Initial options generation and development

Online OMR Options

The OMR was analysed to identify existing problems and generate options to increase resilience of this temporary diversion route. The options generated were grouped into four categories: operational improvements; geotechnical hazard mitigation; flooding mitigation; and infrastructure integrity.

Operational Improvements

The following potential improvement measures were identified to address operational issues due to width constraints, significant vertical gradients and tight horizontal geometry at the northern section of the existing OMR:

• Increasing the radius of existing horizontal radii that are below Desirable Minimum for a Design Speed of 50kph (R=180m), where feasible.
• Curve widening on the three bends, in the northern section of the OMR, highlighted as particularly problematic by the Operating Company BEAR Scotland.
• Decreasing the vertical gradient to a maximum of 8% (a DMRB permitted relaxation), where feasible.
• Increasing the length of two-way working beyond the current 900m length at the southern end.

More discrete measures were also identified, including edge protection in the form of kerbing or hazard markers, safety fencing at high-risk areas, appropriate signage and road markings for clearer delineation.

Geotechnical Hazard Mitigation

The following measures were identified as potential options to increase resilience of the OMR to the risk of landslide and debris flow events:

• Raise the OMR level (embankments) at critical locations.
• Install further lengths of (HESCO) debris bunds at critical sections.
• Install lengths of rockfall catch fences at critical locations.
• Install lengths of debris catch fences at critical locations.
• Expedite short-term mitigation works.
• Reduce the A83 Trunk Road to single lane working and install or extend trunk road centre line barrier.
• Re-profiling of identified critical channels above the OMR.
• Bridging flow paths over the OMR.
• Install further catch fences on A83 Trunk Road for currently unprotected channels.

Flooding Mitigation

The following measures were identified as potential options to increase resilience by reducing the likelihood of closure due to fluvial flooding at the southern end of the OMR.

• Flood bunds
• Change vertical alignment / raise road at this location
• Re-align / relocate junction
• Install flood relief culverts.

The following measures were identified as potential options to increase the resilience by reducing the likelihood of closure of the OMR due to surface water flooding:

• Improvements to existing or provision of new side of road drainage and / or slope drainage in key locations to intercept flows.
• Attenuation ponds, catch pits or sacrificial areas for collecting surface water and debris before the road.
• Upslope bunds that capture out of bank flows.
• Strategic up-sizing of culverts.

Infrastructure Integrity

As part of the existing OMR analysis, the need for assessment of the existing infrastructure was identified, to establish if any remedial or improvement works are required, including:

• Soil nail slopes, rock blankets, and retaining walls between the A83 Trunk Road and OMR.
• Road pavement, i.e., carriageway reconstruction, re-surfacing or high friction surfacing may contribute to increased resilience of this temporary diversion route.
• Three existing bridges: two masonry arch bridges and one reinforced concrete.
• Numerous culverts and pipes of varying diameter and condition.

Offline OMR Options

Three potential offline OMR options, Options 1, 2 and 3, were identified. All three options avoided privately owned land and provided full two-way working. The three Offline OMR Options developed are presented in Figure 2.

Option 1 (the red route in Figure 2) follows the OMR from its southern junction with the A83 Trunk Road for approximately 700m before heading west and crossing the Croe Water. Option 1 then follows the existing contours on the western slopes of the glen, before tying back into the northern section of the OMR, avoiding the hairpin bend. The gradient of the existing terrain, at this northern section tying back into the OMR, is approximately 40%.

Option 2 (the turquoise route in Figure 2) follows the existing OMR from its southern junction with the A83 Trunk Road for approximately 250m before crossing the Croe Water and following the existing contours near the foot of the western slope of the glen. Option 2 then ties back into the B828 Glen Mohr local road, rather than the OMR. The gradient of the existing terrain, at this northern section tying back into the B828 Glen Mohr local road, is approximately 30%.

