Protecting the newly discovered remains of Britain's only Roman amphitheatre while basement construction carried on around and beneath the stonework was the task for one specialist contractor in London recently.
Treasures located directly underneath the Corporation of London's new head office at Guildhall were underpinned by Abbey Pynford, to safeguard the remains and allow two basement levels to be built.
An innovative system of underpinning was devised by the project team, which included project manager DY Davies, consultant Oscar Faber and Abbey Pynford.
Abbey Pynford's head of special contracts Tim Jolley says: 'The work was not intended to take the strength of the new building, but to entirely support the remains. The method of underpinning used was custom designed to suit the various ground conditions encountered, including a layer of Thames Ballast which contains very fine sand and gravel.'
The amphitheatre remains - constituting the arena's eastern gate - had already been encased within timber and foam 'boxes'. Supporting these boxes called for chemical grouting of the ballast immediately below, before tunnelling beneath this stabilised material to make room for a reinforced concrete slab.
This slab would eventually be incorporated into the new building's first basement level slab, prior to top down construction of a further two levels of basement.
'We started by digging pits alongside the remains so that we could access the ballast for grouting,' Jolley says. Each pit was about 1m wide and big enough to allow six steel pipes to be jacked horizontally under the Roman remains. From the pipes, chemical grouts were injected tube a manchette style to consolidate the ground.
When the ballast was firm, each pit was then lowered so that a 900mm high heading could be driven under the consolidated area, the heading 'roof' being supported by lintels on adjustable props. Grout bags were employed on top of the lintels where there was overbreak between treated ballast and the excavation.
Finally, reinforcement cages were inserted and the excavated area filled with concrete.
'We dug about 70 pits in all, to complete the underpinning,' Jolley says. The pits were excavated
out of sequence to avoid the possibility of a collapse.
'The contract differed enormously from our traditional underpinning work. It involved up to 40 workmen at one stage, with the whole process taking more than six months to complete,' says Jolley.