The adverse influence of geology and groundwater on the behaviour of London Underground railway tunnels near Old Street Station

A number of cast iron tunnel lining segments have cracked in two sections of the London Underground between Old Street and Moorgate Stations that are located in the Palaeocene Woolwich and Reading Beds facies. There are also several slow seepages of highly corrosive acidic groundwater. Investigation of the ground has shown that water derived from the consolidation of the overlying London Clay is collecting in a lens of sand which is crossed by the tunnels. Such water, together with air from the tunnels, is oxidising pyrite in the sand lens to yield a sulphate-rich acidic water which is damaging the tunnel, both by corrosion of the cast iron lining and by producing expansive pressures from the precipitation of gypsum crystals, causing the tunnel to be squeezed horizontally and move up at the crown.