The Southeast Collector Trunk Sewer Project involves the twinning of an existing gravity trunk sewer that was built by the Province of Ontario more than 30 years ago and services more than one million people. The new sewer will support provincially-mandated growth and will allow wastewater from the existing trunk sewer to be diverted, so that a detailed condition assessment and rehabilitation of the existing trunk sewer can take place. This sewer project is the first in Ontario to undergo an individual environmental assessment process — the most rigorous process of its kind in the province, with a total of CAD 25M (USD 25.5M) being spent on studies to determine the preferred solution. A number of options were considered and a peer review team supported the conclusion that a new Southeast Collector Trunk Sewer is required.
Project description
The Southeast Collector Project consists of the construction of 15km of gravity trunk sewer and six related facilities. The sewer will travel in a southeasterly direction from the intersection of Ninth Line and Rouge Bank Drive in the City of Markham to the intersection of Finch Avenue and Valley Farm Road in the City of Pickering and will be able to convey a maximum flow of approximately 14cu.m/s. The sewer has an inside diameter of 3m and will be constructed using four, 3.62m diameter EPBMs. The new sewer also includes two, 20m baffle drop structures and two connections to the existing sewer, which must be made in live flow conditions. Two flow metering chambers will be constructed to measure flows entering the system from Durham Region.
Managing odours that are sometimes created as part of normal sewer operation is integral to the project design. Odorous air generated within the portion of the sewer that is downstream of shaft nine, will be forced upstream within the headspace of the sewer from the downstream end of the project, counter-current to the sewage flow. The air will be conveyed around two baffle drop structures to an odour control facility (OCF) where it will be treated — a journey of nearly 9km. Two air handling facilities, located near the drop structures at shaft four and shafts six and seven, will draw air from downstream of the drop structure and will push this air upstream of the baffle structure toward the OCF. Odours generated within the portion of the sewer that is upstream of shaft nine will flow naturally, co-current with the sewage flow, to shaft nine. An air block will be located within the sewer at shaft nine to separate the upstream and downstream air streams. Air will be drawn from either side of the air block at shaft nine and conveyed across the street to the OCF where it will be treated. The OCF will treat the odorous air using three bio-scrubbers and biofilters as well as two carbon filters for polishing. A corrosion control facility, located at the upstream end of the new pipe, will facilitate the addition of hydrogen peroxide into the sewage stream. This will reduce corrosion within the system as well as the potential for odours.
The sewer route was selected to avoid environmental and social impacts. However, numerous additional measures have been implemented to reduce potential impacts even further. These include securing a specific soil disposal site in advance of tendering the contract. This disposal site will be operated and maintained by the contractor for the duration of the contract. And procuring TBMs in advance of tendering. The owner will maintain ownership of the TBMs throughout construction.
Installing more than 5km of 6in (152mm) HDPE discharge piping via HDD in order to carry discharge water from the mining compounds to the existing sanitary sewer.
Installing permanent remote noise and vibration monitoring stations at each of the compounds, which are being monitored continuously and in real time. Additional vibration monitoring is conducted on the surface immediately above the TBMs.
Geotechnical conditions
Surficial soils in the project area, deposited during the Quaternary period by glaciers and associated glacial lakes and rivers, consist predominantly of glacial till, glaciolacustrine and glaciofluvial sand, silt, clay deposits, of alluvium are found in river and stream valleys and their flood plains. The Quaternary soil deposits overlie Ordovician-age bedrock of the Georgian Bay Formation that consists predominantly of shale with interbedded limestone and siltstone. The distribution of all overburden in the upper 30 to 40m of the study area is attributed to the latter stages of Wisconsin glaciation. Successions of glacial deposits were laid down under the massive ice sheets or were transported into ice-marginal ponds and lakes by melt water streams. The latter deposits generally occurred during warmer periods when the ice-front started to recede. The loading of the ice sheets has preconsolidated the glacial deposits.
The glacial till and outwash/deltaic deposits that comprise the majority of the overburden sequence along the tunnel alignment range in thickness from 15m at Valley Farm Road on the southeast part of the route, to more than 60m at Box Grove on the northwest portion of the route. Shafts and tunnels are constructed to a maximum depth of 49m below surface. The majority of the tunnel was located within low permeability, dense clayey silty sand till deposits with some portions of the deeper tunnel drives located within glaciolacustrine deposits originating from the Quarternary period. In the southeast portion of the tunnel alignment, west of Valley Farm Road, glaciolacustrine silts and clays, and glaciofluvial sands and gravels are encountered that generally have limited lateral extent and are subject to rapid compositional change by the nature of their origin. This bedrock will not be encountered while mining but is utilised at some of the deeper shaft locations for groundwater cut-off.
