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Filling the freight gap

Ground engineering Birkenhead roro: A port development under way in the UK's north west is set to transform shipping links across the Irish Sea. Diarmaid Fleming reports on some innovative engineering

Liverpool's connections with Ireland run deep.

Twinned with Dublin, the port is a key artery between the cities while also the gateway for Belfast to England shipping. But currently, docking in Liverpool involves complex manoeuvres for ships inside the enclosed facilities at Brockelbank and Portside Docks, increasing turnaround times.

By August this year, however, completion of the $39M Twelve Quays project at Birkenhead will allow two freight ferries to discharge their cargoes of HGVs simultaneously out in the River Mersey. A giant pontoon will be linked to shore by an 86m long linkspan, while the 19th century 52,000m 2Wallasey Dock will be filled in to create a new trailer park.

'Apart from adding to the capacity of the port, the new terminal will knock around one and a half hours off the current cycle of 25 hours. Keeping the cycle under 24 hours makes it easier for haulage firms to organise their businesses, ' says Mersey Docks & Harbour Company (MHDC) chief engineer Tim Bownes.

Wallasey Dock was used primarily as a berth for the import of livestock from Ireland, before being closed in 1967.

However increased trade across the Irish Sea in the 1990s prompted the MHDC to earmark the site for a roll-on roll-off facility which gained approval in 1997.

Looking at the derelict site, the immense scale of the task is clear. At one stage two separate contracts were considered - one to fill the dock before handing over the total site as one land mass. Both tasks were priced and negotiated separately before a single contract under the New Engineering Option C form was awarded to Amec Capital Projects last February.

At present, four pumps running at 8m 3/s for eight hours a day take water from the Mersey to fill the both the Wallasey Dock and a system of landward sister docks. 'It is still the only way to fill the docks, so we had to continue providing a way for water to flow in, ' says Bownes.

A substantial underground twin-celled culvert tunnel, 8.3m wide and 2.6m deep, was the chosen solution. Construction of the 550m long insitu concrete structure was the first task undertaken by Amec.

The culvert has been designed to maintain a flow of water even if the pumps fail in the event of a power shutdown, with a surge basin to force water through the system. A sheetpiled closure wall prevents water seepage into the infilled Wallasey dock.

Infilling poses considerable geotechnical challenges. The dock bed consists of silt up to 9m deep, much of it accumulated from the water pumped in from the Mersey. 'The unpredictability of the settlement has given us a lot to think about, ' says Bownes.

'We looked at taking out the material, but calculated the amount would be huge - around 400,000m 3. Our environmental impact assessment told us it was not worth it.'

Project consultant Bullen devised the solution to place a huge geotextile sheet on to the silt and hydraulically fill the dock with 250,000m 3of dredged sand, won around 8km away at the mouth of the Mersey within the MDHC's statutory dredging limits.

Rolls of 15m wide factorystitched permeable polypropylene sheets are stitched together on site by teams of two on rafts.

'With wick drains incorporated in the design at 1.5m centres, it's really like a big sponge, ' says Amec project manager John Sherman. 'We are allowing a 1.5m surcharge above the final level of the dock for settlement.'

Around 90% of settlement is predicted within three months of filling.

Continued settlement after completion has been factored into projected maintenance costs for the facility. 'We are making allowance in the budgets for maintenance between the hard and soft areas, ' says Bownes.

On top, the new trailer park will sit on 80mm thick blocks over 50mm sand, on a road base of 180mm cement bound material (CBM) and 800mm thick subbase of sand stabilised to 80% CBR (California Bearing Ratio).

The general circulation area built on terra firma will be surfaced with 300mm roller compacted concrete, one of the first large-scale uses of the material in the UK. A total of 126,000m is being created.

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