Published in Symposium and Workshop on Time Domain Reflectometry in Environmental,
Infrastructure, and Mining Applications held at Northwestern University, Evanston,
Illinois, September 17-19, 1994 (Washington, D.C.: U.S. Bureau of Mines, 1994),
pp. 494-506.
USBM special publication SP 19-94
by Kevin M. O'Connor, Civil Engineer
gtdr29@mail.idt.net and
Lewis V. Wade, Research Director
U.S. Bureau of Mines, Minneapolis, MN
The U. S. Bureau of Mines (USBM) is continuing to actively investigate and promote the application of time domain reflectometry (TDR) to mining. USBM research started in the 1960's when TDR was primarily used to locate breaks in electrical power cables, and the diversity of applications has been expanded tremendously. This paper primarily concentrates on applications in which TDR has been used to monitor rock mass response to underground and surface mining. Technological benchmarks are provided by starting and ending the discussion with a brief summary of USBM efforts to automate the acquisition and interpretation of data. The intent is to provide an overview of information that has been obtained using TDR, and significant advancements that have been implemented.
For any monitoring technique to be useful in the mining industry, it must be economical (installation cost, data acquisition cost, etc.), provide reliable information in real time, and be resistant to sabotage. There are often tradeoffs when attempting to satisfy these requirements, and techniques have been developed using a variety of mechanical, chemical, and electrical principles. The principle of TDR represents another tool in the arsenal available to researchers and practitioners in the mining industry, and the range of applications is evident by the variety of papers presented at this symposium.
In one of the earliest USBM projects, a TDR system was developed to locate faults that occur in trailing electrical cables on mining equipment. Almost simultaneously, TDR technology was adapted by the USBM to monitor caving of rock induced by underground mining. TDR research has advanced to the point where it is now possible to not only locate rock mass displacements, but also to distinguish shear deformation from tensile deformation and to quantify the magnitude of movement.
Technological advances in both hardware and software have made it possible for the USBM to implement remote continuous monitoring of rock response to mining. In addition, the TDR records obtained are compiled as animated sequences and replayed with a computer or recorded for video display. These developments have made it possible to both visually and quantitatively analyze rock mass behavior.
Trailing Cable and Wire Rope
Electrical faults can occur in the trailing electrical cables used by mining
equipment, which can prevent use of the cables until appropriate repairs are made.
These faults can appear as shorts between conductors, open circuits, or resistive
faults. Short circuits often allow precise determination of fault location, but open
circuits and resistive faults may not produce any superficial damage that can be
used to pinpoint the fault location. Faults that produce very few clues regarding
their location must be found by other means. TDR provides a means to accurately
describe each significant fault location and relative magnitude so that correct
repairs can be made.
State-of-the-art TDR hardware in 1974 was not compatible with use in a
mine environment and required interpretation of waveforms by experienced
operators. Carnegie-Mellon University undertook a project to simplify acquisition
and interpretation of TDR waveforms (2, 3,
16). At modification of hardware was
accomplished by the addition of a microprocessor unit to commercially available
TDR equipment. The microcomputer was programmed to perform all analysis of
the TDR waveform and displayed the relative magnitude of the electrical fault, and
its distance down the cable, on a simple numerical display. This modification
eliminated the need for an oscilloscope. The user could read distance to a fault from
the display, measure this off on the cable, and effect repairs. Unfortunately,
electrical malfunctions in logic timing circuitry caused false indications, and it was
not possible to calibrate the TDR units for accurate fault placement. USBM funding
for development of this unit was discontinued since improved TDR units were
becoming available commercially.
Recently, Virginia Polytechnic and State University has investigated the
application of TDR for monitoring the integrity of large-diameter wire ropes (21).
Wire rope specimens were prepared with embedded coaxial cable and other
sensors. TDR was used to detect locations of radial pressure-induced deformation
of the coax cable. A change in TDR waveform was recorded as the rope was
subjected to tensile loading, but additional research will be needed to correlate TDR
reflection characteristics with changes in rope integrity.
When openings are excavated in a rock or soil mass, displacements occur
as unstable blocks move and stresses adjust. Coaxial cables can be grouted into a
rock mass, as shown in FIGURE 1, to monitor such displacements.
