Geotechnical Mapping for a Bypass SegmentThe Oregon Department of Transportation (ODOT) is planning a bypass route for Highway 99W and OR-18 to improve traffic near Newberg and Dundee and to integrate it with an existing bypass around McMinnville. Cornforth Consultants (CCI), as a member of the Parametrix team, was retained to assist ODOT with the planning process and environmental impact statements.
Geologic reconnaissance and preliminary subsurface explorations were performed. The new highway route crosses low-lying areas underlain by silt and sand soils. The groundwater is relatively shallow with seasonal highs near the ground surface. CCI evaluated the relative risks and consequences of geologic conditions along the proposed route. Challenges for construction and operation of the highway include:
- Saturated conditions can liquefy during seismic events, which could require ground improvement mitigation where the soils are relatively loose at bridge crossing locations.
- Excavations for the roadway would be impacted by high groundwater and seepage, and unusual drainage systems would be required for depressed roadway sections.
- The reuse of excavated wet soils would require extraordinary treatment to be placed in roadway embankments. Imported fill could be necessary and excess wet soil quantities would need disposal sites.
- Existing landslides and old landfill and sawdust fill areas would need to be mitigated or avoided.
- Slope stability near stream crossings is marginal and therefore earthwork setbacks and ground improvement methods are likely to avoid causing slope failures.
- Construction of tall embankments could cause unacceptable settlements and marginal slope stability, and therefore mitigation could be necessary if heights exceed 20 feet.