GB2082655A - Method of anchoring a reinforcing element in a rock substrate - Google Patents

Description

SPECIFICATION Method of anchoring a reinforcing element in a rock substrate This invention relates to the art of rockbolting and is concerned with a method of anchoring a reinforcing element such as an anchor bolt in a rock substrate, for example an underground mine roof or gallery. Two principal non-mechanical methods of anchoring a reinforcing element in a borehole in a substrate have hitherto generally been utilised, one based on hydraulic cement anchoring systems and the other on hardenable synthetic resin anchored systems, generally an unsaturated polyester resin system. Both anchoring systems have been employed as cartridged systems, in which the reactive components of the system are both packaged in a frangible cartridge, generally in separate compartments, and the cartridge is introduced into the borehole and then broken and the contents mixed in situ by means of a rotating reinforcing element to form a self-setting anchoring composition around the reinforcing element. The foregoing cement and resin anchoring systems have certain disadvantages, for example an absence of elasticity of the anchorage, the problem of shrinkage, and the problem of the quality of the anchorage. More particularly, if the rock has cracks, with known cartridges either the cracks are not filled sufficiently, and the quality of the anchorage suffers as a consequence, or the cracks are overfilled with the result that there is loss of anchoring composition in these cracks. Moreover, given that it is necessary to use a cartridge containing the various constituents of the anchoring composition, production is lengthy and difficult, whilst, on account of the nature of the composition itself, at the moment of use there are encountered problems relating to solvent toxicity, risk of flammability and inconvenience of use, e.g. necessity of wetting certain inorganic cement systems before use, and absence of rigidity of some cartridges. It has now been found that, surprisingly, it is possible to use as an anchoring material a solid hot-melt adhesive which can be introduced into the borehole before supplying it with the amount of heat required to melt it, whereupon it is left to solidify around the reinforcing element to be anchored which is introduced, whilst rotating, into the borehole and brought into contact with the hot-melt adhesive so as to cause the latter to melt as a result of the heat produced by the rotating reinforcing element. It has been observed that the anchorage produced in this manner retains a certain elasticity which is essential in the application considered; displays substantially no shrinkage, contrary to current fillers; and fills the cracks in the rock in precisely the right amount, since the hot-melt adhesive will penetrate the cracks when it is in the molten state, but will solidify quickly enough so that it does not penetrate too far. Losses, or the opposite disadvantage of insufficient penetration of the cracks, are thus prevented. Hot-melt (or thermofusible) adhesives have been known for approximately 20 years. Their use in rock-bolting has been advocated, but only in the molten state, that is, the hot-melt adhesive is introduced in the molten state directly into the borehole, by a feed tube which is generally electrically heated. This method of rockbolting is described in, for example, German Auslegeschrift 1,116,619 and French Patent Specifications Nos. 1,285,087 and 1,253,773. This known technique involves expensive and complex equipment, inconvenience of use at the bottom of an underground mine, and problems arising from solidification of the hot-melt adhesive to a certain extent before the reinforcing element is introduced, whilst the cracks are not filled correctly on account of the liquid character of the injected adhesive and on account of the high pressure which must be applied, which results for the most part in unnecessary adhesive loss in the cracks. According to one aspect of the invention, there is provided a method of anchoring a reinforcing element in a rock substrate, which comprises introducing into a borehole drilled in the rock substrate a frangible cartridge comprising a solid hot-melt adhesive and thereafter bringing a rotating reinforcing element into contact with said cartridge so as to cause the adhesive to melt as a result of the rotation of the reinforcing element and thereby form a molten grouting adhesive in the annular space between the reinforcing element and the wall of the borehole which when allowed to solidify anchors the reinforcing element