retaining walls and geotechnical design to eurocode 7

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 SchoolSchool of Engineering and the Built Environment

Edinburgh Napier University


EUROCODE 7


This evening’s presentation

1. Introduction to the Eurocodes2. Overview of Eurocode 7, EN 19973. Basis of Geotechnical Design4. Geotechnical Design by Calculation5. Retaining Wall Design6. Conclusion


EUROCODE 7


The Structural EurocodesWhat are the What are the EurocodesEurocodes??

The structural Eurocodes are a European suite of codes for structural design… developed over… 25 years

By 2010 they will have effectively replaced the current British Standards

They will be used as an acceptable basis for meeting compliance with UK Building Regulations and the requirements of other public authorities

from:National Strategy for Implementation of the Structural Eurocodes: Design Guidance

D. Nethercot et al, Institution of Structural Engineers (April 2004)


EUROCODE 7


Objectives of the Eurocodesas a means to prove compliance of building and civil engineering works with the essential requirements of mechanical resistance and stability and safety in case of fire;

a basis for specifying contracts for construction works & related engineering services;

a framework for drawing up harmonised technical specs for construction products.

In addition, the Eurocodes are foreseen to:

• improve the functioning of the single market for products and engineering services by removing obstacles arising from different nationally codified practices for the assessment of structural reliability;

• improve the competitiveness of the European construction industry and its professionals and industries, in countries outside the European Union.


EUROCODE 7


The Structural EurocodesEN 1990 Basis of Structural Design

EN 1991 Eurocode 1 Actions on Structures

EN 1992 Eurocode 2 Design of Concrete Structures

EN 1993 Eurocode 3 Design of Steel Structures

EN 1994 Eurocode 4 Design of Composite Steel & Concrete Structures

EN 1995 Eurocode 5 Design of Timber Structures

EN 1996 Eurocode 6 Design of Masonry Structures

EN 1997 Eurocode 7 Geotechnical Design

EN 1998 Eurocode 8 Design of Structures for Earthquake Resistance

EN 1999 Eurocode 9 Design of Aluminium Structures

EN 1997 Eurocode 7 Geotechnical Design

Part 1: General Rules

Part 2: Ground investigation and testing


EUROCODE 7


The Structural Eurocodes


EUROCODE 7


Publication of Eurocodes

1975: ECC identify need to improve functioning of the single market for products and engineering services

1989: ECC issue Council Directive 89/106/EEC- known as Construction Products Directive

Passed to CEN for development

Eurocode Programme overseen by Technical Committee 250 (CEN/TC 250)

Each Eurocode produced by separate sub-committeee.g. Eurocode 7 : CEN/TC 250/SC 7

Each Eurocode and National Annex published by national standards bodies, e.g. BSI in UK

Eurocode programme developed by the Comité Européen de Normalisation (CEN) – the European Committee for Standardisation.


EUROCODE 7


CEN Member States

• Austria• Belgium• Cyprus• Czech Republic• Denmark• Estonia• Finland• France• Germany• Greece• Hungary• Iceland• Ireland• Italy

• Latvia• Lithuania• Luxembourg• Malta• Netherlands• Norway• Poland• Portugal• Slovakia• Slovenia• Spain• Sweden• Switzerland• UK

(Comité Européen de Normalisation)


EUROCODE 7


CEN committee structure

CENCEN

TC 250TC 250 TC….TC…. TC…TC…

SC 0SC 0 SC 7SC 7SC…SC…SC 1SC 1 SC…SC…

e.g. Eurocode 7 : CEN/TC 250/SC 7


EUROCODE 7


Eurocodes Timeline

1989 – 1999ENVs produced

1975 1980 1985 1990 1995 2000 2005 2010

• All European public-sector clients have been legally required to commission Eurocode-compliant structural designs since March 2010.

• Private sector clients can continue to use any effective design methods. But, as most existing codes will be withdrawn, Eurocodes will be only recognised codes available.

