Paerata Stream Bridge

Client: PF Olsen Ltd

Rapid re-engineering of a critical forestry bridge

With just eight weeks until the scheduled removal of 50,000 tonnes of logs from a remote forestry block, one major hurdle stood in the way for forestry service provider PF Olsen. The only access for log trucks was a deteriorating steel and timber bridge, with no existing design information to establish its intended load-bearing capacity.

Complex conditions, tight timeline

Recommended by PF Olsen’s project partner, Busck Prestressed Concrete, GRIT was called in to assess the existing structure and support the delivery of a workable solution. Finding that the bridge was under-designed for forestry loads meant a replacement was needed - and fast. However, the abutment foundations were in very soft ground, and the bridge approaches at both ends were collapsing, so immediate replacement of the bridge wasn’t possible. The abutments would first need to be strengthened, and the embankments rebuilt and stabilised.

The ideal solution for bridging the stream would have been to use full-span, pre-stressed concrete beams. But the main stream embankments were up to 16m apart, and the longest pre-stressed beams available within the timeframe were just 12m in length. 

Efficiency through innovation and collaboration

An innovative approach was needed, and GRIT rose to the challenge, working with the project team to develop a solution that would provide a safe and compliant access point for logging crews. It would also deliver tangible cost and environmental benefits, and crucially, could be achieved within the tight timeframe.

A fast and economical solution was developed for the abutments, using mechanically stabilised earth (MSE) to meet the load bearing requirements. GRIT’s established partnership with precast beam supplier Busck also reduced design time, creating further efficiencies.

Another key innovation was to design a solution that utilised forestry equipment already on site to carry out the construction work. This allowed for work to progress quickly, without the additional cost of bringing in specialised equipment to the remote location. This could only be achieved thanks to close collaboration among the project team.

Keeping it local delivers environmental and cost benefits

Prioritising locally available materials and manufacturers also eliminated delays. With the bulk of materials - including precast concrete solutions, gravel, abutments and bridge beams - all manufactured within 50km of the site, transport costs and environmental impacts were minimised.

Care was taken to minimise disruption to the stream, and using precast concrete elements meant no concrete was poured on site, further reducing the environmental impact.

Thanks to the efforts of the entire project team, including PF Olsen, Northland Geotechnical Specialists, Quattro Engineering and Busck Prestressed Concrete, the new bridge was completed within the target eight weeks. Designed to NZ codes and built to NZ highway standards, it will provide safe and compliant access for years to come.