Foundation Design: Definition of Bearing Pressures.

The site investigation, laboratory analysis, established principles of soil mechanics, and most importantly the
engineer’s own judgement are used to assess the allowable bearing pressure which the soil can support – this assessment is covered previously.

This allowable bearing pressure is required to provide a sufficient factor of safety against failure in terms of bearing capacity (i.e. ultimate collapse failure), usually taken as 3, and against settlement (i.e. serviceability). This allowable bearing pressure is assessed in one of two forms:

(1) Total allowable bearing pressure. The maximum pressure which can be applied at the soil/foundation interface by the foundation and the loads acting upon it.

(2) Net allowable bearing pressure. The maximum increase in pressure which can be applied at the soil/foundation interface (i.e. the difference in pressure after the foundation is loaded compared with that in the soil before construction is started).

The difference between these two cases in its simplest form is shown in Fig. 10.15. It is common practice for simple foundations such as axially loaded pads and strips to be designed on the basis of checking the net allowable bearing pressure against the load from the superstructure ignoring the weight of the foundation. This is a valid method given that the weight of the foundation is typically of the same order of size as the weight of the soil it replaces (24 kN/m3 for concrete compared with 20 kN/m3 for soil). Providing the surcharge remains the same the error involved is minimal compared with the inaccuracies of basic soil mechanics.

It is, however, fundamentally important that the total allowable bearing pressure is not confused with the net allowable bearing pressure. If, for example, a soil has a total allowable bearing pressure of 80 kN/m2 at a depth of 2 m, the load which the foundation can support is

When considering wind loading conditions the total allowable bearing pressures are increased by 25% in line with  the factor used in permissible stress design codes used for structures prior to the introduction of the limit-state design codes.

It should not be forgotten that unless specific reference has been made to the contrary the allowable bearing pressure is usually based on the ultimate bearing capacity  (typically with a factor of safety of 3) without an assessment of settlement as this will be dependent on the type, size and actual applied bearing pressure adopted in the design. It  is therefore necessary for the engineer to make a separate assessment of the allowable bearing pressure in relation to settlement criteria and to ensure that the site investigation provides the necessary information to make that assessment.

Fig. 10.15 Definition of loads and pressures – simple case.