For tunnel between Licun Station and Junfeng Road Station of Qingdao Metro Line 3, “pile support” and “pile anchor” are adopted for the foundation pit in the open excavation section. Among them, the pile-anchor support adopts 73/59mm self-drilling anchor bolt and the drilling depth is 24 m. Why use self-drilling anchor bolt for support? The following are its advantages:

Bolt support is to change its mechanical state by working together with surrounding rock, so as to achieve the purpose of stabilizing the pit wall. According to the theory of maximum horizontal stress, under the action of maximum horizontal stress, the surrounding rock will have interlayer dislocation, which will cause foundation pit deformation. The role of anchor bolt is to restrain its shear dislocation along the axial strata. Therefore, the pile-anchor supporting form of the foundation pit mainly considers several aspects:

1) In terms of force, the advantages of piles and large-diameter self-drilling anchor bolts are high strength, rigidity and shear resistance;

2) The anchor bolt has better support effect, and with characteristics of saving materials, simple construction, convenient mechanized operation, and fast construction speed;

3) The anchor bolt adopts self-drilling construction technology, which effectively solves the problem of difficult hole formation in weak surrounding rock formation;

4) It provides a large space for the construction of the foundation pit internal structure and avoids the safety risks caused by inverted support. It can effectively ensure the foundation pit stability in the construction process.

Using large diameter self-drilling bolts for construction requires equipment with large torque, while such equipment is short of supply in the market. This makes it difficult to directly drill with self-drilling anchor bolt, so the contractor adjusted the construction plan according to the actual situation:

1) The rotary drill bar is used to lead the hole first, and then use the self-drilling anchor bolt to drill, so as to ensure its effective drilling depth.

2) In order to meet the reinforcement requirements of hollow bar and the surrounding disturbed soil, the secondary grouting technology is adopted after drilling to ensure anchoring effect.

The practice shows that this technology can effectively improve effective drilling length of large-diameter or super-large diameter self-drilling anchor bolt when equipment with large mechanical torque is insufficient in the domestic market. At the same time, the successful application of self-drilling anchor bolt provides a safe and reliable space for foundation pit excavation and structural construction and avoids the operation of inverted support. This not only effectively saves the project cost, but also reduces the construction risk and ensures the construction period.