EP2961928B1

EP2961928B1 - Device for detecting the presence of one or more cavities in a borehole

Info

Publication number: EP2961928B1
Application number: EP14757785.2A
Authority: EP
Inventors: Leif Gustafsson; Sture ÅBERG
Original assignee: Rock Safety Sweden AB
Current assignee: Rock Safety Sweden AB
Priority date: 2013-02-28
Filing date: 2014-02-28
Publication date: 2018-03-28
Anticipated expiration: 2034-02-28
Also published as: EP2961928A4; CL2015002411A1; SE1350239A1; SE537019C2; EP2961928A1; WO2014133449A1

Description

The present invention concerns an arrangement for detecting the presence of one or more cavities in a borehole
A large number of reinforcement systems are today available to stabilise and reinforce a rock structure during the building of tunnels, mining operations, tunnelling, etc. Such a reinforcement system involves the drilling of a large number of boreholes in the wall or roof that is to be reinforced, the subsequent filling of these boreholes with grout, and the subsequent introduction of bolts into the boreholes, to be cast in place in the boreholes, by which means the wall or roof is reinforced. One example of a bolt for casting into a reinforcement system is what is popularly called the "kiruna bolt", which consists of a reinforcement bar with a slotted end with a wedge. Another example is a cable bolt, which consists of a 7-stranded twisted steel thread. These types of bolt have lengths of 3-7 metres.
The rock reinforcement is carried out through a portion of the grout being injected into a borehole by a nozzle, normally a tube. The borehole is filled from the deepest part of the borehole bottom, after which the tube is withdrawn, as filling continues. Rock material may be constituted in different ways: cracks and natural cavities are sometimes present that are filled by an injected portion of filler material. This means that the portion of grout that has been injected may be insufficient to fix the rock bolt, which may have as its consequence that the reinforcement system is deficient and has an insufficient load-bearing capacity. When the rock bolt is introduced into what appears to be a filled hole, there may, therefore, arise empty spaces or cavities, along the bolt that are difficult to detect. These cavities, in particular, usually arise at the extreme end, at the deepest part of the borehole. A serious problem with these reinforcement systems, therefore, is that it is not possible to be certain that any individual bolt is completely cast and well-anchored. The greatest problem arises if the uppermost part of the bolt is not covered by hardening grout. If this is the case, a part of the load-bearing capacity of the bolt is lost. Furthermore, the risk for corrosion of the bolt increases, since the rock in itself may be wet.
One efficient method of detecting directly the presence of a cavity in a borehole with cast bolts is revealed by the document SE 533769. This method makes it possible to detect whether any cavities remain after a hardening grout and bolt have been introduced into the borehole.
SE 533769 reveals a reinforcement means, a rock bolt, that has a passage for the introduction of a medium into the borehole. The passage is provided with radially directed holes, openings, through which the medium flows if there is a cavity at the outer surface of the passage. This results in a change in pressure that can be measured, and that indicates that a cavity is present in the immediate vicinity of the reinforcement means. When using the method and the system according to the said document, the reinforcement means is introduced into a borehole shortly after the borehole has been filled with unhardened filler material that risks penetrating through the holes, the openings, in the passage wall. This is solved by the holes, the openings, being provided with a covering surface layer, a cover of a thin layer of a material, a rubber sheet, a plastic film, or a thin membrane of paint or tape. During the supply of the pressurised medium, the cover is pressed away from the holes, and penetrates out if a cavity is present. The protective cover is normally destroyed, independently of whether there is a cavity present or not. This means that if a cavity is not present, and the protective cover has been destroyed by being put under pressure, the filler material can penetrate into the opening. Thus the filler material risks clogging the passage, which cannot then be used for the intended purpose. This risks influencing the measurement result.
There is thus a need to design the passage such that the problem with the penetration of filler material through the holes, the openings, is avoided.
One purpose of the present invention, therefore, is to achieve an arrangement for the detection of the presence of a cavity in a borehole, which arrangement solves the problems with filler material that penetrates into the measurement passage.
This purpose of the invention is solved through an arrangement according to claim 1. Other distinctive features and advantages of the invention are made clear by the non-independent claims.
Advantages and positive effects of the invention will be described below based on embodiments of the invention and with reference to the drawings, which show:

Fig.1a shows schematically an arrangement according to the invention in a condition in which supply of medium under pressure is not taking place;
Figure 1b shows the arrangement according to Figure 1 a in a condition in which supply of medium under pressure is taking place.

