materials
Article
Evaluation of the Pullout Behavior of Pre-Bored Piles
Embedded in Rock
Kyungho Park 1 , Daehyeon Kim 2, * , Gyudeok Kim 3 and Wooyoul Lee 3
1
2
3
*
Citation: Park, K.; Kim, D.; Kim, G.;
Lee, W. Evaluation of the Pullout
Behavior of Pre-Bored Piles
Embedded in Rock. Materials 2021, 14,
5593. https://doi.org/10.3390/
Department of Civil Construction Engineering, Chosun College of Science & Technology,
Gwangju 61453, Korea; parkgeo@cst.ac.kr
Department of Civil Engineering, Chosun University, Gwangju 61452, Korea
Department of Construction, KL Engineering Co., Jangsung 57241, Korea; kgd125@hanmail.net (G.K.);
ewoo10@naver.com (W.L.)
Correspondence: dkimgeo@chosun.ac.kr; Tel.: +82-062-230-7607
Abstract: The subject of this study is dry process caisson tube method cofferdam (hereinafter called
C.T cofferdam). This C.T cofferdam is designed to use the skin friction of the drilled shaft embedded
into the rock for stability of buoyancy. A pre-bored pile embedded in the bedrock was pulled out
due to the buoyancy of the C.T cofferdam at the pier (hereinafter called P) 2 of the OO bridges under
construction, to which this was applied. In this study, in order to solve this problem, the adhesion
force applied with the concept of skin friction and the pre-bored pile of drilled shaft according to
domestic and foreign design standards were identified; the on-site pull-out load test was used to
calculate the pull-out force; and the skin friction of the drilled shaft and pre-bored pile embedded into
the bedrock were compared and analyzed. In addition, the pull-out behavior of the pre-bored pile
embedded in the bedrock was analyzed through numerical analysis. The adhesion strength tested in
the lab was 881 kN for air curing of concrete and 542 kN for water curing of concrete, and the on-site
pull-out test result was 399.7 kN. As a result of the numerical analysis, the material properties of
the grout considering the site conditions used revealed that the displacement of the entire structure
exceeded the allowable limit and was unstable. This appears to have lowered the adhesion strength
due to construction issues such as ground complexity and both seawater and slime treatment, which
were not expected at the time of design.
ma14195593
Academic Editors: Eddie Koenders
Keywords: dry process caisson tube method cofferdam; skin friction; pre-bored pile; pull-out load
test; grout
and Tomasz Sadowski
Received: 30 June 2021
Accepted: 13 September 2021
Published: 26 September 2021
Publisher’s Note: MDPI stays neutral
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Copyright: © 2021 by the authors.
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conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
1.1. Research Background and Purpose
End-bearing and skin friction behavior of piles subjected to axial loads are important
factors in predicting the bearing capacity of structures. However, the ISO 19902 [1] regulations on offshore steel structures do not suggest an accurate calculation equation for
the evaluation of friction-bearing capacities at the tip and circumferential surface of piles
embedded in bedrock.
The Korean structural foundation design standard (Ministry of Land, Infrastructure,
and Transport [2]) briefly mentions the end-bearing capacity for compression but does not
suggest any formula for calculating the skin friction of the pile foundation embedded in the
rock. Therefore, when designing offshore structures, the equation for calculating the skin
friction of drilled shafts embedded in the bedrock, which is the most similar mechanism to
the required calculation, is applied.
Due to the fact that the bearing capacities of a drilled shaft and pre-bored pile are very
different, it is easy to overestimate or underestimate the skin friction of the pile. Therefore,
due to incorrect evaluation, the safety of the structure may be impaired and errors may
occur in the prediction of the behavior of the pile foundation. In order to construct the
Materials 2021, 14, 5593. https://doi.org/10.3390/ma14195593
https://www.mdpi.com/journal/materials
Materials 2021, 14, 5593
2 of 21
structure safely and economically, it is necessary to study the support behavior of the
pre-bored pile embedded in the bedrock.
C.T cofferdam is designed to use the skin friction of drilled shafts embedded in
the rock to ensure buoyancy stability. However, due to the buoyancy of C.T cofferdam,
a problem occurred in which the pre-bored pile was pulled out. To solve this problem,
firstly, we analyzed the skin friction of the drilled shaft according to domestic and foreign
design standards. Secondly, a lab test was performed on the adhesion force, in which the
concept of a pre-bored pile was applied. Thirdly, the pull-out force was calculated through
the pull-out load test (ASTM D3689-07 [3]) and both the skin friction of the drilled shaft
and pre-bored pile embedded in the bedrock were compared and analyzed. Finally, the
pull-out behavior of the pre-bored pile embedded in the bedrock was analyzed through
numerical analysis.
1.2. Research Trends
Many researchers have been conducting studies on the bearing capacity of piles for
a long time. In the case of soft ground and weathered rock, the basic design theory is
established based on the results of field tests. However, past studies on pile foundations
embedded in bedrock mainly focused on drilled shafts and there are very few studies on
pre-bored piles.
When calculating the ultimate bearing capacity of a drilled shaft embedded into rock,
design standards and bearing capacity calculation equations that were proposed overseas
(FHWA [4]; FHWA [5]; and Canadian Geotechnical Society [6]) are applied.
In Korea, overseas design standards and bearing capacity calculation equations were
modified to match domestic standards and were applied to the structural basic design
standards and road bridge design standards (Ministry of Land, Infrastructure, and Transport [2]).
Previous overseas studies on skin friction of pre-bored piles embedded in bedrock
were reviewed. Maertens [7] evaluated the performance of piles by installing steel pipe
piles on weathered rocks but they only evaluated limited field conditions.
Shakir-Khalil [8], Shakir-Khalil [9], and Nezamian et al. [10] studied the adhesion
strength and adhesion behavior between concrete and steel pipes when steel pipes are
filled with concrete.
Moon et al. [11], a previous study in Korea, determined the skin friction of steel pipe
piles embedded in bedrock by the skin friction between the surface of the steel pipe and
the grout. Moon et al. [12] confirmed that the adhesion strength of the skin friction of the
pre-bored pile embedded in the bedrock was about 1 MPa through a model test. Moon and
Park [13] also evaluated the effect of the grout mixing ratio on the skin friction of steel pipe
piles embedded in bedrock. Regardless of W/C, in the case of grout, the adhesion strength
was about 540~560 kPa.
Kim et al. [14] proposed that the load-settlement curves and axial load distributions
of piles, as well as the load-transfer curves (t-z curves), are obtained on the main surface
of a steel pipe pile. In addition, the N-value obtained from the standard penetration test
for the strata, the compressive strength of the bedrock, and the soil-cement formed by the
injected grout were confirmed to be affected by uniaxial compressive strength.
Kim and Kim [15] evaluated the skin friction characteristics of the pre-bored pile
embedded in bedrock through the dynamic load test of the pile foundation. Chae et al. [16]
proposed an equation for calculating the axial bearing of SDA (separated doughnut auger)
pre-bored piles embedded in bedrock.
According to the previous domestic and foreign studies related to the calculation
of the pull-out force of pre-bored piles embedded in bedrock, the pull-out force of the
pre-bored pile is mostly related to the adhesion force of the grout. However, the related
design standards are not clear, thus, in practice, the pull-out standards of drilled shafts are
applied. In addition, most of the previous studies were limited to the skin friction of steel
pipe piles embedded in bedrock among pre-bored piles.
Materials 2021, 14, 5593
Materials 2021, 14, 5593
3 of 21
3 of 22
Therefore, in this study, theories and empirical equations similar to the behavior
of
pre-bored piles were reviewed among the design standards for drilled shafts penetrated
into bedrock. What sets this study apart from previous studies is the analysis of the pull-out
standards
are not
clear, thus, piles
in practice,
thesteel
pull-out
of drilled concrete),
shafts are apforce
behavior
of composite
(upper:
pipe,standards
lower: reinforced
in which
plied.
In addition,
most
of the previous
wereinlimited
to the
skin the
friction
of steelpiles.
the
lower
reinforced
concrete
piles are studies
embedded
bedrock
among
pre-bored
pipe piles embedded in bedrock among pre-bored piles.
2. Design-Bearing
Capacity
of Pilesand empirical equations similar to the behavior of
Therefore, in this
study, theories
pre-bored
pilesofwere
among the
design standards for drilled shafts penetrated
2.1.
