9. South Corridor Option 2
At 5.1km in length, South Corridor Option 2 provides a direct link between the proposed replacement bridge and the M9 north of Winchburgh. An indicative layout of this option is provided in Appendix 7.
Departing a new junction with the M9 situated west of Junction 1a, the proposed South Corridor Option 2 mainline climbs on embankment crossing the B9080 and the Falkirk-Fife Railway Line. Cresting on approach to Swine Burn, the mainline carriageway descends into cutting passing to the east of Westmuir Riding Centre.
Continuing north along the boundary of Dundas Estate the mainline remains in cutting, passing beneath Builyeon Road, on approach to the A904. It is proposed that the mainline passes beneath the A904 to mask the corridor’s presence, the A904 being carried on structure above. To provide access to the proposed replacement crossing for non-motorway traffic and local traffic, a new junction is proposed at this location.
On approach to the proposed replacement bridge, the earthworks associated with the vertical geometry of the mainline transition from cutting to embankment, facilitating a connection with the proposed replacement bridge.
South Corridor Option 2 has an impact on the oil pipeline, with a crossing required within the grounds of Dundas Estate, south of the A904.
In the provision of this option, connectivity enhancements can be considered to existing roads’ infrastructure with the reconstruction of Scotstoun Junction facilitating traffic movements in all directions between the A90 and M9 Spur.
In the consideration of South Corridor Option 2, two mainline motorway cross sections were considered, D2M and D3M.
9.2 Engineering Issues
A number of engineering constraints were identified which require assessment to allow further development should this option be progressed. Principally these are:
- Carriageway Cross Section
- Route Corridor, Structural Requirements and Topographical Effects
- Junction Provision and Future Transport Developments
- Geotechnical Assessment
9.2.1 Carriageway Cross Section
In considering the most appropriate cross section for implementation, the scheme objectives dictate that two lanes of general traffic in each direction are required. It was recognised that there may be an opportunity to consider additional High Occupancy Vehicle (HOV) lanes in the early years following construction, prior to a possible future introduction of a public transport multi-modal facility.
In light of this opportunity, a dual three lane motorway (D3M) was therefore taken as the working assumption over the extent of South Corridor Option 2 with a design speed of 120kph, with the flexibility to scale back the scope of the upgrade as necessary during scheme development.
9.2.2 Route Corridor, Structural Requirements and Topographical Effects
Situated offline and at a length of 5.1km, South Corridor Option 2 has a greater effect on existing topography than South Corridor Option 1. Situated for the most part along the western boundary of Dundas Estate, the corridor has been placed in cutting to mask its presence to the surrounding area.
The provision of a new offline carriageway allows a fully DMRB compliant horizontal and vertical geometry design to be considered, subject to existing topographical constraints, junction provision and the requirement for crossings of Builyeon Road and the A904 on approach to the proposed replacement bridge.
The implementation of South Corridor Option 2 will require a number of significant structures to be constructed in the provision of junctions to existing routes. The provision of these structures will require careful planning and significant traffic management during the construction period. New structures will also be required where the corridor crosses the Falkirk-Fife Railway Line and local access roads. A single structural crossing of the oil pipeline will be necessary, south of the A904.
As with South Corridor Option 1, initial consultations have been held with BP Oil UK Ltd over the position of the pipeline. Further consultations shall be held if this option is progressed to DMRB Stage 2 Corridor Assessment.
9.2.3 Junction Provision and Future Transport Developments
In the provision of South Corridor Option 2, a new junction to the M9 is required to facilitate access between the proposed replacement bridge and the central Scotland motorway network. In addition, the reconstruction of Scotstoun Junction can be considered, which would provide full connectivity between the A90 and the M9 Spur. In the provision of non-motorway access/egress to the proposed replacement bridge, a new junction to the A904 would be provided.
(a) M9 Junction
The provision of a new junction on the M9 is required to maintain cross-Forth links between central Scotland and the north. Encompassing the existing M9 Junction 1a, the new junction would provide connectivity between the proposed mainline, M9 and M9 Spur.
In maintaining access to the M9 Spur, any future proposals for the use of the Forth Road Bridge can be realised, through the use of this existing route, Scotstoun Junction and the A90.
West facing connectivity to the M9 would be a feature of the junction, providing access to the proposed mainline and the M9 Spur, a movement which is not available within the existing M9 Junction 1a arrangement.
The proximity of the M9, M9 Spur, B9080 and Falkirk-Fife Railway Line to South Corridor Option 2 requires a significant number of structures to be considered in the provision of a junction at this location. Multiple crossings of the M9 will be required, providing full connectivity between the M9, M9 Spur and proposed mainline. As such a significant period of traffic management will be required on both the trunk road and local road networks during the construction period.
