The southeast collector (SEC) Trunk Sewer is a 15kmlong, 3m-diameter wastewater tunnel designed by Hatch that twins an existing section of the 2.6m-diameter York-Durham Sewage System (YDSS) tunnel. Both tunnels are located north of the City of Toronto. The SEC provides capacity for the rapidly growing York Region population and supports development. Construction of the SEC allows the twinned section of the YDSS to be bypassed in order to clean, inspect and perform essential maintenance when required.
The project included construction of 19 shafts and chambers with depths ranging from 8 to 48m and seven facilities that support the operation and maintenance of the sewer. In 2011 the construction contract was awarded to Strabag for CAD 291M (USD 217M). Hatch and Aecom created a joint venture team to provide construction management services.
The YDSS tunnel was completed in 1979 and constructed using openshielded TBM technology with ribs and lagging for temporary support. This was followed by installation of 380mm-thick unreinforced cast-in-place (CIP) concrete for the final lining. The YDSS sewer system has many connections that collect flows from various urban areas within York Region. The section of the YDSS that was twinned by the SEC sewer flows under gravity conditions and is the main and critical connection between the upstream sewer systems and the downstream Duffin Creek Water Pollution Control Plant.
SOUTHEAST COLLECTOR TUNNEL
The SEC tunnelling was carried out by using four ownerprocured Caterpillar Tunneling Canada 3.6m-diameter EPBMs, which utilized a single-pass, 200mm-thick, steel fibre-reinforced, six-segment universal precast concrete tunnel lining (PCTL). The EPBMs mined through full and mixed-face conditions consisting of plastic till, non-plastic till, silt, sand, cobbles and boulders with overburden depths ranging from 5 to 45m. While tunnelling, the EPBMs were required to operate with earth pressure balance pressures of up to 3.0bar.
Given the sewer’s harsh operating environment and the minimum 100-year design life of the tunnel, special considerations for the PCTL were made. Steel fibre-reinforced concrete (SFRC) was utilized to ensure long-term structural durability in the event of section loss over prolonged exposure of the tunnel lining to hydrogen sulfide (H2S). This was the first application of SFRC tunnel lining for York Region. In addition, the concrete mix specified very stringent permeability requirements to mitigate section loss due to the anticipated corrosive environment. Permeability tests incorporated the use of pressure cells and uniaxial water flow measurements under pressure across the concrete sample.
B_ TUNNEL DRIVE
The SEC tunnel is comprised of four distinct drives. Tunnel drive B2 was the easternmost drive and was the first to launch. The connection chamber lies at the eastern downstream terminus of tunnel drive B2 where the YDSS and SEC flows combine back into the downstream portion of the YDSS tunnel. Tunnel drive B2 launched from shaft one and passed through the future SEC-YDSS connection chamber shortly thereafter; this drive encountered the most challenging and varied conditions of all four drives.
The EPBM and trailing gear was completely assembled prior to launch because the contractor was able to take advantage of the space provided by the launch portal configuration. The contractor decided to install the cutterhead with a full face of ripper tools. Immediately westwards after the connection chamber there were 850m of mining through fine sands and mixedface clayey silts below the groundwater table with only 5 to 7m of ground cover beneath the busy road right-of-way of Finch Avenue. This presented extremely challenging conditions for settlement control. Settlement monitoring points were installed at a frequency of 10m. Average settlements were recorded as 15mm in this zone of the tunnel drive. Upon break-out into the first service shaft of 14m ID (S2) the EPBM was turned 90 degrees and re-launched to the north under Liverpool Road and tunnelled for approximately 550m in similar challenging ground conditions. This tunnel stretch finished when repairs to the EPBM cutterhead and cutting tools were undertaken within a pre-constructed emergency secant pile headwall. The headwall was installed after damaged ground conditioning system lines within the EPBM cutterhead chamber were identified. The headwall served as a safe haven for the repairs.
With repairs completed over the course of approximately four days, drive B2 then continued from the headwall and passed beneath the existing YDSS under live flows with 9m of ground cover and only 1.3m clearance between the EPBM and the bottom of the YDSS. Jet grouting ground improvement had been performed around the YDSS invert in advance of the TBM passing to mitigate settlement induced by tunnelling. A maximum of 2mm settlement was registered on the YDSS. (To read more about the grout programme see Tunnels & Tunnelling North America, August 2015, page 34.) Approximately 50m past the YDSS crossing the EPBM crossed under a watercourse at shallow cover of only 6m. During this section the upper EPB target pressure limits were watched carefully and environmental controls were on-site to mitigate any frac-out incidents that could have occurred.
