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Tunnelling in Montréal’s limestones: a
geotechnical characterization.
Véronique Boivin & Brigitte Gagné
Exp Services inc, Montréal, QC, Canada
André Campeau, Martin Tremblay & Sébastien Dubeau
City of Montréal, Infrastructures, Transportation and Environment, Montréal, QC, Canada
ABSTRACT
The City of Montréal, in the province of Québec, will build a 4 km-long watermain to supply the Rosemont Reservoir. A
former quarry and a subway tunnel crossing above the proposed alignment of the 3.6 m tunnel offer a certain challenge.
Geotechnical fieldwork performed in 2012 provided the essential data for the tunnel’s design. A detailed geological
description of the rock cores allowed establishing stratigraphic benchmarks along the tunnel alignment and predicting
fault zones with important vertical displacements. Other captured areas where rock conditions are likely to be
problematic include thick igneous intrusions bearing physical properties radically different from the host rock. The
rockmass characterization allowed formulating recommendations on the tunnel location and proposing optimal strategies
to use during boring. The large amount of generated information undoubtedly constitutes a useful database for any future
deep rock excavation in the Montréal metropolitan area.
RÉSUMÉ
La remise en service du réservoir Rosemont à Montréal, Québec, nécessite la construction d’une conduite d’aqueduc
principale de 4 km. Le passage du tunnel proposé de 3,6 m de diamètre sous une ligne de métro et une ancienne
carrière constitue un des défis du projet. Les investigations géotechniques réalisées en 2012 ont fourni les données
nécessaires à la conception du tunnel. Une description géologique précise des carottes de forage a permis d’identifier
des repères stratigraphiques le long du tracé et d’interpréter d’importants rejets de failles. D’autres zones interceptées
où la condition du roc pourrait être problématique incluent d’épaisses intrusions ignées, dont les propriétés diffèrent
radicalement de la roche encaissante. La caractérisation du massif rocheux a permis de formuler des recommandations
sur la position du tunnel et les stratégies à employer lors du creusage. L’importante quantité d’information obtenue
constituera assurément une base de données utile pour tout autre projet d’excavation profonde dans le roc du Montréal
métropolitain.
1
INTRODUCTION
In the fall of 2012, exp was retained by the City of
Montréal to carry out a geotechnical investigation of the
rockmass on the layout of a proposed 4 km-long
watermain tunnel. The objective was to provide the
geotechnical data and recommendations in terms of
design for the excavation of the tunnel by TBM and the
entry and exit shafts located at both ends of the tunnel.
The entry point of the planned tunnel is located near
the intersection of Sainte-Catherine and Omer-Ravary
streets in Montréal, QC, Canada. With a gentle ascending
slope (0.5% to 1.18% grade), the tunnel will be bored
approximately 40 m under residential and commercial
zones. A subway tunnel and a former quarry, now filled
and transformed as an urban park, are also present above
the tunnel alignment. The exit shaft connects to the
Rosemont reservoir. The total change in the tunnel’s
elevation is 36 m over the 4 km alignment. An overview of
the proposed tunnel layout is shown in Figure 1.
Twenty-seven diamond drill holes were bored during
the 2012 geotechnical fieldwork for a total of 1 251 m
drilled, including 1 033 m cored in the bedrock. A detailed
geological and geomechanical description of the rock
cores was performed, including the descriptions of 3 181
open structures. In situ permeability tests (Lugeon tests)
and laboratory tests on core samples completed the
rockmass characterization.
The study results yielded to a geological model along
the tunnel layout and recommendations on its location
and optimal boring strategies. Stratigraphic benchmarks
were established in the model and allowed for the
identification of fault zones with significant movement.
Other captured areas where rock conditions are likely to
be problematic include large intrusions bearing physical
properties radically different from the host rock.
Olympic
Stadium
Rosemont
Reservoir
Former
quarry area
A subway line
intersects the
tunnel here.
Figure 1. Proposed tunnel layout in Montréal, QC.
The results were reported to the City of Montréal in
February 2013 (Boivin & Gagné, 2013) and compiled in
the city’s geotechnical database (Geotec®). An overview
of the factual data is presented hereafter. This venture
opens up a large database for future projects in the
Montréal metropolitan area.
