Engineering Assessment

Introduction

The engineering assessment considers the engineering advantages, disadvantages, opportunities and constraints associated with the route options under the key engineering features listed below.

  • Walkers, Cyclists and Horse-riders (WCH);
  • Lay-bys and Rest Areas;
  • Relaxations and Departures from Requirements;
  • Geotechnics and Earthworks;
  • Hydrology, Hydrogeology and Drainage;
  • Structures;
  • Public Utilities; and

General Scheme Information

The proposed A9 will be a Dual 2-lane All-Purpose (D2AP) Road (sub-category c) (formerly known as Category 7A), as defined in the DMRB (CD109: Highway link design), which means it will have no gaps in the central reserve and no at-grade minor junctions. It is also recommended that only grade separated junctions are provided on the route for safe access and egress to the A9. Isolated left-in left-out accesses may be provided in exceptional circumstances. Compact grade separated junctions and at-grade roundabouts are not recommended for D2AP (sub-category c) dual carriageways.

The proposed A9 will have a Design Speed of 120 kilometres per hour (kph) (70 mph) over its entire length. However, the Community’s Preferred Route Option (Option ST2A) includes a section that has a Design Speed of 85kph (50mph) due to forward visibility constraints within the 1.5 kilometre cut and cover tunnel.

The typical cross-section for the proposed A9 provides a 2.5m central reserve, 2 x 1m offside hardstrips, 2 x 7.3m carriageways, 2 x 1m nearside hardstrips, 2 x 2.5m verges as shown in Figure A. Where necessary the central reserve and verges have been widened to ensure the necessary Stopping Sight Distance (SSD) is achieved. It is noted that the cross-section varies within the cut and cover tunnel for the Community’s Preferred Route Option (Option ST2A).

Option ST2A requires structural walls, formed with bored piles, to form the cut and cover tunnel. Similarly, Option ST2B also requires structural walls, formed with bored piles, to form the underpass structure at Dunkeld & Birnam Station. These walls will be up to approximately 8 metres maximum height. Option ST2C requires three short lengths of low height retaining walls to avoid encroachment towards residential properties adjacent to the southbound merge slip road that forms part of Dunkeld Junction. More significantly however, Option ST2C includes a retaining wall up to 14 metres high alongside Dunkeld & Birnam Recreation Club.

The variation between route options is considered to be a differentiator, with Option ST2D the most favourable and Options ST2A and ST2B the least favourable.

Hydrology, Hydrogeology and Drainage

Road drainage for the proposed cut and cover tunnel included in Option ST2A is complex, primarily due to the lowered road alignment, existing topography and narrow corridor. Within the tunnel section, sump tanks will be required, which will need to be emptied via a manhole within the tunnel. As such, one direction of the tunnel would need to be closed for this operation, utilising bi-directional traffic in the other half of the tunnel. Bi-directional traffic in a tunnel is not desirable due to the risk of head-on collisions, and reduced speed limits would likely be employed.

At the northern cut and over tunnel portal within Option ST2A, further complications exist, which requires surface run-off, collected outwith the tunnel, to be passed through the tunnel to reach Inchewan Burn. To limit the potential impact on the tunnel during flood events, an additional sump tank with pump is required.

Option ST2B incorporates an underpass in the locality of Dunkeld & Birnam Station. Filter drains on the A9 dual carriageway would continue through the underpass structure with a Hydrodynamic Vortex Separator (HVS) included beneath carriageway level to treat run-off. A geocellular/modular system is also required below the carriageway, which extends the depth of excavation and introduces a requirement for maintenance, albeit this is infrequent.

Options ST2C and ST2D are generally at-grade throughout, albeit Option ST2C is raised at Dunkeld Junction. Both options include filter drains and detention basins throughout, however Option ST2C includes a geocellular/modular system at Dunkeld Junction, to attenuate surface run-off.

The variation between route options is considered to be a differentiator, with Options ST2C and ST2D the most favourable and Options ST2A and ST2B the least favourable.

Structures

Option ST2A includes a 1.5 kilometre cut and cover tunnel that will be a two span structure, constructed using 1.2 metre diameter bored piles. Option ST2B incorporates a 150 metre long underpass structure that, given the site constraints, would also be formed utilising 1.2 metre diameter bored piles. Both options include a drop structure and culvert at Inchewan Burn. As a result of the A9 dual carriageway lowering works, a further structure is necessary for Options ST2A and ST2B across Birnam Glen and Inchewan Burn to the west of Dunkeld & Birnam Station to provide access to properties on Birnam Glen.

The grade separated junctions at Murthly/Birnam and Dalguise for all options include structures and the grade separated junction included in Option ST2C at Dunkeld requires an additional structure. All options include structural works to lengthen the existing Inver and Inch Rail Underbridges and a new structure alongside the existing River Tay crossing.

The total number of structures for each route option is summarised below.

