Geotechnical due diligence for a development site

Geotechnical due diligence is testing the ground assumptions a development feasibility rests on (founding conditions, earthworks balance, fill, groundwater and slope stability) while the price, the yield or the decision to walk away can still change. It runs from a desktop review of mapped and historical records to a site investigation scoped to the questions that carry money, and it has to keep up as the design changes.

In the 1990s the University of Adelaide picked the most thorough of three tenders, not the cheapest, to investigate the ground for a new engineering building on its North Terrace campus. Then cost estimates came in high, the building's lower-level computer suites were cut and its footprint moved about 20 m west. The report had warned against relying on it if the building's size or location changed, but no new investigation was done and nobody reviewed the piles. The bored piles settled unacceptably under load testing, the job switched to jacked precast piles, a technique then untried in Australia, and the change added about $170,000 and four to six weeks. Mark Jaksa (2000), who sat on the project team, blamed several things together: an investigation normal for South Australia but minimal for such variable ground, no fresh advice once the building moved, and a geotechnical consultant left out of later stages.

What happens at each stage?

Stage	Decision being made	Evidence used
Before the offer	Is the site worth a due diligence period	Desktop: geology, terrain, historical imagery, nearby bore logs, hazard mapping
During due diligence	What to price, what to test, what to walk from	Desktop findings plus a scoped investigation
Concept and yield	Lot layout, levels, basement extent, earthworks balance	Investigation results against the ground model
Tender	What contractors are being asked to price	Factual data, stated assumptions, residual risks

What should you check before making an offer?

All desktop work, and an afternoon's worth.

• The whole holding. Part lots, easements and road reserve change the developable area before the ground does.
• The mapped geology. One unit across the site is a different risk from three, and a mapped boundary marks where conditions change.
• Terrain and old imagery. Breaks of slope often mark an old gully, a fill edge or a rock shelf; quarries, dams, stockpiles and old pads on historical photos point to fill.
• Nearby borehole logs. Depth to clay, depth to refusal, water levels, and how much neighbouring logs disagree.
• Hazard layers. Flood, landslip, acid sulfate soils, contamination and mine subsidence each bring an approval pathway and a cost line.

What should happen during the due diligence period?

This is where screening becomes priced risk.

• Write down the three ground assumptions the feasibility depends on. Usually founding depth, cut and fill balance, and whether the site supports the basement or parking design.
• Test each against the desktop evidence. An assumption consistent with the mapped geology and nearby logs can be priced. One that conflicts with them needs field data before settlement.
• Scope the investigation to those questions. Andrews (2006), from 201 reports by 27 Sydney Basin firms, showed that a small scope does not save money: a designer working from thin data designs conservatively, and the cost reappears in construction.
• Decide deliberately who carries latent conditions. With a preliminary investigation only, the consultant stands behind the factual data and the developer keeps the risk of changed strata and unexpected conditions.
• Check the approval-stage requirements. On land zoned landslide-prone, the planning scheme can require a landslide risk assessment, and approval conditions can carry inspections and certification through construction (Walker et al. 2007), which costs time as well as money.

What has to be checked before the layout is locked?

• Lay the lot layout over the ground model. Lots over the deepest fill or steepest fall are likely to need engineered footings; do not price them as standard.
• Check the earthworks balance against the material. Rock excavation carries a different rate. Reactive clay is acceptable as fill only where it is placed and tested as controlled fill, and the site classification must still account for it.
• Check what the platform does to the footings. A level pad can put every footing on fill. Under clause 2.5.3 of AS 2870-2011, controlled fill other than sand deeper than 0.4 m makes a lot Class P unless an engineer reclassifies it by calculation, and uncontrolled fill makes it Class P unless all footings go through it to natural ground. The platform design decides how many lots need engineered footings.
• Set floor levels against flood, drainage and retaining together. A level change at concept stage is a redraw; after approval it is a resubmission.
• Re-check the investigation whenever the footprint moves. Adelaide is what happens when it is not.

What should go to the earthworks and piling tenderers?

• The ground model you hold. Collingwood (2003), writing from the piling side, names unforeseen ground as the most common cause of delay on piling jobs. Tenderers who cannot see the data price the uncertainty as contingency or qualify their tenders.
• What is measured, inferred and untested. A desktop model presented as fact invites a claim; presented as a hypothesis with its evidence, it lets a contractor price the risk.
• The residual risks. Seasonal groundwater variation, disagreement between logs, corestones in weathered rock, and fill of unknown origin.

How much should you spend on investigation?

There is a range, not a rule. Jaksa (2000) notes spending sometimes as low as a few tenths of one percent of project cost against a recommended 3 percent, yet judged 3 percent unrealistic for his own project and argued quality and timing matter more than the sum. Kelly et al. (2020) found large infrastructure projects spent 0.5 to 1.5 percent, with diminishing returns beyond a few percent. In practice: spend early on the questions that decide the transaction, and scope the field work to the assumptions the feasibility cannot carry, not to a standard fee. Typical fees are in the geotechnical report cost guide.

What can desktop work not settle?

Founding levels, design parameters and site classifications, and it is not engineering advice. A desktop assessment gives an evidence-based expectation of the ground and shows where it is most likely to be wrong, so the investigation drills in the right places to the right depths. The boundary is set out in what a geotechnical desktop study covers.

Common questions

What is geotechnical due diligence?

The process of testing the ground-related assumptions behind a property or development decision before committing to it. It combines desktop review of mapped geology, terrain, historical imagery, bore records and hazard mapping with a site investigation scoped to the questions that carry commercial risk.

When should a geotechnical investigation happen in a development?

Desktop review belongs before the offer, so that the due diligence period starts with known questions. Field investigation belongs inside the due diligence period where the answer could change the price or the decision, and before detailed design in every case.

Who carries the risk of unforeseen ground conditions?

It depends on the contract and on the scope of investigation. With a preliminary investigation only, the developer generally retains the risk of changed strata and latent conditions. A larger investigation transfers less risk to construction, which is the trade-off Andrews (2006) describes.

To build the desktop evidence for a site before the due diligence clock starts, generate a desktop report for the address, or see a sample report to check what it resolves.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria and South Australia.

Sources

• Jaksa, M. B. (2000). Geotechnical risk and inadequate site investigations: A case study. Australian Geomechanics, Volume 35, Number 2 (Jun 2000). geomechanics.org.au
• R. B. Kelly, M. Drechsler and R. Goldsmith (2020). Connecting Geotechnical Investigations with Project Risk. Australian Geomechanics, Volume 55, Number 1 (Mar 2020). geomechanics.org.au
• Peter V. Andrews (2006). The hidden cost of geotechnical investigations. Australian Geomechanics, Volume 41, Number 4 (Dec 2006). geomechanics.org.au
• Bruce Walker, Warwick Davies and Grahame Wilson (2007). Practice Note guidelines for landslide risk management. Australian Geomechanics, Volume 42, Number 1 (Mar 2007). geomechanics.org.au
• Ben Collingwood (2003). Geotechnical Investigations For Piling Projects - The False Economy Of A Cheap Site Investigation. Australian Geomechanics, Volume 38, Number 1 (Mar 2003). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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