Mitigation of Chronic Bluff Retreat with an Engineered Riprap Revetment BNSF Bellingham Subdivision, Whatcom County, Washington

The BNSF Bellingham Subdivision traverses coastal bluffs along Puget Sound in Whatcom County, Washington. In 1998, an approximately 50-foot-wide landslide involving an entire 75-foot-high bluff face developed about 3 miles north of Bellingham, initiating chronic bluff retreat to within 15 feet of edge-of-tie the following year. In 1999, Shannon & Wilson installed a proprietary vertical movement detector (VMD) instrumentation system, an owner-serviceable monitoring system connected to the BNSF signal to alert traffic to vertical shoulder movements or catastrophic shoulder failures. In early 2016, precipitation caused renewed bluff retreat to within 10 feet of edge-of-tie, in turn approaching, but not activating the VMD system. In response, Shannon & Wilson performed an emergency geotechnical site evaluation including geologic mapping, review of existing subsurface information, evaluation of slope stability, and development of conceptual mitigation options. The bluff face exposes a bedded sequence of glacial deposits including (from top to bottom) glacial outwash deposits consisting of dense to very dense, cross-bedded sand and gravel with interbedded silt and clay layers overlying glaciomarine deposits consisting of stiff to very stiff massive clay with silt, sand, and gravel. The bluff includes at least two perched groundwater tables in coarse-grained outwash deposits above relatively impermeable clay beds in the glacial outwash and the glaciomarine clays. The perched groundwater horizons issued from the bluff face in two spring lines. A large industrial facility adjacent to the site likely increased seepage volumes due to effluent from a septic system drain field, and increased surface runoff from a large area of pavement that infiltrates upgradient of the site. Erosion of coarse-grained soils at the bluff face spring lines undermines the overlying soils, causing repeated debris flows. Coupled with wave erosion at the base of the slope, these debris flows steepened the face to an approximate average angle of 45 degrees after the storms, causing a cycle of bluff retreat and prompting renewed stability concerns. Following analysis, Shannon & Wilson and BNSF determined that an engineered riprap revetment would provide an effective and economical means to arrest bluff retreat and buttress the slope. In addition to accommodating wave erosion forces, Shannon & Wilson’s revetment design required considerations to accommodate environmentally sensitive tidal flats and constructability concerns due to the lack of a staging area between the railroad and the bluff face. Environmental concerns were accommodated by constructing a relatively steep (1.25H:1V) revetment slope. This slope angle was constructed by individually placing interlocking riprap boulders to form a fortified revetment face. The lack of staging area was accommodated by employing a large crane to transport equipment and revetment materials over the rail line.


  • English

Media Info

  • Media Type: Digital/other
  • Features: Figures; Photos; References;
  • Pagination: 22p
  • Monograph Title: Proceedings of the 70th Highway Geology Symposium (HGS 2019)

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Filing Info

  • Accession Number: 01735594
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Apr 2 2020 9:42AM