Rock instrumentation for performance study of fill material in an underground metalliferous mine

1. s
Practices of Rock Mechanics Instrumentation
Rock Instrumentation for Performance study of Fill
material in an Underground Metalliferous mine
Prepared by:
ROSHAN KUMAR PATEL
M.Tech, Part-1, Semester-2
Roll No.- 16152015
Indian Institute of Technology Varanasi

2. What is Rock Instrumentation ??
• Instrumentation is the art and science of measurement and control of
process variables within a production, laboratory, or manufacturing
area.
• It is nothing but making use of instrument to measure a physical
parameter at a desired level of accuracy and sensitivity.
• Rock instrumentation are mainly associated with the intact rock, rock
mass as well as rock structure.
• It measures the response of rock with the help of integration of sensors,
data acquisition system and data logger depending on the mechanics.

3. Fill Materials used in Underground
Metalliferous mines
• Rock fill: Stone, gravel, soil and Industrial solid waste using
manpower, gravity or machinery equipment to fill underground mined
voids and production of compressed backfill body.
• Hydraulic fill: Water is used as the transportation medium to convey
the materials such as waste tailings, water hydrophilic slag, mountain
sand, river sand, and crushing sand to fill underground mined voids
like stopes etc.
• Novel silica alumina-based fill: It composed of coal refuse and fly
ash.
• Cemented paste fill: It is a non-homogenous material made by
mixing waste tailings, water and cement.

4. Steps of Rock Instrumentation Plan
• Identification of objectives: Determine the objective of using
instrumentation in mines and to ascertain the purpose of each rock
instrument.
• Selection of Instruments: Select the proper instrument and deduce
the number of instruments required to fulfil our desired objectives.
• Installation and Monitoring: Install different no. of instruments
at desired locations or instrument stations in a panel and monitor
those instruments in a proper manner.
• Analysis of Data: Data of different physical parameters are then
examined after monitoring of an instruments at regular intervals.

5. Comparison between the Case Studies
Comparison Parameters BUICK Copper Mine CHIMO Gold Mine
Objectives To evaluate the effect of backfill on mine
stability and to observe backfill performances
during pillar removal
To study in situ performance behaviour of
mine backfill, in response to mining induced
stresses with special reference to die
phenomenon of rockburst
No. of Rock Instruments installed in the
mine
Embedment Strain gauges: 10
Earth Pressure Cell: 11
Vertical Extensometers: 03
Hydraulic pressure cells: 6
No. of days taken to analyse the
performance behaviour of fill material in
the mine
600 200
Stress Measurement In first 200 days, Stress increase due to
temperature dependent stress. After 500 days,
stress remains positive and constant.
After 60 days, Stress increase due to
temperature dependent stress. After 180 days,
stress remains constant.
Strain Measurement Similar trend is observed as the stress changes Similar trend is observed as the stress changes
Average stress and strain after backfilling in
comparison to total initial stress and strain
35% and 28% respectively 42% and 37% respectively
Backfill is stable or unstable Stable Stable

6. Case Studies based on Rock Instrumentation
Plan in Underground Metalliferous mines
• BUICK Mine, Missouri, USA
The Doe Run Co.'s Buick Mine is
located 195 km southwest of St.
Louis, MO, in a deposit called the
New Lead Belt or Viburnum Trend.
The mine produces lead, zinc and
copper ore using a room-and-pillar
mining method at depths ranging from
335 to 366 m. The Buick ore body is
60 to 120m wide with ore thicknesses
ranging from 2.4 to 36.6 m.

7. Objectives and Types of Rock Instruments used
in Buick mine, Missouri, USA
Objectives of this research:
 To evaluate the effect of backfill on mine stability.
 To observe backfill performances during pillar removal.
Rock Instruments used such as:
 Earth pressure cells to identify loading trends in the backfill
 Borehole extensometers to measure relative displacement changes
in the mine roof and support pillars
 Biaxial stressmeters to measure stress changes in several support
pillars and abutments.

