3. SCHEME DESCRIPTION 3.1. Introduction 3.2. Scheme summary 3.3. Bridge specification 3.4. Road improvements 3.5. Construction 3.6. Significant and disruptive works stages 3.7. Assumptions and uncertainties


3.1. Introduction

The bridge is to be constructed on Rannoch Moor (Figure 2.1), which is a large, broad and gently undulating plateau of moderate elevation supporting an extensive area of western blanket and soliginous/valley mire. The bridge spans the River Ba between Lochan na Stainge and Loch Ba within a SSSI, SAC and adjacent to an SPA that supports an internationally important breeding population of Black-Throated Divers. The site also supports other European protected species as well as lying within the Glen Coe and Ben Nevis NSA.

3.2. Scheme summary

The new bridge will be constructed on the same alignment occupied by the existing Ba Bridge. The bridge will be wider than at present to accommodate a 6 m carriageway, two 1 m wide hard strips and two 1.5 m wide verges. The new bridge will cross the watercourse in three spans using the two existing intermediate piers as central supports. The intermediate piers shall be taken down by approximately 1 m to facilitate a new approximately 400 mm deep pre-cast reinforced concrete capping beam, two free sliding pot bearings (per pier) and the new deeper deck. New reinforced concrete abutments shall be constructed behind the existing masonry abutments to minimise the construction activity adjacent to the watercourse. The existing abutments shall be taken down by approximately 1.25 m and hence shall not be connected to the bridge deck. The wing walls are to be shaped to suit the 1.25 m cut-down abutment and the wider deck.

During construction, traffic flow will be maintained throughout on a temporary bridge, situated within the existing embankment footprint as much as is practically possible, however, it may be necessary to extend the works slightly beyond the existing footprint in order the accommodate the carriageway cross-section.. The traffic is to be moved to the temporary bridge to allow the demolition of the deck. The new deck, pier tops and abutments are to be constructed as shown in Figure 2 (Appendix A) and upon completion the traffic is to be moved onto the new structure, allowing the removal of the temporary bridge and the construction of the remaining portion of deck and abutments. During all stages of demolition and construction, a crash deck or similar structure shall be in place to protect the watercourse from any construction debris.

Since the new deck is to be wider than the existing deck, small approximately 2.5 m high, 5 m long reinforced concrete retaining walls are to be provided at the four bridge corner tie-ins.

Provision will be made for pedestrians on the temporary bridge and maintained throughout the construction phase. (Figure 2, Appendix A).

During construction, the Contractor’s offices may be located in a lay-by on the northbound carriageway, approximately 150 m north of the bridge and at the informal lay-by opposite this on the southbound carriageway. The site extents should mostly be within the existing embankment footprint so as to minimise the overall area of peatland to be impacted, however, it may be necessary to extend the works slightly beyond the existing footprint in order the accommodate the carriageway cross-section.. The temporary bridge alignment will be located immediately west of the existing bridge (Figure 1, Appendix A).

On completion of the scheme the area shall be landscaped using seed mix of local provenance to reinstate it, as near as possible, to its existing condition. Peatland turves removed and stored during construction will also be reinstated. There is an area adjacent to the southbound carriageway opposite the existing lay-by which contains much old road debris. It is proposed that this area will be made into a formal lay-by and landscaped sympathetically as part of an additional scheme and is not considered further within this ES.

3.3. Bridge specification

3.3.1. Bridge

The new bridge will cross the watercourse in three spans of approximately 11.5 m, 8.5 m and 11.5 m, spanning from the new abutments (behind the existing) onto modified existing intermediate piers. The skew angle of the existing and new bridge is 0 degrees. The bridge deck shall comprise a reinforced concrete slab deck on 6 longitudinal ‘weathering steel’ girders with identical transverse girders provided at each of the four supports. The weathering steel is designed so that in its early service life a protective patina of oxidation (‘rust’) forms on its surface and thereby provides corrosion protection to the girders. This obviates painting of the steel. The patina develops over a number of months and gives the appearance of a slightly textured typically deep brown/purplish colour that is intended to visually blend with the environment. The soffit of the girders shall be at a level of approximately 2.5 m above water level. The construction sequence for the bridge is detailed in Table 3.1.

Table 3.1 Anticipated Key Construction Sequence


Anticipated Key Construction Sequence


Establish site compound


Place sediment traps and matting on the edges of the water course and then temporarily remove peat from areas where temporary bridge foundations will be laid.


Construct foundations ensuring minimal disturbance to neighbouring habitats, place the temporary bridge ensuring no in-stream working and divert traffic to temporary bridge.


Demolish existing structure. Part demolish intermediate piers to level to accept new capping beams. Part demolish abutments to level necessary to provide adequate clearance to permanent superstructure. Demolition is to be carried out using controlled methods and adherence to environmental restrictions, including no in-stream working, provision of a catch deck and netting to catch debris.


Behind existing abutments, excavate to formation level, removing any loose or fractured rock head ensuring that material does not enter the watercourse or smother surrounding vegetation.


