Cave-In. Live rail tracks are enclosed with arches. (Photo by Peter Reina for ENR)

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. Freyssinet’s subsidiary Reinforced Earth Co. Ltd. (REC) supplied arch segments to its TechSpan design under a subcontract. REC also specified the erection method.

Nagle believes fully filled and compacted sections are safe. "Once stable, it’s stable," he says. But Network Rail officials say some nearby arches seem unsafe.

Subcontractor PCE Ltd., Atherton, began erecting arches in two halves during overnight track closures last October. The halves are unified by a cast-in-place longitudinal beam tied to segments with shear steel. Segments sit in retaining grooves on continuous ground beams founded on some 2,000 bored piles, according to REC. The tunnel is structurally complete, with 75% of fill, up to 6 m deep, in place.

Spanning 20 m, the 35-cm-thick precast arches are among the largest of their type ever built, claim officials with Ex-strado Consulting Engineers. The South African firm did detailed design on the segments for REC, says owner Markus Hausen. The arch span widths increased to 20 m from the originally planned 14 m for future track widening.

ritish Health and Safety Executive officials are focusing on construction methods among possible factors in the June 30 collapse of a 30-meter stretch of railroad tunnel being built 40 kilometers northwest of London. The cave-in dropped thousands of tonnes of debris onto live tracks, but nobody was injured, according to line owner Network Rail Ltd., London.