retaining walls and geotechnical design to eurocode 7 summary

7/25/2019 Retaining Walls and Geotechnical Design to Eurocode 7 Summary

http://slidepdf.com/reader/full/retaining-walls-and-geotechnical-design-to-eurocode-7-summary 1/43

ICE Teesside Branch, NGG and IStructE

EUROCODE 7

Dr Ian Smith, Edinburgh Napier University

Retaining Walls and Geotechnical

Design to Eurocode 7

Dr Ian Smith

Head of School

School of Engineering and the Built Environment

Edinburgh Napier University




EUROCODE 7


Geotechnical design by calculation

Processes involved:

Establish design values of actions and

geometrical data

Establish design values of ground

properties and resistances

Define limit that must not be exceeded

(e.g. bearing resistance)

Perform relevant geotechnical analysis

Show, by calculation, that limit will not be

exceeded




EUROCODE 7



Actions:

• An action is given the general symbol, F.

• Actions can be permanent (persistent) or variable

(transient), accidental, or seismic.• Persistent actions are denoted by FG. Transient actions are

denoted by FQ.

• Persistent actions can be either “favourable” or

“unfavourable”.• Transient actions are always considered as unfavourable.




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Ground properties:

• Geotechnical parameters should be established with consideration

given to published data and local and general experience…

• Clauses 2.4.3(3) to (6) give guidance on how the parametersshould be considered in the design process.

• Material properties are given the general symbol, X.

• Characteristic values of material properties are given the general

symbol, Xk.




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Partial factors of safety

Provided in EN 1997-1

Nationally Determined Parameters (NDPs) provided in National Annexe

Symbols:

Actions: General: F Permanent: G

Transient: Q

Materials: General: M Soil properties: cu, , etc.

Resistance: General: R Bearing resistance: Rv

NB geotechnical engineers already use “” for unit weight (weight density).




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Design values

These are obtained by combining the characteristic value with the

appropriate partial factor of safety.

i.e.

characteristic value

design value

partial factor of safety




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Multiplied by F values

Representative action Fk

Design action Fd Design material property, e.g. c'd

Characteristic material property, e.g. c'

Divided by M values

Geotechnical Analysis

Design effect of actions, Ed Design Resistance, Rd

Verify

Ed ≤ Rd

Actions: (loads, forces etc.) Material Properties (c, tan , etc.)and

The design is all about




EUROCODE 7


Characteristic

action

representative

action

design

action

design effects of

action

(Fk

) (Frep

) (Fd

) (Ed

)

Design values of actions

Correlation

factor, Partial factor

of safety, F

i.e. Frep = Fk ( 1.0; = 1.0 for persistent actions)

Fd = Frep F




EUROCODE 7


Design values of geotech params

i.e.

M

k d

M M

Partial factor of

safety, M

Characteristic geotechnical

Parameter

(Mk)

Design geotechnical

Parameter

(Md)




EUROCODE 7


Design effects of actions (i)i) During the verification of geotechnical strength (i.e. GEO limit state) some effects of

the actions will depend on the strength of the ground in addition to the magnitude of

the applied action and the dimensions of the structure. Thus, the effect of an action in

the GEO limit state is a function of the action, the material properties and the

geometrical dimensions.

i.e.

Ed = E{Fd; Xd; ad}

where

Ed is the design effect of the action, and

Fd is the design action;

Xd is the design material property;

ad is the design dimension,

and where

E{…} indicates that the effect, E is a function of the terms in

the parenthesis.




EUROCODE 7


Design effects of actions (ii)

During the verification of static equilibrium (i.e. EQU limit state) some effectsof the actions (both destabilising and stabilising) will depend on the strength

of the ground in addition to the magnitude of the applied action and the

dimensions of the structure. Thus, the effect of an action in the EQU limit

state, whether it be a stabilising or a destabilising action, is a function of the

action, the material properties and the geometrical dimensions.

i.e.

Edst;d = E{Fd; Xd; ad}dst

where

Edst;d is the design effect of the destabilising action, and

Estb;d = E{Fd; Xd; ad}stb

where

Estb;d is the design effect of the stabilising action.




