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공학석사 학위논문

Assessment of Load Transfer Mechanism of

Compression Anchor Using Finite Element

Analysis

유한요소해석을 통한 압축형 앵커의 하중전이

메커니즘 평가

2016년 2월

서울대학교 대학원

건설환경공학부

주 혁 준


Compression Anchor Using Finite Element

Analysis

지도교수 정 충 기

이 논문을 공학석사 학위논문으로 제출함

2016 년 2 월

서울대학교 대학원

건설환경공학부

주 혁 준

주혁준의 석사 학위논문을 인준함

2016 년 1 월

위 원 장 박 준 범 (인)

부위원장 정 충 기 (인)

위 원 조 완 제 (인)

i

Abstract


Compression Anchor Using Finite Element Analysis

Joo, Hyeok Jun

Department of Civil and Environmental Engineering

The Graduate School

Seoul National University

Ground anchors are mainly used for excavation works of earth

retaining wall. According to the load transfer methods, ground anchors are

divided into pressure type anchor, friction type anchor and hybrid anchor.

Friction type anchor resists the pull-out load by skin friction between grout

and soil, and friction type anchors are separated the tension anchor and

compression anchor.

Compression anchor has benefits comparing to the tension anchor.

For the tension anchor, tensile failure can occur on the grout material and

tensile failure leads to progressive failure of ground anchor, but it is not for

the compression anchor. And steel tendons at the tension anchor cannot be

removed after excavation works, while steel tendons at the compression

anchor are removable. Therefore, removable compression anchor is used

widely in the urban excavation works.

However, researches on the compression anchor are insufficient

compared to tension anchor and even when designing compression anchor,

ii

design methods for tension anchor are used recently. Furthermore,

compression anchors are divided into load concentrative compression anchor

which has single anchorbody and load distributive compression anchor which

has multi anchorbodies. Depending on the number and spacing of

anchorbodies, load distributive compression anchor has complex load transfer

mechanism and it is hard to predict load transfer mechanism of load

distributive compression anchor.

In this study, general purpose finite element analysis program 'Midas

GTS NX' is used to evaluate the load transfer mechanism of compression

anchor. Reliability of the FEM techniques which are applied to this study is

secured through a comparative analysis of the existing studies carried out for

the tension anchor, compression anchor and load distributive compression

anchor. Load distributive compression anchors which are anchored on

weathered soil, weathered rock and soft rock are simulated and spacing of

anchorbodies varies 1 m to 2 m for evaluating load transfer mechanism of the

compression anchor.

Keywords: Compression anchor, Load distributive compression anchor,

Finite element method, Pull-out load, Spacing of

anchorbodies, Weathered residual soil, Weathered rock, Soft

rock

Student Number: 2014-20545

iv

Contents

Chapter 1 Introduction........................................................... 1

1.1 Background .......................................................................... 1

1.2 Objectives ............................................................................ 3

1.3 Dissertation Organization ..................................................... 4

Chapter 2 Literature Review.................................................. 6

2.1 Ground anchor………… ...................................................... 6

2.1.1 Types of ground anchor ............................................... 6

2.1.2 Load distribution of tension anchor.............................. 8

2.1.3 Load distribution of compression anchor ..................... 9

2.2 Load distributive compression anchor ................................ 11

2.2.1 Components of compression anchor .......................... 13

2.2.2 Load distribution of LDCA anchor ............................ 15

2.3 Previous numerical applications for ground anchor ............ 16

2.3.1 Tension anchor .......................................................... 16

2.3.2 Compression anchor .................................................. 20

2.3.3 Load distributive compression anchor ........................ 22

v

Chapter 3 Numerical Modeling for Ground Anchor ............ 28

3.1 Introduction ....................................................................... 28

3.2 Modeling case .................................................................... 28

3.2.1 Tension anchor .......................................................... 28



3.3 Modeling methodology ...................................................... 31

3.3.1 Material parameter .................................................... 35

3.3.2 Interface parameter .................................................... 39

3.4 Results of the numerical modeling .................................... 43

3.4.1 Tension anchor .......................................................... 43



Chapter 4 Numerical Simulations for Load Distributive

Compression Anchor ............................................. 53

4.1 Introduction ....................................................................... 53

4.2 Numerical simulation according to ground conditions ...... 55

4.2.1 Modeling methodology .............................................. 56

4.2.2 Results of simulation ................................................. 57

4.3 Numerical simulation according to ground conditions and

spacing of anchorbodies ..................................................... 62

4.3.1 Modeling methodology .............................................. 63

4.3.2 Results of simulation ................................................. 64

4.4 Summary and conclusions .................................................. 72

vi

Chapter 5 Conclusions......................................................... 74

Reference ............................................................................ 76

Abstract (Korean) ................................................................ 78

vii

List of Tables

Table 2.1 Soil material parameters examined in the finite element analyses

(Kim et al., 2007) ....................................................................... 18

Table 3.2 Material parameters for tension anchor and compression anchor

(case 1 and case 2) ..................................................................... 36

Table 3.3 Material parameters for load distributive compression anchor

(case 3) ...................................................................................... 37


(case 4) ...................................................................................... 38

Table 3.5 Interface parameters for tension anchor and compression anchor

(case 1 and case 2) ..................................................................... 40

Table 3.6 Interface parameters for load distributive compression anchor (case

3) ............................................................................................... 41

Table 3.7 Interface parameters for load distributive compression anchor (case

4) ............................................................................................... 42


(both simulations) ...................................................................... 54

viii

List of Figures

Figure 2.1 Classification of ground anchor ..................................................... 7

Figure 2.2 Schematic load distribution near ultimate load of tension anchor

(Briaud et al., 1998) ..................................................................... 8

Figure 2.3 Pull out load test of tension anchor (Katsura, 1987) ....................... 9

Figure 2.4 Pull out load test of compression anchor (Katsura, 1987) ............. 10

Figure 2.5 Schematic load distribution of compression anchor ...................... 11

Figure 2.6 Schematic friction stress of the load concentrative compression

anchor and load distributive compression anchor ........................ 13

Figure 2.7 Schematic diagram of load distributive anchor (Jason et al., 2015)14

Figure 2.8 Schematic load distribution of load distributive compression

anchor ........................................................................................ 15

Figure 2.9 Elevation and components of test anchor (Kim et al., 2007) ......... 17

Figure 2.10 Finite element mesh of tension anchor (Kim et al., 2007) ........... 18

Figure 2.11 Predicted load transfer on tension anchor : (a) load in strand, (b)

load in grout, (c) load resisted by soil, and (d) load transfer

distribution (Kim et al., 2007) ................................................ 19

Figure 2.12 Finite element mesh of compression anchor (Kim et al., 2007)... 20

Figure 2.13 Predicted load transfer on compression anchor : (a) load in grout,

(b) load resisted by soil, and (c) load transfer distribution (Kim et

al., 2007) .................................................................................... 21

Figure 2.14 Arrangement of anchorbodies and strain gauge

(Naganuma et al., 1997) ............................................................. 22

Figure 2.15 Ground conditions for pull out load test (Naganuma et al., 1997)23

Figure 2.16 Grout axial stress distribution expected (Naganuma et al., 1997) 24

Figure 2.17 Grout axial stress distribution observed (Naganuma et al., 1997) 25

Figure 2.18 Ground conditions for pull out load test (Naganuma et al., 1997)26

Figure 2.19 Grout axial stress distribution on the hard rock

ix

(Naganuma et al., 1997) ............................................................. 27

Figure 2.20 Grout axial stress distribution on the soft rock

(Naganuma et al., 1997) ............................................................. 27

