Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Stress Changes due to Shape Effects in the Construction Process of Pile Walls
Shinj TAENAKA
Chief Researcher, Steel Structures Research Laboratories, Nippon Steel & Sumitomo Metal Corp., Chiba, Japan
Associate Professor, Kanazawa University, Kanazawa, Japan
Email: Taenaka.2m2.shinji@jp.nssmc.com

David J. WHITE
Professor, Faculty of Engineering and Environment, The University of Southampton, Southampton, United Kingdom

Mark F. RANDOLPH
Professor, Centre for Offshore Foundation Systems, The University of Western Australia, Perth, Australia

ABSTRACTThe shaft friction of displacement piles in sand depends on the change in the horizontal stresses on the pile shaft during
installation. Recent research has revealed that the sectional shape of a pile has a strong influence on the horizontal
stresses, leading to great difference of the pile shaft friction. Based on these findings, an optimization idea has been
proposed, referred to as the “switched-on mechanism”. The main emphasis in this paper is given to understanding thechanges in horizontal stresses measured during “switching-on” from open-section (installation individually) to closed(load testing for whole pile walls) in cross section. In order to model the construction and load test process for pilewalls, a dual installation system was installed into the beam centrifuge. This paper discusses the stress changes throughthe whole construction process and the efficiency of the switched-on mechanism, and highlights how physicalmodelling has allowed this highly complex problem to be quantified.
Key words: Stress change, shape effects, Pile walls, Centrifuge modeling

1. Introduction
The shaft friction of displacement piles in sand(Fig. 1) is able to generate high vertical arching due tode-pends on, amongst other things, the change in theeach H-shaped pile is driven individually. Fig. 2 shows aclose cross section, resulting in high load capacity, even if

horizontal stresses during installation. Although there
have been few studies examining the effects of pile crosssectional shape, the authors has investigated the effects –i.e. shape effects– using centrifuge modeling andproposed a theoretical estimation based on verticalarching theory so far (Taenaka et al. 2008; Taenaka et al.
2010). Recent research has revealed that the sectional
shape of a pile has a strong influence on the horizontal
stresses, leading to great differences in the pile shaft
friction.
Based on these findings, an optimization study basedon this effect will be presented in this paper. An
optimization idea has been proposed by White (2002), as“switched-on mechanism”. A continuous wall of H-piles- 387 -

Fig. 1

Continuous H-pile wall (White et al., 2007)

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Reaction piles: loadedin tension by silent piler

Jacking
force

idea for optimizing pile foundation systems in which a

Working load

pile should be driven easily but should mobilize highaxial capacity.
In order to model the construction and load test
process for pile walls as Fig. 2, a dual installation system
was installed into the beam centrifuge. The centrifuge

Direction ofConstruction
sequence

testing program compared the

behavioroftwodifferent-shaped piles, which were prepared in respect tothe optimization of the cross sectional geometry of a pile.
The main emphasis is given to the changes in horizontal
stresses measured during “switch” in cross section. The

Minimal plugging in pile pan
due to open section

Fig. 2

measured stress changes were due to the shape effect of a

Full plugging in enclosed box
due to close section

single pile, the effect from the switch of the cross section

Continuous H-pile wall and the construction sequence

as well as interaction effects. This paper discusses the(White et al., 2003 & White et al., 2007)

stress changes through the whole construction process andthe efficiency of the switched-on mechanism, andhighlights how physical modeling has allowed this highly

Installation

Working loads

complex problem to be quantified.
ts

s’hv

ts

ts

ts

s’hv

ts

s’hv+ ds’hv

s’hv+ ds’hv

Opened-arching

Closed-archingts

2. Background2.1. Shape effectO’Neill (2001) showed that the stress transfer

tsmechanism under compressive loading can lead to ameasurable increase in the vertical stress around a pile,Fig. 3 Vertical force equilibrium during installation and inwhile the inverse occurs under tensile loading. In the sandunder working loads

in the vicinity of a circular pile, the cumulative downward
schematic drawing during installation and under working

vertical stresses (caused by the shaft resistance) can beloads. The first installed H-shaped pile, where the crossdissipated rapidly, with support provided by the far fieldsectional shape is open as shown in the upper figure,soil. Therefore the local stresses may not rise significantly.

