Design and Installation of Micropile Foundations for a Suspension Bridge

A new 620-ft single-cable suspension bridge for bicycles and pedestrians was constructed south of downtown Chicago, IL to replace an aging, existing structure. The bridge spans four (4) electric railroad tracks, two (2) conventional freight railroad tracks, and eight (8) lanes of a divided highway to connect a residential area with an existing lakeside park. The bridge is founded upon five (5) separate foundations, four (4) of which are supported by drilled micropiles. Most notably, the pier foundations, near the ends of the bridge, are required to resist the large dynamic loads imposed by the suspension cable supporting the bridge deck. The pier micropiles are designed with compression and tension capacity to adjust to the varying load directions and installed with a substantial permanent pre-stress to achieve very little movement and prevent slack in the suspension cable. The remaining micropile foundations are designed to support the large vertical loads imposed by the long-span structure, including the center pylon which bears the vertical load of the suspension cable. The geotechnical exploration revealed the presence of a broad mix of urban fill, sand, silt, and clay underlain by low to medium quality dolomitic limestone bedrock. The micropiles for this project were designed to be socketed into the dolomitic limestone bedrock. The micropiles were load tested to verify the capacity and expected movement of the piles. This paper details the steps taken during design and construction to meet the performance requirements of the structure and deliver a quality product for the owner.