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Stornoway jetty: Hard case to solve on Lewis

Stornoway ferry terminal on Lewis is a key link for the Outer Hebridean island. Paul Thompson reports on how efforts to extend its pier are progressing.

For centuries the fishing fleet was the mainstay of the local industry in the Outer Hebridean Islands, but in recent years tourism has become a vital part of their economy.

An expansive project at Stornoway’s cruise ship and ferry terminal highlights the importance of the town’s port to the wider islands.

Since the beginning of May, Bam Ritchies has been working on a scheme that involves the extension of the quayside at the Stornoway ferry terminal, which accommodates ferries from the mainland as well as the multitude of cruise ships that frequent the islands.

Under the £9M contract, parent company Bam Nuttall is building a 30m extension to the existing pier as well as strengthening work and the creation of a new marshalling yard and car parking area.

Geotechnical work

Stornoway pier

Forceful: Maximum working load in the piles is 2,600kN

As part of that contract, Bam Ritchies has been carrying out the geotechnical work to strengthen the raking piles along the existing pier and also those installed on the new infill section.

They are using a stranded ground anchor system threaded down through the raking piles and anchored into the sea bed, to help resist the forces exerted on the extended structure. Bam Ritchies senior geotechnical engineer Gilson Gaston is heading up the team charged with installing the ground anchors in an operation that involves drilling down through the centre of the tubular steel piles before threading the strand anchors through them and socketing them into the conglomerate rock which categorises the sea bed in the area.

“We have both existing and new raking piles that need to be able to provide a level of tensile restraint. Initially, we thought of a dowel design but that would have been very difficult, so we have refined those initial thoughts and developed the strand anchor system,” he says.

The real difficulty in this sort of solution is ensuring the anchors run directly through the centre of the piles, particularly the existing raking piles where “as built” information might not be as accurate as required.

“Here we have one hole through the pile. There are no second chances. We have to get it right first time”

Gilson Gaston, Bam Ritchies

“The as built information is not great and the drill string needs to be centralised. We have developed a guided method that will help ensure we manage that,” says Gaston.

Hydro-demolition experts have used super high pressure jets of water to blast out the existing concrete deck at the pile head, exposing the steel column of the pile itself and the reinforcement tie into the deck. Now Gaston and his team are using a guide casing to help them drill the length of the 914mm diameter piles.

This 500mm diameter conductor casing guides installation of the 406mm diameter temporary casing down to rock level before a 355mm diameter permanent casing ensures the 295mm bore for the rock socket, which can be as much as 10m deep.

“We didn’t really want to exceed a 10m socket. The bedrock is conglomerate, but its matrix is not particularly strong. Typically for this sort of installation we would be looking at a 250mm diameter anchor, but the conditions here meant we had to look at 355mm diameter,” Gaston explains.

The number of strands in the anchors vary between 14 and 18 in those installed in the new infill section, while those in the existing piles typically number between eight and 12 strands. Maximum working load is 2,600kN.

Rock quality

To generate that level of load resistance depends on the quality of the rock and diameter of boreholes it is feasible to drill. In Stornoway, thanks to the relatively poor nature of the conglomerate, the site team needed to use the larger 355mm diameter anchor.

“That means we have to use larger plant. There is a decision to be taken before the size of the rig required to carry out this work becomes unfeasible and we need to look at a piled solution instead of the strand anchors,” says Gaston.

Stornoway pier

Packing a punch: Hydro-demolition was used to blast out the existing deck at the pile head

The team is using a high strength 52.5N cement grout for the anchors, which is slightly higher than required, but Gaston feels it is worthwhile taking the extra strength to be certain of meeting the final design strength of 40N. “I like to be sure that I have taken any variables out of the equation and that we will always hit that required 40N. Here we have one hole through the pile. There are no second chances. We have to get it right first time,” he says.

That high strength cement grout has to be brought onto the island and is batched using colloidal mixers to feed the one rig the team is using on the site. During peak summer in these far-flung northern reaches of the British Isles, the skies are never quite fully dark, even in the early hours of the morning.

The site team will use every hour available to hit its deadline this month.

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