mobilizes a weak arching effect, leading to a gentle

However, the phenomenon is likely to depend on theincrease in local shaft friction from “open arching”,

cross-sectional shape. The opposite extreme of stressperhaps without plugging. After the second pile istransfer is the inside of the tubular pile. In this case theinstalled, the cross section becomes closed between

cumulative vertical stresses develop rapidly, often leading

adjacent pairs of piles, resulting in the strong arching

to soil plugging inside the pile.

effect when both piles are loaded simultaneously. This

To investigate the shape effect, Taenaka et al. (2008)

strong arching effect perhaps leads to full plugging

proposed the estimation method based on the verticalbetween close-section of two H-shaped piles and exhibits

force equilibrium as shown in Fig. 3 according to themuch higher end-bearing capacity under working loads,

simplified arching theory proposed by Janssen (1985),

shown in the bottom figure, in comparison to thegiving a shape factor to describe difference in the crossinstallation resistance. This concept means that the axial

sectional shape of a pile. Following the proposed theory,

capacity of a pile can be varied if the effective crossfor example, an H-shaped pile developed much greater

section of the pile is changed from under construction tostress level within the concave area than an X-shaped pile,under working loads, and is therefore a quite fascinating

which has been observed in the centrifuge modeling by
Taenaka et al. (2010), as shown in Fig. 4.

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Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Horizontal Stress, s’hm
0

100

200

300

400

effect perhaps leads to full plugging and exhibits much
500

600

higher end-bearing capacity under working loads. Indeed,0

it is reported that the capacity of each pile within the walls
Instrument depth (mm)20

is, around 50%, greater than single pile from the field tests(White et al. 2003). These results confirm that enhanced

TF

40

axial capacity can be activated after the installation of an
H-pile wall due to switched-on in the cross sectional

60
Increase withincrease of
shape factor

geometries.

80

This switched-on mechanism could be explained by
LH

the theory of the shape effect. Furthermore, this concept

100
SQ

means that the axial resistance in a pile could be variable if

SH
RC

120

LX

SX

the cross section of the pile be changed from construction

Fig. 4 Profiles of horizontal stresses measured near the pile tip

to service stages. However, there have been few

during installation (after Taenaka et al., 2010)

investigations in detail for the mechanism. The centrifuge
modeling, which would be the first trial, is described in

Pile

this paper.
Local shaft friction
Closed-arching is mobilised by
the downward shaft friction,
leading to plugging sometimes.

3. Test Methodology
3.1. Dual pile installation systems
The University of Western Australia geotechnicalbeam-centrifuge was used in this investigation at 20 g

Depth

Opened-arching is
weakly moblised by thebalance of the downward
shaft friction and upward
shear friction.

Fig. 5acceleration. Sand samples in strong box (internal
dimensions of W390 x B650 x H300 mm) were preparedby air pluviation, resulting in medium sand layer (Silica

Activation

sand: Gs = 2.65; D50 = 0.20 mm; Dr ~ 0.55). To modeling

Switched-on mechanism
occurs due to change
from open-section to close-section“switch”, two actuators was mounted in parallel acrossthe width of the strong box as shown in Fig. 6. The

Comparison of the shaft friction distribution on a single

model piles were placed adjacently between two

H-pile and H-pile walls and vertical force equilibrium

actuator-bases by the arm parts mediated for shifting theat each stage

positions, although a model pile usually grasped by theactuator directly. The three pressure sensors on each
instrumented pile were set face-to-face to measurechange in horizontal stresses. The instrumented pile with

2.2. Switched-on mechanism
An individual H-pile is installed immediatelypressure sensors was designed in the same way as

adjacent to the previous. Fig. 5 shows a schematic

addition, a settlement bar was used to quantify settlementdrawing for shaft friction distribution during installationof the adjacent soil relative to the model piles during

and under working loads. The installed single pile, where

testing via laser transducer above the top of thethe cross sectional shape is open as shown in the left Fig. 3,

settlement bar.

