UK Soil Types & Bearing Capacity: Foundation Design Guide

Quick Answer: Soils are classified to BS EN ISO 14688-1:2018 by particle size (gravel, sand, silt, clay) and behaviour (cohesive vs non-cohesive). Bearing capacity is determined by site investigation under BS 5930:2015 and Eurocode 7 (BS EN 1997-1). For domestic foundations, typical allowable bearing pressures are 200 kPa for dense gravel/sand, 100–150 kPa for firm clay, and as low as 50 kPa for soft clay. Building Regulations Part A and NHBC Standards Chapter 4.2 set the minimum foundation depth and width based on soil type.

Summary

Bearing capacity is the soil's ability to carry the load of a foundation without excessive settlement or shear failure. Two values matter: the ultimate bearing capacity (at which the soil fails) and the allowable bearing pressure (with a safety factor, typically 3, applied). Foundation design uses the allowable value.

UK ground is varied — from London Clay to Glasgow boulder clay to Cornish granite — and the same building specification can need a 750mm strip in one location and a piled foundation in another. The most common cause of subsidence is foundations on shrinkable clay close to trees, but soft alluvial silt, made ground (fill) and karst limestone all present hazards.

For domestic work the site investigation typically consists of trial pits dug by the groundworker to identify strata, plus laboratory testing if sulfate or contamination is suspected. NHBC Standards Chapter 4.2 includes ground category tables that allow many small projects to proceed without full Eurocode 7 analysis.

Key Facts

Quick Reference Table

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Soil Type Description Typical SPT N Allowable Bearing (kPa)
Dense gravel Well-graded, compact >30 250–400
Medium dense sand Round to angular 10–30 150–250
Loose sand Easily penetrated <10 50–100
Stiff clay Firm; thumbnail leaves slight mark 15–30 200–300
Firm clay Indented with thumb 8–15 100–200
Soft clay Easily moulded with fingers 4–8 50–100
Very soft clay Squeezes between fingers <4 <50 — pile or raft required
Peat / soft organic Black, fibrous n/a Not suitable for shallow foundations
Made ground / fill Variable Variable Treat with caution; investigation required
Chalk (Grade I–V) Soft to firm Variable 150–500 (Grade I)
Sandstone (weak) UK common bedrock Refusal 400–1000+

Detailed Guidance

Identifying soil type in a trial pit

A trial pit 1.0–1.5m deep gives a quick first reading of the ground profile. Look for:

Field tests for clay:

For sand and gravel:

Standard Penetration Test (SPT)

For larger sites a borehole investigation by a specialist contractor uses SPT to characterise strata. The test drops a 63.5kg hammer through 760mm onto a split-spoon sampler, counting blows for each 75mm of 450mm drive — the central 300mm gives the N value. N correlates to bearing strength via published tables and Eurocode 7 design methods.

Cohesive vs non-cohesive

Foundation behaviour differs:

Shrinkable clay and trees

The most common UK foundation problem. In London Clay (PI >40%) and similar high-shrinkage soils, mature trees draw moisture out, the clay shrinks, and foundations settle. In a wet summer or after tree removal, the clay swells and lifts foundations — even more damaging.

NHBC Standards Chapter 4.2 includes the influence distance based on:

For high-VCP clay and a high-demand tree, foundations may need to be 1.5–2.5m deep, even with the tree at 10m+ distance. Where the tree has recently been removed, foundations may also need additional depth for moisture rebound (heave).

Mitigation methods:

  1. Deeper trench fill or piled foundation
  2. Compressible layer (e.g. Claymaster) on the inside face of the foundation to absorb heave pressure
  3. Slip membrane to allow settlement movement without dragging foundation

Made ground (fill)

Any ground that has been disturbed by previous excavation and re-tipped is "made ground". Common in urban infill plots, former gardens with rubble, old industrial sites. Characteristics:

Investigation is essential. Where the made ground is shallow (≤1m), foundations can be taken through to undisturbed strata below. Deeper made ground typically needs piles to undisturbed bearing strata, or a raft foundation spanning the variable surface.

Sulfate-bearing ground

Most UK clays contain sulfate. The concentration is reported as DS-1 (negligible) to DS-5 (very high). Common in Lower Lias clay (West of England), Mercia Mudstone, Coal Measures, and some industrial fill. Drives the choice of concrete mix:

DS Class Sulfate (mg/L water-soluble) Concrete Mix
DS-1 <500 Standard (GEN 3 / RC25/30)
DS-2 500–1500 FND2 (sulfate-resisting cement)
DS-3 1500–3000 FND3
DS-4 3000–6000 FND4 + protection
DS-5 >6000 FND4 + barrier membrane + reduced cover

A simple field acid test indicates sulfate but laboratory analysis to BRE Special Digest 1 is the standard.

Frost protection

UK frost penetration is typically 450–600mm in southern England, deeper in Scotland. Foundations must bear below the frost line to prevent uplift from ice lensing. This is one of the reasons for the 750mm minimum strip foundation depth in most of the UK.

Bearing capacity calculation (basic)

Eurocode 7 / BS 8004 use Terzaghi's bearing capacity equation:

qu = c'Nc + qNq + 0.5γBNγ

where:
  qu  = ultimate bearing capacity (kPa)
  c'  = effective cohesion (kPa)
  q   = surcharge above founding level (kPa)
  γ   = unit weight of soil (kN/m³)
  B   = width of foundation
  Nc, Nq, Nγ = bearing capacity factors (depend on φ')

For domestic foundations this is rarely calculated explicitly — the NHBC tables and Approved Document A provide direct answers for normal conditions.

Site investigation specification

For a typical domestic extension:

For a new build or larger extension:

Frequently Asked Questions

How deep should my foundations be in clay?

In normal clay away from trees, 750mm strip foundation depth is typical (NHBC). In shrinkable clay close to trees, 900–2500mm depending on tree species, height and proximity. Always check the NHBC tree depth tables and Local Authority Building Control will accept these depths if site conditions match.

Do I need a soil report for a small extension?

Not always — many extensions proceed on the basis of a trial pit and standard NHBC depth tables. A formal soil report (Geotechnical Investigation) is required where ground is suspected of being made/contaminated, on shrinkable clay near trees, where the soil type is unclear, or where the structural engineer requests one.

What is "trench fill"?

A foundation method where the strip trench is filled with concrete almost up to ground level, rather than building a masonry footing inside the trench. Most common in UK new build because it removes the need for masonry below ground and is faster. Trench fill is usually filled to within 150mm of finished ground level.

How do I tell if my ground is made up?

Look for: rubble, glass, plastic, cinders, or unnaturally mixed strata in a trial pit. Old maps (NLS / Promap) showing past industrial use, former pits or fishponds. Check whether the plot was once a road, lane, garden of an older building, or a tip. If in doubt, dig a trial pit before submitting Building Regs.

What's the difference between settlement and subsidence?

Regulations & Standards