Project Details

Description

Scour repair works are required at the A90 Finavon Bridge (Structure 510), hereafter referred to as the Bridge, a three span structure that carries the A90 (northbound) dual carriageway over the River South Esk.

Background to the Works

Storm Babet (October 2023) generated extreme river flows across Scotland, particularly in areas in the northeast. Gauges in the South Esk Catchment observed record river levels and significant flooding occurred. Following the storm, inspections were carried out on trunk road structures to assess for damage. An inspection found considerable scour and damage had occurred along the riverbed and foundations of the Bridge. Such damage impacted the structural stability of the Bridge and as a result, emergency measures were put in place to temporarily protect the structure from further damage, until a longer-term solution could be put in place.

Emergency repair works consisted of the placement of rock mattresses and rock bags to reinstate the eroded riverbed levels and scour holes on the northwest bank. The undermined section of the north abutment was infilled using flowable concrete. Rock bags were placed to protect the north side abutment from increased water levels, and additional rock bags placed over the north underpass to stabilise the ground slip failure surfaces and improve the bearing capacity of the Bridge’s abutment.

Permanent works are therefore now required to repair and provide long-term protection of the Bridge. Failure to undertake the works would further compromise the structural stability of the Bridge and its use as a key network route.

Proposed Works

Grey Bank Reinforcement

Permanent repair and scour protection designs involve the installation of concrete retaining walls adjacent to the north and south abutments of the Bridge. These will be concrete secant pile walls and will stretch a total of 44m along the riverbank, with 22m on each bank.

Bed Reinforcement

To protect the Bridge foundation from future high flow events, a concrete mattress will be installed slightly below the existing bed level and supplemented by rock nets at the termination points. The mattresses will have baffles to help control the flow of the channel. Rock mattresses and rock nets will be installed over the mattress and covered over with excavated riverbed material. The bed reinforcement activity also includes the reuse of suitable, undamaged rock nets and rock mattress units already on site from the previous temporary works. Bed reinforcement activities will cover a maximum length of 63m measured parallel to the riverbanks, with the area of bed affected being 1,722m2.

Sediment Management

To facilitate bed reinforcement, sediment management activities will also be required, which involves excavation of the riverbed to the desired formation level. Reintroduction of this removed sediment will be undertaken following the bed reinforcement, with 300mm of excavated sediment deposited over reinforcements to reduce the likelihood of localised scour and overall improving the effectiveness of the scour protection measures. The sediment will also help support the re-establishment of aquatic habitats within this highly sensitive water environment.

Removal of Bank Modifications

To facilitate the bed and bank reinforcement, historic gabion baskets along the north and south banks are required to be removed. The total length of modification removed as measured along the banks will be 53m.

To enable installation of the retaining wall along the north bank, the existing, failed wall will need to be removed. The maximum length of bank affected for this removal will be up to 16m of the north bank.

Other Construction Activities

Other site works include repairing of guardrail along the Bridge’s underpass. Additionally, vegetation cut back will be required to facilitate the works and provide access routes to the riverbank and works area.

Programming and Traffic Management

The works are anticipated to be undertaken in two phases, due to seasonal ecological constraints with the in-water working window between June and September.

Phase 1 includes works to the north abutment and will commence in mid-June 2026, for a three-month period. Phase 2 includes works to the south abutment and is expected to commence in 2027 during the in-water working window (start month to be confirmed). 

The works will be undertaken during day-time working hours only, with a reduced traffic flow required during the construction period. Traffic management (TM) will be a contraflow system, ensuring that either the A90 Finavon northbound, or A90 southbound bridge remain open at all times, with traffic temporarily diverted over the opposite bridge. This will maintain traffic flow along this route, with no diversion routes required.

Construction Methodology

An ingress/egress ramp will be formed on the upstream side of the north abutment to allow plant and machinery to access the river channel. This will require localised vegetation clearance and excavation of the riverbank to create a stable working slope. A site compound will be established in the northwest field, making use of existing matting from the 2024 emergency works.

To create a dry working area at the north abutment, a temporary cofferdam using either portadam units or bulk bags lined with plastic sheathing will be installed, diverting flow to the south channel. Depending on water levels and velocities, additional plastic sheeting may be required along the channel edge to manage flow around the temporary works.

Once the dry zone is established, the riverbed and adjacent footpath material will be levelled to form a platform for the piling rig. A secant pile will then be installed between the failed retaining wall and the north abutment, followed by excavation of up to 2m in front of the existing wall to allow its removal. Where full removal is not feasible, the remaining section will be partially removed and buried.

The riverbed will be excavated to the required formation level for the new scour protection measures, including removal of historic gabion baskets and mattresses downstream of the Bridge. An in-situ concrete pile cap and retaining wall will be cast along the top of the secant pile line to provide a sealed interface for the scour protection.

The riverbed will then be prepared for installation of the concrete mattresses. A fabric formwork will be laid out on the riverbed and pumped with concrete to form the mattress, which will be installed across the north half of the channel during Phase 1, and the south half in Phase 2. Mattress levels will be locally adjusted to avoid obstructing existing outlet pipes and the protection will extend approximately 12m upstream and 20m downstream. At both upstream and downstream extents, the mattress will be recessed into the bed to prevent undermining.

Rock mattresses and nets will be installed over the recessed edges and tied into the existing riverbanks using suitable as-dug material. Approximately 300mm of acceptable riverbed material will be placed over the concrete mattress to restore a naturalised bed surface. Along the temporary edge of the concrete mattresses between Phase 1 and 2, a geotextile membrane and rock nets will be installed to protect the exposed edge until the second phase is completed. Undamaged rock nets and rock mattress units already on site, used for the holding repairs will be re-used where possible. The nets will be covered over with suitable as-dug riverbed material.

On completion of all works, the cofferdams will be removed, the access ramp reinstated and the site compound returned to its previous condition.

Engagement with affected landowners regarding land access and compound arrangements is ongoing. Amey has made enquiries; however, the Contractor is responsible for securing all necessary access permissions.

The following plant, machinery and materials will be required:

  • Crane;
  • Large lorries and Heavy Goods Vehicles (HGVs);
  • Excavators (various sizes);
  • Dumpers and tracked dumpers;
  • Concrete mixer;
  • Concrete pump;
  • Vibrating soil compactors;
  • Piling rig;
  • Water pumps;
  • Generators; and
  • Hand tools.

Location

Finavon Bridge is located along a rural stretch of the A90 carriageway, at Finavon, Angus at the following National Grid Reference (NGR) NO 49305 57263.

The scheme area is less than one hectare (ha).