Environmental Engineering Reference
In-Depth Information
- The initial settlements in the case of a lava low
of 20 m of thickness, may vary in a range of
<0,3 cm to >3.92 cm. Geology and geophysics
suggest that the total settlements can reach 19
m, once the effect of subsidence caused by the
actual lavas is finished. in a larger scale, the set-
tlement generated by the growth of the arenal
volcano during the last 7000 years, can be calcu-
lated at a height of 45 m.
- a lava low would generate consolidation on
a pyroclastic deposit of 10 m of thickness in
10 years. if we extrapolate this for the entire
historic lava field of the arenal Volcano on
a package of consolidatable tephras of 75 m
(thick), we will require between 132 (50% of
consolidation) and 568 years (90% of consolida-
tion), respectively.
- The deformations shown in the dry inclinom-
eters can be observed 5 km away from the flow
axis.
- The results and analysis of mechanic of rocks
and soils indicate that the arenal cone and its
foundation have a poor to medium mechanical
quality. Therefore, to analyze the cone and its
foundation stability, we applied several meth-
odologies. if we consider the one provided by
the Buckingham Π Theorem, we have that the
arenal volcano is in an incipient stage of defor-
mation by dispersion of its base and a clear cone
instability, in continuous growth. in general,
all these loads applied during a long time may
be producing continuous deformations known
as creep. The lateral deformations during con-
solidation represent around 15% of the vertical
ones. Both processes (creep and vertical defor-
mation) involve the cone spreading.
- When we use the siGMa/W Program, plas-
ticized sector (that is to say areas that already
failed or moved their maximum resistance) in
the flanks, volcano core and with a less impact
on its foundation. however, since the cone is
mainly made by loose fractured or fragile mate-
rials, it is not easy to see incipient sliding mate-
rials. however, it is interesting to see that the
superior-medium slope of the volcano (cone D)
is highly failed.
- We conclude that the new cone c of the arenal
volcano is prone to the formation of sectorial col-
lapses with debris avalanches towards the occi-
dent, while the old cone D, given its high grade
of fragmentation and the overweight provided
by the new cone c could be prone to the forma-
tion of rockslides towards ne, an area consider
with relatively low hazard until now. as a matter
of fact, the occidental flank of the arenal shows
an unexpected situation, since the global stabil-
ity of the massif (foundation + cone) is lower
than the stability of the cone, even yet, the factor
of safety for this one is the lowest one (0.6) for
a pseudo-static condition, what indicates that in
case of a telluric event with enough magnitude
to induct important accelerations in the region,
there is a risk of big slides (0.03-0.75 km
3
)
towards the occident. a debris avalanche of this
type may reach distances between 5 and 10 km
from the arenal summit.
- on a lower scale, the stability estimations on cone
c and its old lava pool showed that the width of
the crater wall, the height of the lava lake and the
continuous explosions are the main factors that
control its stability. nowadays, cone c (attached
lava domes) is taller than its predecessor, cone D.
Both domes and lava pools, given their instabil-
ity, when they collapse, produce pyroclastic flu-
ids. The continuous growth of the c cone over
the D cone is increasing the overweight on its
predecessor, as well as the possibility of pyro-
clastic flows and rockslides towards an area that
not many years ago was consider relatively pro-
tected, that is to say the oriental flank, right were
the commercial and housing activity increases
every day.
acknoWleDGeMenTs
To Mario arias, Benjamín van Wyk de Wries, Julio
Macías, carolina sigarán and Francisco cervantes
that contributed with valuable comments during
the final study phase. The
Laboratorio Geotécnico
del ICE
and the
Laboratorio Nacional de Materiales
y Modelos Estructurales
(lanaMMe) from the
University of costa Rica, helped with laboratory
tests. The translation of this text from the spanish
version was done by Melisa Quirós echeverría.
ReFeRences
1. alvarado, G.e., (2002), “análisis de la estabilidad del
cono y comportamiento de la Fundación del Volcán
arenal (costa Rica) mediante el uso de parámetros
adimensionales de acuerdo con el teorema Π de
Buchigam”,
VIII Seminario Nacional de Geotecnia,
3er Encuentro Centroamericano de Geotecnistas:
“Geotecnia en la Prevención de Desastres en el
Entorno”. San José
: 267-275.
2. alvarado, G.e., (2003), “Diagnóstico de la estabi-
lidad del cono y comportamiento de la fundación
debido al crecimiento del edificio volcánico del
arenal (costa Rica)”, -xv + 138 pp. Univ. costa
Rica,
M.Sc Thesis.
3. alvarado, G.e. & soto, G.J., (2002), “Pyroclastic
flow generated by crater-wall collapse and outpour-
ing of the lava pool of arenal Volcano, costa Rica”,
Bull. Volcanol
., 63: 557-568.
