Subsurface profile of the northbound alignment Subsurface Conditions The s ou t he rn embankmen t i s approximately 115 ft (35 m) high and is constructed primarily of blast-rock fill obtained from an I-91 rock cut section. A core of granular fill within the blastrock fill shell extends the entire height of the embankment; this arrangement below the existing southern abutments allowed for driving of steel H-piles during the original bridge construction. The embankment was constructed over bedrock or a thin layer of glacial till above bedrock. Layout for I-91 bridges' foundation elements The relatively level area between the southern and northern embankments where the GMR track and river channel reside is underlain by relatively thin layers of fill (below the track) and alluvium over glacial till over bedrock throughout, with a gentle slope downward from south to north. Between the embankments, bedrock depths vary from approximately 20 to 80 ft (6 to 24 m). The northern embankment is approximately 125 ft (38 m) high and constructed of dense granular fill. Alluvial deposits and glacial till underl ie the embankment wi th bedrock depths of approximately 180 to 210 ft (55 to 64 m). Seismic and Design Life Criteria VTrans designated the replacement bridges as " critical " bridges due to the potential importance of I-91 as an emergency travel route after a large earthquake. A critical bridge is required to remain in service following a design earthquake that is larger than what the American Association of State Highway and Transportation Officials (AASHTO) typically requires of bridge design (that standard is a 1,000-year return period earthquake). A preliminary seismic hazard evaluation was performed and provided with the bid documents. The design-build team conducted their own site-specific geotechnical seismic hazard evaluation and ground motion study to develop a design response spectrum based on an earthquake design event with a 2,500-year return period. The piles and embankment slope were designed to account for this design earthquake load instead. A 100-year design life was also specified for the bridges. A corrosion evaluation was performed on representative soils samples and a 1/16 in (1.6 mm) corrosion loss allowance was developed for pile design. 86 * DEEP FOUNDATIONS * JULY/AUG 2021