Redundant dock infilled using Geolon PP120S to stabilise silt
28/11/2001
Wallasey Dock is to be infilled as part of a major redevelopment scheme which will transform the Twelve Quays area of Birkenhead into a new state-of-the-art Ro-Ro ferry terminal. Geolon geotextile material supplied by Maccaferri Ltd, specialists in retaining structures, slope reinforcement and erosion protection, has been specified to create a barrier between the mud and silt in the dock basin and the granular infill material.
Measuring approximately 500m x 100m with a depth of between 13m and 2m, Wallasey Dock has been redundant since the early 1970s. It presently contains a mix of silt and mud to a depth of up to 11m, which needs to be stabilised and filled with granular material to bring it up to ground level. Once this is completed, site owners the Mersey Dock and Harbour Co intend to redevelop the area as a lorry park to service the new ferry terminal.
Geosynthetic material was required to ensure separation between the silt and the granular fill, to reinforce the initial layer of fill and prevent localised failure of the silt. It will also assist in controlling any silt displaced in front of the filling operation. However, main contractor, AMEC and consultant engineers, Bullen Consultants also had to take a number of operational factors into account, as a spokesperson at AMEC, acknowledged:
“For ease of installation, we needed a geotextile material which would float on the water in the dock. Conventional narrow strips with overlap joints were ruled out as being difficult to hold in position as the dock is filled and the geotextile sinks. We needed a material which could be joined into one large panel with a high strength across the joints.”
As the specification of the geotextile material was dependent on the final design, filling material, sequence and the orientation of the panels, Maccaferri worked in close consultation with AMEC and Bullen in proposing a solution. As the UK supplier for Ten Cate Nicolon a company with a world-wide reputation in the manufacture of geotextiles for the marine and reinforced soil engineering markets, Maccaferri called on Nicolon’s special expertise to present a complete solution which could fulfil all the functions required.
Geolon material supplied by Maccaferri and manufactured by Ten Cate Nicolon is currently being installed in strips across the dock. Each strip is 120m - 160m in length and 15m wide, manufactured specifically for the Wallasey Dock project from three widths of the standard 5.2m width stitched together in the Nicolon factory in the Netherlands. Utilising specially designed seaming techniques, factory stitching ensures a tensile strength in the joints of up to 80% of the tensile strength. The 15m strips are laid across the dock on top of the water and stitched together in situ by teams of workers working from pontoons.
Successful trials were undertaken in a small area of the dock (20m x 20m) in July and the full project got underway in September. Infilling is expected to take up to seven weeks. Following installation of the Geolon material, wick drains will be inserted into the silt to assist with water drainage from the dock. To ensure drainage of the water from the dock through a weir, a settling lagoon will be created and up to 1.5m settlement is anticipated.
Geolon PP120S is a woven geosynthetic manufactured from polymetric materials to enhance the performance of a variety of soil and hydraulic structures. Designed to offer the ideal characteristics of high tensile strength, low elongation and low creep Geolon PP is chemically inert to alkalis and acids, with a reliable long term tensile strength.
The UK arm of the Maccaferri Industrial Group, headquartered in Italy, Maccaferri Ltd specialises in gabions, geotextiles, geogrids and geomats. In addition to a full range of products for retaining structures, slope reinforcement and erosion protection, the company offers an in-house design and consulting service.
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