Soil Bearing Capacity Estimation

How it calculates

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.