Articles
Skye Bridge:
Celebrating 30 Years

By Darren Donaldson IEng MICE

The Skye Crossing, linking mainland Scotland to the Isle of Skye, comprises road approaches and two bridges with a combined length of approximately 2.4km. The principal structure, known as the Skye Bridge, serves as the primary connection between the Isle of Skye and Eilean Bán and is notable for its size and architectural significance. The bridge features two distinct structural components forming a continuous five-span arrangement with an overall length of about 568m.

The initial approach structure consists of two continuous spans measuring 31m and 37m, respectively. This bridge's superstructure incorporates an insitu post-tensioned concrete deck supported by two post-tensioned concrete spine beams. The main structure traversing Loch Alsh comprises three continuous spans: two back spans of 125m and a central span of 250m. Its superstructure consists of a single-cell post-tensioned concrete box girder.

Additionally, the Carrich Viaduct, often receiving less attention, connects Eilean Dubh—now effectively part of Eilean Bán due to a land bridge constructed during development—to the Scottish mainland and extends 197m. Its deck is formed by a continuous prestressed reinforced concrete beam supported by seven single-stem reinforced concrete piers.

It should be noted that while there was considerable public debate regarding tolls on the bridge, this article focuses on the engineering achievements of the bridge’s construction and its significance in facilitating access to the Isle of Skye.

Key Data

Location

A87,

Highland

Designer

Miller DYWIDAG

and Arup

Contractor

Skye Bridge Ltd.

Construction

July 1992 to

October 1995

Cost

£27 million (£57 million in 2025)

Planning

The construction of the Skye Bridge in 1995 marked the third instance in which linking the Isle of Skye to the mainland was seriously considered. The initial proposal emerged in 1938 but was abandoned due to the outbreak of the Second World War. Further discussions occurred in the 1960s as increased tourism during the 1950s and 1960s led to greater traffic, significantly straining the ferry service. Although the upgrading of the A87 to Kyle further intensified interest in improved connectivity, continuation of the ferry service was ultimately deemed more viable at that time.

In 1985, Highland Regional Council commissioned a feasibility study to assess engineering options for a fixed crossing. The findings supported the construction of a bridge, highlighting persistent issues such as prolonged queues on the ferries, particularly during peak tourist seasons. Ongoing increases in traffic rendered the ferry service increasingly unsustainable, causing substantial delays and disruptions for local businesses on both sides of Loch Alsh—a situation left unresolved even after larger ferries were introduced.

Further studies conducted by the Scottish Office concluded that a permanent crossing would yield substantial benefits. Nevertheless, financial constraints precluded the immediate implementation of such a project, postponing construction until a much later date. As a result, a plan was developed whereby a private enterprise would undertake the bridge’s construction and financing, with the provision that the company would be permitted to impose tolls on the crossing.

This was the first private finance initiative (PFI) project undertaken in Scotland, providing a good example of the benefits and consequences of such a project, by combining public and private sectors in the delivery of a large infrastructure project.

Skye Bridge - Ferries at Kyle (Pre-Bridge Construction).

The judgement of the Regional, District and many community councils was that, having a bridge built sooner but with the exchange of it being a toll bridge, so long as it was guaranteed to be limited duration for the tolls would be acceptable. In October 1989, the Highland Regional Council asked the Secretary of State to undertake a project on this basis.

The road orders were published on the 1st of Novembers 1991 and were accompanied by an environmental statement which was produced in accordance with European and Scottish legislation, designed to ensure that the likely effects caused by the construction on the environment were fully understood, so that any potential adverse effects could be avoided or reduced to acceptable levels.

It was proposed that construction of the bridge would begin in early summer of 1992, with a view to complete in the winter of 1994 or the spring of 1995.

From the beginning of the planning process, the following points would be of primary importance in the tender:

Integration in the surrounding landscape and seascape; appearance from near and distant viewpoints.
Durability.
Long term maintenance.
Environmental impact.

The Tender Process

Several potential routes for the crossing were considered, with two identified for further review. The first option was a central alignment extending directly from Kyle to Kyleakin. The second was a western alignment from Plock of Kyle to the promontory west of Kyle House, which would take advantage of Eilean Bán. Due to its higher costs and greater environmental impact, the central route was not selected; instead, the western route emerged as the preferred alternative.