Estimation
The reviewed
Bearing Capacity
of Piles
into bedrock. What sets this study apart from previous studies is the analysis of the pullAs shown in Equation (1), the bearing capacity of the pile consists of the end-bearing
out force behavior of composite piles (upper: steel pipe, lower: reinforced concrete), in
capacity and the skin friction force, and the resistance force of the two elements occurs
which the lower reinforced concrete piles are embedded in bedrock among the pre-bored
completely
at the same time.
piles.
Q = Qs + Q p = f s As + q p A p
u
2. Design-Bearing Capacity of Piles
(1)
2.1. Estimation
Theskin
Bearing
Capacity
of (kN/m
Piles 2 ); A is the major surface area of pile (m2 ); q
where
f s is theofunit
friction
force
s
p
shown
in Equation
(1), the(kN/m
bearing2 );
capacity
consists
of the end-bearing
is theAs
unit
end-bearing
capacity
and A pofisthe
thepile
pile
cross-sectional
area (m2 ).
capacity
and the skin
friction(1)force,
and the resistance
of the was
two elements
occurs
Theoretical
Equation
for obtaining
bearingforce
capacity
developed
by many
completely
the same
scholars
foratsafer
and time.
more economical construction after Rankine published the earth
pressure theory. Although the understanding
through
=
+
=of the+overall pile has been improved(1)
these efforts, there are many variables affecting
the
bearing
capacity,
thus
there
is
a
limit to
is the major surface area of pile (m2);
where is the unit skin friction force (kN/m2);
deriving a reliable bearing capacity calculation
equation.
is the pile cross-sectional area (m2).
is the unit end-bearing capacity (kN/m2); and
In classifying the bearing capacity calculation equations proposed so far, there is the
Theoretical Equation (1) for obtaining bearing capacity was developed by many
static-bearing capacity calculation method and the method by static loading and dynamic
scholars for safer and more economical construction after Rankine published the earth
loading
static-bearing
capacity equation,
basedpile
on has
experience
or theory, is
pressuretests.
theory.The
Although
the understanding
of the overall
been improved
mainly
used
during
the
design
process,
although
its
reliability
is
somewhat
through these efforts, there are many variables affecting the bearing capacity, thus low.
there In the
construction
process,
the most
reliable
on-sitecalculation
in situ test
methods, such as the static load
is a limit to deriving
a reliable
bearing
capacity
equation.
test and
dynamic load
test, are
mainly
applied, equations
although proposed
it takes a so
great
time and
In classifying
the bearing
capacity
calculation
far, deal
thereof
is the
are
costly.
static-bearing
capacity calculation method and the method by static loading and dynamic
loading tests. The static-bearing capacity equation, based on experience or theory, is
2.2.
Skinused
Friction
of Pile
mainly
during
the design process, although its reliability is somewhat low. In the
construction
the skin
most friction
reliable on-site
in situ
such
as the static
Figure 1process,
shows the
acting on
the test
skinmethods,
of the pile
embedded
inload
the sandy
test and
test,
arefriction
mainly applied,
it takes
great deal of
andon the
soil.
Thedynamic
ultimateload
unit
skin
force is although
calculated
usinga Equation
(2)time
based
are
costly.
theory considering the friction generated on the surface of a rigid body.
2.2. Skin Friction of Pile
f s = Kp0 tanδ
(2)
Figure 1 shows the skin friction acting on the skin of the pile embedded in the sandy
where
K isultimate
the horizontal
earth
pressure
effective(2)
overburden
payload
soil. The
unit skin
friction
force coefficient;
is calculatedpusing
Equation
based on the
0 is the
theory 2considering
friction
generated
on angle
the surface
(kN/m
); and tan δthe
is the
internal
friction
(◦ ). of a rigid body.
Figure 1. Skin friction force on the pile.
Figure 1. Skin friction force on the pile.
Materials 2021, 14, 5593
4 of 21
Assuming that the sand between the ground and pile is in the ultimate fracture state,
the friction angle was estimated to be the same as the residual friction angle ∅τcs of the
sand rather than the unit weight of the ground and the material of the pile. This value is
not significantly different from the value according to the method of Vesic [17].
Meyerhof [18] determined the coefficient of earth pressure. The horizontal earth
pressure coefficient considers only the horizontal displacement of the pile foundation. This
horizontal displacement affects the surrounding ground so that the soil is compacted and
the maximum displacement occurs at the point of contact between the pile and the soil.
Meyerhof [19], Poulos and Davis [20], Vesic [21], and Berezantzev [22] proposed
a theoretical hydrostatic-bearing capacity calculation method that divided end-bearing
capacity and skin friction force. In order to calculate bearing capacity according to the
theoretical equation, various ground characteristics are required. In addition, both lab
and field tests are required to calculate this. Values that cannot be obtained from lab
and field tests should be assumed and applied. Therefore, when using the theoretical
equation, uncertainty exists because it assumes variables that cannot be obtained from lab
and field tests. It is economically disadvantageous because various soil tests are required.
There is also the problem of poor accuracy when applying it to the design. Table 1 shows
the theoretical equations of the hydrostatic-bearing capacity currently applied in Korea
and abroad.
Table 1. Physical properties of reinforcements used for tests.
Classification
Skin Friction
End-bearing capacity
Equations
Proposers
Applicability
f s = ac + qKtanδ
Tomlinson (1972)
Pre-stress and clay soil
f s = Kq tanδ = βq
Jennings and Burland (1973)
Effective stress amnd clay soil
f s = λ(q + 2su )
McClelland and
Focht (1967)
Mixing method and clay soil
f s = acu
Meyerhof (1976)
Clay soil
f s = Ks σv tanδ
Meyerhof,
Colye-Castello,
Vesic, API etc
Sandy soil
q p = p0 Nq ≤ 5Nq tanφ
Meyerhof (1976)
Sandy soil
q p = cNc
Meyerhof (1976)
Clay soil
q p = cNc + σ0 N0
Vesic (1977)
Cavity expansion theory
q p = Ak γB + Bk q T (q T = a T γD )
Berezantzev (1961)
The skin friction of clay soil was calculated by the α coefficient method and β coefficient method (Ministry of Land, Infrastructure, and Transport, 2018). In this study, the skin
friction of the pile foundation was calculated by applying the α coefficient method under
the undrained condition.
Since the α coefficient method is applied when the pile is embedded in the saturated
clay layer, it is φu = 0 in the undrained condition, thus δ = 0. At this time, the skin friction
of the pile is expressed as the adhesion force between the pile and the ground.
f s = cs = αcu
(3)
where cs is the cohesion force (kN/m2 ); α is the cohesion force coefficient; and cu is the
cohesion force in undrained conditions (kN/m2 ).
The α value varies depending on the hardness of the clay layer, the type and construction method of the pile, the stratum condition, and the size of the pile.
Materials 2021, 14, 5593
2 α is the cohesion force coefficient; and
where is is
the
cohesion
force
(kN/m
the
co2); );
where
the
cohesion
force
(kN/m
α is the cohesion force coefficient; and is is
the
co2).
hesion
force
in
undrained
conditions
(kN/m
2
hesion force in undrained conditions (kN/m ).
5 ofcon21
The
valuevaries
variesdepending
dependingononthe
thehardness
hardnessofofthe
theclay
claylayer,
layer,the
thetype
typeand
and
The
αα
value
construction
method
the
pile,
the
stratum
condition,
and
the
size
the
pile.
struction
method
ofof
the
pile,
the
stratum
condition,
and
the
size
ofof
the
pile.
2.3.
C.T
Cofferdam
Method
2.3.
C.T
Cofferdam
Method
The C.T cofferdam method creates acut-off
cut-offspace
spaceusing
usingprefabricated
prefabricatedsteel
steelcaisson
caisson
The
C.T cofferdam method creates a cut-off space
using prefabricated
steel caisson
and rubber materials when installing,
repairing,
reinforcing
underwater
structures.
and
repairing,
oror
reinforcing
underwater
structures.
rubber materials when installing, repairing,
or
reinforcing
underwater
structures.
There
are
rubber
tubes
and
compression-type
rubber
packing
materials
used for the
There are rubber tubes and compression-type rubber packing materials used
for the
cut-off.
