February 16, 2015In 2012, the City of Saskatoon, Saskatchewan embarked on a two-year design-build project to improve the sanitary and storm sewer system in the City’s rapidly growing East Sector.
The McOrmond Drive Sanitary and Storm Sewer Trunks project, in the southeast quadrant of the City, addresses the anticipated arrival of an estimated 80,000 residents to the East Sector, as well as future development in that quadrant.
In a proactive approach, the installation of 1,500-m (4,920-ft) long parallel sanitary and storm trunk sewers with diameters of 1,200 mm (48 in.) and 2,400 mm (96 in.), respectively, were required. Following a pre-qualification and request for proposal, the project owner (the City of Saskatoon), along with its design-build engineer CH2M HILL, selected the team of Michels Canada Co., Stantec Consulting Ltd., Koziak Engineering Limited and Thurber Engineering Ltd. to complete the work, valued at $33.5 million.
One main advantage of a design-build project delivery approach is the involvement of the constructor from the beginning of design. Doing so allows the project to be geared toward the actual construction approach from the start, reducing the need for lengthy re-design and encouraging a lowest cost construction strategy. Another significant benefit is the capability for construction to be under way before a full design is completed.
Design risks are also transferred from the owner to the design-build consortium.
During the design period, three significant constraints identified by the Michels team included: The fact that locations provided for the existing sanitary and storm trunk stubs were only approximate, the existing sanitary and storm stubs only had an estimated 1-m clearance between them at the tie-in point and that the downstream section of the trunks followed a curved alignment. As a result of these complications, the Michels team had to make several decisions and take many factors into account at the time of bid, prior to being awarded the project.
Time for the TBMs
Due to the depth of the proposed trunk sewers at 14 m (46 ft) and their alignment paralleling a busy existing arterial roadway through a residential subdivision, a trenchless installation was necessary. Based on the geotechnical baseline report (GBR) provided during the pre-qualification process, the minimally available installation grade and the size of the trunks, Michels opted to use a two-pass conventional tunnelling approach with a tunnell boring machine (TBM).
Due to a number of reasons, including cost and connections to existing systems, the choice was made to construct two tunnells with 2-m clear separation distance between the outside of each tunnell. After a thorough review of the pros and cons, it was decided that the new tunnels would share launch and retrieval shafts with an intermediate shaft constructed on each tunnell near the midpoint of the alignment to provide the City with future access to the new utilities.
After award of the project, and in conjunction of the development of the risk register and after lengthy internal discussions with project management and experienced TBM operators, Michels decided to use larger TBMs than those that were included in the original bid submission. Using larger machines allowed for greater face access to deal with anticipated boulders and also provided a more spacious internal working area for the crew. The upsized TBMs reduced risks to the crew, who were required to work inside the tunnell for extended periods of time, as well as to the machines themselves, which would be able to process larger boulders, reducing mining time and maintenance/repair downtime.
A 2.6-m (104-in.) Lovat TBM was used to excavate the sanitary trunk sewer, which was the first tunnell to be constructed. Based on lessons learned from excavating the first tunnell, namely difficult unforeseen geological conditions, a 3.3-m (130-in.) Lovat TBM was used for excavating the storm trunk sewer.
Initial ground support was provided by a steel rib and wood lagging system, with the carrier (HOBAS) pipe being installed and grouted into place following the completion of tunnelling operations. Construction of the project began in November 2012 and is expected to conclude by February 2015. The first tunnell was completed in April 2014, and the second tunnell was completed in early December 2014.
There have been several challenges presented during the construction phase of the project, stemming from environmental, geological, and work force issues.
Maintaining production through two unseasonably harsh Canadian prairie winters, with the coldest day recorded in 2013 at a bone-chilling -62C (-79.6 F), including wind chill, forced the team to develop ways of mitigating the seasonal onsite conditions. Erecting a large tent to shelter the workspace and installing several wind breaks were effective strategies.
The wet, sticky, hard clay contained a significant amount of boulders and cobbles, which greatly exceeded the number identified in the GBR. The ground conditions impacted production rates and generated excessive damage to the TBM cutter head. In one instance, an estimated 3.6-m or larger (more than 12-ft) boulder was encountered, so Michels had to blast it and proceed to walk the TBM through the boulder to continue tunnelling without ever fully ingesting it.
A high groundwater table was another concern and required constant monitoring. Shaft sump pumps and dewatering wells were used at shaft locations and in the tunnell. Operating as a union contractor proved problematic, as retaining a skilled workforce was another on-going difficulty throughout construction. Booming potash mining and oil production in the region attracted many prospective qualified workers out of town.
While there were many complications experienced by Michels throughout the job, numerous aspects contributed to the project’s success. Timely input from the construction team allowed for effective decision-making when issues such as an under-performing TBM or unexpected geological conditions arose. Public outreach was also a key component of the project’s success. Information sessions were held, as well as two successful site tours that allowed guests a firsthand look at the tunnelling process.
Overall, the McOrmond Drive project has been a success from both the owner and contracting side. Knowledge gained from what was experienced in the sanitary tunnell was helpful during construction of the storm tunnell as the construction team was able to incorporate modifications and changes to the process and equipment.
Working with an owner that understands the possible issues that may arise during trenchless construction and having a strong collaborative team has made for quick problem solving and precise decision making throughout the design-build and construction process.
With the McOrmond Drive Sanitary and Storm Sewer Trunks project complete, Saskatoon will be able to accommodate an influx of new residential development in the southeast section of Canada’s fastest-growing city.
Craig Vandaelle is the tunnelling manager for Michels Canada Company and the acting design-build project manager on the project.
Dan Willems is the special projects manager for the City of Saskatoon and overseeing the project for the City.