2
GEOTECHNICAL INVESTIGATIONS
Previous fieldwork took place in 1976 during which 38
boreholes were drilled. Black and white photographs of
the rock cores are available with basic description of the
lithology. No laboratory testing was done on rock cores.
After a thorough analysis of the 1976 data, exp
produced a preliminary geological profile along the tunnel
alignment and recommended a first time to lower the
depth of the tunnel axis from the original 1976 design. The
rockmass crown pillar went from 2.5 m to 6 m at the
southern entry point of the tunnel, where it is the thinnest.
The 2012 drilling campaign proposed by exp was
aimed to complete the work started in 1976 and provide
detailed geological and geomechanical logging of the rock
cores as well as all in-situ and laboratory testing
necessary for the rockmass characterization. The 2012
geotechnical study would then supply a more complete
database, particularly along sensitive zones such as:
 the start and end points of the tunnel and tunnel shafts
where no data was available;
 areas where the 1976 boreholes did show high
fracturing in the bedrock and the presence of breccia;
 the former quarry area, in order to confirm the
maximum depth reached during quarry excavation and
any effect the quarry operations could have had on the
rockmass quality in the vicinity of the proposed tunnel.
The 2012 geotechnical fieldwork took place between
October 11 and December 2, and included:
 geotechnical and environmental soil sampling in the
shafts areas;
 27 NQ size diamond drill holes in the rockmass,
including 22 inclined holes with core orientation;
 44 Lugeon tests at the proposed tunnel elevations;
 135 laboratory tests on rock core samples;
 vibrating wire piezometers installed in 4 drill holes
near the proposed tunnel elevation;
 groundwater monitoring wells at the shafts locations;
 groundwater sampling for corrosivity testing.
Truck-mounted, track-mounted and rail-base CME-55
and BBS-18 drill rigs equipped with 5-foot length doublewall NQ core drilling bits (76 mm-diameter holes and 48
mm-diameter rock cores) were used to core runs of 1.5
metres. A total of 1 251 metres were drilled, including
1 033 metres cored in the rockmass.
3
NATURE AND PROPERTIES OF THE ROCKMASS
3.1
Lithological and structural framework
The proposed tunnel, as shown in Figure 2, traverses a
Paleozoic sequence of sedimentary rocks of the Lower StLawrence Lowlands with near horizontal bedding planes,
varying from 4° to less than 10° dipping East. The tunnel
is located on the east side of the Villeray Anticline, which
plunges slightly to the Northeast (Clarke, 1972). Regional
and local geology including the lithological description are
well summarized by Globensky et al (1993).
In the project area, the underlying sedimentary rocks
belong primarily to two formations of the Trenton Group,
and, to a minor extent in the Southeast area, to the shales
from the Utica Group. In addition to the sedimentary
sequences, several igneous satellitic rock bodies (dikes
and sills) originating from the Mount Royal intrusion are
common throughout.
The Villeray Anticline is sectioned to the south by the
east-west White-Horse–Rapids fault system, with the
main fault located approximately half a kilometre south of
the entry shaft of the proposed tunnel. Two other fault
systems (Northwest-Southeast and Northeast-Southwest)
are also present. Their apparent vertical movement vary
from less than a metre to sixty metres or more. Geological
structures associated to these major faulting zones could
potentially affect the boring of the tunnel.
Figure 2. Simplified geological model along the tunnel alignment, based on the 2012 geotechnical fieldwork results.
The majority of the rocks along the tunnel axis belong
to the Trenton Group. In general, the sedimentary beds
are mainly composed of fine grained limestone with
argillaceous interbeds, rarely fossiliferous and of
fossiliferous fine-to-coarse grained crystalline limestone
with or without shaly partings.
The Montréal Fm, made up of two members, has been
identified only in the last 800 m of the tunnel alignment.