Cast blinding for new abutment pier foundations.


Install pre-cast pier-capping beams and cast abutment foundation bases and columns. Cast plunge columns into position to top of abutment piers.


Erect steel framework adjacent to the works away from the watercourse, because full penetration butt welding is required and ground-level working is most beneficial.


Backfill and compact behind existing abutments/wing walls until formation level for wing walls is reached, ensuring no material enters the l watercourse.


Crane entire steel framework into position ensuring that no material enters the watercourse.


Cast in situ reinforced concrete end diaphragms.


Cast remaining in situ reinforced deck elements.


Cast blinding layer for wing walls and cast in situ reinforced concrete wing walls.


Conclude backfilling operations.


Install pre-cast parapets and complete carriageway/bridge.


Remove temporary bridge and reinstate peat in areas of temporary bridge foundations


Complete landscaping, replacement of peat and reseeding where required.


Remove site compound and clean site-compound area.

3.3.2. Parapets

The aim of the parapet provision is to provide similar parapets on all the bridges being replaced in Rannoch Moor and Glencoe. The parapets on Ba Bridge shall be the same as that provided on the south side of Lairig Eilde Bridge. The parapet comprises a part height concrete barrier with single top tubular rail.

3.3.3. Drainage

The existing bridge drains via down pipes directly into the watercourse. In the new scheme a combined kerb and drainage system will be provided. Ba Bridge and the northern tie-in section drains to kerb drainage units across the bridge which outfall at the south end of the bridge into 2 catch pits through 225mm diameter pipes. The tie-in to the south is kerbed for a length of approximately 30m with kerb drain units at the southern extent to catch run-off. These discharge to the 2 catch pits through 225mm pipes. The catch pits are 1050mm diameter with a 300mm sump. The 2 catch pits discharge through a masonry or precast headwall to a grass swale, 450mm (base width) x 300mm (depth), which follows the existing ground slope at approximately 1:250.  This then discharges to the River Ba.

3.3.4. Footways

1.5 m footways are to be provided at each side of the new bridge. These are to be of the same construction as at Lairig Eilde bridge and shall meet the widened verges at the bridge extents.

3.4. Road improvements

The replacement bridge is on the same alignment as the existing. There will be some minor road improvements required north and south of the bridge to accomplish the increased carriageway width at the bridge and subsequent tie in.

3.5. Construction

The construction period required for the entire scheme will be in the region of 10 - 12 months. It is anticipated that the contractor shall work normal construction hours, which will be from 0700 hours to 1900 hours during summer months and may be less in winter, seven days a week. Night time working will be kept to a minimum in order to prevent disturbance to sensitive nocturnal species such as otter and bat.

3.6. Significant and disruptive works stages

3.6.1. Traffic management

It is likely that one-way traffic management will be continuous throughout the construction period.

  • In addition to normal construction works, disruption for site access and transport of materials is anticipated.

3.6.2. Temporary bridge

Following a survey of a protected species of freshwater invertebrate by the Scotland TranServ Environment Team during summer 2008 (confidential Appendix D), it was advised that the temporary bridge should ideally be located on the eastern, downstream side of the existing bridge to minimise the impacts on the species which are prevalent on the upstream, western side. However, ground conditions on the eastern side of the bridge were deemed unsuitable. Instead the temporary bridge will have to be sited on the upstream, western side of the existing bridge. It will be placed as close as possible to the existing bridge in order to minimise potential disruption to the protected freshwater invertebrate species. Ideally the drainage should be collected from the temporary bridge to maintain water quality, however, due to the nature of the construction of the temporary bridge this has been deemed unfeasible by the scheme designer, The temporary bridge will however be raised in relation to the existing road, therefore run-off entering the watercourse will be minimized to that coming from the bridge itself and not from the road.

In order to construct the temporary bridge without temporary closure of the A82, the main span of the bridge must be constructed on-site, adjacent to and level with its final location so that it can be pushed into place. In order to achieve this, temporary trestle structures are required on either side of the main span. The temporary trestle structures shall require adequate foundations. A crane hard standing area is required on both sides of the bridge to assist in the launch of the main span. Once the main span of the temporary bridge is in position, the temporary trestle structures are to be dismantled and the approaches are to be constructed. The approaches and temporary bridge itself require adequate foundation arrangements.

The foundations for the temporary bridge are expected to be precast mass concrete blocks but there may be additional gabions and fill. Granular fill may be required in the base of the foundations to provide a level surface for the precast blocks. It is proposed that the peat at the foundation locations is carefully excavated and stored for later replacement. The precast mass concrete foundations shall be craned into the excavations and shall bear directly onto the underlying bedrock, approximately 2.5 m below the existing ground level. Upon removal of the temporary bridge, the foundations are to be taken out of the excavations and the peat is to be restored and landscaped as far as possible to its existing condition.

