Cone Penetration Testing 2022 – Gottardi & Tonni (eds)

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Open Access: www.taylorfrancis.com, CC BY-NC-ND 4.0 license

A CPT-based method for monotonic loading of large diameter monopiles

in sand
S. Bascunan
Ramboll, Hamburg, Germany

K. Kaltekis
Fugro, Nootdorp, The Netherlands

B. van Dijk
Arcadis, Amersfoort, The Netherlands

K. Gavin
Delft University of Technology, Delft, The Netherlands

ABSTRACT: A joint academia-industry project, the Pile Soil Analysis (PISA) project, resulted in an empir­
ical method for assessing the monotonic lateral loading response of large diameter monopiles. The method
predicts four soil reactions, namely the distributed load and the distributed moment along the pile shaft, the
pile base shear and the pile base moment. The method considers pile load test data and 3D numerical model-
ling. A 1D framework allows prediction of the four soil reactions. In this paper, a CPT-based approach is
proposed to derive the four soil reaction components for use in a 1D model for conceptual design of mono-
piles in sand subject to monotonic lateral loading. The approach relies on results from 3D finite element ana­
lyses that were performed considering soil conditions for a sand site used in the PISA project (Dunkirk site).
The results are compared to pile load test data from the PISA project, showing good agreement, particularly
for load levels related to the serviceability limit state.

1 INTRODUCTION A joint academia-industry project, the Pile Soil

Analysis (PISA) project, resulted in an empirical
Monopiles are commonly used as foundations for off­ method for assessing the monotonic lateral loading
shore wind turbine generators (WTGs). The current response of large diameter monopiles. The method is
trend in the ever-growing offshore wind energy sector based on conventional models for caisson design, pre­
is for WTGs to becoming bigger which evidently dicting four soil reactions, namely the distributed load
leads to requirements for monopiles with large diam­ and the distributed moment along the pile shaft, the
eters up to 10 m to support the superstructure. It is pile base shear and the pile base moment (Figure 1).
expected that the ratio of embedded length to diam­ The PISA schematisation excludes torsional foundation
eter, L/D (or slenderness ratio) of monopile founda­ loading (Burd et al., 2020). The empirical method con­
tions for the 10 MW+ next-generation wind turbines siders pile load test (PLT) data and 3D numerical mod­
could be in the range between 2 and 6 (Panagoulias elling. A 1D framework allows prediction of the four
et al., 2018). Such structures are categorised as inter­ soil reactions and requires, for sands, profiling of three
mediate foundations according to ISO (2016). soil parameters, namely the relative density, the verti­
An industry standard approach for assessing mono- cal effective stress and the shear modulus at small
pile lateral response was a p-y method for long slender strain.
piles, adjusted to large diameter monopiles. The In this paper, a CPT-based approach is proposed to
p-y method is based on the Winkler assumption accord­ derive the four soil reaction components for use in
ing to which the soil surrounding the pile is modelled a 1D model for conceptual design of monopiles in
as a set of uncoupled, non-linear, elastoplastic springs sand subject to monotonic lateral loading. The
which define the lateral pressure (p) applied to the pile approach relies on results from 3D finite element (FE)
at a given depth, as a function of the lateral displace­ analyses that were performed considering soil condi­
ment (y). The method, however, does not capture the tions for a sand site used in the PISA project (Dunkirk
physics of the monopile behaviour accurately. site). The results are compared to PLT data from the