Prior to
installation, the cable is crimped to provide reference reflections in the cable at
known physical locations in the rock mass. After crimping, the cable is attached to
an anchor, lowered down a borehole, and bonded to the surrounding rock with an
expansive cement grout that is tremied into the hole. At locations where progressive rock
movement is sufficient to fracture the grout, cable deformation occurs that can be monitored with a TDR cable
tester (15, 30). Significant developments in the interpretation of
TDR signatures are presented
by Dowding, Su, and O'Connor (14) with respect to quantifying changes in
reflection magnitude and wavelength due to cable deformation and distinguishing
cable shear from cable extension.
A variety of situations in which TDR has been used to monitor rock and soil
movement is listed in TABLE 1. The list is not exhaustive, but does
demonstrate the
diversity of TDR applications. Details are provided in the references, but some
examples can be briefly summarized to illustrate the evolution of this technology.
One of the earliest applications of TDR to monitor rock mass deformation
was accomplished by Panek and Tesch (30) in which coaxial cables and
other
instrumentation were installed in boreholes drilled into the walls and roofs of adits
at a block caving mine in Arizona. In this mining technique, large zones of
ore-bearing rock are drilled, blasted, and allowed to cave into lower adits where the
caved material is loaded into ore cars and transported out to a materials-handling shaft.
Holes were 43 to 87 m deep and
varied in orientation from -30 degrees to +60 degrees. Although changes in TDR waveforms
were
documented, the correlation between rock mass response and cable deformation
was essentially limited to locating major rock fractures that developed as block
caving progressed.
One cable was laid out 187 m along the floor of an adit against the wall and
covered with grout. At locations where fractures were observed in the grout, it was
possible to compare their aperture and offset with TDR reflections along the cable.
Based on these observations, it was concluded that there must be a correlation
between cable deformation and TDR reflection characteristics, but no attempt was
made at that time to quantify this correlation.
Another of the early applications of TDR to monitor rock mass deformation
involved installation of coaxial cables and other instrumentation in boreholes
drilled from the surface down to coal seams being mined (32, 39).
In high extraction coal mining, 70% to 100% of the coal is extracted in blocks 100 to 300 m wide and
up to 1,500 m long so that caving and fracturing propagates up through the
overlying rock. The TDR records shown in FIGURE 2 were
obtained as a mining face
advanced toward and past a cable installed in a hole 159 m deep (13). TDR
reflections which developed at depths of 31.4, 48.0, 56.9, 66.5, 90.9, 115.9, and
134.5 m are highlighted These are a few of the locations where cable deformation
could be attributed to shearing along horizontal discontinuities that had been
identified in core samples obtained when the hole was drilled (33).
The usefulness of such TDR profiles has been extended beyond
identification of locations where displacements occurred in the rock mass.
Correlations between shear displacement and TDR reflection magnitude have been
developed by subjecting grouted coaxial cables to direct shear in a laboratory (14, 15, 34). Using such calibrations,
it is possible to state that at least 35 mm of shear
displacement occurred at a depth of 48 m before the cable was severed (face
position of +21 m in FIGURE 2). Consequently, TDR allows an
overall view of rock
mass behavior as well as a detailed history of displacement at each location where
cable deformation occurs.
Room-and-Pillar Retreat Potash Mining
When it was established that TDR could be used to quantify rock movement
as well as distinguish shear and tension (14), the applicability to mining was
enhanced. This capability was particularly pertinent to monitoring of roof strata
movements during pillar extraction in a potash mine (29). The immediate
roof
consisted of halite layers, separated by clay partings which presented a major
challenge for ground control. Forty-six pillars were split along their long axis using
the undercut and blast technique, which left long strip remnant pillars and increased
the extraction ratio from 58% to about 84%. As these remnant pillars yielded, the
immediate roof deformed into an asymmetrical bowl and shearing occurred at a
depth of 3.6 m, in the vicinity of a clay parting at a depth of 3.1 m, as indicated by
changes in the TDR signatures shown in FIGURE 3.
This mode of deformation implies that points in the immediate roof
experienced two horizontal components of displacement in addition to the
vertical component. Convergence measurements were used to define roof
deflection profiles along north-south and east-west directions. Assuming simple
bending of a beam 3.6 m thick, horizontal displacement at the clay parting was
computed using slope of the deflection profiles and distance (3.6 m / 2 = 1.8 m)
from the neutral axis. The displacements computed for the north-south and
east-west profiles are plotted versus time in FIGURE 4.
