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Dams and weirs

25-75m

vertical sections

Special bridges, stacks, machinery foundations

2-6 per foundation

Guidance on spacing is available from other sources.5 However, all such guidance is provided as a starting point for assessing the scope of investigations and will need modification to account for site specific requirements. In particular the spacing of exploration points will need to reflect the expected variation in the underlying geology of the site as well as the type and size of structure.

4.2.3 Depth of investigation points

Annex B.3 of EN 1997-2 provides recommendations for the minimum depth of investigation below the lowest point of high-rise structures and civil engineering projects; rafts; embankments and cuttings; linear structures such as roads, airfields, trenches, pipelines; tunnels and caverns; excavations; cutoff walls; and piles. Figure 4.5 illustrates some of these recommendations.

Figure 4.5. Minimum depth of investigation below (top-left) roads, (top-right) trenches, and (bottom) tunnels and caverns

The minimum depth of excavation za for roads (and airfields) is: > 2m for trenches (and pipelines): za > 2m and za > 1.5b and for small tunnels and caverns:

za > 2m where b is the width of the structure as defined in Figure 4.5.

Recommendations for other geotechnical structures are presented in Chapters 10 to 14.

The selection of investigation depth needs to consider factors other than geometrical criteria, as does the spacing of exploration positions. It is not generally necessary to penetrate rock strata by more than 5m unless the anticipated loading is exceptionally high or the geology suggests that significantly weaker strata may be underneath. Further, it is normal to limit investigations to depths at which the expected increase in stress due to foundation loading will be less than 10% of the existing overburden pressure.

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