Application of time domain reflectometry (TDR) for block and sublevel caving...

Report 2008:P3 ISSN 1653-5006

Swedish Blasting Research CentreMejerivägen 1, SE-117 43 Stockholm

Luleå University of TechnologySE-971 87 Luleå www.ltu.se

Application of time domain reflectometry (TDR) for block and sublevel caving mines – State-of-the-art and preliminary laboratory shear tests

Användning av tidsdomän reflektrometri (TDR) för block-och skivrasgruvor – dagsläget i industrin och en första serie labbskjuvförsök

Matthias Wimmer, SwebrecFinn Ouchterlony, Swebrec

Universitetstryckeriet, L

uleå

Swebrec Report 2008:P3

Application of time domain reflectometry (TDR) for block and sublevel caving mines - State-of-the-art and preliminary laboratory shear tests

Användning av tidsdomän reflektrometri (TDR) för block- och skivrasgruvor - dagsläget i industrin och en första serie labbskjuvförsök

Matthias Wimmer, Swebrec

Finn Ouchterlony, Swebrec

Luleå April 2008, revised May 2, 2008

Swebrec - Swedish Blasting Research Centre

Luleå University of Technology

Department of Civil and Environmental Engineering • Division of Rock Engineering

i

Application of TDR for block and sublevel caving mines Swebrec Report 2008:P3

SUMMARY

The objective of the present report is to summarize the state-of-the-art of the application of time

domain reflectometry (TDR) with regard to its application to block and sublevel caving mines.

A second objective is to report on preliminary laboratory shear tests in which a grouted cable was

exposed to a controlled shear movement in a plane inclined relative to the cable.

TDR is an essential part of successful cave management for block caving mines. It is used to verify

cave initiation, the subsequent progress of the caving zone and the effect of caving on rock mass

fracturing. This enables operators to adjust production to the caving rate and allows remedial action to

encourage caving to be implemented if stagnant or delayed areas exist within the cave. Continuous

TDR monitoring also supports the identification of caving mechanisms such as gravity, stress, or

structure controlled caving. In contrast, monitoring initiation and caving by using TDR in sublevel

caving mines is limited to few applications.

The TDR tests are part of a test program named “full-scale in-situ burden instrumentation test”

(FIBIT). The tests focus on the verification of the blast function in the upper part of the ring and

comprise the drilling of boreholes from the longitudinal drifts in the footwall. These boreholes pass

through the interfaces of the intact pillar and the succeeding blast rings.

In the TDR measurements of the FIBIT project, coaxial cables will be grouted into several of these

boreholes and long-term monitored. This monitoring determines the actual burden and verifies that the

planned breakage was complete and if overbreak occurred. In addition, the TDR system could be

useful in monitoring displacements and thus blast related damage behind the actual blasted ring in

order to verify the initial situation as well as functionality of charge columns in the next pre-charged

rings. The laboratory tests focus on this and the following factors:

Correlation between TDR reflection magnitude and shear displacement,

Effect of different shear angles and failure modes,

Influence of cable length (attenuation and resolution),

Influence of upstream cable deformations (different sizes and positions).

As an extension of the present study, field tests preceding the FIBIT tests are planned in the Kiruna

mine to verify the laboratory findings. Another purpose of these tests is to see whether the blast

induced dynamic failure of a coaxial cable could be distinguished from a geologically induced one.

ii


SAMMANFATTNING

Syftet med denna rapport är först att ge dagsöverblick av hur tidsdomän reflektrometri (TDR) används

i block- och skivrasgruvor. Det andra syftet är att rapportera en första serie med laboratorieförsök där

en ingjuten koaxialkabel utsatts för styrd skjuvrörelse i ett plan snett kabeln.

TDR utgör en viktig metod för att kontrollera rasets utbredning i en blockrasgruva. Den används för

att verifiera att raset börjar, hur raset utvecklas liksom hur raset påverkar bergmassans uppsprickning.

Detta gör det möjligt för gruvan att anpassa produktionen till rasutbredningshastigheten och att

tillgripa åtgärder som skyndar på raset om man träffar på områden där raset stagnerat eller dess

hastighet avtagit En kontinuerlig TDR-mätning kan också hjälpa till att identifiera rasmekanismen som

varande främst tyngdkrafts-, spännings- eller strukturdriven. Däremot har TDR bara använts i ett fåtal

fall för att studera initieringen av raset och dess senare utveckling i skivrasgruvor.

De laboratorieförsök med TDR som rapporteras utgör en del av ett försöksprogram med namnet ”full-

scale in-situ burden instrumentation test” (FIBIT). De är inriktade mot att verifiera rassalvans funktion

i den övre delen av skivraskransen och i dem ingår borrning av mäthål genom den kvarstående

malmpelaren in i skivraskransar från en fältort i liggväggen

I TDR-mätningarna i FIBIT-försöken kommer koaxialkablar att gjutas in i flera av borrhålen och

övervakas under en längre tid. Huvudintresset inriktas mot att bestämma kransytans läge och att

bestämma om lossbrytningen blivit fullständig samt om någon bakåtbrytning skett. Dessutom kan

TDR-mätningarna visa sig användbara för att detektera förskjutningar och sprängskador bakom

skivras-kransarna. På så sätt kan de sprängtekniska förutsättningarna för laddningarna i nästa

förvägsladdad skivraskrans bestämmas. De redovisade laboratorieförsöken har inriktats mot frågor av

vikt för FIBIT-försöken enligt följande:

Sambandet mellan den reflekterade TDR-signalens amplitud och skjuvrörelsens storlek

Inverkan av olika skjuvvinklar mot kabelriktningen vid skjuvning med drag eller tryck

Kabellängdens inverkan på signaldämpning och upplösning

Inverkan av klämpunkter uppströms ett skjuvplan

Som en fortsättning innan FIBIT-försöken planeras försök i Kiruna-gruvan där labbresultaten om

möjligt verifieras. Dessutom skall undersökas om ett spränginducerat dynamiskt kabelbrott kan

särskiljas från ett mer statiskt, geologiskt inducerat brott.

iii


Contents

1 INTRODUCTION ..................................................................................................... 1

2 MONITORING MOVEMENTS NEAR CAVING STOPES ........................................ 2

3 EXPERIMENTAL WORK ........................................................................................ 5

3.1 Objectives ................................................................................................................................ 5

