Paper Chacabuquito-LosQuilos

Castro S. O. et al. Dealing with Expansive Rocks in the Los Quilos and Chacabuquito Water Tunnels Andes Mountains of Central Chile
ISRM 2003–Technology Roadmap for Rock Mechanics, South African Institute of Mining and Metallurgy, 2003.
Dealing with Expansive Rocks in the
Los Quilos and Chacabuquito Water Tunnels Andes Mountains of Central Chile
Castro S.O.(*), Van Sint Jan M. (**), González R.E. (***),
Lois P.V. (****), Velasco L. E.(*****)
Edic Ingenieros Ltda.(*), Pontificia Universidad Católica de Chile(**), REG
Ingenieros Ltda.(***), Hidroeléctrica Guardia Vieja S.A.(****), Ingeniería del
Conocimiento Ltda.(*****)
Abstract
Several zones with expansive rocks were encountered during construction of two tunnels of
the water conduction system for the Chacabuquito Hydropower Plant, located in the
Aconcagua river valley of the Chilean Central Andes Mountains, at elevation 1.075 m a.s.l.,
and 65 km north-northeast of Santiago. The expansive behavior was associated to three
different geological features. Firstly with the crossing of important north-south trending
faults, with montmorillonitic and caolinitic fillings. Secondly to the presence of zones of
rock mass showing a penetrative presence of clay minerals (montmorillonite, illite and
caolinite) and penetrative thin veins of ceolites. And thirdly to the hydrothermal alteration
of the rock and the consequent generation of clay minerals (predominantly montmorillonite,
sericite and minor caolinite). The geological and mechanical characteristics of the
expansive rock domains, as well as the field and laboratory reconnaissance and
characterization program, are described in the paper together with the bases for design and
the guidelines for construction and definitive tunnels support.
Resúmè
Pendant l’excavation des deux tunels du système d’adduction d’eau pour alimenter la
centrale hidroélectrique Chacabuquito, plusieures zones de rochers expansifs ont été
découvertes. Ces tunnels sont dans la vallée du fleuve Aconcagua de la Cordillère des
Andes du Chili Central, à une altitude aproximée de 1.075m s.m.n. et à 65km vers le nordnord-ouest de Santiago. Cette conduite expansive est associée à trois différentes
caractéristiques géologiques du massif rocheux. Un premier lieu, à l’intersection de
plusieures zones de failles, d’orientation nord-sud et subverticales, avec remplissage de
rochers désintégrés, montmorillonite, caolinite et sérite. En seconde lieu, a la présence de
zones de rochers expansifs, avec des nombreux minces veines de céolite-calcite et aussi des
minéraux argileux (montmorillonite, illite et caolinite). Et, dans troisième lieu, dût à la
présence de zones d’altération hidrothermale du massif rocheux et la conséquente
génération de minéraux de remplacement argileux (surtout sérite, montmorillonite et en
plus petit degré, caolinite). Les caractéristiques géologiques et géomécaniques des rochers
expansifs, ainsi que la reconnaissance du terrain et les résultats du programme d’essais de
caractérisation géomécanique du laboratoire sont décrits dans ce travail, ainsi que les bases
de dessin et recommandations constructives pour placement du support définitifs pour ces
tunnels
Zusammenfassung
Während der ausgrabung der zwei tunnels der wasserleitungssystem, um die Chacabuquito
Wasserstromzentrale zu besorgen, wurden verschieden ausgedehnte gesteinadern gefunden.
Die beiden tunnels befinden sich im Aconcaguatal, wo der Aconcagua Fluss fliesst, auf
1
einer Höhe von 1.075 m über dem Wasserspiegel und ca. 65 Km Nord-Nordest von
Santiago entfernt, in mittleren Anden Cordillere (chilenische Gebirgskette).
