UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9565 REPORT OF INVESTIGATIONS/1995

Stability Analysis of a Backfilled

Room-and-Pillar Mine

By D. R. Tesarik, J. B. Seymour, T. R. Yanske, and R. W. McKibbin
U.S. Department of the Interior

Mission Statement

As the Nation’s principal conservation agency, the Department of the

Interior has responsibility for most of our nationally-owned public

lands and natural resources. This includes fostering sound use of our

land and water resources; protecting our fish, wildlife, and biological

diversity; preserving the environmental and cultural values of our

national parks and historical places; and providing for the enjoyment

of life through outdoor recreation. The Department assesses our

energy and mineral resources and works to ensure that their

development is in the best interests of all our people by encouraging

stewardship and citizen participation in their care. The Department

also has a major responsibility for American Indian reservation

communities and for people who live in island territories under U.S.

administration.
Report of Investigations 9565

Stability Analysis of a Backfilled

Room-and-Pillar Mine

By D. R. Tesarik, J. B. Seymour, T. R. Yanske, and R. W. McKibbin 

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES

Rhea Lydia Graham, Director
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ........................................................................................... 1

Introduction ........................................................................................ 2

Buick Mine ........................................................................................ 2

Test area ........................................................................................ 2

Instruments ...................................................................................... 6

Stress determination before pillar removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Material property determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Numeric models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Instrument response to mining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

ILLUSTRATIONS

1. Location of New Lead Belt ....................................................................... 3

2. Generalized stratigraphic column in area 5 ........................................................... 4

3. Plan view of area 5 .............................................................................. 5

4. Location of instruments installed in backfill .......................................................... 6

5. Location of instruments installed in rock ............................................................ 7

6. Vertical backfill extensometer ..................................................................... 8

7. Vertical section through trapped pillar showing two-point vertical extensometer installed from lower

access drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. Microstrain changes in trapped pillars calculated from measured and predicted displacements . . . . . . . . . . . . . . . . . 12

9. Stress changes recorded by earth pressure cells in north section of area 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Stress changes recorded by earth pressure cells in south section of area 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

11. Strain measured by embedment strain gauge at midheight in backfill and centered on pillars 102, 103,

111, and 112 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

12. Strains measured by embedment strain gauges and vertical extensometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

13. Measured and predicted vertical strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

14. Horizontal strain measured between downhole anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

15. Vertical strain measured by borehole extensometer in mine roof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

TABLES

1. Pillar extraction sequence ......................................................................... 3

2. Instrument location .............................................................................. 9

3. Measured in situ principal stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Measured in situ stress components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5. Material properties of host rock determined from laboratory tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6. Material properties used in program BESOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7. Laboratory and estimated material properties for UTAH2 analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Maximum compressive stresses recorded by earth pressure cells after installation . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

9. Maximum microstrain recorded by embedment strain gauges after installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Elastic modulus values calculated from backfill instrument readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

11. Measured and predicted vertical stress changes in pillars 102 and 103 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter m square meter 2

cm square centimeter MPa megapascal 2

kg/m kilogram per cubic meter pct percent 3

km kilometer t/d metric ton per day

m meter

Reference to speific products does not imply endorsement by the U.S. Bureau of Mines.

Disclaimer of Liability

The U.S. Bureau of Mines expressly declares that there are no warranties expressed or implied that apply to the software

described herein. By acceptance and use of said software, which is conveyed to the user without consideration by the Bureau of

Mines, the user hereof expressly waives any and all claims for damage and/or suits for or by reason of personal injury, or property

damage, including special, consequential, or other similar damages arising out of or in any way connected with the use of the

software described herein.
Mechanical engineer, Spokane Research Center, U.S. Bureau of Mines, Spoakne, WA. 1

Mining engineer, Spokane Research Center. 2

Mine engineer, The Doe Run Co., St. Louis, MO. 3

Stability Analysis of a Backfilled Room-and-Pillar Mine

By D. R. Tesarik, J. B. Seymour, T. R. Yanske, and R. W. McKibbin 1 2 3 2

ABSTRACT

Displacement and stress changes in cemented backfill and ore pillars at the Buick Mine, near Boss, MO, were

monitored by engineers from the U.S. Bureau of Mines and The Doe Run Co., St Louis, MO. A test area in this

room-and-pillar mine was backfilled to provide support when remnant ore pillars were mined. Objectives of this

research were to evaluate the effect of backfill on mine stability, observe backfill conditions during pillar

removal, and calibrate a numeric model to be used to design other areas of the mine.

Relative vertical displacements in the backfill were measured with embedment strain gauges and vertical

extensometers. Other types of instruments used were earth pressure cells (to identify loading trends in the

backfill), borehole extensometers (to measure relative displacement changes in the mine roof and support pillars),

and biaxial stressmeters (to measure stress changes in several support pillars and abutments).

Two- and three-dimensional numeric codes were used to model the study area. With information from these

codes and the installed instruments, two failed pillars were identified and rock mass properties were estimated.
2

INTRODUCTION

Historically in room-and-pillar mines in the United States, pillar recovery operations (Udd, 1989). Recently, the West

ore pillars have been left to support the mine roof. Although Driefontein Mine, located approximately 60 km west of

this mining method is structurally sound, it decreases the life of Johannesburg, South Africa, designed a modified room-andthe mine because approximately 25 pct of the resource is not pillar mining method to mine a reef dipping 25E to 30E

used. The Doe Run Co., St. Louis, MO, developed a method in (Stilwell, 1983).

which cemented backfill was used in mining pillars in a test Advances in backfilling in the United States have resulted in

section of its Buick Mine, near Boss, MO. Rock mechanics new mining methods for room-and-pillar or slot-and-pillar

consulting services were supplied by Golder Associates, mines that recover nearly 100 pct of the ore deposit. The

Burnaby, BC. To evaluate this mining method, engineers from Cannon Mine, near Wenatchee, WA, uses an overhand benchthe U.S. Bureau of Mines (USBM) and The Doe Run Co. and-fill technique to mine 8-m-wide by 24-m-high by 45-minstalled instruments in the backfill and host rock to measure long stope blocks (Brechtel and others, 1989a; Brechtel and

stress and displacement changes during pillar mining. The in others, 1989b; Tesarik and others, 1983; Tesarik and others,

situ stress state was measured in a barrier pillar next to the test 1989). The American Girl Mine near Yuma, AZ, uses a similar

area before mining was initiated in the pillars. method in a narrow, shallow-dipping ore body.

