WO2015128831A1

WO2015128831A1 - Grouted rock support testing apparatus and method

Info

Publication number: WO2015128831A1
Application number: PCT/IB2015/051437
Authority: WO
Inventors: Quinton Lennox GRIX; Jhean-michael VAN DER MERWE
Original assignee: BARNARD, Andries Jacobus
Priority date: 2014-02-28
Filing date: 2015-02-26
Publication date: 2015-09-03
Also published as: AP2016009458A0

Abstract

The invention relates to apparatus and a method for testing the grout quality around a rock anchor (100) grouted into a drill hole (102). First and second conductors are grouted into the hole with exposed conductive portions supported in known spaced apart. The second conductor is preferably provided by an elongate body (108) of a rock anchor and the first conductor by an insulated wire (12) with an exposed portion at an inner end. An electrical current through the conductors and the resistivity of the grout between the conductors is calculated using a suitably calibrated measuring device (34). The resistivity readings provide an indication of the grout integrity of the anchor installation.

Description

GROUTED ROCK SUPPORT TESTING

APPARATUS AND METHOD

FIELD OF THE INVENTION This invention relates to an anchor testing apparatus. In particular, but not exclusively, this invention relates to an apparatus for testing the integrity of an installed rock anchor, such as a grouted cable anchor, for example.

BACKGROUND TO THE INVENTION

The use of rock anchors in supporting rock strata is well known, particularly in an underground mining industry, and a wide range of different anchors are available. These anchors are also referred to as rockbolts of roof bolts as they are typically used in the mining industry to support rock strata in a hanging or sidewall of an underground working or mine stope.

One type of rock anchor commonly used in the mining industry is known as a cable anchor and includes a (cable or) tendon which is, in use, grouted in a hole drilled into the hanging or sidewall of the stope using a cement or resin grout. In some embodiments the cable anchor may also include a mechanical anchor such as an expansion shell at its end which is, in use, located at the blind end of the borehole. In its common application, a leading or inner end of the cable anchor is inserted into the hole in and the mechanical anchor is fixed in place. The tendon is of such a length that its trailing or outer end extends from the hole. A faceplate (which is also referred to as a grout retaining washer) is then placed against the face of the rock body. The tendon extends through a hole in the faceplate so that the outer end protrudes from the faceplate. With the inner end fixed in the hole the tendon may be pre-tensioned. Next, grout is pumped into the space in the hole surrounding the tendon. The grout may, in some embodiments, be pumped through a grout injection tube running through the faceplate and a breather tube may be used to allow air to escape as the grout is pumped into the hole. A person skilled in the art of mine supports will be familiar with installations that do not use injection and breather tubes. Proper grouting of anchors is crucial in the anchor installation process as it extends the life of the anchor by preventing oxidation. It has also been found that the grout provides resistance against the shear-movement of the rock supported along the various discontinuities within the surrounding rock mass. Where an anchor is pre-tensioned, the grout also acts to maintain tensional forces in the anchor well after its mechanical anchor has been dislodged or damaged. The tension is also retained in the anchor after the grout has set even though the faceplate may have become inefficient.

However, a problem experienced with the grouting process is that it is generally difficult to determine the quality of the grouted connection and, accordingly, the quality of support offered by the grouted anchor. The quality control on the anchor installation process is critical for the safety of the miners in the underground workings. Any defects or irregularities in the consistency of the grout or the quantity of grout pumped into the hole could jeopardise the integrity of the anchor and its associated installation. Quality assurance is therefore a critical step in the installation of the anchors. One known method of quality assurance is to perform direct supervision during the entire installation process of each anchor. A person familiar with mining operations will know that this is not always practical. Due to the large volumes of anchors installed in a mining operation as well as the sporadic intervals and locations of such installations, quality control methods are commonly limited to weekly or bi-weekly inspections. A problem with this quality assurance method is that problematic or substandard anchors are often only picked up after rock faces have advanced substantial distances, sometimes as much as tens of metres, resulting in a reactive instead of proactive approach.

