Underground Segment of Montreal Réseau Express Métropolitain (REM)

The Réseau Express Métropolitain (REM) is an electric and fully automated, light-rail transit network designed to facilitate mobility across the Greater Montreal Region in Canada (Nasri, 2022). This new transit network will link downtown Montreal, South Shore, West Island, North Shore, and the airport (Figure 1). The project consists of 67 km of twin tracks over four branches connected to downtown Montreal. The REM system will connect with existing bus networks, commuter trains and three lines of the Montréal Metro. Once completed, the REM will be fourth largest automated transportation systems in the world after Singapore, Dubai, and Vancouver.

The project includes 26 stations with 3 underground stations in downtown Montreal. One of the underground stations was built using the NATM and the two others with the cut and cover approach. The project also included the rehabilitation and enlargement of the Mont Royal Tunnel. This 100-year-old double track tunnel is about 5 km long. The REM also consisted of 3.6 km new TBM tunnel connecting downtown to the Montreal International Airport through saturated soft ground and karstic rock.

The project is currently under construction by a joint venture of SNC Lavalin, AECON, Dragados, EBC, and Pomerleau and the final design was performed by a joint venture of SNC Lavalin and AECOM. Its construction will be completed at the end of 2024. To deliver this major project, several underground works were undertaken. This paper presents the major underground developments of the REM and the solutions used for the successful achievement of the underground construction objectives which included ensuring safety and stability of the opening during construction and for its full-service lifetime, minimizing impact and disturbance to the surrounding environment, meeting Owner’s technical requirements, and minimizing cost, duration, and risk of underground construction.

Geological Context

Montreal geology consists of a variety of sedimentary horizons dating from the Precambrian, Cambrian and Ordovician periods. The main associated lithologies are limestone and shale. Intrusive rocks dating from the Mesozoic/Cretaceous period are also encountered throughout Montreal, intersecting the sedimentary horizons.

The strata are generally relatively sub-horizontal layers of sedimentary rock. However, events such as faulting, folding, glaciation and isostatic movement have shaped the strata differently in certain regions of the island. Faults are generally considered inactive in the region.

Solutions that were put forth for the successful completion of the REM underground works were selected to best match project constraints and local ground conditions. Located in different parts of the city, intersecting different strata, a total of four different types of underground works were undertaken.

Deep Underground Station

To connect the deeply sitting REM track to an existing metro station located closer to the surface, an underground station accessible by an approximately 70 m deep vertical shaft was designed (Edouard Montpetit Station, EMP). Figure 2 shows the 3D model of the station with the main entrance shaft, the side platforms constructed by enlarging the existing Mont Royal Tunnel, concourse tunnel, ventilation tunnels and shafts and vertical circulation tunnels and shafts. Figure 3 shows the excavation of the main entrance shaft and tunnels. This station configuration was optimized to minimize the rock excavation volume. With practically no overburden present in the area, the interchange station is almost entirely located within the Trenton formation which consists of interbedded limestone/shale packages and argillaceous limestone. The station is also located near the intrusive Mont Royal formation which consists of gabbro, monzonite, and breccia, resulting in a significant number of hard dikes and sills in the area.

In this area, probably resulting from the contact metamorphism due to the close proximity of the Mont Royal intrusive, the sedimentary rock shows hard rock properties, with uniaxial compressive strength (UCS) varying between 125 and 180 MPa and Young’s modulus varying between 75 and 88 GPa. Because of the nature of the work, the quality of the rock and the lower initial cost of the method, controlled drill and blast was selected to sink the deep vertical shaft and excavate the underground station.

The excavation took place in a densely populated area with major infrastructure in the near vicinity. Among these is one of the main University of Montreal campuses. These buildings house lecture theaters and laboratories including a recently completed state of the art and highly sensitive acoustic laboratory located within a few meters of the excavation. Hence, several engineering control mechanisms were put in place to minimize impact and disturbance including line drilling technique along the full entrance shaft excavation perimeter and for its entire depth, use of a maximum blast round length of 2.5 m, blasting sequences and patterns designed for low impact (specific blast hole, loading and delay patterns). Table 1 shows the City of Montreal allowable peak particle velocity for different types of building. Type 1, 2 and 3 are for commercial, residential, and historical buildings respectively. The line drilling was performed using a DTH drill with the holes diameter of 140 mm placed at 250 mm center to center. The holes center was put at 200 mm from the excavation line to account for the vertical deviation of the drilling operation. To complement this effort, a comprehensive monitoring plan, counting over 150 instruments, was used.

