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Fit for a king

A new visitor centre and car park on the shore of Loch Ness requires a concrete structure to be built within a new embankment. Margo Cole reports.

The picturesque ruins of Urquhart Castle have always been popular with tourists travelling in the Highlands of Scotland. But parking space is very limited on the site, and many just pass by when they realise they have no chance of finding a space.

That will change this autumn, with the opening of a 114-space car park and coach park.

Tourists who stop will discover even more improvements in the shape of a visitor centre containing a restaurant, shop and audio visual theatre.

Dating back more than 600 years, Urquhart Castle sits on a rocky promontory flanked on one side by Loch Ness. On the other, a meadow at the base of the rocky outcrop leads to a steep hillside with the A82 trunk road cut into it. The existing car park, accommodating just 40 cars, sits alongside the road. Historic Scotland, which owns the castle, held a design competition in 1994 for the new car park, and asked engineers to indicate where they thought a visitor centre could be built.

The winner was Invernessbased consultant Ken Wilson Associates, with a plan to build a parking platform on top of a new embankment founded on the slope below the existing car park. Mindful of the client's desire to minimise impact on the castle, the engineer proposed making the slope of the embankment as steep as possible to prevent too much encroachment on the meadow.

Local opposition led to a fouryear delay and a public inquiry, with Wilson's design eventually approved in 1998. The agreed location for the visitor centre was within the embankment, with the car parking platform forming the building's roof.

The two-storey structure has its public facilities on the ground floor and plant room and storage in the basement. Entrance from the car park is via an elliptical rotunda at roof level.

Both visitor centre building and embankment have been designed as if they were highway structures. Wilson explains: 'The main loading on the building is the earth pressure from the embankment and traffic on the top. We designed it as an integral bridge structure because the roof has to be designed for road loading and the sides to take the earth pressures. That makes it a civil engineering structure.'

Ground beams formed in rock trenches cut into the underlying weathered sandstone support an insitu concrete basement slab.

Insitu beams and columns make up the main frame. All the visible columns on the ground floor are circular and are 600mm in diameter. At basement level, where there will be no public access, the columns are 500mm square.

The ground floor is made of 200mm deep precast, prestressed concrete units spanning up to 8m one way. A structural reinforcing mesh on top of the units is covered with a 100mm thick screed, which in turn is topped with another 120mm thick layer incorporating an underfloor heating system.

The floor finish consists of 25mm Caithness stone slabs in most areas, with timber flooring in the restaurant.

Exterior roof and walls form an integral box structure and have been designed in much the same way as an integral bridge - without any movement joints - and are formed from insitu concrete. Although the walls are only 300mm thick, the box structure is stiffened by key elements incorporated in the structural frame, such as the basement floor and the 600mm deep main beam at ground floor level.

Further stiffening is provided by additional external columns, the rotunda structure, main stairs and fire escape stairs.

As the structure was designed as an integral box, it cannot carry the loading until all the key structural elements have been completed. Contractor Morrison Construction was therefore required not to backfill around the structure until these elements are in place.

Most of the building will eventually be buried beneath the new embankment, leaving just a curved facade at ground level opening out on to a small terrace formed in the new fill. Wilson originally adopted a 'belt and braces' approach towards waterproofing, opting for an external membrane in addition to relying on the structure itself to keep water out. However, the membrane was value engineered out to meet the client's tight budget, so the concrete walls now form the sole barrier.

A C40 concrete was specified, enhanced with a Sika 1 waterproofing additive. This should ensure watertightness, but there is also a system for preventing leaks at the construction joints in the walls. Every joint contains a length of hose with strips of hydrophilic material contained within it. If the hose gets wet, the material will expand to fill the gap. The hollow hose - part of the Sika Injectaflex system - can also be injected with a waterproof compound to provide extra protection if it is needed.

A drainage membrane around the walls links to an extensive drainage system designed to stop build-up of water. This - together with land drains incorporated in the new embankment - should ensure the building structure does not have to withstand loading caused by excessive hydrostatic pressure.

Morrison is sourcing the concrete from a Bardon plant at nearby Beauly, using a mixed gravel aggregate consisting of granite, quartzite and microgranite. Project manager Raymond MacKeddie says: 'The concrete mix design needs to be right, and it's quite high strength. It hasn't got a very long shelf life - probably because of the super-plasticiser that makes it more workable. It means we have to be very regimented about taking the cubes and then using it as soon as it comes in.'

Most of the pours have been relatively small - only 12m crane. But the roof pours are up to 70m 3, so Morrison has brought in a concrete pump for those.

MacKeddie admits to finding the rotunda a challenge. 'That thing was sent to try us, ' he jokes. 'Everything on it is a oneoff.' It was built in 3m high lifts, using a curved formwork system from EFCO.

The main visitor centre structure is nearing completion, and Morrison has laid just over half of the 80,000m 3material needed for the new embankment. Once the building is weathertight, the internal fit-out will start, ready for an autumn opening.

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