Impact of Eurocode 7 on steel piles - Decoding the Eurocodesgeocentrix.typepad.com/.../impact_of_eurocode_7_on_steel_piles.pdf · Impact of Eurocode 7 on steel piles Dr Andrew Bond

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Impact of Eurocode 7 on steel piles

Dr Andrew Bond(Geocentrix)

Jun-08 Decoding Eurocode 7 ©2005-8 Geocentrix Ltd. All rights reserved 2

Contents of EN 1997-1:General rules

2


Contents of EN 1997-2: Ground investigation and testing


EN 14731

EN 15237

EN 1536

EN 1537

EN 1538EN 1206

3

EN 12715

EN 12716

EN 12699

EN 14199

EN 14475

EN 14679

Execution

standards

Geotechnical

investigatio

n

and testin

g

standards

ISO 22476ISO 224

75

ISO 22282

ISO 17892

ISO 14689

ISO 14688

3

2

12

6

12

13EN 19

98

EN 1999

EN 1997

5

5

ISO 22477

13

2

EN 1990

EN 1991

EN 1992

EN 1993 EN 1996

EN 1995

EN 1994

EN Eurocodes

3

2 26

341

3


Execution of special geotechnical works

Design of sheet pile walls


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Contents of EN 1997-1 Sections 8 and 9

Section 8 AnchoragesSection 9 Retaining structures

§x.1 General (12/6 paragraphs)§x.2 Limit states (1/4)§8.3 Design situations and actions (2)/§9.3 Actions, geometrical data and design situations (26)§x.4 Design and construction considerations (15/10)§8.5/§9.7 Ultimate limit state design (10/26)§8.6/§9.8 Serviceability limit state design (6/14)

Section 8 only§8.7 Suitability tests (4)§8.8 Acceptance tests (3)§8.9 Supervision and monitoring (1)

Section 9 only§9.5 Determination of earth pressures (23)§9.6 Water pressures (5)


Scope of EN 1997-1 Sections 8 and 9

• Section 9 covers retaining structures supporting:– Soil, rock or backfill (i.e. ground)– Water

• Material is retained if it is ‘kept at a slope steeper than it would eventually adopt if no structure were present’

• Silos are NOT covered (see EN 1991-4 instead)

• Section 8 covers temporary and permanent anchorages to:– Support retaining structures– Stabilize slopes, cuts, or tunnels– Resist uplift forces on structures

• Soil nails are NOT covered (see BS 8006 and EN 14490 instead)

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Spacing of investigation points

Annex B.3 of EN1997-2 provides outline guidance on the spacing of investigation points for geotechnical investigations

Along vertical sections25-75mDams and weirs2-6 per foundation

20-200m60m15m to 40mSpacing

bridges, stacks, machinery foundations

Special

-roads, railways, channels, pipelines, dikes, tunnels, retaining walls

LinearGridLarge-areaGridHigh-rise and industrialArrangementStructure/example


Depth of investigation points forretaining walls

Groundwater below formation:za ≥ 0.4 hza ≥ (t + 2 m)

Groundwater above formation:za ≥ (H + 2 m)

and (if stratum is impermeable):za ≥ (t + 2 m)

or (if all strata are permeable):za ≥ (t + 5 m)

For cut-off walls:za ≥ 2 m

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UK National Annex to BS EN 1997-1

• National Annex published November 2007• References to non-contradictory complimentary information (NCCI)

cites:– BS 8002, Code of practice for earth retaining structures– CIRIA C580, Embedded retaining walls – guidance for

economic design• Future of BS 8002 is uncertain:

– Basic philosophy (critical state soil mechanics with mobilization factors) contradicts Eurocode 7

– Code won’t be re-written– Possible PD containing “left-over” clauses covering gravity walls

• Highways Agency intends to commission minor update to CIRIA C580 to remove any contradictory information


Application of partial factors and tolerances

d F repF Fγ=

kd

M

XX

γ=

{ }, ,d E d d dE E F X aγ=

{ }, ,d d dd

R

R F X aR

γ=

Actions

Resistances

Effects of actions

Material properties

d noma a a= ± Δ

Geometrical parameters

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Design Approaches for STR/GEO

• §2.4.7.3.4.1(1)P The manner in which equations [above] are applied shall be determined using one of three Design Approaches– Design Approaches apply ONLY to STR and GEO limit states– Each nation can choose which one (or more) to allow

• UK National Annex, NA.4 …only Design Approach 1 is to be used in the UK• In simplest terms, the design approaches apply factors to the following…

Structural actions or effects& material properties

Actions or effects& resistances

Material properties

ActionsCombination 2Combination 1

321Design Approach


National choice of Design Approach

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Partial factors for limit states GEO/STR (DA1)– footings, walls, and slopes

γReEarth resistance (Re)