Foreign mines have been using backfill to achieve total ore Other than pillar robbing, complete recovery of previously

extraction for a number of years. The Keretti Mine in Finland developed room-and-pillar mines is limited in the United States.

(Koskela, 1983) uses a modified room-and-pillar method and At the Magmont Mine near Bixby, MO, uncemented cycloned

Mount Isa Mines, Ltd., Mount Isa, Australia, achieves total ore mill tailings were used to confine the bottom one-third of the

extraction using an open stoping method with backfill (Bloss pillars in a narrow section of the mine. The upper two-thirds of

and others, 1993). Cemented fill was introduced in the the pillars were extracted by retreat mining (Tesarik and

Canadian Sudbury mines in the early 1960's in cut-and-fill and McKibbin, 1989).

4

BUICK MINE

The Doe Run Co.'s Buick Mine is located 195 km southwest these enclosures were not filled with backfill and served as

of St. Louis, MO, in a deposit called the New Lead Belt or blasting release areas (free faces) when the pillars were blasted.

Viburnum Trend (figure 1). The mine produces lead, zinc, and Dolomite waste rock was quarried underground for use as

copper ore using a room-and-pillar mining method at depths aggregate in the backfill mix, crushed to minus 12 cm and

ranging from 335 to 366 m. The Buick ore body is 60 to 120 mixed with about 4 pct cement at a portable batching plant.

m wide with ore thicknesses ranging from 2.4 to 36.6 m. A Backfill was transported from the batching plant to the test area

generalized stratigraphic column is shown in figure 2. The by a conveyor, transferred throughout the test area with frontmine is divided into two sections, the North Mine and the South end loaders, and spread in 0.3- to 0.6-m lifts with a wheel

Mine. The North Mine has the highest grade ore, but most of dozer. Backfilling rates ranged from 1,000 to 2,000 t/d. The

the developed reserves are existing support pillars in the South maximum aggregate diameter for the top 3-m lift was 5 cm and

Mine. was placed with front-end loaders and slinger trucks. The gap

TEST AREA and was less than 1.3 cm in most areas.

The test area was approximately 107 by 69 m and contained several pillars north and south of this area is listed in table 1.

15 pillars with heights ranging from 14 to 19 m. The pillars Pillars in the drift northeast of the test area were extracted prior

were approximately 9 m per side with 9.8-m-wide rooms to January 20, 1992, the date when the instrument system was

yielding an extraction ratio of 77 pct. Before the pillars were functioning. Pillars 101 through 104, referred to as trapped

extracted, a fill fence was constructed around the perimeter of pillars, were drilled, blasted, and mucked from a drift excavated

the pillars (figure 3). The steel-reinforced shotcrete fence was beneath the test area.

constructed sequentially, from the floor to the roof, as lifts of

cemented rock fill were placed. In addition, 1.5-m shotcreted 2

cyclone fences were constructed from the floor to the roof on

the north side of pillars 101, 102, 103, and 104. The inside of

left between the backfill and mine roof ranged from 0 to 2.0 cm

The pillar extraction sequence for the test area and for

Paper presented at the 97th annual meeting of the Northwest Mining 4

Association, Spokane, WA, entitled "American Girl Underground Mine" by

R. K. Towner an J. W. Keifer, 1991, 12 pp.
3

Table 1.—Pillar extraction sequence, days after January 20, 1992

Pillar Days Pillar Days Pillar Days

97, east half . . . 0 5, north half . . . . . . . . . . 123 111 . . . . . . . . . . 212

96 . . . . . . . . . . . 17 113, bottom 12.5 m . . . . 123 110 . . . . . . . . . . 212

87, south half . . 17 6, west 3.7 m . . . . . . . . 123 93 . . . . . . . . . . . 221

97, west half . . . 17 113, top 4.3 m . . . . . . . . 127 92 . . . . . . . . . . . 233

95 . . . . . . . . . . . 52 5, south half . . . . . . . . . 177 101 . . . . . . . . . . 389

86, south half . . 52 14, north half . . . . . . . . . 177 102, 7 holes . . . 444

94 . . . . . . . . . . . 59 112 . . . . . . . . . . . . . . . . 192 102, 20 holes . . 451

105 . . . . . . . . . . 85 4, north half . . . . . . . . . . 192 104 . . . . . . . . . . 515

106 . . . . . . . . . . 85 4, south half . . . . . . . . . 199 103 . . . . . . . . . . 695

114 . . . . . . . . . . 101 13, north half . . . . . . . . . 199
4
5
6

INSTRUMENTS the dataloggers collected data from three Commonwealth

All the instruments installed in the backfill and support inclusion cells, a Yoke gauge, a USBM deformation gauge, and

pillars in area 5 were manufactured by Geokon, Inc., Lebanon, two biaxial stressmeters. These instruments were installed in

NH. These instruments contained vibrating-wire sensors the barrier pillar to monitor long-term stress changes.

consisting of a tensioned steel wire and a coil and magnet Instrument readings were taken automatically every 2 hours by

assembly. When a pulse generated by a datalogger is sent the dataloggers, but this time interval was reduced to several

along the instrument cable and applied to the coil, the wire minutes before and after some blasts.

