Environmental Engineering Reference
In-Depth Information
of them corresponds to a solid lava, basalt rock
specifically.
relationship with compressive strength in this type of
rock. Be taken into account the possible anisotropy
of the rock by the presence of flow structures.
The size and shape of block, using the block
volume (Vb), less influenced by the direction as the
RQD.
The presence of voids, the cave index can be
used in caves.
The characteristics of the discontinuities, being
primarily responsible behavior of the massif, with
regard to the aperture, roughness, presence of fill
and alteration of the walls.
The presence of water must also be evaluated.
Below is a summary table that shows the impor-
tance of each parameter.
The importance of each parameter, assigned a
priori, must be evaluated carefully. The historical
case studies should lead to the quantification of
the relative importance to obtain reliable and use-
ful classification in the geotechnical work.
3.1.3
GSI
This classification only evaluates the degree
of jointing of the massif and the state of the
discontinuities.
There are more parameters that influence the
behavior of massifs and that this classification
does not account, in particular the presence of
holes and the shape of the blocks.
it also ignores the simple compressive strength
of rock.
The heterogeneity of this type of solid Gsi
makes the classification difficult and often inad-
equate. however, the amendments made by Mari-
nos, P.
et al.
(2001) for type flysch massifs and
hoek, e.
et al.
(2005) for solid molasses type can
be much more appropriate.
3.2
P
yroclastic rocks
The resistance of the pyroclastic massifs does not
depend on the discontinuities but on the resistance
of the rock matrix (Gonzalez-Gallego, J. 2008).
Geomechanical classifications, most commonly
used today (RMR, Q and Gsi) are designed for the
study of rock masses composed of high strength rock
controlled therefore by the properties of the fractur-
ing system. These classifications do not consider
adequately the strength of the rock matrix which is
the most important parameter in pyroclastic beds.
3.3.2
Pyroclastic rocks
Previously we have seen that the behavior of these
massifs depends primarily on:
compaction degree. it is the most influential fac-
tor in the resistance of the massif. The best method
is to determine the in-situ density of the material.
The degree of welding and the imbrication of par-
ticles. This parameter can be evaluated using the clas-
sification proposed by serrano, a.
et al.
(2002).
The degree of alteration.
The presence of water may decrease in up to
30% the strength of the rock (Vásárhelyi 2002).
The intrinsic strength of the particles presents
difficulties in its extent, then it may be advisable to
disregard this parameter.
Discontinuities. although not normally present
in these materials, their possible presence must be
taken into account.
Below is a summary table that shows the
importance of each parameter.
3.3
Recommendations for the design
of new classifications
in view of all previous data are discussed below
the basic features to follow for the proper design
of geomechanical classifications to study volcanic
massifs.
3.3.1
Lavatic rocks
The parameters to be evaluated:
The compressive strength of the rock matrix,
which should be evaluated by laboratory tests mainly,
discourage the use of point load test given his difficult
Table 2. Degree of imbrication and welding (iW)
(serrano
et al.,
2002).
Welding
ex-
tremly
Welded
>45%
Table 1. Parameters to be considered in the classifica-
tion of lavatic massifs.
not very
welded
5-15%
loose
<5%
Welded
15-45%
inbrication
Parameter
importance
low
<8 contacts
iW 1-1
iW 1-2
iW 1-3
iW 1-4
intact rock compressive strength
+
Block size and shape
++
Medium
8-12 contacts
iW 2-1
iW 2-2
iW 2-3
iW 2-4
cave index
++
Discontinuities characteristics
+++++
high
>12 contacts
iW 3-1
iW 3-2
iW 3-3
iW 3-4
Water presence
+
