80
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
fault throw
Figure 21. The main fault surface about 400 m N-W of the Cathedral
It lies on a Pliocene formation of strongly overconsolidated
marly clays whose thickness is about 100 m at the centre of
Pienza and increases to over 800 m southwards (Brogi et al.
2005). The substrate of the Pliocene sediments consists of
Mesozoic, carbonate-siliceous formations of the “Tuscan
Series”. The discontinuities that border the scarp, already
identified in 1956, are a set of locally vicarious faults having a
WNW-ESE direction and southward dip. They are crossed by
minor, approximately perpendicular discontinuities.
The geotechnical investigations showed that both the
sandstones and the underlying marly clays have high strength
and negligible compressibility.
In 1983 a periodical levelling was started by installing many
benchmarks, uphill and downhill from the scarp and from the
set of faults (Fig. 22). The measurements, repeated every year
until 1992 (Guidi 1986) then at various intervals between 1994
and 1999 and resumed recently, show that the whole area
covered by the bench-marks downhill from the scarp has a
constant non uniform settlement of between 1 and 2 mm per
year (Figs. 23, 24). Minor effects of this phenomenon are
visible in other buildings in the same area (Costantini and
Lazzarotto 2010). The lack of uniformity of the settlement rate
shows that the Pliocene marly clay is split by the sets of
discontinuities; the main vicarious fault is the main, but not the
only source, of the soil displacement downhill from the scarp.
Figure 22. Ground settlement contours from June 1983 to January 1992.
Horizontal displacements are null or non measurable. The
steady and extremely small rate of the movement, detectable
only by a high precision levelling over a long term campaign
explains why the phenomenon was never detected in the past.
Then it was finally stated that the apse settlement is not due
to the deformation of the foundation soil but to the constant
lowering of the area downhill from the set of faults.
a)
!20$
!18$
!16$
!14$
!12$
!10$
!8$
!6$
!4$
!2$
0$
1983$
1984$
1985$
1986$
1987$
1988$
1989$
1990$
1991$
1992$
1993$
1994$
1995$
1996$
1997$
1998$
1999$
2000$
2001$
2002$
2003$
2004$
2005$
2006$
2007$
2008$
2009$
2010$
2011$
abbassamen' (mm(
CS$23$
CS$24$
b)
Figure 23. Settlements of two significant points close to the Cathedral
apse: a) plan view; b) settlements
vs.
time
0.0 ‐ 0.4 mm/a
0.4 ‐ 0.8 mm/a
0.8 ‐ 1.1 mm/a
1.1 ‐ 1.5 mm/a
> 1.5 mm/a
Figure 24. Settlement rate contours in the area south of the Cathedral.
At the present time the masonry block constituting the
underpinning of the apse built at the beginning of last century,
whose internal edge lies uphill from a fault plane, while the
external part is downhill, has a rigid downhill rotation which
involves the overlying apse. Since the existence of an active
fault should be ruled out, the only hypothesis that would
account for the continuous settlement is a deep seated
gravitational slope deformation within the marly clay formation,
influenced by the shape of its bed and by the discontinuity
surfaces (Genevois and Tecca, 1984, Calabresi 1992, Calabresi
et al. 1995, Calabresi et al. 1988, Sciotti and Calabresi 2004).
A recent seismic investigation along a longitudinal section
measuring more than 1000 metres has highlighted a significant
anomaly in the P-wave velocity contours under the Cathedral
apse and a depression in the bed of the Pliocene deposits in the
zone where the surface movements are largest, thus confirming
that the faults detected at the surface involve also the underlying
