The new £2.3bn Forth bridge is a major project in every sense, including in its site investigation, says NCE geotechnical correspondent and GE reporter Gemma Goldfingle
The iconic Forth Road Bride structure has suffered much wear and tear in its short 45 year lifespan and so Transport Scotland has taken the decision to construct a new bridge to the west of the original as a traffic relief measure.
With a project of this size and importance comes an immense amount of site investigation, so much in fact that not only did Transport Scotland phase the projects, it divided the work between three contractors.
Norwest Holst is undertaking the northern section of the route, Bam Ritchies the southern section and Glover Site Investigation the marine investigation.
Detailed site investigation took place between March and August last year to determine deep ground conditions and boreholes were drilled to depths of 90m across the site, which stretches over 4km from the A90 at Halbreath in the north to the M9 in the south.
“With a project of this size and importance comes an immense amount of site investigation, so much in fact that Transport Scotland divided the work between three contractors.”
The results from this batch of tests were fed back to the Arup/Jacobs joint venture tasked with designing the new dual carriageway. Results of this investigation have helped narrow down the route alignment and enable more focused ground investigation to take place later this year.
North of the Firth of Forth, Norwest Holst drilled 102 rotary core holes, to a maximum depth of 50m using a combination of T6116 sized holes and PW casing. To investigate surface material, trial pits were excavated to depths between 260mm and 4.8m.
South of the Firth, Bam Ritchies used four cable percussive rigs and five rotary rigs to drill boreholes to 80m using a combination of cable percussion boring followed by rotary drilling, with extensive use of rotary wireline drilling.
Bam Ritchies’ investigation eased concerns that former colliery land would be too contaminated to carry the new approach road.
This first round of investigation narrowed down the route alignment, however consultation with residents in the Edinburgh region has been sought before deciding on the locations of key road junctions.
“Bam Ritchies’ investigation eased concerns that former colliery land would be too contaminated to carry the new approach road.”
After a seven month hiatus the contractors began the next round of investigations in May, this time to determine the position of key structures. As well, the condition of the A90, which runs north to south via the current Forth Bridge and the M9 and M9 spur in the southern section, also needed to be assessed.
Testing was complete in August and findings will be reported to Transport Scotland this month. The construction contract is out to tender. Two consortia remain in the race to build the bridge. They are Forthspan, comprising Morgan Est, Bam Nuttall, Balfour Beatty and Vinci Construction Grands Projets; and a joint venture between Hochtief, Morrison Construction, Spanish contractor ACS and United States firm American Bridge.
The contract is expected to go to tender before the year’s end with construction due to start in 2011.
- Gemma Goldfingle is NCE geotechnical correspondent and GE reporter