U.S. 101/Ralston Avenue Interchange, San Mateo County
The Project
MTCo completed the Project Study Report, Project Report, and Final Design PS&E under Caltrans Encroachment Permit for reconstruction of the U.S. 101/Ralston Avenue/Marine Parkway Interchange to accommodate increased traffic from the growth of commercial business on the east side of U.S. 101 in the Cities of Belmont and Redwood City. The most prominent of these businesses is the corporate headquarters of Oracle Corporation. The interchange was converted from a traditional, full “cloverleaf” design to a more efficient partial cloverleaf.
A major and unique design element of the project included rerouting and grade-separating a local street over a freeway on-ramp and connecting it directly to Ralston Avenue at a new signalized intersection within the interchange. The local road was elevated via a 360-foot long concrete bridge structure and two mechanically stabilized earth (MSE) retaining walls.
Additional design elements included widening the existing on-ramps and off-ramps, adding ramp metering equipment, elevating roadways using MSE and standard retaining walls, widening and improving local streets and sidewalks, installing new storm drain facilities, relocating a 12-inch water main and 8-inch sanitary sewer force main, identifying and relocating high voltage electric, telephone, and fiber optic facilities, staging the improvements to accommodate full traffic use at all times during construction, and use of wick drains to expedite consolidation of Bay mud below major embankments.
The Challenges
In the preliminary design phase the biggest challenge was figuring out how to route traffic from the Island Park area of Belmont to the interchange for efficient access to the freeway since it was cut off from the interchange by Belmont Slough, a protected water of the San Francisco Bay. During final design one of the challenges was how to construct large embankments on Bay mud such that the ultimate settlement of the mud would not cause damage to the roadway.
The Results
The traffic routing challenge solution turned out to be a direct connection into the center of the interchange in lieu of crossing over the slough and routing traffic through other local streets. The solution for construction on Bay mud included installation of wick drains to expedite compression and settlement of the Bay mud and phased construction of the embankments with required, minimum settlement periods (generally 6 to 9 months) to allow the settlement to happen prior to construction of the final roadbed.