Environmental Engineering Reference
In-Depth Information
a very high volume of earthwork. Given the envi-
ronmental uniqueness of the island, which was
declared a Biosphere Reserve in 2000, it is impor-
tant that the balance of earthwork be adjusted as
much as possible, so it was decided to carry out a
partial pre-loading using the volume of excavated
material. The planned pre-loading consisted in
moving some 100,000 m
3
of earth inside “la cal-
dera” in different stages, to eventually cover the
whole of the surface of the reservoir. This solution
tends to take very long to execute, as it requires
waiting for the settlement to take place before the
preloading embankment is moved.
By placing the embankment prior to the fill-
ing of the reservoir, the filling will correspond to
a reloading process. The reduction of settlement
obtained with this pre-loading compared to the esti-
mated settlement if no treatment was undertaken
corresponds to the quotient between the reloading
deformation modulus with respect to that of the
virgin compression, but affected by a coefficient
α
, which corresponds to the relationship between
the increase in stress at depth and the load that
is spread at the surface. This coefficient enables a
comparison to be made between the effect of the
complete preload (
α
= 1) and the partial preload.
The average value of α in the 30 m of compressible
material worked out to 0.98. Therefore, this partial
preload would achieve a similar effect to that of
the complete preload.
Before analysing the trial embankment there was
no information regarding the possible moduli for
the unloading or reloading of the ground. in any
case, it was supposed that the relationship between
the deformation modulus for the virgin compres-
sion line and that corresponding to recompression
would be in the order of 4-5.
it was thus considered that the settlements
would roughly be a quarter to a fifth of those that
would result if there were no treatment, and that
the total expected settlement would therefore be
about 6-8 cm, which would be admissible.
The trial embankment is the best means for
obtaining the ratio between the deformation mod-
ulus for virgin compression and that correspond-
ing to recompression, with the latter tending to
equate that of the unloading stage. at the same
time, it provides the velocity of the response from
the foundation material and enables the waiting
times during the preloading to be adjusted.
a settlement of 56 cm was estimated for the
trial embankment, which was located in the most
unfavourable area (overlying a deformable ground
40 m thick), considering that the upper five meters
5.1
Results of the preloading test
The graphic in Figure 6 shows the average settle-
ments measured on the settlement plates during
the loading stage, the stabilisation of the settle-
ment (waiting period) and during the unloading
stage.
The average settlement during the tests was
between 40% and 60% greater than that estimated
at the design stage, with a deferred settlement aver-
aging 22 cm.
The ratio between the settlement of the founda-
tion and the height of the embankment has been
close to 0.1, a very high value, with a settlement of
99 cm being recorded in the centre plate (n° 5) and
an average value of 87.5 cm.
These results could be due to two factors:
- a deformability of the foundation material
larger than expected,
- or the material considered to be a “rigid layer”
being actually deformable.
There was scarcely any rebound (between 4 and
8 cm) when the unloading was carried out, which
indicates a permanent deformation of the ground,
thus confirming that the type of treatment chosen
for the foundation is appropriate.
5
eVolUTion oF The TRial
eMBankMenT
in order to complement the studies relating to the
deformability of the upper layers of the ground,
the project included the construction of a trial
embankment in the southern sector of “la cal-
dera”, where the greatest thickness of loose mate-
rials had been estimated (location and dimensions
nine settlement plates to measure the response
of the foundation to load variations and varia-
tions in the time taken for the foundations to
consolidate.
Figure 5.
location and definition of the trial embankment.
