Apparatus, method and system for optically measuring the adhesive strength of a rock bolt adhered to a bore

Abstract

The present invention relates to an apparatus, method and system for optically measuring the adhesive strength of a rock bolt adhered to a bore. The apparatus comprises an external unit that can be coupled to an outer portion of the rock bolt by means of mounting elements, said external unit comprising a rotation device connected to a rotary shaft; and a rotary measuring device which is connected to the rotary shaft and can be inserted inside the rock bolt, said rotary measuring device comprising at least one sensor, wherein said at least one sensor comprises a light-emitting source and a photocell.

Description

APPARATUS, METHOD AND SYSTEM FOR OPTICALLY MEASURING THE RESISTANCE OF Adhesion of a rock bolt to a borehole DESCRIPTIVE MEMORY FIELD OF INVENTION

[0001] The present invention is aimed at the rock bolting industry for mining and civil works, particularly in systems for in-situ measurement of the condition of a rock bolt installed in a borehole. BACKGROUND OF THE INVENTION

[0002] Rock bolts are one of the primary means of supporting and reinforcing rock and soil in a wide range of engineering fields, including hydraulic engineering, mining, transportation, and ports. They are typically used to reinforce unstable rock strata so that they can support and stabilize the rock in engineering structures such as walls, tunnels, foundations, or underground excavations.

[0003] One of the most relevant aspects of rock bolt use relates to safety. Historically, accidents caused by rockfall have been significant in the mining industry. For example, data indicates that 50% of coal mine fatalities have been caused by rockfall, resulting in an approximate total of 45,000 deaths in the coal mining industry alone. These types of accidents can cause injuries, disabilities, and deaths, while also impacting mining company production due to operational disruptions, downtime, and equipment breakdowns. However, thanks to the use of rock bolts to increase rock stability, as well as technological advances in the industry, the frequency of accidents caused by rockfall has been progressively reduced.

[0004] In this industry, there are three types of bolts most commonly used. The first is the mechanically anchored rock bolt, either the split-and-wedge or expansion type. The second is friction-type, such as Split Set or Swellex, which are anchored by friction between the bolt and the surrounding rock. The third type is anchored with concrete or resin, relying on the strength of the bolt-rock or grout-rock interface.

[0005] Despite differences in technology and anchoring systems, all of these systems can be prone to problems and deficiencies, which are primarily related to load capacity, anchor quality, and bolt integrity. Some studies indicate that corrosion is the primary cause of bolt failure, in addition to the bending and cracking caused by it.

[0006] One of the most common methods for determining the condition of a rock bolt installed in situ is the pull-out test, which determines the axial forces in order to evaluate the anchor's performance. Other types of tests that seek to determine the same objective are the drop hammer test, the tensile test, the Split Hopkinson Pressure Bar test or the shaking table test, but they are time-consuming and are usually carried out in a single test. They are carried out under laboratory conditions. For these reasons, rock bolt tests are typically performed sporadically, which means that the current state of the installed support is not adequately known, and without a statistically representative population, making it impossible to determine its actual load-bearing capacity.

[0007] Patent literature proposes some innovative solutions for measuring the load, stress or adhesion of rock bolts, such as the one disclosed in WO 2020231321 which consists of a device for measuring stress in a rock bolt, the device comprising a housing, at least one optical sensor, at least one light source arranged to emit light through an interior space of the housing and a wire arranged to move freely within a sheath. The wire has a proximal end extending to a distal end of the housing and a distal free end arranged to join the bolt at a distance from the housing such that longitudinal deformation of the bolt causes displacement of the proximal end of the wire relative to the housing. The optical sensor is configured to measure the displacement of the proximal end of the wire by measuring light emitted from the light source.

[0008] For its part, EP 1003978 describes a rock bolt that passes through a hole drilled into a material so that a support plate can be pressed against the face of the material by turning a nut. Applying tension to the bolt by turning the nut produces a compressive load on a cylinder, which can be measured from the resulting number of fringes that appear on it, which can be observed with the aid of a projected light beam and a telescopic viewing device.

