U.S. patent application number 11/699347 was filed with the patent office on 2007-12-27 for rock bolt with grout flow geometry.
Invention is credited to Steven Cotten, Luis Giraldo.
Application Number | 20070297862 11/699347 |
Document ID | / |
Family ID | 37873068 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070297862 |
Kind Code |
A1 |
Giraldo; Luis ; et
al. |
December 27, 2007 |
Rock bolt with grout flow geometry
Abstract
A rock bolt having a modified tip geometry for spreading
grouting material along the bolt or rupturing a grouting material
container, or both, and method of using such a rock bolt for
substrate support.
Inventors: |
Giraldo; Luis; (Fairfax,
VA) ; Cotten; Steven; (Dumfries, VA) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
37873068 |
Appl. No.: |
11/699347 |
Filed: |
January 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60763370 |
Jan 31, 2006 |
|
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Current U.S.
Class: |
405/259.6 ;
405/259.1 |
Current CPC
Class: |
E21D 21/004
20130101 |
Class at
Publication: |
405/259.6 ;
405/259.1 |
International
Class: |
E21D 20/00 20060101
E21D020/00 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] The U.S. Government may have certain rights in this
invention pursuant to its funding under contract No. R01 OHO7727,
awarded by NIOSH (National Institute for Occupational Safety and
Health).
Claims
1. A substrate reinforcement device, comprising: a bolt having a
tip comprising a geometry configured to transport grouting
materials from the tip, along a length of the bolt.
2. The substrate reinforcement device of claim 1, wherein the
geometry of the tip promotes mixing of the grouting materials.
3. The substrate reinforcement device of claim 1, wherein the
geometry of the tip promotes flow of the grouting materials around
the tip.
4. The substrate reinforcement device of claim 1, wherein the
geometry of the tip promotes flow of the grouting material through
an annulus formed between the bolt and a wall of a pilot hole.
5. The substrate reinforcement device of claim 1, wherein the
geometry comprises an auger shape.
6. The substrate reinforcement device of claim 5, wherein the auger
shape is configured in shape and pitch relative to a diameter of
the bolt, a pilot hole diameter, a viscosity of the grouting
materials, a bolt insertion rate relative to the pilot hole, and a
bolt rotation rate relative to the pilot hole.
7. The substrate reinforcement device of claim 1, wherein the
geometry is further configured to rupture a grouting materials
container residing in a pilot hole.
8. The substrate reinforcement device of claim 7, wherein the
geometry comprises an extreme tip having a smaller surface area
than a cross section of the bolt.
9. The substrate reinforcement device of claim 1, wherein the
geometry is further configured to create a groove in a pilot hole
wall.
10. The substrate reinforcement device of claim 9, wherein the
geometry comprises a protrusion to create a groove in the pilot
hole wall.
11. A substrate reinforcement device, comprising: a bolt having a
tip comprising a geometry configured for rupturing a grouting
materials container residing in a pilot hole.
12. The substrate reinforcement device of claim 11, wherein the
geometry comprises a chisel shape.
13. The substrate reinforcement device of claim 11, wherein the
geometry comprises a plurality of piercing features.
14. The substrate reinforcement device of claim 11, wherein the
geometry promotes rapid release of a grouting material within the
container upon contact with the container.
15. The substrate reinforcement device of claim 11, wherein the
geometry of the tip promotes rapid release of a grouting material
within the container upon rotational contact with the
container.
16. A rock bolt, comprising: a shaft comprising a tip end, the tip
end comprising at least one feature selected from the group
consisting of an auger shape, and an extreme tip having a smaller
surface area than a cross section of the shaft.
17. A method of stabilizing a substrate, comprising: providing a
pilot hole in a substrate; providing a grouting material within the
pilot hole; inserting a bolt into the pilot hole and into the
grouting material, the bolt having a tip with a geometry configured
to transport grouting materials from the tip, along a length of the
bolt.
