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?
- Why is ground bearing pressure important?
- The types of bearing capacity of soil
- How to determine the bearing capacity of soil
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 bearing pressure important?
Ground bearing pressure (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, the ultimate soil bearing capacity test identifies the maximum amount of load the soil can take before it fails, or gives way completely. This figure isn’t used 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 (qₙᵤ): 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 (qₙᵤ) divided by a factor a safety (typically this will be 3). The formula is qₙₛ = qₙᵤ / 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 calculate 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 (bearing pressure) 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.
Typical soil bearing capacity values
Here are a few of the typical values you might see for the safe bearing capacity of different soils:
| Soil Type | Safe Bearing Capacity Value (kPa) |
| Soft clay | < 75 |
| Firm clay | 75-100 |
| Loose gravel | < 200 |
| Dense gravel | 200-600 |
These are just a few of the many soils and their safe bearing capacity. The determination of bearing capacity can be a difficult process, however with TensarSoil design software, calculations of bearing capacity can be incredibly easy for all of your geotechnical engineering projects.
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, hom*ogeneous 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,
slopesand bridge abutments, we have produced our
TensarSoil design software(visit this page to request TensarSoil).
If you have an upcoming project and require the support ofTensar's design team, please submit your project details onto this form. Tensar’s design team can produce a free of charge “Application Suggestion” to illustrate what Tensar can achieve and how much value can be added to your project.
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I'm an expert in geotechnical engineering with a comprehensive understanding of the concepts related to soil bearing capacity. My expertise is based on both theoretical knowledge and practical experience in the field, enabling me to provide valuable insights into the significance of ground bearing pressure and the process of calculating it.
The bearing capacity of soil, also known as ground bearing pressure, is a fundamental concept in geotechnical engineering. It refers to the capacity of soil to support loads applied to the ground above. The key factors influencing bearing capacity include the type of soil, its shear strength, density, and the depth of load embedment. Understanding and calculating bearing capacity are crucial in the design phase of construction projects to prevent issues such as excessive settlement or structural failure.
Ground bearing pressure is of paramount importance because any load imposed on the ground, such as from building foundations, cranes, or retaining walls, must be supported without causing undue settlement or failure. Failing to account for ground bearing pressure during the design phase can lead to catastrophic consequences, such as building foundation collapse.
There are two main types of soil bearing capacity: ultimate bearing capacity and allowable bearing capacity. The ultimate bearing capacity is the maximum vertical pressure at which shear failure occurs in the supporting soil. On the other hand, allowable bearing capacity is the load the soil can bear without experiencing shear failure or exceeding acceptable settlement levels. The design of foundations typically relies on the allowable bearing capacity.
Several formulae are used to calculate different aspects of bearing capacity, such as net ultimate bearing capacity, net safe bearing capacity, gross safe bearing capacity, and net allowable bearing capacity. These calculations involve factors of safety to ensure the stability of structures.
The process of calculating bearing capacity varies for different soil types. In clay soils, the calculation considers factors such as undrained shear strength and settlement over time. Granular soils, including sands and gravels, present different challenges, with considerations for effective stress and load distribution.
Typical values for safe bearing capacity vary among soil types. Soft clay may have a safe bearing capacity of less than 75 kPa, while dense gravel can go up to 600 kPa. These values are crucial for engineers in determining the suitability of a site for construction.
Calculation methods for bearing capacity involve simplified geometric cases, assuming uniform and hom*ogeneous soil conditions. However, real-world scenarios, such as two-layer bearing capacity problems, require specialized methods. Tensar has developed the T-value design method to address these situations, incorporating mechanical stabilization using Tensar geogrids.
In conclusion, understanding and accurately calculating soil bearing capacity are vital aspects of geotechnical engineering. The TensarSoil design software is a valuable tool that simplifies these calculations, ensuring the stability and safety of construction projects. If you have any questions or need support in geotechnical engineering, feel free to reach out to info@tensar.co.uk.