  • Option ST2A: 10;
  • Option ST2B: 11;
  • Option ST2C: 11; and
  • Option ST2D: 10.

The variation between route options is considered to be a significant differentiator, with Option ST2D the most favourable and Option ST2A the least favourable.

Public Utilities

The route options interact with underground and overground public utility apparatus owned by BT Openreach, Scottish Gas Networks (SGN) Scottish and Southern Energy (SSE) and Scottish Water. The total number of interfaces with utility apparatus is summarised below.

  • Option ST2A: 98;
  • Option ST2B: 98;
  • Option ST2C: 87; and
  • Option ST2D: 85.

The impacts between route options are broadly comparable. Options ST2A and ST2B include lowering works in the locality of Dunkeld & Birnam Station, which will add complexity to diversion works. However, it is envisaged that post construction, measures can be employed to divert the utility apparatus.

The variation between route options is not considered to be a significant differentiator.

Constructability

Construction of the 1.5 kilometre cut and cover tunnel for Option ST2A, and the underpass for Option ST2B, in such a constrained and sensitive corridor, will be complex. As insufficient space exists for an open excavation, the walls that form part of the cut and cover tunnel, and the underpass, would be constructed using large diameter bored piles (1.2 metre diameter) to retain a height of approximately 10 metres. Installation will require heavy plant in close proximity to residential properties, Dunkeld & Birnam Station, the Highland Main Line railway and the Category A Listed station building. Construction will generate noise and vibration, with the potential to affect residential properties immediately adjacent to the proposed A9 as bored piles are formed over a significant length. Maintaining bi-directional traffic flows on the A9 throughout construction will be challenging, and reduced speed limits and narrow lane widths will be required.

For Options ST2A and ST2B, construction works will be undertaken approximately 2.5 metres from the Category A Listed station building, with potential risk of accidental damage. It is not anticipated that the works will have a structural impact on residential properties. However, before commencement of piling works, Pre-Construction Condition Surveys may be undertaken

Works to lower Inchewan Burn for Options ST2A and ST2B are complex and will require permanent and temporary bored and sheet piling works, as well as the erection of a temporary A9 bridge, to allow the existing A9 structure to be demolished. The burn itself would be diverted through a temporary culvert while the permanent drop structure and box culvert are constructed.

As Option ST2C is above existing carriageway levels, it requires retaining walls of less than 2 metres high for short lengths on the east side adjacent to residential properties. These walls will likely be simple in-situ concrete or pre-cast L-shaped gravity walls, which are relatively simple to construct. No retaining wall solutions are required on the west side alongside the Highland Main Line railway as a natural earthwork slope can be accommodated, subject to the existing railway bund being removed.

Option ST2D is largely at-grade and while there will be some construction challenges, they are not as significant as the other options.

Anticipated construction durations for the route options are shown below. This assumes a 6-day working week (Monday to Friday, 7am to 7pm, Saturday 8am to 1pm, with no night-time, Sunday and Bank Holiday working).

  • Option ST2A: 4 ½ to 5 years;
  • Option ST2B: 4 to 4 ½ years;
  • Option ST2C: 2 ½ to 3 years; and
  • Option ST2D: 2 ½ to 3 years.

The variation between route options is considered to be a significant differentiator, with Option ST2D the most favourable and Options ST2A and ST2B the least favourable.

Figure A.3: Proposed A9 Dual Carriageway Cross-section, as described in the the text above
Figure A.3: Proposed A9 Dual Carriageway Cross-section

The proposed A9 is a designated high load route. As a result, the headroom clearance for new structures over the dual carriageway will be a minimum of 6.45 metres, in accordance with the DMRB (CD127: Cross-sections and headrooms). The headroom clearance for structures under the proposed A9 and at side roads will be 5.3 metres.

It is noted that, for safety reasons, pedestrians, cyclists, motorbikes (with engines less than 50cc), animals and animal drawn vehicles are not permitted to use the cut and cover tunnel (Option ST2A).

Engineering Assessment Summary

The engineering assessment is summarised below.

WCH

There is a significant network of WCH routes within the Pass of Birnam to Tay Crossing section of the A9 Dualling Programme, comprising Core Paths, Rights of Way, National Cycle Network (NCN) Routes and Regional Cycle Network (RCN) Routes. The route options impact WCH routes at various locations and suitable mitigation to maintain connectivity is necessary.

One of the most significantly impacted routes is NCN Route 77, which is currently segregated from vehicular traffic between the existing left/right staggered priority junction with the B867 and Perth Road at Birnam and Dunkeld & Birnam Station. The route will be diverted onto the realigned B867/Perth Road for all options, however with Option ST2A there is an opportunity that the route can be placed on top of the cut and cover tunnel.

