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A typical strategy would be to first investigate the ground conditions to establish the depth range and plan extent of the mined ground. Given that mine tunnels can be narrow in places (<2m) it is important that close centred grids of exploratory holes are located appropriately for this purpose. Subject to the ground conditions present, dynamic probing or rotary probe drilling offer suitable investigation approaches to defining the mine workings.

Depending upon the geology sequence, the depth and condition of the mine workings (open voids or collapsed, backfilled tunnels) geophysical surveying can also provide useful methods for detecting the presence of mine workings. Other methods of inspecting the condition and extent of mining, present as open voids, can include borehole cctv surveys and downhole laser surveys.

Most mine workings reveal their presence after the entry shaft or tunnel roof collapses. On occasions it may be possible to enter the mine and survey its condition, but frequently the health & safety considerations and difficulty of access rule this out.

With time there is a tendency for old chalk mine workings to become increasingly unstable as the ground around and above the mine “relaxes” and loses strength. This situation can be exacerbated by changes to ground loading and drainage, leaking water-carrying services and long term downward percolation of water from rainfall. In most situations the only feasible way to stabilise the mine workings is by grouting to infill open voids and densify the overlying relaxed ground. Sometimes, where access allows, supporting structures can be built within a mine tunnel to improve the mine stability.

Subsidence Index | Chalk Mining Index | Diagnostic Characteristics | Geographic Occurrence | Investigation & Mitigation | Key Contacts & Expert Advice | Photo Gallery | Essential References & Further Reading | Definitions & Glossary |