Long-term stability of a backfilled room-and-pillar test section at the Buick

Mine, Missouri, USA

J.B. Seymour a D.R. Tesarik , a, ,

a NIOSH-Spokane Research Laboratory, 315 E. Montgomery Avenue, Spokane, WA 99207

* 

* Corresponding author. Tel.: +1509 354 8052; fax: +1509 354 8099.

E-mail address: det4@cdc.gov (D.R. Tesarik).

T.R. Yanske b

b The Doe Run Company, Buick Mine/Mill, HC1 Box 1390, Highway KK, Boss, MO 65440

abstract

Rock mechanics instruments have been providing data in a backfilled room-and-pillar test section of the

Buick Mine near Boss, Missouri, USA, for nearly 16 years. Host rock instruments include borehole

extensometers installed in the mine roof and pillars, and biaxial stressmeters used in pillars and

abutments. Embedment strain gauges, extensometers, and earth pressure cells were installed in the

cemented backfill. The instruments monitored stability of the test section for two years while the pillars

were extracted, and 14 years after pillar extraction to monitor long-term stability. Of the transducers

that were not mined out when the pillars were extracted, 68% still function. Data from these

instruments demonstrate that backfill improves long-term underground safety by supporting the mine

roof and maintaining the strength of support pillars. For example, backfill significantly limited the

dilation of a remaining support pillar by providing confinement on one side of the pillar. Post-mining

stress and strain in the backfill account for 35% and 28% of the total stress and strain that was measured,

respectively. Earth pressure cell stress measurements confirmed visual observations that the backfill

remained stable. Post-mining stress measurements from the earth pressure cells fit natural log

equations as a function of time with r-squared values ranging from 0.76 to 0.98. Natural log equations

also described post-mining strain behavior of the backfill with r-squared values ranging from 0.30 to

0.99. Stresses calculated for the backfill by a three-dimensional numerical model of the test area were

consistent with those that were measured by earth pressure cells.

1. Introduction

Several important factors affect the stability of underground

excavations including in situ stress, rock mass properties, opening

spans, and time since excavation [1]. With the loss of confining

forces on the rock mass after mining, ground conditions are

subject to deterioration, such as pillar spalling, caving of the mine

roof, and roof-to-floor or wall-to-wall convergence. This process

continues with time and threatens long-term stability. Backfill is

placed in underground openings prior to pillar extraction to slow

down this time-dependent failure by providing confinement and

support for the rock mass, thereby limiting the amount of

convergence. In many cases, long-term mining would be impossible without the use of backfill because it also provides

regional stability [2,3].

Backfill can also be subject to long-term loading as stresses are

gradually transferred from the host rock with time. Compaction,

settlement, and movement of backfill from this additional load

can affect regional mine stability. Long-term regional stability of

the backfill is essential when an access way must remain open

adjacent to a backfill-supported area or when there are active

mine workings near the backfilled section. Geotechnical instruments provide an important means of monitoring and quantitatively evaluating the stability of backfilled mine sections so that

appropriate actions can be taken to address stability concerns [4].

The duration of a rock mechanics instrumentation project is

determined in part, by the goals of the project, long-term

accessibility of the instrument cables, and susceptibility of

instruments and their cables to damage from mining activity.

The following citations are examples of backfill-related monitoring programs lasting less than six years. McNay and Corson [5]

measured wall closure and stress in a cut-and-fill vein mine for

over five years to quantify the effectiveness of hydraulic sandfill

on wall closure. Hergot and Munroe [6] monitored backfill

stresses and roof deformation in a potash mine for nearly two

years to assess the time required for waste salt placed as backfill

to provide roof support. Beddoes et al. [7] monitored backfill

stresses in mill tailings and wall deformations in a cut-andfill stoping system for approximately one year at a potash mine to

determine the consolidation and strength development of mill

tailings and the effect of backfill on wall closure. A monitoring

program was performed by Gu ̈ rtunca et al. [8] for nearly two years
to quantify the performance of cemented tailings on hanging wall

closure and compare the results to the performance of timberpack support systems. Pore-pressure was measured for approximately two years by Zhurin [9] in tailings deposited in an

underground iron ore deposit to estimate drainage time and

determine interaction between stopes. Donovan et al. [10]

instrumented three sill mat sections with multiple-point borehole

extensometers to measure vertical displacement of overlying

paste fill when it was undercut, earth pressure cells to measure

horizontal pressure, and contractometers along with extensometers to measure wall closure for about one year.

Rock mechanics programs with the primary objective to

evaluate long-term behavior of host rock and backfill necessarily

require a longer data collection period. De Sousa et al. [4]

monitored a grout and backfill system to control brine inflow

and monitor long-term stability for 10 years. Maleki et al. [11]

analyzed nine years of convergence measurements collected at a

proposed site for an underground repository for the permanent

disposal of radioactive waste to determine long-term stability of a

section of the repository.

Backfill was first tested at the Buick Mine in an instrumented

test section to determine if the support pillars that remained after

primary extraction could be mined safely and economically.

Quantifying the residual strength of failed pillars surrounded by

backfill, or ‘‘trapped’’ pillars, was of particular interest because of

their potential to stabilize the mine roof and reduce loads on

surrounding pillars [12]. The instruments that remained after

mining the pillars in the test section were read for an additional

14 years to monitor long-term stability and quantify time

dependent behavior in the mine roof and backfill.

1.1. Buick Mine

As shown in Fig. 1, the Buick Mine is one of six underground

mines in the New Lead Belt, or Viburnum Trend, that is owned and

operated by the Doe Run Company, St. Louis, Missouri.

Fig. 1. Operating mines in the New Lead Belt (Viburnum Trend), Missouri, USA.

