Surface subsidence will gradually form when coal is extracted over a wide area from a moderate to great depth of cover. The factors which influence subsidence include the height if the caved area over the extraction and the degree of bed separation within the overlying strata. The amount of subsidence is also affected by the size and design of of coal pillars that are left to provide support.

The amount of ground subsidence that occurs is dependant upon the width of the underground extraction. The degree of subsidence that occurs has been classified into three categories, these are :

• Subcritical Width
• Critical Width
• Supercritical Width

CRITICAL WIDTH

The diagrams depicted show the surface and strata affects for a critical width of extraction.

SUBCRITICAL WIDTH

The diagrams depicted show the surface and strata affects for a subcritical width of extraction.

Supercritical Width

The diagrams depicted show the surface and strata affects for a super critical width of extraction.