Geotechnical reports and what drives the cost

What is in a geotechnical report

A report has to do two things: record what was found, and say what to do about it. The findings are the durable part, and the recommendations are what the designer and the council act on.

• What was investigated, where, and by what method, with the logs themselves
• The ground model: the layers, their strength, and where groundwater sits
• The hazards that apply to this site, which may include liquefaction, settlement, slope instability or expansive soil
• Design parameters a structural or civil engineer can use directly
• Foundation and earthworks recommendations, and the conditions they depend on
• What was assumed, and what remains unverified

That last line is the one to read first in anyone’s report. A report that states its limits is usable. One that reads as though the whole site is known from four holes is not.

What drives the cost

Fieldwork is usually the largest single item, and fieldwork is driven by access and by how much uncertainty the project can carry. Those two, more than the size of the building, set the fee.

• How much investigation the decision needs. A single dwelling on flat ground is not a nine-lot subdivision on a terrace edge.
• Which methods the ground allows. Gravel or shallow rock rules out a cone and pushes you to drilling, which costs more per metre.
• Access. A rig that can drive to each location is cheap. Hand-carried gear on a steep or landlocked site is not.
• Travel and mobilisation, which is why the nearest office matters on a small job.
• Laboratory scheduling, and whether settlement or liquefaction analysis is needed.
• Whether the site is on the HAIL, which brings contamination work alongside the geotechnical scope.

LDE quotes per site rather than from a rate sheet, because the same building on two sections 200 metres apart can need materially different work. Ask for a fee proposal with the investigation scope written into it, and compare scopes before you compare numbers.

Keeping the cost down

Most of the savings available are in what you hand over at the start, not in negotiating the fee.

• Give a fixed brief. A moving footprint means repeat visits.
• Hand over any existing report, even an old one on a neighbouring lot.
• Say what the site can tolerate. Knowing that a lawn cannot be dug changes the method before it changes the price.
• Book one visit, not two. Splitting geotechnical and contamination sampling doubles mobilisation.
• Ask early. An investigation that lands before the design is fixed can change the design; one that lands after can only confirm or contradict it.

How long it takes

The programme is usually set by two things outside anyone’s control: when a rig and crew are free, and how long the laboratory takes. Fieldwork on a straightforward residential site is often a single day. Logging, analysis and reporting follow, and laboratory turnaround runs in parallel where the schedule allows it.

If a date is fixed, say so when you ask for the fee rather than after the job is booked. Sequencing is where most of the flexibility lives.

When you need one

A council will generally want geotechnical input where the ground governs the design or where a hazard is mapped or suspected. Subdivision, building on fill, building on or below a slope, liquefaction-prone land, and any site where a specific design replaces a standard solution.

What a given council requires varies, and the requirement attaches to your site rather than to a general rule, so check the district plan and the consent pathway for the property before assuming either way. The methods behind the report are set out on the geotechnical investigation page, and the wider service on the geotechnical engineering page.