Cavern Detection and Remediation
Caverns, i.e., cavities under the ground of various dimensions, pose a potential risk when planning infrastructure projects, construction activities or any other interventions. This is precisely why their timely detection and precise mapping is crucial for the safety and stability of future structures.
Data collection and initial analysis
The initial phase of cavern detection typically involves analyzing and reviewing available documentation, which includes:
- geological and speleological maps;
- historical terrain data (old mines, collapses and landslides);
- archival data of previous investigations (boreholes or soil tests); and
- local experiences and field observations (e.g., collaboration with speleologists or geotechnical engineers).
The goal is to identify potentially high-risk areas that require further investigation.
Geophysical investigations
If there is suspicion of cavern presence, a common next step is conducting geophysical investigation. These methods are beneficial because of their flexibility – their suitability for inaccessible or undeveloped terrain and their ability to cover large areas.
Among the most commonly used geophysical methods are:
- ground penetrating radar (GPR): uses electromagnetic waves to detect changes in soil and rock mass and is very good for shallow zones up to ~10 m;
- seismic methods (e.g. seismic refraction): used for deeper zones, they use waves that bounce or refract at the boundaries of materials of different densities; and
- electrical tomography (ERT): it measures the electrical resistance of soil and rocks – caverns often appear as very low-resistance zones (because of water or clay fill) or as very high-resistance zones (because of empty space).
These methods help define unstable zones and estimate the dimensions and extent of cavities – something often not achievable with exploratory drilling alone.
Exploratory drilling
Based on the data from geological and geophysical analyses, a drilling plan is created that defines the number, depth and locations of boreholes.
Drilling may be:
- rotary drilling with core sampling; or
- prospecting drilling with full-bit drilling (without core recovery).
In the case of prospecting drilling, it is essential to monitor the material being brought to the surface. For example, the presence of clay may indicate the presence of a clay-filled cavity. Other indicators include changes in drilling resistance, free-running of the drill, equipment collapse or sudden loss of drilling material – all of which may suggest a void.
In core drilling, the situation is somewhat simpler, as the cavity characteristics can be assessed by analyzing the extracted core.
Video logging
Cavern analysis through video logging involves the use of specialized cameras lowered into boreholes to visually inspect the condition of the rock mass. This method allows for a detailed view of geological structures, fractures and potential voids that are not always clearly detected by drilling or geophysics. The video logging is used to accurately locate and determine the dimensions of caverns, as well as assess their shape, contents and the presence of groundwater.
Remediation of caverns
Cavern remediation depends on their characteristics and engineering requirements. The most common methods include:
- cement grouting: filling the cavity with a liquid mixture that hardens and stabilizes the terrain;
- micropiles or anchors: for additional reinforcement of the overlying rock mass;
- installation of concrete plugs: for shallow cavities near the surface; and
- load redistribution: using structural solutions to avoid applying loads directly above the cavity.
In some cases, if the cavities are large and easily accessible, mechanical backfilling or even controlled collapse followed by refilling may be applied.
Read more: Video logging in boreholes
