Barrett Byrd Associates

Hidden obstructions and seams of hard ground are no match for a durable foundation system that uses tungsten cutting heads attached to threaded steel tubes, reports Mike Walter

Frenzied demand for new property in tourist hotspots beside the Mediterranean is leading developers to build on tricky coastal sites as the more favoured plots of land fill up fast. One such example is the British colonial outpost of Gibraltar, where apartment blocks are going up on sites with difficult ground conditions that require the installation of durable foundations.

Piling specialist Roger Bullivant Ltd (RBL) was asked earlier this spring to provide support for five apartment blocks at the Ocean Village development, under construction on reclaimed land. The ground contains layers of marl, limestone, stiff clays and the odd bolder which precluded the use of both precast and conventional augered piles.

RBL’s answer was the drill bar pile: a ballistic bit formed from tungsten carbide, attached to sections of threaded steel tube installed in both a percussive and rotary bored fashion. Each ballistic bit is the size of a large clenched fist and is shaped to punch through tough ground with ease. Holes in the cutting head allow for a flush of grout to flow down the pile shaft when it reaches full depth and fill the surrounding ground to form a durable foundation.

Seven hundred drill bar piles are currently being installed at the Gibraltar site to depths of between 12m and 15m using a versatile 11t rig that repeatedly applies a rotary hammered ‘drifter’ load. Each pile is designed to accommodate a load of 800kN and in such difficult ground, around a dozen are being completed each day. Output compares favourably to the two piles a day likely to be achieved on the site using a conventional rotary bored technique.

RB Spain’s technical manager Gerry Cooper said: “We needed to specify a pile that is capable of quickly drilling through very tough material. Use of rotary bored piles could have proved much slower and there would have been no guarantee they would have coped in these ground conditions.”

Speed associated with the drill bar technique is partly explained by the method of installation used. Once the threaded steel shaft reaches full depth, it remains in situ and the rig operator can move on to the next pile position. RBL’s technical piling manager Zac Bastin says: “We can install a drill bar pile in around a third of the time it takes to complete a conventionally augured pile. This is because obstructions in the ground are overcome quickly with this system and there is no need to retrieve sections of threaded bar from 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 prior to a flush of grout. Mr 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.

Use of vertical steel tubes as opposed to a continuous flight auger cuts down on the amount of spoil brought to the surface and the robust cutting head keeps vibration and noise generated on site to a minimum. Manual handling of equipment is also reduced. Sections of flight and the cutting head are only handled prior to their installation and not a second time as the apparatus remains in the ground.

Two versions of cutting head have been developed by product supplier Specialist Tube Supplies for use with the drill bar pile in a variety of sizes. A domed 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.

Each cutting head is designed to be sacrificial, to be used once and to remain in the ground. “The heads do not need to last forever, once they have created their hole they are no longer needed,” adds Mr Bastin.

Use of drill bar piles as a foundation for new property is a concept only recently fully exploited, following RBL’s successful use of the technique to support retaining walls and to underpin buildings.

An example of this is a project in Scotland where drill bar piles are being installed beneath a block of 20 apartments in the town of Hamilton to correct subsidence caused by failed foundations. The block had moved away from an adjoining building and dropped by around 150mm, causing a large crack to appear at the front and rear.

RBL was called in by the NHBC (National House Building Council) to design an underpinning solution and chose to install 360 drill bar piles beside internal and external walls to support new reinforced concrete ‘needle’ beams to take load from the walls. A domed shaped button bit was specified in areas of the site where obstructions in the ground were likely and a cross bit casting was used in slightly softer ground. A mini piling rig was specified to gain access through doorways.

Piling started in February and is due to finish in May, before the company reinstates timber floors and redecorates the interior.

The company’s underpinning manager Paul Doyle says every drill bar pile installed to date on site has achieved full depth. “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.”

Piles extend up to 12m in length and are founded in bedrock. Alternative foundation techniques such as auguring down into the ground and installing permanent steel casings were rejected as taking muck away from site had to be kept to a minimum.

Elsewhere, an extension to a rail bridge beside the West Coast Main Line in Staffordshire was piled last autumn using RBL’s drill bar technique. One hundred piles installed to a depth of 7m were completed in ground containing sandstone and reduced what would have been a two week contract to five days.

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