Figure 1.—Plan view showing 3200-level drifts, reef, and nearby

major faults. The Horseman and Lake faults are thrust faults. The

South Prairie Fault is located near the JM Reef on the hanging wall

side, and both are north of the footwall drift of the 3200 level.

Stress measurement of a jointed rock mass during drift development 

J. C. JOHNSON, M. K. LARSON, T. M. BRADY, & J. K. WHYATT

 Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, WA, USA

R. B. LANGSTON, Stillwater Mining Company, Nye, MT, USA

H. KIRSTEN, Steffen Robertson and Kirsten, Johannesburg, South Africa

ABSTRACT: An experiment is described in which hollow inclusion cells (HICells) were used to measure stress in a jointed rock mass

during drift development at the Stillwater Mine, Nye, MT, USA. Two HICells were installed in the footwall approximately two drift

widths ahead of the face. One HICell was overcored and then immediately tested in a biaxial chamber. The other HICell was left in place

and monitored as the drift was excavated in three advances, each one-half the width of the drift. Strains were recorded hourly by a

datalogger. Results show that there was little stress change during the first and second drift advances, but that a large stress change

occurred after the third advance. The measurements also showed that the in situ principal stress directions lined up with regional geologic

structures. Stress changes during face advance were likely caused by deformation along localized joint sets and inelastic deformation near

the face.

1. INTRODUCTION

Safety and health are prime concerns of the mining industry.

Because ground falls continue to be a major contributor to

underground injuries and fatalities across the nation, many mining

companies conduct design and stability analyses to reduce or

minimize these problems. Investigators at the Spokane Research

Laboratory of the National Institute for Occupational Safety and

Health (NIOSH) are studying methods to validate computer models

of a rock mass in order to advance designs for a safer work

environment for miners. One such method is to compare field

stress to calculated stress. 

An experiment was conducted in the Stillwater Mine, Nye, MT,

in which stresses were measured during the advance of the 3200-

level footwall drift. Hollow inclusion stress cells (HICells)

(Mindata, [1976]) were installed in two holes 6.1 m (20 ft) and 7.2

m (23.5 ft) (about two drift widths) ahead of the face at the extreme

eastern end of the drift. In the first hole, a HICell was installed,

overcored, and immediately tested in a portable biaxial chamber on

site to determine the elastic properties of the rock. In the second

hole, which was drilled from a crosscut, a HICell was installed 2.6

m (8.5 ft) from the overcore site to measure stress change during

drift advance. The purpose of this paper is to describe the

instruments, the installation procedure, and the measured stresses.

2. APPARATUS

The HICell1

 was invented by Worotnicki and Walton (1976). In

the 12-gauge version used in this study, three strain gauge rosettes

and three single gauges were embedded at various orientations in

an thin, hollow epoxy shell. 

The HICell was then installed in an EX-size (38.1 mm [1.5 in])

borehole. During installation, a piston forces epoxy from the

center cavity into the space between the instrument and the

borehole wall. The curing reaction is exothermic, resulting in

thermal strains that come to equilibrium before 16 hr has passed.

Although the instrument is known to creep over long periods of

time, creep effects were considered to be small because the

monitoring period for the second HICell was only 1 week.

3. LOCATION

The Stillwater Mine is a platinum-palladium mine located in southcentral Montana about 145 km (90 miles) southwest of Billings,

MT. The test site in the mine lies in the middle of a small, narrow

valley flanked by large mountains. Mining takes place within a

layered igneous body known as the JM Reef in the Stillwater

complex. Figure 1 is a plan view of the major geologic features in

which the 3200 level is projected to the surface. 

Figure 2 is a plan view of the 3200 level. The test area lies near

the eastern end of the footwall drift. Actual installation took place

when the eastern end of the drift was developed only to the

installation area. Figure 3 shows the location of the two HICells.

