Environmental Engineering Reference
In-Depth Information
We assumed that the fault might be a:
a) Global fault of the massif or b) cone fault.
The first one has a deep fault surface and can go
through the volcano's foot or deeper. The second
case has a superficial fault surface and it does not
regularly get to the volcanic cone's foot.
The analysis was made for three conditions:
- static condition: Models the normal stage of the
massif.
- static condition with an overload: Models the
situation that can be presented with a copious
lava eruption and with the presence of an over-
load due to the material accumulation on the
massif. For this case, a uniformly distributed
load of 900 kPa was used (60 m of lava). it was
modeled for the occidental flanks were the lava
effusions are frequent, but also for the oriental
flank to see how critical the situation is if the
cone c erupts towards this section.
- Pseudo static condition: Models the situation
in case of an earthquake. according to regional
probabilistic estimations, there was obtained,
that for a return period of 500 years, the val-
ues of peak acceleration, where the volcano is
located, are around 0.34 g and the intensities
are MM Vii to Viii (a. climent, com. verbal,
2003). The seismic forces are supposed to be
pseudo static and proportional to the sliding
mass weight, and the coefficients kh and kv that
represent the acceleration produced by the seis-
mic movement. since it is a highly seismic zone
and the materials to be analyzed are similar to an
earth dam, we used, a horizontal acceleration of
kh = 0.15 g and a vertical acceleration of kv = 0 g
(25, 38, 55).
in both profiles, the regional water table is
shown. The four geotechnical levels shown in
Table 6 are used. We also did tests with different
friction angles and cohesion values, recording
variations in the factor of safety, which express
the need of a better geomechanical calibration to
refine the stability of these slopes that genetically
unstable.
- stability for the occidental lank: The results are
shown on Fig. 3a and Table 7. The critical fault
surfaces match for all the load states (static, with
an overload and pseudo static), for the global
stability analysis of the massif as well as for the
cone stability. The cone stability analysis presents
safety factor slightly higher than the global anal-
ysis, close to 1. The most critical case is in the
pseudo static stage, where the factor of safety
is around 0.7. The talus global stability presents
the lowest factors of safety. For the static case,
the safety factor for the global stability is close
to 1.1, this is why it might be considered that this
fault surface is closed to the critical stage. in case
Table 6.
Geotechnical model used for the stability
analysis.
Dry
volumetric
weight
(kn/m
3
)
humid
volumetric
weight
(kn/ m
3
)
angle of
friction
Ø
cohesion
(kPa)
soil
i
15
20
0
40-44°
ia
20
25
0
33°
ii-iii
14
18
90-92
12°
iV
17
22
200-400
44-45°
Table 7.
Results of the stability analysis for both profiles with an overload of 900 kPa.
stability
condition
seismic
coefficient
overload
(kPa)
Minimum
safety factor
Volume
(km
3
)
Flank
occidental
Global
-
1.09
0.05-0.1
-
900
1.06
0.05-0.1
0.15
0.64
0.05-0.1
cone
-
1.18
0.03-0.07
-
900
1.14
0.03-0.07
0.15
0.77
0.03-0.07
oriental
Global
-
2.16
0.04-0.1
900
2.03
0.01-0.03
0.15
1.28
0.03-0.07
cone
-
1.51
3.5 × 10
-3
-
900
1.11
1.1 × 10
-3
0.15
1.5
3.5 × 10
-3