After passing beneath the watercourse the EPBM tunnelled 700m through primarily full-face sandy silt till conditions before reaching a pre-installed freeze block utilised as an entrance block for maintenance shaft one. Mining through the frozen ground proved to be challenging as it was discovered early on that the ripper tools were severely worn. The EPBM, however, eventually broke-out into the shaft with minimal groundwater inflow. Once the replacement of all the cutterhead tools, as well as the inspection and maintenance of the ground conditioning system and trailing shield brushes, was finished the EPBM continued mining the final 750m of the 2.9km drive through a complete range of glacial till conditions with depths of cover ranging from 20 up to 30m.
Upon completion of tunnel drive B2, construction of the connection chamber commenced and required the existing YDSS to be supported across the chamber by both a hanging system and a cradle, and bypassed using a hydraulically-extensible steel flume. This allowed construction of the benching and final CIP chamber structure to be completed without disruption of flows within the YDSS. Using the experience gained from the live bypass within the connection chamber this same operation was performed in the diversion chamber located at the western upstream portion of the SEC tunnel.
ConneCtion Chamber design
The connection chamber is the critical point in the sewage systems where flows of the SEC and YDSS combine and are conveyed directly to the Duffin Creek WPCP. The chamber incorporated sluice gates at all inlets/outlets allowing selective bypass of flows from either sewer coming into the chamber. It was paramount that all construction activities related to the chamber did not interrupt live flow conditions within the existing YDSS at any time.
Unreinforced knock-out panels were included in the final chamber wall design for potential future sewer connections and the rooftop structure incorporated precast concrete panels that are removable for sluice gate maintenance and access for future works. A combination of CIP concrete as well as custom-formed stainless steel was specified to facilitate curved benching profiles. Hatch designers had specified the complex chamber excavation arrangement, hanging system of both the YDSS and flume bypass as well as the support of excavation in the contract documents. This decision helped mitigate the risk of interruption of the YDSS flow during chamber construction. A sequence of secant piles with embedded steel soldier piles was specified as the support of excavation to create a sealed shaft. Fibrereinforced plastic (FRP) soldier piles were also integrated into the soft tunnel eyes of the chamber support of excavation walls for the SEC tunnel. In addition to the secant piles, there were two levels of struts and walers included in the design, as well as steel hanging system used to support and hang the existing YDSS sewer and the flume bypass across the connection chamber excavation.
monitoring program
Due to the inability of a complete diversion of YDSS flows and the critical and complex sequencing of the construction, an extensive monitoring program was carried out that included a CCTV feed in the live sewer and an automatic and continuous precision monitoring system. The data for both systems were accessible online by all parties. The system used a robotic total station that measured Leica prism targets on a continuous basis at 15-minute intervals. The review and alert levels defined for the measured movement of the sewer were specified in the design and triggered automatic emails to key contacts upon exceedance. Vibrating wire strain gauges were also installed on the vertical rods of the hanging system.
ConstruCtion sequencing
Prior to tunnelling, the section of the connection chamber perimeter secant pile wall for the SEC tunnel was installed, confirming a reliable annulus seal between the secant piles and SEC tunnel during passage of the TBM. A minimum clearance of 500mm was preserved between the existing YDSS and the secant piles. This led to “windows” of unsupported ground around the YDSS tunnel that were shotcreted once they were exposed during the excavation sequence.
Once the tunnel drive was completed, excavation of the connection chamber commenced. The chamber excavation proceeded to springline of the YDSS tunnel and allowed the erection and installation of the steel hanging system to take place. The contractor performed some coring and concrete testing of the YDSS final lining at this stage to confirm compressive strength and to verify the as-built lining thickness. The compressive strength of the concrete cores tested to 30 MPa and the YDSS tunnel lining thickness was determined to be equal to or greater than the original design. The existing concrete was observed to be of good quality and did not have any visible deterioration due to time elapsed or exposure since installation.
Once the hanging system installation was completed, the precision monitoring system was installed, tested and baselined. The excavation then proceeded for the north end of the chamber, facilitating the demolition of the exposed SEC PCTL. Exposing the SEC within the connection chamber revealed continuous annulus grout and high-precision ring build, which confirmed TBM and inspection records.