vibrates at its resonant frequency. This frequency is induced in Cables were strung from the instruments to the dataloggers

a pickup coil and transmitted back to the datalogger for through steel pipes placed on the backfill. A slot cut with a

processing and/or storage. Because strain in the wire is directly cutting torch along the pipe's longitudinal axis facilitated cable

proportional to gauge frequency, linear calibration equations placement in the pipe. These pipes were covered with used

for instruments containing these sensors can be used to relate vent bag material to keep rock fill out of the slots. A larger

frequency readings to displacement or pressure changes. diameter pipe cut in half served as a protective cover for the

Three Campbell Scientific dataloggers were used to monitor cable at the open joints between the slotted pipes. Backfill was

all the instruments in this study (figures 4-5). In addition, placed to a depth of about 0.5m over the pipes and

Scientific and Industrial Organization (CSIRO) hollow
7

and was left to cure for 1 day before heavy machinery was 0.64-cm aggregate. This facilitated vertical alignment during

driven over it. installation and provided protection when the wet backfill was

Earth pressure cells, embedment strain gauges, and vertical first placed over the gauge.

extensometers were placed in the cemented backfill (figure 4) Earth pressure cells having a maximum load capacity of 6.9

to identify loading patterns and to measure relative MPa were used to identify loading trends and were not relied

displacement changes when the rock pillars were extracted. upon for precise measurements. The 22.9-cm-diam instruments

Most of the earth pressure cells and embedment strain gauges were precast in a form slightly larger than the cell using the

were installed when the test area was backfilled to midheight same backfill mix that was used to cast the embedment strain

(9 m). Some of these instruments were placed near the top of gauges. This form was removed before the instrument was

the backfill, 1 to 2 m below the roof, under vertical borehole placed in the stope.

extensometers in the rock. The purpose of using these Three vertical extensometers were constructed to measure

instruments was to identify when the backfill began to load. relative displacement in the cemented backfill. The distance

The embedment strain gauges are 25.4 cm long with 5.1-cm- between anchors for these instruments ranged from 4.2 to 16.5

diam steel flanges at each end. A steel wire-and-spring m (figure 6) and the top anchor of all three instruments was

assembly is tensioned between the flanges in 2.54-cm-diam positioned approximately 4.5 m below the mine roof. A 5-cm

steel tubing and provides up to 0.64 cm of relative steel pipe coupler was welded to a plate that served as the

displacement. Prior to installation, the gauges were precast in bottom anchor for these instruments. Sections of steel pipe

wood forms using cemented backfill mix with minus were threaded together with couplers as the backfill height
8
9

increased. Similarly, sections of steel rod were coupled inside and south abutments to monitor stress changes and relative

the pipe to connect the bottom anchor plate to the top anchor

and transducer housing. The protective steel pipe was greased

during construction to help reduce friction between the pipe

wall and the cured backfill. An aluminum extension rod

screwed into the top of the transducer was used to thread the

transducer rod into a tapped hole in the last section of steel rod.

The transducer rod was pulled up approximately 9 cm to

account for expected backfill compression and was secured to

the top anchor with a compression fitting. A steel cover was

bolted to the top anchor to protect the extension rod,

compression fitting, and transducer wires. The lengths and

locations of these instruments are listed in table 2.

Borehole extensometers and biaxial stressmeters were

installed in test area pillars, the mine roof, and the north

displacements (figure 5). The angled extensometers were

placed in boreholes having dip angles from 45E to 69E with the

hole collar positioned approximately 3 m from the floor. The

horizontal extensometers were located at midheight on the

pillars, at the same elevation as most of the backfill instruments.

The vertical extensometers were installed from an access drift

3 to 7 m beneath the test area (figure 7). The collar anchors for

these instruments were placed in the roof of the access drift,

and the two uphole anchors were placed approximately at the

top and bottom of the trapped pillars. The location, borehole

dip, and anchor depths for the instruments installed in rock are

also shown in table 2.