In an attempt to address the shortcomings of a visual inspection to determine the quality of grout inside a hole, it has been suggested to use a device that can measure the vibrations or shock waves of an installed anchor (or its tendon) to calculate a frequency thereof. The frequency of the installed anchor is then compared to empirical frequencies of a range of anchors, some of which were intentionally installed in a substandard manner. Apart from the system being complicated and requiring some sort of expert interpretation to analyse the results, in order to get an accurate comparison between the measured frequency and the empirical frequencies at least two of the tension, anchor length and grout efficiency, parameters must be known. Seeing that the anchor pre-tension and the length of the anchor vary considerably between different installations it is unlikely that their values can be estimated with sufficient accuracy for comparative purposes. This known device is therefore far from ideal.

It is to be understood that the terms "anchor" or "rock anchor" will, in addition to rockbolts and cable anchors, extend to dowels (such as re-bar), cables, pins and any other anchors that are grouted in place. These anchors are also used in civil engineering and the application of the invention is not limited to the mining industry.

OBJECT OF THE INVENTION It is an object of this invention to alleviate at least some of the problems experienced with existing devices and methods used to test and determine the integrity of a grouted anchor installation and, in particular to determine the quality of a grout surrounding the anchor. It is a further object of this invention to provide an apparatus and method that provide an alternative to existing devices and methods used for such testing.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided apparatus for testing the grout around a rock anchor grouted into a drill hole comprising a first and second conductor for location within a grouted drill hole, at least the first conductor being insulated with an exposed contact securable in spaced apart relationship to the second conductor within a body of grout material, and the conductors extending to a position outside of the hole for measurement of grout resistivity readings.

Further features of the invention provide for apparatus as defined: in which the second conductor is provided by an elongate body of a rock anchor; in which the first conductor is an insulated wire with an exposed portion at an inner end providing the contact; in which about 0.5 mm of a conductive core of the wire is exposed to provide the contact. Further features of the invention provide for apparatus as defined: which includes a spacer with a collar securable to the body of the rock anchor which engages the first conductor and supports the contact in spaced apart relationship; in which the spacer includes a retaining formation to releasably engage the first conductor; in which the retaining formation includes a passage through which the first conductor is threaded; in which the passage has an entrance opening through which the insulated wire is inserted and an exit opening from which the exposed portion of the insulated wire protrudes; in which the passage includes a first portion which is supported substantially parallel to an anchor body located within the collar and a second portion which is extends laterally from the anchor body located within the collar to resist withdrawal of the insulated wire; in which the parallel portion of the passage is provided adjacent the entrance opening and the lateral portion is provided adjacent the exit opening; and in which the collar includes a resiliently deformable sleeve with a longitudinal opening to receive the body of the anchor.

Further features of the invention provide for apparatus as defined: in which the anchor is a cable anchor; and in which a tendon of the cable anchor provides the elongate body.

Further features of the invention provide for apparatus as defined: in which a plurality of first conductors are provided each with a contact spaced apart along a length of the second conductor; and in which each of the first conductors is respectively secured to the second conductor by a spacer with a collar securable to the body of the rock anchor which engages the first conductor and supports the contact in spaced apart relationship to the second conductor. Further features of the invention provide for apparatus as defined: in which the second conductor is insulated with an exposed second contact; in which the contact of the first conductor and the second contact are supported spaced apart at substantially the same depth within the hole; and in which the contact of the first conductor and the second contact are longitudinally spaced apart at different depths within the hole.

A further feature of the invention provides for apparatus as defined: which includes an electrical lead with a fitting configured to engage a rock anchor in an electrically conductive manner.

A further feature of the invention provides for apparatus as defined: in which the first and second conductors and installed with leads electrically conductive leads having free ends that are releasable connectable to a resistivity measuring device.

The invention further provides for a measuring device for use with the apparatus as claimed in any one of the preceding claims, the device having a pair of terminals for connection to the first and second conductors and configured to: a. apply an electrical current through a circuit established by the first and second conductors with grout providing a bridge between the first and second conductors;

b. measure the ratio of voltage to current when the electrical current is applied to the circuit; and

c. provide a reading based on recorded resistivity of the grout between the first and second conductors.

Further features of the invention provide for a device as defined: which includes an indicator to display a measured resistivity reading of the grout; in which the series of indicators is an array of light emitting diodes (LEDs); and which includes a pair of terminals for releasable connection to the first and second conductors.