An outline brief was prepared and an invitation to submit proposals was published in the Official Journal of the European Communities (later the OJEU). Over 50 companies expressed an interest in tender for what was Scotland’s first package scheme to “Design, Build, Finance and Operate” (DBFO). Ten proposals were received from six construction consortiums, including five from European companies. The choice of route and form of the crossing had been left open, but all the returned submissions proposed a bridge on the western alignment.

The six submissions were:

1. Lilley/GTM Entrepose with Babtie Shaw & Morton, with two options.
2. Morrison Construction with Acer Consultants, gave one design.
3. Miller Construction/Dywidag with Ove Arup Consultants, with two options.
4. Sir Robert MacAlpine/Monberg Thorsen with Bullen Consultants & CowiConsult, provided three designs options.
5. Trafalgar House with Halcrow Consultants, one design.
6. Norwest Holst/SOGEA with Randall Palmer & Tritton Consultants, had one design.

An assessment of the tender returns concluded that three of them were suitable for further development. In February 1990, the three consortia were invited to prepare tenders: the Miller/Dywidag joint venture, the Morrison Construction Group and the Trafalgar House Offshore and Structure Limited.

On behalf of the Scottish Office and the Highland Regional Council, JMP Consultants Limited with consultation of both parties, prepared a detailed performance brief which was distributed to the three shortlisted consortia. This brief, in addition to technical, contractual and financial matters, focused specific attention on the environmental impacts of the construction, placing a priority on managing environmental effects.

During the tender period, consultations were held with environmental bodies, the Royal Fine Art Commission for Scotland, the National Trust for Scotland and the Countryside Commission for Scotland in addition to local Community Councils. Skye and Lochalsh District Council and Highland Regional Council. The comments received from these consultations assisted the tenderers in developing their proposals further.

Tenders were formally submitted in November 1990 and the views of the Royal Fine Art Commission for Scotland, the Countryside Commission for Scotland and the National Trust for Scotland among others, were again sought through several presentations and discussions before, in April 1991, the Miller/Dywidag submission was announced as the preferred scheme.
Submission 1 withdrew during the tender periods. Submission 2 the Cable stayed bridge was rejected due to financial difficulties within the DBFO agreement, along with discussions over its visual appearance and the total cost.

The draft road orders to allow work to commence were published on 1st November 1991. The Scottish office entered an agreement with Skye Bridge Ltd. to design and build the crossing through a fixed price contract with Miller-Dywidag. The developer would then operate as a toll bridge to recoup the costs of construction; the tolls should be no more than the prices on the existing ferry service.

The Finalists

Submission 1 - Trafalgar House Offshore & Structural Ltd

Engineering Consultants: Sir William Halcrow & Partners
Financial Advisors: The British Linen Bank & Peat Marwick McLintock
Design: Steel Haunched Box Bridge

Submission 2 - Morrison Construction

Engineering Consultants: Acer Consultants
Financial advisor: The Royal Bank of Scotland & Caisse Nationale de Credit Agricole
Design: Concrete Cable Stayed Bridge

Skye Bridge - Concrete Box Girder Option.

Submission 3 - Miller Construction & Dyckerhoff & Widmann AG (Dywidag)

Engineering Consultants: Ove Arup & Partners
Financial Advisors: Bank of America
Design: Prestressed Concrete haunched box bridge

Design

As outlined in the introduction, the new crossing linking the Isle of Skye to the mainland extends over 2,400 metres and comprises approach roads as well as two bridges.

The Skye Bridge

The principal crossing to Skye is the renowned Skye Bridge, whose design was selected by the Miller/Dywidag joint venture as a continuous structure comprising two distinct structural elements. The bridge deck, in both forms, accommodates a single 7.3-metre-wide carriageway with 2-metre footways on either side.

The initial structural configuration of the bridge, commencing on Eilean Bán, comprises two spans (spans 1 and 2) formed by a continuous post-tensioned concrete deck with dual post-tensioned spine beams measuring 31 metres and 37 metres respectively. The substructure supporting these spans includes a bearing chamber anchored by a reinforced concrete abutment on Eilean Bán, as well as two reinforced concrete piers; the second pier also serves as a support for the subsequent primary section of the bridge.

