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Technical Note: Making the earth transparent – Using engineering geophysics to help manage uncertainty and reduce project cost

Rod Eddies, Fugro onshore geophysics manager for the Africa-Caspian-Europe region, and Ray Wood, executive vice-president, Fugro Consultants

Successful site characterisation is defined as deriving sufficient information to build a representative ground model. This is one where subsurface ground conditions and features have been identified and characterised by a combination of desk study, intrusive and geophysical approaches, with reasonable confidence.

Ideally, the ground model will capture all subsurface variability of interest to the development fully, with minimal cost and within the shortest timescale. However, economic and practical factors dictate that the ground model resulting from a conventional “boreholes only” ground investigation will inevitably be an under-sampled, smoothed representation of the ground.

In the specific context of foundation design, and as a result of the great variability in ground conditions and their behaviour, industry adopts reliability-based design methods calibrated to deliver safe designs at very low levels of failure.

Greater use of full-scale or semi-full scale foundation testing to failure or close to it, provides a site-specific calibration of the design method. Such calibrated design methods often lead to significant savings in foundation cost and, because of the investment made in understanding how the site truly behaves, these cheaper foundations are paradoxically safer. There is, however, little point in good quality foundation testing if variations in ground conditions have not been extremely well characterised for the site.

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