Alexandria, VA

Contractor - John Driggs Co.
Geotechnical Engineer - Engineering Consulting Services, Ltd.

Two hundred 3-story townhouses were planned for a acre site in Alexandria, Virginia. During construction of I-495 in the early 1960’s, the site had been filled with up to about 20 feet of poorly-compacted miscellaneous fill soils, primarily silty and clayey sand, occasionally mixed with inorganic construction debris. Groundwater levels were typically between 8 and 10 feet below the existing ground surface. Due to the risk of excessive total and differential settlement within the uncontrolled fill soils, shallow foundations bearing on the existing fill soils were not considered appropriate. Total excavation and replacement of the fill soils was considered, but rejected due to the groundwater level. Pile foundations were deemed too costly. Dynamic compaction was chosen to improve the fill soils in place, allowing the use of shallow foundations.

The building footprints were compacted using a 15 ton weight falling 50 feet. Two phases of compaction were required, plus a final ironing pass. Compaction was performed within 50 feet of an existing office building, and within 20 feet of existing roadways and underground utilities, with no damage reported.