CA1335937C - Method for installing rock bolts - Google Patents

Abstract

A pipe bolt having a central bore is inserted into a hole formed in an underground excavation. A high density grout is injected into the central bore of the pipe bolt to cause the high density grout to travel through the bore, out of the pipe bolt and past the sides of the bolt filling the space between the bolt and the hole. The high density grout allows for improved bonding strength concomitant with reduced porosity.

Description

J--METHOD FOR INSTALLING ROCK BOLTS

TECHNICAL FIELD

The instant invention relates to underground excavations in general, and more particularly, to a method for instfllling pipe bolts into the roof of the excavation.

BACKGROUND ART

Modern mining techniques include the utilization of rock/roof bolting for reinforcement.
Rock bolts are commonly used to secure the ground by binding together a discontinuous rock mass. Essentially, the bolts are inserted into long holes previously drilled into the rock. The bolts may additionally include an anchoring device, disposed at the end, inserted fully into the hole to hold the bolt within the hole.
A bearing plate is disposed about the end of the bolt extending out of the hole and placed against the rock surface. Tubular rock bolts are also employed. A bonding agent such as a relatively liquid, cementitious material (such as low density grout) or a resin may be forcefully inserted into the annular area between the rock bolt and $~

the hole. The bonding agent eventually exits from the hollow interior of the bolt. A plug prevents the grout from dripping out from the hole.
Often the bolt is tensioned within the hole before the introduction of the bonding agent. By placing the bolt in tension usually by the action of a nut affixed to the extended end of the bolt, the ground is placed in compression binding the discontinuous rock mass together. The addition of the bonding agent between the bolt and the hole protects the bolt from corrosion and provides an additional anchoring effect along the entire length of the bolt.
In addition to providing structural integrity to the roof, the bolts are often used with screens to provide protection from the inevitable falling rock.
The vitally important key to a properly secured grouted rock bolt is the absence of voids between the bolt and hole.
Unfortunately, most current rock bolt designs, by initially inserting the common relatively liquid grout up around the exterior of the bolt, may leave sizeable voids. The operator knows that when the grout begins to flow out through the center of the hollow bolt, he may cease pumping the grout. However, due to the watery nature of the grout, circling upwardly about the exterior of the bolt, it has been determined that dangerous pockets of air (voids) may form, thereby reducing the integrity of the bond between the bolt and the hole. There have been repeated instances where bolts have fallen out of the hole due to poor adhesion. Often over a period of time, a previously bolted and screened roof needs to be reconditioned because the conventional mechanical bolts and screens are corroding.
This necessitates a second pass of new bolting and screening - an expensive undertaking.
As alluded to beforehand, conventional bolts may additionally use a mechanical anchor to grip the rock. In other instances grout or resin is injected about the exterior of the bolt and allowed to exit through a hollow bore therein. The main drawback here is that voids form between the bolt and the rock. There are other bolt designs that have attempted to introduce conventional thin grout or resin through the interior of the bolt and then allow it to flow out about the exterior of the bolt. See, for example, CAn~di~n Patent 504,917; U.S. Patents 4,461,600; 2,682,152, U.K. Patent 831,740; German Offenlegungschrlft 3529850Al and Japanese Patent 59-179927. However, as far as appllcant ls aware these methods have not experlenced wldespread acceptance slnce, as before, the grout ls too thln to have an acceptable bondlng strength.
SUMMARY OF THE INVENTION
Accordlngly, there ls provlded a method for securlng roofs ln underground excavatlons by ln~ectlng a thlck, hlgh den-slty grout lnto the central bore of an anchor bolt. The hlgh denslty grout forms a certlflably secure adheslve bond, wlth few volds, between the bolt and the hole. The method easlly lends ltself to one pass developments slnce the method develops rela-tlvely qulck holdlng power.
The lnventlon may be summarlzed as an lmproved method for securlng underground excavatlons by lnsertlng mechanlcal bolts lnto holes drllled lnto the surface of the excavatlons, the lm-provement comprislng: a) lnserting a smooth walled plpe bolt having a plate at a proxlmal end of the plpe bolt lnto a hole, a dlstal end of the pipe bolt located at a predetermlned depth wlth-ln the hole, an annular space formed between the plpe bolt and thewall of the hole; b) lntroducing the plate lnto substantlal con-tact wlth the surface of the excavatlon ln the vlclnlty of the openlng of the hole; c) ln~ectlng 0.3/1.0 water/cement ratlo hlgh denslty grout lnto the plpe bolt to flow through the plpe bolt and to exlt therefrom, the hlgh density grout flowlng back towards the proxlmal end of the plpe bolt and slmultaneously fllllng the space 3a 61790-1622 between the bolt and the hole; and d) contlnulng the ln~ectlon of the hlgh denslty grout lnto the plpe bolt until the high density grout extends about the plate.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross sectional elevatlon of a pipe bolt in accordance wlth the lnstant lnventlon.
Flgure 2 is a cross sectional elevation of alternative embodiment of a pipe bolt ln accordance wlth the lnstant lnven-tlon.
Flgure 3 ls a cross sectlonal elevatlon of an alterna-tlve embodlment of a pipe bolt in accordance with the instant invention.
~ ~K~ MODE FOR CARRYING OUT THE INVENTION
Figures 1, 2 and 3 show alternative pipe bolt designs for use with the instant lnventlon. Common components shall bear the same numerical designations when possible.
Taking each pipe bolt in order. Figure 1 depicts a tubular pipe bolt 10 disposed within a hole 12. The pipe bolt 10 includes a tapered end 14 and an indentatlon 16. A plate 18 registered against the indentation 16 is initially placed flush against the opening 20 of the hole 12. A central bore 22 extends through the bolt 10. The plpe bolt 10 ls designed for applica-tions where a tensioned bolt is not required.

