Kiachnish Viaduct: Tidal Scour Protection Scheme
Client: Story Contracting
Location: Scotland
Year: 2022
Background:
Kiachnish Bridge was built in 1933 to carry the busy A82 trunk road over the River Kiachnish. The bridge structure is a three span-reinforced concrete arched beam, and located approximately 5km south of Fort William. Inspections identified significant scour holes at the pier bases and emergency works were required to achieve structural integrity.
The Kaymac team are experts in delivering scour protection solutions, particularly in the Rail sector. This one was a bit different, this scheme was a highway structure and the repairs required were reactive.
Key Challenges:
- Tidal Structure
- We had to Avoid Road Closures
- Difficult Access
- Winter working
- Marine consent constraints
Following the identification of large scour holes around the three piers and abutments, the asset owners, Transport for Scotland, instructed their consultants to urgently manage reactive repairs to restore the bridge’s structural integrity and protect it from further damage.
The repairs and protection measures were required for all three abutments and piers. Kaymac expertise was on to visit the site and develop a plan to deliver the project and overcome the specific challenges.
Once we received the official instructions, the Marine licence was already in place, and the project was permitted to be delivered between the winter months of October and January. On other occasions, we highly recommend delivering this type of work in the summer when river levels are historically lower and less susceptible to flooding.
This was due to the tidal watercourse that required controlling to carry out the works safely. This meant that the bespoke temporary works system that our engineers designed would need to withstand the massive ebb and flow of the tidal range, as well as unpredictable flood events caused by heavy rainfall and snow that melted on the highlands during the winter months. This would prove to be a significant challenge; as we could continually monitor the weather, we could not predict the surge of melting snow that would rapidly increase flows.
Solution
- 2 Phases
- 2 Bespoke Temporary Works Containment Dam Designs
- 10 Week Program
- 1200m2 of Proserve Mattresses
- 160m3 of micro concrete
- A Team of Divers
- A Team of civils and plant operatives
Benefits
The Pump filled in situ formwork produces an interlocking concrete slab over the protection area, offering a unique performance. The concrete slab effectively distributes hydrodynamic forces, requiring protection up to seven times thinner than traditional rock armour. This scour protection solution does not suffer from rolling or sliding and can resist flows up to 12.5m/s. It has over 50 years of proven performance.
The Outcome
SEPA regulated the works, and representatives were present on-site during the project. The Project was completed successfully with zero health and safety or environmental incidences.
