Design of foundations and retaining structures to Eurocode 7€¦ · IntroducFon#to#Eurocode#7#...

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Design of foundations and retaining structures to Eurocode 7

Dr Andrew Bond (Geocentrix) Chairman TC 250/SC 7

DESIGN O

F FOUNDATIO

NS &

RETAINING STRU

CTURES TO EC7

Outline of talk IntroducFon to Eurocode 7 General rules Design of spread foundaFons Design of pile foundaFons Design of retaining structures Ground invesFgaFon and tesFng Summary of key points

7-15 2 Design of foundations and retaining structures to Eurocode 7 ©2015 Geocentrix Ltd. All rights reserved

INTRODUCTION TO EUROCODE 7

MODIFIED AFTER B

OND AN

D HARRIS (2008)

The 33 members of CEN

Austria Greece Poland

Belgium Hungary Portugal

Bulgaria Iceland Romania

CroaFa Ireland Slovakia

Cyprus Italy Slovenia

Czech Republic

Latvia Spain

Denmark Lithuania Sweden

Estonia Luxembourg Switzerland

Finland Malta F.Y. Republic of Macedonia

France Netherlands Turkey

Germany Norway United Kingdom


BOND AN

D HARRIS (2008)

The Eurocode family


BOND AN

D HARRIS (2008)

EN 1997-‐1 Geotechnical design: General rules


BOND AN

D HARRIS (2008)

EN 1997–2: Geot. design: Ground invesEgaEon and tesEng


NaEonal Annexes differ from country to country


Germany Denmark France

Netherlands Austria United Kingdom

‘EC7 for geo. design – a model code for non-‐EU countries?’ Paper published at 17th Int. Conf. On Soil Mechanics and Geotechnical Engineering, October 2009 Subjects covered

Eurocode 7 ‘Geotechnical design’ Applicability of Eurocode 7 Part 1:

design examples from the 2005 Dublin Workshop

Comments of non-‐European countries

Authors B. Scuppener, A.J. Bond, R. Frank, T.L.L. Orrr, G. Scarpelli, and B. Simpson Paper is available online from:


GENERAL RULES

BOND AN

D HARRIS (2008)

Overview of limit state design


VerificaEon of limit states Limit states may be verified by: •   CalculaFon •   PrescripFve measures •   Experimental models and load tests •   The ObservaFonal Method

This lecture concentrates on design by calculaFon


EN 1997-‐1 §2.4.7.1(1)P

VerificaEon of strength VerificaFon of strength is expressed in Eurocode 7 by:

Ed = design effect of acFons; Rd = design resistance corresponding to that effect Applies to GEO: “Failure or excessive deforma2on of the ground, in which the strength of soil or rock

is significant in providing resistance’ EN 1997-‐1 §2.4.7.1(1)P

…and to STR: “Internal failure or excessive deforma2on of the structure or structural elements … in

which the strength of structural materials is significant in providing resistance” EN 1997-‐1 §2.4.7.1(1)P


d dE R≤

EN 1997-‐1 §2.4.8

VerificaEon of serviceability limit states VerificaFon of serviceability limit states: Values of parFal factors for SLS should normally be taken equal to 1.0 A limi2ng value for a par2cular deforma2on is the value at which a serviceability

limit state … is deemed to occur in the supported structure. This limi2ng value shall be agreed during the design of the supported structure

EN 1997-‐1 §2.4.8(5)P

7-15 Design of foundations and retaining structures to Eurocode 7 ©2015 Geocentrix Ltd. All rights reserved 14

≤d dE C

BOND AN

D HARRIS (2008)

VerificaEon of strength for STR/GEO (geotechnical design)

Verification

Verification of strengthMaterial propertiesActions

Representative actions

Frep

Characteristic material

properties Xk

Design actionsFd

Design material properties

Xd

Design effect of actions

Ed

Design resistance

Rd

strength

VerifyEd ≤ Rd

gE gR

Geometrical parameters

Nominal dimensions

anom

Design dimensions

ad

Da

Characteristic actions

Fk

y i

weight density

Or a

pply

to e

ffect

s

gF

Or a

pply

to re

sist

ance

gM

Calculation model


VeriricaEon of strength expressed mathemaEcally VerificaFon of strength Ed = design effect of acFons; Rd = design resistance Gk = characterisFc permanent acFon; Qk = characterisFc variable acFon ψ = combinaFon factor γG, γQ, γM, and γR = parFal factors

