Course plaxis

May 7, 2017 | Author: Omar Elio | Category: N/A
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Efficient modelling of pile foundations in the Finite Element Method Ronald B.J. Brinkgreve Plaxis / Delft University of Technology DFIMEC 2014

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Outline •

Introduction



Embedded pile (3D)



Embedded pile row (2D)



Applications of embedded piles



Ongoing research



Conclusions

DFIMEC 2014

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1

Introduction Finite Element Method (FEM) in geotechnical engineering: •

Numerical solution of boundary value problems: - Deformation (stress, strain) analysis (SLS) and ULS design - Groundwater flow analysis - (Geo)thermal analysis - Thermo-Hydro-Mechanical coupling



Realistic simulation of soil, structure, soil-structure interaction and construction process DFIMEC 2014

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DFIMEC 2014

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Introduction Dancing Towers, Dubai

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Introduction FEM modelling piles: •

2D: - Axisymmetry: Axially loaded single pile - Plane strain: Pile (beam) becomes a wall - New: Embedded pile row in 2D



Most practical applications involving pile foundations require a 3D model !

DFIMEC 2014

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Introduction Modelling options of piles in 3D FEM: •

Solid elements:  ‘Expensive’  Poor mesh quality  No structural forces



Beam elements:  No pile volume  No surface area  Unrealistic pile-soil interaction DFIMEC 2014

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Introduction

?

(Courtesy of Prof. H.F. Schweiger) DFIMEC 2014

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Embedded pile (3D) Efficient 3D modelling feature: Embedded pile elements Pile as beam elements Pile-soil interaction (shaft friction, end bearing) • Arbitrary crossing of soil elements • •

pile

t skin

Ffoot

DFIMEC 2014

soil

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Embedded pile (3D)

kt kn kt

pile

kn

ks

kt t skin

kn Ffoot

soil

t

ks

ks

Skin stiffness: tmax ks : axial stiffness Kn ,kt : lateral stiffness

k 1

Skin tractions: ts = qs/length = ks (uspile-ussoil) ≤ tmax tn = qn/length = kn (unpile-unsoil) tt = qt/length = kt (utpile-utsoil)

urel

s Base stiffness: kb : base/foot stiffness

t kb (Engin et al, 2007)

n

Base/Foot force: Fb = kb (ubpile - ubsoil) ≤ Fmax

DFIMEC 2014

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Embedded pile (3D) Embedded pile: • •

Beam nodes: Interface nodes:

Real nodes; 6 d.o.f.’s per node (ux uy uz rx ry rz) Virtual nodes, 3 d.o.f.’s per node (ux uy uz), expressed in volume element shape functions

DFIMEC 2014

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Embedded pile (3D) Bearing capacity = ½ (Ttop+Tbot)×Lpile + Fmax Ttop

Lpile

Tbot

Fmax DFIMEC 2014

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Embedded pile – Deformation behaviour Pile bearing capacity is input and not result of FEM calculation F

t

Global

Local

tmax

Specified bearing capacity

k 1

urel

Global pile response from soil modelling and pile-soil interaction

F Fmax k 1

u DFIMEC 2014

urel

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Embedded pile – Elastic region • •

Around shaft Around foot

Soil stress points inside elastic region are forced to remain elastic DFIMEC 2014

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Embedded pile – Output Displacements, bending moments, axial forces, shaft friction, foot force

N

u

Ts

C B A

DFIMEC 2014

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Embedded pile – Validation by TUGraz

DFIMEC 2014

(Tschuchnigg, 2009)

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3D model - volume piles: 70 mm

Embedded pile – Validation

2D model: 72 mm

3D model - embedded piles: 74 mm

DFIMEC 2014

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Embedded pile – Validation by TUDelft

(Dao, 2011)

Lateral movement of pile in horizontal soil slice:  Embedded pile almost behaves as volume pile due to elastic region

DFIMEC 2014

Embedded pile – Validation by TUDelft

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(Dao, 2011)

Lateral force at pile top:

DFIMEC 2014

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Embedded pile (3D) Conclusions embedded pile: • • • •

Efficient 3D modelling of pile foundations (bored piles, piled rafts) Realistic pile-soil interaction (shaft friction, end bearing, group effects) Pile capacity is Input (not a result) Since 2005 many applications in practice (pile foundations, ground anchors)

DFIMEC 2014

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Embedded pile row (2D) How to model a row of piles (out-of-plane) in 2D ?

