Based on engineering geological investigation report, slope stability should be analyzed to determine following design parameters: slope rate of stable slope, slope rate of natural slope, unit weight of each stratum, internal friction angle, cohesive force and compressive resistance, etc. 

Anchorage force is analyzed according to slope stability and determined by sliding force of slope unstable system. So, calculating self drilling rock bolt’s anchorage force, sliding force of slope unstable system should be firstly counted. 

Anchorage body design of self drilling rock bolt is rather complex, which is mainly determined by the particularity of its structure. Anchorage body is composed of self drilling hollow anchor bolt and grouting body. Choice of self drilling rock bolt’s model is based on the size of design anchorage force. Grouting body is formed by injected and agglutinated cement paste.  

After confirmation of grouting parameter, bonding strength between grouting body and anchor bolt, shearing strength between grouting body and hole wall are determined by rock character. Generally, broken stratum, sand and gravel’s grouting, self drilling rock bolt’s bonding strength and hole wall’s shearing strength is higher than sand and soft clay. 

In the construction process of the River Bay Tunnel exit , the first is to design the big pipe-shed according to the drawing, using the DTH Drill with superior performance, high efficiency, and large drilling capacity and with an eccentric drill bit. But in the actual operation, the maximum drilling depth is less than 3m. The main factor is sticking, resulting in deviating from the design direction in the construction. Big pipe-shed advanced support scheme was thus stranded. After the analysis of above reasons, considering the features of self drilling rock anchor, the contractor decided to use the 4m R25N rock anchor for short pipe-shed advanced construction. Self drilling rock anchor has the following advantages for advanced support:

(1) Self drilling rock anchor and drill rod can be merged into one, with a small diameter drill bit and without eccentric block. It’s easy to drill;

(2) Drilling the hole directly, no need to be pulled out and no collapse and blockage  phenomenon;

(3) The anchor bar is hollow, which can be effectively grouting and bear large load, ensuring the grouting diffusion radius and the anchoring effect of surrounding rocks;

(4) The pipe-shed is short and easy to install, easy to operate, and can work at multi-points simultaneously.