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Rehabilitation of County Bridge R-24, Mill Road over Rockaway Creek

Location: Readington Township, Hunterdon County, New Jersey

Client/Owner: Hunterdon County

Phase: Design

Status: Final Design Underway


KMA Consulting Engineers, Inc. was retained by Hunterdon County to provide Design phase services for the reconstruction of County Bridge R-24, Mill Road over Rockaway Creek.  Built circa 1905, the structure is a riveted Pratt half-hip pony truss bridge that is supported on a fieldstone substructure.  The bridge is situated within the New Jersey Register-listed Whitehouse-Mechanicsville Historic District and is individually eligible for listing in the National Register of Historic Places.  Despite a rehabilitation in the year 2005, Country Bridge R-24 is classified as functionally obsolete and, as a result, Hunterdon County proposed to reconstruct the bridge and its approaches.  The reconstruction alternative was presented to the public and was not well-received; as an alternative, the County agreed to pursue a rehabilitation of the truss in order to achieve acceptable inventory ratings of the structure.  KMA’s scope of work included survey/mapping, subsurface utility engineering, geotechnical engineering, environmental services, an hydraulic and hydrology (H&H) analysis, and right-of-way engineering.

As part of initial Design phase efforts, KMA environmental scientists performed a wetland delineation in accordance with the Federal Manual for Identifying and Delineating Jurisdictional Wetlands (1989), submitted a Freshwater Wetland Line Verification Letter of Interpretation (LOI) to the NJDEP for approval, prepared and submitted a NJDEP Freshwater Wetland General Permit #1, and provided documentation to the NJDEP in support of Flood Hazard Area Permit-by-Rule #2 and Flood Hazard Area Permit-by-Rule #41 applications.  Because of the bridge’s situation in the Whitehouse-Mechanicsville Historic District, KMA also managed the Section 106 process, and provided oversight of the Historic Architecture and Phase 1A Archaeological Survey and Application for Project Authorization.

A comprehensive H&H analysis was also part of KMA’s design work.  Inputs for the existing conditions model included cross-sectional topography, selection of Manning’s roughness coefficient, bridge geometry (measurements of span, rise, abutments and piers), and roadway profile elevation.  Stream cross-sections were developed for at least 500 feet upstream and downstream of the bridge and wide enough to encompass the floodplain. A proposed conditions model was created for each alternative considered, each incorporating any proposed revisions to the hydraulic opening, changes to piers, abutments, or parapets, and/or changes to the roadway profile.  A scour analysis was also performed.

Additional design services included field survey and project base mapping; preparation of a soil boring exploration plan to identify the number, depth, and approximate locations of soil borings; laboratory soil testing required to design bridge foundations; identification of the type, size, location, and owner of all existing utility facilities within the project’s limits; and establishment of the existing right-of-way and property lines within the project area using deed information, tax maps, and property corner locations.