Typically, the Saint-Michel Mb, at the base is composed of
fossiliferous fine-to-medium grained crystalline limestone
in beds 5-30 cm thick separated by thinner black
argillaceous and very fossiliferous micrite, with beds from
2 to 5 cm in thickness. Bioturbation is frequent. It
comprises a second, non-fossiliferous, very hard
crystalline limestone facies that can be followed for the
last several hundred meters of the tunnel layout. The
Saint-Michel Mb is overlain by the Rosemont Mb,
composed of thinly bedded crystalline limestones with
wavy shaly partings. The limestones vary from finely to
coarsely crystalline (micrite to sparite) and from pure to
slightly argillaceous. Beds are lenticular, sometimes
nodular and separated by thin argillaceous black shale
partings and depict bioturbation. The presence of this rock
unit for approximately 500 metres beneath a former
quarry and further north is controlled by two normal faults
(interpreted during the 2012 study) with strong vertical
displacements.
The Tétreauville Fm, overlying the Montréal Fm, is
found in more than half of the tunnel alignment, starting at
the south end and ending near the former quarry. The
rock is characterized by two interbedded facies
comprising a regular sequence of non-fossiliferous
limestone made up of fine-grained dark grey limestone
(beds 2 to 15 cm thick) interbedded with an almost black
micrite (2-8 cm). The second facies is a “cloudy” textured,
nodular fossiliferous limestone (calcareous mudstone)
without regular bedding planes containing both crystalline
and micritic limestones and is quite resistant to breaking.
The Utica Gp overlies the Tétreauville Fm and is found
in drill holes at the bedrock surface in the south end of the
tunnel alignment but never at the tunnel elevation. The
black shales are host to a dark green sill of mafic nature.
All but one of the drill holes intercepted intrusive rocks
of various nature and size. The rocks range from felsic to
mafic. They occur as dikes and sills and originate mainly
from the Monteregian intrusions (gabbro and syenite), and
other magmatic episodes (Clarke, 1972). The dykes and
sills vary in thickness from 1 cm to nearly 10 m. The
thickest and most important intrusion, a sill, has been
identified in several consecutive drill holes in the central
area of the tunnel alignment. In an area just south of the
former quarry, the sill is cut by other intrusive rocks.
Larger intrusions have indurated the host limestones
and shales to near marble and argillite states, sometimes
obturating the bedding patterns. The resulting rock is finegrained, massive, very hard, often developing a complex
fracture pattern. Breccia was also present in the drill holes
near intrusive rocks, or generally near inferred fault zones.
Geomechanical logging of the rock cores has identified
open structures of several types. A total of 3 181 open
structures were described:
 71% (2 270 structures) were open bedding planes;
 24% (754 structures) were random open joints;
 2% (65 structures) were random open structures along
veins or veinlets;
 2% (56 structures) were open structures along
contacts (limestone/shale or sedimentary/igneous);
 1% (36 structures) was random fractures.
From these results, it seems obvious that the joints
associated with bedding planes are the ones likely to
control in large part the stability of the excavation, mainly
in the crown of the tunnel. Except rare joints filled with silt
(3 cm thick in average) along the limestone bedding, the
joint planes generally show little or no signs of weathering,
and are occasionally re-crystallized.
The other joints (sub-vertical and oblique), and those
associated with intrusive rocks are also likely to affect the
excavation. However the underrepresentation of these
structures during the present campaign did not allow
identifying regular joint spacing or orientation.
Nevertheless, vertical structures striking NorthwestSoutheast and Northeast-Southwest were observed in
most quarries on the island of Montréal and the tunnel
may encounter these joints.
3.2
Material Properties
Core samples were sent to the rock mechanics laboratory
of Ecole Polytechnique de Montréal for testing. The
specimens were prepared and tested in accordance with
ASTM standards for the following tests:
 62 uniaxial compressive strength tests with strain
measurements;
 20 triaxial compressive strength tests;
 28 splitting tensile strength tests (Brazilian test);
 6 direct tensile strength tests;
 19 abrasiveness determination tests (CERCHAR).
All test results were compiled per geological formation.
The limestones are represented by a fair amount of
samples. The various types of igneous intrusions were
treated as one formation for the needs of this project. The
Utica shales results are provided but should be used with
caution considering very few samples were tested.
Unit weights are provided in Table 1. Test results for
the uniaxial compressive strength (UCS) of intact rocks
are summarized in Table 2 and compiled in Figure 3
according to the ISRM suggested categories of strengths
(ISRM, 1981).
Table 1. Unit weights.