3.2 Test procedure ......................................................................................................................... 6

3.3 Description of TDR system ..................................................................................................... 9

3.4 Discussion of results .............................................................................................................. 10

3.4.1 Characteristic waveform signatures ...................................................................................... 10

3.4.2 Evaluation of reflection coefficients ..................................................................................... 11

3.4.3 Influence of distance on a single shear reflection.................................................................. 14

3.4.4 Effects of upstream cable deformations ................................................................................ 15

4 CONCLUSIONS AND RECOMMENDATIONS ......................................................18

5 ACKNOWLEDGEMENTS ......................................................................................21

6 REFERENCES .......................................................................................................22

7 APPENDICES ........................................................................................................26

iv


APPENDICES

Appendix 1. Drawings of steel pipes for shear tests at different angles. ............................................... 26

Appendix 2. Documented data for all tests. .......................................................................................... 28

Appendix 3. Images of shear faces for all tests. .................................................................................... 32

FIGURES

Figure 1. 3D view of TDR cables located around DOZ (Deep Ore Zone) block cave (Szwedzicki et al.,

2004). .................................................................................................................................. 3

Figure 2. Transverse section of SLC rings showing installation of TDR cables (Krekula 2004). .......... 4

Figure 3. Conceptual test layout for the instrumentation of the burden of blasted SLC rings (not to

scale). .................................................................................................................................. 5

Figure 4. Test set-up for laboratory shear tests at Complab, LTU. ......................................................... 7

Figure 5. Crimps in mm, cable type P3 750 JCA. ................................................................................... 8

Figure 6. Crimps in mm, cable type CA 519 J. ....................................................................................... 8

Figure 7. Distinction of different shear failure modes. ........................................................................... 8

Figure 8. Testing machine, sample III-C-30deg loaded, Complab, LTU. ............................................... 9

Figure 9. Example of characteristic waveform signatures, II-S-0deg. .................................................. 10

Figure 10. Reflection coefficient versus shear displacement, 40 m distance, without crimps upstream.

.......................................................................................................................................... 12

Figure 11. Comparison of width-to-magnitude ratios. .......................................................................... 13

Figure 12. Reflection coefficient versus position, III-C-0deg. .............................................................. 14

Figure 13. Reflection coefficient versus position, III-C-45deg. ............................................................ 14

Figure 14. Reflection coefficient versus shear displacement, I-S-45deg, without upstream crimp. ..... 15

Figure 15. Reflection coefficient “small” crimp (1) / no crimp, 40m distance. .................................... 17

Figure 16. Reflection coefficient “medium” crimp (1) / no crimp, 40m distance. ................................ 17

Figure 17. Reflection coefficient “large” crimp (1) / no crimp, 40m distance. ..................................... 17

Figure 18. Suggested, improved design of a shearing apparatus. .......................................................... 20

TABLES

Table 1. Properties of used smooth and corrugated coaxial cables. ........................................................ 7

Table 2. Mixture for used mortar. ........................................................................................................... 7

Table 3. Level of noise for different tests. ............................................................................................. 11

Table 4. Influence of upstream crimps with respect to position. ........................................................... 16

1


1 INTRODUCTION

The concept of time domain reflectometry (TDR) was initially developed and used as a measurement

technique to locate and evaluate discontinuities in coaxial transmission lines by observing reflected

waveforms (Moffitt, 1964). The concept was first extended to the measurement of material properties

such as soil moisture in which coaxial cables have been embedded (Topp et al., 1980). TDR has also

been applied to monitor progressive, localized rock mass movements and failure (Wade & Conroy,

1980; Panek & Tesch, 1981; O`Connor & Dowding, 1984) as well as soil deformations (Dowding &

Pierce, 1994; Dussud, 2002).

Rock mass movements deform coaxial cables grouted into boreholes, which produces localized

deformations. This changes the cable capacitance which in turn causes reflections of injected pulses,

whose waveform depends on the nature of the change. It has always been of particular interest to

determine a quantitative measure of the magnitude and type of the rock mass deformation, both

empirically and numerically (Su, 1987; Dowding et al., 1989; Aimone-Martin & Francke, 1997;

Bordas, 1998).

Regarding the operating principles of TDR it can basically be viewed as a remote and real-time

sensing electrical measurement technique in which a fast rise-time step voltage pulse is emitted out to

be reflected by any kind of deformation along the coaxial cable. By assuming a constant pulse

propagation velocity (vp), the distance to the detected deformation is proportional to the elapsed time

between initiation of the voltage pulse and the arrival of the returned pulse. The magnitude of the

reflection coefficient ρ defines the impedance change (severity due to the deformation) as the ratio of

the reflected voltage Vr to the incident voltage Vi. The shape of the reflection, i.e. the waveform

signature, defines the type of cable deformation.

An extended review on TDR technology and in particular its application to geomechanics is provided

by O`Connor & Dowding (1999).

2


2 MONITORING MOVEMENTS NEAR CAVING STOPES

To maintain a successful caving operation in terms of production and safety understanding of rock

mass behavior under caving conditions is necessary. Hence regular monitoring, which provides

information on cave initiation, propagation and caving rate is essential for actual production

management. Furthermore this supports and allows correct decision-making to avoid operational risks

that can alter the continuity of the production process. Optimization of the current design makes the

mine cost-effective during the later stage of mine life. Monitoring becomes even more vital today as

several operations mining deep low-grade but massive orebodies are being opened, in which stresses

normally become greater and rock masses may be more competent than in previous operations.

Four general types of measurement methods (Brown, 2003) are commonly used to monitor the

initiation and development of caving, namely a) measurements in open holes, b) cavity monitoring

systems, c) microseismic systems and d) time domain reflectometry (TDR). In practice, most of these

methods are often used in combination and they complement one another. Progress was made in

estimating the shape of the cave back by using gravity measurements at El Teniente mine (Gaete et al.,

2007). The following review focuses on the application of TDR.

TDR was widely used for monitoring cave progression and concomitant phenomena among others in

DOZ (Rachmad & Sulaeman, 2002; Szwedzicki et al., 2004), El Teniente (Rojas et al., 2000; De

Nicola Escobar & Fishwick, 2000), Henderson (Stewart et al., 1984; Rech & Watson, 1994), Kiruna

(Henry & Dahnér-Lindqvist, 2000; Krekula, 2004), Northparkes (La Rosa & Chen, 1997), Palabora

(Bartlett, 2008), Ridgeway (Trifu et al., 2002) and San Manuel (Panek & Tesch, 1981) mines.

