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

Originally built in 1856, the existing bridge carries 5 railroad tracks servicing MNRR and AMTRAK and spanned over Atlantic Street in Stamford, CT providing two (2) lanes of traffic beneath the bridge.  Replacement of the bridge was driven by several factors including age, traffic improvements to widen the narrow underpass to service 5 lanes of traffic and increase headroom beneath the bridge to reduce restrictions on larger vehicular traffic.  The proposed pile supported bridge abutments were designed to be constructed while keeping all 5 railroad tracks in service, creating a need for individual jumpspan girder systems beneath each track.  A temporary excavation support system (SOE) design was included in the Contract  and consisted of drilled H-piles soldier piles and bracing with bolted connections.  GTR re-designed the SOE system to accommodate low headroom restrictions and constructability issues associated with track outages and site logistics.  Several key factors were to be considered for the SOE system: (1) headroom restrictions beneath catenary systems, (2) train axial, lateral, braking and traction forces, (3) headroom during pile cap construction, (4) integrating the SOE system to accept the “jump span” bridge components, and (5) Phased construction considerations.  To address these factors, GTR designed an SOE employing drilled micropile load bearing soldier piles with internal bracing to tunnel beneath the active railroad traffic.  This SOE scheme was successfully installed in low headroom conditions beneath existing  catenary lines in five (5) phases and included an innovative pre-fabricated bearing connection “top hat” to accept the jump span bridge.  Each phase of work was successfully installed and completed to support each active railroad track in five - 9-day outages. To achieve the required excavation support, a detailed phasing plan was required to support each adjacent track as subsequent jump span bridges were installed.  Although tedious excavation and bracing installation process was required, supporting the railroads required a robust system to ensure the safety of the workers and rail traffic alike.  After the SOE and jump span bridges were installed, the abutment foundations were constructed beneath the jump span bridges and between the SOE’s. Careful analysis and planning allowed for the creation of an accurate and precise system to support the axial and lateral train pressures while performing the required construction under the rail traffic.  This paper presents a unique case study of the “tunneling” beneath active railroad tracks utilizing a temporary support of excavation system to construct proposed pile supported abutments.