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Structure Foundations Structure Foundations

AS/NZS 7000:2010 AS/NZS 7000:2010

Section 9 & Appendix L Section 9 & Appendix L

Henry Hawes

FIEAust, RPEQ, CPEng.Consultant

hhawes@bigpond.com

S E C T I O N 9 F O U N D A T I O N S

• 9.1 DESIGN PRINCIPLES

• Foundations for structures and the anchor of

any stays or guy wires shall be capable of

withstanding loads specified for the ultimate

strength limit state and serviceability limit

states conditions.

• Foundation design should be based on

appropriate engineering soil properties.

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Where soil test information is not available, an estimate of soil parameters should be

made based on an appraisal of site conditions, soil types and geological structure.

• Construction personnel shall be made aware of the

assumed parameters and guidelines should be

issued that will allow recognition of soils not

conforming to the adopted design parameters.

• In calculating the strength of foundations,

recognition should be given for the different

strength characteristics of soil under short-term

and long-term loads, and the difference in saturated

and dry properties of the soil.

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For distribution lines• The consequences of partial foundation failure for the

typical distribution pole or structure are not normally

as severe.

• Designers should assess the cost of providing

foundations that will remain elastic for all design loads

versus the cost of straightening poles

Eg. Pole foundation materials will yield under saturated

soil conditions and overload (controlled failure ?)

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Soil design parameters

• In the absence of better information from soil

investigations, the soil parameters provided in

Appendix L may be used as a guideline for

design.

• However it should be confirmed by inspection or

testing, during construction, that the soil parameters used are appropriate.

9.3 BACKFILLING OF EXCAVATED MATERIALS

• When backfilling is used, sufficient compaction shall

be carried out to ensure foundation actions can be

developed as designed.

• In certain circumstances, a possible reduction of

consistency of cohesive soils should be taken into

account in the calculations if compaction standards are

to be relaxed.

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9.4 CONSTRUCTION AND INSTALLATION

• Designs of foundations should include

consideration of the method of construction and

installation of foundations to ensure the

assumed or designed geotechnical parameters

are able to be realised.

APPENDIX L

STRUCTURE FOOTING DESIGN AND

GUIDELINES FOR THE

GEOTECHNICAL PARAMETERS OF SOILS AND ROCKS

(Informative)This Appendix addresses fundamental

performance criteria and the design methods

associated with overhead line footings and

their foundations

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• Several alternative approaches can be used

for the design of footings and the

interpretation of the foundation conditions,

the designer should exercise sound

engineering judgment in determining which

method is most appropriate for the standard

of construction required.

Australian Panel B2 –Overhead Lines Seminar –AS/NZS 7000:2010 Overhead Line Design Sydney 28 – 29 March 2011

• The designer also has the option to design

each footing for site-specific loadings and

actual subsurface conditions

or to

• Develop standard designs that can be used

at sites within application guidelines for

various possible sub soil conditions.

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Geotechnical Parameters Soils And Rocks

• On major transmission lines it can be expected that a

higher level of specialist engineering will be applied to

the geotechnical design of footings and their

foundations and hence some form of subsurface

investigation could be expected to be carried out

• It may not always be practical to do subsoil

investigations and simplified assessments may be

required to establish some indicative yet conservative

parameters.

• In distribution line construction simple subsurface

application design guidelines are commonly applied

Soil and Rock Design Parameters

• Cohesive soils

• Non-cohesive soils

• Soft rock

• Medium –Hard Rock

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Generally, to determine the foundation ultimate load

carrying capacity the shear strength of soil is

required.

s = c + σn tan ϕ . . . L1

where

s = shear strength

c = cohesion

σn = normal stress

ϕ = angle of internal friction

• Cohesive soils can generally be expected to

resist design loads for a short duration of

time without experiencing significant

movements

• Long term loads applied over the service life

of the structure most probably will result in

excessive displacements

Cohesive Soils

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Ref: Tomlinson +

Guideline For Typical Cohesive soil properties

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Non Cohesive Soils• Non Cohesive /Granular soils are normally

firmer in composition and have similar

properties under short-term and long-term

loading conditions

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Guideline For Typical Non Cohesive soil properties

Ref: Tomlinson +

Rock • Table L3 of AS/NZS 7000 can be used as a

conservative guide to typical rock types

• Data has been confirmed by multiple field tests using micropiles

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Easily Bored(just able to be drilled

with air drill)

Rock Auger

(with some

difficulty)

Rock Anchor

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Design should consider available construction plant

L3 Pole Foundations• The Brinch Hansen methodology provided in this

clause and other methods referenced such as

Broms (ASCE 1964), while applied in some areas for

major pole or single bored pier footings they have

not been commonly used for directly embedded

pole type distribution overhead lines.

