For most people, the Port of Dover is a gateway into or out of the UK. With France just 34km away, it’s our nearest port to mainland Europe; it’s also one of the world’s busiest passenger ports with 16 million travellers passing through every year.
And that’s the point – they’re passing through. The Port of Dover is not really a destination in itself (though plenty of visitors linger to explore the mediaeval castle perched high above on the famous white cliffs).
But now, thanks to a £250m redevelopment programme, part-financed by the European Union, Dover’s famous Western Docks are being transformed into what the client hopes will be a bustling new waterfront with a new marina and pier and a host of shops, bars, cafes and restaurants to attract visitors arriving by both land and sea.
The redevelopment also includes a new cargo terminal, centred on what was once the hoverport from where Anglo-French firm SeaSpeed operated its high-speed hovercraft service to and from Calais. The new terminal and distribution centre will create more space and ease congestion for ferry traffic at the Eastern Docks.
One of the main features of the redevelopment is the creation of a new marina, a haven for yachts that extends out into the main harbour area behind a new public access pier. The new 500m pier comprises 360 individual precast concrete units - manufactured by Macrete (Ireland) in County Antrim - with a combined weight of 13,350 tonnes. These precast units are supported on a line of 1.8m-diameter tubular steel piles driven into the harbour floor with the aid of a specially-designed hydraulic piling gate (see box).
Prior to the redevelopment, pleasure craft were accommodated in the Wellington Dock, a sheltered basin behind the sea wall, accessed via the inner harbour – most of which has now been filled in to provide space for the new cargo terminal.
Marine access to Wellington Dock will now be provided from the new marina via a 120m-long navigation channel through the narrow neck of land, with a pair of lock gates to isolate the dock from the tidal marina and outer harbour. Road and pedestrian access across the cut will be provided by a 15m-span lifting bascule bridge.
To create the navigation channel, VSBW – a joint venture of Volker Stevin and Boskalis Westminster which has the £115m marine structures contract – had to demolish part of the 19th century harbour wall and install a sheet pile cofferdam prior to excavation commencing and the reinforced concrete walls and the base slab being cast.
A major feature of the 8m-deep, 20m- wide excavation is the extensive structural support required. Specialist ground support hirer Groundforce was brought in to provide its hydraulic waling beams and props to shore up the sheet-piled walls of the excavation, which was subject to high and very variable loads.
“The navigation channel is a fundamental part of the marine and structures package for this project,” says Patrick Leahy, VolkerStevin’s site agent responsible for the piling works. “And as a marine excavation, this part was complicated by poor ground and groundwater problems,” he adds.
Despite the high chalk cliffs rising up behind the port, the ground in which VSBW was working was loose sand and gravel. Twenty-metre-long steel sheet piles were driven 6m below formation level and two levels of hydraulic props were installed sequentially as the excavation progressed. Total dig depth was around 15m.
The navigation channel cuts directly through the only available land access to the main site in and around the former inner harbour and so it was decided to construct it in phases so that access could be maintained at all times.
Originally there were to be two phases – Phase 1 comprising the inner section, extending from the Wellington Dock to roughly half-way, and Phase 2 starting within the cofferdam installed in the new marina and progressing inland towards the complete first phase. In the end, it was decided to complete Phase 2 in two sections for logistical reasons.
“If we’d opened Phase 2 out in its entirety it would have restricted movements around the site, so we started Phase 2 at the seaward end and then completed the cut with the central section,” explains Mark Whitmore, major projects general manager with Groundforce.
The company provided its hydraulic Mega brace, a modular frame that is adjusted and braced with integral hydraulic rams, to provide the waling beam along the line of sheet piles.
The waling beam supporting the Phase 1 excavation was propped with MP150 tubular props spanning the excavation from side to side. These props each have a maximum load capacity of 150 tonnes; two additional MP250s (250-tonne capacity) were placed as raking props – that is, positioned at an angle of 45o to the vertical – against the end wall to support the load from the Phase 2 ground.
Phase 2a – the seaward section of the cut – was the next section to be excavated. Because this section adjoins the new marina it is therefore exposed to the full effects of the 7m tidal range of the outer harbour and hence subject to high and very variable loadings.
Here, VolkerStevin employed Groundforce’s biggest frames and props to cope with the pressures exerted by the rising tides. The Super-Mega brace is believed to be the strongest proprietary hydraulic brace available.
Modular sections ranging from 3m to 12m in length are combined with a hydraulic unit designed to give 1.5m of fine adjustment within the largest excavations. The high load capacity of the Super Mega brace allows clear spans in excess of 21m. Groundforce fabricated bespoke reverse jointing brackets to connect sections of the waling beam where the excavation widened.