18. The method of claim 17, wherein the tip comprises at least one
auger shape for the transport of grouting materials.
19. The method of claim 18, further comprising inserting the bolt
into the pilot hole at an insertion rate and at the same time
rotating the bolt at a rotation rate, wherein the auger shape is
configured in shape and pitch relative to a diameter of the bolt, a
pilot hole diameter, a viscosity of the grouting materials, the
bolt insertion rate, and the bolt rotation rate.
20. A method of stabilizing a substrate, comprising: providing a
pilot hole in a substrate; providing a grouting material within the
pilot hole, the grouting material being within a container;
inserting a bolt into the pilot hole and into the grouting
material, the bolt having a tip with a geometry configured to
rupture the container for the release of the grouting material.
21. The method of claim 20, further comprising rotating the bolt
during the inserting.
Description
RELATED APPLICATIONS
[0001] The current application claims the benefit of Provisional
Patent Application No. 60/763,370, filed on Jan. 31, 2006 in the
United States Patent and Trademark Office, the disclosure of which
is hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0003] The present invention relates to substrate supporting bolts
and methods of using such bolts.
BACKGROUND
[0004] Underground mining has one of the highest fatal injury rates
of any industry in the United States, more than five times the
national average compared to other industries. Despite
technological advances and industry-wide efforts, roof falls in
mines continue to be one of the greatest safety hazards encountered
in underground mines. During the past decade, approximately 50% of
all fatalities in underground mines have been due to ground falls.
Furthermore, as the most easily accessible coal reserves are
depleted, mines are forced to satisfy coal demand by working in
areas with more challenging geologic and associated ground control
conditions.
[0005] Grouted and mechanical expansion anchor rock bolts have been
by far the most common means used to secure and stabilize mine
roofs and ribs, together comprising over 99% of rock bolts used in
coal mines in the United States. Rock bolts typically support mine
roofs either by beam building (the tying together of multiple rock
layers so they perform as a larger single beam), suspension of weak
fractured ground from more competent layers, formation of a
pressure arch, or support of discrete blocks. Both grouted and
mechanical expansion anchor rock bolt support techniques involve
drilling pilot holes in the rock and establishing anchorage in
those holes. A decline in the use of mechanical bolts and an
increase in grouted bolts is attributed to the fact that grouted
rock bolts distribute their anchoring load on the rock over a
greater area and therefore generally have superior anchorage
capacity. However, the application of grouted rock bolts for ground
control is not without problems, several of which are exaggerated
in the presence of mechanically weak rock.
[0006] FIG. 1A provides a schematic representation of a grouted
rock bolt anchoring technique. FIG. 1B provides a schematic
representation of a mechanical expansion rock bolt anchoring
technique. The grouted bolt 101 shown in FIG. 1A is a rebar bolt
having a threaded end 109 protruding from the pilot hole 118. The
rebar 103 is surrounded by resin, or grout, 105 and is fashioned
with a face plate 107 held in place by a nut 111. The mechanically
anchored bolt 102 shown in FIG. 1B has a threaded tip 106 and a
threaded end 108. The tip 106 is screwed into the mechanical anchor
110, which expands during the process. The threaded end 108 is
fashioned with a face plate 112, washer 116, and nut 114. Other
styles of such bolts may have a forged end shaped like a nut (208,
FIG. 2), instead of a threaded end 108.
[0007] FIG. 2 illustrates the cylindrical geometry of the blunt
insertion tip 206 of a typical grouted rock bolt 202 with a forged
head 208. As shown, the rock bolt's tip 206 has no modifications
for the bolt 202 to interact with grouting material in any
significant way. FIGS. 3A through 3C schematically illustrate the
sequence of events related to installation of such a rock bolt 202
for grout anchoring in a mine roof 313 (however, such a process and
bolt could be used to secure any substrate).
