ROOF BOLT LOAD AND DIFFERENTIAL SAG MEASUREMENTS

Special Research Report SR-53 The results of an underground test installation and laboratory study are reported, Research to determine the effectiveness of roof bolting in the No. 20 Mine of the Barnes and Tucker Coal Company revealed some interesting results. The test arrangement permitted the measurement of bolt loads and bed separation simultaneously. The beds began to separate almost immediately after roof exposure, and blasting had an adverse affect. One separation plane occurred in the bolt zone of influence but two major separations were located above the plane of anchorage. With bolt load loss, the top broke and the place became very hazardous to work. A.s a result of the field work, it became apparent that a bolt load cell was needed .that would combine accuracy and sensitivity with economy and simpncity. Since such a device was not available, it was decided to develop one. A cell was constructed on the same principle as a proving ring. It is essentially a steel ring with two holes drilled diametrically opposite to allow the insertion of the bolt. This ring is placed between the head of the bolt and the plate during installation. Upon torquing the bolt, the ring deflection is measured by a precision dial gage and the bolt load is revealed precisely. Twelve ring gages have been constructed and calibrated and are now ready for field use. Roof bolts today are the major method of supportin coal mines. Nevertheless, it is rare when instrumentation exists to evaluate the bolt load upon installation or anytime thereafter. The torque wrench at best gives only a crude approx.imation and can be in great error. To the authors 1 knowledge, no one in Pennsylvania is attempting to measure the effectiveq.es s of bolting by studying bed separation simultaneously. Yet, unless the effectiveness of bolting can be evaluated, it is impossible to improve anchorage efficiency. The type of research program described in this report is very s ignifi- cant and could lead to the scientific selection of a bolting system instead of the trial-and-error techniques employed to date.