(b) Scotstoun Junction
In the provision of South Corridor Option 2, Scotstoun Junction could be utilised in its current form, providing limited functionality between the M9 Spur and A90. Alternatively, it could be upgraded to provide full connectivity.
If reconstruction was to be considered, the new free flow, all movements junction would provide the following:
- a priority routing of traffic between the M9 Spur and the A90 to the east
- a new slip road arrangements between the M9 Spur and the A90 to the west
- new slip road arrangements maintaining A90 through traffic connectivity between Edinburgh and local destinations via the A904/A8000.
The provision of such measures would also cater for any future proposals for the use of the Forth Road Bridge.
(c) A904 Junction
In the provision of non-motorway connectivity and to maintain local access to the proposed replacement bridge, a new junction is required to the A904 west of South Queensferry. The form of this junction would be simplistic with only north facing slip roads being provided to the proposed mainline.
As with South Corridor Option 1, the A904 is considered the most practical place to terminate the multi-modal aspect of the project.
9.2.4 Geotechnical Assessment
The ground conditions in this area comprise glacial till overlying bedrock. Rockhead is within 5m of surface close to where the corridor coincides with the South Corridor Option 1 alignment, however it deepens to the south, resulting in between 10m and 20m of drift deposits. Based on the available information there would appear to be no significant geotechnical issues, with the exception of any alterations to Scotstoun Junction, which would necessitate construction of links and structures over an area of recorded oil shale workings.
9.3 Environmental Considerations
South Corridor Option 2 provides a predominantly offline mainline carriageway which would have the greatest land take in relation to agricultural land of all southern route corridor options. South Corridor Option 2 would cross the most watercourses and footpaths and is therefore considered to have the greatest potential for impacts on the water environment and on non-motorised users.
South Corridor Option 2 diverts traffic away from the A90 at South Queensferry which would be expected to result in large noise reductions to a large number of properties in this area. However, the alignment between the M9 and A904 would likely result in large noise increases affecting a small number of rural properties.
Cultural Heritage impacts for South Corridor Option 2 are envisaged to be comparatively less than for other southern route corridor options due to lower impacts on designated landscapes.
Ecology, landscape and visual impacts are also expected to be greater for South Corridor Option 2. This is because the mainline is offline for a comparatively longer length than South Corridor Option 1 and therefore has greater potential to impact on habitats. In addition, South Corridor Option 2 would cross between the Swineburn, Muiriehall and Carmelhill woodland complex, resulting in the fragmentation of ecological habitats. With regard to landscape, the mainline carriageway cuts through existing woodland and fields near Dundas Estate, isolating this character area. The significant cuttings through the landscape as well as extensive earthworks required in the implementation of the mainline carriageway would also increase visual impacts.
The provision of a reconstructed Scotstoun Junction would require land take and thus, potentially, give rise to additional environmental impacts. Consequently, impacts on land use, geology, the water environment, ecology, landscape, visual impacts, traffic noise and vibration, air quality are expected to be slightly increased for the area to the southeast of South Queensferry.
9.4 Transportation Considerations
The provision of a D3M cross section encompassing two lanes of general traffic and the possibility of an HOV lane in tandem with the provision of new junctions with the M9 and A904 and the possible reconstruction of Scotstoun Junction is expected to provide the following benefits:
- an increase in the cross-Forth levels of service provided for private, road-based modes of travel to/from the central Scotland motorway network.
- a deterioration in network performance at locations across West Edinburgh (e.g. M9 Junction 1, Newbridge Roundabout)
- an improvement in network performance for cross-Forth traffic to/from Falkirk/Linlithgow
- improved journey time reliability between central Scotland and Fife (excluding the Edinburgh area)
- possible detrimental effects to journey time reliability to/from north Edinburgh along the A8/A90.
- improved accessibility and social inclusion through the provision of complementary transport measures.
- minimal impact on the effective operation of the transport network during times of maintenance.
9.5 South Corridor Option 2 Recommendation
An indicative layout of South Corridor Option 2 is contained within Appendix 7. If carried forward to DMRB Stage 2 Corridor Assessment this route corridor option could encompass the following:
- a dual three lane motorway (D3M) mainline cross section;
- a new M9 Junction providing full interconnectivity between the M9, M9 Spur and proposed mainline; and
- a new junction to the A904.
In addition, the following junction improvements could also be considered:
- the reconstruction of Scotstoun Junction