In accordance with another aspect of the invention there is provided a method of testing quality of grout around a rock anchor grouted into a drill hole comprising: a. grouting a first and second conductor into a drill hole;

b. at least the first conductor being insulated with an exposed contact supported in spaced apart relationship to the second conductor;

c. applying an electrical current through the conductors; and

d. measuring the resistivity of the grout between the contact of the first conductor and the second conductor. Further features of the invention provide for a device as defined: in which an elongate body of a rock anchor is used as the second conductor; in which the first conductor is secured to the body of the anchor using a spacer which supports the contact in spaced apart relationship to the second conductor; in which a plurality of first conductors providing a plurality of contacts are supported spaced apart along the body of the anchor and resistivity readings are taken respectively from a circuit created between each of the first conductors and the body of the anchor; in which the second conductor is insulated with an exposed second contact and the contact of the first conductor is supported in spaced apart relationship to the second contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a partial cross-sectional view of apparatus and a measuring device for testing the integrity of an anchor grouted in a hole in accordance with the invention;

Figure 2 shows a front perspective view of a spacer of the apparatus of

Figure 1 which, in use, connects a conductive wire to a tendon of the anchor;

Figure 3 shows a rear perspective view of the spacer of Figure 2;

Figure 4 shows a partial cross-sectional view of an apparatus for testing the integrity of an anchor grouted in a hole in accordance with the invention at multiple locations along the anchor; and Figure 5 shows a circuit diagram of the measuring device in Figures 1 and

4. DETAILED DESCRIPTION OF INVENTION Referring to the drawings, in which like numerals indicate like features, a non- limiting example of apparatus for testing the integrity of a grouted rock anchor installation is generally indicated by reference numeral (10). Figure 1 shows a partial cross-sectional view of a rock anchor (100) grouted into a hole (102) drilled into a hanging wall (104) of an underground mine working or stope. In the accompanying drawings the anchor is shown as a cable anchor (100) including a mechanical anchor (106), a tendon (108), a faceplate (1 10) and a tensioning device (1 12). In use, an end of the tendon (108) which is its inner end (1 14.1 ) is inserted into the hole (102) and the mechanical anchor (106) is fixed or secured in place. A person skilled in the art will understand the mechanisms/means through which this engagement can be achieved.

From Figure 1 it can be seen that an end of the tendon (108) which is, in use, its outer end (1 14.2), extends from the hole (102) beyond the face of the hanging wall (104). The outer end (1 14.2) runs through the faceplate (1 10) which abuts the face of the hanging wall (104) to close off the hole (102). As shown in Figure 1 , a cement or resin grout is pumped into the hole (102) so as to fill the space between the sidewall of the hole (102) and the tendon (108). Although not illustrated in the accompanying drawings, the cable anchor (100) would typically also include a grout injection tube and breather tube running through the faceplate (1 10).

Referring still to Figure 1 it can be see that the apparatus (10) includes a first conductor (12) which, in use, runs from a position within the hole [or point of interest] (14) along the tendon (108) to a second point or position (16) located outside the hole (102). The conductive member (12) is preferably in the form of an insulated wire (12) which has a conductive inner core surrounded by a layer of insulation.

In this embodiment, the tendon (108) provides an elongate body of the rock anchor (100) and this component is used as a second conductor (108). The first conductor or wire (12) is, in use, attached to the second conductor or tendon (108) at least in the region of the point of interest (14) using an attachment device (18). The attachment device (18) can be seen in greater detail in Figures 2 and Figure 3 and will be described with reference to these figures.

In the accompanying drawings the attachment device is provided as a spacer (18) that includes a collar (22) which locates about the tendon (108) when attached thereto. The spacer (18) has a body (20) through which a cylindrical passage extends to provide the collar (22). In use, the spacer (18) is secured to the tendon (108) such that the longitudinal axis of the tendon is substantially parallel to the axial direction of the passage that provides the collar (22). To facilitate the attachment of the spacer (18) to the tendon (108), the collar (22) is provided as a sleeve with a longitudinal slot providing two resiliently deformable, curved jaws (24.1 ) and (24.2). The resilient jaws allow the spacer (18) to be attached to the tendon (108) without the need to thread the tendon through the collar (22). Instead, the spacer (18) is attached to the tendon (108) by simply forcing jaws (24.1 ) and (24.2) over the tendon (108) in a direction substantially perpendicular to the longitudinal axis of the tendon (108). Although the resilient jaws (24.1 ) and (24.2) will grip the tendon (108) in order to hold the spacer (18) in place, it is envisaged that securing means, such as a cable tie for example, could also be used to facilitate secure engagement of the spacer (18) to the tendon (108).

