National Mall Sinkhole

For more than 200 years, the National Mall has served as America’s greatest civic outdoor space. The historic swath of green is bookended by the U.S. Capitol and the Lincoln Memorial and flanked by Smithsonian museums and other famous landmarks. Schnabel provided geophysical services to investigate the cause and extent of 11 sinkholes that developed beneath the Mall following a turf restoration project.

The sinkholes developed beneath the Mall over the Washington Metropolitan Area Transportation Authority subway tunnel after the turf project. Based on field observations of steel H-piles exposed in the sinkholes, the on-site contractor theorized that voids associated with the excavation support system used for the 1970s construction of the tunnel caused the cave-in.

For our geophysical exploration, we employed three noninvasive technologies: ground penetrating radar (GPR), magnetometer gradiometer (Mag), and multichannel analysis of shear waves (MASW). Mag results confirmed the presence of a steel structure along the perimeter of the original cut-and-cover tunnel. We used this information to guide a second phase of exploration using MASW and GPR. MASW, a seismic geophysical method, effectively located deep areas of soft material or voids, and GPR enabled our evaluation of the shallow subsurface for anomalies associated with existing voids and features that indicate imminent collapse.

The geophysical data we collected at Panel 19 confirmed the on-site contractor’s theory that the steel support of the excavation structure was contributing to settlement. GPR and MASW results identified several areas at risk for future subsidence and provided insight into possible causes. Based on study results, we provided geotechnical recommendations for conceptual remediation options.