The construction of the Skydome Stadium will provide Toronto with the first multipurpose stadium in the world with a retractable roof. The foundation for the structure, with a diameter of about 210 m and a height of about 86 m, will be supported by several hundred columns, about 100 of which will have loadings up to 50 000 kN. The foundation rock is a shale with numerous highly weathered zones and clay seams. Laboratory tests to investigate the strength and deformation properties of these weak layers were performed. Field plate loading tests at two locations with three different size plates were also carried out to study the size effect on the mass modulus of the rock. This paper presents the results of detailed geological mapping and field and laboratory tests together with design considerations for the foundations. The field acceptance criteria, developed to ensure that the differential settlements between foundations are within specified limits, are also discussed. Key words: Skydome, shale, clay seams, plate loading tests, rock-mass modulus, size effect, layered rocks.