Table 2.—Instrument location

eg Location Instrument Dip, d1 Distance from collar or base, m

Anchor No. 1 Anchor No. 2

Southeast of pillar 93 .............. VBX2 90 16.46

Northwest of pillar 103 . . . . . . . . . . . . . VBX 90 11.89

North of pillar 104 . . . . . . . . . . . . . . . . . VBX 90 4.18

Pillar 93 . . . . . . . . . . . . . . . . . . . . . . . . . BX 0 4.57 8.84 3

Pillar 93 . . . . . . . . . . . . . . . . . . . . . . . . . BX 45 7.62 14.63

Pillar 94 . . . . . . . . . . . . . . . . . . . . . . . . . BX 0 7.92

Pillar 94 . . . . . . . . . . . . . . . . . . . . . . . . . BX 52 8.08 14.94

Pillar 95 . . . . . . . . . . . . . . . . . . . . . . . . . BX 0 6.71

Pillar 96 . . . . . . . . . . . . . . . . . . . . . . . . . BX 0 9.14

Pillar 96 . . . . . . . . . . . . . . . . . . . . . . . . . BX 49 8.38 16.76

Pillar 101 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 3.81 5.05

Pillar 101 . . . . . . . . . . . . . . . . . . . . . . . . BX 90 7.01 24.08

Pillar 102 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 7.62

Pillar 102 . . . . . . . . . . . . . . . . . . . . . . . . BX 90 3.66 24.69

Pillar 102 . . . . . . . . . . . . . . . . . . . . . . . . SM !5 6.49 4

Pillar 103 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 4.72 7.92

Pillar 103 . . . . . . . . . . . . . . . . . . . . . . . . BX 90 5.13 26.82

Pillar 103 . . . . . . . . . . . . . . . . . . . . . . . . SM !5 4.62

Pillar 104 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 3.73 6.10

Pillar 104 . . . . . . . . . . . . . . . . . . . . . . . . BX 90 4.88 26.21

Pillar 113A . . . . . . . . . . . . . . . . . . . . . . BX 0 4.39 7.24

Pillar 113A . . . . . . . . . . . . . . . . . . . . . . BX 52 5.79 10.97

Pillar 110 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 2.74

Pillar 111 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 3.15 4.78

Pillar 111 . . . . . . . . . . . . . . . . . . . . . . . . BX 58 6.40 12.50

Pillar 112 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 3.66

Pillar 112 . . . . . . . . . . . . . . . . . . . . . . . . BX 69 6.86 13.11

Pillar 113 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 3.96

Pillar 114 . . . . . . . . . . . . . . . . . . . . . . . . BX 0 4.11 6.71

Pillar 114 . . . . . . . . . . . . . . . . . . . . . . . . BX 56 7.77 14.33

Centered on 93, 94, 102, 103 . . . . . . . . BX 90 11.18

Centered on 95, 96, 104, 105 . . . . . . . . BX 90 10.87

Centered on 102, 103, 111, 112 . . . . . . BX 90 11.18

Centered on 104, 105, 113, 114 . . . . . . BX 90 11.18

North abutment . . . . . . . . . . . . . . . . . . . SM !5 6.12

South abutment . . . . . . . . . . . . . . . . . . SM !5 3.54

Barrier pillar, west face . . . . . . . . . . . . . SM !3 6.10

Barrier pillar, south face . . . . . . . . . . . . SM !3 6.10

horizontal (positive). Dip Angle up from the 1

VBX Vertical backfill extensometer. 2

BX Borehole extensometer. 3

SM Biaxial stressmeter. 4

NOTE.—Blank spaces indicate no data were collected.
10

Table 3.—Measured in situ principal stresses

Stress, MPa1 Azimuth, deg2 Dip, deg3

!12.48 .................. !236.1 !76.9

!6.55 ................... !226.8 12.9

!2.76 ................... !317.2 2.0

1

Minus sign indicates compressive stress.

Azimuth Angle clockwise from north (positive). 2

Dip Angle up from horizontal (positive). 3

Table 4.—Measured in situ stress components

Type of stress Direction Amount, MPa

Normal ....... North-south .............. !4.60

Normal ....... East-west ............... !5.01

Normal ....... Vertical ................. !12.18

Shear ........ North-south, east-west ..... !2.03

Shear ........ East-west, vertical ......... 1.18

Shear ........ Vertical, north-south ....... 0.63

Note.—Minus sign indicates compressive stress.

The borehole extensometers were installed in B-size

diamond boreholes with an enlarged collar that allowed the

transducer heads to be recessed from the face, protecting them

from fill rock. By inflating copper bladder anchors with

hydraulic oil to a pressure of 9.5 MPa, the instruments were

secured in the borehole. Fiberglass rods connected the

downhole anchors to transducers at the collar anchor.

Biaxial stressmeters measure radial deformation of a

borehole with three-wire sensors extended across the diameter

of a steel cylinder. These wires are oriented 0E, 60E, and 120E

from vertical. Secondary principal stress change and direction

can be calculated using the frequency change in each ofthese

wires. The stressmeter can be equipped with an extra set of

radial sensors for backup measurements and a temperature and

longitudinal sensor for more accuracy.

To install a stressmeter, a high-strength, nonshrinking grout

is first pumped into the bottom of a slightly downward-dipping

borehole. With an installation rod equipped with a leveling

device at the collar end, the stressmeter is pushed down the hole

into the grout. After the level is used to orient one of the wire

sensors vertically, a cable-activated pin is pulled to release a

snap-ring anchor. The snap ring holds the stressmeter at the

correct orientation while the setting rods are removed and more

grout is pumped into the hole. Some of the grout is poured into

cylindrical molds, cured at 100-pct humidity for 28 days, and

tested for unconfined compressive strength. The average

strength of the grout for this work was 44 MPa and the average

value for modulus of deformation was 15,400 MPa.

STRESS DETERMINATION

BEFORE PILLAR REMOVAL

Three-dimensional stress was measured in the barrier pillar

before pillars were mined in the test area. Both CSIRO hollow

inclusion cells and USBM borehole deformation gauges were

used. These gauges were set and overcored at depths between

3.3 and 6.7 m down four boreholes for a total of five

measurement sets. Principal stresses and stress components are

shown in tables 3 and 4.
F ' F ' <

&<

F

11

Table 5.—Material properties of host rock determined

from laboratory tests

Young's 

modulus, 

MPa 

Unconfined

 compres-

 sive

 strength,

 MPa

Tensile

strength,

 MPa

Specific

 weight,

 kg/m3

Poisson's

 ratio 

Waste rock . . 87,940 140 11.3 2,611 0.26

Ore . . . . . . . . 80,550 109 9.0 4,709 0.24

Table 6.—Material properties used in program BESOL

Modulus of elasticity, rock mass, MP 1 a ............. 17,240

Modulus of elasticity, pillars, MPa .................. 84,830

Modulus of elasticity, backfill, MPa . . . . . . . . . . . . . . . . . 790

Poisson's ratio, rock mass ....................... 0.25

Poisson's ratio, pillars ........................... 0.26

terial in the mining horizon. All material excluding the ma 1

MATERIAL PROPERTY DETERMINATION

BX-size (4.13-cm) core samples from the barrier pillar, test

area pillars, and north abutment were used for material property

tests. The length of each specimen prepared for unconfined

compression tests had a 2:1 length-to-width ratio, and the ends

were ground to meet American Society for Testing Materials

(ASTM) standards for parallelism. Strain gauges were glued

laterally and longitudinally on each specimen to determine

Poisson's ratio. Specimens having a thickness of approximately

2.54 cm were also prepared from the rock core for Brazilian

tensile tests. Average values from 24 unconfined compression

tests and 23 Brazilian tensile tests are listed in table 5. The data

have been separated into two categories based on a specific

gravity value of 2.76. Specimens having a specific gravity less

than or equal to this value are categorized as waste rock, and

specimens having a specific gravity greater than this value are

categorized as ore. All but one specimen categorized as ore

were obtained from test area pillars that were eventually mined.