DOI: 10.1201/9781003308829-120

812
PISA project, showing good agreement, particularly soil reaction curves were extracted via MoDeTo at
for load levels related to the serviceability limit different load steps and pile depths.
state (SLS).
3.2 Soil model
The Dunkirk test site was characterised using
a range of in situ tests and advanced laboratory test­
ing (Zdravković et al., 2020). Several CPTs were
performed next to the test pile locations and other
key locations. Figure 2 presents the average cone
resistance at the site. The general soil stratigraphy is
shown in Table 2. The water table is found approxi­
mately at 5.4 m below ground level.
The Hardening Soil small strain model (HSsmall)
was used as soil constitutive model. The model was
calibrated against available soil data from the Dun­
kirk site, including CPTs, seismic CPTs and labora­
tory tests such as triaxial tests with bender element
measurements. The calibration process included
study of several CPT-based and empirical parameter
formulations from the literature (e.g. Robertson and
Cabal, 2015; Brinkgreve et al., 2010), investigation
Figure 1. (a) Schematised soil reaction components acting of parameter interdependency and performance of
on a laterally loaded monopile; (b) 1D design model. (after single element test predictions.
Burd et al., 2020). The focus of the CPT-based approach was accur­
ate representation of the SLS, according to which the
2 DATABASE allowance for the total permanent tower axis tilt rota­
tion is 0.5° (DNVGL, 2016). By analysing the data
Several piles driven into dense sand at the Dunkirk obtained from the PISA project, this limit is reached
site were tested during the PISA project in order to at approximately 30% to 50% of the maximum hori­
investigate the effect of different design aspects such zontal load applied to the monopiles during pile load
as pile geometry, load ratio, unloading/reloading testing; hence only that portion of the horizontal
behaviour and creep. In this paper, the results from load-deformation curve was considered for the
three PLTs on medium diameter piles, D = 762 mm HSsmall calibration process.
(i.e. DM3, DM7 and DM4; see Table 1) were com­ Table 3 shows an overview of the soil parameter
pared to results from 3D FE analyses. This allowed, values for the calibrated HSsmall soil model.
using the FE-derived resistance components, devel­
opment of a CPT-based method.

Table 1. Geometry of PISA piles considered in this study

(Taborda et al., 2020).

Diameter Length Slenderness Wall Thick­

Pile (m) (m) ratio (-) ness (mm)

DM3 0.762 6.1 8.0 25

DM4 0.762 4.0 5.3 14
DM7 0.762 2.3 3.0 10

3 FINITE ELEMENT MODEL

3.1 General
The commercial software packages Plaxis 3D and
Plaxis Monopile Design Tool, MoDeTo (Plaxis BV,
2018), were used to perform the FE analyses and
extract the soil reaction curves. Through the latter,
the monopile was modelled and then the FE analysis Figure 2. Cone resistance profile at the Dunkirk site
was performed in Plaxis 3D. Finally, each of the four (Zdravković et al., 2020).

813
 Figure 3 illustrates the comparative results
between the measured horizontal load-displacement
responses from the PLTs and the predicted responses
from the performed 3D FE analyses. A fairly good
match is observed at the initial part of the curves,
rendering the prediction of the stiffness response,
which was of primary interest, satisfactory.
Additional (fictional) piles were considered in
order to expand the database and check the influence
of pile geometry on each of the four soil reaction
components. Table 4 shows an overview of the add­
itional piles considered for the sensitivity analyses.

Figure 3. Comparison of ground level load-displacement

4 SOIL REACTION CURVES for three piles tested during the PISA project (see Table 1
for details). Solid lines represent the results of the pile load
4.1 Distributed lateral load (p-y) tests (after Taborda et al., 2020), dashed lines represent the
results of the 3D FE calculations.
The relationship between p and y along the pile shaft
has been widely studied. In recent years several for­
mulations for p-y curves have been developed by
taking into consideration the cone penetration test
and considering the link between cone resistance Table 4. Geometry of additional (fictional) piles con­
sidered in the study.
(qc) and in situ horizontal effective stress of the soil
(Houlsby and Hitchman, 1988). An overview of Diameter Length Slenderness Wall Thick-
some of those formulations together with their cor­ Pile (m) (m) ratio (-) ness (mm)
responding authors is shown below:
DM3A 1.0 6.1 6.1 25
Table 2. Soil stratigraphy at the Dunkirk site (Zdravković DM3B 1.2 6.1 5.1 25
et al., 2020). DM3D 2.0 6.1 3.1 25
DM7B 0.762 3.0 3.9 10
Depth DM7D 0.762 4.7 6.1 10
(m) Material Description DM7E 0.762 3.8 5.0 10
PL1 0.762 15.0 19.7 20
0-3 Hydraulic Sand dredged from offshore Flan- PL2 0.5 15.0 30.0 25
fill drian deposits
PL3 0.6 21.0 35.0 30
3 - 30 Flandrian Marine sand deposited during three
sand local transgressions
> 30 Ypresienne Eocene marine clay located beneath
clay the southern North Sea

Table 3. Summary of soil parameters for HSsmall model.

