Deep Mixing: An Owner's Perspective

The proposed $2.4 billion replacement of the Woodrow Wilson Bridge, which carries Interstate I-95 across the Potomac River, will require significant realignment and widening of approach interchanges in Virginia and Maryland. Just west of the bridge on the Virginia side, the footprint of the I-95/Route 1 Interchange will be doubled. Maintenance of traffic flow and a looming completion deadline require rapid construction of new embankments over thick deposits of very soft ground. Deep soil mixing will be used to make soil-cement columns for support of the new highway embankments, to protect existing features, and to provide a staging area for subsequent interchange bridge construction. During the design phase, the project team considered a variety of solutions to address design and construction requirements. The design phase investigation included typical subsurface explorations, laboratory soil tests, bench-scale mixing tests, field test columns, and a test embankment built over an array of field test columns. The reliability of in situ ground improvement was a significant concern to the owner. Findings from the design phase investigations were used with simple reliability estimates to reduce uncertainties and provide the data needed to make practical design decisions. This paper describes design alternatives, the bench scale tests, the field trial columns, and the test embankment. The impact of these activities on design and construction of the approach embankments is also presented.