All the props used in the Phase 2a excavation were 250t-capacity MP250s, the longer props employing 1.22m-diameter ‘super tubes’ for added rigidity.
Phase 2b – the central section of the cut, also used the Super Mega brace with a mixture of MP150 and MP250 props as well as one MP500 (at 500t-capacity, the largest prop of its kind currently available) to support the sides.
Using more hydraulic props could have provided a rigid enough structure to support the excavation, says Whitmore, but that was not an ideal solution. “The Super Mega brace needs fewer props and we needed to keep the prop spacing as wide as possible to provide space for VolkerStevin to work in the excavation,” he explains.
“As the sheet piles need to be propped all the time we remove and replace props due to load changes as the concrete works progress,” explains Whitmore.
“The smaller MP250s in Phase 2b are being swapped for an MP150 once the upper level of 2a is removed,” he adds.
Dividing the excavation into three phases complicated the temporary works, which required temporary sheet piled partitions to separate the three excavations. But the biggest challenge was the landward pressure of the rising and falling tides.
“As the water level rises, the loadings increase along the line of piles as water soaks through the sand and gravel,” explains VolkerStevin’s Patrick Leahy; “The sea pushes the frame inland.”
Consulting geotechnical design engineer Tony Gee & Partners “had to look at that in great detail”, says Whitmore. “They estimated the loadings and gave us the specification. We couldn’t do those calculations – they went beyond our PI [professional indemnity] cover,” he explains.
Nevertheless, Groundforce was required to carry out its own temporary works design under separate contracts.
“This is unusual because we normally provide a design service as part of the hire contract,” says Whitmore. “Here though, the design element represented a higher than usual risk and the main contractor wanted accountability.”
Consequently, Groundforce signed three separate short-form NEC design subcontracts, one for each phase of the works, in addition to its main £600,000 hire contract.
“This is becoming more prevalent, especially on major projects,” says Whitmore. “In the past 12 months we’ve had half a dozen jobs where the client has requested a separate design subcontract in addition to the hire contract. As long as we get paid, we don’t have a problem with that.”
Work on the navigation channel began last spring, with the first hydraulic supports delivered in September. “We’re fairly well on with it now,” says Whitmore. “Phase 1 is out of the ground and most of Phase 2a is also out.”
The base of the navigation channel comprises mass concrete, to a depth of about 0.5m with a 1.8m-deep reinforced concrete base slab and retaining walls above. “Reinforcement was complicated and the corners of the base are chamfered, which also required a lot of space beneath the props,” comments Whitmore. “Once the base was in, however, the lower frame could come out.”
All the piling on the project is now completed, says VolkerStevin’s Patrick Leahy, and the contractor is currently completing the capping beams. The VSBW joint venture will have finished all the marine civil engineering work by the end of this year.
Walking piling gate
Time, as always, is of the essence on this project, and the client was keen to find anything that could speed up the installation of the 711 massive 1.8m- diameter tubular piles required for the new pier and sea defences.
VolkerStevin’s response was to develop and manufacture a new type of ‘walking’ piling gate that would speed up the alignment and installation of the piles.
The gate – which weighs about 50 tonnes - comprises a frame that clamps onto the previously installed piles and is equipped with hydraulic roller boxes to guide the piles accurately into position.
After driving a pile, the gate glides forward autonomously from the crane and hydraulically adjusts its position to prepare for the next pile. Being able to reposition and level the piling gate without using the crane shortens the critical path significantly as it can be achieved while the crane is busy upending and pitching a new pile. In addition to piling in straight lines, the new piling gate can also undertake smooth, wide curves, which has proved invaluable with the marina’s curved design.
Without the walking gate, VolkerStevin calculated an average rate of 2.5 units could be installed per day. With the gate, however, the firm achieved around five piles per day and has achieved as many as 10 units.
Away from the new navigation channel into Wellington Dock, the joint venture has been reclaiming the old inner harbour to create new cargo berths.
Boskalis Westminster, a dredging specialist, has removed a million cubic metres of silt and overburden underneath the reclamation areas and within vehicle manoeuvring areas and imported 950,000m3 of hydraulically-placed reclamation fill to form land behind the quay walls to provide a development platform.
Two berths have been built, one primarily for reefer vessels and another designed primarily for container and cruise vessels.
VolkerStevin’s package also involves construction of a 25,000m2 concrete apron and 2.3km of combi-pile quay walls incorporating associated concrete capping beams, cathodic protection, quay furniture and a fendering system.
This article was first published in the June 2018 issue of The Construction Index magazine, which you can read for free at http://epublishing.theconstructionindex.co.uk/magazine/june2018/
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