Sinkhole repair starts with identifying the cause of the sinkhole. After identifying the cause, determining the best type of repair is pretty straightforward.
Different Types of Sinkholes
There are generally two causes of sinkholes–geologic and human activity. Geologic sinkholes result from conditions underground, such as Karst. If you live in an area with lots of caves, there’s a higher likelihood of sinkholes being geologic in nature. Everywhere else, the vast majority of sinkholes are a result of human activity. This human activity can be everything from mining to installation of underground utilities. Generally, for a sinkhole to form, the soil that “disappears” has to go somewhere. In geologic sinkholes, it erodes or washes away into cavities of dissolved or eroded underlying rock. In the case of human activity related sinkholes, soil erodes or washes into or away through leaking pipes or other underground structures.
Some examples of sinkholes caused by human activities include:
- abandoned cisterns and septic tanks that develop leaks
- misaligned joints in large diameter culverts
- leaks in manholes and drainage structures
- corrosion of stormwater pipes leading to piping & leaks
Sinkhole Repair Methods
Sinkholes are generally repaired in two different ways: excavation, or grouting. The preferred methodology is generally driven by the cause, and the depth.
Excavation is most common for sinkholes that originate close to the surface, and when there isn’t anything expensive up top that would need to be demolished to torn up as part of the excavation. For geologic sinkholes, excavation typically involves digging down to the “throat” or narrow part of the sinkhole at the bottom, and “choking” it with material that isn’t susceptible to erosion. The excavation is then backfilled. Depending on if there’s future anticipated water movement, the backfill material may be designed to either let water pass through it easily, or to prevent water from passing at all.
Excavation for fixing sinkholes that result from human activity generally involves exposing deteriorating underground structures in order to fill them, or repairing/replacing failed utilities. For example, old cisterns and tanks are usually pretty shallow. Digging one up to fill it with soil or stone is usually pretty straightforward. For utility-related sinkholes, if it’s a pressurized pipe or small diameter sewer, excavation is most common. For gravity structures, such as stormwater pipes or large diameter sewers, trenchless repair methods are most common, but if the pipe is relatively shallow, or completely failed excavation and replacement may be the only option.
Grouting is a very common sinkhole repair method, particularly when the cause of the sinkhole is relatively deep, if it affects a large area with no single distinct throat, or if there’s a sensitive structure or environment around the sinkhole that can’t be disturbed by excavation. This often includes sinkholes in dams, under or around roads or buildings, or in streams.
Grouted sinkhole repair most commonly uses grout injection tubing driven to specific depths on a grid pattern, determined during the original investigation phase. Grout is then pumped through the tubing into the area to fill voids and seal up leaks, whether they be geologic in nature, or in an underground structure. Sometimes, cement-based grout is best. CJGeo mostly uses geotechnical polyurethane grouts to grout sinkholes. CJGrout 35NHV61 is probably most common–it’s significantly stronger than most soils, and is certified for use around potable water.
Grouting is also done from inside leaking structures, like on this 14′ diameter stormwater pipe, in Asheville, North Carolina. Sometimes, grouting is done from both inside the leaking structure, and then also down from the surface. This is particularly helpful if the structure is relatively deep. It’s usually easier to do high volume grouting from the surface instead of from inside a pipe, so it’s pretty common to grout the leak from inside first, then switch to grouting the soil from the surface.
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