Eurocode 8 En1998-5 2003_faccioli Lecture

Workshop Eurocodes – Background and Applications Brussels, February 18-20, 2008

Eurocode 8 Part 5: Foundations, retaining structures and geotechnical aspects (EN1998-5: 2003) Ezio Faccioli

([email protected])

Outline

• Object and salient characteristics of EN1998-5:2003 • Ground properties (strength, stiffness and damping parameters) • Design seismic action and its dependence on ground type • Requirements of construction site and of foundations soils • Foundation system: shallow and deep foundations • Earth retaining structures Ezio Faccioli

Object of EN1998-5:2003 The norm:

• “rules for the siting and foundation soil of structures for earthquake resistance”, and

• “covers the design of different foundation systems, .. of earth retaining structures ……under seismic actions”

From Part 1:”…. It shall be verified that both the foundation elements and the

foundation soil are able to resist the action effects resulting from the response of the superstructure without substantial permanent deformations. ”

Ezio Faccioli

Salient characteristics and innovative aspects • Complementarity with Eurocode 7 (EN 1997e), which does not cover earthquake resistant design.

• Introduction and use of dynamic soil properties (τcyc, shear wave velocity vs and damping) in addition to standard static properties (tan φ′, cu, qu).

“4.2.2 (2) The profile of the shear wave propagation velocity in the subsoil shall be regarded as the most reliable predictor of the site dependent characteristics of the seismic action at stable sites”

• Different approaches to safety and strength verifications, depending on seismicity level and type of soil.

“4.1.2.3 (8) Simplified methods (of slope stability analysis), such as the pseudo-static ones, shall not be used for for soils capable of developing high pore water pressures or significant degradation of stiffness under cyclic loading”.

• In situ investigations, or gathering of reliable equivalent data, to determine the elastic design response spectrum.

• Recognition of seismically-induced permanent ground deformations as a design criterion. Ezio Faccioli

Ground properties • Static undrained

parameter values (cu, tanφ’ ) can generally be used for strength verifications,

with recommended values of material partial factors (γm).

•

For cohesionless soil the strength parameter is the cyclic undrained shear strength τcy,u which should take the possible pore pressure build-up into account.

•

The soil shear modulus:

w 2 G = vS g

and the soil damping ratio are introduced, for use in SSI calculations, as well as their dependence on the seismic shear strain in the soil (through the design ground acceleration).

•

For the evaluation of the liquefaction potential, the cyclic soil resistance against liquefaction (based on field performance in past earthquakes) is used, which depends on SPT blowcount or cone penetration resistance. Ezio Faccioli

Design seismic action: dependence on ground type The reference ground motion model at a point on the surface is the acceleration elastic response spectrum:

Se (T) = ag f (T; S,TB ,TC ,TD ) The dependence is established through: • The constant soil factor S, which does not modify the spectrum shape but only amplifies it, and takes a single value for each ground type ; • The “control periods” TB ,TC ,TD which modify the shape by enlarging the spectral plateau Remark: it would be desirable to make S a smoothly varying function of Vs30, e. g.: S = (VS 30 / Va ) bV with bv and Va constants Ezio Faccioli

Identification of ground types VS,30

30 = hi ∑ VS i=1, N i

Weighted VS

∑t i

Depth [m]

i=1, N

hi

Particular cases: Types S1,S2

Ezio Faccioli

Special studies

ti

Seismic action: dependence on ground type In seismic geotechnical verifications, such as:

• Slope stability • Liquefaction hazard occurrence • Stability of retaining works the sesmic action is directly determined by the design ground acceleration agS, multiplied by a reductive coefficient, as follows: •(0.5 agS) /g seismic coefficient for pseudo-static slope stability verifications • 0.65 agS effective acceleration for checking the liquefaction potential in a saturated sand deposit • [(ag/g)S)]/r seismic coefficient for computing the dynamic thrust in the pseudo-static verification of a retaining work.

