Page 492 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.92 THE WORK

FIGURE 9.80 Seismitron.

portable battery-powered amplifier increases the level to a condition that enables the microseisms
to be heard by means of simple earphones.
The number of microseisms per minute which occur when the rock is at the failure point, or
about to burst, is determined by monitoring the heading (of a tunnel, for instance) immediately
after a blast. The microseismic rate of decay under such a state is the same as the rate of increase
prior to a rockfall. This is the failure point.
Six-foot-deep holes are drilled and spaced every 100 feet apart in the suspected area. Each of
these stations is monitored each week for microseismic activity. This is accomplished by placing
the geophone in the hole and sealing the opening with waste. The microseisms heard are manually
counted for a 15-minute period and the rate per minute versus dated time plotted on graph paper
at a later time.
A history is thus built up for each station. A low, little-changing rate of microseismic activity
is indicative of stable conditions. A continually increasing rate indicates suspect conditions, espe-
cially when 120 to 180 microseisms occur per minute. A doubling of the rate within any 24-hour
period indicates imminent collapse.
An excellent prediction of collapse, sometimes as much as 45 days ahead of time under ideal
conditions, can be made by extrapolation of the plots on the graph to the rock failure point. The
curvature becomes hyperbolic as failure approaches.
If the active rock, which is sometimes hidden by gunnite, should become stabilized either nat-
urally or by roof bolting or timbering, the decaying microseismic rate would immediately indicate
the tendency toward safe conditions.
The instrument in this manner becomes a very useful tool for judging the necessity for, or the
effectiveness of, expensive roof bolting. The Seismitron is used in earth drilling to determine