Proceedings of Pile 2013, June 2-4th
2013
STRESS AND DISPLACEMENT MONITORING OF AUGER DISPLACEMENT PILES
M.D. Larisch1, M. Arnold
2, M. Uhlig
2, E. Schwiteilo
2, D.J. Williams
1 and A. Scheuermann
1
ABSTRACT: Auger displacement piles have been used for decades worldwide as foundation elements for structures
and embankments. The system has become increasingly popular and very successful in recent years as it can achieve
high production rates and spoil creation is minimal. The advancement of an auger moves and compacts the ground
laterally during penetration, which can result in increased shaft friction of the completed pile. However, this technique
has the potential to damage already completed piles, as well as adjacent structures or underground services, due to
lateral or vertical soil displacements during pile construction. The behaviour of the soil surrounding the auger during
penetration, extraction and concrete pumping is not well understood to date. As a result, designs have been either too
conservative or pile settlements have been above the specified design criteria and excessive, resulting in damage. The
paper describes auger displacement behaviour in fine-grained soils. A fully instrumented test site in Brisbane, Australia,
has monitored soil behaviour (ground stresses and displacements) during the penetration and extraction of two auger
different displacement pile types. The field results are compared with Finite Element model predictions, enabling them
to be validated and calibrated.
Keywords: Displacement, displacement piles, Finite Element model, heave, pile monitoring, stresses, soil interaction
INTRODUCTION AND SCOPE
Auger displacement piles (ADP) are a rotary drilling
technique that can be used to construct concrete piles
and columns. During the penetration of the auger, the
soil is displaced laterally into the surrounding ground,
and the spoil created by ADP installation is minimal.
Concrete is pumped through the hollow stem of the
auger and the piling methodology is very similar to
Continuous Flight Auger (CFA) piling. Due to high
production rates, the technique can be very economical,
which has led to its increased global usage during the
last two decades. The typical ADP installation process is
described in Figure 1 (after Bottiau, 1998) and below:
- Set up auger at pile position and install cap to
close concrete outlet at auger tip.
- Install auger by rotating clockwise and applying
vertical pull down force.
- Drill auger to design depth; the displacement
body of the auger pushes the soil cut by the
auger tip into the surrounding ground.
- Pump concrete through hollow auger stem and
extract auger while rotating clockwise, always
maintaining concrete pressure positive and auger
embedded in fresh concrete.
- Install reinforcement into fresh concrete, if
required.
Fig. 1 Installation process of ADP (after Bottiau, 1998)
Depths of up to 30 m can be achieved with standard
piling equipment. Different auger types and shapes are
available, and typical diameters range from 200 to
550 mm. Skin friction and end bearing behaviour of
different piles can vary, depending on the soil conditions,
and the geometry and shape of the auger (Vermeer 2008,
Larisch et al. 2012).
Soil behaviour during the entire construction process
has, particularly in cohesive soils, not been investigated
in detail. ADPs have the potential to damage adjacent
structures or freshly cast piles due to displacement
effects and heave, as has been observed on many
projects.
The theoretical background of these phenomena has
not been investigated in detail. The scope of this research
project, led by The University of Queensland (UQ) in
collaboration with Piling Contractors, is to investigate
the changes in stresses and displacements in fine-grained
soil during the installation of ADPs.
UQ entered into a cooperation with the Technical
University of Dresden, Germany, in 2010 to develop a
suitable numerical model for the penetration and
extraction of a typical ADP in fine-grained soil, using a
hypo-plastic constitutive model for clay (Larisch et al.
2013), and to validate the numerical model by field test
results to be carried out in Lawnton, QLD, Australia.
FIELD TEST SITE
In February 2011, two cone penetration tests (CPT)
and two undisturbed, continuous soil samples were taken
to bedrock at the proposed test pile locations. The soil
samples were taken about 500 mm from the CPTs in
order to get a close correlation between CPT values and
the soil profile. Index values were determined in the
laboratory for each of the different soil layers
encountered and these are summarised in Table 1.
