AVALANCHE AND DRAINAGE MITIGATION PROJECT, BNSF SNOWSHED NO. 7, ESSEX, MONTANA

摘要

Avalanches have historically damaged railways traversing the watershed chutes in the John F. Stevens Canyon, between BNSF Railway mile posts 1165 and 1159, just east of Essex, Montana. To protect the railway, snowsheds have been constructed to divert the avalanches over the track structure. However, snowsheds were not designed to accommodate surface water flow. In 2013, BNSF and Jacobs Associates (the authors) designed and constructed a unique drainage basin and culvert system that captures surface water, conveying it over the snowsheds yet allows avalanches to pass over the snowshed unimpeded. The BNSF Railway near Essex, Montana, follows the historic Great Northern Railroad (GNR) route along the mountainous, scenic southern boundary of Glacier National Park. The railway traverses multiple avalanche chutes that drain large watersheds. In the early 1900's, GNR constructed Snowsheds Nos.1 to12 to protect the railway. Surface water was conveyed across the tracks via hand-dug ditches draining into culverts. Over the years, the ditches have filled with debris, resulting in the surface water being diverted over the snowsheds. Water leaking through the snowsheds has fouled the ballast beneath the tracks; the pumping action of passing trains jeopardizes the integrity of the track structure. Storm water and freeze/thaw cycles also scoured concrete from the snowshed structure, exposing rebar in the concrete walls and destabilizing foundation footings. Storm water plunging from the snowshed roofs has eroded embankments and access roads. Snowshed No. 7 has the most severe drainage problems with spring runoff washing out the bank and access road every year. The authors designed and provided construction oversight for this project including a reinforced earthen berm and graded catchment area to direct flow to a culvert intake and three reinforced, 3-foot diameter steel pipes to convey flow over the snowsheds into an existing creek. To counter the expected impacts of the avalanches, the design called for extra thick steel culverts to be installed over the snowshed, parallel to the flow of the avalanches, in addition to a low profile, rip-rap armored earthen berm which was anchored with soil nails and Tecco-Mesh.