EVALUATION OF POTENTIAL NEW ROCK FALLS AT NATURAL BRIDGE STATE PARK, VIRGINIA, USING GEOPHYSICS, UAS 3-D PHOTOGRAMMETRY, LIDAR, AND VIBRATION ANALYSIS

Natural Bridge, located in Rockbridge County, Virginia, is a limestone rock structure spanning the 90-foot wide Cedar Creek Gorge. U.S. Route 11 crosses Cedar Creek approximately 215 feet above the stream bed via this rock formation. It is the only known natural rock structure of this size to carry a major highway.

In October 1999, while still a privately owned natural attraction, a rock slab fell from the rock bridge killing a tourist. In September 2016, Natural Bridge became a state park, managed by the Virginia Department of Conservation and Recreation (VDCR) in affiliation with the National Park Service. The Virginia Department of Transportation (VDOT) retains responsibility for maintaining the roadway. In 2018, VDOT contracted with the Radford University Geohazards Research Center for the engineering geologic evaluation of the rock formation to facilitate decisions regarding safety of the traveling public and to aid in preserving the unique feature as a natural resource.

Three aspects of the natural rock bridge were investigated in detail: (1) internal geologic structure; (2) external morphology along with the resulting stability controlled by rock discontinuities; and, (3) the effects of vibrations from traffic and from loudspeakers used for evening music and lightshow entertainment, on potentially unstable areas. A variety of geophysical and remote sensing tools and techniques were employed. Included were electrical resistivity, seismic refraction, ground penetrating radar, LiDAR, 3-D photogrammetry from unmanned aerial systems (UAS), and vibration monitoring.

Geophysics reveals that the natural rock bridge contains internal solution cavities. External 3-D modeling indicates that the formation includes rock masses that pose rock fall hazards to visitors. Vibrations from both traffic and loudspeakers have been modeled demonstrating that those vibrations can accelerate strength reduction factors, thereby decreasing stability over time at sensitive locations. As a result, VDOT is planning to relocate Route 11 away from Natural Bridge as a means to help preserve it and to improve the aesthetics for visitors. VDCR is now exploring options for prioritizing hazardous locations and developing strategies for protecting visitors beneath the bridge.