What do we actually mean by ‘the bearing capacity of soil’?
What does “bearing capacity of soil” mean? It’s important to know this, 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.
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, either 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.
There are two levels of soil bearing capacity that are considered:
- Ultimate bearing capacity: the maximum vertical pressure that can be applied to the ground surface, at which point a shear failure mechanism develops in the supporting soil
- Allowable bearing capacity: this is the ultimate bearing capacity divided by appropriate factors of safety; the factors may be further increased in order to limit settlements in an appropriate way.
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 it’s rarely critical but 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.
Calculation methods for bearing capacity
The calculation methods for both soil types are derived for the simplified geometric case of an infinitely long strip load with a vertical load and horizontal ground surface. Numerous 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.
To view the recent Tensar vlog, entitled “What is the bearing capacity of soil”, visit: Bearing Capacity of Soil (tensar.co.uk)
Author: Andrew Lees, Tensar