Herrick Street Bridge 

 

Herrick Street Bridge

The list of possible uses for segmental retaining wall (SRW) systems continues to grow. The latest success story comes out of Rensselaer, New York. The city of Rensselaer needed a site solution that would not only be aesthetically pleasing, but would also create additional roadway access to one of the busiest rail stations in the state of New York. The design called for an off ramp bridge that would lead traffic from a major highway into the parking lot of the Rensselaer Rail Station. Due to the versatility, durability and aesthetics of SRW units, the product was chosen to provide the perfect solution.  The project would require a combination of a non-flexible bridge structure, with a segmental block retaining wall that has become a success because  of its flexibility. This, coupled with the additional needs of a roadway, would make the project a sizeable task.

PLAN
Time and performance were valuable. To meet the requirements of the heavy traffic flow, the off ramp became a necessity. The site plan called for two walls that would hold up the ramp and would also allow for traffic and pedestrians to flow underneath it. Therefore, in addition to designing a ramp, the plan also called for a tunnel design. It was decided that the SRW units would be built on either side of the ramp, with the tunnel running through it. The seams between the SRW system and the transportation system needed to be flawless, therefore the layout tolerances were kept to a minimum. Engineers at the at every stage of the manufacturing process joined forces with the engineer of record; Russ Reeves C Eng., and the reviewing team of engineers at Ryan-Biggs Associates to make the project a triumphant one.

DESIGN
The stiff requirements for the roadway made the design of the off ramp a challenge. The retaining wall design needed to meet the specifications of the American Association of State Highway and Transportation Officials (AASHTO). The city requires that all retaining walls with state roads above them must be designed according to these standards. AASHTO design specifications require that the minimum geogrid reinforcement lengths must be 70% of the wall height.
    These length requirements are much longer than the typical retaining wall industry standards. The design of the walls also included seismic activity analysis. Rensselaer, New York is generally not considered a seismically active area; therefore seismic analysis would typically not be required.

BUILD
The construction of the Herrick Street Bridge needed to be very well thought-out. The surveying team spent many hours laying out the placement of the walls and accounting for the block setback. This was especially critical since the design required the walls on each side of the roadway to have 31 feet (9.45 m) of spacing between them at the top. During the placement of the units there were several things that needed to be considered.Since there were manholes and storm drains behind the wall, the geogrid reinforcement could not be laid in the soil in the typical fashion. AASHTO required that the geogrid be fastened to all the manholes and storm drains. 
    The geogrid also needed to be cut and placed around and behind the obstructions. In addition, the construction crew had to build around the tunnel that went through the retaining walls.      
    In constructing the wall, the top two courses of SRW units were grouted together, the top course of geogrid reinforcement extended from one wall to the other, and the cap blocks were added to keep the step downs less than six inches (15 cm). A Texas barrier and sidewalks were also constructed at the top of the retaining wall followed by paving the road. The cooperation and dedication of the many individuals involved in the project made the Herrick Street Bridge a success.
CMD

Images courtesy of Allan Block.