3.6.3. Bridge demolition

The demolition of the existing Ba Bridge is a two-stage, controlled process. Prior to any demolition work, a suitable crash deck or similar structure shall be provided in the adjacent area to protect the watercourse from falling debris and other waste arising. In order to minimise dust and debris, the existing bridge deck shall be removed by the formation of a number of fragments formed by stitch drilling/coring from above with dust removal by vacuum.

3.6.4. Bridge construction

The bridge specification has been provided in section 3.3. The abutments and retaining walls are to be formed in situ from vehicle-delivered, ready-mixed concrete. The deck is formed by placing the prefabricated longitudinal and transverse girders onto the abutments and intermediate support bearings. Permanent corrugated glass reinforced plastic (GRP) formwork spans between the longitudinal girders and forms the support for the concrete deck slab. The edge sections of the deck, which cantilever away from the outermost longitudinal girders (and are thus unsupported by any permanent formwork), shall be formed using contractor designed methods. No such construction activity is to take place without the appropriate crash deck or similar structure provision.

The new bridge construction will incorporate the following mitigation measures:

  • In-river working will be restricted to the immediate vicinity of the existing bridge with no machinery access at this point. In-river working upstream of the bridge will not be permitted in order to safeguard the population of protected freshwater invertebrate species.
  • Measures will be taken to prevent sediment run-off in surface water and prevent sediment entering the watercourse. Measures include the use of temporary sheeting over exposed materials, including soils and use of silt traps. The requirements for such measures shall be incorporated into the specification for the works.
  • Measures will be taken to ensure that in situ concrete is placed accurately within a sealed off area ensuring that concrete pumps, including their wash-down, do not discharge into the river. Preventative measures will include temporary scaffolding screens.
  • The design of the bridge incorporates stainless steel reinforcements. The use of stainless steel is preferential to high-yield steel (used on most bridges) as it eliminates the need for hydrophobic pore lining impregnates (silane or siloxane). These impregnates are normally used to prevent reinforcing bars in the concrete corroding over time but are toxic in their liquid form during placement and require re-application at approximate 20 year intervals; and
  • The bridge deck has been made integral with its abutments to prevent the requirement for bearings at these locations thus reducing the requirement for future maintenance and hence access to underneath the bridge. The bearings on the intermediate piers are unavoidable and shall be specified as stainless steel to provide additional durability, thus reducing the maintenance requirements.

3.6.5. Imported materials

  • Concrete: estimated at 215m3 ready mixed structural concrete plus an additional 60m3 for concrete kerb haunching;
  • Reinforcement: 68.5 tonnes of stainless steel and stainless-steel products;
  • Weathering steel: 53 tonnes, prefabricated bridge girders;
  • Road construction materials (excluding that required for temporary diversion works) – approximately 350 tonnes including Type 1 granular material, bituminous bound and asphaltic materials;
  • Structural fill material: approximately 550m3, 1500 tonnes. This may vary depending on ground conditions;
  • Waterproofing material: bridge deck waterproofing to cover approximately 390m2, bituminous and waterproofing paint to cover approximately 210m2;
  • Other materials include: 90m length of metallic components for parapets, 120m of safety fences, clay drainage pipe products, 200m of uPVC ducting in the bridge, approximately 25m2 drainage composite fabrics for retaining walls;
  • Miscellaneous small part materials, such as nails, screws, scaffolding, plywood and polythene sheeting, quantities unknown; and
  • Additional materials, quantities unknown, will be brought in by the Contractor for the temporary works.

3.6.6. Material storage

The restrictive site boundary means that storage space is limited on site. It is anticipated that the contractor shall adopt a logistical plan utilising the ‘Just in Time’ delivery system, which involves bringing materials to site as they are required rather than unnecessarily storing material on site, thus reducing the requirement for storage space. Where the requirement for space is unavoidable, it may be necessary for the contractor to use the non-trafficked area of the road together with a built-up area of the embankment. The contractor shall not be allowed to store bulk material directly on the peatland. However, if this is unavoidable, advice will be sought from Scotland TranServ Environment Team.

3.6.7. Waste

Different wastes will be handled by the following means:

  • All waterborne waste will be intercepted and contained before reaching watercourses or groundwater;
  • Airborne particles will be extracted by vacuum where appropriate e.g. drilling dust;
  • All other waste will be disposed of by a licensed contractor; and
  • There will be no waste disposal on site.

3.6.8. On-site welfare provision

The site compound and lavatories are expected to be located within the confines of the site compound. Waste water will removed from site by tanker. Fresh water will be stored in tanks.

3.7. Assumptions and uncertainties

It is not possible to know the number of construction staff, vehicles and plant that will be on site as the construction will be carried out by a contractor who will have their own approach to addressing the construction and mitigation measures set out in this report. However, based on numbers of construction staff at the nearby Achnambeithach Bridge, a rough estimate would be between 6 and 20 staff at any one time depending on the specific operation. It is also not possible at this stage to determine the extent and location of temporary and permanent signage at the site.