The New

Lead Belt is a flat-lying, tabular ore deposit containing lead, zinc,

and copper that lies under 150–360 m of overburden. The ore

body is 1–40 m thick, 10–600 m wide, and has a trend length of

65 km. Fig. 2 illustrates the rock types using a generalized

stratigraphic column.

Fig. 2. Generalized stratigraphic column of Area 5.

The mining horizon consists of competent

dolomite with a roof composed of interbedded dolomite, shale,

and dolomitic mudstone. A 43-m-thick layer of Davis shale lies

above these beds followed by competent dolomite to the

weathered surface. The deposit has two main joint sets and one

set of horizontal bedding planes with discontinuities spaced from

13 cm to 2 m. One joint set trends N451E and dips 70–901 to the

southeast, and the other trends N451W and dips 70–901 to the

southwest. The dolomitic host rock has an average unconfined

compressive strength (UCS) of 108.9 MPa and a rock mass rating

(RMR) of 72 [13].

Mining equipment used for primary room development at the

Buick Mine includes two-boom drill jumbos, 7- to 9-ton-capacity

loaders, and 27- to 45-ton-capacity haul trucks. Secondary mining

includes back, bottom, undercut, and overcut passes. In sections to

be backfilled, either 1.8- to 2.4-m-long split sets or 1.8- to 2.4-mlong resin-grouted rebar bolts are installed, depending on ground

conditions [14].

Cemented rockfill (CRF) is used during pillar extraction to

provide area stability. To contain the CRF, uncemented 1.8-m-high

waste rock berms are first constructed between pillars around the

perimeter of the area to be backfilled. Several CRF lifts are then

placed between the berms to a height of 1.8 m. Fill fences

consisting of 13-mm steel cables and #4-gauge welded wire

panels are constructed from this CRF platform. The steel cables are

looped through angle brackets secured to the pillar ribs with

expansion anchor bolts. If the fence height exceeds 9 m, two

evenly spaced tieback anchors are placed in the fill to support

each support cable of the steel fence. Additional 1-m lifts are

placed to within 1 m of the top of the fill fence. This construction

procedure is repeated until the CRF is 0.6 m from the mine roof if a

slinger truck is used to fill the final gap. In this case, the top-off
material is a mixture of minus 5-cm waste rock, cement, fly ash,

water, and cycloned mill tailings. If a grout pump is used, the final

gap is 15 cm and filled with a mixture of mill tailings, water,

cement, fly ash, foaming agent, and a retarder [15,16]. After the

area is backfilled, ore pillars on the perimeter of the backfilled

section are mined, and trapped pillars are either left for support or

mined from a sublevel drift.

1.2. Backfill test section

A section called Area 5, shown in plan view in Fig. 3, was the first

place that backfill was used at the Buick Mine. Area 5 was chosen

because this section of the mine was surrounded by barrier pillars

that could arrest redistributed loads of a cascading pillar run if it

occurred during the test.

Fig. 3. Plan view of Area 5.

These pillars were approximately 8–12m

wide per side and 18m high. The entire backfilled area measured

88m x 37m. Rooms were approximately 9.8m wide. Fig. 4 is a

photograph of the south side of the test section.

Fig. 4. Pillars in Area 5.

The test area was backfilled with CRF that was produced by

combining minus 1-m run-of-mine waste rock with waste rock

that had been crushed to minus 13 cm and mixed at an underground batch plant with cycloned mill tailings and about

4% cement [17]. The dolomite waste rock that was used for

CRF aggregate was quarried underground [18]. The CRF was

transported to the backfill section by conveyor and spread in

0.3- to 0.6-m lifts using wheeled dozers. The CRF was placed in

contact with the pillar ribs and the rib of the west barrier pillar. As

lifts of backfill were placed, a steel-reinforced shotcrete fence was

constructed sequentially from the floor to the mine roof between

the perimeter pillars to contain the CRF. One of the completed fill

fences is shown in Fig. 5.

Fig. 5. Steel-reinforced shotcrete fill fence.

In addition, square shotcreted cyclone

fences with 1.5-m sides were constructed sequentially from floor

to roof on the north sides of the four trapped pillars which were

confined by CRF on all four sides. The inside of these enclosures

were left empty to serve as free faces when the pillars were

blasted. The top 3 m of backfill had a maximum aggregate diameter

of 5 cm and was placed with front-end loaders and slinger trucks

[15]. From visual inspections, approximately 50% of the backfill was

in contact with the mine roof, and the maximum gap between the

top of the backfill and remaining area was about 1 cm.

Table 1 contains the mining sequence and it is shown

graphically in Fig. 3. In general, the pillars in the drift northeast

of the backfilled section were mined first, followed by the pillars

along the perimeter of the backfill, and then pillars 101–104

which were surrounded by backfill. The trapped pillars were

drilled and blasted from an access drift approximately 6 m below

the test area. About 3.5 years after the last trapped pillar was

mined, eight additional pillars were mined south of this area

between the south barrier pillar and east abutment [19].

Since 1991, 12.7 million tons of pillar ore have been mined at

Doe Run’s mines in the New Lead Belt, using 5.4 million tons of

CRF. Pillar extraction currently accounts for 26% of rock mined

annually. However, because of their high grade, the mined pillars
currently account for 38% of the lead, 34% of the zinc, and 24% of

the copper metal that is produced annually. Doe Run has 17.2

million tons of pillar reserves.

Table 1

Pillar extraction sequence, elapsed time in days.