[0009] Finally, WO 2018032106 proposes a method for determining a change in the condition of a rock bolt by means of the propagation of ultrasonic and shear waves along it.

[0010] The state-of-the-art solutions described above are primarily focused on measuring the tension of mechanical anchor bolts, i.e., those that are equipped at their ends with a mechanical expansion anchor with the rock.

[0011] However, there is no current solution for optically measuring the adhesion strength of friction pins in situ. In this context, the present invention seeks to provide a more efficient alternative to the costly and cumbersome tests based on state-of-the-art mechanical systems, such as the direct tensile test or pull-out test frequently applied to split-set friction pins.

[0012] These friction bolts consist of a tubular steel plate a few millimeters thick and fitted with a slot with a certain opening or spacing along its entire length. As the bolt is inserted into the borehole, the slot closes, and the further it closes, the greater the bond strength between the friction bolt and the rock surface of the borehole.

[0013] Direct tensile testing is typically performed on test bolts, which are generally shorter than those used to support the excavation. Furthermore, to be able to secure the bolt with the testing equipment, a steel ring must be included on each bolt to be tested. This prevents testing of other bolts already installed, except for those specifically selected for testing, which must include this steel ring.

[0014] For the tensile test, a tripod with a support bar and a coupling for the Split Set bolt is installed, and the bolt is pulled with a hydraulic jack. The tensile load can then be determined using a pressure gauge, and the frictional resistance is obtained as a result. However, this method is destructive, labor-intensive, and time-consuming, both in preparation and for the test itself.

[0015] It is therefore an objective of the present invention to overcome these drawbacks by indirectly measuring the bond strength of friction bolts that have been installed in rock, in order to control the quality of their installation.

[0016] Another objective of the present invention is to facilitate obtaining measurements of the bond strength of friction bolts and to increase the volume of data collected per unit of time in each field inspection, by means of a compact, lightweight and easy-to-operate device. DESCRIPTION OF THE INVENTION

[0017] According to a first aspect of the invention, the invention provides an apparatus for optically measuring the bond strength of a rock bolt to a borehole. The apparatus comprises an external unit attachable to an exterior of the rock bolt by means of mounting elements, said external unit comprising a rotation device connected to a rotary shaft. The apparatus also comprises a rotary measuring device connected to the rotary shaft and insertable inside the rock bolt, the rotary measuring device comprising at least one sensor, wherein said at least one sensor comprises a light-emitting source and a photocell.

[0018] Using the proposed device, it is possible to indirectly measure the adhesion strength of Split Set rock bolts without applying mechanical stress to the bolt that would require its removal or destruction, thus surpassing state-of-the-art direct tensile tests.

[0019] Another advantage of the proposed measuring device is that, since it does not require hydraulic systems for its operation, it can be installed by a single person to monitor several rock bolts in a short period of time, thus allowing a significant amount of data to be obtained and thereby improving the ability to predict rock support behavior on a large scale.

[0020] Furthermore, the device has a compact and lightweight design, comprising a rotating measuring device that can be inserted into the rock bolt and is removable and portable. Said device comprises multiple sensors distributed axially along a rotation axis concentric to the rotation axis. Preferably, the sensors have a modular construction, wherein the rotating measuring device comprises tubular segments distributed axially along the rotation axis, such that each tubular segment connects two adjacent sensors or a sensor to the rotation axis.

[0021] The modular configuration of the sensors advantageously allows them to be easily replaced or the number of sensors installed on the rotating measuring device to be modified depending on the characteristics of the bolt to be evaluated.

[0022] According to a preferred embodiment, the connection between a sensor and a tubular segment comprises electrical contact between one or more of its components, which allows minimizing the use of cables inside the rotating measuring device and at the same time facilitate the connection and disconnection of the modular sensors.

[0023] According to a preferred embodiment, the light source and the photocell are located within a respective cavity in the sensor, said cavities being separated by a small distance. These cavities protect the light source and the photocell from damage caused by contact with the rock or the inner wall of the bolt when they are inserted into it.

[0024] Preferably, the external unit of the measuring device comprises a housing inside which a rechargeable battery, an electronic board, a microcontroller, a wireless transmitter, indicator lights and a battery charging port are housed.

