What is Soil Bearing Capacity?

Soil bearing capacity is the ability of soil or rock to support loads applied by a foundation without experiencing bearing failure or unacceptable settlement. It influences how a structure is supported and whether shallow foundations, piling or ground improvement are appropriate.

Ultimate bearing capacity refers to the pressure at which the supporting ground fails. Foundations provide a margin against this failure and consider settlement separately. The usable support therefore depends on both ground strength and acceptable movement.

Bearing capacity values describe different limits and use different reference points. Understanding these distinctions helps explain how engineers move from assessing ground failure to selecting a suitable foundation pressure.

Ultimate bearing capacity is the pressure beneath a foundation at which the supporting ground reaches bearing failure. The soil can no longer sustain the load without substantial deformation.

Allowable bearing capacity is a working pressure established by applying a factor of safety to the ultimate value. Settlement is also checked and may require a lower pressure to keep movement within acceptable limits.

Gross and net bearing capacity distinguish how pressure is expressed. Gross values include the total pressure at foundation level. Net values account for the pressure previously exerted by the soil above that level, describing the increase associated with construction.

These terms address different aspects of the assessment. A value can therefore be both ultimate and net, or allowable and gross.

Bearing capacity depends on the ground and the arrangement transferring load into it. Principal influences include:

• Soil and rock properties: shear strength, density and geological structure affect the ground’s ability to resist loading.
• Foundation dimensions: width, shape, and depth influence the volume of ground supporting the load.
• Layer thickness: a strong surface layer may overlie weaker deposits that affect performance.
• Groundwater conditions: groundwater levels and pore water pressure can influence the effective stresses within the soil and therefore its bearing resistance.
• Load position and direction: inclined or offset loads create a different pressure distribution from a central vertical load.
• Nearby slopes and excavations: changes in ground geometry can reduce the support available around a foundation.

These factors explain why a bearing value relates to particular foundation conditions rather than being a fixed property of an entire site.

Ground investigation identifies the strata beneath proposed works and provides information about their engineering properties. Boreholes, trial pits, SPT and CPT testing, and laboratory results help develop the ground model used in the assessment.

Engineers combine this evidence with foundation dimensions and loading to calculate resistance. They also examine the depth of ground affected by construction and the movement likely to develop.

A plate loading test can provide additional information about responses beneath a test area. Its interpretation accounts for the plate size and the depth of material influence.

The resulting assessment informs foundation sizing, founding levels and the need for treatment or deeper support.

Bearing capacity describes the ability of the ground to resist loads without failure, while bearing pressure is the pressure applied to the ground by a foundation. Engineers assess the pressure imposed by the foundation against the resistance available from the ground, while also considering settlement and other performance requirements.