Alan Reid and Dr John Taylor, geotechnical engineers, South Lanarkshire Council. This paper was first published in GE’s July 2010 issue.
The standard penetration test (SPT) is one of the most commonly used insitu tests to determine density and subsequently the insitu strength of granular soils for use in bearing capacity analysis.
Due to its simplicity and cost effectiveness the use of the SPT as part of insitu testing for ground investigations of fine soils has become common practice in the UK. Several researchers, (Stroud 1974; Stroud & Butler 1975; Charles 2005; Tomlinson 2001) state that an approximation for the undrained shear strength and the coefficient of volume compressibility can be obtained from a standard penetration test N value correlated to the plasticity index of the soil.
Consequently, it has become common to multiply the SPT N value by a factor of five (Charles 2005) to provide a very approximate value of undrained shear strength. While this correlation is widely used in geotechnical engineering the geographical spread of the data from which the correlation was derived is significantly limited as the research is predominantly English based.