Figure 1 shows schematically an arrangement 1 according to the invention. The arrangement is intended to be used for the detection of the presence of a cavity 2 in a borehole 3 that has been made in a rock structure 4 during tunnel construction, mining, tunnelling or similar according to the method revealed in SE 533769. The borehole 3 is intended for the reception of a reinforcement means, a rock bolt 5.
The reinforcement means, the rock bolt 5, comprises a separate measurement tube 6. The measurement tube has a passage 7 that is intended to introduce a pressurised medium 8 into a borehole that has been filled or partially filled by hardening grout 16. The measurement tube 6 is constituted by a thin, hollow tube that is open in its longitudinal direction, made from, for example, semi-rigid flexible plastic, or semi-rigid flexible rubber that has a certain elasticity. The term "semi-rigid" is here used to describe a flexible material that has the ability to regain essentially its original form after the effect of a force on the material has ceased.
The measurement tube may, for example, be made from polyamide or polyurethane, or a combination of these. Such materials are particularly suitable for this purpose, but are not limiting for the invention. The semi-rigid aspect relates to the transverse stability, when the measurement tube is bent or influenced in its transverse direction it afterwards regains its original form. The measurement tube has a tube wall 9, the thickness of which is selected such that an externally applied load from hardening grout 16 or similar does not influence the function of the passage 7.
The measurement tube 6 with the passage 7 is provided with a passage opening at at least one tube end, where the passage opening at the surface of the rock structure is intended for the introduction of a medium into the borehole.
The diameter of the tube is considerably less that the diameter of the reinforcement means, the rock bolt, but it has essentially the same length, or is longer than the rock bolt. The length of the tube thus exceeds the depth of the borehole.
The measurement tube has an inner end 10 that is located in the vicinity of the bottom end 11 of the rock bolt and the bottom 12 of the borehole, and it has a contact end (not shown in the drawings) that extends out from the borehole. The measurement tube is attached to the reinforcement means with one or several fastening arrangements 13. The fastening arrangements 13 are, for example, regularly distributed at points along the length of the rock bolt and the tube. The fastening arrangements consist, for example, of ties, tape, steel wire or similar.
The measurement tube 6 with the passage 7 may be attached to the rock bolt 5 during manufacture of the bolt, in particular if the reinforcement means is an independent rock bolt, for example a kiruna bolt. If the reinforcement member 5 is a twisted bolt, the length of the bolt is adapted immediately before the rock reinforcement operation is carried out. The bolt is manufactured from metal, for example steel. The measurement tube 6 with the passage 7 is then attached to the rock bolt just before the rock bolt with the measurement tube is introduced into, pressed into, the borehole filled with hardening grout. After anchoring, the contact end of the measurement tube is available for its connection to measuring equipment (not shown in the drawings) for the supply of medium under pressure and the detection of a change in pressure in the medium in the event of the presence of a cavity along the rock bolt, for example such equipment as is shown in SE 533769.
The measurement tube 6 with the passage 7 is provided with at least one means 14 for the supply of a measurement medium 8 to the borehole 3, where the means 14 acts as a valve and has a valve function that is activated by the application of pressure, when measurement medium under pressure is supplied to the passage 7.
The means 14 is designed as a cut, a section,that penetrates the wall 9 of the tube, through the side surface of the passage. The means 14, the cut, has two cut surfaces 15a, 15b that make contact with each other. In their uninfluenced condition, the cut surfaces 15a, 15b are in contact with each other, and seal the cut. No material has been removed from the tube in the cut surfaces. The measurement tube consists of a plastic material of a semi-rigid type, and this has the effect that the means, the cut, acts as a valve. The means 14 is in this way arranged to be opened when placed under pressure from the inside of the passage 7.
When the measurement tube 6 with the passage 7 is supplied with a medium 8 under pressure the means 14, the cut, opens and the cut surfaces 15a, 15b are pressed apart such that an opening, a passage, arises between the cut surfaces, as is shown in Figure 1 b. The passage is united with the outer surface of the wall of the tube when the pressurised medium is supplied. If a cavity is present at the outer surface of the measurement tube, the medium 8 under pressure will be pressed out, and this results in a change in pressure that can be measured, which indicates the presence of a cavity, an empty space, along the rock bolt 5.
One advantage of the cut surfaces 15a, 15b being pressed apart is that, if there is a thin membrane of solidified grout outside of the measurement tube 6, this membrane can be burst by the pressure in the measurement medium 8 that has been supplied. The force that influences the cut corresponds to the pressure multiplied by the area of the passage. The cut has a considerably greater area of passage than, for example, the holes or perforations that have been previously suggested. The force, and thus the bursting ability, is for this reason greater.
When the passage 7 is not being supplied with medium under pressure, the means 14, the cut, remains closed. The means is thus arranged to be closed in a condition in which measurement medium under pressure is not supplied. This has the effect that unhardened grout is essentially fully prevented from pressing in through the means and sealing the passage 7. In contrast to previously suggested solutions, it is not necessary to cover the opening with any cover, which saves a lot of time and will be significantly more cost-efficient.
The means 14 has in this way a valve function that, furthermore, essentially allows the transport of medium in only one direction. The means thus acts as a non-return valve, and has an effect as that of a non-return valve that allows measurement medium 8 supplied under pressure to be transported out from the passage 7 through the open means 14, but prevents unhardened grout from entering the passage 7 when the means 14 is closed and the passage 7 is not being supplied with a medium 8 under pressure.
By selecting material and adapting the number of means 14 and the length of the cuts, it is achieved that the means opens only when the pressure in the pressurised medium 8 that is supplied exceeds a certain limiting value. The possibility of controlling the means is in this way obtained.
It is preferable that the measurement tube be provided with several means 14, cuts, distributed along its complete length and circumference. The measurement tube is provided with several means along its length, in particular in the region close to its inner end, since it is more common that cavities arise at the inner end, close to the bottom of the borehole.
The contact end of the measurement tube is connected during measurement to measuring equipment of the type that is described in SE 533769 (not shown in the drawings).

