Environmental Engineering Reference
In-Depth Information
The main parameters are the angles γ: limit angles determining the influence
zone and β, “rupture angles” determining the zone where the damage actually occurs
(beyond this, they are negligible). They are empirically measured and vary from coal
basin to coal basin, as a function of the geological conditions. The angles increase
with increasing resistance of the layers. Generally in carboniferous terrains, for a
horizontal dip, the order of magnitude of γ is 55°, β being about 70°. According to
the dip value of the layer worked, these figures can be increased by about 10°. In
soft soils β values decrease, and can reach 30° in sandy terrains, corresponding to
the greater extent of mining influence.
The maximum settlement is generally computed using formulae similar to:
S
max
=
m. e
[7.1]
with:
S
max
: maximum settlement value;
e
: thickness of the worked layer; and
m
: empirical coefficient depending on the possible closing of walls as a function
of the type of work (hydraulic or air backfilling, etc.) on one hand and on the other
depending on factors such as the depth of stope, existence of previous works, etc.
The values of
m
range from 0.1 to 0.9.
The duration of subsidence is difficult to determine. According to the working
conditions, stope depths, presence of overburden, etc., it can vary from one year to
several (tens of) years. Generally, we consider that settlements induced by coal
mines come to an end between three and 10 years after the termination of
exploitation. Some motions can occur, however, after 20 and even 30 years.
The coal extraction areas are sensitive zones, even after exploitation ends. The
end of the dewatering (pumping of underground waters in order to exploit coal) can
lead to water saturation of rocks, inducing a change in their mechanical properties
and the effective stress state. The backfilling of stopes can also be influenced by
raising the water table, as can the superficial layers. In these conditions, the end of
the dewatering is expected, in some cases, to induce instabilities and to reactivate
the settlements.
This phenomenon easily can be explained using the basic BBM. The explanation
will be given after a brief presentation of the BBM in section 7.3.
