The IIW team revived the historic limestone retaining wall supporting Grove Terrace within the nationally recognized Historic District in the thriving community of Dubuque, Iowa.
Among the wall’s signature features was a limestone arch that a town trolley traveled through in the early 1900s. While the retaining wall, originally constructed in 1887, needed significant restoration, the City of Dubuque looked to the IIW engineering team to keep it reminiscent of its early days, a focal point in the community and compatible with the Secretary of the Interior’s Standard for Historic Preservation.
On April 1, 2008, with the initial retaining wall restoration design nearly completed, a large rainfall led to the substantial failure of an adjacent section of retaining wall support to the limestone arch for the former trolley.
As a result, the City expanded the scope of the project to include reconstruction of the failed portion of the existing wall. In order to provide a cost-effective repair that met the tight aesthetic standards of the historic district, a limestone retaining wall was determined to be the desired retaining wall alternative.
An analysis revealed that a mass limestone retaining wall would be cost-prohibitive; therefore IIW developed a three-tiered approach to the design. Due to the proximity of the wall to existing ledge rock, the lower portion of the wall was constructed using a limestone façade, with solid concrete poured between the limestone and the ledge rock.
Because the ledge rock sloped away from the proposed face of the wall, the mid-height portion was constructed as a limestone-concrete geofoam composite system, with geofoam fill used both as a form for the concrete and to reduce the lateral pressure on the wall. The upper portion was constructed as a cantilevered limestone retaining wall where stone would be structurally sound. Additional measures were taken to resist vehicle impact.