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• Simple design methods have been in use for

distribution pole overhead lines throughout

Australia and New Zealand and overseas for

many years and these overhead lines have

performed well over time.

Distribution Pole Footing Design

There are several commonly used methods1. American Society of Agricultural Engineers ANSI/ASAE EP486.1 OCT 00

Shallow Post Foundation Design

Sbd

MaVa

86 +Where d =

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Australian Panel B2 –Overhead Lines Seminar –AS/NZS 7000:2010 Overhead Line Design Sydney 28 – 29 March 2011

Assumed

tip loading

position

d t

dgl

db

hr

Tip

Butt

Ground level200mm

L

Assumed critical cross sectionfor design dgl

Pole planting depth LGLLGL

LGL = Min[(1 + 0.1 × hr) × (dg/250).3.6] for hr <18

LGL = Min[(1 + 0.1 × hr) × (dg/330).4.8] for hr ≥18

2. Empirical Design Formula

Pole dia. atGL (mm)

Height from GL (ground line) to conductor (m)

6 7.5 9 10.5 12 13.5 15 16.5 18

150 1.0 1.1 1.1 1.2 1.3 1.4 1.5 1.6 1.3

175 1.1 1.2 1.3 1.4 1.5 1.6 1.8 1.9 1.5

200 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 1.7

225 1.4 1.6 1.7 1.8 2.0 2.1 2.3 2.4 1.9

250 1.6 1.8 1.9 2.1 2.2 2.4 2.5 2.7 2.1

275 1.8 1.9 2.1 2.3 2.4 2.6 2.8 2.9 2.3

300 1.9 2.1 2.3 2.5 2.6 2.8 3.0 3.2 2.5

325 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.4 2.8

350 2.2 2.5 2.7 2.9 3.1 3.3 3.5 3.6 3.0

375 2.4 2.6 2.9 3.1 3.3 3.5 3.6 3.6 3.2

400 2.6 2.8 3.0 3.3 3.5 3.6 3.6 3.6 3.4

425 2.7 3.0 3.2 3.5 3.6 3.6 3.6 3.6 3.6

450 2.9 3.2 3.4 3.6 3.6 3.6 3.6 3.6 3.8

475 3.0 3.3 3.6 3.6 3.6 3.6 3.6 3.6 4.0

500 3.2 3.5 3.6 3.6 3.6 3.6 3.6 3.6 4.2

550 3.5 3.6 3.6 3.6 3.6 3.6 3.6 3.6 4.7

600 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 4.8

MINIMUM EMBEDMENT DEPTH LGL (m)

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LGL = Min[(0.6+ 0.1 ×××× hr) for hr <17

LGL = Min[(0.6+ 0.1 ×××× hr) - 0.1 for hr ≥≥≥≥17

3. Alternative Empirical Design Formula(Old C(b) 1 and Queensland Regulations SECQ M1-1977 )

4 . ASCE Method (EX AS/NZS 4676)

C

CMHHD

2

2.1696.126.3 2RR ++

=

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Class Very soft Soft Firm Very firm Hard

Soil descriptionSilty clays andsands; loose drysands

Wet clays; siltyloams; wet or loosesands

Damp clays; sandyclays; damp sands

Dry clays; clayeysands; coarse sands;compact sands

Gravels; dry clays

Strength (fb) kPa fb ≤ 60 60 < fb ≤ 100 100 < fb ≤ 150 150 < fb ≤ 240 240 < fb

The above values are based on foundation deformations of approximately 12 mm under serviceability loads on building structures.For poles supporting services that are sensitive to displacements at their supporting points (e.g. microwave antennas), this degree ofdeformation might be inappropriate. Therefore, suitable reduction of these values may be necessary. This may be achieved byincreasing the embedment depth, or the footing diameter, or both, which will reduce the bearing pressures and, consequently, thedeformations.

