Design/Build Replacement of Critical Vermont Bridge
GEODesign is the geotechnical engineer for the Design-Build team for the Interstate I-91 Rockingham, VT Bridges Replacement Project. The project replaces the northbound and southbound bridges over the Williams River and the Green Mountain Railroad. HDR, Inc. is the lead designer and Reed & Reed is the prime contractor.
VTrans designated the bridge as a critical bridge. A critical bridge must remain open to all traffic after the design earthquake and must be usable by emergency vehicles and for security/defense purposes immediately after a large earthquake.
The replacement bridges are four-span, approximately 850 feet long with spliced concrete beams and concrete bridge deck. The bridge elevation decreases by approximately 40 feet from the south to the north. The bridge is 100 to 150 feet high above the Williams River and GMR track. The replacement bridges and approaches are wider than the previous roadway, requiring widening of the bridge approaches.
Key geotechnical aspects of the project include:
- North Abutment Piles Foundations – The north abutments (northbound and southbound) were constructed on existing fill embankments over 100 feet in height and of variable density, over natural alluvial and glacial till deposits with bedrock approximately 200 feet below the roadway surface. Advancing piles through dense embankment fill was challenging. After driving piles for the northbound abutment, the team redesigned the southbound pile foundations taking advantage of Pile Driving Analyzer (PDA) pile testing data. Using the PDA data, the team was able to utilize a larger number of lower capacity piles to significantly reduce overall driving lengths, driving difficulty and number of splices.
- Southbound Central Pier Spread Footing Foundation – The central southbound pier is supported by a spread footing directly on bedrock. GEODesign provided bearing and sliding resistance recommendations as well as recommendations related to subgrade preparation. Due to the footing depths and proximity to the river, a cofferdam and tremie seal were utilized to excavate and construct the foundation. GEODesign performed field observation of the prepared bedrock subgrade.
- River Channel Subsurface Characterization – GEODesign used geophysical seismic refraction and GPR measurements to obtain geological data in the river channel for scour analysis. The geophysical work saved time and money compared to performing conventional soil borings from a river barge or from the bridge deck above.
- Seismic Design – GEODesign performed a site-specific geotechnical seismic hazard evaluation including a ground motion study to develop a design response spectrum and liquefaction evaluation. The ground motion study was based on an earthquake with a 2,500-year return period event due to the “critical bridge” designation.
- Slope Stability – GEODesign evaluated static and seismic slope stability of the existing and widened embankments. Boring data was supplemented with design and as-built information regarding the extents and composition of the existing embankment fill materials which resulted in analyses showing adequate global stability safety factors without the need for additional mitigation measures initially considered during preliminary evaluations.
GEODesign’s ability and commitment to providing timely and practical design and construction recommendations have been crucial for a fast-paced design-build project such as this.
