Improvements over undermined areas have
resulted in a variety of serious problems including structural
damage, swails in rods, and damage to utilities. Although
catastrophic failures have occurred, normally deterioration of the
roof of the mine and subsequent mine subsidence takes place over
many years.
Development of the surface areas over
abandoned mines is becoming financially more feasible because the
prime sites have already been developed. The cost to prevent mine
subsidence by stabilization is determined by a number of factors and
may be as simple as installing settlement monitors.
There are several methods of mine subsidence
prevention. The most common method is bulk filling of the void. This
is performed by drilling from the surface with 3 to 6 inch diameter
holes that intercept the mine. Mine maps can be useful in
determining the locations for drilling that give the best
opportunity to intercept the mined out areas. If an isolated portion
of the void is to be filled, bulkheads are constructed with a lower
slump mix placed in openings. The remainder of the void is filled
with high slump, low strength grout. If the mine is flooded, it may
be necessary to control the water displaced by the grout. (Above,
building columns in shallow mine.)
Another common method of mine subsidence
prevention is construction of grout columns in the mine. In areas
where the mine is open, low slump grout is pumped into the mine. As
the column builds, the injection pipe is withdrawn until it is even
with the ceiling. The volume of grout injected is determined by the
height of the mine, the desired surface area contact with the roof
and the natural angle of repose of the grout.
If the mine is highly rubbelized, it is
necessary to have a grout that is fluid enough to penetrate the
rubble. The grout must also be able to stack up in open areas. This
is accomplished by varying the viscosity of the grout with
thixotropic admixtures injected at the well head. Injection pressure
is closely monitored to determine the amount of injection needed.
