Out of sight - and out of most people's minds - below the northern approach to London's Waterloo Bridge are 16 brick arches, each spanning 5m and up to 10m high. They were built under the direction of John Rennie in about 1815 to carry the approach road to his original Waterloo Bridge across the north bank of the Thames, then low lying flood plain.
Nearly two centuries of natural ageing and some major interventions had left the structure in a parlous state.
Westminster City Council inherited responsibility for the arches from the former Greater London Council - but not ownership of the space beneath. Long-term monitoring showed the viaduct to be in increasingly urgent need of strengthening. The challenge was to find a way of doing this without either causing any significant closure of the road above, damage to the listed facades beside the basement area facing Somerset House, or encroaching significantly on the spaces beneath the arch soffits.
Work was put out on a design and build basis as the minimum risk option for the client. After some trial work contractor Balvac and its engineer Gifford & Partners opted for two-stage shotcreting to build up skins of heavily reinforced concrete 200mm thick, which would relieve all arch and bending loads on the brickwork with the absolute minimum loss of air space below.