The surrounding rock of the tunnel is not only the source of the load but also one of the main materials of the support. When building a tunnel, shotcrete and tunnel rock bolts are two important supporting methods. Shotcrete can quickly seal the surface of the surrounding rock after excavation, and the rock bolt can improve the bearing capacity of the surrounding rock from the inside.
Rock bolts in the tunnel include local bolts and system bolts. The former can locally anchor the local unstable rock blocks or rock masses; the main function of the system rock bolts is to anchor the loose surrounding rock area around the tunnel excavation, and the surrounding rock around the auxiliary tunnel bears. However, in the weak surrounding rock, the role of ordinary bolts is limited, and it is even more difficult to construct. Only the self-drilling rock bolt can solve this problem.
According to the anchoring mechanism, the rock bolts can be divided into mechanical bolts (such as expansion rock bolts), full-length bonded bolts (such as cement mortar or resin bolts), friction bolts (such as swellex rock bolts), and other types. Different types of tunnel rock bolts can anchor the surrounding rock from the inside of the surrounding rock, thereby improving the self-bearing capacity of the broken surrounding rock around the tunnel.
In the case of high stress, layered cracks and deformations appear in the surrounding rock. At this time, the role of the tunnel system bolt is equivalent to strengthening several layers of thin plates into a composite beam, and then strengthening the surrounding rock in the radial range around the tunnel to improve the stress conditions of the surrounding rock.
Obviously, in the absence of bolts, the layered structure will produce large tensile cracks, showing uncoordinated deformation, which in turn leads to the limited bearing capacity of the surrounding rock. In the case of a systematic bolt, the bearing capacity of the composite beam will be greatly increased due to the restraint effect of the bolt on the deformation of the surrounding rock. Therefore, in the case of high stress, the system bolt can significantly improve the bearing capacity of the surrounding rock and prevent secondary damage to the surrounding rock caused by the rock burst.
First of all, the length of the bolt is not as long as possible. There is a critical anchoring length in the bolt support, that is, the ultimate anchoring length of the anchored structural rod in the geotechnical medium, beyond this length, the bearing capacity will not increase significantly.
It is closely related to the type of bolt, and there are great differences in the construction technology of different types of bolts.
The construction process of the ordinary hollow bolt and mortar bolt is: drilling - installing bolt - installing stopper - installing backing plate and nut - grouting.
Due to the use of a disposable drill bit in the construction of self-drilling hollow bolts, drilling, grouting, and anchoring are carried out simultaneously, that is, the drilling and the rod body are installed in place at one time. This kind of bolt is especially suitable for construction under the condition of broken and weak surrounding rock structures. Self-drilling hollow bolts mainly have the following advantages:
Suitable for broken rock and conditions which are difficult to drill holes. Casing pipe is omitted in the construction, which greatly improves the construction efficiency.
Has a rich variety of drill bits, which can be selected according to the properties of rock and soil to improve the drilling efficiency.
Can fill cracks, consolidate rock mass, and soil layer through pressure grouting, and has good grouting spreading radius and reliable anchoring quality.
The hollow anchor bar can be cut arbitrarily and lengthened by the coupler. Therefore, it is suitable for construction in a narrow space that the large equipment cannot enter.
Bolts are widely used in various projects at home and abroad. The performance and reliability of this kind of support have been confirmed, but its durability is an important factor restricting its long-term performance. In recent years, in order to solve the durability problem of traditional steel bolts, research on the corrosion resistance of bolts and the application of new material bolts have been carried out at home and abroad.
The bolt is under the combined action of stress corrosion and environmental corrosion during the service period, and its corrosion process is very complicated. At present, some anti-corrosion anchors in use at home and abroad include GFRP rock anchors, stainless steel rock bolts, epoxy-coated rock bolts, and duplex coating rock bolts.
In view of the effect of system bolt support, it is generally believed that in loess tunnels, the system bolt can hardly play a role in strengthening the surrounding rock in mechanism, and due to the application of the rock bolt, the closing time of the lining to form a loop is delayed, which in turn makes the surrounding rock difficult. deformation increases. However, for tunnels under general surrounding rock conditions, system bolts are necessary and effective.
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