Table 1: Typical soil profile, index values and soil
classification of Lawnton clays
Soil
description
Stiff, light
brown
ocher clay
Stiff, grey
clay,
inclusions
Stiff, grey,
sandy clay,
inclusions
Depth (m) 0.00 to -
1.50
-1.50 to -
6.20
-6.20 to -
7.20
Fines content
< 15 mm (%)
79 91 57
Liquid Limit
(LL) (%)
43 55 44
Plastic Limit
(PL) (%)
25 21 19
Plasticity
Index (PI)
(%)
18 32 25
Classification
DIN18196
Medium
plasticity
High
plasticity
Medium
plasticity
Classification
USC
Low
plasticity
High
plasticity
Low
plasticity
For this research project, only the top three soil
layers are relevant. The underlying gravel with clayey
inclusions has no significance to the research as the
proposed test piles were to be founded 4.00 m below the
original ground level, within the clay layers. The scope
of this research is the investigation of auger
displacement pile behaviour in fine-grained soil and the
authors wanted to ensure that the piles would “float” in
Lawnton clays, without any influences from the gravel
layer below. Typical cone penetration test (CPT) profiles
for the field test site are displayed in Figure 2.
Fig. 2 Typical CPT at the test field in Lawnton, QLD,
Australia
The second clay layer, which was defined as grey
clay, is the material in which the ADPs were to be
founded. Characteristics of this grey clay layer are
assumed to be critical for the ADP performance.
Laboratory oedometer tests and consolidated
undrained (CU) triaxial tests were carried out on
undisturbed and remoulded soil specimens to determine
the basic five hypo-plastic soil parameters for use in the
FE model (Larisch et al. 2013).
NUMERICAL MODEL
For the numerical simulation of the penetration and
extraction of an ADP, the Finite Element (FE) code
Abaqus Standard was used (Larisch et al. 2013). Hypo-
plastic soil behaviour for fine-grained soil (clays) after
Mašín (2005) was implemented using a UMAT routine.
An axisymmetric, two-dimensional FE model was
developed to simulate the pile construction process. The
model greatly simplifies the real geometry and
construction process. Due to the complexity of the
process, the model does not take auger rotation, soil
cutting, soil transport, soil disturbance or soil
compaction into account.
The ADP auger is modelled as a cone-shaped rigid
body, with a 60 degree cone angle and a total auger
height of 1.5 m, representing a typical ADP auger (lower
screw section). To avoid excessive mesh distortion at the
beginning of the penetration process, the cone is partly
pre-installed into the soil and the soil and the cone are
modelled to be in full contact.
The expected deviatoric stress field of an auger
displacement pile during the installation process in
Lawnton clay is displayed in Figure 3 and the expected
horizontal soil displacements during this process are
shown in Figure 4.
Fig. 3 Expected deviatoric stress field (von Mises stress
in kPa) of ADP to be installed in Lawnton clays
modelled using Abaqus Standard
In the past (Cudmani 2001, Henke 2010), a particular
method called the “zipper technique” has been used
successfully to model pile penetration into a soil
continuum. A smooth rigid tube with a diameter
d = 1 mm is discretised at the axis of penetration. The
cone-shaped, rigid ADP body slides over the rigid tube
and separates the soil from the tube. The cone establishes
contact with the soil and is able to deform the meshed
continuum, thus simulating penetration and the resulting
soil displacement. The surface-to-surface contact
between the penetrating object (ADP auger) and the
surrounding soil is based on the master-slave principle.
The friction coefficient between the deformable soil and
the rigid piling auger is assumed to be tan (φc/3), based
on Coulomb’s friction law.
The diameter of the displacement auger head is
modelled to be 450 mm and the penetration depth into
the soil continuum is taken to be 4.00 m. Pile installation
is modelled using constant penetration and extraction
rates of 0.03 m/s. However, since the constitutive model
is rate-independent, the rates are of no impact. Soil
behaviour is assumed to be undrained during penetration
(constant volume), as penetration occurs too rapidly to
allow substantial drainage.
Fig. 4 Expected horizontal soil displacement (in meters)
of ADP to be installed in Lawnton clays modelled using
Abaqus Standard
FIELD TESTS
The field test site is located at Lawnton, Queensland,
Australia close to the South Pine River. A 300 mm thick
working platform was installed on the topsoil surface to
ensure safe operations during pile installation. The
platform comprised crushed rock with a maximum
aggregate size of 75 mm. All pile locations, CPT,
dilatometer (DMT) and spatial time domain
relectiometer (TDR) locations were pre-excavated or
pre-drilled through the working platform to minimise
disturbance from displaced rocks during pile installation.
Two test piles were installed, both auger
displacement piles 450 mm in diameter. One test pile
was installed with a progressive displacement auger,
referred to as “test pile C”. The second test pile was
installed with a rapid displacement auger and is referred
to as “test pile D”. More details about the different auger
types are presented in the following section. Another aim
of the research project is to compare the behaviour of
different auger types in similar ground conditions using
similar installation parameters, hence the use of different
auger types.