As
shown
in
Figure
2a,
the
adhesive
rubber
tube
attached
to
the
end
the
steel
the
steel
cut-off. As shown in Figure 2a, the adhesive rubber tube attached to the end ofof
steel
caisson
is
in
close
contact
with
the
leveling
concrete
and
creates
a
pressure
greater
than
concrete
and
creates
a pressure
greater
thanthan
the
caisson is
is in
in close
closecontact
contactwith
withthe
theleveling
leveling
concrete
and
creates
a pressure
greater
the
water
pressure
to
block
the
external
inflow.
As
shown
in
Figure
2b,
the
compressionwater
pressure
to block
the external
inflow.
As shown
in Figure
2b, the
the water
pressure
to block
the external
inflow.
As shown
in Figure
2b,compression-type
the compressiontype
rubber
packing
compresses
the
rubber
packing
attached
to
the
thesteel
steel
rubber
packing
compresses
the rubber
packing
attached
to the bottom
ofbottom
the steel
caisson
type
rubber
packing
compresses
the rubber
packing
attached
to the bottom
ofofthe
caisson
with
the
caisson’s
own
weight,
which
is
greater
than
the
water
pressure,
to
block
with
thewith
caisson’s
own weight,
which which
is greater
than the
pressure,
to block
the
caisson
the caisson’s
own weight,
is greater
thanwater
the water
pressure,
to block
theexternal
external
inflow
theleveling
leveling
concrete.
thisstudy,
study,
close-fitting
typerubber
rubber
tube
external
inflow
to the
leveling
concrete.
In this
study,
a close-fitting
typetype
rubber
tubetube
C.T
the
inflow
toto
the
concrete.
InIn
this
a aclose-fitting
C.T
cofferdam
was
used.
cofferdam
was was
used.used.
C.T
cofferdam
(a) Compression-type rubber tube
(b) Close-fitting-type rubber packing
(a) Compression-type rubber tube
(b) Close-fitting-type rubber packing
Figure 2.
Schematic of
C.T cofferdam
method.
Figure
Figure 2.
2. Schematic
Schematic of
of C.T
C.T cofferdam
cofferdam method.
method.
Review
Design
Data
3.3.
Review
ofof
Design
Data
3.
Review
of
Design
Data
3.1.
Site
Overview
3.1.
Site
Overview
3.1.
Site
Overview
InIn
the
case
of of
OO
Bridge
P2,P2,
thethe
pre-bored
pilepile
waswas
pulled
out out
by about
0.17 0.17
to 0.34 0.34
m
the
case
OO
Bridge
pre-bored
pulled
about
In the
case
of OO
Bridge
P2, the
pre-bored
pile was
pulled
out bybyabout
0.17 toto0.34
due to
thetobuoyancy
generated
at the at
bottom
of the leveling
concreteconcrete
and the and
C.T cofferdam
due
thebuoyancy
buoyancy
generated
thebottom
bottom
theleveling
leveling
theC.T
C.Tcofcofmmdue
to the
generated
at the
ofofthe
concrete and
the
was
levitated,
causing damage
todamage
the structure.
ferdam
was
levitated,
causing
to
the
structure.
ferdam was levitated, causing damage to the structure.
Figure
3 shows
the
pull-out
ofof
the
C.T
cofferdam
at at
P2 where
thethe
problem
occurred.
Figure
3 shows
the
pull-out
the
C.T
cofferdam
where
problem
occurred.
Figure
3 shows
the
pull-out
of the
C.T
cofferdam
at P2P2
where
the problem
occurred.
(a) P2 C.T cofferdam 90° putout (0.17 m)
(a) P2 C.T cofferdam 90° putout (0.17 m)
(b) P2 C.T cofferdam 270° pull-out (0.34 m)
(b) P2 C.T cofferdam 270° pull-out (0.34 m)
Figure 3. Cont.
Materials 2021, 14, 5593
Materials 2021, 14, 5593
6 of 21
6 of 22
(c) Site overview of P2 C.T cofferdam 270°
(d) Pull-out status of P2 C.T cofferdam
Figure3.3. Pull-out
Pull-out view
viewof
ofC.T
C.Tcofferdam
cofferdamat
atP2.
P2.
Figure
3.2.
3.2. Analysis
Analysis of
of Pull-Out
Pull-Out Force
Force of
of Pre-Bored
Pre-Bored Piles
PilesAccording
Accordingto
toDesign
DesignCriteria
Criteria
With
foreign
design
standards,
thethe
pull-out
forces
of
Withreference
referencetotovarious
variousdomestic
domesticand
and
foreign
design
standards,
pull-out
forces
pre-bored
pilespiles
and and
drilled
shafts
embedded
in rock
and compared.
When
of pre-bored
drilled
shafts
embedded
inwere
rock calculated
were calculated
and compared.
calculating
the design
of the C.T
cofferdam,
the pull-out
force of the
pile
When calculating
the structure
design structure
of the
C.T cofferdam,
the pull-out
force
ofapplied
the pile
was
600
kN.
It
was
checked
whether
this
pull-out
force
satisfies
the
design
criteria,
with
applied was 600 kN. It was checked whether this pull-out force satisfies the design criteria,
the
theoretical
equation
substituting
the actual
insertion
depthdepth
of theofpile
1.92 m).
with
the theoretical
equation
substituting
the actual
insertion
the(about
pile (about
1.92
m).
(1) C.T Cofferdam Design Structural Calculation Results
(1) As
C.Tshown
Cofferdam
Design
Calculation
Resultswas 3665.250 kN, the water load
in Table
2, the Structural
total weight
of C.T cofferdam
As
shown
in
Table
2,
the
total
weight
of
C.T
cofferdam
was 3665.250
kN,
the water
inside the tank was 5078.34 kN, the water load inside the watertight
part was
548.311
kN,
load
inside
the
tank
was
5078.34
kN,
the
water
load
inside
the
watertight
part
was
548.311
and the weight of the leveling concrete was 7591.710 kN. When calculating the load in the
kN, and
the weight
of the leveling
concrete
was 7591.710
kN.
load
design
structural
calculation
sheet, the
pile resistance
was 600
kNWhen
(at thecalculating
time of thethe
design)
in
the
design
structural
calculation
sheet,
the
pile
resistance
was
600
kN
(at
the
time
of
the
and the stability of the buoyancy was calculated to be 1.4. Since it is higher than 1.2, the
design)
and
the
stability
of
the
buoyancy
was
calculated
to
be
1.4.
Since
it
is
higher
than
buoyancy was considered stable.
1.2, the buoyancy was considered stable.
Table 2. Stability review for the buoyancy of C.T cofferdam.
Table 2. Stability review for the buoyancy of C.T cofferdam.
Classification
Classification
C.T cofferdam weight
C.T cofferdam weight
Water load inside the tank
Resistance Load
Resistance
Load
3665.250
3665.250
5078.340
Water load inside the tank
5078.340
Water load
inside watertight part
Leveling concrete weight
7591.710
Water load inside watertight part
Leveling concrete
weight
Resistance
of piles (600
kN per pile)
Resistance
piles (45.2
(600 kN
kNper
perpile)
pile)
Pileof
weight
Buoyancy
Selected Load (kN)
Selected Load (kN)
Pile
weight
(45.2 kN
per pile)
Leveling
concrete
bottom
buoyancy
548.311
548.311
7591.710
21,600.000
21,600.000
1630.000
1630.000
29,929.973
Buoyancy
Fs (Satety rate)
+5078.34
+548.311+7591.71+21,600 29,929.973
Leveling concrete
bottom
buoyancy
FS = 3665.25
= 1.4 > 1.2
29,929.973
3665.25 + 5078.34 + 548.311 + 7591.71 + 21,600
Fs (Satety rate)
= 1.4 > 1.2
FS =
29,929.973 the pull-out force of a pre-bored
(2) Review of the theoretical equation for calculating
pile according to the design criteria
(2) Review of the theoretical equation for calculating the pull-out force of a pre-bored
Since
most of the
force
of pre-bored piles is generated from the skin friction
pile according
to pull-out
the design
criteria
of the piles embedded in the bedrock (excluding the sedimentary section at the top of the
Since most of the pull-out force of pre-bored piles is generated from the skin friction
of the piles embedded in the bedrock (excluding the sedimentary section at the top of the
Materials 2021, 14, 5593
7 of 21
bedrock), the skin friction of the piles embedded in the bedrock according to each design
standard was calculated.
Considering no design standards for estimating the skin friction of pre-bored piles
embedded in bedrock have been presented domestically and abroad, referring to the drilled
shaft design standards, the skin friction of the bedrock was calculated.
