Environmental Engineering Reference
In-Depth Information
The strength of the rock matrix can be determined
from triaxial compression tests (only for highly
important works due to its cost), uniaxial
compression tests (the most commonly made),
point load tests or tensile tests. The stiffness can
be determined in laboratory by uniaxial compres-
sion tests or seismic tests, and in situ by plate load
tests, dilatometer tests, flat jack tests or seismic
tests.
classification methods, such as the hoek-Brown
criterion (in development since 1980), have made
clear that the strength of the jointed rock mass
depends not only on the properties of the rock
matrix, but also on the freedom of the intact rock
pieces to rotate and translate among them under
certain stress conditions (hoek
et al
., 2002). This
movement is controlled by the geometrical and
mechanical characteristics of the joints. The Geo-
logical strength index classification was developed
to describe the global behavior of the rock mass
(Marinos & hoek, 2000).
From the mechanical properties of the matrix
rock and Gsi (and since recently, the massif 's per-
turbation induced by the construction process) it is
possible to estimate the design parameters of the
rock mass by the hoek-Brown criterion. This crite-
rion is only applicable to the rock masses in which
the joint pattern is reduced (and therefore the mass
behavior is controlled mainly by the rock matrix)
or fractured rock masses with a random distribu-
tion of joints so the rock mass can be considered
homogenous and isotropic.
in addition to this criterion, other geomechani-
cal classifications can be found in the literature. in
accordance with the assessment of a limited number
of geotechnical parameters which, in general, can
be determined from simple laboratory tests, from
the field observations or from the visual inspec-
tion of the samples obtained from the borehole,
the rock mass is classified into different classes.
among these, one of the best known is the Rock
Mass Rating (RMR), proposed by Bieniawsky in
1973 and revised in 1989 (Bieniawsky, 1989), which
allows the evaluation of the rock mass quality and
the estimation the rock mass strength and stiffness
through correlations. other common classification
is the quality classification system for rock masses,
also known as Q system, proposed by Barton,
lien and lunde (1974). it is based on the experi-
ence obtained in more than 200 underground work
cases, and also aims to provide the classification of
massives for later use in tunnels's design.
The mechanical behavior of soils is nor-
mally assessed by in situ tests, such as the stand-
ard penetration tests, cone penetrometer tests,
pressuremeter tests, and by laboratory tests, such
as identification tests, direct shear tests and triaxial
tests.
5
GeoMechanical PRoPeRTies
oF Volcanic Rocks oF MaDeiRa
5.1
Collected data
a considerable amount of data was collected from
48 tunnels' designs made in the last 20 years in the
volcanic complexes β
1
and β
2
. The greater amount
of laboratory tests was performed in β
2
complex
mainly because of the great difficulty in obtaining
good quality samples in the β
1
complex, due to its
high level of alteration.
after reviewing all the design's data, it was set
the goal to systematize the information by dividing
the basalts into three categories: i) Fresh to slightly
weathered basalts; ii) Moderately weathered, par-
tially vacuolar basalts; iii) highly weathered vac-
uolar basalts. Rocky pyroclastic materials were
divided into: i) Fresh to slightly weathered brec-
cias; ii) Moderatelly weathered partially vacuolar
breccias; iii) highly weathered vacuolar disaggre-
gated breccias.
some typical properties are proposed afterwards
for each category in Table 1.
5.2
Uniaxial compression tests results
a set of 14 uniaxial compression tests were per-
formed in samples from complex β
1
and 136 tests
from complex β
2
. The test results are presented
in the first chart of Figure 7. a set of 65 uniax-
ial compression tests were made in breccias from
analysing all the available information it
was established criteria to define the basalts
and breccias categories based on their uniaxial
compressive strength (σ
ci
). Basalts were divided
into the following categories: i) fresh to slightly
weathered basalts with σ
ci
> 100 MPa; ii) mod-
erately weathered, partially vacuolar basalts with
10 ≤ σ
ci
< 100 MPa; iii) highly weathered, vacuolar
basalts σ
ci
< 10 MPa. each category is equivalent
to 43%, 49% and 8%, respectively of the tested
rock cores (
Fig. 8
). For the rocky pyroclastic
materials it were established the following catego-
ries: i) fresh to slightly weathered breccias with
10 ≤ σ
ci
< 100 MPa; ii) moderatelly weathered par-
tially vacuolar breccias with σ
ci
< 10 MPa. each
category is equivalent, respectively, to 63% and
37% of the tested rock cores. it is noted that no
tests were made in the highly weathered, vacuolar
disaggregated breccias due to the impossibility of
sampling intact cores for testing.
it is also noted that in 64% of the test results
analysed the value of poisson's ratio of the rock
matrix lies between 0.20 and 0.25. in breccias, for
62% of the tests the value of poisson's ratio lies
between 0.15 and 0.25.
