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

Behavior of a Large Diameter Piles Subjected to Moment and Lateral Loads
Vijayakanthan KUNASEGARAMPhD Student, Dept. of Civil and Environmental Engineering, Tokyo Institute of Technology, Tokyo, Japan
Email: vijayakanthan.k.aa@m.titech.ac.jp, heshuvijay@gmail.com

Wei Hsuan HSIAO
Master’s Student, Dept. of Civil and Environmental Engineering, Tokyo Institute of Technology, Tokyo, Japan

Sakae SEKI
Technician, Dept. of Civil and Environmental Engineering, Tokyo Institute of Technology, Tokyo, Japan

Jiro TAKEMURA
Associate Professor, Dept. of Civil and Environmental Engineering, Tokyo Institute of Technology, Tokyo, Japan

ABSTRACTThis paper discusses the influence of embedment depth and embedded medium on the behaviour of laterally loaded large
diameter steel tubular piles with relatively large moment loads based on centrifuge model tests under cyclic lateral loading.
Embedded media tested were Toyoura sands with 80% and 95% relative densities, and a soft sand rock. From the loading
tests, two failure modes of the pile foundation were observed, i.e., ground failure and pile structural failure depending on
the embedment depth and ground strength. Due to the large moment load and rigidity in the embedded portion, the latter
failure mode tends to dominate in hard mediums. For the pile with relatively small embedment depth, the increase ofembedment depth can increase the lateral and moment resistance. However, as the embedment depth increases, the effectof the depth becomes less significant, especially for ultimate resistance due to the pile structural failure. “Optimum
embedment depths” over which the increase of horizontal resistance becomes less are, depending on the initial lateral
stiffness and ultimate bearing capacity, shallower for that of the stiffness than the ultimate bearing capacity. From theabovementioned observations, it was confirmed that the determination of the rational embedment depth considering given
conditions is the most critical step in the design of this type of pile foundation.
Key words: Large diameter piles, Embedment depth, Soft rock, Dense sand, Centrifuge model, Cyclic lateral loads

1. Introduction
Wind turbines are one of the promising sustainable

to massive moment loads has not been well understoodenergy sources in the 21st century. Among various types

observe the behaviour of piles, which are controlled byof foundations, mono-pile foundation, namely a large

several influential factors, such as pile factors (diameter,

diameter pile, is a desirable option for a wind turbine due

stiffness, embedment depth), ground factors (strength andto its simplicity in construction. Thanks to high structural

stiffness), and loading factors (loading height, monotonic

stiffness and strength, large diameter steel tubular piles

and cyclic). On the other hand, conventional design

can be applied for various large geotechnical structures,

methods based on p-y curves often yield a large

such as a mono-pile foundation and a large height self-

installation depth with a high margin of safety, which

standing retaining wall. A typical loading condition to themight not be an economical solution for the large diametermono-pile foundation is relatively large moment loads due

piles. Therefore, the development of a rational design

to high elevation of lateral loading from the turbine.

method for the very stiff large diameter pile foundation is

However, the behaviour of large diameter piles subjected

required, in which several factors and potential criticaldue to the difficulties of conducting large scale tests to- 133 -

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

behaviours are taken into account. For the application of

Bending strains were measured by using full bridgethese structures with rational embedment depths, thecircuits along the pile, while at the pile tip a pair of halfstability against the imposed lateral and large moment

bridge circuits were utilized to measure the axial strains atloads must be secured by the resistance from thethe loading side and the opposite side independently. For

embedded medium.

the sand model, model grounds with 390 mm thickness

This paper reports the behavior of single steel tubular

were made using Toyoura sand with relative densities ofpiles subjected to lateral and moment loads, based on 50g

80% (Model-1) and 95% (Model-2) by air-pluviation as

centrifuge model tests. Influences of embedment depths,

shown in Fig. 1. Upon completion of the model groundembedded mediums and the loading history on thewith desired densities, three model piles with different

behavior of single piles embedded in stiff grounds wereembedment depths (de=130mm, Pile-S; 180mm, Pile-M;

investigated.

