WEATHERING THE STORM

Construction workers are a tough breed, and those employed on the controversial Skye Crossing have faced more than the usual tests of strength over the past couple of winters. They have battled on, however, often around the clock, while buffeted by lashing rain and gale-force winds.

Inclement conditions during the past two winters have caused more than a few problems for the joint venture contractor, comprising Miller Civil Engineering and German firm Dyckerhoff and Widmann (Dywidag). It is now about six months behind its original schedule and praying for a mild winter.

The Scottish/German jv is building the 2.4km crossing, which joins Skye to the mainland to the west of the existing ferry route, under a Design, Build, Operate and Transfer (DBOT) contract. Employed by the jv, Dywidag's sister company, Dywidag Design, is responsible for the design.

The northern end of the works, a 200m-long low-level bridge linking two adjoining islands to the mainland at Plock of Kyle, and a section of new road from here to Kyle of Lochalsh, is progressing well. The contractor managed to position all seven sets of cylindrical piers for this bridge, now called the Carrich Viaduct, in just one week using a floating crane.

The initial design called for in-situ piers, but this was abandoned in favour of the pre-cast units which were secured into pre-excavated sockets using stressed anchors. The prestressed concrete deck was then incrementally or continuously push-launched from a shed on the mainland side of the channel.

The main feature of the crossing is a 500m-long (570m including the northern approach ramp) balanced cantilever concrete box girder bridge that crosses the main navigational channel and links the aforementioned works to Skye. With a clear span of 250m, the bridge will be the longest of its kind in the northern hemisphere. It is also the most weather-sensitive component of the project.

In June 1992, the contractor took over the dry dock at the Kishorn pre-casting yard 15km away and started constructing the two 17m-diameter, 14m-high caissons for the main bridge. This was six months after signing the contract for the crossing.

A public inquiry set the programme back about three months from the start as the National Trust for Scotland insisted that the design of the bridge be altered to minimise its visual impact. Each deck unit, originally designed as a plain box, now has an indented profile near the soffit, pointing inwards by a short 7.5¡ section and then splaying outwards by 45¡, in a bid to make the deck appear shallower.

The 107 separate deck units reach their maximum thickness of 12.5m at each pier, thinning towards the centre and at each end. This, together with a cast-in-situ ribbing, is meant to make the entire structure look like two giant seagulls in flight.

It also created a big headache for the contractor, as standard adjustable shuttering and falsework was no longer suitable for casting the decks units.

But the contractor's first main challenge, to construct the two 28m-high bridge piers presented its own set of problems. The 1,700 tonne units were completed on time at Kishorn, in September 1992, but not towed to the site until the following spring. The necessary preparation of the sea floor was hampered repeatedly by rough sea conditions.

The main site compound for the bridge is located just to west of Kyleakin - from where the ferry crosses the stretch of water known as Kyle Akin - in a gravel quarry operated by Redland.

The contractor set up offices here in September 1992, but had already started drilling and blasting for the 7m-deep main pier foundations in July. This, however, was three months after the initial April start date, thanks to the public inquiry, and meant that only six to eight weeks remained before the end of the planned marine works window.

Excavating the hard Torridonian Sandstone was quite a challenge in the choppy 20m-deep water of the channel. Divers working on the foundations were frequently hindered by currents of up to 5 knots.

Additionally, geologists had to check that the blasted surface of the heavily jointed rock was free of fissures and capable of supporting the 2,300 tonne weight of the finished caissons.

The delayed start meant that dredging had to be carried out in the winter period, with the contractor working right through the planned three-month shutdown.

'We procured one of the biggest backhoe excavators available and managed to get the bulk of the dredging completed by the end of November,' says jv projects manager John Henderson.

The north caisson was eventually placed in March 1993 and its southern counterpart in June, during periods of improved weather. Each was lowered onto three pre-positioned steel and concrete landing pads and externally rock-anchored.

'Once in place and anchored down we were able to start building the piers immediately, using a climbing formwork, in a dry, stable environment,' says Henderson.

Miller/Dywidag spent last summer building the piers and worked on through its second proposed winter shutdown. Downtime, because of bad weather averaged 40% in the early part of this year, and more than 60% in March, and the contractor has worked around the clock throughout this summer.

The tower cranes that serve each of the pier tables provides the only access for deck erecting equipment and these 'cut out' when the wind speed reaches 35 to 40mph.

The contractor must have a clear six-hour wind-free period in order to jack forward and temporarily stress down the twin rail-mounted steel frame gantries from which the falsework and shuttering for each segment is suspended. Movement of the gantry at each pier top is forbidden when wind speeds exceed 33mph.

Fortunately, the weather conditions have been good since early summer and a total of 14 deck units have now been formed, 10 from the northern pier and four from the southern.

The Peri shutters are lined with a special wood-effect rubber formliner which provides the required ribbed finish. The contractor trims them down with each successive pour (to produce the progressively shallower deck units). The splayed section is provided by timber box cut-outs, each one of which is different to the next, and these are made in a workshop on the site.

Temporary concrete props at each pier are designed to take the out-of-balance moment of the structure as the segments are cast and stressed.

Each segment is cast in a single 90m3 pour, using high, early strength 60N/mm3 concrete, which is batched on site and pumped to the pier tables from truckmixers transported by two former landing crafts that were previously owned by the German army.

The length of the segments varies with its depth, ranging from a minimum of 3.25m and gradually increasing as the deck height decreases, to a length of 5m.

If everything goes to plan, the two deck sections should meet in about May of next year, says Henderson.

'The final gap will be the width of one of the travelling forms, so we will just remove one and complete the structure,' he adds.

A calm day will be needed to complete the final stressing run, after which the dense network of tendons that have been used to stress back successively formed units will become redundant.

The finished bridge will have a 30m navigation clearance at high tide and is designed to withstand the impact of a 10,000 deadweight tonne vessel impact, in line with Scottish Office requirements.

It is also designed for seismic events of up to 5 on the Richter scale, and events of a magnitude of about 3 are fairly common in this part of Scotland.