heave

In mining and extraction, heave is the vertical or lateral displacement of a rock stratum, ore vein, or geological feature caused by tectonic stress, fault movement, or subsidence. When a miner encounters a heaved vein, the mineralized layer no longer lies where historical geology or earlier survey data indicated it should be. The vein has shifted its position in three-dimensional space, complicating extraction and forcing miners to relocate their workings.

Heave occurs through several mechanisms. Fault movement is the primary cause: where two blocks of rock have shifted relative to each other along a plane of weakness, any vein crossing that fault gets displaced. A vein that runs straight at one side of the fault may be offset by meters or tens of meters at the other side. Vertical heave (throw) is measured perpendicular to the fault plane; lateral heave (strike slip) occurs horizontally along the fault. In coal mining, heave also describes the upward buckling of the floor, where soft strata beneath the coal seam swell as overburden pressure is removed by extraction, pushing the floor upward by tens of centimeters or occasionally meters.

Detecting and managing heave

Miners detect heave through systematic drilling and sampling during development. If an expected ore body is absent from drill holes, or assays from successive boreholes show the vein at different depths, heave is suspected. Underground surveying then confirms the displacement. In coal mines, floor heave becomes evident as the mine face or roadway settles unevenly or begins to close; timbers crack, and roof bolts go slack. The displacement may be sudden and dramatic, or gradual over months.

Heave affects mine planning and safety. A mining operation must trace the heaved vein or coal seam through additional drilling to keep extraction economically viable. In confined seams with severe heave, the cost of following a displaced vein may exceed its value, forcing abandonment of that section. Floor heave in coal mines creates mechanical damage and can trigger roof falls by altering the load distribution on support systems. Ventilation ducts and conveyor lines must be realigned, and workers face increased hazard as the working environment becomes irregular and unstable.

The term heave is also used in civil and geotechnical engineering to describe ground movement beneath structures, but in mining vocabulary it refers specifically to the displacement of geological features that miners are attempting to extract. It is distinct from subsidence, which is the general settling of ground after extraction, and from creep, which is slow plastic deformation of soft rock under load.