Date of completion April 2021
Crooked River Bridge No. 2 Strengthening for Grey District Council
This project started off as an investigation and report to identify what was causing the rotation and movement in either end abutment. We were tasked to site measure and draw the existing structure and river profile for the Council. During this, we were to identify and report on any damage or abnormalities in the structure, e.g., shorn off hold down bolts, broken or spalling concrete. From there, we carried out excavations around the abutment and piers to detail if piles and anchor points were present and quantify what was present. We worked with the Council and John Mackenzie of Elmac to prioritise critical repairs and put solutions for each repair in place.
A new abutment was precast for the west side of the bridge. During a 10-day road closure, we jacked the existing span, removed the old abutment, excavated existing tie backs, drilled then drove new H piles to retain abutment, installed the new precast abutment over driven piles and grouted into position, retro-fit the abutment to the existing anchor points. When then backfilled and compacted behind the abutment to road height and reformed the roadway. Significant corrosion was noticed during this replacement process on the end of the span and the end bracing. With the Council's approval, we used filter fabric to contain the area around the span, abrasive blasted the affected areas and applied a two-coat paint system to prevent further deterioration.
Once stage one was completed, we moved into the river bed, where we diverted the river from around the piers, drove H piles at the outer extremities of the existing piers, excavated to below water level, and poured a mass footing approx. 1m in depth to be just above water level. A reinforcing cage was then tied around the existing pier; this included core drilled through concrete of the existing pier to ensure the connection between new concrete to existing, the concrete extended approx. 1.5m up existing piers, underpinning of the pier was to protect the structure from further scour.
After underpinning was complete, we moved to the east abutment. During a weekend road closure, we removed the existing deck panel and drove H piles down the face of the existing abutment. We then reinstated the deck panel and opened the road. From here, we excavated down 3m below the bottom of the existing abutment, below water level. We poured a mass concrete footing over approx. 1/2m to above water level then formed a reinforcing cage anchoring to the existing abutment for the entire height of the abutment to the underside of the span, approx. 5m in height. This was done to reduce movement of the abutment.
Once the initial strengthening work was carried out, we then fabricated and installed a jacking frame to pier one, which allowed us to jack the bridge spans and repair the concrete pier cap and hold down bolts that the movement in the abutments had damaged.
Further to this, the underside of the deck was cross-braced. We designed a bracing system that could fabricate and coated in our workshop then installed onto the bridge spans working from the riverbed with crane and access machinery.
This project was conducted in stages as funding became available.