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Objection! Sustained

A city centre office development near a London landmark has posed some interesting technical and logistical challenges for its project team. John Chantler and David Sivyer report from a site where history is making its presence known today.

On a site next to London's famous Old Bailey Central Criminal Court, workers have had to cope with archaeological remains lying beneath a new office development. This is because a Roman city wall lies beneath the east boundary and to the north sits the site of the notorious Newgate Prison.

Client for the project is Marylebone Warwick Balfour and Bowen Construction is main contractor. Pell Frischmann is responsible for foundation, structural and drainage design on the new £21M building and during demolition of old buildings, Geotechnical and Environmental Associates (GEA) came on site to carry out a ground investigation.

Original site owner Corporation of London believed there was the potential for the survival of remains between old foundations beneath the single basement slab of the existing building. As a consequence, the Museum of London Archaeology Service (MOLAS) was brought in to investigate proposed new coring areas and check for remains.

Elsewhere, the team decided as far as possible to form the new foundations on old ones to reduce the impact on the remaining archaeology.

This limited the pile-cap size so only a single reinforced pile per column was planned. The team decided that ground beams would be used only in places where piles could not be located under columns.

In the last hole it drilled, GEA found the horizon of London Clay 12m below the level found in previous boreholes. Site workers therefore had to drill a further six boreholes to delineate the feature and determine the nature and strength properties of its infill, and whether there was any effect on the surrounding clay.

This infill material comprised potentially water-bearing gravel in the upper layers, which grades into brown silty gravelly clay. A plot of the clays across the site showed a considerable scatter, but the infill material and the surrounding clay were not found to significantly differ in strength.

In addition, soft clays on site were found to contain glacio-sedimentary structures derived from the last Ice Age, which has created geomorphologic anomalies in the soil (where remnants of ice trapped in the clay melted creating small voids) that may have affected ground stability.

As a result rig operators formed a test pile, while installing early contract piles, to a safety factor of three until the results of the design could be verified.

In addition, while carrying out a desk study, Pell Frischmann discovered the presence of scour hollows on the nearby Paternoster Square development. It was concerned that bores for the piling could become unstable or permit water ingress in the area. As such, its piling specification called for a system with the ability, if required, to place a temporary casing to a depth of 16m.

Martello Piling won the foundations contract and used its WP5000 machine, rigged with a normal kelly bar and casing drilling, to install 58 bored, cast insitu piles with diameters ranging from 600mm to 900mm. This machine has a high torque capability for spinning in segmental casings, yet occupies a small footprint of 2.9m by 6.6m and needs headroom of only 6m – ideal for the confines of a city centre site.

Bowen project engineer Alan Goddard provided ramp access to allow the rig to work off the existing basement slab. "Re-use of this slab made a good contribution to sustainability and the size and capability of Martello's rig enabled the working area to stay clean and tidy," he says.

The site's city centre location and its proximity to buildings meant the team had to be careful regarding noise and vibration from demolition and construction works. Hours of work were limited and because of next door neighbour the Old Bailey, architects and engineers had to ensure works did not interfere with legal proceedings.

The proposed seven-storey structure plus basement level will be contained within the footprint of the current building. It will go up next to the Roman city wall and straddle Roman and medieval ditches. Piling had finished by the beginning of this year and the main structure is expected to be completed later in 2008.

John Chantler is geotechnical director and David Sivyer is structural engineer at consultant Pell Frischmann.

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