230mm, Pile-L) were installed vertically in the model
ground with the help of pile guide. The section and

2. Centrifuge models and test procedures
Three different ground conditions were made in themechanical properties of piles which were used in Model1 and Model-2 are identical.

centrifuge models, such as medium dense sand, dense
Pile cap

sand and soft sand rock. Setups of centrifuge model tests
for piles embedded in Toyoura sand with two different

Loading jack
VH

LDTs
VH

VH

relative densities (80% & 95%), and soft rock are shown130

in Fig. 1 and Fig. 2 respectively. Detailed mechanical
2@30

2@50

properties of Toyoura sand were reported by Tatsuoka etal. (1986) including the effects of density, and thedescribed in Kunasegaram et al. (2015).

8@40

400

mechanical properties of model soft rock used was

5@25

de=180
5@30

de=230

50 G

20
20

Different containers were used for the sand models

γd=15.6kN/m3

20

and the rock model, with the internal dimensions of

de=130

200

200

200

200

802mm length, 400mm depth and 250mm width, andBending moment strain gauge

Dr = 80% & 95%

700mm length, 500mm depth and 150mm width

Axial strain gauges

All dimensions are in mm

respectively. Model tubular piles used in this study were

800
Experiment set up for piles embedded in Toyoura

thin wall pipes with 40mm outer diameter and 0.5mm

Fig. 1

thickness, made of stainless steel (SUS304) having thesand Model-1 and Model -2

young’s modulus (E) of 193 GPa and yield stress (σy) of

Potentiometer
Load cell

Pile cap

VH

255 MPa. At the pile top, aluminum made solid circularpile cap with 30mm socked depth was tightly fixed to form

VH

Pile-L

Pile-M

Pile-S

de=80

60

40

PL

LoadingjackhL=130a solid loading head. Sectional and mechanical propertiesof steel tubular piles are described in Table. 1. The model
190

piles were equipped with bending strain gauges on bothγd= 16.8 kN/m3
Sand rockqu=1.4 MPa

Wheatstone circuits were made with the help of bridge

200

150

200

Bending moment strain gauge
Tension /Compression strain gauges
Transverse strain gauge

so that the piles could experience the stresses below the

All dimensions are in mmyield strain. Once the calibration completed, the strain
gauges were coated with a thin film of epoxy resin in order700

to prevent the damaging of strain gauges under highFig. 2compressive stresses at 50g centrifugal acceleration.

Model-3- 134 -

Es=400 ~ 600 MPa
150

Acrylic plates

300

piles were calibrated with known weights and a lever arm

500

sides of the pile along the loading direction andboxes. Up on completion of the bridge arrangement, test

50 G

Experiment set up for piles embedded in soft rockProceedings of the First International Conference on Press-in Engineering 2018, Kochi

Table 1.

Experiment conditions and properties of steel tubular pile ( Model scales are given in parenthesis)

EmbeddedmediumProperties of
embedded medium

Pile
Notation

Embedment
depth (m)Model-1
Toyoura sand
Dr=80% (MS)γd=15.5 kN/m3∮’=40oModel-2
Toyoura sand
Dr=95%
Model-3
Soft sand rock(SR)γd=16.0 kN/m3∮’=43o

MS_S
MS_M
MS_L
DS_S
DS_M
DS_L
SR_S*
SR_M
SR_L

6.5 (130 mm)
9 (180 mm)
11.5 (230 mm)
6.5 (130 mm)
9 (180 mm)
11.5 (230 mm)
2 (40 mm)
3 (60 mm)
4 (80 mm)γd=16.8 kN/m3
qu=1.3-1.4 MPa
Es= 400-600MPa

Loadingheight (m)Piledimensions

Φ=2 m
t = 25 mm
6.5(130mm)

Φ=40 mm
t=0.5 mm

EI (GNm2/m)
My & Mp (MNm)
EI=14.6(2.34x10-6)
My=19.3(0.154x10-3)
Mp=24.8(0.198x10-3)

