Za d and za m

The depth za may be reduced to 2m if the embankment or cutting is built on competent strata1 with 'distinct' (i.e. known) geology. With 'indistinct'

fi.e. weaker strata are unlikely to occur at depth, structural weaknesses such as faults are absent, and solution features and other voids are not expected.

geology, at least one borehole should go to at least 5m. If bedrock is encountered, it becomes the reference level for za. [en 1997-2 §B.3(4)]

Figure 9.1. Recommended depth of investigation for embankments and cuttings

Greater depths of investigation may be needed for very large or highly complex projects or where unfavourable geological conditions are encountered. [en 1997-2 §b.3(2)note & B.3(3)]

For many sites where slope stability is an issue, the natural ground is not level and deeper investigations may be required. The simple guidelines above should be read with that in mind and the required depth of investigation determined from the expected conditions.

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