1484
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 GEOTECHNICAL FIELD INVESTIGATIONS AND
GEOLOGICAL MODEL
The subsoil of the Po levees is an alluvial succession originated
by the depositional activity of the Po river itself and, locally, of
his Apennine tributaries. This sequence consists of a cyclic
alternation of coarse soils, prevalently sands in its lower part
and silty, sandy and sometimes clayey soils in the upper part.
The deposits, of Holocene period (< 12,000 years), have a
typical thickness of about 10-15 m and lie on a layer of
Pleistocene channel sands, very extended laterally, generally
saturated, of thickness ranging from 20 to 50 m. Closer to the
embankment, these sands can also be found in the Holocene
part of the succession. The groundwater is always at few meters
depth from the ground level. The geological substratum of the
alluvial sequence typically consists of coastal and marine sands
of inferior-middle Pleistocene age. A simplified model of
depositional environment of a river system in an alluvial plain
area is shown in Figure 2. In order to obtain a homogeneous
spatial distribution of the geotechnical information, the survey
was initially planned according to a grid of test points
distributed at regular intervals (Martelli
et al.
, 2011). The
stratigraphic variations were defined through sets of in situ tests
aligned perpendicularly to the embankment, each set typically
consisting of one test in the floodplain area, one on top and one
at the toe of the embankment (downstream). The tests in the
floodplain area and at the toe, were located as close as possible
to the embankment. Additional investigations were carried out
subsequently, to improve the knowledge in some particular
areas.
1: riverbank
2: river channel
3: alluvial fan
4: floodplain
The following field investigations were carried out within
the project:
- 70 continuous coring boreholes, between 30 and 50 m deep.
24 of these boreholes were equipped by piezometers up to 50 m
for the monitoring of the first confined aquifer; 37 (of which 1
to a depth of 70 m) were equipped for down-hole testing;
- 25 open holes, generally up to about 10 m depth at the toe of
the embankment, downstream, equipped with piezometers for
groundwater monitoring;
- 4 pairs (1 to a depth of about 75 m on top and 3 to a depth of
about 150 m at the toe of the embankment) and 1 triplet of
borings (to a depth of about 75 m on top of the embankment),
equipped for cross hole testing;
- 298 piezocone tests, to a depth of about 35 m;
- 20 seismic cone penetration tests, to a depth of about 30 m;
- 26 down-hole on top and 11 down-hole tests at the toe of the
embankment, together with 4 cross-hole tests (one with three
holes);
- 3 seismic refraction profiles by MASW and ReMi tests;
- 10 electrical resistivity tests across the embankment;
-
about 400 single station recordings of environmental
vibrations, half on top and half at the toe of the embankment.
A total of 107 sections (99 cross-sections and 8 longitudinal
to the embankment), intercepting when possible, the available
tests, were selected and drawn at a scale of 1:2,000 with 5 times
vertical exaggeration (Martelli
et al.
, 2011). The main
lithological units corresponding to sedimentary facies and
depositional environments are represented in the sections up to
a depth of about 50 m. A typical cross section of the
embankment with the relevant geological model is shown in
Figure 3. The stratigraphy of the embankment-subsoil system in
the studied area can be briefly described as follows. The
embankment (Unit Ar*) consists of landfill organized in
alternating layers, various thickness, of different soils including
sands, silty sands, sandy silts and clayey silts, with sporadic
presence of brick fragments. Strong similarity between the
material composing the embankment and the underlying natural
levee soils were evidenced from continuous core drilling
boreholes; therefore, the location of the stratigraphic boundary
is often uncertain in the absence of specific elements (brick
fragments). The subsoil of the embankment frequently consists
of a layer of natural levee environment characterized by sandy
silts alternating to fine and very fine silty sands including
centimetric or decimetric more sandy and clayey-silty levels
(Unit B). In other cases the subsoil consists of clayey and silty
deposits of floodplain environment (Unit C), with centimetric
and decimetric levels of peat and blackish frustules of organic
material or fine to very fine silty sands and sandy silts. River
side, the accumulation of floodplain deposits (Unit D) was
favoured by the presence of the embankments. Two main facies
can be identified in this Unit: a mainly fine (D1) and a sandy
facies (Unit D2). The sequence continues downward with
prevailing sands attributable to fluvial channel environment
(Unit A). The grain size distribution of such sandy deposits,
generally of 20-30 m thickness or greater, vary from medium
fine to coarse and very coarse, with local presence of gravel,
their thickness being. An important aquifer, generally confined,
is there located; however, near the levees and the floodplain
area, the two aquifers (phreatic and confined) sometimes merge.
Figure 2. Depositional environment of a river system in an alluvial
plain.
3 IDENTIFICATION OF SIGNIFICANT SECTIONS
A specific methodology was developed in order to select a
number of significant sections for the stability analyses. Five
geographical “macro-areas” were first identified in the 90 km of
embankments investigated. The relevant significant sections for
the subsequent stability analyses were then selected following a
criterion of representativeness and uniform distribution along
the river, hence not only the most critical. The embankment
features considered to identify the typological groups are:
Figure 3. Typical geological cross-section on the Po river embankment
(Martelli
et al
., 2011).
U105SR
q. 14.05
0
0
-5
-10
25
20
15
10
-15
-20
5
U106SR
q. 19.65
U107SR
q. 14.37
0
5
15
25
30
35
20
10
0
10 20 30
Qt (MPa)
0
5
15
25
30
35
20
10
10 20
10 20
0 10 20 30
Qt (MPa)
5
15
25
30
35
20
10
Qc (MPa)
B
B
C
A
Ar*
A
B
A
B
D1
F.Po