Pillar Day

97, East half 0

96 17

87, South half 17

97, West half 17

95 52

86, South half 52

94 59

105 85

106 85

114 101

5, North half 123

113, Bottom 12.5 m 123

6, West 3.7 m 123

113, Top 4.3 m 127

5, South half 177

14, North half 177

112 192

4, North half 192

4, South half 199

13, North half 199

111 212

110 212

93 221

92 233

101 389

102, 7 Holes 444

102, 20 Holes 451

104 515

103 695

23 1974

22 1984

32 2000

31 2010

41 2048

40 2109

38 2179

39 2181

2. Instrumentation

Borehole extensometers and biaxial stressmeters were installed in the pillars and abutments of Area 5. Extensometers were

also installed in the mine roof. Embedment strain gauges and

earth pressure cells were placed at mid-height and near the top of

the cemented backfill as shown in Fig. 6.

Fig. 6. Instruments in Area 5. A: Instruments originally installed; B: instruments

functioning 16 years after installation.

Full convergence

measurements were not taken because these instruments and

their cables could not be properly protected from mining

equipment and blast damage during backfilling or pillar removal.

The purpose of the instruments was to monitor short- and longterm stability, record stress redistribution to pillars during

mining, provide input to calibrate a numerical model, and

calculate rock mass modulus and strength. Measures taken to

ensure that components of the measurement system would cause

minimal error are discussed below.

2.1. Transducers

All of the instruments used in this study, except for the backfill

extensometers, were manufactured by Geokon, Inc.1

1 Mention of any company or product does not constitute endorsement by the

National Institute for Occupational Safety and Health.

Vibrating

wire transducers were used in all of the instruments because of

their proven longevity, stability, and minimal zero drift over long

time periods [20–22]. Another important feature of vibrating wire

instruments is that their readings are not affected by a change in

resistance of the lead wires. This eliminates the need to retrieve

and recalibrate the instrument if broken wires are spliced. None of

the instruments that were installed in this study were retrieved

because they were either covered with backfill, grouted in

boreholes, or located in inaccessible areas after pillar mining

was completed.

Geokon, Inc. is conducting an on-going long-term test with

eight 4500-series pressure transducers set at different initial

frequencies to determine if zero drift develops with time. The

operating principle of these pressure transducers is similar to that

of the earth pressure cells that were used in Area 5. Each

transducer is equipped with a vibrating wire element attached to

a pressure sensitive diaphragm. As fluid pressure acting on the

diaphragm changes, it causes the diaphragm to deflect, which in

turn produces a change in tension and thus a change in the

frequency of the vibrating wire [23]. Results from Geokon’s

laboratory test, presented in Fig. 7, indicate that all of the
transducers have insignificant drift and repeatable readings over a

period of 20 years.

Laboratory tests with vibrating wire displacement transducers

also demonstrate that these transducers provide reliable longterm readings. The displacement transducer consists of a vibrating

wire that is connected in series with a spring and a connecting

rod. Movement of the rod changes the tension in the spring and

consequently produces a corresponding change in the tension and

the resonant frequency of the vibrating wire. A displacement

transducer tensioned to 50% of full scale for 1,215 days exhibited

no significant zero drift, and repeatability was well within 70.5%

full scale of the gauge [20]. These tests along with those

conducted by Geokon, Inc. indicate that instrument measurements in Area 5 are a function of material behavior and not

instrument error.

Fig. 7. Long-term stability data for Geokon, model 4500 series, pressure transducers (courtesy of Geokon, Inc.).

2.2. Cables

To protect the transducer signal from electrical noise, each pair

of lead wires in the instrument cable is wrapped in Mylar tape

with an aluminum foil shield. A bare copper wire is routed with

each pair of lead wires to drain any induced currents to ground.

An additional drain wire is added to the bundle of pairs and

wrapped with another layer of Mylar tape [21]. Direct-burial

cables with thick polyurethane jackets are used to protect the

encased wires from sharp edges of the aggregate used in the

backfill. For additional protection, the instrument cables were

placed in slotted steel pipes and covered with ventilation fabric.

About 0.5 m of backfill was placed over the pipes, earth pressure

cells, and embedment strain gauges and allowed to cure for at

least one day before heavy machinery was driven over them.

2.3. Dataloggers

Three Campbell Scientific, Inc. dataloggers were used to

synchronize the instrument readings. A shed was placed near

the west barrier pillar to protect the dataloggers from blast fly

rock and mining equipment. Data collection began after backfilling was completed and all the instrument cables were

connected to the dataloggers, which was 101 days after the first

instruments were installed. Instrument readings were automatically recorded every 2 h except before and after some pillar

blasts; for these cases, the interval was shortened to 10 min to

capture immediate pillar response to the blast. Manual readings

for instruments that were not mined out or damaged during

pillar mining were recorded every one to two years, for the next

14 years.

Fig. 8. Stress versus time for earth pressure cells in north drift.

2.4. Temperature effect

Laboratory tests [24], field investigations [25], and theoretical

calculations [26] indicate that temperature change can significantly affect the magnitude of stress that is recorded by an earth

pressure cell. For example, a temperature rise of 34 1C resulted in a

temperature-induced stress increase in an earth pressure cell

equal to 830 kPa when the earth pressure cell was covered with a

thin layer of shotcrete [25]. Correction factors are available from

manufacturers but these factors account for a small percentage

of the temperature effect on the instrument because they

compensate only for the transducer and not the entire instrument

body [25].

In this study, cooling of CRF is characterized by apparent

decreases in the stress or strain measured in the backfill between
blasts as shown in Figs. 8–11. Based on previous field, laboratory,

and theoretical studies, these decreases are assumed to be

primarily temperature induced and were not included in calculations of total stress or strain change measured in the backfill. The

total stress or strain change in the backfilled area caused by pillar

mining was calculated by summing the change caused by the

weight of backfill placed over the instruments, the change that

was recorded immediately after each pillar was mined, the

positive time-dependent change between blasts, and the abrupt

changes not associated with a mining event. This sum does not

account for time-dependent loading from the mine roof that

was masked by temperature-induced decreases in readings.

