Case Study - MNRR Jump Span Temporary Support of Excavation for Proposed Foundation Installation-Paper

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The Atlantic Street Bridge in Stamford, Connecticut, carried five Metro-North Railroad and Amtrak tracks over a congested roadway. Its replacement was needed to widen the underpass, increase vehicle clearance, and accommodate a future railroad track. Because trains had to remain in service, new pile-supported abutments were built beneath the tracks using temporary “jump span” bridges.<br /><br />The contract excavation-support design proved difficult to install within the restricted headroom, tight work areas, and nine-day track outages. Its large drilled soldier piles and numerous bolted connections also posed constructability and scheduling risks. Geosciences Testing and Research redesigned the system using small-diameter, drilled micropile soldier piles with internal bracing. The micropiles could be installed through existing track ties and bonded into bedrock to carry the jump-span bridge loads. A prefabricated “top hat” connection transferred loads from the bridge girders to the piles and simplified installation.<br /><br />The design accounted for soil pressures and train-related loads, including Cooper E60 axle loading, impact, braking, and traction forces. It also addressed the axial loads from the jump-span bridges and the changing support conditions during phased excavation. Construction proceeded in five phases, with each track’s support system and jump span installed during its allotted outage. Carefully sequenced excavation and intermediate bracing maintained support for adjacent tracks as work advanced.<br /><br />Once all jump spans were installed, trains resumed operating over them while crews excavated below and constructed the new abutment foundations between the support systems. The project demonstrates how coordinated design, detailed staging, and micropile technology enabled safe foundation construction beneath active railroad tracks while minimizing disruption to rail service.

Keywords

Atlantic Street Bridge

Stamford Connecticut

Metro-North Railroad

Amtrak tracks

micropile soldier piles

jump span bridges

railroad foundation construction

track outage

temporary abutments

bedrock anchorage