in the borehole. According to another aspect of the invention, there is provided for use in the foregoing anchoring method a cartridge comprising a solid hot-melt adhesive in substantially cylindrical form. Thus, in the method according to the invention, solid hot-melt adhesive, preferably in circular cylindrical form, is used; the adhesive is placed in position at the bottom of the borehole where it can conveniently remain for a certain time (which is not the case with the prior method), after which the reinforcing element to be anchored, rotating at a high speed, penetrates as far as the bottom of the adhesive, the rapid rotation transmitted to the reinforcing element inducing, in proportion to the forward motion of the latter, melting of the hot-melt adhesive, which is thus caused progressively to fill the cracks, to surround the element and then to solidify in situ when the element ceases to rotate. It will be noted that, with the method according to the invention, there is no risk of the reinforcing element being blocked by any premature solidification of the adhesive before reaching the bottom of the hole on account of the fact that the faster the rotation, the more rapid is the fusion. Thus, with the method of the invention it is always certain that the element will be driven completely into the borehole. Conversely, in the prior methods, the element could be blocked through premature hardening of the anchoring material, which then made it necessary to saw off that portion of the element projecting from the mouth of the borehole, quite apart from the fact that the quality and the mechanical resistance of the anchorage were naturally greatly affected. Another considerable advantage of the method according to the invention is that it is possible to select annular spaces between the reinforcing element and the wall of the borehole in the rock face which are smaller than in the case of the previous methods on account of the particular penetration characteristics of the element in the hot-melt adhesive-based anchoring material, penetration which, as has been sen above, is a great deal easier than in the previous methods. The fact that the size of this annular space is reduced is of considerable importance for the user when the cost of the anchorage is considered. In one embodiment of the invention, the hot-melt adhesive is employed in the form of a cartridge comprising a hollow tubular casing containing the solid hot-melt adhesive, which may be in particulate form, e.g. granules or pellets. The method according to the invention can be applied with the aid of hot-melt adhesives belonging to various chemical classes, such as:- ethylene/vinyl acetate copolymers, polyamides, polyesters, and polyolefins. Good results have been obtained with a hot-melt adhesive commercially available from Societe Adhesifs et Insonorisants Modernes (S.A.I.M.) of Cosne-sur-Loire, (France) under the trade name "DYNAFUS 669". This is a hot-melt adhesive based on ethylene/vinyl acetate polymers (E.V.A.). Its density at 20[deg]C is 0.94, its viscosity at 180[deg]C is 100 to 120 poise, and its fusion value at 125[deg]C is 140 to 180. The method according to the invention can be used in all types of rocks and with the aid of reinforcing elements conventionally used, such as metallic anchor bolts, or bolts of polyester plastics material or epoxy reinforced with glass fibres, or wooden dowels. According to the invention, particular anchoring compositions which give clearly better results and which are admirably adapted to the specific problems relating to the technique in question have also been developed. More particularly, these new compositions allow much faster rockbolting cycles, and can be made to be unbreakable or not readily breakable, these two characteristics being of considerable industrial importance. Moreover, the fillers which are advantageously present therein enable the cartridges comprising said anchoring compositions to be produced more economically. The improved anchoring compositions are hot-melt adhesive-based compositions, optionally filled, for the anchoring in rapid cycles of reinforcing elements such as anchor bolts in for example mine galleries, characterised in that they comprise: (a) a mixture of at least two hot-melt adhesive materials selected from: (i) thermoplastic ester resins, and (ii) polymers of vinyl acetate and ethylene, the mixture containing at least one material from each category, and (b) optionally a mineral filler, which may be present in an amount of up to 2/3 of the total weight of the components (a) and (b). The following products have been used as components

