7. North Corridor Option 3
At 7.0km in length, North Corridor Option 3 is a combined option requiring the upgrade of the existing A90 between Ferrytoll Junction and Admiralty Junction and the provision a new offline section of carriageway north of Admiralty. An indicative layout of this option is provided in Appendix 5.
As with North Corridor Option 2, the off-line nature of the northern end of the corridor means that this corridor option also has limited potential for optimising the extent of its improvement within its full extent.
The North Corridor Option 3 mainline would be constructed to motorway standard. 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 (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 to a point just south of Admiralty Junction, turns on a north easterly bearing towards Belleknowes Industrial Estate. Continuing north, the corridor climbs out of the valley containing Belleknowes Industrial Estate intersecting the M90 north of Masterton Junction. The tie in to the existing M90 is achieved east of Duloch Farm. The scheme terminates at Halbeath Interchange.
In the consideration of North Corridor Option 3, two mainline motorway cross sections were considered, D2M and D3M.
7.2 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.
7.2.1 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. The existing A90/M90 between Admiralty and Masterton could be utilised as a local distributor road, similar to Route Corridor Option 2.
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 3 with a design speed of 120kph, with the flexibility to scale back the scope of the upgrade as necessary during scheme development.
7.2.2 Route Corridor, Structural Requirements and Topographical Effects
Using the existing A90/M90 route corridor and replicating its existing horizontal and vertical geometry imposes a number of substandard design elements have been inherited. 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.
North of Admiralty Junction, the provision of an offline mainline carriageway has allowed a fully DMRB compliant horizontal and vertical geometry to be considered, subject to topographical constraints and junction design.
Existing structures will be adopted for use, where possible. However, the reconstruction of Ferrytoll Junction and the provision of a widened cross section would require a number of structures to be widened or replaced. New structures of substantial length will be required in the provision of the mainline carriageway, carrying the corridor over existing local roads and railway lines whilst also providing connectivity to the proposed replacement bridge.
7.2.3 Junction Provision and Future Transport Developments
The provision of an offline mainline carriageway from north of Ferrytoll and the retention of the existing A90/M90 only requires a new junction at Ferrytoll with the existing arrangements at Admiralty and Masterton being retained for local access.
(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.
As with North Corridor Option 1, Ferrytoll Junction is considered the most practical place to terminate the multi-modal aspect of the project.
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) Local Access Provision
To satisfy the scheme objectives, it is necessary that connectivity be maintained to both local and national routes. Therefore, the retention of the existing A90/M90 and the junctions at Admiralty and Masterton is necessary to facilitate access to Rosyth, Inverkeithing and Dunfermline. The south facing slips at Admiralty will require to be extended to connect to the proposed mainline.
North facing slip roads are proposed between Masterton Junction and Halbeath Interchange to facilitate access between the proposed mainline and the existing M90.
7.2.4 Geotechnical Assessment
The recent ground investigation found that the ground conditions associated with North Corridor Option 3 predominantly comprise variably weathered glacial till overlying bedrock consisting of dolerite or sedimentary rocks including sandstone, siltstone, mudstone, limestone and coal seams. Based on the available information there would appear to be no significant geotechnical issues.
7.3 Environmental Considerations
North Corridor Option 3 is an intermediate option for a number of environmental impacts. With regards to air quality, this corridor has the highest number of properties that may experience a deterioration in local air quality.
7.4 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 provision of a new junction at Ferrytoll and the limited use of Admiralty Junction and Masterton Junction is expected to provide the following benefits:
- increased levels of service for private, road based modes of transport on the mainline carriageway
- a reduction in the performance to the wider road network through reduced connectivity
- improved journey time reliability on the mainline carriageway only, with detrimental effects to journey time reliability on local roads with reduced connectivity
- limited improvement to accessibility and social exclusion through the provision of complementary transport measures
- disruption to the effective operation of the transport network during times of maintenance through a reduction in junction connectivity.
7.5 North Corridor Option 3 Recommendation
An indicative layout of North Corridor Option 3 is contained within Appendix 5. This option is the least favoured of the northern route corridor options. However, if carried forward to DMRB Stage 2 Corridor Assessment this corridor option could encompass the following:
- a dual three lane motorway (D3M) mainline cross section;
- a reconstructed Ferrytoll Junction, providing:
- connectivity between the new mainline and existing local roads; and
- connectivity for future transport modes;
- provision of north facing connectivity between the proposed mainline and the existing A90/M90 north of Masterton facilitating limited local access; and.
- extension of south facing slips at Admiralty to provided connectivity between the proposed mainline and the existing A90/M90.
In addition, the following junction improvements could also be considered:
- connectivity to the Forth Road Bridge.