Environmental Engineering Reference
In-Depth Information
at the top in the case of low confining pressures.
These loops are rare and characteristic of the fail-
ure of bonded soils (Maccarini, 1993), and dimin-
ish in the stress path for failures at higher confining
pressures until they eventually disappear. stress
path plots also show an exceptional increase in
pore pressures for high effective confining pres-
sures, where after exceeding the yield strength they
aims for the origin.
Peak and residual strengths values obtained
from both drained and undrained tests are divided
into four groups: peak strength from drained test
(cD peak), residual strength from drained test (cD
residual) and the same for undrained test—cU
peak and cU residual respectively. Fitting kf-
lines through p-q(q') values of the four groups
provides the angle of internal friction values in
Table 2. These values indicate a decrease in the
angle of internal friction with increasing confining
stress.
additionally in the case of undrained tests an
apparent friction angle (ϕ'
ap
) can be calculated as
proposed by sassa (1992):
4
The Role oF ResiDUal soils in
Flank anD secToR collaPses
low shear strength values have been measured for
residual soil units present in the island of Tenerife.
if these units have spatial continuity along the
island or large volumes are present forming the
core stratovolcanoes, this would have serious
implications in the future for the generation of
large-scale deformations and landslides.
X-ray difractograms of the tested samples show
a clear evolution of the residual soils from the par-
ent phonolitic material. There is a clay component
from secondary mineralisation that shows the first
significant difference between the weathered and
the hydrothermaly altered soils. samples of the
former show halloysites and the latter alunite and
kaolinite. While the chemical composition of the
secondary clay minerals is the same, there is an
important structural difference. halloysites are
tubular clays and hence have a well-evolved three-
dimensional structure. alunite crystals are pseudo-
tetrahedral minerals formed by combinations of
two trigonal pyramids that hence form a micro-
three dimensional structure. kaolinites, on the
other hand, are platy, two-layered clays—a very
weak structure. These observations are supported
by shear tests that reveal decreasing shear strength
with weaker clay structure. it therefore appears that
the small fine fraction of the volcanic residual soils
ultimately controls their shear strength properties.
examination of normalised stress-strain curves,
from shear tests, only shows a clear difference in
the case of low normal loads for Rs1. This is proof
of the presence of bonding on the weathered soils.
as expected, this bonding is created by the thermal
alteration of the residual soil as the overlying layer
(lava in this case) is emplaced. in spite of the clear
stress-strain behaviour given by the bonding, our
tests show that its yield strength is surpassed at the
higher normal load end of our test settings, which
corresponds to relatively shallow depths relative to
the depth of large volcanic landslide decollments.
in the triaxial tests, the enormous reduction from
the drained angle of friction of 30° to the appar-
ent friction angle of less than 20° (
Table 3
) verifies
the significance of the pore water pressure during
undrained loading. Moreover, the large values in
the pore water pressure parameter attained at the
high end of the confining stress in the test set-up,
approach one, and this can be associated with a
loose structure that collapses upon the application
of a load. This opens a window into understanding
the impact this could have in the behaviour of the
soil units during large volcanic landslides as they
may act in this manner after fast, undrained load-
ing, if the confining pressure is greater than about
1 MPa.
q
p
ini
max
φ
′
(1)
=
tan
ap
′
where q
max
is the maximum shear strength and
p'
ini
is the initial effective stress of the undrained
triaxial test. The results are given in Table 3. in
this way the decrease in angle if internal friction
for increasing confining stresses is accentuated.
another parameter that picks up the variation
dependant on the confinement stress is the pore
pressure parameter, a, of lambe and Whitman
(1979).
Table 2.
angle of internal friction from triaxial tests [°].
cU
cD
confining stress
[kPa]
Peak
Residual
Peak
Residual
low (300-900)
38.66
33.57
33.59
32.58
high (1550-4500)
30.76
33.24
31.25
29.91
Table 3.
angle of internal friction from triaxial tests [°].
ϕ'
ap
a
effective confining stress [kPa]
[º]
[-]
300
38.24
0.34
900
33.98
0.41
1550
20.68
0.97
4500
18.75
0.99
