Soil Classification for Groundworks: Clay, Sand, Made Ground, Load-Bearing Capacity and Strip Foundation Implications

Quick Answer: UK soils are classified under BS 5930:2015 (Site Investigation) and BS EN ISO 14688. Bearing capacities range from 75–100 kN/m² for soft clay up to 300–600 kN/m² for dense gravel and bedrock. Strip foundation depth and width are determined by soil type, load, and tree proximity — clay soils near trees routinely require depths of 1.0–2.5m under NHBC Chapter 4.2 guidelines.

Summary

Soil classification is fundamental to groundworks pricing and design. Get it wrong and your foundation depth may be inadequate, causing settlement, or your strip widths may be over-specified, doubling excavation cost. In the UK, the most common soil types encountered in domestic and light commercial groundworks are cohesive soils (clay, silt), granular soils (sand, gravel), and made ground — each with drastically different bearing capacities, drainage behaviours, and seasonal movement characteristics.

Clay soil presents the most challenges for UK builders. High-plasticity clays (particularly London Clay, Gault Clay, Weald Clay, and Oxford Clay) are susceptible to volumetric change with moisture content — swelling when wet, shrinking when dry. This shrink-swell behaviour drives the deepest strip foundations required in domestic construction, particularly where trees are present. The 2022 drought highlighted this risk sharply: many insurance claims from that period involved heave or subsidence linked to clay shrinkage.

Made ground (imported or previously disturbed material) is encountered on brownfield sites, former gardens, and anywhere that has been built on or heavily landscaped. Its bearing capacity is highly variable — often poor — and it may contain contaminants, voids, organic material, or demolition rubble. Made ground should never be assumed to be load-bearing without investigation and testing.

Key Facts

• BS 5930:2015 — the primary UK standard for site investigation; defines soil classification by particle size, consistency, and origin.
• BS EN ISO 14688-1 & -2 — European standard for identification and classification of soils; aligned with BS 5930 in the 2015 revision.
• BS 8004:2015 — Code of practice for foundations; gives guidance on allowable bearing pressures and foundation design.
• NHBC Chapter 4.2 — prescriptive guidance on strip foundation depth near trees on shrinkable (clay) soils; widely used by building control and NHBC warranty inspectors.
• Bearing capacity — firm clay: approximately 75–150 kN/m²
• Bearing capacity — stiff clay: approximately 150–300 kN/m²
• Bearing capacity — loose sand: approximately 100–150 kN/m²
• Bearing capacity — dense sand/gravel: approximately 200–600 kN/m²
• Bearing capacity — soft clay / silt: below 75 kN/m² — typically unsuitable for strip foundations without widening or piling.
• Bearing capacity — made ground: highly variable; treat as 0 until tested. Avoid using as founding stratum.
• Standard strip foundation: Building Regs Approved Document A minimum — 225mm wide × 225mm deep concrete strip on ground with bearing capacity of 100 kN/m²+ (trench fill is more common in practice).
• Trench fill: concrete filled to within 150mm of surface level; typically 450mm–1.0m deep depending on soil and frost.
• NHBC shrinkable clay categories: High (Volumetric Shrinkage Potential >40%), Medium (VSP 10–40%), Low (VSP <10%).
• Tree zone of influence: NHBC Table 4.2(D) — influences strip foundation depth based on tree species, height, and distance from wall. Can require foundations of 2.0–3.0m in high-shrinkage clay near large trees.
• Sulphate attack: some clays contain sulphates; concrete specification must be upgraded to ACEC class where sulphates >0.5g/l in ground water (BS 8500-1).
• Heave risk: swelling pressure in high-plasticity clays can exceed 100 kN/m²; slip membranes required on foundation sides in high-heave zones.
• Made ground depth detection: trial pits (BS 5930 methodology) to 2–3m are the most cost-effective investigation method on domestic sites.
• Soil investigation requirement: required by NHBC, most structural engineers, and best practice before designing any foundation on unknown ground.

Quick Reference Table

NHBC Tree Proximity Foundation Depth (Clay — Medium Shrinkage Potential)

Depths increase further on high-shrinkage clay (VSP >40%). Consult NHBC Chapter 4.2 Tables for full matrix.

