$36-million sewer upgrade is a microtunnelling first for Calgary
A city’s growth is often limited by the services needed to handle the extra population that puts higher demands on existing infrastructure. That is certainly the case in Calgary, where development in the city’s northwest quadrant has been hindered due to constraints on current sewer facilities, which are considered inadequate to match increased development in that area.
Indeed, torrid population growth over the past few years has placed tremendous strain on Calgary’s existing infrastructure, to the point where in 2013, the city had to stop any further development in 16 northwest communities, including Bowness, Hawkwood, Tuscany and Royal Oak. Nature was the other canary in the coalmine signalling the need for improvements. Major rainfall in the spring of 2011 caused flooding and many sewer backups in the northwest, followed by the Calgary floods of 2013, which accelerated demands for increased sewer capacity during heavy rain and flood events.
Since 2013, the City has rolled out an ambitious plan to upgrade the sewer system. The Bowness Sanitary Trunk initiative is a $36-million project that is expected to accommodate growth in northwest Calgary through to 2026.
The construction of 1.9 kilometres of new sanitary trunk infrastructure was awarded in two separate contracts to Ward & Burke Microtunnelling Ltd. The first contract, 1-A, installed a twin siphon crossing underneath the Bow River. The second contract, 1-B, includes construction of six microtunnels: five 1,800-mm tunnels west of the Bow River, and one 1,800-mm tunnel east of the Bow River and south of the TransCanada Highway. All work is being done via microtunnelling.
Ward & Burke, a microtunnelling specialist, for the past 10 years has been using the technology extensively in Toronto, where the Irish civil engineering company established an office in 2011. However the technique – a trenchless construction method used to dig small-diameter tunnels – has been less prevalent in Western Canada. The Bowness Sanitary Trunk project is the first time that Calgary has chosen microtunnelling over traditional tunnelling techniques.
According to John Grennan, director of Ward & Burke Microtunnelling Ltd., the City and local designers found that they were running into issues with changing ground conditions, with a lot of glacial till and high water tables, and thus decided to use microtunnelling, which uses a pressurized tunnelling system, versus a tunnel boring machine (TBM) that employs an unpressurized open face.
“Over the two kilometres, the ground changes from poorly-graded sands and gravels, to stiff glacial till, to silt-stones and mud-stones. You’ve got a large variety of different ground conditions on the job that change regularly over the length of the contract,” said Grennan, adding that the high water table is another obstacle to overcome. At three to four metres deep, the water table is encountered and with microtunnelling occurring about 10 metres deep, all of the work is being performed six to seven metres below groundwater. Normally that would mean a lot of dewatering, but because microtunnelling is a sealed system, no dewatering is necessary. That gives the technique a major advantage over traditional TBMs, where groundwater can unexpectedly flow into the tunnel.
With microtunelling, slurry fluid is pumped through the tunnel boring machine to pressurize the cutting face, thereby controlling the volume of muck being excavated. The muck is then pumped to the surface where the solids are separated from the slurry fluid, which is then pumped back underground in a continuous cycle.
“The equipment is a lot more advanced and able to deal with challenging ground conditions because you always have the ability of supporting the excavation face,” says Grennan. “It’s essentially a submarine. The machine is able to work below high water heads and keep the tunnel dry at all times.”
Ward & Burke used an AVN 1200 tunnel boring machine from Herrenknecht, the German TBM manufacturer, for the twin siphon section under the Bow River, and an AVN 1800 for the second phase of the project – the installation of the 1,800- mm ID sanitary lines.
“Both the 1200 and 1800 machines were purchased new for the job so they are the very latest in technology from Herrenknecht and they offer superior torque over other equipment, in terms of size,” Grennan said. “We put the best possible tunnel boring machine in the ground.”
On the surface, the solids are removed from suspension using a screening plant and decanting centrifuge supplied by another German company, ITE GmBH.
To launch and receive the microtunnel boring machine (MTBM), Ward & Burke is constructing circular reinforced concrete shafts using the wet caisson method. The proprietary formwork system allows for a complete circular shaft section of wall, up to 2.5 metres high, to be constructed in one monolithic pour, thus eliminating weak vertical joints from the structure. The company says a 2- to 2.5-metre section of shaft can be formed, poured and sunk in three to four days. Ward & Burke is building eight caisson shafts of varying size and depth for the purpose of launching and receiving the MTBM.
In terms of piping, Ward & Burke has selected the use of reinforced concrete jacking pipe with a high density polyethylene (HDPE) liner, manufactured by Munro Limited out of Barrie, Ontario.
The 1,800-mm pipe is capable of withstanding axial compressive forces of up to 14,000 kN or 1,525-tonne force, according to Ward & Burke. The larger AVN 1,800 MTBM jacking frame is capable of developing up to 800 tonnes of thrust, so the reinforced concrete pipes are able to withstand induced jacking forces. The U.S. Army Corp of Engineers recommends a design life of 70 to 100 years for precast concrete pipe, and the Munro pipes being used for the Bowness Sanitary Trunk project are designed to last 100 years.
As construction continues into the summer, Ward & Burke is progressing on the second phase of the project, having completed the siphon section under the Bow River as outlined in contract 1-A. So far five tunnels have been constructed, including the longest drive on the project, a 460-metre long tunnel with curves. The manholes that will tie into the sanitary sewer are also being built.
The tunnelling portion of the project is scheduled for completion by late November, with an overall finish date set for early 2016.
“We’re hoping that the project achieves two things,” Grennan summarized. “Number one, we deliver a quality product to the client that services the northwest side of Calgary. Number two, the use of innovative equipment and the latest in trenchless technology has allowed what would have been historically a very difficult project to be constructed efficiently and without much hassle.”