A bridge replacement project in Rochester, NY, encountered difficulties when it was
determined that rock below one pier was sloping downward toward the river at an angle
of 42 degrees versus anticipated 12 degrees. Cofferdam design and construction
became very difficult. The unanticipated subsurface conditions delayed the project by
eighteen months and increased the cofferdam cost by almost $3.0 million ($US) when
direct costs and associated delays were taken into account. The initial approach after
discovery of the problem was to use the same type pier design and modify the
cofferdam accordingly. This resulted in a cofferdam 12 meters (39.4 feet) deep on the
shore side and 18 meters (59.0 feet) deep on the water side. The design used a master
pile system to attain required wall strength and used rock anchors to prevent the
cofferdam from moving toward the river. The pier foundation was then redesigned by
the owner using revised seismic parameters. The original stepped footing carved into
rock was replaced by caissons socketed into the rock beneath a large tremie concrete
placement. The revised pier and cofferdam designs caused a shift from a mostly land
based cofferdam, excavation, and rock removal project into an almost entirely water
based cofferdam, caisson drilling, and tremie construction project. These design
changes resulted in a reliable, safe, and unusual cofferdam and construction method.
All of this was accomplished within a very narrow worksite with a railroad on one side,
and the river with its public and commercial freighter traffic on the other. In addition,
the existing two-leaf drawbridge was to be maintained in place which put further
limitations on the water borne equipment. The engineering and construction
considerations, problems, and solutions related to these cofferdams and pier
foundations are presented.
Author: Herbert F. “Buck” Darling III, Herbert F. Darling, Inc. & Richard J. Hartman, Hartman Engineering
Publisher: DFI