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The filling of the underground vacuums left by the mining leads to two kinds of surface disorders:

the depressions , without opening of cavity on the surface,
the collapses in the contrary case.

The phenomenon of depression is relatively slow (it can last of the years) and progressive. It occurs when the grounds are rather plastic and that the depth of exploitation is important compared to the thickness of the size. It generally results from a deliberate choice of exploitation per caving (total exploitation of the Minerai using a going formwork). This phenomenon is observed for example in the Field of the Nord-Pas-de-Calais.

Collapse, on the contrary, is a brutal rupture of the roof of the exploitation leading to an instantaneous variation of the local Topographie (subsidence trough) even, when the depth of the excavation is low relative with its thickness, to an open hole on the surface described as “subsidence” (not to be confused with the Doline S due to the natural phenomenon of dissolution Karstique). Collapse can be deliberated but it is generally the accidental consequence of the evolution of the rooms with pillars after the abandonment of the exploitation (case of certain Lorraine mines or the underground careers of Craie in Normandy for example). The capacity of resistance of the pillars is degraded indeed appreciably in the long run: their surface of supporting can decrease by peeling and their mechanical properties can change in the presence of water.

Influence water on the mechanical stability of the mining sites to the abandonment

The exploitation of a mine very often requires to have recourse to a pumping of draining (pumping out) in order to fold back the Ground water and to avoid the flood of the galleries and other excavations. At the end of the exploitation, if the pumping out is not continued, the mine is ennoyée: the level of the tablecloth goes up and a new state of balance hydrogeologic is established slowly (the time necessary with the rebalancing of the phreatic level will be able to vary between a few months and a few years).

The effect of water on the mechanical stability of abandoned mining is a complex phenomenon, which can be translated as well positively as negatively. It is established that a rock saturated with water loses its resistance, in proportions which differ largely from one rock to another (in a general way, the materials Siliceux are more sensitive to water than the materials Calcaire S). On the other hand, the hydrostatic pressure can play a part of support of the roof (lift-off) which relieves the pillars, provided that this effect is not counterbalanced by the loss of horizontal containment inside the pillars. The principal risks meet however during the transitional stage of submergence, during which strong water pressures can increase the fracturing of the medium appreciably.

Coal basin of the Nord-Pas-de-Calais

The carboniferous exploitation related to veins low thickness (a few meters to the maximum) and great depth (several hundred meters). The majority of the mining companies worked according to the technique of caving, for reasons of economy, the costing embankment too expensive. The result was the creation of vast basins reaching up to 17 meters of depth (in general, it is estimated that total depression reached 80% cumulated thicknesses of the various exploited veins).

These deformations of ground did not have notable consequences on the constructions (dwellings, works of art, etc) located in the middle of the zones depressions. On the other hand, those which were in periphery, at the same time movements of rocker and requests of traction underwent (due to the stretching of the grounds) involving serious damage and often their ruin.

The same mechanism also disturbed the operation of the networks of Assainissement going until the inversion of their direction of flow and often causing their rupture, reinforcing the Pollution basement and tablecloths. The bad persistent quality of the Ground water in the old field of the Northern and the Pas-de-Calais and in particular its strong content of Nitrate S comes mainly from this phenomenon. That more especially as, conscious of the vulnerability of the networks and their lack of effectiveness in similar circumstances, the mining companies had been often exempted to lead some apart from the urban centres.

Generally, mining depressions are at the origin of irreversible modifications of the flows of surface which required the construction of collecting stations of waste water and rain. This situation complicated the urban development since the populations are exposed to the Inondation S in the event of failure of the equipment.

For Noyelles-under-Lens, it was necessary to raise several times the banks of the channel of Lens (the Deûle) to maintain its flow in spite of mining depressions. The close grounds are from now on six meters in lower part of the level of the channel. A station of raising had to be built; it pumps not only the rain water which do not have any more a gravitating discharge system, but also the Ground water now resurgent in this sector and thus potential source of Inondation it too.

The situation is identical on other bank, with Loison-under-Lens where exists another station of raising.

The hydraulic problems in old the field of the Nord-Pas-de-Calais are still sufficiently present so that the Master development plan and management of water (SDAGE) devotes a specific shutter to him. They are at the same time concerns related to mining depressions, water surface (stations of raisings) and “gone up” of ground water but also of concerns related to the connection of the tablecloth of the Carbonifère with the ground water of chalk.

Iron-bearing basin of Lorraine

Finished mining from now on left, because of method employed " by rooms and piliers" great volumes of abandoned galleries. These galleries are with depths varying, according to the zones of a few meters with nearly two hundred meters of depth. The mining method was based on the principle that, by leaving pillars of sufficient size, these pillars would be able to resist in time, without crumbling. The experts of the time had however underestimated the loss of resistance of these pillars in time.

Recent disasters, as from 1996 in particular, brought to reconsider the problem completely and to study the risk of movement of ground. Studies undertaken since 1997 made it possible to characterize several types of risks:

The subsidence

Progressive depression

Progressive depression can occur above a bord and pillar system. It results in the formation on the surface of a basin of a few tens to a few hundreds meters in diameter. In the center of the basin the grounds go down vertically. On the edges, the grounds are put inclined with a stretching on the external edges (opening of fractures) and a shortening on the interior edges (appearance of pads). The depression of surface occurs generally gradually in a few days or a few months according to a dynamics suitable for the mining and geological context. The buildings on the surface are sensitive to the inclined setting of the grounds like to the effects of extension in the zone of stretching and compression in the zone of shortening. The effects are all the more high as the amplitude of depression in the center of the basin is large and that the depth of mining work is low. The buildings are all the more vulnerable as they are long and slim.

Brutal collapse

In certain cases, the ruin of the mining building is not done gradually but one observes collapse in block of the whole of the grounds ranging between the bottom and surface. The collapse of surface occurs then in a dynamic way, in a few seconds. A strong earth tremor is felt. The edges of the affected zone are more abrupt than in the case of the subsidence trough, of the cracks opened there appear. So that a brutal collapse occurs, two conditions at least must be filled: - The underground work must be very fragile (strong proportion of extraction, slim pillars): This constitutes the geometrical criterion. - A thick and resistant bench must exist in covering. The rupture of this bench which protected the pillars from the weight of the grounds starts the process of collapse. This constitutes the geological criterion.

Risk management

The risk of brutal collapse is considered relatively stabilized in the coal basins in France, without absolute certainty, but the movements of ground in the iron-bearing basin can still occur. They thus constitute risks which the State must manage: that results in particular in precautionary measures of monitoring and control of town planning.

There does not seem to be problem of acidification in France, but this phenomenon can have serious consequences where there exists (on a site in the USA, an extremely acid lake (pH 1) was thus formed, the acidity of the medium supporting the circulation of the Heavy metals Toxique S).

Remain problems involved in the pollution of the grounds and thus of the tablecloths, and on the fact that depressions often fractured the drinking water distribution networks and the sewerage systems which flee abundantly.

Finally in the coal basins of which that of the Nord-Pas-de-Calais, a continuous production of Méthane (CH₄, known as Firedamp in mining zone) will perdurera a long time. In the North of France (larger underground field exploited world), a part is recovered by Méthamine and is injected into the gas network, but at the ends is and western basin, the gas flees in the air, but methane is 21 times more active for the greenhouse effect than CO₂.

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