Officials Suspect Compaction In British Tunnel Collapse
The $35-million project at Gerrards Cross is to cover an open trench carrying the London-Birmingham railroad so that Tesco Stores Ltd., Cheshunt, can build a 1.8-hectare plot for a new supermarket. Design-build contractor Jackson Civil Engineering Ltd., Ipswich, enclosed 320 m of track with 340 precast concrete arch segments, each about 7 m tall. It had placed several meters of power plant ash over most of the segments to level off the plot above. The collapse occurred some 80 m from one tunnel end, which is still being covered with ash. Initial investigations target the compaction sequence. Jackson officials declined comment.
"You need fully compacted soil [around] the arch to function properly," says Patrick Nagle, managing director of Freyssinet Ltd., Telford, claiming no knowledge of the collapse cause. Freyssinets subsidiary Reinforced Earth Co. Ltd. (REC) supplied arch segments to its TechSpan design under a subcontract. REC also specified the erection method.