Case Studies

Sinkholes, areas of ground settlement caused by the removal of underlying soil, are common phenomena typically classified into two main categories: geologic sinkholes and utility failures. Both types result from the erosion of the soil substrate by water, leading to the formation of a void beneath the surface and subsequent ground collapse.

Geologic sinkholes form when limestone, also known as Karst—a rock formation made from fossilized shells and organic matter—dissolves in rainwater over time. This process creates caverns, voids, and sometimes underground streams and rivers. During heavy rain, the surface materials such as asphalt and soil are washed away, leaving a void underneath. Eventually, the surface material bridging this void collapses, resulting in a sinkhole. These geologic sinkholes vary in size from small, isolated depressions to large caverns capable of swallowing entire cars, buildings, and infrastructure.

Utility failures, the second primary source of sinkholes, occur when soil and stone surrounding a pipe are displaced into the pipe itself, creating a void. Over time, the surface materials collapse into this void, forming a sinkhole. Repairing utility-derived sinkholes is generally more straightforward, involving fixing or replacing the breached utility, backfilling the void, and restoring the area. At 800 Willowbrook, the sinkhole resulted from both utility failure and Karst formation, necessitating a combined approach for an effective repair.