Environmental Engineering Reference
In-Depth Information
2.4 Continuous shale alternations—cherts with
dolerites and lavas (Sh-do)
The formation consists of red argillaceous shales
with local interlayers of cherts, sandstones and
thin-bedded limestone, in continuous alternations
with dolerites and lavas. The formation appears
highly tectonized, folded and sheared and often
appears in the form of mélange.
The overall permeability is low although more
permeable layers (dolerites and lavas) are present
in continuous alternation with impermeable layers
(shales-cherts). The mechanical properties depend
on the layer with more presence.
Figure 3 shows the histogram with the distribu-
tion of different geological units as they appear in
boreholes Bs2o-1 to s2o-8.
and also on the article “ Determination of residual
strength parameters of jointed rock masses using
the GSI system ” by M. cai, P.k. kaiser, Y. Tasaka
and M. Minami and published in the international
Journal of Rock Mechanics and Mining sciences
in september 2006.
The assumptions made in this study are the
following:
- The result of the unconined compressive test
is equivalent, with a reasonable degree of accu-
racy, that obtained by multiplying the Point
load Test (is) by 24, where is = P/D 2 (where P is
the load and D is the distance between them).
- it has been considered as more representative
the results of those Point load Tests in which
the load was applied normal to the direction
of the stratification and/or schistosity.
- the Geological strength index (Gsi) has been
estimated using the following expression:
3
GeoMechanical classiFicaTion
oF The Rock Mass
Gsi = RMR′ 89 -5
3.1 Criteria and methodology employed
The estimation of the Geological strength index
(Gsi) of the different geological units that will
be encountered during the excavation of the tun-
nels has been based on the analysis of the samples
recovered from B2o-1 to B2o-8, the results from
the Uniaxial compressive strength Tests (Ucs)
and on those of the point load tests (PlT) as well
as on the information given in the GiR report pre-
pared by ok.
This analysis has been based among others
on the recommendations given by e.hoek and
P.Marinos in their publication “ Variability of the
engineering properties of rock masses quantified by
the geological strength index: the case of ophiolites
with special emphasis on tunneling (July 2006) ”,
where RMR′ 89 is the Bieniawski index according
to his 1989 proposal for solid for dry rock masses.
- When evaluating the condition of the discon-
tinuities, it has been assumed that their condi-
tion has been affected by the drilling operations
which tend to disturb their natural condition as
well as to increase their aperture.
- Representative samples of the geological unit
sch-Do have been considered when evaluating
the Gsi from samples recovered from boreholes.
sub-division of the formation in homogenous
section from a lithilogic point of view has been
avoided.
- Due to the complexity of the ophiolitic forma-
tion, it has been decided to reduce the number
of geological units. This has been done in order
to avoid a classification with many different for-
mations which may lead to confusion and mis-
taken interpretation.
- The RQD and the spacing between the discon-
tinuities have been evaluated in accordance with
the latest recommendations and methodologies
proposed by Bieniawski.
- The guidelines given by hoek and Marinos in
2001 have been used when evaluating the Gsi
from boreholes.
3.2 Geotechnical characterization
3.2.1 Geotechnical Unit Do and p-g
The Do unit consists of dolerites and basalt. Dol-
erites often have serpentinized sections and fre-
quent presence of shale and chert within this unit
as intercalations, especially in the vicinity of the
tectonic contacts.
Figure 3. Distribution of different lithotypes in the
boreholes B2o-1 to B2o-8.
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