Engineers recommend $2.8 million precast modular replacement for failing retaining wall on Hannibal Hamlin Parkway

At the April 22 meeting the Infrastructure Committee reviewed an evaluation by DuBois and King of a failing granite block retaining wall on Hannibal Hamlin Parkway. Staff said the report focuses on roughly 100 linear feet of the worst-condition section between Central and State streets and that visual inspection and ground-penetrating radar identified voids and missing mortar that undermine the wall's stability.

Jeff Davis, the city engineer, summarized the consultant's findings and the alternatives considered. "The consultant concluded that the full wall replacement is the only engineering solution that addresses our stability requirements and ensure public safety is moving forward," he said. The consultant and city engineers presented five conceptual options and recommended a precast modular (precast concrete block) system because of constructability, quality control and the ability to create an appearance compatible with the existing stone. The consultant's cost estimate for the recommended alternative was about $2,800,000 for the section reviewed.

Casito, the DuBois and King engineer on the report, described the precast system's assembly benefits and aesthetic options: "They are stable, they're structural, and also value the opportunity to create different appearances," he said, noting manufacturers can vary block size, color and texture to better resemble existing granite. Committee members confirmed the replacement would be full-height for the affected span (engineers cited a wall height of roughly 26 to 28 feet) and were told the wall sits on bedrock in the area under review.

Staff said the city would catalogue and store salvaged granite for reuse in future projects. They also warned a parkway closure would likely be required during construction, and that work would probably be phased over multiple sections so each completed section ties into the next. Given those impacts, staff asked the committee for direction to proceed with detailed design and pricing for the precast modular alternative; the committee recorded a motion to proceed and asked staff to return with design documents and firm cost estimates.

The committee did not vote to approve construction funding tonight; staff will return with designs, refined cost estimates and scheduling information for future committee and council review.