Site Classification Investigations for Subdivisions

Aim

For medium-to-large developments at the feasibility or consenting stage, it is essential to establish a site-wide understanding of the subsurface conditions, geology, and groundwater levels. This knowledge ensures project pricing reflects the true ground conditions and helps avoid both under- and over-design of infrastructure.

On this 60-hectare site, the client was particularly interested in identifying the depth to groundwater and the depth to bedrock (basalt), two factors that significantly influence earthworks, drainage design, and foundation solutions.

Approach

To deliver reliable insights, we collaborated closely with our geotechnical and civil engineers, as well as our IANZ-accredited sister company, GeoCivil. Together, we developed a comprehensive solution that integrated geophysical and geotechnical investigations.

A gridded approach to geophysical testing was adopted, bridging the gaps between invasive investigations and enabling us to capture how subsurface conditions change both horizontally and vertically across the site.

During the investigation, our geophysical team collected:

• 10 km of resistivity data to map electrical contrasts between soil and rock layers

  
  

• 1.5 km of seismic P-wave data for stratigraphy and material stiffness

  
  

• 32 shear wave velocity profiles to characterise soil behaviour under loading

Results

The datasets were processed into detailed models of underground resistivity and seismic velocity.

These were then calibrated against boreholes and Cone Penetration Tests (CPTs) undertaken across the site. By combining geophysical and invasive data, we were able to interpret subsurface conditions with greater confidence.

Outputs & Value

The outcome was a suite of high-value deliverables:

• Interpolated groundwater and bedrock depth maps across the full site

  
  

• Geological cross-sections derived from interpreted geophysical profiles

  
  

• An integrated geotechnical model combining invasive and non-invasive results

  
  

This approach provided far denser site-wide coverage than geotechnical point testing alone. The client was able to de-risk the pricing phase by better understanding horizontal and vertical variations in geology, particularly the depth to basalt relevant for excavation.

By reducing reliance on extensive intrusive testing, we saved time and cost while still delivering a robust dataset. Ultimately, the client gained the confidence to make informed design and investment decisions, with a lower risk of costly surprises during construction.

An integrated geophysical and geotechnical investigation provides broader site-wide understanding, reduces uncertainty, and enables more efficient and cost-effective subdivision design.