Ground Investigation
/ ɡraʊnd ɪnˌvɛstɪˈɡeɪʃ(ə)n /
Also known as: site investigation, soil investigation, geotechnical investigation, ground survey
Definition
A ground investigation is a structured programme of fieldwork and laboratory testing to determine the nature, properties, and hazards of the ground beneath a construction site before foundations are designed. It typically includes trial pits (shallow excavations for direct soil inspection) and/or boreholes (drilled samples from depth), followed by lab testing and a geotechnical report with foundation recommendations. It is the essential first step for any non-trivial new building or extension.
In practice
For most domestic extensions, a programme of two to four trial pits dug by a mini-excavator along the proposed foundation line, logged by a geotechnical engineer, with soil samples sent for Atterberg limits and particle size testing (to assess shrinkability on clay sites) is sufficient. The geotechnical report is typically 10-20 pages and is submitted to Building Control with the foundation drawings as evidence that the design is appropriate for the site conditions.
Where contamination is suspected (former garage forecourts, industrial uses, in-filled ponds), a Phase 1 desk study (reviewing historical maps and records) is followed by a Phase 2 intrusive investigation with soil and groundwater sampling for chemical analysis. Findings are assessed against human health criteria and site-specific risk assessment. Remediation of contaminated ground may be required before foundations can be built, and a verification report submitted to Building Control to confirm the remediation was effective.
Building Regulations
Approved Document A requires that foundations must be appropriate for the site conditions, which implies that conditions must be known before design. Building Control bodies routinely request ground investigation reports for new buildings and for extensions where the ground conditions are not straightforward. Approved Document C (Resistance to contaminants and moisture) requires that sites where contamination is possible are investigated and that any significant contamination is remediated. BS 10175 (Investigation of potentially contaminated sites) is the relevant standard for contamination investigations.
Full Building Regulations guidance