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Bearing capacity

A foundation cannot be loaded so much that the soil below fails and the foundation sinks through. For clay under fast loading the bearing capacity is calculated from the undrained shear strength. With a factor of safety we find the allowable load.

q_u \approx 5.14\,s_u

bearing capacity of a strip footing on clay

q_{allow} = \frac{q_u}{F}

allowable load

Symbols

$q_u$	bearing capacity	kPa
$s_u$	undrained shear strength	kPa
$F$	factor of safety

Example

$s_u = 25$ kPa:

$q_u = 128.5$ kPa, and with $F = 3$ we get $q_{allow} \approx 43$ kPa.

Use undrained strength for clay under fast loading, and the friction angle for sand.

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Part of Geotechnical Engineering: Earth pressure, bearing capacity and stability.