Ezio Faccioli

Requirements for siting and foundation soils Topographic factorsliquefaction (ST) in the event of OBJECTIVE: minimise hazards causedamplification by ground rupture, instability, an earthquake Amplitude response (averaged over frequency) along slopes of different geometry

Type of Sketch GROUND STABILITY VERIFICATIONS: topographic profile • •

Average slope angle,α

ST

Slope limit state = excessive permanent displacements of ground mass

Isolated cliffamplification and Topographic coefficients for the seismic action to be used for slope angles > > 15° 1.2 α 15° slope

• Pseudo-static verification of stability is allowed but:

− Cyclic Δu and strength degradation in saturated soils must be accounted for in areas with

15° to 30°

design ground acceleration > 0.15 g

1.2

Ridge withof crest − Development cyclic Δu and cyclic stiffness degradation are not high

width significantly α base width • less Limit than state condition can be checked by simplified dynamic model (rigid block) > 30° Ezio Faccioli

1.4

Example of simplified 3D representation of a settlement on a crest: Baiardo α1 = 22.6

Transv. cross section

1

α1

1000

α2

α2 = 22.2

900 y = 0,3946x + 565,43 800 y = -0,3873x + 1184,8

h[m] 700

600

500 0

200

400

600

800

1000

1200

1400

1600

1800

d [m]

α1

Long. cross section

2

α2

1200 1000

1

α1 = 4.3 α2 = 17.2

2

y = -0,0759x + 928,52

1

800

2

600

y = -0,3009x + 1171,7

h[m] 400 200 0 0

200

400

600

800

1000

1200

1400

1600

1800

2000

d [m]

Ezio Faccioli

Requirements of the construction site: Proximity to seismically active surface faults “(1) Buildings of importance classes II, III, IV defined in EN 1998-1:2004, 4.2.5, shall

not be erected in the immediate vicinity of tectonic faults recognised as being seismically active in official documents issued by competent national authorities. (2) An absence of movement in the Late Quaternary may be used to identify non active faults for most structures that are not critical for public safety. (3) Special geological investigations shall be carried out for urban planning purposes and for important structures to be erected near potentially active faults in areas of high seismicity, in order to determine the ensuing hazard in terms of ground rupture and the severity of ground shaking..”

Ezio Faccioli

Izmit, Turkey, M 7.3 earthquake of August 1999 Fault rupture (double stranded) near Golcuk

Ezio Faccioli

Izmit, Turkey, M 7.3 earthquake of August 1999 Details of fault rupture

Ezio Faccioli

Izmit, Turkey, M 7.3 earthquake of August 1999 Damages to buildings

Ezio Faccioli

Requirements for siting and foundation soils POTENTIALLY LIQUEFIABLE SOILS Detailed guidelines and charts (Annex B) are provided for evaluating the liquefaction susceptibility of saturated cohesionless foundation soils through the well known empirical method based on NSPT or CPT resistance. The guidelines are not unduly conservative, because evaluation of liquefaction susceptibility can be omitted if: The sandy soil layer or lens lies at more than 15 m depth from ground surface The design ground acceleration is less than 0.15 g and, at the same time: NSPT is sufficiently high, or the content of plastic fines in the soil is sufficiently high.

Relationship between stress ratios causing liquefaction and N1 (60) values for clean and silty sands for M=7,5 earthquakes.

A minimum safety factor of 1.25 (in terms of shear stresses) is recommended. Ezio Faccioli

Ground investigations and studies OBIECTIVES: Determine average subsoil profile for selecting design response spectrum and dynamic soil properties

CRITERIA:

•

The vs profile at the site is taken as “the most reliable predictor of the site dependent characteristics of the seismic action at stable sites”

•

Estimating the vs profile by empirical correlations (e. g. with NSPT) is allowed, provided the inherent scatter is taken into account 0.8 0.7 Catania, 13 dec. 1990 - NS

(ground type C/D)