Formation
No.
Utica shale
Trenton limestone
Tétreauville
Near marble
Rosemont
Saint-Michel
Intrusive rocks
4
65
34
3
11
17
12
Unit weight (kN/m3)
Min
Mean
Max
St.dev.
26.45
22.66
26.16
22.66
26.38
26.34
24.36
26.54
26.46
26.41
25.54
26.65
26.60
27.53
26.65
27.11
26.90
27.11
27.01
27.00
30.75
0.08
0.53
0.17
2.50
0.23
0.25
2.25
Table 2. Uniaxial Compression results for intact rock.
Formation
No.
Utica shale
Trenton limestone
Tétreauville
bedded
nodular
Near marble
Rosemont
Saint-Michel
foss.
non- foss.
Intrusive rocks
4
45
23
13
10
3
7
12
6
6
12
Young’s modulus E (GPa)
UCS (MPa)
Min
Mean
Max
St.dev.
Min
Mean
Max
129.08
43.85
43.85
56.22
43.85
101.43
119.12
64.60
64.60
83.86
107.86
163.30
142.78
145.32
164.77
120.03
133.28
140.84
141.41
124.63
158.19
227.78
218.12
306.28
306.28
306.28
195.04
178.53
174.36
212.70
185.19
212.70
431.71
40.10
55.31
67.98
79.17
41.01
40.26
19.35
49.51
48.58
48.56
109.69
16.34
7.03
7.03
7.03
22.72
39.60
23.95
21.87
21.87
38.01
25.84
22.88
54.77
48.97
51.59
45.58
72.84
59.09
58.84
53.34
64.34
71.84
38.87
91.67
76.46
76.46
71.01
91.67
76.25
77.50
77.50
75.41
112.67
100%
TÉTREAUVILLE
80%
ROSEMONT
SAINT-MICHEL
UTICA
60%
INTRUSIFS
40%
20%
0%
25-50MPa 50-100MPa 100-250MPa >250MPa
(R3)
(R4)
(R5)
(R6)
Figure 3. Distribution per ISRM strength class of the UCS
values obtained for each rock unit.
The dispersion in the UCS results is notable in all
formations. But the mean values are fairly similar between
the Trenton Gp formations (see Table 2) with a global
mean of 143 MPa (R5 grade, or very strong). The data
was also compiled according to the rock facies for the
Tétreauville and Saint-Michel Fms. The results indicate
that samples of well-bedded Tétreauville Fm are stronger
(165 MPa) under uniaxial loading than samples of nodular
Tétreauville Fm (120 MPa). Similarly for the Saint-Michel
Fm, the non-fossiliferous samples seem to be stronger
(158 MPa) than the fossiliferous samples (125 MPa).
The intrusive rocks scored the highest values of UCS,
with a mean of 228 MPa (R5 grade, or very strong) and a
maximum of 432 MPa (R6 grade, or extremely strong).
This last result indicates that boring may be difficult if
these rocks are intercepted in the projected tunnel. The
contrast between the host rocks’ and the intrusive rocks’
UCS is important when considering the cutting tools cost
on the TBM.
The effect of the anisotropic nature of the sedimentary
rocks was not verified on the UCS. However, for the
tensile strength, direct tensile strength testing was
Poisson’s ratio ν
St.dev.
10.73
21.20
20.08
22.48
17.00
28.87
20.75
20.39
25.02
14.70
21.98
Min
Mean
Max
St.dev.
0.09
0.04
0.04
0.04
0.13
0.30
0.17
0.11
0.11
0.21
0.14
0.13
0.23
0.21
0.22
0.19
0.34
0.25
0.25
0.23
0.27
0.28
0.16
0.41
0.31
0.31
0.29
0.41
0.31
0.30
0.30
0.29
0.49
0.03
0.08
0.08
0.09
0.06
0.06
0.06
0.06
0.08
0.03
0.08
performed in addition to common Brazilian tests (indirect
or “splitting” tensile strength) to allow estimating lower and
upper limits, as shown in Table 3. Considering the
boreholes were roughly perpendicular to the bedding, the
direct tests were done by pulling perpendicularly to the
laminations or weakness planes in the core samples,
while the indirect tensile tests were done splitting the core
samples approximately along their strongest direction.
