COVER STORY NFL Stadium, Santa Clara, Calif. Pile Foundations for the San Francisco 49ers NFL Stadium The new San Francisco 49ers Levi’s Stadium is part of the next generation of sporting facilities. Constructed in Santa Clara, Calif., the new structure has 68,500 open-air bowl seats and includes a 7- story tower housing more than 150 suites. The overall project includes the stadium structure, elevated open pedestrian walkways, commercial community space, three new pedestrian bridges and improvements to the adjacent light rail station. Sustainable design for the project includes a green roof, photovoltaic panel and a ground source geothermal system. Stadium-wide next-generation Wi-Fi capability is another feature. The design-build project team included HNTB (project architect), Magnusson Klemencic Associates (MKA, structural designer) and a joint venture of Turner Construction and Devcon Construction (TDJV, general contractor). During preconstruction, the 49ers’ original design team and the design-build team formed later worked together to facilitate communication and provide continuity. This helped to meet the project’s aggressive schedule. Working directly for the 49ers, with the design-build team, Treadwell & Rollo (T&R) served as the geotechnical engineer of record for the project from preliminary to final design and through construction. Although not contracted directly to TDJV, the stadium ownership encouraged direct interaction between T&R and TDJV and improved responsiveness to geotechnical issues during design and construction. 22 • DEEP FOUNDATIONS • MAY/JUN 2014 AUTHORS Subsurface Conditions The new stadium lies within the Santa Clara Valley, a deep alluvium- filled basin resulting from historic movement on the active and potentially active strike-slip and thrust faults that bind the valley. The valley is filled with Holocene age (approximately 11,000 years old to present) alluvium, consisting of levee and basin deposits, with bedrock more than 750 ft (229 m) beneath existing ground surface. The stadium site is between two of the highly-active faults in California, the Hayward Fault 6.8 mi (11 km) northeast, and the San Andreas Fault about 11.8 mi (19 km) southwest. T&R conducted a geotechnical investigation consisting of 14 borings and 14 cone penetration tests to depths up to 122 ft (37 m) below the existing ground surface. Results showed the site was underlain by a veneer of fill, in turn underlain by medium stiff to very stiff clay interbedded with thin and discontinuous sand and gravel layers. The clays were compressible in the near surface and increased in strength with depth. The sand and gravel lenses were medium dense to very dense with variable amounts of silt and clay. Because of the moderately compressible near-surface soils and high column loads, the team concluded that shallow foundations supported on the clay would experience significant and potentially erratic settlement behavior. Therefore, the stadium should be supported on a deep foundation system such as driven precast, prestressed concrete piles or, alternatively, drilled displacement Scott A. Walker, P.E., G.E., Gabriel J. Alcantar, Ramin Golesorkhi, Ph.D., P.E., G.E., Treadwell & Rollo, a Langan Company, and Brian Zuckerman, P.E., Berkel & Company Contractors
