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Nuttall gets tight with Japanese innovators

PLNT

Concerns over fossil fuels and a shortage of land ll space have prompted much thought on new ways of treating waste. An innovative power plant being built in Berkshire to address this is providing Nuttall with rather more down to earth challenges in engineering and logistics.

Alasdair Reisner reports

YOU HAVE to admire the simplicity of it. A new power plant being built for waste firms Grundon and Viridor at Colnbrook in Berkshire is a stone that kills two birds. Not only will it burn thousands of tonnes of refuse every year, reducing the requirements for such waste to be landfilled, but the energy produced will be harnessed to produce electricity, reducing the requirements of the national grid to use fossil fuels.

But although this provides a neat solution to environmental issues, it also presents a challenging engineering task for Japanese joint venture Itochu/ Takuma, which is main contractor for the £180 million project, and Nuttall, which has a £32 million package to carry out civil engineering works.

The firms have to work together on a very tight site.

With up to 400 workers at peak this calls for a rigorous approach to logistics to ensure that the companies are not stepping on each other's toes.

'At the moment we have weekly co-ordination meetings with Takuma, ' says Nuttall project manager Peter Golden. 'As we have more workers on site we will have to have daily meetings to ensure that we don't clash. Those meetings will have to make absolutely sure that everyone knows where everyone else is working.'

During the early stages of the project Nuttall has the site largely to itself. The firm is responsible for providing a building into which the Japanese joint venture can then install the highly technical boilers and equipment required to make the plant function.

One of Nuttall's main tasks is to build a 32 m tall bunker with walls 600 mm thick, rising f rom 11 m below ground level, to store the waste prior to burning.

In order to build it the firm has drafted in Austrian slipforming experts Gleitbau.

'The usual way to build these bunkers has been to install a secant pile wall with temporary propping and then build a steel frame in two or three phases. You would then have to shutter up to the underside of the steel frame and wait for the concrete to go off, ' says Mr Golden.

Instead, Nuttall has chosen to build a capping beam at ground level, which measures 4 m by 2 m and surrounds the bunker. It is heavily reinforced with T40 rebar and will require the use of 1,000 cu m of concrete.

'That means we don't have to have any props so we can dig straight down to the minus 11 m in a single hit.

We can then slipform from the ground floor minus 11 up to ground f loor level with a single sided slipform, ' says Mr Golden.

On reaching ground level the team will pick up a second outer shutter and carry on up to the 21 m level.

Mr Golden says the decision to use slipforming rather than secant walls was dictated by tight programme requirements, with Nuttall having to hand over the ground f loor slab for the boiler hall by the end of its 14 months on site.

'It's a decision purely on time. If we went for a traditional system we would never have achieved that deadline. If we had used props in the basement we'd have to tie the walls back to the ground f loor slab for 10 m either side of the piles before we could remove the props, ' says Mr Golden.

'By putting in the capping beam we don't need to tie the ground floor slabs, which means we can build it in total isolation and are not reliant on the ground floor slab being completed before we can proceed with the walls.'

The jacking system is designed to rise at a rate of 35 mm every four minutes using a system of hydraulic jacks. Previously slipforming presented problems because only relatively small panels could be used to form the concrete, leaving the resulting concrete with streaks marking the overlap of the plates. This was where the Austrians' expertise was important.

'This project uses much longer sheets so we don't have score marks on the wall. Gleitbau has already done four or five towers like this in Europe. To them this is bread and butter, ' says Mr Golden.

The complete slipform pour will run continuously for 12 days. But the speed of slipforming will slow at certain points such as when the former reaches ground level and the external shutters are attached, or when casting inserts are put in place.

'The concrete will be quite high cement content but we will be adjust ing this when there is call to slow down the process. The concrete mix will control the speed, ' says Mr Golden By the time the concrete emerges from the bottom of the slipformer it should be around four hours old and should have started to go off. Cubes are taken to ensure its integrity but Mr Golden points out there is also a rough and ready test to ensure it is looking right.

'When it is coming out of the bottom you try to push your finger into it. You shouldn't be able to. You should just be able to leave a nail print.'

The final finish of the concrete needs to be as close to perfect as possible as any imperfections will cause waste and dust to hang up in the bunker.

The team is currently installing the piles the building will sit on. It gives an idea of the massive scale of the plant that more than 1,600 piles will be required. Of these around 450 will be needed in the bunker area.

'We have continuous flight auger piles going down to 23 m to take the compressive load. There are also tension piles that are in the bottom of the bunker, ' says Mr Golden.

'They have gone down 40 m because you have 30 m of pile and 10 m of open bore that is lost when the bunker is dug out.

'There are 67 tension piles that stop the structure floating due to water pressure between the slab and the ground.'

Nuttall completed piling by late June and the ground slab is t imetabled for complet ion by November in order to leave time for installation of plinths and holdingdown bolts for the process equipment.

The firm will also be building the external envelope of the plot, starting at the end of the platform where it has installed the bunker and tipping hall.

This leaves extra time for the Japanese to install the process plant before the frame and cladding continues over the top.

The plant is due to open on June 28 2008, the end of a 33-month programme that the construction team have planned to the letter. Mr Golden has an added incentive to ensure the works are finished on time.

'On June 29 my pass to get on site will no longer work , ' he grins. With such an exact ing approach to logistics you would expect this not to be a problem.

Slowly does it

THE COLNBROOK plant has a history that goes back 15 years. In 1991 Grundon looked at building an energy from waste plant but the high cost deter red it from tak ing the plan forward. A recycling facility that still operates on another section of the site was built instead. But in 1999 the firm submitted a planning application for the new plant.

Through a concerted effort at community liaison, keeping locals informed about the implications of the plant, planning was granted in 2000 with just eight letters of opposition. Despite this, it still took until last year to get the finances to stack up, helped by the rising cost of landfill. The firm joined forces with rival Viridor in a 50/50 partnership called Lakeside Energy f rom Waste to build the plant.