To estimate Hoek-Brown failure criterion parameters
for the intact rock (Hoek & al., 2002), triaxial testing was
performed on limestone samples, in addition to the
uniaxial compressions and the tensile strength tests, to
obtain better failure envelops. 20 tests were done with
confining pressures of 10 and 20 MPa. RocData© 4.0
from Rocscience was used to estimate the Hoek-Brown
parameters (see Table 4).
Table 3. Direct and indirect tensile strengths of intact rock.
Formation
Utica shale
Test
type
No.
Indirect
4
Min
Mean
Max
22.40
16.99
13.11
2
1.46
0.84
0.21
Indirect
Direct
18
4
19.63
3.36
11.58
1.76
7.00
0.46
Tétreauville
Indirect
Direct
10
3
19.63
3.36
12.72
2.13
7.00
0.46
Near marble
Indirect
2
9.57
9.28
8.98
Rosemont
Indirect
2
10.40
9.83
9.25
Indirect
4
13.66
10.74
8.20
Direct
1
0.64
0.64
0.64
Indirect
6
26.25
16.28
9.21
Trenton
limestone
Saint-Michel
Intrusive rocks
Direct
Tensile strength (MPa)
Table 4. Estimated parameters of the Hoek-Brown failure
criterion for intact rock.
Formation
Tétreauville
Rosemont
Saint-Michel
No.
σci
mi
s
a
45
13
21
136.71
119.37
140.56
8.61
11.37
10.27
1
0.5
Except for the Saint-Michel Fm, the uniaxial
compressive strengths σci estimated with the Hoek-Brown
failure envelop are more conservative than the UCS
results. But the mi parameters obtained are in accordance
with those commonly reported. For instance:
 micritic limestones: 8 ± 3 (compared to Tétreauville
and Saint-Michel);
 sparitic limestones: 10 ± 5 (compared to Rosemont);
 crystalline limestones:12 ± 3 (compared to Rosemont
and Saint-Michel)
Lastly, the laboratory determination of abrasiveness is
summarized in Table 5. All intrusive rock samples and 2
near marble samples showed high abrasiveness.
Limestone samples showed medium abrasiveness.
m/s) in more fractured zones, around the area of major
intrusion south of the former quarry and the former quarry
area itself. Also, severe loss of drilling water was
observed at those locations.
The water level was also measured in each drill hole
and was generally found near 5 m below surface.
Vibrating wire piezometers were installed in 4 drill holes at
the tunnel level or right above the tunnel crown in order to
measure water pressures. These instruments are located:
1) where the constructor could dig a temporary shaft at
mid-way on the tunnel; 2) where a major igneous intrusion
was intercepted south of the former quarry; 3) the former
quarry area; and 4) next to the reservoir Rosemont. The
water pressures (varying between 252 and 304 kPa) were
not stabilized during first readings. The instruments are
available for monitoring before and during the tunnel
construction to detect any abnormal pressures when the
TBM approaches the instrumented areas.
Table 5. CERCHAR Abrasiveness Index results (CAI).
Formation
Number CAI Mean (min, max) Classification
Intrusive
rocks
9
2.88 (2.50, 3.33)
High
abrasiveness
3.4
Limestones
(all Fms)
10
1.89 (1.47, 3.27)
Medium
abrasiveness*
As a preliminary evaluation of the rockmass quality, the
Rock Tunnelling Quality Index, Q (Barton et al. 1974) was
used to describe the rock along each drill hole. The
classification’s parameters assigned are summarized
hereafter based on the geomechanical logging results.
* 2 high abrasiveness values obtained in near marble samples.
3.3
Hydrogeology
As a preliminary characterization of the rockmass
permeability around the proposed tunnel, Lugeon tests
were performed in 21 drill holes. Two 4.5 m-long test
intervals were used in each drill hole at near tunnel
elevations (above and below the tunnel axis to cover the
floor, front and crown). The resulting hydraulic
conductivities are shown graphically along the tunnel
alignment in Figure 4 along with critical benchmarks.