Changes in the TDR reflection magnitude were converted to shear
displacement using the calibration presented by Dowding, Su, and O'Connor
(14). The rate of TDR shear displacement was initially consistent with that
of
the north-south component of horizontal displacement then became consistent
with the east-west component. The magnitude of TDR shear displacement
attained a peak value of 7.3 mm on December 28, 1988 and then decreased to
6.2 mm on December 30, 1988 as shown in FIGURE 4. This
decrease in
magnitude has been observed during laboratory calibration tests (34), and it
can
be attributed to tensile deformation of the coaxial cable as the roof sagged and
strata separation occurred. Consequently, detailed analysis of each TDR
reflection provides valuable information about rock mass response.
The distinct spike at a depth of 3.6 m in FIGURE 3 is associated
with
localized shearing of the cable. The trough-like reflection at a depth of 0.6 m
corresponds with tensile deformation of the cable as strata separation developed
along a clay parting on December 20, 1988. This gap was exposed at one
location and it could be monitored by direct measurement. Its aperture
increased from 25 mm to 112 mm during the period December 20-29, 1988. It
is characteristic of cable extension that the TDR reflection increased in width
but not in magnitude. Attempts to correlate changes in reflection width with
tensile displacement have not been fruitful (34). A reliable correlation
between
tensile deformation and changes in TDR reflections will be required before
further analysis of this data is possible.
The pits, sags, and troughs that develop on the surface over abandoned
underground mines are the ultimate result of subsurface strata movements. In
the United States, an estimated 25% to 50% of the 3.2 million hectares (8
million acres) that have been undermined have developed these features, and
most of the rest will likely be effected in the future (5). In the case of
important
structures located over abandoned mines, it is not adequate to say that
subsidence may occur. A means must be provided to indicate if strata
movements are occurring (and the rate at which they are occurring) beneath
these structures so that appropriate measures can be taken to mitigate damage.
The Illinois State Geological Survey has used TDR to monitor strata
movements over an abandoned coal mine (4), and the USBM is actively
implementing this technology at other sites. The Mining Research Laboratories
of the Canadian Center for Mineral and Energy Technology (CANMET) have
embraced TDR for monitoring subsidence of crown pillars over abandoned
gold mines in Nova Scotia and Ontario (1, 8,
9, 14).
In an in situ mining application, TDR was used to monitor the
development of fractures and rock mass movements induced by the Frasch hot
water process of mining strata-bound sulfur (24). A Frasch production well
is
drilled with a conventional oil well drilling rig, and large quantities of
super-heated water are pumped into the sulfur-bearing formation. This liquefies
the sulfur which is then lifted to the surface with compressed air. A production
well consists of a series of four (or five) concentric pipes to pump hot water
into the formation and return molten sulfur to the surface. An existing well on
the western edge of the mine was made available for TDR cable installation to
monitor strata movement as mining progressed. The 73-mm diameter drill pipe
to which the cable was attached and the 244 mm diameter casing made this
installation similar to an actual production well, so any strata movements
detected by TDR would also be significant in terms of their effect on
production. Increasing TDR reflections at a depth of 68 m indicated shear
deformation of the casing and cable within a breccia zone overlying the sulfur-
bearing limestone. This provided valuable information to the mining company,
and demonstrated that applicability of TDR for monitoring rock mass behavior
in an adverse environment.
Syncrude Canada Ltd. operates an oil sand mine in northern Alberta (20).
The oil sand is mined by large draglines, which operate adjacent to the edge of
a highwall that varies in height from 40 to 60 m. Coaxial cables were installed
in vertical holes at three highwall locations in the immediate vicinity (less than
10 m) of slope inclinometers so that a comparison could be made between the
two types of instrumentation (23, 31). A cable also was
installed in a horizontal
hole drilled into a tailings embankment in the immediate vicinity of a
horizontal extensometer. The objective of these installations was to assess the
ease or difficulty of installation, suitability to field conditions, ease or
difficulty of data acquisition, comparison with existing monitoring procedures,
and sensitivity of TDR to slope movements.
