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Geotechnical News • March 2013
43
equivalent to the application of a uni-
form and controlled stress (or strain)
path to a soil element. This assumption
may however complicate the assess-
ment of the stress-strain properties if
the results of different kind of tests
on the same soil are not compared in
a single framework. In particular the
results of triaxial and torsional shear
tests are traditionally plotted in terms
of (E:
ε
a) and (G:
γ
), respectively, since
this facilitates simplified computa-
tions of stress-strain behaviour of
the ground through equivalent linear
analyses. In this case two different
criteria may be adopted to represent
the results of both set of tests in the
same scale (e.g. [14]): one which
equates the maximum values of shear
stress an strains (equality of Mohr’s
circles diameters), another which
equates the deviatoric stress and shear
strain invariants ([13]). The results of
calibration may be different, depend-
ing on which criterion is followed. For
such a reason the participant groups
were asked to calibrate their constitu-
tive model on the results of both sets
of tests and to discuss the possible
drawbacks at the level of laboratory
test interpretation, before attempting
to model the boundary problem of the
centrifuge tests.
A number of interesting points of
discussion arose about the differ-
ences among numerical predictions
performed with several numerical and
constitutive models of various degrees
of complexity, as well as between
predictions and experimental mea-
surements. Overall, the acceleration
time histories were well-predicted by
all the groups, and the accumulation
of internal forces during the events
was qualitatively reproduced by those
models which were able to simulate
the dynamic densification of sand.
Nevertheless, on the average the cal-
culations under-predicted the observed
cumulated deformations and related
lining force increments.
The general feeling at the workshop
was that the work that was started by
blind predictions should be better con-
tinued by making use of the available
centrifuge results to improve the reli-
ability of numerical predictions. This
procedure could reduce uncertainties
arising from calibrating the constitu-
tive models on the basis of the labora-
tory “soil element” tests, as discussed
above. Therefore, the participants
were recently allowed to access to the
whole T4 test data, and to calibrate
the constitutive models on the basis
of both T3 and T4 test results. Obvi-
ously, such further numerical predic-
tions cannot be considered as blind but
rather as back-calculations. A com-
parative analysis of the results of both
blind predictions and back-analyses
will be published once the latter will
be completed.
References
[1] Bilotta E., Lanzano G., Russo G.,
Santucci de Magistris F., Aiello V.,
Conte E., Silvestri F., Valentino M.
(2007). “Pseudostatic and dynamic
analyses of tunnels in transversal
and longitudinal direction”. Proc.
4th International Conference on
Earthquake Geotechnical Engi-
neering (Pitilakis ed.), Thessa-
loniki, Springer.
[2] Visone, C., Bilotta, E., Santucci de
Magistris, F. (2010). “One dimen-
sional ground response as a pre-
liminary tool for dynamic analyses
in Geotechnical Earthquake Engi-
neering”. Journal of Earthquake
Engineering, 14(1) 131-162.. doi:
10.1080/13632460902988950
[3] Aiello, V., Boiero, D., D’Apuzzo,
M., Socco, L.V., Silvestri, F.
(2008). “Experimental and numeri-
cal analysis of vibrations induced
by underground trains in an urban
environment”. Structural Control
and Health Monitoring, 15 (3), pp.
315-348.
[4] Owen, G.N., Scholl, R.E. (1981).
“Earthquake engineering of large
underground structures”, Report
no. FHWA/RD-80/195. Federal
Highway Administration and
National Science Foundation.
[5] Lanzano, G., Bilotta, E., Russo, G.,
Silvestri, F., Madabhushi, S.P.G.
(2010). “Dynamic centrifuge tests
on shallow tunnel models in dry
sand”. Proc. VII International
Conference on Physical Modelling
in Geotechnics (ICPMG 2010),
Zurich, pp. 561-567. Taylor &
Francis.
[6] Lanzano G., Bilotta E., Russo G.,
Silvestri F., Madabhushi S.P.G.
(2012). “Centrifuge modeling
of seismic loading on tunnels
in sand”. Geotechnical Test-
ing Journal 35(6). doi: 10.1520/
GTJ104348.
[7] Bilotta, E., Silvestri, F. (2012).
“A predictive exercise on the
behaviour of tunnels under seismic
actions”. Proc. IS-Roma 2011 7th
Int. Symp. Geotech. Aspects of
Underground Construction in Soft
Ground. CRC Press.
[8] Visone, C., Santucci de Magistris,
F. (2009) “Mechanical behaviour
of the Leighton Buzzard Sand
100/170 under monotonic, cyclic
and dynamic loading conditions,”
Proc. XIII Conf. L’Ingegneria Sis-
mica in Italia, ANIDIS, Bologna,
Italy.
[9] Papa, V., Silvestri, F., Vinale, F.
(199). “Cyclic/dynamic simple
shear tests: recent developments”.
Proc. XII ICSMFE, Rio de
Janeiro,1989. Vol. 1, pp. 83-88.
[10] D’Onofrio, A., Silvestri, F.,
Vinale, F. (1999). “A New Tor-
sional Shear Device”. Geotechni-
cal Testing Journal, 22 (2), pp.
107-117.
[11] Santucci De Magistris, F., Sil-
vestri, F., Vinale, F. (1998). “The
influence of compaction on the
mechanical behaviour of a silty
sand”. Soils and Foundations, 38
(4), pp. 41-56
[12] Chian, S. C., Stringer, M. E.,
Madabhushi, S. P. G. (2010). “Use
of the automatic sand pourers
for loose sand models”, Proc.
VII International Conference on