Just how accurate are the correlations between SPT N-Value and basic geotechnical strength parameters?

The Standard Penetration Test (SPT) is arguably the most commonly used in situ test in Australia. It is quick, straightforward, and many relationships between the results (SPT N-Value) and geotechnical strength parameters have been published. However, there are a few downfalls with the SPT test; it is not repeatable in the field, drillers often use a convenient rod size even though there is a “standard size” adopted in Australia, N-Values are often used in their uncorrected form and the correlations to geotechnical parameters are not site specific.

SMEC has recently completed the geotechnical investigation for Project Energy Connect in southwestern New South Wales, which included over 1500 investigation data points, including boreholes, cone penetration tests, geophysics, and laboratory testing. These data points have been used to assess a localised, site-specific correlation between SPT N-Value and geotechnical strength parameters, notably undrained shear strength (C_u) of cohesive soils. This paper presents the correlation of SPT N-Value to C_u for the alluvial deposits of southwestern NSW and compares this correlation with those typically used within the industry.