Building technical editor Andy Pearson witnessed the Arup secret tests last Wednesday. Here's what happened.
It is 5.30pm on a cold Wednesday evening in central London, and the long-awaited cure of one the most high-profile engineering cock-ups of recent times is about to be put to the test.

In the darkness, the rain-scoured banks of the Thames are almost deserted. Any passer-by that did happen along would be alerted to the crucial event that is about to take place by the yellow huts that have guarded the southern entrance of the bridge since its closure 18 months ago. These are usually deserted at this time. Tonight, however, light streams from the windows as men in fluorescent yellow jackets dash from one hut to another.

On the river, the drone of a boat engine draws attention to a police launch bobbing up and down on the water. Meanwhile, an inflatable rescue boat has moved into position downstream and a team of St John Ambulance Brigade volunteers has gathered, just in case.

By 5.45pm there are signs of activity on the St Paul's approach to the bridge. A group of about 100 figures start to walk across the bridge. A second, similarly sized group makes its way across, followed by a third and a fourth. Each is led by a figure in an orange jacket sporting what appears to be a flashing red headband.

As each band reaches the south bank, it stays for a few moments before returning in the same disciplined fashion.

Standing beneath the bridge, one thing is apparent – the deck is not moving. The dull thud of footsteps indicates that the army above is still marching. But viewed from below, the deck appears to be stationary.

The only clue that there are people above is the slight rattle of the remnants of a scaffold still suspended beneath the bridge deck – a legacy of the rush to fix the final few dampers. By 6.30pm, the crowd has dispersed and the bridge is deserted again.