Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 256

mechanically or hydraulically carrying out fi ne particles. The origins and
evolution of these processes can, additionally, be directed or aggravated
by human actions.
The existence of a cavity at depth causes a redistribution of parameters
in its surroundings, and, in particular, creates tension and shear stresses in
the overlying terrain (expansion). The increase in vertical constraints and
the washing out of crumbled soil maintain this mechanism, which gradually
evolves and increases. In certain cases, its progression is stopped by self-
fi lling cavities (expansion of the affected terrain or hydraulic infi lling), but
it frequently reaches the surface, where it causes collapses or subsidence.
Subsidence is a slow, gradual phenomenon, which manifests itself
at the surface as a shallow topographic depression with a wide radius
of curvature. It marks the end point of a fl exible deformation, due to the
continuous rearrangement of the fallen material below ground and the
resorption of cavities.
Collapses are the brutal opening of a localized rupture, generally
cylindrical, with a variable depth (from a few meters to a few tens of meters).
These punctures in the soil are often termed sinkholes, and are a result of
the sudden rupture of a rigid horizon which, due to its intrinsic resistance,
created a stable roof under which a cavity was able to form and grow.
The surface effects of such phenomena are similar to those occurring
above old human constructions below ground (quarries, mines, tunnels).
They cause signifi cant stability problems for the built structures within the
area they infl uence, and can lead to serious catastrophes.
3.2 Natural evolution
3.2.1 Dissolution effects
The main soluble rocks subject to such mechanisms present highly variable
solubilities, characterized by the following orders of magnitude for pure
water at normal temperatures:
• 0,3g·L -1 for calcium carbonate (limestone and dolomite),
• 2g·L -1 for calcium sulfate (gypsum and anhydrite),
• 360g·L -1 for sodium chloride (rock salt or halite).
The phenomena are essentially differentiated by their kinetics of
evolution, as the general creation process of underground networks remains
the same, with initial dissolution action along the length of fractures
allowing the infi ltration of water, followed by increased participation
by the mechanical action of water as the passageways are progressively
enlarged. The gradual organization of groundwater circulation enables the