Providing Design Standards for Frictional Compression Ground Anchors

Geotechnical & Geoenvironmental Lab., Seoul Natl. Univ. Master Course Student

Joo, Hyeok-Jun

Contents

1. Introduction : What are Ground Anchors?

2. Literature Review

3. Research Objectives

4. Summary and Conclusions

1. Introduction

A ground anchor is a tension element used to apply a restraining force to a structure by anchoring the distal end of the anchor in the ground.

Applications of Ground Anchors

Support of excavation Slope stability Retaining walls Resist buoyancy forces

Bridge abutment walls Tunnel portal walls Resistance to overturning moments for towers and dams

Ground An-chor

1

Ground Anchor Classi-fication

Tensile force

Ground pressure type

Frictional tension type

(load concentrative type )

Frictional compres-sion type

(load distributive type )

Frictional compres-sion type

(load concentrative type )

Tensile force

Tensile force

Tensile force

Pressure resistance

Anchor body

Anchor body

Anchor body

1. Introduction2

1. Gripping force of the bond length

2. Tensile strength of the free length steel strand

3. Bearing capacity of the anchor head

Ground anchor de-signs contain uncer-tainties->Possibility of ex-cessive design

12

3

Bond Length

Free LengthAnchor Head

Components of the Ground Anchor

- Load-bearing capacity of ground anchors

1. Introduction3

Ground anchors are commonly installed at angles of 15 to 30 degrees. When the axial force is P, and inclined degree is ,

2.1 Ground Anchor Design

Design load

2. Literature Review

1. Allowable tensile strength of steel tendons() > Design load() 2. Allowable bond strength between steel tendons and grout( > Design load() 3. Allowable pullout resistance between grout and ground( > Design load()

Design requirements of ground anchors

For the design of ground anchors, 1, 2 and 3 must be satisfied

4

=

Allowable bond stress between steel tendons and grout: Diameter of tendon, : Bond length of tendon

=

: Diameter of tendon: Allowable tensile stress (= 0.80 (Temporary anchor) or 0.75 (Permanent anchor)): Yield stress of tendon

1. Allowable tensile strength of steel tendons()

2. Allowable bond strength between steel tendons and grout(

2.1 Ground Anchor Design

2. Literature Review5

=

Ultimate pullout resistance: Satefy factor

In the case of structural failure of ground anchors, tensile strength of steel tendons and bond strength between steel tendons and grout are not important factor.

Predicting ultimate pullout resistance is the most important thing for design ground anchors

2.1 Ground Anchor Design3. Allowable pullout resistance between grout and ground(

2. Literature Review6

=

= Unit pullout resistance, =Diameter of boring, =Bond length

4. Ultimate pullout resistance(

Unit pullout resistance()

Littlejohn(1990) The Japanese Geotechni-cal Society

==Cohesion=Coefficient of earth pres-sure=Friction angle

Proposed by field tests

Note Theoretical equation Using SPT-N value

2.1 Ground Anchor Design

2. Literature Review7

Ground anchor manuals

Design /maintenance of ground anchors

(Korea Ministry of Land, Infrastructure, and Transport)

Eurocode 7 : Geotechni-cal design

De-sign

Method

-Limit state design-Deciding pullout resistance by pullout test-Investigation test, Suitability test, Acceptance test

-Limit state design-Deciding pullout resis-tance by pullout test-Investigation test, Suit-ability test, Acceptance test

Note For the tension anchor For the tension anchor Existing design manuals are about tension ground anchors and there are no spe-cific standard for frictional compression ground anchors

2.1 Ground Anchor Design

2. Literature Review8

2.2 Problems of a Design Method for Ground Anchors1. Unclear methods for bond length estimations

2. 정착장의 제작 , 시공 사이의 비효율성

3. 강선 및 정착체 등의 부적절한 사용

4. 띠장 정착부의 비효율성

Bond length for frictional compression type anchor should be designed

Above equations are for frictional tension type anchor

Criteria Bond Length Formula

1. Frictional force be-tween

grout and ground

(T: Designed axial force, =safety factor, =Frictional resistance)

2. Adhesion between grout and anchor

(T: Designed axial force, =allowed adhesion, =circumference of strand, n=the number of strand)

3. Minimum length L = 4.5m

Bond Length = Longest length among 1,2,3

2. Literature Review9

2.2 Problems of a Design Method for Ground Anchors1. Unclear methods for bond length estimations

2. Excessive use of strands and anchor bodies

Over 4 sets of strands and anchor bodies are used for anchor constructions these days, not taking into consideration the ground conditions and the applied loads

The appropriate number of strands has to be decided depending on the ground conditions and the applied loads.

2. Literature Review10

2.3 Frictional Compression Ground Anchor1. Pullout behavior of frictional compression anchor

Pullout test results of compression ground anchor(Katsura, 1987)

Pullout test results of tension ground anchor(Katsura, 1987)

𝑓 𝑠=∆𝑄𝑝∆ 𝑙

𝑓 𝑠=∆𝑄𝑝∆ 𝑙

For the tension anchor, progressive failure occursLow possibility of progressive failure on the compression anchor

2. Literature Review11

Compres-sion

Anch

or

body

4th

3rd

2nd

1st

2. Distribution of the pullout stresses on the load distribution anchor

2.3 Frictional Compression Ground Anchor

2. Literature Review

1. Stresses distribution expected

2. Stresses distribution observed

Ten-sion

Compression

Stress distribution of compression anchor (Naganuma, 1996)

Both compressive stress and tensile stress are applied on compression anchor

12

2. Distribution of the pullout stresses on the load distribution anchor

Stress distribution depending on stiffness of ground(Naganuma, 1997)

2.3 Frictional Compression Ground Anchor

2. Literature Review

Stress distri-bution

Stress distri-bution

Hard rock

Soft rock

Different behavior of pullout stress distribution around anchor bodies depending on stiffness of ground

13

Compres-sion

Anch

or

body

4th

3rd

2nd

1st

2. Distribution of the pullout stresses on the load distribution anchor

2.3 Frictional Compression Ground Anchor

2. Literature Review

1. Stresses distribution expected

2. Stresses distribution observed

Ten-sion

Compression

Stress distribution of compression anchor (Naganuma, 1996)Both compressive stress and tensile stress applied on compression anchor

14

1. Investigation of materials related to the design of ground anchors

2. Modifying the number of strands depending on design load

3. Figuring out load-transfer mechanism of compression anchor and establish design method

4. Providing design standard and maintenance manual for compression anchor

3. Research Objectives

> Design standard, related literature reviews, construction data

> Caculating effective tension of steel strand appropriate for design load

> Understanding magnitude and distribution of skin friction resistance

> Providing reasonable design standard and construction method

5. Investigation by field tests and 3D numerical analysis

> Lab tests, field test, and numerical analysis by Midas GTS program

15

4. Summary and Conclusions

1. Search for design methods of frictional compression anchor.

> A study on existing research paper about ground anchors

2. Research on the improvements of design methods for frictional compression anchor

> A study on actual construction data of the ground anchors

3. Analysis using numerical analysis program(Midas GTS)

> Procuring reliability of FEM program by comparison with field tests

Figure out the load-transfer mechanism of frictional compression anchor and es-tablish a design method and construction manual through field tests and 3D nu-merical analysis.

Ultimate Research Objective

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2015-1 Wednesday Seminar

Thank you