DFIMEC 2014

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Embedded pile row (2D) ‘Conventional’ 2D options: •

Beam (plate):  Continuous out-of-plane  Prevents ‘soil flow’ between piles



Two-node spring (N2N anchor):  No bending stiffness  No pile-soil interaction

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DFIMEC 2014

Embedded pile row (2D) Ls

New 2D modelling option: •

Embedded pile row:  Continuous ‘soil’ mesh  Pile as a superimposed beam element (axial stiffness, bending stiffness)  Pile and soil can move independently  Pile-soil interaction (interface) (shaft friction, end bearing)  Out-of-plane spacing (Ls) DFIMEC 2014

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Embedded pile row (2D)

(Sluis, 2012) 23 / 40

DFIMEC 2014

Embedded pile row (2D) Calibration of interface stiffness from 3D calculations

(Sluis, 2012) DFIMEC 2014

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Embedded pile row (2D) Calibration of interface stiffness from 3D calculations

(out-of-plane)

(Sluis, 2012) 25 / 40

DFIMEC 2014

Embedded pile row (2D) N

10m

150 kN/m

DFIMEC 2014

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Embedded pile row (2D) Case study: Bridge abudment Piled abutment

Bridge deck

Road/railway freeboard

Embankment

Soft layers (peat/clay)

Deep sand (foundation layer) 27 / 40

DFIMEC 2014

Embedded pile row (2D) Case study: Bridge abudment Comparison 2D vs. 3D

2D Q 2d emb [kN]

10

M 2d emb [kNm] N 2d emb [kN]

5

N 3D [kN]

vertical height [m]

M_2 3D [kNm] -600

detail 3D

Q_13 3D [kN] -400 -200

0 0

200

400

-5

-10

-15

-20

First pile row: M/Q/N DFIMEC 2014

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Embedded pile row (2D) Conclusions embedded pile row: • • • • • •

Efficient 2D modelling of pile rows (out-of-plane) Pile and soil can move independently Realistic pile-soil interaction (shaft friction, end bearing) Calibration of interface stiffness, based on out-of-plane spacing (Ls) Successful validation Since 2012 several applications in practice (piles and ground anchors)

DFIMEC 2014

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Applications of embedded piles Quay wall

DFIMEC 2014

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Applications of embedded piles Foundation of high-rise building in Frankfurt (Japan Centre)

(Courtesy of Prof. Y. El-Mossallamy) DFIMEC 2014

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Applications of embedded piles Foundation of high-rise building in Singapore

DFIMEC 2014

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Applications of embedded piles Railway station in Vienna

~ 500m

~500 m

~ 400m

~400 m

47464 elements (Courtesy of Prof. H.F. Schweiger) DFIMEC 2014

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Applications of embedded piles Railway station in Vienna Model without soil (bottom view)

615 Piles  Different pile lengths  Different pile inclinations (Rest is modelled as blocks) DFIMEC 2014

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Applications of embedded piles Railway station in Vienna

axial force

shaft friction DFIMEC 2014

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Applications of embedded piles Excavation in Monaco (Odeon Towers)

(i.c.w. Terrasol, France; Plaxis Bulletin 29, 2011)

DFIMEC 2014

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Ongoing research Research on installation effects of driven piles at TUDelft: • Idea: Impose modified stress and density on ‘wished-in-place’ pile

(Engin, 2013)

DFIMEC 2014

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Ongoing research Research on large deformation analysis (MPM) due to pile installation

DFIMEC 2014

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Conclusions •

• • • •

Efficient modelling of piles in FEM: - Embedded pile row (2D) - Embedded pile (3D) Realistic pile-soil interaction (shaft friction, end bearing) Pile capacity is Input (not a result) Meanwhile many applications in practice (piles and ground anchors) Ongoing research: - Installation effects - Pile penetration using MPM

DFIMEC 2014

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References 1. Engin H.K., Septanika E.G. and Brinkgreve R.B.J. (2007). Improved embedded beam elements for the modelling of piles. In: G.N. Pande & S. Pietruszczak (eds.), Int. Symp. on Numerical Models in Geomechanics – NUMOG X, 475-480. London: Taylor & Francis group. 2. Engin H.K., Septanika E.G., Brinkgreve R.B.J., Bonnier P.G. (2008). Modeling piled foundation by means of embedded piles. 2nd International Workshop on Geotechnics of Soft Soils - Focus on Ground Improvement. 3-5 September 2008, University of Strathclyde, Glasgow, Scotland. 3. Septanika E.G., Brinkgreve R.B.J., Engin H.K. (2008). Estimation of pile group behavior using embedded piles, the 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG), 1-6 October, 2008, Goa, India. 4. Tschuchnigg F. (2009). Embedded piles – 1. Report. CGG_IR021_2009. Technische Universität Graz. 5. Tschuchnigg F. (2009). Embedded piles – 2. Report. Improvements. Technische Universität Graz. 6. Dao T.P.T. (2011). Validation of PLAXIS embedded piles for lateral loading. MSc thesis. Delft University of Technology. 7. Brinkgreve R.B.J., Engin E., Dao T.P.T. (2012). Possibilities and limitations of embedded pile elements for lateral loading. IS-GI Brussels. 8. Sluis J. (2012). Validation of embedded pile row in PLAXIS 2D. MSc thesis. Delft University of Technology. 9. Engin H.K. (2013). Modelling pile installation effects – A numerical approach. PhD thesis. Delft University of Technology. DFIMEC 2014

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Efficient modelling of pile foundations in the finite element method Ronald B.J. Brinkgreve DFIMEC 2014

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