The project, located on the I-95 / I-395 Express Lanes from Prince William to Fairfax County, consisted of three stretches of cantilever post and panel retaining walls along the wall alignment. In The RW-830 wall (Prince William County on I-95) is 240 foot long and ranged in height from 7 to 15 feet and had no soundwall on top. The RW 1540 section (Fairfax County on I-395) is 655 ft long and had retaining walls ranging from 5 to 22 feet high and sound walls from 8 to 18 feet tall. The RW-1546 section (Fairfax County on I-395), is 1186 feet long and consists of retaining walls ranging in height from 7 to 16 feet tall with sound walls from 10 -16 feet tall.
The major challenge for the project was achieving the required factor of safety for global stability. The walls were underlain by Potomac formation soils consisting of elastic silts and fat clays with very low friction angles. In addition the challenges of limiting top of wall deflections to maintain wall alignment for placement of the soundwall on top of varying height walls required the use of tiebacks placed in varying strata of sand and fat clays…
Soil conditions along the alignment of the wall typically consist of a maximum of 0 to 10 ft of fill soil, underlain by 14 to 22 ft of terrace deposits. The Terrace deposits were interbedded with fat clays which created challenges for tieback installation and these were underlain by Potomac clay formation soils.
For walls RW830 and RW 1546, cantilever walls using steel posts and concrete panels on 10-foot centers was sufficient to limit deflections and for RW1546 support soundwalls. Drilled shaft lengths of 20 to 28 feet provided the necessary global stability factor of safety for these walls. For wall RW1540, which ranged in height up 22 feet, tiebacks were required for walls above 15 feet and drilled shafts up to 40 feet deep were required to meet the global stability requirements. For walls RW1540 and 1546, 20 foot wide Soundwall panels were used.