3326
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
project aimed at thoroughly analysing the stress-strain-time
response of the predominantly silty sediments forming the
Venetian lagoon subsoil.
The estimated settlements are compared with vertical
displacement measurements provided by a very accurate
monitoring system, based on an advanced technique known as
Persistent Scatterer Interferometry.
The verification of empirical relationships using
experimental data obtained from a different site of the Venetian
lagoon is likely to constitute an important contribution to the
practice of geotechnical engineering in this area.
2 THE VENETIAN LAGOON SUBSOIL
Over the last decades, the shallow Pleistocene sediments
underlying the Venetian lagoon have been thoroughly
investigated. First in the 1970s, in relation to the regional land
subsidence and then in the 1990s, following the extensive site
investigation programme related to the MOSE project.
From the large amount of data assembled over
approximately 40 years, it turned out that the Venetian subsoil
conditions consist of a complex assortment of interbedded
normally consolidated or slightly overconsolidated silts,
medium-fine silty sands and silty clays.
Despite grain size heterogeneity, research has shown that
these sediments have a common mineralogical composition and
that their mechanical behavior is mostly controlled by
intergranular friction. Furthermore, as a consequence of their
predominantly silty nature and high heterogeneity, undisturbed
soil sampling is rather difficult to achieve, hence geotechnical
characterization must essentially rely on in situ testing.
More recently, a new extensive research programme was
carried out at the Treporti Test Site (
TTS
), located in the
mainland beside Lido Inlet, with the aim of having a better
understanding on the mechanical response of these intermediate
sediments (Simonini 2004).
The valuable experience gained from the overall analysis of
the data collected at
TTS
, including a large number of piezocone
tests (CPTU) and subsoil strain measurements beneath a full-
scale test bank, showed significant limitations of the existing
approaches for the characterization of the predominantly silty
sediments of the Venetian lagoon, thus suggesting a critical
review of empirical and theoretical formulations with regard to
their applicability to such soils (Tonni and Gottardi 2011).
It was also observed that such intermediate soils are often
characterized by permeability values within the range in which
partial drainage phenomena are likely to occur during cone
penetration (Tonni and Gottardi 2010) and that the identification
of this effect is of fundamental importance for a proper
interpretation of CPTU measurements.
Furthermore, field observations showed that in these soils
the decay of excess pore pressures is in general rather rapid and
thus secondary compression plays an important role in the
whole deformation process. As a result, the proper evaluation of
the relevant parameters is crucial in settlement predictions.
3 EVALUATING SECONDARY COMPRESSION FROM
CPTU
Unlike clayey deposits, the estimate of secondary
compression behaviour of sandy and silty deposits is not
routinely taken into account in the classical settlement
calculation, although there is experimental evidence that time-
dependent behavior of granular soils is not negligible. At low
confining stresses the deformations are caused to rearrangement
over time due to sliding and rolling between sand particles,
whilst at high confining pressures the deformations are
associated to continuous fracturing and deformation of grains
(Augustesen
et al.
2004).
Figure 2. View of the Malamocco Inlet and location of the piezocone
tests and radar reflectors (Persistent Scatterer, PS).
Secondary consolidation is typically characterized by the
slope of the straight line portion of the vertical strain (
z
) –
logarithm of time (log
t
) curve obtained from oedometer tests,
giving the secondary compression index
C
α
:
t
C
z
Log
(1)
In recent studies (Bersan
et al.
2012, Tonni and Simonini
2012), empirical, site-specific correlations, obtained from
calibration on the
TTS
field data, have been proposed in order to
estimate the secondary compression coefficient from cone
resistance
q
t
. The approach is based on the experimental
evidence that, in Venetian soils, frictional response governs
both cone resistance and secondary compression, hence
empirical correlations between
C
and
q
t
are likely to be a
useful alternative on the classical laboratory tests for the
estimate of creep characteristics.
Log regression analyses performed on the available data
provided the following more significant relationships, both
expressed in terms of the dimensionless normalized cone
resistance
Q
tn
:
89.0
03.0
tn
Q
C
(2)
74.0
0
14.1
1
077 .0
v
tn
u
Q
C
(3)
Here, an iterative nonlinear stress normalization procedure
(Robertson 2009), accounting for the stress level and the soil
class effects, was applied to the corrected cone resistance
q
t
in
order to determine
Q
tn
.
It is worth mentioning that, according to the analyses based
on the
TTS
data, the regression including a dependence on the
stress-normalized excess pore pressure (Δ
u
/
σ'
v0
) apparently gave
a slightly better fit in comparison with eq. (2). Indeed, such
additional independent variable allows accounting in some way
for the different pore pressure response of soils in relation to the
partial drainage conditions around the advancing cone.
4 CASE STUDY APPLICATION
Accurate measurements of long-term displacements of the
coastal structures built along the Venetian coastline, next to the
three lagoon inlets, have provided an opportunity to evaluate the
predictive capability of the relationships described by eqs. (2)
and (3).
Indeed, movements of coastal defense structures have been
M4
M3
M2
M1
PS10
PS11
Piezocone Test
Radar Reflector
