GEOLOGICAL FACTORS EFFECTING SLOPE INSTABILITY IN THE VIRGINIA COASTAL PLAIN: OBSERVATIONS FROM THE 2018 NOTTOWAY LANE ROTATIONAL LANDSLIDE, KING WILLIAM COUNTY

In the early morning hours of June 3, 2018, after a period of heavy rain and thunderstorms (100 mm/24-hours), residents along a portion of Nottoway Lane in King William County, Virginia awoke to find a large collapse feature behind their houses. The King William County Building Department inspected the area on June 19, concluded that there was an actual and immediate danger to the residents, and condemned two houses. After repeated inspections indicated that the feature was continuing to propagate downslope, local emergency management requested technical assistance from the Virginia Department of Mines, Minerals, and Energy (DMME). On site, DMME geologists determined that the feature was a rotational debris slide-flow. From the crown to the toe, the entire slide body is approximately 60-meters long and up to 52-meters wide. An arcuate, 6-meter high, near-vertical scarp was identified within 3-4 meters of the two houses; additional scarping and tension cracks were found within centimeters of one residence. The landslide location was examined on 2011 LIDAR (1-meter resolution) and found to be within a larger, older landslide complex. Further examination of the LIDAR indicates that there are two additional landslides in the subdivision and multiple rotational landslides south of this location. These landslides appear to have preferentially failed either within the Miocene-aged Eastover Formation, a well-jointed, silty marine clay, or at the contact between the Eastover and the overlying sandy, Pliocene-aged Yorktown Formation. Eastover Formation sediments were observed at the base of the headscarp of the Nottoway Lane landslide. When exposed with a hand shovel, ground water flowed readily from joints in the clay. In 2004, a similar landslide occurred in the City of Richmond along Chimborazo Hill after heavy rains associated with Tropical Depression Gaston. This rotational debris slide also had a rupture surface within the Eastover Formation. Carter and Berquist (2005) found tension cracks and scarps that may have developed along preferentially-oriented fracture and joint sets. More research is needed to assess whether the Eastover Formation is particularly prone to producing slope movements in the Virginia Coastal Plain, especially during heavy rain events.