The Short Answer
A geotechnical report is a written engineering document that describes the subsurface conditions at a site — the soil and rock layers, groundwater, and their measured properties — and provides recommendations for foundations, earthwork, and site preparation. Prepared under the direction of a licensed engineer, it is the authoritative reference that structural engineers use to design foundations and that building departments require before permitting many projects. It converts raw field and lab data into practical, code-compliant engineering guidance.
What's Inside a Geotechnical Report
Site and Project Description
The report opens with the site location, the proposed construction, and the scope of the investigation. This establishes what was studied and what the recommendations apply to — a report prepared for a single-story warehouse may not cover a later multi-story addition.
Field Investigation Findings
This section documents the borings or test pits, including their locations, depths, and the soil and rock encountered. Boring logs describe each layer, sample recovery, and groundwater observations. This is the factual record of what was found underground.
Laboratory Test Results
Samples are tested to measure moisture content, density, strength, plasticity, gradation, and other properties. These results support the engineering analysis and confirm the classification of each soil layer.
Engineering Recommendations
The heart of the report. It specifies the recommended foundation type, allowable bearing pressure, estimated settlement, required over-excavation and fill placement, compaction criteria, lateral earth pressures for walls, and — where applicable — seismic design parameters and pavement sections. These recommendations drive downstream design.
Who Uses a Geotechnical Report
The structural engineer uses it to select and size foundations. The civil engineer uses grading and pavement recommendations. Contractors use the earthwork and compaction requirements to prepare the site. Building officials review it to confirm the design accounts for actual site conditions. Owners and lenders may reference it during due diligence. Because so many parties rely on it, a clear, complete report prevents confusion and costly field changes.
When Do You Need a Geotechnical Report?
A geotechnical report is commonly required for new buildings, additions with new foundations, retaining walls, pavements, and projects on challenging soils. Many jurisdictions and most structural engineers require one before foundation design is completed. If your project involves any new load being transferred to the ground, a report is usually warranted.
Frequently Asked Questions
How long is a geotechnical report valid?
There is no universal expiration, but reports can become outdated if site conditions change, the design changes significantly, or considerable time passes. An engineer can review an older report and advise whether it still applies.
Can I reuse a neighbor's geotechnical report?
Generally no. Subsurface conditions vary over short distances, and a report is prepared for a specific site and project. Recommendations must reflect your own site's investigation.
Who stamps the report?
A licensed professional engineer who directed the investigation reviews the data and seals the report, accepting professional responsibility for the recommendations.