UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9560 REPORT OF INVESTIGATIONS/1995

In Situ Stress at the Lucky Friday Mine 

(In Four Parts)

2. Analysis of Overcore Measurement

 From 5300 Level
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 9560

In Situ Stress at the Lucky Friday Mine

(In Four Parts)

2. Analysis of Overcore Measurement

 From 5300 Level

By J. K. Whyatt, F. M. Jenkins, and M. K. Larson 

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

Field measurement ............................................................................... 2

Overcore method .............................................................................. 7

Site geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Mining experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Stress field estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Evaluation of strain data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Stress field solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Stress field characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Doorstopper-scale assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Site-scale assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Local stress variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Appendix A.—Mining-induced stress at overcore site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Appendix B.—Drilling notes on geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Appendix C.—Rock properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Appendix D.—Rock-burst elastic energy release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Appendix E.—Doorstopper cell local solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

ILLUSTRATIONS

1. Location of Lucky Friday Mine .................................................................. 3

2. Plan view of 5300-level site and vicinity ............................................................ 4

3. Vertical section of 5300-level site showing excavation state during measurement ............................... 5

4. Detailed plan view of 5300-level site .............................................................. 6

5. Overcore measurement sites ..................................................................... 8

6. Schematic of CSIR doorstopper cell in borehole ...................................................... 9

7. Tested sections, geologic formation, and stress solution at 5300-level site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8. Damage caused by rock burst on January 17, 1990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9. Crosscut after removal of debris from January 17, 1990, rock burst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

10. Local stress field on end of borehole for each doorstopper cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

11. Local stress field on end of borehole summarized from results plotted in figure 10 .............................. 24

TABLES

1. Overcore borehole orientations ................................................................... 2

2. Results of overcore measurements with dorestopper cells ................................................ 7

3. Estimated rock properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Ranking criteria for quality designation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5. Measurement quality as classified by range screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6. Measurement quality as classified by strain screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7. Comparison of screen results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

Metric Units

cm centimeter MJ/m megajoule per cubic meter 3

deg degree mm millimeter

g/cm gram per cubic centimeter MPa megapascal 3

GPa gigapascal pct percent

km kilometer t metric ton

m meter t/m metric ton per cubic meter 3

m cubic meter μ, microstrain 3

MJ megajoule EC degree Celsius

U.S. Customary Units

ft foot psi pound per square inch

ft cubic foot st short ton 3

in inch EF degree Fahrenheit

Tables—Continued

Page

8. Stress field estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

9. Best estimate of stress field in map coordinate system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

10. Location and number of good outlying overcore strain measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

11. Stress estimates exploring good outlying strain measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

12. Stress estimates developed from strain-screened data from various combinations of three of four boreholes . . . . . . . . . . . . 23

13. Stress estimates from high stress and average stress data sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

14. Best estimate of average stress conditions at measurement site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

A-1. Mining-induced stress at measurement site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

C-1. Rock type and results of uniaxial compression tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

C-2. Measured rock properties of sulfide-altered quartzite, 4250 level, Lucky Friday Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

C-3. Properties of Revett Formation rocks from various locations in Coeur d'Alene Mining District . . . . . . . . . . . . . . . . . . . . . 31

E-1. Principal stress solutions for stress on end of borehole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

E-2. Range of individual doorstopper cell solutions for stress on end of borehole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Reference to specific products does not imply endorsement by the U.S. Bureau of Mines.
In Situ Stress at the Lucky Friday Mine 

(In Four Parts)

2. Analysis of Overcore Measurement From 5300 Level

By J. K. Whyatt, F. M. Jenkins, and M. K. Larson 1 1 1

ABSTRACT

The U.S. Bureau of Mines conducted an overcore stress measurement on the 5300 level of the Lucky Friday

Mine, Mullan, ID, to investigate the stress regime around an experimental stope in a rock-burst-prone mine. The

result indicated a stress field with unexpected magnitude and orientation [F1 . 135 MPa (19,600 psi) oriented S 80E

W]. An unusual amount of rock-burst activity was encountered during subsequent excavation. A careful review

of the overcore data suggested the overcore strain measurements were valid. Moreover, concentrated rock-burst

activity suggested that the measured stress field did exist, but was localized and therefore did not represent the farfield in situ stress field.

Mining engineer, Spokane Research Center, U.S. Bureau of Mines, Spokane, WA. 1
2

This design was described in a pape r, "Underhand Stoping at the Lucky 2

Friday Mine," by R. R. Noyes, G. R. Johnson, and S. D. Lautenschlager,

presented at the 94th annual meeting of the Northwest Mining Association, Dec.

2, 1988, in Spokane, WA.

INTRODUCTION

The in situ stress measurement described in this Report of (Whyatt and Beus, 1995). The resulting reanalysis provided a

Investigations (RI) was undertaken as part of a field trial of an modified estimate of in situ stress magnitude, but in general,

underhand longwall mining method for mechanized mining of confirmed stress orientation. It also suggested that the measurerock-burst-prone ground. The U.S. Bureau of Mines (USBM) ment site was not as homogeneous as previously assumed.

conducted this study as part of its rock-burst safety research pro- While the stress field in the Coeur d'Alene Mining District was

gram. The trial took place in a stope, known as the Lucky Friday being reexamined, the LFUL stope was developed as planned.

underhand longwall (LFUL) stope, at the Lucky Friday Mine in An investigation of the ground control aspects of LFUL stope

the Coeur d'Alene Mining District of northern Idaho (figure 1). performance has been completed (Pariseau and others, 1992;

This mine has a long history of rock-burst activity (McMahon, Scott, 1993; Whyatt and others, 1992a; Whyatt and others,

1988), and the underhand longwall method was specifically 1992b; Williams and others, 1992), based on the existing

designed to control rock bursting in mine pillars. knowledge of in situ stress in the district. During the interim, 2

This measurement was undertaken between October 1986 and excavation continued at the 5300-level measurement site, setting

January 1987 to improve the existing estimate of in situ stress in off heavy rock-burst activity. The bursts severely damaged the

the Coeur d'Alene Mining District (Whyatt, 1986), which in turn opening and forced its abandonment, lending credence to the

would be used in an analysis of the LFUL stope. However, an accuracy of the 5300-level stress measurement.

examination of the measurement data indicated a stress field This report, the second in a series covering measurement and

significantly different from previous measurements, especially a interpretation of in situ stress at the Lucky Friday Mine, is limited

similar one undertaken by Allen (1979) on the 4250 level of the to description and analysis of the stress measurement at the 5300

same mine, and estimates of overburden stress. The unexpected level. The first report (Whyatt and Beus, 1995) provides an

results of the 5300-level measurement cast doubt on its accuracy. updated reanalysis of an overcore measurement obtained on the

Further examination of these discrepancies led to the recognition 4250 level of the Lucky Friday Mine. The third report (Whyatt

that there were significant shortcomings in the analytical and others, 1995a) provides a reanalysis of an overcore meastechniques applied to earlier stress measurements in the district and urement from the nearby Star Mine. The final report [part 4

that furthermore, significant errors existed in some stress estimates. (Whyatt and others, 1996b)] seeks to integrate these overcore

This erosion of confidence in existing in situ stress information measurements of in situ stress with other indications of stress field

led to a reanalysis of the measurement from the orientation into an overview of the mine-wide in situ stress field.

4250 level, which is reported in part 1 of this series

FIELD MEASUREMENT

Site-selection criteria for this stress measurement were very pct of in situ levels (appendix A). The crosscut was sufficiently

similar to Allen's (1979) when he chose his site on the 4250 level developed to allow future extension without interfering with

(reviewed in the first report of this series), except that a location haulage operations but was far short of its planned length.

in the vicinity of the experimental LFUL stope was desired. The The crosscut stub was mapped for geologic structure, and four

common criteria included finding competent rock to provide boreholes were laid out (figure 4).

adequate diamond drill core recovery in a mine notorious for

poor core recovery and avoiding mining-induced stress to

provide a clear picture of the natural in situ stress state.

The end of the 101 crosscut stub on the 5300 level was chosen

as the measurement site. This site was about 45 m

(150 ft) from the vein and 60 m (200 ft) below active mining

(figures 2-3). Mining-induced stress was estimated at less than 5

Table 1.—Overcore borehole orientations

Borehole Azimuth, deg Dip, deg

1 . . . . . . . . . . 126.4 !3.5

2 . . . . . . . . . . 216.5 !71.0

3 . . . . . . . . . . 173.0 !4.5

4 .......... 80.5 !2.7
3
4

Figure 2.—Plan view of 5300-level site and vicinity.
5
6

Figure 4.—Detailed plan view of 5300-level site.
7

The horizontal boreholes were drilled with a slight inclination potential location was identified, the end of the borehole was

to aid in keeping the boreholes clean and drained. Each borehole polished to accept the doorstopper cell and inspected with a

was driven through the crosscut's stress concentration zone borehole television camera. The location was eliminated if frac-

(approximately one crosscut diameter) before investigators tures or other features were observed that would distort stress

looked for potential doorstopper installation sites. Borehole measurements. This procedure minimized the number of cell

orientations are listed in table 1 and doorstopper cell locations are installations that were invalidated by the presence of undetected

shown in figure 5. fractures.

