The spectacular new Queensferry Bridge spanning the Firth of Forth is unlikely to be finished in time for its scheduled opening in 2016, civil engineering experts have told the Sunday Herald.

The probable delay to the road bridge, which will have no cost implications to the Scottish taxpayer, comes after a 15-month hiatus caused by difficulties in setting the foundations of the fixed-price £1.5 billion structure.

The slow start to the erection of the bridge appears to have cost contractors two summer seasons out of a six-year building project, leaving them with only two years to complete the entire 210-metre three-towered superstructure. The connecting road network is largely complete.

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According to the project's original schedule, motorists on the existing Forth Road Bridge (and viewers of Transport Scotland's live webcams) should by now be able to see three giant piers rising at least 50 metres out of the Forth, at which height the suspending of 2.7km of deck sections would be imminent or under way.

Instead, only a single pier, the one built on the central Beamer Rock, is under way, rising to a height of about 20 metres.

The north and south piers remain cylindrical stumps, clearing the water by about three metres at high tide. The elegant, pencil-like towers that will support the deck via overlapping diagonal cables, have yet to emerge from vast steel "caissons" plugging the structure into the Forth's bedrock.

In December 2014, Transport Minister Keith Brown hailed the bridge's "fantastic progress over the last 12 months" and Scottish Government agency Transport Scotland, which is "closely monitoring progress on the construction programme" has told the Sunday Herald: "The FRC [Forth Replacement Crossing] project remains on schedule to be open by the end of 2016."

The agency has insisted that "the timescales for completing the project have remained the same since construction began in 2011".

But the Government body has not denied reports of problems placing the caissons, and has declined to comment on why this process of setting the foundations of the tallest structures ever built in Scotland, prior to pouring concrete, only took place in September 2013, a full 15 months after foundation work got under way, according to information initially supplied to the press.

Later, in December 2012, STV chief reporter David Cowan, following a Transport Scotland briefing, gave no inkling that the "six-week process" of setting the foundations had not gone to schedule, and claimed: "By the summer of next year [2013] the towers will be rising from the caissons."

In fact, it was to be the best part of a further year before the towers had anything solid to rise from.

Civil engineering experts all agree that the riskiest part of the entire bridge project has been completed. "Forming our underwater foundations is the most challenging part of the entire bridge build programme," Carlo Germani, project director for Forth Crossing Bridge Constructors (FCBC) told New Civil Engineer magazine in June 2012.

"We must manage a host of significant but known risks including seabed geology, deep tidal waters and fast-changing weather conditions."

But the same article confidently predicted that the foundation concrete would be poured over eight days early in October 2012: "A continuous 16,000 cubic metre underwater concrete pour, tremied down from barges, will form a 25m-thick solid plug over the bedrock".

This concrete pour - the largest ever recorded - did take place as described, but it was in September 2013 after an 11-month pause, during which specialised marine plant, including barge-borne cranes and supply boats costing hundreds of thousand pounds a week, was standing by largely idle, at untold cost to the consortium.

With much of the critical work on the bridge structure suspended while consortium member Hochtief worked to set the caissons on the notoriously treacherous and low-visibility estuary bed, the delay ate away at - and possibly exceeded - the programmed "slippage time" that the FCBC (also including Dragados, American Bridge and Morrison Construction), built into the tight six-year construction schedule.

Despite the experience and professionalism of the consortium, there is now a question mark over whether the "critical path" of the project has been critically affected.

While working under the Firth of Forth is ­difficult, working above it is not that much easier. During bad weather, the environment is akin to that of the open North Sea. In these conditions, realising the innovative, super-elegant design of architects and engineers Jacobs-Arup within two years looks a formidable challenge.

Anything but exceptional meteorological good luck means that low temperatures and high wind speed will make work impossible for weeks or months at a time.

Construction of the Forth Road Bridge from 1958 to 1964 was delayed for the best part of a year due to bad weather. The decision to press ahead in poor conditions led to corrosion problems that shortened the life of the bridge.

Having experienced Scottish weather on site since 2011, it seems unlikely that the FCBC or Transport Scotland will be counting on three long summer building seasons to make up for lost time.

The potential knock-on effects of the delay are not being spelled out to the public by Transport Scotland, either because project director David Climie believes the lost time can be made up, or because Scottish ministers and Transport Scotland, previously criticised by parliamentarians for its low-transparency culture, are restricting themselves to good news bulletins.

An arch-critic of TS is John Carson, the civil engineer who was team leader on the Skye Bridge. He is a sceptic about the case for the new Forth bridge and said: "In the run-up to the referendum the Scottish Government don't want any bad news getting out, particularly anything that questions their ability to run major projects.

