We began design for this new off-ramp and modifications to the existing on and off-ramps at I-205 Mill Plain Boulevard I/C using a 3D laser scanning survey system.  A Leica HDS3000 was used to map the project site and roadway topography.  The speed of data collection allowed our surveyors to collect the necessary information in a shorter period of time than with conventional survey equipment.  The survey took place in close proximity to high-volume traffic which created safety concerns.  The laser system offered improved worker safety, as well as significant time and money savings.  The high degree of accuracy and short data collection time allowed design to begin ahead of the original schedule.

Geotechnical testing, analysis and design was used to support environmental documentation, surfacing design and structural design.  Three reports were prepared to document the analysis, 1) a Geotechnical Report, 2) a Surfacing Report and 3) an Infiltration Report.

Once base mapping was complete, preliminary design was begun by developing horizontal and vertical alignments with lines, grades, sections and cut/fill lines, all to WSDOT and FHWA interstate highway design standards.  Critical to this project was assurance that the adjacent and overlapping project would mesh correctly in the final condition.  When completed, the two projects together will provide a braided ramp system with this project spanning over the future project.  It was imperative that horizontal and vertical clearances and sight distances be set correctly to accommodate the future conditions.  Geometric design was concluded with final design/PS&E.

The project footprint identified with the preliminary design effort allowed us to proceed with environmental documentation.  This project required a NEPA Documented Categorical Exclusion and accompanying SEPA Determination of Non-Significance.  Discipline studies were undertaken to address air quality and noise impacts, environmental justice and visual impacts.  A local agency Environmental Classification Summary and Biological Assessment were also prepared to satisfy City, State and Federal requirements.

With receipt of traffic modeling provided by the Southwest Washington RTPO (RTC) via the City of Vancouver, we began operational analysis to determine appropriate lane configurations and intersection modifications.  A new signal controlled intersection was designed to connect the NW Ramp extension with existing NE 112th Avenue.  The new signal was interconnected with the N-E and W-N ramp termini at Mill Plain Boulevard and the signalized intersection at Mill Plain Boulevard and NE Chkalov Avenue.  An intersection Plan for Approval was prepared and submitted to the City and WSDOT as was an Interchange Plan for Approval to document the I-205/Mill Plain Boulevard I/C modifications.  Operational analysis and traffic design, signing, striping, illumination, construction staging/work zone traffic control was concluded with final design/PS&E.

Stormwater/hydraulics design was a major undertaking on this project.  Two Threshold Discharge Areas (TDA’s) were involved with this project, the Columbia River to the south (only quality treatment required) and Burnt Bridge Creek to the north (both quality and quantity treatment required).  Stormwater collection, conveyance and treatment were designed in accordance with WSDOT’s 2006 Highway Runoff Manual including the requirements for enhanced treatment.  The topography combined with limited right-of-way required the use of a very large underground vault for quality treatment prior to the water being infiltrated into the underlying soils.  Pre-treatment was provided prior to water entering the vault system.  Stormwater/hydraulics design was concluded with final design/PS&E.

Two bridge structures were designed for this project, a single span bridge to cross the NW Ramp extension to NE 112th Avenue and a 6-span structure to take the W-N On-ramp from Mill Plain Boulevard across a future off-ramp the NE 18th Street and onto I-205.  In addition, numerous retaining walls were required because of the limited right-of-way available and the close proximity of the existing on and off-ramps.  One of the retaining walls was a soil-nail wall that allowed for more width beneath the Mill Plain Boulevard Bridge across I-205.  The other calls were either conventional concrete cantilever walls or geotextile “bag” walls with concrete facings.  Structural design was concluded with PS&E.

A public involvement effort was begun early in the project by preparing a Public Involvement Plan to chart the planned activities throughout the life of the project.  Open houses, post card announcements and newsletters were used to impart information to the public and request/receive feedback on the project.  Design visualization graphics were prepared for presentation and a public contact database was developed to track telephone and email contacts and the resulting feedback.