Hydraulic and earth engineering – Earth treatment or control – Rock or earth bolt or anchor
Reexamination Certificate
2000-07-25
2002-06-11
Bagnell, David (Department: 3672)
Hydraulic and earth engineering
Earth treatment or control
Rock or earth bolt or anchor
C405S259600, C405S302200
Reexamination Certificate
active
06402433
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A “MICROFICHE APPENDIX”
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to mine roof bolts, and more particularly relates to tensionable mine roof bolts constructed of multi-strand steel cable.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. §1.97 and 37 C.F.R. §1.98
In the art of mine tunnel roof support, there are two major categories of bolting systems wherein mine roof bolts are anchored in bore holes drilled in the mine tunnel roof, the bolts' purpose being to reinforce and stabilize the unsupported rock formation above the mine tunnel. These two categories of mine roof bolting systems are: (1) tension-type systems, and (2) passive-type systems. In each system, it is common practice to, first, drill a hole through the mine tunnel ceiling into the rock formation above to a depth appropriate for the type of rock formation to be supported. A mine roof bolt and roof plate are then anchored in the bore hole to support the mine roof and maintain the rock formation in place.
In a common tension-type mine roof bolt system, an expansion shell type anchor is installed on the threaded end of the bolt. The bolt and expansion shell anchor are inserted up into the bore hole until the roof plate is against the mine roof. The bolt is then rotated to thread a tapered plug section of the expansion shell down toward the bolt head, in order to expand the jaws of the expansion shell against the interior wall of the bore hole to thereby hold the mine roof bolt in place within the bore hole, the mine roof bolt functioning to support and stabilize the rock formation above the mine tunnel.
In passive-type mine roof bolt systems, the passive-type bolt is not attached to an expansion shell or similar anchor at the free (upper) end of the bolt, but rather is retained in place within the rock formation by a rapid-curing resin adhesive material that is mixed in the bore hole as the bolt is rotated and positioned within the bore hole. In theory, the resin adhesive bonds the bolt to the rock formation along the total length of the bolt within the bore hole in the rock formation. It is also common practice to use resin adhesive with a tension-type mine roof bolt to retain the bolt within the mine roof bore hole, at least along the upper portion of the bolt. Again, when the rock formation shifts, certain types of tension system bolts can be retightened by rotating the bolt or nut.
In passive-type and some tension-type mine roof bolting systems, one or more resin cartridges are inserted into the bore hole prior to (ahead of) the mine roof bolt. Forcing the mine roof bolt into the bore hole while simultaneously rotating the bolt ruptures the resin cartridge(s) and mixes the resin components within the annulus between the bolt shaft and bore hole wall. Ideally, the resin adhesive mixture totally fills the annulus between the bolt shaft and bore hole wall at least along the upper portion of tension-type bolting systems, and along the total length of the bolt shaft and bore hole wall in passive-type systems. The resin mixture is forced into cracks and crevices in the bore hole wall and into the surrounding rock formation to adhere the bolt to the rock formation.
When extremely long mine roof bolts are necessary, it is common practice to attach two or more bolt shaft sections together by couplers to result in a “roof bolt” of sufficient length appropriate for the particular type of rock formation. These couplers between bolt sections, being of a larger diameter than the bolt shafts, prevent the mixed resin adhesive from flowing downwardly (resin return) within the bore hole annulus from the first (upper) bolt section to the lower section(s). Therefore, the effective anchoring of the bolt to the bore hole wall within the rock formation is, essentially, only along the length of the first (upper) bolt section wherein the resin adhesive totally fills the annulus between the bolt section and the bore hole wall.
To alleviate this problem, it has been common practice simply to drill a larger bore hole in the rock formation that will enable the resin adhesive to flow around the coupler(s) as the bolt is being inserted into and rotated within the bore hole to mix the resin. Although this does effect the desired result (resin return around the coupler(s) within the annulus between the bolt shaft and bore hole wall), it creates another problem that, depending on the type of rock formation, may be more dangerous than the problem that is corrected by a larger bore hole. Specifically, bonding of the resin adhesive material to hold the mine roof bolt in place within the bore hole is considerably weakened by virtue of the increased distance between the bolt shaft and bore hole wall, and the sheer volume of resin adhesive material necessary to totally fill the annulus. Additionally, by virtue of their specific makeups, mine roof rock formations that actually require long (fifteen feet or longer) mine roof bolts are more susceptible to movement and shifting within the rock formation, than are more solid rock formations that require only shorter mine roof bolts.
Another common problem with using mine roof bolt sections coupled together in such rock formations that require longer (coupled) mine roof bolts, this shifting of the rock formation (shear) causes the bolt couplers to fracture. When this happens, of course, the effective holding length of the mine roof bolt is instantly decreased. In many instances, there is no or very little resin adhesive material around the broken bolt shaft to help stabilize the rock formation. Therefore, in almost all instances, this shortened mine roof bolt is ineffective to safely prevent the mine roof rock formation from further shifting and potential collapse.
In response to these problems, so-called “cable bolts” have been devised for both tension-type systems and passive-type systems. A common design for a tension-type (tensionable) mine roof bolt has a bolt “head” formed of a steel rod having an axial bore in one end and an externally threaded shaft at the other end. This “head” is swaged down upon the cable to define a rigid threaded end (tension head) of the mine roof bolt for receiving a tensioning nut. The threaded section of the bolt head permits tensioning of the bolt within the rock formation above the mine tunnel after the resin mixture has set, and also permits subsequent re-tensioning of the mine roof bolt when the bolt loosens as a result of shifts in the rock formation.
In theory, the tensionable cable bolt alleviates the problems inherent in using solid mine roof bolt sections in a shifting rock formation above the mine tunnel. In practice, however, the above-described tensionable cable bolt has introduced a different problem. Specifically, because the cable is not solid, it tends to twist or torque as it is being initially tensioned upon installation and solidification of the resin material, and also as it is periodically re-tensioned when necessary. This twisting or torquing of the cable has two effects, depending on whether the tightening torque applied to the nut is in the same direction as the cable twist or in the opposite direction of the cable twist. If the nut-tightening torque is in the opposite direction of the cable twist, of course, the cable will unwind and separate. If the nut-tightening torque is in the same direction as the twist of the cable, the cable will torque and twist within the hole, and will actually draw up (shorten) in length between the anchor and the tension head and nut, due to the spiral orientation of the twisted cable. The effective “shortening” of the free length of cable as it is being tensioned or re-tensioned, therefore, causes the cable to become taut prematurely, due to its artificially shortened length. Therefore, when the tensioning torque applied to the nut is released, the twisted cable is permitted to relax
Bagnell David
McCully Michael D.
Stephenson Daniel P
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