NUMERIC MODELS

The three-dimensional, boundary-element program BESOL

(Crouch Research, Inc., 1986) was used to model backfilling

and pillar extraction in area 5. The modeled area in plan view

represented a 275- by 275-m section of the mine with area 5

near the center. The modeled area was divided into square

elements that represented 1.5 m of rock on each side. The pillar

height was assumed to be 18.3 m with an overburden thickness

of 366 m.

Initial vertical stresses in the model were calculated using

2,307 kg/m as the density of the overburden. Initial shear 3

stresses were assumed to be zero. Initial horizontal stresses

were calculated using equation 1, because in situ information

on these stresses was not available. The effect of a large initial

horizontal stress field on pillar stresses and factors of safety is

discussed in Tesarik and others (1989).

(1)

where F = normal stress component in the x (hori- xx

zontal) direction, MPa,

F = normal stress component in the y (hori- yy

zontal) direction, MPa,

< = Poisson's ratio,

and F = normal stress component in the z (vertical) zz

direction, MPa.

The predicted vertical stress in the barrier pillar before

support pillars were extracted ranged from !11.6 to

!23.4 MPa. The predicted vertical stress at the location of the

in situ stress measurements was !12.6 MPa. This value

contrasted to the measured vertical stress value of !12.2 MPa,

indicating that gravity loading with an overburden density of

2,307 kg/m was a reasonable assumption. 3

All materials were assumed to be isotropic and linearly

elastic and are listed in table 6.

The relatively low modulus value for the rock mass represents the dolomitic mudstone and Davis Shale layers (Farmer,

1968) that are deposited above the competent dolomite seam.

The modulus of elasticity for the pillars was obtained from

unconfined compressive tests and the modulus of elasticity for

the backfill was based on in situ measurements in cemented

backfill of similar composition at the Cannon Mine (Tesarik

and others, 1983).

To determine the modulus for the pillar rock mass, strains

calculated from measured displacements in the trapped pillars

were plotted against model-predicted strains (figure 8). Data

collected after the pillar 5 blast were not used in this plot

because predicted strain changes in pillars 102 and 103 were

1.3 and 6.6 times larger than measured strain changes,

indicating that possibly these pillars were no longer behaving

elastically. A line was fit to the data using regression analysis.
),

), '

, ' ,

,

, '

' '

( ' ( %

' % '

12

The slope of the line was 0.45, and the correlation coefficient

for the data was 0.9. An adjusted rock mass modulus was

calculated using equations 2 through 5.

(2)

For predicted and measured stresses to be equal,

(3)

and

(4)

Thus, the adjusted modulus is

(5)

Regression analysis for measured stress changes in the

backfill and stress changes predicted by BESOL resulted in a

correlation coefficient equal to 0.52. The maximum predicted

stress in the backfill at the locations of the earth pressure cells

was 1.6 MPa. This backfill stress was predicted between pillars

94, 95, 103, and 104. With an adjusted pillar modulus

of 38,170 MPa, the maximum predicted backfill stress did not

increase.

A north-south cross section of area 5 was modeled using the

two-dimensional, finite-element program UTAH2. For this 5

model, it was assumed that there were three rows of pillars with

abutments north and south of the backfilled area. The

modeling sequence consisted of mining the crosscuts,

backfilling these crosscuts, mining pillars 92 through 96, and

mining pillars 110 through 113. To account for threedimensional mine geometry, overburden weight was increased

to 4,819 kg/m using equation 6 (Pariseau, 1979). 3

 (6)

where (2D = adjusted specific weight of overburden used

in the two-dimensional analysis, kg/m ,3

( = specific weight of overburden used to develop the initial in situ stress state, kg/m ,3

W = crosscut width, m, c

and L = pillar side length, m. p

The ANSYS (DeSalvo and Gorman, 1989) preprocessor

was used to define mine geometry and automatically mesh the

cross section into 8,260 elements. Elements representing the

pillars and the backfill had a length and width of approximately

1.5 m, and elements at the rock-backfill interface had a width

of 0.3 m. Assigning weak material properties to these elements

allowed them to deform plastically in a vertical direction and

allowed the modeled backfill to develop self-loading stresses.

The mesh represented overburden up to and including the

mine's surface, and the side and lower boundaries represented

a distance approximately two times the width of the backfilled

area.

An elastic, perfectly plastic model was used for all materials. The yield criterion used was Drucker-Prager, where

strength depends on all three principal stresses, and associated

flow rules were applied to determine strains in the yielded

elements.