~4~ 1335937 PC-2l99 Figure 2 depicts an alternative tubular pipe bolt 24 that may be tensioned by turning the bolt as shown in the direction of directional arrow A located at the end of the bolt 24. Disposed within the hole 12 is a conventional mechanical anchor 26. At least a pair of apertures 28 passing through the wall of the pipe bolt 24 are preferably disposed towards the end of the bolt 24.
Figure 3 depicts an alternative tubular pipe bolt 30. The bolt 30 includes a threaded lower section 32 and a correspondingly threaded nut 34. The bolt 30 may be tensioned by rotation of the nut 34 in the direction of arrow A. Besides providing tension to the bolt 30, the nut 34 permits the attachment of a screen (not shown) and restraining it in place.
Regardless of the pipe bolt embodiment selected, a high density cementitious grout 36 is injected up through the central bore 22 and either out through the tapered end 14 of the bolt 10 or through the apertures 28 in bolts 24 and 30. The grout 36 is continuously pumped until it begins to protrude from the opening 20.
In this fashion the grout 36 will completely fill the annular space between the bolt and the walls of the hole 12.
In contradistinction to conventional designs and techniques, it is necessary to stipulate that the density of the grout have a ratio of about 0.3 water to 1.0 cement or thicker. It is preferable to utilize Portland 10 cement. The density of the grout 36 should have the consistency of toothpaste or the like. By utilizing this thick high density grout, grouting of the pipe bolt may be repeatedly done with the expectation that dangerous voids will not occur, thereby increasing the integrity of the bolt-grout-hole bond.
The difficulty in prior art designs is that with conventional watery (0.5:1.0 and greater) grout initially introduced about the exterior of the roof bolt, there was always the danger of voids forming. Additionally, with the watery grout dripping down from the bolt a plug was required. No plug or collar is necessary with the instant invention and as a result of tests it has been shown that the formation of debilitating voids has been greatly reduced.
Indeed, if the grout is of less density, it will not stay in the hole leading to a potentially fatal weak bond strength.

.

The invention and the manner of applying it may be better understood by a brief discussion of the principles underlying the invention.
An operator drills a sized hole by any appropriate method.
He then places the pipe bolts 10, 24, 30 into the hole so that the plate 18 is sitting flush with the hole 12. The plate 18 is not a plug per se but serves to hold screening in place as well as acting as means for distributing rock support over a large area.
Screening, if used, is disposed between the rock surface and the plate 18.
The non-tensioned bolt 10 may have a slight bend so as to hold it in place until the grout sets. In the disclosed versions 24, 30, the mechanical anchor 26 is caused to expand against the interior of the hole. If desired the bolts 24, 30 may be tensioned by rotating them in the direction "A".
After physical installation of the bolt into the hole, the necessary grout 36 is continuously injected, via a suitable pump, into the central bore 22 until the operator sees a donut of thick grout emerging from around the plate 18. This is evidence of a full column of self-supporting grout and the displacement of any water in the hole 12. Directional arrows B indicate the flow of the grout 36 through the bolt and the hole. Indeed, once the grout 36 sets, the bond is usually so strong and reliable that the mechanical anchor 26, if it is employed, essentially becomes redundant.
The pipe bolts 10, 24, 30 may be six to ten feet (1.8-3.0 meters) long and be comprised of any suitable hard metal pipe such as schedule 40 black iron tubulars having an outside diameter of about one inch (25 mm).
The grout 36 may be mixed at a ratio of about twelve liters of water to forty kilograms of Portland 10 cement so as to have at least a toothpaste like consistency and the preferred maximum water weight 0.3/1.0 water/cement ratio.
Inasmuch as the grout is quite thick and thixotropic, a suitable pump is necessary. Conventional grout pumps suitable for standard watery grout are not suitable for properly pumping the instant high density grout over long periods of time. They tend to fail because of the stresses involved.

