A new visitor centre in a disused gravel quarry off junction 31 of the M6 might seem an odd concept in itself, but the differences don't stop there - the designers of the facility at Brockholes wetland nature reserve plan to float the buildings in the middle of a lake to give visitors the best possible view of the waterfowl. Adam Khan Architects, who in 2008 won an RIBA competition to help the Lancashire Wildlife Trust, came up with the pioneering concept of the five barn-like structures that will sit less than a foot off the water and provide visitors with a unique watery experience. Floating homes are common in the Netherlands, but in the UK they are rare and the Brockholes project is on an altogether bigger scale than any Dutch domestic development. The initial idea was to create an archipelago of five “islands” to support an education centre, conference facility, cafe, exhibition space and shop, but Khan says there were problems, including maintaining level access between them, and cost. Instead, a single floating deck is being created and the single-storey, timber-frame pavilions will sit on top of this.

The pontoon has been designed to take a live load of 2kN/m2 which equates to around 5,000 people. As the buildings only allow for a maximum of 400 visitors this gives a large factor of safety and even if a group of people should suddenly rush to one corner of the visitor centre the structure should remain stable. In the final design the pontoon will float 300mm higher than the water level - 200mm to allow for the defection from the live load and 50mm for tolerances in the construction, leaving a minimum “freeboard” of 50mm as a factor of safety.

The pontoon is essentially a large concrete box with a series of internal walls that provide stiffness. Because of its size - together with buildings it will weigh in at 5,700 tonnes - it is being built in situ. It is divided up into a number of “super cells” created by the base, outer walls and a number of the key internal walls that provide stiffness. These were built traditionally using temporary formwork.

Each of these super cells contains 14 further cells. When it came to constructing these, the process was speeded up by using dense polystyrene blocks that sit in these cells as permanent formwork. The 3.6m x 2.4m x 1.8m polystyrene blocks were dropped into place, then reinforcing steel was positioned in the voids between the blocks and concrete pumped in.

The volume of concrete used to build the pontoon is slightly more than a traditional foundation, but the type of concrete used came under scrutiny because the team wants to achieve a BREEAM “outstanding” rating. The concrete specification explored the use of a low cement content and recycled aggregate to help hit the target of 25% recycled materials on site. In the end a 31% pulverised fuel ash cement substitution was used to help bring down the embodied carbon of the concrete.

Apart from the fact that the visitor centre needs to float, the Lancashire Wildlife Trust and the architects also wanted to minimise dispruption to wildlife and overall carbon footprint. To that end, the village-like cluster of buildings on the pontoon are all single-storey, lightweight timber structures which will largely be fabricated off-site, minimising waste and disturbance to surrounding wildlife.

The glulam timber portal frames are clad with structural insulated panels to help provide a well sealed, thermally efficient envelope that should outperform Building Regulations requirements. The large pitched roofs will be clad in oak shakes while timber battens will be used to clad the walls.

The services strategy for the buildings has also been designed with sustainability in mind. The glazing is optimised so that the heat loss in winter - and gain in summer - does not outweigh the benefits of high levels of daylighting. The buildings are all naturally ventilated, while heating and hot water are generated using a biomass boiler fuelled by wood chips.

All water comes from an on-site borehole while WCs are flushed using lake water, and wastewater is treated using a reed bed before being released back into the watercourse. BREEAM innovation credits are expected for the daylighting design, low-carbon technologies and smart water metering, which should help it attain its BREEAM “outstanding” target.