Depth [m] 0-3 3-5.4 5.4-9 9-12.2 12.2-15

γ’ [kN/m3] 19.1 20.8 11.0 11.8 9.8

E50,ref [MPa] 250 223 174 202 87
(=Eoed,ref)
Eur,ref [MPa] 751 668 523 605 260
’0 [deg] 46 45 43 42 37
ψ[deg] 15 9 9 9 9
γ0.7 [-] 1e-4 1.3e-4 1.3e-4 1.3e-4 1.3e-4
G0,ref [MPa] 321 285 223 259 111
Rf [-] 0.88 0.91 0.91 0.91 0.91
K0 [-] 0.5 1.0 0.8 0.7 0.7

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where Equation 1 is by Novello (1999), Equation 2
is by Dyson & Randolph (2001), Equation 3 is by Li
et al. (2014), Equation 4 is by Suryasentana &
Lehane (2016), D = pile diameter, γ0 = effective unit
weight of soil, z = depth, Gmax = small strain shear
modulus, pu = ultimate lateral soil resistance (for
more details refer to Suryasentana & Lehane, 2016)
and f(y) = exponential function that depends on lat­
eral displacement (for more details refer to Surya­
sentana & Lehane, 2016).
Equations 1 to 4 were used to derive p-y curves
which were then inserted in a 1D Timoshenko beam
model for modelling of the pile-soil lateral behaviour.
Long slender (fictional) piles (L/D ≥ ~20) were con­
Figure 4. Distributed moment ratio as function of the slen­
sidered so that the influence of the other three soil
derness ratio L/D.
reaction components (distributed moment, base shear
and base moment) to the overall response is negli­
gible (see Table 4; piles PL1, PL2 and PL3). The
results obtained from the 1D model were thereafter
compared with results from 3D FE analyses and it of the rigid pile, the rotation can be obtained from
was found that Equation 2 (Dyson and Randolph, the horizontal displacements. Figure 5 shows the dis­
2001) was providing the better match and was thus tributed moment for various slices along the shaft of
selected to define the p-y component for this study. pile DM4, obtained both from the 3D FEA and the
proposed CPT-based formulation (Equation 6).
4.2 Distributed moment (m-ψ)
The distributed moment (m) is caused by the vertical
shear stresses along the pile shaft due to pile rotation
ðψÞ . It is considered that m is linked to p, which is
acting as a normal force along the shaft, through
consideration of the pile-soil interface friction angle
ðδÞ and the pile geometry (L and D). A fitting param­
eter, Fmψ, was adopted in order to investigate the
relationship between the aforementioned parameters
for the range of pile geometries considered.

where δ = pile-soil interface friction angle taken Figure 5. Pile DM4 distributed moment along each slice.
Solid lines correspond to the results from 3D FE models
as 2=3j0 .
and dashed lines correspond to the results from the pro­
By considering the maximum value of the distrib­ posed CPT-based formulation.
uted moment at every slice along the pile shaft
obtained from the 3D analysis, mmax, the influence of
L/D on the ratio mmax/Fmψ was investigated
(Figure 4) and a formulation for determination 4.3 Horizontal base force (HB)
of m is proposed (Equation 6). The relatively low R2
value is attributed to the small dataset and the fact Due to the applied force at the pile head, the base of
that the proposed linear trend might be less suitable the pile tends to move in the opposite direction, gen­
as L/D increases. erating a horizontal base force (HB). HB was linked
to the base displacement, vb, via a fitting parameter,
FHB, which is a function of the qc at the pile base
and the pile geometry (Equation 7). Figure 6 shows
the relationship between FHB and the ultimate hori­
zontal base force, HB,ult, for all piles in the con­
sidered database.
The distributed load and distributed moment are soil
reactions along the pile shaft, thus the pile was div­
ided into slices and both soil reactions were com­
puted per slice. By considering geometric continuity

815
 from the Plaxis 3D models of the database resulted
in the following bi-linear relationship:

Figure 8 shows the pile base moment reactions

obtained from the 3D FE models and the proposed
CPT-based formulation (Equation 9) for a selection
Figure 6. Fitting parameter FHB versus the ultimate hori­ of piles from the database.
zontal base force, HB,ult.

Curve fitting with results from the Plaxis 3D models

of the database resulted in the following bi-linear
relationship:

Figure 8. Pile base moment reactions. Solid lines corres­

pond to the results from 3D FE models and dashed lines
correspond to the results from the proposed CPT-based
formulation.

