Tunnel, viaduct or canopy? Weighing up options for A83 route

A tunnel and a debris shelter – both rejected by officials eight years ago – are among five options put forward for the new route of the A83 in Glen Croe.

Details of possible solutions for the Rest and Be Thankful were released last week following Transport secretary Michael Matheson’s announcement of the preferred route corridor from 11 alternatives put forward last year.

Millions has been spent in recent years on debris pits, catch fences a and a huge bund – but both the A83 itself and the single-track Old Military Road have still been frequently closed by landslides.

Yet an analysis of a public consultation by engineers Jacobs Aecom states: “A tunnel option and a debris shelter option were both considered previously as part of the A83 route study undertaken in 2013, and discounted at the time in favour of the programme of measures that are currently being implemented.

“Due to the significance of landslide events and related disruption during the summer and autumn 2020, and the need to consider alternative options for the A83, the current work will include reconsidering a tunnel option and a debris shelter option within corridor one at Glen Croe, in addition to a range of other potential options.”

Now the public is being asked to comment on five colour-coded options which have been put forward for the site, which is in Loch Lomond and the Trossachs National Park:

Brown: A debris flow shelter or canopy

This closely follows the line of the current A83. The road would have to be widened though, with a 300-metre viaduct built on the approach to the car park at the top of the Rest.

Transport Scotland says would have less impact on the environment than the other options and less need to acquire new land.

But construction work would cause significant disruption on the current road while the steep hillside would ”resent challenges’ for construction.

Yellow: A 1.8km viaduct on the north-east slopes of Glen Croe

The highest pier of the viaduct would be 37-metres tall, with spans between piers varying between 40 and 70 metres.

Advantages are said to include flexibility for traffic management and workers’ protection, and less potential for soil ‘sealing’ (preventing natural soil processes).

But the steep hillside would present challenges again, and the risk of high winds could affect the design.

And the report adds: “Due to the proposed viaduct, this option could result in significant landscape and visual effects, including effects on the setting of cultural heritage resources.”

Green: A new 4.3km stretch of road

This would be on the opposite side of the valley to the existing A83 and would largely follow an existing forestry access track, but would need a new 300-metre viaduct and ‘landslide mitigation works’ such as debris shelters or fencing would be needed to protect it from future landslides.

This is seen as the quickest option, and advantages include the land largely being already owned by Scottish Ministers, while area is said to be outside the ‘zone of highest landslide hazard susceptibility’.

But woodland would be affected and it would be close to the car park at the top of the Rest, while the steep hillside would again create challenges for construction.

Pink: A 2.9km tunnel

This would mean 4.1km of new road being built, most of it in a tunnel.

The B828 road to Lochgoilhead would probably be extended to the north tunnel portal by using part of the existing A83 the Rest and Be Thankful car park, with a new junction created.

Advantages are said to include greater protection for the workforce and ‘smaller impact on surface-level natural assets than most other options due to the use of the tunnel’.

But the tunnel would create a ‘considerable cost element’ and construction is estimated to take at least a year longer than the other options.

A higher level of greenhouse gas emissions is predicted, and removing the excavated material would create road traffic.

Purple: The base of the valley with a shorter tunnel

This would see a shorter tunnel and a new 3.2km road in the base of the valley, potentially with a viaduct as well.

The tunnel would again reduce impact on the landscape and it seen as one of the better options for worker safety and traffic management.

But this tunnel would also add to construction time and is said to increases greenhouse gas emissions.

And the report adds: “This option has the most potential negative environmental effects in terms of population and human health, use of materials, bedrock geology, peat, cultural heritage and landscape and visual effects – although with careful design and mitigation the landscape and visual effects could be reduced or even made beneficial.”

More information all the options, and how to comment on them, is available on Transport Scotland’s website.