Soil Bearing Capacity Foundations: 5 Essential Checks Before You Design
Soil bearing capacity foundations design is the first calculation any structural engineer must complete before sizing a footing. Whether the project is a ground floor extension pad, a chimney breast underpinning or a new residential strip foundation, the bearing resistance of the soil directly controls the plan area, depth and reinforcement of every element below ground. Get this wrong and you risk undersized foundations, differential settlement and costly remedial works.
This guide covers soil classification, EC7 drained and un-drained bearing capacity equations, bearing capacity factors, partial factors for both load combinations, and a full worked example for a 0.75 m × 0.75 m pad on sand/gravel — verified against both Combination 1 and Combination 2.
Soil Bearing Capacity Foundations: Know Your Soil Type First
Before any calculation is performed, soil bearing capacity foundations design depends on correctly classifying the ground. The five principal soil types each carry distinct bearing characteristics and preferred foundation solutions:
| Soil Type | Key Bearing Characteristics | Typical Foundation |
|---|---|---|
| Rock | High bearing capacity; weaknesses at fissures and weathered zones | Reinforced pad — anchors substructure rather than spreading load |
| Gravel | Non-cohesive, high capacity, low compressibility. Groundwater can halve bearing capacity | Pad foundations; piling rarely needed |
| Sand | High capacity, low compressibility when dense; loose sand risks significant settlement; groundwater is detrimental | As gravel; monitor loose compaction zones |
| Clay | Cohesive; lower capacity than granular soils; long-term consolidation settlement; sensitive to moisture content | Pads to 1–2 storey; piles for heavier loads or where settlement must be controlled |
| Silt | Reasonable capacity when confined; structure breaks down when wet; frost heave risk | Piling through silt into competent strata — direct founding on silt is avoided |
Soil Bearing Capacity Foundations: The EC7 Governing Inequality
BS EN 1997-1 (Eurocode 7) requires that the design bearing resistance Rd is not exceeded by the design vertical load Vd. This single inequality governs all soil bearing capacity foundations checks:
The calculation of Rd divides into two routes: un-drained for cohesive soils under short-term load (clay), and drained for granular soils or clays under long-term conditions. Both equations are covered in full on page 2.
Soil Bearing Capacity Foundations: Un-drained and Drained Equations
cu;d = cu;k / γcu
γcu = 1.0 (M1) or 1.4 (M2)
(π + 2) ≈ 5.14 — the Prandtl factor for a purely cohesive soil. The overburden q adds the beneficial effect of soil surcharge alongside the footing.
Nc, Nq, Nγ = bearing capacity factors (function of φ'd)
sc, sq, sγ = shape factors
φ'd = tan⁻¹(tan φ'k / γφ')
γφ' = 1.0 (M1) or 1.25 (M2)
sγ = 0.7
sc = (sq·Nq − 1) / (Nq − 1)
For rectangular foundations: sq = 1 + (B'/L') sin φ'd; sγ = 1 − 0.3(B'/L'). B' and L' are effective foundation dimensions (reduced for eccentric load).
| Property | M1 | M2 |
| γφ' | 1.00 | 1.25 |
| γc' | 1.00 | 1.25 |
| γcu | 1.00 | 1.40 |
| γqu | 1.00 | 1.40 |
Bearing Capacity Factors Nq, Nc, Nγ
| φ'd (°) | Nq | Nc | Nγ | φ'd (°) | Nq | Nc | Nγ |
|---|---|---|---|---|---|---|---|
| 0 | 1 | 5.14 | 0 | 26 | 11 | 22 | 10 |
| 16 | 4 | 11 | 1 | 28 | 14 | 25 | 14 |
| 18 | 5 | 13 | 2 | 30 | 18 | 30 | 20 |
| 20 | 6 | 14 | 3 | 32 | 23 | 35 | 27 |
| 22 | 7 | 16 | 5 | 34 | 29 | 42 | 38 |
| 24 | 9 | 19 | 7 | 36 | 37 | 50 | 53 |
Soil Bearing Capacity Foundations: Full Worked Example (0.75 m × 0.75 m Pad, Sand/Gravel)
A 0.75 m × 0.75 m × 500 mm thick pad sits on sand/gravel. Footings at 1.5 m below ground level; water table at 3 m depth. Applied bearing pressures: 750 kN/m² (Combination 1) and 385 kN/m² (Combination 2). Soil: φ' = 30°, γ' = 17 kN/m³, c' = 0 — cohesionless, drained approach governs.
