1
w w w . g e o f e m . c o m
Critical soil parameters
Some critical soil parameters and appropriate analysis types for some typical geotechnical
structures:
Application type Analysis type & constitutive model Some critical soil parameters
Tips and pitfalls
Embedded
retaining wall
deflection and
structural forces
Usually plane strain, but 3D for complex
geometries and corners. Axi-symmetric
for vertical shafts.
Drained analysis for design, or coupled
consolidation to predict time-related
values. Undrained analysis for short
term in stiff clays.
LEPP Mohr-Coulomb sufficiently accurate
for design prediction of wall structural
forces (bending moment, shear force,
prop forces).
Strain-dependent stiffness for accurate
prediction of SLS wall deflections and
structural forces.
K0 has a direct effect on
the earth pressures on
the wall.
Shear strength or cu:
even if the wall is not
approaching failure,
shear strengths are still
important for prediction
of limiting active and
passive earth pressures.
Permeability k is
important for accurate
prediction of pore
pressures when
groundwater flow
occurs.
More sophisticated models than
LEPP should improve predictions.
Excavation-
induced ground
movements
Usually plane strain, but 3D for complex
geometries and corners, and installation
effects. Axi-symmetric for vertical shafts.
Drained analysis (conservative),
undrained analysis (short term in stiff
clays) or coupled consolidation analysis
to predict time-related ground
movements.
Strain-dependent stiffness needed for
accurate prediction of settlement profile
behind excavations and heave inside
excavation.
Stress and strain-
dependant stiffness G0
and G.
For assessing building distortions,
more accurate predictions can be
achieved by modelling the weight
and stiffness of the building, rather
than assuming greenfield
conditions, although complex
geometries would require 3D
analysis.
Wall installation may also cause
ground movements but analysis is
complex.
Tunnelling-
induced ground
movements
Plane strain for suitable geometries but
cannot predict effect of advancing
tunnel, also movements during unlined
excavations will be over-predicted. 3D
for complex geometries and interactions
and for predicting effects of advancing
tunnel, tunnel heading stability.
Stiff clays: undrained analysis for short
term, consolidation analysis for long-
term post-construction settlement
behaviour.
Soft clays: consolidation analysis.
Granular soils: drained analysis.
Strain-dependent stiffness needed for
accurate prediction of settlement
troughs over tunnels.
Stress and strain-
dependant stiffness G0
and G.
For assessing building distortions,
more accurate predictions can be
achieved by modelling the weight
and stiffness of the building, rather
than assuming greenfield
conditions, although complex
geometries would require 3D
analysis.
Heave in soft clay tunnels can be
over-predicted, cancelling out
surface settlements.
In consolidation analyses, the
drainage properties of the tunnel
lining are important.
2
w w w . g e o f e m . c o m
Application type Analysis type & constitutive model Some critical soil parameters
Tips and pitfalls
Seepage into
cofferdam
Plane strain for rectangular, axi-
symmetric for circular cofferdams.
Steady-state seepage.
Given the uncertainties in permeability
measurement, linear anisotropic
permeability is probably sufficiently
accurate (as opposed to non-linear,
dependent on void ratio or effective
stress).
Permeability k. Permeability is notoriously difficult
to measure accurately, with values
sometimes varying by orders of
magnitude. This results in outputs
of seepage from the analysis
varying by orders of magnitude.
Therefore, parametric studies to
quantify the range of uncertainty in
analysis outputs is essential.
Cut slopes
Plane strain.
3D if structure such as abutment wall
influences slope or for certain
stabilisation measures or if 3D
mechanism critical.
Drained analysis is conservative for
stability in stiff clays, undrained for
short-term stability in stiff clay, or else
coupled consolidation analysis of clay
slopes to predict time-related swelling
and delayed failure. In soft clays, short
term (undrained) is usually critical for
stability.
LEPP Mohr-Coulomb for granular soils
and stiff clays. Hyperbolic or Modified
Cam Clay for normally to lightly over-
consolidated clays to predict plastic
straining pre-failure.
Soil shear strength or
cu.
