A clever new design and a daring piling technique made recent upgrade work to an Aberdeen port more time and cost effective. Jo Stimpson reports.
Commercial Quay, on the northern side of the River Dee, has provided Aberdeen Harbour with Eurolink roll on-roll off shipping services, long trailer parking and handling of forestry products.
But modern trends in vessel size and cargo handling had outstripped Commercial Quay’s capacity, and after years of service the sheet pile quay wall was showing signs of progressive deterioration from corrosion.
So, as part of Aberdeen Harbour Board’s ongoing £65M development plan, it was decided to refurbish and upgrade the 177m by 40m site to accommodate deeper vessels, primarily by deepening its berth to 7.5m - a 1.5m change - and strengthening the quay wall so it could take larger working loads, both in terms of plant and cargo.
Aberdeen Harbour commissioned a design for horizontal tie-bars over a relieving slab, on steel bearing piles.
However, when main contractor McLaughlin & Harvey came to bid for the work, its engineers believed they could do better. “At tender stage we offered an alternative design,” says McLaughlin & Harvey director John McCarey.
Consultant Arch Henderson designed a more cost-effective solution for the tender, comprising a reinforced concrete relieving slab immediately behind the existing 13m deep, 170m long sheet pile wall, supported on piles and laterally restrained by ground anchors.
The relieving slab - approximately 3m below quay level - was designed to accommodate all vertical, horizontal and impact loadings.
The slab is supported by 144 630mm diameter vertical piles with a safe working load of approximately 2,300kN, accompanied by 66 permanent bar ground anchors at a 45º angle, with a safe working load of around 1,100kN.
Accordingly, the slab includes concrete upstands with 200mm diameter ducts cast at 45º to accommodate the ground anchors’ installation.
The supporting piles were installed in alternate rows at 2.55m centres, with everything connected together via the reinforced concrete relieving slab.
“The helicopter will carry a light cable across the route and this will be used to gradually thread larger and larger cables through until the 50mm diameter cable is in position”
After completion of the piling and ground anchoring works, it took six weeks to refurbish corrosion damage to the sheet piles by removing marine growth and loose material, and repairing any holes by welding steel plates over them, overlapping the hole by up to 500mm to ensure the thickness of the pile is sufficient throughout.
A long-term sacrificial cathodic protection system was installed to prevent further corrosion and extend the sheet pile wall’s design life.
Finally, compacted sub base was placed over the relieving slab and a new reinforced concrete pavement was cast at ground level to complete the works.
A principal advantage of the new design was that it allowed the original line of sheet piles to remain in place - following some refurbishment work - and the quay’s berth to be deepened through dredging, rather than installing an additional outer line of sheet piles which would have encroached into the harbour.
Moreover, the new design removed the need for “quite difficult” directional piling which would have required equipment to be moved at high tide, and the noise of which would have restricted work to business hours.
The less disruptive new design meant work could continue from 7am to 9pm, which contributed to an overall time saving of three months from the original 13-month programme.
Aberdeen Harbour Board assistant engineering director Jamie Christie was “delighted” with the alternative design, as it reduced the amount of cut and cover required, obviated the need for handling, storage and noisy impact driving of steel piles, and ultimately resulted in construction cost savings.
Aberdeen Harbour appointed McLaughlin & Harvey to begin work on the project in January 2010.
McLaughlin & Harvey awarded the piling and ground engineering works subcontract package to Quinn Piling, which employed Byland Engineering for the specialist geotechnical design.
Quinn Piling devised an innovative method for constructing the bearing piles, which combined rotary and continuous flight auger (CFA) piling techniques.
This consisted of installing 10m of 660mm diameter thin-walled permanent steel casing through the 10m of made ground that lay on top of the glacial till strata, and then sealing the casing into the top of the glacial till using rotary drilling techniques.
Then 630mm diameter CFA piles were formed through the casing to a depth of approximately 23m, founding in the top of the conglomerate bedrock.
The 10m of permanent steel casing through the made ground prevented concrete washout in the tidal zone during the concrete curing period.
“Convincing the harbour’s designers to accept the proposed design was a challenge because it had never been done before,” says McCarey.
One of the main concerns was that the CFA piles would not be able to penetrate the glacial till - which contained numerous bands of very dense gravel and cobbles - to the required design depth.
Standard penetration tests recorded “refusal” N-values in several places, when the tests were conducted at the site investigation stage.
“We were always confident that we could do it,” says McCarey. “I think it was just that the designers were not familiar with the technology and the advances in the plant.”
In the end, pre-boring through a bands of very stiff clay around 16m below ground surface was required for some piles, and some piles reached refusal at around 22m depth - roughly 1m short of the intended pile depth. However, all piles were successfully installed, and the average depth was 22.7m.
“I think it was just that the designers were not familiar with the technology and the advances in the plant”
The CFA piles were installed at a rate of three to four per day, which McCarey says is “considerably faster” than would have been achieved using more conventional rotary piling techniques in a marine environment such as this.
McLaughlin & Harvey then began pile trimming and construction of the reinforced concrete relieving slab.
The 30m long ground anchors were drilled using Atlas Copco’s Odex percussive drilling technique, whereby the bore is drilled and cased simultaneously.
The drill rods and hammer were then removed, and a 63.5mm diameter permanent bar anchor with full double corrosion protection was installed.
Neat cement grout was pumped into the base of the bore, and as the temporary casing was extracted McLaughlin & Harvey applied four to five bar of additional pressure at ground level, forcing the fluid cement grout to permeate into and reconsolidate the ground around the end of the casing, producing a stronger bond.
“Normally you would just use the head of pressure on the grout itself,” says McCarey. “To get it extra-adhesive, we put on extra pressure which impregnated the grout into the surrounding ground.”
“ensure Aberdeen Harbour remains one of the UK’s busiest and most efficient ports”
The works were completed last year and commissioned in May this year, and the site teams have now moved on to the redevelopment of Torry Quay on the other side of the harbour, where work includes the construction of 800m of new quay and infilling a ship repair dock.
This project requires more working in the water, and the site team has swelled by around 20 people.
Torry Quay brings new challenges, including environmental restrictions - the River Dee does, after all, flow past royal haunt Balmoral - but the experience of Commercial
Quay will stand McLaughlin & Harvey in good stead to continue the programme that Aberdeen Harbour Board engineering director Ken Reilly says will “ensure Aberdeen Harbour remains one of the UK’s busiest and most efficient ports”.