Preliminary Surface Fault Assessment and Conceptual Fault Crossing Design of Proposed Gas Pipeline, South-Central Alaska

A preliminary geologic and engineering study of fault crossings along a proposed high pressure natural gas spur pipeline was conducted in 2005 for the Alaska Natural Gas Development Authority (ANGDA). The route crosses a number of faults that comprise the eastern Castle Mountain-Caribou fault system in south-central Alaska, which is known to be active within 60 miles (100 km) to the west. The route approaches the faults at mostly subparallel angles, resulting in several long coincident crossings, five of which were judged to be potentially active in this study. Maximum displacements of 7 feet (2.1 m) in both vertical and horizontal directions were conservatively estimated for each crossing based on a maximum magnitude 7.0 earthquake and 700-year return period suggested for the western Castle Mountain fault. Preliminary design permanent displacements were recommended as 2/3rds of the maximum. A conceptual buried crossing design in a sloped wall trench with a double geomembrane liner and loose granular backfill would accommodate both vertical and lateral displacements. Additional geologic studies could potentially substantiate longer return periods and lead to a reduction in the number of crossings, crossing lengths, and displacement values. If design displacements are close to allowable settlement criteria, the fault crossings could be eliminated from requiring special design.