One of the earliest applications of TDR in monitoring rock mass deformation in caving mines was

accomplished by Panek and Tesch (1981). Several experiments were conducted in which coaxial

cables were installed both in boreholes drilled into the walls and roofs as well as laid out on the floor

of a drift against the rib, covered by mortar. These cables were used to detect the outward progress of

cracking around a separate mining block as it was undercut and caved on the level below. Although

changes in TDR waveforms were documented, the correlation between rock mass response and cable

deformation was limited to locate major rock fractures, which developed as the caving progressed

(O`Connor & Wade, 1994). The location of visible fractures observed in the grout allowed comparison

of the aperture and shear displacement with TDR reflections received along the cable.

Following these first trials, TDR measurements were more widely used to verify the cave initiation

process and to serve as a long-term data source on cave growth for determination of the production

schedule. This was mainly done by periodically monitoring the progress of breakage on singular

coaxial cables grouted into former exploration boreholes (Stewart et al., 1984; Rech & Watson, 1994).

3


Great efforts were made to advance the monitoring of cave growth by using TDR as numerous TDR

cables (e.g. DOZ, Northparkes and Palabora mines) were installed in a fan like manner from several

stations to cover a large area, see Figure 1. This allows, besides the verification of cave initiation, an

accurate determination of the actual 3-D shape of the caving zone, i.e. horizontal and vertical

propagation as well as the effects of caving on rock mass fracturing (Szwedzicki et al., 2004).

Figure 1. 3D view of TDR cables located around DOZ (Deep Ore Zone) block cave (Szwedzicki et al., 2004).

Adequate knowledge about the shape of the cave front is an essential part of cave management and

enables one to adjust production in relevant areas to increase or slow down caving (La Rosa & Chen,

1997). Thereby waste dilution at an early stage deriving either from the sides of the orebody or the

surface caused by chimney caving can be mitigated. Furthermore, if stagnant or delayed areas exist

within the cave, remedial actions such as boundary weakening or extending the undercut can be

implemented to encourage caving. TDR by itself, even if combined with production data, cannot

provide accurate enough information on the development of an air gap, i.e. the distance between the

solid cave back and the loose caved rock. However, the actual air gap might simply be monitored in

open holes with the “cable and plug method” (Julin, 1992) or “geophone-type detector with whiskers”

(Chen 2000) or more advanced methods with cavity monitoring systems (Stewart et al., 1984; De

Nicola Escobar & Fishwick Tapia, 2000).

Continuous and thus fully automatic TDR monitoring also aids definition of caving mechanisms such

as gravity-, stress-, or structure controlled caving (La Rosa & Chen, 1997).

Monitoring initiation and caving by using TDR with respect to sublevel caving (SLC) mines is limited

to a few applications (Trifu et al., 2002; Henry & Dahnér-Lindqvist, 2000).

4


There was an investigation on the effect of blasting and mucking on rock mass fracturing behind

blasted SLC rings at the Kiruna mine (Krekula, 2004). TDR cables have been installed parallel with

the rings and measurements have been performed directly after blasting and at different stages of

mucking the material, see Figure 2.

Figure 2. Transverse section of SLC rings showing installation of TDR cables (Krekula 2004).

Possible displacements could be combined with the locally observed joint planes by combining

magnitudes as well as positions of rock displacements at a specific time. The promising results have

shown that the top and central part of the subsequent rings is the part most affected by blast damage

and movements while mucking. Shear movements considerably decreased in number and magnitude

with increased distance to the actual brow. It could also be shown that the intersecting of several waste

lenses of quartz porphyry has caused major, detectable movements. The position of displacements in

the subsequent rings coincides well with the assumed shape and development of the Isolated

Movement Zone (Larsson, 1998). However, the extent of back-breakage caused by blasting to the

subsequent ring could not be studied with this layout of the cable installation.

5


3 EXPERIMENTAL WORK

3.1 Objectives

The implementation of TDR is part of a test program named “full-scale in-situ burden instrumentation

test” (FIBIT) as part of the doctoral thesis “Improved breakage and flow in sublevel caving” which is

being carried out at the LKAB mine in Kiruna, Sweden.

The complete test program focuses on the following:

Verification of actual burden (TDR, part 1)

Study of deformations behind the blasted ring (TDR, part 2)

Measurement of the propagation of breakage inside the burden (fast-sampling TDR)

Study of burden movement and compaction of caving masses (different sensor systems)

All tests will be monitored by new and proven measuring methods so that a relevant judgment of the

results can be made and a better conceptual flow model can be built up.

The conceptual layout of the test program is outlined in Figure 3. The tests focus on the verification of

the blast function in the upper part of the ring and comprise the drilling of boreholes from the

longitudinal drifts in the footwall. These boreholes pass through the interfaces of the intact pillar and

the succeeding blast rings.

Figure 3. Conceptual test layout for the instrumentation of the burden of blasted SLC rings (not to scale).

Concerning TDR measurements as part of this project, coaxial cables will be grouted into several of

these boreholes and long-term monitored. The main interest lies in determining the actual burden and

verifying if the planned breakage has been completed and if back-breakage has occurred. This process

6


of blasting in a confined situation is entirely inaccessible to direct observation. In addition, the TDR

system is useful in monitoring deformations and thus blast-related damage behind the actual blasted

ring to verify the initial situation as well as the functionality of charge columns in the next pre-charged

rings.

The laboratory tests focus on this and the following relationships and factors are:

Correlation between TDR reflection magnitude and shear displacement,

Effect of different shear angles and failure modes,

Influence of cable length (attenuation and resolution),

Influence of upstream deformations (different sizes and positions).

Preliminary field tests are planned to verify the laboratory findings as well as if the blast induced

dynamic failure of a coaxial cable (tensile failure) could be distinguished from a geologically induced

one (shear failure).

3.2 Test procedure

The tests described in the following were carried out within 2 campaigns (I and II) at Complab at

Luleå University of Technology (LTU) and comprise totally 9 cable samples.

