Mine Support System with Anchor Bolt, Chain Link Fence & Spray Mortar

Combined support with anchor bolt, chain link fence and spray mortar can increase the anti-bending and shear capacity of coal mines.

In recent years, combined supporting system of anchor bolt and spray mortar is widely applied in most of coal mines. However, with the increase of mining depth and the constant change of geological condition, the metal support makes the strata exposed too long and the danger of its falling is increasing. For the broken rock zone, the supporting strength of bolt support and spray mortar support can not be solved completely and its rework effort is also increasing, which increases labor intensity and economic losses. Nowadays, some mines begin to apply combined support system with anchor bolt, chain link fence and spray mortar in roadway support.

1. Main principle of anchor bolt, chain link fence and spray mortar support system

Combined support technique with anchor bolt, chain link fence and spray mortar is a new kind of support which adds a layer of diamond galvanized chain link fence as an anchor net on the basis of the existing anchor bolt support and spray mortar support. This technique combining the first two kinds of support well, can not only give full play to the role of anchor bolt, but also utilize the positive effect of spray mortar. At the same time, the chain link fence, after spraying mortar, makes the fracture zone of surrounding rock surface integral, makes the original broken surrounding rocks even and uniform, increases the anti-bending and shear capacity and allows high flexibility and large deformation.

Anchor bolt, chain link fence and spray mortar support system breaks through the old support form and traditional support theory. It is not to exert a certain pressure on the broken surrounding rocks passively to avoid their falling, but to maintain the integrity and stability of surrounding rocks and control the deformation, displacement and crack development of surrounding rocks proactively. By doing this, this system can help the surrounding rock fully play its own supporting role, that is, it lays emphasis on protection, supplemented by supporting. Anchor bolt, chain link fence and spray mortar support system is a kind of reasonable supporting form which is used to reinforce and protect the loose circle.

1.1 Anchor bolt

Anchor bolt supporting is the main body of the combined support system with anchor bolt, chain link fence and spray mortar. It gets into the interior surrounding rocks and works with a certain range of surrounding rocks to support roadway. Under the extruded and reinforced effect of anchor bolt, there forms a good connection between bolt and the surrounding rock to maintain a certain stability of surrounding rock. At that time, the surrounding rock is not only the external load source, but also the supporting structure, which can give full play to the bearing capacity of the surrounding rocks and prevent the loosening and deformation of the upper surrounding rocks.

1.2 Spray mortar support

The roles of spray mortar support are mainly reflected in 4 points:

• Reinforced & weather-proof effect: The spray mortar is injected into the open joint crack at high speed to produce a condensation like a stone wall, improving the strength and internal friction angle of rock and the strength of surrounding rock directly.
• Improving the stress state of surrounding rock: On the one hand, spray mortar support can fill the uneven surface of the surrounding rock and eliminate the stress concentration caused by uneven surface of the rocks. On the other hand, the surrounding rock of roadway whose stress state is transformed from one-way or two-way force state to three direction stress state, improving the strength of surrounding rocks.
• Flexible supporting structure: Spray mortar is high-strength and it can tightly condense with surrounding rocks and work together with them. With a certain degree of flexibility, it can deform together with surrounding rocks and produces a certain amount of radial displacement, forming a certain range of non-elastic area in the surrounding rock to give full play to the support ability of surrounding rocks.
• Interaction with surrounding rock: Spray mortar can make the bonding strength and shear strength of spray layer and rock be strong enough to resist the local damage of surrounding rock and prevent individual dangerous rock movement, sliding or falling. In this way, it can not only maintain the stability of the surrounding rock, but also form a common bearing structure with the spray layer.

1.3 Metal wire—Galvanized chain link fence with diamond mesh opening

Chain link fence has greatly improved the previous supporting form of anchor rod and increase the whole area of anchorage extrusion. When the length and density of the bolt are the same, the effect of the extrusion arch is enlarged and the ability of roadway to resist damage is also enhanced by chain link fence. At the same time, chain link fence connects the spray layer with the tunnel way of the roadway to prevent the separation between them, greatly improving the shear and tensile strength of spray layer. In addition, chain link fence can combine anchor bolt and spray mortar these two supporting methods efficiently, which can improve the supporting strength of loose and broken weak rock stratum.

2. Design and selection of supporting parameters

2.1 Selection of anchor bolt and anchoring agent

The construction of the combined support system with anchor bolt, chain link fence and spray mortar in roadway support has the requirements of being safe and reliable, simple operation, good heat, water, acid and alkali resistance performance and excellent inflammable performance. Therefore, we should take resin anchor bolt as the first choice when selecting the type of anchor bolt. Resin anchor bolt consists of resin cartridge and rod, adopting resin as binder. Under the effect of curing agent and accelerating agent, the head of the bolt is stuck into the bolt hole.

