ABSTRACT
To increase the safety and productivity of underground coal mines, the U.S. Bureau of Mines, through an in house research project beginning in lg75, demonstrated the feasibility of using fast setting hydraulic cements for grouting coal mine roof bolts. The Colorado School of Mines was funded through the Spokane Mining Research Center to select, test, and demonstrate inorganic cements for this application. A grout composed of gypsum cement and potassium sulfate accelerator was selected as having the best properties for roof bolting. This grout is rapid setting, and reaches the yield strength of roof bolts in about three minutes from the time of mixing the cement. This cement system was found through testing of a variety of cements in combinations with several types of accelerators through a series of laboratory tests. The laboratory tests consisted of penetration, shrinkage, cube strength and pull strength tests. Field tests were conducted in Sommerset Mine of the U.S. Steel Corporation. Two intersections were used to test and demonstrate the cement-grouted bolt system. One intersection was supported with only cement-grouted bolts and an adjacent intersection with resin roof bolts in the same bolting pattern. These two intersections were instrumented to measure roof parting. It was found that the roof sag was essentially the same with a magnitude of less than .6 inches for the two intersections. Field visual inspection of these intersections was also conducted and no apparent differences were found. On the basis of this successful field demonstration further testing and development of production installation methods is recommended. Also additional testing will be required before government approval can be expected.
INTRODUCTION
This paper will establish the role of cement roof bolt grouting in coal mines by a brief discussion of rock bolting in general. Then the procedure and results of the cement grout project will be described. A more detailed compilation and analysis of the laboratory and field testing can be found in the final report to the U.S. Bureau of Mines, available soon from the National Technical Information Service.
LABORATORY TESTING
The object of rock bolting is to bind together a discontinuous rock mass to increase the stability around an underground opening. Two types of action have been attributed to rock bolts:
Suspension of incompetent rock from an anchor in competent rock.
Clamping of separate blocks or beds to create friction on the interface which will resist transverse movements. This clamping would tend to reinforce a potential beam or arch.
In coal mines, tensioned bolts with mechanical expansion anchors predominate. Fully-grouted untensioned bolts are used increasingly because of advantages which are inherent in the mechanics of stress distribution associated with their support capabilities.
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The section of hole upon which a mechanical anchor bears is subject to high stress. In soft rock the response is crushing and the anchorage is lost. In hard rock, a transverse crack may form above the anchor (Caverson, 1971), or radial cracks can run from one hole to the next. All of the tensile load in the bolt is transmitted through the plate to the free surface of the roof.