OVERCORE METHOD Society for Rock Mechanics (ISRM) standard test procedures for

The Council for Scientific and Industrial Research (CSIR) did not deviate significantly from ISRM procedures for similar

biaxial strain cell, commonly known as a doorstopper, was overcoring instruments or from the manufacturer's recommended

chosen to measure overcoring strains. The doorstopper cell procedure. Extensions of these procedures were devised in an

consists of a four-element strain gauge rosette (figure 6) mounted effort to improve the accuracy of overcore strain measurements.

in a waterproof package that is glued to the polished end of a A battery-operated data acquisition system (DAS) was deborehole. The cell measures deformations that accompany veloped to monitor strain through an instrument cable carefully

unloading of the rock as the cell is overcored. This type of cell strung through the drill rods, drill head, and water swivel to the

was chosen because it is particularly well suited for sites where DAS. Care had to be taken to maintain a slight amount of

core recovery is a problem, since only about 7.5 cm (3 in) of tension on the cable during drilling to avoid cable damage.

overcore is needed for a successful measurement (Jenkins and Further details on the DAS, doorstopper cell wiring, and cable

McKibbin, 1986). This method also uses a smaller diameter routing were reported by Jenkins and McKibbin (1986). This

borehole than alternative methods. arrangement was used to monitor the complete history of each

Installation of doorstopper cells was limited to sections of the doorstopper strain gauge during overcoring. The strain changes

borehole that produced sufficiently long pieces of core to ensure resulting from overcoring are listed in table 2.

that successful measurements could be made. Once a

The doorstopper cell was not included in the International

overcore stress measurements (ISRM, 1987), but the procedure

Table 2.—Results of overcore measurements with doorstopper cells

Doorstopper cell Strain gauge orientation, μ, Borehole

depth, m

Notes

+45E !45E Vertical Horizontal

Borehole 1:

 3 . . . . . . . . . . . . . . . . 420 927 !369 2,021 7.4 Vitreous quartzite.

 4 . . . . . . . . . . . . . . . . 161 1,856 189 2,127 7.6

 5 . . . . . . . . . . . . . . . . 146 1,390 !155 2,077 7.7

 6 . . . . . . . . . . . . . . . . 324 697 508 522 7.9

 21 . . . . . . . . . . . . . . . 495 555 503 912 12.2

 22 . . . . . . . . . . . . . . . 2,720 958 528 3,798 12.2

 23 . . . . . . . . . . . . . . . 666 489 214 885 12.5 Vitreous quartzite.

Borehole 2:

 24 . . . . . . . . . . . . . . . 85 1,135 748 587 5.6

 25 . . . . . . . . . . . . . . . 483 1,261 1,473 231 5.8

 26 . . . . . . . . . . . . . . . 592 2,014 2,097 663 5.9

 27 . . . . . . . . . . . . . . . 519 2,441 1,151 1,833 6.0

 28 . . . . . . . . . . . . . . . 465 1,383 775 1,092 6.2 Super-vitreous quartzite.

Borehole 3:

 7 ................ !126 1,764 862 1,007 6.4 Hard sericitic quartzite.

 8 . . . . . . . . . . . . . . . . 1,216 3,871 2,597 2,339 6.5

 9 . . . . . . . . . . . . . . . . 288 1,928 713 1,136 6.5

 10 . . . . . . . . . . . . . . . 618 1,154 940 1,094 6.6 Hard sericitic quartzite.

 11 . . . . . . . . . . . . . . . 563 148 928 9 6.7

Borehole 4:

 12 . . . . . . . . . . . . . . . 346 1,196 227 1,156 9.8

 13 . . . . . . . . . . . . . . . 459 1,631 841 1,709 9.9

 NOTE.—Several doorstopper cells failed and are not included in these results. These were doorstoppers 1 and 2 (glue problems),

15 and 17 (electrical problems), and 16 and 18 (face-polishing problems).
8

Figure 5.—Overcore measurement sites. Doorstopper cells were not installed in first 5 m of each borehole to minimize influence of excavationinduced stress.
9

Figure 6.—Schematic of CSIR doorstopper cell in borehole. (H = horizontal; V = vertical.)
10

The considerable contrast in temperature between mine rock [44 and strength. These samples indicated that the overcored portions of

EC (112 EF)] and mine service water [10 EC (50 EF)] at the site boreholes 1, 2, and 3 were drilled in vitreous quartzite, super-vitreous

raised concerns that the drill water would influence overcore strain quartzite, and hard sericitic quartzite, respectively (figure 7). Samples from

measurements. The effect of temperature on strain gauges is widely borehole 4 were not available. Elastic properties for these rocks (estimated

recognized and has been dealt with in a number of ways. For in appendix C) are summarized in table 3.

example, Allen (1979) used a compensating doorstopper cell

mounted on a similar piece of rock in his mounting device.

However, the cool drill water can also induce thermal strains

around the borehole that could distort overcore measurements.

For this measurement, a system was designed to minimize

temperature effects by maintaining drill water at rock temperature.

A thermistor mounted in the cable bundle just behind the

doorstopper cell was used to provide the control signal to a simple

heater system that warmed the drill water to rock temperature.

Cooling of drill water was accomplished by adding cool mine

water. This system was able to maintain the water within about 1

EC (2 EF) of rock temperature (Jenkins and McKibbin, 1986).

SITE GEOLOGY

The crosscut stub was developed into the edge of a hard

quartzite subunit of the Lower Revett Formation (figure 2) to

reveal massive, sulfide-altered vitreous quartzite, called "blue rock"

by miners. The structure of the subunit, as projected from drilling

and nearby openings (see figures 4 and 5 and appendix B), is

summarized in figure 7. Rocks in this subunit ranged from hard

sericitic quartzite to super-vitreous quartzite. Argillite and softer

sericitic quartzites common in other parts of the Revett Formation

are generally absent from this subunit.

A number of workers have reported on the elastic properties of

Revett quartzites, including Skinner and others (1974), Chan

(1972), and Ageton (see report by Beus and Chan, 1980). The

most relevant tests were conducted by Allen (1979) on rock

samples taken from a vitreous quartzite bed of the Revett at his

4250-level stress measurement site. The original test plan

depended on estimates of material properties derived from these

earlier studies and assumed fairly uniform geology throughout the

measurement site.

The subsequent reanalysis of Allen's (1979) 4250-level

measurement showed that variations in properties were systematic

within a geologic structure rather than the result of random error.

Thus, a program of geologic classification and rock testing was

initiated for the 5300 site well after completion of the overcore

measurements. Unfortunately, only five doorstopper cells with

attached rock core had been saved. These rock samples were classified as hard sericitic quartzite, vitreous quartzite, and supervitreous quartzite in order of increasing stiffness

Table 3.—Estimated rock properties

Rock type Young's modulus Poisson's

si ratio GPa 10 p6

Hard sericitic quartzite . . . 48 7 0.2

Vitreous quartzite . . . . . . . 69 10 0.1

Super-vitreous quartzite . 90 13 0.1

Site average . . . . . . . . . . . 69 10 0.1

MINING EXPERIENCE

Excavation in the vicinity of the measurement site ceased until

the nearby 5210-101 crosscut was developed in November and

December 1989. This crosscut was driven approximately 30 m

(100 ft) above the measurement site. Shortly thereafter, in January

1990, excavation in the 5300-101 crosscut stub (the stress

measurement site) resumed as miners attempted to gain access to

the vein.

The first round was taken without incident [approximately 2.5

m (8 ft) of advance], although increased seismic activity was noted.

The following round, fired at the end of the day shift on January

10, 1990, triggered a major rock burst measuring 79 mm on the

mine's seismograph (a local relative measurement of intensity) with

an estimated local magnitude (M ) of 1 1.4. Forty metric tons (fortyfive short tons) of rock collapsed into the 5300-101 crosscut,

creating a 2- by 2- by 6-m (6- by 6- by 20-ft) long void in the roof.

Another 135 t (150 st) of rock collapsed into the 5210-101 ramp

directly above. A map of this damage is provided in figure 8.

After repairs were completed, a third round was shot on January

17, 1990, triggering a burst measuring 31 mm on the mine

seismograph and estimated to be less than 1 M . This burst took 1

out the right rib of the 5300-101 crosscut, releasing 65 t (72 st) of

rock. Subsequent seismic activity continued at high levels. Figure

9 shows the enlarged crosscut after broken rock was removed.

Intense seismic activity continued into the following day,

January 18, causing mine personnel to abandon repair activities in

the 5300-101 crosscut. Shortly after the crosscut was evacuated,

at 8:09 a.m. and without further warning, a rock burst occurred.

