Analyzing Surcharge Needs to Reduce Secondary Compression at Embankment Interfaces

In doubling the width of I-95 as part of the Woodrow Wilson Bridge replacement project, 24 to 36.5 m wide embankment extensions that are 3 to 7.6 m in height are being constructed over very soft alluvial organic clay that is 4.5 to 9.7 m thick. There will be considerable (0.6 to 1.5 m) primary consolidation and secondary compression in the soft organic clays. To hasten the time to the end of primary consolidation, Pre-Fabricated Vertical Drains were installed throughout the mile long project area. However, project criteria limits on long-term settlement which would occur due to secondary compression required extensive design analysis to assess surcharge thickness needed. In 20 years, not more than 5 cm differential settlement at bridge structures, and between the existing roadway and pavement on new embankment is permitted. This paper discusses the methods used to predict secondary compression settlement in the soft organic alluvial clay due to the embankment loads within the compressed construction schedule, and determine requirements for surcharge thickness to meet project differential settlement limit criteria. The paper also demonstrates the use of AAOS in planning transportation construction over soft clay.