What it calculates
Estimate the bearing capacity of soil below a shallow foundation using Meyerhof's general bearing capacity equation, from parameters obtained on site. Considers a single soil stratum for preliminary shallow-foundation sizing.
Who uses this calculator
Australian structural and geotechnical engineers who need an allowable soil bearing pressure for a shallow footing before a full geotechnical report is in hand, working from soil type, friction angle, and cohesion. Intended for preliminary estimates on single-strata soils, not a replacement for a site-specific geotechnical investigation.
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.
