Islington Tunnel, Regent's Canal, Danbury Street

876m long with portals to the west and east built of brick with stone dressings. The interior width of the barrel is 5.2m at normal water level, with the water depth varying between 2.0m and 2.3m, and headroom ranging from 5.4m to 5.6m. Previous investigations revealed that the tunnel lining thickness varies between 2 to 2½ rings of brickwork. The whole structure is covered by two Grade II listings – 'West Portal and Islington Tunnel' (i.e. including the barrel) and the ‘East Entrance to the Islington Tunnel of the Regent's Canal, and Flanking Walls’. Owing to the high consequence of failure and poor structural condition of the tunnel, it is classified as a ‘critical asset’ by its owner, the Canal & River Trust. The tunnel's defects include severe cracking to the west and east portals together with signs of tilting of these structures. There is also severe cracking to the tunnel lining at the entrances. A recent inspection report of the tunnel and portal structures recommended that repairs be undertaken for stability and durability purposes for the long-term safety and serviceability of the tunnel. The repairs are currently programmed to take place during the 2027/28 navigation stoppage season (October 2027 to March 2028). A programme of intrusive investigations of the brickwork is proposed to enhance the understanding of the severity of the defects and associated structural risks, to refine the scoping and design of the proposed remedial works, and, in turn, to reduce the extent of interventions and associated project costs and risks, as well as to minimise intervention into the historic fabric of the Grade II-listed structure. At twelve locations within the length of the tunnel there would be: 1) three 50mm-diameter brickwork cores to the intrados of the tunnel, taken at the crown, shoulder and sidewall; 2) extraction of an individual whole brick sample; and 3) flat-jack testing to measure the masonry stress and stress-strain relationship of the masonry unit. In addition, a trial pit would be excavated adjacent to each of the portals and the foundation drilled to determine the type of footing (e.g. stepped brickwork footing), depth of foundation, and location of the heel and toe of the wall.