Roof control system

Abstract

A roof control system for underground strata has an elongated bolt adapted for insertion into a hole bored into the underground strata. The bolt is securely anchored in the hole at a location where a significant length of the bolt remains between the opening of the borehole and the anchor. A significant length of the bolt is placed in tension by rotating the bolt at a predetermined torque, with a plate mounted on the bolt and located adjacent to the outer surface of the strata and a nut on the end of the bolt for engaging the plate. The friction between adjacent surfaces which rub against each other when the bolt is rotated is selectively adjusted so that the tension/torque ratio of the bolt is selected to match the desired level for a particular type of strata. A array of friction reducing washers with different contact surface areas are provided so that one can be selected for location between the nut and plate.

I claim: 
     
       1. Roof control system for underground strata, comprising: a) an elongated bolt adapted for insertion into a hole bored in underground strata;   b) . .anchor means.!. .Iadd.an anchor assembly .Iaddend.for securely anchoring the bolt in the hole at a location where a significant length of the bolt remains between the opening of the borehole and the anchor . .means.!. .Iadd.assembly.Iaddend.;   c) tension means for placing said significant length of said bolt in tension by rotating the bolt at a predetermined torque, . .torque.!. the .Iadd.tension .Iaddend.means including a plate mounted on the bolt and located adjacent to the outer surface of the strata, and a nut . .means.!. on the end of the bolt for engaging the plate;   d) tension/torque adjustment means for selectively adjusting friction between adjacent surfaces which rub against each other when the bolt is rotated, whereby the tension/torque ratio of the bolt is selected to match the desired level for a particular type of underground strata, the tension/torque adjustment means including .Iadd.a friction reducing washer selected from .Iaddend.an array of friction reducing washers with different contact surface areas, for location between the nut . .means.!. and plate.   
     
     
       2. The system of claim 1, wherein the bolt includes a length of steel rebar or J-bar. 
     
     
       3. The system of claim 2, wherein the anchor . .means.!. .Iadd.assembly .Iaddend.includes an expansion shell with a bail strap for holding the shell on the bolt. 
     
     
       4. The system of claim 2, wherein the expansion shell includes a support nut for holding the shell in the bolt. 
     
     
       5. The system of claim 1, wherein the anchor . .means.!. .Iadd.assembly .Iaddend.includes an expansion shell maintained on the innermost end of the bolt relative to the hole, means for expanding the shell into contact with the walls of the borehole upon rotation of the bolt, and a two-component resin which is mixed upon rotation of the bolt. 
     
     
       6. The system of claim 1, . .wherein.!. .Iadd.with .Iaddend.the nut . .means includes a nut.!. formed on the end of the bolt and a steel washer adjacent to the nut, whereby the tension/torque adjustment means adjusts friction between the washer and plate. 
     
     
       7. A method of mine control for underground strata, comprising: a) placing an elongated bolt into a hole bored in underground strata;   b) securely anchoring the bolt in the hole .Iadd.by an anchor assembly .Iaddend.at a location where a significant length of the bolt remains between the opening of the borehole and the . .anchor means.!. anchor .Iadd.assembly.Iaddend.;   c) placing said significant length of said bolt in tension by rotating the bolt at a predetermined torque, wherein a plate is mounted on the bolt and located adjacent to the outer surface of the strata and a nut . .means.!. is on the end of the bolt that engages the plate;   d) selectively adjusting the friction between adjacent surfaces which rub against each other when the bolt is rotated for adjusting the tension/torque ratio of the bolt to match the desired level for a particular type of underground strata, including selecting .Iadd.a friction reducing washer .Iaddend.from an array of friction reducing washers with different contact surface areas and volumes of washer material and placing . .one of said washers.!. .Iadd.the selected friction reducing washer .Iaddend.between the nut . .means.!. and plate prior to rotating the bolt.   
     
     
       8. The method of claim 7, wherein in step "a" the bolt includes a length of steel rebar or J-bar. 
     
     
       9. The method of claim 7, wherein in step "b" the anchor .Iadd.assembly includes .Iaddend.an expansion shell . .is.!. maintained on the innermost end of the bolt relative to the hole and there is further provided the steps of expanding the shell into contact with the walls of the borehole upon rotation of the bolt, and mixing a two component resin upon . .rotating.!. .Iadd.rotation .Iaddend.of the bolt. 
     
     
       10. The method of claim 9, wherein the expansion shell is a bail-type shell. 
     
     
       11. The method of claim 10, wherein the expansion shell is an expansion shell held in place by a support nut. .Iadd.12. The system of claim 1, wherein the selected friction reducing washer is the friction reducing washer which best matches a desired tension/torque ratio to the strata to obtain an optimum beaming effect..Iaddend..Iadd.13. The method of claim 7, further including the steps of selecting the friction reducing washer by: using a test bolt at various mine roof locations to determine a desired tension/torque ratio;   testing various friction reducing washers from the array with the test bolt and measuring installed load on the test bolt; and   selecting an optimum friction reducing washer from the array, wherein the optimum friction reducing washer is that washer for which pull tests determine the lowest deflection or lengthening of the test bolt to have occurred..Iaddend.