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soft
diaphragm
wall
soft
diaphragm
wall
weak layer
Fig. 6.9. Scheme for improvement of seismicperformance of existing foundation
by introduction of asoft caisson
upward propagating shear waves. In addition, the soft caisson adds the mass of the soil
withinthecaissontothemassofthestructureandthismodifiesthedynamicperformance
by reducing the resonant frequencies. There are evidently potential practical problems
with such a mitigation scheme, not least the need to ensure static stability particularly
against rotation.
Modellingsuchasoftcaissonprovidesitsownchallenges.Theintentionisthattheweak
layer should have a very low angle of friction. Assuming that the soft lateral walls (par-
allel to the direction of shaking) offer negligible shear stiffness, the resonant frequency
for the mitigated system is around 0.9Hz. With the scale factor for frequency, this gives
atargetmodelresonantfrequencyofaround5Hz.Themodelsoftcaissonwascontained
within a plywood box resting on a base layer which was modelled by an array of roller
bearings. These were found to permit sliding at a slope inclination of about 1
8 —close
to the target value. The soft walls were modelled using cylinders of neoprene sponge
sandwiched between the side walls of the plywood box and outer plywood sheets. The
stiffnessofthesystemcouldbetunedbyalteringthenumberandgeometryofthesecylin-
ders: using four cylinders on each wall, with length 57mm and diameter 60mm gave a
system resonant frequency of 7Hz which was deemed to be near enough to the target
frequency.
.
The shear stack was excited horizontally by two earthquake time histories (Figure 6.10):
the Y-Y component of the Friuli-San-Rocco earthquake (15/9/76) and the N-S compo-
nent of the Vrancea earthquake (4/4/1977). Similitude requires time to be scaled by a
factorof5.4(1
/ n
):the16.75sFriuli-San-Roccoearthquakeisreplayedover3.1s;the
40sVranceaearthquake,over7.4s.TheappliedaccelerationsareshowninFigure6.10in
both time and frequency domains. The Vrancea time history (Figure 6.10c, d) is charac-
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