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A quiet monster in the heart of Cardiff

A huge excavation in Cardiff city centre took advantage of a 'silent' piling method which helped keep the neighbours happy.

Project St David’s 2
Cost £280 million
Client Land Securities and Capital Shopping Centres
Main contractor BovisLend Lease
Piling Giken

Nick Wylie, project director with Bovis Lend Lease calls it “a beast of a project”.

And as we survey the monster excavation underway in Cardiff city centre, it’s clear that he is not exaggerating.

Over the course of the St David’s 2 development, which involves building a 89,891 sq m retail and residential scheme just a stone’s throw from the station, some 270,000 cu m of earth will have to be moved.

“If you laid out each cu m of soil consecutively, it would be enough to take you to Piccadilly Circus,” comments Mr Wylie. “The amount of earth that we are moving out of the basement would fill the Millennium Stadium.”

Bovis is design-and-build contractor on the £535 million regeneration project - a joint venture between Land Securities and Capital Shopping Centres. Currently the largest private investment in Wales, it aims to transform the southern part of Cardiff city centre.

The contractor won the £280 million design-and-build contract last year. And after finishing the diversion of services, the contractor and subcontractors have been working to create a basement which will stretch under a large part of the completed project.

“At points we were carrying out piling and demolition simultaneously. The interfaces were very tight,” says Mr Wylie.

Previously the area housed two multi-storey carparks, a 1970s shopping centre, a library and an ice rink. The basement will be completely under a new John Lewis store and halfway under some residential blocks. It will provide 650 carpark spaces, loading bays for 26 articulated lorries, management suites and staff changing rooms.

The bulk of the dig took around six months and around 150 to 160 wagonloads of earth were being shifted a day.

Given that the site, in the heart of Cardiff, could cause a lot of disruption to the city, it is not surprising that not all neighbours welcomed the contractor with open arms at the beginning.

Silent cinema

A cinema abuts the site, and when work was in the early phases, the manager phoned Mr Wylie, concerned that the work on the foundations would cause vibrations which would affect his screenings.

It is easy to imagine Mr Wylie suppressing a wry smile as he asked him, “Haven’t you noticed anything yet? We’ve been piling for the past two months.”

This triumph for community relations was thanks to a silent piling system, used on site by Giken Europe. It used silent piling both for the installation of steel sheet and tubular piles.

Giken site manager Stephen Timmins says that, at £5 million, this was the largest job that the subcontractor, whose parent company is Japanese, had carried out in the UK.

“We have done a lot of work for Bovis using our tubular crush piler,” says Mr Timmins. “Bovis is the only company to use it in the UK so far, although it has been active in Europe.”

The machine operates by gripping previously driven piles to provide a reaction force for pressing down on the next pile. Giken claims to be the only company in the world able to offer silent and vibration-free installation for tubular piles.

“The advantage is that it allows us to install piles into ground that we couldn’t previously go into,” says Mr Timmins.

Using the silent piler method the company managed to reduce noise levels from a typical 104 dB (which would be created by using a pile hammer) to 67 dB at a distance of 6 m.

The piling must withstand unusually high vertical loads - sheet piles have to withstand 500 kN/m while the tubular piles need to stand up to forces of 1100 kN/m. So since piling was completed over the summer, the project team has been hard at work testing the strength of the piles and repressing where necessary.

Now the concrete capping beam is being installed and the raft slab over the basement - which will require 5,000 tonnes of reinforcement - is roughly 40 per cent complete.

Mr Wylie adds that getting Giken on board before the project had even started was very helpful.

“Giken was one of the subcontractors who really helped us in the bid phase,” he says. “And using Giken’s methods meant that we didn’t have to use any temporary props during the excavation.

Digging round obstacles like that could have added another three months to this part of the project.”

He is also relieved that the complicated work has been carried out with virtually no complaints from the neighbours. “Cardiff City Council has complimented us on lack of disturbance,” he adds proudly.

Tubes over sheets

Over the course of construction of the monster basement, Giken installed 3,223 m of tubular piles, 15,891 m of sheet piles, and 501 m of continuous flight auger piles around an excavation, the walls of which would stretch out to 900 m.

Sheet piles were installed at the rate of roughly 6 m a day, whereas tubular piles were installed at the rate of between 3 and 4 m a day.

On the Cardiff project, Giken used its Giken Super Crush Tubular Piler SCP 260 to install 205 tubular piles on the western and eastern side of the excavation to create a cantilevered retaining wall. The piles were 914 mm diameter and ranged from 13.5 m to 15.5 m in length.

Mr Wylie estimates that installing tubular piles was three times more expensive than sheet piles. “This is because they used three times as much steel,” he says.

But he praises the method for being much quieter, more efficient, and avoiding the need for temporary works. Because they are freestanding, tubular piles are particularly useful in areas where there is no room for ground anchors.