Mapping a Buried Landfill Before You Build Over It
- Jul 18, 2024
- 4 min read
Plenty of land now being considered for development was once used for something else, and some of it was landfill. Old tips and filled ground are scattered across New Zealand. They are often poorly documented, sometimes forgotten entirely, and can create serious problems if built over without proper investigation.
Buried waste can lead to uneven settlement as fill decomposes, ground gas, and leachate, the contaminated liquid that forms as water moves through waste, migrating into soil and groundwater.
Before those risks can be managed, you need to understand three basic things that are often surprisingly hard to establish: where the landfill is likely to be, how deep it extends, and where the boundary sits between waste and natural ground.
This article explains how Cook Costello used geophysics and targeted drilling to map a buried landfill on a constrained development site without widespread excavation.
Why buried landfill is hard to pin down
The trouble with old fill is that its boundaries are rarely recorded accurately, and you cannot see them from the surface. A site might have been filled decades ago, with the edges of the tip, its depth and what is in it lost to time.
Digging to find out is slow, can expose workers and the environment to waste, and, as with any intrusive testing, only tells you about the exact points investigated.
What a developer or council needs is a site-wide picture: the likely extent of the landfill, how deep it goes, and where the boundary sits between waste and natural ground. That allows the development to be planned around the buried material and the environmental risks to be managed.
That is where geophysics earns its place. It can help map the buried material across the site with minimal disturbance, then guide targeted drilling or sampling to confirm the interpretation where it matters most.
The approach: two geophysical methods, integrated with drilling
We combined two complementary geophysical techniques with targeted geotechnical boreholes.
Electrical Resistivity Tomography, or ERT, images the subsurface by measuring how easily electrical current passes through the ground. Landfill material often shows as a lower-resistivity zone compared with surrounding natural ground, particularly where waste, moisture or leachate are present. That makes ERT useful for identifying the likely edges and depth of buried fill.
Multichannel Analysis of Surface Waves, or MASW, added complementary information on how ground stiffness changes with depth. This helped distinguish softer or more variable fill from the firmer natural ground beneath it.
On a spatially constrained site, the survey lines were planned to cover as much of the area as practical and to tie into the borehole locations. That meant the geophysics and drilling reinforced each other: the geophysical surveys mapped patterns across the site, while the boreholes confirmed the interpretation at key points.
Using geophysics to guide the drilling also meant fewer boreholes were needed to build a useful ground model, because intrusive testing could be focused where confirmation would add the most value.
What the survey found
The ERT profiles distinguished the landfill as a lower-resistivity zone in the upper few metres, underlain by a higher-resistivity sandy gravel layer interpreted as the natural ground beneath the fill.
Mapping this response across the site showed that the landfill extended to more than 5 m below ground level in places. It also allowed us to produce a contour map showing how the depth of the landfill varied across the site.
Combining the geophysical results with the borehole data produced a defensible subsurface model that clarified the landfill’s likely boundaries and thickness. Importantly, it also highlighted zones with higher potential for leachate migration, helping identify where environmental risk may need closer assessment or management.
What it delivered, and why it matters to developers and councils
For the developer and council involved, the investigation turned a poorly defined buried hazard into a clearer, usable ground model.
Knowing the likely extent and depth of the landfill early meant the development could be planned with that constraint in mind, rather than encountering the waste, and its cost and environmental implications, partway through construction.
Because the geophysics guided the drilling, fewer boreholes were needed to build the ground model, helping keep investigation costs focused. Identifying zones with higher potential for leachate migration also supported proactive environmental planning and the compliance obligations that come with developing former landfill, which in New Zealand can be significant.
In short, the investigation reduced project risk and gave everyone involved a sound basis for decisions about a site that would otherwise have carried a large unknown.
Why it matters beyond this site
The same approach can apply to many developments on or near former landfill, reclaimed ground, filled ground or contaminated land. As easier sites are used up and more brownfield land is redeveloped, this kind of ground risk is becoming a more important part of the development picture.
For a developer assessing feasibility, a council considering consent, or a contractor preparing to work on site, understanding what is buried, and where it is likely to be, is far safer and cheaper than finding out during construction.
Mapping buried fill non-invasively, then confirming the interpretation with targeted drilling, is a practical way to bring more certainty to the planning stage. It helps identify constraints early, focus investigation where it matters most, and support better decisions before significant money is committed.
Where we come in
Cook Costello carried out this investigation using our in-house geophysics capability, working alongside our geotechnical engineers. That meant the survey was not treated as standalone data. It was interpreted into a ground model that could support design, environmental planning and decisions about the site.
Combining geophysics with targeted geotechnical drilling, rather than relying on either method alone, is how we build a more complete site-wide picture efficiently. The geophysics helps map the pattern across the site, while the boreholes confirm the detail at key locations.
If you are assessing a site that may include former landfill, filled ground or contaminated land, tell us about it. We can advise how a combined geophysical and geotechnical investigation could help map what is beneath the site before you commit.
OR