EI: Pile flexural rigidity, My: Bend moment causing pile yielding, Mp: bending moment causing pile plastic failure,∮’: friction angle from Triaxial compression with σ3’=98kPa(Tatsuoka et al, 1986), *Preloaded prior to the test without instrumentation

Centrifuge model arrangement for three single(δt) and rotation (θt) at the pile top was obtained by means

piles embedded (de=40mm, Pile-S; 60mm, Pile-M; 80mm,

of LDTs at two elevations as described in Fig. 1. In Model

Pile-L) in soft sand rock is described in Fig. 2. In the3, due to the less accurate measurements by LDTs, the pile

preparation of soft rock model, 300mm thick acrylic plates

top displacements were only measured by using thestack was tightly placed in the container bottom to reduce

potentiometer attached to the loading jack as described in

the depth to 200mm. A 190 mm thick layer of soft sandFig. 2.

rock was made by compacting sand-clay-cement mixture

Having completed one loading test, the centrifuge

layer by layer; meanwhile the target unit weight of each

was once stopped and the loaded pile was removed.

30mm layer of compacted mixture was confirmed.

Resetting the jack and LDTs to the next pile and the same

Immediately after casting the mixture, the model tubular

horizontal loading was repeated. The loading waspiles were installed vertically into the unsolidified mixture

conducted in the sequence of Pile-S, Pile-M and Pile-L. In

with a pile guide to the specified depth, and fixed the pilethe loading of Pile MS_L of Model 1, due to the trouble ofposition. The casted mixture was cured for 14 days inloading jack, unloading at the small displacements couldorder to achieve the targeted strength (qu) and stiffness of

not be made. While the pile SR_S of Model 3 wasthepreparation

accidently preloaded about 1mm prior to the test without

procedures and mechanical properties of the soft sand rockembeddedmedium.

The

detailed

instrumentation, the stiffness and resistance could not bematerial are reported by Kunasegaram et al. (2015). It is

obtained in the intact condition for the small displacement

important to note that sand and soft rock was filled inside

range. In the following chapter, the test results are shown

the pile up to the ground surface level with the pilein prototype scales.

installation process employed in the model preparation
process, which was confirmed by means of physical

3. Results and discussion

measurements.
After 14 days curing for the soft sand rock or upon

3.1. Load-displacement and failure behaviour
A typical observed cyclic load (PL) - displacement

the completion of sand model, the loading jack and lazar(δt) curve is presented in Fig. 3. In each cycle, certain

displacement sensors (LDTs) were mounted on theamount of residual displacements can be seen after

container. Thereon the container moved to centrifuge

unloading. However, in the reloading processes the P L-δtplatform andcentrifugal

curves return to a unique curve, which is the envelope ofacceleration was increased up to 50g. One way horizontal

cyclic load-displacement behaviour. Here onwards theload cycles were applied by the jack from small to large

envelope curve will be written as a backbone curve.

rigidly fixed,

then thepile top displacements as described in Fig. 3 at 50g

Fig. 4 shows the backbone curves obtained in theenvironment. Applied horizontal load at the pile top (P L)

loading tests of all tested piles (Table 1). Observed

was measured by a load cell and horizontal displacement

structural failures of piles in different mediums are also

- 135 -

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

Applied lateral load (PL) (kN)shown in Fig. 5. The influence of embedded medium andembedment depth under identical loading conditions can
be clearly confirmed from Fig. 4. The deeper theembedment is, the larger the mobilized resistance of pilecan be observed. However, this observation is not true
while comparing the behaviour of piles DS_L and DS_M.
There is no significant difference between the two piles,
implying less effects of embedment depth increase from 9

4000

∆PL

Ei

Imposeddisplacement

3000

PLpi

Backbone
curve

∆δt

2000
1000

m to 11.5 m in the dense sand. As shown in Figs. 5(a), (b)00and (c), structural failures of the pile were confirmed with

50

100

150

200

250

Residualdisplacement Pile top displacement (δt) (mm)clear local buckling point below the ground level for
MS_L, DS_M, DS_L piles. Once the pile failed by theFig. 3300

Typical cyclic load-displacement behaviour

structural buckling, the further increase of embedment
4000

MS_S

sand) could have no significant influence on the lateral

3500

MS_M

3000

MS_L

2500

DS_S

2000

DS_M

Lateral load (PL) (kN)depth (de >11.5 m for medium dense, de >9m for dense
resistance of piles for abovementioned loading conditions.
The reductions of resistance after the peak load are theindication of structural failure, while for MS_S, MS_M,
and DS_S piles, the resistance increased until large pile
top displacement over 50% of pile diameter (Φ) without
showing peak resistance.

