Forestry Bridge Design

Client - PF Olsen Limited Date - 2021

Forestry service provider PF Olsen Ltd had up to 50,000 tonnes of logs ready to be removed from a remote forestry area but had one thing standing in their way: a deteriorating steel and timber bridge.

GRIT was engaged to complete a condition and engineering assessment to see if the existing structure should be maintained, modified, or replaced. Our review found that the bridge was in bad condition, under-designed for forestry use, and the approaches were collapsing into the stream.

There was an eight-week deadline to deliver a finished bridge design with no current information to establish the bridge’s capacity. GRIT provided a solution that involved using mechanically stabilised earth abutments to meet the load requirements.

The bottom abutment foundations were in very soft ground, and the approaches were unstable. Our design had to allow for the removal and reinstatement of the approaches in conjunction with the abutments.

The ideal solution was to have beams reach right across the span of the stream. But, unfortunately, the bridge embankments were too far apart, and the only available prestressed bridge beams were 12m long. Meanwhile, the mainstream embankments are between 15-16m apart.

Using construction equipment already in use from the logging company, we avoided hiring any specialist construction equipment. The bulk of the project’s materials, including the precast concrete, gravel, abutments and bridge beams were sourced within 50km of the site.

Designing to the New Zealand codes, GRIT delivered an innovative, safe and compliant design. Working with Northland Geotechnical Specialists Ltd, Quattro Engineering Ltd and BUSCK Prestressed Concrete Ltd, we saw the completion of the new forestry bridge from initial investigations through to construction.