Terramesh/gabion retaining wall provides secure anchorage for extension to Royal Portbury Dock, Bristol

07/04/2003

Maccaferri Ltd, specialists in retaining structures, slope reinforcement and erosion protection, is working with main contractors John Mowlem & Company Plc and consulting engineers, Waterman Burrow Croker to construct a berthing facility at Royal Portbury Dock, Bristol.

The new facility is an extension to the existing Dock and will see a former tidal reservoir incorporated into the Dock to form a new Berth capable of accommodating cape size vessels.

Owned and operated by Bristol Port Company, the Royal Portbury Dock handles car imports, containers, forest products, animal feed and coal. Vessel access to the Dock is through the largest sea lock in the UK, capable of handling vessels up to 42m beam, 366m long and drawing 14.5m.

Maccaferri became involved in the project when the original plan to construct a tie-back sheet pile retaining wall had to be abandoned as the poor ground conditions (soft alluvial silt, sand and gravel) could result in the anchorage ties pulling away. Instead Maccaferri worked with Waterman Burrow Crocker, Mowlem's consulting engineers, to develop a reinforced earth wall retaining structure sitting over a Basal Reinforced Platform (BRP) all supported on reinforced concrete piles.

The Basal Platform and reinforced soil block were both designed to BS8006. The former using Terram Paralink whilst the latter used Maccaferri woven wire steel mesh. Pre-cast driven piles provided the foundation support for the BRP and a Gabion Terramesh vertical quayside retaining wall some 310 metres in length.

Combining a conventional gabion face basket with an integral variable length mesh tail providing anchorage, this method of construction eliminated the need for any additional anchorage ties and ground treatment. Construction of the whole structure was completed earlier in the summer of 2002.

Specialist sub-contractor Stent installed piles through the silt into load bearing strata 12 metres down. Terram Paralink geogrid (manufactured by Linear Composites) was laid over the pile caps in both transverse and longitudinal directions using a single height gabion at both sides to act as a thrust block to provide bond lengths to resist 'pull out' forces. Raking piles were installed to accommodate the lateral loadings on the structure.

Terram ParaLink strips are high strength, high modulus polyester based geogrid strips. Available in a range of strengths from 100kN/m up to 1250kN/m, ParaLink 300S and 200S with a characteristic strength of 337kN/m was specified for the BRP at Portbury.

Specialist sub-contractor, Caras undertook the construction of the 3.5 metre high Gabion Terramesh wall. Filled with natural local stone, each unit is 2 metre long, 1 metre deep and 0.5 metre high with a mesh tail of between 4 - 6 metres anchored within Class 6I/6J granular backfill. This retaining wall will provide an edge with a jetty facility being built out to provide moorings for tankers and associated tug boats, and also retain a level area behind the berth itself.

During construction the slope behind the Terramesh wall was cut to a gradient of 1in3. A Bidim geotextile was placed over this slope to act as a separation between the granular backfill and the in-situ alluvial soil, its puncture resistance and filtration properties helped to ensure the reinforced soil block would not be contaminated with fines from the retained alluvial deposits.

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 can offer an in-house design and consulting service.

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