DS_L

1500

SR_S*

1000

For the piles embedded in the soft sand rock, 1 m

500

increment in embedment depth can significantly increase

0

SR_M
SR_L
0

500

the ultimate lateral resistance up to de=4m. As a clear

1000

1500

Pile top displacement (δt) (mm)structural failure with local buckling was observed slightly
Fig. 4
Envelope curves observed from cyclic loaddisplacement behaviour (Back bone curves)

above the ground level for SR_L pile with 4m embedment
depth as shown in Fig. 5(f), it can be inferred that further
increase of the embedment could not contribute to theincrease of pile lateral resistance.
The depth over which the effect of embedment
increment cannot be obtained is considered as an(a) MS_L“optimum embedment depth”. The difference of optimum(c) DS_M(b) DS_L

embedment depth in the medium dense sand, dense sand,and soft rock can be attributed to their rigidity or(f) SR_L

confinement. The soft rock can effectively hold the pile at(g) SR_L

shallow depth even for the relatively stiff large diameterS

pile.
No structural failures were observed in SR_S andM(d) DSLL

M

S(e) SR

SR_M piles in the soft rock (Fig. 5(e)), but the load Fig. 5 Pile deformation and failures, and ground failure

displacement curves of the two piles also showed clear
peak resistance and the reduction. The peak load
displacement of SR_S was smaller than that of SR_M.
These behavior of the piles with no pile failure but groundfailure of the soft rock is different from that of the sand.
This can be attributed to the stain softening ofstress-strain relationship of the rock material and smaller
embedment depth of the soft rock model than that of sandmodel. Although the piles with structural failure and thepiles embedded in the soft rock with relatively small d e- 136 -

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

showed

load-

4500

displacement curves, the post peak resistance reduction

strainsofteningbahaviour

in

the

4000

are different between the piles failed by the structural
initiated a sudden reduction of load against thedisplacements can be seen. On the other hand, theobserved post peak behaviour related to ground failures(SR_S*, SR_M) exhibited smaller post peak stiffness and3500

Lateral load (kN)failure and the ground failure. Once the structural buckling

1%Ø
2%Ø
4%Ø
6%Ø
10 % Ø
P_ult

3000

2500
2000
Model-2

1500

much more ductile especially for the deeper embedment

1000

case (SR_M)).

500

Model-3

Model-1003.2. Influence of embedment depth

2

4

6

8

10

12

Embedment depth (de) (m)The influence of embedment depth (de) on lateralFig. 6
Influence of embedment depth (de) on lateralresistance of piles embedded in sand and soft rockresistance of piles embedded in different mediums fromsmall to large pile top displacements (δt) are depicted inFig. 6. It is important to note that the load corresponding

The resistance of SR_L is almost the same and slightly

to the pile top displacements of 50%Ø was considered aslarger than that of SR_M piles at δt =1%Φ and 2%Φ

ultimate resistance (P_ult ) for the piles without peak

respectively. At the small pile displacement the shallower

resistance in the load - displacement relation (Fig. 4). A

depth of the rock (less than 3m) could mainly confine thedistinctive behavior of piles at different displacement

pile by the large stiffness of intact rock. But the softening

levels depending on the stiffness of embedded mediumof rock by the displacement deteriorates the subgrade

can be observed. In Model -1 (MS), as the embedment

reaction of shallow depth and cause the difference in

depth increases, the lateral resistance also increases fromresistance between SR_M and SR_L piles.

small to large displacements. However, the increment

As overall behavior it can be concluded that thetrends of resistance from de =6.5m to 9m depth and de =9m

lateral resistance of pile embedded in sand and soft rockto 11.5m change with imposed displacement. At the small

increases with increasing embedment depth if the grounddisplacement ( δt <2%Φ) the resistance increases linearly

stiffness dominates the resistance.

with de, while at δt =4~10% Φ , the larger resistanceof the embedment depth over the optimum depth has noincrease can be seen for the increment of de from 9m tosignificant contribution.

