Geoscience Reference
In-Depth Information
In conclusion, it appears that the response of piled foundations may be less favourable
than that of rigid mat foundations. However, two significant limitations of the performed
analyses on which these conclusions arepartlybased must be noted here:
•
perfect (“bonded”) contact was assumed between piles and soil and
•
thepiles were modelled as a perfectly elasticmaterial.
As a result of the first assumption, the forces upon the piles by the outward and down-
wardmoving“hangingwall”areexaggerated.Soilslidingaroundthepileswouldreduce
the magnitude of such “drag” forces, thereby leading to smaller pile distress and smaller
cap rotation/displacement. Regarding the second assumption, note that the large bend-
ing moments in the piles would not of course materialize in reality, since their ultimate
structural capacity cannot be exceeded. Prediction of the consequences of the unavoid-
able redistribution of loads among the piles, and among piles and raft, cannot be made
reliably with the resultspresented above forpurely elasticpiles.
2.3.2. Rigid caisson
Thecaissonis10m
10minplanandalso15mindepth.Itcarries10MNverticalload.
Onlyfullybondedcontactbetweenthecaissonandthesoilisconsidered—anidealization
that is likely to lead to aconservative assessment of the caissondisplacement/rotation.
×
Dominant role in the response of a given caisson to fault rupturing underneath plays its
position with respect to the free-field rupture outcropping. Again four such positions are
considered: 1, 5, 9, and 13m. Figures 9.8 and 9.9 portray the deformed mesh with the
distribution of plastic octahedral shear strains for each value of S. Figure 9.8 gives the
plane section
(along the axis) while Figure 9.9 depicts a 3D view (of half the model).
The following conclusions are drawn:
α
(1) ForS
1m,thefaultemergestotheleftofthecaisson,divertedslightly,andforms
adistinctscarpsimilartothatinthecaseofthepiledfoundation.Thecaissondoes
not experience any measurable rotationor displacement.
(2) More significant is the diversion of the rupture path in case of S
=
5m; the fault
now emerges vertically along the side of the caisson. The latter hardly “feels” the
rupture, experiencing a rotation of merely 1
◦
for a vertical component of dislo-
cation h
=
2m. By contrast, recall that the corresponding piled foundation had
developed a rotation and horizontal displacement, while its front row of piles had
been substantially distressed.
=
Notice also that a secondary rupture has begun to form, propagating at an angle of about
30
◦
to the left of main rupture. It is about to reach the ground surface for h
2m, and
the associated graben between the two normal ruptures is (barely) visible in the scale of
the figure.
=
(3) For S
=
9m, the rupture is diffused the caisson rotates substantially to the left
8
◦
for h
(
, and an active state of stress develops on the back side of the
caisson. Clearly this behaviour is not so favourable; for instance it would cause
distress in a framed structure one column of which is supported on such a caisson.
=
2m
)
