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ANCHOR DESIGN & CONSTRUCTION FOR SHORING SYSTEMS

FUNDAMENTALS and AS4678 Civil & Structural Engineering Panel

19 March 2013 Andrew Shirley & Simon Fagg

SHIRLEY CONSULTING ENGINEERS PTY LTD INVESTIGATIVE, REMEDIAL & GROUND ENGINEERING


INTRODUCTION - 1

1. Soil & Ground Anchors for Shoring Systems. 2. Failures due to limited understanding &

reliance on ‘expert contractors’. 3. Some engineers are simplistic in their anchor

specifications, and just ‘call up the load required’.

4. Become litigation ‘targets’ when the anchor / shoring system fails.


INTRODUCTION This Presentation will cover: 1. The Fundamentals of Anchor Design for Shoring

Systems. 2. The differences between soil anchors & anchors and

usual reasons why anchors & shoring systems fail. 3. The requirements of AS 4678 and its implications for

design engineers. 4. Five case studies of failures because the Design

Fundamentals have not been followed. 5. Lessons for Design Purposes. 6. The Risks & Liability Issues for Engineers 7. A pdf version of the PowerPoint Slides will be available

on our website www.shirley.net.au


EXAMPLE OF TYPICAL ANCHOR SPEC.


EXAMPLE OF TYPICAL ANCHOR SPEC. A correct understanding of the way in which the anchors interact with the structural shoring system [viz: soil / structure interaction] is essential.


ANCHORS FOR SHORING SYSTEMS

1. Anchors are NOT the same.

2. Many types of anchor; each type of anchor interacts differently with the structural part of the shoring.

3. Only limited information on the design of shoring wall anchors is generally available [e.g. CIRIA C580].


ANCHORS FOR SHORING SYSTEMS

Anchors fall into three Categories

Rock Anchors • Stiff Anchorage

• No load transferred from the Rock to the Soil

• No interaction between Anchorage Zone and Restrained Zone

Dead Man Anchors • Shallow Anchorage

• Surficial Failure of Dead Man, typically not within the Restrained Zone

• Minimal interaction between Anchorage Zone and Restrained Zone

Soil Anchors • Deformable Anchorage

• Anchorage Zone Interacts with the Restrained Zone

• Where problems are most frequent, and topic of this talk


1. Rock anchors are ‘stiff’ elements.

2. Rock Anchors form Structural Frame with the rock and structural parts.

3. Rock Anchors have a Large Overload Capacity.

ANCHORS WITH AN ANCHORAGE IN ROCK

Anchorages in rock are very common & rarely fail unless they are too short. Why?


SOIL ANCHORS vs ROCK ANCHORS

• Anchors into rock form a ‘Frame’

• Soil Anchors do NOT create a ‘Frame’. Only a Restrained Cantilever

• Soil Anchor MUST be able to ‘self tension’


Why do Soil Anchors Fail?

1. No Structural Frame.

2. Minimal / No Overload Capacity.

3. Incorrect Lock-Off Load.

4. Unable to ‘self tension’ if installed at too steep an angle.

ANCHORS WITH AN ANCHORAGE IN SOIL


SHORING WALL TERMINOLOGY

ACTIVE SIDE

(Adjoining Property / Ground)

PASSIVE SIDE

(Excavation)

Shoring Wall Shoring Wall

Restrained Zone

Anchorage Zone

Anchor


ANCHOR COMPONENT TERMINOLOGY

Tendon

Shoring Wall

Drilled Hole

Shoring Wall

Anchor Head

See AS4678, Clause B2 : Anchor Components

Anchorage Zone


ANCHOR LOAD SPECIFICATION

THE TERM SWL SHOULD NOT BE USED

Symbol Name Description

PW Working Load The Maximum or Ultimate calculated load in the anchor during the construction processes.

PP Proof Load The load to which the ground anchor is to be loaded to check the adequacy.

PLO ‘Lock Off’ Load The load which will be left in the ground anchor when stressed. [i.e. The residual anchor load after the proof loading is complete].

PLT Long-term design load

The long-term residual load in the anchorage following its construction, prestressing, creep and subsequent ground movements.

Note: Whilst AS4678 (2002) describes the Working load as the “load necessary to ensure that the structure behaves in a satisfactory manner”, in the context of a shoring system and as the load changes during construction, the above definition is considered to be more precise.

Australian Standard [AS4678: Clause B4.4] requires the following loads to be specified by a designer:


1. The Anchorage Zone can interact with the Restrained Zone.

2. As Soil Anchors can ‘creep’ and loose tension under load, soil anchors need to be able to ‘self-tension’.

3. Limited Capacity for Overload

ANCHORS WITH AN ANCHORAGE IN SOIL FAILURES & CAUSES OF PROBLEM


SOIL ANCHOR SELF TENSIONING

As the shoring wall moves:

• Really Steep Anchors lose tension.

• Anchors which cross the failure plane at right angles only act in shear.

• Shallow Anchors can ‘self tension’, as movement of the potential failure wedge creates anchor tension.