This rock burst measured 72 mm on the mine seismograph and was

estimated at 1.2 M . The 1 burst expelled 135 t (150 st) of rock from

the r ight rib
11

Figure 7—Tested sections, geologic formation, and stress solution at 5300-level site. A, Plan view; B, vertical east-west section.
12

Figure 8.—Damage (shaded area) caused by rock burst on January 17, 1990.
13

Figure 9.—Crosscut after removal of debris from January 17, 1990, rock burst. Original dimensions were 3 by 3 m.

and back. This burst was particularly dangerous as it occurred (400 st) of rock collapsed during the three rock bursts. At a labon-shift without warning, whereas the previous bursts had oratory density of 2.7 t/m (see appendix C), this rock has a

occurred with blasting, when the mine was evacuated. Mine volume of about 140 m (5,000 ft ). Volume and tonnage

management decided to abandon the 5300-101 crosscut after this estimates from the first rock burst suggest a lower in situ density

final burst and to develop an alternative accessway in a safer of about 1.9 t/m , which would suggest that about 200 m

portion of the rock mass. (7,000 ft ) of rock collapsed. Thus, destressing a volume of rock

The energy released by these three bursts (January 10, 17, equivalent to the volume of collapsed rock could supply enough

and 18, 1990) was not measured directly, but was estimated at energy for the three rock bursts. This implies that only the

85 to 170 MJ using conventional methods (appendix D). If the immediate vicinity of the crosscut was destressed, and it was

source of energy is assumed to be stored elastic strain energy in unlikely that the rock mass was sufficiently destressed to allow

rock loaded to the measured stress level, these bursts would des- completion of the crosscut without additional rock bursts.

tress an estimated 86 to 172 m (3,000 to 6,000 ft ) of rock, or Rock bursts are often associated with pockets of highly 3 3

a sphere between 5.5 and 7 m (18 and 23 ft) in diameter. By stressed rock. For instance, Leighton (1982) reported a

comparison, the damage reports estimate that about 375 t correlation between high local stresses and rock bursting

3

3 3

3 3

3
1 & 1

E

&

&

14

Table 4.—Ranking criteria for quality designation1

Quality Maximum percent of variation

Orientation2 S

3

1 S

4

2

Excellent ......... 1 5 5

Good . . . . . . . . . . . . 3 10 10

Acceptable . . . . . . . . 5 15 15

Poor . . . . . . . . . . . . . 10 20 20

Bad . . . . . . . . . . . . . . >30 >20 >20

for explanation of S d S See text footnote 3 an . 1

1 2

S d S e principal stress components on 3

1 2 an are nonstandard notations for th 

the end of a borehole (standard notations are F1  and F2 ). These nonstandard

notations are needed to emphasize that they are not far-field in situ stress

components. In the equations in table 4, the "min" and "max" subscripts refer

to the range of estimates produced. Four estimates are produced by each

doorstopper cell.

in his study of a stope pillar at the nearby Galena Mine. He and periodically released by a series of rock bursts. The major

measured stress in the pillar with vibrating-wire stressmeters as difference between Leighton's study and this investigation is the

mining narrowed the pillar and concentrated stress in the relative remoteness of the 5300-101 crosscut measurement site

remnant. The concentration of stress was not uniform. from mining.

Localized concentrations of stress were built up during mining

STRESS FIELD ESTIMATE

An accurate estimate of the in situ stress field depends on A simple screen consists of simply solving for the stress field

determining which of the many doorstopper cell measurements

were reliable and then applying statistical procedures and accurate

stress concentration factors to minimize estimate error. Ideally,

the rock mass at the site would be homogeneous and isotropic

and would not be influenced by mining. Such an estimate would

represent the far-field stress loading mine openings.

Stress estimates were calculated from strain data using the

computer program STRESsOUT (Larson, 1992). STRESsOUT

uses a standard set of assumptions to develop estimates of in situ

stress from overcore strain measurements by minimizing the

squared error for each strain measurement. By treating all

measurements throughout the test site equally, it was assumed that

(1) stress and material properties at a site were homogeneous and

(2) the rock mass was linearly elastic and had no discontinuities.

The program is capable of providing—

1. Statistical treatment of data—A least squares routine

ensures equal (or specified) weighting of all data points. This

program runs on 8088 or better DOS-based personal computers

in a matter of minutes, providing the capability to conduct

parametric studies if needed.

2. Improved computations for the induced stress field of a

borehole—Advanced modeling techniques have led to the

development of more exact stress concentration factors that

include the effect of Poisson's ratio. The program uses stress

concentration factors specified by the user.

EVALUATION OF STRAIN DATA

The evaluation of strain data, especially the identification of

invalid measurements, is a critical step in estimating the in situ

stress field. Field notes describing difficulties with the instruments, bad glue joints, or rock defects are the most important

source of information. Further insight can be gained by applying

a number of screens that numerically test the overcore strains

against various criteria.

(S , S , and 1 2 1) measured by each of the various sets of three 3

strain gauges in each doorstopper cell and comparing the results.

A sound doorstopper overcore measurement should produce

substantially the same stress field regardless of which gauges are

chosen. Local solutions for the overcore strains shown in table 2

are derived in appendix E. The relative quality of each solution

was defined by assigning it to one of five arbitrarily defined

groups (table 4) based on the extent of the range of solutions

produced (table 5).

2

180 × 100.

3

 × 100.

4

 × 100.

A related screen checks for the self-consistency of strain

readings in another way. The self-consistency of a doorstopper
15

Table 5.—Measurement quality as classified

by range screen1

Doorstopper cell Percent of variation Quality2

Orientation S 1 S2

Borehole 1:

 3 . . . . . . . . . . . . . . . 4 17 16 Poor.

 4 . . . . . . . . . . . . . . . 3 14 14 Acceptable.

 5 . . . . . . . . . . . . . . . 4 20 19 Poor.

 6 ............... 1 1 1 Excellent.

 21 . . . . . . . . . . . . . . 25 39 30 Bad.

 22 . . . . . . . . . . . . . . 5 18 17 Poor.

 23 . . . . . . . . . . . . . . 3 6 6 Good

Borehole 2:

 24 . . . . . . . . . . . . . . 3 9 10 Good.

 25 . . . . . . . . . . . . . . 1 3 3 Excellent.

 26 . . . . . . . . . . . . . . 2 7 7 Good.

 27 . . . . . . . . . . . . . . 1 1 1 Excellent.

 28 . . . . . . . . . . . . . . 1 1 1 Excellent.

Borehole 3:

 7 . . . . . . . . . . . . . . . 4 12 12 Acceptable.

 8 ............... 2 4 4 Good.

 9 . . . . . . . . . . . . . . . 7 20 19 Bad.

 10 . . . . . . . . . . . . . . 14 17 20 Bad.

 11 . . . . . . . . . . . . . . 6 26 25 Bad.

Borehole 4:

 12 . . . . . . . . . . . . . . 3 12 13 Acceptable.

 13 . . . . . . . . . . . . . . 16 35 35 Bad.

for explanation of S d S See text footnote 3 an . 1

1 2

See table 4. 2

Table 6.—Measurement quality as classified by strain screen

Doorstopper

cell

Strain test

Quality Summation, μ, Difference

±45E Horizontal + vertical μ, pct

Borehole 1:

 3 . . . . . 1,347 1,652 305 15 ( )1

 4 . . . . . 2,017 2,316 299 13 ( )2

 5 . . . . . 1,536 1,922 386 20 ( )1

 6 . . . . . 1,021 1,030 9 1 ( )2

 21 . . . . . 1,050 1,415 365 26 ( )1

 22 . . . . . 3,678 4,326 648 15 ( )1

 23 . . . . . 1,155 1,099 56 5 ( )2

Borehole 2:

 24 . . . . . 1,220 1,335 115 9 ( )2

 25 . . . . . 1,744 1,704 40 2 ( )2

 26 . . . . . 2,606 2,760 154 6 ( )2

 27 . . . . . 2,960 2,984 24 1 ( )2

 28 . . . . . 1,848 1,867 19 1 ( )2

Borehole 3:

 7 . . . . . 1,638 1,869 231 12 ( )2

 8 . . . . . 5,087 4,936 151 3 ( )2

 9 . . . . . 2,216 1,849 367 17 ( )1

 10 . . . . . 1,772 2,034 262 13 ( )2

 11 . . . . . 711 937 226 24 ( )1

Borehole 4:

 12 . . . . . 1,542 1,383 159 10 ( )2

 13 . . . . . 2,090 2,550 460 18 ( )1

ue was above limits. Bad reading; test val 1

Good reading; both test values were below limits. 2

NOTE.—Numbers in bold indicate values outside test limits.

cell can be tested by determining whether all four gauges of a cell

are measuring the same strain field. This "strain test" takes

advantage of the fact that any two perpendicular measurements of

normal strain define the center of a Mohr's circle in strain. Thus,

each of two pairs of perpendicular gauges in a doorstopper cell

should sum to the same total strain. If the sums are drastically

different, the doorstopper cell is failing to measure a single strain

field at the end of the borehole. This failure may be attributable

to a number of factors, including an electrical fault, the presence

of a fracture on or near the face, a poor or nonuniform glue joint,

or improper centering of the cell. However, this method will not

characterize the source of the strain state, including whether or not

isotropic elastic rock is present.

A large difference between the sums for a single cell suggests

that the cell should be considered suspect in estimating the in situ

stress field. However, defining "large" proved to be problematic.