However, the effect11.5m than that of 6.5m to 9m. However, the effect offormer depth increment on P_ult become less than that of

3.3. Bending behaviour of pilethe latter increment. This is a clear evidence that the pile

Measured bending moments along the pile athorizontal resistance or stiffness was mobilized by the soil

different pile top displacements (1%, 4%, 10% & 20% Ø)

in the early stage of loading but controlled by the strengthfor sand and soft rock models are presented in Fig. 7.

and stiffness of pile at the ultimate conditions as discussed

Depths at the maximum bending moment for the piles

in Fig. 4 and Fig. 5(a) for the pile (MS_L).

In Model-2

embedded in sand (MS, DS) are about -0.5 m for pile-S(dense sand), the piles with de=9 m and 11.5 m show

(de=6 m),-2 m for pile-M (de=9 m) and -2.5 m for pile-L

almost no difference in the mobilized resistance from(de=11.5 m). The bending moments of the pile in the sandsmall displacement to ultimate values.

models increase almost linearly with the distance from the

The horizontal resistance of piles embedded in theloading point to the depth of maximum bending moment.

soft rock also increased with de. But the trend is quite

The observation clearly indicates that the confining

different from that in sand.

Large increase of lateralstresses from shallow layers of the sands has lessresistance can be attained by increasing de from 2m to 4m,

significant influence on the lateral resistance of piles. On

almost linearly with d e for all displacements, except

the other hand, the piles in the soft rock showing thethe piles with de over 3m at small displacements.

maximum moment was observed at slightly above- 137 -

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

Model-1 (MS)Height from embedment level (m)

6

My

4

Mp

Model-3 (SR)Model-2 (DS)

My=19.3MNm

My

Mp

My

Mp=24.8MNm

Mp

2
0
-2
-4
-6

1% Ø
4% Ø
10% Ø
20% Ø
Pile_S
Pile_M
Pile_L

-8
-10
-12
0

-5

-10

-15

-20

-25 -30-40-50

the ground surface.

-5

-10

-15

-20

-25

Pile_S

4% Ø

Pile_M

10% Ø

Pile_L

-5

-15

-10

-20

-25

-30

Bending moment (MNm)

Measured bending moment profiles at different pile top displacement

An abrupt change in the bending
Height from embedment level (m)observed. This observed bending behaviour is anotherevidence of very high confinement of shallow layers ofsoft rock.
The depths of local buckling were 1.5m in MS_L,
1.0m in DS_M&L, for the sand model and 0.15m above
the ground surface for SR_L, which were shallower thanthe depth of maximum bending moment. This could

2
0
-2

-4
-6
-8
-10

ground surface. Therefore, without this plugging effect,-12

Fig. 8Up to δt =4% Φ (80 mm), all piles show themedium dense sand exhibit the bending moments slightly
below and about the yielding moment respectively.
However DS_M and DS_L piles in the dense sand withsimilar embedment depths reached the bending moments

DS_M and DS_L had already reached the plastic limit,
therefore this is a clear evidence for the structuraldeformation observed in Fig. 5(a, b &c). The pile SR_L
exhibits the bending moment higher than the plastic limit
at a pile top displacement of 10% Ø.As the stiffness of theground increases from MS, DS to SR, the pile reaches its
yielding and plastic limits at small displacement, which
increases the possibility of structural failure over the soil

-1

-1.5 0

-1

-2

-3

-4

-5

-6

-7

Bending moment (MNm)

Measured bending moment profiles at different

It can be also confirmed that the increasing embedment
depth over the optimum depth in relatively hard
embedment mediums cannot prevent the ultimate failure
since it is caused by structural hinge. In other words, it canbe said that the increasing section modulus can increase
the optimum depth and the ultimate resistance.
Measured bending moment profiles observed at thecorresponding to yielding and plastic hinge (Mp)respectively at δt =10%Φ. At δt =20%Φ, piles MS_L,-0.5

Pile_S
Pile_M
Pile_L

Bending moment (MNm)

have been smaller than the observed resistance.

