PLAXIS -1605 1615 1715 Structural Elements Excavations

CG1 Chile May 15-18, 2006: Structural elements and excavations 1

PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

Structural elements in Plaxis

Dennis WatermanPlaxis BV


Structural elements in Plaxis

• Plates and shells (walls, floors, beams, tunnels)

• Anchors

• Geogrids (geotextiles)

• Interfaces



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 (b=1 m)

• Normal stiffness (b=1 m)

• Element thickness

12

3 bhEEI ⋅⋅=

bhEEA ⋅⋅=

EAEIhd 12==

b

h hb

b = 1 m in plane strainb = 1 meter in axisymmetry



Plate weights

• Compensate for overlap:

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

realsoilconcrete dw ⋅−= )( γγ


Plate weights for tunnels

• Overlap is only for half the lining thickness

rinsideroutside

lining soil

r

dreal

( )outsideinside rrr += 21

( ) ( )realsoilrealconcrete ddw 21⋅−⋅= γγ



strut

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


anchored wall cofferdam

Node-to-node anchors

• To model anchors, columns and rods• Elasto-plastic spring element• Connects two geometry points in

the geometry• Pre-stressing option



Anchor material propertiesNormal 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



geotextile wall ground anchor

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


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



Ground anchors

axial forces in geotextile element

real distribution of axial forces in ground anchorInput geometry

Generated mesh

Axial forces in ground anchors


Interfaces

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

• Soil material properties • Taken from soil using reduction factor Rinter

Cinter = Rinter * Csoiltan(φ)inter = Rinter * tan(φ)soil

• Individual material set for interface



Interfaces

Suggestions for Rinter:• 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 = Rinter≈ 0.9 – 0.5

(foil, textile)


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



Excavations in Plaxis

Dennis WatermanPlaxis 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 E50



Material behaviour: Stress paths

KK=1v

0

passivePoint B K

Kactive

h

III III

Point APoint A

Point B

Construction phases:• I 1st excavation• II Pre-stressing anchor•III Final excavation


Dewatering

• Wet excavation• Impermeable (concrete) excavation floor

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



Dewatering: options

• General phreatic levelApplies to all clusters that have not been separately defined.

• Cluster phreatic levelApplies to one specific cluster.

• Cluster dryMakes a specific cluster dry.

• InterpolateInterpolate pore pressures between clusters above and below.

• User-defined pore pressureSpecify pressure pref at level yref and increase pinc per meter in y-direction.


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



Dewatering: dry excavationUndisturbed 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 lowpermeability soils


Dewatering: dry excavationUndisturbed water table outside excavation

dryinterpolate

GPL



Dewatering: dry excavationDrawdown outside excavation• For every excavation phase do

• Excavate soil• Define boundary conditions (heads)• Perform groundwater flow analysis.

Suitable for long-term excavations or excavations inhigh permeability soils

Simplified alternative:• Draw GPL according to expected groundwater level and

generate pore pressures based on GPL.


Dewatering: dry excavationDrawdown outside excavation

Groundwater flow calculation gives steady-state solution,so for time is infinite!

PLAXIS -1605 1615 1715 Structural Elements Excavations

Documents

plaxisdennis

dry excavationdrawdown

ground anchor

ground anchors

excavation

stressing

axial forces

cg1 chile