A grout pump especially useful for pumping this high density grout is the M.A.I. (trademark) 3500 pump. This pump can supply the grout to the pipe bolt at a pressure of sixty to one hundred pounds per square inch (414-689 kPa) and is capable of extending the grout up through the central bore 22 to the requisite height through the tapered end 14 or the apertures 28 and out about the plate 18.
A number of experimental trials were conducted to ascertain the efficacy of the instant invention.
Relating a set of trials:
To test the flow characteristics of a 0.3/1 weight ratio, plain water/cement grout in point anchored pipe bolts, single and double 1/8 inch (0.32 cm), 1/4 inch (0.64 cm) and 3/8 inch (0.96 cm) holes were drilled near the mechanical anchor. The holes were drilled normal to the pipe axis and where two holes were drilled, they were diametrically opposite one another.
Equipment 1. One 6 foot (1.8 m) long piece of clear plastic tubing with an internal diameter of 1.375 inches (3.5 cm) 2. Two lengths of the proposed pipe bolt pipe I.D. = 0.563 inch (1.4 cm) O.D. = 0.938 inch (2.4 cm) 3. A Spedel~ 6000 grout pump and mixer (for actual applications, the M.A.I. pump is preferred) 4. Portland type 10 cement 5. A protractor to measure the inclination of the clear plastic tube 6. An electric drill and drill bits in sizes 1/8 inch (0.3 cm), 1/4 inch (0.64 cm) and 3/8 inch (0.96 cm) 1335g37 _7_ PC-2199 Procedure 1. The clear plastic tube was plugged at one end and mounted on a wall in a vertical position with the plugged end up. The open bottom of the tube was more than 6 feet (1.8 m) above the floor to permit insertion of trial pipe bolts.

2. One pipe bolt was plugged at one end. A 1/8 inch (0.32 cm) bit was used to drill through the pipe about 3 inches (7.6 cm) below the plugged end to form two diametrically opposed 1/8 inch (0.32 cm) holes.

3. The pipe bolt was inserted into the clear plastic tube and held in place.

4. A batch of 0.3/1.0, water/cement, Portland type 10 grout was mixed and pumped up through the pipe bolt.
The behavior of the grout was observed while the grout extruded out the two 1/8 inch (0.32 cm) diameter holes and filled the clear plastic tube.

5. When the clear plastic tube was full of grout, pumping was stopped. The time to fill the clear plastic tube was recorded. The pipe bolt was then pulled slowly out of the clear plastic tube to examine the grout for air bubbles not observed through the clear plastic tube.

6. The pipe bolt and the clear plastic tube were flushed clean of grout.

7. The clear plastic tube was rotated down to a horizontal position and steps 3, 4, 5 and 6 of the above-referenced procedure were repeated.

` 1335937 8. The clear plastic tube was returned to the vertical position at an angle of 90 degrees above the horizontal.

9. The second pipe was plugged at one end and a 3/8 inch (0.96 cm) hole was drilled through just one side of the pipe at a distance of about 3 inches (7.6 cm) below the plug.

10. The pipe with the single 3/8 inch (7.6 cm) hole was inserted in the clear plastic tube.

11. 0.3/1, water/cement, Portland type 10 grout was pumped up through the pipe bolt. The behavior of the grout was observed while the grout extruded out the single 3/8 inch (7.6 cm) diameter hole and filled the clear plastic tube.

12. When the clear plastic tube was full of grout, pumping was stopped. The time to fill the clear plastic tube was recorded. The pipe bolt was then pulled slowly out of the clear plastic tube to examine the grout for air bubbles not observed through the clear plastic tube.

13. The pipe bolt and the clear plastic tube were flushed clean of grout.

14. The clear plastic tube was rotated to 70, 45, 20 and 0 degrees above the horizontal. At each inclination, steps 10 through 13 were repeated.