Figure 3.1 Finite element mesh of tension anchor (case 1) ............................ 32

Figure 3.2 Finite element mesh of compression anchor (case 2) .................... 32

Figure 3.3 Finite element mesh of compression anchor (hard rock) (case 3) .. 33

Figure 3.4 Finite element mesh of compression anchor (soft rock) (case 4) ... 34

Figure 3.5 Load distribution in the steel strand of tension anchor (case 1) ..... 44

Figure 3.6 Load distribution in the grout of tension anchor (case 1) .............. 44

Figure 3.7 Load resisted by soil of tension anchor (case 1) ........................... 45

Figure 3.8 Friction stress distribution of tension anchor (case 1) ................... 45

Figure 3.9 Load distribution in the steel strand of compression anchor

(case 2) ...................................................................................... 47

Figure 3.10 Load distribution in the grout of compression anchor (case 2) .... 47

Figure 3.11 Load resisted by soil of compression anchor (case 2) ................. 48

Figure 3.12 Friction stress distribution of compression anchor (case 2) ......... 48

Figure 3.13 Load distribution in the grout of load distributive compression

anchor (case 3) ........................................................................... 50


anchor (case 4) ........................................................................... 51

Figure 4.1 Finite element mesh of load distributive compression anchor

(simulation 1) ............................................................................. 55

Figure 4.2 Grout axial force of load distributive compression anchor

(simulation 1) ............................................................................. 57

Figure 4.3 Load resisted by soil of load distributive compression anchor

(simulation 1) ............................................................................. 59

Figure 4.4 Friction stress distribution of load distributive compression anchor

(simulation 1) ............................................................................. 60


x

(simulation 2) ............................................................................. 62

Figure 4.6 Grout axial force of load distributive compression anchor installed

at weathered soil (simulation 2) .................................................. 64


at weathered rock (simulation 2) ................................................. 65


at soft rock (simulation 2) ........................................................... 66

Figure 4.9 Friction stress distribution of load distributive compression anchor

installed at weathered soil (simulation 2) .................................... 68

Figure 4.10 Friction stress distribution of load distributive compression

anchor installed at weathered rock (simulation 2) ....................... 69


anchor installed at soft rock (simulation 2) ................................. 70

1

Chapter 1 Introduction

1.1 Background

Ground anchors are mainly used for excavation works of earth

retaining wall. According to the load transfer methods, ground anchors are

divided into pressure type anchor, friction type anchor and hybrid anchor.

Friction type anchor resists the pull-out load by skin friction between grout

and soil, and friction type anchors are separated by a tension anchor in which

tensile force is applied to the grout at the bonded length and compression

anchor in which compressive force is applied to the grout.

Compression anchor has a steel tendons which are covered by the

sheath tube over the entire length of the anchor and steel tendons are

connected in the lower structure called anchorbody. Through the anchorbody,

pull-out load is transferred to the compressive force on the grout.

Compression anchor has benefits comparing to the tension anchor. For the

tension anchor, tensile failure can occur on the grout material and tensile

failure leads to progressive failure of ground anchor, but it is not for the

compression anchor. And steel tendons at the tension anchor cannot be

removed after excavation works, while steel tendons at the compression

2

anchor are removable. Therefore, removable compression anchor is used

widely in the urban excavation works. However, researches on the

compression anchor are insufficient compared to tension anchor and even

when designing compression anchor, design methods for tension anchor are

used recently.

Compression anchors are divided into load concentrative

compression anchor which has single anchorbody and load distributive

compression anchor which has multi anchorbodies. Depending on the

number and spacing of anchorbodies, load distributive compression anchor

has complex load transfer mechanism and it is hard to predict load transfer

mechanism of load distributive compression anchor.

In this study, general purpose finite element analysis program

'Midas GTS NX' is used to evaluate the load transfer mechanism of

compression anchor. Reliability of the FEM techniques which are applied to

this study is secured through a comparative analysis of the existing studies

carried out for the tension anchor, compression anchor and load distributive

compression anchor. Load distributive compression anchors which are

anchored on weathered soil, weathered rock and soft rock are simulated and

spacing of anchorbodies varies 1 m to 2 m for evaluating load transfer

mechanism of the compression anchor.

3

1.2 Objectives

This dissertation deals with the load transfer mechanism of

compression anchor especially on load distributive compression anchor. This

study focuses on load distribution of the grout and friction stress distribution

of compression anchor by analyzing results of finite element method

simulations. The specific objectives of this study are as follows:

1. Securing reliability of the applied FEM methods by Comparing

previous numerical applications of the ground anchors

2. Simulating load distributive compression anchor subjected to

pull out load

3. Evaluating load transfer mechanism of the anchor according to

the ground conditions and spacing of anchorbodies

4

1.3 Dissertation Organization

This dissertation deals with the assessment of load transfer

mechanism of compression anchor.

Chapter 1.Introduction

Introduction is about research backgrounds and objectives, and

dissertation organization.

Chapter 2.Literature Review

Literature review for the classification and approximate load transfer

of ground anchors, and previous numerical applications.

Chapter 3.Numerical Modeling for Ground Anchor

To secure reliability of the applied finite element method by

comparing previous numerical applications of the ground anchors.

Chapter 4.Numerical Simulations for Compression Anchor

For the assessment of load transfer mechanism of compression

anchor, 2 kinds of numerical simulations are conducted. One is pull out

loading simulation according to the ground conditions and the others are pull

out loading simulation according to the ground conditions and the spacing of

anchorbodies.

5

Chapter 5. Conclusions

Summary and conclusions for this study are described and

recommendations are presented.

6

Chapter 2 Literature Review

2.1 Ground anchor

2.1.1 Types of ground anchor

Depending on the load transfer methods, ground anchors are

generally divided into pressure type anchor, friction type anchor and hybrid

anchor. Pressure type anchor resists the pull out load with the passive

resistance of the ground using pressure board and friction type anchor resists

the pull out load by skin friction between grout and soil. Hybrid anchor uses

both pressure type anchor and friction type anchor. Among them, friction

anchor is widely used for the ground which has low stiffness as weathered

soil.

Friction type anchors are separated the tension anchor in which

tensile force is applied to the grout and compression anchor in which

compressive force is applied to the grout. Tension anchor has a steel tendons

which are divided two parts; bonded length and unbonded length. At the

bonded length, steel tendons are covered by the sheath tube and at the

unbonded length, steel tendons are directly bonded to grout.

7

Compression anchor has a steel tendons which are covered by the

sheath tube over the entire length of the anchor and steel tendons are

connected in the lower structure called anchorbody. Through the anchorbody,

pull-out load is transferred to the compressive force on the grout.

Compression anchor is divided into load concentrative compression anchor

which has single anchorbody and load distributive compression anchor

which has multiple anchorbodies.

Figure 2.1 schematically shows pressure type anchor, tension type

anchor, load concentrative compression anchor and load distributive

compression anchor.

Figure 2.1 Classification of ground anchor

8

2.1.2 Load distribution of tension anchor

Briaud et al. (1996) assumed schematic load distribution near

ultimate load of tension anchor as Figure 2.2. According to this assumption,

pull out load is equal to accumulative load resisted by soil which is sum of

load in the steel tendon and load in the grout. Schematic friction stress of

grout-soil is load resisted by soil over area of frictional surface.

Figure 2.2 Schematic load distribution near ultimate load of tension anchor

(Briaud et al., 1998)

9

For the tension anchor, tensile force is applied to the grout at the

bonded length and pull out load transfers top to bottom of bonded length.

Figure 2.3 shows load distribution at the grout of tension anchor at the

bonded length.