The definition of large was chosen arbitrarily as being a difference

greater than either 300 F, or 20 pct of the largest sum. This

definition was taken from a reanalysis of the 4250-level

measurement in the first RI of this series, where it proved to be

convenient. The strain-screening process and resulting strainscreened data set are summarized in table 6. This screen is

generally consistent with the first screen, except for doorstopper

10, which passes this screen, while failing the first (table 7).

A widely used screen that is included in the STRESsOUT

data-reduction program identifies outlying strains. These are

strains that deviate substantially from the average and greatly

increase the squared error of the least squares fit estimate of the

stress field. The governing assumption in this approach is that

outlier data points are attributable to error and are not real

conditions. Outlier data can be examined by comparing the

results of the screening procedure described earlier with the outlier

elimination routine in STRESsOUT. The strain screen was used

to eliminate the same number of strain gauges as were eliminated

in the strain-screening process, i.e., 7 doorstopper cells, or 28

gauges, about 37 pct of the total. The first 28 strain gauges

selected by the STRESsOUT program as outliers are listed in

bold italics in table 6 for comparison with range- and strainscreen-passed data sets. Only 12 strain gauges were eliminated by

both outlier and strain screens. By comparison, if both of these

screens were essentially random, the expected overlap in

selections would be between 10 and 11 strain gauges. This independence suggests that the outlying strains were valid

measurements.
16

Table 7.—Comparison of screen results

Doorstopper

cell

Strain gauge orientation, μ, Quality

+45E !45E Vertical Horizontal Strain screen1 Range screen2

Borehole 1:

 3 . . . . . . . . . . 420 927 !369 2,021 Bad. Poor.

 4 . . . . . . . . . . 161 1,856 189 2,127 Good. Acceptable.

 5 . . . . . . . . . . 146 1,390 !155 2,077 Bad. Poor.

 6 . . . . . . . . . . 324 697 508 522 Good. Excellent.

 21 . . . . . . . . . 495 555 503 912 Bad. Bad.

 22 . . . . . . . . . 2,720 958 528 3,798 Bad. Poor.

 23 . . . . . . . . . 666 489 214 885 Good. Good.

Borehole 2:

 24 . . . . . . . . . 85 1,135 748 587 Good. Good.

 25 . . . . . . . . . 483 1,261 1,473 231 Good. Excellent.

 26 . . . . . . . . . 592 2,014 2,097 663 Good. Good.

 27 . . . . . . . . . 519 2,441 1,151 1,833 Good. Excellent.

 28 . . . . . . . . . 465 1,383 775 1,092 Good. Excellent.

Borehole 3:

 7 .......... !126 1,764 862 1,007 Good. Acceptable.

 8 .......... 1,216 3,871 2,597 2,339 Good. Good.

 9 . . . . . . . . . . 288 1,928 713 1,136 Bad. Poor.

 10 . . . . . . . . . 618 1,154 940 1,094 Good. Bad.3

 11 . . . . . . . . . 563 148 928 9 Bad. Bad.

Borehole 4:

 12 . . . . . . . . . 346 1,196 227 1,156 Good. Acceptable.

 13 . . . . . . . . . 459 1,631 841 1,709 Bad. Bad.

ble 6. See ta 1

See table 5. 2

Only disagreement between strain and range screens. 3

NOTE.—Numbers in bold italics indicate a STRESsOUT-screened strain reading.

STRESS FIELD SOLUTION The discrepancies between the 4250- and 5300-level measureStress estimates were developed using stress concentration sets developed by all the screening methods. The best estimate

factors developed by Rahn (1984) and laboratory values of rock probably arises from the most impartially screened data set. Thus,

elastic properties [Young's modulus of 69 GPa (10 million psi) the authors have selected the strain-screened data set (estimate F

and Poisson's ratio of 0.1]. Stress field estimates for the full and in table 8) to represent the best solution. This solution is prevariously screened data sets are presented in table 8 along with the sented in map coordinates in table 9.

best stress field estimate from the 4250-level measurement site. 

ments that prompted this study are preserved by solutions for data
17

Table 8.—Stress field estimates

Stress component Magnitude Bearing Plunge MPa psi 

A. 4250-level best estimate:

 F ......................................... 1 91 13,200 N 40E W 13E

 F ......................................... 2 55 7,900 S 41E W 33E

 F ......................................... 3 37 5,400 S 68E E 54E

 F ......................................... v 45 6,600

B. All data:

 F ......................................... 1 126 18,300 S 65E W 23E

 F ......................................... 2 92 13,300 N 11E W 28E

 F ......................................... 3 68 9,800 S 58E E 52E

 F ......................................... v 82 11,900

C. Range-screen-passed data:

 F ......................................... 1 143 20,800 S 83E W 36E

 F ......................................... 2 74 10,700 N 5E E 16E

 F ......................................... 3 67 9,700 S 65E E 49E

 F ......................................... v 94 13,600

D. Strain-screen-passed data (best estimate):

 F ......................................... 1 135 19,600 S 80E W 34E

 F ......................................... 2 73 10,500 N 4E W9E

 F ......................................... 3 69 10,000 S 81E E 54E

 F ......................................... v 90 13,100

E. STRESsOUT-screen-passed data (outlying 28

 gauges removed):

 F ......................................... 1 93 13,600 S 84E W 27E

 F ......................................... 2 56 8,100 S 84E E 63E

 F ......................................... 3 55 8,000 S 3E E5E

 F ......................................... v 64 9,200

F. Strain-screen-passed data with an additional

 eight outlying gauges removed:

 F ......................................... 1 99 14,300 S 78E W 27E

 F ......................................... 2 62 9,000 S 24E E 23E

 F ......................................... 3 56 8,100 N 32E E 54E

 F ......................................... v 65 9,500

NOTE.—Empty cells in columns intentionally left blank.

Table 9.—Best estimate of stress field in map coordinate system

Stress component Magnitude

MPa psi 

F ............................. ns 74 10,700

F ............................. ew 113 16,400

F ............................. v 90 13,100

J ........................... ns/ew 7 1,100

J ............................ ew/v !30 !4,400

J ............................ ns/v !5 !700
18

STRESS FIELD CHARACTERIZATION

The stress field at the measurement site is fully described by

the in situ stress estimate in only the most ideal of cases. That is,

the rock mass and stress field may be considerably more complex

than the assumptions implicit in STRESsOUT calculations at

both doorstopper-cell and measurement-site scales. This section

attempts to go beyond these assumptions to better characterize the

stress field.

DOORSTOPPER-SCALE ASSUMPTIONS ground control problems, including rock bursting, if not

The rock immediately surrounding a doorstopper cell was

assumed to be homogeneous, continuous, isotropic, and linearly

elastic. A uniaxial compression test showed the rock to be

linearly elastic, although some hysteresis at low loads was noted.

The available information was not sufficient to determine whether

there was any anisotropy of elastic properties, but rock samples

inspected appeared to be isotropic and did not show any structure

on the scale of a doorstopper cell. Homogeneity and continuity

were fairly well ensured by carefully inspecting the borehole with

a camera to detect and avoid fractures and bed interfaces. Thus,

the doorstopper cells were probably mounted inside the thicker

beds. However, there would be no way to tell if there were a

fracture in the rock far enough from the doorstopper cell to elude

detection but still close enough to affect the stress field around it.

SITE-SCALE ASSUMPTIONS

Assumptions on the scale of the measurement site are

particularly important when using two-dimensional cells,

which are capable of measuring stress only on the face at the

end of a borehole. This two-dimensional local stress field has

three components, but it is determined by four components

of the in situ stress field. Thus, the in situ components

are underdetermined and additional information from boreholes in other directions are needed before in situ conditions

can be estimated. Data from doorstopper cells in three

nonparallel boreholes are necessary to estimate the threedimensional stress state.

The least squares procedure followed in the previous

section assumes that all doorstopper cells were installed in a

homogeneous material experiencing uniform loading. Any

deviations from the average of measured material properties

or the stress field estimate are considered

random errors. The potential for real variability in stress

field and/or rock mass properties throughout the measurement site raises two important issues: assessment of local

stress field variability and the potential for sampling bias.

Assessment of local stress variability is needed to determine if

deviations from ideal conditions are significant to engineering

design. That is, it must be determined whether the pattern

and/or degree of stress variability is sufficient to create

dealt with explicitly. Furthermore, sampling procedures need

to be evaluated in light of any stress field and/or rock

property variations to ensure that a valid estimate of average

stress conditions is attained.

Local Stress Variability

The degree of local stress variability can be investigated by

examining outlying measurements and the consistency of local

stress measurements. Under ideal conditions, there should be

little variability among doorstopper cells installed in a single

borehole far from the influence of mine openings. Ideal

conditions are rare. Thus, the evaluation process becomes an

investigation of the validity, and reason for, outlying strain

measurements.

Tests for outlying strains and measurement validity were

developed earlier in the course of the average stress field

estimation procedure. During this process, a discrepancy was

noted between the strain and range screens that examined

doorstopper cell self-consistency and the identification of outlying

strain readings. This discrepancy is examined in greater detail in

table 10. Of particular interest are the strain readings that are

clearly self-consistent but also identified as outliers. These strains

can be described as good outlying strains and appear to represent

local conditions. Their existence suggests that real variations are

present at this measurement site. The locations of good outlying

strain measurements are indicated in table 10. Stress estimates for

the set of good outlying strains (estimate B in table 11) and the

remaining strains (estimate C in table 11) show considerable

differences in magnitude but remarkably similar orientations.