The piles were installed with a centre-to-centre
distance of 4.50 m between the piles. This distance was
assumed to be sufficient to ensure that the installation of
one pile would not influence the stress and displacement
state at the other pile. The piles were installed using a
Casagrande C30 piling rig, as shown in Figure 5.
Fig. 5 Piling rig with a rapid displacement auger in
preparation to install test pile D
The installation of both piles was recorded using the
on-board computer of the piling rig and Jean Lutz
software. Displacement piling is a blind process and it is
important to ensure a constant penetration rate by
monitoring changes in the torque readings of the drilling
rig. Higher torque readings combined with normal
penetration rates can indicate stiffer soil layers and it is
advisable to carry out a test drill close to a CPT location
in order to calibrate the drill parameters. For the test site,
it was expected that the torque reading would increase
linearly to a depth of about 2.00 m below the working
platform level. The CPT shows a peak cone resistance at
this level. Below this level, the torque reading should be
constant, as auger penetration should occur as fast as
possible. The installation records are displayed in Figure
6 and it can be observed that the energy input to install
the piles is almost identical for both test piles, as torque,
rotation, and penetration rates are almost similar. The
lifting rate is not critical for the soil penetration process
and indicates the extraction rate of the auger during
concrete placement.
Fig. 6 Pile installation record for test piles C and D show
very similar installation rates and energy input
IN SITU MONITORING
CPT and DMT tests were carried out before and after
the installation of the two test piles. Both tests provide
some measure of the stress state in the soil before and
after the installation of the test piles.
Spatial time domain reflectiometer (Spatial TDR)
pressure sensors (Scheuermann and Huebner, 2009)
were used for the first time to measure stresses in the
ground during pile installation, continuously over a
predefined depth of 5.00 m, covering the length of the
pile to 1 m below the pile toe, and not only at a single
point as for an earth pressure cell. The TDRs were
assembled on site and installed around the test piles, as
shown in Figure 7 and Table 2. The analysis of the TDR
results is still ongoing and is not the subject to this paper.
The working principle, assembly, installation and
measurement results of the TDR for this research project
will be presented in another publication (see also
Scheuermann et al, 2009).
For the monitoring of horizontal soil displacements, a
series of inclinometer tubes was installed around each
test pile, as shown in Figure 5. The tubes were installed
and grouted into 100 mm diameter holes, drilled by a
subcontractor. Three sensors were lowered down each
tube before and after the test pile installation to measure
lateral soil displacements before and after the installation
of both test piles. Measurements were taken every
100 mm over a depth of 6.00 m. It was important to
measure potential changes below pile toe level as the FE
model indicates stresses up to 1.0 m below the proposed
pile toe.
In summary, a series of instruments was located
around each test pile, as shown in Figure 7. The
inclinometer tubes and TDR’s were installed and
maintained in place. The CPTs and DMTs were carried
out at the pile location before pile installation and
outside the pile after completion of the test piles. The
configuration, as shown in Figure 7, is similar for both
test piles C and D.
Fig. 7 Location and direction of different monitoring
equipment from the proposed test piles (not to scale)
Table 2: Distance of different monitoring devices from
centre of test piles C and D of diameter d, location as per
Figure 7
Distance
to centre
of pile
Target
(mm)
Actual
(mm)
Difference
(mm)
CPT 1 0.0 200 200 0
CPT 2 1.0 d 450 440 -10
DMT 1 0.0 200 200 0
DMT 2 1.0 d 450 455 5
Inc. 1 1.0 d 450 465 15
Inc. 2 1.25 d 563 548 -15
Inc. 3 1.5 d 675 670 -5
TDR 1 0.72 d 325 305 -20
TDR 2 1.0 d 450 420 -30
TDR 3 1.5 d 675 605 -70
For the measurement of vertical soil displacements,
survey pegs were used and measurements were taken
before and after the test pile installation to observe soil
heave around the piles.
The augers
For the installation of the test piles, two different
auger types were used:
- Progressive displacement auger for the
installation of one test pile (pile C).
- Rapid displacement auger for the installation of
one test pile (D).
In order to understand the general working principle
of ADPs, it is important to be familiar with the basic
principles of piling auger mechanics. Detailed
descriptions of various screw auger models and the most
accepted theories can be found in the literature (Viggiani
1993, Fleming 1995, Slatter 2000) and are beyond the
scope of this paper. However, it is important to
understand the influence of auger geometry on the
stresses and displacements created in the soil, as well as
the installation parameters and, potentially, the pile load
capacities. All screw auger theories and models are
essentially based on three basic auger actions:
- soil cutting,
- soil transport, and
- soil displacement.
