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Geotechnical News • March 2013
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A round robin test on tunnels under seismic actions
Emilio Bilotta and Francesco Silvestri
Introduction
The seismic behaviour of urban tun-
nels can be predicted by simplified
pseudo-static approaches and dynamic
methods of analysis, i.e. procedures
that take into account the dynamic
nature of the seismic loads and the
cyclic soil behaviour [1]. These latter
methods can either uncouple the anal-
ysis of free-field soil response from
that of the tunnel (‘simplified dynamic
analysis’), or use more complex
procedures (‘full dynamic analysis’)
accounting for soil-structure interac-
tion, which is basically kinematic.
The calibration of all such methods
should require validation against
experimental data, which are seldom
available at the prototype scale. In
fact, experimental measurements of
seismic internal forces on real-scale
structures during earthquakes are very
difficult, firstly because of the random
occurrence of such events. Moreover,
the instrumentation routinely adopted
for static tunnel monitoring gener-
ally has too large sampling intervals
to record seismic time histories. It
measures therefore only the internal
forces before and after the earthquake,
which cannot completely describe the
transient nature of dynamic soil-tunnel
kinematic interaction mechanisms.
On the other hand, the evolution with
time and the peak
increments of
internal forces
in the lining
appear crucial
for the engineer-
ing assessment
of the seismic
performance
of a tunnel. To
such a purpose, finite element or finite
difference methods of analysis can be
the most useful and reliable predic-
tive tools. If suitable constitutive laws
for the soils are well-calibrated on
laboratory and field tests and the most
appropriate geometrical and physical
description of the boundary problem
is performed, FEM and FDM analyses
can provide a reliable evaluation of
both free-field (e.g. [2]) and soil-tun-
nel (e.g. [3]) dynamic response.
Centrifuge modelling is definitely an
alternative powerful tool to produce
‘artificial case histories’ for calibra-
tion, back-analysis or benchmarking
among different analytical approaches.
This paper outlines the main features
of RRTT, a Round Robin numerical
Test on Tunnel centrifuge models,
organized by the Authors and jointly
promoted by three Technical Commit-
tees of ISSMGE.
Reference centrifuge tests
A typical damage pattern due to the
longitudinal and transversal com-
ponents of ground motion is that of
Figure 1. Ovalization during shaking and increments of
internal forces.
Figure 2. Instrumented model tube. Positions of the strain
gauges.