To examine the skin friction of main pile, (1) Korean Geotechnical Engineering [23],
Ministry of Land, Infrastructure, and Transport [24], (2) Cho [25], and (3) Cho [26] were
referred to. As the condition to be reviewed was similar to that of the anchor method, the
study of Kim and Jung [27] was also referred to for review. As shown in Table 3, as a result
of examining the grouting construction data, the grouting depth was found to be about
1.92 m on average and the skin friction value was calculated based on that value.
Table 3. Comparison of the pull-out force applied to the design and design standard values.
Classification
Existing design data
Design standard
Structural Foundation
Design Standard
(2008)
Anchor method
Allowable Skin
Friction of Pile
(pull-out, kN)
C.T cofferdam design structure calculation sheet
(1.0 m embedded in the rock)
600
Pull-out force considering the thread-embedded depth
of the pile
(2.197 m embedded in the rock)
1209
AASHTO (1996)
200.5
Canadian Geotechnical Society (1985)
356.9
NAVFAC (1982)
931.8
FHWA (1988)
Horvath and Kenny (1979)
648.8
FHWA (1999)
Horvath and Kenny (1979)
770.4
FHWA (1999)
Rowe and Armitage (1984)
746.8
Wiliams et al. (1980)
659.6
Rowe and Armitage (1987)
1774,3
Horvath and Kenney (1979)
819.9
Carter and Kulhawy (1988)
770.9
Reynolds and Kaderabek (1987)
3460.5
Gupton and Logan (1984)
2307.8
Reese and O‘Neil (1987)
1730.8
Rosengerg and Journaeaux (1976)
1489.7
Putout resistance of anchor
292.8
Friction of ground and grout
566.7
Maximum adhesive force of tensile material and grout
791.1
Remark
Depth embedded in
the rock
1.92 m
Diameter of the pile
0.6 m
3.3. Adhesive Force Analysis of Pile and Grout
3.3.1. Adhesive Force Concept of Pile and Grout
The skin friction failure of the pre-bored pile embedded in the bedrock appears in four
forms, as shown in Figure 4. These are the shear failures between the pile-filling material
and the pile; the adhesion failure between the rock and the pile-filling material; the shear
failure of the rock mass itself; and finally the shear failure of the pile-filling material itself.
Where the shear failure with the least resistance occurs, it leads to skin friction failure.
Materials 2021, 14, 5593
Since the uniaxial compressive strength of most rocks is generally hig
uniaxial compressive strength of the grout, shear failure usually occurs in th
material before the shear failure of the rock and the skin friction of the pile e
the rock is determined by the adhesion strength between the pile and the pi
8 of 21
terial.
Figure 4. Pull-out fracture modeling of pre-bored piles.
Figure 4. Pull-out fracture modeling of pre-bored piles.
Since the uniaxial compressive strength of most rocks is generally higher than the
uniaxial compressive strength of the grout, shear failure usually occurs in the pile-filling
Moon
etthe
al.shear
[12]failure
studied
thefriction
W/Cof(water/cement,
hereinafter
material
before
of thethe
rockeffect
and theof
skin
the pile embedded in
the
rock
is
determined
by
the
adhesion
strength
between
the
pile
and
the
pile-filling
material.
ratio of the pile-filling material and the fine aggregate mixing ratio on the u
Moon et al. [12] studied the effect of the W/C (water/cement, hereinafter called
pressive
strength, and the uniaxial compressive strength of the pile-filling
W/C) ratio of the pile-filling material and the fine aggregate mixing ratio on the uniaxial
about
7–36 strength,
MPa depending
on the
mixingstrength
ratio. Moon
et al. [11]
measured
compressive
and the uniaxial
compressive
of the pile-filling
material
was about 7–36 MPa
depending
on theafter
mixing
ratio. Moon
et al. [11]
measured
load of 50%
displacement
at the
pile head
injecting
grout
with
a W/Ctheratio
and displacement at the pile head after injecting grout with a W/C ratio of 50% into basalt
perforated with a diameter of BX (BX is a casing standard of out diameter of 5
perforated with a diameter of BX (BX is a casing standard of out diameter of 59.0 mm
for
standard
penetration
tests)
and
inserted
a structural
steel diameter
pipe (outer
dia
used
for standard
penetration tests)
and
inserted
a structural
steel pipe (outer
of
48.6 mm).
skinfriction
friction ofofthe
steel
pipe pipe
pile embedded
in the bedrock
was determined
mm).
TheThe
skin
the
steel
pile embedded
in the
bedrock was de
by
the
adhesion
strength
of
the
steel
pipe
and
the
grout,
and
the
unit
ultimate
skin
friction
the adhesion strength of the steel pipe and the grout, and the unit
ultimate
was in the range of 0.92 to 1.04 MPa.
was in
range
of 0.92
to 1.04
In athe
study
by Moon
and Park
[13],MPa.
in the absence of fine aggregate, the unit ultimate
skin In
friction
was 0.54
0.56 MPa
regardless
of W/C.
Table
4 shows of
ourfine
field aggregate,
conditions
a study
by to
Moon
and
Park [13],
in the
absence
the
(D
=
0.6
m
in
diameter
and
1.92
m
in
rock
penetration
condition)
using
the
unit
ultimate
skin friction was 0.54 to 0.56 MPa regardless of W/C. Table 4 shows our fiel
skin friction values determined in a previous study by Moon et al. [11] and Moon and
(D
= 0.6
diameter
in rock
penetration
condition)
using the
Park
[13].m
Asin
shown
in Tableand
4, this1.92
is themresult
of calculating
the allowable
skin friction.
The allowable
friction
was about 651.1
kN. study by Moon et al. [11] an
skin
friction skin
values
determined
intoa 1109.29
previous
Materials
Materials 2021,
2021, 14,
14, 5593
5593
Materials 2021, 14, 5593
99 of
of 22
22
Park
Park [13].
[13]. As
As shown
shown in
in Table
Table 4,
4, this
this is
is the
the result
result of
of calculating
calculating the
the allowable
allowable skin
skin friction.
friction.
The
allowable
skin
friction
was
about
651.1
to
1109.29
kN.
9 of 21
The allowable skin friction was about 651.1 to 1109.29 kN.
Table
Table 4.
4. Allowable
Allowable skin
skin friction
friction between
between tension
tension member
member and
and grout.
grout.
ClassificaTable
4. Allowable skin friction between tension member and grout.
Classification
tion
Allowable
Allowable Skin
Skin Friction
Friction
Classification
Allowable Skin Friction
Test
Test Methods
Methods
Test Methods
=
= ×
× ×
× ×
×
=
920
×
×
×
1.92
Qu =×f 0.6
×π
D×L
= 920 ×
××
1.92
s0.6
Moon
et
al.
=
3,327.89
kN
π × 0.6 × 1.92
Moon et al.
=Q3,327.89
kN
u = 920 ×
3,327.89
[11]
= 3327.89
kN
Moon et al. [11]
3,327.89
[11]
=
=
=
=
=
= 1,109.29
1,109.29
Qa = Q
= 1109.29 kN
33u = 3327.89
3
FS
Test conditions:
W/C: 50%
Test
W/C:
Test conditions:
conditions:
W/C: 50%
50%
=
= ×
× ×
× ×
×
Qu =×
f 0.6
×π
D×L
=
540
×
×
1.92
s0.6
=
540
×
×
××
1.92
Moon
Moon and
and
Q
=
540
×
π
×
0.6
× 1.92
=
1,953.33
kN
u
= 1,953.33
kN
Moon and Park
Park
=
1953.33
kN
Park
1,953.33
1,953.33
[13]
1953.33
u
=
=
= 651.11
[13]
Qa = Q
= 651.11 kN
=
=
[13]
FS
33 = =3 651.11
Test
conditions:
W/C: 60~120%
Test
conditions:
W/C:
60~120%
Test conditions: W/C: 60~120%
3.3.2.
Lab
Test
for
Measuring
Adhesion
Strength
3.3.2.
3.3.2. Lab
Lab Test
Test for
forMeasuring
MeasuringAdhesion
AdhesionStrength
Strength
The
skin
friction
of
the
pre-bored
pile
embedded
rock
should
be
by
The
pile
embedded
in in
the
rock
should
be evaluated
by the
Theskin
skinfriction
frictionofofthe
thepre-bored
pre-bored
pile
embedded
in the
the
rock
should
be evaluated
evaluated
by
the
adhesion
and
skin
friction
occurring
on
the
surface
of
the
grout
and
pile
foundation,
adhesion
and skin
friction
occurring
on theon
surface
of the of
grout
pile
foundation,
rather
the adhesion
and skin
friction
occurring
the surface
the and
grout
and
pile foundation,
rather
than
of
ground
and
In
during
on-site
than
those
the ground
grout.
