Upload
amy-sanders
View
213
Download
0
Tags:
Embed Size (px)
Citation preview
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
16
2015-1 Wednesday Seminar
Thank you