Here at Howell, we’re always looking for ways to remove any consequences for construction- and learning from relevant cases. We are especially attentive for areas with Karst topography- that’s a term used to describe a landscape with hollowed characteristics, like sinkholes, caves or springs, created from the erosion of the underlying soluble bedrock. This geological condition is scattered throughout the world and makes up roughly 20% of the land in the US.
Where in PA?
Central and southern portions of Pennsylvania, known for limestone bedrock, are susceptible to these features, which impose structural and environmental risks on developers and the public health downstream of improvements.
Howell Engineering actively works to prevent sinkholes from forming from land disturbance:
- Providing positive stormwater drainage away from structures
- Minimizing depressions where ponding may occur
- Utilizing impermeable liners for stormwater BMPs
- Avoiding deep excavations
- Stabilizing and closing excavations as soon as possible
- Ensuring underground pipes are leak-proof
For us, knowing where karst topography is present, understanding the consequences of construction, and the effort it takes to mitigate risks, we address it at the preliminary stages of land development.
Here’s a report of a major incident that proves karst topography is a critical land development concern. Interesting to us for what we can learn, and to see how the Corvette Museum honors great feats of engineering…
Kentucky Case Study
On February 12th, 2014, early in the morning before normal operations began, an unexpected event occurred at the National Corvette Museum in Bowling Green, Kentucky. The aftermath of the sinkhole collapse can be seen in the photo above.
The 69,000-square-foot building was constructed in 1994 and is located in the Pennyroyal sinkhole plain consisting of soluble limestone bedrock. The region in Kentucky is known for having sinkholes from corrosion, which are documented in the Kentucky Geological Survey, as well as unrecorded small-scale sinkhole formations. For greater regional context, this area is home to hundreds of cave systems and Mammoth Cave, the largest known cave in the world.
Karst Regions of Kentucky, USA
Various types of sinkholes can form, but the more uncommon cave roof collapse was the cause in this case. As the limestone corrodes, a cantilevered roof forms like a cohesive soil arch, then suddenly collapses under stress, making it hard to predict failure. The main attraction and location of the failure was the Skydome showroom, which was 65 feet in diameter and reached 130 feet tall. In the center of the Skydome, the sinkhole formed, measuring 36 feet in diameter, when the cave roof collapsed, and the floor structure failed to hold the weight of the building without proper support. Investigations were conducted using a drone to fly into the opening and evaluate the remaining area for voids. Downhole cameras were also used during reconstruction and after water jet drilling to determine the integrity of the ground beneath micropiles.
A team from Western Kentucky University formulated a plan to recover the corvettes from the collapse and stabilize the hole to allow for further investigation. Additional cave tunnels were discovered and mapped during a cave survey before reparations began. The cave underneath the National Corvette Museum measures 220 feet in length and 40 feet in width, with the deepest location roughly 80 feet.
Cross Section of Sinkhole
After choosing a path forward, the reparation plan intended to use micropiles down to stable bedrock to support a new concrete slab floor. The sinkhole walls were reinforced with shotcrete, and a one-foot-thick concrete slab was poured into metal sheet piles in the shape of the hole, acting as a mold to avoid spillage into the other cave passages. It was then filled with 4000 tons of sand. The 69 micropiles used ranged from 130 to 200 feet long to reach suitable bedrock within a grid below the Skydome. A manhole was installed to allow access into the southern cave passage for testing purposes. Now, the Skydome is self-supported by the micropiles and safe from future foreseeable ground movements.
A Micropile Supporting the Skydome Structure
With restoration complete, the National Corvette Museum created an exhibit to showcase the feat of engineering used in the design of the new Skydome floor and educate visitors about karst landscapes. After intensive sinkhole research was done after the failure, it was determined that only the Skydome was most susceptible to this issue. As with some of our sensitive features in Pennsylvania, it is imperative to design and build with safety at the forefront, especially when working in unique regions such as the Bluegrass Region of Bowling Green.
Click here to see the active sinkhole, it's cool.
Sinkhole Security Camera Footage