Drei
verschiedene geologische merkmale dieser massiven gesteine können dieses ausgedehnte
verhalten erklären. Erstens, hier befinden sich verschiedene und auch ganz wichtige
erdbrüche, mit untersenkrechte Nörd-Süd orientierung, diese erdbrüche sind mit
zertrümmerte gesteine (montmorillonite, caolinite und sericite) gefüllt. Zweitens, diese
ausgedehnten gesteine haben durchdringende fine gänge (ceolite and calcite) und tönige
mineralien (monmorillonite, illite und caolinite). Und drittens, hier befinden sich auch
hydrothermale zonen, die diese massive gesteine durch die entstehung von tönigen
mineralien (vorherrschende sericite, caolinite und in zweiten stelle montmorillonite)
vevändert haben. Diese geologische und geomechanische merkmale dieser gesteine, sowie
die erkennung der landschaft, die ergebnisse der charakteristischen labor programmproben,
der entwurf des projekts und die bauempfehlungen für die endgültigen tunnelstütze, werden
in diesen bericht beschrieben.
Introduction
long water conduction system captures the
discharge waters from the Los Quilos
hydroelectric power plant at elevation 1.075m
a.s.l., crosses the Aconcagua river through a
siphon and leads to the power house at an
elevation of 930m a.s.l. The project was designed
for a maximum water flow of 24,3m3 /s and 137m
of hydraulic head, allowing the generation of
approximately 25MW.
The Chacabuquito Hydropower Plant Project is
located in the valley of the Aconcagua River,
65km north-northeast of Santiago, in the Andes
Mountains of Central Chile. The Aconcagua
Valley shows here an abrupt topography, with a
narrow cross section and steep hillsides; the
narrower part of this zone is locally known as
Puntilla del Viento. Figure 1 illustrates some
important features of the project. The 13,3km
Figure 1: Location Map
2
§
Edic Ingenieros Ltda., a Chilean consulting
office, carried out the project for the owner,
Hidroeléctrica Guardia Vieja S.A. The water
conduction system included mainly an open
channel and the excavation of two 4,20m-wide
and 4,35m-high “horse shoe” cross section
tunnels. Tunnel N°1, called Los Quilos, is
approximately 788m long, and Tunnel N°2, called
Chacabuquito, is 2.171m long. Zublin and Tecsa,
a German-Chilean consortium undertook the
excavation of these tunnels; it started in April
2001 and finished in January 2002. The tunnels
were under operation in May 2002.
and clayey, with thick montmorillonitic and
caolinitic soft fracture fillings. This situation
is valid for both tunnels.
§ Zones with thin veins of ceolites and also
some
calcite
and
clay
minerals
(montmorillonite, caolinite and illite). This
condition was encountered only in the
Chacabuquito tunnel eastern front.
§ Hydrothermal alteration of the host andesitic
rock mass due to the contact with an intrusive
monzogranodioritic stock, thus allowing the
presence of clay minerals (predominantly
montmorillonite,
sericite
and
minor
caolinite). This situation was only detected
in the Los Quilos tunnel eastern front.
Nevertheless, some major instability problems
were also encountered under lateral creeks areas,
where the overburden was small or minimum.
These creeks coincide with the occurrence of fault
zones and ground water leakages, so that the rock
mass was more deteriorated and lattice girders
and complementary support were required to
cross them safely. Minimum or no expansive
rocks were there encountered, so such not
included in this paper.
Geology
The tunnels were excavated in rocks of the Los
Pelambres Formation, formerly known as the
Abanico Formation, of Cretaceous age. These
rocks include stratified porphyric and aphanitic
andesite, aphanitic basaltic andesite and basalt,
volcanic breccia and local thin layers of fine
grained sandstone and/or andesitic tuff and
limolites.
The majority of the main faults encountered
during construction were steeply dipping
structures, striking nearly N-S, and thus related
with the regional north-south Pocuro mega fault
that crosses this area approximately 2km
downstream from the western portal of the
Chacabuquito tunnel. A second system, of NWSE trending and sub vertical faults, and isolated
sub horizontal faults coinciding with the
stratification, were also found.