In grout.
addition,
during
on-site
construction,
it should beit
rather
thanofthose
those
of the
the and
ground
and
grout.
In addition,
addition,
during
on-site construction,
construction,
it
should
that
is
due
seawater
in
the
considered
that the adhesion
isadhesion
reduced due
to seawater
inflow
in theinflow
drilling
slurry,
should be
be considered
considered
that the
the
adhesion
is reduced
reduced
due to
to
seawater
inflow
inhole,
the drilling
drilling
hole,
and
aa poor
W/C
Therefore,
in
evaluate
friction
the
and
poor W/C
Therefore,
in order
to evaluate
theto
frictionthe
of skin
the pre-bored
hole,a slurry,
slurry,
andratio.
poor
W/C ratio.
ratio.
Therefore,
in order
order
toskin
evaluate
the
skin
friction of
ofpile,
the
pre-bored
pile,
five
reinforced
concrete
pile
samples
collected
at
the
work
site
were
used.
five
reinforced
concrete
pile
samples
collected
at
the
work
site
were
used.
In
using
this
pile,
pre-bored pile, five reinforced concrete pile samples collected at the work site were used.
In
pile,
grout
underwater
at
70%
the
W/C
ratio
the
the
groutthis
is poured
underwater
at 70%
of the W/C
and
adhesive
strength
of the
In using
using
this
pile, the
the
grout is
is poured
poured
underwater
at ratio
70% of
of
thethe
W/C
ratio and
and
the adhesive
adhesive
strength
of
the
pile
foundation
and
grout
in
the
bedrock
was
inspected
for
28
pile
foundation
and foundation
grout in theand
bedrock
inspected
28 inspected
days by air
curing.
strength
of the pile
groutwas
in the
bedrockfor
was
for
28 days
days by
by air
air
curing.
curing.
(1) Blending process
(1)
process
(1) Blending
Blending
process
In order to
apply a mixing ratio similar to the field conditions, the W/C ratio was set
In
order
to
aa mixing
similar
the
field
conditions,
the
ratio
was
set
In
order
to
apply
mixing
ratio
similarofto
tothe
thepile
field
conditions,
the W/C
W/Cfrom
ratiothe
was
set
to 70% and groutapply
was poured
onratio
the surface
foundation,
collected
field
to
70%
and
grout
was
poured
on
the
surface
of
the
pile
foundation,
collected
from
the
field
to 70%both
and grout
was
poured on conditions,
the surface of
pile foundation,
thepiece
field
under
air and
underwater
asthe
shown
in Figure 5. collected
Similarly,from
a test
under
both
under
both air
air and
and underwater
underwater conditions,
conditions, as
as shown
shown in
in Figure
Figure 5.
5. Similarly,
Similarly, aa test
test piece
piece
was
prepared.
was
was prepared.
prepared.
Table
5 shows the concrete mixing ratio for the lab pull-out test.
Table
Table 55 shows
shows the
the concrete
concrete mixing
mixing ratio
ratio for
for the
the lab
lab pull-out
pull-out test.
test.
Table 5. Mixing ratio for lab pull-out test.
Classification
Water (mL)
Cement (g)
Water curing of concrete: 28 days, 2 EA
490
700
Grout Material (g)
70
Air curing of concrete: 28 days, 3 EA
490
700
70
Materials
2021, 14, 5593
erials 2021,
14, 5593
10 of 21
(a) Compounding material
(b) W/C 70% blend
(c) Air curing of concrete
(d) Test piece
11 o
Figure 5. Test
piece
production
process.
Figure
5. Test
piece production
process.
(2) Test
(2) process
Test process
The lab
was
conducted
toconfirm
confirm
adhesion
strength
of the of
surface
The test
lab test
was
conducted to
thethe
adhesion
strength
of the surface
the pileof the p
foundation
grout.As
Asshown
shown in
6a,6a,
the the
surface
of the of
grout
cut into
cm ×
foundation
andand
grout.
inFigure
Figure
surface
thewas
grout
was4 cut
into 4 cm
4
cm
sizes
and
the
epoxy
was
coated
on
the
surface
of
the
pull-out
equipment;
the
grout
4 cm sizes and the epoxy was coated on the surface of the pull-out equipment; the gro
was as shown in Figure 6b. As shown in Figure 6c, the pull-out force was tested to evaluate
was as shown in Figure 6b. As shown in Figure 6c, the pull-out force was tested to evalua
the adhesion strength. Figure 6d is a view after the test.
the adhesion strength. Figure 6d is a view after the test.
Materials 2021,
2021, 14,
14, 5593
5593
Materials
11
11 of
of 22
21
(a) Cutting the grout surface
(b) Epoxy coating
(c) Pull-out test
(d) After pull-out test
Figure
force.
Figure 6.
6. A
A view
view of
of the
the test
test to
to check
check the
the adhesion
adhesion force.
(3) Test results
(3) The
Testadhesion
results strength of the three points of the air curing of concrete was in the range
of 0.65–0.81
MPa and
the adhesion
of the
twoofpoints
the water
curing was
of concrete
was
The adhesion
strength
of the three
points
the airofcuring
of concrete
in the range
in
range MPa
of 0.43–0.47
Tableof6the
shows
adhesion
of the
member,
of the
0.65–0.81
and theMPa.
adhesion
two the
points
of the strength
water curing
oftensile
concrete
was in
the
and
the calculation
results
of thethe
allowable
friction
of the
test piece.
the grout,
range of
0.43–0.47
MPa. Table
6 shows
adhesionskin
strength
of the
tensile
member, the
Even
the experimental
are different,
the test
results
grout,
andthough
the calculation
results ofmethods
the allowable
skin friction
of the
test applying
piece. air curing ofEven
the concrete
are
similar
to
those
of
previous
studies,
thus
the
results
areapplying
considered
though the experimental methods are different, the test results
air
reliable.
In
the
case
of
the
water
curing
of
concrete,
it
represents
60%
of
the
adhesion
curing of the concrete are similar to those of previous studies, thus the results are considered
strength
curing
of concrete
and when
considering
various variables
(slurry,
seareliable. of
Inthe
theair
case
of the
water curing
of concrete,
it represents
60% of the
adhesion
water
inflow,
etc.)
the field,and
the when
adhesion
strength is
expected
to be lower.
strength
of the
airoccurring
curing ofinconcrete
considering
various
variables
(slurry,
seawater inflow, etc.) occurring in the field, the adhesion strength is expected to be lower.
Materials 2021, 14, 5593
12 of 21
Table 6. Allowable skin friction according to the adhesion between the pile and grout.
Classification
WT-1
Qu = f s × π × D × L
Qu = 430 × π × 0.6 × 1.92
= 1555.4 kN
1555.4
u
Qa = Q
= 518.4 kN
3
FS =
WT-2
Qu = f s × π × D × L
Qu = 470 × π × 0.6 × 1.92
= 1700.12 kN
1700.12
u
Qa = Q
= 566.7 kN
3
FS =
AT-1
Qu = f s × π × D × L
Qu = 810 × π × 0.6 × 1.92
= 2929.99 kN
2929.99
u
= 976.66 kN
Qa = Q
3
FS =
AT-2
Qu = f s × π × D × L
Qu = 730 × π × 0.6 × 1.92
= 2640.61 kN
2640.61
u
Qa = Q
= 880.2 kN
3
FS =
AT-3
Qu = f s × π × D × L
Qu = 650 × π × 0.6 × 1.92
= 2358.72 kN
2358.72
u
= 786.24 kN
Qa = Q
3
FS =
Water curing
Air Curing
Avgerage Allowable
Skin Friction
Allowable Skin Friction
542.55 kN
881.03 kN
3.4. Results of Pull-Out Load Test
There are many design standards for designing C.T cofferdams with a pull-out force
of 600 kN or more, but most of these standards are for drilled shafts. Since this study used
the design standard of pre-bored piles based on empirical equations that have not yet been
clearly established, various reviews are needed.
