Environmental Engineering Reference
In-Depth Information
Table 3.
Geotechnical model for the case of the arenal volcano.
Mechanical properties
compress-
ibility
index
cc
cohesion kPa
Friction
static
deformation
modulus e
10
6
kPa
Unit
weight
γ kn/m
3
layers
Description
c
c'
φ
φ'
Poisson
i
Mainly lavas and
autobreccias
20-25
-
-
40-44º
-
0.28
2.5
-
ia
lava Talus, pyroclastic
deposits with alter-
nate lava flows
15-20
-
-
33-36º
-
-
-
-
iii
alternation of pyro-
clasts and recent
epiclasts
14
145
92
-
12º
(10-42º)
0.37
1.042
0.6
iii
old pyroclastic rocks
and laterites
14
90
75
10º
12º
0.33
2.238
0.67
iV
Rocks (lavas and old
pyroclastic rocks)
17
200-400 -
40-45º
38º-42º
0.33-0.42 13.91
-
4
sTaBiliTY analYses
1998, 1999, 2000 and 2001 (3). an analysis of this
instability problem can be treated with an approach
based on the mechanic of rocks.
4.1
Talus analysis
is the arenal Volcano susceptible to a creeping
process? To test this, a series of analysis that involve
from analytical solutions to the computer programs
siGMa/W and sTaBl 5M was established.
The seismic movements with M ≥ 4.0 (26) could
generate landslides in an active seismic zone like
arenal. The topographic effect of the cone of the
arenal volcano would amplify the seismic signals.
The intense degree of fracture in the lavas and
their auto-breccias, combine with the alternation
of tephra layers makes the rock massif behave as
a unique unit of soil with big blocks. Because of
this, the massif would probably fail through poorly
defined stratification surfaces a) Fault in crater c,
b) Faults in cones c or D and c) Fault in the cone
'
s
foundation, instead of failing through the joints.
The safety factor is affected by different condi-
tions with a wide variety of alternatives, depending
on the media and available time for investigations.
it is also particularly sensitive to judgment errors
like the anisotropy of the media, wrong selection
of the volumetric weight, calculation methods, etc.
also, the factor of safety does not reflect the prob-
ability of failure, because it does not involve any
information about the used values of stress and
resistance. in the present investigation, a safety
factor of 1.3 is used as critical.
- case of a fracture illed with a liquid (i.e. lava
pool): in this case, the rupture could be model
as a plane fault in a block with a pressure gen-
erated by the presence of a lava pool, which
behaves as a huge fracture in our case filled with
lava and due to geometrical issues, its shape is
similar to a square, instead of circular or tabu-
lar. since the fluid is lava, a non-newtonian and
very viscous liquid with an elevated temperature,
gas and crystal content, the pore pressure in the
base of the failing surface must be cero (with-
out water), but another pressure generated by
the gases will exist. some parameters have to be
taken based on studies made of similar deposits.
also, the slopes and weights are estimated based
on the indirect measures taken on field (slid rock
mass), based on observations made by alvarado
and soto (3). The total volume of slid material
during the eruption of august 28th, 1993 was
2.3 ± 0.8 × 10
6
m
3
, assuming a depth of the lava
pool of 80 m, for a γlv = 25 kn/m
3
(due to the
porosity between blocks). The equivalent volume
of dense rock or young non vesiculated magma
was 1.35 ± 0.4 × 10
6
m
3
( = Vlv).
For the instability analysis, an approach of sta-
bility of plane fault with crown fracture can be
used in hoek & Bray (21). The parameters were:
h = 150 m, Z = Zw = 80 m, Ψf = 36°, Ψp = 25°,
γlv = 25 kn/m
3
, c = 600 kPa (obtained through
rugosity) and Φ = 36°. seismic activity and sub-
pressures in the base of the slid block were not
4.1.1
Stability analysis of the cone C
The cone c with its active crater has partially
failed, generating a collapse of its faces combined
with the overflow of the lava pool, producing pyro-
clastic flows in 1975 and 1993; and small ones in