At δt =10%Φ (200 mm), MS_M and MS_L piles in the

MS
DS
SR
0

ultimate resistance of the structurally failed piles could

ML=5 MNm

4

indicate the effect of infilled materials in the pile up to thebending moment smaller than the yielding moment (My).

ML=1 MNm

6

moments above and below the ground surface can be also

failure in hard mediums.

1% ØBending moment (MNm)

Bending moment (MNm)

Fig. 7

0

1% Ø
4% Ø
10% Ø
20% Ø
Pile_S
Pile_M
Pile_L-30 -40 -60 -80 0

moment load of 1 and 5 MNm. The moment load definedby horizontal load times the loading height at the groundsurface is compared for all the piles in Fig. 8. Some strain
gauges of the short piles in the two sand models did notwork. Excluding the short piles, similar bending moment
profiles were measured in the medium and long piles inthe two sands respectively. The difference between the
middle and long piles are more evident, i.e., the longer thepiles are, the lager the moment in the deep portion
becomes. This implies that as the load increases, the- 138 -

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

Residual displacement (mm)resistance mobilization at the deep depth becomes larger
relative to the shallower depth. While in the soft rockmodel, 3m and 4m embedded piles showed almost thesame bending moment from the loading point up to -2mtill the moment load of 5MNm and very small bending
below this depth. Thanks to the large stiffness and strengthof the intact rock, the observation implies that the large
horizontal resistance concentrated at the very shallow

400

MS_S
MS_M
MS_L

300

DS_S

DS_M

200

DS_L
SR_S

100

SR_M
SR_L

0
0

depth against relatively small load.

100 200 300 400 500 600

Imposed displacement (mm)Fig. 9 Relationship between imposed and residual
displacement

3.4. Residual displacement

Residual displacement (mm)Fig. 9 illustrates the observed relationships between
imposed and residual displacements (see Fig. 3). Despite
the embedded medium, almost linearly increasing trend ofresidual displacements against the imposed displacementswith different rates of increment can be observed. The
larger the embedment depth is, the smaller the observed
residualdisplacement

under

the

same

imposeddisplacements. From this result, it can be said that the long
piles behave as flexible meanwhile the behaviour of short

400

MS_S
MS_M
MS_L

300

DS_S
DS_M

200

DS_L

SR_S

100

SR_M
SR_L

0
0

pile is rigid. In the initial stage of imposed displacements,

0.2

0.4

0.6

0.8

1

almost zero residual displacements can be observed for theNormalized peak load (PLp/Pult )piles SR_M and SR_L, which could be attributed to theFig. 10 Relationship between residual displacement andnormalized load

elastic recovery of the piles in soft rock. However, thepiles in sand exhibits certain amount of residualbe said that the displacement should be the dominantdisplacements even at small imposed displacement, thisfactor compared to the ultimate failure for the soft ground,phenomenon can be caused by local densification of soil

while for the stiff embedment medium, securing the factor

behind the pileof safety against failure tends to be a critical condition inwhich could prevent the elastic recovery.

