support technology in recent years is inclined to improve the strength of roadway support and ensure its reliability. Taking the strength improvement of both bolt support rod and its matched steel band as the breakthrough point, while ignoring the support efficiency of auxiliary net, leads to serious deformation of roadway after being formed, roof separation and the formation of net pockets. At the same time, partial leaky roof and caving accidents occurs and the return repair rate of partial dynamic pressure roadway is up to 65%. In view of this supporting weakness, this paper investigates the bidirectional tensile plastic net and carries out industrial test and comparative observation and analysis of mine pressure.

General situation of industrial test

Underground industrial test situation of the bidirectional tensile plastic net: Through the underground industrial test, a complete inspection can be conduct to the feasibility of adopting bolt support in mine crossheading roadway IV and V class unstable surrounding rock, support design and construction technology, in order to further guide and modify the initial design of bolt support in coal mine. Underground industrial test is also an important link of the dynamic design of coal roadway bolting. Through the test and the observation of roadway rock pressure characteristics, we can draw the deformation and failure law of roadway under actual conditions and the actual effect of bolt support, so as to determine the reasonable support type and improve the scientific and reasonable basis. By doing this, the supporting technology of the bolt support can be more reasonable and the roadway support technology can be more mature.

• Conditions of underground industrial test
  The local roof in underground industrial test site of reinforced plastic composite net is a composite roof with thin coal or weak interlayer. Using bidirectional tensile plastic net in this kind of roadway can provide strong engineering construction experience for the mine in the static pressure state and the compound roof roadway, and provide a scientific digital basis for the future construction and support mode of such roadway.
• Technical conditions of production
  The roadway is 4.3 m wide, 2.8 m high. Its net width is 4 m and net height is 2.6 m. The standard height of the roadway baseboard is 315 m and its design length is about 640 m.
• Main parameters of roadway supporting
  • The specification of anchor bolt: Anchor bolt adopt φ20 × 2200 mm holoscrew and identity intensity anchor bolt. The row spacing between the roof bolts is 950 × 900 mm.
  • The type of anchor bolt: Resin lengthening anchorage.
  • Steel strip: Roof support adopts M-type steel strip whose specification (length × width × thickness) is 4000 × 140 × 50 mm. The steel strip on both sides shall also be M-type steel strip whose specification (length × width × thickness) shall be no less than 2200 × 140 × 3.25 mm.
  • Tray: Adopt special tray (10 mm thick) which is matched to the strength model of the M steel band and its area is not less than 110 cm².
  • The layout form and specification of anchor cable: The length of anchor rope is 6.0 m and the length of anchor bolt is 1.5 m, with 12# steel joist which is no less than 0.8 m. The arrangement spacing of anchor rope is 2000 mm × 1600 mm. They are arranged in the composite roof and fault fracture zone.
  • Net: Adopt the new reinforced plastic composite net whose model is matched with tunnel circulation footage. Its mesh size is 50 × 50 mm. The net adopts double connection with 14# iron wire and each connection at a interval of 200 mm.

Main technical parameters

The bidirectional tensile plastic net is a new type of protective net specially used in the support of coal mines. It takes polypropylene as the basic material. After the treatment of the anti-static and flame retardant technology, the integral structure of the grid is formed by the bidirectional stretching method, and the comprehensive performance is improved under the premise of guaranteeing the strength. Compared with ordinary plastic mesh and metal mesh and metal warp and weft net, the utility model has the following characteristics.

• High strength, corrosion resistance, aging resistance, easy to bend and transport.
• The tendons and ribs formed by bidirectional stretching are perpendicular to each other. It enables the pressure energy applied to the net surface to be transmitted to the surrounding tendons and ribs through the nodes effectively, thereby greatly improving the overall pressure bearing capacity of the net surface.
• The plastic net is directional tensile instead of knitting, so its creep variable is small and the mesh size is uniform, which can effectively prevent the drop of broken coal and protect the safety of workers underground.
• The overall structure of the grid has strong bearing capacity in both vertical and horizontal direction. In addition, the color of net is compatible with coal seam color and the roadway is tidy.
• Good flame retardation, the detection value is 2 seconds

The evaluation of use effect

The application of bidirectional tensile plastic net greatly improves the support strength of side support and avoids the vulcanization, oxidation, brittle fracture phenomenon which often happens on diamond-shaped mesh and iron wire net. It can also exert the active support efficiency of surrounding rock, and ensure the reliability of side support and the safety of service stage. The use of the bidirectional tensile plastic net not only greatly improves the roadway support strength and reduces the deformation of the roadway, but also avoids the input of material and manpower for two times due to the serious deformation of the roadway and can not meet the production requirements. in addition, it greatly reduces the comprehensive cost of the roadway.

Through the observations, under the original support form, the average shift of the two sides of roadways in 890 mm. Moreover, in the dynamic pressure roadway, the moving distance of the two sides nearly accounts for 60% of the roadway width. Phenomenons, such as steel bands on the supporting side are torn, anchors are fracture failure and the supporting sides are badly damaged, occur, which make the roadway not working properly. Through the application effect, using a bidirectional tensile plastic net with metal anchor bolts and protective steel band, the closure amount of two sides of roadways is 350 mm–470 mm. The overall stability of the coal body on the both sides lays the foundation for the reliability of roof support.

The kind of net is not only reliable, but also has good flexibility. It can be transported quickly and installed easily in the construction of roadways, thus improving the construction speed and reducing the labor intensity of the workers effectively.

Conclusion

• The deformation of roadway surrounding rock is controlled into the allowable range of bolt support, promoting the stability of roadway surrounding rock. Though the roadway deformation is small, it can meet the needs of production. Therefore, using bidirectional tensile plastic net + anchor + M-type steel combined support is feasible. It lays a foundation for the popularization and application of the combined support of anchor and bidirectional tensile plastic net.
• The labor intensity of the workers is obviously lower. Compared with the previous mode, the construction of bolt support is of high mechanization, which makes workers avoid heavy work, such as transportation, erection and repeated undercover picking of shed beam and shed column. It not only greatly reduces the labor intensity, but also ensures the construction safety and normal production continues.
• The experiment and application of bidirectional tensile plastic net have raised the support technology of coal roadway to a new level. It provides a reference for the support of the roadway and finds a feasible way for strengthening the surrounding rock of the supporting difficult area, especially the section affected by dynamic pressure.