All options improve accessibility to the station, compared to the existing condition. Options ST2A and ST2B provide a direct connection from Station Road by lowering the proposed A9 dual carriageway. Options ST2C and ST2D incorporate a pedestrian underpass structure, with associated lifts and stairs. There is potential to provide Equality Act 2010 compliant access between station platforms, which will be investigated as part of the DMRB Stage 3 assessment, in conjunction with Network Rail and Transport Scotland.

All options impact existing WCH routes in the locality of the River Braan, albeit this impact is heightened for Option ST2C due to the larger land-take associated with the grade separated junction at Dunkeld. The impacts in the locality of the River Tay crossing are common to all options.

The variation between route options is not considered to be a significant differentiator.

Lay-bys and Rest Areas

The options under consideration include junctions at Murthly/Birnam, Dunkeld, The Hermitage and Dalguise. In addition, Options ST2C and ST2D include a left-in left-out junction at Dunkeld & Birnam Station, to provide maintenance access to the station and Highland Main Line railway. As a result of the frequency of junctions within the 8.4 kilometre scheme and considering the required weaving length (distance between junctions) to lay-bys and other geometrical parameters for the design of lay-bys, it is not proposed to provide any parking lay-bys. However, as part of the DMRB Stage 3 assessment, consideration will be given to upgrading the existing lay-bys to the immediate south of the scheme.

Relaxations and Departures from Requirements

In accordance with the DMRB (CD109: Highway link design) Relaxations and Departures from requirements have been considered in the design. This is predominantly to reduce impacts on adjacent environmental and physical constraints and to lessen construction complexity and costs.

Reductions in design standards are assessed as either being Departures from requirements, Relaxations or Deviations from recommendations, as detailed in the DMRB (GG101: Introduction to the Design Manual for Roads and Bridges) and summarised below.

  • Departures from requirements
    • Where the requirements set out in the DMRB are not met, a departure application shall be submitted before the design is finalised. Departures should be submitted where it can be justified, it would not have unintended adverse consequences, innovative methods or materials are proposed, or a requirement not detailed in the DMRB is proposed.
    • All Departures must be approved by the Overseeing Organisation prior to being incorporated into the works.
  • Relaxations
    • Relaxations shall be applied where they are permitted within the appropriate section of the DMRB, with suitable justification for its inclusion recorded.
  • Deviations from recommendations
    • A Deviation is where recommendations contained within the DMRB are not followed. Suitable justification for the use of Deviations should be documented and include a comparison of costs and time of the proposed solution, compared to the approach recommended in the DMRB.
    • A safety risk assessment should be undertaken where a Deviation is proposed.

Where Departures from requirements, Relaxations and Deviations from recommendations are applied, careful consideration must be given to the safety implications and any requirements for mitigation measures, such as additional signage or road markings, to reduce or eliminate potential hazards.

As part of the DMRB Stage 2 assessment, only Departures from requirements and Relaxations have been considered. Deviations from recommendations will be fully considered as part of the DMRB Stage 3 assessment for the Preferred Route Option. It is noted that the most evident Deviation from recommendations within the DMRB Stage 2 assessment is the inclusion of an at-grade roundabout at Dunkeld Junction for Options ST2A, ST2B and ST2D.

The combined total of Relaxations and Departures from requirements for the Whole Route Options are listed below.

  • Option ST2A: 13 Relaxations and 30 Departures;
  • Option ST2B: 21 Relaxations and 28 Departures;
  • Option ST2C: 16 Relaxations and 17 Departures; and
  • Option ST2D: 20 Relaxations and 28 Departures.

The variation between route options is considered to be a differentiator, with Option ST2C the most favourable and Option ST2A the least favourable. Option ST2A is considered the least favourable due to its reduced speed limit, as it includes an at-grade roundabout and as it prohibits certain usage.

Geotechnics and Earthworks

Option ST2A requires significant excavation to form the 1.5 kilometre cut and cover tunnel (approximately 535,000m3). Option ST2B also requires excavation to form the underpass structure at Dunkeld & Birnam Station. While not as significant as Option ST2A, the excavated volume (approximately 168,000m3) is still extensive.

Options ST2A and ST2B also impact the Ladywell Landfill site, located to the west of the Highland Main Line railway, north of Inchewan Burn, as a result of the access road to properties on Birnam Glen, west of Dunkeld & Birnam Station. While the works are largely outwith the known boundary of the site, there is potential for contaminated ground to be encountered in this area, which may require treatment.

Option ST2C is raised above existing carriageway levels north of Dunkeld & Birnam Station to facilitate a grade separated junction at Dunkeld. This requires imported fill material (approximately 287,000m3). Option ST2D is largely at-grade and on-line, therefore there are no significant excavations or embankments.

A summary of the total volume of material required for import and export for each route option is given in Table A.2.

Table A.2: Earthwork Volume Summary
Route Option Total Import (m³) Total Disposal (m³)
Option ST2A 0 698,000
Option ST2B 0 356,000
Option ST2C 287,000 146,000
Option ST2D 0 163,000