[0008] As indicated in FIG. 3A, after a pilot hole 318 of the
proper length and diameter is drilled into the roof 313, a sealed
cartridge 320 of two-component grout 305 is inserted into the pilot
hole 318. The roof bolt 202 is then inserted in the hole 318. The
roof bolt 202 is then rapidly rotated and simultaneously advanced
into the hole 318. At the bolt insertion point 325 indicated in
FIG. 3B, the advancing blunt tip 206 of the bolt compresses the
sealed grout cartridge 320, thereby expanding the cartridge wrapper
322 until it has completely filled the end of the pilot hole 318.
Due to the inherent rupture strength of the grout cartridge wrapper
322 and the confinement provided by the walls of the pilot hole
318, the bolt 202 may penetrate several inches into the volume
occupied by the grout cartridge 320 before the cartridge wrapper
322 fails and releases the contained grouting material 305,
building up significant pressure within the grout 305. Once the
wrapper 322 bursts, the rotating bolt 202 continues to advance
through the grout 305, mixing its components and pushing the mixed
grout back along the length of the bolt 202 through the narrow
annulus 315 formed between the roof bolt 202 and the wall of the
pilot hole 318.
[0009] As indicated in FIG. 3C, the grout 305 then at least
partially surrounds the fully inserted bolt 202 and, upon curing,
bonds the bolt 202 to the roof material 313 with the intent of
enhancing the overall integrity of the mine roof 313. Less than the
complete length of the bolt 202 may be surrounded by the grouting
material 305 due to insufficient mixing and transport of the
grouting material 305 by the bolt 202. The relatively smooth
surface, even in a textured rebar (103, FIG. 1A), and blunt tip 206
are not configured to perform this mixing and transport.
[0010] Very weak roof conditions are increasingly being encountered
in underground coal mines. For instance, as discussed in Zhang et.
al., Abstract: "Design Considerations of Roof Bolting under Very
Weak Roof Conditions", to be presented at the 2006 SME Annual
Meeting and Exhibit technical presentation, the disclosure of which
is incorporated by reference herein, it was found that in the
Illinois Basin, the more easily mined reserves with more competent
roof rock are rapidly being depleted, and the higher quality, lower
sulfur coals are more strongly associated with weaker, laminated
roof rock. Roof bolting under less competent roof conditions in
underground coal mines often encounters difficulties not only
because the roof has very low inherent mechanical strength but also
because it composed of thin laminations of different rock
types.
[0011] Recent studies in mines with weak roof rock have shown that
traditional bolting procedures using standard fully grouted rebar
bolts may cause hydraulic fracturing of the roof rock due to the
build up of pressure in the grout exerted just prior to the rupture
of the grouting materials container, as shown in FIG. 3B. Examples
of such studies include Pile. J., et al., "Short-encapsulation Pull
Test for Roof Bolt Evaluation at an Operating Coal Mine",
Proceedings: 22nd International Conference on Ground Control in
Mining; WV, Aug. 5-7, 2003; Compton. C., et al., "Investigation of
Fully Grouted Roof Bolts Installed Under In Situ Conditions",
Proceedings: 24th International Conference on Ground Control in
Mining; WV, Aug. 2-4, 2005; and Campbell. R. N., et al.,
"Investigation into the Extent and Mechanisms of Gloving and
Un-mixed Resin in Fully Encapsulated Roof Bolts" Proceedings: 22nd
International Conference on Ground Control in Mining; WV, Aug. 5-7,
2003, the disclosures of which are incorporated by reference
herein.
[0012] As a consequence of hydraulic fracturing, grout may be
injected laterally into the roof external to the pilot hole (also
known as grout migration), thereby separating the rock layers and
reducing the length of bolt encapsulation within the grouting
material.
