Once a hub for international shipping, Liverpool has been off the mariner's chart for the last three decades. On 21 September, the great transatlantic liner QEII will dock in Liverpool to mark her 40th year afloat.
And the QEII's visit will be the first of several by cruise ships this year, a sign that Liverpool may once again have become a desirable port of call.
Liverpool is fast reinventing itself, remodelling its centre with a frantic burst of construction activity and injecting economic vitality back into its depleted veins (NCE 22 February). Its waterfront at Pier Head and old commercial district were recently designated a UNESCO World Heritage Site and next year Liverpool will be European City of Culture.
Ships like the QEII, which measures 300m long, weighs 70,000t and has a draught of nearly 10m, cannot simply slide up the River Mersey and alongside Pier Head's quay wall, however. The river is not deep enough at its banks and its Victorian walls will not take the kinds of loading imposed by vessels this large.
Work is therefore proceeding at full steam to deliver a new pontoon which will allow big ships to moor in deeper water.
There is already a landing pontoon in front of Pier Head, used by the Isle of Man ferry.
The cruise liner pontoon will be built immediately downstream, and connected to the existing facility by a short, wide bridge.
The new and old pontoons are reinforced concrete cellular structures, but their means of anchorage are entirely different.
Where the old pontoon is held in position by chains weighted to the river bed and by booms connecting back to the shore, the new pontoon will be xed in place by tubular steel piles.
Horizontal movement allowed by the old pontoon's chains is in the order of 5m to 7m to cope with tidal variations. But for the new pontoon, the size of the vessels it will handle means 'you don't want things moving about more than they have to, ' says John Williams, project manager for 2020 Liverpool, the city council's agent for the procurement and supervision of construction schemes.
Eight 2m diameter piles will anchor the new pontoon. These will allow vertical movement, so that it can fall and rise with the tide, but keep the pontoon xed on the horizontal plane.
Steel fabricator Cimolai supplied the piles and subcontractor Commercial Marine & Piling inserted them, working from a spud leg barge attended by a jack-up as its working platform. Commercial cored 2.5m diameter cased sockets through 3m of fractured and weathered rock and up to 11m into the underlying competent sandstone.
Balfour Beatty Civil Engineering won the £19M design and build contract for the pontoon with designer Gifford by answering the client's brief for a steel pontoon. But it also submitted an alternative, reinforced concrete proposal.
'That decision was made because of the volatility of the steel market - we couldn't guarantee price with an all steel design. There were also thoughts that Lloyds Register [which classies and assigns risk proles to marine structures] preferred concrete, and concrete's long term durability was perceived to be better, ' comments Balfour Beatty project manager Paul Brown.
The 258m new pontoon has been broken down into four sections of 64.5m each.
Sections will be coupled with specially designed male-female joints, enabling the pontoon as a whole to ex with the swell of waves. Central sections of the pontoon are 19m wide, and the end sections are 26m. Height is 4.7m to 5m - deeper sections are needed to provide extra buoyancy under high loadings.
Brown says that the pontoon is designed to cope with significant ship impacts, so it is surprising to learn that concrete is not especially tough, with a strength of C40/50. Nor is it very thick, with walls and slabs 150mm deep.
Dividing the pontoon sections into watertight cells stiffens them as well as ensuring that, if the external wall is punctured, the section won't sink.
But strength also comes from the three layers of dense reinforcement in the walls. Bars are up to T32. To guard against rebar corrosion, concrete cover is a minimum of 45mm.
'To ensure concrete penetration, especially at junctions, we decided to use a self-compacting mix, ' explains Brown. Self compacting concrete also saved us a lot of time - probably two to three weeks over conventional concrete, because the contractor did not need to use vibrators and could scale down concreting gangs from four members to two.
Time and manpower savings made the £10/m 3 extra cost of self compacting concrete economic, he adds.
Even in a city built on shipping, finding a dry dock in which all four pontoon sections could be cast was not easy.
Down river from Pier Head, in the old Port of Liverpool, Balfour Beatty secured a lease on Canada Graving Dock, which was once used to carry out repairs on the Cunard liner Lucitania. It had been out of service for 20 years and was full of water.
Before it could be put into use, the dock had to be dewatered and dredged.
'At the bottom, we found 900mm of silts to dispose of.' Granular ll was placed to create a level floor to the dock, and it was then blinded with no fines concrete.
Brown says that building the pontoon sections on a slipway and launching them into the river was briefly considered. But there were doubts about whether the structures could cope with temporary impact and bending loads as they hit the water.
Casting got under way last autumn and was completed at the end of May. The pontoon sections were floated out of the dock to an adjacent berth in midJune, for fit-out with elastomeric fenders, a covered walkway and a pair of buildings.
The pontoons stand only 40m from Pier Head quayside, meaning that the gradient of the existing link span bridge is extremely steep at low tide. QEII passengers will transfer to shore by bus, so a shallower gradient structure is needed.
For this, Balfour Beatty is installing an 80m 400t warren truss structure of double the length to allow far gentler transition between sea and land.
This will be delivered in sections later this month.
Client: Liverpool City Council, with Mersey Docks & Harbour Co, Peel Holdings, Northwest Regional Development Agency, Liverpool Vision, City Focus, Government Office North West and the Mersey Waterfront Regional Park.
Client's agent: 2020 Liverpool (Mouchel Parkman) Main contractor: Balfour Beatty Civil Engineering Design engineer: Gifford Piling subcontractor: Commercial Marine & Piling Steelwork fabricator: Cimolai Safety facts Lost time accidents: none Safety innovation: Balfour Beatty has introduced a behavioural safety initiative called 'take care', across the company. Care stands for 'conscious actions reduce errors'.