Geoscience Reference
In-Depth Information
fessor J. Bray (2005): deep-rooted trees being torn apart by a fault rupturing directly
underneath, apparently as a result of their roots being pulled inopposite directions.
Two typical foundation systems are examined here in order to highlight the interaction
between a deep foundation and anemerging fault rupture:
•
a3
×
3 capped pilegroup and
•
asquare rigidembedded foundation (“caisson”).
A3Dfiniteelementmodelwasdevelopedforeachcase,usingeight-nodedelements,and
employing the same soil and interface constitutive models as described in the preceding
section. Figure 9.6 presents a plane section of the complete model. In both cases the soil
deposit consisted of dense sand, of total thickness H
20m. Needless to say, the choice
of this limited depth was motivated solely by the desire for the smallest possible size of
this 3Dmodel.
=
2.3.1. Piles
ThepilesareoflengthL
p
=
15m,diameterd
p
=
1mandarespaced4mapart(fromaxis
toaxis).Theircapis10m
10minplanand2.5mthick,andcarriesastructuralvertical
load of 10MN. A rigid connection is assumed between cap and piles (fixed-head piles).
Only ideally elastic pile behaviour is considered at the present time, although the neces-
sity for accounting for pile inelasticity will become apparent (if a realistic assessment of
the response of thesystemto largefault offsets is needed).
×
Aiming at giving a first picture of the possible straining to be experienced by the piles,
Figure9.7portraysthedeformedfinite-elementmeshwiththedistributionofplasticshear
strains. Four positions of the pile group with respect to the outcropping fault in the free-
field are examined: S
1, 5, 9, and 13m, where S is measured from the edge of the pile
cap (which lies 1m to the left of the nearby pile axis). Then Figure 9.8 presents detailed
results (deformations and internal forces) for the case of S
=
=
5m, only. Several trends
are worth noting inthesefigures:
(1) For S
1m, when the fault emerges near the left edge of the pile group (and
the group is therefore almost all in the footwall), a slight diversion of the rupture
pathtothelefttakesplace.Averydistinctscarpisformedimmediatelynexttothe
piles. The scarp forms a slope of about the same angle
=
α
as that of the triggering
basement rupture, and is appreciably larger than the imposed base offset,
.The
pile group, however, remains almost intact: there is no displacement or rotation of
the pile and only the piles in the front row experience some (rather minor) distress
(bending moments of the order of 300kNm).
(2) For S
5m, the fault would have emerged at the centre of the foundation in the
free-field. The presence of the axially loaded piles makes the rupture path: (i) to
partly divert to the left and emerge just at the edge of the front of piles, and (ii) to
become diffuse in the region between the piles. Substantial rotation and horizontal
displacement of the pile cap take place. The front row of piles is being “pulled”
outwardanddownwardbythe“dropping”hangingwallofthefault;asaresultvery
=