16 Design of foundations and retaining structures to Eurocode 7 ©2015 Geocentrix Ltd. All rights reserved

≤d dE R

7-15

( ) ( ) ( )γγ γ γ ψ

γ> >

+ + ≤∑ ∑, , ,1 ,1 , ,0 1

k MG j k j Q k Q i i k i

j i R

f XG Q Q

EN 1997-‐1 §2.4.7.3.4.1(1)P

Design Approaches for STR/GEO “The manner in which equa2ons [for GEO/STR] are applied shall be determined using

one of three Design Approaches Design Approaches apply ONLY to STR and GEO limit states

Each na2on can choose which one (or more) to allow” EN 1997-‐1 §2.4.7.3.4.1(1)P

Design Approach

Parameter

Input Output

AcFons Material properFes

Effect of acFons Resistance

DA1

DA2 DA2* DA3


Design Approaches expressed mathemaEcally Design Approach 1 Design Approach 2 Design Approach 3

18 Design of foundations and retaining structures to Eurocode 7 ©2015 Geocentrix Ltd. All rights reserved 7-15

( )

( )

,, ,1 ,

0 1

,, ,1 ,

0 1

tan 1.0, 1.01.35 1.5 1.5

1.0and

tan 1.25, 1.41.0 1.3 1.3

1.0

k u kk j k k i

j i

k u kk j k k i

j i

f cG Q Q

f cG Q Q

ϕ

ϕ

> >

> >

× + × + × ≤

× + × + × ≤

∑ ∑

∑ ∑

( ), ,1 ,0 1 ,

, ,1 ,0 1

1.35 1.5 1.5tan 1.25, 1.4

1.01.0 1.3 1.3

STR STR STRk j k k i

j i k u kGEO GEO GEOk j k k i

j i

G Q Qf c

G Q Qϕ> >

> >

⎫× + × + ×⎪≤⎬

+ × + × + × ⎪⎭

∑ ∑

∑ ∑

( )

( )

,, ,1 ,

0 1

,, ,1 ,

0 1

tan 1.0, 1.01.35 1.5 1.5

1.4or

tan 1.0, 1.01.35 1.11 1.11

1.4

k u kk j k k i

j i

k u kk j k k i

j i

f cG Q Q

f cG Q Q

ϕ

ϕ

> >

> >

× + × + × ≤

⎛ ⎞× + × + × ≤⎜ ⎟⎝ ⎠

∑ ∑

∑ ∑

AFTER B

OND (2013) SPECIAL G

EOTEC. P

UBL.

NaEonal choice of Design Approach for shallow foundaEons


DESIGN OF SPREAD FOUNDATIONS

MODIFIED FRO

M BOND AN

D HARRIS (2008)

VerificaEon of bearing resistance N (= V in EN 1997-‐1) = normal/verFcal load (or component of total acFon) acFng normal to foundaFon base N includes weight of foundaFon (W) and backfill and earth pressures on side R = resistance to verFcal load Short-‐term and long-‐term values of R must be checked


d d d d Ed RdV R E R q q≤ ≡ ≤ ≡ ≤

γ γ γ γ ψ> >

⎛ ⎞+ + +⎜ ⎟⎜ ⎟

⎝ ⎠=ʹ′

∑ ∑, , ,1 ,1 , ,0 1

G j Gk j G Gk Q Qk Q i i Qk ij i

Ed

N W N Vq

A

EN 1997-‐1 A

NNEX D

Method for calculaEng bearing resistance Annex D provides the following expressions for drained resistance:

…and undrained resistance: A’ = effecFve base area c’, cu = soil’s effecFve cohesion and undrained strength, respecFvely q’, q = effecFve and total overburden pressure, respecFvely γ’ = effecFve weight of the soil = (γ -‐ γw) Nx = bearing capacity factors bx, sx, ix = base inclinaFon, shape, and load inclinaFon factors Depth factors dx and ground inclinaEon factors gx are missing!


γ γ γ γγʹ′ ʹ′ ʹ′ ʹ′= + + 0.5c c c c q q q qR A c N b s i q N b s i BN b s i

( )2 u c c cR A c b s i qπʹ′ = + +

BOND AN

D HARRIS (2008)

Drained bearing capacity factors


MODIFIED FRO

M BOND AN

D HARRIS (2008)

VerificaEon of sliding resistance H = horizontal load (or component of total acFon acFng parallel to foundaFon base), including acFve earth force on foundaFon (Pa on diagram) R = resistance to horizontal load from the underside of the foundaFon Rp = resisFng force caused by earth pressure on side of fooFng (Pp on diagram)


d d pd d d Ed RdH R R E R τ τ≤ + ≡ ≤ ≡ ≤

γ γ γ γ ψ

τ> >

⎛ ⎞+ + +⎜ ⎟⎜ ⎟

⎝ ⎠=ʹ′

∑ ∑, , , ,1 ,1 , ,0 1

G j Gk j G a Gk Q Qk Q i i Qk ij i

Ed

H P H H

A

Drained sliding resistance Eurocode 7 gives the following expression for drained sliding resistance:

V’d = design effecFve verFcal acFon; δd = design angle of interface fricFon A’ = effecFve base area; V (= N) verFcal/normal acFon; U = water thrust (upwards) γG = parFal factor on unfavourable permanent acFons (γG,fav = on favourable) γQ = parFal factor on unfavourable variable acFons γϕ = parFal factor on coefficient of shearing resistance γRh = parFal factor on horizontal sliding resistance Subscripts ‘d’ and ‘k’ denote design and characterisFc values; ‘G’ and ‘Q’ permanent and variable acFons


δʹ′= × tand d dR V

ϕ

δδ

τγ γ

γ γ γδ

γ γ

⎛ ⎞+ − ×⎜ ⎟

× ⎝ ⎠= =ʹ′ ʹ′

⎛ ⎞+ −⎜ ⎟

⎝ ⎠= ×ʹ′

∑∑

∑

,,

, , , ,

tantan

tan

Gd j Gd Gd kjd fav d

RdRh Rh

G fav j Gk j G fav Gd G Gkj k

Rh

N W UN

A A

N W U

A

DESIGN OF PILE FOUNDATIONS

MODIFIED FRO

M BOND AN

D HARRIS (2008)

VerificaEon of compressive resistance P = axial compression load W = self-‐weight of pile Rs = shar resistance Rb = base resistance For pile groups, must check: •   Individual failure of the piles •   Block failure of piles and soil between

them •   Design resistance is lower of two values


γ γ γ γ ψ

τ> >

⎛ ⎞+ + +⎜ ⎟⎜ ⎟

⎝ ⎠=ʹ′

∑ ∑, , , ,1 ,1 , ,0 1

G j Gk j G a Gk Q Qk Q i i Qk ij i

Ed

H P H H

A

, ,c d c d d d Ed RdF R E R N N≤ ≡ ≤ ≡ ≤

CalculaEon of compressive resistance Design compressive resistance (Rc,d) may be obtained directly from tesFng using: Rm,min, Rm,mean = minimum/mean pile resistance measured in field tests ξmin, ξmax = correlaFon factors depending on number of tests γt = parFal factor on total resistance of pile (or) Rc,d may be obtained from calculaFon using ground parameters: Rb,d, Rs,d = design compressive base/shar resistances Ab = area of base; As,i = surface area of shar in stratum qb,k = characterisFc unit base resistance; qs,i,k = characterisFc unit shar resistance γb, γs = parFal factor on base/shar resistance; γRd = model factor


γ γ γ γ

⎛ ⎞⎛ ⎞ ⎜ ⎟= + = +⎜ ⎟ ⎜ ⎟× ×⎝ ⎠ ⎜ ⎟

⎝ ⎠

∑ , , ,

, , ,

s i s i kb bk i

c d b d s dRd b Rd s

A qA q

R R R

ξ γ ξ γ

⎛ ⎞ ⎛ ⎞= ⎜ ⎟ ⎜ ⎟

× ×⎝ ⎠ ⎝ ⎠

,min ,m n,

min m

smaller of or m m eac d

t ean t

R RR

BOND AN

D HARRIS (2008)

Recommended correlaEon factors for pile foundaEons


AFTER B

OND (2013) SPECIAL G

EOTEC. P

UBL.

NaEonal choice of Design Approach for pile foundaEons


UK N

ATIONAL A

NNEX TO BS EN

1997-‐1 UK NaEonal Annex values for pile resistance factors Set StaEc load tests on piles

loaded to… Model factor (γRd)

Resistance factors Bored/CFA Driven Total

(γt) Tens-‐ion (γst)

Base (γb)

Shar (γs)

Base (γb)

Shar (γs)

R1 (All cases) 1.4 1.0 1.0 1.0 1.0 1.0 1.0

R4 No explicit SLS check 1.4 2.0 1.6 1.7 1.5 As base

2.0

> 1% constructed piles taken to 1.5 x representaFve load (or sevlement no concern)

1.7 1.4 1.5 1.3 1.7

Maintained load test to calculated unfactored ulFmate load

1.2


DESIGN OF RETAINING STRUCTURES

BOND AN

D HARRIS (2008)

Earth pressures on a reinforced concrete wall


BOND AN

D HARRIS (2008)

Is passive earth pressure an acEon or a resistance?