[0013] Loss of grout through lateral grout migration also has the
effect of reducing the length of the grout column, which has a
significant effect on the design assumptions and stability of mine
openings. Additionally, grouted bolts with reduced encapsulation
due to reduced length of the grout column may allow the body of the
bolt to come in contact with the mine environment with the
potential for corrosion and eventual degradation of the roof
support system. In some instances, as is the case of mines with
high levels of hydrogen sulfide inherent within the roof rock, the
corrosive effects are accentuated and the need for full
encapsulation of the bolts becomes even more important.
[0014] A field test program by the Inventors using different grout
types, insertion speeds, and annulus sizes was specifically
designed to characterize the forces required for standard, blunt
end bolt insertion. The tests consisted of pushing bolts having
blunt ends at constant speed, without rotation, into grout-filled
pilot holes in a mine roof (the substrate). A load cell was
installed between the drill head and the bolt to measure load. An
extensometer was used to measure bolt displacement. Values of load
and displacement were simultaneously measured.
[0015] The test plan called for three bolting systems employing
standard rebar bolts, two grout types and two insertion speeds.
Twelve combinations of these parameters are possible and two (2)
tests were to be performed for each combination for a total of 24
tests. The bolt systems were: (a) a #6 (0.75 inch diameter) bolt in
1.03'' diameter hole; (b) a #6 (0.75 inch diameter) bolt in 1.25''
diameter hole; and (c) a #7 (0.875 inch diameter) bolt in 1.375''
diameter hole all using 6-foot long standard headed rebar bolts.
The grout types tested were Minova LIF and Fasloc low viscosity,
both with a two (2) minute set time. Grout cartridges of 0.9,
1.125, and 1.25 inch diameters and appropriate total length were
used to match each of the bolting systems. The insertion speeds of
the bolts into the pilot holes were 4.5 and 7 inches per second.
These tests allowed measurement of insertion force and demonstrated
how the test parameters interact to generate the pressure front
ahead of the bolt tip. As expected, the force required to push the
bolt into the grout-filled borehole increased with the depth of
bolt insertion. The load curves observed were similar for the two
types of grout employed, and no significant difference in the load
ranges were recorded during the tests. However, in some instances,
the early generation of higher pressure triggered hydraulic
fracturing of the roof followed by resin loss, which in turn
reduced the observed length of bolt encapsulation.
[0016] Load (force) of insertion vs. depth was plotted for each of
the tests. All of the plots exhibited a common behavior, and three
distinct load regions were identified as indicated in FIG. 4. The
initial insertion load increased at a constant and relatively low
rate up to around 20 inches of insertion (Region I of the graph).
At this point, the load increased at an accelerated rate for a
short interval (Region II of the graph) after which the load rate
declined to a rate slightly greater than the initially observed
rate (Region III of the graph). Region I was well defined in most
of the tests. Regions II and III exhibited more variability and in
some cases overlapped.
[0017] The graph of FIG. 4 suggests that the following effects take
place. In Region I, there was a compression of the intact grout
cartridge with a Poisson effect on the cartridge. That is, as the
length of cartridge was compressed, it expanded within the hole
until the first region transition was reached. Since the cartridge
had now filled the hole, the pressure increased until the rupturing
strength of the cartridge wrapper was exceeded. Once the wrapper
ruptured, fluid flow of the grout began similar to flow of water in
a pipe, albeit the grout is much more viscous than water. The flow
rate remained constant since the speed of insertion was maintained
constant and the load increased proportionally to the length of
bolt insertion. In some instances, the early generation of higher
pressure triggered hydraulic fracturing of the roof followed by
resin loss, which in turn reduced the observed length of bolt
encapsulation.
[0018] The possibility of reducing grout viscosity to reduce
internal pressure during installation is not a practical solution
because a low viscosity grout could leak out of the hole during
bolt installation therefore negating any benefits. Additionally, a
grout with a viscosity lower than what is currently used would
contain a higher percentage of the most expensive grout components
and therefore not be considered as an economical solution to the
grout pressure reduction problem.
