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Underpinning contractor Abbey Pynford

Underpinning contractor Abbey Pynford used a novel system to protect the remains of Britain's only Roman amphitheatre while basement construction was being carried out beneath the Corporation of London's new head office at Guildhall recently (ABOVE).

The system, developed with project manager DY Davies and consultant Oscar Faber, involved first encasing the remains - the ancient arena's eastern gate - in timber and foam boxes. Seventy pits were then dug into the underlying Thames Gravel alongside the remains, allowing six steel tube

a manchette pipes to be horizontally jacked underneath in each. Chemical grout was then injected into the gravels and allowed to cure.

The excavations were progressed below the consolidated gravel layer and propped to allow a 900mm heading to be formed and for reinforcement cages inserted. A concrete slab was cast, forming part of the new building's first basement level slab. Top down construction of two further levels of basement was then able to proceed.

Pits were excavated out of sequence to reduce the chance of collapse. At one stage, 40 workers were on site for this unusual technique, which took over six months to complete.

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