Shallow foundation bearing capacity calculations estimate the ground resistance available beneath a footing for a defined failure mechanism. A complete foundation assessment must also consider settlement, eccentricity, sliding, groundwater, construction conditions and global stability.
Topic guide: For background, method selection and related checks, see Bearing Capacity of Soil and Foundations.
This page presents a general calculation framework. It does not prescribe bearing factors, correction factors, resistance factors or factors of safety.
General bearing-capacity framework
A commonly used effective-stress form is:
$$ q_{\mathrm{ult}} = c'N_c s_c d_c i_c+ q'N_q s_q d_q i_q+ \frac{1}{2}\gamma'BN_\gamma s_\gamma d_\gamma i_\gamma $$| Symbol | Meaning |
|---|---|
| \(q_{\mathrm{ult}}\) | Ultimate gross bearing pressure |
| \(c'\) | Effective cohesion intercept selected for the analysis |
| \(q'\) | Effective vertical surcharge at foundation level |
| \(\gamma'\) | Effective unit weight used in the third term |
| \(B\) | Footing width used by the selected method |
| \(N_c,N_q,N_\gamma\) | Bearing-capacity factors |
| \(s,d,i\) | Shape, depth and load-inclination factors |
Additional factors for base inclination, ground slope or other effects may apply under the selected method.
The equation is a framework. The definitions and equations for every factor must come from one consistent design method. Mixing factors from different methods can produce an invalid result.
Gross, net and design values
The calculation record should distinguish:
- ultimate gross bearing pressure
- ultimate net bearing pressure
- applied gross contact pressure
- applied net foundation pressure
- design or allowable value derived under the nominated design framework
Do not label a value simply as "bearing capacity" without stating which quantity it represents.
Illustrative worked example
Assume an illustrative strip-footing calculation with:
- \(c'=0\) kPa
- \(B=1.5\) m
- foundation depth \(D_f=1.0\) m
- soil unit weight \(18\) kN/m3
- \(q'=18\) kPa
- selected \(N_q=18.4\)
- selected \(N_\gamma=15.7\)
- all displayed correction factors set to 1.0
This arithmetic verifies the formula implementation only. The selected factors are illustrative and are not recommended design inputs.
Groundwater and effective unit weight
Groundwater can affect:
- effective surcharge at footing level
- the unit weight used below the footing
- soil strength parameters
- settlement
- excavation and construction conditions
Any bearing-capacity calculation should show the groundwater model and intermediate effective stresses explicitly, rather than silently substituting a submerged unit weight.
Eccentric and inclined loading
Eccentricity reduces the effective loaded area. Inclined loading can reduce available resistance. These effects should be handled using the selected method rather than by applying an unexplained percentage reduction.
Use the Footing Pressure and Eccentricity guide to review the basic contact-pressure calculation.
Input and method checks
A calculation should retain:
- foundation geometry and embedment
- load direction and eccentricity
- ground and base inclination
- soil profile and groundwater
- drained or undrained basis
- strength parameters and their source
- effective surcharge
- bearing-factor equations
- every correction factor
- gross and net conventions
- design framework and method version
Block impossible geometry, negative effective dimensions and undefined factors.
What the calculation does not prove
A satisfactory bearing calculation does not prove that:
- settlement is acceptable
- the founding layer is continuous
- excavation conditions will preserve the assumed soil
- sliding or overturning is satisfactory
- a nearby slope is stable
- the structural footing is adequate
- construction-induced softening will not occur
Related resources
- Footing Pressure and Eccentricity
- Effective Stress in Soil
- Consolidation Settlement
- Retaining Wall Stability
- Soil Shear Strength
Authoritative reference
Final calculations require a selected design method, verified parameters, applicable standards and review by a suitably qualified geotechnical professional.