Rock mass
bedding is sub horizontal, gentle dipping to the
east or to the west.
Geologic mapping carried out during excavation
revealed that the rocks of the Chacabuquito tunnel
eastern front were predominantly of good to
regular geotechnical quality, and a wide an
important zone and three isolatedand thinner
zones of poor to very poor quality, according to
Grimstad E. and Barton N. (1993); no major
problem were detected in the western front of this
tunnel. The rock mass found in the eastern front
of Los Quilos tunnel was in general of only
regular geotechnical quality, deteriorated at the
expansive rocks area, where a poor to very poor
geotechnical quality was observed.
As
Chacabuquito tunnel, no major problems were
encountered in the western front.
The most serious problems were related to rocks
exhibiting an expansive behavior, which was
mainly associated to three different geological
features:
Crossing of important north south trending faults,
where the rock is strongly fractured
Expansive Rock Zones in both Tunnels
Eastern Front of the Chacabuquito Tunnel
A wide expansive rock zone extends from PK
0.287 to PK 0.550, where andesitic-basaltic lava
flows were excavated. These rocks are strong to
partially soft, generally dense but intense to
strongly
fractured.
Consequently,
their
geotechnical quality is poor to locally regular;
joint planes infilling materials include a
predominant presence of iron oxides, clay
minerals (sericite, illite, caolinite and minor
montmorillonite)
and
mainly
ceolites
(predominantly laumontite, and secondarily
chabazite and stilbite) and calcite.
The situation was far more critical at three northsouth trending fault zones, located at PK 0,340 to
0.347, PK 0.440 to 0.447 and PK 0.535 to 0.550,
where the rock mass is soft, decomposed, with
hematite
and
argillized
(predominantly
montmorillonite), thus having a very poor
geotechnical quality. Expansion pressure values
varied from less than 0.3kg/cm2 to 1.40kg/cm2
(Table I).
They were three other critical fault zones,
encountered at PK 0.170 to 0.182, PK 0.660 to
0.670 and PK 1.230 to 1.243, where crushed
rocks with hematite and a highly plastic red
montmorillonite were predominant; some strongly
3
decomposed rock blocks fell down here during
excavation, generating over-excavations of up to
3.5m and requiring a strong initial support to
stabilize these zones.
Nevertheless, no important deformation of the
unsupported and wet tunnel floor was observed
during excavation, except at the PK 1.230-1.243
fault where some deformation was observed.
alteration and the presence of pyrite, sericite,
montmorillonite and minor caolinite-illite, were
there encountered. The rock mass consisted of
strongly to intensely fractured and intensely
oxidized yellowish-brownish soft rocks, mixed
with grayish relatively stronger rocks, of regular
to poor geotechnical quality, being very poor at
the crossing of some fault zones. These rocks
showed in general a good stability during
excavation, except at the isolated and relatively
thin fault zones crossings (less then 5m
thickness), where highly plastic and expansive.
Figures 2 and 3 show the geologic map and
profiles along both tunnels.
Eastern Front of Los Quilos Tunnel
A wide critical zone of expansive rocks extends
from PK 0,230 to PK 0,420. Layers of andesitic
lava flows, showing a noticeable hydrothermal
Figure 2: Geologic Map and Profile of Los Quilos Tunnel
Figure 3: Geologic Map and Profile of Chacabuquito Tunnel
4
clay were detected. Hydrothermal alteration was
estimated to be induced by the intrusion of the
granodioritic rocks that were encountered at the
final portion of this tunnel, approximately 200m
from the western portal. This hydrothermal
alteration is less noticeable near the contact zone,
probably due to some structural control and/or to
the fact of having granodioritic rocks close to the
tunnel invert as proposed in the following Figure
2.
Typically, such rocks tend to disintegrate under
the presence of water or just natural humidity,
especially those that were oxidized.