Figure 2.3 Pull out load test of tension anchor (Katsura, 1987)

2.1.3 Load distribution of compression anchor

For the compression anchor, compressive force is applied to the

grout at the overall lengths and pull out load transfers bottom to top. It is

because strand is connected to anchorbody which exist bottom of the anchor.

Figure 2.4 shows load distribution at the grout of compression anchor by pull

out load test (Katsura, 1987). Figure 2.5 shows schematic load distribution

of compression anchor. When the pull out load is subjected to steel tendon,

firstly, load(p) is applied to steel tendon which is protected by sheath pipe

10

overall length. And assuming grout-soil sufficiently resists pull out load,

load in the grout approaches 0 and load resisted by soil approaches load(p) at

the specific depth. Also, friction stress of grout-soil is assumed constant

regardless of depth.

Figure 2.4 Pull out load test of compression anchor (Katsura, 1987)

11

Figure 2.5 Schematic load distribution of compression anchor

2.2 Load distributive compression anchor

Generally, compression anchor is divided into load concentrative

compression anchor which has single anchorbody and load distributive

compression anchor which has multiple anchorbodies. For the load

concentrative compression anchor, pull out load can be concentrated on the

single anchorbody. However, for the load distributive compression anchor,

pull out load can be divided according to the number of anchorbody.

12

Through the calculation of earth pressure applied to the earth

retaining wall, designing anchor force is determined. When installing the

ground anchor, generally 3 elements should be considered. First, the pull out

load applied to steel strand should be less than allowable load of the steel

strand to prevent failure of steel strand. Second, the internal stress of the

grout due to the pull out load should be less than allowable stress of the

grout to prevent failure of the grout. Third, skin friction stress between

grout-soil should be less than ultimate skin friction stress of the soil to

prevent failure between grout-soil interface.

For the load distributive compression anchor, applied pull out load

is distributed to each anchor using multiple anchorbodies. Therefore, applied

load to steel strand, the internal stress of the grout and skin friction stress

between grout-soil can be decreased in allowable range. Through the load

distribution, it is possible to secure design anchor force and grout

compressive failure can be prevented for load distributive compression

anchor. Figure 2.6 shows schematic friction stress of the load concentrative

compression anchor and load distributive compression anchor.

13

Figure 2.6 Schematic friction stress of the load concentrative compression

anchor and load distributive compression anchor

2.2.1 Components of compression anchor

Load distributive compression anchor is composed to anchor head,

wall system, steel strands which is connected to each anchorbody,

anchorbodies and grout. Figure 2.7 shows schematic diagram of load

distributive compression anchor (Jason et al., 2015).

14

Figure 2.7 Schematic diagram of load distributive anchor (Jason et al., 2015)

15

2.2.2 Load distribution of load distributive compression anchor

Figure 2.8 shows schematic load distribution of load distributive

compression anchor. It is assumed that grout-soil sufficiently resists pull out

load, load in the grout approaches 0 and load resisted by soil approaches

load(p) at the specific depth at the above the 3rd anchorbody. Also, friction

stress of grout-soil is assumed constant regardless of depth.

Figure 2.8 Schematic load distribution of load distributive compression

anchor

16

2.3 Previous numerical applications for ground anchor

There are several previous numerical analysis for ground anchor.

Park (2010) studied ground displacements when the pull out load is applied

to tension anchor at the weathered rock using finite difference method and

Park (2012) evaluated load transfer of tension anchor using finite difference

method.

For the compression anchor, Kim (2001) studied load transfer of

compression anchor using ABAQUS which is general purpose finite element

program and Kim et al. (2007) conducted numerical analysis and beam-

column analysis for the tension anchor and compression anchor. From these

researches, by comparing results of the field tests and results of numerical

analysis, load transfer behaviors of the tension anchor and compression

anchor are schematized.

For the load transfer of load distributive compression anchor,

Naganuma et al. (1996) conducted numerical analysis for the 4 anchorbodies

load distributive compression anchor and compared with field test results.

From this result, Naganuma evaluated load distribution of grout inner

stresses depending on the ground conditions.

In this section, 4 researches for tension anchor, compression anchor

and load distributive compression anchor are introduced.

2.3.1 Tension anchor

17

Kim et al. (2007) conducted finite element modeling and beam-

column modeling of ground anchor to investigate the load transfer

mechanism of tension anchor. In this research, the numerical predictions for

tension anchor were compared with the result of the field test. The program

ABAQUS was used for numerical analysis. Figure 2.9 shows the ground

condition and schematic components of test anchor, Table 2.1 is soil material

parameters used in the finite element analysis.

Figure 2.9 Elevation and components of test anchor (Kim et al., 2007)

18

Table 2.1 Soil material parameters examined in the finite element analyses

(Kim et al., 2007)

Material

( / )

( )

(°)

(°)

( )

Fill 18.6 0.3 15,000 25 0.6 1.0 4 5

Sandy Clay 17.6 0.3 20,000 30 0.5 1.0 4 10

Weathered soil 19.6 0.3 45,000 38 0.4 1.0 6 20

Kim et al. (2007) simulated 2D four-noded axisymmetric brick

elements and Drucker-Prager failure criterion was applied to soil material.

Figure 2.10 is the finite element mesh of tension anchor.

19

Figure 2.10 Finite element mesh of tension anchor (Kim et al., 2007)

Figure 2.11 is the measured and predicted load distribution of the

tension anchor. As shown below, at the grout material, tensile force is

applied to bonded length and compressive force is applied to unbonded

length. Friction stress was calculated as the procedure Briaud et al. (1998)

suggested.

In this results, Kim et al. (2007) evaluated that finite element

analysis and beam-column analysis can predict the load transfer of tension

anchor properly and both analyses give a good prediction for the ground

anchor.

20

Figure 2.11 Predicted load transfer on tension anchor : (a) load in strand, (b)

load in grout, (c) load resisted by soil, and (d) load transfer distribution

(Kim et al., 2007)

2.3.2 Compression anchor

Load transfer mechanism of compression anchor was investigated

by Kim et al. (2007) using finite element modeling and beam-column

modeling. As the researches about tension anchor, the numerical predictions

for compression anchor were compared with the result of the field test.

Figure 2.9 shows the ground condition and schematic components of test

21

anchor, Table 2.1 is soil material parameters used in the finite element

analysis. Figure 2.12 is the finite element mesh of compression anchor.

Figure 2.12 Finite element mesh of compression anchor (Kim et al., 2007)

Figure 2.13 is the measured and predicted load distribution of the

compression anchor. As shown below, at the grout material, there is only

compressive force at the overall length. Friction stress was calculated as

assumed procedures that Briaud et al. (1998) suggested and friction stress

decreases gradually from the bottom to the top of the compression anchor.

22

Figure 2.13 Predicted load transfer on compression anchor : (a) load in grout,

(b) load resisted by soil, and (c) load transfer distribution (Kim et al., 2007)

2.3.3 Load distributive compression anchor

Naganuma et al. (1997) conducted finite element modeling of load

distributive compression anchor to investigate the load transfer mechanism

of load distributive compression anchor. In this research, 4 anchorbodies

load distributive compression anchor was used for investigation as shown

23

Figure 2.14. And evaluation of load transfer of anchor is limited to load

distribution at the grout body. They pulled out anchor with 18 ton for each

anchorbody (design load of load distributive compression anchor was

assumed 72 ton), and this pull out method is called each tension. For the

field test, totally 8 steps of test loads were applied and anchorbodies were

installed at hard rock. Total ground conditions are as shown Figure 2.15.