Specification of anchor bolt:

• The type of anchorage agent: Z2335.
• The type of anchor bolt: strong thread anchor bolt.
• The size of anchor bolt: 16 × 1600 mm.
• The arrangement of anchor bolts: each row of 3 rods with row spacing of 1000 mm and spacing from 500 mm to 800 mm.
• Anchorage agent resin cartridge: Φ23mm × 350mm, 2 rolls per rod.
• Pallet: Square pallet with diameters of 160 mm and thickness of 10 mm.
• Drill: Its diameter is 28 mm and depth is 1700 mm.

2.2 Selection of spray mortar

Spray mortar requires high early strength, fast setting and hardening. Thus, Portland cement, whose strength grade shall not be less than 32.5, should be selected by preference, instead of agglomerated cement. Mixing sand for spray mortar should adopts hard clean medium sand or coarse sand, whose fineness modulus is better to be greater than 2.5.

Notice: Fineness modulus is an important index to measure the quality of artificial sand, which directly affects the workability, strength, impermeability and economic index of mortar.

The specification of the sand

According to the fineness modulus, sand is mainly divided into three degrees: coarse, medium and fine sand. The fineness modulus of coarse sand is from 3.7 to 3.1; the fineness modulus of medium sand is from 3.3 to 3 and fine sand is from 2.2 to 1.6.

The strength of spray mortar shall not be less than 15 MPa. In order to make the spray mortar be quickly stuck to the wall, users need to add quick-setting additive in it. The proportion of quick-setting additive is according to its type, while generally, the setting time of spray mortar should not be longer than 10 min. If it is needed, users can add other cement modifying agent to spray mortar, such as thickening agent, water reducer, early strength agent and so forth, which can improve the supporting performance of spray mortar.

2.3 Chain link fence

Chain link fence is also called hanging net or diamond mesh. In the selection of chain link fence, users need to meet the strength and stiffness requirements of net bars, which have a certain degree of shear and bending capacity. Manpower can make chain link fence deform with the tunnel arch and be close to the tunnel wall. Thus, mesh size of chain link fence can not be too small, avoiding appear empty holes when spraying mortar.

According to these requirements, the ideal specifications of chain link fence are as followings:

• Material: high-quality galvanized low carbon steel wire.
• Dimensions: 1.2 m × 10 m per roll.
• Diameter: 4 mm.
• Mesh size: 75 mm × 75 mm.

Chain link fence has the advantages of low cost, light weight, good corrosion resistance high strength and stiffness, which can not only prevent the loose rock falling, but also can increase the overall effect of the anchor bolt supporting effectively. So, it is suitable for roadway support with large deformation and high stress.

3. Construction technology in practical projects

In the construction of a project, there is too much pressure on the mine. Coupled with the effect of original geological conditions and fault fracture zone, the surrounding rock here is in the broken loose circle, the phenomenon of roof collapse occurred repeatedly in the roadway. Therefore, we choose a 28-millimeter-diameter drill punching holes in tunnel wall, with row spacing of 800 mm × 800 mm. The depth of each hole is 1800 mm. After punching holes, the construction team fix chain link fences on the top of the arch or the wall of the roadway, where they were drilling at and press these chain link fences into the wall of the roadway to make them close to it. At the same time, do temporary supporting work well. And then fix the resin anchorage agent and anchor rod through locking them in a tray. When the installation of each anchor bolt is complete, Twist and test them with pneumatic wrench timely to ensure that the anchoring force and torque of each bolt torque and anchoring force reach the standard. Finally, the construction team spray the mortar and complete the whole process. In the construction of spraying mortar, the thickness of the spraying layer should reach 50 mm and there will be a second spraying process if the first spray layer does not meet the thickness requirements.

4. The advantages of combined support system with anchor bolt, chain link fence and spray mortar in coal mine roadway support

As a new reform in coal mine roadway supporting technology, anchor bolt, chain link fence and spray mortar support system has a very strong application in the excavation and maintenance of roadway. Based on its using situation in some coal mines, there are some advantages as follows:

• Simple supporting technology: Its labor intensity is low and the construction time is short.
• Low-cost supporting technology.
• High-strength supporting technology: This supporting system makes anchor bolt, chain link fence and spray mortar work together to form an active support. The roadway is a kind of flexible support, having a certain shrinkage. The roadway support is also high-strength, while the metal shed support and masonry-lining support are passive support, which is easy to compression deformation and has large maintenance works.
• Small drilling section: The utilization rate of roadway is high and the ventilation resistance is small. Because the drilling section of the masonry-lining support and metal roof support is much larger than the mesh section, it increase the amount of digging into the gangue. However, combined support system with anchor bolt, chain link fence and spray mortar in roadway support has small drilling section and its ventilation resistance is smaller than other support.
• Strong applicability: Anchor bolt, chain link fence and spray mortar support system is not only suitable for using in the shaft, the parking lot, the street and many other main roadways, but also suitable for applied in transport lane and return airways. At the same time, its advantages can be displayed in the maintenance of roadways. Adopting brickwork and metal roof support, there will appear roof caving caused by mining-induced influence, which needs to be brushed again. It has a great influence on the normal operation of the roadway, thereby influence the normal production of the mine. Adopting combined support system with anchor bolt, chain link fence and spray mortar in roadway support, it does not affect the normal passage of the original roadway and ensure the normal working of production.