Permanent CT bolts and shotcrete reinforced with steel fiber (Dramix 3D) were used as both initial and final liners for all shafts and tunnels. A layer of 5 cm of flashcrete was applied first for safety, the bolts were installed, spray on waterproofing membrane (BASF Masterseal 345) was added, and then another 5 cm of steel fiber reinforced shotcrete was applied. The liner was designed for a 125-year service life per the contract requirements. During development of the underground excavation, rock mapping was undertaken after each exposure of the final wall. Permanent rock bolting pattern was adjusted based on the ground condition and the need for additional rock bolting was assessed on site to ensure the overall stability of the excavation. In addition to durability, the shotcrete mix was designed specifically for the cold weather application in Montreal.

The EMP Station was built within the existing double track Mont Royal Tunnel (MRT). The side platforms were built by enlarging the existing tunnel. Figure 4 shows the side platform enlargement allowing one track to remain in operation.

Mont Royal Tunnel Rehabilitation

The Mont Royal Tunnel is a railway tunnel in operation since 1918, third longest in Canada, which connects the city’s Central Station (Gare Centrale), located Downtown Montreal, with the north side of the Island of Montreal and Laval, passing through Mount Royal (Figure 5). The REM project will use this existing double track horseshoe tunnel (5060 m long, 9.6 m wide and 4.4 m high, and a constant 0.6% grade) and two of the project stations were built inside this tunnel by enlarging it from a double track tunnel to side platform station at the location of these stations. To accommodate the new track system and to ensure the tunnel is to current safety standards complying with NFPA 130 fire life safety requirements, the existing tunnel conditions were assessed. Various solutions including the installation of a center wall and boring a parallel egress tunnel on one side of the existing tunnel and connected to the existing tunnel through cross passages at regular spacing were evaluated. Adding a center wall to the existing double track tunnel was selected as the preferred solution.

To accurately evaluate the current conditions and define the tunnel enlargement needs based on the new train envelops, a high-resolution laser and optic scanning was performed by Dibit. Using this information, the current conditions of the existing tunnel and accurate clash analyses and interfaces requirements assessment was performed. Results from such analyses were used for the development of the optimal solution to minimize the volume of enlargement excavation.

Cut and Cover Stations

To connect the financial district of Montreal to the REM tracks located approximately 15 m below grade, a major station was planned on McGill Avenue (Figure 6). In this area, due to previous underground works for building the MRT and the Montreal Metro Green Line, the first 15 m of ground consists of backfill. The rock located below this layer of backfill was observed to belong to the Tétrauville formation, which consists of an interbedded limestone/shaly limestone.

Given the local stratigraphic column and low water table, the soldier piles and lagging wall solution was selected as the support of excavation for this station. Drilled soldier piles were socketed into the rock and steel fiber reinforced shotcrete were used for the lagging. The soldier piles and shotcrete lagging support of excavation walls were considered as the permanent station walls. Once the rock is reached, controlled drill and blast took place to cut the rock to the design level.

The station is located between two major high-rise buildings and is connected to shopping centers inside these buildings. An existing underground commercial passageway connecting these two high-rise buildings is just above the station and were kept in place during the station construction. The station is at the intersection of McGill Avenue with two of the most important streets in Montreal and therefore the maintenance of traffic and utility relocation were among the main challenges of this station construction. In addition, the site limits of this station were at the edge of the adjacent buildings and the existing metro tunnel and therefore robust design of support of excavation in soil, controlled drill and blast in rock, and comprehensive instrumentation and monitoring program were required to prevent damage to the neighboring structures. The station new tracks were installed at the location of the existing Mont Royal Tunnel tracks and therefore the excavation had to be sequenced in a way to keep the existing tunnel liner in place as long as possible in order to minimize the existing tunnel closure.

The Technoparc Station, located at the northeast corner of Alfred-Nobel Boulevard and Albert-Einstein Street, is an underground station on the Airport branch. A method of construction using secant piles for the sidewalls was selected as the preferred structure for this station that serves as both temporary and permanent support of the excavation. Wide-flange permanent roof beams act as struts during excavation. This method is used for the approach ramp, the cut-and-cover tunnel structures and the Technoparc Station. The secant piles were drilled to rock and socketed into it. The excavation was drained during the construction and the final structures were tied down to the bedrock for buoyancy control during the permanent condition. Minimum 100 mm shotcrete liner was installed on the inside face of the secant pile wall. Sprayed on water proofing system was integrated in this liner which provides the water tightness required for the permanent condition.