To study the influence of the shear angle on the TDR response, steel pipes (ST 52.0) with a length of

1.5 m and cut at three different angles (0, 30 and 45 deg) at half-length have been prepared. The

diameter of the pipes, Ø 127 × 6.3 mm, has been adjusted to the standard dimension of boreholes that

are drilled in Kiruna. To enable the griping of the sample by the servo-controlled dynamic testing

machine (Dartec, 600 kN capacity) precisely aligned steel plates have been welded to the pipes. To

facilitate the mounting of the sample in the machine the pipe halves have been dot welded. Drawings

of the pipes are shown in Appendix 1. Two coaxial cable types, i.e. smooth Al- and corrugated Cu –

outer conductor (P3 750 JCA“S” and CA 519 J “C”), with different mechanical and electrical

properties (see Table 1) were tested. They have almost the same outer dimensions, Ø 21.08 and 20.10

mm respectively. Both cable types had an impedance of 75 ohm.

The cables have been centrically grouted into the pipe. To increase the bond between cable and

concrete the plastic jacket of the cable has been roughened. The concrete was mixed according to the

standard recipe used by KGS (LKAB`s contractor for concrete), see Table 2. The curing time was at

least 28 days (DIN 1164).

7


Table 1. Properties of used smooth and corrugated coaxial cables.

Table 2. Mixture for used mortar.

Figure 4 shows the complete test assembly. The cable ends have been prepared with suitable cable

preparation tools and then interconnected using suitable splice connectors as well as reducers with low

return loss and high shielding efficiency. If the splice connectors are well screwed together no

influence on the signal is detected.

Figure 4. Test set-up for laboratory shear tests at Complab, LTU.

type OD type ID OD OD

- mm - mm mm mm mm N N/m pf/mOhm

s% nom Ohms/km

P3750JCA Al-Cu 4,24 Al 17,2 19 21,1 203 3002 3,86 50 ± 3 75 ± 2 87 2,55

CA519J Cu 3,90 Cu 16,70 17,20 20,10 145 1606 5,37 50 ± 3 75 ± 2 88 4,22

* CommScope; both cables ahave a micro-cellular foam polyethylene dielectric and polyethylene jacket

Attenuation @68°F [dB/100m] versus frequency, see corresponding data sheets at: www.commscope.com

conductor

inner outerjacket

imped

ance

min

ben

din

g

radiu

s

velo

city o

f

pro

pag

ation

capacitan

ce

vendor id

Electrical characteristicsm

ax D

C lo

op

resistance

@68°F

Properties of coaxial cablesMechanical

characteristics

max

pullin

g

tensio

n

weig

ht

Contents Specification / trade name mass- %

sand 0-8 mm 72,61

cement CEMII/A-LL 42.5R, Maxit 17,66

water - 8,48

silica sand Grade 920-D, Elkem Microsilika 1,18

plasticizer Glenium 51, BASF 0,07

8


In order to study the effect of upstream deformations on signals reflected downstream, crimps with

different sizes (campaign I and II) and positions (campaign I) have been simulated. The crimps have

been made in a controlled and repeatable way and are shown in Figure 5 and Figure 6. The depth of

the crimp is set by adjusting the separation of the squeezing grips in a bench vice. The length of the

crimps is determined by the grip’s rounded width (half of a steel rod with Ø = 40 mm).

Figure 5. Crimps in mm, cable type P3 750 JCA. Figure 6. Crimps in mm, cable type CA 519 J.

In campaign I the attenuation as a function of cable lengths (30, 40 and 50 m) was also studied. The

frequent changing of cables according to the set-up chosen required the use of special low-resistance

cable connectors. In campaign II the effect of the actual shearing mode was studied, see Figure 7. The

shearing was then associated with tension acting on the cable whereas for campaign I the cable was

being compressed while sheared off. According to this the shear angle could also be linked to the

actual shearing mode if the angle was defined between the centre line of the pipe and the shear plane

at the sliding, with the sign depending on the direction of movement (downward or upward).

Figure 7. Distinction of different shear failure modes.

The following three tests have been affected by hydraulic system problems: I-S-0deg (at 6 mm; stop at

14 mm shearing), II-S-30deg (0 - 10 mm shearing deformation not captured) and II-S-45deg (19 - 20

mm shearing deformation not captured, slipping of machine grips). Here S refers to the cable type P3

750 JCA with smooth Al outer conductor and C to cable type CA 519 J with a corrugated Cu outer

conductor.

9


Figure 8 shows the testing made at Complab, LTU. Appendix 2 contains the documentation of all tests

carried out.

Figure 8. Testing machine, sample III-C-30deg loaded, Complab, LTU.

3.3 Description of TDR system

The Campbell Scientific`s TDR100 time domain reflectometry system consisting of the TDR100

reflectometer, the SDMX50 coaxial multiplexer as well as the PCTDR software were used in these

tests. The system had the following features (TDR100 – manual, 2001):

Pulse generator output: 250 mV

Output impedance: 50 ohms ± 1 %

Time response of combined pulse generator and sampling circuit: 300 ps

Pulse generator aberrations: ± 5 % within first 10 ns, ± 0.5 ns after 10 ns

Pulse length: 10 ms

Timing resolution: 12.2 ps

Waveform sampling: 20 to 2048 waveform values over chosen length

Distance (vp = 1)

o Range: 2 to 2100 m (i.e. 0 to 7 ms)

o Resolution: 1.8 mm (i.e. 6.1 ps)

Waveform averaging: 1 - 128

The reflectometer was highly sensitive to electrostatic discharge since no transient protector is present.

The pulser should thus not be used without an appropriate protector or solely in combination with the

multiplexer. In addition to this the pulser should be grounded in a proper way.

10


3.4 Discussion of results

3.4.1 Characteristic waveform signatures

Figure 9 shows typical composite characteristic waveform signatures, in which the reflection

coefficient ρ is plotted versus the cable length. Totally 2048 waveform values have been sampled over

a length of 60 m and averaging 4 values at a given distance before collecting values at the next

distance increment.

For a short distance in the beginning the influence of the multiplexer as well as the patch cable (RG

59) was noticeable. Afterwards the signal was stable with ρ around 0.2 and could be explained by the

mismatch of the TDR pulser with 50 ohm output impedance connected to a 75 ohm cable which

created a nominal Reflection Coefficient Rc = (Z1 + Z2) / (Z1 + Z2) = (75 - 50) / (75 + 50) = 0.2.