In addition to the field study, an extensive laboratory test program was
implemented to correlate TDR reflection magnitude with shear deformation of
grouted cables. It is significant that the laboratory calibration indicated a
change in TDR reflection magnitude of 20 mrho per mm of shear displacement,
while the field correlation with inclinometer data indicated only 0.3 mrho per
mm of shear displacement. It was concluded that TDR represented a promising
technology for slope monitoring, but modifications must be made to increase
its sensitivity in oil sands and stiff clay soils. Furthermore, the direct shear of
encapsulated coaxial cable performed for purposes of laboratory calibration
tests was not representative of the development of shear zones which were
observed the highwall soil slopes (31).
REMOTE, REAL-TIME MONITORING AND
ANALYSIS
TDR has been used successfully for monitoring rock mass displacements,
but there have been limitations due to the frequency with which cables could
be interrogated, and the manual techniques used for analysis of signature
changes caused long delays in data interpretation
(1, 6, 9, 17,
19, 20, 22, 23,
24, 27, 28, 29,
30, 31, 32, 38, 39). Data was often lost when cables were sheared
in advance of active longwall mining faces where it was expected that the rock
mass would deform elastically.
TDR cable testers are now available that allow for serial communication
via an RS232 module (36). Data can be acquired using a personal computer
connected directly to the module or remotely with a modem connected to the
module (13). Systems also are available (7) that use a
data logger with the cable
tester to acquire and store data that can be downloaded via telemetry. This
capability allows for acquisition of digitized TDR signatures and facilitates
analysis of signature changes. The USBM has implemented remote monitoring
of cables grouted into the strata over longwall coal mines, and software has been
developed to allow visual and quantitative analysis of signature changes (18).
The USBM creates animated sequences of changes in TDR records.
Waveforms, such as those shown in FIGURE 2 and
FIGURE 3, are overlain as a series of
slides and compiled as an animation (25, 26). The
animation can be replayed
using a computer, or recorded for video display, which allows visual analysis of
rock mass response to mining activities.
The principle of TDR has proven to be economical, efficient, and
resistant to sabotage for monitoring rock mass response at active and abandoned
mines. The total number of locations where TDR has been applied has grown
from 1 in 1969 to at least 30 by 1993, and the total is expected to increase as
technology advances and costs for TDR hardware decrease. Significant
improvements have been in identifying cable types that reduce costs; the
database of TDR records and experience with interpretation of TDR reflections
has increased; digital TDR records can be obtained; telemetry for data
acquisition is available; and development of software for data analysis and
interpretation is continuing. The applications have included rock mass response
to underground mining, strata movements for ground control, rock mass
response to in situ mining, rock movement over abandoned mines, and slope
movement in large surface mines.
1. Aston, T., and F. Charette. Installation and Monitoring of Three
Abandoned Mine Crown Pillar Sites, Timmons, Ontario: August 1992 to October 1992.
CANMET Div. Rep. MRL 93-020 (CL), Min. Res. Lab., Ottawa, June, 1993, 61
pp.
2. Bartel, E. W., M. Fox, E. Borgoyn, and P. Wingfield. TDRM Testing
(contract J0377021). BuMines OFR 60-82, 1980, 135 pp.
3. Bartel, E.W., P. Wingfield, and M. Leister. TDRM Testing (contract
J0377021). BuMines OFR 83-82, 1981, 17 pp.; NTIS PB 82-243940.
4. Bauer, R. A., C. H. Dowding, D. J. Van Roosendaal,
B. B. Mehnert, M. B. Su, and K. O'Connor. Application of Time Domain Reflectometry to
Subsidence Monitoring. Office of Surface Mining, Pittsburgh, 1991, 48 pp.; NTIS PB
91-228411
5. Breslin, J. J. and R. J. Anderson. Current American, British and West
German Underground Coal Mining Practices. Battelle Energy Program Rep., Battelle Mem. Inst., Columbus, OH, 1976,
pp. 77-89.
6. Brutcher, D. F., B. B. Mehnert, D. J. Van Roosendaal, and R. A. Bauer.
Rock Strength and Overburden Changes Due to Subsidence Over a Longwall Coal Mining Operation in Illinois. Paper in Rock Mechanics Contributions and Challenges (Proc. 31st U.S. Sym. on RockMech., Golden, CO), Balkema, 1990, pp. 563-570.