Really steep anchor Loses Tension

Steep anchor Only acts in shear

Shallow anchor Self Tensions


LOAD TRANSFER DURING EXCAVATION

Soil Material

Passive Pressure

Where does the removed passive resistance go?

• Deformation of the Retained Soil

• Deflection of the Wall.

• Increased Anchor Load

• Increased Passive Pressure at the Toe of the Wall


1. An anchor tensioned to the Rankine active load doesn’t work, particularly when the tension can ‘relax’ due to Anchorage creep.

2. Rankine analysis not suitable – must use finite element methods [e.g. Wallap].

3. The selection of the right ‘lock-off’ load is a trade off of deflection and anchor load – the Optimal Range’.

ANCHOR LOCK-OFF LOADS & DEFLECTIONS


ANCHOR LOCK-OFF LOADS & DEFLECTIONS – (2)

Anchor Lock-off Load [kN]

Max

imum

Wal

l Def

lect

ion

[mm

]

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400

6m Excavation

5m Excavation

Optimal Range

NOTE: THIS DIAGRAM IS NOT A DESIGN CHART. IT IS AN EXMAPLE BASED ON A SPECIFIC SITES GEOTECHNICAL CONDITIONS, WALL AND ANCHOR PROPERTIES.


0

50

100

150

200

250

300

350

1 2 3 4 5 6

ANCHOR LOCK-OFF LOADS VS EXCAVATION DEPTH

Excavation Depth [m]

Anc

hor L

oad

[kN

]

Opt

imal

D

efle

ctio

n Lo

ck-o

ff

For a 6m Excavation

PLO = 150 kN

PP = 256 kN

1.25x PW = 206 kN

NOTE: THIS DIAGRAM IS NOT A DESIGN CHART. IT IS AN EXMAPLE BASED ON A SPECIFIC SITES GEOTECHNICAL CONDITIONS, WALL AND ANCHOR PROPERTIES.


1. Failure does not mean ‘Collapse’. 2. Principal causes of failure:

• Wrong Specification by the designer [e.g. the angle of installation is too steep and can’t self tension, etc.].

• Anchors too short. • Incorrect load testing procedures & wrong

lock-off load. 3. Construction Problems.

ANCHORS WITH AN ANCHORAGE IN SOIL FAILURES & CAUSES OF PROBLEMS


MONITORING OF SHORING & ANCHOR PERFORMANCE

MONITORING: Checks Anchor Performance wrt design. Must perform monitoring because Factor of Safety normally 1.2 to 1.25. Good anchor load test records. Monitoring Points must be continued down the shoring wall, and accurate to 2 mm. Includes visual ‘walk around’ the site every day Enables action to avert failure


INCIDENCE & EXAMPLES OF ANCHOR INDUCED SHORING FAILURES

Incidence of Anchor Failures Common in Sands & Soils

Occasional in Clay Rarely in Rock

Examples – by Simon Fagg Global Failures

Anchorage & Pull-out Failures


CASE STUDY 1 EASTERN SUBURBS, SYDNEY

This project involved: • a 5m deep excavation in sandy alluvial foundation material. • the installation of steel sheet piles and two rows of screw

anchors. • the excavation was to support a historical 3 storey building

and associated historical hall.




CONSEQUENCE OF THE FAILURE Construction • A requirement to backfill against part of the collapsing shoring

system, resulting in delays in Construction. • The need to demolish, and then reconstruct, a heritage listed

building; • Several other neighbouring buildings ‘walking’ across the site

boundaries.

Long Term • The Builder [over 100 years in operation] incurred significant losses

on the project, and was placed in Liquidation 3 years later. • The Piling Contractor was placed in liquidation 4 years later. • The Engineer who designed the system has now retired.


CASE STUDY 2 NORTHERN BEACHES, SYDNEY

This project involved: • a 12m deep excavation in sandy alluvial foundation material. • a high ground water level. • a sheet pile shoring system with four rows of ‘bar’ type anchors. • several neighbouring single & double storey commercial

premises.


GLOBAL FAILURES NORTHERN BEACHES, SYDNEY



Design Deflection

Warning Limit

Action Limit


CASE STUDY 2 NORTHERN BEACHES, SYDNEY CONSEQUENCE OF THE FAILURE

Construction • Partial Backfilling was required to stabilise the wall; • Damage was caused to six neighbouring buildings; • One building being unrepairable and not able to be replaced

without a four level basement; three years after the completion of construction this building is still unable to be occupied.

Long Term • Significant rental losses to the neighbouring building owners. • Two local businesses failed due to the disruption and cracking. • The Builder was placed in liquidation 3 years after the works. • The Piling Contractor was placed in liquidation 3 years after the

works. The engineer has now retired


CASE STUDY 3 SOUTHERN SYDNEY

This project involved: • a 6m deep excavation in sandy foundation material. • a contiguous concrete pile shoring system with two rows of

anchors. • a high ground water level above the base of the excavation. • an arterial road with extensive services, including a water main.



