Concrete Society Awards - Civil Engineering Category - Certificate of Excellence

Lakeside Energy from Waste, Waste Bunker, Slough

Main contractor Edmund Nuttall is undertaking construction of the heavy civil engineering elements of the Lakeside Energy from Waste (EfW) facility under a design and construct contract. The key integral component of this EfW plant is the waste storage bunker, which was constructed in reinforced concrete using slipform techniques.

The incinerators at the Lakeside EfW plant in Slough, Berkshire are specified to burn 400,000 tonnes per year. For this reason, it requires a front end material recycling facility to handle the waste prior to incineration. The plant will have a combined electricity output of 34MW per hour This includes the clinical waste incinerator, which has an incineration capacity of 1.25 tonnes per hour.

The concrete solution

The concrete waste bunker is 22 x 40m in plan and extends 10m below ground level, with walls rising to 21m above ground level. This structure forms the initial reception and storage area for the waste prior to incineration, storage cells for the resulting ash from the incineration process and provision of the fire water storage tanks. The waste bunker does not only store waste, it also acts as the building's structural core, carrying the imposed loads from the steel frame and cladding.

The internal environment of the waste bunker is chemically aggressive, and the structure must also resist impact loads from the waste crane grab. These consideration dictated that the structure should be reinforced concrete.

Edmund Nuttall's project strategy provided the client with a construction programme three months shorter than other tender offers; this was a very valuable saving against the financing costs for the project. The programme saving was generated by constructing the waste bunker, which lies on the project critical path, using slipform techniques as opposed to traditional formwork or jumping formwork systems.

Construction phase

Waste bunker construction

For slipforming to be viable, the construction of the waste bunker below ground level could not incorporate any temporary propping as this would impede the rising of the slipform formwork. Edmund Nuttall, in partnership with design consultant Royal Haskoning, developed a design for the below ground waste bunker using a box formed with secant piles, with the bunker base slab and capping beam at ground level designed to act as props. The piles and capping beam were designed to allow excavation of the bunker to formation level without any temporary propping.

The use of reinforced concrete piles provided the most efficient engineering solution for creating the waste bunker box in the prevalent ground conditions. In addition, the construction process had to comply with noise constraints that were stipulated on the site. Bored piles were used to comply with these.

The concrete piles in the secant wall required 2% steel reinforcement by volume in 1.200m diameter piles. These piles were installed as either rotary bored or continuous flight auger (CFA) piles, to a depth of 24m before ground level.

Slipforming the bunker superstructure

Edmund Nuttall's proposals for slipforming the bunker structure were supported by Austrian based slipform specialist Gleitbau, Collaboratively working with both Gleitbau and the concrete supplier, these proposals were developed into a workable process to create the complex waste bunker structure with a continuous slipform process.

The slipforming process was a new challenge for all the project team and a number of meetings were organised between Gleitbau, the design, construction and client teams to ensure the process was understood by all the stakeholders. This understanding allowed consideration to be given to the practicalities of construction, such as the scheduling of the steel reinforcement. All the reinforcement was fixed as the formwork rose and consideration was given to ensure that the laps in the vertical reinforcement were staggered to maintain a consistent work load for steel fixing. In addition. to enable manual handling bar lengths were scheduled in terms of weight and practical lengths.

The slipform was planned over ten months with the complex shape requiring significant temporarv works The bunker walls below ground level required the use of a single sided slipform. This presented all the inherent problems of single-sided formwork with the added complication that, being slipformed, it moved. The temporary works design used a quantity of forty five 12m long running guide beams fixed at the bunker base to provide support for the single-sided slipform formwork. As the formwork reached cap beam level, the guide beam fixings were removed and replaced below the formwork to allow it to pass.

The slipform commenced in August 2006 and was successfully completed four weeks later, taking one week longer than planned but still one month ahead of the contractual programme

Post-slipform works

On completion of the waste bunker structure the follow on works were planned using precast concrete manufactured by Macrete in Northern Ireland. This strategy has allowed the critical path to be maintained and proprietary precast ‘Y’ beams to be used to create the tipping hall floor, giving a very high standard of finish where columns and beams are exposed.

The floor on the +21m level has been constructed using proprietary Omnia formwork panels. which simultaneously created a safe working platform for construction at height.

Concluding remarks

Throughout the design and construction of the Lakeside EfW project, the properties of concrete have been used in a variety of forms to provide an efficient solution. By using bored piling and slipform techniques, both the end use and the essential construction programme were met.

Following completion of the bunker, the use of precast, reinforced concrete elements enabled the programme to be minimised and allowed safe early access to areas at height

Article courtesy of Concrete Magazine (November 2007)

Further Info

For further information, contact Peter Bishop, Head of Public Relations at:
Edmund Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk