Geotechnical Illustrated: What is your CPT Test Really Telling You?

In many cases with offshore filling construction, such as building and forming artificial islands in the middle of a body of water, borrow areas of suitable seabed materials must be first identified. A comprehensive soil investigation will subsequently help assess the suitability of the potential seabed borrow material to be dredged and used for engineering purposes.  Some of the seabed in areas around the world consist of carbonate sand that may have shell fragments within its matrix composition. Shell fragments contribute to carbonate content that may be in the order of 90% or more. This specific soil composition can influence the measurements obtained by in-situ soil testing techniques.

One of the most commonly used in-situ techniques to assess soil strength, especially in offshore applications, is cone penetration tests (CPT). The most important measurement obtained from CPT is the cone penetration resistance, which is the force required to push the tip of the cone through the soil of interest and is defined in ASTM D: 5778.  However, most of the standardized in-situ testing techniques, such as CPT, have been commonly calibrated to interpret soil properties for soils found onshore, such as silica sands.

After reclamation of an artificial island using carbonate sands, CPT can be used to assess its strength to carry loads for engineering purposes. However, a correction factor is required to adjust the measured tip resistance so that correlations developed for silica sands can be used for the carbonate deposits. This correction factor is called the Shell Correction Factor (F shell) and has been extensively studied by researchers around the world.  The Shell Correction Factor is the ratio between cone tip resistance of silica sand to cone tip resistance of carbonate sand. F shell commonly varies from 1.2 to 1.6 depending on the relative density of the sand.

So, don’t just pick up some standard correlations to interpret your CPT results. Make sure you understand the soil’s composition and its effects on engineering properties. 

Assem Elsayed is the Vice President and Practice Area Leader of GeoStructural Engineering at Geocomp. Assem has extensive experience with waterfront and marine structures, design of monopiles for wind farms, and support of deep excavation.

Published by Geocomp Blog

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