Franklin Falls – The Toughest Trenchless Crossing in New Hampshire

摘要

As part of construction of a new water treatment plant, the City of Franklin, New Hampshire,recently completed installation of two 12-in. HDPE pipelines below the Pemigewasset River. The crossing locationis situated to the southeast of the Franklin Falls Dam, which is operated by U.S. Army Corps of Engineers. At thislocation, the river includes two separate channels, each about 300 to 500 feet wide, separated by an approximately560-ft wide wooded peninsula. To minimize disturbance to the river, the crossings were completed using horizontaldirectional drilling (HDD) and four (4) separate drill paths; two per river channel.Initial land-based test borings, and geophysical survey suggested the bore paths would encounter granular alluvialand fluvial deposits. However, an unfortunate surprise was in store: an historic armoring was present below theriver bottom along two of the bore paths. This armoring consisted of 10 to 20-ft diameter stone blocks, carefullyplaced to form a channel raceway/spillway during initial dam construction (circa 1920). Ultimately, this armoringwould create havoc for an HDD contractor expecting a soil-based alignment, on a schedule driven project. Thispaper details the design and planning elements of the project, and the construction phase modifications that wererequired to complete the installation below this immovable obstruction.