0.5 SA [g]

Vs profile at Catania strong motion station on deep sediments

0.6

0.4 0.3 0.2 0.1 0 0

Ezio Faccioli

0.5

1 T [s]

1.5

2

Ground investigations and studies Indicative reduction factors for vs or Gmax, and for internal damping in the soil are provided as a function of shear strain amplitude (through the peak ground acceleration) 35

1

30

G/Gmax

25 0.6

Shear Modulus

20

Damping Ratio 15

0.4

10 0.2 5 0 0.0001

0 0.001

0.01

0.1

Shear Strain (%)

Ezio Faccioli

1

10

Damping Ratio (%)

0.8

Foundation system BASIC RULE Only one foundation type to be used for the same structure, unless this consists of dynamically independent units. E. g. use of piles and shallow foundations in the same structure must be avoided, except for bridges and pipelines. DESIGN ACTION EFFECTS Action effects transmitted to the foundations are evaluated according to capacity design considerations for dissipative structures (high ductility), while allowable seismic action combination applies for non-dissipative structures (essentially responding in the elastic range). DIRECT FOUNDATIONS (Footings) Stability of footings is to be checked against sliding failure, i. e. Vsd ≤ Nsd tan δ shear friction force resistance

+ Epd lateral resistance

and against bearing capacity failure (Annex F, see following two slides). Factors to be taken into account include: Inclination and eccentricity of structural loiad, inertia forces in the soil, pore pressure effects, non-linear soil behaviour. Ezio Faccioli

Foundation system Bounding surface of external loads A single expression has been obtained for both cohesive and granular soils; it is introduced in Annex F of Part 5 HD e/B = 0 LD

HD

e/B = 1/6

LD

Cross-sections for vanishing F and different load eccentricities

3D view of bounding surface

Ezio Faccioli

Foundation system Piles and piers Should be designed to resist both: (a) Inertia forces from the superstructure, and (b) Kinematic forces due to the eartquake-induced soil deformations. The latter apply if all of the following conditions occur: b1. Class D, S1, or S2 soil profile with consecutive layers of sharply contrasting stiffness b2. Design ground acceleration > 0.10 g, and b3. The supported structure is of importance category III or IV. Although piles will generally be designed to remain elastic, they may under certain conditions develop plastic hinges at their head.

Ezio Faccioli

Kinematic forces on piles Kinematic forces induced by earthquake on piles in “stable” soils

Design earthquake

s(z): seismically induced horizontal displacements in the soil

Ezio Faccioli

Kinematic forces on piles Kinematic forces induced by earthquake on piles in “unstable” loose soils Stable soil Horizontal displacements induced in soil by eqk (“flow failure”)

Unstable loose sandy soil

Stable soil

Design earthquake

Ezio Faccioli

Effects of kinematic forces on piles

(NISEE slide collection) Ezio Faccioli

Earth retaining structures General requirements and considerations • Permanent displacements/tilting may be acceptable, provided functional or aesthetic requirements are not violated • Structural choice is based on static loads , but seismic action may lead to different solution • Build-up of significant PWP in backfill or supported soil is to be absolutely avoided • Methods of analysis should in principle account for: -

inertial and interaction effects between structure and soil (even non-linear)

-

hydrodynamic effects in the presence of water (in the soil, and on outer face of wall)

-

compatibility of deformations of soil, wall, and free tendons)

NB: After introducing requirements for general methods of analysis, the code only provides prescriptions for pseudo-static verifications Ezio Faccioli

Experimental observations (flexible retaining walls) 1) Effects on retaining walls in historical earthquakes - Liquifiable soils (harbour facilities)

Collapse (0.6 – 4m displacements)

Loma Prieta (California, 1989) – MW=7.1 Kobe (Giappone, 1995) – MW=6.9 Bhuj (India, 2001) – MW=7.6

- Non-liquifiable soils

Good behaviour (<10cm displacements)

Northridge (California, 1994) – MW=6.8 Kobe (Giappone, 1995) – MW=6.9 Taiwan (1999) – MW=7.6