[0019] Mines have used an oversized borehole to prevent pressure
build-up. However, this solution is not optimal because additional
grout is necessary and the anchorage capacity of the bolt is
reduced. A method and apparatus is therefore needed to overcome the
difficulties of the prior art.
SUMMARY
[0020] The invention relates to modifications to and improvements
on existing grouted bolt design and practice, which have the aim of
improving bolt anchorage performance in all circumstances,
particularly where rock with low compressive strength or laminated
structure is encountered.
[0021] An exemplary embodiment of the invention includes a modified
geometry in the tip of a rock bolt first inserted into a pilot
hole. The modified geometry provides a physical means to facilitate
the flow of grout past the end of the bolt, promote distribution of
the grout in the annulus formed between the bolt and the pilot hole
and/or facilitate the rupture of the grout material container in
the pilot hole.
[0022] In another exemplary embodiment of the invention,
improvement in grouted rock bolt system performance is achieved by
modifying the tip of the rock bolt to have an auger shape, which
facilitates grout flow and mixing in the borehole allowing
increased anchorage capacity of each bolt by providing a longer
grout column, increases the effective thickness of the structure
formed by the bonding of the bolt in the supported roof, and
reduces the potential for corrosion of the bolt by reducing the
length of bolt exposed to the mine environment.
[0023] In another exemplary embodiment of the invention, the rock
bolt tip has a geometry modified to have a physical means for
facilitating rapid rupture of a sealed grout cartridge and thereby
reducing pressure build up of the grout within the cartridge.
Reducing internal grout pressure during bolt installation allows
reduction of the potential for hydraulic fracturing of the roof
rock, increases the effective length of bolt encapsulation by
preventing loss of grout into the roof rock, reduces bolt "gloving"
by preventing cartridge expansion caused by internal grout pressure
during installation, and improves bolt anchorage capacity as a
result of reduced bolt gloving and increased bolt
encapsulation.
[0024] The above and other structures, techniques and advantages of
the invention can be better understood based on a reading of the
following description in view of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A is a schematic representation of a grouted rock bolt
and anchoring technique.
[0026] FIG. 1B is a schematic representation of a mechanical
expansion rock bolt and anchoring technique.
[0027] FIG. 2 is a representation of the blunt insertion end of a
conventional grouted bolt.
[0028] FIGS. 3A, 3B, and 3C show steps stages in a conventional
installation of a grout anchored bolt in substrate, such as a mine
roof.
[0029] FIG. 4 is a graph of insertion load (force) vs. depth of
insertion representing a series of field test observations using
conventional rock bolts.
[0030] FIG. 5 shows a rock bolt having a modified geometry in
accordance with the invention.
[0031] FIG. 6 shows a rock bolt having a modified geometry in
accordance with the invention.
[0032] FIG. 7 shows a rock bolt having a modified geometry in
accordance with the invention
[0033] FIG. 8 shows a rock bolt having a modified geometry in
accordance with the invention
[0034] FIG. 9 shows a rock bolt having protuberances for improved
holding capacity, but without a modified tip geometry in accordance
with the invention.
[0035] FIG. 10 shows a rock bolt having protuberances for improved
holding capacity and a modified tip geometry in accordance with the
invention.
[0036] FIG. 11 is a graph showing grip factor vs. bolt type
illustrating the improved grip factor of rock bolts, both
conventional and in accordance with the invention.
[0037] FIG. 12 shows a rock bolt having protuberances for improved
holding capacity and a modified tip geometry in accordance with the
invention.
DETAILED DESCRIPTION
[0038] In the following detailed description, reference is made to
the accompanying drawings, which form a part hereof and show by way
of illustration specific embodiments in which the invention may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to practice the invention, and it
is to be understood that other embodiments may be utilized, and
that the structural, logical, and other changes may be made without
departing from the spirit and scope of the present invention. The
progression of method steps described is exemplary of the
embodiments of the invention; however, the sequence of steps is not
limited to that set forth herein and may be changed as known in the
art, with the exception of steps necessarily occurring in a certain
order. The terms "resin" and "grout" are used interchangeably
herein. While the invention is discussed primarily in relation to
mine roof reinforcement, it is suitable for reinforcing or
anchoring any drillable substrate and may be adapted in size
therefore.
