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 excavationSlope stabilityRetaining wallsResist buoyancy forces Bridge abutment wallsTunnel portal wallsResistance to overturning moments for towers and dams Ground Anchor 1

Ground Anchor Classification Tensile force Ground pressure type Frictional tension type (load concentrative type ) Frictional compression type (load distributive type ) Frictional compression type (load concentrative type ) Tensile force Pressure resistance Anchor body 1. Introduction 2

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 designs contain uncertainties ->Possibility of excessive design Ground anchor designs contain uncertainties ->Possibility of excessive design Bond Length Free Length Anchor Head Components of the Ground Anchor - Load-bearing capacity of ground anchors 1. Introduction 3

2.1 Ground Anchor Design Design load 2. Literature Review Design requirements of ground anchors For the design of ground anchors, 1, 2 and 3 must be satisfied 4

2.1 Ground Anchor Design 2. Literature Review 5

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 Design 2. Literature Review 6

Littlejohn(1990) The Japanese Geotechnical Society Proposed by field tests NoteTheoretical equationUsing SPT-N value 2.1 Ground Anchor Design 2. Literature Review 7

Ground anchor manuals Design /maintenance of ground anchors (Korea Ministry of Land, Infrastructure, and Transport) Eurocode 7 : Geotechnical design Design Method -Limit state design -Deciding pullout resistance by pullout test -Investigation test, Suitability test, Acceptance test -Limit state design -Deciding pullout resistance by pullout test -Investigation test, Suitability test, Acceptance test NoteFor the tension anchor Existing design manuals are about tension ground anchors and there are no specific standard for frictional compression ground anchors 2.1 Ground Anchor Design 2. Literature Review 8

2.2 Problems of a Design Method for Ground Anchors 1. 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 CriteriaBond Length Formula 1.Frictional force between grout and ground 2. Adhesion between grout and anchor 3. Minimum lengthL = 4.5m Bond Length = Longest length among 1,2,3 2. Literature Review 9

2.2 Problems of a Design Method for Ground Anchors 1. 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 Review 10

2.3 Frictional Compression Ground Anchor 1. Pullout behavior of frictional compression anchor Pullout test results of compression ground anchor(Katsura, 1987)Pullout test results of tension ground anchor(Katsura, 1987) For the tension anchor, progressive failure occurs Low possibility of progressive failure on the compression anchor 2. Literature Review 11

Compression Anchor body 4 th 3 rd 2 nd 1 st 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 Tension 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 distribution Hard rock Soft rock Different behavior of pullout stress distribution around anchor bodies depending on stiffness of ground 13

Compression Anchor body 4 th 3 rd 2 nd 1 st 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 Tension 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 establish a design method and construction manual through field tests and 3D numerical analysis. Ultimate Research Objective 16

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