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Why has the construction of secant piled walls for a road underpass in Stoke turned one site into 38?

David Hayward has the answer.

Steve Mallinson's workload looks awesome.

As a Bachy Soletanche contracts manager he is in charge of nearly 40 piling sites in Stoke on Trent alone.

Each one needs its own method statement plus risk and environmental assessments. Each must cope with repeated, sometimes daily, changes in programming, design and surrounding traffic management.

But at least he does not have to travel far to visit them. All 38 sites involving, he says, piling for some 45 structures, lie on the same 1.4km stretch of busy A500 dual carriageway in central Stoke.

To the casual visitor, Mallinson's project list appears, to say the least, exaggerated. Providing 2,500 piles to create secant walls for underpasses beneath two roundabouts about 1km apart, plus abutment foundations for four new overbridges, is - in fact - just one £4.5M contract.

And, given a clear run at all 3km of piling, it would indeed be just that. There are, though, rather a lot of 'obstructions' in the way of that utopian 'clear run' A canal, two rivers, myriad live services and - above all - frequently realigned through and cross traffic that must never be delayed, dissect that clear piling run into numerous short lengths, some offering space for just half a dozen piles.

'It's like a game of chess.We are constantly shuffling plant and materials between these isolated areas as they become available, ' Mallinson explains.'We really do treat each as a separate site with individual structures, many of which must be revisited several times; and this contract involves by far the most complex programming I have ever faced.'

His logistical challenge is shared by main contractor Edmund Nuttall which must convert, for client the Highways Agency, this major traffic bottleneck of two choked central roundabouts into free flowing, grade separated interchanges.

Lowering the main road into two separate 500m long underpasses, sweeping it beneath both roundabouts, that are then converted into grade separated junctions by adding two new cross traffic overbridges and two canal crossings, should solve motorist's frequent peak hour delays of half an hour.

Bachy Soletanche's task is to provide two rows of secant piling either side of the road and right through the roundabouts. The piled walls will later be excavated to form watertight sides for each 30m wide underpass.

Fortuitously for everyone, Nuttall's £40M design and build contract also bears the label Pathfinder. Back in spring 2001, the project became the Highways Agency's first Early Contractor Involvement scheme (ECI) allowing Nuttall, its consultant Hyder and prospective subcontractors a luxuriously long time to brainstorm and value engineer every aspect.

Initially, foundations contractor Keller was brought in to debate possible designs. But discussions ended somewhat abruptly after the company decided to pull out of the large diameter rotary piling market (GE November 03).

When the invitation was handed to Bachy Soletanche, the firm and Hyder value engineered the original conceptual design into a significantly different detailed alternative that has saved time and cut costs.

Underpass sides are now being formed of hard-firm cantilevered secant piled walls 18m deep, with the sunken roadway running at a depth of 6m. The hard 'male' piles, originally designed as 900mm diameter, have been beefed up to a maximum 1,050mm, with shorter, unreinforced interlocking firm 'female' piles just 600mm diameter.

These closer spaced male piles need less reinforcement, while the smaller females reduce overall concrete volumes. Although this solution increases total pile numbers by about 10%, the redesign is claimed to save several hundred thousand pounds. Use of continuous flight auger piling added considerable time savings to the list of value engineered benefits, with CFA piles being at least three times quicker to complete than standard rotary versions.

Installing over 2,500 piles in short lengths around, between and over so many obstacles to create continuous watertight walls places a considerable proportion of the project's overall risk in the ground. Both sides felt it 'appropriate' that a geotechnical specialist, more experienced at assessing this ground risk, should accept a higher proportion of that risk than the main contractor.

A decision, during negotiations between Nuttall and Bachy Soletanche, to change the piling subcontract form from lump sum to target cost, helped the company to win through as the chosen foundations subcontractor, claims Mallinson.

The switch produced a formula where an up to 10% overall saving or overrun in the foundations contract will be shared equally between Bachy Soletanche and Nuttall.

With Bachy confirmed as foundations subcontractor in May this year, the benefits of its six months ECI deal have borne fruit.

'We have never had such a long lead time before a contract, ' Mallinson claims. 'Though with this job's considerable logistics problems, it would have been a major struggle to develop solutions in just the normal few weeks tender period.'

'I am in favour of the ECI approach even though we committed two senior engineers to it full time for the six months without either any payment or guarantee of winning he work, ' he comments, adding with a wry smile: 'This stretches any contractor's goodwill to the limit and it would seem more appropriate to be paid during the ECI period.'

Nuttall agrees. 'It is highly beneficial having an experienced subcontractor like Bachy on board as early as possible, ' says structures agent Jacques Le Sueur. 'I wish they could have been involved even earlier and, in future, prospective subcontractors should be paid for their time.'

Praise all round, though, does not in itself convert a well engineered design into problem free construction.

While Nuttall was busy diverting and rediverting three sizeable watercourses - the River Trent with its Fowlea Brook tributary, plus the Trent and Mersey Canal, all of which repeatedly weave across the site, Bachy Soletanche was equally busy avoiding not only these waterways but also frequently changing road diversions and a maze of similarly live underground services.

Bachy's two CFA rigs - a Soilmec CM70 and a CM700 - have already, just 40% through the company's 72 week contract, notched up over 15 major movements around site.

A dozen times they have been derigged for transfer by low loader while numerous ramps and haul roads have been needed.

There remain many more logistics challenges ahead before Bachy completes its underpass piling by November next year. The programming chart for the overall project, that occupies a 5m length of Nuttall's office wall, features over 1,000 operations - many of which change weekly or even daily.

'Thanks to good teamworking we remain on schedule and within target cost, ' states Mallinson.'And we intend to remain so no matter how complex the programming.'

Pile planning

Unable to secure a single length of unobstructed ground through which to install its two rows of 500m long secant piled underpass walls, Bachy Soletanche's programming for over 2,500 piles centres on occupying 38 varying sized separate areas. Many of these will be revisited several times during the contractor's 17 month contract.

With watertightness a priority in the saturated sands and silts, the 18m deep hard-firm secant walls must be as accurately bored as if formed in a single run.

In most areas, piling crews do not have the conventional luxury of completing several of the smaller 600mm diameter unreinforced firm female piles before boring the 1,050mm diameter hard male interlocking piles between them several days later.

Detailed planning is needed to ensure the female piles are broken into as early as possible.

Cut too early and there is a risk of damaging the pile: Cut too late and there could be a struggle drilling it out.

To ensure slow early strength gain, the concrete mix contains a high 75% blast furnace slag content as cement replacement.

Each short length of piling must end with a female. But it could be several months before a rig can regain access to this mini site to continue the wall, by which time these female piles will be far too strong to break into.

The solution at Stoke is to end each run with a dummy male pile instead. This is bored into the adjacent female as if a full male pile, but is not then reinforced or concreted. Instead it leaves only a circular bite in the female to act as a template for the real male installed sometimes months later.

Completion of this piling jigsaw late next year will leave Mallinson relieved, but with his fingers still crossed. 'Our piled walls will not be exposed to form the underpasses until the end of 2005 - over 18 months after initial sections were piled, ' he says. 'Whether they have retained their required watertight seal all this time will only be confirmed when excavation begins.'

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