In recent decades the City of Greater Sudbury has experienced a rapid rate of growth and development in it’s “South End” (the general region south of Walford Road and Laurentian University). The public works department has had many challenges in providing municipal services to this growing area of the city, sanitary sewers in particular.
The existing sanitary collection system in this area consists of a series of local sewers, lift stations, force mains and trunk sewers – all required due to the hilly character of the South End. Many of these components were added piecemeal to the system as a result of the high rate of growth and development in this area. Excessive inflow and infiltration to the sewers during periods of wet weather have periodically over-taxed the collection system, causing basement flooding (due to sewage backups) and environmental degradation (due to emergency overflows/by-passes).
For these reasons, the City of Greater Sudbury completed a number of studies and investigations to identify the technical problems and constraints with the wastewater collection system in the South End, and provide an overall concept for the improvement and expansion of the system. The concept of extending the existing sewage rock tunnel, along with additional infrastructure and operations & maintenance measures, emerged as the Preferred Alternative as chosen through the Environmental Assessment process in 2001. This concept offered the City the opportunity to save on the long term maintenance costs of its wastewater collection system, as well as to handle all of the growth potential in the South End.
The severity of the above problems was such that in a 2003 report Don Belisle (then General Manager of Public Works) noted, “if the rock tunnel is not approved, then [the City] will have to put a hold on all new development and retroactively cancel some existing building permits” in the South End, an area that holds significant future growth potential.
Dennis Consultants, a division of R.V. Anderson Associates Ltd., was commissioned by the City of Greater Sudbury in 2003 to design, tender and manage the construction of the project.
Sewage Tunnel Concept
Sudbury is unique in that it is situated in an area of generally shallow soils and hard bedrock. These geologic conditions, combined with the highly variable topography of the surface, have rendered the use of rock tunnels beneath the City ideal for the transportation of sanitary wastewater.
This concept was initiated in the early 1960s when the first sewage rock tunnel was built beneath the City from Kelly Lake Road through the Downtown area to New Sudbury. The tunnel has since been expanded to the Minnow Lake and Lockerby areas. Currently, there are approximately 15 km of tunnel beneath the city streets (ranging in depth from 18 to 30 m below the ground) used exclusively for the conveyance of sanitary wastewater.
Sewage flows through shallow sewers in the city streets and is then diverted into the rock tunnel system through “drop shafts” at key areas or low points (the drop shafts are cased boreholes drilled into the bottom of maintenance holes). Sewage then flows through the tunnels (which are openings excavated in the bedrock using standard hardrock mining methods) under the force of gravity alone.
The tunnels have been constructed at a shallow vertical grade such that the low point of the system is directly beneath the City’s Wastewater Treatment Plant on Kelly Lake Road. Sewage flows to this point, is then pumped to surface, treated in the plant and discharged to the environment. This system has proved effective for the handling of wastewater in the City since its inception.
South End Tunnel Extension
The South End Tunnel project is an extension of the existing tunnel system into that portion of the City. The existing sewage collection system in the South End ultimately conveys sewage to the southernmost point of the existing tunnel network. The proposed tunnel will begin at this point and extend into the South End, branching off in a number of areas (Walford, Paris and Lo-Ellen) to intercept key low points in the surface sewage system. Approximately 6.5 km of additional tunnel are to be constructed as part of this project.
Once complete the tunnel will be a minimum 1.5 m wide by 2.1 m high opening in the bedrock (matching size of existing tunnel) roughly 25 to 30 m below the ground. Standard “drill & blast” hardrock mining practices are being followed, since the use of tunnel boring machines was determined to be poorly suited to the ground in this area. No liner is required to support the ground in the tunnel; rather rockbolts are being used, as well as screen and shotcrete in some areas. Prior to completion, the floor of the tunnel will be lined with concrete and sloped to the center to assist in the overall hydraulics and limit the buildup of solids once in use.
Three vertical shafts are required for access to the tunnel system both during construction and for regular inspection of the system once operational. The size of the shafts varies between roughly 3.0 m x 3.0 m (Burwash Shaft) to 4.0 m x 5.5 m (Yale and Green Shafts). The shafts are all located on City property and will be capped once complete to prevent access by unauthorized personnel.
Once complete a total of seven drop shafts will be constructed to divert sewage from the surface collection system to the tunnel. In some areas, the drop shafts are being built at low areas where lift stations are currently used to pump sewage in force mains (allowing for decommissioning of the stations and significant savings to the City). In other areas, the drop shafts are being built to divert a portion of the sewage to the tunnel and reduce the amount handled by the existing collection system. In all areas the design of the drop shafts allows for efficient delivery of the wastewater to the tunnel and recirculation of air.
Design of the project was completed in the spring of 2005 and the project was tendered by the City in April, 2005. The successful Tenderer was McNally Construction Inc. from Hamilton, ON. McNally is making use of a number of local subcontractors, such as Tera North Construction & Marathon Drilling. Construction has been underway since it was awarded, and is currently about one third complete.
The contractor has elected to stage construction of the project from all three sites (at each access shaft). Construction of two of the three access shafts is complete, with the third underway. At the central site the contractor has also driven a temporary ramp to provide access to the underground workings for rubber-tired equipment.
All tunneling activity is being done using hardrock mining drill & blast techniques. Hand-held pneumatic drills (“jack-legs”) are being used to perform the drilling, and battery-powered locomotives on rail load and haul blast rock from the tunnel headings. At a central rock re-handling station, a diesel-powered LHD (load-haul-dump vehicle) hauls the rock to surface via the ramp at which point it is stockpiled. Rock is then loaded and hauled by legal dump trucks to the designated surplus fill area.
In addition to the underground works, the South End Tunnel project requires sanitary sewage works in all areas where drop shafts are being constructed. These works are being completed as the tunnel is advanced. Reinforced concrete intake chambers for the drop shaft structures are being constructed, including vertical boreholes (drilled into the bedrock and cased) that will serve as the sewage drop shafts and air return shafts.
At the time of writing of this report, approximately one and a half years have elapsed since construction began. Most of the major surface works are complete, two access shafts are complete and approximately 20% of the tunnel has been excavated. By advancing tunnel headings from three work sites, the contractor anticipates completing the project on schedule.
In addition, as work proceeds on the project local residents have become more aware of the works and more vocal about complaints related to construction, such as noise, dust and truck traffic. The contractor and design consultant have been required more and more frequently to deal with complaints from citizens and mitigate their concerns given that the construction is underway so close to residential areas.
Other upcoming challenges include connecting the new tunnel to the existing tunnel system, and driving the tunnel headings through areas that have been identified by the geotechnical sub-consultant (Golder Associates) as having potentially limited head cover or poor ground conditions.
As these issues present themselves, the engineering team at Dennis Consultants will work diligently with the contractors and the City’s engineering staff to ensure that the project is completed on schedule and within the budget.