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 surface subsidence takes place over many years. (At right,
column construction in rubble using variable viscosity grout.)
DEVELOPMENT
Development
of the
surface areas over abandoned mines is becoming financially more feasible
because the prime sites have already been developed. The cost to stabilize the mine is
determined by a number of factors and may be as simple as installing settlement monitors. (At right, bulk void filling.)
METHODS
Stabilization
of undermined areas can be
performed with several techniques. 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 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withdrawn until it is even with the ceiling.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.
