May/June 2022 - 94

Locations of main profiles
C
B
A
This called for a third soil investigation
campaign, consisting of 22
additional CPTs, in an even denser grid,
locally extending the investigated depth
to 50 m (165 ft) below ground level.
Only after this third campaign was it
possible to distinguish three separate
areas, each having their own consistent
soil profile, and choose suitable pile toe
levels for the design. Soil profile A had a
clear bearing stratum, with the top
sloping down from 17 m (51 ft) to 26 m
(85 ft) below ground level towards
profile B. Soil profile C also had a clear
bearing stratum, with the top constant
around 26 m (85 ft) below ground level.
Soil profile B, located between profiles A
and C, concerned a kind of gulley,
without a clear bearing stratum.
Based on these soil profiles and
corresponding depth (or absence) of the
bearing stratum the foundation design
was divided in 6 separate zones. Zones I,
II and III concerned soil profile A, with
pile lengths 23 m (75 ft), 25 m (82 ft) and
27 m (88 ft) respectively, following the
sloping top of the bearing stratum.
94 * DEEP FOUNDATIONS * MAY/JUNE 2022
Zones IV and V concerned soil profile B,
with pile 27 m (88 ft) m and 30 m (98 ft).
Zone VI concerned soil profile C, with
pile length 30 m (98 ft). In zones IV and V
it was accepted that the piles would
have low capacity and small stiffness,
which was mitigated by increasing the
number of piles and dilating all zones in
the superstructure. Because of the
flexibility for (further) adjusting pile
lengths on site, the construction was
founded on cast in-situ piles. Pile
installation was executed without any
problems or unexpected events.
Lessons Learned
An interesting question is what could
have happened if the pile design had
been made according to the traditional
design approach described in the article
introduction, using only the first
available 2 CPTs, perhaps in combination
with load testing on trial piles
next to one of them.
CPT 1 is similar to soil profile A, but
with the top of the bearing stratum a
couple of meters higher. CPT 2 is similar
to soil profile C, but with the (top of the)
bearing stratum less clear and less
strong. If the pile design were based on
CPT 1, most piles would have been too
short. This would have resulted in
lower pile capacity than anticipated,
possibly resulting in failure of the
foundation. If the pile design were
based on CPT 2, most piles would have
been longer than needed. This would
have resulted in an overdesigned
foundation and perhaps problems or
delays during pile installation, because
driving resistance would have been
(substantially) higher than anticipated.
Soil profile B would have been missed
anyway, so in that area the pile design
would not have been adapted to the
local absence of the bearing stratum.
This would have possibly resulted in
(local) failure of the foundation.
Foundation design

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
May/June 2022 - 7
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