Geoscience Reference
In-Depth Information
A parametric investigation revealed the need for a long
and very refined mesh
(elementsizeof0.5-1.0m)alongwithasuitableslip-linetracingalgorithmintheregion
of soil rupture and foundation loading. An elastoplastic constitutive model with the
Mohr-Coulomb failure criterion and isotropic strain softening was adopted and encoded
in the ABAQUS finite element environment. Similar models have been successfully
employed in modelling the failure of embankments and cut slopes (Pottset al., 1990).
Modelling strain softening was shown to be necessary; it was introduced by suitably
reducing the mobilised friction angle
(
B
=
4H
)
ψ
mob
with
increasingplasticoctahedralshearstrain.Withalltheabovefeatures,theFEformulation
iscapableofpredictingrealisticallytheeffectoflargedeformationswiththecreationand
propagation of shear bands.
ϕ
mob
and the mobilised dilation angle
The foundation, modelled with linear elastic beam elements, is positioned on top of the
soil model and connected to it through special contact elements. The latter are rigid in
compression but tensionless, allowing detachment of the foundation from the bearing
soil (i.e. gap formation beneath the foundation). The interface shear properties follow
Coulomb's friction law, allowing for slippage. Both detachment and slippage are impor-
tant phenomena for arealisticfoundation model.
A typical result elucidating the interplay between loose
soil, rupture path,
andaperfectlyrigidfoundationcarryinga4-storeystructureisgiveninFigure9.2.Abase
rock dislocation of 2m (5% of the soil thickness) is imposed. The structure is placed
symmetrically straddling the free-field fault breakout (i.e. the foundation is placed with
its middle coinciding with the location where the fault would outcrop in the free-field).
Yet, a distinct rupture path (with high concentration of plastic shearing deformation and
a resulting conspicuous surface scarp) is observed only in the free-field. The presence of
the structure with its rigid foundation causes the rupture path to bifurcate at about the
middle of the soil layer. The resulting two branches outcrop outside the left and the right
corner of the foundation, respectively. The soil deformations around these branches are
far smaller and diffuse than in the free-field, and the respective surface scarps are much
milder.Thankstothesubstantialweightofthestructureandtheflexibilityoftheground,
the structure settles and rotates as a rigid-body. The foundation does not experience any
loss of contact with the ground; apparently, the foundation pressure is large enough to
eliminate any likely asperities of theground surface.
As a result of such behaviour, the structure and its foundation do not experience any
substantial distress,while their rotation and settlement could perhaps be acceptable.
(
D
r
=
45%
)
The main factors influencing FR-SFSI are:
the style of faulting (normal, thrust, strike-slip), the angle of dip and the offset
(dislocation) at the basement rock,
•
the total thickness (H) of the overlying soil deposit, and the stiffness (G), strength
(
•
ϕ
, c) and kinematic
(ψ)
characteristics of the soil along the depth,