Green data points show results for test intervals
located in limestone with a mean value of 2.4E-7 m/s,
whereas red data points show results for intervals partially
or entirely in intrusive rocks with a mean value of 5.8E-7
m/s. The results suggest that the rockmass has a medium
permeability in general, with localised highs (up to 2.7E-6
high
permeability
1,0E-05
The Rock Quality Designation index (RQD) and the Joint
Number parameter, Jn, used in Q index calculation were
determined for each 1.5 m drilled runs.
Mean RQD values are given in Table 6, whereas
Figure 5 shows the distribution of the results per RQD
class. More than 70% of the runs drilled in limestone got a
RQD value higher than 90%. Typically, the lowest values
were obtained in runs drilled near surface or at the contact
with intrusive rocks. The Saint-Michel Fm looks slightly
better than the other limestones’ formations; however this
is likely due to its absence in surface in this project.
Subway
tunnel
Major
intrusion
Former
quarry
medium
permeability
1,0E-06
Mainly limestone
Includes intrusive rocks
3.4.1 Rockmass structures
1,0E-07
1,0E-08
low
permeability
Hydraulic Conductivity, K (m/s)
1,0E-04
Rockmass Classification
1,0E-09
Station along tunnel project (km)
Figure 4. Estimated hydraulic conductivities in drill holes along the project alignment.
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
-0,5
1,0E-10
100%
100%
TÉTREAUVILLE
TÉTREAUVILLE
ROSEMONT
80%
ROSEMONT
80%
SAINT-MICHEL
SAINT-MICHEL
INTRUSIF
INTRUSIF
60%
60%
UTICA
40%
40%
20%
20%
UTICA
0%
0%
0-25 %
25-50 % 50-75 % 75-90 %
≥ 90 %
RQD
Figure 5. Distribution per RQD class of the RQD values
obtained for each rock unit.
Table 6. Mean RQD values per rock formation.
Formation
Utica shale
Tétreauville
Rosemont
Saint-Michel
Intrusive rocks
No. of
runs
10
431
175
48
106
RQD
mean
80
93
86
98
83
Rock Quality
(based on RQD only)
Good
Excellent
Good
Excellent
Good
0.5
2
3
4
6
20
Jn
Figure 6. Jn parameter distribution for each rock unit.
The distribution of the Jn parameters assigned to each
drilled run is shown in Figure 6. Most often, a value of 2 or
3 was given, i.e. one joint set or one joint set plus random
structures. The bedding joint set represented 71% of the
logged structures. The drilling orientation did prevent from
verifying the presence of sub-vertical or oblique joint sets.
They appeared as randomly oriented.
Joint spacing was also estimated based on the count
per meter of open joints logged along the drill holes. The
resulting apparent spacings are given in Table 7. An
estimate for the spacing between open bedding structures
could be obtained. This information will be useful when
modeling the rockmass for stability analyses of the tunnel.
Table 7. Estimate of joint spacing.
3.4.2 Roughness and frictional characteristics of joints
All open structures
Formation
Utica shale
Tétreauville
Rosemont
Saint-Michel
Intrusive rocks
No.
runs
10
431
175
48
106
Mean
spacing (m)
0.19
0.51
0.34
0.47
0.35
Bedding structures
only
No.
Mean
runs
spacing (m)
6
0.19
378
0.68
170
0.37
46
0.52
-
Table 8. Jr and Ja values for joint condition descriptions.
Formation
Utica shale
Tétreauville
Bedded
Nodular
Near marble
Rosemont
Saint-Michel
Intrusive rocks
Q system
Jr
Ja
1
1 – 1.5
1 – 1.5
1-2
1-2
1 – 1.5
1-2
1-2
1-2
1-2
1-3
1-2
1-3
1-3
1-2
1-4
The joint roughness and the joint alteration parameters, Jr
and Ja, were determined for each geological structure
during the geomechanical logging. Assigned values of Jr
ranged from 0.5 to 4 and values of Ja ranged from 0.75 to
10. A value was then assigned to each drilled runs. In
absence of structures, a run was given a Jr of 4 and a Ja
of 0.75. The most often used values for drilled runs are
summarized in Table 8 for each formation.
3.4.3 Stress parameters
Stress parameters for Q calculation include the joint water
reduction factor, Jw, and the stress reduction factor, SRF.