The secant pile wall was designed to resist the lateral earth and hydrostatic pressure. To support lateral pressure, two levels of permanent struts were provided at station area. These struts and their walers were integrated as the main structural elements of the station carrying the lateral ground loads and the vertical loads of the station (Figure 7). 0.5% verticality was assumed for the construction tolerance of these secant piles. The wide flange section, the main pile reinforcement, installation tolerance was considered 25 mm horizontal and 1 degree rotation at top of the pile.

Airport Tunnel

The REM connects downtown Montreal to the airport, requiring the development of an entirely new underground line that runs below the international airport airstrips. The overall underground stretch consists of approximately 3.6 km. Along the tunnel alignment, bedrock elevation varies significantly, and a constant grade is observed at surface, resulting in an overburden thickness varying between 12 and 20m. The overburden consists of layers of backfill, granular material and glacial till, going from grade to bedrock. The bedrock consists of interbedded limestone/shaly limestone, belonging to two different formations: the Tétrauville formation and the Montreal formation.

Within the Tétrauville formation, two different members are expected to be intersected by the underground works. The upper horizon would consist of a good quality micritic shale with UCS values varying between 75 and 185 MPa, Young’s modulus varying between 25 and 65 GPa and Cherchar abrasivity index varying between 0.8 and 1.8. The lower horizon would consist of softer shaly limestone with UCS values varying between 60 and 80 MPa, Young’s modulus varying between 35 and 40 GPa and Cherchar abrasivity index varying between 0.3 and 0.6. Within the Montreal formation, only the Rosemont member is expected to be intersected. This member is expected to be of good quality with UCS values varying between 55 and 145 MPa, Young’s modulus varying between 45 and 65 GPa and Cherchar abrasivity index varying between 0.8 and 1.8.

To align the REM system with the Airport station designed by others, the REM double-track descends from surface to approximately 40 m below grade. Given the significant constraints related to developing a tunnel underneath an international airstrip, the main portion of the underground works were performed using a hybrid tunnel boring machine (TBM). From the surface, the REM started its descent and entered a cut and cover underground station (Technoparc Station). Heading out of the station, the cut and cover portion continued and widened over a 13 m length at about 125 m away from the station to serve as the TBM launch pit. At this point, the tunnel invert was at approximately 14 m below surface, in the overburden soil.

The hybrid TBM was launched in the overburden material and continued its descent and progress in this material for about 300 m. The ground was improved at the break-out over the first 10 m of the drive to allow watertightness and TBM control at the launch. Once the TBM reached the bedrock, it continued its descent over a course of about 300 m, down to 40 m below grade. From there, it progressed at this constant elevation, totalizing approximately 2.7 km of excavation within the rock. The hybrid TBM was able to progress within the overburden loose material in Earth Pressure Balance (EPB) mode and in open mode during its progression through competent rock.

As the TBM was advancing, precast segmental lining was installed, ensuring the stability of the excavation and the safe development of the tunnel. Routine probing ahead of the face was also performed to assess ground mechanical and hydraulic conditions prior to advancement. Depending on ground conditions, pre-excavation grouting performed ahead of the face was required to improve mechanical and hydraulic properties of the ground before the TBM excavation.

Architectural Design

The architectural signature of the stations was based on three main themes: movement, identity, and transparency. A language expressing the kinetic movement of the train and the landscapes that follow one another was integrated through the various components of the network. The identity in the signature was constructed through a set of common elements and by considering the context introducing the notion of variability. Finally, transparency was implicitly reflected in the stations by design guidelines such as natural lighting, security, and the relationship with the context (Figure 8).

Conclusion

This paper presents the main design and construction aspects of the underground elements of the REM mega project in Montreal. It discusses the details of one of the underground stations built using the NATM. It also explains the rehabilitation and enlargement of the existing Mont Royal Tunnel and the construction of the airport TBM tunnel. Several innovative and cost-effective design and construction methods used for the REM project were discussed in this paper.

REFERENCES

Nasri, V., De Nettancourt, X. and Rey, A. 2022. Construction of REM Project in Montreal. World Tunnel Congress 2022, Copenhagen, Denmark.
Ramirez, H., Lotfi, R., Bhargava, A. and Nasri, V. 2022. Design and Construction of McGill Station in Downtown Montreal. World Tunnel Congress 2022, Copenhagen, Denmark.
Lee, J., Gupta, S., Bhargava, A. and Nasri, V. 2022. Design and Construction of Technoparc Station in Montreal. World Tunnel Congress 2022, Copenhagen, Denmark.
Nasri, V. 2021. Design and Construction of a Deep Underground Station in Urban Area. Rapid Excavation Tunneling Conference 2021, Las Vegas, NV, USA, 1078-1088.

Verya Nasri is Chief Tunnel Engineer, SVP, with AECOM, New York, NY, USA.