The Kings Waterfront development in the heart of Liverpool will be an integral part of the city's European Capital of Culture 2008 celebrations. GE looks at the dual piling solution for the high-profile project.
One of the first events of Liverpool's year as European Capital of Culture will be the opening of the £400M Kings Waterfront arena and conference centre on New Year's Day 2008.
Since the 2003 announcement of Liverpool's cultural coup, the city has seen a marked rise in construction activity, and the Kings Waterfront redevelopment is one of the biggest projects on Merseyside.
Situated next to Albert Dock and the world-famous Liver Building, it will provide the city with a new landmark and a cultural focal point.
Given the scale of the development, its location on the banks of the Mersey and the site's history of industrial use, foundation contractor Bachy Soletanche knew it was not going to be a simple job.
But after conducting a test programme last summer, Bachy says it was able to fine-tune its scheme.
'The majority of the difficulties involved with the ground conditions were identifid during the testing programme, ' says Bachy contract manager Steve Mallinson.
The initial design proposed CFA piling throughout, as a fast and economic solution. But there was some uncertainty that the method could be used at all because of the sandstone beneath the site.
However, the contractor was confident that CFA would work with heavy duty, Bauer BG 22 rigs, which each boast a 22t torque.
The test programme included 12 trial bores and installation of three test piles. Various CFA drilling tools were tried to identify which were most effective in penetrating the sandstone.
Mallinson says: 'Five years ago, CFA might never have been considered for this job, but a combination of our experience of piling in the Liverpool area and the capabilities of the heavy duty CFA rigs has meant that it can be used on a majority of the Kings Waterfront site very easily.' However, the test programme did show that the proposed methods needed to be reappraised.
'Because the old Kings Dock that used to be on part of the site had been hydraulically filled with sand, when we constructed the test piles we discovered CFA piling wouldn't work in this area due to problems with flighting, ' Mallinson says.
Flighting is the process of ground loss caused by ground being drawn out during the augering process. At Kings Waterfront, the sand used to hydraulically fill the Kings Dock was being drawn into the CFA rig's auger in a similar process to Archimedes' screw.
'When we tried to drill into it we got ground loss that was undermining the rig and the platform was becoming unstable, ' Mallinson explains.
'In geological terms it was placed very recently. If the material had been laid down thousands of years ago it would probably have been compact and might have been OK.' To combat the problem Bachy decided to use cased large diameter auger piling (LDA) as well as CFA.
Discovery of the sand in the test programme meant the firm was able to change the piling in the old dock area without incurring the problems of a mid-project redesign.
CFA is still being used for two thirds of the project, with LDA piles within the old Kings Dock area.
The typically 15m long steel casing is vibrated through the sand with a heavy duty casing vibrator, penetrating the sandstone beneath.
Full piling work began in November, but the need to build more than 370 LDA piles put added pressure on an already tight programme.
Mallinson says: 'Building LDA piles is far more time consuming than CFA, vastly reducing daily pile productivity. So to combat any impact this would have on the programme we introduced an additional rotary rig to the project.' Upon completion of the £2.2M contract, the company will have constructed 768 CFA and 377 LDA piles to provide support for the huge arena and conference centre that will transform the Liverpool waterfront.
The size of the buildings means some loads will be as high as 400t, with a number of piles catering for wind loading and to support large retaining walls. Given these loads, the company says the beam jacked pile testing has been rigorous.
The project's multi-level buildings combined with the underlying sandstone's varied depth - it slopes down towards the Mersey - has resulted in pile depths ranging from 8m to 19m.
The development site is a hive of activity. Bachy alone had two rotary rigs and three large hydraulic CFA rigs on site, and construction work by other companies has already started on the multi-storey car park and further foundation work for the arena.
A large amount of work is being carried out simultaneously under planning devised by main contractor Bovis Lendlease. Given the strict 2007 deadline, it is unlikely the furious activity will slow down until completion.
As GE went to press, Bachy was on schedule to leave site by the middle of February.