FIELD OF THE INVENTION The invention relates to a rock bolt with means to measure the strain on the rock bolt as it yields under load.

BACKGROUND OF INVENTION

[0002] To improve mine safety, it is important to be able to assess the integrity of rock wall support, such as installed rock bolts.

[0003] Passive and dynamic forces are imposed on a rock bolt by the surrounding rock mass to which the bolt offers load support. These forces strain the rock bolt. A rock bolt is prone to catastrophically fail when the bolt is subjected to load that exceeds a maximum tolerance of the bolt. This event is termed“overload”.

[0G04J ft would be desirable to be able to monitor a rock bolt and to detect when the rock bolt is approaching overload or maximum strain. The bolt can then indicate that it is approaching its load or strain limit and action can be taken to avoid a catastrophic failure event. However it is difficult to measure strain on an installed bolt,

[0005] A strain gauge typically is used to measure strain. The device measures a change in resistance over the device as a function of the strain of an element to which the device is attached. o

[0006] Use of a strain gauge is limited in the mining context as it is difficult to adhere the device to the bolt with any degree of permanence and, even if this problem was overcome, the device is designed to measure sma!l changes in resistance, proportional to small strain Furthermore, complexity in the electronics to measure the change comes with small changes in an electronic parameter. With complexity comes cost and size, neither of which lend to a high volume, commoditised product

[0007] A rock bolt is designed to elongate significantly over its length. The magnitude of the strain would render inutile a strain gauge or the like.

[0008] The invention at least partially solves the aforementioned problems.

SUMMARY OF INVENTION

[0009] The invention provides a strain measuring rock bolt which includes: a yielding rod of a malleable material which extends between a distal end and a proximal end; a bore which extends through the rod between the distal and proximal ends; an strain sensor comprising an elongate member of a conductive elastomeric material, which is fixed to the rod within the bore to elongate as the rod elongates under load, and a power source which is electrically connected to the member in a power circuit; a processing module electronically connected to the power circuit; wherein the processing module is adapted to measure a change in an electrical property of the elongate member, as the member elongates under strain; and wherein the processing module is adapted to calculate the strain on the rod based on the change and to communicate with an output means based on the calculated strain.

[0010] The conductive elastomeric materia! of the elongate member preferably is a silicone carbon composite material.

[0011] The electrical property maybe resistance, capacitance or inductance. [0012] The output means may be a means that visually, audibly or by data communicates the difference.

[0013] The output means may be an LED, an audible alarm or a transmitter.

[0014] The processing module may communicate with the LED, alarm or transmitter, to illuminate, sound or transmit data respectively, when the calculated strain exceeds a predetermined limit.

[0015] The LED may be visible from the proximal end of the rod.

[0016] The processing module may be electronically interposed between the power circuit and the output means.

[0017] The processing module may include a microcontroller with an Analog-to- Digital Converter ADC”)

[0018] The power source may be a battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention is described with reference to the following drawings in which Figures 1 and 2 are diagrammatic representations of a strain measuring rock bolt in accordance with the invention and an electronics module of the rock bolt respectively

DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] The invention provides a strain measuring rock bolt 10 which is illustrated in Figure 1

[0021] The rock bolt includes a yielding rod 12, of a suitable malleable metal material, which extends between a distal end 14 and a proximal end 16. The rod has a bore 18 which extends the entire length of the rod, between the ends (14, 16). The rod is partially externally threaded along a proximal end portion 20. A nut 22 is engaged to the threaded portion.

[0022] The rock bolt 10 further includes a strain sensor 24, comprising an elongate member 26 of a suitable conductive elastomeric materia! which is fixed to the rod 12 within the bore 18, a battery 28 and a resistor 30. The member, battery and resistor are electrically connected in series in a power circuit. The battery and the resistor are part of a PCB 32.

[0023] The PCB 32 is housed in a housing 33 which is attached to the proximal end [0024] An LED 34 is partially contained within the housing. The PCB includes a processing unit 36 which is electronically interposed between the power circuit of the sensor 24 and the LED.

[0025] The elastomeric elongate member 20 extends between a first end 38 and a second end 40. The member extends within the bore, fixed to the rock bolt in any suitable way so as to allow the member to elongate proportionally to the rod when the rod elongates, in use, under load in this example, the member is fixed at the first end 38, to the distal end 14 of the rod 12. At the opposed second end 40, the member 26 is attached to the RGB 30 in electrical communication. The member is a transducer, to the processing unit 36, of the strain of the rock bolt.

[0026] The processing unit 36 includes a microprocessor 42, an ADC 44 and a memory unit 48, such as ROM or RAM. input into the memory unit are at least the following values: resistance (“RT) of the resistor 30, voltage (“Vin") of the battery 28, resistance ("Rx”) of the elongate member 26 before elongation and the physical dimensions of the elongate member.

[0027] As the rod 12 experiences strain, elongating under load when in use, inserted into a rock hole, the elongate member 26 elongates proportionally, The elongation of the elongate member changes its resistivity and increases its resistance to an electricai current flowing through the power circuit. [0028] As the processing unit 26 is connected to a centre tap between the resistor

30 and the elongate member 26, it can measure the voltage across the elongate member (“Vout”) to determine the strain (difference in length) of the yielding rod 12 This measurement can occur after a loading event or it can occur periodically.

[0029] Calling on the invariable values from the memory unit 48, with the Vout fed from the ADC 44, the microprocessor 42 can compute the resistance (“Ry") of the elongate member 26 after the event or period. From R _y, strain can he calculated from resistivity to length data pertaining to the particular elastomeric material.

[0030] The processing unit 36 can be pre-set with a stress limit. If the measured strain exceeds the stress limit, the microprocessor can cause the LED 34 to energise to emit light as a visual signal. As the LED is in the housing, and the housing protrudes from the rock hole in which the rock bolt 10 is inserted, the visual signal is easily seen by a mine worker as an indication that the rock bolt is overloaded.