8. Test Area in Buick mine
 The test area was approximately 107 by 69 m and contained 15 pillars with
heights ranging from 14 to 19 m.
 The pillars were approximately 9 m per side with 9.8-m-wide rooms.
 A fill fence was constructed around the perimeter of the pillars before the
pillars were extracted. The steel-reinforced shotcrete fence was constructed
sequentially, from the floor to the roof, as lifts of cemented rock fill were
placed.

10. Installation of Rock Instruments used in Buick
mine
Earth pressure cells, embedment strain gauges, and vertical
extensometers were placed in the cemented backfill to identify
loading patterns and to measure relative displacement changes when
the rock pillars were extracted.
Most of the earth pressure cells and embedment strain gauges were
installed when the test area was backfilled to mid height (9 m). Some
of these instruments were placed near the top of the backfill, 1 to 2 m
below the roof, under vertical borehole extensometers in the rock.
The purpose of using these instruments was to identify when the
backfill began to load.

11. 1. Embedment Strain gauges:
 The embedment strain gauges are 25.4 cm long with 5.1 cm. diameter steel flanges
at each end.
 A steel wire and spring assembly is tensioned between the flanges in 2.54 cm.
diameter steel tubing and provides up to 0.64 cm. of relative displacement. Prior to
installation, the gauges were precast in wood forms using cemented backfill mix
with minus 0.64 cm. aggregate.
 This facilitated vertical align first placed over the gauge alignment during
installation and provided protection when the wet backfill was first placed over the
gauge.
2. Earth Pressure Cell:
 Earth pressure cells having a maximum load capacity of 6.9 MPa were used to
identify loading trends and were not relied upon for precise measurements.
 The 22.9-cm-diam instruments were precast in a form slightly larger than the cell
using the same backfill mix that was used to cast the embedment strain gauges. This
form was removed before the instrument was placed in the stope.

12. 3. Vertical Extensometers:
 Three vertical extensometers were constructed to measure relative displacement in
the cemented backfill.
 The distance between anchors for these instruments ranged from 4.2 to 16.5 m and
the top anchor of all three instruments was positioned approximately 4.5 m below
the mine roof.
 A 5 cm. steel pipe coupler was welded to a plate that served as the bottom anchor
for these instruments. Sections of steel pipe were threaded together with couplers
as the backfill height increased.
 Similarly, sections of steel rod were coupled inside the pipe to connect the bottom
anchor plate to the top anchor and transducer housing.

15. Number of Instruments installed in Buick mine
There are three types of instruments which are mainly installed on backfill in the
test area of Buick mine. The required number of instruments for proper monitoring
and study the performance of backfill are as follows:
• Embedment Strain gauges: 10
• Earth Pressure Cell: 11
• Vertical Extensometers: 03

16. Instrument Result installed in Buick mine
For this study, compressive loading recorded by backfill instruments and tensile
displacements measured by extensometers are reported as positive.
Backfill Stress:
Earth pressure cell measurements versus time are plotted. For approximately the
first 200 days, the general trend for the measurements is an immediate stress
increases followed by a temperature-dependent stress decrease that can be
attributed to cooling of the backfill mass.
After day 200, the temperature effect is less prominent between blasts, and some
of the instruments recorded time-dependent loading and abrupt changes not
directly associated with the blasts due to uneven redistribution of stress as the
mine roof settled on the backfill.
After day 500, the slope of the lines are generally positive, indicating that the
effect of temperature is small compared to time-dependent loading.

