2015 Northwest Trenchless Project of the Year


Congratulations to the City of Calgary, AECOM, and Ward and Burke Microtunneling on being awarded the 2015 Northwest Trenchless Project of the Year for the Bowness Sanitary Offload Trunk!

2015 POTY Poster


AECOM was selected, in a Qualification Based Selection process to provide Preliminary and Detailed Design and Contract Administration services to the City of Calgary, for the Bowness Sanitary Offload Trunk (BOS-T) project.
The BOS-T project is being constructed to provide additional sanitary sewer capacity for Calgary’s northwest to meet future wastewater loads. The schedule was critical from concept to commissioning, as a moratorium was placed on upstream land development until the Trunk was in service, initially scheduled for Q4 2016.
The project was tendered in 2 phases in Q3 of 2014, with Ward and Burke Microtunnelling Ltd (WBML) being successful on both, for a combined construction value of approximately $34 Million.

Preliminary Design

The Conceptual Design Report (CDR) provided with the Request for Proposal, outlined construction of approximately 3,700m of 1,650mm sewer through the mature Bowness community that is comprised of residential, recreational, commercial and institutional development, and also included a proposed twin 900mm siphon crossing of the Bow River. The CDR alignment included two mainline CPR crossings and crossing of the Trans-Canada Highway and proposed construction largely by conventional open cut methods, with the depth of the excavation in the range of 8-10 metres.

AECOM envisioned an opportunity to assess trenchless construction technologies to successfully deliver this project at the same or lower cost and with considerably less impact on the public and the environment. The use of Microtunnelling Boring Machine (MTBM) technology was seen as a viable solution to address project challenges including:

• Minimizing disruption to the public and the environment.
• Opening up alternative alignments through alternate corridors not suitable for open cut techniques, (less length and less cost – see Figure 1).
• Tunneling under the Bow River could be completed without impacting the environment which reduced the CDR schedule by a year due to an expedited Department of Fisheries and Oceans approval process.

Development of the design revealed that a minimum of three major infrastructure trunks were in direct conflict with the CDR profile, including the 1980mm BearsPaw feedermain, the City’s largest water supply trunk. These features dictated that the sewer be located at depths of over 15m for most of the route.
During final hydraulic modeling, it was determined that adoption of the proposed alignment east of the CPR ROW provided an opportunity to utilize the upstream existing trunk as a parallel conveyance element, which resulted in both downsizing of approximately 1/3 of the trunk and deferment of this section of construction for approximately 20 years.
In accordance with the City of Calgary’s Triple Bottom Line (3BL) policies, and to address any public concerns regarding implementation (in an area still recovering from catastrophic river flooding in June 2013), a comprehensive public engagement program was initiated with the local community through a series of open houses during the design stage which allowed for seamless transition to construction.

Detailed Design
A comprehensive geotechnical and hydrogeological program was undertaken. These investigations confirmed that complex ground conditions would be encountered including a high groundwater table and highly variable soils (e.g. fluvial river deposits, boulders and cobbles, and a minimum of three classes of bedrock ranging from extremely weak to extremely strong). As a means of equitably distributing construction risk between the client and the contractor, a Geotechnical Base Line Report (GBR) (Figure 2) was prepared for the project and incorporated into the Contract.
The high groundwater table with mixed ground conditions necessitated the use of “sealed” shaft construction techniques and a pressurized face MTBM to control ground loss to acceptable levels and avoid any adverse impacts to adjacent structures and utilities.
As this was the largest MTBM project to date in Calgary, a contractor prequalification 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.
Alignments and micro-tunneling worksites were selected to minimize community disruption and maximize effects of 3BL considerations. This included securing suitable temporary easements on CPR right-of-ways and completing HIRA archeological assessments on riverbanks and other areas.
Detailed 3D finite element analysis (FEA) modeling was also conducted to assess the effects of construction on critical infrastructure, including the BearsPaw Feedermain (Figure 3). This exercise facilitated a reduction in clearances with critical infrastructure at increased confidence levels.
Other design features incorporated into this project included:

• MTBM drive lengths of up to 460m.
• Double curved drive alignments to enable full and intelligent use of a narrow (6m) right-of-way for pipe installation.
• Utility and surface monitoring to assess effects of micro-tunneling on adjacent infrastructure.
• Pre-assessment of existing structures and homes adjacent to ROW to document existing conditions.

Construction commenced in December of 2014. WBML selected state-of-the-art Herrenknecht AVN MTBM technology (Figure 4) and use of 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.
WBML also utilized cast in place concrete caisson methodology (Figure 5) 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.
Construction is approximately 85% complete, with 7 of the 8 MTBM drives successfully completed, with the final drive under the Bearspaw commenced. Commissioning will be completed in Q4 of 2015, nearly one year ahead of initial schedule. Restoration will be completed in 2016.
Project successes include:

• Monitoring points installed on critical utilities, roadways and adjacent houses have shown no significant movement to date.
• The technical approach combined with the public engagement program has virtually eliminated complaints/concerns through the construction phase of the project.
• Demonstrated success of matching the correct micro-tunneling 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).
• Use of GBR to equitably manage and distribute risk.
• Completion of a time sensitive project in an expedited manner.