In general, the pile load test for the bearing capacity test was performed according to
the standard after the pile construction was completed and the dynamic load or static load
test was performed within 5% of the number of piles. However, since there was no similar
guideline in the pull-out load test, the pull-out force for the pre-bored pile at the point was
measured to examine the pull-out problem that occurred at the research site.
The most accurate measurement of pull-out force was 217 kN in the pull-out load test
performed on the T1 pile of P2 at the OO Bridge in April 2018. However, this was tested 13
days after the grout was placed and 78.7% of the measured value was set as the allowable
pull-out force by referring to the Basic Structure Design Standards and Explanations, Road
Bridge Design Standards, and other references (Table 7).
Table 7. Re-examination of allowable pull-out force according to the results of the pull-out load test.
Classification
Re-examination of Allowable Pull-Out Force
Allowable
Pull-Out Force
1 Qu = 651.1 kN (at Offset Point)
On-site pull-out test
P2
Qu = 651.1 ÷ 0.787 = 827.3 kN
Qu = 827.3 ÷ 2 = 413.6 kN
2 Qu = 629.2 kN (at 0.25 in)
Qu = 629.2 ÷ 0.787 = 799.4 kN
Qu = 799.4 ÷ 2 = 399.7 kN
399.7 kN
The correction value was calculated by referring to the ACI 208R-92 standard and
estimating the uniaxial compressive strength according to the material age. The on-site
ultimate pull-out test value was 629.2 MPa and the safety side’s age of 12 days was applied
in consideration of the curing time and pull-out test time. Therefore, the correction factor
according to the age of the concrete for 12 days was 0.828 and here, again, 5% was corrected
for the water curing of concrete; the final correction factor was 0.787. The calculated pull-
Materials 2021, 14, 5593
13 of 21
out force was 629.2 MPa/0.787 = 799.95 MPa and the short-term load safety factor of 2 was
applied here; the final pull-out force was calculated as 399.7 kN. The allowable pull-out
force reviewed based on the on-site pull-out load test was 399.7 kN, which is very different
from the pull-out force of 600 kN applied to the design. It is agreed that poor W/C ratio,
due to the inflow of seawater, slurry, and excessive inflow of seawater into the drilling hole
at the site, is the cause of lowering the adhesion between the pile and the grout.
A total of 399.7 kN from the pull-out load test is about 60% of the allowable pull-out
force f 600 kN applied to the design and therefore it is agreed that the pre-bored pile was
pulled out due to the lack of the design load. In addition, in comparing the results of the
pull-out load test with the theoretical results, the results are most similar to those of CGS
(1985). This study compared the uniaxial compressive strength of rock and concrete, which
is similar to the concept of estimating the skin friction of a pre-bored pile embedded in
bedrock, thus it seems to show similar results to our experiment.
3.5. C.T Cofferdam Stability Review
In order to estimate the skin friction of the pre-bored pile embedded in the rock,
the original design, analysis of the research data, review of the design criteria, adhesion
strength test of the pile foundation and grout, the results of the field pull-out load test were
reviewed. Based on this, the stability of C.T cofferdam was reviewed by recalculating the
pull-out force similar to when calculating to the field conditions. As a result of examining
the stability of C.T cofferdam as shown in Table 8, the minimum safety factor is satisfied
when the pull-out force of the pile foundation is at least 500 kN.
Table 8. Results of C.T cofferdam stability review according to the recalculation of the pull-out force.
Classification
Safety Factor
(Standard 1.2)
Remark
Original design
(1.0 m rock-bottom
embedded)
600
1.4
stable
Estimated value
Grout
air curing of concrete
(1.92 m rock depth)
881
1.7
stable
Site conditions
lacking
Grout placed in water
(1.92 m rock depth)
542
1.3
stable
Only the conditions for
placing it in water were
taken into account
On-site
experiment
On-site pull-out test
(1.92 m rock depth)
399
1.1
instable
Test methods have
high reliability
Design
data
Designer review
(2.197 m rock depth)
1210
2.0
stable
Cast-in-place pile
standards
Various research standards
(1.92 m rock depth)
680~1160
1.4~2.0
stable
Air curing of concrete
Various design standards
(1.92 m rock depth)
220~1700
0.8~2.6
stable~
instable
Cast-in-place pile
standards
Existing
design
Lab
experiment
P2
Pile Pull-Out
Force (kN)
Previous
research
Materials 2021, 14, 5593
14 of 21
While the pull-out force of the original design was designed based on 600 kN, as a
result of the on-site experiment test, it was found to be unstable at 399 kN. In addition,
when reviewed by various design standards, it was unstable at 220–1700 kN. Therefore,
considering various variables (slurry, seawater inflow, etc.) occurring at the site, the
adhesion strength can be further lowered, thus the depth of penetration of the rock and the
W/C ratio should be sufficiently considered when designing.
4. Reviewing the Amount of Pull-Out of Pre-Bored Piles through Numerical Analysis
4.1. Analysis Conditions
In order to identify the amount of pull-out of the pile foundation, depending on
leveling the concrete base buoyancy, the shape of the pile and field ground conditions were
considered, and soil properties of the field were estimated based on the site investigation
reports and empirical equations.
In this study, two boring investigations were conducted. The sites are composed of
sedimentary layers, weathering soils, weathered rocks, and soft rock layers from the top,
and the ground water table was GL(-) 8.3 to 8.5 m in the upper plate of the temporary
bridge, being distributed in sedimentary layers. Therefore, this ground condition was
modeled for numerical analysis.
4.1.1. Analysis Program
In this study, Midas GTS (Midas IT. Co.,Ltd., Gyeonggido, Korea), a finite element
analysis program, was employed. Midas GTS can perform modeling to consider the ground
shape and construction process as much as possible during ground analysis, and can reflect
actual field conditions in the light of non-linearity and in-situ ground stress states of various
materials used for numerical analysis.
For analyzing complex non-linear behaviors of the ground, Midas GTS was equipped
with various constitutive elasto-plastic models, as well as with automatic mesh generation.
4.1.2. Drilling Section
The ground on the upper plate was investigated from the 270◦ direction to the 90◦ direction of C.T cofferdam to collect the ground data to be applied to the model. Sedimentary
layers were confirmed in the current surface, the thickness was distributed as 12.2–16.3 m,
and the composition consisted of neutral and fine yarns. The N value measured in the
standard penetration test was 1/30 to 12/30 (times/cm) and the relative density was in a
very loose to moderately dense state. The weathered soil layer was distributed in the BH-1
hole with a thickness of 1.3 m, the composition was of silt sand, and the condition was
dense. The weathered rock layer was in a state of severe weathering and was distributed
at a thickness of 4.8–6.4 m, wherein the composition was of silt sand. As a result of the
standard penetration test, the N value was 50/10 to 50/3 (times/cm) and the relative
density was very dense. The soft rock layer was distributed at a thickness of 2.0 m in the
entire borehole and it was observed with the naked eye that the rock condition was severely
to moderate weathering. In addition, the rock strength was weak to moderately strong.
Figure 7 is a stratum section view used for the analysis section.
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Figure 7. Stratum section view.
Figure 7. Stratum section view.
4.1.3. Calculation of the Material Properties of the Ground
4.1.3. Calculation of the Material Properties of the Ground
The Mohr–Coulomb (MC) model was used for the natural ground and the composite
(MC)
modelthe
was
used
for pile
the natural
composite
pileThe
wasMohr–Coulomb
modeled both by
applying
steel
pipe
SKK490ground
and byand
the the
reinforced
conpile
was
modeled
both by
applying
thestructure
steel pipeofpile
by the of
reinforced
concrete
pile
specification.
The
geological
the SKK490
site was and
composed
a sedimentary
crete
pile
specification.
The geological
of the
a sedimentary
layer,
weathered
soil layer,
weatheredstructure
rock layer,
andsite
softwas
rockcomposed
layer, andofits
structure was
layer,
weathered
soil
layer,
weathered
rock
layer,
and
soft
rock
layer,
and
its structure
relatively simple; very soft sand was distributed in the upper original ground.
was relatively
simple;
very
soft
sand
was
distributed
in
the
upper
original
ground.
In order to determine the design constant of the ground, the characteristic values
In order
to determine
the design
constant
theelastic
ground,
the characteristic
valueswere
for
for the
cohesion
force, internal
friction
angle,of
and
modulus
of each stratum
the
cohesionbased
force,on
internal
friction
angle,
and elastic
modulus
of each
were calcalculated
the ground
survey
report,
empirical
equations,
andstratum
other references.
culated
based9 on
survey
report,
empirical
andanalysis.
other references.
