Coastal defence work is a race against the changing tides, requiring heavyweight engineering answers and on-site technical support.
On the Humber Estuary a £7.7M project is underway to protect up to 10,500 people from rising sea levels. Led by the Environment Agency, the Humber Estuary Flood Risk Management Study has identified an initial package of five schemes with a total value of £45M, to deliver long-term protection for up to 400,000 people across the estuary. For main contractor Birse Coastal, it requires concrete products that provide long-term durability and time savings to maximise productivity in a limited tidal window.
Stallingborough is the first of the five schemes and will see the replacement of 1.6km of aging flood wall protection on the shoreline between Grimsby and Immingham Docks, North East Yorkshire.
High fibre, high productivity
Tarmac is supplying around 3,000m3 of its Topforce concrete and 2,000m3 of conventional concrete for the construction of the new sea defence at Stallingborough. Birse Coastal and Tarmac previously worked together on sea defence schemes in Blackpool. For the Stallingborough scheme, Birse has specified Topforce, a high strength, fibre-reinforced concrete, ideally suited to coastal defence work. According to Alan Smith, site agent at Birse Coastal, the product is already proving its worth.
Smith says: “Typically, mesh fabric is used on sea wall defence schemes, but the macro-synthetic fibres within Topforce have allowed us to reduce the use of steel mesh fabric on the overall scheme by around 35 per cent. Mesh can easily get exposed and can quickly corrode, so using a material with added fibre is beneficial.”
Saving time was a key consideration of Birse Coastal’s product specification decisions. “Every day we are working against nature’s clock and only have a six hour tidal window in which to operate. Using Topforce has delivered both cost and time savings, because using mesh fabric typically requires significant onsite preparation time. I believe we are saving around a week compared with the time we would have had to set aside for mesh-based material. We have also found that the concrete delivers an even distribution when discharged,” he adds.
High technical standards and rigorous testing are vital, both at Tarmac’s concrete batching plant in nearby Immingham and on-site to assess the delivered product. This ensures that the product offers the optimum strength and workability that the contractor team requires.
In order to offer long-term strength and durability, the Topforce used at Stallingborough consists of gravel aggregate from Tarmac’s Langford quarry, near Newark, combined with a specialist blend of fibre, Portland cement and ground granulated blast furnace slag.
“We are currently delivering between 30m3 to 60m3 of concrete a day to site and our technical team will conduct regular slump tests as well as taking cube samples of cured concrete to check the product strength,” says James Freeman, national special products manager. “Long-term durability of the sea wall is critical and the concrete has to deliver a strength of over 50N/mm2.
“Because of chlorides in the sea water we have to ensure that the concrete can meet an exposure class of XS3,” continues Freeman.
The Stallingborough project is set to be completed by October 2009. The next project in the Humber package of works is the replacement of the flood wall at Swinefleet, near Goole. The project team will then move to Halton Marshes, near East Halton, Donna Hook, near North Somercotes and onto a location on the north bank of the Humber.
It is estimated that the five projects within the Humber Estuary Flood Risk Management Strategy will be completed by July 2011.