The A30 is due to be fully closed between the Boxheater junction and Chiverton Cross roundabout, to enable the installation of a new Tolgroggan accommodation bridge near Zelah.
The work was originally scheduled to take place last weekend, before being moved to the coming weekend of February 10 to 13, the start of half term.
However, it is now being moved a week on again, to instead take place on the weekend of February 17 to 20 – the end of half term.
National Highways said the ground is still not firm enough to accommodate the 600-tonne crane needed to lift the 137-foot bridge.
Neil Winter, National Highways’ senior project manager for the A30 Chiverton to Carland Cross scheme, said: “We’ve taken advice from our principal contractors and although ground conditions are improving they are still not perfect, and we have taken the decision now to move the operation further back.
“We allowed for contingencies in the programme, and feel it only right to reschedule to allow site preparations to be exactly how we want them and to be sure we can complete the lift under one weekend closure.
“Adverse weather over the past two months, a mixture of heavy rainfall and low temperatures, has thrown up circumstances out of our control, but a little extra time will ensure the ground has firm footings required for what is a large-scale and off-road crane lift.
“It won’t impact on our programme or future road opening date, and, based on traffic modelling from last year’s half-term, the weekend of 17-20 will cause lesser disruption to local communities and people making holiday journeys in Cornwall.
“And should strong winds and adverse weather interfere with that weekend, we also have a contingency in place for February 24-27.”
This road will now be closed from 8pm on Friday, 17 February, and reopened to traffic by 6am on Monday, 20 February.
Clearly signed diversions will be in place via the Boxheater Junction and Chiverton Cross roundabout, with eastbound traffic diverted via the A3075 to Goonhavern and the B3285 to rejoin the A30.
The westbound diversion will operate in reverse, with the B3284 remaining open under temporary traffic signals north and south across the Chybucca roundabout.
Mr Winter added: “We apologise for any confusion caused by the rescheduling but by announcing now, we hope this will give people more time to plan ahead for the closure, while making sure everything is 100% on site for the lifting operation.”
The installation operation will see the new 137ft (42-metre) structure being lifted into place before the dismantling of the old accommodation bridge later in the year.
The new bridge weighs 275 tonnes, the equivalent weight of 23 double decker buses – or 887,097 standard Cornish pasties – and once completed it will span both the new dual carriageway and the old A30, which will be detrunked for improved local connectivity and traffic.
As part of the multi-million upgrade, the new A30 carriageway is scheduled to be open in winter 2023 and further information on the scheme is available at National Highways’ A30 Chiverton to Carland Cross web page.