Snohomish County, Centennial Trail Extension Phase 1 and 2
An abandoned rail corridor protects natural resources and creates a popular recreational trail
Completed cribwall and restored, planted embankment
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Completed cribwall and restored, planted embankment
Expertise
  • Geologic
  • Geotechnical
  • River Science & Engineering

Market
  • Transportation

Location
  • Arlington, Washington

Overview

The Centennial Trail stretches more than 25 miles from the City of Snohomish to the Skagit County border in Snohomish County, Washington. The trail lies on what was once the Seattle, Lake Shore and Eastern Railway constructed in the early 1900s, which has since been abandoned. The County purchased the alignment and began redevelopment in 1989 during Washington State’s centennial year.

Centennial Trail consists of a 10-foot-wide paved multipurpose trail for bicyclists and pedestrians, along with a parallel six-foot-wide natural surface path for equestrian users.

In 1995, GeoEngineers began providing geotechnical recommendations for the first Centennial Trail extension, a 10-mile section from Lake Stevens north to Arlington. This section included a bridge at Lauck Road, crossings at the Main and North Fork of Quilceda Creek and a boardwalk and dock at Lake Cassidy.

When the County began Phase 2 of the project, a seven-mile extension from Arlington to the Snohomish/Skagit County Line, GeoEngineers contracted to provide a geologic-hazard evaluation and geotechnical engineering services for the trail, including three bridge crossings. This trail extension crosses the Stillaguamish River, a large building-material supplier’s driveway and Pilchuck Creek.

Approach

The GeoEngineers project team provided geotechnical design and engineering, geomorphic and hydraulic design, construction oversight and overall project management for multi-discipline subconsultants. To complete the project, we:

  • Explored subsurface soil and groundwater conditions along the 25-mile trail sections including drilling in difficult access areas near the 100-year-old bridge piers. We also provided recommendations for suitable wall types, reinforced slope design and drainage considerations at numerous locations.
  • Completed dynamic PIT (pile integrity testing) of the original timber piles supporting the piers and developed recommendations to restore the foundation.
  • Provided construction monitoring services and observation for, earthwork and subgrade preparation along the trails, reinforced slopes and wall construction, individual pile evaluations and restoration recommendations for the original timber piles beneath the bridge pier, and construction monitoring for driven piles for the bridges, boardwalk and dock projects during Phase I.
  • Designed a 200-foot log cribwall to be placed at the toe of an eroding trail section along the Pilchuck Creek during Phase 2.
  • Designed in-stream mitigation for temporary impacts to endangered salmonids

Innovation


GeoEngineers utilized an innovative approach and design to stabilize the toe of a failing section of the trail along the top of a bluff adjacent to the Pilchuck River:

  • Modeled both the hydraulic forces and slope instability using a combination of two widely used models, but were able to synthesize the output from each to create a global stability model
  • Designed a cribwall structure that was anchored into a bedrock shelf, thereby reducing costs substantially from a typical cribwall design.

The cribwall and slope stability measures were constructed to improve fish habitat locally, gain quick project approvals from regulatory agencies and provide a long-term stable surface for the regional trail

Results

The Centennial Trail Extension:

  • Connects Snohomish County communities with a well-designed and functional transportation corridor
  • Reduced the threat of landslides, flooding and other risks to pedestrians and trail infrastructure