The construction of Serendib Flour Mills' mill buildings and silos at the northern end of Colombo Port in Sri Lanka required the installation of 634 32-inch piles. The contractor, Sincon International Ltd., commenced the project using a Bauer BG22 bored pile machine, which successfully completed 269 piles at good production rates. It became apparent that the boulders, which were encountered at other pile locations, were harder and tougher than anticipated, and the BG22 refused at those locations. After consideration of various methods to overcome the boulders, the Super Jaws method was selected. A suitable piling rig was available at New Zealand contractor Daniel Smith Industries Ltd., and was converted there to DTH and shipped to the project.

Cores from boulders at three pile locations were tested at the University of Melbourne's rock mechanics lab. The results confirmed the boulders were hard (160, 122 and 108 Mpa), however, the distinguishing characteristic was that all three cores were extremely abrasive - more abrasive than 186 MPa Barre granite tested in the same lab.

The site location presented another challenge - the mill is located directly beside the breakwater at the northernmost end of the port. The pile design called for reinforced concrete piles, but seawater could be seen surging through cavities in the breakwater and into the site, and DTH cuttings included an assortment of seashells and lead fishing sinkers.

It was concluded that the wave action must have been surging through a large part of the site, which was confirmed by core drilling the DTH piles already completed. The decision then was made to case every DTH pile on the project, and the piling was successfully completed using this method.

The overall project production rate based on the total feet drilled divided by the compressor engine hours was approximately 18 feet per hour.

Project Particulars

Application: piles for a dockside flour mill

Hammer: NUMA Champion 240

Bit: Super Jaws T710 under-reaming bit

Casing Size: 30-inch, 0.374-inch and 0.402-inch wall

Formation: hard and abrasive granite gneiss boulders, unconsolidated fill containing occasional scrap steel

Rig Type: Nippon Sharyo DH508-105M piling rig converted to DTH

Compressors: (3) IR XHP1070WCAT - 1,070 cfm/350 psig