Figure 2.14 Arrangement of anchorbodies and strain gauge

(Naganuma et al., 1997)

24

Figure 2.15 Ground conditions for pull out load test (Naganuma et al., 1997)

25

Naganuma et al. (1997) predicted load distribution of load

distributive compression anchor as shown Figure 2.16. They predicted that

equally distributed compressive force is applied on the grout of compressive

anchor, but results of the field test drew different load distribution as shown

Figure 2.17. In this figure, there are not only compressive force but also

tensile force on the grout body. They concluded that load distributive

compression anchor is resisted by complex distribution of both compressive

force and tensile force, not only by compressive force as predicted before.

Figure 1.16 Grout axial stress distribution expected (Naganuma et al., 1997)

26

Figure 2.17 Grout axial stress distribution observed (Naganuma et al., 1997)

In a later studies, pull out tests of compression anchor installed at

the hard rock and soft rock was performed. Figure 2.18 is the ground

conditions of second field test and anchorbodies are located throughout 3

layers which have lower stiffness than first field test ground.

Figure 2.19 is grout axial distribution on the hard rock and Figure

2.20 is grout axial distribution on the soft rock. In the Figure 2.20, there is

only compressive force on the grout of load distributive compression anchor.

From this results, they concluded that load distribution of compression

anchor is different according to ground stiffness and if ground stiffness is

high, both compression and tension occur on the grout body of compression

anchor. In addition, they conducted finite element analyses and compared

27

FEM results with measured results from field tests as shown Figure 2.19 and

Figure 2.20.

28

Figure 2.18 Ground conditions for pull out load test (Naganuma et al., 1997)

29

Figure 2.19 Grout axial stress distribution on the hard rock


Figure 2.20 Grout axial stress distribution on the soft rock


28

Chapter 3 Numerical Modeling for Ground Anchor

3.1 Introduction

In order to secure the reliability of numerical modeling methods, I

apply numerical modeling method to previous numerical applications and

field test results. For the numerical modeling, general purpose finite element

program ‘Midas GTS NX’ for geotechnical engineering is used.

Finite element method is applied to 4 cases which are ground

anchors subjected pull out load; tension anchor, compression anchor and

load distributive compression anchor. Each cases were introduced at the

chapter 2 literature review.

3.2 Modeling cases


Case 1 is the tension anchor. As shown in the chapter 2.3.1, Kim et

al. (2007) conducted finite element modeling and beam-column modeling of

tension anchor and compared modeling results with field test results. In this

study, general purpose FEM program ‘ABAQUS’ was used and tension

anchor was installed at the weathered soil. Details of the modeling elevation

and components is shown at Figure 2.9.

29

The ground anchor was modeled as axisymmetric and mesh

consisted of 6147 nodes and 1981 elements. Ground depth was 20 m below

the ground surface and ground diameter was 20 m laterally. For the soil

element, Drucker-Prager failure criterion was applied. Strand was treated as

a linear elastic material and the diameter of strand was 12.7 mm, elastic

modulus of the strand was 2.07 × 10 / . Grout material was treated

as a linear elasto-perfect plastic material and the cross sectional area of the

grout was 20,888 , compressive strength of the grout was 20 MPa, the

tensile strength of the grout was 2.0 MPa and elastic modulus of the grout

was 2.1 × 10 / . Soil-grout and grout-strand interface surface model

was considered using the Coulomb friction model in ABAQUS. Finally, pull

out load was applied sequentially up to the design load (657.3 kN) (Kim et

al., 2007).


Case 2 is the compression anchor. Finite element modeling and

beam-column modeling of compression anchor was conducted by Kim et al.

(2007). Using general purpose finite element program ‘ABAQUS’,

compression anchor was installed at the weathered soil. Details of the

modeling elevation and components is shown at Figure 2.9.

Mesh consisted of 6672 nodes and 2152 elements for the

compression anchor, ground depth was 20 m below the ground surface and

ground diameter was 20 m laterally. The properties of soil elements, strand

30

and grout was same as the tension anchor described 3.2.1. Soil-grout and

grout-strand interface surface model was considered using the Coulomb

friction model in ABAQUS. Finally, pull out load was applied sequentially

up to the design load (657.3 kN) (Kim et al., 2007).


Case 3 and case 4 are load distributive compression anchor. As

shown chapter 2.3.3, Naganuma et al. (1997) conducted finite element

modeling of load distributive compression anchor which had 4 anchorbodies

and compared modeling results with field test results. In this study, load

distributive compression anchors are installed at the hard rock (case 3) and

soft rock (case 4). Ground conditions of case 3 is as shown Figure 2.15 and

case 4 is as shown Figure 2.18.

The ground anchor was modeled as axisymmetric and soil element

was treated elastic material. Grout material seems to be treated elastic

material and elastic modulus of the grout was 1.22 × 10 / . In the

case of these studies, specific values of material properties and information

about FEM program which was used were not reported at the paper.

31

3.3 Modeling methodology

Using Midas GTS NX, tension anchor installed at the weathered

soil, compression anchor installed at the weathered soil, load distributive

compression anchor installed at the hard rock and load distributive

compression anchor installed at the soft rock are modeled to secure

reliability of finite element method used in this study.

Case 1 is the tension anchor installed at weathered soil and it is

modeled as shown Figure 3.1 and Case 2 is the compression anchor installed

at weathered soil and details of modeling is shown as Figure 3.2. For the

both cases, ground depth is 20 m below the ground surface and grout

diameter is selected as 13.5 cm and ground extended 10 m laterally as the

modeling of Kim et al. (2007).

Case 3 is the load distributive compression anchor installed at hard

rock and Figure 3.3 is the details of modeling. Due to the insufficient details

of ground elevation from previous study, ground depth is decided as 35 m

below the ground surface. Total length of the ground anchor is 21 m and

grout diameter is selected as 13.5 cm

Case 4 is the load distributive compression anchor installed at soft

rock and Figure 3.4 is the details of modeling. Ground depth is decided as 25

m and total length of the ground anchor is 17.5 m, and grout diameter is

selected as 13.5 cm

In addition, bottom boundary and side boundary of ground are fixed

and displacements at boundaries are constrained.

32

Figure 3.1 Finite element mesh of tension anchor (case 1)

Figure 3.2 Finite element mesh of compression anchor (case 2)

33

Figure 3.3 Finite element mesh of compression anchor (hard rock) (case 3)

34

Figure 3.4 Finite element mesh of compression anchor (soft rock) (case 4)

35

3.3.1 Material parameter

Selecting material input parameter is important to evaluate load

transfer of ground anchor using finite element method. In this section,

material parameters which are applied to FEM program will be shown. It is

obvious that there are several limitations to simulate 4 cases exactly.

Because field tests of 4 cases were performed by previous researchers and

there were omitted material parameters and ground conditions.

Material parameters are about steel strand, grout, anchorbody and

ground conditions. In this study, reasonable values for material parameters

are applied to model 4 types of ground anchor if there are not given values.

In addition, for the case 3 and 4 which are modeled load distributive

compression anchor, soil parameters are partially adjusted as previous

studies. Soil material was assumed to be elasto-perfectly plastic materials

obeying the Mohr-Coulomb model and steel strand was assumed to obey

Von mises model, and other materials were assumed to be elastic. For the

unknown factors of soil parameter, applied soil material parameters are

referred to the ‘Ground investigation manual of Seoul (2006)’.

Table 3.2, Table 3.3 and Table 3.5 are soil material parameters for

Case 1, Case 2, Case 3 and Case 4.

