Date of Award
Master of Engineering Science
Civil and Environmental Engineering
Dr. Hesham El Naggar
Dr. K.Y. Lo
A 20.5 metre deep excavation with a footprint of 17 700 m2 was completed in downtown Calgary for The Bow, EnCana office complex. The excavation support system required to protect the neighbouring structures was monitored in accordance with Peck’s Observational Method. The maximum observed horizontal deformation of the shoring wall was 153 mm.
Geotechnical site investigation indicated that the bedrock was predominantly weak Porcupine Hills Formation mudstones with more competent siltstones and sandstones present in thin interbedded layers. Seismic survey results, point load test analysis, and inclinometer readings during construction indicate that a weak zone of mudstone is present within the bedrock. This zone is responsible for large horizontal deformation and should be considered during design of excavation support. The shear band phenomenon responsible for large deformations in previous downtown excavations was observed to be highly sensitive to excavation activity. Movement of the shear band occurred when excavation activity was 12 m above the layer. Detailed and experienced geotechnical investigation is required to accurately determine the location of potential shear bands. Testing and monitoring results indicate that time dependent swelling in the order of 0.25 to 0.50 %/log cycle be considered during design of structures in the bedrock.
A finite element analysis indicates that the large movements observed during excavation are due to the weak zone of mudstone, the shear band layer, and an elevated in-situ stress. The combination of the above factors results in a large zone of influence due to excavation. The estimated horizontal in-situ stress is 1.3 MPa. Further exploration and testing of the in-situ stress is recommended.
Lardner, Thomas Rychard Alexander, "BEHAVIOUR OF THE SHORING AND BEDROCK DURING THE EXCAVATION FOR THE BOW" (2011). Digitized Theses. 3278.