March2010

Implmtn.

2002ENs start to appear

1989Programme

passed to CEN

1975EEC initiateprogramme

1999

2011Today


EUROCODE 7


Fundamental requirements

The structure and structural members should be designed, executed and maintained in such a way that they meet the following:

• Serviceability requirement – the structure during its intended life, with appropriate degrees of reliability and in an economic way, will remain fit for the use for which it is required.

• Safety requirement – the structure will sustain all actions and influences likely to occur during execution and use.

• Fire requirement – the structural resistance shall be adequate for the required period of time.

• Robustness requirement – the structure will not be damaged by events such as explosion, impact or consequences of human errors, to an extent disproportionate to the original cause.


EUROCODE 7


National Annex

EN Title Page

EN Annexes

EN Text

National Title Page

National Foreword

Structure of a Eurocode Document


EUROCODE 7


EN Annexes

EN Annexes are either Normative or Informative.

Normative – contains information that must be followed.

Informative – contains supplementary information that may be followed.


EUROCODE 7


National Annexes• The “link” between Eurocode and national standards for member state.• Contain rules and NDPs to ensure safety remains a national, and not a

European, responsibility.• Foreword of each Eurocode lists paragraphs in which national choice

is allowed. However, the National Annex has limited overriding authority to the Eurocode.

A National Annex cannot change or modify the content of the EN Eurocode text in any way other than where it indicates that national choices may be made by means of Nationally Determined Parameters.

Guidance Paper L: § 2.3.4


EUROCODE 7


National Annex

The National Annex flavours each Eurocode to each country’s needs.

A National Annex exists for each Eurocode Part.

National Annexes provide:• Nationally Determined Parameters (NDPs)• Country specific data• Procedure to be used, where choice is offered• Guidance on the informative annexes• Reference to non-contradictory, complementary information (NCCI)


EUROCODE 7



1. Introduction to the Eurocodes ✔2. Overview of Eurocode 7, EN 19973. Basis of Geotechnical Design4. Geotechnical Design by Calculation5. Retaining Wall Design6. Conclusion


EUROCODE 7


Eurocode 7: Geotechnical design• Part 1: General rules• Part 2: Ground investigation and testing

Published

December 2004Published

November 2007


EUROCODE 7


National Annexes• Part 1: Published November 2007• Part 2: Published December 2009


EUROCODE 7


Development of Eurocode 7• Agreement for geotechnical design more “challenging” than for

EN 1990, EN 1991 and material Eurocodes.

• EN 1997 was one of the later codes to be published.

• Unique in that some national practices maintained within design process, e.g. the 3 Design Approaches


EUROCODE 7


Soil properties8 features considered by drafters of Eurocode 7:

1. Soil properties determined by investigation, EN 1997 Part 2

2. Undrained and drained conditions to be considered

3. Property characteristic value is “cautious estimate” of mean value

4. Soil variability is high, judgement required for ‘k’ values

5. Strength related to normal stress , care required when applying partial factors of safety to geotechnical loads

6. Soil can redistribute loading from weaker to stronger zones

7. Soil is compressible, SLS usually controls design, though ULS calculations usually performed in design

8. Soil stress-strain behaviour is complex, few calculation models provided in EN 1997


EUROCODE 7


Contents of Eurocode 7 Part 1Foreword

1. General2. Basis of Geotechnical design3. Geotechnical data4. Supervision of construction, monitoring and maintenance5. Fill, dewatering, ground improvement and reinforcement6. Spread foundations7. Pile foundations8. Anchorages9. Retaining structures10. Hydraulic failure11. Overall stability12. Embankments

Annexes A – J 167 pages


EUROCODE 7


Contents of Eurocode 7 Part 2

Foreword1. General2. Planning of ground investigation3. Soil and rock sampling and groundwater measurements4. Field tests in soil and rock5. Laboratory tests on soil and rock6. Ground investigation report

Annexes A – X

196 pages


EUROCODE 7


Contents of Eurocode 7 Part 2Scope:

EN 1997-2 is intended to be used in conjunction with EN 1997-1 and provides rules supplementary to EN 1997-1 related to:

• planning and reporting of ground investigations;• general requirements for a number of commonly used laboratory and

field tests;• interpretation and evaluation of test results;• derivation of values of geotechnical parameters and coefficients.

