800 Willowbrook
Unearthing the Hidden Hazard:
Sinkholes, or areas of settlement, are a fairly common occurrence and typically can be grouped into two categories: 1) naturally occurring or geologic sinkholes and 2) utility failures. In both instances, the soil substrate is carried away by water leaving a void and an area of settlement.
In the case of geologic sinkholes, limestone also known as Karst, a rock formation made from fossilized shells and organic matter, slowly dissolves in rainwater creating caverns, voids, and in some cases underground streams and rivers. During rain events, asphalt, soil, or other material near the surface is washed away causing a void under the surface. Over time, the surface material bridging the void collapses causing a sinkhole. These sinkholes can range in size and depth from very small isolated contained areas of settlement to very large, interconnected caverns that can swallow cars, buildings, and infrastructure in their entirety.
The second primary source of sinkholes is utility failures. This kind of subsidence occurs due to utility failures, wherein soil and stone surrounding an existing pipe are displaced into the pipe itself, creating a void. Over time, the asphalt, soil, or stone bridging the void collapses, and a “sinkhole” is formed.
Typically a utility-derived sinkhole is an easier and more straightforward repair than a Karst sinkhole, with the repair consisting of repairing or replacing the breached utility, backfilling, and restoring the area. Karst-derived sinkhole repairs are often more complicated, including working at the direction of an engineer and using specialized construction techniques including inverted rock filters, flowable fill, and compaction grouting.
In the case of 800 Willowbrook, the cause and the approach to the repair of the sinkhole fell into both categories: utility failure and karst.
Beneath the Surface:
In response to an initial report of a sinkhole, our team swiftly mobilized to the site, arriving the same day to assess the situation and provide temporary safety measures. After our initial assessment, Caliber removed the asphalt and concrete around the depressed area to access the disturbed soils beneath. Shortly after excavation had begun we discovered an underground retention basin. After consulting with experts in the field, including the original basin installer, manufacturer, and our engineering partner, a determination was made to remove a section of the underground retention basin. After removal of the basin section was completed Caliber and their engineering partners conducted an examination of the soil beneath the basin, in the general location of the area of subsidence. While we encountered soft soils there was no clear evidence of limestone within the footprint of the excavation. Our team proceeded with the excavation at the direction of the onsite geotechnical engineer.
Filling the Void:
After finding no evidence of competent rock or clearly defined sinkhole throat, it was determined that the origin of the sinkhole was most likely beneath the limit of a reasonable excavation. To minimize the need for further disruption, the suggestion was made to utilize the compaction grouting method to stabilize the subsurface soils.
Compaction grouting is a minimally invasive repair technique that utilizes the injection of cementitious material under pressure to densify the underlying soils stabilizing the limestone-susceptible areas. Compaction grouting repair methods are designed and implemented in close conjunction with the onsite geotechnical engineer.
Featured Team Member
Michael Grizzle
Operations & Strategic Project Manager