36

Table 3.2 Material parameters for tension anchor and compression anchor

(case 1 and case 2)

Material Model

( / )

c

( / )

∅

(°)

E

( / )

Soil (0-4 m) Mohr-Coulomb 20 10 33 0.80 × 10 0.32



Grout Elastic 21 - - 2.09 × 10 0.30

Strand Von mises 77 - - 1.56 × 10 0.28

Anchorbody Elastic 77 - - 2.00 × 10 0.30

37


(case 3)

Material Model

( / )

c

( / )

∅

(°)

E

( / )

Clay

(0-2.75m) Mohr-Coulomb 19 50 28 35,672 0.30

Boulder

(2.75-6.15m) Mohr-Coulomb 21 150 32 137,200 0.25

Black schist

(6.15-7.20m) Mohr-Coulomb 20 100 30 49,000 0.30

Green schist

(7.20-11.00m) Mohr-Coulomb 22 300 35 196,000 0.25

Green schist

(11.00-13.25m) Mohr-Coulomb 24 450 35 294,000 0.25

Green schist

(13.25-21.00m) Mohr-Coulomb 24 800 37 2,940,000 0.20

Green schist

(21.00-30.00m) Mohr-Coulomb 24 800 37 2,940,000 0.20

Grout Elastic 21 - - 2.00 × 10 0.30

Strand Von mises 77 - - 2.00 × 10 0.28


38


(case 4)

Material Model

( / )

c

( / )

∅

(°)

E

( / )

Fill

(0-0.8m) Mohr-Coulomb 18.5 50 28 27,440 0.45

Loam

(0.8-5.4m) Mohr-Coulomb 18.5 50 28 27,440 0.45

Green schist

(5.4-6.5m) Mohr-Coulomb 20 100 30 49,000 0.40

Green schist

(6.5-7.5m) Mohr-Coulomb 23 300 35 196,000 0.30

Green schist

(7.5-13.4m) Mohr-Coulomb 20 100 30 49,000 0.40

Green schist

(13.4-15.0m) Mohr-Coulomb 25.5 1200 38 98,000 0.20

Green schist

(15.0-25.0m) Mohr-Coulomb 23 300 35 49,000 0.30

Grout Elastic 21 - - 2.00 × 10 0.30

Strand Von mises 77 - - 2.00 × 10 0.28


39

3.3.2 Interface parameter

It is necessary to configure the interface elements in addition to the

material parameters in order to model the ground anchor subjected pull out

load. Interface element was developed to describe the interface behavior

between the homogeneous materials or heterogeneous materials. It is based

on the Coulomb’s friction law and mainly used to simulate the interface

between piles and soil, earth retaining walls and foundations.

At the ‘Midas GTS NX’ program, the interface elements between

steel strand – grout, grout – soils are governed by Coulomb’s friction law.

Input parameters consist of vertical rigidity modulus (Kn), shear rigidity

modulus (Kt), cohesion and friction angle. Vertical rigidity modulus is

generally 1 to 10 times of smaller oedometeric modulus between materials

and shear rigidity modulus is generally 1 to 10 times of smaller shear

modulus between materials. However, determining vertical rigidity modulus

(Kn) and shear rigidity modulus (Kt) depends upon to empirical method.

In this study, vertical rigidity modulus and shear rigidity modulus

of interface elements are adjusted differently. Through the trial and error,

vertical rigidity modulus and shear rigidity modulus of interface elements

are determined as shown Table 3.5 to Table 3.7.

40

Table 3.5 Interface parameters for tension anchor and compression anchor

(case 1 and case 2)

Materials

( / )

( / )

c

( / )

∅

(°) Remarks

Strand-grout

(unbonded) 0 0 - - Case 1

Strand-grout

(bonded) 20,900 20,900 - - Case 1

Grout

-soil

Soil (0-4 m) 8,000 80,000 10 33 = 1 ×

= 10 ×

Soil (4-8 m) 15,000 150,000 10 35 = 1 ×

= 10 ×

Soil (8-20 m) 36,000 360,000 10 35 = 1 ×

= 10 ×

41

Table 3.6 Interface parameters for load distributive compression anchor

(case 3)

Materials

( / )

( / )

c

( / )

∅

(°) Remarks

Grout

-soil

Clay

(0-2.75m) 35,672 356,720 50 28

= 1 ×

= 10 ×

Boulder

(2.75-6.15m) 137,200 1,372,000 150 32

= 1 ×

= 10 ×

Black schist

(6.15-7.20m) 49,000 490,000 100 30

= 1 ×

= 10 ×

Green schist

(7.20-11.00m) 196,000 1,960,000 300 35

= 1 ×

= 10 ×

Green schist

(11.00-13.25m) 294,000 2,940,000 450 35

= 1 ×

= 10 ×

Green schist

(13.25-21.00m) 2,940,000 29,400,000 800 37

= 1 ×

= 10 ×

42

Table 3.7 Interface parameters for load distributive compression anchor

(case 4)

Materials

( / )

( / )

c

( / )

∅

(°) Remarks

Grout

-soil

Fill

(0-0.8m) 27,440 27,440 50 28

= 1 ×

= 1 ×

Loam

(0.8-5.4m) 27,440 27,440 50 28

= 1 ×

= 1 ×

Green schist

(5.4-6.5m) 49,000 49,000 100 30

= 1 ×

= 1 ×

Green schist

(6.5-7.5m) 196,000 196,000 300 35

= 1 ×

= 1 ×

Green schist

(7.5-13.4m) 49,000 800,000 100 30

= 1 ×

= 16 ×

Green schist

(13.4-15.0m) 98,000 98,000 1200 38

= 1 ×

= 1 ×

Green schist

(15.0-25.0m) 49,000 49,000 300 35

= 1 ×

= 1 ×

43

3.4 Results of the numerical modeling

In this chapter, results of the applied numerical modeling is shown

comparing with previous application of each ground anchor. Material

properties and interface properties were described at the chapter 3.3.

For the tension anchor and compression anchor, there are load

distribution in the steel strand, load distribution in the grout, load resisted by

soil and finally friction stress distribution. However, for the load distributive

compression anchor, there are only load distribution in the grout. It is

because previous studies about load distributive compression anchor by

Naganuma et al. (1997) suggested load distribution in the grout only.


Figure 3.5 shows load distribution in the steel strand of tension

anchor and results from applied finite element method are ‘Midas GTS NX’

line. Measured line is empirical data of previous study and predicted line is

results of applied numerical analysis. Likewise, Figure 3.6, Figure 3.7 and

Figure 3.8 show load distribution in the grout of the tension anchor, load

resisted by soil of tension anchor and friction stress distribution of tension

anchor in sequence. As shown at each Figure, results of applied finite

element method well fit the previous experimental data and previous applied

numerical method results.

44

Figure 3.5 Load distribution in the steel strand of tension anchor (case 1)

Figure 3.6 Load distribution in the grout of tension anchor (case 1)

45

Figure 3.7 Load resisted by soil of tension anchor (case 1)

Figure 3.8 Friction stress distribution of tension anchor (case 1)

46


Figure 3.9 shows load distribution in the steel strand of

compression anchor. From previous studies, results of load distribution in the

steel strand was omitted. Figure 3.10, Figure 3.11 and Figure 3.12 show load

distribution in the grout of the compression anchor, load resisted by soil of

compression anchor and friction stress distribution of compression anchor in

sequence. Measured line is empirical data of previous study, predicted line is

results of applied numerical analysis and results from applied finite element

method are shown as ‘Midas GTS NX’ line. As shown at each figure, results

of applied finite element method well fit the previous experimental data and

previous applied numerical method results for compression anchor.