From table at φ' = 30°: Nq = 18, Nγ = 20
Shape factors (square): sq = 1 + sin 30° = 1.5; sγ = 0.7
= (25.5 × 18 × 1.5) + (½ × 17 × 0.75 × 20 × 0.7)
= 688.5 + 89.25 = 778 kN/m²
778 > 750 kN/m² → ✓ Combination 1 PASSES
By interpolation at 24.8°: Nq ≈ 10, Nγ ≈ 8.5
sq = 1 + sin 24.8° = 1.42; sγ = 0.7
= 362.1 + 37.9 = 400 kN/m²
400 > 385 kN/m² → ✓ Combination 2 PASSES
Both EC7 combinations satisfied. The 0.75 m × 0.75 m pad is adequate for this sand/gravel site.
Soil Bearing Capacity Foundations: Site Investigations and Geotechnical Categories
All soil bearing capacity foundations calculations depend on reliable ground data. EC7 classifies structures into three Geotechnical Categories (GC) that define the minimum scope of investigation required before any foundation can be designed:
Soil Bearing Capacity Foundations: 4 Mistakes That Cause Foundation Failure
In sand and gravel, groundwater within the influence zone of the foundation can cut effective bearing capacity by up to half. Where the water table is above foundation level, the submerged unit weight γ' must be used in place of the bulk unit weight — typically dropping from 17–20 kN/m³ to 9–11 kN/m³. Using the bulk unit weight for a waterlogged granular site significantly overestimates Rd and can produce an unsafe soil bearing capacity foundations design.
EC7 requires both load combinations to be verified. Combination 1 applies higher load factors with unfactored soil properties. Combination 2 applies lower load factors but reduces the angle of friction via γφ' = 1.25, which significantly reduces Nq and Nγ. On medium-strength granular soils, Combination 2 frequently governs. A soil bearing capacity foundations check that runs only Combination 1 is incomplete and non-compliant.
Appendix values of presumed bearing capacity — 50 kN/m² for soft clay, 100 kN/m² for firm clay, 200 kN/m² for dense gravel — are useful for preliminary sizing only. They are not a substitute for site-specific geotechnical data and should not appear as the basis for final soil bearing capacity foundations calculations submitted to Building Control. Where ground conditions are unknown, a trial pit with laboratory samples is the minimum acceptable investigation for a GC2 residential project.
Bearing capacity and settlement are separate limit state checks. A clay foundation can satisfy Vd ≤ Rd yet still undergo unacceptable long-term consolidation settlement as pore water pressures dissipate over months or years. The EC7 rule of thumb — if Rd / Vk,char ≥ 3, no formal settlement analysis is required for clay — provides a useful screening check. Where the ratio falls below 3, or where differential settlement must be controlled (e.g. extensions to existing buildings), a consolidation settlement analysis is required.
Soil Bearing Capacity Foundations: Frequently Asked Questions
What is a typical soil bearing capacity for residential foundations in the UK?
Typical indicative values are: competent gravel 100–200+ kN/m², stiff clay 75–150 kN/m², firm clay 50–75 kN/m², soft clay 25–50 kN/m². These are starting points only — soil bearing capacity foundations calculations must use site-specific geotechnical investigation data. Two sites on the same street can have very different bearing capacities depending on historical use, groundwater depth, depth of made ground and weathering.
Do I need a site investigation for a residential extension?
For a small single-storey extension on well-understood ground (GC1), a desk study with trial pits may satisfy Building Control. However, where ground conditions are uncertain, there is adjacent existing structure at risk of differential settlement, or the extension is two storeys or more, a proper site investigation with boreholes and laboratory testing is strongly recommended. Soil bearing capacity foundations design without reliable ground data risks undersized foundations that will not satisfy EC7 requirements.
What is the difference between drained and un-drained bearing capacity?
Un-drained bearing capacity applies to cohesive soils under short-term loading, where pore water pressures carry the applied stress before dissipating over time. Drained capacity applies to granular soils or clay under long-term conditions, where inter-particle friction — characterised by angle of friction φ' — provides resistance. For most soil bearing capacity foundations checks in UK residential practice, both conditions are assessed and the critical case governs.
How does groundwater level affect my foundation design?
Groundwater within the zone of influence of the foundation reduces effective stress in granular soils, cutting bearing capacity by up to half. It also triggers buoyancy checks for basement and retaining structures. In clay soils, moisture content is a primary control on undrained shear strength. The piezometer test during site investigation establishes the water table depth and is one of the most important data points feeding into soil bearing capacity foundations design.
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