Option 3 (the dark blue route in Figure 2) follows the existing OMR from its southern junction with the A83 Trunk Road for approximately 820m before heading west and crossing the Croe Water. Option 3 then follows the existing contours, near the lower forestry track, on the western slope of the glen, before tying back into the northern section of the OMR, bypassing the hairpin bend and looping around to follow contours and avoid the steep gradients at the northern end. The gradient of the existing terrain, to climb up to the lower forestry track, is approximately 22%.

Further Online (OMR) Option Development

Additional options involving modifications to the existing OMR were also considered and three Online (OMR) Options were developed. These all provided a width suitable for two-way traffic and considered the potential for a maximum vertical gradient of 8%. The three Online (OMR) Options developed (Existing OMR Option, Option G and Option K) are described below.

An option was also considered that provided full two-way working along the line of the existing OMR over its full length, adopting the existing horizontal and vertical geometry. The widening required to achieve two-way working resulted in major engineering challenges, particularly in relation to extensive earthwork cuttings, resulting in substantial additional costs and making it extremely difficult to keep the OMR available as an emergency diversion route. This was not considered a proportionate response for a medium term solution. Consequently, this option was not pursued.

Existing OMR Option

The Existing OMR Option is approximately 4.0km in length and was developed based on a maximum vertical gradient of 8% over the majority of the route but using the existing sub-standard horizontal alignment. These vertical alignment improvements resulted in large embankments of over 58m in height at the northern end. The Existing OMR Option is shown below in Figure 3.

Option G

Option G is approximately 4.0km in length and was also developed based on the existing sub-standard horizontal alignment but re-aligned at the hairpin bend. A relaxation of the 8% maximum to a 10% gradient was also applied to the climbing section in an effort to reduce the large embankment in the Existing OMR Option. However, even with a steeper gradient the height of the embankment was still over 40m and large cuttings of over 32m in height were present at the hairpin bend. Option G is shown below in Figure 4.

Option K

The final online (OMR) option is also approximately 4.0km in length and was designed to realign the northern section of the OMR horizontally to follow the existing contours at a 10% gradient and improve the geometry of the hairpin bend.  Whilst the realignment helped to reduce the size of the embankment, to approximately 30m in height, the realigned hairpin bend requires a steep embankment of over 80m in height and large cuttings of over 45m in height. Option K is shown below in Figure 5.

Further Offline Option Development

Following initial consideration of the options, further development of the offline route options was taken forward. A variety of high-level options, based on the criteria list below, were developed. The Offline Options developed (Options A to F) are described below.

Offline route option development was carried out using the following design criteria:

• DMRB Design Standards using 50kph design speed.
  • Desirable Minimum Horizontal Radius R=180m
  • Desirable Minimum Crest Curve K=10
  • Desirable Minimum Sag Curve K=9
  • Minimum Vertical Gradient = 0.5%
  • Maximum Vertical Gradient = 6% (permitted relaxation of up to 8%)
• Cross section of 3.65m lane width, 1.0m verge width and 1:2 cut/fill slopes.

Option A

Option A is approximately 3.3km in length and follows the bottom of the Glen before making the climb up towards the RABT car park by looping along the contours to tie into the end of the hairpin bend. Figure 6 includes a plan of Option A.

It was not possible to maintain the permitted relaxation of 8% vertical grade due to the large level difference of the Glen and the RABT. A maximum gradient of 14% was required to climb the steep slope. This option produces large earthworks quantities and geotechnical measures would be required to strengthen the slopes and existing ground.

Option B

Option B is approximately 3.1km in length and, again, follows the bottom of the Glen for the first half before heading towards the western side of the hillside and looping round to tie in at the RABT car park. Figure 7 includes a plan of Option B.

The same issue occurs here with the vertical grade. Again, the maximum gradient is approximately 14%. Although less than Option A, there is still a significant volume of earthworks which would require geotechnical solutions to strengthen the slopes and existing ground.

Option C

Option C is approximately 3.0km in length and follows the same alignment as Option B but deviates from the Glen sooner to try and relax the maximum 14% gradient. Figure 8 includes a plan of Option C.

The maximum vertical grade on Option C is 13%, which is only a slight improvement on Option B and deviating earlier increases the height of the embankment on the uphill climb.

