Maungaturoto Solar Farm

New Zealand needs substantially more renewable generation, and many new solar farms are being developed on rural land that was never intended to carry such infrastructure.

The 20 MW solar farm near Maungaturoto is a case in point. The site combined moderately sloping terrain, very soft and expansive clay near the surface, and the high-intensity rainfall Northland regularly experiences. Those conditions had to be understood before thousands of panel foundations, access tracks and drainage features could be built.

Cook Costello provided the geotechnical engineering, civil engineering and surveying for the project, from site investigation through to the consent package and construction monitoring.

The ground challenge: thousands of foundations in soft, expansive clay

A solar farm creates an unusual geotechnical problem. The individual structures are relatively light, but there are thousands of foundations spread across a large area, all relying on the near-surface ground.

At Maungaturoto, that ground included very soft, expansive clay. These soils shrink and swell as moisture changes and were found to be very weak near the surface in places. To reliably generate energy, the solar-array foundations needed to perform in the long term.

The main risk was not one dramatic failure. It was distributed movement across the site, with foundations potentially heaving, settling or leaning as the clay responded to Northland’s ongoing cycle of experiencing droughts and high-intensity rainfall events.

Our geotechnical investigation combined in-situ testing across the site with laboratory testing of the clay. This established its strength, expansiveness and variability across the sloping land. It also helped to indicate how acidic the soils are in terms of degrading and damaging any structural elements embedded in the ground.

The results provided the geotechnical parameters and recommendations used by the foundation designer, suited to the soft ground, seasonal shrink-swell behaviour and natural hazards identified at the site, without adding unnecessary conservatism for conditions that were not present.

The water challenge: intense rainfall across modified slopes

Northland rainfall can arrive quickly and in large volumes. A solar farm also changes the way a rural site sheds water, as access tracks, hardstands and rows of panels alter runoff patterns.

On sloping ground underlain by expansive clay, water management matters doubly. Stormwater needs to be controlled to manage flooding and erosion, while changes in soil moisture can also affect slope stability and foundation movement.

Our civil engineers designed the site’s stormwater and flood-management system around the existing topography. The design controlled how water moved across and left the site, helping protect the arrays, access tracks and downstream environment during high-intensity rainfall.

The layout challenge: placing 20 MW accurately

Solar farms depend on accurate geometry. Arrays need to be positioned and aligned for installation and generation performance, while the overall development must remain within legal boundaries and match the approved layout.

Our surveyors mapped the site topography, providing the base information for the civil design and array layout. They then set out the arrays to the required installation accuracy and resolved boundary issues identified during the work.

The consent application was supported by a coordinated drawing package bringing together the topography, solar layout, civil design and cadastral information.

One connected model of the site

For such a project, geotechnical, civil and surveying inputs could not be treated separately.

The topography influenced the stormwater design. Water management and expansive clay affected foundation performance. Legal boundaries and array geometry constrained the layout around them.

Delivering these services through one team meant the investigation, design and set-out were based on the same site information, with fewer gaps between disciplines.

As New Zealand expands renewable generation, more infrastructure will be built on sites that are sloping, soft, wet or otherwise difficult. The Maungaturoto Solar Farm shows the value of understanding those conditions early and treating the ground, water and layout as one connected engineering problem.