Monitoring Marks



CONSEQUENCE OF THE FAILURE Construction • Bursting of a high pressure water main. • The construction site was flooded for several weeks. • Backfilling was required against the shoring system to ensure

collapse did not occur; This caused significant delays in this area. • A probably excessive replacement shoring system was installed

before construction was allowed to continue in this area. Long Term • Main traffic artery closed for several months, with a significant cost

to the community. • Legal proceedings lasting for 4 years.


CASE STUDY 4 NORTH WESTERN SYDNEY

This project involved: • a 7m deep excavation in Silty Clay and weathered shale

materials. • a concrete pile shoring system with concrete infill panels • one row of anchors, ‘kept inside the boundary’.



‘Greasy backs’


CASE STUDY 4 NORTH WESTERN, SYDNEY

CONSEQUENCE OF THE FAILURE

Construction • Collapse of the excavation, with consequent backfilling delaying

construction. • Loss of parking and limited access for the neighbouring residential

building. Long Term • Delayed project delivery due to the backfilling and additional

stabilisation works required. • Increased future insurance costs for the Builder on the project.


CASE STUDY 5 MONA VALE

This project involved: • a 2.5m deep excavation in recent sand deposits. • a steel sheet pile shoring system. • one row of screw anchors.



Grout

Anchor Shaft


REVIEW OF CAUSES What were the causes of these failures?

Other Causes: Design Errors • Using ‘Flexible’ retaining walls to support sensitive structures. • Failure to determine what services might be affected by the

shoring wall and anchors. • Failing to consider locally deeper excavations

[ie. Don’t forget the lift pits!]. • Using a Computer Program for design without understanding

how it works.

THE PRINCIPAL CAUSE WAS CUTTING CORNERS IN THE DESIGN PROCESSES!


REVIEW OF CAUSES What were the causes of these failures?

Other Causes – Specification & Construction Errors Anchor Specification Errors • Specifying anchors only in terms of the SWL • Specifying short anchors • Specifying steep anchors in sand

Construction Errors • Not following the geotechnical warnings given • Ignoring the warning signs that something was wrong


LESSONS FOR DESIGN

Before Starting your Design, you need to at least know: • Site Geotechnical Conditions

[i.e. A Geotechnical Report] • A Survey of the Site & Land surrounding the site

• Details of Services surrounding the site [Dial Before You Dig]

• Details of the Footing System of Buildings within the ‘Zone of Influence’ of the Anchors.

• Confirmation of the Permission to anchor under the neighbouring property.

DESIGN IS NOT CALCULATIONS (although calculations are a part of it)


LESSONS FOR DESIGN

The Design Process should at least include: 1. Determination of design limits based on the

surrounding structures & services. 2. Determine the design depth of the excavation

[e.g. including lift pits, footings, over excavation etc.]. 3. Evaluation of the best type of shoring wall to be used. 4. Calculations for the Shoring Wall & Anchors. 5. Documentation [i.e. The Drawings and / or Specification].

DESIGN IS NOT CALCULATIONS (although calculations are a part of it)


LESSONS FOR DESIGN

THE SPECIFICATION MUST SUMMARISE THE DESIGN

For Ground Anchors AS4678, Clause D4.6 requires: a) Type of Anchor (ie. Temporary or Permanent) and the Design Life. b) Expected Anchorage Zone material. c) Minimum free length. d) Ultimate Design Load. e) Test proof load and test load duration. f) Lock-off Load.

In addition, the Installation Angle is very important.


LESSONS FOR DESIGN Specification : Monitoring Requirements

In addition, SCE recommend specifying the monitoring requirements for all shoring installations. This should include at least: 1. A daily ‘walk around’ of the site and regular photography. 2. Regular, accurate survey of monitoring points located both

along and down the face of the shoring wall. 3. Timeframe for the provision of monitoring results to the

engineer. [e.g. results to be provided with 48 hours]. 4. Warning and Action [e.g. backfilling] deflection limits.


SOME LESSONS FROM THE FAILURES

1. AS 4678 is important and should be followed; non-compliance leads to failure.

2. A Failure to consider overall & global stability of the shoring system.

3. If the monitoring reveals a problem, it is a warning; so get independent engineering advice quickly – and preferably not from the anchoring / shoring designer.

4. An early independent review can usually avoid a catastrophe – why not ‘phone a friend’.


SOME LESSONS FROM THE FAILURES (2) (EXPERTISE)

1. Appropriate expertise essential. 2. Interaction between Structural & Ground

Engineers – if poor, failure inevitable. 3. Ground & Geotechnical Engineers – don’t

assume that someone who can drill & test understands design processes.


ENGINEERING RISK & LIABILITY

Professional Issues 1. Code of Ethics & Relevant Expertise – don’t design

what you don’t understand. 2. Soil / Structure Interaction & Ground Engineering. 3. Comply with Standards [e.g. AS 4678, CIRIA C580]. 4. Be involved in the Contract Inspection & Review

processes. Commercial Issues • Can you afford to be sued? • What is your reputation worth?


THANK YOU Any Questions?

A copy of this presentation is available on our website.

www.shirley.net.au, ‘Links & Downloads’, SCE Papers & Publications. http://www.shirley.net.au/index.php?page=links_downloads&id=2&table=downloads

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