Understanding Your Geotechnical Report: A Sydney Homeowner's Guide

What a geotechnical report is designed to explain

A geotechnical report translates site observations, subsurface investigation and any relevant testing into information for the proposed development. Depending on the agreed scope, it can inform footing design, founding conditions, excavation, retaining-wall design, slope assessment or construction verification.

The report is specific to the site information, investigation locations and proposed works available when it was prepared. It is not a guarantee that conditions between every investigation point will be identical.

The sections commonly found in a report

A report usually describes the site and proposal before explaining the fieldwork, ground profile, relevant observations and engineering recommendations. The exact content varies with the investigation brief.

• Project description, site address, available plans and investigation scope.
• Site observations including terrain, drainage, access and existing development.
• Investigation locations, methods, depths and any laboratory results.
• Description of soil, fill, weathered material, rock and observed groundwater.
• AS 2870 site classification where relevant to residential footing design.
• Founding, excavation, earthworks or other recommendations within scope.
• Limitations, assumptions and situations requiring review or inspection.

Reading borehole logs and groundwater observations

Borehole or test-pit logs describe the materials encountered at particular locations and depths. A log is a record of what was observed at that point, not a promise that the same layer continues unchanged across the entire property.

Groundwater observations also relate to the investigation conditions at the time. Water levels can respond to rainfall, drainage, excavation and seasonal conditions, so a single observation may not represent the highest or lowest future level.

Site classification, bearing and footing recommendations

Where an AS 2870 classification is provided, the result helps the structural or footing designer consider expected moisture-related ground movement and relevant site features. Bearing or founding recommendations, where included, should be read alongside the proposed structure, investigation depth and stated assumptions.

A geotechnical recommendation does not replace structural design. If the report identifies an inspection requirement, that inspection should be coordinated before work is covered or concrete is placed.

Questions to ask before relying on an older report

Check whether the proposed building footprint, excavation, slope works or loading have changed since the report was prepared. Demolition, imported fill, regrading or nearby works may also affect whether earlier findings remain applicable.

If your builder, engineer, council or certifier has requested additional information, provide the original request and drawings. Sydney Geotechnical can review the project context and identify whether an updated investigation or targeted assessment is appropriate.