Both estimates show a strongly biaxial stress field, with the maximum principal stress (F1) oriented roughly perpendicular to

bedding. Since many doorstopper cells produced strains for both

data sets, the spatial significance of this result is not clear.
19

Table 10.—Location and number of good outlying overcore strain measurements

Location Rock type 1 Total number

of gauges gauges, pct

Strain-screenpassed screen-passed passing both outlier

gauges gauges screens gauges

STRESsOUT- Gauges Good Good outlier

1a . . . . . . . . 16 8 7 5 2 29 Vitreous quartzite.

1b . . . . . . . . 12 8 2 2 0 0 Vitreous quartzite.

2 . . . . . . . . . 20 0 6 0 6 100 Super-vitreous quartzite.

3 . . . . . . . . . 20 8 9 3 6 67 Hard sericitic quartzite.

4 . . . . . . . . . 8 4 4 2 2 50 Unknown.

ure 7. See fig 1

Table 11.—Stress estimates exploring good outlying strain measurements

Stress component Bearing Plunge Magnitude

MPa psi 

A. Strain-screen-passed data:

 F ...................................................... 1 135 19,600 S 80E W 34E

 F ...................................................... 2 73 10,500 N 4E W9E

 F ...................................................... 3 69 10,000 S 81E E 54E

 F ...................................................... v 90 13,100 

B. Outlying strains only:

 F ...................................................... 1 188 27,300 S 73E W 34E

 F ...................................................... 2 93 13,500 N 49E E 55E

 F ...................................................... 3 76 11,100 S 22E E7E

 F ...................................................... v 85 12,300

C. Strain-screen-passed data with outlying strains removed: 

 F ...................................................... 1 90 13,100 S 84E W 21E

 F ...................................................... 2 55 7,900 S 34E E 51E

 F ...................................................... 3 53 7,600 N 7E E 31E

 F ...................................................... v 59 8,500

NOTE.—Empty cells in columns intentionally left blank

The spatial variation of stress can be examined more clearly by terms). These variations may be showing changes in stress.

examining data from each borehole, and each doorstopper cell, as Alternatively, they may be showing a homogeneous stress field

a unit. The influence of each borehole on the stress estimate can with variations in elastic properties along the boreholes, or some

be evaluated by taking advantage of the extra borehole at the site combination of these alternatives. In any case, the assumption of

to estimate stress using data from various combinations of three a uniform rock mass under uniform loading is violated. The

boreholes (table 12). The results show that removing data from variation in borehole 2 is especially significant because it is

boreholes 1, 2, and 4 (estimates B, C, D, and E, respectively, in repeated by two successive doorstopper cells. Local variations in

table 12) nudges the estimates up, but removing borehole 3 data stress direction are also evident in boreholes 1 and 2, but these

reduces the estimate. Significantly, though, the difference is variations do not correspond to variations in stress magnitude.

primarily in the magnitude of stresses, not in rotation of the If all of the highly stressed doorstopper cells sample stress

maximum principal stress direction. conditions caused by local but common mechanisms (hard

The spatial variation of stress can also be examined within a inclusion effect in a stiff bed, etc.), a composite stress estimate can

borehole. The stress field on the end of the borehole at each be developed. A similar approach might provide a view of the

doorstopper cell location (S , S , and 1) can be estimated general background stress regime. For instance, doorstoppers 8, 1 2

independently (appendix E). These solutions are plotted 22, 26, and 27 would be a solvable high stress data set while the

individually in figure 10 and summarized in figure 11. Large remainder of cells could be assigned to an average or background

local variations in stress magnitude are evident in three of the four set. Doorstopper 12 could be added to the high stress data set to

boreholes, although the high stress doorstopper cell in borehole include measurements from all four boreholes.

1 failed to make the strain screen (in absolute, but not relative,
20

Figure 10.—Local stress field on end of borehole for each doorstopper cell. Plots show orientation of principal axes of stress ellipse.

Boxes show ranges of magnitudes and orientations for each axis calculated from various combinations of three of four doorstopper strain

gauges.
21

Figure 10.—Continued. Local stress field on end of borehole for each doorstopper cell. Plots show orientation ofprincipal axes of

stress ellipse. Boxes show ranges of magnitudes and orentations for each axis calculated from various combinations of three of four

doorstopper strain gaugues.
22
23

Figure 10.— Continued. Local stress field on end of borehole for each doorstopper

cell. Plots show orientation of principal axes of stress ellipse. Boxes show ranges of

magnitudes and orientations for each axis calculated from various combinations of

three of four doorstopper strain gauges.

Table 12.—Stress estimates developed from strain-screened data from various combinations of three of four boreholes

Stress component Bearing Plunge Magnitude

MPa psi 

A. Strain-screen-passed data:

 F ........................................................... 1 135 19,600 S 80E W 34E 

 F ........................................................... 2 73 10,500 N 4E W 9E 

 F ........................................................... 3 69 10,000 S 81E E 54E 

 F ........................................................... v 90 13,100 

B. Strain-screen; borehole 1 data removed:

 F ........................................................... 1 145 21,000 S 75E W 44E 

 F ........................................................... 2 81 11,800 S 50E E 31E 

 F ........................................................... 3 59 8,600 N 20E E 31E 

 F ........................................................... v 106 15,400 

C. Strain-screen; borehole 2 data removed:

 F ........................................................... 1 161 23,400 N 84E W 28E 

 F ........................................................... 2 113 16,300 S 6E E 20E 

 F ........................................................... 3 81 11,800 N 53E E 54E 

 F ........................................................... v 103 15,000 

D. Strain-screen; borehole 3 data removed:

 F ........................................................... 1 113 16,400 S 72E W 18E 

 F ........................................................... 2 68 9,900 S 32E E 14E 

 F ........................................................... 3 57 8,300 N 31E E 67E 

 F ........................................................... v 63 9,100 

E. Strain-screen; borehole 4 data removed:

 F ........................................................... 1 140 20,300 S 85E W 36E 

 F ........................................................... 2 77 11,200 N 21E E 31E 

 F ........................................................... 3 61 8,900 S 41E E 39E 

 F ........................................................... v 93 13,400 
24

NOTE.—Empty cells in columns intentionally left blank.
25

Figure 11.—Local stress field on end of borehole summarized from results plotted in figure 10. Underlined numbers indicate

stopper cells that passed strain screen. Circled numbers indicate number of good outlying strains for each doorstopper cell.
26

The resulting stress estimates (table 13) show a surprising Sampling

decoupling of stress fields and the only marked change in stress

direction noted in any of the alternative stress estimates. The high

stress estimate, with and without doorstopper 12, has rotated F1 to

the northwest-southeast while dramatically increasing stress

magnitudes.

The background or average stress estimate has rotated further to

the southwest-northeast and is lower in magnitude. The lesser

principal and vertical stress components are still significantly

greater than the 4250-level estimate, but the greatest principal stress

estimate at this level is not supportable by the background stress

estimate. That is, the northwest component of the background

stress estimate (F ) is about 25 pct less than the northwest compo- 2

nent of the 4250-level stress estimate (F1). However, a weighted

average of these two stress field estimates could easily carry the

necessary stress.

It is clear that significant local variations in overcore strains were

measured. Moreover, these strains had a significant impact on the

stress field estimate. The next logical step in this analysis might be

to develop a test site model as was attempted in part 1 of this series

(Whyatt and Beus, 1995) for the 4250-level measurement site.

However, there is not enough physical property information to

differentiate between variations in overcore strain arising from

changes in rock properties from those variations reflecting a true

change in stress regime.

Estimation of a true average stress field for this site is seriously

complicated by the presence of real variations in stress and/or rock

properties. The spatial distribution of measurements was not a great

concern as long as each measurement could be considered an

independent, randomly selected data point. But now that spatial dependence of stress and/or material properties has been established, increased

attention must be paid to the actual position of measurements.

The measurable and unmeasurable sections in each borehole are shown

in figure 5. Doorstopper core recovery requirements disqualified a large

portion of each borehole. Whether fractures in the core were preexisting

features or were caused by drilling is unclear. (Core discing was ruled out

by the shape of core fragments.) In either case, the unmeasured sections

probably represent zones of relatively weak, soft rock with relatively lower

levels of stress.

Obviously, then, measurements at this site were concentrated in space.

For example, seven measurements in borehole 1 were concentrated in two

short sections comprising only 0.8 m (2.5 ft) of the available 10 m (30 ft).

Furthermore, a plot of the competent borehole sections on a geologic map

suggests that the five measured borehole segments actually measured

conditions in only three stratigraphic intervals (figure 7). Thus, this

measurement was clearly biased by concentrated sampling. This biased

sampling could easily account for at least part of the unusually high stress

levels, especially the unusually high magnitude of the vertical component.