The fluidity value enables the relative viscosity of these two products to be determined (ELVAX 4310 being more fluid than ELVAX 4260).

This is a copolymer of vinyl acetate (39-42%) and ethylene (EVA). Fluidity value 48-66 g/10 mins. As the mineral filler (b), there has been used NE 34 sand, commercialized by the SIFRACO company, having a grain diameter of approximately 75-425 . Other suitable fillers are waxes, e.g. microcrystalline waxes, and chalk. Whilst it is not essential to use a filler, nevertheless the latter improves the thermal phenomena when the anchor bolt is introduced into the cartridge placed at the bottom of the drilled borehole, and enables the cost of the anchoring composition to be greatly reduced. In this respect, it is surprising to find that the proportion of filler which can be present can be in the region of up 2/3 of the weight of the composition without the latter becoming unusable, since it is well known that when hot-melt adhesives are filled, they must not be overfilled for fear of losing the adhesive properties. Hence the Du Pont guide relating to their ELVAX products recommends only 10 to 30% of fillers.

It is also surprising to ascertain that by varying slightly the relative proportions of components (a) (i) and (a) (ii), it is possible considerably to modify certain properties of the anchoring compositions, more particularly to make them almost or completely unbreakable. The following Examples illustrate the invention. In these the general anchoring technique is the same. EXAMPLE 1 Laboratory bolting test The aim of this test was to examine the quality of the anchorage. A high density polyethylene tube having an internal diameter of 28mm was used. Three cylindrical cartridges of adhesive (diameter 25mm, length 300mm) in the solid state, based on DYNAFUS 669 were introduced intothetube. The metallic bolt which was then introduced had a diameter of 20 mm and was a castellated rod 2.20m in length ("WEMA" bolt). A pneumatic drill (type MEUDON 860) (7 bars)) was used with a guide (type MONTABERT) which enabled 800-1000 r.p.m. to be reached with a 90 kg thrust. Under the above conditions, the rotating bolt took approximately 2 minutes to reach a depth of 2 m. The quality of the anchorage was then noted by cutting the lateral wall of the polyethylene tube, which exposed the anchorage. After 30 minutes, it was established that the bolt was encased in the plastics tube in a homogeneous manner. EXAMPLE 2 Pulltests by means of a jack This was also a laboratory test which produced a quantitative measurement. There was used in place of the plastics tube a metallic tube 28/34 mm in diameter and 1 m in length, having at the open end a plate for seating a jack. As in the preceding Example, the boltwas a "WEMA" bolt 20 mm in diameter, castellated, but only 1.15 m in length. Two cartridges based on DYNAFUS 669 (diameter 25 mm; length 300 mm) were used. Placing was carried out by means of a MEUDON 860 P (7 bar) pneumatic drill; the thrust was 50 kg. The bolt took approximately 3 minutes to penetrate the two cartridges at a speed of approximately 950 r.p.m. A tensile test was carried out, 24 hours after the bolt had been placed, by means of a hollow hydraulic jack. The results of this test are shown in the table below. In this table, the percentage of elongation (which is in fact due to an unknown proportion not only to the elongation of the bolt, but also to a certain slipping of the bolt relative to the filler) of the bolt relative to its initial length as a function of the extraction load is set forth.

The test had to be abandoned towards 11,000 kg when the metallic tube was subjected to too great a torsional strain. EXAMPLES 3 TO 8 The same procedure was used in this Example as in Example 1, although different types of hot-melt adhesive were used. Table I which is set out hereinafter shows the essential characteristics of these adhesive resins, as well as the results obtained. It can be seen that all the results obtained are comparable with that obtained in Example 1 above. No problem of shelf life was encountered, which is decisive advantage for the skilled worker. On the other hand, several weeks after the bolt had been installed the anchorage still displayed remarkable elasticity, whereas this characteristic is lacking in the polyester and cement fillers currently used. It should also be noted that it was possible to produce in the laboratory small anchorages of 2 to 100 cm, but that, of course, the invention can also be used in commercial practice with, for example lengths which may be between 1 and 5 m. The composition of the hot-melt adhesive composition can be modified by inorganic fillers, preferably glass fibres, or mineral fillers such as chalk, kaolin, microspheres, or aluminium powder. Such fillers more particularly mineral fillers, by increasing the friction, enable the liberation of heat to be increased. However, care must be taken not to introduce too may fillers which could make the hot-melt adhesives brittle and hence lower its softening point to a value which could become incompatible with the high temperature which prevails in underground mines, and which may be in the region of 40[deg]C.Moreover, this is, without a doubt, one of the reasons why, although they have been known for 20 years and have been used in the anchoring of dowels and anchor bolts whilst in the heated and molten state, there has until now been no thought of using them in the solid state in spite of the obvious advantages obtained which are, moreover, surprising in themselves, more particularly with regard to the resistance to extraction. The following Examples relate to improved adhesive compositions.