Note: Establishment of characteristic values is covered in EN 1997-1.


EUROCODE 7


Contents of Eurocode 7 Part 224 Annexes:• Annex A List of test results of geotechnical test standards• Annex B Planning of geotechnical investigations• Annex C Example of groundwater pressure derivations based on a model and long term measurements• Annex D Cone and piezocone penetration tests• Annex E Pressure meter test• Annex F Standard penetration test• Annex G Dynamic probing test• Annex H Weight sounding test• Annex I Field vane test• Annex J Flat dilatometer test • Annex K Plate loading test• Annex L Detailed information on preparation of soil specimens for testing• Annex M Detailed information on tests for classification, identification and description of soil• Annex N Detailed information on chemical testing of soil• Annex O Detailed information on strength index testing of soil• Annex P Detailed information on strength testing of soil• Annex Q Detailed information on compressibility testing of soil• Annex R Detailed information on compaction testing of soil• Annex S Detailed information on permeability testing of soil• Annex T Preparation of specimen for testing on rock material• Annex U Classification testing of rock material• Annex V Swelling testing of rock material • Annex W Strength testing of rock material• Annex X Bibliography


EUROCODE 7


Contents of Eurocode 7 Part 2• Reminder (Scope):

EN 1997-2 is intended to be used in conjunction with EN 1997-1 and provides rules supplementary to EN 1997-1.

• Part 2 does not cover standardisation of the geotechnical tests.

• Several ISO Technical Specifications play a part…

Eurocode 7 Geotechnical Design –Part 2: Ground investigation and testing

EN ISO 22476Field TestingParts 1 – 13

EN ISO 14688EN ISO 14689

Identification and classification of soil and rock

EN ISO 22475Sampling and groundwater

measurements

CEN ISO/TS 17892Laboratory tests

Parts 1 – 12


EUROCODE 7


Contents of Eurocode 7 Part 2

2.4.1 (2) It should be considered that knowledge of the ground conditions depends on the extent and quality of the geotechnicalinvestigations. Such knowledge and the control of workmanship are usually more significant to fulfilling the fundamental requirements than is precision in the calculation models and partial factors.

EN 1997-1:2004 §2.4 Geotechnical design by calculation

In other words…Design to EN 1997 depends as much on Part 2 as Part 1.


EUROCODE 7


European Geotechnical Codes

European Standards for the Execution of Special

Geotechnical Works

Other structural Eurocodes

e.g. EN 1998, EN 1993-5

Geotechnical Design

(Eurocode 7: Parts 1 & 2) & NAs

Eurocodes:

EN 1990 Basis of Structural Design

EN 1991 Actions on Structures

Test Standards and Technical Specs for ground properties

ISO/CEN Standards for identification &

classification

Geotechnical Projects


EUROCODE 7


Using Eurocode 7Key aspects

• Limit state design to ensure serviceability limit states not exceeded

• Principles and Application Rules

• Characteristic values of geotechnical parameters

• Partial factors of safety• Characteristic values → design values

• The 5 ultimate limit states

• GEO/STR limit states - Design approaches

• Serviceability limit state


EUROCODE 7


Limit state designServiceability limit states: (EN1990 §1.5.2.14)

“States that correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met”

Ultimate limit states: (EN1990 §1.5.2.13)

“States associated with collapse or with other similar forms of structural failure”


EUROCODE 7


Principles & Application Rules

The Principles (preceded by the letter P) comprise general statements and definitions for which there is no alternative, as well as requirements and analytical models for which no alternative is permitted unless specifically stated.