Referring to Figure 2 in particular, it can be seen that the spacer (18) has a retaining formation in the form of an internal passage (26) running through its body (20). In use, the passage (26) receives a portion of the wire (12) therein in order to secure the wire to the spacer (18) and, accordingly, retain the wire at the point of interest (14) along and spaced apart from the tendon (108). The passage (26) has an entrance opening (28) through which the wire (12) is, in use, received and an exit opening (30) which is open to the hole (102).

The internal passage (26) includes a first portion which is supported substantially parallel to an anchor body or tendon (108) and a second portion which is extends laterally from the tendon (108). The passage (26) in this embodiment is arranged so that the axial centreline of the exit opening (30) is substantially transverse, preferably perpendicular, to the axial centreline of the tendon (108) - and thus extends laterally away from the tendon (108). The axial centreline of the entrance opening (28) is substantially parallel to the axial centreline of the tendon (108). It is believed that the change in orientation between the entrance (28) and exit (30) openings help secure the wire (12) in the passage (26) and, accordingly, to the spacer (18). The spacer (18) further includes a shield (32) which at least partially covers the exit opening 30. As illustrated in Figure 2, the shield (32) is dome-shaped and protrudes from the body (20) of the spacer (18). In use, the collar (22) of the spacer (18) is attached to the tendon (108) in an orientation so that the dome- shaped shield (32) is open towards the opening of the hole (102). This arrangement allows the wire (12) to run from the spacer (18) along the length of the tendon (108) and out the hole (102).

The wire (12) is inserted into the passage (26) so that a portion thereof that extends from the exit opening (30). This portion of the wire (12) that extends from the exit opening (30) is stripped of its layer of insulation to expose its inner conductive core. In the preferred embodiment, at least 0.5mm of the conductive inner core of the wire (12) is exposed.

It should be understood that by using the spacer (18) the end of the wire (12) is held at a predetermined, fixed distance (spaced apart) from the tendon (108). This is an important aspect when measuring electrical conductivity or resistivity between the exposed portion of the wire (12) and the tendon (108). More about this feature is set out below.

Returning now to Figure 1 it can be seen that the apparatus (10) further includes a device (34) for measuring and displaying the resistivity of the grout located between the exposed portion of the wire (12) and the tendon (108) at the position within the hole (102) corresponding to the point of interest (14). A portion of the wire (12) that extends from the hole (102) provides a first lead with a connector (38) at its free end for connecting the first conductor (12) to the measuring device (34). The device (34) has two terminals (36.1 ) and (36.2) which are releasably connectable to the connector (38) provided at the end of the wire (12) and a connector (40) provided at the end of a second lead (42) connected to the tendon (108) at a location outside the hole (102). From the above description it should be understood that by connecting the terminals (36.1 ) and (36.2) to the connectors (38) and (40), an electrically conductive circuit is created. The grout located between the exposed portion of the wire (12) and the tendon (108) at the position within the hole corresponding to the point of interest (14) (provides a grout bridge which) completes the electrically conductive circuit to allow the device (34) to measure the electrical resistivity of the circuit. The device (34) also includes a series of indicators (44) which is used to indicate the measured value.

In the preferred embodiment of the apparatus (10), the series of indicators (44) is in the form of an array of light emitting diodes (LEDs). Each LED represents a predetermined value so that an assessment of the integrity of the grout inside the hole can be made based on the number of LEDs that are illuminated during the testing procedure (when the current is applied through the circuit). The device (34) further includes an indicator in the form of an LED (46) which lights up when an open circuit is detected. The device (34) also includes an indicator in the form of an LED (48) to indicate when a low battery level is detected. In the preferred embodiment of the device (34) the voltage supply of its battery is measured to detect a low battery level.

The device is accordingly configured to: a. apply an electrical current through a circuit established by the first and second conductors with grout providing a bridge between the first and second conductors;

It will be appreciated that the device will be calibrated based on the arrangement of the first and second conductors and other parameters relating to the particular grout requirements of an installation. While a numerical resistivity data reading will be taken by the device, the array of LEDs (44) can be provided to indicate whether the grout quality (based on the reading) falls within an acceptable range.