To identify amplitude minima within the signal it was necessary to refer to the actual Rc (=offset),

which was averaged from the waveform values 500 to 1000.

Figure 9. Example of characteristic waveform signatures, II-S-0deg.

As can be seen from the graph, the crimp of size 12.2 mm upstream of the actual shearing, then results

in a relative ρ of 0.063. The subsequent actual shearing, in the present example 22 mm, is clearly

visible at a distance of 40 m yielding a ρ of 0.138. Exceeding the maximum shear displacement for the

chosen cable type and set-up typically results at first in a short circuit (27 mm, present case). Further

displacement will then finally shear off the cable completely, being observed as open circuit (35 mm).

The open end should normally look like a 100 % mismatch and it is thus interesting to actually have

had a Rc > 1. A possible explanation might be that – because of the relatively short length of the cable

– one might get a reflected signal from the mismatch which could add back in phase with the original

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60

Cable length, m

Ref

lect

ion

coef

fici

ent

ρ, -

II-S-0deg, crimp 12.2mm(1), def = 22mm



Multiplexer,

patch cable

Crimp 12.2 mm,

position (1)

def = 27 mm

Open circuit

Short circuit

75 ohm coaxial cable

11


signal from the TDR as this could add around 10 % (20 reflected back then 20 % reflected forward)

back into the equation thus giving an apparent Rc of 1.1.

Comparing the two different cable types, smooth Al- versus corrugated Cu- outer conductor, and with

regard to the noise level, it could be concluded that there was practically no difference between the

two types. Table 3 summarizes the calculated standard deviation from the reflection coefficients for

both cable types. The noise level was typically around 1 mρ. The evaluation thus considered reflection

coefficients > 5 mρ as being significantly above the noise level.

Table 3. Level of noise for different tests.

test id ρ stdev def steps

I-S-0deg 0.1899 0.00078 7

I-S-30deg 0.1992 0.00070 5

I-S-45deg 0.1996 0.00109 9

II-S-0deg 0.1927 0.00122 27

II-S-30deg 0.1922 0.00117 30

II-S-45deg 0.1923 0.00110 64

II-C-0deg 0.1879 0.00100 22

II-C-30deg 0.1878 0.00102 24

II-C-45deg 0.1881 0.00090 40

Note:

Reflection coefficients averaged from waveform values

500-1000, 40 m cables, without crimps upstream.

3.4.2 Evaluation of reflection coefficients

Figure 10 plots the reflection coefficient magnitude ρ versus the amount of shearing for all tests done.

It should be noted that test II-S-45deg should be excluded since the conductor began to slide facilitated

by the adhesive film between the outer conductor and the plastic jacket. This caused the cable only to

be stretched within the shear zone (after 45 mm shearing).

At first view a distinct separation between the curves is obvious. The relationship between ρ and

shearing is certainly not linear. Especially in the beginning the amplitude/shearing ratio is strongly

reduced and the TDR system was therefore less sensitive. This supports recent findings by Singer et al.

(2006) and Krekula (2004), but is in contrast to those found earlier by Dowding et al. (1989). It is

assumed that the sensitivity for a small shear displacement is dependent on the cable type and

dimension and also on the response of the actual measurement instrument.

The separation of curves can be related to the different shear angles and thus also the larger effective

cross section of the cable being involved in the shear process. In fact, the larger the shear angle the

smaller is the reflection coefficient at a given step of deformation. This circumstance that the

reflection coefficient might be different when the cable is not sheared perpendicular to its centre line

12


has not been mentioned in previous studies (Dowding et al., 1989; Pierce et al., 1994; Aimone-Martin

& Francke, 1997). However, oblique shearing is likely to occur in field measurements.

Figure 10. Reflection coefficient versus shear displacement, 40 m distance, without crimps upstream. 1

It is important to stress that the deformation mode affected by the shear zone width (i.e. gap between

pipe halves) could also have an essential effect on the TDR signal. Localized shearing causes a

distinct, large increase in the reflection amplitude, whereas a longitudinal extension of the cable

produces only a small but increasingly wide reflection with little increase in amplitude (Su, 1987;

Dowding et al., 1988). It thus becomes essential to verify to what extent this circumstance influenced

testing. One reason is that with shear angles 30 and 45° and larger shear displacement it was not

possible to control the shear zone width. In addition to the mode of grout fracturing (giving an ill-

defined shear plane) the rotation of the grips due to the eccentric load (Appendix 1) caused a certain

separation of the pipe halves. The actual gap widths are documented in Appendix 2 and images of the

shear planes for each test are shown in Appendix 3. This separation effect is the most plausible

explanation that shears larger than the actual cable dimensions could occur.

O`Connor & Dowding (1999) suggested that the “width to magnitude” ratio of the reflection

coefficient could be used to verify the actual shearing mechanism and failure of the cable since

bending of the cable through large shear zone should give greater ratios that localized shear within

small gaps.

1 Annotation: I-S-0deg, problems at 6 mm; I-S-0deg & II-S-45deg, cables not sheared off; see

Appendix 2.

13


Figure 11 shows the width to magnitude ratios for the tests carried out. There is no tendency towards

larger ratios with increasing shear angle. Nor is there any observable increase in the ratios as the tests

near completion at larger shear displacements. It can be thus concluded that the decreased sensitivity

(ρ/mm) for increased shear angles (Figure 10) is mainly attributable to an effect by the angle itself.

The shape of the reflection coefficient curves does not contain any information about the shear angle.

For instance, an increased shifting of the peak values with progressing shearing which resulted in a

more systematic skewing of the curves is not noticeable, see the comparison in Figure 12 and Figure

13. The only observation might be that the minima of the reflection coefficient curves are not that

precisely positioned at the shearing plane for shear angles > 0 deg.

Figure 11. Comparison of width-to-magnitude ratios.2

2 Box and whisker plot showing mean (white bar), average (red-white rhomb), 1

st and 3

rd quartiles

(gray boxes), whiskers (perpendicular line segments; as far as the farthest point within 1.5 inter-

quartile ranges below the 1st or above the 3rd

quartile) and outliers.

0

2

4

6

8

10

12

14

I-S-0deg I-S-30deg I-S-45deg II-S-0deg II-S-30deg II-S-45deg II-C-0deg II-C-30deg II-C-45deg

Test id

Wid

th-t

o-m

ag

nit

ud

e ra

tio

, m

/-

14


Figure 12. Reflection coefficient versus position, III-C-0deg.

