A full 24-hour road closure near Talerddig, between Llanbrynmair and Carno, will begin from 9am on Thursday, October 31. It is expected to reopen at 5pm on Friday, December 20.
A huge traffic diversion, which is estimated to take almost two hours if the 70-mile route was completed in full, will be in place while work is carried out to build foundations for a new five-metre tall retaining wall near the River Iaen after the previous wall partially collapsed in October last year.
The Welsh Government said other options had been explored, including building a temporary embankment to allow traffic through, however “ecological restrictions” made the option “unviable”.
The diversion is listed as driving along the A470 to Caersws before joining the A489 towards Newtown, then the A483 to Welshpool and A458 to Mallwyd before rejoining the A470 at Glantwymyn and proceeding towards Llanbrynmair.
Recommended diversions are put in place on similar roads, meaning the official route takes in A-roads only.
Diversion map
The Welsh Government said work will be carried out during the winter to avoid travel disruption at the busiest time of the year.
“Unfortunately, this will have an impact on school transport,” the government’s traffic body said.
“We are liaising with the relevant transport authorities and will provide a further update in due course.
“The closure period will coincide partially with the October/November half term dates.”
Emergency vehicles will also not be able to travel through the closure.
Some temporary weight restriction will be introduced along country roads in the area to deter motorists from using alternative diversion routes that are not suitable for their vehicles.
Contractors will be working on site from 7am to 6pm seven days a week. Pedestrians and cyclists will be able to travel through the road closure but may be asked to wait before being guided through the site by marshals.
Two-way traffic lights will be reinstalled at the site from 5pm on Friday, December 20, until 5pm on Friday, February 14.