Claims

An arrangement (1) for the detection of the presence of a cavity (2) in a borehole (3) intended for the reception of a reinforcement means comprising a measurement tube (6) with a passage (7) whereby the measurement tube (6) comprises at least one means (14) for the supply of a measurement medium (8) to the borehole (3), wherein the means is characterised by comprising a cut arranged through a wall (9) of the tube, said cut having two cut surfaces (15a, 15b) that make contact with each other, where the means (14) functions as a valve that is activated by the application of pressure when measurement medium (8) is supplied to the passage (7), and wherein the means (14) is opened only when the pressure in the supplied medium exceeds a threshold value.
The arrangement according to claim 1 where the means (14) is arranged at the tube wall (9) of the measurement tube and is opened on the supply of medium (8).
The arrangement according to claim 1 or 2, where the means (14) is arranged to be closed when no measurement medium (8) is supplied.
The arrangement according to any one of claims 1-3 where the means acts as a non-return valve where the means (14) is opened on the supply of measurement medium (8) under pressure and allows the measurement medium under pressure to be transported out through the open means from a passage (7) arranged in the measurement tube (6), and that the means is closed when measurement medium under pressure is not supplied, whereby grout is prevented from entering the passage (7).
The arrangement according to any one of claims 1-4, where the material of the measurement tube (6) comprises a semi-rigid plastic material.
The arrangement according to any one of claims 1-5, where the measurement tube is provided with a large number of means (14) distributed along the complete length of the measurement tube (6).