The River Witham Phase 2/3 contract in Lincolnshire comprises a £10M package of works being carried out for the Environment Agency in partnership with Bullen Consultants and May Gurney Construction.
Using the Engineering and Construction Contract Option C (Target Cost), open plan colocated offices, a true team work approach, a project extranet and the retention of core project staff for over five years, the project demonstrates best practice, workable solutions and cost savings compared to other traditional contracts.
The integrated team enables innovative solutions to be identified and tried as alternatives, for example, use of local borrow pits, waterborne transport, 'water vole friendly' erosion protection and most notably an alternative engineering design using waste car tyres as an economic, innovative fill material for strengthening an unstable river bank.
As a result, the project not only boasts local construction gains and regional environmental benefits but provides a disposal route for one of the least degradable waste products.
A 1,600m long section of embankment between the River Witham and Branston Island flood storage area provided the team with a challenge to find an alternative design solution.
The embankment was in a poor condition, with a narrow crest and unstable side slopes and is underlain by a highly compressible peat layer with low shear strength. The crest had to be widened and stabilised. The traditional method would have involved placing clay fill on the landward side to widen the embankment.
At the Branston Island site this was not practical due to the close proximity of an 11kV overhead power line and a soke dyke (drainage ditch) running parallel to the embankment.
Any solution involving increasing the embankment footprint would have meant relocating the overhead power line and soke dyke - both costly and environmentally damaging. It was therefore imperative that an alternative solution was found.
Several solutions were considered:
Constructing the embankment on piles or sand/stone/lime columns l Reinforcing the embankment with geotexile and soil nailing l Introducing staged construction to allow soil consolidation l Improving the foundation soils, for example, by accelerated consolidation with vertical drain systems and preloading l Improving the foundation soils by chemical mixing l Using lightweight fill materials in the embankment construction.
Tyre bales provided the best solution on technical, economic and environmental grounds. They offer lower driving forces due to their low unit weight and also provide high internal friction, allowing the embankment to be built with a steeper side slope.
Their use minimised the embankment footprint area as well as reducing primary and secondary consolidation settlement. Addition- ally, lorry movements were minimised and waste car tyres safely disposed of.
An average of 100 tyres were strapped together to form each of the 1.4m long rectangular bales, which were then laid two deep in stepped benches cut into the landward side of the embankment.
A geotextile membrane, placed around the bales, prevents soil from infilling holes between the tyres, ensuring the bundles remain relatively light.All 12,000 bales are buried under a layer of backfill/ topsoil to create a steeper than normal 1 in 2.5 slope.
The unit cost of tyre bales is similar to clay (by volume). However, savings were made by avoiding knock-on costs of having to move the powerline and relocating the adjacent soke dyke.
Intangible savings were also realised by avoiding environmental damage and reducing the number of deliveries to the site. A further benefit is that construction using tyre bales could take place during winter, giving the client an improved flood defence - a year early.
With the implementation of the European Landfill Directive banning the disposal of whole tyres to landfill, their use in river engineering has potentially paved the way for future re-use in this sector.
The project team has worked together in the spirit of partnering for over four years and the project is being heralded by the Environment Agency as best practice and an excellent example of how it wants to procure and run contracts in the future.
Andy Yarde, Bullen Consultants Project team Client: Environment Agency Anglian Region Designer: Bullen Consultants (contact Andy Yarde) Contractor: May Gurney Construction (contact Malcolm Bear)