As the city of calgary, Alberta, experiences rapid population growth the lack of sewer infrastructure brought development to a halt in the northern part of the city.

"Maximising protection of the environment through the upgrading of their sanitary sewer systems remains a key focus area for the cities of Calgary and Edmonton, which continue to grow at a rapid rate due to the healthy state of the economy in the Province of Alberta," explains TAC regional director Heinrich Heinz in his 2013 report. He makes note of three projects in the north and northwest of the city, including the Bowness Offload Sanitary Trunk (BOS-T). With home-building on hold,

maintaining schedule on the BOS-T project was critical. The initial route and plans called for a 2017 completion date. Construction started in late 2014 after revising the project’s design for open trench construction of 8-10m depth, to instead use microtunnelling, including tunnelling under the Bow River. Commissioning is expected to start in March.

The results not only reduced construction time, but also reduced the space in which the pipeline was constructed and the price of the project. Additionally, construction was less disruptive to the community and the environment — earning an Award of Excellence from the Consulting Engineers of Alberta (CEA) under the Sustainable Design category in February.

"The West Memorial Trunk was a critical project that affected multiple communities in Ward 1, as houses and businesses could not move forward until the tunnel was completed," says Ward Sullivan of the Calgary City Council. "Thanks to the hard work of the West Memorial Trunk team and the contractors, this important project was completed almost two years ahead of the first proposed date."

NASTT-NW awarded its 2015 Northwest Trenchless Project of the Year award to the BOS-T at its annual conference on November 19, 2015. Its project write-up explains that the original open cut design included in the RFP outlined construction of approximately 3,700m of 1,650mm sewer through a mature community that comprises residential, recreational, commercial and institutional development, and also included a proposed twin 900mm siphon crossing of the Bow River. The original alignment also included two mainline CPR crossings and crossing of the Trans-Canada Highway.

The designer, Aecom, assessed trenchless construction options for the project anticipating it would minimise disruption to the public, open up alternative alignments that were previously not considered suitable for open cut techniques. Furthermore, tunnelling under the Bow River could be completed without impacting the environment, which reduced the original schedule by a year due to an expedited approval process.

Selecting microtunnelling located the sewer at depths of more than 15m for most of the route, avoiding infrastructure such as the city’s largest water supply trunk.

"As this was the largest MTBM project to date in Calgary, a contractor prequali_ cation process was initiated to ensure that an MTBM contractor with relevant experience and equipment would be engaged to address the unique requirements of the project," according to the NASTT-NW Award write-up.

The project was tendered in two phases 2014, with Ward & Burke Microtunnelling being successful on both, for a combined construction value of approximately CAD 34M (USD 26M). The majority of this project involved the construction of approximately 2km of 1,650mm sanitary pipe with two 1,050mm sanitary pipes crossing under the Bow River.

Ward & Burke selected a Herrenknecht AVN MTBM and used direct design reinforced concrete pipe to successfully implement this project. This included deployment of a VMT navigation and monitoring system to successfully navigate the long, double curved drive to within millimetres of the design profile and alignment.

Ward & Burke also used a cast in place concrete caisson methodology to meet the sealed shaft project requirements. Twelve caissons ranging from 6m to 11.5m in diameter and over 20m in depth were successfully constructed without any appreciable ground loss.

Geotechnical and hydrogeological investigations confirmed that complex ground conditions would be encountered including a high groundwater table and highly variable soils i.e., fluvial river deposits, boulders and cobbles, and a minimum of three classes of bedrock ranging from extremely weak to extremely strong).

The high groundwater table with mixed ground conditions necessitated the use of "sealed" shaft construction techniques and a pressurised face MTBM to control ground loss to acceptable levels and avoid any adverse impacts to adjacent structures and utilities.

Design features incorporated into this project included MTBM drive lengths of up to 460m and double curved drive alignments to enable full and intelligent use of a narrow (6m) right-of-way for pipe installation.

The project demonstrated the success of matching the correct microtunnelling equipment to deal with a wide range of complex ground conditions and installation requirements (driving curved alignments to tight tolerances– completed within 20mm of alignment, with VMT curve < 4mm), according to the NASTT-NW award, highlighting the project’s successes.

The CEA judges commented that "technically difficult, environmentally challenging, time bound and high profile — this project had it all, and came in ahead of expectations."

Other project partners include Rural Road Construction Ltd., Earth Drilling, Lifeways of Canada Limited and Decast Ltd. (formerly Munro)