The second structural component of the Skye Bridge encompasses three major spans positioned above Loch Alsh, traversing an approximately 400-metre tidal channel. This segment consists of two 125-metre back spans and a central span of 250 metres, constructed using the in-situ cast balanced cantilever method with travelling formwork.

The superstructure for this portion features a single-cell, post-tensioned, in-situ-cast concrete box girder. The depth of the superstructure transitions from 12.5 metres at the piers to 4.7 metres at mid-span of the centre span, and further reduces to 3.25 metres at the terminal supports.

The substructure for the main section of bridge consists of two piers comprised of grade 60 reinforced concrete, build directly off the strong Torridonian sandstone which permitted a flat foundation beneath the piers. Cast insitu within 17m, diameter vertically post-tensioned hollow caissons, each founded at a depth of around 12m below the water surface.

An interesting problem that was mitigated in design was the issue of long-term shortening of the superstructure, caused by creep, concrete shrinkage and prestress losses. If the superstructure were too rigidly restrained by the piers by being too stiff, the piers would generate undesirable horizonal forces and bending stresses in both the piers and the deck. The solution the designers employed was to minimise the stiffness of the pier legs and increase the free length of the piers to 27.9m, by founding them on the foundation slabs and leaving a gap between the piers and the mass concrete with the caisson.

The gap between the pier and the mass concrete of the caissons is filled with a bentonite clay, which prevents the space from filling with salt water, while also allowing the piers to be socketed into the caissons to reduce the stiffness of the piers as mentioned above.

At the Skye end there is the second abutment in the form of a reinforced concrete bank seat arrangement. Due to the nature of the concrete box design, there is a hollow walkable area that runs the whole length of the three main spans of the bridge. This hollow area is to reduce the total dead load of the bridge by creating a cavity where there is no need to structural capacity.

This walkable area allows for inspectors to routinely check the internal concrete surfaces from cracking and for the heads of the post-tensioned cables to be accessed if they need to be broken out and checked. Interestingly the two pier heads have walls which climb high within the structure, likely to provide lateral rigidity, it creates the requirement for a ladder and platform to traverse between each span, with an internal chamber between the pier head walls creating 5 internal sections within the box girder.

The development in the construction of concrete bridges of this type in the specific features of the Skye Bridge can be best seen when compared with similar bridges built in Germany before the construction of the Skye bridge. The first is the Bendorf Bridge over the Rhine built in 1962 and the second is the Schweich Bridge over the Mosel built in 1972.

To accommodate the longitudinal displacements due to shrinkage, a movement joint was located at mid-span in the case of the Bendorf, a movable bearing under a pier in the case of the Schweich. The avoidance of such joints or bearings in the exposed marine environment was an important feature of the Skye Bridge design and only possible by using a slender monolithic frame structure. From the first preliminary design to the last construction drawing, the design of the Skye Bridge was a great challenge for everybody concerned and it has pushed free cantilever concrete bridge construction to a new limit.

Skye Bridge - Aerial View of Construction (1993).

Carrich Viaduct

The second structure which forms the Skye crossing is the 197m long Carrich Viaduct, which connects the mainland of Scotland to the small island of Eilean Dubh by crossing a 170m tidal channel (once adjacent to Eilean Bahn).

The Carrich viaduct comprises eight continuous simply supported spans, the two end spans are 20.5m and the six intermediate spans are 26 metres. As with the Skye bridge, the deck accommodates a 7.3m single carriageway and two 2m wide footways either side of the carriageway.

The superstructure of Carrich is a reinforced post-tensioned concrete box girder with cantilevered edge slabs. At the edges of the cantilevered edge slabs are the longitudinal edge beams which support the aluminium vehicle restraint barrier with stainless steel anchorages.

The substructure of Carrich consists of seven reinforced concrete piers, built directly on the bedrock, each pier has two elastomeric bearings which provide the accommodation of movement between the spans of the bridge.

There are two reinforced concrete abutments at each end of the viaduct which are also built off of the bedrock at the foreshore, with rock armour revetments to protect the abutments from scour due to the tidal environment.

Construction

The Skye Bridge

The construction of the Skye crossing took around 3 years to complete. At the time of the bridge’s completion, it was the world's longest span balanced cantilever bridge.

The first construction operation for the Skye Bridge was casting the foundation caissons, these were hollow open top cylinders, which would eventually form the marine supports of the main bridge.

