PLAXIS -1605 1615 1715 Structural Elements Excavations

Structural elements in Plaxis Dennis Waterman Plaxis BV

PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

Structural elements in Plaxis • Plat Plates es and and shel shells ls (walls, floors, beams, tunnels) • Anchors • Geog Geogri rids ds (geo (geote text xtil iles es)) • Interfaces


CG1 Chile May 15-18 2006: Structural elements and excavations

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Plates and shells • • • •

3 or 5 noded line elements 3 degrees of freedom per node Elastic or elastoplastic behaviour To model walls, floors, tunnels


Input parameters for plates • Flexural rigidity • Normal stiffness • Element thickness

EI = E ⋅

h3 ⋅ b

(b=1 m)

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EA = E ⋅ h ⋅ b

(b=1 m)

EI d = h = 12 EA

h

h b b = 1 m in plane strain b = 1 meter in axisymmetry

b PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES


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Plate weights • Compensate for overlap: w = (γ concrete − γ soil ) ⋅ d real

• For soil weight use: γunsat above phreatic level γsat below

phreatic level


Plate weights for tunnels d real r inside

r

r outside lining

soil

• Overlap is only for half the lining thickness w = (γ concrete ⋅ d real ) − r = 1

2

(

γ soil ⋅

1 d 2 real

)

(r inside + r outside )



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Fixed-end anchors • To model supports, anchors and struts • 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

strut PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

Node-to-node anchors • To model anchors, columns and rods • Elasto-plastic spring element • Connects two geometry points in the geometry • Pre-stressing option

anchored wall

cofferdam



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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]


Pre-stressing of anchors • Defined in Staged construction phase • Both tension (grout anchor) or compression (strut) possible



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Geogrids • 3 or 5 noded line element • Linear 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

geotextile wall

ground anchor


Ground anchors • Combination of node-to-node anchor and geogrid • 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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Ground anchors

Input geometry

real distribution of axial forces in ground anchor

axial forces in geotextile element

Axial forces in ground anchors

Generated mesh PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

Interfaces • Soil-structure interaction • Wall friction • Slip and gapping between soil and structure

• Soil material properties • Taken from soil using reduction factor R inter Cinter = R inter * Csoil tan(φ)inter = R inter * tan(φ)soil • Individual material set for interface



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Interfaces Suggestions for R inter: • Interaction sand/steel

= Rinter ≈ 0.6 – 0.7

• Interaction clay/steel

= Rinter ≈ 0.5

• Interaction sand/concrete

= Rinter ≈ 1.0 – 0.8

• Interaction clay/concrete

= Rinter ≈ 1.0 – 0.7

• Interaction soil/geogrid = Rinter ≈ 1.0 (interface may not be required) • Interaction soil/geotextile (foil, textile)

= Rinter ≈ 0.9 – 0.5


Interfaces • Try to omit stress oscillations at corners of stiff structures Inflexible corner points, causing bad stress results

Flexible corner points with improved stress results PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES


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Excavations in Plaxis Dennis Waterman Plaxis BV


Material behaviour • Unloading due to excavation • Primary loading due to pre-stressing • Hardening Soil model is preferred • Non-linear elastic unloading/reloading behaviour

• Sometimes better to use Mohr-Coulomb with known parameters than HS with unverified parameters. • When using MC: Eur should be used rather than E 50



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Material behaviour: Stress paths Construction phases: • I 1st excavation

• II Pre-stressing anchor •III Final excavation 

v

Point A

K active K 0 Point A II I

Point B

K=1 III

K Point B passive



h


Dewatering • Wet excavation • Impermeable (concrete) excavation floor

• Dry excavation • Undisturbed water table outside excavation • Drawdown outside excavation



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Dewatering: options • General phreatic level Applies to all clusters that have not been separately defined.

• Cluster phreatic level Applies to one specific cluster.

• Cluster dry Makes a specific cluster dry.

• Interpolate Interpolate pore pressures between clusters above and below.

• User-defined pore pressure Specify pressure pref at level yref and increase p inc per meter in ydirection.


Dewatering: wet excavation • Excavate without changing water conditions (in stages or at once) • Apply stabilising weight at the bottom • Set excavated area dry • Use “cluster dry” option or • Use “cluster phreatic line”

• Pore pressures outside excavated area remain unchanged PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES


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Dewatering: dry excavation Undisturbed water table outside excavation

• For every excavation phase do • Excavate soil • Set excavated area dry • Define area just below excavation floor as interpolate between lines or clusters Suitable for short- term excavations in low permeability soils


Dewatering: dry excavation Undisturbed water table outside excavation

dry interpolate

GPL



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Dewatering: dry excavation Drawdown outside excavation • For every excavation phase do • Excavate soil • Define boundary conditions (heads) • Perform groundwater flow analysis. Suitable for long-term excavations or excavations in high permeability soils

Simplified alternative: • Draw GPL according to expected groundwater level and generate pore pressures based on GPL. PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

Dewatering: dry excavation Drawdown outside excavation

Groundwater flow calculation gives steady-state solution, so for time is infinite! PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES


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PLAXIS -1605 1615 1715 Structural Elements Excavations

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