Excavation then continued for the south end of the shaft from springline of the YDSS. The underside of the YDSS was exposed in increments of 2m across the length of the connection chamber. This allowed inspection of the existing steel rib where gaps were found at the invert ribs. New connecting plates were designed and welded on the ribs to ensure continuous support. The full span of the YDSS was eventually suspended 11m across the excavation by the hanging system.
Settlement review levels were activated on two prisms at mid-span springline of the suspended sewer but showed no further movement upon full exposure of the YDSS and no other alarms were triggered. Excavation then proceeded to the base elevation and the final base slab was completed. During this phase a temporary concrete cradle was poured between the YDSS and the base. The cradle allowed for the removal of the YDSS hanging system, temporary walers/struts and pouring of two concrete wall lifts for the final chamber walls.
Design and installation of the flume
Utilizing a flume was required to facilitate demolition of the existing YDSS tunnel within the chamber while maintaining flows across the chamber. When the flume was hanging it also provided the needed space to build the benching to channel flows through the chamber. The contractor was responsible for the design and manufacture of the flume. The contractor performed Lidar surveying to determine the interior dimensions of the YDSS. The design utilized a three-piece steel cylinder assembly and each end used hydraulic jacks to independently extend and retract using control switches. Custom, inflatable Multiflex seals were fabricated for the flume ends along with a secondary grease sealing system to seal off the annular gap between the flume and YDSS intrados that varied in depth. Continuous pressure was applied to the seals through an air supply system that included low-pressure warning sensors and backup generators.
On the day of the flume installation, pre-cut segments of the upper portion of the YDSS were removed by crane leaving the bottom half of the cradle-supported sewer and exposing live sewage flow. Once the upper half was removed, the retracted flume was lowered slowly into place, and the flume ends were extended and seals inflated. The flume achieved all functional requirements with practically no detectable seepage. A steel hanging system was installed to hang the flume, and the temporary cradle and remaining half of the YDSS were demolished and removed.
Implementing the hanging system for the flume permitted the installation of the final benching and sluice gate to commence. Once finished, the flume hanging system was removed after the flume was supported from below with temporary steel legs. FRP service walkways and ladders were then installed and the precast concrete roof slabs placed, leaving a lifting window open for flume removal. The flume was removed by depressurizing the seals, retracting the ends and then finally lifted out introducing live sewage flows into the connection chamber for the first time. Removal of the flume concluded the sewer bypass process and permitted the completion of the shaft roof and FRP platforms/ladders for the chamber.
In the event of a failure of the hanging system or bypass process a risk mitigation plan was established. Although they were not used, additional safety measures were implemented and equipment was mobilized and available at the work site as a precaution. Three high-powered cameras were also placed within the work site and chamber and provided continuous real-time footage available for mobile viewing.
CONCLUSION
Construction of the connection chamber provided a unique opportunity to uncover and compare tunnelling methods installed 35 years apart. The YDSS on the south of the connection chamber used traditional open-shielded TBM tunnelling with rib and lagging temporary support before a second pass of CIP unreinforced completed the final lining. While the SEC to the north used a sophisticated EPBM incorporating a single pass state-of-the-art PCTL. After uncovering the tunnels it was obvious that both the YDSS and SEC demonstrated characteristics of effective design and high quality construction. The effectiveness of EPBM technology was clear from the challenges faced and overcome on the B2 tunnel drive as it tunnelled through a range of conditions. Although open-shielded technology is suitable for tunnelling in more cohesive till materials, along with dewatering when necessary – EBPM technology is a proven system capable of handling the full range of glacial till ground conditions found in Southern Ontario providing exceptional settlement management without needing dewatering.
The connection chamber construction and YDSS bypass were successful with uninterrupted flows within the existing YDSS. These high-risk activities were determined early on in the design stage and led to prescriptive design and phasing for the work including well-defined expectations within the contract specifications for monitoring and contingency planning. The requirements of the contract were stringently applied during the undertaking, and the contractor’s team was involved and dedicated to completing this sensitive part of the project without incident. Using the experience gained from the hanging and bypass operations at the construction chamber a similar exercise was executed successfully at the western terminus of the SEC tunnel in the diversion chamber. Successful collaboration and communication between the designer, owner, construction management team and contractor was crucial to the success of both the connection chamber construction and tunnelling efforts.
