Environmental Engineering Reference
In-Depth Information
The present basis for the design and definition
of these activities go through a classification of the
ground to be excavated, so that we can assume its
behavior.
all rock mass classifications regarding the
excavation of a tunnel are based on the assump-
tion that the ground is homogeneous around the
section concerned. This is so from the oldest by
lauffer, Terzaghi, Protodiakonov, etc. to the cur-
rent by Bieniawski, Grimstad and Barton etc. The
latter are usually used along with failure criterion
(hoek-Brown type or similar).
a number of parameters linked to the quality
of the material to be excavated have to be defined
as a unit: cohesion, friction angle, resistance to
compression, fractures, families and guidance of
diaclases, filled joints, shaped edges, presence of
humidity.
added to that are the characteristics of the work
to be performed: dimensions, types and possible
consequences of their defects (water works, roads,
caves, nuclear plants, etc.).
on these premises, supporting is defined theo-
retically or empirically, backed by practice and the
experience of well known authors.
as soon as an alteration of key assumptions
such as homogeneity and flatness occur (eg prox-
imity of the mouth, opening galleries across, cross-
ing a field with a certain “homogeneity” to another
with a different one, crossing faults or fractures,
etc), the methodical rules become evanescent and
a series of recommendations such as reducing
advances, measuring and controlling convergence
appear. according to them, the density or type of
bolts, thickness and type of support, etc, will have
to be redefined.
in short, everything will depend upon the judge-
ment of the technicians, their experience and other
conditioning such as financial circumstances,
deadlines, etc.
in a volcanic environment, there is a succession
of basaltic lava flows, from 1, 3 or 5 m thick, very
variable in dips and directions along the drive.
along with them, there are layers of scoriaceous
material of different thickness coexisting in a sin-
gle advancing front.
also, too often unfortunately, these discon-
tinuous layers of basalt-scoria are interrupted by
large bags of scoriae or pyroclasts. in some cases,
these layers show some welding which gives them
some “cohesion”, but it can easily disappear with
the vibrations of the excavation.
as it is easily understood, rock mass classifica-
tions provide very little information for these areas.
This does not apply if the thickness of basaltic lava
flow layers are very large related to the size of the
work and loosely cover the entire section of the
excavation and its surroundings. Moreover, scoriae
and pyroclasts are rather similar to grounds with
high friction and little or no cohesion (welding).
now we will approach the methodology used to
advance in these areas. it has been applied in other
projects such as Parador Tunnel in el hierro in
1990, la cumbre Tunnel in la Palma in 1998, los
Roquillos Tunnel in el hierro in 2004, a tunnel in
Timijiraque variant in el hierro, which so far are
having a satisfactory performance.
in the case we are dealing with we will only face
two situations:
4.1
Progress in succession of basaltic lava flows
(hard and soft elements)
Barton and Bieniawski Geomechanics classifica-
tions were used to massive areas of basaltic lava,
from data obtained from rock drillings. We assume
that the thickness of the lava flow can be taken
into account for resistance. considering the width
of this excavation (∼13 m), it will be considered a
10% of that width. With these classifications, we
strengthen and improve the bearing capacity of
these strata using empirical methods of supporting
on their recommendations:
• systematic bolting will sew the possible discon-
tinuities, providing a strong “uniformity” and
even some unstable element may be fixed.
• Fibre reinforced or unreinforced shotcrete, of
various thicknesses depending on the charac-
terization of materials.
This system, known as the new austrian
Tunneling Method naTM (and gives a bit of blush
keep using the term “new”), takes into account
the self-bearing capacity of the ground, plus the
supports, to stabilize the stress and load that it
transmits to the surrounding areas, and reaching
stability, which is verified by measuring the conver-
gences. They show if the thickness of shotcrete are
sufficient or should be increased.
however, though with this method the excava-
tion allows a smooth advance through this kind
of grounds, bolting and shotcrete guarantee that
those uplayer lava flows support the loads over
them: in this case the weight of the scoriae between
a lava flow and the next, loose elements among
them, and so on.
shotcrete, besides being a resistant element
(whose main mission is to avoid an initial and
uncontrolled decompression), provides stability
to the walls against the active stress (usually very
small in these cases) caused by the decompression
and protects areas resulting from excavation, roof
and walls, from the alteration that can lead to deg-
radation of the bearing capacity.
of course, although not very often, it may be
that the lava flows are so fragile and weak that we