Fig. 9. Stress versus time for earth pressure cells in south drift.

Fig. 10. Strain versus time for embedment strain gauges and vertical fill extensometer in north drift.

Consequently, reported values may be smaller than the actual

stress or strain that occurred in the backfill during mining.

Readings recorded by the borehole extensometer installed in

the mine roof centered on pillars 95, 96, 104, and 105 indirectly

show that temperature did not change significantly between days

451 and 515. Strain is plotted as a function of time for this

borehole extensometer in Fig. 12. Because strain caused by tensile

displacement in the mine roof and strain induced by CRF cooling

are both in the positive direction for this instrument, small

changes in readings between blasts indicate that the tensile strain

in the mine roof has stabilized by day 451 and imply that the

temperature change in the immediate roof is negligible.
Fig. 11. Strain versus time for embedment strain gauges located mid-height to

pillars in south drift.

Fig. 12. Strain versus time during mining for extensometer in mine roof centered on pillars 95, 96, 104, and 105.

3. Instrument placement

When the CRF was mid-height to the pillars, it provided a

working platform for installing horizontal extensometers and

biaxial stressmeters in the support pillars and placing earth

pressure cells and embedment strain gauges in the north and

south drifts at the mid-height level of the backfill.

3.1. Backfill instruments

Model 4800E earth pressure cells were used to measure

vertical stress change in the backfill. These 23-cm-diam instruments have a maximum load capacity of 6.9 MPa [23]. Before the

earth pressure cells were placed in the CRF, they were cast in

wood forms with minus 0.64-cm cemented aggregate and cured

for several weeks. The forms were removed before they were

placed in the backfilled drifts.

The model VCE-4210 embedment strain gauges were 25.4 cm

long with 5-cm-diam steel flanges at each end. A steel wire-andspring assembly is tensioned between the flanges in 2.54-cmdiam tubing and custom-built to provide up to 0.64 cm of relative

displacement, or about 25,000 me [23]. They were cast in nominal

15-cm-diam x 30-cm-long cardboard cylinders with the same

material as the earth pressure cells. Casting the instruments prior

to their installation in the backfilled drifts helps protect them

from the large aggregate in the CRF and also helps maintain their

alignment when they are covered with wet backfill.

To measure relative vertical backfill displacement over a large

distance, three 4.2-, 11.0-, and 16.5-m-long vertical extensometers

were assembled as the backfill was placed [18]. A 46-cm-square

steel plate was used as the bottom bearing surface of the

instrument to which a 5.1-cm-diam pipe coupler was welded. A

section of steel pipe was threaded into the coupler, and additional

sections were added as the height of the backfill increased.

Sections of steel rod were coupled inside the steel pipe to connect

the bottom plate to the vibrating wire transducer and top bearing

plate. A steel cover was bolted to the top plate to protect the

transducer. The top anchor of all three extensometers was

positioned about 4.6 m below the mine roof.

3.2. Host rock instruments

Model A-6 borehole extensometers [23] with fiberglass

measurement rods and hydraulic anchors were installed in all

but one of the pillars in the backfilled section to measure

horizontal strain, identify when the skin of pillars began to spall,

and compare strain on the backfilled side of the pillar to the strain

on the side that was not backfilled. The extensometers were

installed in B-size diamond drill holes with a shallow, largediameter hole, counterbored at the collar to recess the anchor and

protect the transducers from equipment and fly rock.

Four multiple-point, model A-6, vertical borehole extensometers were installed in the mine roof to measure roof sag and

strata separation. Earth pressure cells and embedment strain

gauges were placed in the backfill near these extensometers to

verify roof sag and to measure the load applied to the backfill by

the mine roof. Borehole extensometers were installed in the

exterior side of the perimeter pillars at inclinations from 451 to

691 up from horizontal with the borehole collar approximately

3 m from the floor. Vertical extensometers were installed in the

trapped pillars from a sublevel access drift about 6 m beneath the

test area.

To monitor and evaluate stress changes in the host rock, model

4350 biaxial stressmeters [23] were installed at the mid-height of

trapped pillars 102 and 103, the north abutment, and the west and
south barrier pillars. A description of this instrument and an

explanation of its installation procedures and use in other mining

applications is provided in [27].

4. Instrument results

For this study, compressive loading recorded by backfill

instruments, and tensile displacements measured by extensometers in the mine roof and pillars are reported as positive.

After nearly 16 years, 68% of the transducers installed in the

backfilled area are still providing data. This percentage does not

include instruments installed in pillars that were mined or biaxial

stressmeters installed in the abutments and barrier pillars.

Functioning instruments are shown in Fig. 6B. The location,

orientation, and anchor lengths for working extensometers are

listed in Table 2.

Table 2

Location, orientation, and anchor lengths of extensometers functioning after 16 years.

Instrument Location Dipa (deg) Distance from collar or base (m)

Anchor 1 Anchor 2

VFXb Northwest of pillar 103 90 11.89

BXc Pillar 113A 0 4.39 7.24

BX Pillar 113A 52 5.79 10.97

BX Centered in mine roof on 95, 96, 104, 105 90 10.87

a Dip Angle up from the horizontal (positive).

b Vertical backfill extensometer.

c Borehole extensometer.

Cables for the biaxial stressmeters installed in the

north and south abutments were cut by mining equipment shortly

after the instruments were installed and safe access could not be

provided to repair the cables. Time in service for instruments that

quit working are listed in Table 3.

Table 3

Time in service for instruments currently not working.

