Safe bearing capacity (SBC) is the maximum pressure the soil beneath a foundation can carry safely — without shear failure and without excessive settlement. Get it wrong and foundations crack, tilt or sink; get it right and you can size footings economically. This guide explains what safe bearing capacity means, gives typical values for common soils and rocks, and shows you how to calculate it step by step with a worked example.
What is safe bearing capacity of soil?
Safe bearing capacity is the load per unit area that a foundation can transmit to the ground while keeping a comfortable margin against failure. It is obtained from the soil’s ultimate bearing capacity by applying a factor of safety, and it must also satisfy a limit on settlement.
In practice, two conditions must both be met:
- Shear criterion — the soil must not fail in shear under the applied load.
- Settlement criterion — total and differential settlement must stay within acceptable limits (commonly 25 mm for isolated footings on sand).
The lower (governing) of the two values controls the design.
Ultimate vs safe vs net bearing capacity
These terms are easy to confuse. Here is how they relate:
- Ultimate bearing capacity (qu) — the pressure at which the soil fails in shear.
- Net ultimate bearing capacity (qnu) — ultimate capacity minus the overburden at foundation level: qnu = qu − γDf.
- Net safe bearing capacity (qns) — net ultimate divided by a factor of safety: qns = qnu / F.
- Safe bearing capacity (qs) — the net safe value with the overburden added back: qs = qns + γDf.
- Allowable bearing pressure — the final design value that satisfies both shear and settlement. This is what you actually design to.
Typical safe bearing capacity values (table)
The table below lists presumptive (indicative) bearing values for common ground types. Use them for preliminary sizing only — always confirm with a site-specific geotechnical investigation before final design.
| Soil / rock type | Safe bearing capacity (kN/m²) | Approx. (t/m²) |
|---|---|---|
| Soft clay, soft silt or made-up fill | 50 or less | ~5 |
| Medium (firm) clay | 100 – 150 | 10 – 15 |
| Stiff clay | 150 – 250 | 15 – 25 |
| Loose sand | 100 – 150 | 10 – 15 |
| Medium dense sand | 200 – 250 | 20 – 25 |
| Dense sand and gravel | 400 – 450 | 40 – 45 |
| Soft / weathered rock | 450 – 900 | 45 – 90 |
| Hard sound rock | 1,650 – 3,300+ | 165 – 330+ |
Note: 1 kN/m² = 1 kPa ≈ 0.0102 t/m². Values are indicative and vary with moisture, density, footing width and depth.
How to calculate safe bearing capacity (Terzaghi’s method)
The most widely used method is Terzaghi’s bearing capacity equation. For a shallow strip footing, the ultimate bearing capacity is:
qu = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ
For square and circular footings, shape factors change the cohesion and width terms:
- Square footing: qu = 1.3·c·Nc + γ·Df·Nq + 0.4·γ·B·Nγ
- Circular footing: qu = 1.3·c·Nc + γ·Df·Nq + 0.3·γ·B·Nγ
Where:
- c = cohesion of the soil (kN/m²)
- γ = unit weight of the soil (kN/m³)
- Df = depth of foundation (m)
- B = width of footing (m)
- Nc, Nq, Nγ = bearing capacity factors that depend on the soil’s angle of internal friction (φ)
Worked example
Size a square footing 2 m wide founded at 1.5 m depth in soil with c = 10 kN/m², φ = 20° and γ = 18 kN/m³. From Terzaghi’s tables (general shear) at φ = 20°: Nc ≈ 17.7, Nq ≈ 7.4, Nγ ≈ 3.6.
qu = 1.3(10)(17.7) + (18)(1.5)(7.4) + 0.4(18)(2)(3.6)
qu = 230.1 + 199.8 + 51.8 = 481.7 kN/m²
Net ultimate: qnu = 481.7 − (18 × 1.5) = 454.7 kN/m²
Applying a factor of safety of 3: qns = 454.7 / 3 = 151.6 kN/m²
Safe bearing capacity: qs = 151.6 + 27 = ≈ 179 kN/m²
Factor of safety
A factor of safety of 2.5 to 3.0 is normally applied to the net ultimate bearing capacity, with 3 being common for shallow foundations. The margin covers natural variability in soil properties, uncertainty in loads, and workmanship.
Field methods to determine bearing capacity
- Plate load test — a rigid steel plate is loaded in a test pit and settlement is recorded; best suited to granular soils.
- Standard Penetration Test (SPT) — the blow count (N-value) is correlated to soil strength and bearing capacity.
- Cone Penetration Test (CPT) — gives a continuous profile of cone resistance with depth.
- Laboratory tests — triaxial and unconfined compression tests provide the cohesion (c) and friction angle (φ) used in the equations above.
Factors that affect bearing capacity
- Soil type, density and shear strength (c and φ)
- Width and depth of the foundation
- Position of the water table — a high water table can roughly halve the capacity of granular soil
- Type of foundation and nature of loading — eccentric or inclined loads reduce capacity
Frequently asked questions
What is a good bearing capacity for a house?
For low-rise homes, a safe bearing capacity of about 150 kN/m² or more (medium to stiff soil) is generally comfortable for conventional strip or pad footings. Softer soils may need wider footings, a raft or piles.
What is the difference between safe and allowable bearing capacity?
Safe bearing capacity guards against shear failure only. Allowable bearing pressure is the final design value that also limits settlement — it is the smaller of the two, and it is what you design to.
How does the water table affect bearing capacity?
When the water table rises to foundation level, the effective unit weight of the soil drops, which reduces the frictional terms in the bearing capacity equation. For sandy soils, the capacity can fall by up to about 50%.
Conclusion
Safe bearing capacity ties together soil strength, footing geometry and an appropriate factor of safety. Use presumptive values for quick preliminary sizing, but base any final foundation design on Terzaghi’s method (or a modern equivalent) using parameters from a proper soil investigation — and always check settlement as well as shear.
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