Figure 13. Reflection coefficient versus position, III-C-45deg.

Comparing the two different cables used with each other it can be observed that the corrugated Cu-

cable is considerably more sensitive (mρ/mm) than the smooth-walled Al-cable. On the other hand,

this also implies that the displacement limit before the cable is finally sheared off is substantially

lowered. Both cable types show a minimum shearing displacement limit before any movement can be

detected of around 5 mm.

Part of this test program was used to verify if differences in the actual shearing mode are detectable

(see chapter 2.2). For this purpose the curves for the test I-S (shear + compression) and II-S (shear +

tension) should be directly compared in Figure 10. Test I-S-0deg had machinery problems at 6 mm

and for both tests I-S-0deg and II-S-45deg the cable could not be sheared off completely (Appendix

2). Despite this, a preliminary interpretation is that the curves for different shearing modes are quite

congruent with each other. Noticeable though is that a test in which shear was associated with tension

could take remarkably larger shear displacements than a test associated with compression (II-S-30deg

compared to I-S-30deg). In this respect, there are no observable differences in the width-to-magnitude

ratio of the tests in campaign I (shear + compression) and in campaign II (shear + tension), see Figure

11.

3.4.3 Influence of distance on a single shear reflection

Figure 14 shows the decrease in magnitude of the reflection coefficient curves with increasing cable

length. For example, shearing the 21.1 mm smooth-walled aluminum cable by 20 mm produces a

reflection coefficient of 36 mρ at a distance of 30 m, 31 mρ at 40 m and only 27 mρ at 50 m. Around

10 - 15 % attenuation in the reflection coefficient can thus be anticipated per 10 m of cable length,

progressively increasing with larger shear displacements.

The decrease occurs rapidly and the minimum shear displacement needed to detect a 1 mρ change in

the waveform magnitude increases with distance from the TDR instrument. This reduction in

reflection magnitude results from an attenuation of the signal along the cable as well as a signal

0.0

0.1

0.1

0.2

0.2

0.3

43.2 43.4 43.6 43.8 44.0 44.2 44.4 44.6-0.1

0.0

0.1

0.1

0.2

0.2

0.3

43.2 43.4 43.6 43.8 44.0 44.2 44.4 44.6

15


dispersion, i.e. increase in rise time (Su, 1987). This dispersion also has a major influence on the

resolution (Taflove, 1995), i.e. smallest distance between two local cable deformities that can be

resolved before the discontinuities appear as one. Besides this, the resolution is dictated by the

specified timing capabilities of the TDR instrument (see chapter 3.3). An important factor is also the

increasing width of the reflected signal amplitude with progressive shear displacement on the cable.

This would in the end have a bearing on the possibility to identify narrowly spaced deformities.

Figure 14. Reflection coefficient versus shear displacement, I-S-45deg, without upstream crimp.

Within one and the same test at constant cable length, but with different deformities the position of the

minima of the reflection coefficient always occurred within ± 3 cm. This resolution could probably be

increased if, unlike the present case, only a certain cable length of interest (e.g. a few metres behind

the burden) is surveyed for any deformities with the available waveform sampling rate (2048

waveform values over chosen length).

3.4.4 Effects of upstream cable deformations

Figure 15, Figure 16 and Figure 17 show the ratio of reflection coefficient amplitudes with upstream

deformations of different sizes (ρsmall, ρmedium and ρlarge) to amplitudes without any upstream crimps

(ρ0). All upstream crimps are located 2 m (position 1 in Figure 3) ahead of the actual deformation at 40

m cable length.

The effect of upstream crimps on signals reflected downstream is minimal. A clear effect is not visible

until the upstream deformity is very large as for the crimp made on the smooth-walled Al- cable with a

reduction in diameter of 15.2 mm, i.e. down 72 % from the actual cable dimension. In this case the

cable is almost short-circuited, but a highly resistant bond is still left between the inner conductor and

the dielectric, see Figure 5 and Figure 6. Concerning the corrugated Cu-cable for which the upstream

16


crimp is reducing the cable dimension by 62 %, only a slight decrease in the reflection coefficient

amplitude is obtained.

The recent findings are consistent with those by Pierce et al. (1994), which have also shown that even

multiple deformities upstream have a marginal effect. The application of artificial crimps as “location

markers” to enhance the position accuracy of TDR measurements for the identification of actual

deformities could thus be worth consideration. “Small” crimps that cause a reflection coefficient

amplitude of about 20 - 25 mρ are easily discernible from noise along the cable and do not affect the

signal reflected from downstream shear displacements. However, the number of crimps should be

minimized to reduce the energy loss that occurs as the pulse passes through these multiple deformities.

If the location of interest is relatively well identified, a single crimp may be placed near this location

and its position accurately measured to reduce the errors deriving from cable and borehole

misalignment.

The influence of upstream crimps with respect to the position relative to the downstream shear

displacement was investigated in campaign I. Table 4 compares the amplitudes of ρ for an upstream

crimp at position 1 to one at position 2, i.e. 2 m and 5 m in front of the downstream shearing (see also

Figure 4).

Table 4. Influence of upstream crimps with respect to position.

test id ρsmall (1) / ρsmall (2) ρmedium (1) / ρmedium (2)

average stdev data average stdev data

I-S-0deg, 30m 1.001 0.011 5 1.101 0.178 6

I-S-30deg, 30m 1.095 0.182 4 0.960 0.088 4

I-S-45deg, 30m 1.050 0.069 6 0.971 0.081 6

I-S-0deg, 40m 1.008 0.049 5 1.081 0.149 5

I-S-30deg, 40m 0.979 0.050 3 0.936 0.051 3

I-S-45deg, 40m 1.009 0.076 5 0.989 0.041 4

I-S-0deg, 50m 1.002 0.046 5 0.953 0.061 5

I-S-30deg, 50m 1.032 0.041 3 0.965 0.023 3

I-S-45deg, 50m 0.950 0.081 6 0.975 0.085 6

Note: Reflection coefficients from > 0.005 considered

It is therefore concluded that the exact position of the crimp is irrelevant and does not affect the

amplitude of the later reflected signal. The minimum distance relationship needed to resolve multiple

deformities is still valid though (see chapter 3.4.3).