Instrument Location Time in service (yrs) Comments

VFXa N of 104 0.39

VFX SE of 93 1.24 Damaged when pillar 102 mined

EPCb Centered near top of backfill on 103, 104, 112, 113 1.24 Damaged when pillar 102 mined

EPC Centered near top of backfill on 104, 105, 113, 114 1.24 Damaged when pillar 102 mined

VFX Centered in mine roof on 93, 94, 102, 103 1.24 Damaged when pillar 102 mined

VBXc Centered in mine roof on 102, 103, 111, 112 1.24 Damaged when pillar 102 mined

VBX Centered in mine roof on 104, 105, 113, 114 1.24 Damaged when pillar 102 mined

ESGd Centered near top of backfill on 93, 94, 102, 103 1.29 Damaged shortly after pillar 102 mined

ESG Centered mid-height in backfill on 92, 93, 101, 102 6.68

ESG Centered mid-height in backfill on 94, 95, 103, 104 8.18

ABXe Pillar 113A, short anchor 11.82 Transducer located on exposed side of pillar

a Vertical backfill extensometer.

b Earth pressure cell.

c Vertical borehole extensometer.

d Embedment strain gauge.

e Angled borehole extensometer.

The biaxial stressmeters

installed in the west abutment still provide data, but the stress

changes obtained from these instruments became inexplicably

high during mining of the trapped pillars. An analysis of the

biaxial stressmeter data is not included in this paper. The earth

pressure cell and embedment strain gauge centered mid-height in

the backfill on pillars 92, 93, 101, and 102 and pillars 94, 95, 103,

and 104 were previously reported as nonfunctional after five years

because of the negative stresses and displacements that were

recorded [19]; but this effect has since been attributed to

decreasing temperature in the backfill. Transducer survivability

is 90% for instruments installed mid-height in the backfill as

opposed to 28% for those located in or near the mine roof. All but

one of the instruments with cables strung in the backfill near the

mine roof quit providing data when pillar 102 was mined, which

suggests that the blast likely damaged their cables. The angled

extensometer in pillar 113A was installed on the south side of the

pillar with the transducers located at the borehole collar. A sealed

cap was not used to protect the transducers from humid mine air,

and consequently, one of them possibly failed because of

corrosion.

Changes in instrument readings caused by blasts were

identified by examining the data in Excel spreadsheets. A stress

or strain reading that deviated from the trend on the day of the

blast was considered to be caused by a blast. The selection was

verified visually on plots of stress or strain versus time.

4.1. Backfill stress

Earth pressure cell measurements versus time are plotted in

Figs. 8 and 9. For approximately the first 200 days, the general

trend for the measurements is an immediate stress increase

measured within 10 h after each blast, followed by a temperaturedependent 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. A possible reason for the abrupt changes,

such as negative stress change, is 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. This is
consistent with temperature data collected in CRF that was about

23 m wide, 5 m high, and 31 m long. In general, temperature

change in this rockfill sill was negligible after about 400 days [25].

Stress changes measured by the earth pressure cells during

various time periods are compared in Table 4.

Table 4

Stress changes measured during and after pillar extraction.

Earth pressure cell

location

Calculated

initial stress

(kPa)

Sum of changes

measured within 10 h

after each pillar

extraction (kPa)

Sum of changes

measured between

each pillar extraction

(kPa)

Change from

installation to end of

mining (kPa)

Post-mining

change

(kPa)

Change from

installation to last

reading (kPa)

93Ma 190 379 12 581 1229 1810

94M 190 600 195 985 643 1628

95M 190 758 122 826 208 1034

96Tb 63 296 157 516 243 759

96M 190 1082 111 1383 1043 2426

101M 190 152 146 488 208 696

102M 190 1096 257 1543 126 1669

103M 190 938 69 1197 339 1536

104M 190 37 141 368 498 866

105M 190 655 158 1003 312 1515

Average NA 599 112 889 485 1374

Standard deviation NA 377 106 403 377 547

a Centered midheight in backfill on pillars 92, 93, 101, 102.

b Centered near top of backfill on pillars 95, 96, 104, 105.

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 average total stress increase that was

measured between blasts is 19% of the average total stress

increase recorded immediately after each pillar blast. The 30-

day UCS for 15-cm-diam samples cored from backfilled stopes

with similar constituents is 5.65 MPa, and the 28-day UCS for

laboratory-prepared specimens of this backfill mix is 8.3 MPa [28],

indicating that the backfill remained in the elastic range. Visual

observations confirm instrument data that the backfill remained

in a stable condition during mining, as illustrated in Fig. 13.

Fig. 13. Stable backfill after pillar extraction.

The stress measured by the earth pressure cells after the pillars

in the backfilled area were mined, or ‘‘post-mining’’ period, are

plotted versus time in Figs. 14 and 15. Regression lines for these

data are described by

sðtÞ 1⁄4 A lnðtÞ B, (1)

where s is compressive stress in kPa, t is time in days, and A and B

are parameters calculated from regression analyses. R-squared

values for these plots range from 0.79 to 0.98. Stresses recorded on

day 1917 for two earth pressure cells, which are plotted between

the dashed lines on the graphs, appear to be erroneous and are not

included in the regression data set. Mining the eight pillars

between the south barrier pillar between days 1974 and 2181 had

no apparent effect on the long-term trend of the stress-versustime data.

The average post-mining stress change is 35% of the total stress

change. This figure may be slightly high because some of the timedependent loading between blasts was likely masked by temperature-induced stress decrease. For two of the earth pressure

cells, post-mining stress change is greater than the total stress

change during mining. This emphasizes the importance of

incorporating time-dependent loading into long-term stability

design as well as incorporating a factor of safety for nonuniform

loading.

The maximum stress change from the time the instruments

were installed to the last reading that was taken, approximately

16 years later, is 2.4 MPa. This value is 42% of the average UCS of

specimens prepared from core samples of a placed backfill with

similar constituents [28], indicating that the backfill remained

stable. Periodic visual observations confirm this conclusion.