Detailed Guidance

Identifying Soil Type on Site

Clay: Smooth, plastic when wet, forms a ribbon when rolled between fingers. Sticks to boots. Colour varies (blue-grey London Clay, brown/orange Oxford Clay, grey Weald Clay). Can be cut cleanly with a spade. Dries to hard, cracked blocks. Shrink-swell test: a hand specimen should be noticeably harder when air-dried for 24–48 hours versus when freshly excavated.

Sand: Granular, non-cohesive, falls apart when wet. Loose sand collapses in trench walls. Dense compacted sand may mimic clay in colour but lacks plasticity. Test: a handful squeezed hard forms a temporary ball that crumbles easily on release.

Gravel: Clearly visible coarse particles (>2mm). May be mixed with sand (sandy gravel) or clay (gravelly clay). Well-graded gravel with sand is excellent founding material.

Made ground: Look for mixed layers of rubble, topsoil, ash, brick, glass, or organic material. Layer boundaries are irregular. Often found from 0 to 1.5m depth. The giveaway is variability: no made ground has uniform colour, composition, or consistency. Made ground depth can be identified by probing (rod probe test) or trial pit.

Silt: Similar to clay in cohesion but finer and more reactive to water. Often found in river floodplains. Dilatancy test: shake a wet sample — silt releases water to the surface, clay does not.

Strip Foundation Design — Soil-Specific Rules

On clay (standard domestic extension, no tree influence):

• Minimum 450mm below ground surface in frost-free zone — but in practice 600–900mm is usual in clay.
• Trench fill concrete to within 150mm of formation level is now almost universal — it avoids hand-digging and reduces labour cost.
• Where trees existed and have been removed within 10 years (heave risk), specify 1.0–1.5m depth and consult SE.

On sand and gravel:

• 450–600mm depth typically sufficient on dense sand/gravel if water table is below formation level.
• Check water table: if within 600mm of proposed foundation level, allowable bearing pressure reduces and temporary dewatering may be needed.
• Fine loose sand below water table: risk of running sand (trench collapse without support). Use hydraulic shoring or closed-face excavation.

On made ground:

• Never found on made ground without investigation. Options: remove made ground to virgin soil, install raft foundation (if loads permit), or use pile-and-beam.
• Made ground depth varies: commission trial pits (typically 3 pits minimum on a domestic plot) before design.

On chalk:

• Generally good bearing capacity but chalk dissolves in water (karstic voids). Inspect trial pit base carefully for soft spots or voids.
• Frost-susceptible if fine-grained chalk within 450mm of surface — ensure foundation is below frost line.

Clay Heave and Slip Membranes

When founding in stiff, high-plasticity clay on a site where trees have been removed, the soil will absorb moisture over time and expand (heave). This upward movement can lift foundation concrete and crack superstructure walls.

Where heave is a risk:

• Install a compressible slip layer (Clayboard or equivalent — typically 25–75mm compressible void-former) around the sides of the foundation, between the concrete strip and the clay trench walls.
• This allows the clay to expand against the compressible layer without transmitting heave forces into the concrete.
• The void former must be rated for anticipated heave pressure (often >100 kN/m²).
• Do not use slip membranes as a substitute for adequate depth — both are needed where heave risk is high.

Contaminated and Made Ground: What to Look For

On brownfield or infill sites, the ground investigation should be extended to include chemical testing:

• Sulphate content (SO₄): affects concrete specification. >0.5g/l → ACEC class DC-2 minimum; >1.5g/l → DC-3 or higher. Sulphate-resistant cement (SRPC or ggbs/pfa blend) is required at higher classes.
• pH: acidic ground (pH <5.5) can attack concrete. Specify protective concrete or ground treatment.
• Hydrocarbon contamination: may require specialist remediation before building.
• Radon: certain geologies (granite, certain shales) produce elevated radon. Check UK Health Security Agency radon map. Part C Building Regs addresses radon-protective measures.