15. A 1/4 inch (0.64 cm) bit was used to enlarge the two 1/8 inch (0.32 cm) holes in the first pipe.

16. The flow characteristics through double 1/4 inch (0.64 cm) holes were observed for clear plastic tube inclinations of 90, 70, 45, 20 and O degrees above the horizontal, following the procedures outlined in steps 10 through 13.

17. A 3/8 inch (0.96 cm) bit was used to enlarge the two 1/4 inch (0.64 cm) holes in the first pipe.

18. The flow characteristics through double 3/8 inch (0.96 cm) holes were observed for clear plastic tube inclinations of 90, 70, 45, 20 and O degrees above the horizontal, following the procedures outlined in steps 10 through 13.

19. With respect to the two 1/4 inch (0.64 cm) holes and the two 3/8 inch (0.96 cm) holes, the above procedures were repeated with a non-clear plastic tube. After the high density grout had set, the entire unit (tube and bolt) was sliced lengthwise and checked for air inclusions. No significant air inclusions were present in the samples tested.

Observations Pump 25Number/Size Inclination Above Horizontal Time Holes 90 70 45 20 0 secs 2 - 1/8 (0.32)Air Included -- -- --Air Included 10 Air trapped, Too much back pressure 1 - 3/8 (0.96) Good Good Good GoodGood 8 Too much back pressure -Number/Size Inclination Above Horizontal Time Holes 90 70 45 20 0 secs 2 - 1/4 (0.64) Good Good Good Good Good 3 Very little back pressure 2 - 3/8 (0.96) Good Good Good Good Good 3 Very little back pressure 1. The flow of the grout in the clear plastic tube was not as good coming from the side holes as was previously observed in earlier tests where the grout was free to flow out of the end of the pipe bolt. The sideways injected grout showed a tendency to pick up a little of the air trapped above the grout outlets. It was felt that the amount of air should not affect overall grout performance except for hole sizes less than 1/4 inch (0.64 cm).

2. The centering action of the grout, observed in earlier tests with end injection, was not evident with side injection. However this is not considered a difficulty.

Conclusions and Recommendations 1. Side injection of the grout through either double 1/4 inch (0.64 cm) or double 3/8 inch (0.96 cm) holes presents little pumping difficulty.

2. Side injection of the grout does entrap some air. It is felt, however, that the limited amount of entrapped air will not significantly affect bonding performance.

3. The inclination of the hole has no bearing on the flow characteristics of the grout.

133~937 4. The optimum grouted pipe bolt, from a pure grout quality standpoint, should use end injection of the grout rather than the side injection method used in these tests when the mechanical anchor is not deemed necessary.

5. It appears that pure cement is preferable to cements with expanding agents and curing accelerators. Tests indicate that fast setting cements cause high temperatures and swelling which causes the grout to fail. It is preferred to grout the pipe bolts within a few days of their installation and before any significant ground activity may be expected to occur.

While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

Claims (10)

1. An improved method for securing underground excavations by inserting mechanical bolts into holes drilled into the surfaces of the excavations, the improvement comprising:
a) inserting a smooth walled pipe bolt having a plate at a proximal end of the pipe bolt into a hole, a distal end of the pipe bolt located at a predetermined depth within the hole, an annular space formed between the pipe bolt and the wall of the hole;

b) introducing the plate into substantial contact with the surface of the excavation in the vicinity of the opening of the hole;

c) injecting 0.3/1.0 water/cement ratio high density grout into the pipe bolt to flow through the pipe bolt and to exit therefrom, the high density grout flowing back towards the proximal end of the pipe bolt and simultaneously filling the space between the bolt and the hole; and d) continuing the injection of the high density grout into the pipe bolt until the high density grout extends about the plate.

2. The method according to claim 1 wherein the high density grout exits from the distal end of the pipe bolt.

3. The method according to claim 1 wherein the high density grout exits from at least one aperture extending through the side of the pipe bolt.

4. The method according to claim 1 wherein the high density grout is comprised of water and cement.

5. The method according to claim 1 wherein the high density grout is thixotropic.

6. The method according to claim 4 wherein Portland cement is mixed with water to form the high density grout.

7. The method according to claim 4 wherein an anchor, disposed at the distal end of the pipe bolt, is expanded against the interior of the hole prior to the introduction of the high density grout into the pipe bolt.

8. The method according to claim 1 wherein the pipe bolt is tensioned within the hole before the high density grout is introduced into the pipe bolt.

9. The method according to claim 8 wherein a nut is rotated about the pipe bolt.

10. The method according to claim 1 wherein screening is disposed between the surface of the excavation and the plate.