Environmental Engineering Reference
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velocity that ends for a strain level around 0.1. This
behavior corresponds to the more concave upward
part of the stress vs strain curve. after this, the
P wave velocity increases at a much lower rate till
a plateau or slightly increasing trend part in the
P wave vs strain curve. The point where this plateau
begins is approximately coincident with the onset
of the rectilinear part of the stress vs strain curve.
in some cases a slight decreasing trend is observed
just before of the peak stress level.
5
conclUsions
in this paper, we have presented the initial results
regarding the physical mechanical behavior of
a lava affected by different degrees of alteration.
alteration induces strong changes in physical
properties and behavior of any rock material, and
this is particularly true for volcanic rocks subjected
to intense hydrothermal alteration.
all the lava specimens were characterized
by their total porosity (see companion paper).
The total porosity for the tuff specimens ranges
between approximately 6% and 32% according to
the Micro cT study.
The uniaxial compressive strength decreased
non-linearly with increasing total porosity with a
minimal to moderate data dispersion that is quite
reduced for high alteration grade samples.
Uniaxial compressive strength decreases by
almost an order of magnitude passing from fresh
to totally weathered conditions. Tensile strength
decreases of about 5 times to 1 order of magnitude.
in general, a strong decrease in properties is
observed passing from grade 2 to grade 3 (15% to
20% increase in total porosity) whereas a very small
increase in strength is sometimes observed when
moving to grade 5 (total alteration). in any case
this does not seem to be a systematic behavior.
if we compare both the tensile and compressive
strength with respect to the fractal dimensions com-
puted in the companion manuscript we observe an
increase with fractal dimension. again, this trend
is not visible in the most altered sample (sl5).
Furthermore, results suggest a notable decrease
in stiffness with the level of degradation. This could
be associated to a most diffuse argilization of the
matrix and the increasing frequency of medium to
large pores. The increase in large pores frequency
seems associated with the increasing alteration
grade till grade 5. For this last grade we observe
a decrease in large pores and a relative increase of
very small pores. as mentioned in the companion
paper, this trend is associated with the increasing
content in amorphous minerals resulting from
hydrothermal deposition.
Figure 4. Dependency of elastic modulus et
50
, uniaxial
compressive and tensile strength on total porosity as from
Micro cT image analysis. Plotted data are average (ave)
values. Data shows higher variation of modulus at third
grade of alteration (sl3 sample). Best fitting exponential
relationships are shown together with their coefficient of
determination (R
2
).
Figure 5. example of the relationship between recorded
P-wave velocity values at different strain levels and the
stress vs strain curve for one sample characterized by
alteration grade 5.
values for the uniaxial compressive strength, tensile
strength and et
50
are compared with the average
porosity value. The computed best fitting exponen-
tial curves are characterized by values of the coeffi-
cient of determination, R
2
, that are relatively high
and ranging between 0.85 and 0.90.
These relationships follow a general trend pre-
sented in the technical literature for other lithologies
at varying degree of weathering/degradation. The
most rapid rate of decrease is observed for the et
50
and the compressive strength values between values
of the total porosity ranging from 10 to 20%.
Finally, we show in Figure 5 the relationship
between the recorded P wave velocity and strain. as
we can see there is an initial step increase in P wave
