Ground Investigation and Site Assessment

What This Requirement Covers

Ground investigation determines the subsurface conditions that affect foundation design, basement construction, retaining wall design, and overall building stability. Eurocode 7 (BS EN 1997) requires that ground conditions are adequately investigated before the geotechnical design is carried out.

Key Requirements

When Is Ground Investigation Required?

1. For all new buildings (the level of investigation depends on the complexity and risk)
2. For extensions where ground conditions are uncertain or where the extension foundations may differ from the existing building
3. For basements, retaining walls, and any construction below ground level
4. When building on or near former industrial land, landfill sites, or areas of known ground instability

Types of Investigation

1. Desktop study: Review of historical maps, geological maps, mining records, flood risk data, and previous site investigation reports
2. Walk-over survey: Visual inspection of the site and surrounding area for signs of ground instability, contamination, or unusual features
3. Trial pits: Shallow excavations (typically up to 3-4 metres) to expose the soil profile and allow direct inspection
4. Boreholes: Deeper investigation (10-30+ metres) using rotary or cable percussion drilling to obtain soil and rock samples
5. In-situ testing: Standard penetration tests (SPTs), cone penetration tests (CPTs), and vane shear tests carried out within boreholes
6. Laboratory testing: Soil classification, strength testing, chemical analysis (for contamination), and compaction testing

Geotechnical Design Report

1. The results of the ground investigation are presented in a Geotechnical Design Report (GDR) to BS EN 1997
2. The GDR includes: ground model, design parameters (bearing capacity, settlement estimates, groundwater level), and recommendations for foundation type and design
3. The GDR forms the basis for the structural engineer's foundation design

Contamination

1. Where land contamination is suspected, a preliminary risk assessment (PRA) and, if necessary, a detailed assessment must be carried out
2. Contamination assessment follows the framework in CLR11 (Model Procedures for the Management of Land Contamination)
3. If contamination is found, a remediation strategy must be agreed with the local authority

Practical Compliance Tips

1. Commission a ground investigation before finalising the building design; late discovery of poor ground conditions causes delays and cost overruns
2. For simple domestic projects on established ground, trial pits may be sufficient; for larger or more complex sites, boreholes are necessary
3. Check the BGS (British Geological Survey) geological maps and the Environment Agency flood maps as part of the desktop study
4. If the site is in a former mining area, obtain a coal mining report from the Coal Authority
5. Ensure the ground investigation is carried out by a suitably qualified geotechnical company (members of the Association of Geotechnical and Geoenvironmental Specialists, AGS)
6. Provide the ground investigation report to the structural engineer, Building Control, and (if applicable) the warranty provider (NHBC, LABC Warranty)
7. Do not commence foundation excavation until the ground investigation results are available and the foundation design is confirmed