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.

Description

FIELD OF THE INVENTION

The present invention relates to substrate supporting bolts and methods of using such bolts.

BACKGROUND

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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

FIG. 1A is a schematic representation of a grouted rock bolt and anchoring technique.

FIG. 1B is a schematic representation of a mechanical expansion rock bolt and anchoring technique.

FIG. 2 is a representation of the blunt insertion end of a conventional grouted bolt.

FIGS. 3A, 3B, and 3C show steps stages in a conventional installation of a grout anchored bolt in substrate, such as a mine roof.

FIG. 4 is a graph of insertion load (force) vs. depth of insertion representing a series of field test observations using conventional rock bolts.

FIG. 5 shows a rock bolt having a modified geometry in accordance with the invention.

FIG. 6 shows a rock bolt having a modified geometry in accordance with the invention.

FIG. 7 shows a rock bolt having a modified geometry in accordance with the invention

FIG. 8 shows a rock bolt having a modified geometry in accordance with the invention

FIG. 9 shows a rock bolt having protuberances for improved holding capacity, but without a modified tip geometry in accordance with the invention.

FIG. 10 shows a rock bolt having protuberances for improved holding capacity and a modified tip geometry in accordance with the invention.

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.

FIG. 12 shows a rock bolt having protuberances for improved holding capacity and a modified tip geometry in accordance with the invention.

DETAILED DESCRIPTION

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.