17. In North drift region In South drift region

18. The total stress increase for the earth pressure cells, from the time of their
installation to the end of mining of the backfilled area, ranges from 370 kPa to 1.5
MPa, with an average equal to 900 kPa. The standard deviation for the total stress
changes during this period is 400 kPa, indicating that the stress distribution on the
backfill was not uniform.
The 30 day UCS for 15 cm. diameter samples cored from backfilled stopes with
similar constituents is 5.65MPa and the 28 day UCS for laboratory-prepared
specimens of this backfill mix is 8.3 MPa, indicating that the backfill remained in the
elastic range.
Visual observations confirm instrument data that the backfill remained in a stable
condition during mining. The maximum stress change from the time the instruments
were installed to the last reading that was taken, approximately 2.4 MPa. This value
is 42% of the average UCS of specimens prepared from core samples of a placed
backfill with similar constituents, indicating that the backfill remained stable.
Periodic visual observations confirm this conclusion

20. Backfill Strain:
Embedment strain gauges and the vertical fill extensometer during pillar mining
was similar to that of the earth pressure cells. The instruments responded to each
pillar blast with compressive strain, followed by temperature-induced tensional
strain.
Strain measured by the embedment strain gauge centred mid-height on pillars
follows the same trend as the other embedment strain gauges, but is significantly
larger in magnitude.
A possible cause for this apparent tension is aggregate that bridged over the
embedment strain gauge and shielded the instrument from loading, combined with
the effect of backfill cooling.

21. In North drift region In South drift region

23. Conclusions regarding Performance of Backfill
in Buick mine
Post-mining stress and strain measured in the backfill are significant compared to
total measured stress and strain. The average stress change measured in the backfill
after mining is 35% of the total average stress change. The average post-mining
strain in the backfill is 28% of the total strain that was measured.
Accounting for this time-dependent loading into the design of backfilled sections
could improve the assessment of long-term stability. In this study, the strength of
the backfill was adequate to carry this additional load, and remained stable.
Backfill can be used as primary long term ground support and subsidence
abatement in hard rock mines provided that the initial design strength of the
backfill accounts for time-dependent loading. In this study, the strength of the
backfill was adequate to carry time-dependent loading and remained stable

24. Case Studies based on Rock Instrumentation
Plan in Underground Metalliferous mines
• CHIMO Mine, Quebec, Canada
Chimo Mine is located 50 km east of Val D'Or, near Louvicourt, Quebec.
Native gold is found associated to quartz and sulfides (arsenopyrite and
pyrrhotite).The deposit is part of a sequence of massive volcanic rocks,
surrounded in the north by a sequence of meta sedimentary and volcanic
rocks, in the south by sedimentary rocks, grauwackes and pelitic schists.
About six ore bearing zones have been identified in Chimo Mine. These ore
zones are generally striking east-west and dip steeply (around 70°) to the
north. The ore body between levels 16 and 17 included zones B, F and A
with 4 meters wide and an average of 90 m long.
Mining method is open stoping longhole with 70 m height of stopes.
Hanging wall and foot wall are generally strong except in some situations
where they contain a graphite bearing schist material. Blasting holes are 4
inches diameter.

25. Objectives and Types of Rock Instruments used
in Chimo mine, Quebec, Canada
Objectives of this research:
 To study in situ performance behaviour of mine backfill, in response to
mining induced stresses with special reference to die phenomenon of
rockburst.
Rock Instruments used such as:
 Hydraulic pressure cells which were placed in the backfill to measure
backfill pressure.
 Earth pressure cells, electronic piezometers and convergence monitors
can be used to study the mechanical behaviour of the fill under load.

26. Installation of Rock Instrument used in Chimo mine
• The instrumentation at Chimo Mine consisted of rock mass
instrumentation and backfill instrumentation.
 In the rock mass, Extensometers and Vibrating wire stressmeters were
used to measure the displacements and change of stresses respectively.
 In the backfill, Pressure cells were installed in the stopes prior to
backfilling to measure the actual pressure within backfill.
• In order to choose the instrumentation location in the mine, a
preliminary numerical modelling was conducted to study the
displacement and mining induced stresses in the rock mass and backfill
during the mine and fill sequence. The results of numerical modelling
were used to determine the stopes where significant build up of backfill
pressure could be expected. The mine and fill sequence were also
studied to determine when instruments had to be installed in order to
respect the mining schedule.

