Case study: Low-level submersible bridges provide customised and reliable access for harvesting

The Bridge It NZ team recently showcased their strong problem-solving skills and collaborative approach with two new low-level submersible bridge builds for Aratu Forests Limited, at their Kopua Forest in Maraetaha, Gisborne.

The need

An urgent, customised, and cost-effective bridging solution was needed across two streams with high catchment areas in the Kopua Forest to enable Aratu Forests Limited to effectively carry out its harvest operations, on-time and year-round.

Both streams are prone to flash flooding during heavy rain and the existing fords did little to provide reliable access for day-to-day operations. The bridges needed to withstand significant flood loads and allow the connecting forestry roads to be constructed at practical gradients for heavy forestry machinery.

Aratu Roading Operations and Harvest Planner, Scott Western-Arnold says they wanted a solution that had longevity — that wasn’t going to get washed away so easily.

An effective solution

After site visits, Bridge It NZ and design partner, Tiaki Engineering Consultants, suggested Aratu invest in a customised low-level submersible concrete bridge — rather than an arch culvert, simple wooden crossing, large high-level clear span bridge, or a closed bottom drift deck.

“When the Bridge It NZ team reverted with the low-level submersible bridge option for the Kopua North site, we could see it better suited the characteristics of the catchment, with its rock boulder nature and high gradient,” says Scott.

“It was a no brainer that, 12 months later, we also went for the same bridge in Kopua South.”

Bridge It NZ Project Engineer Craig Langsford says the team has extensive experience building crossing solutions for forestry companies needing effective and timely access to trees for harvesting.

“We collaborated closely with Aratu to understand their complete infrastructure needs and were able to deliver a customised crossing solution for both areas, on-time and on-budget.”

A customised build

In both Kopua North and Kopua South, the low-level submersible bridges needed to be built to certain specifications to handle regular, heavy forestry loads. 

In Kopua North, Bridge It NZ constructed the 10.5 metre low-level submersible bridge with skewed abutments to allow it to be perpendicular to the stream and help channel flood water through the spillway. The cast in-situ abutments used 40m3 of concrete, weighing approximately 100 tonnes. The bridge beams consist of approximately 14m3 of heavily reinforced concrete weighing 40 tonnes. In total, the bridge weighs approximately 140 tonnes. 

In Kopua South, Bridge It NZ constructed the 10-metre low-level submersible bridge with a seven percent grade applied to it to assist with track gradient for heavy forestry equipment. Specifications for the cast in-situ abutments, bridge beams, and weight for were similar to that for the Kopua North site. 

The Bridge It NZ team poured the precast bridge deck units for both bridges at their yard in Mount Maunganui. Each bridge was then constructed on site, in Kopua Forest, over only two and a half weeks. 
 

 
Left: Precast bridge beam reinforcing. | Right:  Abutments at Kopua North

Beam installation at Kopua North

Total project management

The Bridge It NZ team looked after the whole process for Aratu – from design to consent to construction.

“Having an engineer on site conducting the catchment analysis, looking after the resource consent process and enabling a smooth resource consent authorisation gives us confidence that the jobs have been done right,” says Scott.

“The beauty of Bridge It NZ is they’re essentially a one-stop shop, with cost-effective, ‘no surprises’ pricing, and at the same time they allow us room to collaborate and work in partnership with them.”

Collaboration, and supporting local, are important to Bridge It NZ. The team worked with Aratu’s roading contractor, BBL Contracting, to use their excavators to assist with bridge beam installation. BBL Contracting also completed the concrete block retaining.

Future-proofing our client’s operations

Aratu now have reliable access to the forest for first harvest, and can carry out their operations uninterrupted, all year round. 

“We’re stoked with the results for both bridges —Bridge It NZ has provided us with permanent bridging solutions that helps us future-proof our operations,” says Scott.

“The team had a positive, can-do attitude and no problem was too big. They were professional and efficient and Aratu will 100 per cent be working with the Bridge It NZ team again.”
 

   
Left: Low-level submersible concrete bridge at Kopua North.  |  Centre: Kopua North bridge in use during harvesting. |  Right: Crossing Kopua North bridge at low flood flows.

Read more about Bridge It NZ’s low-level submersible bridge here. For advice, guidance or a quote for your next bridging project, get in touch.

View Issue 18 of HarvestTECH News featuring this case study.