This
phenomenon was detected from PK 0.230 to PK
0.420, but it was critical from PK 0.335 to PK
0.400. The expansion pressure ranged from less
than0.3 kg/cm2 to a maximum of 3.3kg/cm2 , in
one case where a fault zone was also cooperating
to deteriorate the rock mass; generally, it was less
than 2kg/cm2 (Table I).
During construction, the clayey rocks in the
tunnel floor softened due to the combined effect
of saturation and heavy traffic, resulting in a
maximum 0.7m of floor sinking. The tunnel
invert was repaired several times by adding muck
made up of much sounder rock fragments.
Additionally, where the alignment crossed some
faults (PK 0.258, 0.375 and 0.388), relatively
minor rock falls of no more than 1 to 2m3 fell
down during construction in spite of a 5cm layer
of shotcrete and systematic rock bolts that had
been installed as initial support.
carried out reliable uniaxial compression or point
load tests.
Table I: Summary of Laboratory Test Results
PK
Density
IP
wa
Drock
Expansion
Test
F. E.
E. P.
Sl
D
Chacabuquito Tunnel
0.630
0.447
0.540
13
10.9
2.02
2.67
-
1.33
<0.01
1.321.42
-
0.535
0.518
0.496
0.480
0.454
0.435
0.417
0.405
0.354
0.342
0.302
0.295
0.283
1.236
1.238
20
14
11
21
27
23
13
7.0
27.2
4.2
2.28
1.51
2.50
2.70
2.72
2.63
2.73
2.70
2.78
2.81
2.77
2.69
2.60
-
0.30
1.42
Null
0.84
1.40
0.36
0.31
0.10
Null
0.6
0.92
97
91
96
78
88
-
0.81
3.32
(1)
1.141.95
-
Los Quilos Tunnel
Test Results and Geomechanical
Analysis
0.346
0.388
19
21
-
1..6
-
2.17
-
-
0.375
23
-
-
2.50
9.16.3
0.390
17
-
-
2.57
0.03
0.030.08
66
-
0.428
12
2.58
2.0
0.13 90
0.250
2.66
0.280
13
2.67
0.21 95
0.310
14
2.66
0.07 87
(1) Clay of the fault zone core; it was considered not
representative for the tunnel rock support definition.
Laboratory Test Results
Systematic rock samples were collected from the
tunnels’ floor and walls using a geologist hammer
and other hand tools.
The laboratory tests
included regular U.S.C.S. classification tests, free
swelling, swelling pressure and slake durability
tests. Whenever possible, swelling tests were
carried out on small pieces of undisturbed rock
fragments, but more often they were carried out
on samples recompacted to the maximum dry
density of the Modified Proctor test. Such
recompacted samples were air dried before
testing.
The laboratory test results are summarized in the
following Table I. Due to the fact that the rock
mass was in general decomposed and strongly
fractured at these zones, it was not possible to
PK
D
Drock
F.E.
IP
E.P.
Sl
wa
=
=
=
=
=
=
=
=
tunnel kilometer
apparent density
rock density
free expansion (%)
plasticity index
expansion pressure (kg/cm2)
slake durability index
water content
The decomposed rock behaves more like a soil, so
a grain size distribution analysis was carried out.
It classified as clayey sand, SC, according to
Casagrande (1948).
On the other hand, in the case of Chacabuquito
tunnel, the petrographic description and X-ray
analysis allowed the detection of volcanic
breccias, andesites and andesitic tuffs, which
showed a predominant ceolites/calcite and
5
hematite presence, together with secondarily
occurrence of sericite and caolinite, minor illite
and interstitial montmorillonite, as fracture
infilling materials.
Table II: Expansion Curve Parameters A and B
Expansivity
Laumontite is clearly predominant among the
ceolites, presenting a minor content of water
molecules. Nevertheless, at the relevant fault
zones, hematite and montmorillonite content is
predominant as fractures infilling minerals. On
the other hand, andesitic/dacitic lava flows and
volcanic breccias showing a remarkable
hydrothermal alteration were detected at the
initial eastern front of the Los Quilos tunnel.
