May/June 2022 - 92

Modern CPT truck
operator working in a cabin sheltered
from weather conditions. For these
reasons, CPT has also become a very
flexible and productive type of soil
investigation: depending on site
conditions, execution of 5-10 CPT tests
to depths of 25-30 m (80-100 ft) per day
are often achieved. Evolution of the CPT
cone has also resulted in the possibility
of monitoring other parameters, for
instance inclination (for check on
verticality), groundwater pore pressure
and magnetic fields (for detecting
subsoil objects).
But the most important result of any
CPT is always the continuous record (as
a function of penetration depth) of cone
tip resistance, sleeve friction and friction
ratio (sleeve friction as percentage of
cone tip resistance). Because experience
and research have proven that CPT data
First available CPTs 1 & 2, differences below NAP -18 m
provide a reliable identification of the
soil behavior type (Robertson, 1990), any
CPT simultaneously provides an
accurate soil profile combined with
corresponding soil strength and density.
This makes CPT very suitable for various
geotechnical designs, like prediction of
pile capacity. Also, the first interpretation
is relatively straightforward:
higher cone resistances (> 10 MPa)
usually correspond to denser (sand)
layers with more bearing capacity and
lower cone resistances (< 4 MPa)
correspond to softer layers (clay or peat)
with little capacity. So, a first glance at a
CPT graph already provides a good
impression of the required pile toe level.
Pile Design Based on CPT
Results
For decades, the standard practice in
Dutch design code has been based on the
calculation method developed by
engineer A.W. Koppejan in 1948. The
calculation method converts the
resistance measured with the small
diameter cone to the corresponding
resistance of a larger diameter pile based
on the ratio between penetration depth
(in the bearing stratum) and the
diameter. Thanks to decades of extensive
experience with behavior of installed
pile foundations, execution of load tests
on trial piles and/or working piles is not
required for pile foundations designed
by using CPT, according to the code.
Also, thanks to decades of extensive
CPT with cone resistance (left), friction ratio (middle) and soil type identification (right)
92 * DEEP FOUNDATIONS * MAY/JUNE 2022
experience with executed load tests, the
CPT-based calculation method in the
Dutch code can also be used to calculate
expected load-displacement behavior
of the pi le head, depending on
installation characteristics of the
considered pile type. This result is
use ful for the des i gner o f the
superstructure, because it provides an
estimate of (the range of) the spring
stiffness of the piles.

May/June 2022

Table of Contents for the Digital Edition of May/June 2022

TOC
May/June 2022 - Intro
May/June 2022 - 1
May/June 2022 - 2
May/June 2022 - TOC
May/June 2022 - 4
May/June 2022 - 5
May/June 2022 - 6
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