BOND AN

D HARRIS (2008)

Fixed vs free earth condiEons/gross vs net pressures To verify rotaFonal stability: If we treat passive earth pressure as a resistance: Giving FO ≥ 1.35, 1.0, 1.89, and 1.35 for DAs 1-‐1, 1-‐2, 2, and 3 respecFvely Whereas, if we treat passive earth pressure as a favourable ac2on: Giving FO ≥ 1.35, 1.0, 1.35, and 1.35 for DAs 1-‐1, 1-‐2, 2, and 3 respecFvely Finally, if we treat passive earth pressure as an unfavourable ac2on: 7-15 35 Design of foundations and retaining structures to Eurocode 7 ©2015 Geocentrix Ltd. All rights reserved

, , , ,d dst d stb a d a p d pM M P L P L≤ ⇒ × ≤ ×

,,, , ,

, , ,

p k pk stb GG a k a G fav p k p O

k dst a k a G fav

P LMP L P L F

M P Lγ

γ γγ

×× × ≤ × × ⇒ = = ≥

×

, ,,, Re

Re , ,

p k p p k pk stbG a k a O G

k dst a k a

P L P LMP L F

M P Lγ γ γ

γ

× ×× × ≤ ⇒ = = ≥ ×

×

,,, ,

, ,

1.0p k pk stb GG a k a G p k p O

k dst a k a G

P LMP L P L F

M P Lγ

γ γγ

×× × ≤ × × ⇒ = = ≥ =

×

Sign is wrong in paper

GROUND INVESTIGATION AND TESTING

MODIFIED AFTER B

OND AN

D HARRIS (2008)

Obtaining design material properEes


BOND AN

D HARRIS (2008)

Obtaining design material properEes – detailed Other relevant

data

Derived values of geotechnical parameters X

Correlation Theory Empiricism

Test results

Characteristic value Xk

Well-established experience

Statistical methods

Standard tables of characteristic

values

Design value Xd

Interpreted appropriately for the limit state considered

Interpreted appropriately for the limit state considered

Calibration factorsfrom EN 1997-2

95% confident value Very cautious estimate

Cautious estimate

Partial factors gM


BOND AN

D HARRIS (2008)

If we can’t fix the characterisEc value..


Understanding how cauEous you have to be


XXk,inf

Numberof testresults

1.64σ

Mean100

10

2

Standarddeviation, σ

v ar ian c e u n k n o w n

5 0 % fr ac t ile s

5 % fr ac t ile s

v ar ian c e k n o w n

30

50

3

5

WWW.DECO

DINGEU

ROCO

DE 7.COM

‘Decoding Eurocode 7’ by Bond and Harris (2008) Book published August 2008 Key features

Covers ENs 1997-‐1 and -‐2, plus relevant parts of other Eurocodes

Also covers associated execuFon and tesFng standards

Explains key principles Illustrates applicaFon rules with

real-‐life case studies Material extensively tested on

training courses over 5 years Authors Andrew Bond (Geocentrix) and Andy Harris (ex-‐GeomanFx) Published by Taylor and Francis in hardback, with colour secFon ISBN: 9780415409483


SUMMARY OF KEY POINTS

Design of foundaEons and retaining structures to Eurocode 7 IntroducFon to Eurocode 7

•   ENs 1990 – 1999 now adopted in 33 European countries •   … and in many other countries (Singapore, South Africa, Russia, China, …)

General rules •   Eurocodes based on limit state principles (ULS and SLS verificaFons) •   Eurocode 7 made complicated by three different Design Approaches

Design of spread foundaFons •   VerificaFon of sliding exposes problems of design based on buoyant weights

Design of pile foundaFons •   VerificaFon by tesFng or calculaFon •   ParFal factors almost exclusively applied to resistance

Design of retaining structures •   Treatment of passive earth pressure must invoke ‘single source principle’

Ground invesFgaFon and tesFng •   SelecFon of ‘the’ characterisFc value is problemaFc •   An understanding of reliability theory will be essenFal in the future


A second generaEon of Eurocodes is coming Future development of Eurocode 7

•   Work on the 2nd generaFon of Eurocodes started in January 2015 •   EN 1997:2020 will have 3 Parts:

•   Part 1: general rules •   Part 2: Ground invesFgaFon •   Part 3: Geotechnical construcFons

•   Design Approaches will be replaced by Design CombinaFons •   Plus many more improvements …

Thank you for your avenFon!


Geocentrix – pu.ng you at the centre

Design of foundations and retaining structures to Eurocode 7 www.geocentrix.co.uk www.decodingeurocode7.com

Design of foundations and retaining structures to Eurocode 7€¦ · IntroducFon#to#Eurocode#7#...

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