"The bid involving caissons might have been the cheaper option but anyone who knows the complex geology and currents of the Forth knows that that the decision came with substantial risk. It now looks like it has backfired. Opting for caissons was bad engineering management."

FCBC was designated preferred bidder for the principle contract for the new crossing in March 2011, beating rival consortium Forthspan. The latter group was early the favourite and comprised Balfour Beatty, Morgan Est, BAM Nuttall and MT Hojgaard.

If the early-stage problem does breed further delays, Transport Scotland may regret not ­choosing the other consortium for the contract, despite the fact that its price was £300 million higher.

Forthspan's bid proposed a different method of laying the foundations that would have substantially lowered the risks of foundation-building.

Instead of caissons, the rival team would have laid the foundations by drilling three clusters of six-metre thick steel-and-concrete piles into the river bed from a "jack-up barge", a platform on solid legs like a temporary oil rig. The base of the bridge piers would then be built around the piles just above sea level

Sam Whitaker of Seacore, the Falmouth-based company that would have set the bridge foundations had Forthspan succeeded, told the Sunday Herald that anticipation of caisson problems caused them to involve his company.

Seacore has experience in the Forth, having laid the foundations for Transport Scotland's Clackmannanshire Bridge, which also shunned caissons in favour of piles driven from above water level, a method the company has used in Canada and the US.

Whitaker told the Sunday Herald: "The big plus of this method is that we stay above water, everything is done from the platform. We construct piers and you are dry-working, out of the current. The sooner you are not reliant on boats and marine work the better.

"With caissons and cofferdams, there are so many things that can go wrong not just with weather conditions, but all the loading and unloading.

"Essentially you're trying to create a nice, clean, dry place in the middle of a river, which is extremely difficult. When you are trying to pour top-grade concrete to get the best possible foundations, you constantly have the problem of waves coming over, water, silt and mud seeping in. There are fissures in rock where water will find its way in especially with the water pressure at 10-15m down."

With all the known risks in the estuary, all the years of expensive investigative preparatory work commissioned by Transport Scotland, and all of the combined global experience of the consortium members, there will be questions as to why Transport Scotland chose a construction method that exposed the project to substantial delays.

According to Whittaker it is not because caissons were substantially cheaper than the alternative, though other ex-Forthspan engineers say otherwise.

Whether and when the inside story of the early-stage problems emerges depends on whether the project can make up lost time. It also depends on whether the Scottish public now simply expects major capital projects to be delivered late, and how interested it remains in explanations when they are.

The curse of the caissons

Caissons - giant cylinders - sometimes with closed tops, are contraptions that are manhandled into water to allow dry working. They have a long and chequered history in the Forth.

In the 1580s, the ancestor of the present Earl of Elgin and Kincardine, Sir George George Bruce, sank a cofferdam or caisson to connect with his coal mine near Culross. "It was his invention and it was most ingenious," Lord Elgin told the Sunday Herald. "Because he understood the need for ventilation and pioneered the use of fires to draw in oxygen, it was also very safe."

More relevant to the new Queensferry Crossing is the example of the Forth [Rail] Bridge (1882-1890) which pioneered the use of caissons for foundations on an unprecedented scale.

Parallels between the old Forth Bridge and the new Queensferry Bridge span across the centuries.

In both cases this specialised engineering solution, beyond the experience of Scotland's own civil engineers, was led by a foreign expert. In the case of the Victorian bridge it was M Coiseau of Paris and Antwerp, leading a team of Italian, Belgian, Austrian and German labour. With the new bridge it is Ralf Wiegand of Hochtief, a construction multinational based in Essen, part of the FCBC consortium comprising Spanish, German, American and British companies.

We have not yet been told the cause or extent of the 2012-13 caisson problem that dragged out what was meant to be a "six-week operation", but there was more transparency in Victorian times when the public was permitted to follow the drama in the press.

On January 1, 1885, one of the set of four caissons used to allow workers to set foundations of the South Queensferry pier, weighing 2640 tonnes (with concrete and stone ballast), sank unevenly into the mud. It then slid 20ft due to combination of very high and low spring tides. This meant the rim of the cylinder was about 2m under water at low tide, and had to be built up over the waterline. During pumping out, external water pressure caused some of plates to tear, killing two men, and had to be stiffened with timber.

The delay caused a hiatus of 10 months on this pier, shorter than the delay that appears to have occurred 128 years later. The differences in health and safety standards, however, make comparisons unhelpful.