previous similar studies (e.g., Lehane & White, 2005). In

mobilize the weak arching effect, leading to a gentle

During the switched-“on” mechanism, the first pileincrease in local shaft friction, perhaps with no plugging.

fixed to the actuator-1 was normally installed at 0.2

A completed wall of H-piles consists of square box

mm/sec to specified depth and then halted at this depth

sections (close section) between adjacent two piles,(Fig. 7). Subsequently, the second pile fixed to theresulting in strong arching effect. This strong archingactuator-2 was installed to the same depth without the- 389 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

9.0

4.0

1.5

Wing
plates.5
10Wing plates
Instrumental pile30.0

H-shaped pile
Pressure sensors

Closed section
while pile moving together

two H-shaped piles

Fig. 8Fig. 6

23.5

40°

1.5

13.0

10.0

Instrumental pileWing
plates4.0
25.0

X-shaped pile
Pressure
sensors

Closed section
While pile moving together

two X-shaped piles

Cross sectional shapes of model piles

Setting of dual pile installation systems for theplates were bolted to both sides of the plate pile (Fig. 8).

switched-on tests

The pressure sensors were placed at 10 mm (h/B = 1),
60 mm (h/B = 6) and 120 mm (h/B = 12) behind the pile
tip on the 1st pile and at 10 mm (h/B = 1), 30 mm (h/B = 3)
and 60 mm (h/B = 6) behind the pile tip on the 2nd pile tomeasure changes in the horizontal stresses within the
concave area. For the pile wall tests, the face where thepressure sensors were mounted facing to each other asshown in Fig. 8.
In addition, X-shaped piles were prepared formodeling the switched-on mechanism only. It is because
X-shaped piles can prevent increase in the horizontal
Fig. 7stresses during a single pile installation, which has been

Test sequence in the switched-on tests

observed by Taenaka et al. (2010). It is therefore expected
first pile moving. Then the load test was performed at
0.02 mm/sec together. On the other hand, theswitched-“off” mechanism is the reverse process, where
the cross section changes from close to open. Thisthat X-shaped piles could be effective to gain more
activate change during the switched-on mechanism (i.e.,lower installation resistance and then higher bearing
capacity).

process can be modeled in the dual pile installation
systems, by which the two piles were installed together
first, and then one pile was halted while another pile was
kept moving.

4. Comparison of open- vs close-section piles
Before investigation of the switched-on mechanism,
the installation tests for a double H-shaped pile (DP) are

3.2. Model piles
Two plate piles with 4 mm in thickness (t) and 10 mm
in width (B) were used as instrument piles, which haveminiature total pressure cells embedded on the pile shaft
surfaces. These piles made it possible to change the crosssectional geometries by attached extra pieces to the side
wall, for example, to model an H-shaped pile, both thincompared to those for a single H-shaped pile (SP). The
wide plates connected two instrument piles, forming one
rigid double H-shaped pile. Settlement of ground adjacentto the pile or in the enclosed area during installation wasmeasured as shown in Fig. 9. Settlement started to occur
at 25 mm in pile installation and the measurement
settlement in the double H-shaped pile was greater than in- 390 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Fig. 9

Soil plug progression within the internal and concave

Fig. 10Comparison of horizontal stresses with the height ofsoil column between open- vs close-section

area in double H-shaped pile and single H-shaped pile
respectively

single H-shaped pile due to downward drag force caused

rapid increase was measured as the pile tip of 2nd pileby the vertical arching effect.

approachedtheinstrumented

level.

The

detailed

Fig. 10 shows variation of horizontal stresses formechanism behind a stress change will be discussed later.

average value at h/B = 1 and measurements at h/B = 3 for

Measurements were plotted in the same way for X-shaped

open- and close-section piles. In this figure, thepiles in Fig. 12. The general features observed were very

instrument height was expressed in vertical axis, using thesimilar to H-shaped piles.

measurements of settlement. The pro-files of horizontal

The maximum stress measurements were different

stress change were very similar to the schematic drawing

between two tests. For the H-shaped piles in Fig. 11, theof local shaft friction in Fig. 5. It is very clear that the

maximum horizontal stress during installation process

close section in the double H-shaped pile developed

was recorded in the pressure sensor on the 2nd installinghigher horizontal stresses due to strong arching effect,

pile (i.e. on the moving pile), where the maximum

while the horizontal stresses were governed by weakhorizontal stress was developed due to amplification byarching effect in open section of the single H-shaped pile.

shape effects from H-shape. On the other hand, for
X-shaped piles, the maximum horizontal stress wasmeasured in the pressure sensor on the 1st pile (i.e.