2) Dynamic experimental tests - Shaking table - Dynamic centrifuge tests

Ezio Faccioli

Earth retaining structures Simplified (pseudo-static) analysis: the seismic action can be reduced by kind of ductility factor r:

kh = ag γI S/r kv = ± 0,5 kh (Spectrum Type 1)

kv = ± 0,33 kh (Spectrum Type 2)

Values of reduction factor r and residual displacement Type of retaining structure

r

Acceptable residual displacement, dr (mm)

Free gravity walls that can accept a displacement

2

300agγI S / g

1,5

200ag γI S / g

1

0

As above, but less”tolerant” Flexural reinforced concrete walls, anchored or braced walls, reinforced concrete walls founded on vertical piles, restrained basement walls and bridge abutments

For loose, saturated granular soils, r = 1 and FS against liquefaction not less than 2. The provisions for pseudo-static analysis follow a standard approach (Mononobe and Okabe), given in Annex E. Ezio Faccioli

Earth retaining structures Resistance and stability verifications • Foundation soil The following need be verified: − Stability of slope − Stability w. r. to failure by sliding and loss of bearing capacity, for shallow foundation. Design actions: permanent gravity loads, horizontal thrust Ed , seismic action. •

Anchorages

They shall assure equilibrium and have a sufficient capacity to adapt to the seismic deformations of the ground . The distance Le between the anchor and the wall shall exceed the distance Ls, required for non-seismic loads :

Le = Ls (1 + 1.5 S ag) • Backfill material must be immune from liquefaction. • Structural strength under the combination of the seismic action with other possible loads, equilibrium must be achieved without exceeding the strength of any structural element:

Rd > S d

Rd : design resistance of the element, evaluated as for the non seismic situation Sd : design value of the action effect, as obtained from the analysis.

Ezio Faccioli

Examples of numerical analyses: simple flexible wall (from M. Eng. Thesis of O. Zanoli, Politecnico di Milano, Dec. 2007) vs [m/s] 400

200

600

800

1000

0

h

10 Profondità [m]

Soil profile properties Ground categories B, C, D: governed by Vs,30 Coarse grained, dry, γS=20kN/m3, Φ=34°, c’=0, ψ=0°, δA=0°, δP= Φ RC flexible wall Unit weight γRC=25kN/m3, νRC=0.3, ERC=28GPa Excavation depth h=3, 5, 7m + Embedment (d)

0

20 30

ΔPaE,d 40

Pa,d

50 60

PpE,d

Ka,d

Ka,k Suolo D

Suolo C

K Suolo BpE,d

d KpE,k d2

Pre-dimensioning (EC 8) 1) Design strength : tan φd=tan φ /1.25 2) Seismic coefficients

3)

⎧k v = 0 ⎪ 1 a gR ⎨ k = ⎪ h rS g ⎩

1 ⎧ = + Δ = P P P γ (h + d ) 2 K A + γ (h + d ) 2 ΔK AE , , , AE d A d AE d ⎪⎪ 2 Thrusts ⎨ 1 2 ⎪P ⎪⎩ PE ,d = 2 γd K PE

4) Rotational equilibrium w. r. to base 5) 20% increase in embedment d Ezio Faccioli

h3d5

h5d8

h7d11

Excavation d. [m]

3

5

7

Embedment [m]

5

8

11

H total [m]

8

13

18

Examples of numerical analyses: simple flexible wall

(from

M. Eng. Thesis of O. Zanoli, Politecnico di Milano, Dec. 2007)

Material models Soil: coarse grained, homogeneous, Vs=156m/s, elastic-plastic non associated constitutive model, Mohr-Coulomb rupture criterion(f=32°). Flexible wall: 13m height, 5m excavation, linear elastic behaviour, E=28 GPa, ν=0.3.

