What is a soils report? Geotechnical investigations explained

A soils report, also called a geotechnical report or geotechnical investigation, describes the ground under a planned building and tells the designer how to found it. It is prepared by a geotechnical engineering firm and sealed by a Professional Engineer (P.Eng.) licensed in the province or territory where the site is located.

Why it matters

Foundations transfer the weight of the building to the soil. Soft clay, loose fill, organic soil, a high water table, sloping ground or soils that swell and shrink can all lead to settlement and cracks if the foundation is not designed for them. Building codes based on the National Building Code of Canada require foundations suited to the soil conditions, and the building official may ask for a report when those conditions are uncertain.

How the investigation is done

1. Desk study: the engineer reviews geological maps, nearby reports and the project drawings.
2. Field work: boreholes drilled with a rig or test pits dug with an excavator, at locations and depths chosen for the project. The engineer logs each soil layer and takes samples.
3. Groundwater: water levels are noted in the holes, sometimes with a standpipe left in place for later readings.
4. Laboratory tests: moisture content, grain size, plasticity, strength or consolidation tests, depending on the soils found.
5. Report: the findings and the engineer's recommendations.

What the report usually contains

• a site plan with the borehole or test pit locations;
• a log for each hole, describing the layers and the groundwater;
• the laboratory results;
• recommendations for the type and depth of foundation, the bearing values to use for design, drainage, frost protection and backfill;
• notes for construction, such as excavation, dewatering or inspection of the foundation subgrade.

The exact scope depends on the building, the site and what the municipality requires. A small house on well-known ground may need a short letter; a building on soft clay or a slope may need a full investigation.

How to prepare

• Have the site plan and, if possible, the preliminary foundation drawings.
• Have buried services located before any drilling: contact your provincial one-call or utility locate service.
• Make sure the rig or excavator can reach the building footprint.
• Ask the firm what is included: number and depth of holes, laboratory testing, report format and timing.

Next steps: check when a soils report is needed for a building permit, or describe your project in the form and local geotechnical firms get back to you.