OFR 47(2)-80. Interpretation of Rock Mechanics Data: A Guide to 5

Using UTAH2 by W. G. Pariseau.
13

The same stratigraphic layers as shown in figure 2 were used INSTRUMENT RESPONSE TO MINING

in the model, but it was assumed that dolomite, crystalline

dolomite, and dolomite with shale had the same material In general, the earth pressure cells recorded compressive

properties. The layer having shale interbedded with thin beds stress increases after each pillar blast, followed by stress relief

of dolomite was given the same material property values as the that lasted until the next blast (figures 9-10). This decrease in

Davis Shale stratum. The values for modulus of deformation, stress was possibly caused by lateral movement of the backfill

unconfined compressive strength, and tensile strength for the at the fill fence. As shown in table 8, readings from the earth

dolomite were determined in the laboratory tests described pressure cells indicated that the maximum and average stresses

above. Properties for other rock types and overburden material after installation were only !0.98 and !0.41 MPa, respectively.

were based on published values (Farmer, 1968; Sowers, 1979), The calculated weights of the backfill on the cells positioned

and cemented backfill properties were obtained from laboratory 9.1 and 16.8 m from the floor are approximately !0.19 MPa

tests. Laboratory and estimated material property values are and !0.032 MPa, respectively. When these values are subgiven in table 7. tracted from the total stress values, then the maximum

UTAH2 was calibrated by reducing the modulus of and average stresses caused by mining are !0.79 and !0.28

deformation values of all rock types to 45 pct of their MPa. Several instruments recorded total stress changes smaller

laboratory values based on results from BESOL. The strengths than the stress calculated using only the weight of the backfill.

of these materials were also reduced until plastic zones The smaller measurement could be caused by resistance of the

developed in pillar 103. The adjusted, unconfined compressive fill to shear along vertical planes at the rock-backfill interface

strength for the dolomite using this method was 38.2 MPa. (Bloss and others, 1993).

Table 7.—Laboratory and estimated material properties for UTAH2 analysis

Material

Elastic

modulus, compressive strength,

MPa strength, MPa MPa

Uniaxial Tensile Poisson's

ratio kg/m 

Density, 3

Dolomite ............. 84,830 127.4 9.8 0.26 3,492

Dolomitic mudstone .... 34,483 53.8 5.4 0.25 3,492

Davis Shale ........... 17,241 53.8 5.4 0.25 2,195

Sandstone ............ 41,690 93.1 6.4 0.25 2,307

Overburden . . . . . . . . . . . 19 0.7 0.03 0.30 2,082

Cemented backfill ...... 3,793 8.3 2.1 0.30 2,114

Table 8.—Maximum compressive stresses recorded by earth pressure cells

after installation, megapascals

Cell location Distance from

floor, m

Stress change

 Total Minus backfill weight

92, 93, 101, 102 ............ 1 9.1 2

!0.08 0.00

93, 94, 102, 103 . . . . . . . . . . . . . 9.1 !0.30 !0.11

94, 95, 103, 104 . . . . . . . . . . . . . 9.1 !0.29 !0.10

95, 96, 104, 105 . . . . . . . . . . . . . 9.1 !0.66 !0.47

95, 96, 104, 105 ............. 16.8 !0.36 !0.33

101, 113A, 110 . . . . . . . . . . . . . . 9.1 !0.28 !0.09

101, 102, 110, 111 . . . . . . . . . . . 9.1 !0.98 !0.79

102, 103, 111, 112 . . . . . . . . . . . 9.1 !0.71 !0.52

103, 104, 112, 113 ........... 16.8 !0.28 !0.25

103, 104, 112, 113 . . . . . . . . . . . 9.1 !0.13 !0.00

104, 105, 113, 114 ........... 16.8 !0.42 !0.39

104, 105, 113, 114 . . . . . . . . . . . 9.1 !0.44 !0.25

 Average .................................. !0.41 !0.28

1

Gauge centered on pillar s 92, 93, 101, and 102.

Minus sign indicates compressive stress. 2
14

Table 9.—Maximum microstrain recorded by embedment strain

gauges after installation

Cell location Distance

from floor, m

Microstrain

change 

......... 92, 93, 101, 1021 9.1 87 !2 2

93, 94, 102, 103 . . . . . . . . . . 9.1 !201

93, 94, 102, 103 .......... 16.8 !168

94, 95, 103, 104 . . . . . . . . . . 9.1 !308

95, 96, 104, 105 . . . . . . . . . . 9.1 0

101, 113A, 110 . . . . . . . . . . . 9.1 !165

101, 102, 110, 111 . . . . . . . . 9.1 !345

102, 103, 111, 112 . . . . . . . . 9.1 !1,087

103, 104, 112, 113 . . . . . . . . 9.1 !4,119

1

Gauge centered on pillar s 92, 93, 101, and 102.

Minus sign indicates compressive strain. 2

The maximum measured backfill stress is less than the

average unconfined compressive strength of Buick Mine

backfill specimens composed of 17.7 pct cycloned tailings, 71-

pct minus 7.6-cm crushed rock, and 2.7 pct cement. This

backfill mix had an unconfined compressive strength of !1.39

MPa, indicating that the backfill remained in the elastic state.

A similar loading and unloading pattern was recorded by

most of the embedment strain gauges. This pattern is illustrated

in figure 11 using data from the embedment strain gauges

installed midheight on pillars 102, 103, 111, and 112.

Maximum compressive strains recorded after these instruments

were installed are listed in table 9.
15

After 250 days, strains measured by vertical fill exten- (table 10). Total stress at each measurement site was obtained

someters exceeded strains recorded by embedment strain by adding compressive stress increases recorded by an earth

gauges in the same area (figure 12). This difference could have pressure cell after every pillar blast. This value was divided by

occurred because the top bearing plates for all the fill the total compressive strain recorded by the closest embedment

extensometers were positioned approximately 4.5 m below the strain gauge or backfill extensometer. Based on measurements

mine roof, and backfill strains are likely to be larger at the top, in backfill with aggregate and 6-pct cement, (Brechtel and

rather than at midheight, of the fill. (Two of the three others, 1989b) the values exceeding 6,900 MPa were not

embedment strain gauges were installed midheight in the representative of in situ conditions. The average elastic

backfill.) Maximum strain values for all the instruments in the modulus without these two values is 1,913 MPa.

north section of area 5 were of the same order of magnitude, The vertical extensometers in the trapped pillars recorded

ranging from approximately !0.00015 to !0.000375. compressive strain after most blasts. This response was usually