Table 13.—Stress estimates from high stress and average stress data sets

Stress component Bearing Plunge Magnitude

MPa psi 

A. Strain-screen-passed data:

 F .............................................. 1 135 19,600 S 80E W 34E 

 F .............................................. 2 73 10,500 N 4E W 9E 

 F .............................................. 3 69 10,000 S 81E E 54E 

 F .............................................. v 90 13,100 

B. High stress data set only:

 F .............................................. 1 299 43,300 N 22E W 48E 

 F .............................................. 2 231 33,500 S 60E W 7E 

 F .............................................. 3 70 10,100 S 35E E 41E 

 F .............................................. v 200 29,000 

C. High stress data set plus data from doorstopper 12:

 F .............................................. 1 214 31,000 S 63E E 8E 

 F .............................................. 2 167 24,300 N 12E W 63E 

 F .............................................. 3 120 17,400 S 31E E 26E 

 F .............................................. v 159 23,100 

D. Average stress data set:

 F .............................................. 1 100 14,500 S 48E W 25E 

 F .............................................. 2 73 10,600 N 35E W 15E 

 F .............................................. 3 55 8,000 N 84E E 60E 

 F .............................................. v 64 9,300 

NOTE.—Empty cells in columns intentionally left blank.
27

Table 14.—Best estimate of average stress conditions

at measurement site 

Stress component Magnitude Bearing Plunge

MPa psi 

F .................. 1 135 19,600 S 80E W 34E

F .................. 2 73 10,500 N 4E W 9E

F .................. 3 69 10,000 S 81E E 54E

F .................. v 90 13,100

NOTE.—Empty cells in columns intentionally left blank.

DISCUSSION AND CONCLUSIONS

The stress estimate derived from the 5300-level overcore for the development of a complex natural stress field. Energy

measurement site has been considered suspect, at first glance, by calculations show that the extent of local stress concentrations

a number of researchers active in the Coeur d'Alene Mining District, could be quite limited, on the order of only a hundred or so cubic

including the authors, for three reasons. First, the large magnitude meters, and still contain sufficient strain energy to power the three

of F1 does not fall in line with previous measurements in the

district. Second, the rotation of F1 from the generally accepted

northwest quadrant to essentially east-west does not correlate with

previous measurements in the district or the generally accepted view

of recent right-lateral movement on the east-west trending Osburn

Fault (Hobbs and others, 1965). Third, the estimate of vertical

stress is greater than twice the overburden weight.

However, careful review and screening of the overcore strain

data demonstrated that the measurements were valid and accurate.

Furthermore, the unusual concentration of rock bursts suggested

that the rock originally contained an unusually high density of

elastically stored energy. This is possible considering the unusually

strong, stiff rock at the site. At the same time, however, these

stress levels do not appear to be representative of the overall stress

condition at the mine.

The crux of the problem with this measurement, then, is proper

interpretation of these good overcore strain measurements.

Application of the conventional approach to analysis of these data

encounters some serious problems. For instance, there is

substantial evidence that the site was not homogeneous in either

material properties or stress field. Furthermore, the apparent

sampling bias toward strong, stiff rock probably increased stress

estimate magnitudes. Integration of a more sophisticated rock

property and in situ stress model into the analysis could account for

these factors. Two such models were proposed and used to

explore alternative stress estimates for the 4250-level measurement

site in part 1 of this series (Whyatt and Beus, 1995). Unfortunately,

expectations of a uniform site for this measurement led to

insufficient attention to geologic information, and laboratory testing

of rock samples was neglected. Thus, it is not practical to pursue

alternative models for this site.

Based on the available information, the best estimate of stress

conditions at the measurement site is given in table 14.

All indications are that these measured stresses did exist, at least

locally, and were the driving force behind the unusually severe rockburst activity. The contrasts in rock elastic properties and the

proximity of the site to the axis of the Hook anticline and a fault

provide more thansufficient opportunity

major rock bursts on January 10, 17, and 18, 1990. The stress field

rotation was remarkably stable throughout the screening and sorting

processes, and may well exist throughout the site. However,

decomposition of the strain data into high stress and average stress

data sets, and apparent rotation of stress orientations in two of the

boreholes suggest that stress orientation is locally variable, at least

to a small degree. This apparent rotation of the overall stress field

should be apparent in the geomechanical behavior of surrounding

mine openings. Some indications of a stress field rotation have

been noted in bored raises near the axial plane of the Hook

anticline. These indications and the relationship of this stress

measurement to mine-wide stress conditions are explored in part 4

(Whyatt and others, 1995) of this series of reports.

This overcore stress measurement suggests that localized stress

concentrations in a natural stress field were associated with a

concentration of rock-burst activity that interfered with excavation

of development openings. Similar observations between natural

and excavation-induced stress concentrations and rock bursts have

been reported throughout the world. Further work needs to be

done to identify which geologic structures and strata are prone to

naturally concentrating stress to a degree sufficient to cause rock

bursts at the Lucky Friday Mine. This information could then be

used to place development openings in relatively rock-burst-safe

portions of the rock mass. Finally, it is clear that overcore stress

measurements in the Lucky Friday Mine should not be considered

to be routine tests. Rather, each measurement should be

approached as a geomechanical experiment in a complex rock

mass.
28

Interpretation of in situ stress at the site was hampered by the the tremendous difference between measured and expected values.

limited information collected on rock properties as part of the Furthermore, stress estimation techniques for doorstopper cells

measurement effort. But the information indicates that some and other two-dimensional cells need to be developed in which

assumptions implicit in the stress estimation procedure were more sophisticated site models are incorporated to allow for

violated. In the future, widespread and thorough testing of rock variations in physical properties and stress. Additional research

properties and mapping of geologic structure should be carried on the type and degree of natural stress variations that can be

out, especially if a heterogeneous site is suspected. Analysis of expected in typical geologic settings would also be useful for

available information showed that while violations of underlying developing site models and evaluating the difficulty of estimating

assumptions were sufficient to erode confidence in the stress stress at potential overcore sites.

measurement, they were not sufficient to explain

ACKNOWLEDGMENTS

The authors wish to express their appreciation to the staff of We also acknowledge the invaluable contributions of our

Hecla Mining Co., Lucky Friday Unit, Mullan, ID, and especially

to George Johnson, mine manager; Steve Lautenschlager, mine

superintendent; Rich Appling, mining engineer; and Ed Wilson,

head of the safety department, for providing a location for the

field work, logistics support, and a safe working environment.

The authors also wish to thank those at the Spokane Research

Center (SRC) who assisted in the drilling and measurement work

at the Lucky Friday Mine: Mike Jones, Gene Stone, and Mike

King, electrical engineers; Bob McKibbin, general engineer; and

Ted Williams and Jeff Johnson, mining engineers. Jeff Johnson's

detailed field notes and laboratory tests were particularly useful.

USBM colleagues Brad Seymour, mining engineer, and Priscilla

Wopat, technical publications editor, in reviewing and improving

this manuscript. The assistance of Mark Larson, mining engineer,

and Kylan Kracher, engineering technician, both of SRC, were a

great help in conducting computer analyses of overcore strains

and mining-induced stress. The guidance and support of Charles

Fairhurst, professor of civil engineering, University of Minnesota,

Minneapolis, MN, and Mel Poad, group supervisor, SRC, are

also gratefully acknowledged.

REFERENCES

Allen, M. Determining the In-Situ Stress Field on the 4250 Level in the Jenkins, M., and R. McKibbin. Practical Considerations of In Situ Stress

Lucky Friday Mine: Using the CSIR Biaxial Strain Gage and Structural Determination. Paper in Proceedings, International Symposium on Application

Geologic Mapping. M.S. Thesis, Univ. ID, Moscow, ID, 1979, 73 pp. of Rock Characterization Techniques in Mine Design, ed. by M. Karmis. Soc.

Beus, M. J., and S. S. M. Chan. Shaft Design in the Coeur d'Alene Mining Min. Eng., 1986, pp. 33-39.

District, Idaho—Results of In Situ Stress and Physical Property Measurements. Larson, M. K. STRESsOUT—A Data Reduction Program for Inferring

USBM RI 8435, 1980, 39 pp. Stress State of Rock Having Isotropic Material Properties: A User's Manual.

Chan, S. S. M. A Case Study of In-Situ Rock Deformation Behavior for USBM IC 9302, 1992, 163 pp.

the Design of Ground Support System. USBM OFR 44-73, 1972, 130 pp.; Leighton, F. A Case Study of a Major Rock Burst. USBM RI 8701, 1982,

NTIS PB 221 880/AS. 14 pp.

Goodman, R. E. Introduction to Rock Mechanics. John Wiley, 1980, pp. McMahon, T. Rock Burst Research and the Coeur d'Alene District.

121-123. USBM IC 9186, 1988, 45 pp.

Gutenberg, B., and C. F. Richter. Magnitude and Energy of Earthquakes. Pariseau, W. G., J. K. Whyatt, and T. J. McMahon. Rock Mechanics InvesAnn. Geofis. (Rome), v. 9, 1956, pp. 1-15. tigations at the Lucky Friday Mine (In Three Parts): 3. Calibration and ValiHedley, D. G. F. Rockburst Handbook for Ontario Hardrock Mines. dation of a Stope-Scale, Finite-Element Model. USBM RI 9434, 1992,

CANMET Spec. Rep. SP92-1E, 1992, 305 pp. 16 pp. 

