WorldWide Drilling Resource

32 NOVEMBER 2022 WorldWide Drilling Resource® Blasting for a Scenic Bridge Over “Grand Canyon of the East” Compiled by Amy White, Associate Editor, WorldWide Drilling Resource® In 1875, a single-track iron and steel truss bridge was constructed across the Genesee River Gorge in Portageville, New York. The area is located in Letchworth State Park and known as the “Grand Canyon of the East” due to its scenic views. The Portageville Bridge serves as a lifeline along Norfolk Southern’s Southern Tier Line. The old bridge was one of the oldest rail bridges in the country. Due to structural instability, it became necessary to impose speed and weight limits for trains crossing the bridge. This significantly slowed rail traffic and began to cause a bottleneck in activities. A new bridge was proposed to improve rail traffic and safety. The new bridge included a 483-foot steel arch above the river which runs through a canyon more than 200 feet deep. The arch foundations were positioned in the canyon walls and required blasting for installation. Blasting involved several challenges to overcome. During construction of the new bridge, the old bridge remained in service 75 feet away. Vibrations from blasting excavation had to be limited to keep the 1875 structure safely open for rail traffic. Blasting fragments also had to be kept from entering the river below. Overbreak had to be minimized and the final rock face needed to be stabilized. Along with these challenges, nearby park trails and facilities, the Civil War Era Cascade House Historic Site, and nesting American Bald eagles were located within the bridge site, calling for further care to be taken during blasting. Maine Drilling & Blasting (MD&B) served as a specialty contractor to prime contractor American Bridge. Brierley Associates provided independent blasting consultation and worked with MD&B to oversee explosives use for drilling and blasting, compliance, and overseeing rock containment, fragmentation, and vibrations. To mitigate potential issues in the sensitive environment, preconstruction condition surveys were completed of the area’s surroundings. Vibrations and air blast overpressures were monitored with seismographs. Along with setting precise vibration limits, contract specifications for blasting limited bench heights to 15 feet, and two to three blast rounds per bench. Given these tight specifications, blast designs called for double- and triple-checked holes with powder factors just under one pound per cubic yard. Cranes had to be used to lower the drill and workers to the blasting area. LiDAR (light sensing and ranging) technology was used to confirm crane placements, layouts, quantities, and locations for rock excavation for foundations inset at angles in the gorge walls MD&B drilled and installed rock anchors every 15 feet to maintain the rock face’s stability. Following each blast, the construction crew lowered an excavator, filled its bucket, and hoisted material out to clear the lift for further use. Rubber tire blasting mats were used for containment for a total of 13,000 cubic yards of rock. This bridge project also included features such as micropile foundations, concrete piers and abutments, and site grading and ballast. Following two years of construction, the updated bridge has been open for rail traffic since late 2017. Following completion of the new arch bridge, the iron and steel bridge from 1875 was dismantled. Although the old bridge represented a historical marvel of engineering, the new bridge enables safe movement for double-stacked rail cars without added delays or weight restrictions. The old Portageville Bridge was viewed as an icon. The new bridge was carefully designed on a parallel alignment 75 feet south of the old bridge. It is the first true arch bridge constructed for the rail industry since the 1940s. Photos courtesy of American Bridge. EXB Groundwater Week Booth 712

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