4.2. Backfill strain

As shown in Figs. 10 and 11, the general trend for the

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 centered

mid-height on pillars 104, 105, 113, and 114 follows the same trend

as the other embedment strain gauges, but is significantly larger

in magnitude. The earth pressure cell located near this instrument

is in the same range as the other earth pressure cells, indicating

that the large embedment strain gauge readings were caused by

some mechanism limited to a small volume of backfill around the

instrument, but the mechanism is unknown. There also appears to

be a local effect on the embedment strain gauge located midheight to pillars 95, 96, 104, and 105 exhibited by apparent tensile

strain. 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.

Fig. 14. Post-mining stress versus time for earth pressure cells in north drift.

Fig. 15. Post-mining stress versus time for earth pressure cells in south drift.

Table 5 compares strain changes measured by the embedment

strain gauges during various time periods. The total strain

increase recorded by the embedment strain gauges, from the

time that they were installed to the time when the last pillar in

the backfilled section was mined, ranges from 140 to 3514 me, with

an average equal to 746 me and standard deviation equal to

1,135 me. Hooke’s Law was used to calculate initial strain. For this

calculation, initial stress was obtained by multiplying the specific

weight of the backfill by the depth of the backfill over the

instrument, and the deformation modulus was calculated from

earth pressure cell and embedment strain gauge readings that

were recorded during mining. The average total strain increase

that was measured between blasts is 42% of the average total

strain increase recorded within 10 h after each pillar blast.

A natural log function describes post-mining strain-versustime data, as shown in Figs. 16 and 17, with r-squared values

ranging from 0.30 to 0.99. The average post-mining strain in the

backfill is 28% of the total measured strain. Knowledge of this

general trend in time-dependent behavior could be useful for

assessing the long-term stability of other backfilled areas.
Table 5

Microstrain changes measured during and after pillar extraction.

Embedment strain

gauge location

Calculated

initial strain

(me)

Sum of changes

measured within 10 h

after each pillar

extraction (me)

Sum of changes

measured between

each pillar extraction

(me)

Change from

installation to end of

mining (me)

Post -

mining

change (me)

Change from

installation to last

reading (me)

94a 101 1 38 140 31 171

96 101 111 100 312 55 257

101 101 0 60 161 282 443

102 101 60 42 203 92 295

103 101 422 43 566 598 1164

104 101 155 132 388 323 711

105 101 2325 1088 3514 983 4497

103VFXb 69 581 33 683 28 711

Average NA 457 192 746 285 1031

Standard deviation NA 783 364 1135 353 1437

a Centered midheight in backfill on pillars 93, 94, 102, 103.

b Vertical fill extensometer northwest of pillar 103.

Fig. 16. Post-mining strain versus time for embedment strain gauges and vertical fill extensometer in north drift.

4.3. Pillar and roof strain

Measurements from the horizontal two-point borehole extensometer in pillar 113A demonstrate the effect of confinement

provided by backfill on long-term pillar dilation. Backfill is placed

on the north side of the pillar and the south side is unconfined. As

shown in Fig. 18, the extensometer recorded a higher strain rate

on the confined side of the pillar until day 515 when pillar 104 was

mined. A possible cause for this high initial strain rate, despite

confinement by the backfill, is the highly fractured condition of

the north side of the pillar’s skin that was observed during the

installation of the extensometer. The trend reversed on day 515,

and by day 1207 the horizontal strain rates on each side of the

pillar were equal. By day 5689, the total measured strain on the

unconfined side was over twice the strain on the confined side.

Strain rate for the last 290 days, calculated by using the final two

readings, was 0.17 me/day for the unconfined side and 0.025 me/

day for the confined side; therefore, the strain rate for the

unconfined side of the pillar is 6.8 than the rate for the confined

side during the final 290 days of measurements.

Strain calculated from displacements recorded by the angled

extensometer installed from the south side of pillar 113A is

plotted against time in Fig. 19. The extensometer recorded

compressive strain between the two deepest anchors during

mining, indicating that the mine roof loaded the pillar. Strain,

measured between the middle and collar anchors, did not

substantially change during this time period. The general trend

for both segments of the extensometer after mining, was

increasing tension at approximately the same rate, indicating

that the pillar was dilating. Another extensometer, installed from

the north side of the pillar at the same angle as the one installed

from the south side, would be necessary to compare the effect of

confinement by the backfill.

Fig. 12 shows that the vertical borehole extensometer in the

mine roof that is centered on pillars 95, 96, 104, and 105 measured

compressive strain after each pillar was mined, followed by timedependent tensional strain, up to day 233. A large component of

the time-dependent strain is likely temperature-induced because

the backfill mass was cooling. Pillar blasts had little effect on

measured strain after day 233, until day 515 when support for the
mine roof near the vertical extensometer was reduced by mining

pillar 104. The extensometer responded with tensional strain,

followed by time-dependent tensional strain. A similar pattern

was recorded in response to the extraction of pillar 103 on day

695. Post-mining measurements plotted in Fig. 20 indicate that

the instrument measured tensional strain until day 1914, after

which strain change was insignificant. This further confirms that

temperature-induced tension had ceased.

Fig. 17. Post-mining strain versus time for embedment strain gauges located midheight to pillars in south drift.

Fig. 18. Horizontal strain versus time for extensometer in pillar 113A.

Fig. 19. Strain versus time for angled extensometer in pillar 113A.

Fig. 20. Strain versus time for extensometer in mine roof centered on pillars 95,

96, 104, and 105.

