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The right combination

Barry Slocombe of Keller Ground Engineering and Chris Adam of site investigation specialist Fugro describe recent projects which have successfully combined CPT investigation and vibro ground improvement techniques.

Every practising geotechnical engineer will be familiar with the concept that you pay for a site investigation whether you have one or not. Even where a site investigation has been performed, the site may have been sold on for another use for which the original investigation may not be appropriate. In any case the original work may not have fully examined a particularly important factor for the proposed foundation solution, especially when the solution involves vibro ground improvement techniques.

Often the client is reluctant to commit to further expense on site investigation, but once the engineering need is accepted, the client usually expects accurate information quickly. Many in the site investigation industry find these demands incompatible with the investigation process, but fortunately cone penetrometer testing and its variants can provide investigation data custom designed to specific requirements.

Cone penetrometer testing has seen many improvements over the past few years. Developments in computer, electronic and strain gauge technology have resulted in systems which are accurate and reliable with improved data acquisition and presentation capabilities. Screen displays of cone data during the test provides real time characterisation of soil stratigraphy and its associated geotechnical properties, particul- arly strength and relative density.

Direct correlation of these two properties to measured cone data makes the cone penetrometer an excellent tool for quantifying the effectiveness of ground improve- ment schemes.

Such schemes can be influenced by subtle variations in ground conditions. For instance the presence of clay pockets within sandy materials may reduce the effectiveness of the technique .

The cone penetrometer is capable of identifying features as thin as 20mm if a piezocone is used. More recent adaptations of the cone penetrometer include the seismic cone, which provides a measure of insitu shear wave velocity, from which shear modulus can be assessed, and the full displacement cone pressure- meter for obtaining a measure of elastic modulus strength. CPT can therefore provide a detailed picture of the ground conditions before and after improvement - a vital factor in ensuring the work is fit for its intended purpose.

While daily production depends upon factors such as accessibility or ground conditions, a daily rate for conventional cone testing of between 100m to 200m can be achieved, fulfilling the client's needs for speed and cost efficiency.

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