17


Figure 15. Reflection coefficient “small” crimp (1) / no crimp, 40m distance.

Figure 16. Reflection coefficient “medium” crimp (1) / no crimp, 40m distance.

Figure 17. Reflection coefficient “large” crimp (1) / no crimp, 40m distance.

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5


Tests: "small" crimp (1) / no crimp @ 40m

Ref

lect

ion

coef

fici

ent

ρsm

all/ρ

0, -

0.6

0.8

1.0

1.2

1.4

1.6

1.8


Tests: "medium" crimp (1) / no crimp @ 40m

Ref

lect

ion

co

effi

cien

t ρ

med

ium

/ρ0,

-

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

II-S-0deg II-S-30deg II-S-45deg II-C-0deg II-C-30deg II-C-45deg

Tests: "large" crimp (1) / no crimp @ 40m

Ref

lect

ion

co

effi

cien

t ρ

larg

e/ρ

0,

-

18


4 CONCLUSIONS AND RECOMMENDATIONS

The following conclusions may be drawn:

There exists a non-linear relationship between the reflection coefficient and the actual shear

displacement. Especially in the beginning the reflection amplitude to shear displacement ratio

is quite small and the TDR system is therefore insensitive. It may be assumed that the

sensitivity at small deformities is dependent on the cable dimension and on the response of

the measurement device.

The position of the reflection coefficient versus shear displacement curves depends on the

shear angles involved. The larger the shear angle, the smaller the reflection coefficient is for a

given amount of displacement. This dependence of the reflection coefficient on the shearing

angle has not been reported in previous studies. Oblique shearing probably occurs more

frequently than perpendicular shearing in the field. A connection line between the shearing-

off points on the curves for different shear angles may have some practical interest but these

loci were not investigated here.

The actual set-up could not avoid the development of a gap in the shear zone with

progressing shear displacement. The importance of this effect has been verified by

determining the width of reflection coefficients to magnitude ratio. It may be assumed that

localized shearing causes a narrow and large increase in the reflection amplitude whereas

extension produces only a minor but increasingly wide reflection with little increase in

amplitude. Analyzing the actual data does not show any tendency to greater width to

magnitude ratios with increased shear angles. Nor is there an increase in the ratio when it

comes to larger deformations. It can thus be concluded that the decreased sensitivity (ρ/mm)

for increased shear angles is mainly attributable to an effect of the angle itself.

The shape of the reflection coefficient curve does not reveal any information about the shear

angle.

Different signal responses to the actual shear failure mode (down- or upslip, i.e. shear

associated with compression or tension) may exist. If so, they can – at least within this test

series – not be clearly identified.

Cable length has a significant and quantifiable effect on TDR reflections. Around 10-15%

attenuation in the reflection coefficient can be expected per 10 m cable extension,

progressively increasing with larger shear displacement.

This signal dispersion due to longer cables also affects the overall resolution since the width

of the reflected signal will also increase. Besides this, the width increases with progressive

shear displacement on the cable, which finally will also influence the possibility to resolve

narrowly spaced cable deformities.

19


A clear effect of upstream crimps on downstream deformities is not readily identifiable

unless the upstream deformation is very large. Thus artificial crimps (in the range of 10 - 25

mρ) should be considered to enhance the position accuracy of TDR measurements for the

identification of natural shearing displacements.

The exact position of an upstream crimp is irrelevant and does not affect the amplitude of the

signal reflected downstream. However, deformities situated too close to each other cannot be

individually resolved.

Comparing the two different cables used with each other one can observe that the corrugated

Cu- cable is considerably more sensitive (mρ/mm) than the smooth-walled Al- cable. On the

other hand this also means that the displacement range before the cable is finally sheared off

is correspondingly smaller. Both cable types need a minimum shear displacement before any

movement of around 5 mm can be detected. The level of noise is around 1 mρ for both cable

types. In a single case (II-S-45deg, cable type P3 750 JCA), the conductor began to slide

facilitated by the adhesive film between the outer conductor and the plastic jacket. This

caused an unwanted stretching of the cable within the shear zone.

The actual set-up has been effective in the sense that the concrete embedded the coaxial

cables and prevented a slippage of the cable. No such slippage could be detected. However,

the separation of the pipe halves and widening gap at the shear zone, with a bending of the

cable through this zone could not be avoided.

The most important outcome of the present test program is that the monitoring of the reflection

coefficient with progressive shear displacement (i.e. monitoring time) contains information about the

shear angle of the actual rock movement. If this information is combined with that from a borehole

imager a-priori to the cable installation a 3D model with respect to the caving propagation or blast

damaging zone behind SLC rings could be constructed.

The correlation between different shear angles and TDR response found in the present test campaigns

is definitely worth further study. A continuation of laboratory tests should then focus on this and

eliminate all other possible interference effects. The cables should then be mounted between two rigid

guide plates, see Figure 18. Steel rings fastened to the outside of the cables on both sides would keep

the cable in position and avoid any lengthwise extension of the cable. Easily interchangeable steel

plates with different angles drilled for the insertion of cables would be prepared. In addition the effect

of shear failure associated both with tension and compression could be simulated.

As an extension of the present study, preliminary field tests are planned in the Kiruna mine in block

12, level 691, to verify the laboratory findings. Another purpose of these field tests is to see whether

the blast-induced dynamic failure of a coaxial cable could be distinguished from a geologically

induced failure.

20


Figure 18. Suggested, improved design of a shearing apparatus.

21


5 ACKNOWLEDGEMENTS

Hjalmar Lundbohm Research Centre (HLRC) of Luleå University of Technology, endowed by LKAB,

is thanked for the continuing support within the PhD project “Improved breakage and flow in sublevel

caving”.

The present report also serves as project work for the course “Block Caving Principles” at the

University of the Witwatersrand, South Africa. I thank HLRC for financial support which enabled me

to attend this course.

I also thank colleagues at Complab, LTU for their invaluable practical support as well as for

discussions concerning the actual test set-up and results.

CommScope Europe and Corning Cablecon are thanked for the generous provision of coaxial cables

and connectors as well as tools. I also thank Richard Harris and Johan Holmberg, both of CommScope

Europe, for most valuable discussions.

22


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26


7 APPENDICES

Appendix 1. Drawings of steel pipes for shear tests at different angles.