Depending on the auger shape, geometry, and the
main installation parameters (penetration rate, torque,
auger rotations), the influence of the three auger actions
is different in granular and fine-grained soils.
The augers shown in Figures 8 and 9 are all defined
as full-displacement piling augers and generally fall in
the group of long displacement auger systems (Larisch et
al. 2012), which all have the same basic geometrical
components, as shown in Figure 8 and Table 3:
Fig. 8 Basic components of long displacement augers
used for the installation of pile C (left) and pile D (right)
The different basic geometry of both auger types is
also shown and sketched in Figure 9. Progressive and
rapid displacement augers are both designed with longer
flighted sections and the lower auger sections are used
for cutting and transporting the soil to the displacement
body of the auger. The counter screw sections, located
above the displacement body, re-displace soil that has
collapsed into the cavity behind the auger during the
extraction process. The auger geometry of both augers
seems similar, and visually both augers seem comparable.
However, one auger is a progressive displacement auger,
as the diameter of the lower auger section progressively
increases towards the displacement body. During
penetration, soil is displaced progressively along the
lower auger section and finds it peak at the location of
the displacement body.
The overall height of the rapid displacement auger is
about 500 mm greater than the height of the progressive
displacement auger, due to the longer lower screw
section of the rapid displacement auger. Both augers
have identical outer diameters (450 mm), but the taper
from the displacement body to the smaller inner diameter
at the auger base is different. The progressive
displacement auger is tapered over three pitches (about
750 mm) and the rapid displacement auger is tapered
over only two pitch heights (about 500 mm), which
results in a more rapid displacement process for the
former auger.
Table 3: Typical dimensions of progressive and rapid
displacement augers (DA)
Auger Details Progressive DA Rapid DA
Outer Diameter (flights
and displacement body)
450 mm 450 mm
Inner Diameter (inner
tube at the bottom)
Height:
Lower Section
Displacement Body
Counter Screw Section
250 mm
1,000 mm
1,000 mm
500 mm
250 mm
1,500 mm
1,000 mm
500 mm
Total
2,500 mm
3,000 mm
Height Flight Pitch 250 mm 250 mm
Bottom Flight Pitch 100 mm 100 mm
For rapid displacement augers, the displacement
body has a larger diameter than most sections of the
inner auger stem (the flights have the same diameter as
the displacement body). Soil displacement occurs rapidly
at the displacement body and only minor displacements
are expected below it. Potential soil loosening might
occur (in granular soil) along the lower section of the
auger, if penetration rates are too slow and not optimised
for the ground conditions encountered.
Fig. 9 Progressive displacement auger (left) was used for
the installation of pile C – for pile D the rapid
displacement auger type was used (right)
Counter screw
section
Displacement
body
Lower screw
section
Auger tip with
end cap
TEST RESULTS
CPT and DMT
For measurements after pile installation, the CPT was
located 450 mm (1.0 d) from the centre of the pile. The
tests were carried out typically 1-2 days after pile
installation. All figures in this section display the stress
changes in the soil after test pile installation.
Pile C (Progressive Displacement Auger)
Fig. 10 Pile C (progressive displacement auger) –
Changes in base resistance and sleeve friction (CPT)
Significant stress changes for both cone resistance Qc
and sleeve friction Fs occurred in the top 2.5 m below
working platform level, indicating stress reductions after
pile installation in this depth range. Ignoring a peak
reading close to the surface, which could have been
caused by a displaced fill boulder, the peak reduction is
around 2.00 m below existing platform level with a value
of -4.0 MPa (-2.0 MPa on average for this region) for Qc
and about -200 kPa for Fs (average of -100 kPa),
respectively.
The depth range from 2.50 to 4.00 m below working
platform level shows increased stresses after pile
installation, of about +1.0 MPa on average (+2.0 MPa at
the peak) for Qc, and +80 kPa on average (+200 kPa at
the peak) for Fs.
In the region of the pile toe, both graphs show some
stress reduction between 200 m and 300 mm above and
below the pile toe averaging -1.0 MPa for Qc and about -
40 kPa for Fs, which indicates soil distortion. The
stresses up to 1.0 m below the pile toe increase by the
same magnitude as in the depth range from 2.5 to 4.0 m
below the surface.
