On September 11, 2001, 33 passengers and seven crew members fought back against four hijackers who were intent on using United Airlines Flight 93 as a weapon in a terrorist attack on Washington, DC. These efforts caused the aircraft to crash. On the site of this crash, the National Park Service constructed a memorial to those patriots who saved hundreds, if not thousands, of lives. The memorial encompasses approximately 2,262 acres of land. Much of this property rests upon reclaimed bituminous coal strip mines which, following the completion of mining operations, were backfilled primarily using mine spoils that generally consisted of five to eight ton boulders the size of small automobiles. Because of the subsurface conditions, foundation design for the memorial was going to require highly detailed and accurate information.
Froehling & Robertson, Inc. was hired with the responsibility to conduct the necessary field exploration geotechnical engineering activities. Drilling through the existing mine spoils proved difficult due to the irregularity of the fill and the oversized materials. However, F&R was able to verify the depths and quality of the mine spoils within the project area and develop ground improvement recommendations that allowed for the construction of the memorial on shallow foundations – an approach that was significantly cost effective rather than the alternative deep foundation system that was considered earlier.
During the final phase of construction, F&R was also retained to provide extensive materials testing related to the site’s earthwork. F&R performed exploratory soil borings to verify the deep dynamic compaction ground improvements related to deep dynamic ground improvement, utility construction, site lighting, landscaping, parking, and paving.