5. Switched-on testing

stopped pile) during the subsequent pile coming as shown

5.1. Measured dataset
The switched-on testing of H-shaped piles andin Fig. 12. This maximum stress in X-shaped piles was

X-shaped piles are discussed in this section, in which the

installation rather than by the stress amplification due tocross section of piles was initially open during installation

shape effects.

caused probably by interaction effect by the adjacent pileand then turned to close form after completing installation.
testing time in H-shaped piles. The horizontal stresses at

5.2. Switched-on mechanism
Regarding the switched on mechanism, the stressh/B = 1 on both 1st and 2nd pile increased while the pilechange from installation to load testing needs attention.

itself was installed. In particular, a drop in the horizontal

Ignoring the jump in stress measured on the 1st installedstpile, the horizontal stresses in the load testing withndclose-section was over the values during pile installation

Fig. 11 shows changes in the horizontal stress against thestress at h/B = 1 of the 1 installed pile was initiallymeasured in a while after 2 pile installation. Afterward, a- 391 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

1,600

1,600

1,400

1,200

h/B=1

2nd pile
installingst

2nd installed pile

1,400

1,200

1 pileinstalling

1,000

Horizontal stress, s’h (kPa)Horizontal stress, s’h (kPa)

Load testing

2nd installed pile

1st installed pile

2nd pile
installing

1st installed pile

h/B=1

800

600
h/B=3

400

1,000

Load testing

1st pile
installing

800

h/B=1

600

h/B=3

400

h/B=6

h/B=6

200

200
h/B=12

h/B=12

0
0

1,000

2,000

3,000

4,000

0

5,000

0

1,000

2,000

Testing time (sec)Fig. 11

3,000

4,000

5,000

Testing time (sec)Changes in horizontal stresses during two H-pileFig. 12installations and in the load testing

installations and in the load testing(s’h)avg /qc

the normalized horizontal stresses under the load testing in
X-shaped piles went up rapidly and well over the
maximum value measured during each pile installation,
due probably to a switch of the cross section from open to
close. This result successfully demonstrated the activationof the switched-on mechanism.
However, no big difference was observed between

Normalised displacement, d/B, at load testingin X-shaped piles as shown in Fig. 13. In this view point,
this figure focused on the load testing stage. It is clear that

installation (open-section) and load testing (close-section).
It is expected that even in the single H-pile may develop

Changes in horizontal stresses during two X-pile

0.00
-5.0

0.10

-4.0

0.20

X-piles(h/B=1)

-3.0

0.30

0.40

H-piles(h/B=1)-2.0-1.0

Installationof each pile

0.0
1.0
2.0

Load testing
on double piles

3.0

Fig. 13

Average values in horizontal stresses at h/B =1

similar stress level to the double H-shaped piles, implying

normalized by the cone tip resistance during installationthat the single pile could reach the limit value in stressand in the load tests for H-shaped piles and X-shaped

level form a soil plug.

piles

5.3. Interaction effect
The changes in horizontal stresses at h/B = 1 and 6 on

the X-shaped was small. This is not specific to thethe existing pile (i.e. 1st installed pile) were plottedinvestigation in this paper. Chow (1996) investigated pileagainst distance to the tip of the 2nd pile during
installation in Fig. 14. The horizontal stresses recordedinteraction by recording radial stress changes on an
existing pile during adjacent pile installation, although the

0.29 and 0.18 at h/B = 1 in the H-shaped pile and thepiles were just rod-shape. Careful examination of Chow’s

X-shaped pile respectively during the 1st installation. In

data reveals that there were small stress drop observed

the subsequent releasing process of the head load to zero

before increase of horizontal stresses as shown in Fig. 15.

