Environmental Engineering Reference
In-Depth Information
Table 4. average geometry and mechanical variables of volcanoes in general (31, 37, 49) and the case of the
arenal volcano.
Value
arenal
Variable
Definition
Volcanoes
h (m)
height of the volcano
0.6-12 × 10
3
1.1 × 10
3
R (m)
Radius of volcanic cone
4-600 × 10
3
4 × 10
3
D (m)
Thickness of brittle substratum (Foundation)
0-40 × 10
3
1-2 × 10
2
i (m)
Thickness of the weak substratum (Foundation)
20-125
0.5-100 × 10
3
ρv (kg m
-3
)
Density of volcanic cone
2.5-2.8 × 10
3
2.8 × 10
3
ρs (kg m
-3
)
Density of substratum
2.0-2.5 × 10
3
2.65 × 10
3
T (s)
Time of span for deformation
5 × 10
9
-1.5 × 10
12
2.2 × 10
11
Viscosity of the weak substratum
10
17
-10
22
10
19
µ (Pa ⋅s)
Ø
angle of internal friction
15°-40°
33°
slope angle
2°-35°
30°-34°
α
Gravity acceleration
9.8
9.81
G (ms
-2
)
c (Pa)
cohesion
10
4
-10
7
10
5
Unit density (volumetric weight)
19-24
24
γ (knm
-3
)
the old cone D, even though, the asymmetry in
the pyroclastic thicknesses in the western part of the
cone might also be a conditioning factor for the
dispersion towards the arenal lake reservoir.
The arenal Volcano is in a construction stage,
of at least 7000 years, and it has formed a new cone
(c) since 1974, that has increased at least 270 m
from 1460 m to approximately 1730 m.a.s.l. also,
between crater a (the inferior one formed in 1968)
and crater c, there have been erupted 0.6 km
3
of lava
since 1968 to 2009, that are deforming the ground
due to its weight. on the other hand, the volcano is
basically constitute of a huge lava flow body, which
was build on a basement made of volcanic rocks,
the oldest ones deeply fractured lavas, old pyroclas-
tic rocks with a paleo-surface of laterites.
Based on the model of lagmay et al. (27) and
the cases quoted on that paper, we could hypoth-
esize that the occidental flank of the volcano (new
cone c) is the most prone to the formation of a
partial collapse through the formation of a debris
avalanche or rockslide. however, if we assume
the sigmoid model, with the thrust fault (occi-
dental pseudo-border of the caldera), where the
faults nnW have a dextral component and the
Danta fault is dip towards the east and of normal
type (2, 5, 6), then, we have that the volcano is
unstable to the formation of a sectorial collapse
leaded ne.
also, the surrounding parts of the craters of
c and D cones posses fumarolic manifestations
that chemically affect the rocks, combined with its
high grade of fracture (also see (15)), particularly
that unchain debris avalanches generally occur in
dilated sectors rich in fluids (52).
-limit equilibrium analysis
Figure 5. Π graphic that shows the limits between the
stability domains.
develops around the spreading volcano: folding
and thrusting. For Π
3
<< 1, a fold belt develops
The values of Π
3
, Π
4
and Π
a
suggest that the
arenal Volcano is in an incipient and critical stage
of spreading due to dispersion (material displace-
ment on its base) and cone instability based on the
results of Π
1
and Π
2
. however, obvious evidences
that support dispersion are not shown; unless
the dry tilt meters indicate the phenomenon in a
very slow way (for example. inclinometer e). We
might think that the dispersion tends to occur in
a direction towards the regional slope, for exam-
ple towards la Fortuna, combined with the fact
that the new cone c generates extra weight on
