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520 Trail Grade Separation at NE 40th St 

City of Redmond Public Works

Redmond, Washington
190 LF Trail Tunnel, plus transitions
Estimated Cost:
$10.7 million
Value Analysis
Market Sector:
Facility Specialty:
Highways, streets, parking lots


This project contributed significantly to the increased fluidity of pedestrians and cyclists through the intersection at NE 40th Street. The tunnel structure includes retaining walls that will convey the shared use of the SR 520 Regional Trail beneath NE 40th Street.

The tunnel section consists of soldier pile walls with concrete fascia on the tunnel side, and supporting continuous concrete caps on top. Pre-stressed concrete slabs are supported on the cap, and covered with a cast-in-place concrete topping slab.

Tunnel design criteria includes:

  • Pedestrian Loading in Tunnel: 90 Pounds per Square Foot Pedestrian Live Load

  • Traffic Loading on NE 40th Street: AASHTO NL-93 Truck Loading

  • Ramp and Sidewalk Railings: 54-Inch-Tall Minimum

  • Stairway Railings: 42-Inch-Tall Minimum

  • Maximum Trail Ramp Gradient: 5% Maximum

  • Tunnel Service Life: 75-Year Minimum

  • Tunnel Clear Width: 14 Feet Minimum

  • Tunnel Minimum Vertical Clearance: 10 Feet

  • Additional site work includes paved walkways and stairs, requirements of the Americans with Disabilities Act (ADA), utility relocations, lighting and restoration of disturbed surfaces.

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