shield was employed on the north side where the ground conditions were better. The first shield was only able to be steered in a straight line, whilst the second one could be steered through gentle curves. The tunnelling was constructed under compressed air and the river-bed lined with puddle clay. No construction workers were killed owing to the extensive safety precautions undertaken; a rarity at that time for a project of this size and complexity. The entrance arches have been formed from cutting edges from the second tunnelling shield, which measured in diameter, forming in effect a loading gauge for the tunnel. Physical characteristics The tunnel consists of a single bore, long, carrying a two-lane carriageway below the high-water level of the Thames, with a maximum depth of below the surface. Four shafts were sunk alongside the tunnel to aid construction, later ventilation and entrance shafts. The shafts are numbered 1 to 4, south to north. Shaft 1 is located on the corner of Brunel Road and Canon Beck Road in Rotherhithe. It contains ventilation fans that supply fresh air to the tunnel. It is a single storey circular redbrick building, originally constructed without a roof. It now has a high roof containing three large vents. Also within the shaft is a floodgate dating from 1939 which protects the nearby London Overground. Shaft 2 is located along the river front close to Rotherhithe Street Bascule Bridge on the south side of the river. Shaft 3 is the corresponding shaft on the north side located by the river in King Edward VII Memorial Park. They are both single storey circular redbrick buildings with ornate ironwork grilles in the large side openings and granite dressings. Each contain an iron spiral staircases that were originally pedestrian access points but were closed in the 1970s due to low usage and security concerns. The shaft buildings were originally capped with a dome glass roof (similar to that at Greenwich Foot Tunnel) however these were removed in the 1930s in a bid to improve ventilation within the tunnel. New roofs were added in 2007 to a different design. The staircases having been open to the elements for more than 70 years suffered extensive corrosion and were fully refurbished the same year. The shaft buildings and staircases for these two shafts were Grade II heritage listed in 1983. Both shafts contain ventilation fans which are primarily used