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Design resistances

Equation 6.6 in EN 1990:2002 indicates that the design resistance depends

on material properties and the structural dimension. However, in geotechnical

design, many resistances depend on the magnitude of the actions and so EN1997-1:20042.4.7.3.3 redefines Equation 6.6 to include the contribution

made by the design action. The clause actually offers three methods of

establishing the design resistance,

or or

Annex B of Eurocode 7 Part 1 offers guidance on which of the 3 formulae

above to use for each design approach.

d d d d a X F R R ;;

R

d k d

d

a X F R R

;;

R

d d d

d

a X F R R

;;




EUROCODE 7


The five ultimate limit states

Eurocode 7 lists five ultimate limit states to consider:

• Verification of static equilibrium (EQU)

• Verification of (structural) strength (STR)• Verification of (ground) strength (GEO)

• Verification of resistance to uplift (UPL)

• Verification of resistance to heave failure due to seepage (HYD)




EUROCODE 7


Ultimate limit states

Loss of static equilibrium

EQUEQU UPLUPLUplift by water pressure

HYDHYDHydraulic heave/erosion

GEOGEOFailure of the ground

STRSTRInternal failure of structure

ULS for Stability:ULS for Stability:

ULS for Strength:ULS for Strength:




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Equilibrium (EQU) limit stateLoss of static equilibrium

Limit state is satisfied if the sum of the design values of the effects of destabilising actions

(Edst;d) is less than or equal to the sum of the design values of the effects of the stabilising

actions (Estb;d) together with any contribution through the resistance of the ground around

the structure (Td),

i.e. Edst;d ≤ Estb;d + Td.

EQU: loss of equilibrium of the structure or the

supporting ground when considered as a rigid body

and where the internal strength of the structure and

the ground do not provide resistance.




EUROCODE 7


Geotechnical (GEO) limit stateFailure of the ground

This limit state is satisfied if the design effect of the actions (Ed) is less than or equal to the

design resistance (Rd),

i.e. Ed ≤ Rd

GEO: failure or excessive deformation of the ground,

where the soil or rock is significant in providing

resistance.




EUROCODE 7


Structural (STR) limit stateInternal failure of structure

As with GEO limit state, the STR limit state is satisfied if the design effect of the actions (Ed)

is less than or equal to the design resistance (Rd),

i.e. Ed ≤ Rd

STR: failure or excessive deformation of the

structure, where the strength of the structural

material is significant in providing resistance.




EUROCODE 7


Uplift (UPL) limit stateUplift by water pressure

This limit state is verified by checking that the sum of the design permanent and variable

destabilising vertical actions (Vdst;d) is less than or equal to the sum of the design stabilising

permanent vertical action (Gstb;d) and any additional resistance to uplift (Rd).

i.e. Vdst;d ≤ Gstb;d + Rd.

UPL: the loss of equilibrium of the structure or the

supporting ground by vertical uplift due to water

pressures (buoyancy) or other actions.




EUROCODE 7


Hydraulic (HYD) limit stateHydraulic heave/erosion

This limit state is verified by checking that the design total pore water pressure (udst;d) or

seepage force (Sdst;d) at the base of the soil column under investigation is less than or equalto the total vertical stress (stb;d) at the bottom of the column, or the submerged unit weight

(G'stb;d) of the same column.

i.e. udst;d ≤stb;d or Sdst;d ≤ G'stb;d.

UPL: hydraulic heave, internal erosion and piping in

the ground as might be experienced, for example, at

the base of a braced excavation.




