Estimate Soil Bearing Capacity

This sheet estimates the ultimate bearing capacity of a shallow foundation subjected to low to moderate loading conditions.

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Worked example

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Design Example (Example 3.2, B. M. Das, 2011- Principles of Foundation Engineering)

A square foundation is 2 m x 2 m in plan. The soil supporting the foundation has a friction angle of 25 deg and effective cohesion of 20 kPa. The unit weight of soil is 16.5 kN/m3. Determine the allowable gross load on the foundation with a factor of safety (FS) of 3. Assume that the depth of the foundation (DF) is 1.5 m and that general shear failure occurs in the soil.

1. Input from the user - Footing Geometry:

2. Input from the user - Base soil properties:

3. The bearing capacity parameters is then calculated based on the friction angle of base soil, foundation dimensions and angle of load inclination.

4. Effective stress at foundation base and bearing capacity of foundation is then calculated as shown below:

5. The allowable load at foundation will then be calculated using the user defined factor of safety and foundation base area.

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Method & scope

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General Notes

• This sheet should be used only to estimate the bearing capacity of shallow foundation (e.g. column footing, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations and earthbag foundations).
• The ultimate bearing capacity of shallow foundation is the maximum load per unit area in foundation soil at which the shear failure occurred in the foundation.
• Meyerhof’s bearing capacity equation has been used for estimation of bearing capacity of shallow foundation.
• Weight of the foundation is ignored in entire calculator.
• Effect of water table should be considered for the sites where permanent or seasonal water table is located.

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Assumptions and Limitations

• In the present sheet only single soil strata has been considered.
• Water level could be as high as near to ground (H = 0 m) and much below the ground (H > footing depth).
• No lateral load or moment have been considered at anywhere in the calculations.
• In the case of footings in marine environment or heavy loads acting on the footing, case specific design should be carried out.

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References

• B. M. Das (2011) Principles of Foundation Engineering.

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Calculation method

The calculator estimates the ultimate and allowable bearing capacity of the soil beneath a shallow foundation using the general bearing capacity equation. It works from parameters that can be obtained on site, then applies a factor of safety to give a factored capacity and a maximum allowable load.

General bearing capacity equation

The ultimate bearing capacity is built from three contributions: a cohesion term, an overburden (surcharge) term driven by the effective stress at the founding depth, and a foundation-size term driven by the soil self-weight below the footing. Each term is the product of a bearing capacity factor and a set of correction factors for shape, depth, and load inclination. The effective stress at the bottom of the foundation is computed first, since it scales the overburden and size contributions.

Bearing capacity factors

Three bearing capacity factors are derived from the effective internal angle of friction of the foundation soil: one for the cohesion part, one for the overburden part, and one for the foundation-size part. These increase sharply with the friction angle and are the primary driver of capacity in granular soils, while the cohesion term dominates in clays.

Shape, depth, and inclination factors

Each of the three terms is corrected by a shape factor, a depth factor, and an inclination factor. The shape factors use the footing length and width (so a square footing and a strip footing give different results). The depth factors reward the confining effect of soil above the founding level. The inclination factors use the angle of load inclination to reduce capacity when the applied load is not vertical.

Effective stress and the water table

The depth of the water table below ground level determines whether bulk or saturated (submerged) unit weights govern the effective stress at and below the founding depth. A high water table reduces the effective stress and therefore the overburden and size contributions to capacity, which the calculator accounts for using the entered unit weight of water.

Factored capacity and allowable load

The ultimate general bearing capacity is divided by the entered factor of safety to give the factored bearing capacity. Multiplying by the footing plan area gives the maximum allowable load that the soil can support beneath the shallow foundation.

Assumptions

The estimate uses the Meyerhof (1956) general bearing capacity equation, considers only a single base soil stratum, and applies only to the soil below shallow foundations. It is intended as an on-site estimate and should be confirmed by a geotechnical investigation for final design.

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How to use it

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Common questions

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Next steps