1.0

1.4

(0)

1.0(0)

1.51.0

1.35A1

Combination 1

1.0

M1

1.0

R1

γRhSliding resistance (Rh)

γcEffective cohesion (c’)γcuUndrained shear strength (cu)

1.0γRvBearing resistance (Rv)γγWeight density (γ)γquUnconfined compressive strength (qu)

1.25γφShearing resistance (tan φ)

Symbol

(0)-Favourable

1.0γAUnfavourableAccidental action (A)

(0)-Favourable

1.3γQUnfavourableVariable action (Q)

(γG,fav)Favourable

1.0γGUnfavourablePermanent action (G)

R1M2A2Combination 2Parameter


Verification of strength for GEO/STR (DA1-1)

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Verification of strength for GEO/STR (DA1-2)


Unplanned excavation

• Design geometry shall account for anticipated excavation or possible scour in front of the retaining structuread = anom + Δa

• For normal site control– Cantilever: Δa = 10% of retained height– Supported wall: Δa = 10% of height below lowest support – Maximum Δa = 0.5m

H

ΔHΔH

H

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New formulation for active and passive earth coefficients

β ϑ ϕϕ ϕϕ ϕ

± + − −⎫ ± × ±=⎬ × ±⎭ ∓

2( )tan1 sin sin(2 )1 sin sin(2 )

t wa m mw

p t

K m eK m

β ϕ βϕ

− ⎛ ⎞−= −⎜ ⎟±⎝ ⎠

∓1 sin2 cossintm

δ ϕ δϕ

− ⎛ ⎞= ⎜ ⎟

⎝ ⎠∓ ∓1 sin2 cos

sinwm


Charts of earth pressure coefficients from from Bond & Harris (2008)

Charts given for:• Angle of shearing resistance φ = 0 to 45°• Angle of interface friction δ = 0 to 30°• Ground inclinations tan β = flat, ±1:10, ±1:5, ±1:4, ±1:3, ±1:2.5, ±1:2, and ±1:1.5• Angle of wall inclination θ = 0 °

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Angle of interface friction

Eurocode 7 allows δd to be determined from the soil’s design constant-volume angle of shearing resistance φcv,d

Values of k are:• 1 for soil against cast in-situ concrete• ⅔ for soil against precast concrete

The UK National Annex states:It might be more appropriate to

select the design value of φcvdirectly

Perhaps it is better to use γφ,cv < γφ to determine φcv,d?

,1,

tantan cv k

d cv dk kϕ

ϕδ ϕ

γ−⎛ ⎞

= = ⎜ ⎟⎜ ⎟⎝ ⎠

,1,

,

tantan ?cv k

d cv dcv

k kϕ

ϕδ ϕ

γ−⎛ ⎞

= = ⎜ ⎟⎜ ⎟⎝ ⎠


Should water pressures be factored?

For ultimate limit states (ULSs)…design values [of groundwater pressures] shall represent the most

unfavourable values that could occur during the design lifetime of the structure

For serviceability limit states (SLSs)…design values shall be the most unfavourable values which could

occur in normal circumstances[EN 1997-1 §2.4.6.1(6)P]

Design values of ground-water pressures may be derived either by applying partial factors to characteristic water pressures or by

applying a safety margin to the characteristic water level…[EN 1997-1 §2.4.6.1(8)]

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Possible ways of treating water pressures

a) Design water levels for ULS and SLS design situationsb) Characteristic water pressures for SLS design situationc) Characteristic pressures for ULS = design pressures if no factor appliedd) Design pressures for ULS with factor on permanent actions (1.35)e) Design pressures for ULS with factor on permanent actions (1.35) applied to

normal water level and factor on variable (1.5) applied to rise highest possible water level

f) Design pressures for ULS with factor on variable actions (1.5)


Passive earth pressure:resistance or action?

Unfavourableaction

Favourable action or resistance?

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Possible outcomes influenced by the ‘single source principle’

Unfavourable (or destabilising) and favourable (stabilising) permanent actions may in

some situations be considered as coming from a single source. If … so, a single partial factor may be applied to the sum of these

actions or to the sum of their effects

EN 1997-1 §2.4.2(9) NOTE

Design of bearing piles


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Contents of EN 1997-1 Section 7

Section 7 Pile foundations§7.1 General (3 paragraphs)§7.2 Limit states (1)§7.3 Actions and design situations (18)§7.4 Design methods and design considerations (8)§7.5 Pile load tests (20)§7.6 Axially loaded piles (89)§7.7 Transversely loaded piles (15)§7.8 Structural design of piles (5)§7.9 Supervision of construction (8)

Scope• Section 7 applies to end-bearing piles, friction piles, tension piles, and

transversely loaded piles– but not to settlement reducing piles in piled-raft foundations