Tables
andthe
10 ground
show the
ground
constants
used equations,
for numerical
The material
Tables applied
9 and 10toshow
the ground
constants
for numerical
properties
the grout
and injected
into used
weathered
rock andanalysis.
soft rockThe
werematerial
selected
properties
applied
to
the
grout
and
injected
into
weathered
rock
and
soft
rock
were
in full consideration of the site conditions. The strength of 399.7 kN in the pull-out load setest
lected
in full45.4%
consideration
of theof
site
conditions.
Thecuring
strength
of 399.7 kN
the was
pull-out
was about
of the strength
881
kN in the air
of concrete,
thusin40%
used
2 , which
load
test was
about Therefore,
45.4% of the
strength
of 881
kN inisthe
airofcuring
of concrete,
40%
for safety
reasons.
928,000
kN/m
40%
the concrete
elasticthus
modulus
2
2, which is
of 2,320,000
kN/mreasons.
, was determined
the elastic
of40%
the of
grout,
considering
the
was
used for safety
Therefore, as
928,000
kN/mmodulus
the concrete
elastic
2, was
effects ofofseawater
slime,
anddetermined
was applied
the
numerical
analysis.
modulus
modulus
2,320,000and
kN/m
asto
the
elastic
modulus
of theThe
grout,
consid-of
elasticity
is a factor
that greatly
amount
of elastic
and the
modulus
ering
the effects
of seawater
andaffects
slime,the
and
was applied
tosettlement
the numerical
analysis.
Theof
elasticity
of
weathered
rocks
and
soft
rocks
is
not
the
same.
The
modulus
of
elasticity
modulus of elasticity is a factor that greatly affects the amount of elastic settlement andof
themodulus
grout was
toof
both
the weathered
rocksoft
androcks
the soft
rock
consideration
of the
the
of applied
elasticity
weathered
rocks and
is not
theinsame.
The modulus
conditions
the
application
ofboth
the grout.
offield
elasticity
of theafter
grout
was
applied to
the weathered rock and the soft rock in consideration of the field conditions after the application of the grout.
Table 9. Material properties applied to the numerical analysis.
Classification
Wetting
unit weight
(kN/m3)
Saturation
unit weight
(kN/m3)
Cohesion
c
(kN/m2)
Sedimentary Soil
Weathered
Rock
Soft
Rock
Leveling
Concrete
Rebar
Concrete
Steel
Pipe
Pile
17
23
25
25
25
78
18
23.5
25.5
25.5
25.5
79
13
30
50
-
-
-
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Table 9. Material properties applied to the numerical analysis.
Classification
Sedimentary
Soil
Weathered
Rock
Soft
Rock
Leveling
Concrete
Rebar
Concrete
Steel Pipe
Pile
Wetting
unit weight
γt
(kN/m3)
17
23
25
25
25
78
Saturation
unit weight
γsat
(kN/m3 )
18
23.5
25.5
25.5
25.5
79
Cohesion
c
(kN/m2 )
13
30
50
-
-
-
Internal
friction angle
φ
(◦ )
0
33
35
-
-
-
Elastic modulus
coefficient
E
(kN/m2 )
4000
167,500
2,180,000
2,320,000
2,320,000
2.1 × 108
Poisson’s ratio
ν
0.38
0.3
0.26
0.2
0.19
0.3
Table 10. Applied material properties of the grout injected into the weathered rock and soft rock.
Classification
Weathered Rock
(Injection Material)
Soft Rock
(Injection Material)
Wetting
unit weight
γt
(kN/m3 )
23
25
Saturation
unit weight
γsat
(kN/m3 )
23.5
25.5
Cohesion
c
(kN/m2 )
30
50
Internal
friction angle
φ
(◦ )
33
35
Elastic modulus
coefficient
E
(kN/m2 )
928,000
928,000
Poisson’s ratio
ν
0.3
0.3
4.1.4. Modeling
The internally excavated composite pile modeled was composed of a steel pipe pile
with a diameter of 600 mm and thickness of 14 mm from the leveling concrete to a depth of
6.5 m, which was a reinforced concrete pile with a diameter of 600 mm from the end of the
steel pipe pile to the tip. The stratum structure was modeled as shown in Figure 8 based
on the BH-2 drilling column.
In the numerical analysis, the composite steel pipe pile and reinforced concrete pile were
modelled as elastic materials and a leveling concrete base buoyancy of 29,929.973 kN/m2 was
applied to the upper leveling concrete upwards.
Figures 9 and 10 present the analysis results and show the maximum value of the
vertical displacement in each node. The pile end had the maximum displacement in the
nodes, as referred to in the boxes in Figures 9 and 10.
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Table 11. Numerical analysis results.
Displacement (cm)
Stability
(based on 1 inch)
Pile tip
0.22 to 0.29
0.29 ≤ 2.54_stable
Total displacement
0.70 to 0.77
0.77 ≤ 2.54_stable
Pile tip
0.64 to 0.89
0.89 ≤ 2.54_stable
Total displacement
2.37~2.61
2.61 ≥ 2.54_unstable
Pile tip
0.22 to 0.29
0.29 ≤ 2.54_stable
Total displacement
0.70 to 0.77
0.77 ≤ 2.54_stable
P2 bridge direction
Pile tip
Underwater grout
Totalsection.
displacement
Figure 8. Analysis
Consider material properties
0.64 to 0.89
0.89 ≤ 2.54_stable
2.37~2.61
2.61 ≥ 2.54_unstable
Classification
P2 lateral direction
Soft rock at 1.92 m
P2 lateral direction
Underwater grout
Consider material properties
P2 bridge direction
Soft rock at 2.21 m
Figure 8. Analysis section.
4.2. Numerical Analysis Review
In the case of the P2 lateral direction (90°→270°), as shown in Figure 9a, the pull-out
displacement of the tip of the pile foundation was 0.22 to 0.29 cm and the total pull-out
displacement of the structure was 0.70 to 0.77 cm during the integral movement, as shown
in Figure 9a. It is within safe limits because it is smaller than the safety standard of 1 inch
(2.54 cm). Figure 9b is the result of the on-site pull-out load test and the numerical analysis
reflects the field conditions. For the modulus of the grout injected in the weathered rock
and soft rock, 40% of the modulus of the elasticity of concrete was applied. As a result of
the numerical analysis, the pull-out displacement at the tip of the pile foundation was 0.64
to 0.89 cm and the overall pull-out displacement was 2.37 to 2.61 cm. It was higher than 1
inch (2.54 cm) and outside of safety standards.
In the case of the direction perpendicular (0°→180°) to the P2 pier, as shown in Figure
10a, the pull-out displacement of the tip of the pile foundation was 0.22 to 0.29 cm and the
overall pull-out displacement was 0.70 to 0.77 cm, which is within the safe range. However, as depicted in Figure 10b, considering the field conditions, it was found that the total
pull-out displacement of the structure was 2.37 to 2.61 cm, similar to the lateral direction
and thus unstable.
As a result of the numerical analysis, the integral behavior of the pile foundation and
the ground appeared to be stable when the site conditions were not sufficiently considered, but in the numerical analysis considering the site conditions, it was found that the
(a)
pull-out displacement exceeded the standard of 1 inch and was unstable. Therefore, when
designing the skin friction
(pull-out
Figure
9. Cont.force) of the pre-bored pile embedded in bedrock, the
site conditions should be sufficiently considered.
Table 11 summarizes the numerical analysis results.
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19 of
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22
19
(b)
(b)
Figure
P2P2
direction,
(a)(a)
SoftSoft
rock
condition
at 1.92
m and
the integrated
behavior
of both
Figure 9.
9. Numerical
Numericalanalysis
analysisresult
resultinin
inthe
the
direction,
rock
condition
at 1.92
1.92
m and
and
the integrated
integrated
behavior
of
Figure
9.
Numerical
analysis
result
the
P2 direction,
(a) Soft
rock
condition
at
m
the
behavior
of
the
pile
foundation
and
ground;
(b)
The
soft
rock
insertion
condition
of
1.92
m
and
the
on-site
pull-out
load
test
results
are
both
the
pile
foundation
and
ground;
(b)
The
soft
rock
insertion
condition
of
1.92
m
and
the
on-site
pull-out
load
test
both the pile foundation and ground; (b) The soft rock insertion condition of 1.92 m and the on-site pull-out load test
reflective
this.
results are
areofreflective
reflective
of this.
this.
results
of
(a)
(a)
Figure 10. Cont.
