A steep challenge

Alasdair Reisner, Construction News, 03/05/2007

WHEN IT comes to building iconic bridges it is fair to say Newcastle-upon-Tyne has done more than its fair share. From the low-lying swing bridge to the High Level bridge, the Tyne Bridge which acted as a model for the Sydney Harbour Bridge and the more recent 'winking-eye' Gateshead Millennium Bridge, the Toon has been a showcase for engineering innovation.

But those who have to build in the shadow of these icons know that some of the bridges tower over them for a reason. Almost as soon as you step back from the banks of the Tyne the ground rises steeply on both sides of the river. Newcastle is built on the side of a bloody great hill, presenting an engineering challenge to anyone who wants to build there.

The latest firm to take on th is bat tle with gravity is May Gurney. The firm is at work in the west end of the quayside, building a fully cantilevered wall to make space for a 13-storey apartment tower block and eightstorey mixed-use block for developer Mandale.

The team had to install a contiguous bored pile wall across the back of an exist ing lower retaining wall on the very tight site to create more room for the development and protect it from any movement in the bank side, according to May Gurney northern area manager Nick Sharp.

"We came up with the idea that this could be a fully cantilevered system which would not only allow Mandale to excavate to a fully retained height of 8 m so they could construct their building, but it also negated any need for any anchors to be installed to support and retain this structure, " says Mr Sharp.

"The alternative would have been to sheet pile the area in quest ion or install a smaller retain ing wall with a series of anchors going back into the land behind the site. Given that this land included a cement plant and the East Coast Main Line, such anchors would have restricted future development on these sites, potentially throwing a planning hand-grenade into our project if these neighbouring sites refused permission for the anchors.

"A fully cantilevered retained system allows Mandale great options in terms of speed of construction and also negates these planning issues with problems next door."

But just because the team were not straying from their site boundaries does not mean the project was easy. The site boundary pinches on all sides even before you try to put a piling rig, crane, excavator, concrete holding drum and pump onto it. And that gradient of the hillside compounds the problem.

"The construction of the piles is pretty straightforward , " says Mr Sharp. "It is managing the logistics of getting in here that is difficult. The level of the existing ground which will become the main loadbearing area is at a datum of 7 but the actual piling platform level is at 16.5 AOD. To get up to those levels over a maximum of 50 m, maybe just 40 m, was a problem."

The only solution was the simple one: to import 8,000 tonnes of material to create ramps and the piling platform.

"That had to be compacted and tested for the rig loads we had. Because it is so high up and steep-sided they have had to put a scaffold hand rail to stop anyone from falling off the edge of it, " says Mr Sharp.

Before any piling work, the team first installed a guide wall to ensure the piles went in true.

"The guide wall keeps the position of the rig and the pile we are putting in as required. It is a pre-formed shuttered cast in-situ wall with a little bit of reinforcement, " says Mr Sharp.

"It just gave a line to let the guys know where their position was at all times and where they are going.

When you are drilling with such a long auger it can veer off a little bit. The guide wall keeps you exactly where you are meant to be and allows you to stay on line so that verticality is kept true."

The crane, rig and attendant serving plant could then be driven up to the top of the site. Given the size and steepness of the site and the limited space for the piling platform the various parts of the rig had to be put together after it was in its working position.

Once assembled the piling rig worked its way down the line of the contiguous wall.

"The piles are done on a hit or miss basis because we don't want them to interlock between each pile wh ile the concrete is still green. We try to do four to six each day, allowing a single pile spacing between each pile. The piles go off overnight, then we come back the next day and fill in the gaps, " says Mr Sharp.

The formation of the piles involves drilling down through about 2 m of clay into at least 6 m of dense sandy clay. Once through that the team hit a dense gravelly sand or glacial till into which the piles are founded.

"Once we have drilled to depth the concrete is pumped into the system, a flat valve opens at the bottom of the auger under pressure from the concrete and we retract slowly using a CFA technique to fill the bore we have created. As we retract and the arisings come out there is an attendant 360 excavator that sweeps them away, " says Mr Sharp.

Once the auger is fully retracted with concrete poured right to the top the crane is used to lift the reinforcement cages. The 4-tonne cages, made up of t40 bars by local firm ROM Reinforcement, are delivered six at a time daily to the site to meet storage capacity and the demands of the piling team. By prefabricating the cages rather than assembling them on site the team not only saves time but can be sure they will be st rong enough and will not deform when they are placed into position. When this is done by the crane, the cages are slowly allowed to sink under their own weight into their final position.

"But we do have a vibrator on site we can vibrate the cages with if they come across any problems. It pushes the last metre or so, " says Mr Sharp.

Seven days after the last pile has gone in the groundworks team will come in, break out the guide wall and install a capping beam, already designed by WSP. This will tie all the piles together to provide its integral strength and allows the team to excavate behind it to make way for the piles for the development itself. May Gurney is also pitching for this work and the successful delivery of the retaining wall should help the firm's cause.

Mr Sharp sums up what he feels have been the successes of the scheme: "We wanted to come up with a fully cantilevered system to allow Mandale to carry on construction without any problems. Our system has been quicker. It might have been a bit more costly because you have to put in a slightly wider diameter pile with more cage but in programme terms we have allowed them to get in, do the caps and get on with erecting the steel frame far more quickly than if they had to work around a load of props and anchors."

With advertisements on the site hoardings announcing the opening of the apartments this autumn, you can see the attraction of the May Gurney approach.
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