5. Numerical modeling

A three-dimensional, finite-element program, UTAH3 [29], was

used to model the excavation and backfill sequence [13]. A

continuum model was chosen because Area 5 did not contain

major faults that could dominate material behavior. The modulus

of deformation was reduced from its laboratory value to account

for the joint sets. The yield criterion in UTAH3 is Drucker–Prager,

in which strength is dependent on all three principal stresses, and

the associate flow rules are applied for determining strains in

yielded elements. Elastic perfectly plastic material behavior was

used for the rock mass because of the confinement provided by

the backfill. This material property was also chosen for the backfill

because it was assumed that the large volume of material would

provide self confinement; also, laboratory triaxial tests on rockfill

specimens, similar in composition to the backfill in Area 5,

indicate that the stress-versus-strain relationship can be idealized

with an elastic perfectly plastic material model [30].

The finite-element mesh contained 252,000 elements representing a rock mass 271 x 271 x 535 m with a 18.3-m-thick mining

horizon 366 m below the surface and 151 m above the bottom of

the rock mass block. Fig. 3 shows the model’s boundary in plan

view. Rollered boundary conditions were used on all sides of the

block except the top, which was free to move vertically. Cubes

4.6 m on a side simulated the rock mass from 9 m below to 46 m

above the mining horizon. The remaining elements were

4.6 x 4.6 x 9.1 m with the longest dimension along the vertical

axis. The entire mesh is shown in Figs. 21 and 22 depicts a

discretized section containing a room and two pillars. Vertical

coordinates of the nodes in the finite element mesh were adjusted

to coincide with the boundaries between rock types from a

generalized stratigraphic column of Area 5. The idealized excavation shape was realistically represented by using these brickshaped elements because pillar boundaries nearly coincided with

the faces of the bricks. Node coordinates for pillars 93, 102, 103

and 104 were adjusted to conform to pillar boundaries more

accurately. Each pillar blast or excavation step that was modeled

required about 12 h to run on a Dell Precision 530 work station.

Laboratory material properties for the rock types used in the

model are listed in Table 6. Properties for dolomite were obtained

from unconfined uniaxial compression tests on specimens

prepared from BX core recovered from the instrument holes.

The specific weight of these specimens was averaged for input

into the model. Dolomitic mudstone and shale property values
were based on published laboratory tests [31]. The deformation

modulus for the backfill was calculated using data recorded by

pairs of earth pressure cells and embedment strain gauges placed

at the same location in the backfill by dividing the sum of the

stress changes measured immediately after a pillar blast by the

sum of the corresponding strain changes. Strength values for the

backfill were estimated from UCS tests conducted on laboratoryprepared specimens and core samples extracted from a backfilled

stope with constituents similar to the Buick Mine backfill [28].

In situ stresses that were used as the initial stress state in the

numeric model are listed in Table 7. The were obtained by

overcoring four hollow inclusion stress cells (developed by

Australia’s Commonwealth Scientific and Industrial Research

Organization) in the north part of the Buick Mine. Measurements

were taken at borehole depths between 13.4 and 16.5 m in an

abutment of a room-and-pillar section approximately 72 m wide

and 305 m long. Cube-shaped pillars in this area measured 7.6 m

on each side and rooms were 10.7 m wide [32].

Fig. 21. Finite-element mesh.

Fig. 22. Discretized room, pillars, mine roof, and mine floor.

UTAH3 [29] was calibrated using blast-induced displacement

readings recorded by the vertical borehole extensometers in the

trapped pillars [13]. Model-predicted strains were plotted against

strains calculated from measured displacements and a line was fit

to the data using regression analysis. The initial modulus of

deformation for dolomite, equal to 37.2 GPa, used in the model

was based on results of previous calibrations using a threedimensional, boundary-element program and a two-dimensional

finite-element program [18]. This value is approximately 45% of

the laboratory deformation modulus. Deformation modulus

values for dolomitic mudstone and shale were also reduced by

45%.

The slope of regression line for calculated versus measured

strain was equal to 0.85, indicating that the initial modulus could

have been further reduced by multiplying it by 0.85 to achieve a

slope equal to one for the regression line. However, this would

reduce the difference between calculated and measured strain for

only a few pair of points with large strain and increase the

difference for the majority of pairs. For this reason, the initial

modulus was not reduced.

The UCS of dolomite was reduced by comparing modelcalculated changes in vertical strain in trapped pillar 103 to

measured strain. On day 192 when pillars 112 and 4 were blasted,

measured strain was nine times that of elastic calculated strain,

indicating that pillar 103 failed. Iterative computer runs were

made with decreasing UCS values for dolomite until the model

produced failed elements in pillar 103 when pillars 112 and 4 were

mined. This value is 43 MPa or about 40% of the laboratory UCS

value of dolomite. Tensile strength was also reduced by 40%.

Reduced deformation modulus and unconfined strength values

obtained by calibrating UTAH3 compare favorably to those

calculated using empirical methods. A rock mass modulus equal

to 44 GPa, or 52% of the average laboratory deformation modulus,

was calculated using the following empirical relationship [33].

EM 1⁄4 2 x RMR 100, (2)

where EM is the rock mass deformation modulus in GPa.

The Hoek–Brown generalized failure criterion for jointed rock

[34] was used to estimate the UCS of the rock mass. This formula

is

s0

1 1⁄4 s03 þ s ðmbs03=sci þ sÞ

a ci , (3)

where s1

0 and s30 are the maximum and minimum effective

stresses at failure, mb is the value of the Hoek–Brown constant for

the rock mass, s and a are the parameters that depend upon rock

mass characteristics, and sci is the UCS of the intact rock. For an

estimate of the UCS of the rock mass, Eq. (3) reduces to

s0 1⁄4 s a cis , (4)

where

s 1⁄4 exp1⁄2ðGSI 100Þ=9 1⁄2, (5)

where GSI is the geological strength index developed in [35,36] to

estimate the reduction in rock mass strength for different

geological conditions; it is presented in tabular form in [37].
Table 6

Average laboratory rock properties and in situ backfill properties.

