Scheme options

The image is of the wider Glen Croe valley with Ben Luibhean on the right and Ben Donich on the left. The 5 Scheme Options considered as part of the DMRB stage 2 assessment for the permanent, long term solution are illustrated running top to bottom, with north pointing towards the top of the image. The Green Option is 4.35km long with two viaducts of 435m and 300m and a 2.3km debris flow shelter. The Yellow Option is a 1.8km viaduct, the Brown Option is a 1.4km debris flow shelter, the Purple Option is a 1.5km viaduct and 1.1km tunnel and the Pink Option is a 2.9km tunnel. The study area is shown by red dashed line.

The following headings provide details of the options considered as part of the DMRB stage 2 process and how they were assessed against a range of criteria.

Green option

The Green Option is located on the western side of Glen Croe, on the lower slopes of Ben Donich. Our assessment has highlighted the potential for landslips on the lower and upper Ben Donich slopes. Therefore, a debris flow shelter with catch pit has been included along the majority of the Green Option to protect road users from future debris flow and landslides, similar to that of the Beinn Luibhean slope. The total route is 4.35km long with two viaduct structures to cross the Croe Water at the southern end and the steep ravine at the northern end.

Yellow option

The Yellow Option is predominantly situated away from the existing A83 on the lower slopes of Beinn Luibhean, below the existing A83 and the Old Military Road.

The route is 2.5km long, with 1.8km of the route on a viaduct up to 90m above the ground. The remaining lengths to the north and south of the viaduct, are on embankment. Some of the viaduct piers require deflection structures to protect the pier from debris flow and landslides in the future.

The Yellow Option has been moved to the lower lying ground below the Old Military Road following design refinement that has considered the impacts resulting from construction and an assessment of resilience. The lower slopes of Beinn Luibhean, where the Yellow Option is now located, are less steep and less susceptible to ground movement. This makes the construction of the piers more efficient and reduces the extent of the temporary works.

Brown option

The Brown Option is generally located on

the existing A83. To protect the road users from debris flows and landslides, a structure known as a debris flow shelter is required to cover part of the road considered to be at higher risk.

The form of the Brown Option has evolved to include a catch pit on the uphill side of the flow shelter structure. The purpose of the catch pit is to channel landslip material, allowing it to be removed safely and efficiently, without adversely impacting the road user or the downstream slope.

The Brown Option is 2.4km long, with the flow shelter and catch pit extending to 1.37km. The catch pit and protection wall extends a further 180m to further protect road users below the Beinn Luibhean slope and area of highest vulnerability.

Purple option

The Purple Option is predominantly situated away from the existing A83 on the lower slopes of Beinn Luibhean, below the existing A83 and the Old Military Road where adjacent ground is less steep and less susceptible to movement.

The route is 3.7km long, with around 1.48km of the route on a viaduct up to 52m above ground level. There is also a 1.2km tunnel located beneath the Old Military Road, existing A83 road and slopes of Beinn Luibhean.

As a result of the tunnel, the Purple Option bypasses the existing B828 Glen Mhor local road junction and Rest and Be Thankful Viewpoint car park. Therefore, a new junction is required to maintain access. This involves using a section of the existing A83 and

creating a new junction to connect the B828 Glen Mhor local road with the Purple Option (A83), north-east of Loch Restil.

Pink option

The Pink Option is located mostly away from the existing A83. The route is 3.94km long, located to the east of the existing A83 road, beneath the western slopes of Beinn Luibhean.

The route includes a tunnel which is 3km long.

Similar to the Purple Option, the Pink Option bypasses the existing B828 Glen Mhor local road junction and Rest and Be Thankful Viewpoint car park, requiring a new junction to maintain access.