Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to the newest version of your browser.

Your browser appears to have cookies disabled. For the best experience of Construction News, please enable cookies in your browser.

Welcome to the Construction News site. As we have relaunched, you will have to sign in once now and agree for us to use cookies, so you won't need to log in each time you visit our site.
Learn more

A for tune rests on firm foun ndations

PILING - Rocketing real estate prices usually put off first-time buyers and are in danger of causing the housing market to stagnate. So how have they positively triggered a development in Cardiff? Paul Thompson investigates

DAVID Stacey looks like the cat that got the cream - and rightly so.The managing director of developer Deanminster is staring out of the window of a site office, surveying his domain.

Eventually this former brickwork and clay pit in Cardiff 's north-western suburb of Birchgrove will become a bustling residential estate with 177 houses, sheltered accommodation for 44 people and a medical centre.

That it currently resembles nothing more than a quagmire, owing to heavy rain, is not dampening Mr Stacey's ardour for the Cardiff Grove project.

He talks excitedly through the history of planning applications for the site and how eventually the soaring property market made the scheme viable.

'Developers had been interested in this site for years but the cost of remediation had made it uneconomical.Now higher property values have allowed us to take another look at it, ' he says.

The site itself has always interested developers but most have been put off by the high costs of remediation.

Many of the plans for its redevelopment have been pushed in front of Ian Statham, head of engineer Arup's geotechnical division in Wales.

He has carried out initial viability studies for the site at least four times during his time and is convinced other consulting engineers in the area will also have carried out some work on it.

'It's been across my desk probably four or five times over the years and it would be naive to think that others had not taken a look at it too, ' he says.

One option was to cap and dynamically compact the site in an effort to make the ground good enough for the contractor to put in foundations for the development.

But for Mr Statham and his team the calculations did not add up.

'Geotechnically that idea just did not stack up.There was just too much risk, the compaction would not reach the depths we needed it to, ' he says.

During its time as a brickworks, clay had been excavated to a depth of 30 m at its deepest point.Uncontrolled fill from demolition projects has been placed over the years and a final capping layer brings it up to its current level.The depth and the unknown nature of the ground made it difficult for the Arup engineers, although they knew piles and ground beams would be their preferred option.

Initially, bored piles were considered but concerns about drilling through contaminated land and the cost of shipping off the arisings to landfill sites eventually put an end to this idea.

'As the developers we had concerns over the cost and these were starting to add up.We were also worried that we could bore through the water table, ' says Mr Stacey.

Driven piles became the obvious solution but Mr Statham was worried about using precast concrete piles in case they hit an obstruction at depth.

How would they know they had reached the bedrock?

'We would be piling so deep into poor ground that if we had a refusal at say 15 m, we couldn't be sure if we'd hit bedrock or a piece of demolition waste, ' he says.

When piling contractor Aarsleff 's driven steel tube method was mentioned in a meeting the team knew they were on to something.

'Being an open tube it meant we could confirm we had hit bedrock and also they have the ability to flex a little around obstructions. It's a very robust system, ' says Arup project engineer Aled Phillips.

Aarsleff piling manager Phillip Chippindale echoes these views.'Steel tubes can deal with obstructions. Precast concrete will hit an obstacle, take the bending moment placed on it and then break.This is far more robust, ' he says.

Even so the initial proposal that Aarsleff put together was a mixture of both precast concrete and steel tubes, but that was quickly abandoned in favour of an all-steel approach.

Two different sizes of tube are being used on the scheme: 244 mm and 273 mm diameters reflect the different pile loads of 425 and 625 kN placed on them and also the depths to which they are driven.The Arup team then designed the piling layout for the scheme, no easy task when there is little, if any, repeat work across the site.

'It is all about maximising the working load for each pile.We were given 13 plot drawings from the house builder Westbury Homes and for each one had to produce a pile layout and ground beam design, ' says Mr Phillips.

On site though, the Aarsleff team are racing through the work according to Sir Robert McAlpine project manager John Evans.

'The Aarsleff lads work really well.They will carry out the work in two phases because of the constraints of the site and the quantity of capping material we have on here, ' he says.

The bulk of that work is in the first phase, which will take three months to complete.Aarsleff will then come back later in the year with its Banute 700 rig to carry out the second phase, a step that will take around three weeks.Then the whole site will be handed over to Westbury for it to transform into the bustling estate it promises to be.

Driven pile lengths of up to 30 m are not the norm in housing schemes such as this and Mr Stacey knows it.

'If I had said to a developer that houses would need piles driven to more than 25 m they would have thought I was daft. But now we are showing an economic return on the amount of investment we have spent on the site under the surface, ' he says.

That may or may not be developer-speak for 'We stand to make a fortune from this site'If this is the case, fair play to Mr Stacey, Aarsleff and Sir Robert McAlpine for going where others feared to tread.