Reading a Whole Site Before You Design It: Site Characterisation for Subdivisions

At the feasibility stage of a subdivision, the biggest risks are often the ones you cannot see yet. How deep is the groundwater? How far below ground is the rock layer, and where does it rise close to the surface? How much do the ground conditions change from one corner of the site to another?

Those questions drive earthworks, drainage and foundations requirements, and ultimately cost. Getting the answers wrong, or waiting until later to find answers, is how a development ends up with unexpected excavation costs, redesigned drainage or foundation changes no one budgeted for.

The traditional way of answering those questions is through boreholes and other point-specific testing. That information is essential, but on a large site it only samples selected locations. This article explains how Cook Costello used geophysics and targeted geotechnical investigation to build a site-wide picture across a 60-hectare subdivision before the design was locked in.

Why site-wide understanding matters at feasibility

On a large subdivision, ground conditions rarely stay the same across the entire site. Rock can be deep in one area and close to the surface in another. Groundwater levels can vary. Soft or variable zones can sit between the points selected for testing.

Boreholes and CPTs are essential, but they only confirm conditions at the locations tested. If those are the only data points available, the ground model depends heavily on interpolation and assumptions between them. Where the ground changes between test locations, the surprise can show up later as unexpected excavation cost, redesigned drainage or foundation changes partway through the project.

For a developer pricing a project, or a planner and council assessing what a site can support, that uncertainty is expensive. It can force conservative assumptions into the budget and design, while still leaving room for costly surprises. What is really needed is a site-wide picture, and that is what geophysics adds to traditional drilling and testing.

On this 60-hectare site, the client’s key questions were the depth to groundwater and the depth to bedrock, in this case basalt. Both strongly influence earthworks cost, drainage design and foundation requirements.

Basalt was particularly important. Where it rises close to the surface, excavation can become expensive quickly. Knowing where that occurs across the site changes both the design strategy and the pricing risk.

The approach: geophysics across the site, calibrated by drilling

We combined geophysical surveys with geotechnical investigation, working alongside our geotechnical and civil engineers and our IANZ-accredited testing partner, Geocivil.

Rather than relying only on isolated boreholes, we used a gridded geophysical approach to help bridge the gaps between test locations. This allowed us to capture how the ground changed both across the site and with depth.

The geophysical team collected a total of:

• 10 km of resistivity data, mapping electrical contrasts between soil and rock

• 1.5 km of seismic P-wave data, helping interpret layering and material stiffness

• 32 shear-wave velocity profiles, characterising how the ground responds under load

These results were checked against boreholes and Cone Penetration Tests, or CPTs, across the site. That meant the broader geophysical model was tied to direct measurements at key locations.

The geophysics mapped the pattern across the site. The drilling and CPTs grounded that interpretation in verified data.

What it produced

The result was a site-wide interpreted model rather than a set of isolated readings. It included interpolated maps of groundwater and bedrock depth across the site, geological cross-sections built from the interpreted geophysical profiles, and an integrated geotechnical model combining both non-invasive and invasive data.

In practical terms, the client could identify where basalt was interpreted to rise close to the surface, where excavation was likely to be more costly, how groundwater varied, and how the geology changed across the site.

That level of coverage could not have been achieved with boreholes alone without a much larger number of holes and significantly higher cost. By combining geophysics with targeted drilling and CPTs, the investigation produced a denser and more useful ground model for feasibility, pricing and early design.

Why it matters to developers, planners and councils

For the developer, the investigation reduced risk at the pricing stage. Understanding the horizontal and vertical variation in the ground, particularly the depth to basalt, meant the project could be costed against the conditions identified across the site rather than a conservative assumption. It also reduced the chance of excavation or foundation costs escalating later.

For a planner or council, a site-wide ground model helps show what the land can realistically support and where the main constraints lie. That information can feed into feasibility, layout, consenting strategy and the decisions around the development.

Because the geophysics reduced the amount of intrusive testing needed, the investigation was faster and more cost-effective than covering the site with boreholes alone, while still producing a defensible dataset. The client was able to make design and investment decisions using evidence, with far less risk of major ground constraints emerging during construction.

More broadly, this is what site characterisation is for: understanding a site early and across its full extent, so the big decisions around feasibility, layout, pricing and detailed design are based on measured evidence rather than broad assumptions.

On a large or variable site, that early, whole-site picture is one of the highest-value things you can have before committing.

Where we come in

Cook Costello brings geophysics, geotechnical engineering, civil engineering and testing together in one team. For investigations like this, that means the work is designed, carried out and interpreted as one connected exercise rather than handed between separate firms.

The geophysics maps the pattern across the site. The drilling, CPTs and testing confirm the detail at key locations. Our geotechnical and civil engineers then turn those findings into a ground model the design and budget can use.

It is the same measure-don’t-assume approach applied at subdivision scale: understand the ground early, identify the major constraints, and make layout, pricing and design decisions on evidence rather than broad assumptions.

If you are assessing or planning a subdivision and want to understand the ground before the design and budget are set, tell us about the site. We can advise how a combined geophysical and geotechnical investigation could help.

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