Standard penetration tests in clays derived from weathered Jurassic mudstones in central England

Abstract

Standard Penetration Tests (SPTs) are widely used in the UK for ground profiling and empirical correlation with geotechnical properties and parameters. In particular, Stroud (1974) relates SPT N-values to undrained shear strength through an average coefficient at individual locations. Coefficients have been published for clay-rich tills and mudstone formations. However, few studies have utilised statistically useful datasets (>30 samples) or quantified the variability of the observations relative to the average coefficient.
This paper investigates these issues with reference to large datasets that were obtained from outcrops of weathered Jurassic mudstones in central England as part of a commercial ground investigation for the High Speed Two (HS2) railway. Data pairs from triaxial tests and SPTs located in close proximity (+/-1 m vertically, in the same borehole) were used to compare undrained shear strength and SPT N60-values, and explore the variability of the observations relative to the average coefficient.
The results show that the average coefficients derived for each geological formation were close to published relationships. However, statistical analyses showed dispersion about the mean. Locally low values in the 5% fractile were between 15% and 50% of the mean value. Therefore, average coefficients are informative, but they should be used with caution.

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Identifiers

ORCID for Kevin Briggs: orcid.org/0000-0003-1738-9692

ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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