A tunnelling shield, based on Marc Isambard Brunel's design used for the Thames Tunnel a century earlier, was used to dig the two tunnels. The tunnels were cut perfectly circular with the road deck sitting a third of the way up. Under each road deck is the pedestrian/cycle tunnel and ventilation ducts. 16 miners operated the shield working shifts in a compressed air environment to ensure that the rock and the river above did not collapse into the tunneling area. The digging itself would be done only with great difficulty due to the geology of the area, hard rock sitting under a soft silt layer beneath the river.