Statistical review of CPT data and implications for pile design

Natural soil variability can result in a range of pile capacities at a specific
test site. Soil variability is ideally determined using in-situ testing techniques
such as the CPT. This paper presents a series of CPT profiles from a dense sand site
in Wicklow, Ireland. The qc value is detrended using a quadratic profile with depth
and the scatter in the remaining data is quantified using the coefficient of variation
(COV). In addition, the autocorrelation of each CPT trace is quantified using the scale
of fluctuation. The measured values from the site, including the trend, COV and
scale of fluctuation are subsequently incorporated into a Monte Carlo simulation that
generates multiple realisations of the CPT data from an underlying lognormal distribution.
These realisations are applied to a simple pile design model to calculate the
variability in capacity that arises from the site specific variability. Spatial averaging is
shown to be highly influential in reducing the uncertainty in pile resistance, with long
piles and small scale of fluctuations demonstrating the lowest COV in the calculated capacity.