What is a soil investigation report and who commissions it?

Foundations transfer a building's weight into the ground, so the designer needs to know what the ground can carry. Soil varies from plot to plot, and even within a plot, which is why guesses based on a neighbouring site are risky.

A typical report covers the number and location of test points, a description of each layer encountered, the depth at which water was found, laboratory results on strength and settlement behaviour, and notes on anything unusual such as filled ground or soft clay. It ends with recommendations on the type of foundation, and the allowable pressure the ground can take, which the structural engineer uses in design.

The usual order of events is:

• The owner or project manager appoints a geotechnical agency, often on the structural engineer's advice, with a testing plan based on the building type and footprint.
• The agency carries out fieldwork and lab tests, then issues the report with recommendations.
• The structural engineer designs the foundation using the findings.
• The contractor prices and builds against that design.

Who pays is normally the owner, since the information serves the design and protects the whole project. Contractors sometimes include it in their scope, but then the owner should still receive the full report, not just a summary.

Owners should ask for the report to be signed by a qualified geotechnical engineer, shared with the designer and kept in the project file. Retain it for future extensions and for any dispute about foundation performance. The design basis and testing method follow applicable codes and local requirements.