EXAMPLES A, B, C, D Compositions (1)to (4) shown in Table II below were prepared by mixing, fusion and cooling. These compositions are of the preferred fillertype. An anchoring test, the parameters of which also appear in Table II, was carried out. A pull test was then carried out. This test was carried out in a known manner with the aid of a jack. Measurement was carried out 1 hour after the bolt had been placed. It can be seen that the four products have excellent substantially homogeneous properties.

EXAMPLE E Composition (5) shown in Table II below, of the non-filled type, was manufactured as in Example 1. The same placing and extracting operations were carried out. It can be seen that the extraction characteristic is appreciably superior. However, if the cost of the composition is also taken in consideration, composition (4) will be preferred.

EXAMPLES F AND G Laboratory tests for bolting into concrete test pieces were carried out on compositions (4) and (5) of Table II, which contained a mixture of adhesive materials. The results are collated in Table III below. These results reveal the shortness of the installation time relative to the cartridges hereinabove described which contained a single adhesive material, if reduced to comparable values. This represents a decisive industrial advantage which is not obtained if the components based on (a) (i) or (a) (ii) alone are used, with or without a filler.

EXAMPLE H An anchor bolt was placed using a cartridge of composition (4) under the following conditions:-

A pull test after 24 hours resulted in the bolt breaking under 18 tonnes.

EXAMPLES I, J, K, L, M By modifying slightly the proportions of components (a) (i) and (a) (ii), itwas possible to modify to a considerable, and hence surprising, extent certain physical properties of the cartridges, more particularly to make them semi-flexible or completely flexible and therefore substantially unbreakable.

A slight decrease in the proportion of component (a) (i) can be seen to be expressed by an unrelated increase in flexibility and also by a reduction in the adhesion factor. It can also be seen that with the same amount of FORAL 85, a decrease in the amount of ELVAX 4310 and an increase in that of ELVAX 40 P increases the flexibility of the composition. EXAMPLES N AND 0 The anchoring compositions according to the invention have the advantage of being usable in the form of a mixture of granules, the composition then being restricted to a simple mixture of components of type (a) (which are products which are usually granulated) and optionally type (b). In spite of the particulate nature of the composition, when the bolt is introduced into the cartridge there is obtained a correct mixture and hence the properties desired for anchorage. N: The following components were mixed in a glass tube 10 mm in diameter and 100 mm in length: STAYBELITE ESTER 10 (granules) 50% by weight ELVAX 4260 (granules) 50% byweight. An installation test was carried out as indicated in Table II. The adhesion factor was 64 kg/cm . O : The procedure described in Example N was followed using the following composition: STAYBELITE ESTER 10 (granules) 67% by weight ELVAX 4260 (granules) 33% by weight Introduction time 2 minutes 5 seconds Adhesion factor 64 kg/cm By way of comparison with all the above tests, the adhesion factor of the concrete before slipping, on the steel, is in the region of 30 kg/cm . Returning to the above tests, it can be seen that component (a) (i) allows the mechanical resistance and also brittleness on impact as well as the characteristics of ease of penetration of the bolt to be controlled, and that component (a) (ii) allows the final flexibility of the product to be controlled.