before the 2nd pile installation, the horizontal stresses

On the other hand, stress field investigations around an

dropped and kept the residual value initially for the 2nd

installation pile have not reported such a stress drop (e.g.

pile installation because the pile tip was too far away from

Leung et al., 1996; Gavin & Lehane, 2003), so that thestress drop was probably specific trend to the pile

the pressure sensors.
The stress drops were observed at h/B =1 both theinteraction. This interaction effect could link to a

H-shaped pile and the X-shaped pile, in which the drop in

difficulty of centrifuge modeling for the switched- 392 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

0.40
Maximum value measured
during 1st H-pile installation

0.35

H-piles(h/B=1)0.30
Maximum value measured
during 1st X-pile installations’h /qc

0.25

H-piles(h/B=6)

0.20
Stressdrop

0.15

X-piles(h/B=6)

X-piles(h/B=1)

0.10
0.05

Stress drop

0.00
-12

-10

-8

-6

-4

-2

0

2

4

Distance normalized by the pile widthFig. 14

Fig. 16

Relative position for two H-piles after load testing

Changes in normalized horizontal stresses on an
existing during an adjacent pile installation

dramatic rise in horizontal (radial) stress on the adjacentpile occurred when the installed pile tip was passing.
Indeed, it was observed that the magnitudes of rise inhorizontal stresses due to interaction effect wereStress drop

sometimes higher than the stress amplification due toshape effects as shown in Fig. 12. Accordingly, the stressfield around the existing pile was expected to beimbalanced with a large difference between the stresslevel occurring between the side facing on installation pileand the opposite side. Fig. 16 shows a photo taken afterFig. 15

Changes in normalized horizontal stresses on ancompleting the switched-on testing. A gap between two

existing during an adjacent pile installation (after Chow,piles was observed because the first pile was subject to

1997)high horizontal pressure due to interaction effect, which
was sufficient to overcome the rotational stiffness of themechanism, which will be summarized in the next section.
After the stress drop, the measurements in horizontal

pile head connection and induce bending in the pile,leading to lateral movement.
As known widely, centrifuge modeling is a powerfulstresses trended upwards to reach the peak stresses a little
bit before the pile tip reached the instrument level. Similar
trends were also reported by Leung et al. (1996) and
Gavin & Lehane (2003). The peak stresses at h/B = 1 weresimilar between the two cross sectional shapes, although
the amplification stresses measured on installation piles
were quite different between H- and X-shaped piles. It
suggested that the maximum horizontal stresses were notdue stress amplification by shape effect, rather determinedby cavity expansion stress from the tip of an adjacent pile

tool to replicate in-situ stress level in terms of soil
behavior depending on stress level and stress history.
From this view point, the switched-on testing has been
well performed and the stress level enhanced by the shapeeffect could be reproduced as similar to the stress level inthe prototype scale. However, the very high lateral loads
were unexpected and the stiffness of the pile headarrangement in the centrifuge model was not adequate tomaintain verticality of both piles at ground level. Also, thestiffness of the model structures did not replicate theduring installation.

stiffness at prototype scale due to the scaling of the small
model piles. In reality, the point of fixity is also lower

5.4. Modeling challenges in the switched-on condition
The interaction effect of the adjacent pile on thedown, close to the ground level, at the piling machine.

existing pile during installation is too great to ignore. A- 393 -In addition, pile wall systems in practical use has

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

interlocks to connect adjacent pile each other (see Fig. 1),

installation system and the instrumented model piles.

which should be utilized to keep the cross section even if
an existing pile is subject to high pressure from themoving pile, at least preventing it from a lateral

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6. Conclusion
The paper focuses on the horizontal stress change

Janssen, H.A. 1895. Versuche uber getreidedruck in

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7. Acknowledgements
The experiments were performed at UWA, and theWhite, D. J. & Deeks, A. D. 2007. Recent research into

authors gratefully acknowledge the excellent technicalthe behaviour of jacked foundation piles. Advances

support provided during the development of the dual pilein Deep Foundations, Yokosuka, Japan, 3-26.

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