1558
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
It must be noted that the all available test results consistently
show that
D
r
> 45% throughout the dam foundation.
4.5
Progressive failure
The risk of progressive failure was estimated for
D
r
= 30% and
D
r
= 35%. It was assumed that the indicator of progressive
failure is the proportion of the sand layer yielding beyond the
phase transformation plateau (where
s
u
is defined) and reaching
the critical state (where
s
u,c
is defined).
Using this concept, if
s
u,c
is only reached at few points within
the sand layer, the soil mass is yet far from it's critical state and
the risk of progressive failure is nil, while if
s
u,c
is reached at a
significant proportion of the dam base, progressive failure
requires a detailed analysis. The ratio of the length at the critical
state
L
cr
compared to the total length of the foundation (440m)
is shown in Table 2.
Table 2. Factors of safety computed for various relative densities.
Relative density
30%
35%
L
cr
[m]
55
25
L
cr
/ length of base
14%
6%
The analysis could not be run for
D
r
= 40% and 45% because
the failure modes do not include sliding through the undrained
sand layer. The results shown in Table 2 demonstrate that the
risk of progressive failure is negligible and that a detailed
analysis is not necessary.
5 SUMMARY AND CONCLUSIONS
The case study of the port-earthquake behavior of Potrerillos
CFRD dam has been presented. The dam and the existence of
weak materials embedded in the alluvial foundation soils were
briefly described. Those weak materials were not encountered
during the design investigation phase but found during the
construction of the diaphragm wall and piezometers. Client
engineers feared that the dam would not be stable under
earthquake loading and ordered the construction of a
downstream buttress embankment to improve it's sliding
stability. The contractors obeyed but started a claim which
escalated to the Supreme Court of Justice in Mendoza.
Núñez was appointed as one of the expert witnesses by the
Court. He evaluated the sliding stability of the dam using simple
rigid-body procedures and demonstrated that the dam was stable
without the buttress, ending the judicial discussion. Numerical
computations performed after the incident proved that
progressive failure - a subject that could not be addressed using
simple assumptions - did not change the conclusions of the
analyses.
Three well known lessons were confirmed after this incident: i)
at design stage, the use of the best available geotechnical
investigation techniques is crucial for the success of large
projects; ii) where weak non-plastic materials are found, all
available efforts must be made to get a full, comprehensive and
reliable characterization of their mechanical properties; and iii)
simple yet sound procedures for analysis suffice to support an
expert opinion; more elaborate approaches can be employed to
enrich the knowledge, but do not substitute the need for a basic
comprehension of the problem.
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