BEARING STRENGTH OF SOILS AT THE SERVICEABILITY LIMIT STATE

Effectiveheight h(m)

Embedment depth ( D) (Note 1) m, for horizontal force ( H) kN

H = 1.5 H = 3.0 H = 6.0 H = 10b=0.3

0.45 0.60 0.30 0.45 0.60 0.3 0.45 0.6 0.75 0.9 0.3 0.45 0.6 0.75 0.9

3.0 0.8 0.7 0.6 1.0 0.9 0.8 1.4 1.2 1.0 0.9 0.9 1.8 1.5 1.3 1.2 1.1

4.5 0.9 0.7 0.7 1.2 1.0 0.9 1.6 1.4 1.2 1.1 1.0 2.1 1.7 1.5 1.4 1.2

6.0 1.0 0.8 0.7 1.3 1.1 1.0 1.8 1.5 1.3 1.2 1.1 2.4 1.9 1.7 1.5 1.4

7.5 1.1 0.9 0.8 1.4 1.2 1.1 2.0 1.7 1.4 1.3 1.2 2.6 2.1 1.8 1.7 1.5

9.0 1.1 1.0 0.9 1.6 1.3 1.1 2.2 1.8 1.6 1.4 1.3 2.8 2.3 2.0 1.8 1.6

10.5 1.2 1.0 0.9 1.7 1.4 1.2 2.3 1.9 1.7 1.5 1.4 3.0 2.4 2.1 1.9 1.7

12.0 1.3 1.1 1.0 1.8 1.5 1.3 2.4 2.0 1.8 1.6 1.5 3.2 2.6 2.2 2.0 1.8

13.5 1.3 1.1 1.0 1.8 1.5 1.3 2.6 2.1 1.8 1.7 1.5 3.3 2.7 2.4 2.1 1.9

15.0 1.4 1.2 1.0 1.9 1.6 1.4 2.7 2.2 1.9 1.7 1.6 3.5 2.8 2.4 2.2 2.0

16.5 1.5 1.2 1.1 2.0 1.7 1.5 2.8 2.3 2.0 1.8 1.7 3.6 3.0 2.6 2.3 2.1

18.0 1.5 1.3 1.1 2.1 1.7 1.5 2.9 2.4 2.1 1.9 1.7 3.8 3.1 2.7 2.4 2.2

19.5 1.6 1.3 1.2 2.2 1.8 1.6 3.0 2.5 2.2 1.9 1.8 3.9 3.2 2.8 2.5 2.3

22.0 1.6 1.4 1.2 2.3 1.9 1.6 3.2 2.6 2.3 2.1 1.9 4.1 3.4 2.9 2.6 2.4

POLE EMBEDMENT DEPTHS FOR SOILS WITH fb = 150 kPa

Tabulated depths include the 0.2 m additional depth

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[1]

Comparison of Pole Foundation Design MethodologiesFoundation Design

Formula Failure Criteria Advantages Disadvantages Comme nt

Brinch-Hansen

Precise calculation 0.5 °at tip Considers: •Multi layered soil properties•Soils with both friction and cohesion•Variable water table•Variable bearing widthsBased on the ultimate lateral soil resistance of the soils

Complex, requires soil modelling. Iterative analysis approach required. Considers free head situation only. Stiff clays.

Strength factor of 0.65 appropriate.

Broms Precise calculation 0.002 to 0.006 radians at ultimate capacity

Relatively simpleBased on the ultimate lateral soil resistance of the soilsApplicable for short and long piles.Considers both fixed and free head restraint

Cannot be used in complex soils or variable shaft sizes [i.e. non-uniform soils, water table]. Not appropriate for high eccentricity situations. Very conservative.

Appropriate for non-cohesive and cohesive soils. Broms suggested strength factor of 0.7

AS/ NZS 4676

Empirical formulaAppendix L

Unknown Relatively simpleRelated to Scala penetrometer

Caters for uniform soils and specific configurations [i.e. directed buried and blocked only].

Need to assess soil prior to calculating depth. Based on simplifying assumptions.

C(b) 1 –pre 1992 [Working Stress Design]

1/10 pole length + 0.6m

2D or 12mm at ground line

Simple Based on working stress method and FOS=4

Applies to firm soil and medium sized conductors (≈18 mm) associated with free standing intermediate poles up to 150m spans, 24m long poles. FOS = 4.0

C(b) 1 –2006 [Limit States]

1/12 pole length + 1.4m [loose sands]1/10 pole length + 0.8m

2D or 12mm at ground line

Simple Based on working stress method and FOS=3.

