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Approach to site investigation challenged

A LEADING authority on ground property characterisation says the whole approach to site investigation needs to be changed.

Speaking at Imperial College's Geotechnics in the New Millennium symposium in London last month, Dr David Hight of Geotechnical Consulting Group said: 'It is no longer sufficient to bang a tube into ground and if it is clay, put it in a triaxial apparatus and run quick undrained compression test, or if it is sand pour it into a shear box and run a drained test.'

'To make use of the developments in soil models and the analysis of boundary value problems, we need to change our whole approach to site investigation, urged Hight.

'We need to first establish far more carefully stratigraphy and level of heterogeneity, and then characterise the soils. We should be asking what is the operational strength, stiffness and permeability? How anisotropic are these properties? How non-linear, brittle and viscous are they?'

Hight recommended taking a restricted number of truly undisturbed samples, whose quality was measured.

'And we use insitu continuous penetration tests and geophysics, not laboratory testing, to determine variability and heterogeneity,' he added.

'While methods of assessing sample quality already exist, developments are obviously needed in sampling.'

Hight also singled out three areas of research for closer examination: the importance of creep on sands in determining the performance of foundations and their serviceable life; the anisotropy of strength and stiffness of soils; and the improved description of soil fabric and its link with soil behaviour.

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