Geotechnical Design- Eurocodes

GEO-E2080 Foundation Engineering and Ground Improvement

Geotechnical Design-Eurocodes30st October 2019Henry Gustavsson

(92Concrete, 93Steel, 94Composite, 95Timber, 96Masonry,99Aluminium.

Annex A: RecommendedSafety factors

Connection between Eurocode 7 and associated European and International standards

The Eurocode systemThe Eurocode is the European standard which specifies how structural design should be conducted within the EU. Geotechnical design is covered in Eurocode 1997 and consists of two parts:- SFS-EN 1997-1, Eurocode 7: Geotechnical design - Part 1: General rules - SFS-EN 1997-2, Eurocode 7: Geotechnical design - Part 2: Ground investigation and testing

In addition to Eurocodes, every country has right to make National Annexes (NA) to the Eurocodes.

Eurocode 1997-1 has a “recommended” set of safety factors in Appendix A. In NA’s e.g. safety factors used in that country and also some other nationally chosen parameters and design principles/guidelines can be presented.

In Finland we have 2 different National Annexes to Eurocode 1997.

Ministry of Environment (YM) has made NA’s that are applied for buildings and civil engineering construction works design that need building permit (luvanvarainenrakentaminen). Finnish guidelines are presented in RIL 207-2017 Geotekninensuunnittelu, which includes NA’s and New Guidelines by Ministry of Environment (YM) 10.1.2018: Rakenteiden lujuus ja vakavuus-Pohjarakenteiden suunnittelu 2018. Finnish: In English: www.ym.fi/download/noname/%7B66367332-E64B-4A59-B5BD-9B004FC8C81C%7D/134895

Ministry of Transportation (LVM) has made NA’s that are for applied for public roads, railways, sea-lanes and related construction works (incl. bridges) design (Väylävirastonkohteet). Finnish NA’s with guidelines are presented in Eurokoodin soveltamisohje -Geotekninen suunnittelu - NCCI 7 13/2017.

Geotechnical design according to Eurocodes in Finland.Ministry of Transportation Ministry of Environment

This used for public roads, railways etcThese are used for houses and stuctures that need permission from municipal authorities (Luvanvarainen rakentaminen, rakennuslupa).

Limit states

ULS

SLS

Settlements, deformations etc.

Symbols:Q= VariableG= PermanentA= Accidental

Above safety factors are ”recommended” values for Equation 6.10 (Not used in Finland!)

Foundations, piles, cellar walls etc.

Ultimate limit states (ULS) of strength

STR

GEO

STR

GEO

GEO

GEO

Ground characterization: Test results-> Characteristic values -> Design values

DA1, DA2, DA3

Finland has chosen to use:- DA2 mainly for geotechnical design, but - DA3 is used for Slopes and overal stability

Design Approach (other than Slopes: Finland DA2)

Design Approach for Slopes. (Finland DA3)

Resistance

gM values =1,0gF values >1,0

gR values >1,0

Material properties

gR values =1,0

gM values >1,0

gF values >1,0

Combinations of actions in DA2 Most of European countries do the combination of actions using Equation 6.101,35 KFI Gkj,sup + 1,0 Gkj,inf + 1,5 KFI Qk,1+ 1,5 KFI S y0,i Qk,I

Finland has chosen to make the combination in DA2 using 2 equations:6.10a (only permanent actions) and 6.10b (permanent and variable actions). Ministry of Environment’s NA (YM):

Use of equation 6.10 and combinations 6.10a and 6.10b, according to EN1990

Combinations of actions in DA2 (LVM, Traffic Agency)

Normal,average

Consequence classes (CC1....3), Reliability classes (RC1..3), and Load factors (KF1..3)

• EN1990-1 and Finnish values from Rakenteiden lujuus ja vakaus - Kantavien rakenteiden suunnitteluperusteet (YM-2016)

”Serious Consequences (CC3):a) - More than 8-storey buildings,- Concert halls, theaters etc- Buildings with high loads...”

”CC3: Slopes and embankmentsIf environment is sensitive For deformations; especially in areas with fine graded soils”

Consequencies are average (CC2) if they are not serious (CC3) or small (CC1)

Consequence classes (CC1....3), Reliability classes (RC1..3), and Load factors (KF1..3)

• EN1990-1 and Finnish values from Rakenteiden lujuus ja vakaus-Kantavien rakenteiden suunnitteluperusteet (YM-2016)

CC3->CC2->CC1->

Reliability classesConsequence classes Load factors

EN1990-1: Combination factors for transient/variable actions

EN1990-1: LVM-NCCI7 (Finnish)Combination factors for GEO/STR in road bridges

DA2 and DA2*In Finland, Design Approach 2 is used in the design of spread foundations, pile foundations, anchorages and retaining structures. In the design of slopes and overall stability, Design Approach 3 is used.

Note: Design Approach 2 can be applied in two ways denoted as DA2 and DA2*. In DA2 the actions are factored at their source and the bearing resistance is calculated using factored values of actions. In DA2* the bearing resistance is calculated using characteristic values of actions, and partial safety factors are applied only at the end of calculation in verifying the ultimate limit state condition (see Designers’ guide to EN 1997-1 - EUROCODE 7: Geotechnical design - General rules. Thomas Telford 2004; ISBN 0 7277 3154 8 ).

When using the design approach DA2* special attention shall be given to the verification of the stability of a foundation structure.

DA2*

DA2

Difference between DA2* and DA2

ed>ek, because horizontal actions (wind etc.) often are variable and gQ=1,5 > gG=1,15.

