Hull wastewater flow transfer tunnel: tunnel collapse and causation investigation

The Kingston upon Hull wastewater tunnel was 80% complete in 1999 when a collapse occurred 200 m behind the tunnel boring machine. The majority of the tunnel was in glacial soils, although the collapse occurred in a relatively short length of tunnel in alluvium. There were no obvious reasons for the collapse, so an investigation was undertaken to examine systematically all credible factors. It was concluded that the primary factors leading to the collapse were an extensive layer of single-sized fine sand under considerable water pressure beneath the tunnel, and a leak large enough to admit sand particles. The leak was most likely caused by differential movement between the tunnel and an access shaft immediately adjacent to the seat of the collapse, a consequence of relatively compressible peat at the crown of the tunnel that was probably adversely affected by the stress relief of boring the tunnel. The paper describes the works under construction, the collapse and the events leading up to it, and the investigation after the event. The most likely reasons for the collapse are presented, together with the steps taken to ensure that the remainder of the tunnelling progressed safely.