Base excitation

30m

55m

2 groups of 7 accelerograms on ground type A Zone II (amax=0.25g)

Average response spectra matching EC8 elastic spectrum

10 Acc. spettrale [m/s 2]

Zone I (amax=0.35g)

150m

8 6 4 2 0 0.0

0.5

1.0

1.5

2.0 T[s]

Ezio Faccioli

2.5

3.0

3.5

4.0

Results of EC8 pseudo-static vs. 2D dynamic (FEM) analyses (from the M. Eng. Thesis of O. Zanoli, Politecnico di Milano) Bending moment profiles in retaining wall Eurocode 8:

pseudo-static analyses with assigned kh values for r = 1, 2;

Dynamic analyses: average Mmax values over groups of input accelerograms -3500

-1800

amax=0.35g

(Both pseudo-static and dynamic FEM analyses performed with commercial software FLAC)

-2500

-2000

r=1

-1500

dyn

-1000

-500

amax=0.25g

-1600

Momento flettente massimo [kNm]

Ground type B

Momento flettente massimo [kNm]

-3000

r=2

-1400

r=1

-1200 -1000 -800 -600 -400

r=2

-200 0

0 2

3

4

5 h (m)

Valori massimi Maximum Mmaxdinamici values

Ezio Faccioli

6

7

8

Pseudo-statico Pseudo-static r r=1= 1

2

3

4

Pseudo-statico =2 Pseudo-static r r=2

5 h (m)

6

7

8

Media dinamici Averagevalori dynamic values

Results of EC8 pseudo-static vs. 2D dynamic (FEM) analyses

(from the M. Eng. Thesis of O. Zanoli, Politecnico di Milano) Bending moment profiles in retaining wall

pseudo-static analyses with assigned kh values for r=1, 2;

Eurocode 8:

Dynamic analyses: average Mmax values over groups of input accelerograms -5000

-3000

-4500

amax =0.35g

(Both pseudo-static and dynamic FEM analyses performed with commmercial software FLAC)

-3500

Momento flettente massimo [kNm]

Ground type D

Momento flettente massimo [kNm]

-4000

-3000 -2500 -2000 -1500

r=1

dyn

-1000

amax=0.25g

-2500

-2000

-1500

-500

r=2

-500

r=1

dyn

-1000

r=2

0

0

2

3

4

5

6

7

8

2

3

4

h (m) Valori dinamici Maximum Mmaxmassimi values

5

6

7

8

h (m)

Pseudo-statico r=1 Pseudo-static r =1

Ezio Faccioli

Pseudo-statico =2 Pseudo-static rr =2

Media valori dinamici Average dynamic values

Examples of numerical analyses Complexity of wave propagation phenomena in soil – flexible wall systems (other example with amax = 0.15g) Snapshots of displacement field at different instants 5

2.0

1.5

1

Accelerazione [m/s²]

1.0

0.5

0.0

-0.5

-1.0

-1.5 0

2

4

6

8

10

12

Tempo [s]

Nonlinear dynamic analyses performed with FEM code Gefdyn

2

3

4

180 m

Ezio Faccioli

Final Displacement

Examples of numerical analyses Acceleration profile showing amplification of soil-wall seismic motion from depth to surface (example with amax = = 0.23g) 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3

0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3

Amax = 0.279 g, z = 0.00 m

Amax = 0.187 g, z = - 2.53 m

Amax = 0.180 g, z = - 4.30 m

Amax = 0.126 g, z = - 6.96 m

Amax = 0.105 g, z = - 8.42

Amax = 0.120 g, z = - 11.42

Amax = 0.141 g, z = - 13.47

Ezio Faccioli

Suggested reference:

Designers’ Guide to EN 1998-1 and EN 1998-5 Eurocode 8: Design of structures for earthquake resistance. General rules, seismic actions, design rules for buildings, foundations and retaining structures Michael N. Fardis, Eduardo Carvalho, Amr Elnashai, Ezio Faccioli, Paolo Pinto and Andre Plumier. Published by Thomas Telford, UK, 2006

Ezio Faccioli