Applies to loose sands and larger conductors associated with intermediate poles up to 150m spans.

New Zealand

Pole length / 6 Unknown Simple Not appropriate for wea k soils. Based on working stress method.

Applies to firm soil and medium, sized conductors (≈15 mm) associated with intermediate poles up to 120m spans

[1]

Typical Concrete Pole Footings

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Wood Pole Foundation

Reinforcing (Nailing)

• Appendix N Cl N7.2.1

• Design to be based on propriety systems

when installed

• Estimated 200,000 reinforced wood poles in

Australia with potentially questionable

strength

• Needs to be carefully evaluated over time

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Tower Foundations

• Lattice tower footings are typically designed

for vertical forces (uplift or compression)

combined with horizontal shear forces.

• Some of the more commonly used

foundation capacity calculation methods are

presented in Appendix L

• All are well documented in Cigre TB’s, ASCE

and other references.

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Australian Panel B2 –Overhead Lines Seminar –AS/NZS 7000:2010 Overhead Line Design Sydney 28 – 29 March 2011

Three basic models

Straight sided shaftNo undercut at base

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L5.3 Rock Anchors

• Design principle that the applied loads

(compression and tension) are being transferred to

the soil or foundation material by a number of soil

or rock anchors via a load transfer cap.

• Generally if you can drill the rock, small diameter

grouted rock anchors can provide an economical

solution.

• Post-tensioned ground anchor systems can also be

used

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• Failure mode of anchors is normally associated with the

progressive de-bonding of the anchor tendon with

increasing load due to elastic extension of the tension tendon

Anchorage capacity is normally based on a shear failure model

along the grout column as in Figure L10

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L5 GUY ANCHORS

• L5.1 Cast in situ anchor blocks

Modified Figure L19

Where S1 = LG γstanδ for Drained Condition and

= αcu for Undrained Conditions

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L5.2 Bored pier anchors

• Normally single tension tendon in soil or rock

• Anchorage capacity is normally based on a shear failure model

along the grout column as in Figure L10

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L4.6.4.2 Design of base plates

• Base plate design should generally be based on

ASCE 10-97 recommendations, except when

modified by AS 4100 (e.g. shear stress on bolts)

and AS 3600 requirements for bolt anchor

length.

L4.6.4.3 Design of stubs

• Most of the stub axial force is resisted by shear

connections. Some force is transferred by bond.

• The normal method is to provide bolted or welded

cleats or studs attached to the lower end of the leg

stub in sufficient number and spacing to transfer

the total force in shear and bearing

• Need to check for punching shear under both

maximum compression and uplift loads on base

slabs

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L4.6.2 Deep piled footings• Deep piled foundations are used where weaker soil

strata is encountered.

• Can be based on concrete cast in situ piles, steel

driven or screw piles or precast concrete driven pile

systems.

• Piling design and installation should comply with the

requirements of AS 2159.

• The design of the screw piles shafts should be based

on Eurocode 4.

Miscllaneous Provisions

L6 FOUNDATION TESTING

• Tests of the driven piles and other foundation

types can be performed generally in accordance

to AS 2159.

L7 CATHODIC PROTECTION

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Foundation PerformanceApproximate total number of structures in Australia

and New Zealand

Steel Towers 100,000

Timber Poles 5,100,000

Concrete Poles 450,000

Steel Poles 160,000

Stobie Poles 660,000

Major Failure Events over last 60 yrsEvent Type Number of Events

C –Cyclone 4

D- Downdraft 36

T- tornado 4

F- Foundation (Bored -1; mass concrete -2;

Grillage -2)

5

FIRE -Fire Storm 1

G -Gale force winds 4

W -Wake Turbulence 3

O- Other (Structural weakness -2;

Construction overload-1;

Ice/snow/wind -1 )

4

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Lattice Tower Foundation FailuresLimited number (5 significant events recorded ) – very low probability

Plus a number of partial failures – mainly corroded grillage foundations or older excavated mass

concrete with leached concrete.

Distribution pole ‘foundation failures’

• Estimated that 70% of wood poles were installed during

period 1945 and 1965

• Localised partial failures (leans) are generally isolated

events , but common during heavy seasonal rains, flooding

and storms with total failures with debris overload.

• When footings ‘partially fail’ – most are simply straightened

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Questions?