[0025] According to a second aspect of the invention, a method is proposed for optically measuring the bond strength of a rock bolt to the interior of a borehole, using the measuring apparatus described above. The method comprises the steps of: a. inserting the rotary measuring device into the rock bolt; b. fixing the external unit to an exterior part of the rock bolt; c. rotating the rotary measuring device in a first direction and at a preset advance angle; d. activating the light-emitting source of the at least one sensor and recording the refractive index using the photocell of said sensor; e. sequentially executing step d for each sensor of the measuring device of the rotating device; f. repeating steps d and e until a preset number of refractive index measurements are completed. g.Perform steps c and e through f until the rotary measuring device has been rotated to a preset angle. h. Rotate the rotary measuring device in a second direction and at a preset pitch angle; i. Perform steps d and g in the second direction; j. Determine the arc distance of a rock bolt slot based on the average of the refractive index measurements performed; and k. Compare the result of step j with a reference value of the relationship between the arc distance of a known rock bolt slot and its tensile strength.

[0026] The proposed measurement method relies on averaging a large set of individual signals in multiple stages due to the high level of refractive index noise within the borehole. This way, the measured data are read sequentially (one sensor at a time) and spaced out in time (e.g., 2 ms) so that the microcontroller has time to process the data and information from the next sensor.

[0027] According to a preferred embodiment, step j comprises determining the opening arc distance by measuring the angle between two inflection points of a curve representing the refractive index. by the feed angle of the rotating measuring device, where said inflection points represent the change in the infringement index between two different materials. For example, metal and rock.

[0028] Preferably, the first direction of rotation is clockwise or vice versa and the second direction of rotation is counterclockwise to the first direction.

[0029] The advance angle can be preset in the range between 0 o and 2 o , preferably 0.5°. The preset rotation angle in each direction is 180°.

[0030] According to a third aspect of the invention, a system is proposed for monitoring the quality of a rock bolt installed in a borehole using the measuring apparatus described above. The system comprises at least one user device and a wireless network, wherein the at least one user device is in communication with the measuring apparatus via the wireless network. The user interface is configured to control, calibrate, and / or configure the measuring apparatus by means of data transmitted to it by the user device and via the wireless network. Furthermore, the at least one user device further comprises a user interface configured to provide a report to the user based on data obtained and transmitted by the measuring apparatus.

[0031] According to one embodiment, the wireless network is generated by the wireless transmitter of the measuring device, and the user interface comprises real-time information obtained from the measuring device. Preferably, said real-time information is an estimate of the rock bolt's opening arc. DESCRIPTION OF THE FIGURES

[0032] As part of the application, the following representative figures of the invention are presented, which show preferred configurations thereof and, therefore, should not be considered as limiting the definition of the claimed subject matter. Figure 1 illustrates a schematic of a state-of-the-art Split Set type bolt. Figure 2 illustrates the installation sequence of a bolt like the one in Figure 1 into a borehole. Figures 3a and 3b illustrate the proposed rock bolt bond strength measuring apparatus. Figure 4 illustrates a sensor of the apparatus of Figures 3a and 3b. Figure 5 illustrates a schematic diagram of the operation of the measuring apparatus of the present invention. Figure 6 illustrates an example of the representation of measurements performed by the measuring apparatus of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0033] Figure 1 shows an exemplary rock formation 1, which is to be fortified by means of a rock bolt 10 of the state of the art, which is configured to be inserted into a perforation or borehole 2 previously made in the rock for these purposes.

[0034] The rock bolt 10 illustrated is a split-set friction bolt, consisting of a steel tube slotted along its length. One end of the tube is tapered, and the other typically has a welded ring to retain a plate 11, which rests against the rock formation 1.

[0035] As seen in the sequence in Figure 2, the rock bolt is inserted into borehole 2 by its tapered end using a thrust and percussion method using a drilling machine. Borehole 2 has a substantially smaller diameter than the bolt, which generates radial pressure along the entire length of the bolt against the borehole walls. This causes the bolt's longitudinal groove to partially close during installation. Thus, the pressure generated by the bolt inside the borehole will oppose the movement or separation of the surrounding rock, thereby increasing the load-bearing capacity of the rock formation.

