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 :