7. Campbell Scientific, Inc. ( Logan, UT).
Time Domain Reflectometry for Measurement of Rock Mass Deformation. Product brochure, July 1991, 2 pp.
8. Charette, F. Installation and Monitoring of Three Abandoned Mine Crown Pillar Sites, Cobalt, Ontario: February 1992 to July 1992. CANMET Div. Rep. MRL 92095(CL), Min. Res. Lab., Ottawa, Mar. 1993, 64 pp.
9. Charette, F. Results of the Monitoring of Three Crown
Pillar Sites in Cobalt, Ontario. CANMET Div. Rep. 92-101 (CL), Min. Res. Lab., Ottawa, Mar.
1993, 31 pp.
10. Dames and Moore. A Demonstration of Longwall Mining - Appendix
(contract J0333949, Old Ben Coal Co.). BuMines OFR 86(2)-85, 1983, 371 pp.
11. Doepken, W. G. Climax
Completing $40 Million Modernization of Henderson Mine and Mill. Min. Eng., Nov. 1991, pp. 1315-1316.
12. Dowding, C. H. (Northwestern University). Internal report, 1983;
available upon request from C. H. Dowding, Dept. of Civil Eng., Evanston, IL.
13. Dowding, C. H. and F.-C. Huang. Telemetric Monitoring for Early Detection of Rock Movement With Time Domain Reflectometry. J. Geot. Eng., Am. Soc. Civ. Eng., v. 120, No. 8, 1994.
14. Dowding, C. H., M. B. Su and K. M. O'Connor. Measurement of Rock Mass Deformation with Grouted Coaxial Antenna Cable. Rock Mech. and Rock Eng., v. 22, 1989, pp. 1-23.
15. -----. Principles of Time Domain Reflectometry Applied to Measurement
of Rock Mass Deformation. Int. J. Rock Mech. and Min. Sci., v. 25, No. 5, 1988, pp.
287-297.
16. Dworak, R. A., A. G. Jordan, and J. S. Thorne. Time Domain
Reflectometer Microcomputer (contract H0346138). BuMines OFR 106-77, 1977, 113 pp.
17. Hill, J. D. Monitoring of Surface Crown Pillar Deformation at
Goldenville, N. S. using Time Domain Reflectometry. Final Rep., DSS contract 23440-0-9245/01-S2, CANMET, prep. by Dept. of Min. and Metall. Eng., Tech. Univ. Nova Scotia,
Halifax, Mar. 1993, 94 pp.
18. Huang, F.-C., K. M. O'Connor, D. M. Yurchak, and C. H. Dowding.
NUMOD and NUTSA: Software for Interactive Acquisition and Analysis of Time Domain
Reflectometry Measurements. BuMines IC 9346, 1993, 42 pp.
19. Janes, J. R. A Demonstration of Longwall Mining (contract J0333949,
Old Ben Coal Co.). BuMines OFR 86(1)-85, 1983, 105 pp.
20. Lord, E., D. Peterson, G. Thompson, and T. Stevens. New Technologies for Monitoring Highwall Movement at Syncrude Canada Ltd. Pet. Soc. of CIM and AOSTRA, Tech. Meet. preprint 9197, Banff, Apr. 1991, pp. 97-1 to 97-8.
21. May, R. G., R. O.
Claus, and K. A. Murphy. Preliminary Evaluation for Developing Smart Ropes
Using Embedded Sensors. Paper in Proceedings of the First European
Conference on Smart Structures (Glasgow, May 12-14, 1992), Inst. of Physics
Publishing, Bristol, 1993, pp. 155-158.
22. Mehnert, B.B., D. J. Van Roosendaal, R. A. Bauer, D. Barkley, and E. Gefell. Final report of Subsidence Investigations Over a High-Extraction Retreat Mine in Williamson County, Illinois. BuMines Cooperative Agreement CO267001, Illinois State Geological Survey, Sept. 1992, 91 pp.
