Initial short listing
Initial Option Sifting
All remaining options were categorised into 3 groups: Online OMR Options; Glen Options; and Forestry Track Options and considered at an initial option sifting stage, as summarised below.
Online OMR Options
- Existing OMR Option
- Option G
- Option K
Offline Options within the Glen
- Option H
- Option J
Offline Options aligning with the Forestry Track
- Option L
- Option M
- Option N
Assessment Approach
A simple colour based (Red-Amber-Green or RAG) traffic light reporting system was developed to demonstrate how each option performed against each of the following twelve assessment criteria:
- Topography and Alignment Considerations
- Geology/Geomorphology Considerations
- Structures Considerations
- Drainage and Flooding Considerations
- Constructability Considerations
- Environmental Considerations
- Need for Third Party Land
- Safety Considerations
- Operational Considerations
- Financial Considerations
- Construction Timeline
- Programme to Procurement.
Initial Options Sifting Assessment Summary
The assessment for the eight options presented is summarised in Table 5.1.
| Options | Topographical & Alignment |
Geology/ Geomorphology |
Structures Considerations |
Drainage & Flooding |
Constructability Considerations |
Environmental Considerations |
Need for Third Party Land |
Safety Considerations |
Operational Considerations | Financial Considerations | Construction Timeline | Programme to Procurement |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Existing OMR | R | R | G | R | R | A | R | A | G | A | A | A |
| G | R | R | G | A | R | A | R | A | G | G | G | A |
| K | R | R | G | A | R | A | R | A | G | G | G | A |
| H | R | R | G | A | A | R | A | A | R | A | A | A |
| J | R | R | G | G | A | R | A | A | R | A | A | A |
| L | A | R | G | R | R | A | G | A | G | A | A | A |
| M | A | R | G | R | R | R | G | A | G | R | R | A |
| N | A | R | G | R | R | R | G | A | G | R | R | A |
At the initial options sifting stage there was concern on the extent, impact and cost of all the offline options due to the magnitude of the earthworks required.
The outcome of this stage was that in addition to targeted interventions on the existing OMR, a single optimum Glen Option and a single optimum Forestry Track Option were taken forward for further development and assessment. Option L was chosen as the Forestry Track Option to be taken forward because options M and N costs were considered prohibitive. Option H was chosen as the Glen Option to be taken forward because it was considered to be the better alignment. The remaining options, G, K, Existing OMR, J, M and N, were not taken any further.
Glen Option (H) – Further Development and Assessment
Various options for refining the alignment were considered in the further development of Option H, but the option has significant constraints, including:
- Steep gradients at 8% over significant lengths which will affect vehicle speeds uphill and safety for vehicles travelling downhill, particularly where the steep gradients coincide with the switchback section of the alignment at the north-western end. This means this option would potentially require a convoy, as on the current OMR diversion, as a safety measure.
- Earthwork cuttings interfacing with the Green Option currently being considered as a possible route option for the LTS which would present significant constructability challenges for the Green Option, should it be taken forward as the LTS preferred option. There are limited opportunities to realign the Green Option to avoid this effect without significant adverse effects on its alignment, earthworks, and the overall scheme footprint.
- Embankments up to over 80m high on the downslope side are required which would be extremely complex to construct and present stability issues due to the underlying ground, inevitably requiring extensive geotechnical or structural measures to reduce the risk of embankment instability.
Overall, it was not considered that Option H would offer any significant advantages compared to Option L.
Given the impact of the option on the Glen, likely high costs and earthwork cuttings interfacing with the Green Option currently being considered as a possible route option for the LTS, Option H was therefore not recommended for further consideration.
Forestry Track Option (L) – Further Development and Assessment
In order to minimise the long-term impact on the Glen and minimise abortive works, the Forestry Track Option was further developed taking into account its interaction with the LTS Green Route. Two interaction scenarios were considered:
- Scenario 1 – an alignment where the MTS follows the line of the Green Option; in this case the MTS could potentially then be upgraded to become the LTS.
- Scenario 2 – an alignment that keeps the MTS independent of the Green Option; in this case the MTS could either remain as an access route through forestry land once the LTS is provided or be removed.
The differing scenarios relate to constructability of the Green Option, should it be selected as the preferred option for the LTS. If it is not possible for the MTS to remain open as a diversion route during the upgrade to become the LTS (Scenario 1), then an additional diversion route would be needed at that time, likely to involve upgrading the OMR. For Scenario 2, the MTS could remain as the diversion route during construction of the Green Option.
Whilst construction is expected to be complex, it is considered feasible to maintain a diversion route during construction of the Green Option. That being the case, in order to avoid the construction of two separate new roads on the southwestern slopes of the Glen, Scenario 2 was not recommended for further consideration.
Following consultation with the A83 Campaign Group, a single lane option of Scenario 1 was also taken forward and referred to as the Single Lane Forestry Track Upgrade in order to consider whether the significant earthworks and associated impacts, costs and construction implications could be reduced.
Agreed Short List
From the assessments undertaken, the following options were taken forward:
- Single Lane Forestry Track Upgrade;
- OMR Interventions; and
- Developed Option L - Two Way Offline MTS along the line of the ‘Green Route’.