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Oil's well that ends well


A basal reinforcement platform and gabion baskets allowed construction of a dock wall on very soft ground for a new oil tanker berth in Bristol. Max Soudain reports.

Royal Portbury Dock is the largest in the Bristol and Avonmouth area. Here massive ships from around the world offload thousands of cars and containers as well as forest products, oil and coal.

Built in the 1970s, 20km downstream from Bristol, the dock has undergone a renaissance under owner Bristol Port Company since the 1990s. New facilities include a huge forest products terminal and last year 9.5km of the Portishead Branch Line was reopened for freight, linking the port to the UK rail network and saving more than 400 lorry movements a day.

The dock is being extended to form a new oil terminal for 120,000t tankers. Berth 7 is being built in a lagoon next to the main dock. At the moment the lagoon is used to balance water levels in the dock and its lock entrance. The lock is the largest in the UK, able to handle vessels up to 41m beam, 290m length and 14.5m draft.

John Mowlem is the main contractor for the extension, working for dock owner and operator the Bristol Port Authority. The scheme was designed by consultant Waterman Burrow Crocker.

The project involves deepening the lagoon to allow for the draft of the big ships, construction of a new dock wall and a monopile dolphin structure 40m in front of it and removal of the peninsula between the dock and the lagoon, creating a continuous dock area while preserving the line of existing berth facilities.

A small pumping station that regulates water levels in the lock will be replaced by a larger one to allow the lock and the dock to be replenished from the River Severn estuary.

Ground conditions are a variety of estuarine alluvial silts, sands and gravels overlying a strong marl. While much of this material is natural, the lagoon and surrounding area has been used as a dumping ground for soft alluvium excavated from other parts of the docks.

'The challenge was to create a vertical retaining quay wall on these soft alluvial soils, ' says Maccaferri area engineer Niall Corney.

The plan had been to build a tie-back sheet pile wall with a concrete capping beam, as had been done elsewhere in the docks.

But the poor ground here meant there was a chance that anchors would pull out, causing movement and settlement of the wall.

To prevent this, the alluvial deposits would have had to be stabilised, with extra cost.

Instead, Maccaferri proposed a basal reinforced platform using Terram Paralink grid to reinforce fill placed on top of precast driven piles. This in turn supports the 310m long, 5m high Terramesh gabion retaining wall.

The system combines a conventional gabion basket with a continuous mesh tail forming an anchor element, removing the need for tie backs and ground treatment.

Piling contractor Stent Foundations installed three rows of piles through the silts to found on the marl 12m down. All the piles are 270mm 2, cut-off to be typically 9m long and topped with individual, 1m 2, 0.5m thick pile caps. The front two rows, nearest the water, are raked at 1:8 towards the dock to resist lateral movement and the back row is vertical.

Originally, the piles were to be installed on a 2.75m square grid and two layers of Paralink 200S used. 'This has a nominal 200kN/m strength and a characteristic strength of 225kN/m, ' Corney says.

However, Maccaferri proposed a 3m spacing along the wall using higher strength Paralink 300S with a characteristic strength of 337kN/m - 'the clear span between the piles dictates the load on the strips, ' Corney explains. While this material is more expensive, costs were offset by opening up the pile spacing - 'Five or six rows of piles could be omitted, ' he says.

The 6.25m wide basal reinforced platform was designed to BS8006 and sits directly on top of the pile caps. The first layer comprises a grid of Paralink 200S polyester strips laid at 90° to the line of the wall, with an overlap of 2.75m of grid at the back and a 3.25m at the front of the platform.

Two rows of 0.5m square gabions are placed on top, one along the front and one along the back of the platform, to act as anchoring blocks for the Paralink.

Next, a grid of Paralink 300S is laid directly on the first layer with the strips running along the line of the wall. Grids are overlapped by 2.75m along the platform. This is then covered with 250mm of 6I rated fill. The extra Paralink 200S is then pulled around the gabions and folded back in to the platform, before being covered with another 250mm of 6I fill that is compacted.

The Terramesh wall is made up of 2m long, 1m deep and 0.5m high units, delivered to site in 'flat pack' form. Each is made from a double-twist hexagonal mesh of PVC and Galfan galvanised steel wire and is assembled by forming the basket and lacing the sides together with lacing wire or high tensile steel fixing rings.

The Terramesh gabions' 'tails' are an integral part of the basket, acting as a tie into the fill behind and as a reinforcement element, Corney explains. Seven rows of baskets are being used at Portbury Dock, with 6m tails at the bottom of the wall and 4m tails at the top. The gabions are filled with 200mm and 50mm limestone gravel sourced from a local quarry.

The wall is built up behind the front row of gabions in the basal reinforcement platform, which will be used to secure a concretefilled scar mattress that will protect the top of the 1:3 slope below the toe of the wall.

A battered 1:3 slope has been formed about 8m behind the Terramesh wall and a non-woven needle punched polyester Bidim geotextile laid to stabilise it and to act as a separator between the granular backfill to the wall and the insitu alluvial material. This reduces the risk of contamination and prevents fines being washed out, while still allowing groundwater to flow.

The completed wall will be topped with concrete and the berthing dolphin joined to it by a concrete jetty platform. Tug boats used to manoeuvre the massive tankers in the dock area will tie up to the wall and a car parking area will be created between the wall and the estuary.

When GE visited site last month, piling had finished and more than half of the dock wall had been built by subcontractor Cares. Earthworks contractor Vertase was hard at work in a pumped-out area of the lagoon excavating through the soft estuarine silts.

When this process reaches the underlying sand and gravel, the lagoon will be flooded and Westminster Dredging will excavate down to the marl that will form the dock floor.

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