Fig. 11 Pile C (progressive displacement auger) –
Changes in undrained shear strength (DMT)
The post-installation undrained shear strength,
estimated by the DMT, confirms the results of the CPT
for test pile C. The top 2.5 m showed a reduction in the
average shear strength of about -50 kPa (peak reduction
of -85 kPa). The depth range from -2.5 to -4.0 m
indicates an increase in the average undrained shear
strength of about +30 kPa (peak increase of +50 kPa).
The region around the pile toe showed a strength
reduction of about -10 kPa (on average) and the soil
below the toe region indicated an increased strength of
up to +50 kPa about 0.5 m below the pile toe. All results
are summarised in the last section.
Pile D (Rapid Displacement Auger)
Fig. 12 Pile D (rapid displacement auger) – Changes in
base resistance and sleeve friction (CPT)
The stress changes after pile installation for test pile
D, installed with a rapid displacement auger, appear
slightly different to the stress changes around test pile C.
The stress reduction in the top 2.5 m is not as marked,
and for Qc the changeover point from reduced to
increased stresses is located at about 0.5 m deeper at
3.00 m below the working platform surface. Both, Qc
and Fs show a “zig-zag” response in the upper 2.5 to
3.0 m depth, with both positive and negative stress
changes. However, the stress reduction is less significant
and the magnitude is only 60% compared to pile C,
showing a peak in Qc of -2.8 MPa (average of -1.6 MPa)
and a peak in Fs -120 kPa (average of about -40 kPa).
The depth range below this zone to above the pile toe
shows comparable stress changes to pile C for Qc (peak
of +1.9 MPa and average of about +0.8 MPa) and
reduced, but positive stress changes for Fs (peak of
+105 kPa and average of about + 60 kPa).
The stress reduction around the toe of pile D extends
to 1.0 m below pile toe level. The average value for Qc
(-0.8 MPa) and Fs (-40 kPa) are almost similar for both
pile types. Below this region of stress reduction, stresses
increase again. However, as this region is deeper than1.0
m below pile toe level, it is assumed not to be due to the
effects of the pile.
Fig. 13 Pile D (rapid displacement auger) – Changes in
undrained shear strength (DMT)
The undrained shear strength, estimated by the DMT,
also confirms the results of the CPT for test pile D. The
strength reduction within the upper 3.0 m is more
significant for pile D and shows peak values of up to -
160 kPa (average about -50 kPa). Increased strength can
be observed over the lower 1.0 m section of the pile
reaching an average of +30 kPa and peak of + 50 kPa,
very similar to that for pile C. The region below pile toe
level shows reduced shear strength as well, more than
50% lower than for pile C for both the average (-15 kPa)
and peak (-30kPa) readings. The region below the toe
experienced a strength increment of about +55 kPa on
average (+105 kPa peak). This region is located about
0.5 m below the toe of the pile, which corresponds to the
observations made at pile C (progressive displacement
auger). All results are summarised in the last section.
PILE DISPLACEMENTS
Horizontal Displacements
Horizontal displacement changes were measured with
inclinometers before and after pile installation. The
horizontal displacements measured using inclinometers
are shown in Figures 13 and 14, using the convention
positive displacements away from the pile.
Unfortunately, a few tubes were damaged during pile
installation and only two comparable readings at 1.25 d
from the centre of each test pile are available for a
realistic displacement comparison.
For test pile C, maximum horizontal displacements of
about 26 mm were measured about 2.5 m below the
working platform. These displacements decrease almost
linearly over the next 1.5 m depth to 20 mm at pile toe
level. At 1 m below pile toe level, displacements are
only 6 mm and the increased values measured at 6.0 m
depth could be the result of a measurement error or local
geological conditions.
Fig. 13 Pile C (progressive displacement auger) –
Change of horizontal displacements 338 mm from the
edge of the pile shaft or 563 mm from the centre of the
pile
The inclinometer-based horizontal displacement
measurements for pile D are shown in Figure 14. At
about 2.5 m below the working platform surface,
maximum displacements of about 30 mm were measured.
These displacements decrease almost linearly over the
remaining 1.5 m of the pile length to 26 mm at pile toe
level. At 0.5 m below pile toe level, displacements are
only 6 mm and reduce linearly to zero at 5.5 m depth.
Fig. 14 Pile D (rapid displacement auger) – Horizontal
displacements 338 mm from the edge of the pile shaft or
563 mm from the centre of the pile
Surface Displacements
Surface displacements were measured for both piles
using survey pegs. For both test piles, soil vertical heave
and horizontal displacements were observed during the
installation of the piles.