5 PILE LATERAL RESPONSE

The four soil reaction components, as computed

with the use of the proposed equations, were entered
in a 1D Timoshenko beam model for modelling of
the general monopile response under lateral loading.
Results for the piles of Table 1 are shown in Fig­
ures 9 and 10. The predictions of the CPT-based
method show in general good agreement with the
Figure 7. Pile base horizontal reactions. Solid lines corres­ PLTs and the 3D FE analyses for the initial part of
pond to the results from 3D FE models and dashed lines the load-displacement curve, i.e. until approximately
correspond to the results from the proposed CPT-based
formulation.
half the ultimate lateral load (Figure 9). The initial
stiffness response of the monopiles is, therefore,
fairly captured. Figure 10 depicts the pile deflections
Figure 7 shows the pile base horizontal reactions below ground level at different loads, all with mag­
obtained from the 3D FEA in comparison to the reac­ nitude lower than half of the ultimate lateral load.
tions from the proposed CPT-based formulation Again, a satisfactory agreement between the results
(Equation 8) for a selection of piles from the database. of the CPT-based method, the 3D FE analyses and
the PLTs is observed.
Figure 9 shows that after a certain level of
4.4 Base moment (MB) ground level displacement, the response obtained
The base moment is caused by rotation of the pile from the proposed CPT-based method is stiffer
toe. Similarly to the base horizontal force, the base than the response obtained from the 3D FE ana­
moment relationship contains a first linear portion lyses and the PLTs. Therefore, a cut-off point
followed by a plateau. Curve fitting with results needs to be defined beyond which the proposed

816
 response in which the displacements at ground
level are not larger than 2% to 3% of the pile outer
diameter. This level of deformation generally cor­
responds with the serviceability limit state of
monopiles used in the offshore wind industry.

6 DISCUSSION AND CONCLUSIONS

The paper presents a CPT-based method for predict­

ing the response of laterally loaded monopiles in
a sand setting. In order to verify application of the
method to full-scale monopiles and until further
Figure 9. Comparison of ground level load-displacement experience is gained with the use of this method,
for three piles tested during the PISA project. Solid lines a FE analysis, considering typical soil conditions of
represent the results of the pile load tests; thinly dashed the investigated site and expected pile geometry, is
lines represent the results of the 3D FE calculations; thickly recommended as a minimum.
dashed lines represent the results of the CPT-based method. The method allows for performing monotonic
conceptual design calculations for monopile founda­
tions supporting WTGs in a time-efficient manner,
requiring only CPT data. Total computing time can
be reduced by up to 90 % with respect to performing
3D FE analyses.
The proposed soil reaction formulations were cali­
brated against soil data from the PISA sand site in
Dunkirk and consider a specific limit state, i.e. the
SLS. Therefore, applicability of the method to
marine sites with significantly different soil condi­
tions (e.g. in terms of strength, stiffness, sand type)
than the ones at Dunkirk and/ or for different limit
states should be carefully checked. In these occa­
sions, FE analyses are required prior to implementa­
tion of the approach shown in this paper to develop
a site-specific CPT-based method. Alternatively, the
PISA ‘numerical-based method’ can be employed
(Byrne et al., 2017).
The curve fitting process considered the individual
soil reactions from the 3D FE analyses and not the
actual PLTs, since modelling of each individual soil
reaction component based on measured PLT data has
been shown to be problematic (Foursoff, 2018).
The proposed CPT-based method provides
a representation of the global monopile response
under monotonic lateral loading, although the individ­
ual soil reactions at a local level can differ consider­
ably between the FE analyses and the CPT-based
Figure 10. Comparison of deflection at and below ground formulations. The latter can be attributed to factors
level for three piles tested during the PISA project. Solid
such as imperfect curve fitting and inherent limitations
lines represent the results of the pile load tests; thinly
dashed lines represent the results of the 3D FE calculations; of the 1D model which cannot accurately represent all
thickly dashed lines represent the results of the CPT-based mechanisms of soil-pile interaction at a local level.
method. Values within brackets denote the applied load in The CPT-based method should be employed in its
kN. entirety, i.e., individual soil reaction components
should not be excluded from the analysis or used
method is less accurate. This point was defined by independently.
analysing, for all piles of the database, the differ­
ence in stiffness magnitude between the 3D FE
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