Environmental Engineering Reference
In-Depth Information
To investigate the geotechnical properties of the soils, a borehole about 300 m
deep was drilled and continuously sampled. In this way the soil profile has been
obtained together with the reconsolidation stress, oedometric modulus, one-
dimensional consolidation coefficient and temperature as function of depth.
5.1.5.2.
The mathematical model
The vertically averaged mathematical model, referred to in section 5.1.3, has
been employed for the analysis of the alluvial cover, where the subsidence takes
place. This technique allows the problem to be studied without the use of expensive
three-dimensional models or oversimplified two-dimensional solutions, e.g. plane
stress, plane strain or axisymmetric ones. In this case there is no symmetry in the
geological setting. Measurements performed over a time span of more than 30 years
indicate an almost constant temperature distribution value in the reservoir, so a fully
saturated isothermal model was chosen that employs the balance equations
presented in equation [5.5].
The lateral extension of the numerical model was assumed from the presence of
vertical fractures. The aquifer has a variable thickness (see Figure 5.18) and presents
an irregular distribution of the wells. Furthermore, each well presents a peculiar
exploitation history.
The unknowns of the numerical model are: the mean values over the thickness of
the horizontal displacements and pore pressure, together with the value of
compaction. The numerical model comprised 432 parabolic finite elements in a
horizontal plane, involving 1,381 nodal points and 5,524 degrees of freedom. The
following mechanical data were used (average values for the alluvial cover, which
comprised aquitards and aquifers):
Young's modulus of the skeleton:
2,200 MPa
Poisson's ratio:
0.4
10
-9
m s
-1
Permeability coefficients:
Porosity:
0.5
The reservoir is exploited through 63 boreholes that may be grouped in clusters.
The pumping rates range from 150 to 600 dm
3
min
-1
. The variable aquifer thickness
is shown in Figure 5.18 along with the irregular distribution of well clusters. The
pumping rate has been assumed to be constant throughout the year and a trial-and-
error procedure was used to determine the amount of water drawn from the cover as
compared to total water drawn from the system.
