Instrumented King Wire for Monitoring Cable Bolts

Objective

A cable bolt can be a strain-measuring device as well as a
supporting device in underground mines, Placing instruments on
a cable bolt allows certain characteristics of a rock mass, such as
high strain and load, to be evaluated during mining and provides
a means for mining engineers and mine inspectors to forecast a
potential roof fall.

Background

Thousands of cable bolts are installed in U.S. mines each year.
Studies performed by NIOSH researchers show that support loads
can exceed the yield point of the steel, which can result in unstable
ground conditions that pose direct safety hazards for underground
workers. Rock falls can also create secondary hazards, such as
caved escapeways and blocked ventilation passages, Development
of methods and tools to assist in the evaluation and selection of
rock supports could reduce the number of unplanned rock falls
significantly.

Approach

Instrumented cable bolts developed at NIOSH’s Spokane
Research Laboratory were used along with existing ground control
systems to monitor rock mass loads and movement. Axial and
shear loads on the cables are determined as these loads are
transferred to strain gages embedded in the king wire. In longwall,
room-and-pillar, or underhand cut-and-fill mines, rock mass loads
and movements can be detected down to microstrains of stress or

millimeters of movement. By detecting such small amounts of
ground movement and support load, early intervention to prevent
a roof fall is possible, and miner safety and health are increased.

How it Works

Up to 10 strain gages are embedded in the king wire at whatever
locations a mine ground control engineer may specify. The gages
can either be set in pairs in the same plane to detect shear or set at
90° angles to detect loading in three dimensions. The cables are
up to 5 meters long if they are to be grouted with resin or up to 15
meters if they will be grouted with cement. They are capable of
holding an ultimate load of 214 KN and produce reliable readings
to 178 KN, as shown in the calibration curve. The cables are fitted
with an instrument plug for monitoring the gages and a special
head for insertion into the rock, A data acquisition system, such as
a Vishay strain indicator box, Campbell Scientific, or Omnidata
system, is required to obtain the data for load and displacement
calculations.

Patent Status

A patent has been applied for under the name “Instrumented
King Wire for Monitoring Cable Bolts for Use with Resin Grout,”
application no. 60/076,138.

For More Information

Additional information can be obtained by contacting Lewis
Martin at (509) 354-8077, e-mail lim8@cdc.gov, or Richard Curtin
at (509) 354-8076, Spokane Research Laboratory, Spokane, WA,
99207.

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U.S. Department of Health and Human Services
Public Health Service
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health

CDC

‘CENTERS Fon Disease CONTROL


To receive additional information about mining issues or
other occupational safety and health problems, call 1-800-

35-NIOSH (1-800-356-4674), or visit the NIOSH
Home Page on the World Wide Web at http://www.cde.

gov/niosh

Mention of any company name or product does not
constitute endorsement by the National Institute for

Occupational Safety and Health.

KEY
© Series1 ¢ Series4 4 Series7 © Series
© Series2 = Series5 ¢ Series8 * Series 10
4 Series3 © Series 6
10

a(S

10° MICROSTRAIN
S

0 25 50 75
LOAD, kN

Loads on instrumented cable bolts.

100 125 150 175 200

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: epoxy-coated king wire during

Top: King wire with ga
and rewrapped with cable; bottor
cable rewrapping.

Instrumentation plug and installation he