...without a hitch, but installation of the project’s biggest bridge nearby came close to disaster. In late May 2008, the newly launched 84-m-long Warren truss bridge, just east of the goods yard, slipped off temporary bearings. One end dropped 20 cm, shedding five heavy sections of permanent precast-concrete deck formwork onto live tracks beneath. Nobody was hurt, and no damage was reported.

A BB-C subcontractor had assembled the 1,300-tonne bridge next to the six tracks it would span and slid it into place early that month. Before being lowered onto the permanent level, the bridge rested on temporary steel packing plates over final bearings. Because of the bridge’s 1:30 slope, packing plates under the bridge’s Teflon were wedge-shaped and vulnerable to slippage.

Thermal movements on the bridge are suspected to have ejected the wedges at two piers, toppling a jack supporting the bridge, according to an investigation by TfL. Engineering errors and inadequate supervision were the root causes.

“Two individuals working for a subcontractor did not follow the rules,” says Stuart. “As it happens, it did not end up being very serious.” Internal investigations went “up to managing-director level,” he adds. “There have been lessons learned.” No prosecutions by safety authorities followed. Because the bridge was off the critical path, the schedule was unaffected, adds Stuart. The financial cost is currently being assessed.

With construction largely completed, BB-C’s focus is now on the systems and completing the extension’s four new stations. Two smaller stations at Haggerston and Hoxton are aboveground. But the one at Dalston Junction is “like an inverted cathedral,” says Stuart.

Set just belowground, the stations is covered with a just completed 220-m x 45-m slab consuming over half the site’s 25,000 cu m of concrete. Designed to support apartment buildings with up to 19 stories, the 1.5-m-deep slab sits on bearings big enough for a major bridge above piles up to 2 m in dia., says Stuart.

Designs for the new Shoreditch High Street station, in the goods yard, also cater to future development. As the financial district’s last vacant large plot, the site will likely be developed when markets allow.

To shelter the elevated track from overhead work, 475 m of it is enclosed in a hefty precast concrete enclosure. It overshadows the yard’s remaining grass-covered viaduct, a 166-year-old structure saved from demolition.

BB-C’s design-build contract started with a value of about $600 million but was substantially expanded with extra work, says Kothari. The project’s total budget, including trains and other costs, stands at about $1.6 billion, he adds.

Agreeing on prices for the additional work has been “the easy bit,” says Stuart. But, he notes, getting the work done on time has been trickier because the completion deadline has been “set in tablets of stone.”