23. O'Connor, K.M. Development of a System for
Highwall Monitoring Using Time Domain Reflectometry. Sum. Rep. Syncrude
Research Ltd., Edmonton, AB, Aug. 1991, 75 pp; available upon request from
K. M. O'Connor, BuMines, Minneapolis, MN.
24. -----. Monitoring of Rock Response to Solution Mining (grant 9C-BD-03-90000). State Min. and Miner. Res. Res. Inst., New Mexico Tech, Socorro, June 1989, 26 pp; available upon request from K. M. O'Connor, BuMines, Minneapolis, MN.
25. -----. Overburden Response to Longwall Mining, Emery County, Utah. Autoflix animation, 1992, 370 Kbyte file; available upon request from K. M. O'Connor, BuMines, Minneapolis, MN.
26. -----. Overburden Response to Longwall Mining, Franklin
County, Illinois. Autoflix animation, 1992, 374 Kbyte file; available upon
request from K. M. O'Connor, BuMines, Minneapolis, MN.
27. O'Connor,
K.M., J.E. O'Rourke, and J. Carr. Influence of Rock Discontinuities on Coal
Mine Subsidence (contract J0100087). BuMines OFR 30-84, 1983, 129 pp.;
NTIS PB 84-166370.
28. O'Rourke, J. E., P. H. Rey, and K. M. O'Connor.
Characterization of Subsidence over Longwall Panels (contract J0333953).
BuMines OFR 3-84, 1982, 180 pp.; NTIS PB 84-142918.
29. O'Connor, K. M.
and T. Zimmerly. Application of Time Domain Reflectometry to Ground
Control. Paper in Proceedings of the 10th International Conference on Ground
Control in Mining (Morgantown, June, 1991). WV Univ., 1991, pp. 115121.
30. Panek, L. A. and W. J. Tesch. Monitoring Ground Movements Near
Caving Stopes-Methods and Measurements. BuMines RI 8585, 1981, 109 pp.
31. Peterson, D. Evaluation of Coaxial Cable / MTDR Method for
Quantifying Highwall Movement. Syncrude Can. Ltd., Res. Rep. 93-4, Edmonton, Alberta,
Apr. 1993, 411 pp.
32. Pulse, R. R. Results of the Subsidence Study at Old Ben
No. 21; (OCT 68 JULY 70) Settlement Probe, BPC & TDR Monitoring
Methods. BuMines, Denver, July, 1970, 4 pp; available for inspection at the
Bureau of Mines Twin Cities Research Center, Minneapolis, MN.
33. Siekmeier, J. A., K. M. O'Connor, and L. R. Powell. Rock Mass
Classification Applied to Subsidence Over High Extraction Coal Mines. Paper
in Proceedings of the Third Workshop on Subsidence Due to Underground
Mining (Morgantown, June, 1992). WV Univ., 1992, pp. 317-325.
34. Su, M. B. Quantification of Cable Deformation with Time Domain Reflectometry. Ph.D. Dissertation, Northwestern Univ., Evanston, IL, 1987, 112 pp.
35. Su, W. H., and G. J. Hasenfus. Field Measurements of Overburden and Chain Pillar Response to Longwall Mining. Paper in Proceedings of the 6th International
Conference on Ground Control in Mining (Morgantown, 1987). WV Univ.,
1987, pp. 296-311.
36. Tektronix, Inc. (Redmond, OR). SP232 Serial Extended
Function Module Operator/Service Manual. Jan. 1989, 103 pp.
37. Van
Roosendaal, D. J., D. F. Brutcher, B. B. Mehnert, J. T. Kelleher, and R. A.
Bauer. Overburden Deformation and Hydrologic Changes Due to Longwall
Mine Subsidence in Illinois. Paper in Proceedings of the Third Conference on
Ground Control Problems in the Illinois Coal Basin (Carbondale, June, 1990).
Southern IL. Univ., 1990, pp. 73-82.
38. Van Roosendaal, D. J., B. B. Mehnert,
and R. A. Bauer. Three-Dimensional Ground Movements During Dynamic
Subsidence of a Longwall Mine in Illinois. Paper in Proceedings of the Third
Workshop on Surface Subsidence Due to Underground Mining (Morgantown,
June 1-4, 1992). WV Univ., 1992, pp. 290-298.
39. Wade, L. V., and P. J.
Conroy. Rock Mechanics Study of a Longwall Panel. Min. Eng., Dec. 1980, pp.
1728-1734.
