Op Critical Soil Parameters

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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.

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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

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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


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.






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


Consolidation analysis for time-


LEPP Mohr-Coulomb for group resistance

(total stress analysis for undrained case)

and approximate settlement.


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.






stiffness.

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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


Consolidation analysis for time-


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.


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.

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Op Critical Soil Parameters