SR 520/I-5 Express Lanes Connection Project

SR 520/I-5 Express Lanes Connection Project, a remarkable endeavor in the ongoing SR 520 Bridge Replacement and HOV Program. Spearheaded by the Walsh Group, this project aims to establish a dedicated reversible transit and carpool connection between SR 520 and the I-5 Express Lanes.

The scope of work encompasses the construction of 23 retaining walls, replacement of five ramps along with a complete overhaul of the roadway cross section, utilities, and walls. Moreover, a new three-span steel girder bridge will be erected, and an existing southbound off-ramp will be reconfigured into a reversible structure. This transformation involves a comprehensive replacement, alignment adjustment, installation of concrete barriers, and soldier pile walls. Notably, this onramp intersects with an existing tunnel that connects I-5 and SR520.

Engineers faced a crucial challenge during the planning phase of this project. They were concerned that the additional fill load over the tunnel could potentially lead to overburden stress, jeopardizing the tunnel’s integrity. To tackle this issue, the ingenious solution of employing a construction technique called net zero loading was implemented. This technique involved replacing sub excavated near-surface soil with LCC backfill and fill, ensuring that no net increase in effective vertical stress occurred in the critical sublayer. Consequently, the consolidation settlement/load became negligible, resulting in substantial cost savings by obviating the need for extensive improvements to support the new structure’s added load on the existing tunnel.