Environmental Engineering Reference
In-Depth Information
Table 2.
excavation and support proposed based on Bieniawski 1989.
Reinforcement
RMR
class
steel
sets
excavation
Bolts
shotcrete
iii.
Top heading and bench 1.5-3 m advance
complete support 20 m from face
systematic bolts 4 m long spaced
1.5-2 m in crown and walls.
Wire mesh in crown
5-10 cm in crown
and 3 cm in walls
no
Therefore, it is considered a class iii rock,
medium quality. The time of stability for an unsup-
ported length of 10 m is 1 week. For this type of
excavation (road tunnel) the kind of support to be
Barton Rock Mass classification is based on the
RQD index and on a number of parameters such
as: index of jointing, roughness, degree of altera-
tion, coefficient for the presence of water and sRF
(stress Reduction Factor).
it gives a quality index “Q” which is not lineal
(as RMR) but exponential, ranging from 0.001 to
1000, from very poor rock to exceptionally good.
in this case, to determine the Q of Barton has been
correlated with RMR Bieniawski, by the following
expression: e = e exp RMR-44/9.
The Q value obtained is 1.3, so that it would be
classified as a rock of poor quality. From this value
and with the esR (excavation support Ratio) for
this type of excavation (road tunnel) the kind of
support to be used is 4: systematic bolting, shot-
crete 40-100 mm: B +s.
3.3
Basaltic pyroclastics
in general, the geotechnical characteristics of
pyroclastic deposits depend on the density of the
welding between the particles, their imbrication's
overlap and the degree of alteration of the deposit.
The greater the overlap and welding the higher
particle density and compactness of the deposit
and, therefore, its bearing capacity.
The overlap is the number of contacts of a par-
ticle with surrounding particles. in turn, the weld
is related to the percentage of areas of welded con-
tacts respects to the total area of the particle.
Regarding outcrops, basaltic pyroclasts from
el Golfo-las Playas edifice have a higher degree
of welding that those from the volcanism of the
ridges ss since they are so young that they have
not undergone alteration processes to increase its
compactness.
Given the appearance presented by these mate-
rials in the outcrops, they have been considered a
priori as weak or soft rock or a bad soil accord-
ing to Roman (1980). For these reasons, they will
require support immediately after excavation as
with the scoriae.
These treatments consist of installing a micro-
pile umbrella grouting around the tunnel. if the
area of loose pyroclasts continues after completion
of the first umbrella, it would require the place-
ment of successive umbrellas, with a minimum
overlap between them of 2-3 m.
in areas where basaltic pyroclastics have a
higher degree of welding (more compact) it is
recommended to work with short advance in top
heading (1 to 1.5 m) reinforcing with shotcrete
(12-15 cm) and steel sets to prevent the collapse of
the section.
3.2
Toplayer and downlayer scoriae
From a geotechnical point of view, scoriae are con-
sidered a granular soil with angular and subangu-
lar gravel with prickly rough surfaces involved in
sands. This determines that there is good interlock-
ing between them, giving greater compactness to
the whole.
Geotechnical characteristics depend on its
degree of compaction, associated in most cases
with its geological age. Thus, scoriae from lava
flows from the el Golfo-las Playas edifice show
a high degree of welding, which confers certain
compactness. on the other hand, scoriae from the
volcanism of the ridges, have a low degree of weld-
ing so that they appear almost loose.
according to the classifications of Roman
(1980) they are considered a weak or soft rock or a
bad soil. excavation in these loose materials carries
a high risk of detachment. That is why inmediate
support will be needed. special treatments of rein-
forcement will have to be used.
4
UsUal PRoJecTion oF The sUPPoRT
oF a TUnnel
once the geometry of the tunnel for the use it is
required is defined, its execution and supporting
methods have to be defined, trying to ensure its
safety.
