It is debatable in many cases, however, whether loss of agricultural land in such conditions would of necessity be permanent. Many open east works have been successfully restored to original ground level after use, and with the replacement of top-soil, re-seeding and drainage agricultural land of similar if not better quality than the original can be expected (se Section 6.5). In yet other cases there is little realistic prospect of reclamation for agricultural use. Deep clay pits often become waterlogged after extraction has ceased, and in the absence of large amounts of nat ural soil for backfill such pits often have to be backfilled with sanitary landfill (such as domestic waste, building rubble etc.) which can pollute undorlying groundwater reservoirs (see Chapter 3.3) through infiltration and leaching from the fill materiall5 . Land Subsidence A marked impact of underground extraction is land subsidence in the working areas which is common especially where mining proceeds along several horizontal seams simultaneously. Subsidence can damage buildings, roads and engineering structures (such as bridges and walls), and it can also produce land drainage problems and safety hazards. Coleman notes that slow surface subsidence occurred widely under the concealed Kent Coalfield early in the present century, but that it ceased within about 10 years after underground working stopped16. No structural damage to buildings in the area was noted but there was a significant loss of grazing land occasioned through flooding of the low-lying subsided land. Wallwork has documented similar but more pronounced subsidence in Cheshire triggered off by large scale brine pumping since the last centuryl7 an(j damage has included building subsidence, disruption of ser vices and Communications (such as piped water supply, main drainage, and ruptured road' surfaces), and deterioration of farmland in general and pasture in particular through impairment of natural drainage and subsequent land flooding.