Sinkholes are deep depressions that form due to the collapse of the roofs of underground caverns. They can pose a serious geologic hazard because they can damage overlying structures, drain surface water features, and allow direct infiltration of groundwater contamination.
Sinkholes occur in karst terrains as either cavities, voids, solution channels, or caverns in limestone. They can:
- develop suddenly, in stages, or over extended periods of time;
- be shallow or deep and small or large; and,
- often develop following excessive rainfall in a short period of time, following ground water pumping, or following new construction where water flow is diverted.
Formation of sinkholes is due to the dissolution of soluble carbonate in limestone bedrock accompanied by movement of water that dissolves carbonates in limestone thereby creating subsurface anomalies such as solution channels and voids. Exponent geologists and engineers utilize many tools to investigate sinkholes.
“Sinkholes” also occur due to underground erosion of soil into sewer pipelines, caving of underground mines, collapse of underground structures, hydrocompaction of collapsible soil upon wetting, and other mechanisms. Published geological data, past experience, as well as destructive and nondestructive in situ tests can be used to identify the existence of or potential for the formation of subsurface cavities and sinkholes.
Sinkhole hazard and occurrence is not easy to predict. However, assessment of subsurface conditions prior to development can mitigate the significant risks posed to projects by sinkhole formation. Site-specific evaluations of sinkhole conditions depend on an accurate characterization of buried bedrock topography, soil or sediment stratigraphy, soil mechanical properties, and subsurface drainage relationships.
Subsurface conditions can be investigated by use of geotechnical field testing methods such as: soil borings or cone penetration test (CPT) soundings, both inside and outside of the affected area; geophysical test methods such as ground penetrating radar (GPR), electrical resistivity, Multi-Electrode Electrical Resistivity (MER), or Multi-Channel Analysis of Surface Waves (MASW) around the affected area to assess lateral continuity of subsurface layers; and piezometers to monitor groundwater conditions. The assessment can be designed to produce a two- or three-dimensional “picture” of the subsurface. Key elements in an investigation depend on site conditions and clients’ needs, and may include:
- Sinkhole damage claims investigation
- Sinkhole(s) lineament determination
- Verification of sinkhole activity
- Emergency response and evaluation of imminent hazards to persons and structures
- Assessment of damage to structures and infrastructure
- Evaluation of the boundaries of an affected area
- Evaluation of the relative contributions of multiple factors
- Installation and monitoring of instrumentation for measuring ground movement and groundwater conditions
- Subsurface investigation through test pits, soil borings, CPT soundings, and interpretation of geophysical surveys (GPR, ER, MER, MASW, Seismic Reflection/Refraction)
- Geotechnical laboratory testing for measuring rock and soil engineering properties
- Inspection and pre-condition survey including floor elevation surveys
- Assessment of effectiveness of various grouting methods (Compaction, Chemical, and Pressure)
- Development of conceptual repair recommendations
- Third-party review of geologic and geotechnical investigations (“soil investigations”) as well as repair recommendations prepared by others.