Comparison Of Different Hard Rock Drilling Methods For Bored Piles

Drilling penetration into rock becomes more difficult with increasing hole diameters and rock compressive strength. In piling applications, hard rock formations have to be cut and excavated prior to the installation of the foundation piles and/or piled retaining walls. Commonly, conventional rotary drill tools are used for bored piles in medium to very high strength rocks. For harder rock formations different methods have to be adopted as much larger cutting energy and force input are normally required to break the material at the rock tool interface.

Cluster drilling is a proven method to penetrate rocks with strengths exceeding 100 MPa. The method has been used successfully for decades in America, Asia and Europe for applications in the mining and construction industry with diameters usually ranging from 600 to 2400 mm. In Australia cluster drilling has recently been used for mining and modest construction applications. However, in 2010 Piling Contractors Pty Ltd has started utilizing innovatively designed and built cluster drills for the penetration of extremely high strength rock formations. Since then, more than 1,000 linear meters of extremely high strength rock (most of it with UCS in excess of 200 MPa) was successfully excavated using this technology for the installation of bored piles.

Air roller core barrels were also utilized in the past for various projects involving hard rock drilling. Basic working principles and limitations of these two traditional techniques compared to cluster drilling are identified and discussed in this paper.