It is permissible to use alternative design rules to the Application Rules, provided that it is shown that the alternative rules accord with the relevant principles and are at least equivalent with regard to resistance, serviceability and durability which would otherwise be achieved for the structure.

Note: If an alternative design rule is substituted for an Application Rule, theresulting design cannot be claimed to be wholly in accordance with the Eurocodealthough the design will remain in accordance with the Principles of the Eurocode.

All statements in each Eurocode are either: Principles (must be followed), or Application Rules (offer advice).


EUROCODE 7



1. Introduction to the Eurocodes ✔2. Overview of Eurocode 7, EN 1997 ✔3. Basis of Geotechnical Design4. Geotechnical Design by Calculation5. Retaining Wall Design6. Conclusion


EUROCODE 7


Basis of Geotechnical DesignEN 1997-1:2004

Section 2 Basis of geotechnical design

2.1 Design requirements 2.2 Design situations2.3 Durability2.4 Geotechnical design by calculation2.5 Design by prescriptive measures 2.6 Load tests and tests on experimental models2.7 Observational method 2.8 Geotechnical Design Report


EUROCODE 7


Basis of Geotechnical Design2.1 Design requirements

(1)P For each geotechnical design situation it shall be verified that no relevant limit state, as defined in EN 1990:2002, is exceeded.

§2.1(1)

This section sets the scene for the design situations and identifies aspects to be considered in the design, including: factors to be considered (e.g. site conditions) (§2.1(2)); methods of verifying the limit states (§2.1(4)); and a means of identifying the complexity of the design together with the associated risks (§2.1(8)).

(4) Limit states should be verified by one or a combination of the following:

— use of calculations… (most common)— adoption of prescriptive measures…— experimental models and load tests…— an observational method…


EUROCODE 7


Basis of Geotechnical Design2.1 Design requirements

Expanding on Clause §2.1(8), Eurocode 7 introduces the notion of three Geotechnical Categories to establish the geotechnical design requirements §2.1(10):

Category 1 is for small projects with negligible-risk and where the fundamental requirements will be satisfied on the basis of experience and qualitative geotechnical investigations;

Category 2 is for conventional structures (e.g. foundations, retaining walls, embankments) with no exceptional risk or difficult soil or loading conditions;

Category 3 is for structures not covered by Categories 1 and 2 (e.g. very large structures, structures involving abnormal risks).

Most routine geotechnical design work will fall into Geotechnical Category 2.


EUROCODE 7


Basis of Geotechnical Design2.2 Design situations

(1)P Both short-term and long-term design situations shall be considered.

§2.2(1)

Section 2.2 of Eurocode 7 Part 1 gives guidance as to what to include in the detailed specifications of design situations, such as: the actions, their combinations and load cases, and the general suitability of the ground on which the structure is located with respect to overall stability and ground movements.


EUROCODE 7


Basis of Geotechnical Design2.3 Durability

(1)P At the geotechnical design stage, the significance of environmental conditions shall be assessed in relation to durability and to enable provisions to be made for the protection or adequate resistance of the materials

§2.3(1)

Section 2.3 of Eurocode 7 Part 1 gives brief guidance on designing for the durability of materials (such as concrete, steel and timber) used in the ground.


EUROCODE 7


Basis of Geotechnical Design2.4 Geotechnical design by calculation

Fundamental!