Large diameter piles are being used as mono-pilethe foundation design.

foundation for wind turbines, where one way cyclic loads
by wind forces are the typical loading condition. Due to

3.5. System stiffness and residual displacements

one-way cyclic loads, the accumulation of residual

Observed system stiffness (Ei) of each cycle isdisplacement and permanent movement of foundation andplotted against the applied pre-maximum load (PLpi) in the

super structure could be a major concern for a long term

previous cycle in Fig. 11. It should be noted that in SR_S

stability of such large geotechnical structures.

pile the observed stiffness was affected by the

In order to discuss the ground stiffness and theunintentional preloading, and in MS_L pile the stiffness

influence of embedment depth on the permanent

under small load could not be measured, but only

deformation of mono-pile foundations, the relationshipmeasured after the large loading history.

between residual displacement at the pile top and theeffects of embedment medium and depth can be clearlynormalized applied pre-maximum load in previous cycle

confirmed from the figure. The initial stiffness very much

(PLpi, see Fig. 3) by the ultimate load (Pult) is presented in

depends on the ground stiffness, much higher for the soft

Fig. 10. PLpi /Pult is a kind of load factor. As an overall trend,rock compared to sands, even though the embedment

the residual displacements for the same load factor are

depth of the rock was about one third of that of the sand.

reduced by increasing the ground stiffness from medium

However, the significant deterioration of the stiffness tookdense sand, dense sand to soft rock. From this trend, it canplace for the soft rock, but not in the sand. To scrutinize- 139 -

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

2.5

MS_S

60

Normalized Stiffness (Ei/E0)

Stiffness in ith cycle (Ei) (MN/m)

70
MS_M
MS_L

50

DS_S

40

DS_M

30

DS_L

20

SR_S*

10

SR_M

*: subjected to preloading

SR_L

0
0

1000

2000

MS_S
MS_M

2

MS_L
DS_S

1.5

DS_M

1

DS_L
SR_S *

0.5

SR_M*: subjected to preloading

0

3000

SR_L

0
0.2

0.4

0.6

0.8

1

Peak load in previous cycle (PLpi) (kN)

Normalized peak load (PLpi /Pult )Fig. 11 Variation of stiffness in each cycle with applied peakload in previous cycle

Fig. 12 Relationship between normalized stiffness andnormalized peak load in previous cycle

the change of the system stiffness by the cyclic loading

depth. On the other hand, due to the softening of groundhistory, the normalized stiffness by initial one (E i /E0) is

materials and the change of failure mode from the groundplotted against the normalized peak load (PLpi /Pult) in

failure to pile structural failure, the optimum embedment

Fig. 12. From this figure, the larger increase of thedepth changes depending on the conditions of resistance,

stiffness, in other words the effect of hardening can beshallower for the initial stiffness than the ultimate bearing

seen for the medium dense sand than the dense sand, andcapacity in the soft rock.

also for the short pile than the long piles. While in the soft

From the observations, it can be confirmed that therock, the initial stiffness of middle (de=3m) pile and longdetermination

of

the

rational

embedment

depthpiles (de=4m) are almost the same, but the stiffness

considering given conditions is the most critical step in thedeterioration was more significant for the middle pile thandesign of this type of pile foundation.

the long pile. However, the deteriorated stiffness at P Lpi/Pult =0.5 were about two times larger than that ofincreased stiffness of pile in the sands.

5. Acknowledgements
The authors gratefully acknowledge the invaluable advice

As discussed on the lateral resistance in Fig. 6, overand guidance provided by the members and advisers of thea certain embedment depth, the effect of the depth

IPA TC1 (Committee on Application of Self-retaining

becomes less significant, especially for ultimate resistance

Tubular Pile Wall to Stiff Ground) in connection with thedue to the pile structural failure and “optimum embedment

preparation of this paper.

depths” can be defined. However, the optimum
embedment depth depends on the conditions of resistance,

References

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Association of Steel Pile. 2009. Design manual of self-standing

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4. Conclusions

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embedment depth. A small increment of embedment depthK., Ishihama, Y., and Fujii, Y. 2015. Modeling of soft rockin soft sand rock highly controlled the displacement andfor a centrifuge study, Proc. 12th GeoKanto, pp. 15-19.

failure behaviour of the piles compared to the increment

Tatsuoka, F., Goto, S. and Sakamoto, M. 1986. Effects of somein the sand soil. However, over some depth, the increment

factors on strength and deformation characteristics of sandof pile length becomes less significant on the increase of

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resistance. This depth can be considered as an optimum

114.

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