Option D

Option D is approximately 3.3km in length and introduces a winding alignment along the western hillside to increase the length of the road and hence decrease the maximum vertical grade. It ties back into the RABT car park. Figure 9 includes a plan of Option D.

The winding alignment on the western hillside means that the topography becomes very hilly. In order to minimise the volume of earthworks, the alignment would need to follow the hilly sections. This produces vertical grades of up to 29% and large cut and fill heights.

Option E

Option E is approximately 3.9km in length and introduces a switchback arrangement on the western hillside to increase the length of road and hence decrease the maximum vertical grade. Figure 10 includes a plan of Option E.

Overall, earthworks are kept to a minimum except at the switchback section which would be located on a high embankment. This option does not help improve the maximum vertical grade, with a maximum gradient of approximately 23%.

Option F

Option F is approximately 3.1km in length and deviates from the Glen quite quickly after leaving the existing OMR carriageway to sit on the western hillside with the alignment following the contours of the hill. Figure 11 includes a plan of Option F.

Large earthworks volumes are generated, and the maximum vertical grade is approximately 27%.

Initial Offline Route Options - Sifting

Options A – F were all discounted due to the steep vertical gradients and extensive earthworks, and refinements to these into two new options, Options H and J, were then considered.

Option H

Option A was used as the basis for developing the new Option H. The main refinements from Option A to Option H were:

• reducing the minimum horizontal radii from R=127m to R=50m
• reducing the minimum sag curve from K=20 to K=9
• reducing the minimum crest curve from K=30 to K=10
• reducing the maximum gradient from 14% to 8%
• junction tie in with B828 Glen Mohr local road instead of existing OMR.

Option H is approximately 3.3km in length and avoids any overlap with the current OMR. The embankment present on the switchback section reaches a height of over 76m sitting on the deep slope of the Glen. Figure 12 includes a plan of Option H.

Option J

Option F was used as the basis for developing the new Option J. Figure 13 includes a plan of Option J.  The main refinements from Option F to Option J were:

• reducing the minimum horizontal radii from R=180m to R=50m
• reducing the maximum gradient from 27% to 8%
• junction tie in with B828 Glen Mohr local road instead of existing OMR.

Option J is approximately 3.2km in length with the switchback provided at the south/east end to allow the scheme to follow along the western hillside. The placement of Option J on the western hillside of Glen Croe produces cuttings of over 172m in height.

Offline Options Aligning with the Forestry Track

In addition to the above options, consideration was given to options that followed the existing forestry track alignments, where possible, but limiting the gradient to 8%, these are identified as Options L, M and N.

Option L

Option L is approximately 3.2km in length. It begins at the junction with the A83 Trunk Road and joins the lower forestry track, keeping close to the track for its full length and ending at a junction with the B828 Glen Mohr local road. Figure 14 includes a plan of Option L. Similar to Option J, its location on the western hillside with 1:2 cut slopes produced cuttings of over 169m in height along the majority of the length.

Option M

Option M is approximately 3.2km in length. It begins at the junction with the A83 Trunk Road and joins the lower forestry track, keeping close to the track for its full length while avoiding any overlap with the LTS Green Route currently being developed at the time, and ending at a junction with the B828 Glen Mohr local road. Figure 15 includes a plan of Option M. The positioning of Option M further up the western hillside from Option L increased the maximum height of cutting to over 177m.

Option N

Option N is approximately 3.4km in length. It begins at the old junction with the A83 Trunk Road and crosses the hillside to join the upper forestry track, keeping close to the track for its full length and ending at a junction with the B828 Glen Mohr local road. Figure 16 includes a plan of Option N. The positioning of Option N and the provision of 1:2 slopes produce cuttings of over 150m in height along the majority of its length.

Final Options for Initial Shortlisting

The final options to be considered for shortlisting were the Existing OMR Option and Options G, H, J, K, L, M and N. Original offline OMR Options A, B, C, D, E and F were not considered for shortlisting.