I-90 – SR 18 Interchange Improvements, WA
Image credit: © WSDOT
AAR Testing and Inspection, a SOCOTEC Company, provided construction materials testing, special inspections, and quality assurance services for the I-90/SR 18 Interchange Improvements project, supporting a major transportation infrastructure upgrade in Snoqualmie Valley, Washington.
Key Information
Project Name: I-90 – SR 18 Interchange Improvements
Client: Aecon Pacific Northwest
Owner: Washington State Department of Transportation
Reference Contact: Kathleen Mahon, Project Manager, Aecon
Funding: $125 Million
Location: Snoqualmie Valley, WA
Services Provided: Construction Materials Testing, Special Inspection, QA/QC Oversight
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The Challenge
Supporting a Major Highway Interchange Improvement in Washington State
The I-90/SR 18 Interchange Improvements project is a Washington State Department of Transportation (WSDOT) initiative designed to reduce congestion, improve traffic operations, and enhance safety at one of southeastern King County's busiest highway interchanges.
Due to significant growth in the Snoqualmie Valley, bottlenecks and backups at the existing interchange and along SR 18 had long created travel delays and safety concerns for commuters and freight movements.
The project consists of a strategic design and roadway expansion that replaces the existing interchange with a diverging diamond interchange (DDI) while widening more than two miles of SR 18. Additional improvements include new bridges, upgraded traffic movements, replacement of fish passage barriers with open-channel streams, and enhancements to wildlife habitat connectivity.
Delivering these improvements presented several construction challenges, including maintaining traffic flow adjacent to active interstate lanes, coordinating phased construction activities, and working within terrain and environmental constraints involving multiple creeks and drainage systems.
The Solution
Construction Materials Testing and Quality Oversight Throughout Construction
AAR TESTING AND INSPECTION, a SOCOTEC Company, supported the project by providing construction materials testing, special inspections, and quality assurance services throughout critical phases of construction.
Building on its Design-Build experience during the SR-520 program, AAR implemented an integrated approach to inspection staffing, reporting, and project coordination to support work within a highly constrained interchange environment.
Services provided included:
- Construction materials testing for pavement, structural concrete, and soils
- Quality inspections during bridge widening, structural steel placement, and DDI-related components
- QA/QC oversight during phased construction adjacent to live interstate traffic
- Field density testing, grading verification, and compaction monitoring
- Inspection coverage for stormwater systems, drainage structures, and subgrade preparation
- Daily coordination with contractor leads and WSDOT inspectors to maintain traffic flow
- Verification of geometry, tolerances, and elevations for DDI transitions
- Documentation and reporting aligned with refined Design-Build processes
Project activities were performed during phased construction adjacent to live interstate traffic while supporting coordination throughout the project's evolving construction sequence.
AAR TESTING AND INSPECTION, a SOCOTEC Company, supported the project by providing construction materials testing, special inspections, and quality assurance services throughout critical phases of construction.
Building on its Design-Build experience during the SR-520 program, AAR implemented an integrated approach to inspection staffing, reporting, and project coordination to support work within a highly constrained interchange environment.
Services provided included:
- Construction materials testing for pavement, structural concrete, and soils
- Quality inspections during bridge widening, structural steel placement, and DDI-related components
- QA/QC oversight during phased construction adjacent to live interstate traffic
- Field density testing, grading verification, and compaction monitoring
- Inspection coverage for stormwater systems, drainage structures, and subgrade preparation
- Daily coordination with contractor leads and WSDOT inspectors to maintain traffic flow
- Verification of geometry, tolerances, and elevations for DDI transitions
- Documentation and reporting aligned with refined Design-Build processes
Project activities were performed during phased construction adjacent to live interstate traffic while supporting coordination throughout the project's evolving construction sequence.
The Results
Supporting the Delivery of a High-Impact Transportation Infrastructure Project
Through construction materials testing, inspections, and quality assurance services, AAR TESTING AND INSPECTION, a SOCOTEC Company, supported the delivery of a major transportation improvement project serving the Snoqualmie Valley.
The completed improvements include a new diverging diamond interchange, the widening of more than two miles of SR 18, multiple new bridges, and upgraded roadway infrastructure designed to improve regional mobility.
The project also incorporates environmental enhancements through improved stream crossings and wildlife habitat connectivity while supporting freight, commuter, and recreational traffic throughout the corridor.
By providing inspection, testing, verification, and project documentation throughout construction, AAR supported quality oversight during the phased delivery of this significant Washington transportation infrastructure project.
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