• Section 7 applies to piles installed by driving, jacking, screwing, and boring (with or without grouting)


Depth of investigation points for pile foundations

Depth of investigation points below lowest point of foundation (za):za ≥ 5mza ≥ bg (smaller width of group) za ≥ 3 DF (base diameter)

If built on competent strata with distinct geology, za may be reduced to 2m…

with indistinct geology, at least one borehole should go to za ≥ 5m

See EN 1997-2 Annex B.3

Za

bg

DFDF

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Verification of strength for pile foundations


Design Approaches for STR/GEO for pile foundations

• §2.4.7.3.4.1(1)P The manner in which equations [above] are applied shall be determined using one of three Design Approaches– Design Approaches apply ONLY to STR and GEO limit states– Each nation can choose which one (or more) to allow

• NA.4 …only Design Approach 1 is to be used in the UK• In simplest terms, the design approaches apply factors to the following…

Structural actions (or effects)& material properties

Actions (or effects)& resistances

ResistancesActionsCombination 2Combination 1

321Design Approach

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Partial factors for GEO/STR Design Approach 1 – pile foundations

1.6 [1.7-2.0]1.25 [1.0]γstShaft resistance, tension (Rst)

Bored/CFADriven 1.3 [1.3-1.5]

Values in [brackets] given in NA to BS EN 1997-1

CFABoredDriven

CFABoredDriven

1.3 [1.5-1.7]1.0γtTotal resistance (Rt)

1.3 [1.4-1.6]1.0γsShaft

resistance (Rs)

1.45 [1.7-2.0]1.1 [1.0]1.6 [1.7-2.0]1.25 [1.0]

1.5 [1.7-2.0]1.15 [1.0]1.4 [1.7-2.0]1.1 [1.0]

Not specified in EN 1997-1 [1.2-1.4]

(0)1.0(0)1.51.0

1.35A1

Combination 1

1.0

M1

1.0

R1

γRdModel factor

1.3 [1.5-1.7]γbBase resistance (Rb)

1.0γMAll material properties

Sym-bol

(0)-Favourable1.0γAUnfavourableAccidental

action (A)

(0)-Favourable1.3γQUnfavourableVariable action

(Q)

γG,favFavourable1.0γGUnfavourablePermanent

action (G)

R4M1A2Combination 2Parameter


Design Approach 1 for pile foundationsusing ground parameters

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Safety factors for pile capacity

“… it has been customary to design piles with ultimate axial capacities between 2 and 3 times the required working load. For example …a factor of safety of 2.0 on the combined shaft and base capacity, or, for underreamed piles … unity on the shaft capacity and 3.0 on

the base capacity.“A factor of safety of 2.0 is often deemed sufficient when test piles

have been loaded to failure. However … 2.5 is recommended where only proof loads are applied to working piles.”

Fleming et al., Piling Engineering (2nd Edition), p212


Pile resistance factors for DA1-2 (from NA to BS EN 1997-1)

Shaft (γs)

Base (γb)

1.3

1.5

Shaft (γs)

Base (γb)

1.4

1.6

1.2

2.0As base

1.72.0No explicit SLS check 1.4

Model factor (γRd)

DrivenBored/CFA Tens-ion (γst)

Total (γt)

1.71.51.7> 1% constructed piles taken to 1.5 x representative load (or settlement no concern)

Maintained load test to calculated unfactoredultimate load

Resistance factors from Set R4Static load tests on piles loaded to…

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Global factor of safety for replacement piles(with 25% variable action)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0% 20% 40% 60% 80% 100%

Qs/Qult

F =

Qul

t / Q

a

UK National Annex vs UK traditional

No explicit SLS check

Tests on 1% working piles

Preliminary load tests

Summary of key points


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Conclusion

“… misunderstandings and calculation errors can have significant effects on engineering designs. … factors of safety have an

important role in covering a certain degree of human error. For this reason, the results of new design processes should be checked

against traditional methods. Reductions in overall safety levels …should only be accepted in small increments, and tested in extensive practice before further reductions are considered”

Simpson (2005)


Decoding Eurocode 7

• Book published Autumn 2008 by Spon in hardback with colour section

• Authors Andrew Bond (Geocentrix) and Andy Harris (Geomantix)

• Web: www.decodingeurocode7.com• Key features

– Covers EN 1997-1 and -2, plus relevant parts of other Eurocodes

– Also covers associated execution and testing standards

– Explains key principles and application rules with real-life case studies

– Material extensively tested on training courses over 5 years

• ISBN 9780415409483

Documents

Impact of Eurocode 7 on steel piles - Decoding the Eurocodesgeocentrix.typepad.com/.../impact_of_eurocode_7_on_steel_piles.pdf · Impact of Eurocode 7 on steel piles Dr Andrew Bond