Environmental Engineering Reference
In-Depth Information
saturated near to the volcano and has an aquifer
at its foot.
The foundation is well defined by the geotechni-
cal levels iii and iV. The upper section is consti-
tuted by old tuffs (>3000 years old), with a higher
grade of alteration, along with the presence later-
itic soils that make up layer iii. The water table is
usually found in some level of this unit, and given
its relatively reduced thickness of 10-15 m, it rises
as a takeoff surface for the generation of land-
slides. The layer iii overlies the rocks with a high
lateral heterogeneity as well as vertical (layer iV),
compose of rocks with a moderate permeability
and with a significant and variable grade of frac-
turation in the lava flows, mesobreccias and tuffs.
Table 3 summarizes some of these characteristics.
From the practical point of view, layers i and iV
differ only in their age, degree of compaction and
tectonism, while layers ii and iii are very simi-
lar (soils), increasing their degree of compaction
and fines with their age, and for the analysis, lay-
ers ii and iii are taken as a single layer. The angle
Φ ≠ 0, was modeled as a small value, according to
the results from the tests and to be on the side of
safety. all of this information provides the base to
develop the analysis of stability of the huge natu-
ral edifice of arenal through different models, for
example, instability of the crater c, of the cone or
of the entire volcano and its foundation, or apply-
ing methods, for example: Buckingham, geotechni-
cal, etc. The mechanical properties of the materials
reported on Table 3 for the different geotechnical
layers, are according to the ones reported for other
volcanoes (14, 17, 51, 52, 56).
Table 1. synthesis of the geometrical parameters of the
groups of soils, mainly from the foundation (the values
between parenthesis are average values).
sUcs classification
Ml
liquid limit
25-88 (65.5)
Plasticity index
24-52 (41.6)
specific gravity Gs
2.43-2.88 (2.65)
humid density (kn/m
3
)
14.4-19.1 (16.0)
Void ratio e
0.82-2.6 (1.75)
Dry density (kn/m
3
)
9.6
saturation percentage
30.7-98.1 (68. 0)
Triaxial shear test cD/c', kPa
75-110
internal friction angle/Ø'
11.5-13 (10-42º)
Triaxial shear test UU/cu, kPa
80-190
Permeability (m/s)
10
-5
-10
-7
coefficient of consolidation (cv)
0.17-6 m
2
/año
coefficient of compressibility (cc)
0.2-0.67
Pre-consolidation load (P'c), kPa
260-4000
coefficient of recompression (cr)
0.03-0.076 (0.05)
Figure 3.a. Results of the stability analysis of the occi-
dental flank of the arenal Volcano. a: critical fault sur-
faces found for the talus global stability. B: critical fault
surfaces for the cone stability. The lowest factor of safety
is given for the global fault with pseudo-static loads.
Figure 3.b. Results of the stability analysis of the east
flank of the arenal volcano. a: critical fault surfaces
found for the talus global stability. B: critical fault sur-
faces for the cone stability. The lowest factor of safety is
given for the global fault with pseudo-static loads.
Table 2.
Basic rock parameters at the arenal Volcano.
Tufs
lavas and breccias
lithology condition
healthy
altered
healthy
altered or fractured
Unit weight
14-16.8
17.54-28.34
Gsss
2.33-2.71
G
s
2.22-2.64
G
b
2.39-2.72
absorption
0.08-48
Ø
10°-42°
10º-37º
33°-47°
33º-38º
V
p
0.5-0.9
0.3-0.6
2.1-3.9
1-2.5
Poissons ratio
0.33-0.42
0.33-0.40
0.26-0.42
0.26-0.37
Uniaxial compressive
strength (kPa)
8000-15 000
250-8000
40 000-300 000
10 000-250 000
Permeability (m/s)
10
-5
10
-6
-10
-7
10
-5
10
-5
-10
-6
