5. North Corridor Option 1
At 7.1km in length, North Corridor Option 1 covers much of the existing A90/M90 route corridor between Ferrytoll Junction and Halbeath Interchange. An indicative layout of this option is provided in Appendix 3.
As a substantially on-line corridor, it was recognised that this corridor option had significant potential for optimising the extent of its improvement over its overall extent.
Connection to the proposed replacement bridge is achieved through the provision of a new section of carriageway between the northern bridgehead and Ferrytoll Junction. Descending on viaduct in a north easterly direction towards Ferrytoll Junction, the new section of carriageway clips the eastern edge of St Margaret’s Marsh, a Site of Special Scientific Interest (SSSI), before crossing the B981 east of Dunfermline Waste Water Treatment Works.
The change in bearing associated with the approach to the proposed replacement bridge necessitates the reconstruction of Ferrytoll Junction. In the provision of a new junction at this location, all traffic movements associated with the existing arrangement will need to be maintained, new connections being required between the mainline carriageway, the B980, B981 and Ferrytoll Road.
North of Ferrytoll, the new mainline, retaining the horizontal and vertical geometry of the existing A90/M90, is required to interact with junctions at Admiralty and Masterton where access is provided to the A985, A921 and A823(M). A range of proposals are available for implementation at these junctions ranging from do nothing to full reconstruction. Continuing north, the corridor passes to the east of the properties at Middlebank, before passing beneath Masterton Road and the B981 on approach to Halbeath Interchange where the scheme terminates.
For the purpose of assessment, it was assumed that the carriageway cross section existing A90/M90 route corridor would be upgraded to full motorway standard. A dual two lane motorway (D2M) or dual three lane motorway (D3M) was considered.
5.2 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.
- A90/M90 Route Corridor and Structural Requirements.
- Junction Provision and Future Transport Developments.
- Geotechnical Assessment.
5.2.1 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 North 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.
5.2.2 A90/M90 Route Corridor and Structural Requirements
Using the existing A90/M90 route corridor and replicating its existing horizontal and vertical geometry imposes a number of substandard design elements. Where economically practicable, these would be removed through design development, but a number of substandard elements are likely to remain due to topography, existing structures and existing junction positions.
Existing structures will be adopted for use, where possible. A number of structures may be retained in their current form but many would require widening to accommodate a dual three lane motorway. A number of new structures will also be required, particularly in the junction areas and in connecting Ferrytoll Junction and the proposed replacement bridge.
5.2.3 Junction Provision and Future Transport Developments
North Corridor Option 1 being an online upgrade of the A90/M90 intersects the existing junctions at Ferrytoll, Admiralty and Masterton.
(a) Ferrytoll Junction
The bearing of the proposed replacement bridge dictates that Ferrytoll Junction must be reconstructed. The junction will, as a minimum, replicate the movements of the existing layout.
In future-proofing the scheme for future transport modes such as Light Rapid Transit (LRT), an appropriate entry/exit point must be considered. North of the Firth of Forth, it was determined that Ferrytoll Junction 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.
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.
(b) Admiralty Junction
The existing junction layout at Admiralty provides a typical grade separated junction facilitating all movements. In the provision of the North Corridor Option 1, the following proposals could be implemented:
- provision of minor improvements to all slip road tapers and nosings to enable implementation of D3M motorway cross section;
- provision of minor improvements to south facing slip roads and provision of north facing auxiliary lanes to alleviate weaving issue on mainline carriageway between Admiralty and Masterton;
- provision of minor improvements to south facing slip road tapers and nosings and provision of parallel running lanes between Admiralty and Masterton, removing north facing slip roads; and
- full improvement, encompassing minor improvements to south facing slip roads and closure of north facing slip roads, existing north facing functionality being catered for by a new junction arrangement at Masterton.
(c) Masterton Junction
The existing layout of Masterton provides a free flow junction arrangement between the M90 and the A823(M) both northbound and southbound. In the provision of North Corridor Option 1 the following proposals could be implemented:
- provision of minor improvements to all slip road tapers and nosings to enable implementation of D3M cross section;
- partial reconstruction as a free flow junction with south facing auxiliary lanes to Admiralty Junction;
- provision of a new grade separated dumbbell junction arrangement facilitating all movements with the addition of south facing parallel running lanes to Admiralty Junction; and
- full reconstruction as a free flow junction with new links to the A921 catering for the functionality lost at Admiralty Junction through the closure of the north facing slip roads.
5.2.4 Geotechnical Assessment
The recent ground investigation found that North Corridor Option 1 is mostly situated on variably weathered glacial till overlying dolerite bedrock at St. Margaret’s Hope and in the vicinity of Ferrytoll and Admiralty Junctions, with sedimentary bedrock comprising sandstone, siltstone, mudstone, limestone and coal seams elsewhere. Based on the available information there would appear to be no significant geotechnical issues.
5.3 Environmental Considerations
North Corridor Option 1 is mainly on-line and has the lowest land take of all northern route corridor options. It is anticipated to have the lowest overall impact on watercourses, designated sites of geological importance, ecological impact on habitats and species, landscape, visual, footpaths and community severance.
An increase in the carriageway cross section to D3M would potentially increase the landscape and visual impacts of the corridor. There may also be changes in the air quality, traffic noise and vibration impacts due to the increase in the carriageway width.
With regards to cultural heritage, North Corridor Option 1 has the least potential for impacts on unrecorded archaeology but has the potential to impact on Middlebank Souterrain Scheduled Ancient Monument (SAM), which is located adjacent to the existing M90 at Masterton Junction.
5.4 Transportation Considerations
The provision of a D3M cross section, encompassing two lanes of general traffic and the potential for an HOV lane, in tandem with the continued provision of junction arrangements at Ferrytoll, Admiralty and Masterton 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; and
- minimal impact on the effective operation of the transport network during times of maintenance.
5.5 North Corridor Option 1 Recommendation
An indicative layout of North Corridor Option 1 is contained within Appendix 3. If carried forward to DMRB Stage 2 Corridor Assessment this option could encompass the following:
- a dual D2m or D3M mainline cross section; and
- a reconstructed Ferrytoll Junction, providing:
- - connectivity between the new mainline and existing local roads; and
- connectivity for future transport modes.
In addition, the following junction improvements could also be considered:
- a full improvement to Admiralty Junction;
- a full reconstruction of Masterton Junction; and
- connectivity to the Forth Road Bridge.