[0036] Figures 3a and 3b illustrate the proposed measuring apparatus for measuring the bond strength of rock bolt 10 installed in rock formation 1, by determining the degree of opening of longitudinal groove 12 of the bolt at different points of its extension into the borehole. To this end, the measuring apparatus comprises a rotary measuring device 20 configured to be inserted into rock bolt 10 and an external unit 30 configured to be coupled to plate 11 of the bolt by means of mounting elements 31, as shown in Figure 3a. Said mounting means 31 may be threaded, magnetic or any other appropriate type fixing elements.

[0037] The external unit 30 of the measuring apparatus comprises a housing inside which is arranged a rotation device 32 such as a stepper motor. Said rotation device 32 is connected to a rotary shaft 33 by means of, for example, a transmission belt 34 as seen in the embodiment of Figure 3b.

[0038] The housing also houses a rechargeable battery and all the electronics that control the device, which generally consist of a circuit board and a microcontroller. It may also include a wireless transmitter, indicator lights, a battery charging port, among other things.

[0039] For its part, the rotary measuring device 20 is configured to be coupled to the rotary axis 33 and consists of a tube comprising a plurality of sensors 21 distributed along its length. According to Figure 4, each sensor 21 consists of a module that is coupled to one or more tubular segments 22 of the rotary measuring device, such as PVC pipes. The module can be made of a metal such as aluminum and has a circular cross-section. In addition, it can comprise a recess at each end for snap-fitting with the tubular segments 22 along the axis of rotation 23.

[0040] The sensor comprises a light source 24 and a photocell 25, each located within a respective cavity in the module body and separated from each other by a distance of, for example, 15 mm. The light source 24 and the photocell 25 are in electrical communication with the electronic board of the external unit. To this end, the rotary measuring device may comprise one or more cables that connect to each of the modules and to the electronic board. Alternatively and preferably, communication with the latter may occur by means of electrical contacts in the coupling mechanism between each module and the tubular segments 22.

[0041] In the exemplified embodiment, an arrangement of five light sources 24 corresponding to high brightness LEDs (20mA, 2.4V) and five photocells 25 (5mm, 10MÍ2, 25 Lux) spaced 28cm apart is used.

[0042] Figure 5 illustrates a sectional view of a rock bolt 10 installed in a rock formation 1 with the rotating measuring device 20 inserted inside the bolt. By means of the rotating device of the apparatus (not illustrated), the rotating measuring device 20 is first rotated in a first direction di and then in a second step a rotation is performed in a second direction dz. Due to the noise level of the signal, a multi-step sampling technique is used in order to average a large set of individual signals. Thus, in each step, the photocell 25 records the refractive index of light 26 generated between the light emitting source 24 and the inner wall of the rock bolt 10.

[0043] The angle of rotation carried out at each step is predetermined by the microcontroller, so that through the change in the refractive index that is recorded by the photocell 25 at each step, the opening arc of the slot 12 of the rock bolt 10 is determined at the depth at which each sensor is located.

[0044] The measurement results are illustrated in Figure 6, where the sensor readings can be represented as an inverted parabola since the refractive index determined by the photocells changes depending on the material measured at each angle. That is, the refractive index obtained will be different depending on whether the material illuminated by the light source in each sensor corresponds to the metallic material of the bolt or to the rock visible through the bolt slot.

[0045] By determining the slope change points on the curve, both at the beginning and end of the path, it is possible to determine the width of the rock bolt's opening arc or slot. In the illustrated example, inflection points ii and 12 are used to measure the angular distance of the slot arc 12.

[0046] Thus, a friction bolt with a smaller groove arc than an unloaded bolt necessarily implies greater adhesion to the borehole walls. Therefore, by using a reference table between the opening arc of a known rock bolt and the tensile strength, it is possible to indirectly determine the quality of the friction bolt installation and perform quality control.

