Geoscience Reference
In-Depth Information
in the base rock will likely cause a distinct fault scarp of nearly the same displacement
magnitude. One important finding of the above studies is that the rupture path in the
soil is not a simple extension of the plane of the fault in the base rock: phenomena such
as “diffraction” and “bifurcation” affect the direction of the rupture path, and make its
outcropping location and offsetmagnitude difficult topredict.
Ourinteresthereisnotonthepropagationofarupturewithinthesoil,butonhowastruc-
ture sitting on top of the fault breakout behaves. It turns out that a fascinating interplay
takes place between the propagating fault rupture, the deforming soil, the differentially
displacing foundation, and the supported structure. Two different phenomena take place.
First,thepresenceofthestructuremodifiestherupturepath.Dependingontherigidityof
the foundation and the weight of the structure, even complete diversion of the fault path
before it outcrops may take place. Obviously, the damage to a given structure depends
not only on its location with respect to the fault outcrop in the “free-field”, but also on
whether and by how much such a diversion may occur. Second, the loads transmitted
from the foundation on to the soil tend to compress the “asperities” and smoothen the
“anomalies” of the ground surface that are produced around the fault breakout in the
free-field, i.e. when the structure is not present. Thus, depending on the relative rigidity
(bending and axial) of the foundation with respect to the soil, as well as on the magni-
tude of the structural load, the foundation and the structure will experience differential
displacements and rotation different fromthoseof the free-fieldground surface.
This phenomenon, given the name “Fault-Rupture-Soil-Foundation-Structure Inter-
action” [FR-SFSI] by Anastasopoulos and Gazetas (2007), is briefly elucidated in the
sequel for shallow and deep foundations.
2.2. NUMERICAL ANALYSIS AND RESULTS: SHALLOW FOUNDATIONS
The problem studied here is illustrated in Figure 9.1. We consider a uniform soil deposit
of thickness H at the base of which a normal fault, dipping at an angle
(measured
fromthehorizontal),producesdownwarddisplacement(“dislocation”,“offset”)ofverti-
cal amplitude h. The analysis is conducted in two steps. First, fault rupture propagation
through soil is analysed in the free-field, ignoring the presence of the structure. Then,
a strip foundation of width B carrying a uniformly distributed load q or a multistory
frame-structure is placed on top of the free-field fault outcrop at a specified distance S
(measured from its corner), and the analysis of deformation of the soil-structure system
due to the same base “dislocation” h is performed. The analyses are conducted under
2D plane-strain conditions—evidently a simplification, in view of the finite dimensions
of a real structure in the direction parallel to the fault. The relative location of outcrop-
pingisvariedparametricallythroughthedistanceS.Comparingsoilandground-surface
deformations inthe twosteps gives a firstpicture of the significance of SFSI.
α
Among several alternatives that were explored, the FE model shown in Figure 9.2 pro-
duced results in excellent accord with several centrifugal experiments conducted at the
University of Dundee for both steps of the analysis (Anastasopoulos et al., 2007a,b).
