Shotcrete supporting function
Shotcrete has two construction techniques: "dry spray" and "wet spray". The dry spraying method is to mix cement, sand and stones with a maximum particle size of less than 25 mm in a certain proportion, then load them into a spraying machine, and use compressed air to transport the dry mixture along the pipeline to the nozzle, mix with water and High-speed jet of 40-60 m/s onto the working surface. In the wet spray method, the raw materials are mixed with water before spraying. During the construction of shotcrete, due to the impact of cement particles and aggregates, continuous extrusion, and the use of a smaller water-cement ratio, the concrete has sufficient compactness, higher strength and better durability.
1. Supporting role
Shotcrete has certain physical and mechanical performance requirements, among which compressive strength is one of its main properties. Generally, the compressive strength can reach 20MPa, so it can play a role in supporting the ground pressure. The concrete formed by layer-by-layer injection and pounding has dense structure, and its compressive and shear strength are higher than that of concrete poured under the same conditions. During spray construction, a certain number of accelerating agents are mixed to make the concrete set fast, with high early strength, can closely follow the tunneling face, play a role in supporting the surrounding rock in time, and relax the pressure of the surrounding rock caused by the tunneling blasting There should be no excessive development.
2. Filling effect
The dry-mixed material is conveyed by compressed air and has a high speed at the outlet of the spray gun, which can fill the fissures and joints of the surrounding rock well, and can fill the rock surface of the cavity. The concrete filled in the rock cracks or cracks not only has a high cohesive force, but also increases the friction between the rock cracks, acting as a "wedge", so that the originally separated rock masses can be tightly united As a whole, it effectively prevents the relative movement between the rock masses and enhances the supporting ability of the surrounding rock itself.
3. Isolation
The shotcrete is directly and tightly bonded to the rock and soil, so it can completely isolate the air, water and the surrounding rock from contacting, and can avoid the surrounding rock slab and roof fall caused by weathering and deliquescence; at the same time, due to the concrete filled in the cracks of the surrounding rock, The original filling in the depth of the fissure will not reduce its strength due to weathering. Therefore, as a result of the insulation effect, the surrounding rock maintains its original stability and strength.
4. Transformation
The concrete layer formed by high-speed spraying on the rock surface has high cohesive force and high strength. The concrete and surrounding rock are tightly combined and can transmit various stresses on the bonding surface. As a result of the isolation and filling effects, the stability of the surrounding rock and its own supporting capacity are improved, so that the shotcrete layer and the surrounding rock form a common mechanical unity, which can transform the surrounding rock load into a rock bearing structure The role has fundamentally changed the weaknesses of various passive supports in the past.
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