Understanding Site Investigation Reports: What Wellington, Kāpiti Coast, and Wairarapa Homeowners Need to Know Before Building
- romybromley
- Aug 13
- 4 min read
Updated: Aug 19
If you’re planning to build or renovate in the Greater Wellington region, including Wellington City, the Kāpiti Coast, and the Wairarapa, a site investigation is one of the most important steps to get right before construction begins. Also known as a geotechnical report or site investigation report, this process identifies the soil, rock, and groundwater conditions on your property. With this information, your engineer can design safe, cost-effective foundations and retaining walls that match your site’s actual conditions.
What is a Site Investigation Report?
A site investigation report examines the subsurface conditions where you plan to build. The findings are documented in a report that is used to:
Guide structural foundation design.
Identify potential geotechnical hazards such as liquefaction, slope instability, or poor drainage.
Support building consent applications.
In many parts of Greater Wellington, especially hillside suburbs, coastal sections, and reclaimed land, councils will require this report before approving your consent.
The Process Usually Involves
Desktop Study - Reviewing geological maps, council records, and any existing site reports
On-site testing - Collecting soil samples and measuring ground strength.
Analysis and recommendations - Interpreting results so designs meet both engineering requirements and local council standards.
Common Testing Methods in Greater Wellington
Because ground conditions can vary significantly between locations — even within the same suburb — engineers select tests based on site access, soil type, and slope.
Hand Auger – Drills into soft to medium soils to collect samples up to a few metres deep.
Scala Penetrometer – Measures soil strength to estimate foundation bearing capacity.
Dynamic Cone Penetrometer (DCP) – Works in a wider range of soil conditions than the scala.
Test Pits – Excavations that allow direct observation of soil layers.
Laboratory Testing – Determines properties such as moisture content, particle size, and plasticity.
Local Examples: Khandallah vs Lyall Bay
Two Wellington suburbs illustrate how different ground conditions can result in very different geotechnical and foundation design recommendations, even for houses of similar size and construction type.
Khandallah (Subsoil Class B or C)
Many residential sites in Khandallah are located on steep slopes underlain by weathered greywacke rock with a relatively thin layer of colluvium or residual soil. A typical site investigation might reveal:
Soil depth: Often less than 1 metre before encountering competent greywacke rock.
Bearing capacity: High — often exceeding 300 kPa once founded on rock.
Groundwater: Generally low, except where seepage occurs along rock bedding planes.
Slope stability: Localised instability can occur where slopes are over-steepened by excavation or where fill has been placed historically.
Foundation recommendations for these conditions could include:
Shallow reinforced concrete strip footings or pads founded directly onto rock.
Bored concrete piles drilled into greywacke to provide sliding resistance on steep slopes.
Retaining walls designed with appropriate back-drainage (e.g., drainage blankets, subsoil drains) to control hillside water and reduce pore water pressures.
Lyall Bay (Subsoil class D or E)
In the coastal flats of Island Bay, especially reclaimed or dune sand areas, ground conditions are typically much softer and more variable. A site investigation might find:
Soil profile: Loose to medium dense sand near the surface, sometimes underlain by silty sand or marine deposits.
Bearing capacity: Low to moderate — often in the range of 100–150 kPa in upper layers.
Liquefaction potential: Elevated in loose saturated sands, particularly with shallow groundwater tables.
Groundwater: Can be within 1–2 metres of the surface, affecting excavation and foundation options.
Foundation recommendations for these sites could include:
Deepened strip footings to reach denser sand at depth.
Driven timber or steel piles to transfer loads to competent layers.
Screw piles in high groundwater areas where open excavation is impractical.
Ground improvement such as gravel rafts or engineered fill replacement to mitigate liquefaction and improve bearing capacity.
Why this matters
The same building design in Khandallah and Lyall bay could require completely different foundation solutions because of the difference in bearing capacity, slope stability, liquefaction risk, and subsoil class.
The subsoil class affects seismic design under NZS 1170.5 — for example, Class B or C sites in Khandallah would typically have lower seismic amplification factors, while Class D or E sites in Lyall Bay may require higher structural demand considerations.
Similar contrasts exist across Greater Wellington. For example:
Paekākāriki – Predominantly coastal sand (often Class D or E) with liquefaction potential.
Masterton – Alluvial gravel plains (often Class C) with generally good bearing capacity but occasional variability in floodplain areas.
Why Site Investigations Matter for Structural Design and Consent
As shown in the examples above, a geotechnical report directly impacts:
Foundation type and depth.
Retaining wall design.
Drainage and stormwater management.
Seismic resilience.
Without accurate site data, foundations may be over-designed (adding unnecessary cost) or under-designed (increasing risk). In Greater Wellington, most councils require a geotechnical report for new builds, significant alterations, or sites with known hazards.
How much does a Geotechnical Report cost in the Wellington Region?
While prices depend on site complexity, typical costs are:
Standard residential site investigation – $2,000–$3,500 + GST
Complex hillside or coastal sites – $4,000 + GST
This investment helps avoid costly design changes or structural issues later in the project.
Conclusion
For homeowners in Wellington, the Kāpiti Coast, and the Wairarapa, a site investigation is a vital step before building. It ensures your design is tailored to your site’s unique conditions, meets council requirements, and minimises construction risks.
At DTCE Consulting Engineers, we carry out both geotechnical investigations and structural design, ensuring a seamless process from first site visit to final consent. With extensive experience across Greater Wellington’s varied terrain, we provide practical, reliable solutions to help your project start — and finish — on solid ground.
Or call us on on 04 475 7933
About the Author
Lee Huang is a Director at DTCE and a Chartered Professional Engineer with over 15 years of experience in New Zealand. He leads our ongoing effort to expand our Geotechnical capabilities alongside our Senior Engineer, Kyle Nicholson
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