Main locations of subsidence (paragraph common to progressive and rapid subsidence processes)
Accordingly to the geologic conditions which advantage subsidence, it is mainly located in areas:
- Where natural cavities exist, i.e. in soluble bedrocks like limestone, chalk, gypsum, salt, etc. In karst areas the creation of tunnels and cavities can be very rapid when aggressive water (i.e. water with a high content of carbonic acid) dissolves soluble rocks like gypsum or salt, . In such locations the development of large cavities can occur in a few years, whereas dissolution is much slower in limestones or chalks where the development of cavities can take far more than a hundred years;
- Where anthropogenic cavities exist (subterranean quarries or mines), i.e. in the main coal, saline and iron basins (mining), and in many urban zones (quarries) (Link with study case Caen-Carries) because a lot of cities were built with stones or materials extracted in direct vicinity, and urbanization often forced the settlement into the zones of exploitation.
According to Embleton and Embleton (1997) to Maquaire (2005), for some countries in Western Europe the zones prone to subsidence processes are: In Luxembourg in the Walfendigen sector: Some collapses caused by natural dissolution and the mining of gypsum. In The Netherlands near Maastricht and St Pietersberg: Subsidence caused by coal mining from 1900 to the mid 1970s, and by the marl excavation since the seventeenth century. Furthermore, subsidence is still occurring in regions where oil and gas are extracted at or close to the coastline, in particularly near the Groningue basin. Further causes for subsidence in coastal areas of the Netherlands are the compaction of Holocene sediments and a decrease of the water table pressure in recent polders. These polders have been artificially filled with a one meter layer of sand to improve the state of the built-up land (Flageollet, 1988). In Germany in the region of the Hartz Mountains and along the fringes of other central German uplands (Mittelgebirge) in Hesse, Lower Saxony and Thuringia, and also at a few localities in the north German lowlands: Soluble formations in subterranean caves have collapsed (sulphate and chloritic rocks, to a lesser extent calcareous rocks) due to karst processes. Natural karst processes, which seem to have been more active in the early Tertiary and late Pleistocene, actually show only weak effects, but mining engineering, copper and salt mining, and water pumping have intensified and modified these natural processes, sometimes leading to local damage (Garleff et al., 1997). At Lüneburg (Lower Saxony), 169 buildings were demolished between 1949 and 1973 because of subsidence caused by salt mining and the karstification of gypsum (Flageollet, 1988).
In Belgium, there are mass movements and subsidence associated with karst processes in the limestone fringe along the north of the Ardennes, in the Condroz region and also near Doornik and the region of “Pays de Herve” (Heyse, 1997). Furthermore, collapses and subsidence due to marl excavation occur regularly since the seventeenth century in the Muizenberg, associated with the marl excavation in various part of Belgium (areas of Zichen-Zussen-Bolder, Riemst, Kanne and Hoegaarden). Coal mining causes subsidence and considerable damage to buildings, roads and infrastructure in the Campine region as well as in Wallonia, in the Borinage, and in the Liège Basin.
In France, subsidence caused by human activity has been observed in the coal basins, the Lorraine iron and salt mine, above underground quarries (marl, gypsum, chalk…) in the Paris region, the North Pas-de-Calais basin, the Val-de-Loire, the Bordelais and the Touraine, the city of Caen (link Study case) ; the Pays d’Auge and the plateaux in the Seine-Maritime and the Eure. The dissolution of karst also entails subsidence, in the Paris region in the gypsum, in the Orleans region in the chalk, in the Causses du Quercy, in Perigord, etc.
Bibliography:
Embleton, C., and Embleton C. (eds.) (1997), Geomorphological Hazards of Europe. Developments in Earth Surface Processes 5. Amsterdam : Elsevier, 524p.
Flageollet, J. C. (1988), Les mouvements de terrain et leur prévention, Paris : Masson, 224p.
Maquaire, O., (2005). Geomorphic hazards and natural risks, In: Koster, E., A. (ed.), The Physical Geography of Western Europe, Oxford Regional Environments, Oxford University Press, Chapter 18, 354-377.
Ministère de l’Ecologie et du Développement Durable, 2004. Dossier d’information sur le risque Mouvement de terrains, 20 p. (à télécharger sur site du MEDD).
Liens Internet :
http://fr.wikipedia.org/wiki/Subsidence
http://www.lorraine.drire.gouv.fr/mines/g_cadreDomaine.asp?droite=2_ApresMines.asp&bas=g_MinesNavig.asp?DEST=APMINES
http://www.cgm.org/themes/soussol/mines/
http://www.cavite.net
http://www.prim.net/professionnel/documentation/dossiers_info/nat/low/mouvtTerr.pdf
http://www.catp-asso.org/cavites37/pages/missions.htm
http://clamart.cyberkata.org/