Although the method of testing the integrity of a grouted anchor using the apparatus (10) should be clear from the above description, it will now be described briefly. The method starts with the step of providing an insulated wire (12) and attaching it to a tendon (108) of an anchor (100) in the region of or position corresponding to a point of interest (14). The wire (12) is typically connected to the tendon (108) so that it runs from the point of interest (14), along the tendon (108) and to a second point located outside the hole (102). A lead (42), preferably in the form of a conductive wire, is then attached to the tendon (108) at a location outside the hole (102). Next, the wire (12) running from the point of interest (14) on the tendon (108) and the lead (42) running from the second point of the tendon located outside the hole (102) are connected to a measuring device (34) for measuring the electrical resistivity of the electrical circuit. When all of the components are connected as described above an electrically conductive circuit is created which allows the measuring of the resistivity of the grout located between the exposed end portion of the wire (12) and the tendon (108).

More generally, the method of testing quality of grout around a rock anchor grouted into a drill hole comprising: a. grouting a first and second conductor into a drill hole;

c. applying an electrical current through the conductors; and

d. measuring the resistivity of the grout between the contact of the first conductor and the second conductor.

A second embodiment of the apparatus according to the invention (and broader application of the method involved) is illustrated in Figure 4 and indicated generally by reference numeral (50). The only difference between the first and second embodiments is that the apparatus (50) includes a number of spacers (18) which are located spaced apart along the length of the tendon (108) inside the hole (102). Each spacer (18) respectively supports a corresponding first conductor provided as an insulated wire (12) of the kind described above. The use of multiple spacers (18) and first conductors (12) allows the device (34) to measure the resistivity of the grout at multiple points of interest along the length of the tendon (108). It will be understood that the resistivity at the different positions within the hole corresponding to these points of interest is measured by respectively connecting the wires (12) corresponding to the three spacers (18) at a particular point of interest to the device (34) along with the second conductor (108) to close the various circuits which result from these connections. The three wires (12) can be marked with numbered tags or colour coded where they protrude from the hole (102) to indicate at which depth each one takes a reading. In a development of the second embodiment, the use of apparatus (50) with multiple spacers (18) may also provide for the measurement of resistivity between the exposed wires (12). This will be done by connecting two of the wires (12) to the device (34). A second wire (12) connected to the device (34) replaces the lead (42) which would ordinarily be attached to the tendon (108). The arrangement tests the resistivity of the grout in the region between the exposed ends. From the development described, it will be understood that the apparatus may have different versions of first and second conductors which are insulated with contacts at their inner ends to be supported in spaced apart relationship within a grouted anchor hole. Such an arrangement of conductors may be used to test a grout bride across the width of a hole or one that extends along the length of the hole. Various configurations of suitable spacers or support mechanisms that hold the contacts in a position where the space between them is known will be within the design competence of a person skilled in the art.

The inventors have identified that by measuring the electrical conductivity or resistivity of the circuit and, accordingly, the grout between the exposed portion of the wire (12) and the tendon (108), or second wire, the structural integrity of the grouted anchor can be determined. The measured resistivity value is a direct indication of the quality and quantity of grout located between the exposed portion of the wire (12) and the tendon (108) or second wire. The advantage to using the spacer (18) is that the distance between the exposed portion of the wire (12) and the tendon (108) is fixed and therefore known. Additionally positioning several spacers (18) at fixed distances apart, along the length of the tendon (108) will achieve the same result. In view of the fact that other factors, such as the length of the wire (12) and resistivity of the tendon (108), can be controlled the only variable in measuring the resistivity of the circuit is the resistivity of the grout. The apparatus (10) therefore allows for an accurate measurement of the quantity and quality of the grout inside the hole. Substandard anchors can therefore be detected at an early stage before any rock face movement has taken place. It is believed that the apparatus (10) can be used to improve the accuracy and reliability of quality assurance processes in order to improve the safety of underground working conditions.

A person skilled in the art will appreciate that a number of variations may be made to the features of the embodiments described without departing from the scope of the invention. The first and second conductors may, for example, be provided by a pair of rigid insulated rods with exposed inner ends. Such conductors can be supported in fixed relative relationship from positions where they extend through openings in a faceplate. Alternatively, the first conductor may be provided as a rod of this kind and the second conductor by the body of the rock anchor.

