UK Soil Bearing Capacity Calculator 2026
Bearing capacity represents maximum pressure soil can withstand from foundations without shear failure or excessive settlement. Calculated using Terzaghi's bearing capacity equation incorporating soil strength parameters including cohesion (c'), friction angle (φ), and unit weight (γ). UK design follows BS 8004 and Eurocode 7 requiring adequate safety factors (typically 2.5-3.0) applied to ultimate bearing capacity determining safe allowable bearing pressure for foundation design.
Ultimate bearing capacity calculates as: qu = c'Nc + γDNq + 0.5γBNγ where Nc, Nq, and Nγ are bearing capacity factors dependent on soil friction angle φ. These factors increase exponentially with friction angle, making granular soils (sand, gravel) with high friction angles achieve significantly higher bearing capacities than cohesive soils (clay, silt) with low friction angles. Shape factors adjust formula for square or circular footings increasing capacity versus strip footings.
| Soil Type | Safe Bearing Capacity | Typical Use |
|---|---|---|
| Dense Gravel | 600+ kN/m² | Excellent foundation material |
| Compact Sand | 300+ kN/m² | Good for most structures |
| Medium Sand | 100-300 kN/m² | Standard residential |
| Loose Sand | <100 kN/m² | Requires compaction |
| Hard Clay | 300-600 kN/m² | Very stiff clays |
| Stiff Clay | 150-300 kN/m² | Good foundation soil |
| Firm Clay | 75-150 kN/m² | Standard foundation |
| Soft Clay | <75 kN/m² | Poor, requires piles |
Factor of safety (FoS) divides ultimate bearing capacity producing safe allowable bearing pressure accounting for uncertainties in soil properties, loading variations, and construction tolerances. Standard FoS ranges 2.5-3.0 for typical projects with well-investigated soil conditions. Higher factors (3.5-4.0) apply where soil properties uncertain, loads variable, or consequences of failure severe. Lower factors (2.0-2.5) acceptable only with comprehensive ground investigation and controlled construction quality.
Safe bearing capacity (SBC) represents maximum pressure soil can safely support from foundations without risk of shear failure or excessive settlement. Calculated by dividing ultimate bearing capacity by factor of safety (typically 2.5-3.0). UK typical values: dense gravel 600+ kN/m², compact sand 300+ kN/m², stiff clay 150-300 kN/m², firm clay 75-150 kN/m². Ground investigation determines site-specific values through trial pits, boreholes, and laboratory testing per BS 5930.
Bearing capacity calculates using Terzaghi's equation: qu = c'Nc + γDNq + 0.5γBNγ where c' is cohesion, γ is unit weight, D is depth, B is width, and Nc, Nq, Nγ are bearing capacity factors determined from soil friction angle φ. Safe bearing capacity equals ultimate capacity divided by factor of safety (2.5-3.0). Shape factors modify equation for square or circular footings. BS 8004 and Eurocode 7 provide detailed calculation procedures and safety requirements.
Factor of safety (FoS) typically ranges 2.5-3.0 for bearing capacity calculations. Use FoS 3.0 for standard projects with normal ground investigation, FoS 2.5 for comprehensive investigation with proven soil consistency, FoS 3.5-4.0 where soil properties uncertain or loads highly variable. Lower factors acceptable only with detailed geotechnical study, controlled construction, and monitoring. Building Regulations require adequate safety margins ensuring foundations perform satisfactorily throughout building lifespan.
Yes, UK Building Regulations require ground investigation determining site-specific soil properties for foundation design. Investigation includes trial pits or boreholes identifying soil types and depths, laboratory testing measuring shear strength parameters, standard penetration tests (SPT) assessing density and bearing capacity, and groundwater level determination. Presumed bearing values from tables provide preliminary estimates only - final design must use measured soil properties from proper investigation per BS 5930 standards avoiding foundation failures from inadequate soil assessment.