We shall look at this shortly…


EUROCODE 7


Basis of Geotechnical DesignOther sub-sections of EN 1997-1:2004, Section 2

The remaining sub-sections of Section 2 of Eurocode 7 Part 1 are:

2.5 Design by prescriptive measures 2.6 Load tests and tests on experimental models2.7 Observational method 2.8 Geotechnical Design Report


EUROCODE 7



1. Introduction to the Eurocodes ✔2. Overview of Eurocode 7, EN 1997 ✔3. Basis of Geotechnical Design ✔4. Geotechnical Design by Calculation5. Retaining Wall Design6. Conclusion


EUROCODE 7


Geotechnical design by calculation

(1)P Design by calculation shall be in accordance with the fundamental requirements of EN 1990:2002 and with the particular rules of this standard. Design by calculation involves:

— actions, which may be either imposed loads or imposed displacements, e.g. from ground movements;

— properties of soils, rocks and other materials;

— geometrical data;

— limiting values of deformations, crack widths, vibrations etc.;

— calculation models.

EN 1997-1:2004 §2.4.1(1)

Covered in Section 2.4 of Eurocode 7 Part 1


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


Geotechnical design by calculation 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.


EUROCODE 7


Geotechnical design by calculation 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 parameters should 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.


EUROCODE 7


Characteristic values of geotech parameters

(1)P The selection of characteristic values for geotechnical parameters shall be based on results and derived values from laboratory and field tests, complemented by well-established experience.

EN 1997-1:2004 §2.4.5.2(1)


EUROCODE 7


Characteristic values of geotech parameters

Cautious estimate

• Statistical methods not readily applicable to the determination of characteristic values

• Notion of cautious estimate introduced

(2)P The characteristic value of a geotechnical parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state.

EN 1997-1:2004 §2.4.5.2(2)


EUROCODE 7


Derived values

1.5.3 Specific definitions used in EN 1997-2

1.5.3.1 derived value

value of a geotechnical parameter obtained from test results by theory, correlation or empiricism (see 1.6)


EUROCODE 7


Test results and derived valuesF1 F2 L1 L2

Cautious selection

Geotechnical model and characteristic value of geotechnical parameters

Application of partial factors

Design value of geotechnical parameters

1 2 3 4

C1C1 C1C2Information from other

sources on the site, the soils and rocks

and the project.Test results and derived values

Correlations

Type of test (Field, Lab)

EN 1997-2

EN 1997-1


EUROCODE 7


Other means…Statistical methods – can be used if sufficient geotechnical measurements/results exist.

Except on projects where a large amount of high quality ground investigation data is available, it is unlikely that statistical methods would be adopted to select characteristic values of geotechnical parameters.

Standard tables of characteristic values, where available, may be used in the selection of a characteristic value.

(12)P When using standard tables of characteristic values related to soil investigation parameters, the characteristic value shall be selected as a very cautious value.

EN 1997-1:2004 §2.4.5.2(12)


EUROCODE 7


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).


EUROCODE 7


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


EUROCODE 7


Geotechnical design by calculation

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

VerifyEd ≤ 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

kd

MM

Partial factor of safety, M

Characteristic geotechnical Parameter

(Mk)

Design geotechnical Parameter

(Md)


EUROCODE 7


Design values of geometrical data

(2)P In cases where deviations in the geometrical data have a significant effect on the reliability of a structure, design values of geometrical data (ad) shall either be assessed directly or be derived from nominal values using the following equation (see 6.3.4 of EN 1990:2002):

ad = anom± a

for which values of a are given in 6.5.4(2) and 9.3.2.2

EN 1997-1:2004 §2.4.6.3(2)


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 effects of 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

whereEdst;d is the design effect of the destabilising action, andEstb;d = E{Fd; Xd; ad}stb

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


EUROCODE 7


Design resistancesEquation 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 EN 1997-1:2004 §2.4.7.3.3 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.

dddd aXFRR ;;

R

dkdd

aXFRR

;;

R

dddd

aXFRR

;;


EUROCODE 7


The five ultimate limit statesEurocode 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:


EUROCODE 7


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 equal to the total vertical stress (σstb;d) at the bottom of the column, or the submerged unit weight (G'stb;d) of the same column.

i.e. udst;d ≤σstb;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)


EUROCODE 7


EQU limit stateDestabilising 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 stateActions 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 material properties, Xd