27


28


Appendix 2. Documented data for all tests.

Test I-S-0, date 15-16.11.2007

Shear angle 0 deg (+-90deg), cable P3 750 JCA

displ gap load remarks

mm mm kN -

0 - 0.24 - 2 - 11 - 4 - 4 - 6 - 1 6.4mm (30m) / 15.2mm (40+50m, not used files) 8 - 4 new fixation in the machine

10 - 3 - 12 - 4 - 14 - 5 machinery problems, stop

Test I-S-30, date 04.12.2007

Shear angle 30 deg (-120deg), cable P3 750 JCA


mm mm kN -

0 0 3.8 - 5 2.5 6 101kN max

10 3.2 7.4 - 15 3.6 14 - 20 3.8 12 - 25 3.8 16.4 short circuit

Test I-S-45, date 22.11.2007

Shear angle 45 deg (-135deg), cable P3 750 JCA


mm mm kN -

0 0 6 - 2 2.7 11 - 4 4.3 11.5 - 6 5.6-5.9 10.1 - 8 6.1-6.4 14.2 -

12 6.8-6.9 12.3 - 16 7.5-7.8 12.7 - 20 7.8-8 12.4 - 25 8.4 11.6 - 29 8.9-9.1 30 geometrical limits + cable break outside pipe (excessive bending),

open circuit at that position Test II-S-0, date 11.01.2008

Shear angle 0 deg (+-90deg), cable P3 750 JCA


mm mm kN -

0-4 - - - 5 4.2 - - 6 - - - 7 4.9 - - 8 - - - 9 4.9 - -

10-12 - - - 13 5-6 - -

14-15 - - - 16 - - = file 17 (overwritten) 17 5.5-6.6 - -

18-20 - - - 21 6-7 - -

22-25 - - -

26 7-8.5 - short circuit, open circuit (cut cable) at 35mm

29


Test II-S-30, date 18.01.2008

Shear angle 30 deg (+60deg), P3 750 JCA


mm mm kN -

11.2 4.2-5.3 - machinery problems 12.2-13.2 - - -

14.2 5-5.5 - - 15.2-16.2 - - -

17.2 5.1-6.1 - - 18.2-21.2 - - -

22.2 5.5-6.5 - - 23.2-25.2 - - -

26.2 - - slipping of plastic jacket 27.2-31.2 - - -

32.2 5.6-7 - - 33.2-38.2 - - -

39.2 - - unstable signal 40.2 - - - 41.2 6-7.3 - short circuit 43.2 - - short circuit 51.2 - - unstable signal, short-open circuit

Test II-S-45, date 21.01.2008

Shear angle 45 deg (+45deg), cable P3 750 JCA


mm mm kN -

0 2.5-7.5 - - 1-2 - - - 3 2-8 - -

4-6 - - - 7 3.5-9.8 - -

8-10 - - - 11 5.8-12.5 - -

12-15 - - - 16 6-12 - -

17-20 - - - 21 6-12 - -

22-25 - - - 26 6-12 - -

27-29 - - - 30 6-12 - -

31-35 - - - 36 6-12 - -

37-42 - - - 43 6-12 - -

44-49 - - - 50 - - file 50.3 (15.2mm) missing 51 - - - 52 - - file 52-3 (15.2mm) missing

53-59 - - - 60 4-10 - - 63 4-10 - - 66 - - - 69 - - -

72 - - relative slide between outer Al-conductor and plastic jacket; no cable cut (break),

no short circuit

30


Test II-C-0, date 28.01.2008

Shear angle 0 deg (+-90deg), cable CA 519 J


mm mm kN -

0 - - intact concrete 1 - - 37.8kN max

2-4 - -

5 1.5-2.5 - -

6-8 - - -

6-8 - - - 9 3-2 - - 10 - 11-12 -

11-13 - - - 14 2.5-3.5 - = file 15 (overwritten) 15 - - - 16 2.8-3.8 - -

17-20 - - - 21 4-5 - - 22 - - open circuit

Test II-C-30, date 31.01.2008

Shear angle 30 deg (+60deg), cable CA 519 J


mm mm kN -

0 - - - 1 - 48.7 68.3kN max (welding point!) 2 - 19.6 - 3 - - - 4 - 20.5 - 5 3-3.8 - -

6-8 - - - 9 3.7-4.4 - -

10-12 - - - 13 4-5 - -

14-15 - - - 16 - 22 - 17 4.8-5.5 - -

18-20 - - - 21 5.2-5.8 - - 22 - - - 23 - - snap at 23.5mm 24 5.9-6.2 - open circuit

31


Test II-C-45, date 01.02.2008

Shear angle 45 deg (+45deg), cable CA 519 J


mm mm kN -

0 - - - 1 0.8 38.5 - 2 1.6-1.9 15.8 47.6kN max 3 - 25.9 remaining load: friction in the concrete 4 3.3-3.5 16 - 5 - 15.3 - 6 - 15 - 7 - - - 8 5.7-5.9 - - 9 - - file 9.1 (8.5mm),9.2 (10.5mm), 9.3 (12.5mm) missing

10 - - file 10.0 (0mm) missing 11-12 - - -

13 9.5-9.5 - - 14-16 - - -

17 12.5-12.9 - - 18 - - - 19 - - missing files (slipping of machine) 20 - - missing files (slipping of machine)

21.4 14-15.5 - - 22-23 - - -

24 16.5-17.8 - - 25-28 - - -

29 21.5-23.5 - - 32 - - - 35 25.3-26.8 - - 38 - - -

41 - - cable only pulled in the gap

51 - - short circuit 103 - - open circuit

32


Appendix 3. Images of shear faces for all tests.

I-S and II-S series denote tests on cable type P3 750 JCA, II-C series on cable type CA 519 J

I-S-0deg

II-S-0deg

I-S-30deg

II-S-30deg

I-S-45deg

II-S-45deg

33


III-C-0deg

III-C-30deg

III-C-45deg

Report 2007:1 ISSN 1653-5006

Swedish Blasting Research CentreMejerivägen 1, SE-117 43 Stockholm

Luleå University of TechnologySE-971 87 Luleå www.ltu.se

An experimental investigation of blastability

Experimentell bestämning av sprängbarhet

Matthias Wimmer, Swebrec

Universitetstryckeriet, L

uleå

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