South 277 th Street Grade Separation Project
Kent & Auburn, Washington
PanGEO, Inc. assisted SSL, LLC, a WSDOT pre-approved mechanically stabilized earth (MSE) wall supplier, with value-engineering services during design and construction of eight MSE walls over 30 feet in height on this freight mobility grade separation project. Project cost savings resulting from this value-engineering effort was estimated to be in excess of $1 million.

The contract design for all the MSE retained bridge approaches on this project was for two-stage construction to accommodate large estimated total and differential settlements of the alluvial soils underlying the alignment. Total settlements of up to 25 inches had been predicted for the wall and approach fills. While stone columns were installed beneath segments of the approach fills to mitigate potential liquefaction, the presence of these columns would tend to accentuate differential settlement between the improved and unimproved ground, resulting in more difficult conditions for the MSE wall construction. Faced with these challenges, the wall supplier, SSL, consulted with PanGEO regarding the potential of constructing conventional one-stage walls designed with the provision to tolerate the expected settlement.

PanGEO's engineers used actual project settlement data from embankment and MSE wall construction on similar ground conditions near the project site and around the Puget Sound Lowland to refine settlement estimates correlated to the existing subsurface data for the project site. The reduced settlement estimates, coupled with strategically located slip joint panels, allowed the cost-effective substitution of conventional single-stage, precast concrete panel faced MSE walls for the contract specified two-stage wall system. The proposal as devised by the SSL/PanGEO team was accepted by the project designer, contract administrator and the owners and is currently under construction .