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(b)
Figure
perpendicular
direction
of of
thethe
P2 P2
pier.
(a) (a)
SoftSoft
rockrock
condition
at 2.21
m and
the
Figure10.
10.Numerical
Numericalanalysis
analysisresults
resultsininthe
the
perpendicular
direction
pier.
condition
at 2.21
m and
integrated
behavior
of
both
the
pile
foundation
and
ground.
(b)
Soft
rock
penetration
condition
at
2.21
m,
reflecting
the
the integrated behavior of both the pile foundation and ground. (b) Soft rock penetration condition at 2.21 m, reflecting
results
of the
pull-out
load load
test. test.
the results
ofon-site
the on-site
pull-out
For the modeling of pre-bored piles embedded in the rock, material properties of
5. Conclusions
weathered rock and soft rock, where the piles are embedded, were compared on the basis
In this study, the pre-bored pile of OO bridge pier P2 and the pull-out phenomenon
of the material properties of grout materials with a low elastic modulus, as shown Table 9.
observed at C.T cofferdam were reviewed according to the design standards of the prebored
pile reflected
in Review
the design. The stability of C.T cofferdam was reviewed by evalu4.2.
Numerical
Analysis
ating the adhesion strength of the grout and ◦concrete
surface attached to the bedrock. The
In the case of the P2 lateral direction (90 →270◦ ), as shown in Figure 9a, the pull-out
following conclusions
drawn.
displacement
of the tipwere
of the
pile foundation was 0.22 to 0.29 cm and the total pull-out
According
to
the
existing
design
the theoretical
equation
for skin
displacement of the structure was
0.70 todata
0.77and
cm during
the integral
movement,
as friction
shown
embedded
in
bedrock,
most
of
the
allowable
skin
friction
forces
were
more
than
kN,
in Figure 9a. It is within safe limits because it is smaller than the safety standard of600
1 inch
thus
it
was
safe
in
design.
However,
in
the
field,
the
pre-bored
pile
showed
instability.
(2.54 cm). Figure 9b is the result of the on-site pull-out load test and the numerical analysis
This means
that the
skin friction
of the
drilled shaft
during the
design
and therock
prereflects
the field
conditions.
For the
modulus
of theapplied
grout injected
in the
weathered
bored
applied
the construction
were not
the same,
and
it seems
have ocand
softpile
rock,
40% ofduring
the modulus
of the elasticity
of concrete
was
applied.
Asto
a result
of
curred
because
the
pull-out
resistance
of
the
pre-bored
pile
was
the
lower
one
the
the numerical analysis, the pull-out displacement at the tip of the pile foundation wasof0.64
drilled
shafts.
to
0.89 cm
and the overall pull-out displacement was 2.37 to 2.61 cm. It was higher than
As
a result
of the
lab test
adhesion
strength, the strength during the air curing of
1 inch (2.54
cm) and
outside
of for
safety
standards.
◦ ) to the
concrete
was
overestimated
as
881
kN
compared
to◦the
pull-out
force
600 kN
as applied
In the case of the direction perpendicular (0
→180
P2ofpier,
as shown
in
during10a,
the the
design
and displacement
the strength for
the tip
water
curing
of concrete was
was 0.22
542 to
kN,
which
Figure
pull-out
of the
of the
pile foundation
0.29
cm
showed
similarpull-out
results. displacement
However, when
variables
seawater
and
the overall
wasvarious
0.70 to field
0.77 cm,
which(slurry,
is within
the safeinflow,
range.
etc.)
occurring
in
the
field
were
taken
into
consideration,
the
adhesion
strength
was
exHowever, as depicted in Figure 10b, considering the field conditions, it was found that
pected
to
be
lower.
the total pull-out displacement of the structure was 2.37 to 2.61 cm, similar to the lateral
As aand
result
of re-examining
the allowable pull-out force according to the pull-out load
direction
thus
unstable.
test, As
thea pull-out
is 399.7 kN,
which
about 66%
of theofpull-out
force of 600and
kN
result of force
the numerical
analysis,
theisintegral
behavior
the pile foundation
applied
to the
design.toThe
resultswhen
of thethe
on-site
pull-out load
test
the theoretical
equathe
ground
appeared
be stable
site conditions
were
notand
sufficiently
considered,
tioninwere
compared.analysis
The method
of obtaining
theconditions,
skin frictionit of
thefound
pre-bored
pilepulland
but
the numerical
considering
the site
was
that the
the displacement
method of obtaining
both
uniaxialofcompressive
of the rock
and the
conout
exceeded
thethe
standard
1 inch and strength
was unstable.
Therefore,
when
designing
the skin
friction
ofcalculated
the pre-bored
pile embedded
in bedrock,
the
crete pile were
similar,
and(pull-out
the skin force)
friction
by these
two methods
showed simsite
should be sufficiently considered.
ilar conditions
results as well.
Table
11 summarizes
the numerical
analysis
results.
As a result
of the numerical
analysis
review,
in the case of the integral pull-out behavior of the pile foundation and ground, the displacement of the entire structure was
Materials 2021, 14, 5593
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Table 11. Numerical analysis results.
Classification
P2 lateral direction
Soft rock at 1.92 m
P2 lateral direction
Underwater grout
Consider material properties
P2 bridge direction
Soft rock at 2.21 m
P2 bridge direction
Underwater grout
Consider material properties
Pile tip
Displacement (cm)
Stability
(Based on 1 inch)
0.22 to 0.29
0.29 ≤ 2.54_stable
Total displacement
0.70 to 0.77
0.77 ≤ 2.54_stable
Pile tip
0.64 to 0.89
0.89 ≤ 2.54_stable
Total displacement
2.37~2.61
2.61 ≥ 2.54_unstable
Pile tip
0.22 to 0.29
0.29 ≤ 2.54_stable
Total displacement
0.70 to 0.77
0.77 ≤ 2.54_stable
Pile tip
0.64 to 0.89
0.89 ≤ 2.54_stable
Total displacement
2.37~2.61
2.61 ≥ 2.54_unstable
5. Conclusions
In this study, the pre-bored pile of OO bridge pier P2 and the pull-out phenomenon
observed at C.T cofferdam were reviewed according to the design standards of the prebored pile reflected in the design. The stability of C.T cofferdam was reviewed by evaluating
the adhesion strength of the grout and concrete surface attached to the bedrock. The
following conclusions were drawn.
According to the existing design data and the theoretical equation for skin friction
embedded in bedrock, most of the allowable skin friction forces were more than 600 kN,
thus it was safe in design. However, in the field, the pre-bored pile showed instability. This
means that the skin friction of the drilled shaft applied during the design and the pre-bored
pile applied during the construction were not the same, and it seems to have occurred
because the pull-out resistance of the pre-bored pile was the lower one of the drilled shafts.
As a result of the lab test for adhesion strength, the strength during the air curing of
concrete was overestimated as 881 kN compared to the pull-out force of 600 kN as applied
during the design and the strength for the water curing of concrete was 542 kN, which
showed similar results. However, when various field variables (slurry, seawater inflow, etc.)
occurring in the field were taken into consideration, the adhesion strength was expected to
be lower.
As a result of re-examining the allowable pull-out force according to the pull-out load
test, the pull-out force is 399.7 kN, which is about 66% of the pull-out force of 600 kN
applied to the design. The results of the on-site pull-out load test and the theoretical
equation were compared. The method of obtaining the skin friction of the pre-bored pile
and the method of obtaining both the uniaxial compressive strength of the rock and the
concrete pile were similar, and the skin friction calculated by these two methods showed
similar results as well.
As a result of the numerical analysis review, in the case of the integral pull-out behavior
of the pile foundation and ground, the displacement of the entire structure was stable
within the allowable value, but when using the material properties of the grout, considering
the field conditions, the displacement of the entire structure exceeded the allowable value.
This appears to have lowered the adhesion strength due to construction issues such as
ground complexity as well as seawater and slime treatment, which were not expected at
the time of design.
Author Contributions: Conceptualization, K.P.; methodology, software and writing—original draft
preparation, D.K.; writing—review and editing and project administration, G.K. and W.L.; funding
acquisition. All authors have read and agreed to the published version of the manuscript.
Materials 2021, 14, 5593
21 of 21
Funding: This study was supported by Chosun University, 2021.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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