EXAMPLE P Industrial production of the anchoring cartridges according to the invention can be carried out by using a polystyrene thermoplastics tube which constitutes the external casing of the cartridge. This tube has the following advantages:- it does not allow the anchoring cartridges to adhere to one another when packaged together in a box; it allows a certain amount of flexibility in the pneumatic filling of the drilled holes; it allows, if required, pre-heating of the anchoring cartridges, for example, a box of hot-melt adhesive-based anchoring cartridges can be placed on the motor of a bolting device. The advantage of such pre-heating is to reduce the duration of the installation cycle by approximately half. It is also possible to consider pre-heating the anchoring bolt for the same purpose. EXAMPLE Q (comparative) A composition with 25% by weight of FORAL 85 and 75% of sand gives poor results : too great a fluidity, lack of cohesion, too much decantation of sand, mediocre anchorage properties (under the conditions of the installation test of Table II : introduction time 4 mins., pull test: 32 kg/cm ). This is attributable to the lack of combination with a product of the ELVAX type.

EXAMPLE R Mine test There was used a jumbo drill type B26C from the Societe-de-Mecanique de Moutiers having the following performance figures: rotational speed empty : 920 r.p.m. rotational speed loaded : 580 r.p.m. torque : 12.5 mkg at 105 bars thrust: 950 kg. (1) Operating conditions: diameter of drilled hole : 22 mm bolt diameter : 18 mm smooth steel (Artop type) bolt length : 1.80 m Using one "TFI" cartridge (formula 4 of Table II) diameter 18 mm, length 1.06 m, provided with a finned head, per hole. (2) Results: 2.1 -Rotation time: 2.1.1 -with "TFI" at ambient temperature (about 20[deg]C)

2.1.2 - with "TFI" previously exposed to a heat source (about 40[deg]C)

2.2-Tensiletests : "TFI" cartridge 2.2.1. Anchorage to coal No. 1 : 4,500 revs. -1 hour after installation :slipping No. 2 : 4,700 revs. -1 hour after installation: slipping No. 3 : 5,000 revs. - 35 mins. after installation: slipping 2.2.2. - Anchorage to rock (schists) No. 4 : 11,300 revs. -1 hour after installation: holding 2.3 -Tensile tests: Known anchoring cartridges (polyester type) 2.3.1 -to coal:

2.3.2. - rock No. 5 : 10,000 revs. - 24 h. after placing:slipping and elongation In the case of coal, "slipping" signifies that the ground is giving way. On the contrary, in rock, "holding" signifies that it is again within the field of elastic return. Test No. 4 on coal must be viewed as aberrant, the anchorage not having taken.

EXAMPLES S AND T Awax and chalk respectively were used in the formulation of the cartridge filling. The accompanying drawing in which (a) is particle diameter (Micro) and (b) is % by weight, represents the granulometry characteristics of the chalk used. The waxes used were microcrystalline waxes which had the following characteristics:

Table IV below shows a comparison of three compositions according to the invention, which may or may not contain a wax or chalk. Therefore, it can be seen that the compositions according to the invention which are formed by combining in certain proportions certain hot-melt adhesives, and optionally a filler, display a group of surprising properties, which is decisive for industrial application: clear reduction in the length of the bolt installation cycle, capacity for controlling flexibility, lower cost. The cartridges according to the invention also have other advantages, including: they can be stored indefinitely; no risk of an incorrect mixture, in contrast to conventional cartridges; any variation in the annular space (between the bolt and the borehole) has only a very slight effect on the anchorage; the arrival of the bolt at the bottom of the borehole definitely means that anchorage has been effected; no risk of premature blocking of the bolt, as should blocking commence, it then suffices to increase the rotational speed thereof in order to free the bolt; and adaptation to small anchorages, including even the "odd jobs" sector.

