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The Structural Engineer

This paper addresses geotechnical risks in the context of the overall costs for a typical UK building project and summarises the major risks that ought to be addressed by appropriately robust foundation design and construction. The first part of the paper puts geotechnical costs into perspective. It demonstrates that site investigation costs are usually just 2% of the substructure costs or 0.1% of the overall building cost. Yet ground-related problems typically account for about one third to one half of construction programme over-runs, and the average costs of these delays far exceed the original sums expended on site investigation. The second part of the paper draws together the elements that improve the chances of foundations working well and sets out a methodology whereby these elements can be considered together for a particular project to give the required level of robustness. Tim Chapman, BE, MSc, DIC, CEng, MIEI, FICE Associate Director, Ove Arup & Partners Ltd Alain Marcetteau, ENSM, CEng, MICE, FConsE Director, Ove Arup & Partners Ltd

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In a concrete pile or circular column, only a component of the force in a circular or spiral shear link resists the applied shear. However, some approaches to the design of these links make no allowance for their curvature or the asymmetry of spirals. Although the author is unaware of failures of piles or columns designed using these methods, elements so designed are likely to have a reduced factor of safety. This paper recommends alternative expressions that consider the shape of the links more rigorously. It also recommends that the method for calculating shear stress in the Highways Agency’s design manual should be reviewed. Ian Feltham, MA, CEng, MIStructE, MICE Arup Research + Development, London

The Structural Engineer