27. A total of 6 hydraulic pressure cells were installed successfully in two
backfilled stopes as follows:
• Three pressure cells were installed in 20 meters depth at middle of the stope
17-5B-5 in level 17.
• Three pressure cells were installed in 20 meters depth at 1/4 width from the
east side wall at the stope 18-5F-8 in level 18.
Number of Instruments installed in Chimo
mine

28. A special procedure was used to install the pressure cells from the top of an open
stope in a Canadian underground mine. The procedure was as follows:
• A cubic frame of elbow steel with dimension of (2" x 2" x 2") w/s prepared.
• 3 hydraulic pressure cells were fixed in x, y and z planes of the cubic frame.
• A cable was passed by the top of the stope using bow, arrow and fishing line. The
arrow was shot to an access on the other side of the stope from the top access. The
fishing lines was then used to pass cables from the top and other side of the stope.
These cables were then fixed at both sides of the stope.
• The cubic frame together with 3 fixed pressure cells was lowered to the 20 meters
depth from the top of the stope using cables and pulleys. Reflectors positioned on
the corners of the box were used to target the box and align it properly with
surveying equipment once the target level in the stope was reached

29. Performance Analysis of Backfill in Chimo mine
 Backfilling of the stope started after hanging the frame inside the stope. The
pressure cells in North-South (across ore), East-West (along ore) and vertical
directions were monitored and data registered once they were covered with
backfill.
 Field data from the pressure cells in two stopes were recorded over 200 days and
excellent results were obtained. These data showed that a significant amount of
energy was absorbed by paste backfill materials..
 Three pressure cells were installed in the centre of the backfilled stope 17-5B-5
at a depth of 20 meters, on September 13, 1995. Backfill operation started on
October 20 1995 and finished on November 18, 1995 (7 meters of backfill on top
was left for later operation).

30.  When backfill operation was finished, the recorded backfill pressures were as
follows:
 In last day of the operation, backfill was in a liquid situation with hydraulic
pressure. Considering the 7 meters left on top, the vertical pressure on top of the
pressure cell should be:
This is almost the same value that was collected from the field (180 kPa). In
general, after termination of the backfill operation in stope 17-5B-5, vertical
pressure in the backfill was equal to gravity loading.

31.  The ratio of the build up pressure in different directions inside the backfill was
compared with in situ stresses i.e.
This comparison tends to indicate that the ratio of build up pressure in the
horizontal section (or plane) is related to the ratio of the in situ stress in the
horizontal section. Because, if only gravity loading was involved, the stress ratio
in the horizontal plane should have been 1. During the curing period, the
temperature inside the backfill increased. The backfill temperature at the
beginning of the backfill operation was 18° C. During the next 60 days, backfill
temperature increased to the 38° C and then started to decrease very smoothly.

32.  The stress in N-S direction inside the backfill can be calculated as follows:
Stress = £*E
where E İs the Young's Modulus of the backfill (Enominal = 22 MPa) and £ is the
strain of the backfill due to the displacement of hanging wall in N-S direction.
Considering the displacement of the hanging wall prior to me rockburst 10 cm and
the width (vertical distance between two side walls) of the stope 4 meters:
dX = 10 cm
e = d X / L=10 cm / 400 cm = 0.025
. Stress = 22 MPa * 0.025 = 550 kPa
This value is within the range of data collected in the field in N-S direction right
before the rockburst. These results explain that the pressure applied inside the
backfilled stope 17-5B-5 was related to both the displacement of the hanging wall
and the Young's Modulus of die backfill. The fault movement towards backfill
causes higher backfill strain and pressure.