[0039] Embodiments of the invention relate to a rock bolt for
reinforcing a substrate, for example, the roof of a mine. The rock
bolt has a modified geometry at the tip first inserted into a pilot
hole. The modified geometry provides a physical means to facilitate
the flow of grout past the end of the bolt, promote distribution of
the grout in the annulus formed between the bolt and the pilot hole
and/or facilitate the rupture of the grout material container in
the pilot hole. The rock bolts and methods of the invention can be
used with holes drilled and rock bolts formed in accordance with
the subject matter described in U.S. patent application Ser. No.
10/919,271, the entirety of which is incorporated by reference
herein. The invention will now be described with reference to the
drawings.
[0040] Various embodiments of the invention eliminate the blunt
insertion end of typical grouted bolts and utilize a modified bolt
tip geometry. This geometry can provide a smaller cross-sectional
area at the tip of the bolt, a pumping effect, or both, similar to
that of an auger, as the bolt is spun up into the hole during the
normal bolt installation process. This pumping effect promotes the
flow of grout through the annulus between the rock bolt and the
pilot hole thereby reducing the pressure gradient as the bolt is
inserted through the grout material, thereby minimizing the overall
maximum pressure within the pilot hole.
[0041] FIG. 5 shows a rock bolt 502 according to an exemplary
embodiment of the invention that has an auger shaped tip 508. As
this tip 508 rotates through grout material, the auger shape forces
the grout material to mix and travel down past the tip 508. The
grout material is pushed down along the length of the rock bolt 502
as the bolt 502 is rotated deeper into the pilot hole. The extreme
tip 506 of the rock bolt 502 has a reduced cross-sectional
area.
[0042] Another exemplary embodiment of the invention is shown at
FIG. 6. Although the extreme tip 606 of the rock bolt is blunted as
in a conventional bolt, the tip 608 below the extreme end has a
modified geometry. The auger shape of this modified geometry comes
into effect as the tip 608 is inserted into the grout material or
into the container holding the grout material. Once the container
is ruptured, the auger-shaped tip geometry mixes the grout and
pushes it down the length of the rock bolt 602.
[0043] The actual leading edge geometry of the bolt, auger pitch,
and other physical requirements of this rock bolt, as exemplified
in FIGS. 5 and 6, can be configured based on bolt diameter, pilot
hole diameter, grout viscosity, bolt insertion rate, and bolt
rotation rate. The goal of the tip geometry configuration is to
optimize the geometry with respect to these operational parameters
to maximize grout flow around the bolt and minimize the rate of
grout pressure increase within the pilot hole.
[0044] FIG. 9 shows an HRB-E rock bolt 902, such as is described in
U.S. patent application Ser. No. 10/919,271, which has protrusions
910 at its tip 908 for cutting a pilot hole groove, but without an
auger shaped modification. The HRB-E rock bolt 902 has an extreme
tip 906 that is flat. As the HRB-E rock bolt 902 is inserted in a
borehole, the protrusions 910 create a groove in the borehole wall
and produce rock cuttings that mix into the grout.
[0045] FIG. 10 shows an HRB-EP rock bolt 1002 having a tip 1008
modified in accordance with an embodiment of the invention to have
an auger shape for transporting grout material. The HRB-EP rock
bolt 1002 also has protrusions 910 and an extreme tip 1006 that is
flat.