Estimated values of elastic moduli for the cemented backfill followed by time-dependent strain until the next pillar was

were calculated from pressure cells and embedment strain removed. Figure 13 plots strain between downhole anchors

gauges or fill extensometers installed in the backfill caused by pillar removal, starting with pillar 96 on day 17.
16

Table 10.—Elastic modulus values calculated from backfill

instrument readings, megapascals

Cell location Strain instrument Elastic modulus

..... 92, 93, 101, 1021 ESG 2,690

93, 94, 102, 103 . . . . . . ESG 7,000

93, 94, 102, 103 ...... 16.5-m VBX 1,350

93, 94, 102, 103 ...... 11.9-m VBX 710

94, 95, 103, 104 . . . . . . ESG 4,160

94, 95, 103, 104 ...... 4.2-m VBX 2,550

95, 96, 104, 105 . . . . . . ESG 9,240

102, 103, 111, 112 . . . . ESG 2,320

103, 104, 112, 113 . . . . ESG 1,160

104, 105, 113, 114 . . . . ESG 360

ESG Embedment strain gauge.

VBX Vertical backfill extensometer.

Gauge centered on pillars 92, 93, 101, and 102.

1

Predicted pillar strains from BESOL using an elastic modulus

of 38,170 MPa for the pillars are also shown.
17

The plots for pillars 103 and 104 indicate that these pillars 100, well before estimated failure on day 192, which had been

became inelastic after pillar 112 was blasted on day 192. The based on vertical extensometer readings and numeric model

measured strain in pillar 103 on this day was about nine times results. However, the strain rate in these pillars increased

greater than the predicted strain. After day 192, the measured significantly after day 192, helping to confirm that the pillars

strain rate increased with time, but there was little response to became plastic at that time.

subsequent blasts, a result that was contrary to predicted strain On the basis of horizontal strains calculated from downhole

behavior. A further indication that pillar 103 became plastic is anchors and predicted strains calculated by BESOL, there were

that the drill steel stuck when the blast holes were drilled. no indications of perimeter pillar failures. Strains from BESOL

Similar strain behavior was recorded by the extensometer in were added to measured strains to account for excavation of the

pillar 104, but the measured and predicted strains caused by the entries and crosscuts, which took place before the

removal of pillar 112 were nearly equal. extensometers were installed. The maximum measured internal

In general, the horizontal extensometers in the trapped horizontal strain was 0.00014 in pillar 111. If an additional

pillars recorded elastic tensional strains after most blasts, horizontal strain value of 4.6 × 10 estimated from BESOL

followed by time-dependent tensional strains (figure 14). results is added to the measured value, the total horizontal strain

Unlike the vertical extensometers in the pillars, data from these is still less than the estimated failure strain of 0.00026.

instruments did not clearly indicate when a pillar failed. Similarly, the largest measured internal vertical strain added to

Failure strain calculated from a tensile strength value of 11.3 the predicted strain caused by removal of entries and crosscuts

MPa and elastic modulus value of 38,170 MPa is 0.0003. was !0.000083. The average compressive failure strain of

Strains in pillars 103 and 104 exceeded this value before day laboratory specimens was !0.000145.

!7
18

Table 11.—Measured and predicted vertical stress changes

in pillars 102 and 103, megapascals

Location Day Measured Predicted

Pillar 102 . . . . . . . . . . . . . . . . . 392 !3.4 !10.3

Pillar 103 . . . . . . . . . . . . . . . . . 652 !33.4 !16.2

The four vertical borehole extensometers located in the mine initially programmed into the datalogger. After this range was

roof recorded both compressive and tensile strains when the increased, the readings became more consistent. However,

pillars were mined (figure 15). Strain changes became tensile vertical stress changes that were transformed from measured

as more pillars were extracted. None of the extensometers principal stresses in the north and south abutments were

recorded a strain change larger than rock failure strains tensional where compressive stresses would be expected.

measured in the laboratory, but without knowledge of prior Measured and predicted changes in compressive vertical stress

roof movement, an accurate prediction of whether or not the in pillars 102 and 103 are shown in table 11.

roof remained elastic cannot be made. However, on the basis

of the small strain changes, it was apparent that the roof

remained stable during pillar removal.

Secondary principal stress changes calculated from the

biaxial stressmeters were erratic and inconsistent with stress

changes calculated by BESOL. Erratic readings could have

been caused by the narrow excitation frequency range
19

CONCLUSIONS

Readings from instruments installed in cemented backfill at fill was less than 1.3 cm in most areas. The top 3 m

the Buick Mine indicated that the backfill remained in the of backfill had a maximum aggregate diameter of 5 cm and was

elastic range. The average maximum compressive stress caused placed by front-end loaders and slinger trucks. The backfill

by mining and recorded by earth pressure cells was !0.26 MPa. mix and mining method used in this test project could be

This value is much lower than the average unconfined applied to sites having similar pillar geometry, in situ stresses,

compressive strength of backfill specimens derived from and rock mass properties, such as other mines in the New Lead

laboratory experiments. Belt.