Claims (26)

1. A method of anchoring a reinforcing element in a rock substrate, which comprises introducing into a borehole drilled in the rock substrate a frangible cartridge comprising a solid hot-molt adhesive and thereafter bringing a rotating reinforcing element into contact with said cartridge so as to cause the adhesive to melt as a result of the rotation of the reinforcing element and thereby form a molten grouting adhesive in the annular space between the reinforcing element and the wall of the borehole which when allowed to solidify anchors the reinforcing element in the borehole.

2. A method according to Claim 1, wherein the cartridge comprises a hollow tubular casing containing the solid hot-melt adhesive.

3. A method according to Claim 2, wherein the hot-melt adhesive is in particulate form.

4. A method according to Claim 1, 2 or 3, wherein the hot-melt adhesive comprises a polymer selected from ethylene-vinyl acetate copolymers, polyamides, polyesters and polyolefins.

5. A method according to Claim 4, wherein the polymer is an ethylene-vinyl acetate copolymer having a density at 20[deg]C of 0.94, a viscosity at 180[deg]C of 100 to 120 poise and a fusion value at 125[deg]C of 140 to 180.

6. A method according to Claim 4 or 5, wherein the hot melt adhesive comprises a mixture of an ethylene-vinyl acetate copolymer with a thermoplastic ester resin, optionally together with a mineral filler.

7. A method according to Claim 6, wherein the thermoplastic ester resin is an ester of an hydrogenated rosin.

8. A method according to Claim 6 or 7, wherein a mineral filler is present in an amount of up to two-thirds of the total weight of the hot-melt adhesive.

9. A method according to Claim 6,7 or 8, wherein the mineral filler present in the hot-melt adhesive is sand.

10. A method according to Claim 9, wherein the sand has a particle size in the range from 75 to 425 Micro.

11. A method according to any preceding claim, wherein the hot-melt adhesive contains a wax.

12. A method according to any one of Claims 1 to 10, wherein the hot-melt adhesive contains chalk.

13. A cartridge for use in anchoring a reinforcing element in a borehole drilled in a rock substrate, comprising a solid hot-melt adhesive in substantially cylindrical form.

14. A cartridge as claimed in Claim 13, wherein the cartridge comprises a hollow tubular casing filled with a solid hot-melt adhesive.

15. A cartridge as claimed in Claim 14, wherein the hot-melt adhesive is in particulate form.

16. A cartridge as claimed in Claim 13,14 or 15, wherein the hot-melt adhesive comprises a polymer selected from ethylene-vinyl acetate copolymers, polyamides, polyesters and polyolefins.

17. A cartridge as claimed in Claim 16, wherein the polymer is an ethylene-vinyl acetate copolymer having a density at 20[deg]C of 0.94, a viscosity at 180[deg]C of 100 to 120 poise and a fusion value at 125[deg]C of 140 to 180.

18. A cartridge as claimed in Claim 16 or 17, wherein the hot-melt adhesive comprises a mixture of an ethylene-vinyl acetate copolymer with a thermoplastic ester resin, optionally together with a mineral filler.

19. A cartridge as claimed in Claim 18, wherein the thermoplastic ester resin is an ester of an hydrogenated rosin.

20. A cartridge as claimed in Claim 18 or 19, wherein a mineral filler is present in an amount of up to two-thirds of the total weight of the hot-melt adhesive.

21. A cartridge as claimed in Claim 18,19 or 20, wherein the mineral filler present in the hot-melt adhesive is sand.

22. A cartridge as claimed in Claim 21, wherein the sand has a particle size in the range from 75 to 425 It.

23. A cartridge as claimed in any one of Claims 13 to 22, wherein the hot-melt adhesive contains a wax.

24. A cartridge as claimed in any one of Claims 13 to 22, wherein the hot-melt adhesive contains chalk.

25. An anchoring cartridge substantially as described in the foregoing Example R, S or T.

26. A hot-melt adhesive composition for use in an anchoring cartridge as claimed in Claim 13, substantially as described in any one of the foregoing Examples 1 to 8, Ato O and R to T.