Design and Construction of a Sheet Pile Earth Retention System in Soft Clays

Construction of a new multi-story high school in downtown Chicago, Illinois required a 5.5 m (18 ft) deep supported excavation. The 122 m (400 feet) x 30.5 m (100 feet) site was bordered on two sides by city streets and buried utilities, and on the other two sides by existing multi-story structures founded on shallow spread footings. The initial geotechnical exploration revealed the presence of urban fill underlain by very soft clay to depths well below the proposed excavation depth. An earth retention system (ERS) consisting of sheet piles and internal bracing was designed and constructed to protect the surrounding utilities and structures. Additionally, global stability was a concern due to the depth of the very soft clay and adjacent surcharge loads. This paper presents information about the design, construction, and observed performance of the excavation support system.