DA2*

DA2

DA2* gives especially in cases with considerable eccentricity (big vertical actions) smaller size of footing for spread foundations than DA2. Therefore, when using DA2*, the value of eccentricity is limited to less than B/3, when B is the breadth of a rectangular foundation.

The Resultant (R), calculated with permanentactions only , shall stay in the dashed rectanculararea (e<B/6)!

Actions with considerable eccentricity:

DA2: Safety factors for Soil parameters (Set M1) and Resistance factors (Set R2) for spread foundations.

YM LVM

Example of the level of safety for spread foundation (DA2)Figure shows the Overall Factor of Safety for a spread foundation with centric loading as the relative amount of variable and permanent actions vary. The straight line at OFS=2,0 is the level of needed safety according to the national building code (B3: Foundations, Ministry of the Environment 2004). The dotted, inclined, straight line describes the recommended level of safety of EN1997-1. The bended (angular) line describes the level of safety according to EN1997-1 with its Finnish National Annex , when the reliability class is RC2 (KFI = 1,0), obtained using formulas 6.10 a and 6.10b.

Spread foundations, centric loading.

Overall Factor of Safety, OFS

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

0 0.2 0.4 0.6 0.8 1

Q/(G+Q)

OF

S

EC7, gR=1,4

EC7 FIN

6.10a+b,

gR=1,55

B3(F=2,0)

When the amount of variable loads (Q) exceeds 12% of total loads (G+Q), equation 6.10b will be determining.

6.10a 6.10b

Example1,55*1,35=2,1

Example1,55*1,5=2,33

1,4*1,35=1,89

Eurocode DA3 for slopes and overall stability

Eurocode DA3 for slopes and overall stability(Ministry of Environment, YM)

Combination of actions using Equation 6.10:

The partial factors for ground properties; set M2 :YM

Eurocode DA3 for slopes and overall stability(Ministry of transportation, LVM)

AAdditio

Safety factors for Actions

Safety factors for soil parameters

YM uses 1,5

Additional model factors when soil type is clay or silt:

Serviceability limit state analysis• When there is only permanent loadings, the safety will

be formed only of the partial safety factors of the material parameters.

• If undrained shear strength is used, the total safety factor level will be around 1,4..1,5

• In cases of frictional materials safety will be 1,25 • This usually too low for structures• There will also be some shear deformations• Also the service limit state analysis should be done

=numerical analysis• Stability analyses with higher safety level is an

alternative.• All the partial safety factors for actions are then 1,0.

40

Service limit state analysis

41

is applied to tan f’

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Example Slope stability according to EN1997-1

The even load above the slope is gk = 35 kPa (permanent, unfavourable load) in 2 m distance from the edge.

Original level of ground water is at 3 meter below groundlevel. The final shape of the ground water level is drawn in figure.

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The collapse of drained situation is most critical. So effectivestrength parameters (c’, ’)´are used in the long-termanalysis in ULS.

The characteristic properties of clay are:

- Saturated unit weight gk = 20 kN/m3

- Effective friction angle (kitkakulma): k’ = 28˚

- Effective cohesion: ck’ = 10 kPa

- Surface loading gd = gGgk = 1,035 kPa (Table A.3(FI), series A2)

= 35 kPa (permanent, unfavourable load)

Example. Characteristic values.

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Example 3. Design parameters

-Unit weight of clay gd = gggk = 1,020 kN/m3 (table A.4(FI), M2) = 20 kN/m3

-Friction angle d’ = arctan(tank’/g’) = arctan(tan28˚/1,25) (table A.4(FI),

sarja M2) = 23,0˚

- cohesion cd’ = ck’/gc’ = 10/1,25 kPa (table A.4(FI), sarja M2) = 8,0 kPa

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Example. Resistance of Soil

-The resistance of soil

Partial factor gR;e = 1,0 (taulukko A.14(FI), sarja R3)

So in DA3, the principle is to focus the safety to the material

parameters, not to loading or to resistivity.

Example Bishop’s method DA3 partial factors

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F=1,193>1,0 ->OK.If we would make SLS checking in CC2(g=1,65 instead of 1,25), this structure would not be acceptable!

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Example with Factor of Overall Safety (FOS)F = 1,53. (F>1,5, which was the required value in Finnish National codes before Eurocode, if there is no structures near. In our case we have the house, so using FOS it should have been F>1,8).

In FOS analysis characteristic values of actions and strength parametres are used. So FOS<1,0 means that the structure is in failure! The safety is achieved by using higher values of Fneeded=1,5…1,8

References and Links related to Eurocodes- All Eurocodes and SFS-EN-ISO Standards can fe found (pdf) through Aalto’s Library pages-> Search Aalto-FINNA: ”SFS”-> SFS-database -> SFS On Linehttps://Aalto Libray-Finna (in English and Finnish!)SFS also sells printed books with discount (50%) to students (Green student-versions even cheaper)

Publications of Ministry of Environment (YM) can be found: http://www.ym.fi/fi-FI/Maankaytto_ja_rakentaminen/Lainsaadanto_ja_ohjeet/Rakentamismaarayskokoelma/Rakenteiden_lujuus_ja_vakaus

Transportation Agency’s (LVM) publications:https://www.liikennevirasto.fi/julkaisut/ohjeet/2017#.WfmiFXZx1aQ

Eurokoodi Help desk: http://www.eurocodes.fi/Contains material related to Eurocodes (also National Annexes of different countries etc).

RIL books can be bought from RIL (student discount 50%): http://www.ril.fi/kirjakauppa/Books can also be found from Aalto library.