FEATURE ARTICLE Deep Excavation Support at Toronto’s Shangri-La Aerial view of the site at final depth while ramp is being removed The Shangri-La hotel and luxury condominium tower is a landmark of downtown Toronto. Located on prestigious University Avenue, it comprises 81,104 sq m (873,000 sq ft) and has 66 storeys. Eight underground basement levels provide parking; the lower four extending into the shale bedrock. The excavation for Shangri-La was one of the deepest in Toronto’s history. The site, bounded by city streets on three sides (Simcoe on the west, Adelaide on the south and University on the east), shares the block with a 14-storey building (200 University) with 5 under- ground levels, built from 1957-60. Along the east perimeter, seated on the rock surface under University Avenue, is the Toronto Transit Commissions (TTC) University Subway Line, with running track in a triple-box structure built in the 1950s. This was intended to lie only 2.5 m (8.2 ft) from the property boundary at the site’s southeast corner. The proposed excavation was to be 26.0 m (85.3 ft) deep. The surface of the bedrock is relatively flat at about 13.0 m (42.7 ft) below ground level. Earlier buildings on the site resulted in various depths of fill and existing foundations. The bedrock of the Georgian Bay Formation consists of shale containing inter-beds of calcareous shale, limestone and calcareous sandstone. It has very pronounced horizontal jointing typical of shale, but provides a reliable foundation with allowable bearing pressures of 2.4 MPa (25 tons/sf) for footings and 7.2 MPa (75 tons/sf) for drilled shafts. AUTHORS However, the bedrock under Toronto is known to contain high locked-in horizontal stresses around 3.8 MPa (40 tons/sf), much higher than can be resisted by shoring techniques, resulting in inevit- able wall movement when excavating deep into the rock. Vertical joints in the rock are sealed tight by these stresses, but appear as the excavation proceeds, with dominantly east-west orientation. The TTC box structures are sensitive to movement, particularly at the construction joints, typically spaced 12.2 m (40.0 ft) apart. The TTC enforces strict alert levels for such movements because of potential service disruption. The tender drawings indicated one of these joints may be located very near the site’s critical southeast corner, where differential rock movement along existing vertical fissures was expected to be at a maximum. Giving up Underground Space In 2007, Anchor Shoring & Caissons retained Isherwood Associates to provide an alternative design-build solution for its shoring proposal, with a mandate to better address the perceived serious risks to the TTC subway. Isherwood recommended asking the owner to give up space, allowing relocation of the basement wall further away from the critical corner. The owner agreed to abandon all eight basement levels in a 9 m x 35 m (29.5 ft x 114.8 ft) triangular area and thus move the closest approach to the subway from the southeast corner nearer to the center of the wall, providing Brian Isherwood, MICE, FCSCE, P.Eng.; Tara Brown, P.Geo.; and Jenny Earle, EIT, all of Isherwood Associates; and Paul Kreycir, CET, Anchor Shoring & Caissons Ltd. DEEP FOUNDATIONS • MAR/APR 2014 • 77
