Bearing Capacity of Soil - Types and Calculations

by Andrew Lees, on 18-Feb-2021 04:47:19

This guide explains the concept of the bearing capacity of soil (also known as ‘ground bearing pressure’), its significance in geotechnical engineering, the types of soil bearing pressure, and the process of calculating it under different conditions.

Use the links below to jump to the sections you’re most interested in:

What is the bearing capacity of soil?

In a nutshell, bearing capacity is the capacity of soil to support the loads that are applied to the ground above. It depends primarily on the type of soil, its shear strength and its density. It also depends on the depth of embedment of the load – the deeper it is founded, the greater the bearing capacity. 

Where there is insufficient bearing capacity, the ground can be improved or alternatively the load can be spread over a larger area such that the applied stress to the soil is reduced to an acceptable value less than the bearing capacity. This can be achieved with spread foundations composed of reinforced concrete, for example. 

In the case of working platforms for cranes and piling rigs, improved load spread is provided by a granular platform whose performance can be further improved by mechanical stabilisation using Tensar geogrids.

Ground Coffee 'Ask Andrew' Episode 4: Andrew Lees explains what bearing capacity means

Why is ground pressure important?

The ground bearing pressure or bearing capacity of soil is important because whenever a load is placed on the ground, such as from a building foundation, a crane or a retaining wall, the ground must have the capacity to support it without excessive settlement or failure.

This means that it is important to calculate the bearing capacity of the underlying soil during the design phase of any construction project. Failing to understand and account for the ground bearing pressure before beginning the project could have catastrophic consequences, such as a building foundation collapsing at a later stage.

The types of bearing capacity of soil

The most commonly used types of bearing capacity of soil are ‘ultimate bearing capacity’ and ‘allowable bearing capacity’. Let’s take a look at the definitions of these terms first.

What is the ultimate bearing capacity of soil?

The ultimate bearing capacity of soil is the maximum vertical pressure that can be applied to the ground surface, at which point a shear failure mechanism develops in the supporting soil.

In essence, this means the maximum amount of load the soil can take before it fails, or gives way completely. We don’t use this figure on its own in the foundation design process, as it’s also important to consider how soil will settle under pressure, which could affect its ability to support a structure.

What is the allowable bearing capacity of soil?

The allowable bearing capacity of soil is the amount of load the soil can take without experiencing shear failure or exceeding the allowable amount of settlement. This is the figure that is used in the design of foundations.

The allowable bearing capacity is always lower than the ultimate bearing pressure because it takes into account the settlement of soil, not just the load required to cause shear failure.

Bearing capacity types and formulae

The types of bearing capacity of soil are:

    • Ultimate bearing capacity (qᵤ): the maximum vertical pressure that can be applied to the ground surface, at which point a shear failure mechanism develops in the supporting soil.

    • Net ultimate bearing capacity (qᵤ): this is the ultimate bearing capacity minus the weight of soil (𝝲) multiplied by the depth of the foundation (D). The formula is qₙᵤ = qᵤ - 𝝲Df.

    • Net safe bearing capacity (qₙₛ): the allowable bearing capacity  (qₙₛ) is the net ultimate bearing capacity (qᵤ) divided by a factor a safety (typically this will be 3). The formula is qₙₛ = qᵤ / F. The factor may be increased to limit settlements further if required.

    • Gross safe bearing capacity (qₛ): dividing the ultimate bearing capacity by a factor of safety gives you the gross safe bearing capacity (qₛ = qᵤ / F).
  • Net safe settlement pressure (qₙₚ): the maximum load the soil can take before it exceeds the allowable amount of soil settlement.‎

  • Net allowable bearing capacity (qₙ‎ₐ): this is the value used in the design of foundations, and is often simply referred to as the ‘allowable bearing capacity’. The net allowable bearing capacity (qₙ‎ₐ) is equal to either the net safe bearing capacity (qₙₛ) or the net safe settlement pressure (qₙₚ), whichever is the lower figure.

How to determine the bearing capacity of soil

Now that you understand the difference between ultimate and allowable bearing capacity, let’s move on to how we can determine the bearing capacity of soil for use in the design process. The type of soil you are working with is a major factor in its bearing capacity, so the sections below explain the process for clay and granular soils separately.

How to calculate bearing capacity of clay soils

The calculation method depends very much on the soil type. In saturated clays and other fine-grained soils, the incompressible pore water supports applied loads initially, raising the pore water pressure in the soil beneath the applied load. The low permeability of clay means it can take months or years for the pore water to flow, pressures to dissipate, the soil skeleton to compress and the ground surface to settle. This means that clays are generally more vulnerable to bearing capacity failure in the short-term before excess pore water pressures dissipate and effective stress rises. 

Although that all seems quite complicated, the calculation method for short-term bearing capacity in clays is relatively straightforward and linear since a single, uniform value of undrained shear strength, unchanged by the applied loading, is normally assumed. The long-term bearing capacity clays are usually greater – so this is rarely critical – but it can be calculated using the same method as for sands.

How to calculate bearing capacity of granular soils

The bearing capacity of sands and gravels are not normally critical in design because they are relatively strong and because effective stresses within the soil increase immediately under the applied load due to their high permeability. It does not take months or years for this to happen like in a typical clay soil. 

Only loose sands with a high water table under a concentrated load (such as a piling rig) may have an issue with bearing capacity. In most cases settlement governs the design. The calculation of bearing capacity in granular soils such as sands is more complicated because it depends on the effective stress along the assumed failure mechanism, which varies with depth and soil density and due to the applied load itself. Dilatancy in the sand on shearing also complicates matters.

Calculation methods for bearing capacity

The calculation methods for both soil types are derived from the simplified geometric case of an infinitely long strip load with a vertical load and horizontal ground surface. Various factors can then be introduced to take approximate account of other shaped loadings (e.g. rectangular, square, circular), inclined loads and inclined surfaces.

These methods also assume uniform, homogeneous soil conditions but a working platform is a good example of a two-layer bearing capacity problem, i.e. crane or piling rig loads are applied to the surface of dense granular layer overlying a weaker subgrade composed of clay or sand, for example. Conventional calculation methods cannot be applied here but Tensar developed the fully validated T-value design method to take account of this particular situation and to introduce the benefit of mechanical stabilisation using Tensar geogrids in a scientifically rigorous way.

Next steps

This guide has explained what the bearing capacity of soil is, why it’s important for geotechnical and structural engineering, the different types of bearing capacity – differentiating ultimate and allowable bearing pressure – and finally how to determine bearing capacity.

As you may have gathered from the last section, the process of calculating the bearing capacity of soil can quickly become quite complicated. To assist you with the calculations involved in the designs for reinforced soil walls, slopes and bridge abutments, we have produced our TensarSoil design software (visit this page to request TensarSoil).

Got a burning question about geotechnical engineering?

Why not drop us an email at info@tensar.co.uk and the answer to your question may feature on the Tensar blog!

Comments