EUROCODE 7


ULS for retaining structures

(a) Overturning(Eurocode 7 EQU limit state)

(b) Bearing failure(Eurocode 7 GEO limit state)

(c) Forward sliding(Eurocode 7 GEO limit state)

(d) Ground failure

(Eurocode 7 GEO limit state)

(e) Structural failure

(Eurocode 7 STR limit state)




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EQU limit state

Destabilising actions and effects

Representative destabilising

actions, Fdst; rep

Partial factors,

F dst

GEOTECHNICAL ANALYSIS

Design effect of destabilising

actions, Edst;d

Representative stabilising

actions, Fstb; rep

Design effect of stabilising

actions, Estb;d

Verify Edst;d ≤ Estb;d

Stabilising actions and effects

Design destabilising

actions, Fdst;d

Design stabilising

actions, Fstb;d

Partial factors,

F stb




EUROCODE 7


EQU limit state example

Pq

q

PaW

Overturning




EUROCODE 7


GEO limit state

Actions and effects

Representative

actions, Frep

Partial factors, F

GEOTECHNICAL ANALYSIS

Design effect of actions,

Ed

Characteristic material

properties, Xk

Design resistance, Rd

Verify Ed ≤ Rd

Material properties and resistance

Design actions, Fd Design materialproperties, Xd

Partial factors, M




EUROCODE 7


GEO/STR Limit states

Three Design Approaches are offered - to reflect national choice

The design approach followed reflects whether the safety is applied to the

material properties, the actions or the resistances.

Design Approach 1: Combination 1: A1 + M1 + R1†Combination 2: A2 + M2 + R1

Design Approach 2: A1 + M1 + R2Design Approach 3: A* + M2 + R3

A*: use set A1 on structural actions, set A2 on geotechnical actions

† For axially loaded piles, DA1, Combination 2 is: A2 + (M1 or M2) + R4

The UK National Annex states that Design Approach 1 shall be used.




EUROCODE 7


GEO/STR Limit states

DA 1-1: A1 + M1 + R1 DA 1-2: A2 + M2 + R1DA 1-1: A1 + M1 + R1

GEO/STR - Partial factor sets

Parameter Symbol A1 A2 M1 M2 R1 R2 R3

Permanent action (G) Unfavourable G 1.35 1.0

Favourable G 1.0 1.0

Variable action (Q) Unfavourable Q 1.5 1.3

Favourable - - -

Accidental action (A) Unfavourable A 1.0 1.0

Favourable - - -

Coefficient of shearing resistance (tan ') ' 1.0 1.25

Effective cohesion (c') c' 1.0 1.25

Undrained shear strength (cu) cu 1.0 1.4

Unconfined compressive strength (qu)

qu1.0 1.4

Weight density () 1.0 1.0

Bearing resistance (Rv) Rv 1.0 1.4 1.0

Sliding resistance (Rh) Rh 1.0 1.1 1.0

Earth resistance (Re) Re 1.0 1.4 1.0




EUROCODE 7


Representation of degree of safety

Over-design factor:

Degree of utilisation:

d

d

E

R

d

d

R

E




EUROCODE 7


Gfav

Gunfav

Qunfav

Ed

Rd

Gunfav

Gunfav

Qunfav

Ed

Rd

sliding… … and bearing

GEO limit state examples




EUROCODE 7


Retaining wall design

(1)P The provisions of this Section shall apply to structures, which retain

ground comprising soil, rock or backfill and water. Material is retained if it

is kept at a slope steeper than it would eventually adopt if no structure

were present.

Retaining structures include all types of wall and support systems in

which structural elements have forces imposed by the retained material.

EN 1997-1:2004 9.1.1(1)P

Covered in Section 9 of Eurocode 7 Part 1

EUROCODE 7




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The limit states to be considered are listed in9.2(1) and are:

• loss of overall stability;

• failure of a structural element such as a wall, anchorage, wale or strut

or failure of the connection between such elements;• combined failure in the ground and in the structural element;

• failure by hydraulic heave and piping;

• movement of the retaining structure, which may cause collapse or

affect the appearance or

• efficient use of the structure or nearby structures or services, which rely

on it;

• unacceptable leakage through or beneath the wall;

• unacceptable transport of soil particles through or beneath the wall;

• unacceptable change in the ground-water regime.

EN 1997-1:2004 9.2(1)

Limit states

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EUROCODE 7



Gravity walls:

bearing resistance failure of the soil below the base;

failure by sliding at the base;

failure by toppling;

Embedded walls:

failure by rotation or translation of the wall or parts thereof;

failure by lack of vertical equilibrium.