33.  The paste backfill inside the stope with a size of 4m x15m x70m can be assumed
as a large specimen that is tested by a testing machine in laboratory. The major
source of the force was the displacement and bending of die hanging wall due to
the fault separation. The value of strain in the middle of the backfill was obtained
by dividing the displacement of the anchor 5 by the stope wide:
Considering that the angle between the extensometer and stope wall is 70° then,
the value of the stress vertical to the stope side (Force applies in the direction of
extensometer) was calculated as follows:
Having the stress and strain inside the backfill, the strain energy density was
calculated as follows:
Strain Energy Density = 1/2 x stress x strain

35. The field data of pressure cells show that backfill absorbed a large quantity of the energy
from the surrounding rock mass. This energy transfers to the backfill through the work
done by displacement and build up pressure.
Work = Displacement x Force
Force = Stress x Area
To calculate the energy absorbed by pastefill in stope 17-5B-5, with 70 m. height and 15 m.
width:
S = 15x70 = 1,050 m2
The recorded backfill pressure m N-S direction before the rockburst was 1 MPa, therefore:
Force = l Mpa x 1,050 m2 = 106 x l.05
Pascal x m2 =106 x 1.05 Newton
Force =106 x 1.05 Kgf
Considering the 10 cm displacement in hanging wall at the end of rock burst period:

36. Conclusions regarding Performance of Backfill
in Chimo mine
The performance of backfill in the alleviation of rockburst can be concluded as
follows:
 The build up vertical pressure after termination of backfill operation was equal to
hydraulic pressure in the case of no mining activity around the stope. In the case
of mining activities around the backfilled stope, the vertical backfill pressure was
3 times more than hydraulic pressure.
 The ratio of build up backfill pressure in different direction was related to the in-
situ stress in the case of no mining activity around the stope. In the case of mining
activities around the stope, the convergence had a major role in the magnitude of
the backfill pressure in each direction.
 Significant stress redistributions took place due to the mining activities and stress
transfer occurred from the rock toward the backfill.

37.  The pressure inside the backfill was related to both the displacement of the side
walls of the stope and the Young's Modulus of the backfill. The fault movement in
side walls of the stope caused higher backfill strain and pressure. The strain energy
increased significantly in a period of one month before the start of rockburst
activities.
 The rate that strain energy was absorbed into the backfill could be used as an
indicator to study the potential for rockburst. This method needs further
investigation in field sites. The rate of stress increase in the backfill might be used
as an indicator to predict the potential of rockburst.
 Backfill absorbed energy through the compressive strain. The movement of the
side walls compressing the backfill, increased the backfill strain as well as the
pressure inside the backfill. This deformation work done on the backfill by the
convergence reduced the concentration of energy in surrounding rock mass. By
decreasing the concentration of stress in surrounding rock mass, the rockburst
intensity probably reduces.

38. References
• F.Hassani, J.Ouellet & S.Servant; In Situ Measurements in a Paste Backfill: Backfill and Rock
Mass Response in the Context of Rockburst; 17 International Mining Congress and Exhibition of
Turkey- IMCET2001, ©2001, ISBN 975-395-417-4.
• D. R. Tesarik, J. B. Seymour, T. R. Yanske, and R. W. McKibbin; Stability Analysis of a Backfilled
Room-and-Pillar Mine.
• D. R. Tesarik, J. B. Seymour and F. M. Jones; Determination of in situ deformation modulus for
cemented rockfill.
• D.R. Tesarik, J.B. Seymour and T.R. Yanske; Long-term stability of a backfilled room-and-pillar test
section at the Buick Mine, Missouri, USA.
• Landriault D. Backfill in underground mining. In: Hustrulid WA, BullockRL,editors. Underground
mining methods: engineering fundamentals and international case studies. Littleton, Colo: Society
for Mining Metallurgy & Exploration;2001.p.601–14.
• Geokon, Inc.Geokon: the world leader in vibrating wire technology. http://www.geokon.com.