Innovative Design for the Merchants Bridge West Approach Reconstruction for TRRA in St. Louis, MO

Using design elements and lessons learned from the UP-BNSF Colton Crossing Project in Colton, California, the Merchants Bridge Design Team adapted an innovative approach for the Merchants Bridge West Approach reconstruction of the Terminal Railroad Association (TRRA) track in St. Louis, Missouri. The existing west approach consists of an open deck steel trestle supported on steel frame bents. The project begins at existing Bent W6 which was part of a previous partial reconstruction of the Merchants Bridge over Mississippi River in 2005 and extends westerly to the existing concrete abutments. Due to TRRA’s requirement that rail operations be maintained during construction, the existing bridge was fully encapsulated within the lightweight concrete (LCC) embankment. Conventional MSE wall panels were selected to contain the LCC. Auger-cast piles placed underneath the MSE wall panel footings increased the bearing capacity where it exceeded the allowable bearing capacity of the soil. Another issue arose because the project site is located near known active New Madrid and Wabash Valley seismic zones. Liquefaction potential of the existing fine alluvium soil found at the project site was a particular concern. In addition, several existing utilities were present under the footprint of the proposed embankment. Utility bridges were proposed to minimize the impacts on the utilities but construction constraints challenged placement and methods of installation. The design team performed advanced service and seismic load design utilizing finite element numerical analyses as well as other traditional methods. “Hard points” were identified within the embankment and analyzed for increased stresses within the LCC embankment. A reinforced rock mat was used to distribute the load of the embankment to the auger-cast piles to help reduce differential settlement. Liquefaction potential and seismic design was analyzed using the AREMA Level 1, 2, and 3 earthquake event criteria. To fully understand the utility “bridge” concept, many methods were considered and analyzed to estimate the settlement of each utility impacted by the placement of the new LCC embankment fill. The design for Merchants Bridge West Approach Reconstruction has the following advantages. (1) Track operations could be maintained throughout construction. (2) Cellular concrete is lighter and stronger than compacted soil backfill, increasing stability and decreasing settlement impacts. (3) Increased seismic stability due to the embankment behaving like a large block that moves uniformly. (4) Utility bridge solutions work with limited overhead clearance under the existing tracks.