RWE npower, one of the UK's largest electricity suppliers, has purchased two 40-tonne ADTs from Bell Equipment UK, for use at its Aberthaw Power Station near Cardiff.

But these machines are rather different from the standard Bell B40Ds that you will see in quarries and construction sites across the UK.

They have been specifically designed for the unique site requirements at Aberthaw - and are currently the only two of their kind available in the world.

Bell was asked to provide a solution for collecting and transporting thousands of tonnes of water-conditioned ash from three power station silos, despite the silos being too low for a typical 40-tonne truck to drive beneath.

The ADT manufacturer was also asked to come up with a viable method of regularly dampening down any loose ash on site to assist with the power station's strict environmental controls.

The answer was to design an articulated dump truck with a unique but operationally practical dual function. Bell achieved this partly by supplying a custom-designed water tank unit, complete with high-powered water cannon, which could be fitted onto the back of either truck in less than two hours. This can be used to spray water anywhere on the site.

But Bell still faced the challenge of ensuring that the ADTs were capable of clearing the height beneath Aberthaw Power Station's silos.

The power station was built in the 1970s, meaning that it was originally designed with an older generation of transportation in mind. Bell's standard B40Ds measure more than 3.8m in height, but they would need to be just 3.2m to clear the height of the silos.

Gareth Cribb, Materials and Handling Section Head at Aberthaw Power Station, said: "Bell Equipment has come up with an excellent solution to the height restrictions that we were faced with, ensuring that the machines can now drive beneath the silos to collect the water-conditioned ash from the plant.

"I have been impressed with the way Bell Equipment has approached the unusual design brief. In addition, the machines are incredibly reliable and the after-sales support has proved invaluable."

Bell engineers approached the unusual brief from several angles. Firstly, they reduced the height of the bin. But to compensate for this, the trucks' chassis was lengthened in order to retain the same maximum payload as the standard B40D.

As well as lowering and lengthening the bin, they fitted the Aberthaw machines with smaller 26.5 tyres rather than the standard 29.5 size. And the cab was designed with reduced headroom and modified floor plate height, similar to the type of cab used on Bell's underground mining trucks.

Neville Paynter, Managing Director of Bell Equipment said: "We pride ourselves on having a strong reputation for engineering excellence and we lead the field in areas such as fuel efficiency. We are pleased to be working with RWE npower at its Aberthaw Power Station as the experience has provided us with the opportunity to find a specific solution to a series of operational challenges".

When first approaching the project, it was questioned as to whether it would be more practical to buy smaller standard trucks or to raise the height of the silos. However, Gareth Cribb explained the reasons why Aberthaw opted for the bespoke designed 40-tonne trucks:

"We have carefully considered the logistics of the project and decided that going with the bespoke trucks provided the most viable option," he said.

"When the power station was originally designed in the late 1960s, trucks were significantly smaller than they are today. However, it makes more sense to opt for larger trucks and reduce our fleet size. This enables us to move more ash and, in turn, keep operational costs down."

He added: "Bell Equipment has ensured that the water bowser can be fitted separately to either of the new ADTs. This ensures that we are able to obtain maximum efficiency from the Bell machines and, if needed, both trucks can be used to haul ash at times when the site dampening function is not critical."

The weather conditions determine the Bell water bowsers' level of usage. However, the flexibility offered by the water bowser enables it to be fitted or removed in a very short space of time, playing a crucial role in the control of ash at Aberthaw Power Station.

Each Bell ADT at Aberthaw transports around 15,000 tonnes of ash every month, equating to more than 180,000 tonnes per year.

In order to sustain energy production at Aberthaw, the trucks need to maintain an almost continuous cycle of 5-10 minute hauls from the three silos, which are each capable of holding up to 1,500 tonnes of ash.