Gold Mine Road Bridge

Brudis & Associates, Inc. (BAI) provided services for the replacement of Gold Mine Road Bridge over Hawlings River. Most of the project was located on the Maryland National Capital Park & Planning Commission Property (MNCPPC). Therefore, coordination, review, and approval from the park authorities were required during the design process.

Structural Design: BAI provided three (3) replacement alternatives including a single span prestressed concrete slab, a CONSPAN precast concrete arch, and a single cell concrete box culvert for the existing one-lane bridge. The final design included prestressed concrete slabs with a single span length of 55’ and a 44’ out-to-out superstructure width. The design also included 10’-0” reinforced concrete approach slabs.

Roadway/Shared-Use Path Design: The limited sight distance of the existing structure posed safety concerns. BAI’s roadway design services included a geometric alignment analysis of the existing roadway and provided two (2) alternative alignments that met a 25 and 30 mph design speed. To meet horizontal sight distance requirements, the replacement bridge shifted nearly 20 feet upstream to increase the horizontal curvature to be AASHTO compliant. To alleviate frequent flooding, the roadway profile was raised across the bridge by nearly 6’ to pass the 5-year storm event, while not increasing the 100-year flood elevation by more than 0.5 feet. Consequently, nearly 2,000 linear feet of approach roadway needed to be reconstructed to provide full bicycle/pedestrian connectivity. BAI designed and obtained permits associated with the design of an 8’ off-roadside bicycle path that tied into an existing trail at Chandlee Mill Road. The extension of the path cuts into an 8-10’ high embankment and required a retaining wall.

Hydrology and Hydraulics, SWM & Drainage: BAI conducted hydrology and hydraulic analyses, computing discharges at the bridge location. The hydrologic analysis evaluated storm frequencies (2, 10, 25, 50 and 100-year). In addition, the 500-year discharge was determined for scour analysis. BAI analyzed the stream profile for waterway elevations, storm runoff and floodplain limits.

The hydraulic analysis compared existing and post-construction stream conditions based upon structural waterway opening, typical bridge section and stream cross sections.

BAI also provided existing/proposed 100-year floodplain mapping. BAI satisfied Storm Water Management (SWM) requirements by implementing environmental site design (ESD) to the maximum extent practical (MEP) by utilizing shallow gravel wetlands. Due to poor soils, steep slopes, wetlands, and forested areas, most of ESD facilities were not applicable or suitable for this project. The design also included sizing of flat bottom ditches with stabilization matting. A combination of ditches, pipes, and end sections were used to convey all stormwater runoff from the improvements to the shallow gravel wetland for treatment.

Traffic: BAI provided a Maintenance of Traffic Alternative Analysis (MOTAA), that examined maintenance-of-traffic strategies during construction. The report examined two (2) scenarios: a complete bridge closure with a full detour; and maintaining one (1) lane of traffic over the existing bridge during construction. The detour routes required BAI to analyze nearly 14-miles of roadway to determine adequate detour routes.

Public Outreach: Due to the dense residential development within the project limits, BAI provided public outreach and involvement throughout the design process to ensure all stakeholders received information on the proposed design and impacts. BAI provided PowerPoint technical graphics and developed computer renderings of the proposed improvements to serve as visual aids to the non-technical stakeholders.