Claims

CLAIMS 1. An apparatus for optically measuring the adhesion strength of a rock bolt (10) to a borehole (2), comprising: an external unit (30) coupleable to an outer part of the rock bolt (10) by means of mounting elements (31), said external unit (30) comprising a rotation device (32) connected to a rotary shaft (33); a rotary measuring device (20) connected to the rotary shaft (33) and insertable inside the rock bolt (1), the rotary measuring device (20) comprising at least one sensor; and wherein said at least one sensor comprises a light emitting source (24) and a photocell (25).

2. The apparatus according to claim 1, CHARACTERIZED in that the rotary measuring device (20) comprises multiple sensors distributed axially along a rotation axis (23) concentric to the rotation axis (33).

3. The apparatus according to claim 2, CHARACTERIZED in that the rotary measuring device (20) comprises tubular segments (22) distributed axially along the axis of rotation (23), wherein each tubular segment (22) connects two adjacent sensors or a sensor with the rotary axis (33).

4. The apparatus according to claim 3, CHARACTERIZED in that the connection between a sensor and a tubular segment (22) comprises electrical contact between one or more of its components.

5. The apparatus according to any of the preceding claims, CHARACTERIZED in that the light emitting source (24) and the photocell (25) are located inside a respective cavity in the sensor, said cavities being separated.

6. The apparatus according to any of the preceding claims, CHARACTERIZED in that the external unit (30) further comprises at least one of: a housing, a rechargeable battery, an electronic board, a microcontroller, a wireless transmitter, indicator lights and a battery charging port.

7. A method for optically measuring the bond strength of a rock bolt (10) to a borehole (2), using the apparatus of claims 1 to 6, CHARACTERIZED in that it comprises the steps of: a. inserting the rotary measuring device (20) into the rock bolt (10); b. fixing the external unit (30) to an exterior part of the rock bolt (10); c. rotating the rotary measuring device (20) in a first direction (di) and at a preset advance angle; d. activating the light emitting source (24) of the at least one sensor and recording the refractive index by means of the photocell (25) of said sensor; e. executing step d sequentially for each sensor of the measuring device of the rotating device (20); f. repeating steps d and e until a preset number of refractive index measurements are completed. g. performing step c and steps e to f until the rotation of the rotary measuring device (20) is completed through a preset rotation angle. h. rotating the rotary measuring device (20) in a second direction (d i ) and through a preset pitch angle; i. performing steps d to g in the second direction (d i ); j. based on the average of the refractive index measurements performed, determining the opening arc distance of a slot (12) of the rock bolt (10); and k. comparing the result of step j with a reference value of the relationship between the opening arc of the slot of a known rock bolt and its tensile strength.

8. The method according to claim 7, CHARACTERIZED in that step j comprises determining the distance of the opening arc by measuring the angle between two inflection points (ii, ij) of a curve representing the refractive index per angle of advance of the rotary measuring device (20), wherein said inflection points represent the change of the infringement index between two different materials.

9. The method according to claim 7 or 8, CHARACTERIZED in that the di direction is the clockwise direction or vice versa and in that the di direction is opposite to the di direction.

10. The method according to any of claims 7 to 9, CHARACTERIZED in that the advance angle can be preset in the range between 0° and 2°.

11. The method according to any of claims 7 to 10, CHARACTERIZED in that the preset rotation angle is 180°.

12. A system for controlling the quality of a rock bolt (10) installed in a borehole (2) using the measuring apparatus of claims 1 to 6, CHARACTERIZED in that the system comprises at least one user device and a wireless network; wherein the at least one user device is in communication with the measuring apparatus through the wireless network; wherein the user interface is configured to control, calibrate and / or configure the measuring apparatus by means of data that is transmitted to it by means of the user device and through the wireless network; and wherein the at least one user device further comprises a user interface configured to deliver a report to the user from data obtained and transmitted by the measuring apparatus.

13. The system according to claim, CHARACTERIZED in that the wireless network is generated by a wireless transmitter of the measuring device.

14. The system according to claim 12 or 13, CHARACTERIZED in that the user interface comprises real-time information obtained from the measuring apparatus.

15. The system according to claim 14, CHARACTERIZED in that said real-time information is an estimate of the opening arc of the rock bolt (10).

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