10. South Corridor Option 3
At 4.6km in length, South Corridor Option 3 provides a direct link between the proposed replacement bridge and the existing M9 Spur north of M9 Junction 1a. An indicative layout of this option is provided in Appendix 8.
Departing a new junction with the M9/M9 Spur, the proposed corridor follows the line of the M9 Spur before departing northwest into the fields of Humbie Farm, crossing the Falkirk-Fife Railway on embankment. To the north of Humbie Farm, the vertical alignment of the mainline transitions from embankment to cutting, following existing ground topography as closely as possible.
Sweeping north, the mainline continues in cutting, intersecting the south west corner of Dundas Estate, west of Barrencraig Wood, before passing beneath Builyeon Road to the west of Dundas Mains. The corridor, continuing north along the boundary of Dundas Estate, remains in cutting, masking its presence on approach to the A904. It is proposed that the mainline passes beneath the A904, the A904 being carried on structure above. To provide access to the proposed replacement bridge 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 3 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 the existing roads’ infrastructure with the reconstruction of Scotstoun Junction, which would facilitate traffic movements in all directions between the A90 and M9 Spur.
In the consideration of South Corridor Option 3, two mainline motorway cross sections were considered, D2M and D3M.
10.2 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
10.2.1 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 3 with a design speed of 120kph, with the flexibility to scale back the scope of the upgrade as necessary during scheme development.
10.2.2 Route Corridor, Structural Requirements and Topographical Effects
South Corridor Option 3 has a similar effect on existing topography to South Corridor Option 2. 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 3 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.
Similar to South Corridor Options 1 and 2, 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.
10.2.3 Junction Provision and Future Transport Developments
In the provision of South Corridor Option 3, a new junction to the M9 Spur 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 to provide full connectivity between the A90 and the M9 Spur. In the provision of non-motorway access to the proposed replacement bridge and the central Scotland motorway network, a new junction to the A904 will be required.
(a) M9 Junction 1a/M9 Spur Junction
The provision of a new junction to the M9 Spur 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.
Full west facing connectivity to the M9 would be a feature of the junction, providing access/egress to 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 the corridor requires a significant number of structures to be considered in the provision of a junction at this location. Multiple crossings of the Falkirk-Fife Railway Line, M9, and proposed mainline will be required in order to provide full connectivity. As such, a significant period of traffic management will be required on both the trunk road and local road networks during the construction period.
Any junction considered at this location is like to be extremely complex and difficult to construct. The provision of a junction at this location is likely to require significant Departures from Standard in its implementation, reducing its operational effectiveness.
(b) Scotstoun Junction
In the provision of South Corridor Option 3, Scotstoun Junction could be utilised in its current form, which provides limited functionality between the M9 Spur and A90. Alternatively, it could be upgraded to provide full connectivity.
If upgrading 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 arrangement 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, a new junction is required to the A904 west of South Queensferry. The form of this junction would be simplistic, providing north and south facing slip roads 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.
10.2.4 Geotechnical Assessment
The ground conditions in this area comprise glacial till overlying bedrock. 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.
10.3 Environmental Considerations
South Corridor Option 3 is predominantly offline and is expected to have the greatest impacts, compared to the other southern options, in relation to residential property demolitions.
South Corridor Option 3 is an intermediate option for a number of environmental impacts. Ecology, landscape and visual impacts are likely to be greater than for South Corridor Option 1 but less than for South Corridor Option 2.
The construction of the M9 Junction 1a/M9 Spur Junction and A904 Junction could result in land use, landscape, visual, air quality, traffic noise and vibration 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.
10.4 Transportation Considerations
South Corridor Option 3 is deemed to provide similar benefits and disbenefits to that experienced with South Corridor Option 2. 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.
10.5 South Corridor Option 3 Recommendation
An indicative layout of South Corridor Option 3 is contained within Appendix 8. This option is the least favoured of the southern route corridor options. However, 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 1a/M9 Spur 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