A conservative value of 0.5 was used for Jw to take
into account the high anticipated water head above the
tunnel (located at about 40m under surface) and the
relatively high pressures (˃250 kPa) measured in the
installed vibrating wire piezometers.
The in situ state of stresses is assumed to be
gravitational. When comparing the crown thickness above
the tunnel to the UCS, two values of SRF are found: 2.5 is
assigned to drill holes where the rock crown pillar is thin
(at the entry point of the tunnel, below the former quarry
and in the upper portion of the exit shaft) and 1 is
assigned everywhere else.
3.4.4 Rock Tunnelling Quality Index, Q
Using the previously discussed parameters, the Q index
was calculated per run and included in the drill hole logs
and the geological model of the project. The mean values
are summarized in Table 9. The distribution per rockmass
quality class is shown in Figure 7.
5% of all drilled runs (not shown) have a Q index
higher than 40 (very good to exceptional quality
rockmass), 49% have a Q index between 10 and 40 (good
rockmass quality), 33% have a Q index between 4 and 10
(fair rockmass quality) and 13% have a Q index less than
4 (poor rockmass quality). Typically, lower values of Q
were assigned to run drilled in surface. But low Q indices
were also obtained in depth, in fractured zones or with
altered structures, at contact with igneous intrusions, and
in intrusion areas.
A notable difference exists in the Q values between
the Tétreauville Fm and the members of the Montréal Fm.
This is largely explained by the higher SRF factor (2.5)
assigned in areas where the Montréal Fm was absent
(except for the exit shaft).
Table 9. Mean Q indices per geological formation.
Formation
Utica shale
Tétreauville
Rosemont
Saint-Michel
Intrusive rocks
Breccia
No. of runs
10
431
175
48
106
4
Q
(mean)
4.70
23.35
12.71
22.50
15.01
5.90
Quality
(mean)
Fair
Good
Good
Good
Good
Fair
100%
Extremely
poor
Very poor
Poor
Fair
Good
Very good
Extremely
good
Exceptional
80%
TÉTREAUVILLE
ROSEMONT
SAINT-MICHEL
INTRUSIF
UTICA
60%
40%
20%
0%
< 0.1
4 - 10
10 - 40
40 - 100
Rock tunneling quality index, Q
Figure 7. Rock tunneling quality index Q distribution for each rock unit.
4
0.1 - 1
1-4
SUMMARY
OF
THE
GEOLOGICAL
AND
GEOTECHNICAL SECTION ALONG THE TUNNEL
ALIGNMENT
Precise mapping of the geological units in each rock core
has allowed partial reconstruction of the stratigraphy and
structure of the bedrock and the presentation of a cross
section where the correlation between the geological units
was greatly advantageous in evaluating potentially
problematic areas along the tunnel alignment. The
identification of the sedimentary facies and igneous
bodies has also allowed for the selection of representative
samples to submit for testing of rock properties. This
information can then be used for predicting the challenges
that may occur during tunnel excavation.
Southeasterly dipping sedimentary rocks, mostly
carbonates showing medium abrasiveness indices, will
comprise the bulk of the rockmass along the tunnel. The
direction of the dip will be, within a few degrees, subparallel to the tunnel axis. For all but two areas, the thick
rockmass overlying the proposed tunnel should offer good
quality in average. Major changes in sedimentary rock
100 - 400
> 400
formations, brought along by normal faulting, will be
encountered in two areas on the alignment near stations
2+600 m and the 3+150 m. However, all formations, apart
from various intrusions, are composed of various types of
limestone (fine-grained crystalline to coarse–grained
fossiliferous, to shaly) with only few differences as to their
overall composition. Knowledge of the intrinsic rock
properties will help predict if changes in the boring
techniques have to take place.
All along the tunnel, narrow dikes and sills of highly
abrasive intrusive rocks running in random directions will
be intercepted during the excavation. An 8 to 10 m thick
sill, present along the previous course of the proposed
tunnel near mid-distance will be avoided following a
change in slope and depth of the tunnel axis. However,
associated
indurated
limestones
(near
marble),
argillaceous limestones, breccias, and other associated
dikes will still be present and potentially cause a challenge
along at least 10% of the course, at depth reaching
approximately 40-45 m, particularly at a distance of 2 360
to 2 500 m from the south end of the tunnel, where a
structurally complex zone of sills and dikes from various
magmatic episodes coupled with breccias and marble will
be encountered.
