US3379016A - Rock bolt assembly and procedure for use in conjunction with blasting operations - Google Patents

Description

Aprll 23, 1968 c. i. WILLIAMS 3,379,016

ROCK BOLT ASSEMBLY AND PROCEDURE FOR USE IN CONJUNCTION WITH BLASTING OPERATIONS Filed Jan.

INVENTOR. CHESTER W/L L/A M5 United States Patent ROCK BOLT ASSEMBLY AND PROCEDURE FOR USE IN CONJUNCTION WITH BLAST- ING OPERATIONS Chester I. Williams, 347 Greeubriar, SE., Grand Rapids, Mich. 49506 Filed Jan. 11, 1965, Ser. No. 424,657 5 Claims. (Cl. 61-35) ABSTRACT OF THE DISCLOSURE A rock bolt with removable shielding around it along the portion immediately inward from the surface of a rock formation in which the bolt is installed, the shield being operative to prevent the bonding of grout to the shielded portion of the bolt.

This invention relates to rock bolt installations adapted for use in an area where blasting operations are performed. It is common practice to install rock bolts in a ground formation prior to blasting in order to prevent or control the displacement of certain portions of the rock. To develop the full strength of these rock bolts, it is also the usual practice to charge the hole in which the rock bolt is installed with a cementitious composition commonly known as grout to increase the bond between the rod and the surrounding ground formation, and also to protect the bolt against corrosion. To obtain the maximum benefits from the bolt, a pre-stressing operation should be performed which has the effect of stretching the rod sufiiciently to generate stresses approaching the normal working load, so that these forces can be applied to the ground formations to hold them in position. Without pre-stressing, the ground formation must exhibit considerable displacement before the rock bolt is actually able to apply its restraining forces.

When blasting operations are performed, there is usually enough disturbance of the ground formation adjacent the outer end of the rock bolt so that the nut is no longer tight against the bearing plate, thus reducing the effectiveness of the bolt. Re-tightening of the bolt is very difficult to accomplish, if at all possible, without destroying the bond between the bolt and the previously-injected grout. It also frequently happens that the detonation of a blasting charge loosens a suflicient quantity of the ground formation so that some of the area immediately adjacent the bearing plate is removed entirely. In the usual rock bolt having a few inches of threading at the outer end, about the only way in which the rock bolt could be made to continue to perform a portion of its function would be to weld the bearing plate to the rod, and then cut off the projecting useless extremity of the rod. This, of course, would not leave the outer portion of the bolt rod in a stressed condition.

Applicant has devised a rock bolt assembly which can be used in conjunction with blasting operations to permit a secondary prestressing without the necessity of pulling the outer extremity of the bolt rod through a surrounding grout bond as the secondary stressing operation is performed. This assembly also has the desirable characteristic of rendering the outer section of the bolt rod replaceable so as substantial removal of the ground formation can be accommodated by installing a shorter section of bolt rod in place of the original outer portion.

The several features of the invention will be analyzed in further detail through a discussion of the particular embodiment illustrated in the accompanying drawing. In the drawing:

FIGURE 1 is a sectional elevation showing the installed condition of a rock bolt assembly embodying this ice invention, in a condition such as would be encountered prior to a blasting operation.

FIGURE 2 illustrates the condition of the rock bolt assembly after the blasting operation has been performed, and a new outer section of the bolt rod has been installed.

FIGURE 3 is a section taken on the plane 3-3 of FIGURE 1.

Referring to the drawing, the illustrated rock bolt assembly is shown installed in a hole 10 in the rock formation 11. The bolt rod has an outer portion 12 and an inner portion 13 joined in threaded engagement by the coupling 14. The inner end of the portion 13 is provided with an anchor device of any convenient form. In the illustrated modification, a conical expander 15 has threaded engagement with the inner end of the portion 13, and is surrounded by the expansible shell 16. Axial movement of the expander 15 to the left in response to the rotation of the bolt rod will induce expansion of the shell 16, with the thrust resulting from this operation being taken by the ring 17 which remains axially fixed with respect to the bolt rod.

