The A83 Rest and Be Thankful is expected to be shut for up to three further weeks for repairs to landslip damage.
The road has been closed after a landslip brought down 6,000 tonnes of debris amid heavy rain on August 4.
Road management and maintenance organisation BEAR Scotland said the carriageway has been undermined after material was washed away from beneath the road and embankment.
Teams plan to work round the clock to build a concrete wall to support the road and a culvert to carry water under it.
A new temporary catch-pit will be created and the cliff formed below the road reinforced with concrete.
BEAR Scotland said the repairs could take up to three weeks before one lane of the road can safely reopen under traffic light control.
The landslide also blocked a parallel alternative route on the Old Military Road, which reopened later in the week.
It has since shut several times overnight amid heavy rain forecasts and will again shut overnight on Thursday.
Eddie Ross, BEAR Scotland’s north west representative said: “We’ve concluded designs to allow us to safely repair the A83 following last week’s landslip.
“The repairs involve complex engineering methods which involve stabilising the recently formed steep channels on the embankments above and below the carriageway, as well as repairing the road which was heavily damaged by the landslip.
“A catch-pit will also be installed to help provide additional mitigation for the A83.
“With good weather and no unforeseen circumstances we expect these repairs will take up to three weeks to complete, and our teams will work 24 hours a day to get the repairs completed as quickly as we can to allow the A83 to safely reopen.”
He added: “With weather warnings for thunderstorms in place overnight across much of the country we have taken the difficult decision to close the local diversion route again from 9pm due to the restrictions in being able to safely monitor the hillside.”
A diversion is in place from 9pm via the A83, A82, A85 and A819.
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