However, campaigners were pessimistic about the worth of the £1.1 million defensive hole, which they described as a Band-Aid solution while repeating calls for a replacement road to be accelerated.
BEAR Scotland, which maintains the trunk road to Kintyre for Transport Scotland, said it would add “resilience” to the route, which has been forced to close by a series of landslides.
The 120m-long pit, which was completed last week after being started last September, is 12m wide and can hold some 4,500 tonnes of debris – the size of an Olympic swimming poll.
Work started after a major landslip blocked the A83 in August last year.
It is the latest of a series of catch pits, with a total capacity of nearly 20,000 tonnes, which have been excavated beside the A83 so rocks and soil fall into them before reaching the road.
Measures to shore up the steep hillside above the road are continuing, with a further catch pit to be added at the site of landslips last year.
Five options are being considered for a replacement road on the other side of Glen Croe, costing up to £860m and likely to take up to ten years to complete.
Transport minister Graeme Dey said: “Improving the resilience of the A83 Rest and Be Thankful (RABT) is one of our top priorities, so the completion of this additional catch pit is very welcome.
“The Scottish Government continues to treat the issue with the seriousness and urgency it deserves, as we look to implement measures to maintain connectivity on a short, medium and long term basis.”
BEAR Scotland north west representative Eddie Ross said: “The completion of another catch-pit at the Rest and Be Thankful is another step forward in strengthening the landslide mitigation measures for the A83.”
However, John Gurr, chairperson at the RABT Campaign, said: “Over £80m has been spent on mitigation measure over the years and sadly most of those measures have been unsuccessful in keeping the RABT open.
"Last year, despite £1m being spent on mitigation measures just like this one, 20,000 tonnes of debris still fell at the rest and closed the area for over 200 days.
“Our autumn and winter seasons are increasingly wetter, and with 100,00 tonnes of unstable material expected to fall over the coming months, what we need is a permanent fix at the RABT, not a Band-Aid solution which slips off at the first sight of rain.
Dumbarton Labour MSP Jackie Baillie said: “While this new catch pit is welcome, this is the latest in a long line of mitigation measures – and millions of pounds spent – which have all failed to keep the RABT open.
"Last year, despite measures being put in place, the RABT was closed for the majority of the year which hugely disadvantaged the surrounding communities and the local economy.
“We need a lasting solution for the A83 and, in the meantime, we need to consider better interim alternatives so that traffic can keep moving and access to and from Argyll and Bute is maintained.”