Soil Investigation and Trial Pits: Ground Assessment Methods, Depths and Recording Requirements for Domestic Projects

Summary

Ground investigation is the process of understanding what lies below ground before designing and building foundations. For most domestic extensions and small new-build projects, a simple visual assessment of trial pits is sufficient. For larger projects, brownfield sites, or where unusual ground conditions are suspected, more detailed investigation using boreholes, laboratory testing, and specialist analysis is needed.

The consequences of inadequate ground investigation are severe: incorrect foundation design, structural cracking, differential settlement, and costly remediation. Building control will often ask for evidence of ground investigation before approving foundation designs. Structural engineers cannot design foundations without ground data.

Trial pits are the most cost-effective investigation method for domestic work. They can be hand-dug for shallow investigations or machine-dug (using a mini-excavator) for depths up to 3–4m. At each pit, the excavation should be logged systematically.

Key Facts

• BS 5930:2015 — Code of practice for ground investigations; the definitive UK standard
• NHBC Chapter 4.1 — Ground Investigation guidance for housing; required for NHBC Buildmark warranty
• Trial pit depth — minimum 1.5m; aim for 2.0m or to a depth 1.0m below proposed foundation formation level
• Minimum number of pits — no fixed minimum; judge by site variability; typically 4 corners + centre for a house footprint
• Soil description — describe colour, texture, consistency (for clays), density/looseness (for sands/gravels), and presence of organic material
• Groundwater — note depth at which water is encountered; if water table is within foundation depth, dewatering required
• Made ground — any fill, rubble, or disturbed ground must be flagged; foundations in made ground require structural design
• Bearing capacity — assessed in field using shear vane, plate bearing test, or simply from soil description and consistency
• SPT (Standard Penetration Test) — in boreholes; number of blows per 300mm penetration relates to soil density and bearing capacity
• Shrinkage test — clay soil samples sent to lab for Plasticity Index and Linear Shrinkage; determines if clay is high shrinkage
• Log format — trial pit log records: level, depth, soil description, groundwater, remarks, date, logger name; BS 5930 format
• Contamination indicators — unusual smells, staining, ash, tars, foreign material; if found, stop and commission Phase 2 investigation

Quick Reference Table: Trial Pit Assessment Guide

Detailed Guidance

Planning the Investigation

Before digging:

• Check historical maps — Ordnance Survey historical maps at 1:2500 and 1:10560 scales available from local libraries or commercial map providers; show previous uses, ponds, watercourses, clay pits, quarries
• Check planning history — local authority planning portal often shows whether site has previous development
• Check Environment Agency Flood Map — floodplain sites may have high groundwater; alluvial soils (river deposits) often poor bearing
• Review any existing site investigation data — sometimes available from local authority or previous development; may not represent current conditions

Decide pit locations to:

• Cover all corners of the proposed building footprint
• Investigate any areas of concern (low spots, unusual vegetation, old filled areas)
• Investigate the line of proposed drainage runs where soil type affects drainage design

Conducting the Trial Pit

Machine pits: A mini-excavator can dig a trial pit to 3–4m depth safely. Key rules:

• Never enter an unsupported trench or pit more than 1.2m deep (see excavation safety trench support)
• Inspect from ground surface; or shore the pit before entering
• Keep excavated soil at least 1m from the pit edge
• Have someone at surface while any person is in the pit

Logging sequence: As the excavator digs, stop at approximately 300mm, 600mm, 900mm, 1200mm, etc., and log each layer.

What to record at each stratum:

1. Depth — top and bottom of each layer (in metres from ground surface)
2. Colour — e.g. "reddish-brown", "dark grey"
3. Material type — clay, silt, sand, gravel, cobbles, rock, fill
4. Consistency (clays) — very soft, soft, firm, stiff, very stiff (from SPT/shear vane or field test)
5. Density (sands/gravels) — very loose, loose, medium dense, dense, very dense
6. Structure — fissured, laminated, homogeneous
7. Inclusions — gravel, roots, cobbles, made ground materials (brick, concrete, ash)
8. Groundwater — depth at which seepage or standing water first appears
9. Remarks — odour, unusual staining, boulders, roots

Field consistency tests for clays:

• Very soft: easily penetrated by thumb; flows when disturbed
• Soft: easily moulded by hand; indents 25mm under thumb pressure
• Firm: moulded by strong hand pressure; indents 10mm under thumb
• Stiff: cannot be moulded by hand; indented 5mm by thumbnail
• Very stiff/hard: scratched by thumbnail; does not indent

Groundwater Assessment

Groundwater affects:

• Foundation type selection (high water table → sealed raft or piled foundations)
• Drainage design (soakaways won't work in saturated ground)
• Concrete specification (aggressive groundwater can attack standard Portland cement)
• Construction method (dewatering requirement)

Record groundwater depth at time of observation. Note that groundwater levels vary seasonally and may be higher in winter than at time of investigation. The Environment Agency's NRFA (National River Flow Archive) and local boreholes can provide historical groundwater level data.

