Keeping the neighbours happy

Displacement piling work is helping to avoid disturbing neighbours in Cardiff. Alexandra Wynne hears how.

When designing piles for nearly 200 homes in the Welsh capital, designers were asked to come up with a scheme that would minimise disruption to neighbours living as close as 6m from the site.

Traditional pre-cast driven piles were immediately considered to be the most cost effective option but main contractor Bovis Homes was forced to reconsider.

Planning permission restrictions meant that they had to take into account any noise, vibrations and dust that might disturb neighbouring homes. So it was down to the subcontractor, May Gurney to come up with a more appropriate piling design.

"It might be that noise generated by driven piling would have been at an acceptable level but with the constant hammering sound it makes, it's more the perception of how bad it is," says May Gurney south west area manager, Rob Cannon.

"There were also concerns that vibrations would cause adverse effects and even structural damage to nearby homes."

He says the company considered CFA piling as one possible solution to noise and vibration problems, but with that came a new set of issues for local residents.

The Ferry Road site, close to Cardiff Bay, is in an area where other developments have been completed with driven piling, but now there are too many homes close by. With houses near the east and south borders of the site, the company opted for displacement auger piling. Cannon believes this is the first time that displacement piling has been used in Cardiff on a job of this size.

Cannon says that while CFA would have been efficient, it creates a large volume of spoil. The trucks needed to move this to land waste sites would have generated unwanted additional traffic around the city.

"Because you disqualify driven piling the next logical option to consider is bored piling," says Cannon. "But the amount of arisings it generates would be costly to remove from site and the logistics were a problem too."

Displacement piling is environmentally friendlier because it cuts the amount of spoil. "By reducing arisings you're reducing the amount of traffic coming on and off the site which is a big plus. This was simply about saving arisings."

Displacement piling depends upon ground conditions and at Ferry Road they are good. Up to 4.5m of soft made ground sits on soft to firm clays to 13.5m and firm brown silty clay with gravels to 15.5m, before dense sand, gravels and cobbles with occasional boulders and a stiff red marl appear at 19m to 20m.

The CFA auger works like a corkscrew, replacing spoil with a column of concrete. But the displacement pile boring tool is designed to shift soil laterally and downwards. In granular soils it has the effect of compacting soils which generates the load bearing capacity, says Cannon.

The company uses a bespoke boring tool. "The pitch and the angulation of the flights are designed to push material to the sides and down," says Cannon. Once the tool reaches depth, it works in a similar way to CFA, filling the void with pumped concrete through the hollow stem as the auger is retracted.

Displacement piling does generate some spoil as the auger emerges at the surface, but much less than with CFA, says Cannon. "If driven piles are ruled out, we'll always go for displacement instead of replacement. If you take the ground out, you have to find a way to get rid of it."

The company is using two of its largest rigs, a 49t Woltman THW 4019-D and a 78t Llamada P-150-TT, to install 634 piles at 15m to 19m depths depending upon where stronger ground appears. Company foreman at Ferry Road, Steven Anderson, explains the Woltman has the added benefit of being able to turn 360 degrees. This factor is helping to complete one pile about every 15 minutes.

Preliminary pile load tests done in mid-December proved the piling design down to a maximum depth of 19m and to a safety factor of 2 times 550kN -- the maximum load required for any pile.

Cannon sees a lot of future potential for this method. "Because there is now so much pressure on contractors to consider environmental issues, displacement piling will look like a more attractive option and could become the next big thing."

The 350mm diameter piles are strengthened by 6m long steel reinforcement bars with two collars. All of the piling was due to be completed as GE went to press.

Ground works contractor WD Lewis is sub-contracted to pick up where May Gurney leaves off. It has responsibility for completing ring beams before work on external structures can begin.

The development of one and two bedroom apartments and houses is expected to complete by mid-2009.