Trial Pits: Practical Guidance

Trial pits are the most practical way to investigate soil conditions on a domestic site:

• Depth: typically 2–3m, or to the proposed founding stratum.
• Number: minimum 3 on a standard domestic plot (corners and centre); more on irregular sites.
• Documentation: log each stratum by depth, description, colour, and consistency (use BS 5930 descriptors). Note depth to groundwater if encountered.
• Safety: pits over 1.2m deep require shoring or battered sides per HSE guidance. Never enter an unshored pit.
• Cost: £150–£400 per pit typically on domestic sites (JCB/mini-digger hire + operatives).

Frequently Asked Questions

My building control officer wants a soil investigation report. What does this involve?

For a simple domestic extension, a basic site investigation involves 2–3 trial pits logged by a qualified person (engineer or geologist) with a written report describing soil types, depths, groundwater levels, and a recommendation for foundation type and depth. Cost typically £300–£1,000 for domestic scale. Some BCOs accept a competent contractor's trial pit log instead for simple extensions on known soils — check with your local BCO.

What's the difference between bearing capacity and bearing pressure?

Bearing capacity is the maximum pressure the soil can support without shear failure (kN/m²). Bearing pressure is the actual pressure your foundation exerts on the soil (total load divided by foundation area, kN/m²). For safe design, bearing pressure must be less than allowable bearing capacity, which is the ultimate bearing capacity divided by a safety factor (typically 2.5–3.0 for strip foundations).

Can I build an extension on made ground?

Yes, but not with a standard strip foundation. Options are: (1) remove made ground and found in virgin soil below; (2) pile through made ground to load-bearing stratum; (3) raft foundation if loads are low and made ground is stable and shallow. Always investigate made ground depth and composition before committing to a design.

My clay site has had large trees on it. Do I need extra-deep foundations?

Almost certainly yes if the trees were within 1–1.5× their height from the proposed building. After tree removal, clay shrinks initially (roots no longer drawing water) and then swells as moisture returns over 5–15 years. NHBC Chapter 4.2 gives the definitive depth matrix. Your structural engineer should design for both shrinkage (during and after tree removal) and heave (if trees are still present and roots extend to the foundation zone).

What is a VSP test and do I need one?

Volumetric Shrinkage Potential (VSP) is measured by laboratory testing of a soil sample — it quantifies how much a clay will shrink. It's used to classify clay as Low (<10%), Medium (10–40%), or High (>40%) shrinkage potential. NHBC requires VSP classification when designing foundations near trees on clay. A single VSP test costs approximately £80–£150 from a specialist geotechnical lab.

Regulations & Standards

• BS 5930:2015 — Code of practice for ground investigations; soil classification methodology.

• BS EN ISO 14688-1:2018 and -2:2018 — Geotechnical investigation and testing: identification and classification of soil.

• BS 8004:2015 — Code of practice for foundations; allowable bearing pressures and foundation design guidance.

• Building Regulations Approved Document A — minimum strip foundation dimensions by soil bearing capacity.

• NHBC Standards Chapter 4.2 — foundations adjacent to trees: prescriptive depth tables by tree species, height, distance, and clay shrinkage class.

• BS 8500-1:2015+A2:2019 — Concrete: Specification, performance, production and conformity; sulphate class concrete specification.

• HSE CIS54 — Excavations: safe working in trenches and excavations; trial pit safety.

• Building Regulations Part C — site preparation and resistance to contaminants and moisture; radon protective measures.

• NHBC Standards Chapter 4.2 — foundation depth near trees on clay soils (official NHBC site).

• Building Research Establishment: Clay Shrinkage — BRE guidance on clay shrinkage and subsidence.

• BGS GeoScour and Geology Viewer — British Geological Survey soil and geology maps for site investigation.

• ICE Manual of Geotechnical Engineering — authoritative reference for bearing capacity and foundation design.

• HSE: Excavation Safety — CIS54 safe excavation guidance.

• strip foundation design — strip foundation sizing, concrete specification, and design process

• soil investigation trial pits — practical guide to trial pit investigation on site

• excavation safety trench support — trench support and excavation safety

• groundworks near trees — tree proximity rules and root protection zones

• sleeper retaining walls — retaining wall foundation depth and drainage design

• underpinning methods — underpinning options when existing foundations are inadequate