The atwater potable water Treatment Plant (Usine de filtration d’eau d’Atwater) was first commissioned in 1911 and has over the years undergone a series of significant upgrades with the last one being in 1967. The plant is the second largest potable water treatment plant in Canada and it currently provides potable water to approximately 42 per cent of the Montreal Island population. It has a capacity to treat 1,350,000m3 of water per day (356.6 MGD) and its average daily treatment is approximately 650,000m3 of water per day (172 MGD). The plant takes its water from a nearby aqueduct channel that was originally built in 1854 to provide clean water to the south east area of Montreal. The channel has also undergone several upgrades and reconstruction to its current configuration of approximately 40m width and approximately 8m depth.
The channel takes its water from the Saint Lawrence River at approximately 8km upstream and goes through the Lachine rapids some 4km from the plant. The channel ends approximately 400m downstream at the current treatment plant intake structure.
Near the plant, the channel is bounded to the north by a very busy section of Highway 15, which links the Champlain Bridge to the Turcot Interchange, which are both having major reconstruction and replacement works as they have reached the end of their design life. It should be noted that the Champlain Bridge is the most travelled bridge in Canada.
The portion of Highway 15 that borders the channel is set to undergo major works starting mid 2018. Following a Quebec provincial regulation on water withdrawal and protection that went into effect in August 2014 and requires evaluation of risks around all water withdrawals within the province, the Montreal Potable Water Department has undertaken a risk analysis exercise for the Atwater plant. Highway 15 scored the highest among all risks that were identified. In addition, the planned works on that section of Highway 15 may further impact the quality of the water in the channel and lead to the plant closure for a long period of time. With the expected increase of traffic on that stretch of the highway there is an increasing risk of a potential chemical spill into the channel from the highway as it is travelled by trucks carrying dangerous goods and chemical materials. The department therefore recommended securing the current intake from potential risks of contamination from the surrounding environment.
In addition, to alleviate these risks, the City of Montreal is currently undertaking the construction of a new intake structure to permanently secure the water source of the Atwater plant by isolating the section of the channel most at risk from sources of contamination. This will be achieved by building a new water intake within the channel approximately 1km away from the current intake location and conveying the water through a siphon system that will carry water to the existing facilities. The project is being delivered under a fast-track design and construction schedule to meet the nearby project’s schedule starting mid-2018.
The project is funded by the provincial government as well as the City of Montreal.
PROJECT DESCRIPTION
The new intake structure that is being built will connect to the existing plant facilities by two deep shafts and a conveyance tunnel as well as a downstream connecting channel that links to the existing facilities at the plant. The main components of the project are:
¦ An intake structure that is approximately 21m wide and 55m long, of which 10m is in the channel, itself approximately 6m deep. The intake is located in the park area slightly to the west of Galt Street to the north of Champlain Boulevard. The intake structure is being built near a high voltage electric tower. Selecting this location was based on the need to minimise the project cost by optimising the tunnel length as well as to reduce traffic disruption, noise and vibration during construction to the nearby neighborhood. A site restoration plan will be implemented with architectural and landscaping considerations planned by the city to ensure a good insertion of the operating building within this park.
¦ An upstream shaft that is approximately 6m diameter and 35m deep—shaft sinking is in the park area near the Galt Street. Work is being carried out near by a high voltage tower and the contractor had to take appropriate steps to comply with safety measures. For instance, some of the equipment had to be adjusted to ensure that its swing met maximum height requirements. At this location there are major underground power utilities that could not be relocated within the schedule, and had to be suspended and supported by a temporary structure.
¦ A conveyance tunnel that is approximately 877m long and 35m deep that slopes at 0.2 per cent towards the treatment plant. Its inside diameter is 6m and final lining is cast-in-place reinforced concrete. The tunnel runs under a mostly residential area with some commercial and industrial facilities. It terminates near a Siemens facility that houses jet engine testing equipment.
¦ A downstream shaft that is also approximately 6m diameter and 35m deep. The shaft is being sunken past the Siemens facility on the Atwater treatment plant site. There is known soil contamination at this site that was properly disposed based on environmental regulations.
¦ An underground cut and cover portion to connect to the existing valve chamber, which is approximately 9.6m wide and 115m long.
GEOLOGY
The geology consists of soil deposits for the first 10m that overlay bedrock. The soil deposits consist of fill that overlay the natural soils that consist mostly of fine grained till deposits with some traces of silt and sandy silt. There is a bottom layer of the till that contains boulders and cobbles. The till is dense to very dense. The bedrock consists of the Utica shale with some metamorphic intrusions that are associated with the Monteregian formations. The shale has an average compressive strength of approximately 90MPa (13ksi) but it can be as high as 130MPa (19ksi). It is expected to have good to excellent quality of the rock and very tight joints.
The shale is locally known to have time dependent behavior as well as fissile and slaking potential. Even though no explosives or toxic gases were encountered during the geotechnical investigation, the tunnel was classified as potentially gassy given the nature of the shale that is known to have significant concentration of methane and hydrogen sulfate. The compressive strength of the intrusions can reach 250 MPa (36ksi) based on previous observations in Montreal area.
Groundwater table is controlled by the nearby channel. Hydraulic conductivity for the soil deposits is very low as well. Similarly, the shale is also tight and it is expected to have low groundwater inflow but the soil rock interface is expected to be highly permeable given the layer of cobles and boulders that sit on top of the rock.
Sealing of that interface is therefore one of the major concerns for the project to avoid shafts and tunnel structures flooding.
DESIGN AND CONSTRUCTION CONSIDERATIONS
The following are some of the design and construction considerations:
1. The design of temporary support of the excavation for the new intake structure was left to the contractor who elected to use a cofferdam system that is anchored to the rock and injected from the surface. Given the size of the intake structure the uplift was an issue that was compensated by mass concrete of the invert slab.
2. Working in the channel had to be very well planned and coordinated with the City staff to avoid impacting the downstream water intake especially the turbidity effect for the suspended solids being dragged into the plant. A monitoring and warning protocol was established with the treatment plant personnel and trigger levels were specified and implemented successively to avoid the plant shutdown. A series of screens were installed within the channel to hold the sediments that are accumulated on the bottom of the channel.
3. With the highly permeable layer at the soil rock interface, cut off grouting was specified and is currently being carried out from the surface before the rock excavation. The support of excavation system was required to penetrate through the cobbles and boulders that sit on top of the rock and be embedded into sound rock as well for at least 750mm. 4. Given its vicinity to nearby residential area, vibration and air-overpressure criteria were specified in the baselines and are being successively implemented. The City has undertaken a community outreach effort to inform the residents and keep them updated of the progress with the cooperation of local municipality.
5. To meet the tight schedule, working from both ends of the tunnel was required and is being implemented. Innovative techniques such as the use of collapsing formwork were required to speed up the schedule. Given the short length of the tunnel, drill-and-blast or roadheader was recommended. The estimate of production was based on previous subway extension projects that had comparable size within similar ground conditions. The contractor has elected to use drill-and-blast for both shafts and tunnel excavation. He has also proposed to widen some portions of the shafts and some tunnel portion from the original design size to allow him to use bigger equipment and carry the work faster.
6. To avoid the shale degradation during construction due to its slaking behavior, a shotcrete layer was specified to be applied immediately after the blasting and before proceeding to the next round.
