Tunnel excavation must be supported in time to reduce the looseness of surrounding rock and prevent collapse. Construction support is divided into component support and bolt-shotcrete support. Generally, there are wood, metal, steel-wood mixed components and so on. Now more and more people use steel supports.
Shotcrete-anchor support is a kind of support method developed in 1950s, which is characterized by timely, stable and reliable support, certain flexibility, close contact with surrounding rock and large activity space in the construction site. The application of bolting and shotcreting support in some old loess tunnels in China has also achieved success. Spraying concrete technology is divided into dry spraying and wet spraying.
At present, dry spraying method is mostly used, that is, a certain amount of accelerator is mixed into dry-mixed concrete, and the concrete is sprayed from the pipeline with compressed air. Add water to the nozzle and spray it on the rock surface with a thickness of 3 ~ 5 cm at a time. Adding some steel fibers to shotcrete or hanging steel wire mesh on the rock surface can improve the strength of shotcrete and bolt support.
Steel anchor is installed in the borehole on the rock surface, and its length and spacing depend on the nature of surrounding rock, generally 2 ~ 5 meters in length. Usually, the anchorage is carried out with gum and cement slurry along the whole length of the rod body. Only by spraying concrete in good rock stratum can sufficient support strength be obtained. In the hard and stable surrounding rock area, no support can be added. In the weak surrounding rock area, shotcrete and anchor support can be combined, and the anchor rod can be lengthened to strengthen the support force.
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Development trend:
In tunnel engineering, shotcrete and bolt support can replace component support. The main advantages of bolting and shotcreting support are timely, safe and reliable, and can save a lot of wood and steel. In some European countries, after large-section blasting in soft strata, long anchor rod combined with shotcrete is used as support, which has achieved success. China also used shotcrete and bolt support when excavating the old loess tunnel.
After the development of bolting and shotcreting support, full-face excavation can also be carried out for soft rock strata, and local excavation can be changed to full-face excavation. The rock stratum adopts full-face excavation and shotcrete lining.
The quality of this method first depends on smooth blasting. Based on NATM principle and considering the bearing capacity of surrounding rock itself, single-shot or shotcrete-anchor support can be used as the initial support as soon as possible after tunnel blasting, and then the displacement is continuously measured to determine the basic stability time of surrounding rock, and then the secondary support is carried out, so as to build an economical lining structure.
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