Two major fault zones absent from regional maps,
identified at approximately 2 560 to 2 660 m and between
3 100 and 3 200 m from the south end of the tunnel may
offer difficulties during the excavation due to the highly
fractured rock and brecciated zones encountered at
various elevations in these areas. Their nature and
orientation could not be verified but hypothetically could
be related to the White-Horse-Rapids fault system. Other
suspected fault zones with minor displacement are
present (hinted by fractured rock, rock flour, breccia)
along the course, notably near the 1+020 to 1+080 m
stations. The sudden disappearance of the thick sill at the
station 1+900 m combined with fractured rock and a
change in inclination of the strata reinforces the evidence.
Two particular areas of the alignment may call for
caution. In the south end of the tunnel, the vertical shaft to
access the tunnel will be excavated into a thick (>20 m)
sediment cover. It will then penetrate less than 10 m of
rock into soft black shales and intercept a hard igneous
sill before cutting through more shales and limestone. A
relatively thin (<10 m) rockmass over the first 150 m of
tunnel may command extra precaution in terms of stability
for the structure. Another area of such thinness of rock
cover is the area below the former quarry. In this instance,
rock instability may be accentuated by possible deep rock
fracturing. Hydrogeological data collected in the vicinity of
the quarry supports this hypothesis.
5
CONCLUSIONS AND FURTHER WORK
The collected data during the Fall of 2012 allowed a better
understanding of the expected rockmass conditions along
the alignment. The results have enabled identifying
geological structures (faults and folds) absent from the
regional maps and allowed the engineering team to
modify the profile of the proposed tunnel a second time in
order to bypass a very hard and abrasive sill along a
distance of at least 450 m, between stations 1+800 and
2+250 m along the tunnel axis.
Further work includes stability analysis by means of
numerical modeling to develop the ground support
standards to use along the tunnel. Rock cores are also
available for further laboratory testing; joint shear
strengths along bedding planes is another parameter to
be evaluated. The study of the anisotropic behaviour of
the limestones may also be pursued.
More research on the newly identified geological
structures is carried out by geology students and will allow
a better understanding of the potential impact on the
construction work. A thorough research in the city’s
geotechnical database combined with outcrops mapping
may provide enough information to map the structures.
Lastly, during construction, continuous geological
mapping and instrument monitoring will allow for the
timely detection of any unstable condition. All these efforts
will definitely contribute to a better knowledge of
Montréal’s bedrock.
ACKNOWLEDGEMENTS
The writers would like to acknowledge the contribution of
Dr. Ghislain Lessard (Sobek Technologies) to the
configuring of the geomechanical database, and the
contribution of Dr. Robert Corthesy and Dr. Maria Helena
Leite (Ecole Polytechnique de Montréal) to the laboratory
testing program on rock cores.
REFERENCES
Barton, N. et al. 1974. Engineering Classification of Rock
Masses for the Design of Tunnel Support. Rock Mech.
6(4): 189-236.
Boivin, V., Gagné, B. 2013. Rapport factuel, Investigations
géotechniques 2012, Réservoir Rosemont (Lot 1) –
Conduite d’alimentation en tunnel de 2 100 mm de
diamètre. Projet A-209. Ville de Montréal, Québec.
Clark, T.H. 1972. Montréal Area. Geological Report 152.
Ministère des Richesses naturelles, Québec.
Globensky, Y. et al. 1993. Lexique stratigraphique
canadien, Volume V-B. Région des Appalaches, des
Basses-Terres du Saint-Laurent et des Îles de la
Madeleine. Ministère de l'Énergie et des Ressources,
Québec, DV 91-23.
Hoek E, Carranza-Torres C, Corkum B. 2002. HoekBrown failure criterion-2002 edition. Proc. of the 5th
North Am. Rock Mech. Symp. 1, Toronto: 267–273.
International Society for Rock Mechanics Commission on
Testing Methods. 1981. Rock Characterization,
Testing and Monitoring: ISRM Suggested Methods.
Pergamon, Oxford