These rocks showed a predominant presence of
caolinite and also montmorillonite (X-ray
analysis), replacing some of the plagioclase and
fundamental mass of these rocks. Hematite
presence is scarce.
It is possible that the expansion or swelling of the
clay minerals was the reason causing that the
saturated tunnel floor sank under the continuous
transit of heavy trucks and equipment.
A
B
Maximum pressure
(EP) kg/cm2
Very
high
High
5.20
9.97
3.3
2.14
8.36
1.8
Regular
0.60
5.24
1.3
Low
-1.44
4.77
0.5
The above-mentioned formulas allowed the
drawing of the reaction curve for the Los Quilos
tunnel, which is now presented.
Figure 4: Reaction Curve of Los Quilos Tunnel
Geomechanical Analysis
Specialized geomechanical assessment was a key
issue to analyze the rock expansion potential of
these tunnels under operating conditions. A
flexible rock support was adopted in order to
allow some reduction of the ground pressure with
rock expansion. The reduction of the expansion
pressure at the tunnel floor as a function of the
support displacement was computed following the
method of Kovari et al. (1988):
u a (cm) = A - B ∗ log(p a )
[1]
p a = 10(( A-u a ) / B )
[2]
Four alternatives for the position of the expansive
rock mass around the tunnel were analyzed: only
under the floor, only over the crown, just in one
wall and at the whole tunnel perimeter, all of
them plus the effect of equivalent pressure of
loose rock over the crown (P). Following the
recommendations of Kovari et al (1988) the wall
expansion pressure was estimated to be half of
that at the tunnel floor. For design purposes, the
ballast coefficient was estimated from the elastic
rock displacement under a uniformly loaded
rectangular area, given by equation [3]:
or:
Where:
u a (cm)
p a (kg/cm2 )
= rock mass displacement
into the tunnel.
= contact pressure between
rock support “lining” and
surrounding rock mass.
δ =
Where:
d
=
q
=
Values for parameters A and B fitted from the
laboratory test results are indicated in the Table II
according to the rock mass expansivity.
B
I
?
6
=
=
=
(
I ∗ q ∗ B 1 −ν 2
E
)
[3]
surface displacement (cm)
uniform contact pressure
(Kg/cm2 )
width of the loaded area (cm)
shape factor. (I = 1.5)
Poisson’s ratio.
E
rock
=
modulus of elasticity of the
Table III: Measured Expansion Pressure
mass (Kg/cm2 )
E values used for the analysis were the following:
E=
40.000 kg/cm2 , for rock mass qualities
R3 and R4.
E=
4.000 kg/cm2 , for rock mass quality R5.
Considering a width under pressure of B = 0,25
D, where D is the tunnel diameter, the ballast
coefficient results:
q
K =
δ
Name
Rock
Quality
A
R4 = poor
0.3
-
B
R4 = poor
0.8
< 0.8
C
R5 = very
poor
1.4
1.0 – 2.0
Table IV:
The definitive tunnel support at expansive rock
zones was designed for the combined effect of the
equivalent pressure of loose rock blocks over the
tunnel crown, plus the swelling pressure of the
expansive rock mass. The equivalent pressure of
loose rock blocks was estimated using the RMR
criterion of Bieniawski (1986). The following
values were used:
<
20, for rock mass of
very poor geotechnical
quality
RMR
= 35, for rock mass of
poor geotechnical
quality
Rock density
= 2.6 T/m3
Elastic modulus
= 40,000 Kg/cm2
?r
=
Vertical and Expansion Pressure for
Design
Rock
Quality
RMR
R4=Poor
35
R4=Poor
R5=Very
Poor
Vertical
Pressure
Expansion
Pressure (kg/cm2 )
Chacabuquito
Los
Quilos
s v = ?r * Hp
EP=0.3
-
35
s v = ?r * Hp
EP=0.8
EP<0.8
20
s v = ?r * Hp
EP=1.4
EP=1.02.0
The support structure consisted of rock bolts and
lattice girders fully encased in shotcrete lining
with a structural invert. Rock mass interaction
was modeled with the ballast coefficient.