47

Figure 3.9 Load distribution in the steel strand of compression anchor

(case 2)

Figure 3.10 Load distribution in the grout of compression anchor (case 2)

48

Figure 3.11 Load resisted by soil of compression anchor (case 2)

Figure 3.12 Friction stress distribution of compression anchor (case 2)

49


Figure 3.13 and Figure 3.14 show load distribution in the grout of

load distributive compression anchor. Figure 3.13 is for the load distribution

compression anchor installed at hard rock and Figure 3.14 is for the load

distribution compression anchor installed at soft rock. From previous studies,

results of load distribution at the grout were described and other load

distributions were omitted. Measured line is empirical data of previous study,

predicted line is results of applied numerical analysis and results from

applied finite element method are shown as ‘Midas GTS NX’ line. As shown

at Figure 3.13, results of applied finite element method well fit the previous

experimental data and previous applied numerical method results for load

distributive compression anchor installed at hard rock. However, for the load

distribution in the grout of load distributive compression anchor installed at

soft rock, overall results do not fit well. It is considered that modeling

grounds consist various layers of soils especially at the anchor bonded depth

as shown before at Table 3.4.

50


anchor (case 3)

51


anchor (case 4)

52

From the results of numerical modeling, applied finite element

method well fit for tension anchor, compression anchor and load distributive

compression anchor. Therefore, it is considered that reasonable predicted

results for load distribution of compression anchor can be derived using

applied finite element method.

For the numerical modeling, it is obvious that material properties

and interface element properties are the most important factors. For

modeling compression anchor subjected pull out load, it is verified that

among material properties and interface element properties, elastic modulus

of soil and shear rigidity modulus of grout-soil interface element were

critical factors.

From above modeling, elastic modulus of soil for tension and

compression anchor was applied same with field conditions and elastic

modulus of soil for load distributive compression anchor was partially

adjusted as previous studies. For seeing the proper tendencies of load

transfer of ground anchor using finite element method, ground condition

should be simplified and various layers of soils lead to inaccurate finite

element analysis results. In regard to interface elements between grout and

soils, applied vertical rigidity modulus (Kn) was 1 × and shear

rigidity modulus (Kt) was 10 × except for case 4. Therefore, applied

rigidity modulus of interface elements is considered to lead proper

expectations of load transfer of ground anchor.

53

Chapter 4 Numerical Simulations for Load Distributive

Compression Anchor

4.1 Introduction

Based on the chapter 3, tension anchor, compression anchor and

load distributive compression anchor were modeled and applicability of the

proposed numerical model was verified.

In this chapter, 2 kinds of numerical simulations are conducted to

evaluate load transfer mechanism of compression anchor. Firstly, pull out

loading simulation according to the ground conditions are conducted.

Through this simulation, load transfer of compression anchors which are

installed at weathered soil, weathered rock and soft rock will be evaluated.

Secondly, pull out loading simulation according to the ground conditions and

spacing of anchorbodies are conducted. From this simulation, load transfer

of the compression anchor can be evaluated depending on spacing of

anchorbodies.

For both simulations, analysis conditions are applied equally except

the configuration of meshes. Analysis conditions using finite element

method are as follows. Nonlinear axisymmetric 2D analysis is carried out

and pull out load is applied 0.65 × (ultimateloadofstrand) which is

allowable load for removable ground anchor from Ministry of Land,

Infrastructure and Transport of Korea (2010). In addition, pull out loads are

54

simulated as being applied same load for each strand or anchorbody and this

tension method is called each tension. Therefore, 278.2 kN for each

anchorbody is applied. Table 4.1 shows material parameter of both

simulations.


(both simulations)

Material Model

( / )

c

( / )

∅

(°)

E

( / )

Weathered soil Mohr-Coulomb 20 50 30 2.00 × 10 0.45

Weathered rock Mohr-Coulomb 22 200 35 2.00 × 10 0.40

Soft rock Mohr-Coulomb 25 450 40 2.00 × 10 0.30

Grout Elastic 21 - - 2.00 × 10 0.30


* : unit weight, c :cohesion, ∅:friction angle, E: young smodulus, :

*‘Ground investigation manual of Seoul (2006)’ is used for material properties

For both simulations, there were no steel strands and pull out load

is applied directly to the anchorbodies. In these simulations, tension type is

each tension and for the each tension condition it can be considered

reasonable to model load distributive compression anchor without steel

strands. For that reason, load distribution in the theoretical steel strand of

load distributive compression anchor is assumed constant for each pair of

steel strand.

55

4.2 Numerical simulations according to ground

conditions

Using Midas GTS NX, load distribution compression anchor

installed at the weathered soil, weathered rock and soft rock which is

subjected pull out load is simulated. Finite element mesh of simulation 1 is

as shown Figure 4.1. For this simulation, number of anchorbodies is 3 and

spacing of anchorbodies are 1.3 m.


(simulation 1)

56

4.2.1 Modeling methodology

At this simulation, allowable anchor force 278.2 kN for each

anchorbody is applied. Table 4.1 is the material parameter of this simulations.

Ground depth is 20 m below the ground surface and grout diameter is

selected as 10 cm and ground extended 5 m laterally. For this simulation, 3

anchorbodies are used and spacing of anchorbodies is assumed 1.3 m for

each ground condition. In addition, bottom boundary and side boundary of

ground are fixed and displacements at boundaries are constrained.

About application of interface elements, the interface elements

between steel strand – grout are neglected. There is no steel strand and it is

considered reasonable, because steel strand theoretically protected by sheath

pipe which make no friction between steel strand and grout material for

overall length of load distributive compression anchor. For the interface

elements between grout – soils, vertical rigidity modulus (Kn) is defined

same with Young’s modulus of soils adjacent to grout and shear rigidity

modulus (Kt) is defined 10 times larger than vertical rigidity modulus.

Values of vertical rigidity modulus and shear rigidity modulus are verified

reasonable from chapter 3. Cohesion and friction angle of soils are applied to

interface parameter related to Coulomb’s friction.

57

4.2.2 Results of simulation

Figure 4.2 is grout axial force of load distributive compression

anchor installed at weathered soil, weathered rock and soft rock.


(simulation 1)

58

As shown Figure 4.2, for the weathered soil there is only

compressive force on the grout and small decreasing rate of compressive

force relative to weathered rock and soft rock. And large compressive force

is applied to the higher parts; parts of #2 anchorbody and #3 anchorbody.

The reason for large compressive force at the higher parts are considered that

compressive force at the lower parts is not perfectly resisted by soil and

transfer to higher part, and finally lower compressive force overlaps with

pull out load at #2 anchorbody and #3 anchorbody.

Tendencies of load distribution at the grout are different for the

load distributive compression anchor installed at weathered rock and soft

rock. For the weathered rock and soft rock, generally compressive force is

applied to the grout, but tensile force is applied to the bottom of #2

anchorbody and #3 anchorbody. Applied tensile force at the soft rock is

larger than applied tensile force at the weathered rock. Therefore, effects of

tensile force to friction stress of anchor should be investigated. And large

decreasing rate of compressive force relative to weathered soil. In addition,

effects of overlapping compressive force seem to be small. It is because that

compressive force at the lower parts is mostly resisted by soil and remained

compressive force which transfers to higher part is small for high stiffness

soils.

59

Figure 4.3 is load resisted by soil of load distributive compression

anchor installed at weathered soil, weathered rock and soft rock. Load

resisted by soil means accumulative load resisted by grout-soil friction. At

the Figure 4.3, large increasing rate for the ground with high stiffness is

observed.