From a design perspective, rock fill containing 4-pct cement Pillar strains were also monitored and compared with failure

is adequate to maintain roof and pillar stability for mining strains of laboratory rock specimens. There was no evidence

remnant support pillars in area 5 at the Buick Mine. For this that the perimeter pillars failed, but measured horizontal strain

rock fill to provide support, the gap left between the roof and changes in trapped pillars 103 and 104 exceeded laboratory 
20

tensile failure strains, and the extensometers in these pillars indicate that pillar 103 failed. A two-dimensional, finitestopped recording strain changes after one of the support pillars element computer program having an elastic, perfectly plastic

was blasted. These data indicate that these two pillars failed. material model was used along with extensometer readings

Vertical strain changes in the trapped pillars plotted against from the trapped pillars to generate an estimate of unconfined

predicted strain changes from a numerical model resulted in a compressive strength of the rock as 38.2 Mpa.

calibrated elastic rock modulus of 38,170 MPa. These data also

ACKNOWLEDGMENTS

The authors wish to express their appreciation to the Kennecott Corp., Bingham Canyon, UT, for initial design

management of The Doe Run Co., Buick Mine, St. Louis, MO, work on the vertical backfill extensometer; D. P. Roberts,

and especially to W. L. Lane, manager—mine technical senior mining engineer, Golder Associates, Burnaby, BC, for

services; R. W. Roscoe, senior planning engineer; J. S. Hawk, instrument data interpretations; L. A. Atkins and F. M. Jones,

mine engineer; and G. F. Sutton, mine engineer, for providing engineering technicians, USBM, Spokane Research Center

access to the test section of the mine, supplying and (SRC), Spokane, WA, for material property testing and

maintaining instruments and data acquisition systems, and assistance with in situ stress determination; C. C. Clark,

providing mine maps and schedules. The authors also wish to mechanical engineer, USBM, SRC, for assistance with

thank W. G. Pariseau, professor, Department of Mining horizontal borehole extensometer installation and in situ stress

Engineering, University of Utah, Salt Lake City, UT, for determination; and R. R. Backer, group supervisor, USBM,

assistance in data analysis; J. D. Vickery, project mine manager, SRC, for assistance with in situ stress determination.

REFERENCES

Bloss, M. L., R. Cowling, and J. L. Meek. A Procedure for the Design of Pariseau, W. G. 3D Mine Pillar Design Information from 2D FEM

Stable Cemented Fill Exposures. Paper in Minefill 93, ed. by H. W. Glen

(Johannesburg, S. Afr., Sept. 1993). Symp. Ser. S13, S. Afr. Inst. Min. Metall.,

Johannesburg, S. Afr., 1993, pp. 3-8.

Brechtel, C. E., J. F. T. Agapito, and M. E. Mudlin. Stability Evaluation

During Bench Cut-and-Fill Mining of the B-Neath Zone at the Cannon Mine.

Proceedings of the SME Annual Meeting and Exhibit (Las Vegas, NV, Feb.

1989). SME 1989a, pp. 1-16.

Brechtel, C. E., M. P. Hardy, J. Baz-Dresch, and J. S. Knowlson.

Application of High-Strength Backfill at the Cannon Mine. Paper in

Innovations in Mining Backfill Technology: Proceedings of the 4th International Symposium on Mining with Backfill, ed. by F. P. Hassani, M. J.

Scobie, and T. R. Yu (Montreal, PQ, Oct. 2-5, 1989). Balkema, 1989b, pp.

105-117.

Crouch Research, Inc. (St. Paul, MN). Computer Software for

Rock Mechanics, The BESOL System, User's Guide, Part II, ThreeDimensional Programs. Version 1.15. 1986, 68 pp.

DeSalvo, G. J., and R. W. Gorman. ANSYS Engineering Analysis System

Users' Manual. Swanson Analysis Systems, Inc., Houston, PA, v. I, Sec. 3.3,

1989, 118 pp.

Farmer, I. W. Engineering Properties of Rocks. Butler and Tanner,

London, 1968, p. 37.

Koskela, V. A. Consolidated Backfilling at Outokumpu Oy's

Vihanti, Keretti and Vammala Mines. Paper in Mining with Backfill:

Proceedings of the International Symposium on Mining with Backfill, ed. by

S. Granholm (Lulea, Sweden, June 7-9, 1983). Balkema, 1983, pp. 151-160.

Analysis. Int. J. Numeri. Anal. Methods in Geomech., v. 3, 1979, pp. 145-

157.

Sowers, G. F. Introductory Soil Mechanics: Geotechnical Engineering.

Macmillan, 4th ed., 1979, pp. 180-231.

Stilwell, A. W. The Extraction of a Steeply Dipping, Wide Gold Reef

Using Cemented Backfill. Paper in Minefill 93, ed. by H. W. Glen

(Johannesburg, S. Afr., Sept. 1993). Symp. Ser. S13, S. Afr. Inst. Min. Metall.,

Johannesburg, S. Afr., 1993, pp. 147-155.

Tesarik, D. R., and R. W. McKibbin. Instrumentation and Modeling of

the North 140 Section of Magmont Mine, Bixby, MO. USBM RI 9215, 1989,

30 pp.

Tesarik, D. R., J. B. Seymour, and M. E. Mudlin. Analysis of Backfill and

Pillar Performance in the B-Neath Ore Body at the Cannon Mine. Paper in

Minefill 93, ed. by H. W. Glen (Johannesburg, S. Afr., Sept. 1993). Symp.

Ser. S13, S. Afr. Inst. Min. Metall., Johannesburg, S. Afr., 1993, pp. 55-64.

Tesarik, D. R., J. B. Seymour, and J. D. Vickery. Instrumentation and

Modeling of the Cannon Mine's B-North Ore Body. Paper in Innovations in

Mining Backfill Technology: Proceedings of the 4th International

Symposium on Mining with Backfill, ed. by F. P. Hassani, M. J. Scobie, and

T. R. Yu (Montreal, PQ, Oct. 2-5, 1989). Balkema, 1989, pp. 119-128.

Udd, J. E. Backfill Research in Canadian Mines. Paper in Innovations in

Mining Backfill Technology: Proceedings of the 4th International

Symposium on Mining with Backfill, ed. by F. P. Hassani, M. J. Scobie, and

T. R. Yu (Montreal, PQ, Oct. 2-5, 1989). Balkema, 1989, pp. 3-13.

INT.BU.OF MINES,PGH.,PA 30140