Environmental Engineering Reference
In-Depth Information
obtained during the first loading (or initial modu-
lus), with a rheological factor of 1, a factor value
that is commonly used for peat, another material
with a high porosity.
6
conclUsions
The elastic behaviour of materials resulting from
the alteration of pyroclasts can be established as
follows:
- When subjected to stresses that are less than
the yield stress, the soil skeleton has an elastic
behaviour,
- When said stress is exceeded, the particles break
up and the soil undergoes excessive creep defor-
mation due to the high intragranular porosity of
these materials.
The initial settlement was instrumental in car-
rying out an acceptable calibration of instanta-
neous settlement, which has been shown to be
20% greater than expected. however, the creep
value could not be foreseen under these load
levels.
The post-construction settlement value that had
been supposed in the initial analysis (10% of the
instantaneous settlement) establishes an upper
bound for granular materials and was therefore a
conservative value. however, the deferred settle-
ment due to creep has turned out to be in the order
of 33% of the instantaneous settlement, far greater
than that initially supposed.
in addition, it has been seen that the instan-
taneous settlements adjust fairly well to those
calculated through the elastic method, using the
pressuremeter moduli obtained from the first
loading. a rheological factor of 1 has been used
in order to obtain the deformation modulus from
the pressuremeter modulus. This is the value that
is recommended for peat, another material that
is highly compressible and which also presents a
structure with a high porosity.
Ground heave has scarcely been observed in the
unloading stage: rebound oscillated between 4 and
8 cm, which indicate a permanent deformation
of the ground. The carrying out of preloading is
therefore considered to be an appropriate treat-
ment for this type of ground.
Figure 8.
average creep during the waiting stage.
different plates and in average values—has been
made of settlement corresponding to secondary
consolidation due to material creep, which showed
that the deferred settlement is the equivalent of
33% of the instantaneous settlement. This value far
exceeds the 10% settlement that had been assumed
in a first calculation and which constitutes an upper
limit for granular materials.
Deferred settlement tends to be expressed as a
law such as the following:
(1)
s
H
t
t
o
=
α
log
The phenomenon of creep has been studied
during the “waiting stage”, establishing (t
0
) as
day 1, being the day following the completion of
the embankment.
The value
s
corresponds to the increase in settle-
ment that occurs when time
t
is increased tenfold,
and an average value of 18 cm has been obtained.
as the settlement value (
s
) varies according to the
compressible thickness (h), to make it possible to
compare different locations, a post-constructive
index (
α
) is employed, which is the ratio between
s
and
H
(soriano, 1999).
s
180
40
mm
m
α
=
=
=
45
.
o
oo
t
t
o
(2)
H
⋅
log
The index of post-construction settlement
obtained for the embankment support (as can be
seen in equation 2) is much larger than that which
was supposed at the design stage, 1.25‰.
Finally, the creep values measured during the test
have been used to compare the settlement estimated
in the design calculations with those obtained after
moduli used to adjust the calculation of the instan-
taneous settlement during the construction of the
trial embankment were the pressiometric moduli
acknoWleDGeMenTs
This work constitutes a part of the geotechni-
cal conclusions extracted from the investiga-
tion, analysis and calculations carried out for
the “estudio Geologico-Geotécnico del Depósito
superior”, presented by idom internacional in
