Subsidence

Localized collapses or subsidence

The subsidences occur when the benches of the roof not very resistant and are surmounted grounds of mechanical characteristics poor or movable grounds.

The mechanism is initiated by the progressive rupture of the first benches of the roof by inflection or shearing near the face of mass, generally in the middle of the galleries and a fortiori with the crossroads of galleries, at the place where quarry ceiling to the greatest range. The process develops vertically giving place to the formation of the bell of subsidence. One attends the up to date arrival of the subsidence except if the relationship between the thickness of the grounds of covering and the height of the vacuums is such as there is filling of the subsidence by expansion of materials, in which case one does not witness that more or less important decompressions of the grounds on the surface.

These phenomena of subsidence are observable in the coarse gypsum and limestone quarries.

In the Gypsum, the subsidences are supported by:

important heights of exploitation, ten to sixteen meters;

the layout and dimensions of pillars in the old exploitations were often given according to the facility of extraction, without precise rule;

the Gypse is a soluble rock. The ciels and the pillars worsen by dissolution in addition to mechanical erosion.

Generalized collapses.

Generalized collapses can in addition occur. If the phenomenon of ageing, of deterioration of the ciels affects several crossroads, it occurs one unbalanced general of the pillars around being able to involve the depression of a whole zone. It is generally allowed that this type of disorder develops from one or several pillars of the center of the sector concerned, pillars whose maximum resistance is exceeded. These pillars yield and cause a carryforward of the loads on the close pillars which yield in their turn and so on in a movement in chain.

The phenomenon of degradation of a career thus results in envisaging an incidence surfaces some without for as much the date of supervening can be envisaged. By order ascending of gravity, these accidents are:

- a localized decompression of ground without appreciable movement,

- a progressive lowering of the ground on a certain surface,

- the up to date arrival of a subsidence localized from 1 to 20 meters in diameter,

- the collapse of one or more pillars.

External bonds

careers given up in Ile-de-France

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