Judging the safe bearing value of a foundation

The safe bearing value for a given foundation material ought not to be decided by the resident engineer but by the engineer or his specialist advisers.
However, the engineer will not thank the resident engineer for referring to him questions about foundation materials which are obviously satisfactory, such as gravel and rock, where the load thereon is well within the traditionally accepted bearing strength. Standard field descriptions are given in BS 5930 for various materials and BS 8004, Table 1 shows allowable bearing values for such materials for preliminary design purposes.
In clays or silts, or materials having clay bands or organic layers, and other mixtures containing weak layers, special investigations, sampling techniques and sophisticated analyses may be necessary before a safe bearing value can be advised – dependent upon the type of structure the formation is to support.
These matters need to be considered by an experienced geotechnical engineer.
Site tests, such as the ‘standard penetration test’, vane shear tests and permeability tests, may be used but these must be regarded as an adjunct only to more sophisticated investigation techniques. Details of the standard penetration test are given in BS 1377 Part 9: 1990, para 3.3. Its widest use on site is to reveal any weak spots in an otherwise consistent foundation material.