Project Profile – Geotechnical and Geology Expertise for the Calgary Airport Trail Underpass Project

The City of Calgary’s Airport Trail Tunnel Underpass Project at the Calgary International Airport consists of a 620 m long, 6 lane twin cell, 16.3 m wide, cast-in-place concrete structure and was built using the cut-and-cover method.  The nearly 500,000 m3 excavation earthwork and rockwork began in 2011.  By late that year, the 14 m high cut excavation face experienced rock falls due to the complex geological conditions and rock slope design angles.  The rock falls and associated slope instability caused safety concerns stalling the construction which could impact the final delivery schedule and cost of the project.

Terracon was retained by PCL Parsons Dufferin (PPD- JV), a joint venture that worked as the construction manager on behalf of the City of Calgary, to provide geotechnical and drainage expertise in establishing mitigative options and then monitoring the site once safe construction activities were re-established.

Terracon Highlights:

  • Terracon identified the selected mitigative option which included a combination of wire mesh, rock pins, crest hog wire line arrest systems and surface water control coupled with 24 hour visual inspection and monitoring;
  • Terracon provided piling support for the runway where it crossed the tunnel;
  • Terracon provided mitigation support for a deep water control structure which required a significant rock bolting exercise designed by Terracon and installed under our supervision. The bolting was required to address significant fracturing and tensile strains on a highwall rock face;
  • In collaboration with other subcontractors, Terracon designed and supervised the installation of a mesh-anchor-system (MAS) that was applied to the entire cut slope. The MAS combined with an inspection and monitoring successfully mitigated the safety concern of the workers working below the slope in the excavation;
  • Terracon conducted construction engineering re-design of a MSE wall, on site monitoring of the excavation slope during tunnel backfill, and regular slope stability related assessments.

As a result of these measures, the box work concrete tunnel structure was completed on schedule without further uncontrolled rock movement incidents.  The use of the Terracon recommended MAS mitigation option was millions of dollars less than other possible alternative solutions.  The Underpass project was reported to be under budget and on schedule.  The opening ceremony of the tunnel was held in May of 2014.

The Terracon project was jointly led by Paul Xu and Lee Nichols with field work and other project duties conducted by other members of our Terracon Technical Services team.  We’d like to thank everyone for their hard work and efforts on this very important and high profile project in Calgary’s growth.