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Piling and foundations: Straight gate

In Dorset, engineers have used an innovative piling gate to help provide a new quay for Poole Harbour

Poole Harbour Commissioners (PHC) has called in contractor Bam Nuttall to provide a new quay and berthing pocket alongside its existing facilities on the northern shore of the harbour.

“The new facility at South Quay is part of the masterplan that we had drawn up for the development of the port. Initially it was intended to help service the Navitus Bay Wind Farm which was due to be built a few miles offshore. When that was refused planning permission we had to rethink our business case,” explains Phil Armstrong, harbour engineer at PHC.

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The new quay wall comprises sheet and king piles

That rethink indicated a definite desire for better berthing and quay facilities at Poole, albeit pared back from the original loading demands that the wind farm would have brought. It identified a real need for a 200m long quay wall and 40m wide berthing pocket dredged to 9m below chart datum.

“The two ro-ro ferry berths alongside the site of the new quay are the only deep-water berths across the whole harbour. Building a new facility opens up a variety of uses for us,” says Armstrong.

With the permissions in place, the project went out to tender, with Bam Nuttall winning its bid and starting on site in January 2017 under a Royal Institution of Chartered Surveyors design and build contract. Before work began, PHC took the dredging out of the main contract and into the enabling works, trusting its own experience and that of its supply chain to carry out the small amount of demolition work required as well as the berthing pocket and quay footprint dredge and backfill, alleviating any risk of the main contractor being held up by unexploded ordnance or archaeological finds.

“This approached worked well for both parties,” admits Bam Nuttall site agent Tony Davis, “PHC was able to manage that risk, but also we were able to have close dialogue with them about how much backfill we wanted and up to what level it was brought. It meant we could optimise everything for our pile installation process,” he says.

In all, 40,000m3 of peats and silts were dredged from the site, with 80,000m3 of sand fill replacing it. 

By Christmas 2016 dredging was complete, and the Bam Nuttall team was ready to swing into action immediately after the Christmas break. With a 200m by 40m new quay to complete and only 12 months to do it in there was no time to waste.

With the change in loading on the quay, designer Byland Engineering had scaled back the original tubular steel and sheet pile plans for the main cofferdam, instead opting for a solution which utilises 175 HZ-m 880 king piles coupled with AZ-13 700 sheet piles from supplier Acelor Mittal.

The king piles are I-section in plan view, 831mm long and 460mm deep. These connect to the infill sheet piling to create a continuous wall.

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The piling gate prevented the 22m piles from deflecting

“The initial design was for tubular piles, but when the applied loading was reduced thanks to the failure of the wind farm plans, we were able to scale back that design. The HZ-M king pile and AZ-13 sheet pile system is perfect. They are very strong but easy to adjust if the need arises,” he says.

Typically, the piles have been driven to 17.5m below chart datum, leaving a socket length of 8.5m beneath the berthing pocket.

With the top of the capping beam sitting at 4.5m above the chart datum, the 22m typical length can be tricky to handle and can easily go off course.

In a bid to prevent this, the Bam Nuttall team developed an innovative piling gate that enabled the team to drive both types of pile, ensuring they did not stray.

“With a standard combi-wall you would have one gang installing the king piles, while another comes along and installs the sheets. The danger is that the piles may spring out of position. Instead we developed a pile gate that enabled us to hold the king pile in position as the sheet pile is installed,” explains Davis.

The steel gate weighs in at 38t and is 22m long and 2.8m tall with enough room to install eight king piles at 1,927mm centres and eight sheet piles at each location. It anchors onto two previously installed piles at one end and to a spud pile which is located almost by eye at the other end. On top of the spud pile sits a bracket cap which is then connected to the gate. This flexible connecting arrangement ensures there is a large amount of tolerance for the spud pile installation.

“Obviously we try and build in as much tolerance as we possibly can. One of the benefits of using your own in-house temporary works team is that they can deliver a design that gives us those high tolerances,” says Davis.

Working alongside a 50m by 18.8m floating barge, the team used a 160t crane to help install the piles, which have been tied back to one another using typically 68mm diameter tie rods. These are in three section lengths – two of 15m and one of 10m – to complete the 40m span.

“One of the advantages of the HZ system is that the tie rods are very simple to anchor. They are supplied with a bracket that slips through holes cut in the piles then held with a pin. They are very quick and easy,” says Davis.

The sand fill sitting in the berthing pocket has been re-dredged and placed into the quay caisson. Now the project team will use vibro-compaction to speed up the compaction rate of the fill.

Test probes are being carried out before the full compaction sequence is carried out.

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Probing was carried out to ascertain levels of fill compaction

The probe has been fabricated at 20m in length and weighs in a 6.5t. Its steel fins are 15mm thick and will be trimmed down when the final probes are installed at the end of the quay, where the tie rods are closer together. The trials will ascertain just how much settlement and compaction the team can expect when the main rigs arrive.

“The trials will determine exactly how quickly we need to withdraw the probe and what settlement there will be,” says Davis. 

“Theoretically there should be a 6m zone of influence around each probe site. It’s just a way of speeding up the natural settlement. It means final surfacing work can be carried out sooner so that PHC can offer its facilities to the widest range of craft.”

With target completion for the work – without final surfacing – set for mid-November, the site team will hope the good weather and conditions it has enjoyed recently will continue to help steer the project through.

 

Project fact file

Scheme: Port of Poole – South Quay Works

Client: Poole Harbour Commissioners

Main contractor: Bam Nuttall

Contract Value: £7M

Start Date: January 2017

Target completion date: November 2017

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