Manufactured at the Kishorn dry dock, the caissons weighed around 2,300 tonnes each, the dock is a deep natural harbour in Loch Kishorn, it was last employed during the 1970s when Howard Doris used the facility to construct the Ninian Central Platform, at the time the largest concrete oil production platform in the world. The approach by road to the Kishorn Dock was more than 31 miles but the route by sea to the bridge site was only 10 miles.

Interestingly the caissons built for the Skye bridge were named. The northern one was called Lena, after the daughter of one of the German Engineers on the project. The other is called Janette, after a local student who was employed by Miller/Dywidag at Kishorn during construction.

While the construction of the caissons progressed at Kishorn, the seabed at Lochalsh was being prepared to receive them. Initially, a dredger moved bulk rock, and a team of divers completed the final trimming and removal of loose material by hand.

The divers worked in harsh conditions, at depths of 15 metres and with currents of up to 5 knots which made work at times impossible. Having cleared the seabed, the divers installed concrete landing pads using underwater tremie techniques, a special concrete mix was developed to avoid segregation of the concrete in water. Because of the water depths, divers were only permitted to work for one hour under water at a time before returning to the surface. Teams often worked round the clock, diving during the darkness of night and using underwater floodlighting to guide them.

Skye Bridge - Carrich Viaduct Construction.

During spring 1993 the caissons were towed to site and secured on the seabed. This operation required specific tidal conditions and a suitable weather window to be undertaken safely and successfully. The caissons had to be placed on the seabed in a precise and demanding operation which required the structures to be lowered on to the landing pads, using the natural tidal conditions in the Kyle of Lochalsh, to within a tolerance of 100mm. The team managed to place them within 30mm in an operation lasting 24 hours. The placing of the caissons marked the first of many milestones in the construction of the Skye Bridge.

The next scheduled activity was the construction of the piers above the foundation caissons to the underside of the main deck. This was done in sections or 'lifts'. The formwork (shuttering), which formed the mould for the concrete, was raised as each section was completed. In total 620 cubic metres of concrete were poured to construct each pier.

At the same time, auxiliary or temporary piers were being constructed, these would perform a vital function in supporting the bridge deck and provide stability during the superstructure construction, resisting the overturning moment caused by the chosen method of construction, known as 'free cantilevering', it involved building the deck of the bridge out from the supports one section at a time, alternating construction to each side of the pier. Theoretically, this could have rotated the pier table, and the auxiliary piers were necessary to prevent any movement during construction.

The construction of the superstructure began in winter 1993. Pier tables - the section of deck immediately above and spanning the permanent and temporary piers were constructed in four separate concrete pouring operations, two for the base, one for the walls and one for the deck. The concrete had to be pumped 35 metres vertically to where the construction was taking place, up to 300 cubic metres of concrete were poured at any time, in a continuous operation. The pier table structures were designed as reinforced concrete with steel as dense as that used in the construction of a nuclear power station.

The special design features incorporated to enhance the Bridge's aesthetic appearance presented a further challenge. Incorporating these features into the building of the structure required a high degree of control and quality of workmanship.

The technical demands of the construction of the superstructure were a challenge to the highly skilled workforce, but the construction team also had to endure an unpredicted extraordinarily harsh winter. The construction area at that time was only accessible by sea. The high winds, driving rain and rough sea states put an added strain on an operation which was dependent on vital supply line logistics.

The piers, however, were completed within the revised programme in the Summer of 1994 and work commenced on the free cantilevering process which would create the dramatic arch which now spans the Kyle of Lochalsh. The deck of the Skye Bridge grew from each pier table section by section, using two pairs of form travellers to support and form the in-situ concrete bridge deck.

In total there were one hundred and seven pours of concrete, the largest being over 100 cubic metres. The segments varied in length up to five metres. The first segment was poured on 19th of July 1994, and the centre and final section was poured on 29th of July 1995. A degree of rivalry arose between the two teams constructing the north and south sides of the deck as they competed to see which side could complete a section in the shortest possible time.

During the late summer of 1995, the final finishing activities such as footpaths, surfacing, street lighting and landscaping were completed to allow opening to traffic by the Secretary of State for Scotland.

Skye Bridge - Aerial of Construction (Winter 1994/95).

Skye Bridge - Nearing Completion - View from East.

