The renovation work was confined to a 65 m length of the bridge, incorporating three spans and the corresponding piers. It was intended as a follow-up to the 2011 pilot project, to build on what had been trialled before and pinpoint techniques and procedures that could be efficiently applied across the remainder of the structure, as well as firming up time and cost estimates for the full renovation.
The 1.5 km bridge has 68 spans each approximately 21.5 m long, and it carries the single carriageway A9 highway across the Cromarty Firth north of Inverness. Its strategic importance to Scotland is clear, providing the main access to the north of the country, as well as ferry routes to Orkney and Shetland; the nearest alternative crossing of the waterway is more than 60 km away.
Lessons learned from the first phase of repairs to Scotland’s Cromarty Bridge enabled Freyssinet to bring valuable insights to delivery of the subsequent contract, including a custom-designed movable platform for access to the piers.
Each span is formed of five precast, prestressed beams supported on top of capping beams which rest on the piers, and topped by a reinforced concrete deck structure. The renovation contract involved replacement of deck surfacing and waterproofing, hydrodemolition and subsequent reconstruction of the concrete deck, repairs to the concrete of the superstructure and piers, the addition of a cathodic protection system, and replacement of bearings and joints.
A significant challenge was access below the structure, which was necessary for work to the piers and capping beams, and also to encapsulate the deck while hydrodemolition was carried out, to prevent polluted water and debris from discharging into the estuary.
The work crew needed access to the full height of the pier columns for the concrete repairs, and certain operations were restricted to very short windows of opportunity governed by the tides. Anodes for the cathodic protection system had to be attached to the pile caps, which are only exposed during the very low tides that occur a few times a year.
Initial proposals were to use a fixed, continuous platform below the deck with multiple levels, the lowest of which would be submerged under certain conditions. Although this would have provided easy access between all the underdeck area, the tidal range of up to 6m in the estuary meant it would have been necessary to add a lot of stiffening to the temporary structure to resist uplift from wave and hydraulic forces.
To facilitate replacement of the bearings – which was the first major operation of the contract – a fixed platform was erected just below the deck to give access to the capping beams. But once this work had been completed, the fixed access was exchanged for a bespoke mobile platform around the piers which could be lowered down to follow the tide as it receded, and pulled back up when the tide came in. A fixed platform was used for access to the main span soffit.
Containment of operations was particularly critical in the protected environment and demanded complex logistics. For example tanks of potable water had to be brought to the site for the hydrodemolition process; the run-off was subsequently collected and treated prior to controlled discharge.
