Page 491 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.91

FIGURE 9.79 Surface subsidence.

Surface Subsidence. On the planning side, any mining method that allows the roof to cave as
the ore is taken must allow for the effects on the surface. A thin, even bed such as a 6-foot coal
seam may let the surface down evenly, with comparatively little damage to structures. But any
substantial and irregular removal of underground material will result in subsidence pits. These
may be hundreds of feet deep and thousands of feet wide, and are likely to destroy any road or
structure within their reach.
The subsidence of a caving block may be nearly vertical at first, but the cleavage in the over-
burden is likely to fan out widely. The ultimate effect, in a wide variety of rock formations, is
crumbling and sinking inside a slope line of 40° to 45° outwardly from the bottom of the block or
caving operation, with additional slip faults that might extend to a 38° slope. See Fig. 9.79.
All too often ore treatment mills, shaft head structures, and towns are involved in such subsi-
dence, so that very expensive relocation is required.

FORECASTING ROCKFALLS

One of the principal safety problems of underground work is the danger of rockfalls and rockbursts.
Knowledge of where such incidents are likely to occur usually makes it possible to install addi-
tional supports, or if that is not possible, to remove personnel and equipment from the danger area.
The cracking and popping of rock, often heard by personnel working underground, has long
been recognized as a warning of moving, unstable ground. It has now been discovered that sounds
of the same type, so slight as to be heard only by very sensitive instruments, are made by ground
that is under even moderate stress. Increasing frequency and intensity of the sounds, called micro-
seisms, indicates an increasingly unstable condition that is likely to result in collapse.
Microseisms occur within the audio frequency range and, when amplified, sound like “clicks”
akin to the creaking of wooden stairways, floors, and diving boards. Almost any hard material,
such as salt, rock, shales, sandstone, quartz, brick, glass, wood, steel, concrete, sand, and sugar,
will produce microseisms. Microseisms by their clicking nature can be easily distinguished from
sounds caused by foot shuffling, drill rigs, trucks, mucking cars, talking, and other means.
Microseismic apparatus can never be used when vibrations are so great as to overwhelm the
equipment.
The Seismitron (Fig. 9.80) is an instrument based on the microseismic principle and has been
in use since 1951. Its primary purpose is for the prediction of rockfalls in tunnels and mines as
well as for the determination of stability of earth dams and slopes at the sides of mountains and
highways. The apparatus is made by the Walter Nold Company of Natick, Massachusetts, and is
certified (permissible) for use in coal mines by the U.S. Bureau of Mines and by the Department
of Mines and Technical Surveys for Canada.
1
Receiving sensors (geophones) are cylinders 1 ⁄ 4 inches in diameter and 9 inches long. They
contain synthetically grown crystals which, when stressed, generate low-level electric currents. A