Bullivant’s tough bit

Roger Bullivant has developed a new alternative to mini-piles which can cope with difficult ground conditions. ‘Drill bar piles’ use a tungsten carbide ballistic bit attached to a section of hollow threaded steel tube to punch through tough ground.

‘This system provides a speedy alternative to installing driven mini-piles, which may be affected by obstructions in the ground and take around twice as long to complete,’ says Roger Bullivant’s underpinning manager Paul Doyle.

Developed over two years, drill bar piles have been used commercially for the first time this year to provide a foundation beneath five apartment blocks built on reclaimed land in Gibraltar and to underpin a block of flats suffering from subsidence in the Scottish town of Hamilton.

The piles will next be used at the site of Glasgow’s Bishopbriggs Academy where work starts this month. Roger Bullivant will install 126 of the piles to a depth of 16m on behalf of Amec, in a four-week programme of works valued at around £170,000.

The school will be built on land containing sands and gravels that cover sandstone, and hidden within the ground are obstructions such as a sewer and small boulders. Roger Bullivant chose to use the drill bar technique because driven piling may have damaged the sewer and auger bored piling would have found trouble with overcoming the obstructions.

Two versions of cutting head have been developed in a variety of sizes by Specialist Tube Supplies for use with the drill bar pile. A dome-shaped button bit is designed for use in hard rock and sandstone while a cross bit casting featuring three flat serrated edges can be specified for clays and shales.

Holes in the cutting head allow for a flush of grout, water or air to aid penetration and when the pile shaft reaches full depth, grout fills the surrounding ground to form a durable foundation.

Each cutting head remains in the ground. Trying to remove sections of shaft would not only delay progress on site, but run the risk of damaging the integrity of pile positions. Roger Bullivant’s technical piling manager, Zac Bastin, adds that leaving steel bars in situ also helps with the transfer of load. ‘Most of the tension load is accommodated by the threaded steel bar which acts as reinforcement and extends to the full depth of the pile,’ he says.

Leaving the bars, which can be used in both a percussive and rotary bored fashion, also contributes to the speed of installation as the rig operator can move to the next position once the threaded steel shaft reaches full depth.