Environmental Engineering Reference
In-Depth Information
2 m as an acceptable value when an e = 150 su
is assumed, by recommendation of stamatopou-
los & kotzias (41). if we use values of e provided
between 11 and 100 cm. an average value of 1.3 m
may be taken as a representative value for this case
of analysis.
The vertical stress at the foundation in arenal
under the vortex of a conic load under the volcano
summit, at the same depth of 2 km and using the
relations provided by Poulos & Davis (36), is σ = 13
750 kPa. The increase of stresses is still significant
(influence zone; (7)) at a depth of 24 km (about the
crust/upper mantle limit).
5.2.2
Immediate-primary compression using the
compressibility index
When a lava flow runs on a granular soil (thick
ashes to lapilli), the excess in pore pressure (if there
is any) dissipates in a short time and the consolida-
tion would be fast. in low permeability silt and clay
soils, the water flow would be very slow and the
dissipation process will take a considerable amount
of time. in these cases, we may need to predict:
a) The total settlement of the lava flow or lava field
and b) The velocity at which it occurs.
The settlement calculation under the actual
lava field (once the process ends) would be around
7.8-15 m using the compressibility index (cc =
0.6-0.67) according to the methodology explained
by stamatopoulos & kotzias ((41), p. 124-125).
in a larger scale, how the settlement generated
by the growth of the volcanic edifice in the last
7000 years is, using a simplified model shown in
Fig. 7. The final result is a total settlement of 20.0
to 29.7 m.
Through the consolidation coefficient (cv), we
may obtain the time that takes a settlement on the
field, given a layer with a drainage longitude hf.
a lava flow would generate a consolidating effect
on the pyroclastic package of 10 m just in 10 years,
lapse necessary to finish the theory of consolida-
tion due to the effects of overweight. since the
tephra or pyroclastics thickness in the west flank
of the volcano has been estimated as 150 m, from
which 50% can be assumed as thick non cohesive-
granular tephras. if we analyze the whole lava field
of the arenal Volcano (1968-2009), for a 50% and
90% consolidation of a tephra package of 75 m of
thickness and whose drainage conditions are not
very clear, it takes 132 and 568 years, respectively.
From another point of view, the actual lava field,
in the last 41 years, has only reached approximately
18% or more of the consolidation process, so we
can expect the detection of more deformations
by the vigilance inclinometers for one or many
centuries.
Figure 7. shear stress by the load of the arenal, in the
triangular and conic section, obtained from (41). actu-
ally, the compressible tephras, volcanic sands and tephras
are linked.
5.2
Settlements based on the consolidation tests
and the superimposed lava loads
5.2.1
Initial settlements
When there are loads applied on the field, it is sub-
jected to an increase of stresses that produces an
instant deformation and a long term deformation,
with a decrease in the void ratio due to the water
flow towards the exterior of the soil (consolida-
tion). if the instant initial settlement wants to be
analyzed in the case of a lava flow, the theory of
elasticity provides a convenient formula (41):
∂=
00,
ρH
E
(3)
where
ρ
is the average pressure in the soil at the sur-
face, and it is found multiplying the total weight of
the lava flow (25.2 kn/m
3
) by its thickness (20 m);
h is the thickness of the compressible unit (arbi-
trarily taken as 10 m in compressible soils) and
e is the Young modulus (for pyroclastic rocks),
value derived from geophysics (between 0.36 and
3.7 × 10
6
kn/m
2
) or e can also be estimated as 1000
su to 100 su (that is to say between 13 500 and
90 000 kn/m
2
), where su = c (in our case 90 kn/m
2
)
in a UU test (see 41). The initial instant settlements
may vary from 1 m to 4 cm, given that a lot of
values of modulus are considered. The total cone
settlements were not analyzed, since its growth has
taken several millions of years.
Geology and geophysics suggest that en some
areas, volcanic soils can reach thicknesses of 150 m,
particularly under erupted lava between 1968 an
1973. Thereby, the initial estimated settlement
may vary between 30 cm and 4.1 m, considering
