Hull wastewater flow transfer tunnel: recovery of tunnel collapse by ground freezing

The Hull Urban Wastewater Treatment Directive (UWWTD) Flow Transfer Works Contract commenced in January 1998. The scheme required the construction of 10·5 km of 3·6 m internal diameter segmental tunnel alongside the north bank of the Humber estuary between West Hull Pumping Station and a new outfall at Saltend to the east of the city. Completion was programmed for November 2000. In November 1999 the works were ahead of programme and progressing well. However, while tunnelling between shaft T3 on the east side of Hull Marina and shaft T2, some 1146 m to the west, on the opposite side of the marina, a 100 m section of the new tunnel collapsed. Following a four-month period for investigation and review, a recovery scheme was agreed and works commenced in March 2000. This report describes the design of the chosen recovery option and the use of ground freezing with liquid nitrogen to provide a stable environment in which to rebuild the tunnel.