21 Feb 2008

Forensic Geomorphological Study of the April 5, 1987 Collapse of the NYS Thruway Bridge across Schoharie Creek

Mr. Bob Dineen, PA Department of Transportation, Bureau of Construction and Materials

The NYS Thruway bridge crossing the Schoharie Creek in eastern NYS collapsed during the morning twilight hours of April 5, 1987. The NYSGS was requested to study the geologic factors contributing to the collapse by the Governor's office. Dineen mapped the geomorphic features formed by the flood. The geologic/geomorphic and weather factors contributing to the collapse included: The filled embankment of the Thruway confined the stream to one-tenth the area available in pre-NYS Thruway time. The spring flood in the Schoharie Creek was particularly intense because of rapid snow melt, saturated ground conditions, and a spring storm that flooded the Catskill Mountains. The stream gradient near the Schoharie's confluence with the Mohawk River was unusually steep because the Mohawk River was not in flood...the spring storm did not extend into the upper drainage basin of the Mohawk River. The Schoharie flood was a thirty-year event (coincidently, the bridge had been in place for 30 years). The floodplain deposits were 12 feet thick, and the footing of the bridge was placed on the contact between the alluvial deposits and the underlying glacial till. The till was interlaminated with gravel, and was folded. The confined stream raged through the narrow opening under the bridge, eroded the till, and undercut the bridge.

Engineering features contributing to the failure included improperly installed and maintained rip-rap at the base of the bridge piers (the installation of a spread-footing to the base of the floodplain deposits, and the above-mentioned embankment.