Significant inaccuracies in the as-built drawings for the Forth Road Bridge’s cable anchorages are to dramatically increase the cost of corrosion investigations, NCE learned this week.
Examination of the bridge’s southern anchorages - which hold the huge suspension cables in place - is taking much longer than anticipated because they are deeper and steeper than the on the original as-built drawings.
The Forth Estuary Transport Authority (Feta) is investigating the condition of the 48 year old anchorages after a resident engineer’s report raised concerns about the possibility of corrosion in the post tensioned strands within them.
In a capital update report to the bridge authority, Forth Road Bridge chief engineer and bridgemaster Barry Colford says the cost of the inspection is likely to be “significantly higher than the original estimate” of £3.5M.
“It’s a very resource driven contract,” Colford told NCE. “As the rockhead level was higher and there more was concrete [than anticipated] the contractor has had to spend more money.”
The anchorages are four concrete filled tunnels - 80m deep on the south side, 57m deep on the north side - and up to 14m in diameter.
The anchorages each transfer a load of 14,000t from the main suspensions cables into the bedrock.
Each anchorage consists of 114 ducts with four post-tensioned, galvanised, 32mm diameter high tensile steel strands made up of 19 wires in each.
With the original access chamber to the bottom of all four anchorages filled in, the only way to assess the condition of the strands is to dig down and open them up, said Colford.
Contractor Graham began the anchorage investigation on the southern bank in August 2011 under a New Engineering Contract (NEC) 3 Option C target cost contract with a target cost of £3.5M.
But after investigations began, engineers discovered that the ducts were 400mm deeper and were at an angle of 33˚, not 30˚ as recorded on the as-built drawings.
“It’s a significant change to what we expected and it is very disappointing the as-built drawings of a major structure were not correct,” added Colford.
Graham will begin exposing up to nine stands on each of the southern anchorages by the end of the year.
Consultant Fairhurst will assess the strength of the anchorages by the end of 2013.
Colford is in discussions with Graham about the size of the compensation event - the cost of work unforeseen at tender stage - and £220,000 has already been agreed.
The extra funds for the anchorage investigation are contributing to an overall deficit of £3.5M in Feta’s capital budget for 2012/13.
Its capital funding was cut by 58% by Transport Scotland last year.
Colford said budget shortfall would be met by using some of Feta’s £5.8M reserve, as well as delaying or cancelling “non-committed” schemes on the bridge.
He was also seeking additional funds from Transport Scotland.
Cable bolt replacement
Feta is taking legal advice this week after it was forced to replace all cable bolt assemblies just 13 years after they were originally installed.
It wants to know whether Danish contractor Monberg & Thorson - who replaced cable band bolt assemblies along with the hangers in 1998/99 - bears any responsibility for the subsequent failure of the nuts in the assembly earlier this year.
Feta was forced to replace all 944 assemblies in July at a cost of £4M.
Monberg & Thorson was criticised for taking five months longer than originally planned when replacing the suspension bridge’s hangers in 1999.