Figure 4.3 Load resisted by soil of load distributive compression anchor

(simulation 1)

60

Figure 4.4 is friction stress distribution of load distributive

compression anchor installed at weathered soil, weathered rock and soft rock.


anchor (simulation 1)

61

As shown Figure 4.4, the largest magnitude of friction stress is

applied for soft rock and rates of change of friction stress along the depth for

soft rock is large relative to the weathered soil and weathered rock. In

addition, length that friction stress approaching 0 is short for soft rock and

long for weathered soil. For the soft rock, friction stress approaches 0 at the

depth about 4 m, for the weathered rock which has lower stiffness, friction

stress approaches 0 at the depth about 2 m and for the weathered soil,

friction stress does not approach 0 before the depth is 0.

For the weathered soil, friction stress decreases in overall length,

but for the weathered rock and soft rock, friction stress both decreases and

increases around anchorbodies. The reason for both decreasing and

increasing of friction stress around anchorbodies is because of the parts at

which tensile force is applied. From the results of grout axial force of load

distributive compression anchor installed at weathered rock and soft rock,

there were tensile force which is applied to the bottom of #2 anchorbody and

#3 anchorbody. However, the effects of load distribution are valid for each

ground condition concluding weathered rock and soft rock at which tensile

force is applied.

62

4.3 Numerical simulations according to ground

conditions and spacing of anchorbodies

Secondly, load distribution compression anchor installed at the

weathered soil, weathered rock and soft rock which is subjected pull out load

is simulated for different spacing of anchorbodies. Finite element mesh of

simulation 2 is as shown Figure 4.5. For this simulation, number of

anchorbodies is 2 and spacing of anchorbodies are 1.0 m and 2.0 m.


(simulation 2)

63

4.3.1 Modeling methodology

Overall modeling methodology is same with simulation 1.

Allowable anchor force 278.2 kN for each anchorbody is applied. Table 4.1

is the material parameter of this simulations. Ground depth is 20 m below

the ground surface and grout diameter is selected as 10 cm and ground

extended 5 m laterally. Contrary to simulation 1, for this simulation, 2

anchorbodies are used and spacing of anchorbodies is varied 1.0 m to 2.0 m

to evaluate the effects of spacing of anchorbodies. Bottom boundary and side

boundary of ground are fixed and displacements at boundaries are

constrained.

About application of interface elements, modeling methodology for

simulation 2 is same with modeling methodology for simulation 1, as shown

chapter 4.2.1.

64

4.3.2 Results of simulation


anchor installed at weathered soil. Round scatters are for the 1.0 m spacing

of anchorbodies, triangle scatters are for the 2.0 m spacing of anchorbodies.


installed at weathered soil (simulation 2)

65


anchor installed at weathered rock. Round scatters are for the 1.0 m spacing

of anchorbodies, triangle scatters are for the 2.0 m spacing of anchorbodies.


installed at weathered rock (simulation 2)

66


anchor installed at soft rock. Round scatters are for the 1.0 m spacing of

anchorbodies, triangle scatters are for the 2.0 m spacing of anchorbodies.


installed at soft rock (simulation 2)

67

According to the results of grout axial force of load distributive

compression anchor, effects of spacing of anchorbodies are divided two parts.

First part is compression and second part is tension.

For the compression part, when the spacing of anchorbodies

increases 1.0 m to 2.0 m, compressive force of upper side around #2

anchorbody decreases for weathered soil, weathered rock and soft rock. It

means that large spacing of anchorbodies is good for the load distribution of

compression anchor. If applied compressive force is larger than allowable

compressive load of grout, compressive failure can occur. Therefore,

increasing spacing of anchorbodies can be one of the solutions for

preventing compressive failure of grout material and the lower limit of

spacing of anchorbodies should be determined by comparing the applied

compressive force with allowable compressive load of grout.

For the tension part, it is limited to weathered rock and soft rock,

because there is no tensile force for weathered soil. When the spacing of

anchorbodies increases 1.0 m to 2.0 m, tensile force of upper side around #2

anchorbody increases for weathered rock and soft rock. If applied tensile

force is larger than allowable tensile load of grout, tensile failure of grout

can occur. Therefore, increasing spacing of anchorbodies can lead to a

tensile failure of grout and the upper limit of spacing of anchorbodies should

be determined by comparing the applied tensile force with allowable tensile

load of grout.

68


compression anchor installed at weathered soil. Round scatters are for the

1.0 m spacing of anchorbodies, triangle scatters are for the 2.0 m spacing of

anchorbodies


anchor installed at weathered soil (simulation 2)

69


compression anchor installed at weathered rock. Round scatters are for the

1.0 m spacing of anchorbodies, triangle scatters are for the 2.0 m spacing of

anchorbodies


anchor installed at weathered rock (simulation 2)

70


compression anchor installed at soft rock. Round scatters are for the 1.0 m

spacing of anchorbodies, triangle scatters are for the 2.0 m spacing of

anchorbodies


anchor installed at soft rock (simulation 2)

71

When the spacing of anchorbodies increases 1.0 m to 2.0 m, applied

maximum friction stress decreases for all ground conditions. It means that

large spacing of anchorbodies is good for the load distribution of

compression anchor. If applied maximum friction stress is larger than

maximum resistance of ground, failure between grout and soil can occur.

Therefore, increasing spacing of anchorbodies can be one of the solutions for

preventing failure between grout and soil, so that the lower limit of spacing

of anchorbodies should be determined by comparing the applied maximum

friction stress with maximum resistance of ground.

72

4.4 Summary and conclusions

In this chapter, 2 kinds of numerical simulations are conducted to

evaluate load transfer mechanism of compression anchor.

Firstly, pull out loading simulation according to the ground

conditions are conducted. Through this simulation, 3 conclusions are derived.

1) For the weathered soil, only compressive force is applied to the

grout.

2) For the weathered rock and soft rock, both compressive force

and tensile force are applied to the grout.

3) If tensile force is applied to the grout, friction stress both

decreases and increases around anchorbodies, but the effects of

load distribution are valid.

Secondly, pull out loading simulation according to the ground

conditions and spacing of anchorbodies are conducted. Through this

simulation, 3 conclusions are derived.

1) For the compression part, when the spacing of anchorbodies

increases, compressive force of upper side anchorbody

decreases for weathered soil, weathered rock and soft rock.

Therefore, increasing spacing of anchorbodies can be one of the

solutions for preventing compressive failure of grout material

and the lower limit of spacing of anchorbodies should be

determined.

73

2) For the tension part, when the spacing of anchorbodies

increases, tensile force of upper side anchorbody increases for

weathered rock and soft rock. Therefore, increasing spacing of

anchorbodies can lead to tensile failure of grout and the upper

limit of spacing of anchorbodies should be determined.

3) When the spacing of anchorbodies increases, applied maximum

friction stress decreases. Therefore, increasing spacing of

anchorbodies can be one of the solutions for preventing failure

between grout and soil, so that the lower limit of spacing of

anchorbodies should be determined.

74

Chapter 5 Conclusions

The objective of this dissertation is to assess load transfer

mechanism of compression anchor using finite element analysis. Previous

researches have limitations in considering load distribution of compression

anchor. Compression anchor which has multiple anchorbodies, so called load

distributive compression anchor, was dealt with a little previous researcher

and friction stress distribution of load distributive compression anchor was

not dealt with before. Even for the designing of load distributive

compression anchor, the design method of tension anchor was applied.