Around both test piles a circular heave zone was
observed with a radius of about 1.0 m from the edge of
the piles. The height of the soil heave at the edge of the
piles was about 250mm above the working platform
level (the soil was pushed upwards by 250 mm) and the
slope of the heave cone decreased more or less linearly
to zero at the edge of the heave zone, as shown in the
simplified sketch in Figure 15.
Fig. 15 Soil heave was observed for both test piles with
almost similar dimensions of the circular failure pattern
The volume of the displaced soil above the working
platform level was about 0.44 m3, which is about 10%
higher than the volume of the pile shaft over the top
2.5 m and could indicate dilatancy. This approach needs
further refinement as dilatancy and soil loosing due to
the drilling and displacement effect were not taken into
account. On the other hand, the depth of the heave zone
might be deeper than 2.5 m as the stresses and
displacements were measured not right at the edge of the
pile shaft, but about a pile diameter away from the edge
of the shaft.
Further research is required to investigate the failure
mode and pattern of soil heave in fine grained soils
produced by displacement augers. It is important to state
that the shape of the auger seems not to have an
influence on the volume of soil heave.
CONCLUSIONS
For both auger test piles, significant stress changes
occurred during the pile installation process. Tables 4
and 5 summarise the stress changes at the different
sections of the piles. The horizontal soil displacement
curves 1.25 diameters from the centre of the piles show
comparable values for both piles.
The installation records in Figure 6 show that the
external energy input from the piling rig is almost
similar for both piles. Consequently, the stress changes
observed during the installation process are solely
caused by the different shape of the augers.
Over the upper 2.5 m of the piles, significant stress
reductions occurred for both piles, which indicate highly
disturbed ground conditions caused by the displacement
piling process. The authors conclude that the pile
installation caused a circular failure mode around the
piles. With ongoing penetration of the auger, different
slip lines were created, pushing the soil upwards, similar
to a base failure. Maximum horizontal displacements at
2.5 m depth also indicate that the final slip line could be
located at around that depth; soil heave volumes
(including dilatancy) seem comparable to the pile shaft
volume to 2.5 m depth. The progressive displacement
auger performs worse than the rapid displacement auger,
and the reductions in stress and strength are up to 50%
higher. This could be evidence that the rapid
displacement auger performs more soil cutting and soil
disturbance as well as less displacement during the
penetration of the lower auger section. Over the upper
2.5m of the pile shaft the stress changes are lower with
this auger type as the disturbed soil cannot be
(re)compacted as effectively as with progressive
displacement augers. The strength changes for
progressive augers are smaller as the soil will not be
disturbed and distorted as much as with rapid
displacement augers.
Over recent years, the first authors’ experience with
various soil displacement piling projects in Germany and
Australia, indicates that cracks in unreinforced
displacements piles and columns installed in cohesive
soils usually occur in the top 1.5 m section of the pile
(assuming 1 m pile spacing c/c), which is supported by
the findings of this research. Soil heave is commonly
known as a potential risk for auger displacement piling,
but the mechanism of heave in auger displacement piling
in fine-grained soil has not been investigated in detail to
date. The data from the test piles at Lawnton clearly
show the presence of heave; however, further research is
required to define details about the failure mode, the
dependence on soil types, and the influence of auger type.
Table 4: Pile C – Summary of maximum and average
stress changes in Qc, Fs and Su at different depths
0 to
-2.5m
-2.5 to -
4.0 m
Pile toe
region
-4.5 to -
6.0 m
Qc (MPa)
(peak)
-4.0 +2.0 -2.0 +5.0
Qc (MPa)
(average)
-2.0 +1.0 -1.0 +2.0
Fs (kPa)
(peak)
-200 +200 -60 +180
Fs (kPa)
(average)
-100 +80 -40 +100
Su (kPa)
(peak)
-85 +50 -20 +65
Su (kPa)
(average)
-50 +30 -10 +30
Table 5: Pile D – Summary of maximum and average
stress changes in Qc, Fs and Su at different depths
0 to
-2.5m
-2.5 to -
4.0 m
Pile toe
region
-4.5 to -
6.0 m
Qc [MPa]
(peak)
-2.8 +1.9 -1.2 -1.8
Qc [MPa]
(average)
-1.6 +0.8 -0.8 -1.0
Fs [kPa]
(peak)
-120 +105 -80 +120
Fs [kPa]
(average)
-40 +60 -40 +80
Su [kPa]
(peak)
-160 +50 -30 +105
Su [kPa]
(average)
-50 +30 -15 +55
Increased stresses and strength below the heave zone
and above the pile toe were observed for both pile types
and were caused by the pile installation process.
