Repair of Subgrade Failure During a 104-Hour Track Window BNSF Railway Little Falls, Minnesota

摘要

BNSF Railway recently completed an expedited repair of a significant subgrade failure beneath a 300-foot section of new 2nd main line track near Little Falls, Minnesota. The subgrade materials in the area of track distress were comprised of a 3-foot thick shot rock stabilized subgrade underlain by relatively deep, soft organic clays and peat extending to depths of 5 to 18 feet below bottom of tie. Repair options evaluated included 1) an at-grade land bridge, 2) compaction grouting, 3) soil mixing and 4) a column-supported load transfer platform (LTP). The column-supported LTP was chosen for track support in the identified zone of distress due to its cost, flexibility, and speed of installation. Helical piles were used as the columns because conventional driven pile were anticipated to have driving difficulties through the shot rock section, which would likely require predrilling. The LTP for the new 2nd main line repair was completed during a 104-hour continuous track window in which 480 feet of track and 1,100 cubic yards of ballast/subballast and embankment were removed; then 256 helical piles, a 30-inch LTP and subballast/ballast were installed prior to replacement of the track. The repair activities were completed without injuries and on time despite initially inclement weather. No further track movement has been observed since the LTP installation. This paper discusses the original construction, observed distress, geotechnical investigation and design of the LTP, as well as summarizes the lessons learned during the construction activities.