Partial factors, M


EUROCODE 7


GEO/STR Limit statesThree 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 R3Permanent action (G) Unfavourable γG 1.35 1.0

Favourable γG 1.0 1.0

Variable action (Q) Unfavourable γQ 1.5 1.3

Favourable - - -

Accidental action (A) Unfavourable γA 1.0 1.0

Favourable - - -

Coefficient of shearing resistance (tan ') γ' 1.0 1.25

Effective cohesion (c') γc' 1.0 1.25

Undrained shear strength (cu) γcu 1.0 1.4

Unconfined compressive strength (qu) γqu 1.0 1.4

Weight density (γ) γγ 1.0 1.0

Bearing resistance (Rv) γRv 1.0 1.4 1.0

Sliding resistance (Rh) γRh 1.0 1.1 1.0

Earth resistance (Re) γRe 1.0 1.4 1.0


EUROCODE 7


Representation of degree of safety

Over-design factor:

Degree of utilisation:

d

d

ER

d

d

RE


EUROCODE 7


Gfav

Gunfav

Qunfav

Ed

Rd

Gunfav

Gunfav

Qunfav

Ed

Rd

sliding… … and bearing

GEO limit state examples


EUROCODE 7



1. Introduction to the Eurocodes ✔2. Overview of Eurocode 7, EN 1997 ✔3. Basis of Geotechnical Design ✔4. Geotechnical Design by Calculation ✔5. Retaining Wall Design6. Conclusion


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



The limit states to be considered are listed in §9.2(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


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



(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



Must also consider overall stability (Section 11)…


EUROCODE 7


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 the resisting soil should be lowered below the nominally expected level by an amount Δa.

…

— for a cantilever wall, Δa should equal 10 % of the wall height above excavation level, limited to a maximum of 0,5 m;— for a supported wall, Δa 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 of Δa, 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


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

dKpd0

O Ka(h+d0)

q = 10kPa

Kp(h+d)Kad

d0

h0.1h; > 0.5m

Pq1

Pq2

Pa1

Pp2

Pp1

Pa2

Cantilever wall – pressure distribution


EUROCODE 7


Embedded wallsCantilever wall – simplified pressure distribution

Ka(h+d0)

Pp

Pa

R

Kpd0

h+d0 3

Pq


EUROCODE 7


Passive resistance

favGkpdp PP ;;;

Re

;;

kpdp

PP

Favourable action:

or

Resistance:


EUROCODE 7


Passive resistance

Design Approach

1 2 3

Combination 1 Combination 2

G;fav 1.0 1.0 1.0 1.0

Re 1.0 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

Pp

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;fav 1.0 1.0 1.0 1.0

G;unfav 1.35 1.0 1.35 1.0

Re 1.0 1.0 1.4 1.0


EUROCODE 7


Conclusion (Recap…)1. Intro to Eurocodes

2. Intro to Eurocode 7

3. Basis of Geotechnical Design

4. Geotechnical design by calculationActions, Ground properties, Characteristic values of geotechnical parameters, Cautious estimate, Partial factors of safety, Designvalues, Design effects of actions, Design resistances, Five Ultimate limit states of Eurocode 7, Design Approaches (GEO), Over-design factor and the degree of utilisation, single source principle

more…


EUROCODE 7


Conclusion (Recap…)

2.4.1 (2) It should be considered that knowledge of the ground conditions depends on the extent and quality of the geotechnicalinvestigations. Such knowledge and the control of workmanship are usually more significant to fulfilling the fundamental requirements than is precision in the calculation models and partial factors.

4. Geotechnical design by calculation (continued)

In other words…Design to EN 1997 depends as much on Part 2 as Part 1.

5. Retaining Wall Design


EUROCODE 7


Design to Eurocode 7

Many thanks for your attention.

retaining walls and geotechnical design to eurocode 7

Documents