Za t m

If the wall is designed to cut off water flow into the excavation (see right-hand side of Figure 12.1), then the investigation should extend at least 2m into the impermeable stratum.

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

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.

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 and B.3(3)]

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