The shoring and deep foundations design/build contract for this three-tower project was awarded to The Anchor Shoring Group of Companies. Design and construction of subsurface work for these towers, which are up to 36 stories in height with 4 levels of underground parking, involved technical, logistical and regulatory challenges. One key challenge was the stringent vibration limit imposed on areas of the site in the vicinity of existing TTC structures. This drove the consideration of Continuous Flight Auger (CFA) piles as a design solution.
Shoring work was carried out using a combination of Bauer, Watson and Texoma drill rigs. Over 300 vertical holes were drilled for the lagged and caisson wall systems. Tiebacks were installed for additional lateral bracing at lagged wall areas. The caissons were constructed in regions subject to the very stringent TTC shoring deflection limit of 3 mm. To provide lateral support and further improve the stiffness of the caisson wall, an internal system of pre-loaded rakers and walers was used.
The foundations of the three towers are comprised of 130 structural caissons and 1350 CFA piles (totaling 23 000 linear metres) that were installed using a Bauer BG28 and BG40. The Bauer rigs were outfitted with equipment to monitor auger extraction rates, concrete pressures and concrete volumes during installations, verifying the quality of each pile. In areas where CFA pile installations were unfeasible due to excessive drill length requirements, Anchor proposed the drilling of additional boreholes to determine the presence of a potentially higher bearing capacity stratum. A higher bearing stratum was found, allowing caisson diameters to be reduced from 2300mm to 1300mm diameter and eliminating the need for vibratory installation equipment. Further examination of soil conditions permitted 68 of the 130 structural caissons to be reduced from 30m to 18m lengths.
The testing and iteration of design conducted by Anchor in the early stages of the project resulted in value engineering that yielded significant cost savings. The high productivities demanded by the aggressive project schedule were made possible by the motivated work and cooperative efforts of the contractor and consultants.