Sinkholes are naturally occurring geologic hazards that can cause voids/cavities to develop in the subsurface in regions underlain by Limestone/Dolomite geology. Once sinkholes develop, they can cause significant damage to property, pose safety threats, and are typically costly to repair. The purpose of this article is to help individuals identify potential warning signs of when a sinkhole is about to form, methods to reduce the risk of development, and typical measures to repair sinkholes. Read on to learn how to identify the telltale signs of an impending sinkhole and areas that are susceptible.
What Is a Sinkhole?
A sinkhole is an open void that forms on the ground surface in areas underlain by Limestone/Dolomite bedrock. These sinkholes form from soil loss/erosion through open cavities and voids into the underlying bedrock. The underlying Limestone or Dolomite is composed of carbonate rock. Having a calcareous structure, carbonate rock is subject to openings and the solution channels. Carbonate rock is derived from shell fragments and other hard parts of marine organisms that were deposited during the Cambrian period. Carbonate minerals are dissolved in groundwater. When the water is slightly acidic, it allows for the dissolution of the parent bedrock leaving voids in the underlying carbonate rock. The past dissolutions, not a future dissolution of carbonate rock, are the greater hazard. In most cases, the previous dissolution of carbonate bedrock has allowed the development of cracks and caves within the bedrock: similar to a network of plumbing pipes. If water is allowed to flow into these pipes, it will take some of the soil overlying the bedrock. As more and more soil is washed into the voids within the bedrock, the ground surface may become depressed, or collapse altogether, forming a sinkhole.
Types of Sinkholes
This is the type of sinkhole that occurs where limestone is present at the land surface or covered by a thin layer of previous soil. Rainwater runoff causes the surface to subsurface erosion of the soils into the parent bedrock. Solution sinkholes are typically smaller in overall size compared to cover collapse sinkholes. An open “throat” is typically visible in the solution sinkhole.
Cover collapse sinkholes result when the bedrock is covered by a thicker section of cohesive soils that erode from below through underground fractures and caverns. Gradually the subsurface soils erode causing an arching effect. The arching effect will also typically create a surficial depression to form on the ground surface above the eroding soils. Eventually, the arch collapses causing a sinkhole to develop suddenly at the surface. This type of sinkhole may occur overnight, which can result in catastrophic property damage. The following diagram obtained from the Kentucky Geological Survey depicts the typical stages of sinkhole development/collapse. Cover collapse sinkholes are also oftentimes more expensive to stabilize compared to the solution sinkhole.
Areas Where Sinkholes Are Most Likely To Appear
In the United States, there are four main regions where sinkholes are considered a high risk occurrence: Central Florida, southwest Missouri, central Kentucky and Tennessee through the Appalachian Mountains into Pennsylvania.
Regardless of the locations referenced above, there is and always will be a natural risk for sinkhole activity within areas underlain by Limestone and Dolomite geology. The concentration of stormwater runoff is one of the main contributing causes of the sinkholes. Land features such as swales, drainage ditches, and/or basins are particularly vulnerable to sinkhole development during periods of heavy rainfall. Leaking joints or damaged stormwater pipes are also vulnerable areas for sinkhole development.
Some Methods to Reduce the Risk of Sinkhole Development During Construction
- Excavation should be kept to a practical minimum in areas of known or suspected sinkhole or solution activity. In general, the closer excavations get to the rock surface, the greater the potential is for sinkhole development.
- Watertight seals should be provided at all water-bearing utility line connections. All roof drains should be watertight and should connect to the on-site stormwater management systems.
- Site grades should provide positive drainage away from building areas.
- Joints between asphalt paving and concrete curbing should be sealed to reduce water infiltration in these areas.
- The base of all excavations in carbonate areas should be inspected for soft or unusually moist conditions. A visual inspection of the excavated surface, as well as probes of the soil at regular intervals, is recommended. Any soft or unusually moist soil should be further evaluated and a determination of the extent of the problem be made. Remedial measures should then be undertaken as necessary.
- The lining of swales, drainage ditches, and basins with an impervious liner.
Methods to Stabilize Sinkholes
There are three typical methods used to stabilize sinkholes. The three methods are as follows:
- filling in the sinkhole with soil or concrete,
- an excavation and replacement operation and
- a subsurface grouting operation
In general, simply filling in the sinkhole with soil or low strength concrete is typically considered the lowest cost option for repair. However, this approach also generally carries the highest risk for sinkhole recurrence. The excavation and replacement operation can be an effective method to stabilize open sinkholes at your property. In this approach, experienced personnel directs the excavation of the soft/loose soil in the sinkhole to either the rock surface or a depth where suitably dense material is encountered. Upon achieving a dense base, the excavation is typically backfilled with a combination of quarried aggregates and fine grained soils. The third option that is typically the most expensive, but carries the least risk for recurrence is to perform a subsurface grouting operation. In this method, a hollow steel casing is installed to the top or into the rock and a variable mobility grout is injected under pressure to fill open voids/ cavities at the soil/bedrock interface. The grout is injected in ascending stages to a specified depth. The grouting is typically performed in a grid pattern around the sinkhole to reduce the risk for future occurrence.
Protect Your Property
Upon discovery, property owners should contact a qualified geotechnical engineer that is familiar with sinkhole activity to assess the conditions and provide recommendations for repair. Furthermore, if you plan to build your property and make a significant investment in your area, then you may want to hire geotechnical engineering companies in PA to determine if sinkhole activity is a natural risk to development at the site.
Earth Engineering is a highly qualified geotechnical engineering firm that has vast experience as the best sinkhole inspection in PA, with its cost-effective recommendations for sinkhole repair and preventative measures. Contact us today for a consultation.