The progressive displacement auger seems to more
effectively increase the stresses around the pile shaft in
this region (by better displacement action) than the rapid
displacement auger.
At the pile toe, both augers reduced the stresses and
strengths in the ground (soft toe), to different degrees.
The rapid displacement auger seemed to have more
influence in regard to the depth and magnitude of stress
reduction, which could be expected due to the longer
lower screw section and the increased soil cutting and
transport actions in this region. Potential reductions in
pile base capacities caused by rotation of the auger
without sufficient penetration need to be considered in
pile design, particularly for rapid displacement augers.
The stress reductions at the pile toe are less significant
and not as deep for progressive displacement augers;
however a potential reduction in the base capacity also
needs to be considered.
The rapid displacement auger causes about 15%
more horizontal displacement at 1.25 d from the centre
of the pile than the progressive displacement auger,
while vertical displacements are almost similar. It must
be noted that the measured displacements might be
conservative and that the “true” horizontal displacements
might be larger (the FE model predicts about 60 mm
lateral displacement). The inclinometers tubes are made
of PVC and were grouted in the soil, providing a higher
stiffness than the original soil. Consequently, the “true”
horizontal displacements will be higher than the
measured data, which nevertheless give a good
indication about the shape and the minimum magnitude
of the displacements in the ground. Two out of six
inclinometer tubes were destroyed during the tests, due
to excessive soil displacements, and measurements could
not be taken after the pile installation. Further research is
required to develop measurement devices that can
monitor soil displacements more accurately. However,
the measured results provide a good first indication
about the shape of horizontal displacement curves.
The FE model does not allow for any soil distortion
or disturbance from the cutting, transport and
displacement actions throughout the entire installation
process. The FE model greatly simplifies the installation
process and no account was taken of soil heave and the
failure pattern in the top 2.5 m of the pile. Similarly, the
stress reduction at the pile base could not be modelled by
the FE code, due to the complexity of the problem.
Furthermore, the FE model assumes pre-excavated soil
conditions at the surface, to allow full contact between
the auger and the surrounding soil prior to penetration.
This assumption is not correct and does not model the
heave phenomena realistically.
The authors recommend taking the measured in situ
stresses (CPT sleeve friction) from -2.5 to -4.0 m into
account when comparing the field values with the output
data of the FE model. The numerical model predicts
deviatoric stresses of about 75 kPa at a distance of
338 mm (0.75 pile diameters) from the edge of the pile.
The FE model shows good agreement with the measured
average sleeve friction changes of +80 kPa (<10%) for
the progressive displacement auger and over-predicts the
average stress changes after pile installation by 20 kPa
(+30%) for the rapid displacement auger. However, the
model provides a good indication about the stresses in
the ground for progressive displacement augers in
Lawnton clays. Additional research is recommended in
this area to further develop numerical simulations to
model soil behaviour during displacement piling using
different augers. The authors believe that the
development of high performance computers and
improved FE codes in the next few years will greatly
enhance research efforts in this area.
FURTHER RESEARCH
The test field results presented in this paper provide
evidence that different ADP augers cause different stress
changes in similar ground conditions. However, the
results also open new opportunities for further research
in the field of ADP in fine-grained soil conditions.
It is critical to determine the influence of heave on
the load capacities of auger displacement piles. The
reduced stresses and shear strengths over the upper 2.5 m
of the piles can be critical, particularly for short piles and
columns, as the magnitude of reduced shaft capacity in
this section is unclear. Usually, auger displacement piles
are assumed to have increased shaft capacities due to the
displacement effect along the entire pile shaft (including
the upper section). The test results provide evidence that
this is not true for the top 2.5 m of the piles installed in
Lawnton clays.
The authors see the need to carry out pile load tests in
order to investigate the influence of auger shape and
geometry on pile capacity and to correlate with the auger
shape and installation parameters. In particular, the
influence of heave on pile shaft capacities (positive or
negative skin friction) is of great interest, especially for
the design of rigid inclusions (Simon and Schlosser
2006; Plomteux and Lacazidieu 2007; Wong and
Muttuval 2011 ). Research in this field is ongoing.
The installation of Continuous Flight Auger (CFA)
test piles in similar ground conditions would give further
data about the behaviour of different auger types in
Lawnton clays. CFA piles are defined as non-
displacement piles and the potential occurrence of heave
and the amount of stress and displacement changes as
well as load capacities in Lawnton clay, compared to
auger displacement piles, would be of immense interest.
