Inver Viaduct Scour Protection Scheme

Inver Viaduct Scour Protection Scheme

Client: Story Rail

Location: Dunkeld, Scotland

July 2023


Inver Viaduct is a three-span masonry arch viaduct carrying the Dunkeld to Pitlochry line over the River Braan. The structure required an improved scour assessment rating, and as such, Kaymac was contracted to design and develop a temporary works system to carry out the permanent works: the installation of concrete mattresses tied into the existing piers, rock armour toes situated upstream and downstream, and bank protection works on the east and west banks of the watercourse. The watercourse was flashy, susceptible to flooding, and highly regulated by the Scottish Environment Protection Agency (SEPA)

The Solution:

Installing concrete mattresses within the watercourse required a robust method of temporary works to control and mitigate any risk of environmental pollution. Kaymac developed a working method proposal that permitted the works to be undertaken in wet conditions whilst also facilitating the requirement to control and monitor any ecological implications arising from the works, specifically the movement of silt.

Kaymac undertook the scheme in 2 phases over an 11-week programme. Once we mobilised on-site and all service checks were carried out, the team installed pollution protection measures in the form of silt curtains downstream to contain any dispersed silt. Accessing the area from the Southeast bank, the team then installed Phase 1 of Temporary Works, the KayDam, a bespoke containment bund. This temporary works solution was chosen because a standard frame dam system’s expected water depth design parameters would likely be exceeded. The KayDam and its unique design can stay in situ during higher flows, whilst the operatives can carry out the permanent works through diving operations.

Before any permanent works could occur, an Initial part of de-watering of the working area to permit a fish rescue to be undertaken by our licensed and approved supplier. Once any impounded fish are relocated, operatives could then install the sheet membrane on the internal face of the temporary frames. Once fully sealed on both sides of the dam, silt busters were commissioned, and internal flooding and maintenance of the working area could commence.

Using excavators within the bunded areas, the excavation, removal, and storage of existing invert and riverbed materials and levelling existing riverbed to formation level, including toe details, placing and securing the Proserve mattresses, filled with concrete, and backfilling the toes and embankments with rock armour. Finally, reinstating bed material.

Phase 2 was undertaken in the same manner by altering the proposed temporary works to encapsulate the west pier to enable the remainder of the permanent works to be installed in a similar method, with access to the temporary works system from the northwest bank.

Despite significant challenges with unavoidable weather delays and stringent environmental monitoring procedures the project was delivered successfully.