Young’s modulus

(MPa)

Unconfined compressive

strength (MPa)

Tensile strength

(MPa)

Specific weight

(kg/m3

)

Poisson’s ratio

Dolomite 84,800 108.9 10.9 2563 0.26

Dolomitic mudstone 34,500 54.0 5.4 2563 0.25

Shale 17,200 54.0 5.4 2195 0.25

Cemented backfill 1910 6.9 0.7 2114 0.30

Table 7

Major principle stresses from hollow inclusion stress cells.

Stress (MPa) Azimuth (deg)a Dip (deg)b

24.52 240 1.6

7.98 128 85.8

3.44 149 3.9

a Angle clockwise from north is positive.

b Angle down from horizontal is positive.

Based on the existence of two joint sets and one set of bedding

planes in Area 5, the GSI value is about 80. For a GSI value greater

than 25, the parameter a is equal to 0.5. Using Eqs. (4) and (5), the

reduction factor (s

a

) in UCS for a GSA value of 80 is 0.33.

The initial modulus of deformation of the backfill was not

adjusted because it was calculated from in situ measurements.

There is scatter in the measured versus model-calculated stress

changes as shown in Fig. 23, however, these data help explain the

interaction of the backfill and mine roof.

Fig. 23. Calculated versus measured stress in backfill.

The plot contains a

cluster of points in which the calculated stresses are significantly

larger than measured stress changes and five points representing

negative measured stress changes. Inspection of the data containing the large difference between measured and calculated stress

changes reveals that these points were recorded when at least one

of the four pillars surrounding the earth pressure cell was mined.

With loss of local support provided by nearby pillar or pillars, the

roof likely settled on the uneven backfill surface, filling voids left

by the belt slinger, but not significantly loading the backfill. In the

numerical model, the backfill was in complete contact with the

mine roof, enabling the backfill to react immediately to vertical

sag in the roof and produce higher calculated stresses than those

that were measured. Other possible reasons for the discrepancy in

stress magnitude are varying material properties in the backfill

and mine roof, and mine geometry that is not accounted for in the

idealized model.

The negative changes recorded by the earth pressure cells may

have been caused by point contacts between the mine roof and

backfill, yielding and redistributing stresses. Based on this

interpretation of the interaction of the backfill and mine roof,

the magnitude of measured stress changes are realistic.

6. Conclusions

This case study in a backfilled section of a lead/zinc/copper

room and pillar mine suggests the following conclusions:

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 for 16 years.

Backfill limits pillar dilation. The total measured strain on the

unconfined side of a support pillar was over twice the strain on

the side confined by backfill. The strain rate for the unconfined

side of the pillar was 6.8 times larger than the rate for the

confined side for the last 290 days that were measured.

Vibrating wire instruments provide reliable long-term data

provided that the instruments and their cables are adequately

protected. After nearly 16 years, 68% of the transducers that were

installed in the Area 5 backfilled test area, and not mined out, still

provide data. Based on long-term laboratory tests on vibrating

wire transducers performed by other researchers, these data are a

true function of material behavior because vibrating wire

transducers have repeatable readings and insignificant drift.

A natural logarithm function in the form s(t) 1⁄4 A ln(t) B

describes post-mining stress measured by earth pressure cells and

strain measured by embedment strain gauges, as a function of

time. R-squared values for the earth pressure cell plots range from

0.79 to 0.98, and from 0.30 to 0.99 for the embedment strain

gauge plots. Knowledge of this general trend in time-dependent

behavior could be useful for assessing the long-term stability of

other backfilled areas.

Numerous instruments placed throughout the backfill aided in

interpreting the interaction of the backfill and host rock. Data

from earth pressure cells in the backfill indicate that the backfill

was not loaded uniformly by the mine roof. This was verified by

plotting stress changes in the backfill (calculated by a threedimensional, finite-element model of Area 5) versus measured

stress changes in the backfill. Calculated stress was much larger

than measured stress in some cases because the mine roof was

not 100% in contact with the backfill.

Recording temperature of the backfill and host rock would

facilitate data interpretation for backfill studies. For example, in

this study, time-dependent stress increase between pillar blasts

could not be measured because it was masked by decreasing

temperature in the backfill. The time at which backfill cooling
ceased was interpreted from extensometer displacements in the

mine roof.

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 for 16

years.

Acknowledgments

The authors wish to express their appreciation to the following

members of The Doe Run Co. who were instrumental in the

success of the A-5 Study at the Buick Mine, Boss, Missouri: Bill

Lane, Vice President of Mining & Technical Development, for

initiating the research, assisting in the development and implementation of the geotechnical instrumentation plan, and

supplying most of the instruments and data acquisition equipment; Bob Roscoe, Vice President of Mining and Milling Operations in Missouri and Greg Sutton, Mine Manager, for their

support installing instruments and maintaining data acquisition

systems during the initial years of this study. We also wish to

thank the many Doe Run employees who have periodically

collected manual readings from the instruments over the intervening years following completion of mining in Area 5, in

particular, Lori Young, Mine Engineer, and former employee

Lyndon Clark. We gratefully acknowledge the efforts of the

following employees of the NIOSH Spokane Research Laboratory,

Spokane, WA: Mike Jones for collecting manual readings; Bob

McKibbin and Curtis Clark for installing instruments; and Bob

McKibbin and Mike Jenkins for determining in situ stress. We also

wish to thank Tony Simmonds, International Projects Manager,

Geokon, Inc., Lebanon, NH, for providing data from long-term

pressure transducer tests; and Bill Pariseau, Department of Mining

Engineering, University of Utah, for technical suggestions regarding this research.

Disclaimer

The findings and conclusions in this paper have not been

formally disseminated by the National Institute for Occupational

Safety and Health, and should not be construed to represent any

agency determination or policy.

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