For projects with high groundwater, a groundwater monitoring programme (installing standpipes in boreholes and measuring water level monthly over a season) may be required before design can proceed.

Bearing Capacity Assessment

Simple field assessment of soil bearing capacity:

Note: these are approximate assessments only. For structural foundation design, formal testing (plate bearing test, laboratory testing on undisturbed samples, or SPT in boreholes) is required to confirm bearing capacity.

Sending Samples to Laboratory

Where clay soil is found and tree proximity or shrinkage is a concern, collect samples for laboratory testing:

• Atterberg Limits (Plasticity Index) — determines shrinkage classification; high PI = high shrinkage
• Linear Shrinkage — direct measurement of clay volume change with moisture
• Moisture Content — indicator of ground saturation
• Unconfined Compressive Strength (UCS) — for clays; direct measure of bearing capacity

Samples should be collected in sealed polythene sample bags, labelled with pit reference, depth, and date, and sent to a UKAS-accredited geotechnical laboratory.

Trial Pit Log Format

The BS 5930:2015 standard borehole/trial pit log format includes:

• Project name and reference
• Location (grid reference and site description)
• Date excavated
• Depth of pit (or depth to refusal/groundwater)
• Drawn by (logger name and qualifications)
• Stratigraphy table with all observations
• Groundwater observations
• Remarks and summary interpretation

This log becomes part of the project file and should be provided to the structural engineer and building control as part of the foundation design submission.

Frequently Asked Questions

How many trial pits are needed for a domestic extension?

There is no fixed rule. The aim is to understand the ground across the full building footprint. For a typical single-storey rear extension (6m × 4m), two to three trial pits (one at each proposed corner of the extension, plus one in the middle if the ground is variable) are usually adequate. For a new house on a virgin site, four to six pits covering the corners and centre of the footprint, plus additional pits for drainage runs, is typical.

Can I do the trial pits myself?

Yes, for machine-dug pits on your own site, you can operate a hired mini-excavator and log the pits yourself. However, the soil description and bearing capacity assessment must be reliable — an incorrect interpretation could lead to inadequate foundation design. If you are not confident in soil assessment, hire a geotechnical consultant to supervise and log the pits. Their fee is small compared to the cost of getting the foundation wrong.

What if I find something unexpected in a trial pit?

Stop and assess. If you find made ground, very soft soil, contamination (odour, staining, ash), groundwater close to surface, or any condition not described in Approved Document A Table A1, stop the trial pit investigation and seek professional advice before designing the foundations. Never assume that unusual conditions are acceptable; the building control surveyor will ask questions if the foundation design doesn't match the ground conditions.

Do I need formal laboratory testing for every domestic project?

Not necessarily. For straightforward sites in non-clay soils with no indication of contamination or unusual conditions, visual assessment of trial pits may be sufficient. For clay soils where tree proximity is a concern, or for any brownfield site, laboratory testing of Plasticity Index and/or contamination testing adds certainty that's usually worth the cost.

Regulations & Standards

• BS 5930:2015 — Code of practice for ground investigations; site investigation methodology

• Building Regulations 2010 Approved Document A — foundation design; requires adequate ground investigation

• NHBC Standards Chapter 4.1 — Site preparation and investigation for housing

• BS 1377:2016 (Parts 1–9) — Methods of test for soils for civil engineering purposes

• CDM Regulations 2015 — pre-construction information including ground conditions

• BS 5930:2015 via BSI — site investigation code of practice

• NHBC Chapter 4.1 — foundation investigation for housing

• Environment Agency Flood Map — groundwater and flood risk indicator

• strip foundation design — using trial pit data to select foundation type and depth

• raft foundation guide — when poor ground investigation findings require a raft

• cdm regulations groundworks — pre-construction information including site investigation

• groundworks near trees — trial pit assessment for shrinkable clay adjacent to trees