7 1605 1615 1715 Structural Elements Excavations

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G1 Chile May 15-18, 2006: Structural elements and excavations 2

PLAXIS FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

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 =

EA

EIhd 12==

b

hh

b

b = 1 m in plane strain

b = 1 meter in axisymmetry


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Plate weights

Compensate for overlap:

For soil weight use:unsat above phreatic level

satbelow phreatic level

realsoilconcrete dw = )(


Plate weights for tunnels

Overlap is only for half the lining thickness

rinsideroutside

lining soil

r

d real

( )outsideinside rrr += 21

( ) ( )realsoilrealconcrete ddw 21=


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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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Interfaces

Suggestions for Rinter

:

Interaction sand/steel = R inter 0.6 0.7

Interaction clay/steel = R inter 0.5

Interaction sand/concrete = R inter 1.0 0.8

Interaction clay/concrete = R inter 1.0 0.7

Interaction soil/geogrid = R inter 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


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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: Eurshould be used rather than E50


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Material behaviour: Stress paths

K

K=1v0

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


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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 lowpermeability soils



Undisturbed water table outside excavation

dry

interpolate

GPL


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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.


Dewatering: dry excavationDrawdown outside excavation

Groundwater flow calculation gives steady-state solution,

so for time is infinite!

7 1605 1615 1715 Structural Elements Excavations

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