8. South Corridor Option 1 8.1 Description 8.2 Engineering Issues 8.3 Environmental Considerations 8.4 Transportation Considerations 8.5 South Corridor Option 1 Recommendation
8. South Corridor Option 1
At 2.75km in length, South Corridor Option 1 provides a direct link between the proposed replacement bridge and the existing A90 southeast of Echline Junction. In the provision of this route corridor option, best use is made of the existing roads’ infrastructure associated with the Forth Road Bridge including the A90 and the recently completed M9 Spur Extension. An indicative layout of this option is provided in Appendix 6.
The proposed mainline, commencing approximately 250m east of the A8000 overbridge, departs the existing A90 into the fields of Dundas Home Farm to the west where a new junction to the A904 is proposed. Continuing west, the mainline enters Dundas Estate before swinging north and crossing beneath the A904 to the west of South Queensferry. On approach to the proposed replacement bridge, the vertical geometry of the corridor rises to meet the southern bridgehead.
In the provision of this option, further connectivity enhancements are proposed to the existing road network with a sliding scale of improvements available at M9 Junction 1a and Scotstoun Junction. The construction of a new junction to the A904 is also proposed to provide access to the proposed replacement crossing for non-motorway traffic and local traffic.
A major consideration to be taken into account when assessing South Corridor Option 1 was the position of an oil pipeline which runs to the south of the existing A90 and continues west across the fields to the south of Echline and South Queensferry. Protection measures are required where the proposed corridor crosses the pipeline, a major diversion of the pipeline costing an estimated £200 million per day to implement. The interaction between the new carriageway and the oil pipeline was therefore a key consideration in the identification of the preferred South Corridor Option 1 alignment.
In the consideration of South Corridor Option 1, two mainline motorway cross sections were considered, D2M and D3M.
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
8.2.1 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 1 with a design speed of 120kph, with the flexibility to scale back the scope of the upgrade as necessary during scheme development.
8.2.2 Route Corridor, Structural Requirements and Topographical Effects
Due to its close proximity to South Queensferry, a number of measures were considered in the provision of South Corridor Option 1.
To provide protection to the oil pipeline in the fields to the south of the A904, the corridor must be constructed above ground level. In order to provide suitable mitigation and to limit the number of structural crossings of the pipeline, initial consultations have been held with BP Oil UK Ltd to discuss the horizontal and vertical geometry of the corridor. Further consultations shall be held if this option is progressed to DMRB Stage 2 Corridor Assessment.
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 and the requirement to cross the A904 on approach to the proposed replacement bridge. Through the utilisation of the existing A90 and the M9 Spur, a number of substandard design elements are likely to be inherited. Where economically practicable, these would be removed through design development. However a number of substandard elements are likely to remain due to topography, existing structures and existing junctions.
In the provision of the South Corridor Option 1, the requirement exists for the reconstruction of the A8000 overbridge. A new structure will also be required to carry the A904 over the proposed mainline carriageway. Any improvement considered at M9 Junction 1a will also generate significant modifications to enhance the functionality of the existing arrangement.
In future-proofing the scheme for future transport modes such as Light Rapid Transit (LRT), an appropriate entry/exit point must be considered. South of the Firth of Forth, it was determined that the A904 is the most practical place to terminate the multi-modal aspect of the project. Termination at this point will enable connections to be established to a number of local destinations.
8.2.3 Junction Provision and Future Transport Developments
In the provision of South Corridor Option 1, a new junction is required at Echline to maintain access to local communities and as an access for non-motorway traffic. In addition, improvements can be made to M9 Junction 1a and the recently completed Scotstoun Junction, optimising existing infrastructure for use with the proposed replacement bridge.
(a) Echline Junction
The provision of a new junction at Echline is required in order to connect the proposed mainline to the A904 to provide access to the proposed replacement crossing for non-motorway and local traffic.
Connectivity to the Forth Road Bridge must also be considered in the development of this junction, thus catering for any future role for which the existing bridge might have.
In the provision of South Corridor Option 1, the following proposals could be implemented:
- a standard grade separated junction utilising a roundabout to maintain all movements between the proposed mainline and the A904
- a free flow junction arrangement, incorporating improvements to Scotstoun Junction and the use of the existing Echline Junction, providing access between the proposed mainline, the A904 and the A8000.
Should South Corridor Option 1 be taken forward for further assessment, it is deemed appropriate that the design should consider the optimum junction arrangement at Echline and Scotstoun.
(b) M9 Junction 1a
The existing layout of M9 Junction 1a provides partial free flow junction movements between the M9 and the M9 Spur. No west facing functionality is offered by the existing junction arrangement. In the provision of improvements, the following options could be implemented:
- provision of minor improvements.
- full reconstruction of the existing junction arrangement providing a change in traffic priority, the M9 to M9 Spur link becoming the mainline carriageway.
- full reconstruction of the existing junction arrangement, providing full functionality between the M9 and the M9 Spur.
8.2.4 Geotechnical Assessment
The ground conditions in the area of South Corridor Option 1 essentially comprise variably weathered glacial till overlying bedrock. The bedrock comprises mainly sedimentary deposits of sandstones, mudstones and siltstones, with dolerite located at depth beneath the A904 crossing. 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.
South Corridor Option 1 is the shortest of the southern route corridor options under consideration and is therefore expected to have the lowest overall land use impacts in terms of potential land loss, severance and building demolitions. It is likely to cross the least number of watercourses and terrestrial habitats of all of the southern route corridor options and is therefore expected to have the lowest impacts on ecology and the water environment. Landscape and visual impacts are likely to be the lowest as South Corridor Option 1 uses less of the rural landscape.
With regards to traffic noise and vibration, South Corridor Option 1 is likely to affect the greatest number of properties (due to its proximity to South Queensferry), although most are already affected by the A904 traffic.
South Corridor Option 1 may result in significant cultural heritage and landscape impacts on Designed Landscape Area of Dundas Estate and may result in changes to the area’s cultural heritage.
Whilst South Corridor Option 1 breaches the Green Belt, this option is expected to have the lowest impact on policy objectives as all other southern options affect the Green Belt and also have potential impacts on a Designated Area of Outstanding Landscape Quality. It does propose the lowest level of new road / junction construction and has the least properties within 200m and is therefore anticipated to result in the least construction disruption.
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 a new junction at Echline and improvements to the existing junctions at Scotstoun and M9 Junction 1a is expected to provide the following benefits:
- increased levels of service for private, road based modes of transport
- improved performance to the wider road network through improved connectivity.
- improved journey time reliability
- improved accessibility and social exclusion through the provision of complementary transport measures.
- minimal impact on the effective operation of the transport network during times of maintenance.
An indicative layout of South Corridor Option 1 is contained within Appendix 6. If carried forward to DMRB Stage 2 Corridor Assessment this route corridor option could encompass the following:
- a dual D2M or D3M mainline cross section;
- a new Echline Junction encompassing improvements to Scotstoun Junction providing:
- connectivity between the new mainline and existing local roads .
In addition, the following junction improvements could also be considered:
- an enhancement of M9 Junction 1a to provide all ways movements; and
- connectivity to the Forth Road Bridge.