Background to the Project
The existing trunk road bridge dates to the 1930s or 1940s and has been stabilised with temporary props since 2001 (Figure 2.1). It is now considered necessary to replace the structure. Existing land use in the vicinity of the bridge consists mainly of deciduous woodland and rough grazing.
Policies and Plans
The proposed bridge upgrade supports the Highland Wide Local Development Plan (HwLDP) (Highland Council, 2012) to ensure better road connections for residents and tourists. In conjunction with this, the Inverness and the West Highland and Islands Local Plans have been considered. Similarly, the proposed scheme is in line with one of the outcomes supported by the Transport Strategy for the Highlands and Islands (HITRANS, 2008), i.e. to improve journey times and connections. The Local Transport Strategy 2010/11 – 2013/14 (Highland Council, 2010) supports improvements to the trunk road network, for example, in LTS Policy No. 2 “… the Council will continue to lobby for improvements to the strategic trunk road network …”.
The River Basin Management Plan for the Scotland River Basin District (2015) has also been considered to seek to ensure the water environment is protected throughout the works.
The Project Objective
The project objective is to replace the A887 Allt Lagain Bhain Bridge with a structure that meets current Design Manual for Roads and Bridges (DMRB) standards. The work is required to meet the existing standards and provide a long-term solution to structural issues with the bridge; it is not required to address environmental issues per se.
Description of the Project
Works to the Trunk Road Bridge
The proposed scheme will entail replacing the existing bridge with a new structure (i.e. a box culvert) on the existing horizontal alignment of the road. Existing annual average daily traffic flow at the bridge is 965. In August daily flows increase to 1367. There will be no predicted increase in traffic flows as a result of the proposed scheme. The existing bridge is single track (Figure 2.2) while the proposed scheme will be upgraded to a standard carriageway width and allow for the eventual upgrading of the short section of single track road lying immediately to the west.
A drawing of the proposed scheme layout can be found in Appendix B - Drawing 09/NW/1202/001/005. The design working life for the new structure is 120 years.
The proposed scheme will have no effect on the horizontal alignment of the A887. The road centre line will be retained and the road cross section widened out from this point. The vertical alignment of the road will be altered to allow additional clearance through the proposed new culvert. The horizontal alignment of Allt Lagain Bhain will be altered at the culvert so it returns to a more natural alignment. The vertical alignment of the road will be raised to allow the culvert to provide the design flood capacity with an allowance for freeboard.
The proposed scheme to replace the existing bridge entails installation of a buried invert segmental precast concrete box structure with internal dimensions 5.5 m width, 1.9 m height at 11.35o skew. The overall skewed length is approximately 12.5 m and the approximate depth of cover is 0.6 m. A 0.5 m wide otter ledge will be cast on one side and the invert of the box structure will be buried to a depth of 0.3 m and the watercourse bed re-instated to reflect a natural state. This will provide a channel opening of 5 m and height of 1.6 m. In situ concrete upstand walls will be cast to the end of the structure and support the masonry bridge parapets.
The foundation options are either a piled foundation or ground improvement with the preferred option being to improve the bearing capacity by use of a geogrid basal platform. Construction of the foundations and installation of the precast structure will require a temporary cofferdam rather than open excavation.
A reinforced concrete U-shape channel will be installed to Allt Lagain Bhain, for approximately 4.6 m upstream of the inlet and 6.4 m downstream of the outlet of the box structure. This will be an in situ structure, varying in height to provide wingwalls and training walls. The otter ledge will be continued along the channel walls. The watercourse bed will be reinstated on the base of the concrete channel to reflect a natural state. There will be no step in the bed along the whole length of the proposed scheme.
Works to the Old Bridge
In order to accommodate the new structure and facilitate safe working, the historic masonry arch bridge lying directly upstream of the trunk road bridge will need to be demolished. The works will require formation of a temporary diversion route to the north of the bridge and realignment of the Allt Lagain Bhain watercourse to its original line, as its current alignment has been skewed to flow perpendicularly to the A887 trunk road.
The realignment will improve flow and the buried invert will allow a natural stream bed to be formed in place of the current artificial brick laid stone arrangement (Figure 2.3). There will be no permanent lighting associated with the finished bridge. Boulders will be placed on the stream bed and set in concrete to form baffles providing both a low flow channel and cover for fish.
Land Use Setting and Land-take
The extent of land-take is shown in Figure 2.4.
The construction of the proposed scheme, subject to satisfactory progression of the statutory process, is currently programmed for 2019. The construction works will take approximately nine months to complete and will be programmed to ensure works within the watercourse are completed during the summer months (1st June to 14th October inclusive). The majority of the works will be undertaken during daytime hours with the possibility of some limited night-time works. Consequently, the need to light the site during construction is likely to be limited.
Although the detailed design is still in progress, Table 2.1 provides an anticipated high-level construction plan for the main works based on similar bridge replacements undertaken by BEAR Scotland.
Table 2.1 Anticipated construction plan
Install traffic management on the A887, set up site compound and construct temporary diversion - six weeks
Switch to temporary diversion, demolish old structure and construct new - three months
Switch traffic on to new road and remove temporary diversion - one month
In addition, landscaping works entailing appropriate planting with native species will be undertaken once the main works are completed.
During the operational phase, it is expected that routine maintenance will be carried out as required over the new bridge’s lifespan.
Design Options that have been Considered
The location of the existing Allt Lagain Bhain Bridge and the relatively small scale of the likely works, led to the conclusion that an on-line solution to the deterioration of the existing bridge would be the most appropriate. Consideration was given to the following options:
strengthening the existing structure through maintenance repairs; and
complete replacement with a new structure.
The “Do Nothing” scenario is not considered viable as this would eventually entail a weight restriction being placed on the bridge forcing HGVs to take a long detour. It would eventually entail the closure of the trunk road.
The maintenance repairs option would involve stripping the surfacing from the bridge, carrying out concrete repairs to the top of the deck, waterproofing the deck, resurfacing, installing new joints and installing new safety barriers. Concrete repairs would also be required at the bottom flanges of the precast beams that form the soffit of the bridge. The temporary propping would also be replaced. Although this option would improve the condition of the current structure, it would not improve its assessed load-carrying capacity or improve the road cross section. This option was, therefore, discounted because the assessed capacity of the structure is only 17 tonnes and considered not sufficient for a trunk road of this nature.
The preferred option is to replace the bridge with a new structure, designed to current loading standards and this is required to allow the long term availability of this route to trunk road traffic.
Further Design Development
Should there be significant amendments and/or variations to the design, an assessment of environmental effects would be carried out as required. This would include consultation with relevant statutory consultees, and presentation of the assessment in an Addendum to this Environmental Statement.