Barrett Byrd Associates

A displacement pile with superior load capacity has been developed to suit high rise construction on poor ground in Scotland. Mike Walter reports

Difficult ground and large loads on the site of a Glasgow riverside development has led foundation specialist Roger Bullivant Ltd to develop a Continuous Helical Displacement (CHD) pile with enhanced load capabilities. Each pile founded in the soft alluvial deposits has been designed to accommodate a maximum working load of 1250kN, almost double that of Bullivant’s standard CHD pile.

The increase in pile capacity has been developed to reduce the number of piles needed to support a new 10 storey block of luxury apartments on Clyde Street in Glasgow. Main contractor CBC is building the development, known as the Metropole, which will include basement car parking, 94 apartments and 25,000m2 of retail space at ground level.

“There is a great deal of high rise construction being carried out in Glasgow that demands heavily loaded piles, but historically there have been problems with Continuous Flight Auger (CFA) piling in the area and up until now the most cost effective alternative – our CHD system – has been limited to a maximum working load of 700kN per pile,” says Bullivant’s area manager Hugh Sloan.

A total of 535 conventional CHD piles would have been needed to accommodate the large loads at Clyde Street. So Bullivant set about developing a 1250kN pile to reduce the number of piles to a more economical 300.

“Much of central Glasgow is built on fine silty sand overlaying rock some 25m deep,” Sloan says. “In such sensitive soils, the CHD system is ideal for ensuring minimal disturbance of the surrounding ground. CHD piling consolidates ground and enhances the soil around a pile.”

Increasing the capacity of Bullivant’s CHD system led to the trial of two new methods of creating a pile on site at Clyde Street. Conventional CHD piles feature a spiral of concrete up to 600mm in diameter around a 300mm core of concrete reinforced to partial depth by a 160mm steel reinforcing cage.

Bullivant looked first of all to increase the diameter of the auger flight used to create a CHD pile from 600mm to 720mm and the core diameter was increased from 300mm to 400mm. The pile was completed with a reinforcing cage installed, as usual, to partial depth.

The second test pile used a conventional 600mm diameter CHD auger flight with a 300mm core, but this time a reinforcing cage was installed to the full pile depth of 12m. Static load tests carried out on each new type of pile after a three week curing period verified that both could accommodate a working load of 2,500kN without failure.

According to Sloan, Bullivant chose the latter of the two options, to increase the load capacity with full depth reinforcement. “This may seem the obvious choice, but pushing 12m of steel into wet concrete can prove tricky. Reinforcing cages often refuse to enter a concrete pile and it can be a huge effort to force a large cage into the ground,” adds Sloan.

Efforts to overcome such difficulty led to development of a special concrete formula. The new mix is designed to retain its fluidity for a great deal longer than conventional concrete grades and prevents moisture in the concrete from migrating to the surrounding ground. This means that piling operatives can install a cage of four T32 steel bars relatively easily to full pile depth and CHD therefore becomes a viable option for founding large buildings in poor ground.

?Without developing the concrete mix, we would not have secured a contract for CHD piling with steel reinforcement down to 12 metres,? adds Sloan.

In excess of 300 piles were installed at Clyde Street during April at a rate of around 30 a day using a 7002 series rig which delivers 18t/m of torque and a high rotation speed needed for this type of pile construction.

An auger attached to a smooth central shaft was bored into the ground to full depth and concrete was then pumped under pressure through the centre of the shaft. The concrete pressure results in a helical shaped pile being formed as the auger is reversed before the reinforcing cage is lowered to the required depth.

Around 1.8m3 of concrete was used to form each 12m deep pile. “CHD piles are very efficient with regards to their concrete requirement,” says Sloan. “To carry a load of 1250kN in the type of ground at Clyde Street, we are using around a quarter of the concrete compared with an equivalent conventional CFA pile.”

Piles were installed either individually or in groups of up to six where loadings will be greatest. Use of the CHD system meant no spoil had to be taken off site, helping to avoid disruption to traffic and pedestrians in the area and a further inherent advantage of CHD piling is its near total lack of vibration.

“Doubling the working load of each pile has been a considerable challenge,” says Sloan. “It has helped to reduce the number of piles installed on site and has provided a significant saving to the client in terms of substructure costs.”

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