[0046] FIG. 11 is a chart comparing average grip factor (the
anchoring force of an installed rock bolt) to bolt type for an
HRB-E rock bolt 902 as shown in FIG. 9, an HRB-EP rock bolt 1002 as
shown in FIG. 10, and Standard, DP103, and DP125 rock bolts. As
FIG. 11 shows, the HRB-EP rock bolt 1002 has a greater average grip
factor, e.g., 1.01 ton/in, than the other rock bolts. FIG. 11 shows
that the HRB-EP rock bolt 1002 with a tip 1008 having a pump
feature in accordance with an embodiment of the invention produced
more consistent anchorage capacity than the HRB-E rock bolt 902,
which is a similar bolt, but lacks the pump feature. Without
wishing to be bound by theory, it appears that the pump feature
directs the grout flow in a manner that enhances the mixing of the
rock cuttings and produces the observed results during testing.
[0047] The grout pumping feature of the rock bolts according to
various embodiments of the invention can be used in conjunction
with other helical rock bolt enhancements to improve grout flow and
reduce the pressure of bolt insertion ahead of the bolt, which may
cause loss of grout laterally into the strata. These enhancements
may include the addition of a grout cartridge puncturing feature,
as discussed below, and the use of rebar with a thread-like pattern
to promote the flow of grout in the direction of the bolt head and
reduce the pressure gradient within the grout.
[0048] In accordance with another exemplary embodiment of the
invention, the pressure gradient of Region II, shown in FIG. 4, is
reduced or eliminated by replacing the blunt, piston-like end of a
typical grouted rock bolt with a modified tip having an extreme tip
with a geometry that rapidly ruptures the grout cartridge. Earlier
cartridge rupture reduces the maximum pressure attained in Region
II, shown in FIG. 4, which is the interval of most the rapid grout
pressurization. Use of this new geometry helps prevent grout
pressure reaching a magnitude sufficient to fracture the
surrounding rock by rupturing the grout cartridge inside the
borehole. Experiments by the inventors at the San Juan Mine in New
Mexico show that early rupture of the cartridge prevents pressure
buildup and reduces the possibility of hydraulic fracturing of the
substrate into which the rock bolt is inserted, which would cause
grout loss and hinder full encapsulation.
[0049] According to an exemplary embodiment of the invention, the
tip geometry of the rock bolt 702 shown in FIG. 7 has a chisel
shape. This shape facilitates the rapid rupture of a grout material
container within a pilot hole upon insertion of the rock bolt
therein. Such a configuration maintains considerable strength of
the rock bolt tip 708 and can pierce the grout container whether
rotated or not.
[0050] Another exemplary embodiment of the invention is shown in
FIG. 8. The extreme tip 806 of the tip 808 of the rock bolt 802 of
this embodiment has multiple piercing features for a rupturing
geometry. This embodiment can pierce the grout container whether
rotated or not, but is preferably rotated during insertion.
[0051] The leading edge geometry of the rock bolt in accordance
with FIGS. 7 and 8 can be determined by the measured strength of
the grout material cartridge wrapper, the cartridge diameter, the
pilot hole diameter, the grout viscosity, the bolt insertion rate,
and the ability of available manufacturing processes to create a
specific geometry. The goal of these embodiments of the invention
is to optimize the modification of the bolt end geometry with
respect to these operational parameters to accelerate grout
cartridge rupture and minimize the ultimate grout pressure within
the pilot hole.
[0052] Another exemplary embodiment of the invention is shown in
FIG. 12, which shows a rock bolt 1202 incorporating a tip 1208
having an auger shape for transporting grouting material, an
extreme tip 1206 having a rupturing geometry with multiple piercing
features for rupturing a grout material container, and protrusions
1210 at its tip 1208 for forming a groove in a pilot hole wall.
[0053] Various embodiments of the invention have been described
above. Although this invention has been described with reference to
these specific embodiments, the descriptions are intended to be
illustrative of the invention and are not intended to be limiting.
Various modifications and applications may occur to those skilled
in the art without departing from the spirit and scope of the
invention as defined in the appended claims.
* * * * *