1 MODELLING OF EXCAVATIONS USING PLAXIS William Cheang PlaxisAsia PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES Plaxis Vietnam 2008 PLAXIS FINITE ELEMENT CODES 1. 2-D Analysis + Modules • Plaxis Professional 8.x • PlaxFlow 1 x • PlaxFlow 1.x • Dynamics 2. 3-D Analysis • 3D Foundation (Full control of X-Z space) • 3D Tunnel (Full control of X-Y space) 3 Other programs supporting Plaxis PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES Plaxis Vietnam 2008 2 3. Other programs supporting Plaxis 1. Delft Geosystems (M-Series Codes + M-Geometry Input.) [see Http://www.delftgeosystems.nl]
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MODELLING OF EXCAVATIONS USING PLAXIS · PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES Plaxis Vietnam 2008 3 A. Modelling of Excavations in Plaxis The geometry is the representation
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MODELLING OF EXCAVATIONSUSING PLAXIS
William CheangPlaxisAsia
PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES Plaxis Vietnam 2008
PLAXIS FINITE ELEMENT CODES1. 2-D Analysis + Modules
• Plaxis Professional 8.x• PlaxFlow 1 x• PlaxFlow 1.x• Dynamics
2. 3-D Analysis• 3D Foundation (Full control of X-Z space)• 3D Tunnel (Full control of X-Y space)
3 Other programs supporting Plaxis
PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES Plaxis Vietnam 2008 2
Modelling of Excavations: Introduction to methods, tools and procedure.
A. Modelling of Excavations in PlaxisB. Model SpaceC. Continuum, Structural & Membrane ElementsD. Loads and Porewater Pressure BoundariesE. Constitutive ModelsF. Demonstration
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A. Modelling of Excavations in Plaxis
The geometry is the representation of the physical problemproblem
• Consists of points, lines and clusters• Definition of soil layers, structural elements and loads
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B.Model Space-Part 1• Plaxis Professional Version 8.x (2D
code)• Plane Strain• Plane Strain• Axi-symmetry
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B.Model Space-Part 2• 3D Foundation (X-Z space)
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B.Model Space-Part 3• 3D Tunnel (X-Y space)
yy
xz
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C. Elements in Plaxis
• ContinuumS l• Structural
• Interfaces• Membrane (Geogrid)
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C1.Continuum Elements
Mesh6 d d 15 d d t i l l t• 6-noded or 15-noded triangular elements
• Displacements calculate in nodes• Stresses calculated in Gaussian integration points
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C2.Structural elements in Plaxis
• Plates and shells • Anchors• Anchors• Geogrids (geotextiles)• Interfaces
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Fixed-end anchors
• To model supports, anchors and strutsEl t l ti i l t• Elasto-plastic spring element
• One end fixed to point in the geometry,other end is fully fixed for displacement
• Positioning at any angle• Pre-stressing option
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strut
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Node-to-node anchors
• To model anchors, columns and rodsEl t l ti i l t• Elasto-plastic spring element
• Connects two geometry points in the geometry
• Pre-stressing option
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anchored wall cofferdam
Anchor material properties
Normal stiffness, EA (for one anchor) [kN]Spacing, Ls (distance between anchors) [m]Maximum anchor force for compression
and tension, |Fmax,comp| and |Fmax,tens| [kN]
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Pre-stressing of anchors
• Defined in Staged construction phaseB h i ( h ) i ( )• Both tension (grout anchor) or compression (strut) possible
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C3. Membrane Elements (Geogrids)
• 3 or 5 noded line element• Linear elastic behaviourLinear elastic behaviour• No flexural rigidity (EI), only normal stiffness (EA)• Only allows for tension, not for compression• Soil/Geogrid interaction may be modelled using interfaces
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geotextile wall ground anchor
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C4.Interfaces
• Try to omit stress oscillations at corners of stiff structuresstructures
Inflexible corner points, causing bad
stress results
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Flexible corner points with improved stress
results
C4.Interfaces
• Soil-structure interactionW ll f i ti• Wall friction
• Slip and gapping between soil and structure• Soil material properties
• Taken from soil using reduction factor RinterCinter = Rinter * Csoiltan(φ)inter = Rinter * tan(φ)soil
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(interface may not be required)• Interaction soil/geotextile = Rinter≈ 0.9 – 0.5 (foil, textile)
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inter
C5.Combining Elementsg
• Example 1: Ground anchors• Example 2: Thick walls (‘Sandwich’ method)
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Example 1: Ground anchors
• Combination of node-to-node anchor and geogrid• Node to node anchor represents anchor rod (no interaction• Node-to-node anchor represents anchor rod (no interaction
with surrounding soil)• Geogrid represents grout body (full interaction with grid• No interface around grout body; interface would create
unrealistic failure surface
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Combined Elements:Ground anchors
axial forces in geotextile element
real distribution of axial forces in ground anchorInput geometry
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Generated mesh
Axial forces in ground anchors
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Anchor material properties
Normal stiffness, EA (for one anchor) [kN]Spacing, Ls (distance between anchors) [m]Maximum anchor force for compression
and tension, |Fmax,comp| and |Fmax,tens| [kN]
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Pre-stressing of anchors
• Defined in Staged construction phaseB h i ( h ) i ( )• Both tension (grout anchor) or compression (strut) possible
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Example 2: Thick WallsE1=real
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Scaled-down E2=fictitious(Try E2= 0.01 to 0.1 E1, to avoid double value)
Aim: Expedite data extraction like displacements, bending moments, axial & shear forces(but be careful!. Disp.=Correct. But F,S, M (Scale back to the correct value because E2=0.01 to 0.1 of E1)
D B d i Fi it L d d PWPD. Boundaries: Fixity, Loads and PWP
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D1.Input boundary conditions
Default fixities
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D2.Input loads
Point force A & B
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D3.Porewater Generation
• Wet excavationI bl ( t ) ti fl• Impermeable (concrete) excavation floor
• Plastic calculation, Consolidation, Phi/c reduction and Dynamic analysis.Dynamic analysis.
• Updated mesh.• Loading input: Multipliers or Staged Construction.• Changing water conditions• Multiple calculation phases can be pre-defined and executed
at once.
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Stress paths
Construction phases:• I 1st excavation
σK
K=1v0Kactive
III III
Point APoint A
I 1 excavation• II Pre-stressing anchor•III Final excavation
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passivePoint B K
σh
Point B
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Calculation program
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Output program
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View results
• Graphical and tabulated output of displacements, stresses and structural forcesstresses and structural forces
• Output in cross sections• Multiple output windows can be opened
simultaneously (comparison of results)
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Demonstration : Anchored Sheet-pile Wall
• See demonstration
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Thank You!
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