Construction of a new roll-on roll-off ferry terminal on the River Mersey involves stitching a massive geotextile blanket together while it floats in a soon-to-be infilled dock.
In the late 19th and early 20th centuries the Twelve Quays area of Birkenhead and Wallasey Docks on the River Mersey was one of Europe's largest cattle handling facilities, with ships delivering livestock from all over Ireland. But by the end of the 1960s trade had seriously declined, forcing its partial closure.
Trade in the Irish Sea underwent something of a revival in the 1980s and Mersey Docks and Harbour Company (MDHC) identified Twelve Quays as a prime site for a new roll-on roll-off ferry terminal. Work was given the go-ahead in 1997.
The £26M Twelve Quays Terminal is being developed by the Northwest Development Agency, MDHC and main contractor Amec Capital Projects, with consultant Bullen Consultants. The scheme involves construction of terminal buildings and cargo and passenger handling facilities. Massive ferries will dock at a pontoon 140m offshore, linked to the terminal area by a piled bridge.
The project, which will regenerate a large area of derelict land, has attracted nearly £7M in funding from government agency English Partnerships for groundworks and infilling of some of the dock area.
Wallasey Dock, largely redundant since the closure, has been used only to pump water into the Birkenhead dock system. As part of the redevelopment, a culvert has been built to allow the dock to be infilled, creating a large lorry park for a new container facility.
The 500m by 100m basin is between 2m and 13m deep and filled with up to 11m of soft silt and mud. Marine sand dredged from the River Mersey will be used to bring the dock floor up to the required level and water will be drained off as the granular material settles.
A geotextile layer was needed to separate the silt from the sand above, to reinforce the lower layer of the fill, to prevent any localised failure of the silt and to control any silt displaced in front of the filling operation.
The geotextile had to be capable of being easily handled and placed, calling for a material that could be joined into one large panel with high strength across the joints, all while floating on the water surface.
'A floating geotextile can be pulled across the water and soft silt easily, ' explains Amec Capital Projects' David Squires. 'The other criterion was to be able to join the geotextile without fishing around in the soft silt to find and join the edges. '
Once stitched together, the geotextile will be weighed down with sandbags to make it sink as the sand is placed and the water is gradually drained off, via a weir, into an adjacent dock.
Consequently, geotextile strips with conventional overlapping joints were ruled out as having insufficient strength and being impossible to hold in position as the dock is filled with sand.
Squires says geotextile selection required close co-operation between Amec, Bullen and geosynthetics supplier Maccaferri, as geotextile specification depended on the final design, filling material and sequence and orientation of the panels.
Maccaferri proposed using Geolon PP120S geotextile from Dutch based manufacturer Ten Cate Nicolon, because it is easily handled and will float on water while it is securely joined.
Maccaferri, the UK distributor for Ten Cate Nicolon, says the woven polypropylene geotextile offers long term high tensile strength, low elongation and low creep and is chemically inert to alkalis and acids.
The Geolon is being installed in specially manufactured 15m wide and 120-160m long strips across the dock. These are stitched together by teams working from pontoons. The strips are made from three widths of the standard 5. 2m wide sections, stitched together in the Nicolon factory. Special seaming techniques were used to ensure a tensile joint strength of up to 80% of the tensile strength of the geotextile. For transport, each length was folded in half and rolled on to steel tubing.
Small scale trials were successfully carried out on a 20m by 20m area of dock in July 2001. Work began last month and is expected to take up to four weeks. However, work depends heavily on tidal water levels in the docks.
Contractor Westminster Dredging will carry out the dredging for the project, with the sand pumped ashore five times a day. 'There are three sources in the Mersey where suitable sand is available, ' Squires says.
Wick drains will be installed through the fill and geotextile and into the underlying soft silt when the surface is stable enough to speed up the consolidation.
The dock will then be over-filled in anticipation of the expected 1. 5m settlement during consolidation.
'Consolidation settlement has been reduced to about three months by applying a surcharge and decreasing the spacing between the wick drains, ' says Squires.
The Twelve Quays Terminal is expected to be operational in the first half of 2002 when Norse Merchant Ferries will transfer from berths at Liverpool to operate six sailings a day to Belfast and Dublin.
The ferry company expects the all-weather terminal's downstream location to reduce journey times to Ireland by up to and hour.