DEVELOPMENT of Merrill Lynch's new regional headquarters in the City of London is having to take account of Roman and medieval remains underlying the site.
All excavation work, including piling, has been designed and planned with preservation of the Newgate Street site's important archaeological features in mind.
Expanded Piling is responsible for drilling the foundation piles which vary between 900mm and 1,800mm diameter and extend up to 57m below pavement level, this great depth being needed to transfer loads below a series of tunnels at 20m and 30m below ground level.
Positioning of the piles and the accuracy of drilling are critical, not only because of the need to avoid disturbing known features of archaeological significance, but because of the deep tunnels.
To minimise the disturbance during piling, engineering consultant Ove Arup & Partners has developed special on-site procedures for pile construction.
These include, where possible, excavating pile locations in advance of the piling rig to a depth of 8m below ground level with Musuem of London Archaeological Services staff in attendance. Where this has not been possible the piles are constructed with permanent steel casings, twisted down continuously ahead of the boring to avoid disturbance of the ground.
To achieve the demanding vertical tolerances, Expanded developed a special 30m long drilling bar, made up of only two sections welded together on site. Expanded says this has resulted in improved vertical accuracy, enabling a pile to be installed with confidence within 300mm of one of the tunnel side walls.
For the smaller diameter piles, Expanded modified one of its heavy duty auger rigs to twist up to 12m of 1500mm diameter steel casing, fitted with cutting teeth to deal with any obstructions or archaeological features, into the underlying London Clay. For the 1,800mm diameter piles, however, Expanded brought in a German 2m diameter, 25t oscillator, specifically for the contract.
A bentonite plant was established on site, but was used only for the deepest pile which penetrated below the London Clay, through the Lambeth Group (formerly the Woolwich and Reading Beds), and into the underlying Thanet Sand.
Merrill Lynch and its construction manager Mace had to agree to minimise disturbance of archaeological features on site to gain approval for the scheme.
In accepting planning and scheduled monument consents the project developer undertook to carry out an archaeological desk study and site evaluation to the satisfaction of English Heritage and the City of London. Museum of London Archaeological Services was brought in as approved archaeological contractor.
The consents also required the project to use a groundworks design that minimised disturbance. This was achieved by using a small number of large diameter piles, supporting ground beams and slabs positioned well above the Roman remains; and by positioning services within defined zones.
Merrill Lynch undertook to keep English Heritage and the City of London archaeological planning officer informed of the evolving design, the proposed archaeological treatment and resource protection schemes.
The Newgate Street site overlies the original Roman city wall, thought to have been built about 200 AD.
In 886 King Alfred the Great instructed refortification of the wall, which included a deep, wide ditch to deter Viking raiders. Archaeologists today are particularly interested in the material which was used to infill the ditch from the 13th to 15th century.
This included many discarded everyday objects, and the current digs have unearthed numerous worn leather shoes, a fine tooled leather knife scabbard, fragments of pottery jugs, bowls and cooking pots and many species of animal and fish bones.
A section of the wall, together with an attached medieval bastion or tower dating from 1250-1350, has been preserved at the site within a chamber constructed in the basement of a 1909 development.