Carrich Viaduct

The construction of the Carrich Viaduct began with the piers which were prefabricated on a temporary jetty on Skye. It had been proposed that these piers should be cast in position in the sea, but the hostile weather and sea conditions persuaded the constructors that there was a better option. It was to first create the piers and then drop them into place. Again, divers prepared the seabed to receive the piers, and all seven piers were lifted into place within one week during September 1993. The piers weighed up to 380 tonnes and were up to 22 metres in height.

Work on the deck of the viaduct began in October 1993. The segments which would form the deck for the bridge were cast in a covered facility on the Plock of Kyle and incrementally launched over the seven already constructed piers. Each segment was 15m long and required 130 cubic metres of concrete in construction. The operation took place between October 1993 and May 1994. However, the covered working environment provided ideal working conditions even during the rigours of awest coast winter.

The work on the island of Eilean Bán and Eilean Dubh proved to be extremely sensitive because the islands were known to be otter habitats. Extensive research had been carried out to identify where otters lived on the islands and their traditional runs. This research was used to determine how construction on the island should proceed and to incorporate any otter protection measures into the design of the roadway.

During construction, one otter holt which lay in the line of the road had to be relocated. Otters are itinerant creatures, often living in up to 15 different holts. To ensure that this habitat was not occupied at the time, a radio, playing music was left by the holt for 24 hours before work began.

The causeways also incorporated five special otter tunnels. The research had identified all otter habitats and tracks, and the tunnels would allow the otters unimpeded access along traditional routes. One thousand seven hundred metres of dry-stone wall was built to keep the otters away from the main carriageway.

Skye Bridge - Nearing Completion - View from the West (1995).

Environmental Considerations

The Skye Bridge project marked a shift in major Scottish construction projects toward environmental responsibility. Though environmental concerns are now central to such projects, this approach became formally recognised by the Institute of Civil Engineers only in 2007, when it redefined civil engineering as essential for both societal well-being and sustainability.

From the outset, environmental preservation influenced tender applications, given the area's internationally important scenery and wildlife. Miller/Dywidag engaged Holford Associates to manage environmental, architectural, and landscape factors, resulting in an Environmental Statement covering noise, visual impact, ecology, archaeology, and community effects, with input from multiple consultants and Scottish Natural Heritage.

The joint venture developed measures to protect rare plants and the local otter population, including temporary fencing for flora and significant interventions for otters. Otter tracks intersected the road, so tunnels, new holts, and pools were built to minimise disruption and maintain their habitats.

To reduce post-construction otter mortality from traffic, several mitigation methods were devised in consultation with stakeholders. Simultaneously, efforts focused on minimising the visual impact of safety features; a carefully designed stone wall was selected as a multipurpose solution, using local materials to blend with the environment and reduce metal barrier use.

Tolling Controversary

There was widespread support for the construction of the Skye Bridge, however the charging of tolls to cross it was universally unpopular. The Scottish Office hoped that a charge in line with the cost of the ferry service would mean that bridge users would be no worse off than before. The revenue generated by the tolls would also be used to pay back the loans borrowed to pay for its construction. What hadn’t been considered was the strength of local opinion!

The local community felt that the charges being imposed on them were unjust, and it was said that based on the length of the bridge, the tolls were the highest of any bridge in world. Tolls were levied in each direction with cars charged £11.40 (equivalent to over £22 today) for a round trip. Large HGVs were charged almost £60 (over £110 today).

Shortly after the bridge’s completion, a group called SKAT (Skye and Kyleakin Against Tolls) was set up to oppose the charges. Through campaigns, protests and non-payment it was their aim to see the tolls removed. Non-payment saw several of the campaigners given criminal convictions and their protests and the surrounding police action became a very high-profile event.

Ultimately their campaign was successful when the bridge was bought back by the Scottish Government for £27 million in late 2004. The tolls were abolished on the 22nd December of that year leading to much local celebration.

The Bridge Today

The Skye Bridge was officially opened on Monday 16th October 1995 by the Rt Hon Michael Forsyth MP. This date also marked the end of the ferry service between Kyle of Lochalsh and Kyleakin.

The Skye Bridge and its approaches are now owned by the Scottish Government and its operation and maintenance is managed by Transport Scotland.

It is estimated that more than one million vehicles use the bridge each year, confirming its importance as a key link for local people and visitors in the Scottish trunk road network.