Hobbs, S. W., A. B. Griggs, R. E. Wallace, and A. B. Campbell. Geology Rahn, W. Stress Concentration Factor for the Interpolation of "Doorof the Coeur d'Alene District, Shoshone County, Idaho. U.S. Geol. Surv. Prof. stopper" Stress Measurements in Anisotropic Rocks. Int. J. Rock Mech. Min.

Pap. 478, 1965, 139 pp. Sci. Geomech. Abstr., v. 21, No. 6, 1984, pp. 313-326.

International Society for Rock Mechanics. Suggested Methods for Rock Scott, D. F. Geologic Investigations Near an Underhand Cut-and-Fill

Stress Determination. Int. J. Rock Mech. Min. Sci. Geomech. Abstr., v. 24, No. Stope, Lucky Friday Mine, Mullan, ID. USBM RI 9470, 1993, 21 pp. 

1, 1987, pp. 53-74.
29

Skinner, E. H., G. G. Waddell, and J. P. Conway. In Situ Determination Whyatt, J. K., T. J. Williams, and W. Blake. In Situ Stress at the Lucky

of Rock Behavior by Overcore Stress Relief Method, Physical Property Friday Mine (In Four Parts): 4. Mine In Situ Field. USBM RI, 1995b, in

Measurements, and Initial Deformation Method. USBM RI 7962, 1974, press.

87 pp. Whyatt, J. K., T. J. Williams, and M. P. Board. Rock Mechanics

Spottiswoode, S. M., and A. McGarr. Source Parameters of Tremors in Investigations at the Lucky Friday Mine (In Three Parts): 2. Evaluation of

Deep-Level Gold Mine. Bull. Seismol. Soc. Am., v. 65, No. 1, 1975, pp. 93- Underhand Backfill Practice for Rock Burst Control. USBM RI 9433, 1992a,

112. 10 pp.

Whyatt, J. K. Geomechanics of the Caladay Shaft. M.S. Thesis, Univ. ID, Whyatt, J. K., T. J. Williams, and W. G. Pariseau. Trial Underhand

Moscow, ID, 1986, 195 pp. Longwall Stope Instrumentation and Model Calibration at the Lucky Friday

Whyatt, J. K., and M. J. Beus. In Situ Stress at the Lucky Friday Mine (In Mine, Mullan, ID. Paper in Rock Mechanics Proceedings of the 33rd

Four Parts): 1. Reanalysis of Overcore Measurements From 4250 Level. Symposium, ed. by J. R. Tillerson and W. R. Wawersik (June 3-5, 1992, Santa

USBM RI 9532, 1995, 26 pp. Fe, NM). Balkema, 1992b, pp. 511-519.

Whyatt, J. K., M. J. Beus, and M. K. Larson. In Situ Stress at the Lucky Williams, T. J., J. K. Whyatt, and M. E. Poad. Rock Mechanics InFriday Mine (In Four Parts): 3. Reanalysis of Overcore Measurements From vestigations at the Lucky Friday Mine (In Three Parts): 1. Instrumentation of

the Star Mine. USBM RI, 1995a, in press. an Experimental Underhand Longwall Stope. USBM RI 9432, 1992, 26 pp.
30

Table A-1.—Mining-induced stress at measurement site

Stress

component

In situ

stress

Mining-induced

stress without backfill

MPa psi MPa psi 

F ....... xx 84.24 12,200 2.02 290 

F ....... yy 84.24 12,200 !0.20 !30 

F ....... xy 42.12 6,100 1.02 150 

J ....... xy 0 0 !1.36 !200 

J ....... yz 0 0 !1.52 !220 

J ....... zx 0 0 !0.78 !110 

NOTE.—Compressive strength is positive.

APPENDIX A.—MINING-INDUCED STRESS AT OVERCORE SITE

The level of mining-induced stress at the measurement site was

estimated with a MINSIM-D boundary-element model of the

Lucky Friday Mine. Model runs conducted with an arbitrary in

situ stress field and without backfill were used to generate the

estimates of stress presented in table A-1. The magnitudes of

stress components are less than 5 pct of horizontal in situ stress.

Thus, mining-induced stress should not have a major influence

on the measured in situ stress. Backfill, if included in the model, would reduce the estimate of mining-induced stress further.
31

APPENDIX B.—DRILLING NOTES ON GEOLOGY1

The following information was collected during borehole borehole (0 to 30 ft). A fault zone was encountered at about 28

drilling and doorstopper cell installation at the Lucky Friday ft (25 to 30 ft). The borehole had to be cleaned of rock chips

Mine. Measurements are provided in U.S. customary units before installing the first doorstopper gauge. The borehole had

because the original measurements were in inches and feet. been cleaned at the end of the previous week when the initial 30

Borehole 2 (up): The rock at doorstopper 12 (first in this borehole at 32 ft, 2-

Argillite interbed just beyond doorstopper 24 striking about core recovered.

30E to the borehole. A soft "talcy" fault paralleling the borehole was encountered

Borehole 3 (right): Ambient borehole temperatures for overcoring the various

Doorstopper 9 was recovered with only a 1/4-in stub and had

cubic pyrite crystals in the face of the stub. The next 6 in (21 ft, Borehole 4 (left), 100 EF.

4 in to 21 ft, 10 in) were "very fractured with closely-spaced Borehole 1 (center), 110 EF.

micro joints. The rock is quartzite and looks solid." Borehole 3 (right), 110 EF.

Borehole 4 (left):

Audible popping of rock along the borehole was noticed

during pauses during drilling the initial section of the

ft of borehole had been completed.

3/4 in) was described as "good rock" with 1/2- to 1-in pieces of

with doorstopper 20 (36 ft).

boreholes were—

Borehole 2 (up), 106 to 107 EF.

Jeff Johnson, mining engin eer at the USBM's Spokane 1

Research Center, collected the information in this section.
32

APPENDIX C.—ROCK PROPERTIES

Rock properties for the measurement site were estimated from These properties are similar to those reported by Allen (1979)

geologic observations, laboratory tests on suitable surviving

samples, and reports of laboratory testing of similar Revett

Quartzite samples. Five samples saved from the field

measurement program form the foundation for these estimates, all

of which were glued to doorstopper cells. Two of these samples

were large enough to provide test specimens for uniaxial compression testing. Information gleaned from these samples is summarized in table C-1. Observations of rock types are plotted by

location in figures 5 and 7.

for sulfide-altered quartzite at the 4250 level. His results for three

samples are given in table C-2.

Sericitic quartzite samples taken from the Revett Formation in

the Crescent and Lucky Friday Mines were somewhat softer (table

C-3). On the basis of these results, the hard sericitic quartzite was

estimated to have a Young's modulus of 48 GPa (7 million psi)

and a Poisson's ratio of 0.2. Elastic properties for each rock type

and the site as a whole are estimated in table 2. A review of

Coeur d'Alene rock properties, including all of these results, has

been compiled by Whyatt (1986).

Table C-1.—Rock type and results of uniaxial compression tests

Doorstopper cell Rock type 1 Young's modulus Poisson's Density,

ratio g/cm

Strength 3 si GPa 10 p6 MPa psi 

3 . . . . . . . . . . . . . . 69 10 0.10 324 47,000 2.7 Vitreous quartzite.

7 . . . . . . . . . . . . . . ND ND ND ND ND ND Hard sericitic quartzite.

10 . . . . . . . . . . . . . ND ND ND ND ND ND Hard sericitic quartzite.

23 . . . . . . . . . . . . . ND ND ND ND ND ND Vitreous quartzite.

28 . . . . . . . . . . . . . 90 13 0.10 310 45,000 2.7 Super-vitreous quartzite.

 Average . . . . . . 79 11.5 0.10 317 46,000 2.7 

ND No data.

Sample recovered from numbered doorstopper cell. 1

Table C-2.—Measured rock properties of sulfide-altered quartzite, 4250 level, Lucky Friday

Mine (after Allen, 1979)

Sample Young's modulus Poisson's

ratio

Strength

GPa 10 psi 6 MPa psi 

1 .................. 65.5 9.5 0.14 470 68,300

2 .................. 60.7 8.8 NR 415 60,200

3 .................. 68.9 10 NR 460 66,700

NR Not reported.

Table C-3.—Properties of Revett Formation rocks from various locations in Coeur d'Alene Mining District

Site Rock type Reference Young's modulus Poisson's

ratio

Strength

GPa 10 psi 6 MPa psi 

Lucky Friday 4240 level . 54.8 7.95 0.22 310 45,000 Sericitic quartzite. Allen (1979).

Crescent 3300 level ..... 50.0 7.1 NR 185 26,900 Sericitic quartzite. Skinner and others (1974).

Galena ............... 50.3 7.3 0.27 224 32,500 Sericitic quartzite. Chan (1972).

Galena ............... 44.1 6.4 NR 116 16,800 Revett Quartzite. Ageton (in Beus and Chan, 1980).

Galena ............... 60.7 8.8 NR 290 42,000 Competent quartzite. Royea (in Beus and Chan, 1980).

Galena . . . . . . . . . . . . . . . NR NR NR 199 28,800 Incompetent quartzite. Royea (in Beus and Chan, 1980).