The investigation of the shape and general failure
mechanism of heave is another area which requires
additional research, including the influence of soil
parameters and classification data on the failure pattern.
The analysis of the TDR data from the tests described
in this paper is ongoing. The results will be published
separately.
Furthermore, future trials in different soil conditions
will provide evidence of whether or not the behaviour of
auger displacement piles can be generally formalised and
predicted for fine-grained soil conditions other than
Lawnton clays. The authors suggest carrying out field
tests prior to construction in order to get a good
understanding of the stress changes, displacements and
load capacities of auger displacement piles in fine-
grained soils.
ACKNOWLEDGEMENTS
The authors thank the Group of Eight (Go8) and the
DAAD (German Academic Exchange Organisation) for
funding the research collaboration between The
University of Queensland and the Technical University
of Dresden.
Thanks are also extended to the ARC Linkage
scheme and industry sponsors Piling Contractors, Golder
Associates and Insitu Geotech Services for their ongoing
support of the research project.
The further development of the Spatial TDR
technology is supported by a recently granted
Queensland Science Fellowship awarded to A.
Scheuermann.
REFERENCES
Bottiau M. and Meyus I.A. (1998) “Load testing at Feluy
test site: Introduction of the Omega B* pile”.
Proceedings of the 3rd Int. Geotechnical Seminar on
Deep Foundations on Bored and Auger Piles. W. F.
van Impe and W. Haegeman. Ghent, Belgium,
Balkema, A.A., Rotterdam, Brookfield, 1: 187-199.
Cudmani, R. O. (2001). “Statische, alternierende und
dynamische Penetration nicht bindiger Boeden”.
Publications of the Institute of Soil Mechanics and
Rock Mechanics, University of Karlsruhe (Germany),
Issue 152.
Fleming, W. G. K. (1995). “The understanding of
continuous flight auger piling, its monitoring and
control.” Proc. Instn Civ. Engrs Geotech,: 157 - 165.
Henke S. (2010) “Influence of pile installation on
adjacent structures”. International Journal for
Numerical and Analytical Methods in Geomechanics,
34(11): 1191-1210.
Larisch M., Nacke E., Arnold M.., Williams D.J. and
Scheuermann A. (2012) “Load Capacity of Auger
Displacement Piles”, Proceedings of the International
Conference on Ground Improvement and Ground
Control, 30th October – 2nd November 2012,
Wollongong, Australia.
Larisch M., Williams D.J. and Slatter J.W. (2013)
“Simulation of Auger Displacement Pile Installation”,
Int. Journal of Geotechnical Engineering, ANZ2012
Special Edition, (to be published in July 2013).
Mašín D. (2005) “A hypo-plastic constitutive model for
clays”. International Journal for numerical and
analytical methods in Geomechanics, 29(4), 311-336.
Plomteux, C. and Lacazedieu, M. (2007). Embankment
Construction on Extremely Soft Soils using
Controlled Modulus Columns for Highway 2000
Project in Jamaica, Proceedings of the 16th Southeast
Asian Geotechnical Conference, Kuala Lumpur.
Scheuermann, A. and Huebner, C. (2009), On the
feasibility of pressure profile measurement, IEEE -
Instrumentation and Measurement Magazine, 58,
467-474.
Scheuermann, A. et al (2009), Spatial time domain
reflectometry and its application for the measurement
of water content distributions along flat ribbon cables
in a full-scale levee model, Water Resour. Res., 45,
W00D24, doi:10.1029/2008WR007073.
Simon, B. and Schlosser F. (2006). Soil reinforcement
by vertical stiff inclusions in France. Symp. Rigid
Inclusions in difficult subsoil conditions, 11-12 mayo,
Mexico, 22p.
Slatter, J. W. (2000). The fundamental behaviour of
displacement screw piling augers. Department of
Civil Engineering. Melbourne, Monash University.
PhD thesis.
Vermeer P. (2008) “Screw piles: construction methods,
bearing capacity and numerical modelling“,
Bautechnik 85, Heft 2.
Viggiani, C. (1993). “Further experiences with auger
piles in Naples area.” Proceedings Deep Foundations
on Bored and Auger Piles II. Balkema, Rotterdam:
445 - 455.
Wong, P. and Muttuvel, T. (2011). Support of Road
Embankments on Soft Ground using Controlled
Modulus Columns. Proc. Int. Conference on
Advances in Geotechnical Engineering, Perth, 621-
626.