Soil Types and Bearing Capacity: A Groundworker's Reference

Quick Answer: Safe bearing capacity ranges from 50 kN/m² for soft clay to 600+ kN/m² for solid rock. For domestic strip foundations under normal residential loads, most UK soils (excluding soft clays and filled ground) are adequate at depths specified in Approved Document A. Any doubt — cracked walls on adjacent buildings, soft spots in trial pits, made ground, or aggressive soil chemistry — requires a structural engineer and ground investigation before foundation design can proceed.

Summary

Soil classification and bearing capacity are the foundations of all groundworks. Every foundation design starts with an assumption about what the ground can carry. Get it wrong and you either over-engineer (wasting money on unnecessary depths and widths) or under-engineer (inviting settlement, cracking, and potential structural failure).

UK soils are highly variable — London Clay behaves completely differently from the chalk of the North Downs or the alluvial deposits of river floodplains. Clay soils shrink and swell with moisture changes (particularly near trees), creating seasonal heave and settlement that can crack foundations even when the bearing capacity is theoretically adequate. The age and composition of fill material on brownfield sites is often unknown and can include anything from demolition rubble to industrial waste.

For groundworkers, understanding the soil they're working in helps them spot problems early, interpret engineer's drawings correctly, and price groundworks realistically. Discovering that the site has deep fill or soft clay after tendering at strip foundation rates is a commercial disaster.

Key Facts

Quick Reference Table — Typical Bearing Capacity by Soil Type

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Soil Type Typical Safe Bearing Capacity (kN/m²) Notes
Solid rock (granite, sandstone) 500–1000+ Variable by jointing and weathering
Soft rock (chalk, limestone) 200–400 Check for solution features in limestone
Dense gravel (dry) 200–400 High confidence; check for loose zones
Compact gravel 100–200 Good for strip foundations
Dense sand 100–300 Confirm density with SPT
Loose sand 50–100 Settlement risk; check SPT N-values
Stiff to very stiff clay 75–150 Good for strip foundations; check tree proximity
Firm clay 50–75 Adequate for low-rise domestic; inspect in autumn
Soft clay 25–50 Marginal; engineer input needed
Very soft clay (alluvial) <25 Unsuitable for strip foundations
Peat <5–10 Unsuitable as founding stratum
Made ground / fill 0–100 (highly variable) Always investigate before use

Detailed Guidance

Cohesive Soils (Clay)

Clay is the dominant soil type across much of southern and central England, including the London Clay basin, Lias Clay (Midlands and South West), and Kimmeridge Clay (Dorset). Clay behaviour is dominated by:

Shrink-swell characteristics — clay shrinks when dry (low moisture content) and swells when wet. Near trees, the root system removes moisture from clay, causing local drying and shrinkage. Remove the tree and the moisture content gradually recovers, causing heave. Foundation design must account for the seasonal moisture variation zone:

London Clay is a heavily overconsolidated stiff fissured clay. It appears strong in laboratory tests but has natural fissures that reduce its in-situ bearing capacity. The safe bearing capacity is typically 50–150 kN/m², but the fissures make it sensitive to disturbance — do not leave London Clay exposed to rain or frost; concrete same day if possible.

Identifying clay consistency in the field:

Non-Cohesive Soils (Sand and Gravel)

Sand and gravel soils drain freely, have good bearing capacity when dense, and present fewer complications for conventional strip foundations. Key characteristics:

Particle sizes:

Density assessment: In the field, probe the ground with a 50mm diameter rod:

Dense gravels and sands are excellent founding materials. Loose sands (particularly fine sands) are susceptible to liquefaction in earthquake zones (not generally relevant for UK domestic work) and to piping failure under water pressure — watch for sand boils in flooded excavations.

Contamination — sands and gravels are common in river floodplains and glacial outwash deposits. In these areas, brownfield sites may have contaminated groundwater; test if there is any reason to suspect prior industrial use.

Made Ground and Fill

Made ground is any deposited fill material — demolition rubble, domestic refuse, colliery spoil, industrial waste, dredgings, or old building platforms. It is present on virtually every brownfield site and on many greenfield sites near settlement areas.

The bearing capacity of made ground is unpredictable. It may contain voids (from demolished cellars or roof void demolition), combustible material (domestic refuse), chemically aggressive content (concrete from demolished industrial buildings), or organic material (garden soil, vegetation) that will consolidate and settle under load.

Never found a building on made ground without ground investigation. The investigation must:

  1. Define the extent and depth of the fill (trial pits or boreholes at the building footprint corners)
  2. Characterise the fill composition
  3. Test for chemical contamination if there is any indication of industrial use
  4. Provide a foundation recommendation from a structural engineer

Options when made ground is encountered:

Reading a Trial Pit Log

Experienced groundworkers should be able to read a basic trial pit log, as these inform foundation depth recommendations:

The log shows: depth from surface, soil description (colour, consistency, soil type), water table observation level, any significant features (cobbles, made ground, roots, voids).

Example reading:

0.0–0.2m   Topsoil, black, loose
0.2–0.6m   Made ground: mixed brick rubble and brown clay
0.6–2.0m   Firm brown clay with orange mottling (weathered zone)
2.0+m      Stiff grey-brown clay (London Clay); no water table observed

This log tells you: ignore the top 0.6m (made ground), the weathered clay zone to 2.0m is marginal, found at 2.0m+ on stiff London Clay. Expected bearing capacity at 2.0m: 75–150 kN/m².

Sulphate Testing

Certain UK soils contain sulphates that attack Portland cement concrete. The main culprits are:

The BRE Special Digest SD1 defines sulphate classes DS-1 to DS-4+. If any of the above ground types are encountered, soil samples should be taken and tested for sulphate content (water-soluble sulphate; total sulphate; chloride content if relevant). Match the foundation concrete specification to the ground class using the DS classification table.

Frequently Asked Questions

How do I know if the ground is suitable without a ground investigation report?

Approved Document A provides presumed bearing capacity values for domestic buildings of up to two storeys that can be used without a ground investigation, provided: the site is not in a mining area; the ground is not shrinkable clay or fill; there is no evidence of previous development (brownfield); and adjacent buildings have no settlement cracks or foundation problems. If any of these conditions are not met, ground investigation is required.

What's the difference between bearing capacity and settlement?

Bearing capacity failure is a shear failure of the soil beneath the foundation — the soil 'punches' or slides and the foundation moves suddenly. Settlement is gradual compression of the soil under load — the foundation moves slowly downward. For clay soils, even adequate bearing capacity doesn't prevent long-term consolidation settlement. For residential buildings in the UK, settlement governs foundation design more often than bearing capacity.

When should I call a structural engineer about the ground?

If you encounter any of the following during trial pit inspection or excavation: soft or very soft clay; made ground or fill; peat or highly organic material; evidence of prior industrial use; voids or loose material; water at unexpected depth; or soil that smells (indicating organic content or chemical contamination). These are all grounds (pun intended) to stop, document what you've found, and get engineer input before proceeding.

Regulations & Standards