Detecting Voids in a Karstic Environment

Aim

The project aimed to identify and map potential subsurface voids beneath the working floor of an active quarry. These voids posed a significant health and safety risk, with the potential to compromise both worker safety and quarry operations. Understanding their location and extent was essential to managing operational risks and planning safe extraction.

Approach

A grid of Electrical Resistivity Tomography (ERT) surveys was carried out across the quarry floor. ERT is well suited to void detection, as it highlights resistivity contrasts between intact rock, rubble zones, and voids filled with air, water, or mud.

Survey lines were positioned to cover all accessible parts of the quarry, allowing a pseudo-3D subsurface model to be built across the ~15,000 m² working platform. Targeted boreholes were drilled to validate the geophysical interpretation and refine the hazard model.

Results

The ERT profiles revealed strong resistivity contrasts:

• High resistivity (yellow): competent, intact rock.

• Low resistivity (blue): rubble zones at surface and deeper voids filled with clayey, saline mud.

Drilling confirmed over 80% of the geophysical anomalies were in fact zones of karstic voiding and channels. The strong link between the ERT results and borehole evidence provided high confidence in the mapped features.

A key finding was the presence of shallow karst features beneath the working floor. In one instance, pressure release during drilling caused the quarry floor to heave, further highlighting the hazard.

Outputs & Value

A subsurface hazard map was produced integrating the geophysical results with borehole data. This map clearly identified areas where karstic voids and channels were present, enabling quarry managers and mining engineers to:

• Safely restrict the use of heavy machinery in high-risk areas.

• Plan safe extraction sequences for the remaining rock resource.

• Proactively manage health and safety obligations.

The approach provided a cost-effective and site-wide understanding of subsurface conditions that would have been impractical to achieve with drilling alone.

Wider Relevance

Beyond quarry operations, this type of mapping is also highly valuable for subdivision development or infrastructure projects in karst terrain. Identifying voids and dissolution features early in the planning process can reduce geotechnical risk, improve design certainty, and avoid costly surprises during construction.