Analysis of a Deep Excavation In Calgary, Alberta-Article

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The Bow office complex excavation in downtown Calgary was approximately 20.5–21 m deep and extended over 17,000 m². Because nearby buildings and transit infrastructure were sensitive to movement, the shoring system was monitored with inclinometers, extensometers, and survey measurements.<br /><br />Monitoring showed substantial lateral deformation in the bedrock, driven mainly by two mechanisms: bulging within a weak mudstone zone and localized movement along thin shear bands. Some measured deflections reached about 150 mm overall; the weak rock contributed up to 100 mm, while shear-band movement reached roughly 45 mm. Shear-band movement increased as excavation advanced and could occur below the final excavation level. Extensometer results also suggested that deformation extended beyond the immediate shoring area.<br /><br />Laboratory tests on difficult-to-recover mudstone samples indicated very low uniaxial compressive strength, typically 560–1,230 kPa, and an elastic modulus of about 80–180 MPa. Estimated intact strength parameters were cohesion of 340 kPa and a friction angle of 24°; residual values were zero cohesion and a 15° friction angle. The rock also showed swelling potential.<br /><br />A preliminary finite-element model using field and laboratory data reproduced the location and magnitude of movement along the north wall reasonably well. The findings emphasize the need to account for weak rock layers in deep excavations and to use comprehensive monitoring. However, the shear-band mechanism, horizontal in-situ stresses, and some rock properties remain uncertain, requiring further investigation.

Keywords

Bow office complex

downtown Calgary

deep excavation

shoring system

inclinometers

extensometers

bedrock deformation

mudstone

shear bands

finite-element model