May/June 2022 - 93

Case Study
The case study is based on CPT at the
development of a 4-5 story apartment
building, with no basement and a
footprint of approximately 70 x 33 sq m
(230 x 110 sq ft). The project is located
approximately 20 km (12.4 mi )
southwest of Amsterdam, in the same
polder (a tract of lowland reclaimed
from a body of water) as Amsterdam
Schiphol Airport. The polder is the
bottom of the former inland lake,
Haarlemmermeer, which was drained
in 1852, using the biggest steam
pumping stations of that era.
At first, for a preliminary design
there were two CPTs available from an
adjacent project located approximately
60 m (200 ft) away. Despite the relative
short spacing, the two CPT graphs
indicated considerable differences in
soil profile.
The respective soil profiles are
described in table 1.
CPT 1 showed the usual regional
profile, with sand layer [c] being a
suitable bearing stratum for a pile
foundation. CPT 2 showed a similar soil
profile in the top part, but layer [b]
extended until the maximum investigated
depth of 25 m (75 ft) below ground
level. At that level the top of sand layer [c]
was still not (convincingly) encountered.
This implied that, within the
investigated depth range, no suitable
bearing stratum for the pile toe could be
identified in CPT 2.
These CPT results implied that a
couple of important questions needed
to be answered for the foundation
design: (A) Which of the CPTs is
representative for the project? (B) How
abrupt is the transition between both
profiles and where is it located? (C) How
much deeper would the required pile
toe level be in case of CPT 2?
The first soil investigation campaign
within the project plot consisted of 6
CPTs. Considering the result of the
available CPTs, it was decided to extend
the depth range of the new CPTs to 37 m
(120 ft) below ground level. The spacing
between these CPTs varied between 15
CPT 1
CPT 2
top
m BGL (ft)
-11 (33)
17 (-50)
bottom
m BGL (ft)
-11 (-33)
-17 -(50)
< -25 (-75)
Layer
[a]
[b]
[c]
soil type
Soft soils
moderate
sand with
clay
Bearing
stratum
sand
Table 1: soil profiles CPT 1 and CPT 2
m (45 ft) and 25 m (75 ft), as required by
the Dutch design code.
In all 6 CPTs a clear bearing stratum
[c] could be detected, but the CPT results
a l so posed new ques t i ons and
challenges. First, the depth of the top of
this bearing stratum varied between 17
m (50 ft) and 28 m (90 ft) below ground
level. Second, it also became clear that
bearing stratum [c] had a limited
thickness and that it was followed by a
clay layer [d] that was not suitable as
bearing stratum for the pile foundation.
This implied that pile toe level could not
be chosen too deep below the top of
bearing sand stratum [c], because in
that case the pile might end up too close
to clay layer [d] or even in it, rendering
piles with insufficient capacity. This
top
m BGL (ft)
-9 (-27)
??
bottom
m BGL (ft)
-9 (-27)
< -25 (-75)
??
L ayer
[a]
[b]
[c]
soil type
Soft soils
moderate
sand with
clay
Bearing
stratum
sand
Traditionally this transition location
is established by starting from the CPT
with the deepest pile toe level and making
the transition at 2/3 of the distance to
the adjacent CPT with a higher pile toe
level. However, this could not be applied
in this case because of the large differences
in pile toe level and because of the
presence of unsuitable clay layer [d].
It was decided to execute a second
soil investigation campaign, consisting
of 15 additional CPTs in a narrower grid
around the locations of the first
campaign. However, the results of this
campaign only partially answered the
remaining question. Depth of the top and
bottom of bearing stratum [c] could be
identified better in most CPTs but still
showed strong variations of depth of the
Identification of main soil profiles
meant that suitable pile toe levels
needed to be established for each CPT
individually, resulting in strong
differences in pile length (> 6 m = 18 ft)
over short distances (< 15 m = 45 ft).
So, questions (A) and (C) were basically
answered, but important question
(B) remained: where to choose the
transitions from one pile length to
another between CPT locations?
bearing stratum. But more important,
the additional CPTs also revealed the
presence of an area (gulley) where the
top of bearing stratum [c] was encountered
very deep or even seemed to be
absent. Although the adjacent CPT
locations were no more than 6 m (18 ft)
away from this weak location, the top of
bearing stratum [c] was encountered 5-7
m (15-20 ft) deeper in this area!
DEEP FOUNDATIONS * MAY/JUNE 2022 * 93
May/June 2022

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