Environmental Engineering Reference
In-Depth Information
thickness, which allows a certain deformability nec-
essary to the development of the discharge arch. The
shotcrete is reinforced with metallic fiber or wire
net, rockbolts or metallic ribs, with the advantage
that these elements allow the increase of the mass
arch thickness, thus increasing the resistance.
Due to the great heterogeneity, versatile cons-
truction methods have been adopted for a better
adjust to the different geotechnical conditions. When
the formations are ripable the dismount proceeds
mechanically with hydraulic demolition hammers.
The compact basalts and compact breccias are
generally in the limiar of ripability, making the use
of explosives the most economical method to pro-
ceed with the excavation.
The geotechnical class ZG1 is mainly formed
by compact basalt, by vacuolar compact basalt
and by compact breccias. These are highly resist-
ant, homogeneous formations, whose fracturation
resulted from columnar disjunction produced by
slow cooling of the magma and therefore do not
significantly affects the stability. consequently,
class ZG1 is a good quality zone. in the current
cross-sections, primary support includes only local
swellex rockbolts associated with 0,05 m of shot-
crete. Permanent support includes reinforced con-
crete 0,25 m thick ( Fig. 12 ) .
on the whole, class ZG2 is a moderate quality
zone. it is constituted by resistant rocks, fractured
basalts, fractured vacuolar basalts, consolidated
breccias, with lenticular disaggregated breccias
and tuffs and compact pyroclasts (mainly tuffs).
Primary support includes swellex rockbolts with
4 m length spaced about 1,5 m by 1,5 m in the
roof, associated with the immediate installation of
a shotcrete shell 0,05 to 0,10 m thick. in sections
with larger spans, like entry and exit slip roads,
bolts with 6 m long in a 1,5 m by 1,2 to 2 m mesh
have been adopted, associated to shotcrete with
0,15 to 0,30 m thick ( Fig. 13 ) .
The permanent support is a reinforced concrete
lining 0,25 m thick.
class ZG3 is constituted by poor quality forma-
tions. it includes low resistance rocks, weathered
and very weathered basalts, pyroclastic coarse for-
mations with blocks, tuffs with bombs and ash and
compact to medium consolidated soils. Primary
support includes steel arches one meter spaced and
steel fiber-reinforced shotcrete, 0,2 to 0,3 m thick.
class ZG4 includes very poor quality forma-
tions as very weathered fine pyroclastic formations,
slope deposits and sometimes ancient fills, with the
presence of water. in the current cross-sections,
primary support includes jet grout umbrella vault
associated with a face reinforcement similar to pri-
mary support of class ZG3, basic underpinning of
the half cross-section and an invert arch support.
Permanent support is a reinforced concrete lining
0,4 m to 0,5 m thick. in the most unfavourable
cases a definitive reinforced concrete invert arch
has been adopted ( Fig. 14 ) .
Figure 13.
Primary support for sections with large spans.
Figure 12.
Primary support for different geotechnical
classes.
Figure 14.
Permanent support with invert arch.
 
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