Claims

Apparatus for testing the grout around a rock anchor grouted into a drill hole comprising a first and second conductor for location within a grouted drill hole, at least the first conductor being insulated with an exposed contact securable in spaced apart relationship to the second conductor within a body of grout material, and the conductors extending to a position outside of the hole for measurement of grout resistivity readings.

Apparatus as claimed in claim 1 in which the second conductor is provided by an elongate body of a rock anchor.

Apparatus as claimed in claim 2 in which the first conductor is an insulated wire with an exposed portion at an inner end providing the contact.

Apparatus as claimed in claim 3 in which about 0.5 mm of a conductive core of the wire is exposed to provide the contact.

Apparatus as claimed in claim 3 which includes a spacer with a collar securable to the body of the rock anchor which engages the first conductor and supports the contact in spaced apart relationship.

Apparatus as claimed in claim 5 in which the spacer includes a retaining formation to releasably engage the first conductor.

Apparatus as claimed in claim 6 in which the retaining formation includes a passage through which the first conductor is threaded.

Apparatus as claimed in claim 7 in which the passage has an entrance opening through which the insulated wire is inserted and an exit opening from which the exposed portion of the insulated wire protrudes.

Apparatus as claimed in claim 8 in which the passage includes a first portion which is supported substantially parallel to an anchor body located within the collar and a second portion which is extends laterally from the anchor body located within the collar to resist withdrawal of the insulated wire. 10. Apparatus as claimed in claim 9 in which the parallel portion of the passage is provided adjacent the entrance opening and the lateral portion is provided adjacent the exit opening.

1 1 . Apparatus as claimed in any one of claims 5 to 10 in which the collar includes a resiliently deformable sleeve with a longitudinal opening to receive the body of the anchor.

12. Apparatus as claimed in any one of claims 2 to 1 1 in which the anchor is a cable anchor.

13. Apparatus claimed in claim 12 in which a tendon of the cable anchor provides the elongate body.

14. Apparatus as claimed in claim 2 in which a plurality of first conductors are provided each with a contact spaced apart along a length of the second conductor.

15. Apparatus as claimed in claim 14 in which each of the first conductors is respectively secured to the second conductor by a spacer with a collar securable to the body of the rock anchor which engages the first conductor and supports the contact in spaced apart relationship to the second conductor.

16. Apparatus as claimed in claim 1 in which the second conductor is insulated with an exposed second contact. 17. Apparatus as claimed in claim 16 in which the contact of the first conductor and the second contact are supported spaced apart at substantially the same depth within the hole.

18. Apparatus as claimed in claim 16 in which the contact of the first conductor and the second contact are longitudinally spaced apart at different depths within the hole.

19. Apparatus as claimed in claim 2 which includes an electrical lead with a fitting configured to engage a rock anchor in an electrically conductive manner.

20. Apparatus as claimed in claim 1 in which the first and second conductors and installed with leads electrically conductive leads having free ends that are releasable connectable to a resistivity measuring device.

21 . A measuring device for use with the apparatus as claimed in any one of the preceding claims, the device having a pair of terminals for connection to the first and second conductors and configured to: a. apply an electrical current through a circuit established by the first and second conductors with grout providing a bridge between the first and second conductors;

22. A device as claimed in claim 21 which includes an indicator to display a measured resistivity reading of the grout.

23. A device as claimed in claim 22 in which the series of indicators is an array of light emitting diodes (LEDs).

24. A device as claimed in any one of claims 21 to 23 which includes a pair of terminals for releasable connection to the first and second conductors.

25. A method of testing quality of grout around a rock anchor grouted into a drill hole comprising: a. grouting a first and second conductor into a drill hole;

c. applying an electrical current through the conductors; and d. measuring the resistivity of the grout between the contact of the first conductor and the second conductor.

26. A method as claimed in claim 25 in which an elongate body of a rock anchor is used as the second conductor. 27. A method as claimed in claim 26 in which the first conductor is secured to the body of the anchor using a spacer which supports the contact in spaced apart relationship to the second conductor.

28. A method as claimed in claim 27 in which a plurality of first conductors providing a plurality of contacts are supported spaced apart along the body of the anchor and resistivity readings are taken respectively from a circuit created between each of the first conductors and the body of the anchor.

A method as claimed in claim 25 in which the second conductor is insulated with an exposed second contact and the contact of the first conductor is supported in spaced apart relationship to the second contact.