Challenges to the excavation of the ground for the shafts and tunnels are due to small to large-size boulders which are common within the glacial deposits and the dense nature of the glacial deposits. Specifically, the Interstadial Aquifer in the upper sand layers creates a challenge for certain shaft excavations and the lower Thorncliffe Formation aquifer is the criteria for the stability of the deeper shaft bases and lower tunnel drives. Consequently, where interference with these aquifers is expected, shafts are specified to be constructed as ‘sealed shafts’ thereby requiring groundwater cut-off and uplift protection. Due to the significant challenges experienced with dewatering on previous projects in the area, where the groundwater table was substantially influenced by water extraction over longer periods, dewatering is generally not permitted as a ground stabilisation measure on this project.
Shaft construction
Sixteen shafts and one portal access have to be constructed, whereby shafts 10, six and the portal at the shaft one compound, are the starting points for the TBM drives and therefore time critical for the completion of the project. The other shafts are required for the construction of permanent access manholes, tie-ins to the existing trunk sewer, baffle drop structures and an air block. Distances between shafts vary from 760m to a maximum of 2,340m. Intermediate shafts located along the TBM drives will be excavated in advance of the arrival of the TBM and will be utilised for TBM inspections. Shaft diameters range between 4.6m and 14m with depths varying between 11m and 49m.
For the construction of the various shafts, a variety of temporary support measures are used including secant pile walls, slurry walls, ground freezing and shotcrete for the shafts excavated using NATM/SEM principles. Sealed shafts (where groundwater is expected to be encountered) are constructed utilising slurry walls, secant pile walls or ground freezing. Non-sealed shafts (where no groundwater inflow is expected) are constructed using shotcrete. In some shafts, a combination of secant pile wall and shotcrete are used — secant pile wall for the top portion of the shaft where granular water bearing layers are present and shotcrete for the bottom portion of the shaft where the ground has a low permeability.
All shafts have a final lining with cast-in-place reinforced concrete constructed with climbing formworks. The contract documents call for high-performance, dense concrete for the final shaft lining in order to withstand the aggressive conditions created by the sewage. Most of the shaft linings will be constructed with decks separating the active TBM drives and the shaft works to help reduce construction time. The total drop in elevation along the 15km of tunnel is roughly 60m, therefore, two 20m baffle drop structures will be constructed at shaft four and shaft six/ seven to help dissipate energy. The drop structures direct the sewage over a series of concrete baffles in a cascade.
TBM and segments
The owner elected to procure four Caterpillar EPBMs, trailing gear as well as the steel fibre reinforced precast segments in advance of awarding the construction contract. The owner will continue to own the TBMs throughout construction. The reasons for following this model was to be able to select the TBMs for the job, introduce a higher level of owner control over TBM maintenance decisions as well as the schedule savings associated with having the TBMs ready at contract award.
The TBMs are 3.62m excavation diameter with variable frequency electric drive (VFD) and maximum torque of 2,245kNm, designed for the expected ground conditions. The precast concrete segmental universal ring consists of four, 67.5 degree segments and two, 45 degree segments 200mm thick with a 3m internal diameter and a minimum concrete strength of 50MPa. The high-density concrete segments with EPDM gaskets isolate the sewer from the surrounding ground and support external pressures of more than 5MPa.
Progress of the works
The main construction contract was awarded to Strabag in August 2011. The main access shafts for the drives have been constructed and the starter and blind tunnels have been excavated using NATM with sprayed concrete and lattice girders. The first TBM was delivered to shaft one and started mining July 2012. Due to the fact that this was the first TBM launch, it is also used as the training facility for the remaining three TBM drives, where miners, shaft crews and mechanics can gain experience before moving to their tunnel drive.
The second TBM was delivered to shaft 10 in July 2012 and is also mining. More than 60 per cent of the tunneling will occur from shaft 10. At this location, two TBMs will mine in orthogonal directions. The third TBM is being assembled at shaft six and the delivery of the last TBM to shaft 10 is scheduled for the end of September. By the end of October 2012, all four TBMs will be mining.
