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The Forth dimension

The new £2.3bn Forth bridge is a major project in every sense, including its site investigation. With more than 6.7km of new road, three contractors are carrying out three phases of testing before the final design is complete. Gemma Goldfingle reports from Edinburgh.

After only 45 years in operation the Forth Road Bridge is having something akin to a heart bypass. The iconic structure has suffered much wear and tear in its short life and is now showing the scars. Significantly corroded cables have resulted in the loss of 8% of the bridge’s strength.

Weight restrictions loom for the bridge if deterioration continues. But with 66,000 vehicles a day using the bridge, closing this vital artery could send Edinburgh’s transport system into meltdown. So Transport Scotland has taken the decision to build a new bridge to the west of the Forth Road Bridge to ease the load.

The Forth Replacement Crossing is to be a 2.7km cable-stayed bridge with a dual carriageway connecting into key trunk roads either side of the Firth of Forth. Around 4km of connecting roads will be built to tie the new crossing into the network.

A project of this size and importance naturally requires an immense amount of site investigation, so much, in fact, that not only has Transport Scotland split the work into three phases, it has divided it between three contractors.

Norwest Holst is undertaking investigations on the northern section of the route, Bam Ritchies the southern section, while Glover Site Investigation is handling the marine investigation.

“Continuity is crucial to us,” says Transport Scotland geotechnical manager for the Forth Replacement Crossing Paul Mellon. “Our contractors know the route and have been working on the project since the preliminary investigations in 2007.”

The contractors began detailed site investigation between March and August 2008 to determine deep ground conditions, drilling boreholes 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. The results from this initial batch of tests were fed back to the Arup/Jacobs joint venture tasked with designing the new £2.3bn dual carriageway that will become one of the busiest in the Scottish capital. Results from last year’s investigations have helped narrow down the route alignment and allowed more focused ground investigation to take place this year.

In the northern section 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 of 0.26m to 4.8m.

“We were working in some difficult locations with some incredibly steep slopes to navigate,” says Norwest Holst director Phillip Kirkwood. “In some areas rigs simply couldn’t get to the required position. We had to conduct window samples and dynamic cone penetrometer probings instead as it allowed us to use hand-held devices.”

A survey was undertaken to assess the extent and thickness of suspected peat formations in one area of the site. This involved probing at 16 positions, each position using a hand-operated mechanical dynamic Macintosh probe.

In the southern section, BAM Ritchies used four cable percussive rigs and five rotary rigs to drill boreholes to 80m. BAM Ritchies’ investigation assuaged concerns that former colliery land south of the Forth would be too contaminated to carry the new road.

This first round of investigation narrowed down the route alignment. However, residents’ opinions were sought to decide on the location for key road junctions. Transport Scotland held exhibitions throughout the region, whichmore than 2,000 people attended. Feedback has been factored into the design.

“This is the most high-profile project in Scotland, not to mention the most expensive,” says Mellon. “The scheme is transforming the lives of thousands of commuters. We must get it right, which means engaging with the people it affects.”

After a seven-month hiatus the contractors began the next phase of investigation in March this year, this time to determine the position of key structures. The condition of roads on the A90, which runs north to south via the current Forth bridge, and M9 and M9 spur in the southern section, needed to be assessed.

BAM Ritchies carried out 102 road pavement core holes. “Working along existing roads has involved extensive traffic management,” says BAM Ritchies ground investigation manager Andrew Stevenson. “On certain roads we
only have access to conduct ground investigation at night.”

The majority of ground investigation has focused on testing shallower material. In the northern section Norwest Holst formed 33 cable tool holes to 20m depth and 32 rotary cored holes with trial pits. Hard cobbles and boulders present in the bedrock made the material time-consuming to drill.

“The scheme is transforming the lives of thousands of commuters. We must get it right, which means engaging with the people it affects.”

Paul Mellon, Transport Scotland

Steep slopes, both in wooded areas and on the cliffs overlooking the Forth, made the job difficult for the contractor. “We had to drill lots of angle holes. Rigs were replaced with hand-held devices when possible but sometimes it was impossible,” says Norwest Holst’s Kirkpatrick.

“A Boart Longyear 520 was used as it can drill at angles. It was working at 60 degrees in some sections.”

In the southern section 91 machine-dug trial pits were constructed. California Bearing Ratio (CBR) testing was undertaken to assess the strength of shallow material by the M9 and A90.

A cylindrical plunger is driven into the soil at a uniform rate. A reaction load, in this case a vehicle, is used to provide the force to the plunger and CBR press. Tests will also determine the moisture content within the trial pits.

Three terrier rig boreholes to 5m depth were drilled, along with 88 cable percussive boreholes which plunged to 30m at their deepest. Insitu permeability, packer and acoustic televiewer testing was carried out in boreholes.

A further 63 rotary boreholes were drilled to 90m to gain additional information about deep geology. Testing was completed in August and findings will be reported to Transport Scotland this month.

“This phase of investigation is about filling in the gaps. It will further develop the design,” says Mellon. “However, we are anticipating a further phase of investigation in early 2010 when we have a contractor on board.”

The construction contract is out to tender. Two consortia remain in the race to build the bridge: Forthspan, a consortium comprising Morgan Est, BAM Nuttall, Balfour Beatty and Vinci Construction Grands Projets will compete against a joint venture of Hochtief, Morrison Construction, Spanish Contractor ACS and US firm American Bridge. The contract is expected to be awarded before the year’s end with construction due to start in mid-2011.

Marine investigations

Investigation takes place next to the Forth

In the Firth of Forth itself, Glover Site Investigations is undertaking marine investigations. Information about the seabed conditions around the bridge’s proposed north and south towers will help plan construction methodology
and equipment needed.

Between May and August 2008 a team of 25 specialists from Glover worked from two jack-up rigs to extract rock samples from various locations, including the proposed sites of the southern and northern bridge towers and approach piers and from Beamer Rock − the proposed site of the central tower.

The contractor returned to site in May this year to focus on testing the upper seabed sediments, probing the sediment up to 20m below the seabed to measure the soil strength.

Two additional rotary drilled boreholes and three trial pits will be dug on the south foreshore near the Port Edgar Barracks sea wall to supplement the findings in that area. These foreshore works will be carried out by Bam Ritchies during low tides.

Paul Mellon, Transport Scotland’s geotechnical manager for the Forth Replacement Crossing, says: “Last year we carried out marine investigations to establish the deep ground conditions in the estuary, which involved boring rock up to 90m deep. The vast majority of works this year involve testing softer alluvial soils. The works are less intensive, quieter and quicker.”

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