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Going underground

The second of our Bachy features reports how it has strategically placed some very large diameter piles to help avoid problems for London Underground.

Vauxhall was once a neglected area of London with many of the streets destroyed during German bombing in the Second World War, but recently it has transformed. The distinctive MI6 building, and residential and office developments have been central to this transformation.

The St George Wharf residential scheme is one that stretches 275m along the water's edge. Built in a staggered process, the Aquarius Building will be the final block in the project.

To create a load bearing capacity of between 11,000kN and 41,500kN, the original plan was to use an under-ream solution. But these piles can only be constructed in stable soils.

But the site's strata included only a thin layer of London Clay so this option was ruled out. Instead Bachy Soletanche carried out a £1.6M large rotary bore piling subcontract for main contractor J Reddington, between October last year and January. White Young Green is the project's engineer.

The site is flanked by buildings, main roads, the River Thames and Vauxhall Bridge, which proved to be vital considerations in the piling solution. In addition the Victoria tube line runs directly underneath the site.

This meant the team opted to install 10 large diameter auger (LDA) piles, with diameters between 1.8m and 2.4m, to depths of about 60m using the high torque, Bauer BG-40 rig with an extra long kelly bar.

The rig installed the piles by first rotating into the ground a temporary casing followed by a permanent casing inside it, each with a diameter only slightly larger than the pile. Site workers then injected grout in the annulus between the two, before pouring concrete to form a pile within the permanent one.

The permanent casings prevent load transfer into the nearby underground tunnels and escalator shafts – it absorbs the load and transfers it to the pile protruding below – as well as cutting off the high groundwater on site.

The strata is 10m of made ground, London Clay, layers of Lambeth sands, clay, and then Thanet sands. The casings were between 14.9m and 21.5m in length, which took them down to the London Clay, while the piles found in the Thanet Sands.

Because of the size and complexity of the piles, each one took a week to install. The first two days were spent installing the casing and grouting the annulus. On the third day, site workers drilled the core to depths of about 60m under bentonite. The next two days
were needed to clean the base, de-sand the borehole and install the reinforcement.

Once these cages were installed, workers poured the concrete using a tremie pipe running down to the bottom of the pile. The bentonite fluid was displaced by the concrete and pumped out as the concrete gradually rose.

The underground obstructions determined pile locations and as a result there are four on the east side of the site (two 1.8m in diameter and two of 2.4m), three with a 2.4m diameter on the south side, and three with a 1.8m diameter on the west of the site.

"The piles to the south and east of the site were extremely important as these piles fell in line with the Victoria tube line," says Bachy site engineer Vijay Pookat.

"Driving piles very close to and between these tunnels required complete accuracy. There was a huge risk of damage to the tunnels if the work had not been completed correctly."

With the underground tunnels being so close to the piling works, engineers on site used a monitoring system in order to detect any movement or vibration that may affect the tube lines. Client St George South London continually measured the deflection of the escalator shaft within the Vauxhall Station complex. If this had moved more than 2mm, the escalators would have jammed and work on site would have stopped immediately.

Follow-on work is now progressing and the Aquarius Building – comprising one and two-bedroom apartments – is expected to be complete later this year.

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