Sinkhole repair begins with identifying the cause of the sinkhole. Once the cause is identified, determining the best repair method becomes straightforward.
Different Types of Sinkholes
Sinkholes generally fall into two categories: geologic and human activity-related. Geologic sinkholes arise from underground conditions, such as karst landscapes. If you live in an area with numerous caves, the likelihood of encountering a geologic sinkhole increases. In other areas, most sinkholes result from human activity. This activity can range from mining to installing underground utilities. For a sinkhole to form, the soil that “disappears” has to go somewhere. In geologic sinkholes, it erodes or washes away into cavities created by dissolved or eroded underlying rock. In human activity-related sinkholes, soil erodes or washes into or away through leaking pipes or other underground structures.
Examples of human activity-related sinkholes 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 and leaks
Sinkhole Repair Methods
Repairing sinkholes typically involves two methods: excavation or grouting. The choice of method usually depends on the cause and depth of the sinkhole.
Excavation
Excavation suits sinkholes that start close to the surface and when there are no valuable structures above that would need demolition. For geologic sinkholes, excavation usually involves digging down to the sinkhole’s “throat” or narrow part at the bottom and filling it with erosion-resistant material. Then, backfill the excavation. Depending on anticipated water movement, choose backfill material to either allow water passage or prevent it.
For sinkholes caused by human activity, excavation often involves exposing deteriorating underground structures to repair or replace them. For instance, old cisterns and tanks are typically shallow, making it straightforward to dig them up and fill them with soil or stone. For utility-related sinkholes, excavation is common for pressurized pipes or small-diameter sewers. For gravity structures like stormwater pipes or large-diameter sewers, trenchless repair methods are usually preferred, but excavation and replacement may be necessary for shallow or completely failed pipes.
Grouting
Grouting is a common method for sinkhole repair, especially when the cause is deep, the area is large with no distinct throat, or the sinkhole is near sensitive structures or environments that excavation could disrupt. This method often applies to sinkholes in dams, under or around roads or buildings, or in streams.
To grout a sinkhole, technicians use grout injection tubing, which is driven to specific depths according to the investigation phase. Then, they pump grout through the tubing to fill voids and seal leaks, whether they are geologic or from underground structures. Sometimes, cement-based grout is appropriate, but CJGeo primarily uses geotechnical polyurethane grouts, such as CJGrout 35NHV61. This grout is notably stronger than most soils and certified for use around potable water.
Technicians also perform grouting from inside leaking structures, such as a 14′ diameter stormwater pipe in Asheville, North Carolina. Often, they grout from both inside the leaking structure and from the surface. Grouting from the surface is usually more efficient for high-volume applications, so technicians often start by grouting the leak from inside and then continue with surface grouting.
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