Stress and displacement monitoring of auger displacement ... et al... · STRESS AND DISPLACEMENT MONITORING OF AUGER DISPLACEMENT ... is able to deform the meshed continuum, thus

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.

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