South Main Street Bridge, built in 1872, is an arch bridge that spans the Chadakoin River in Jamestown, New York. The bridge is comprised of two stone arches, abutments, a center pier, stone spandrel walls and parapets. The New York State Department of Transportation rehabilitated the bridge in the 1970s and again in 2000. The projects included constructing reinforced concrete abutments, expanding the spandrel walls and parapets followed by installation of a reinforced concrete relieving slab across the bridge, relocating utility lines, installing sheet pile and stone fill upstream and downstream of the bridge, repairing the stone masonry walls, and installing new drainage.

Inspections of the bridge detected deterioration of the stone arches supporting the bridge. The inspections also noted settlement of the upstream end of the center pier. Due to the deterioration, the number of travel lanes were reduced and one pedestrian sidewalk was restricted. GPI was retained to rehabilitate the bridge. GPI requested that McMahon & Mann complete a subsurface investigation of the bridge site and provide geotechnical recommendations for the rehabilitation. McMahon & Mann reviewed historical plans, and prepared and monitored a subsurface exploration program. We prepared a report summarizing the subsurface conditions observed, results of the laboratory testing, and presented geotechnical recommendations for rehabilitation of the bridge.

Several options for the bridge were evaluated including either full replacement or various rehabilitation options. After reviewing the options, rehabilitation of the bridge to preserve the stone arches was selected. This option was developed in a manner in which new reinforced concrete arches would be constructed on top of the stone arches. Based on the collected geotechnical information and the abutment work completed in the 1970s, we recommended that the new concrete arches could be founded on the original abutments and that reuse/modification of the center pier was possible.

Based on our understanding of the center pier construction, our knowledge of the upstream settlement and the chosen bridge structure rehabilitation, we developed a foundation design that would increase load-carrying capacity by installing micropiles and an expanded foundation. In addition, a grouting program was planned to fill voids beneath the existing foundation. As part of design, assumptions were made for soil conditions at the center pier as wells as its original design. During the initial phase of construction, additional soil borings were completed to verify the design. The planned expanded foundation allowed for excavation along the existing foundation so that actual conditions could be observed. Ultimately this information confirmed our assumptions and construction continued according to design.