The Job
Panel 19 of the National Mall in Washington, DC, is a high-traffic, 16,000-square-foot lawn area situated directly above a WMATA metro tunnel. For over a decade, this specific section of the Mall was plagued by chronic sinkhole formation, creating significant safety hazards and maintenance challenges. The instability eventually became severe enough that the National Park Service was forced to close the panel to the public in 2021. Working under general contractor G.M. Hill, CJGeo was brought in to provide a definitive geotechnical solution that would allow for the safe reconstruction and reopening of this iconic public space.
National Mall Sinkholes
The Challenge
The subsurface conditions at Panel 19 were exceptionally complex due to the presence of the metro infrastructure and various utility systems, including irrigation lines and trench drains. Previous repair attempts over the preceding decade had been strictly reactive, using polyurethane grouting to fill individual sinkholes as they appeared. This “spot-treatment” approach failed to address the systemic voiding and soil degradation occurring beneath the surface, allowing new sinkholes to continually emerge adjacent to old repairs. The initial plan for the 2026 reconstruction involved excavating six feet and again spot-treating only visible holes, an ineffective strategy for long-term remediation that would have left the panel vulnerable to future collapses.
The Solution
CJGeo engineered a comprehensive void-filling and compaction grouting solution using CJGrout 20SDB high-density structural polyurethane. Rather than chasing visible surface symptoms, CJGeo treated the entire 16,000-square-foot footprint as a single unit. Crews injected approximately 55,000 pounds of structural foam across a systematic grid to a depth of five feet. This approach allowed the expanding material to seek out and densify hidden micro-channels and voids before they could migrate to the surface. Over the course of 15 days, the crew installed roughly 30,000 cubic feet of expanded grout, thoroughly mitigating the risk of soil migration and providing a stable, uniform base for the subsequent reconstruction of the panel.
Speak With An Expert
Reactive repairs often fail to solve the underlying causes of sinkholes and soil instability. CJGeo’s systematic approach to polyurethane grouting provides long-term stabilization for complex infrastructure and high-profile public sites. If you are managing recurring settlement or voiding issues, contact us today to learn how our engineered grouting solutions can provide a permanent fix for your project.