EN 1997-1:2004 9.2(1)

Plus…

EUROCODE 7




EUROCODE 7



(a) Overturning(Eurocode 7 EQU limit state)

(b) Bearing failure(Eurocode 7 GEO limit state)

(c) Forward sliding(Eurocode 7 GEO limit state)

(d) Ground failure

(Eurocode 7 GEO limit state)

(e) Structural failure

(Eurocode 7 STR limit state)

EUROCODE 7




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Must also consider overall stability (Section 11)…

EUROCODE 7




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Future unplanned excavation

(2) In ultimate limit state calculations in which the stability of a retaining wall

depends on the ground resistance in front of the structure, the level of theresisting soil should be lowered below the nominally expected level by anamounta.

…

— for a cantilever wall,a should equal 10 % of the wall height above

excavation level, limited to a maximum of 0,5 m;— for a supported wall,a should equal 10 % of the distance between the

lowest support and the excavation level, limited to a maximum of 0,5 m.

EN 1997-1:2004 9.3.2.2(2)

(3) Smaller values ofa, including 0, may be used when the surface level is

specified to be controlled reliably throughout the appropriate execution

period.EN 1997-1:2004 9.3.2.2(3)

EUROCODE 7



ICE Teesside Branch, NGG and IStructEDr Ian Smith, Edinburgh Napier University

Gravity wallsWhen Rankine’s conditions do not apply...

Charts for both horizontal and inclined retained surfaces are given in Annex C.

Ka for a horizontal ground surface behind the wall

0.1

1

0 5 10 15 20 25 30 35 40 45

Design values of φ'

Ka

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

δ / φ' = 0

δ / φ' = 0.66

δ / φ' = 1

1.0

EUROCODE 7




Example

Retained fill:

c' = 0; ' = 32

= 18 kN/m34.0 m

2.0 m

2.6 m

1.8 m

1.0 m Foundation soil:

c' = 0; ' = 28

= 20 kN/m3

Surcharge, q = 20 kPa

3

1

2

= 22.4 kPa

= 26.7 kPa

34.1 kPa

= 6.2 kPa

7.4 kPa

haK qaK

Check the overturning (EQU) and sliding (GEO) (using Design Approach 1) limit states.

EUROCODE 7




Embedded walls

EUROCODE 7




Embedded walls

d K pd 0

O K a(h+d 0)

q = 10kPa

K p(h+d)

K ad

d 0

h0.1h; > 0.5m

Pq1

Pq2

Pa1

Pp2

Pp1

Pa2

Cantilever wall – pressure distribution

EUROCODE 7




Embedded walls

Cantilever wall – simplified pressure distribution

K a(h+d 0)

P p

Pa

R

K pd 0

h+d 03

Pq

EUROCODE 7




Passive resistance

favGk pd p PP ;;;

Re

;

;

k p

d p

P

P

Favourable action:

or

Resistance:

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Passive resistance

Design Approach

1 2 3

Combination 1 Combination 2

G;fav1.0 1.0 1.0 1.0

Re1.0 1.0 1.4 1.0

i.e. only concerns Design Approach 2

EUROCODE 7




Passive resistance

but what about for embedded walls?…

Single Source Principle…

NOTE Unfavourable (or destabilising) and favourable (or stabilising)permanent actions may in some situations be considered as coming from a

single source. If they are considered so, a single partial factor may be

applied to the sum of these actions or to the sum of their effects.

EN 1997-1:2004 2.4.2Note to (9)P

EUROCODE 7




Passive resistance

P p

Pa

“uncertainty” in Pp = “uncertainty” in Pa

i.e. if Pa is a permanent unfavourable action, so must be Pp

(Single source principle)

EUROCODE 7




Passive resistance

Design Approach

1 2 3

Combination

1

Combination

2

G;fav1.0 1.0 1.0 1.0

G;unfav1.35 1.0 1.35 1.0

Re1.0 1.0 1.4 1.0

retaining walls and geotechnical design to eurocode 7 summary

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