It is usual practice to surround the bolt rod with a charge of grout generally indicated at 18. To eliminate a bond between the outer extremity of the bolt rod, the illustrated assembly includes a shielding tube 19 preferably extending over the full length of the outer portion 12. This shielding tube may be of cardboard or any other convenient frangible material, the primary function of which is to retain the grout outside of the tube so that it does not come into contact with the bolt rod. A coating may be applied to the rod itself to perform this function, with the characteristic of this coating providing that the rod can move axially to some extent without disturbing the grout. This can be done either by permitting the rod to slip within the protective coating or tube, or by providing that the shield material itself has sufficient plasticity to deform in shear and permit the necessary relative axial movement as the bolt rod is stressed.

The outer portion of the bolt assembly is conventional, and includes the bearing plate 20, the bevel washer 21, and the nut 22. Pro-stressing of the rock bolt assembly is accomplished by a rotation of the nut 22 with sufficient torque to develop a substantial portion of the tensile strength of the bolt rod. The assembly is usually grouted in this pro-stressed condition.

After the blasting operation has taken place, the usual result is a sufficient disturbance of the outer portion of the rock formation to render the nut 22 fairly loose. The prevention of a bond between the grout and the outer portion of the rod will permit the elastic shortening of the rod that is inevitable upon such release of stresses, without disruption of the surrounding grout. This elimination of bond will also permit the nut 22 to be re-tightened so that the rock bolt assembly can be pre-stressed again in the final condition of the ground formation.

In situations where the blasting operations have resulted in the loosening and removal of the rock at the surface, it is possible to disconnect the outer portion 12 of the bolt rod and replace it by another shorter portion 120, followed by the re-tightening of the nut 22 as described above. Under normal conditions, the outer portion 12 can be unscrewed, and the projecting part of the tube 19 cut off generally flush with the new surface of the ground formation. A mass of plastic sealing material indicated at 23 will of course not be present in the new condition, since that portion of the rock formation will have been removed. It is most convenient to extend the shielding tube 9 down to the coupling 14, which should represent approximately the outer four to six feet of the bolt length, at least. The coupling therefore forms a convenient stop establishing the axial position of the shielding tube. The tube also forms a protective guide for the insertion of a new outer portion 12a of the bolt rod, since it will form a conduit leading directly to the coupling 14 which will not be contaminated with rock chips, particles of grout, or other foreign material.

The particular embodiment of the present invention which has been illustrated and discussed herein are for illustrative purposes only and are not to be considered as a limitation upon the scope of the appended claims. In these claims, it is my intent to claim the entire invention disclosed herein, except as I am limited by the prior art.

I claim:

1. A rock bolt assembly, in combination with a mass of grout surrounding said assembly in a hole in a ground formation, said assembly comprising:

a bolt rod;

an anchor device secured to the normally inner end of said bolt rod and said ground formation; and

a shield material of substantially less strength than the material of said bolt rod surrounding exclusively only an outer portion of the said bolt rod whereby said portion is isolated from said grout, said grout being in contact with substantially all of the remaining inner portion of said rock bolt assembly.

2. An assembly as defined in claim 1, wherein said rod has an outer and an inner portion connected by a coupling.

3. An assembly as defined in claim 2, wherein said shield material is a tube extending over said outer portion to said coupling.

4. A method of conducting blasting operations in a ground formation, comprising:

drilling at least one hole in said formation, and installing a rock bolt therein and grouting the same in place, with the outer portion thereof covered by a shield material;

detonating a charge of explosive in the adjacent ground formation; and

tightening said rock bolt after said charge has exploded.

5. A method as defined in claim 4, wherein said rock bolt has inner and outer portions connected by a coupling, and tightening said rock bolt includes replacement of said outer portion with a shorter outer portion.

References Cited UNITED STATES PATENTS 1,883,196 10/1932 Wertz 61-45 X 2,442,113 5/1948 Beij 6145 X 3,226,933 1/ 1966 White 6139 FOREIGN PATENTS 152,879 1953 Australia.

80,182 1963 France. 341,185 11/1959 Switzerland.

OTHER REFERENCES DAS, 1,104,471, 4-1961, German printed application to Eckert.

JACOB SHAPIRO, Primary Examiner.

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