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Piled retaining walls Step by step

Foundations - Cutting into a sloping site to make space for Newcastle apartment blocks called for close team working

Intertwined is the most appropriate description for the relationship between main contractor and foundations specialist creating a site for prestige city centre flats cut deep into the steep sloped banks of Newcastle's River Tyne.

'For over 12 weeks we did little else but repeatedly provide zig-zag access ramps for Bachy Soletanche's piling rig as it worked its way down the hillside, ' explains Ken Dyer, site manager for main contractor PJ Carey.

'Without total and close cooperation between us, this job would certainly have ground to a halt.'

To his client, City Loft Developments, location was the key word in siting the three, up to seven storey high blocks of luxury apartments.

And what better location than close to Newcastle's first £1M flat, overlooking the city's famous cluster of river bridges.

To provide such a location, a large 20m high wedge shaped chunk of the northern river bank has to be removed. And Bachy Soletanche has spent much of this year forming a 115m long multilevel network of contiguous piled retaining walls to create stepped, level sites for the three apartment blocks with their integral underground carpark.

'The plan was to drive our piling rig in at the top of the hill, work our way down forming the walls, and drive out at the bottom, ' recalls Bachy Soletanche contracts manager Rob Howarth. 'But it did not quite turn out like that, and we had to bring in a crane to recover our trapped rig half way down.'

The hillside, with its 30infinity slope, offered a congested location both above and below ground.

Barely 45m wide, the site boasts an important flight of 16th century steps up the centre; the remains of a medieval timber wharf at its base, an electricity substation and two major live sewers running right through the site and down the bank.

It is little wonder engineers for both main and piling subcontractor describe the job as the most complex, challenging project they have yet worked on.

'It is a three dimensional site that has proved much more complicated in practice than any of us realised, ' says Howarth.

The seven different piling levels and 15 rig locations were more than double the numbers planned.

Central to Bachy Soletanche's six month, £600,000 contract is the formation of two separate retaining walls. An upper 30m long wall, of 13m deep CFA piles bored across the top of the hillside, has been excavated to a depth of 6m to provide a retained, level site for the upper of the three apartment blocks.

Along the front edge of this upper site, a larger more complex shaped 85m run of 600mm diameter piles has been bored an impressive 22m deep. Over 30 temporary ground anchors have been drilled in two rows into its front face and each 15m long, multi strand Dywidag tendon stressed to a maximum 750kN.

This 110 pile wall is now being exposed to a depth of 15.6m as the lower site is excavated for an underground carpark and the two remaining blocks of flats.

Cobbles up to 200mm diameter in the glacial clay soil made Bachy Soletanche's Solmec CM700 high torque, CFA rig the most suitable, and it was accepted this large 75t machine would present a manoeuvring challenge as it negotiated the slope. Carey's main job at this stage was to keep the rig moving by repeatedly building ramps across and down the hillside.

Ground conditions dictated a four pile gap between bores, and the awkward site geometry sometimes meant that the rig could complete only one pile a day. Half way down the slope, with the machine wedged into a corner, and no more piling positions readily available, the site team had a rethink.

The most economic solution was to remove the rig altogether from the site for several weeks while Carey provided new piling areas. But the practicality of plucking the stranded machine off the hillside was less straightforward.

There was no room on the road beneath to bring in the vast 500t crane that would have been needed to provide the height and reach required to lift out the complete rig. Instead a smaller 200t mobile version arrived and stripped the rig down to just its undercarriage and tracks.

Now weighing in at less than 45t, this chassis unit could tackle steeper slopes and drove itself down the remaining hillside.

'But it still took us four days to complete a normally four hour operation, ' claims Howarth.

Six weeks later, Bachy Soletanche brought back a much smaller Solmec CM48 rig, with a 8.7tm torque and weight half that of the larger machine.

The gamble paid off as the rig still coped with the cobbled ground and lengthy piles, while proving much more manoeuvrable around site.

With piling complete on schedule a few weeks ago, and the last of the 33 ground anchors now stressed, attention turns to exposure of this impressive retaining wall. Howarth's confidence that the excavated piles will 'look good 'could be tested to the full as the 22m high hillside structure will be very visible to motorists and train passengers crossing any of Newcastle's five nearby river bridges.

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