Permeability k for
coupled consolidation
analysis.
Time to failure in clay
slopes depends also on
K0.
Only use undrained analysis if clay
is fully undrained. This will not be
the case in clays with silt or sand
layers.
Specialised analysis is required to
simulate softening and progressive
failure of cut slopes in stiff plastic
clays. In non-softening analyses,
use c
3
w w w . g e o f e m . c o m
Application type Analysis type & constitutive model Some critical soil parameters
Tips and pitfalls
Single pile
(vertical load)
Axi-symmetric.
Undrained for pile resistance in clays.
Drained for pile resistance in sands and
for long term deformations.
Consolidation analysis for time
dependent deformations.
LEPP Mohr-Coulomb for pile resistance
(total stress analysis for undrained case)
and settlement.
Shear strength or cu.
K0 for shaft friction.
Dilation in a confined problem such
as this causes unrealistic results
and can even prevent failure.
Always set = 0.
The various installation methods of
piles will affect soil and soil/pile
interaction properties in different
ways. For instance, void ratio
changes will affect soil strength and
stiffness.
Single pile
(lateral load)
3D analysis.
Undrained for lateral resistance in clays.
Drained for lateral resistance in sands
and for long term deformations.
Consolidation analysis for time
dependent deformations.
LEPP Mohr-Coulomb for lateral
resistance (total stress analysis for
undrained case) and approximate
deflection.
Hyperbolic model for more accurate
lateral deflection prediction.
Shear strength and
stiffness or cu, G.
K0.
The various installation methods of
piles will affect soil and soil/pile
interaction properties in different
ways. For instance, void ratio
changes will affect soil strength and
stiffness.
Pressuremeter testing is particularly
suited to obtaining parameters for
laterally-loaded piles due to its
horizontal loading.
Pile group
3D analysis.
Undrained for group resistance in clays.
Drained for group resistance in sands
and for long term deformations.
Consolidation analysis for time-
dependent deformations.
LEPP Mohr-Coulomb for group resistance
(total stress analysis for undrained case)
and approximate settlement.
Strain-dependent stiffness for accurate
prediction of settlement and soil-pile
interaction effects.
Small-strain stiffness G0
and G for interaction
effects.
Dilation in a confined problem such
as this with vertical loads causes
unrealistic results and can even
prevent failure. Always set = 0.
The various installation methods of
piles will affect soil and soil/pile
interaction properties in different
ways. For instance, void ratio
changes will affect soil strength and
stiffness.
4
w w w . g e o f e m . c o m
Application type Analysis type & constitutive model Some critical soil parameters
Tips and pitfalls
Shallow
foundation
Strip: plane strain.
Circular: axi-symmetric.
Pad: 3D
Undrained for bearing resistance on
clays.
Drained for bearing resistance on sands
and for long term deformations.
Consolidation analysis for time-
dependent deformations.
LEPP Mohr-Coulomb for bearing
resistance (total stress analysis for
undrained case).
Hyperbolic model (or Modified Cam Clay
for soft clays) for bearing resistance and
accurate settlement.
Strain-dependent stiffness for accurate
prediction of deformations adjacent to
foundation.
Stiffness G for
deformations, shear
strength or cu for
failure.
Anisotropic strength reduces
bearing resistance. If using an
isotropic model, an average shear
strength must be determined.
Dynamic analysis
(low strain)
Single source vibration (e.g. pile driving,
machinery): axi-symmetric.
Soil-structure interaction: plane strain or
3D depending on geometry.
Drained or undrained depending on rate
of loading and soil type. Even sands
might be treated as undrained under
rapid loading.
Constitutive model depends on structure
being modelled and purpose of analysis
(see application types above). Simple
linear elastic model could be used for
basic wave propagation studies, but the
small-strain stiffness (G0) should be
used. Strain-dependent stiffness could
be used for higher strain levels. Note
that for high strain dynamic studies,
volume change characteristics under
cyclic loading need to be considered.
Small strain stiffness G0
and decay of stiffness
with strain.
Damping ratio for soil-
structure interaction
problems.