Design, Construction, and Performance of the Temporary Earth Retention System at No. 9 Walton-Paper

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No. 9 Walton is a 36-story condominium in Chicago’s Gold Coast, built above three levels of parking. Its excavation extended 9.75–12.5 m into soft-to-medium clays, close to a historic church, a 39-story residential tower, city utilities, and subway tunnels.<br /><br />The temporary earth retention system used secant pile walls integrated with drilled shafts that also supported the new building’s exterior columns. Two levels of internal bracing supported the walls during excavation; tiebacks were ruled out because of conflicts with utilities, public rights-of-way, and subway infrastructure. To balance bracing forces, the team connected the system to the neighboring tower’s foundation, coordinating connection locations and using compressible material to limit loads on its caissons. The adjacent church’s shallow foundations were extended and protected with hydraulically installed pipe-pile underpinning.<br /><br />Designers assessed multiple excavation stages and predicted maximum wall movement of up to 4.32 cm (1.70 in). Weekly monitoring included inclinometers and optical survey points on the ground and nearby buildings. Actual maximum lateral wall movement was only 0.95–1.71 cm (0.37–0.67 in), substantially below predictions. Nearby ground settlement was generally 1.27–1.91 cm, with localized areas reaching 4.57 cm; maximum measured settlement of an adjacent building was 0.61 cm. No damage to surrounding facilities was observed.<br /><br />The project demonstrates how collaboration among owners, designers, and contractors, combined with an integrated retention system, underpinning, and close monitoring, can successfully manage excavation risks in a dense urban environment.

Keywords

No. 9 Walton

Chicago Gold Coast

condominium excavation

secant pile walls

drilled shafts

internal bracing

underpinning

wall movement monitoring

urban excavation

subway tunnels