For this objective, reliability of the applied finite element method by

comparing previous numerical applications of tension anchor, compression

anchor and load distributive compression anchor is secured. At the Chapter 3,

numerical modeling was conducted for the 4 cases and results of applied

finite element method well fit the previous experimental data and previous

applied numerical method results. Through this procedure, proper values of

interface element properties were determined as follows. Applied vertical

rigidity modulus (Kn) was 1 × and shear rigidity modulus (Kt) was

10 × .

After securing reliability of the applied FEM method, 2 kinds of

numerical simulations are conducted to evaluate load transfer mechanism of

compression anchor. Through the pull out loading simulation according to

the ground conditions and spacing of anchorbodies, several tendencies were

75

derived. From the simulation 1, for the weathered soil, only compressive

force was applied to the grout and there were both compressive force and

tensile force for the weathered rock and soft rock which has relatively high

stiffness. In addition if tensile force is applied to the grout, the effects of load

distribution were valid. From the simulation 2, necessity of upper limit and

lower limit of spacing of anchorbodies was derived. Upper limit of spacing

of anchorbodies are necessary to prevent tensile failure of grout and lower

limit of spacing of anchorbodies are necessary to prevent compressive

failure of grout and failure between grout and soil. Therefore, for the proper

design of compression anchor, spacing of anchorbodies and induced failure

of grout and failure between grout and soil should be considered.

76

References

Briaud, J. L., Powers, W. F., Weatherby, D. E. (1998). “Should grouted anchors

have short tendon bond length?”, J Geotech Geoenviron Eng, 124(2), pp.

110-119.

Jason, S. H., Greg, M., Michael, P., Saul S. S., Marc, J. G. (2015).

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78

초 록

그라운드 앵커는 흙막이 굴착 공사에 주로 사용되며, 하중 전달

방법에 따라서 지압형 앵커, 마찰형 앵커, 복합형 앵커로

나뉘어진다. 그 중 마찰형 앵커는 그라우트와 지반 사이의

마찰력으로 긴장하중에 저항하며, 마찰형 앵커는 그라우트에

인장력이 발생하는 인장형 앵커와 압축력이 발생하는 압축형

앵커로 구분된다.

압축형 앵커는 강선이 앵커 전 길이에 걸쳐서 쉬스관에 의해

보호되어 있으며, 내하체라는 구조체와 직접적으로 연결되어

그라우트에 압축력을 전달한다. 압축형 앵커는 인장형 앵커와

비교하여 그라우트의 인장파괴로 인한 진행성파괴에 강하다는

장점이 있으며, 강선의 제거가 용이하기 때문에 가설 흙막이 구조물

시공 후 강선의 제거가 필요한 도심지 공사에서 활발하게 사용되고

있다. 하지만 시공 역사가 오래된 인장형 앵커에 비해 압축형

앵커에 관한 연구는 미흡하며, 압축형 앵커 설계시에도 기존 인장형

앵커 설계기준을 따르는 실정이다. 압축형 앵커는 내하체의 개수에

따라서 단일 내하체를 사용하는 하중집중 압축형 앵커와 2개

이상의 내하체를 사용하는 하중분산 압축형 앵커로 나뉘어지며,

하중분산 압축형 앵커의 하중전이는 내하체 개수와 내하체 간격에

따라서 복잡한 거동을 보인다.

본 연구에서는 상용 유한요소해석 프로그램인 ‘Midas GTS NX’ 를

이용하여 하중분산 압축형 앵커의 하중전이 메커니즘을 평가하였다.

첫 번째로 인장형 앵커, 압축형 앵커, 하중분산 압축형 앵커에

대해서 수행된 기존 연구와의 비교분석을 통해서 연구에 적용할

79

유한요소해석 기법의 신뢰성을 확보하였으며, 인발하중을 받는

하중분산 압축형 앵커의 유한요소해석 모델링을 수행하였다.

유한요소해석 모델링은 풍화토, 풍화암, 연암 3종류의 지반에

내하체가 정착된 동일한 인발하중을 받는 하중분산 압축형 앵커를

모사하였으며, 내하체의 간격을 1 m 와 2 m 로 변화시켜 내하체

간격이 각각의 지반에 정착된 하중분산 압축형 앵커의 하중전이에

미치는 영향을 분석하였고, 이를 통해 하중분산 압축형 앵커의

하중전이 메커니즘을 평가하였다.

주요어 : 압축형 앵커, 하중분산 압축형 앵커, 유한요소해석,

인발 하중, 내하체 간격, 풍화토, 풍화암, 연암

학 번 : 2014-20545

Chapter 1 Introduction 1.1 Background1.2 Objectives1.3 Dissertation Organization

Chapter 2 Literature Review2.1 Ground anchor2.1.1 Types of ground anchor2.1.2 Load distribution of tension anchor2.1.3 Load distribution of compression anchor

2.2 Load distributive compression anchor2.2.1 Components of compression anchor2.2.2 Load distribution of LDCA anchor

2.3 Previous numerical applications for ground anchor2.3.1 Tension anchor2.3.2 Compression anchor2.3.3 Load distributive compression anchor

Chapter 3 Numerical Modeling for Ground Anchor3.1 Introduction3.2 Modeling case3.2.1 Tension anchor3.2.2 Compression anchor3.2.3 Load distributive compression anchor

3.3 Modeling methodology3.3.1 Material parameter3.3.2 Interface parameter

3.4 Results of the numerical modeling3.4.1 Tension anchor3.4.2 Compression anchor3.4.3 Load distributive compression anchor

Chapter 4 Numerical Simulations for Load Distributive Compression Anchor4.1 Introduction4.2 Numerical simulation according to ground conditions 4.2.1 Modeling methodology4.2.2 Results of simulation

4.3 Numerical simulation according to ground conditions and spacing of anchorbodies4.3.1 Modeling methodology4.3.2 Results of simulation

4.4 Summary and conclusions

Chapter 5 ConclusionsReferenceAbstract (Korean)

14Chapter 1 Introduction 1 1.1 Background 1 1.2 Objectives 3 1.3 Dissertation Organization 4Chapter 2 Literature Review 6 2.1 Ground anchor 6 2.1.1 Types of ground anchor 6 2.1.2 Load distribution of tension anchor 8 2.1.3 Load distribution of compression anchor 9 2.2 Load distributive compression anchor 11 2.2.1 Components of compression anchor 13 2.2.2 Load distribution of LDCA anchor 15 2.3 Previous numerical applications for ground anchor 16 2.3.1 Tension anchor 16 2.3.2 Compression anchor 20 2.3.3 Load distributive compression anchor 22Chapter 3 Numerical Modeling for Ground Anchor 30 3.1 Introduction 30 3.2 Modeling case 30 3.2.1 Tension anchor 30 3.2.2 Compression anchor 31 3.2.3 Load distributive compression anchor 32 3.3 Modeling methodology 33 3.3.1 Material parameter 37 3.3.2 Interface parameter 41 3.4 Results of the numerical modeling 45 3.4.1 Tension anchor 45 3.4.2 Compression anchor 48 3.4.3 Load distributive compression anchor 51Chapter 4 Numerical Simulations for Load Distributive Compression Anchor 55 4.1 Introduction 55 4.2 Numerical simulation according to ground conditions 57 4.2.1 Modeling methodology 58 4.2.2 Results of simulation 59 4.3 Numerical simulation according to ground conditions and spacing of anchorbodies 64 4.3.1 Modeling methodology 65 4.3.2 Results of simulation 66 4.4 Summary and conclusions 74Chapter 5 Conclusions 76Reference 78Abstract (Korean) 80

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Disclaimer - Seoul National University · 2019. 11. 14. · 'Midas GTS NX' is used to evaluate the load transfer mechanism of compression anchor. Reliability of the FEM techniques