Systematic rock bolts were also incorporated to
the invert in order to compensate the uplift
pressure and so they were modeled as springs.
Finally, a load factor of 1.4 (Norm ACI-318), for
both, the expansion pressure and the vertical load.
This last assumption means that the rock support
design would accept some minor damages, such
as local shotcrete cracks and deterioration of the
rock mass behind, but in such a way that the
tunnel will not collapse. Additionally, these
damages would be easily reparable.
Measured expansion pressure at both tunnels is
exposed in Table III. Then, the equivalent
pressure of loose rock blocks over the flexible
rock support on the tunnel crown was calculated
as:
Where:
Hp
=
Los Quilos
Tunnel
Finally, the vertical and lateral loads on the rock
mass support of Los Quilos and Chacabuquito
tunnels were defined as exposed in Table IV.
Bases for Design
P = γ r × Hp
Chacabuquito
Tunnel
(4)
Final Rock Support
RMR
Expansion Pressure
Kg/cm2
[5]
Definitive Rock Support of both Tunnels
The detailed definitive or final rock support of
both tunnels was defined as exposed in Figure 5
of the next page.
In the case B of Tunnel Los Quilos, a 20 to 60cmthick layer of rock was removed in the areas
where the invert had softened during construction.
A geomembrane (BIDIM OP-20) was placed on
the new floor and the rock removed was replaced
with a fill of sound and clean muck, made up of
4” maximum size and 2.6 T/m3 minimum density
height of loose rock blocks over
the tunnel crown
rock density
7
hard rock fragments, compacted to 65% relative
density.
Figure 5: Final Support at Expansive Rock Zones
Conclusions
§
§
§
Two important expansive rock zones were
encountered during the construction of Los
Quilos and Chacabuquito water tunnels. A
field and laboratory reconnaissance and
characterization program allowed the
definition of bases for design and the
guidelines for tunnel construction and
definitive rock support.
Expansive behavior was mainly due to the
crossing of important north south trending
faults (both tunnels), to zones with thin veins
of ceolites-calcite and clay minerals
(Chacabuquito tunnel), and also to
hydrothermal alteration of the host andesitic
rock mass generated by the intrusion of
monzogranodioritic rocks (Los Quilos
tunnel).
Main faults were steeply dipping structures,
striking nearly N-S, and thus related with the
§
§
regional north-south Pocuro main-fault that
crosses this area approximately 2km
downstream from the western portal of the
Chacabuquito tunnel.
A wide zone where expansive rocks were
detected, extends from PK 0.287 to PK.0.550
of the eastern front of Chacabuquito tunnel,
showing ceolites and clayey minerals as
infilling materials of fractures planes. Some
north-south trending fault zones with
expansive cores were also encountered.
Expansion pressure values varied from less
than 0.3kg/cm2 to 1.40kg/cm2 .
A wide zone of expansive rocks that extends
from PK 0,230 to PK 0,420 was also detected
in the eastern front of Los Quilos tunnel,
showing a noticeable hydrothermal alteration.
Expansion pressure generally ranged from
less than 0.3kg/cm2 to a maximum of
2kg/cm2 .
8
§
§
A heavy flexible tunnel support was designed
for the expansive rock zones above
mentioned. It included systematic rock bolts
and welded steel mesh reinforced shotcrete,
generally combined with the installation of
systematic lattice girders; it also included a
reinforced concrete invert.
Finally, it is interesting to mention that after
one-year operation, no problem has been
detected during the regular inspection of
these tunnels.
Acknowledgements
The authors want to sincerely acknowledge the
authorization and participation of the Chief
Engineer of the Chacabuquito Hydropower Plant
Project, both during study and construction
phases, Mr. Pablo Lois V., co-author of this
paper. We also want to acknowledge to the
technical committee of this congress for allowing
us to present this work, and to Mr. Francisco
Montecinos, who drew all the figures.
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