NR Not reported.

NOTE.—Poisson's ratio not measured in some test programs.
' &

' F % F % F & < F F % F F % F F

33

APPENDIX D.—ROCK-BURST ELASTIC ENERGY RELEASE

The energy released by three rock bursts on January 10, site, assuming a local magnitude of 0.5 for the second burst, gives

17, and 18, 1990, was not measured directly but can be a total seismic energy release of approximately 12.3 MJ. A

estimated from a relationship between local magnitude (Ml) source energy of 19 to 37 MJ can be estimated using a seismic

and energy (W ) (equation D-1) below: efficiency for strain bursts of 30 to 60 pct (Hedley, 1992). k

(D-1)

This relationship was developed for California earthquakes

(Gutenburg and Richter, 1956) and has been confirmed for mine

tremors in the Republic of South Africa by Spottiswoode and

McGarr (1975). Application of this relationship to the three large

bursts at the measurement ••••••••••

The volume of rock destressed by these events can be

estimated from equation D-2 (see below) if the source of energy

is assumed to be primarily stored strain energy (Hedley, 1992).

A strain energy density (μ ) of approximately M 0.17 MJ/m is 3

based on overcore and laboratory measurements of elastic rock

properties. Depending on the seismic efficiency, these events

destressed an estimated 100 to 200 m (3,500 to 7,000 ft ) of 3 3

rock.

(D-2)
34

APPENDIX E.—DOORSTOPPER CELL LOCAL SOLUTIONS

Local deviation in stress field can be examined by developing Goodman, 1980). Local stress solutions are listed in table E-1 for

independent stress estimates for each instrument location. The various sets of three strain gauges for a Young's modulus of 6.9

strain field measured at the end of a borehole by any combination GPa (1 million psi). The range of solutions is summarized in

of three of the four strain gauges can be converted to stress using table E-2 and plotted in figures 10 and 11.

Hooke's law (for example, see

Table E-1.—Principal stress solutions for stress on end of borehole

Stress attributes1 Solution 1 Solution 2 Solution 3 Solution 4

Doorstopper 3:

 1, deg . . . . . . . . . . . 171 173 178 175 

 S , MPa . . . . . . . . . 1 14.37 11.97 14.01 13.68 

 S , MPa . . . . . . . . . 2 !0.13 !0.36 !0.23 !2.07 

Doorstopper 4:

 1, deg . . . . . . . . . . . 157 157 162 161 

 S , MPa . . . . . . . . . 1 17.97 15.47 16.84 16.76 

 S , MPa . . . . . . . . . 2 1.99 1.92 3.12 0.63 

Doorstopper 5:

 1, deg . . . . . . . . . . . 162 163 169 167 

 S , MPa . . . . . . . . . 1 16.23 13.02 15.16 14.95 

 S , MPa . . . . . . . . . 2 0.34 0.22 1.41 !1.71 

Doorstopper 6:

 1, deg . . . . . . . . . . . 136 135 136 137 

 S , MPa . . . . . . . . . 1 5.54 5.48 5.49 5.48 

 S , MPa . . . . . . . . . 2 3.34 3.33 3.39 3.32 

Doorstopper 21:

 1, deg . . . . . . . . . . . !23 !27 18 !2 

 S , MPa . . . . . . . . . 1 7.79 4.74 7.57 6.76 

 S , MPa . . . . . . . . . 2 4.41 4.31 4.63 2.30 

Doorstopper 22:

 1, deg . . . . . . . . . . . 9 17 18 12 

 S , MPa . . . . . . . . . 1 28.57 24.93 30.32 28.20 

 S , MPa . . . . . . . . . 2 8.72 6.77 6.97 3.51 

Doorstopper 23:

 1, deg . . . . . . . . . . . 10 7 5 8 

 S , MPa . . . . . . . . . 1 6.78 7.13 6.70 6.82 

 S , MPa . . . . . . . . . 2 2.70 2.83 2.78 3.14 

Doorstopper 24:

 1, deg . . . . . . . . . . . 131 128 130 134 

 S , MPa . . . . . . . . . 1 9.13 8.37 8.48 8.28 

 S , MPa . . . . . . . . . 2 2.37 2.14 3.03 2.24 

Doorstopper 25:

 1, deg . . . . . . . . . . . 105 106 107 106 

 S , MPa . . . . . . . . . 1 11.50 11.63 11.62 11.83 

 S , MPa . . . . . . . . . 2 3.19 3.40 3.07 3.21 

Doorstopper 26:

 1, deg . . . . . . . . . . . 114 111 111 114 

 S , MPa . . . . . . . . . 1 18.02 17.36 17.40 16.73 

 S , MPa . . . . . . . . . 2 5.77 5.11 6.39 5.74 

Doorstopper 27:

 1, deg . . . . . . . . . . . 145 144 145 145 

 S , MPa . . . . . . . . . 1 18.79 18.59 18.66 18.64 

 S , MPa . . . . . . . . . 2 6.94 6.92 7.07 6.88 

Doorstopper 28:

 1, deg . . . . . . . . . . . 144 144 145 145 

 S , MPa . . . . . . . . . 1 10.89 10.74 10.79 10.77 

 S , MPa . . . . . . . . . 2 5.21 5.19 5.31 5.16 

Doorstopper 7:

 1, deg . . . . . . . . . . . 137 134 138 141 

 S , MPa . . . . . . . . . 1 14.17 12.50 12.84 12.59 

 S , MPa . . . . . . . . . 2 1.92 1.62 3.27 1.52 

See footnote at end of table.
35

Table E-1.—Principal stress solutions for stress on end of borehole—Continued

Stress attributes1 Solution 1 Solution 2 Solution 3 Solution 4

Doorstopper 8:

 1, deg . . . . . . . . . . . 132 134 132 131 

 S , MPa . . . . . . . . . 1 28.51 29.57 29.37 29.65 

 S , MPa . . . . . . . . . 2 14.04 14.29 13.18 14.21 

Doorstopper 9:

 1, deg . . . . . . . . . . . 144 148 141 136 

 S , MPa . . . . . . . . . 1 11.83 14.78 13.86 14.27 

 S , MPa . . . . . . . . . 2 4.12 4.32 2.08 4.83 

Doorstopper 10:

 1, deg . . . . . . . . . . . 140 129 150 154 

 S , MPa . . . . . . . . . 1 11.10 9.21 9.67 9.59 

 S , MPa . . . . . . . . . 2 6.43 6.07 7.86 5.69 

Doorstopper 11:

 1, deg . . . . . . . . . . . 84 80 73 75 

 S , MPa . . . . . . . . . 1 6.74 6.57 7.26 5.39 

 S , MPa . . . . . . . . . 2 1.34 !0.43 0.82 0.74 

Doorstopper 12:

 1, deg . . . . . . . . . . . 162 161 156 156 

 S , MPa 9.29 10.61 9.90 9.94 1

 S , MPa . . . . . . . . . 2 2.63 2.68 2.02 3.35 

Doorstopper 13:

 1, deg . . . . . . . . . . . 151 145 173 165 

 S , MPa . . . . . . . . . 1 16.30 12.57 14.22 14.10 

 S , MPa . . . . . . . . . 2 5.68 5.44 7.77 3.92 

tnote 3. See text foo 1

Table E-2.—Range of individual doorstopper cell solutions for stress on end of borehole

Doorstopper cell Orientation of S , deg Pa Pa 1 S , M1 S , M2

Minimum Maximum Minimum Maximum Minimum Maximum

Borehole 1:

 3 ............ 171 178 11.97 14.37 !2.07 0.23

 4 ............ 157 162 15.47 17.97 0.63 3.12

 5 ............ 162 169 13.02 16.23 !1.71 1.41

 6 ............ 135 137 5.48 5.54 3.32 3.32

 21 . . . . . . . . . . . !27 18 4.74 7.79 2.30 2.30

 22 . . . . . . . . . . . 9 18 24.93 30.32 3.51 8.72

 23 . . . . . . . . . . . 5 10 6.70 7.13 2.70 3.14

Borehole 2:

 24 . . . . . . . . . . . 128 134 8.28 9.13 2.14 3.03

 25 . . . . . . . . . . . 105 107 11.50 11.83 3.07 3.40

 26 . . . . . . . . . . . 111 114 16.72 18.02 5.11 6.39

 27 . . . . . . . . . . . 144 145 18.59 18.79 6.88 7.07

 28 . . . . . . . . . . . 144 145 10.74 10.89 5.16 5.31

Borehole 3: 

 7 ............ 134 141 12.50 14.17 1.52 3.27

 8 ............ 131 134 28.51 29.65 13.18 14.29

 9 ............ 136 148 11.83 14.78 2.08 4.83

 10 . . . . . . . . . . . 129 154 9.21 11.10 5.69 7.86

 11 . . . . . . . . . . . 73 84 5.39 7.26 -0.43 1.34

Borehole 4:

 12 . . . . . . . . . . . 156 162 9.29 10.61 2.02 3.35

 13 . . . . . . . . . . . 145 173 12.57 19.44 2.12 8.92

INT.BU.OF MINES,PGH.,PA 30129
36