1

Mention of specific products or manufacturers does not imply

endorsement by the National Institute for Occupational Safety and Health.
Figure 2.—Plan view of 3200 level showing area where

measurements were collected

Figure 3.—Plan view of eastern end of 3200-level footwall drift

showing location of HICell installations, drift advances, and joints.

4. SIGN CONVENTION

In this paper, tensile strain and stress are indicated with a positive

sign, while compressive strain and stress are denoted with a

negative sign. This practice is standard for mechanics, but may be

different in many rock engineering circles. In rock mechanics,

usually the highest compressive principal stress is labeled the major

principal stress. Using the term major to denote the largest

algebraic principal stress may cause confusion. Therefore, in this

paper, the authors avoided using the terms major and minor

principal stress, but labeled them principal stress PS1, PS2, and

PS3 in order of highest to lowest algebraic magnitude.

5. IN SITU STRESS MEASUREMENT

5.1 Procedure

A 152-mm- (6-in-) diameter borehole was drilled to 5.84 m (19 ft,

2 in), and an EX hole was drilled along the same axis for 0.5 m (20

in) more. The HICell was prepared and installed in the EX hole

according to standard installation procedures (Mindata 1976). Final

placement of the strain gauges was 20.3 cm (8 in) from the EX

collar and 6.05 m (19 ft, 10 in) from the drift. The instrument was

allowed to cure for 19-1/2 hr before overcoring operations began.

The HICell was monitored with three strain indicators. A 10-

channel, switch-and-balance box was connected to the first strain

indicator so that readings from all twelve strain gauges could be

recorded. The drill water was turned on and allowed to run for 10

min to reach a temperature equilibrium. During overcore drilling,

gauge readings were taken at 5-cm (2-in) intervals. Once drilling

ceased, the water was allowed to run until readings stabilized. 

Upon removal from the borehole, the overcore was placed in a

biaxial chamber. The chamber was pressurized in -0.69-MPa (-

100-psi) increments up to -6.89 MPa (-1000 psi) for the first

loading cycle. HICell strain gauges were monitored at each load.

A second loading cycle was begun at -3.45 MPa (-500 psi) and

went to -10.3 MPa (-1500) psi. A third and a fourth loading cycle

went from 0 to -10.3 MPa (-1500 psi) in -3.45-MPa (-500-psi)

increments. 

5.2 Results

Figure 4 shows strains as a function of distance between the drill

bit and the centerlines of the strain gauge rosettes. The first three

scans were taken before drilling began, and the last three scans

were taken after drilling ceased. For convenience, these readings

are shown in Figure 4 at distances of ±27.9, ±25.4, and ±22.9 cm

(±11, ±10, and ±9 in) from the centerlines of the rosettes.

Overcoring strain changes were calculated by taking the difference

between initial and final measurements. Initial measurements were

considered those taken after the drilling water had been running for

10 min but before drilling began. “Final” measurements were

considered to be those collected last as readings stabilized. Table

1 lists initial and final strains and changes in strain during

overcoring. A plot of strain versus applied biaxial pressure is

shown in Figure 5. The figure shows linear, isotropic behavior.

Elastic material constants were fit to biaxial test strains using

the least squares method. Ideally, the strains used in this fit should

be those measured at the stress level that most closely reflected the

expected stress state relieved. Because of equipment limitations,

the maximum applied stress level was only -10.3 MPa (-1500 psi),

which was less than the expected overburden stress. Because a

secant modulus was desired, only strains measured at 0 and -10.3

MPa (-1500 psi) were used to calculate biaxial test strain changes.

These values are shown in Table 2.
Figure 4.—Overcoring strains versus distance of drill bit to

centerline of gauges

Figure 5 .—Applied biaxial pressure versus measured strain in

HICell during biaxial compression test

Elastic constants were determined from the biaxial test with a

least squares fit using the computer program STRESsOUT (Larson,

1992) and checked with the computer program STRESS91

(Mindata, 1990). Young’s modulus was calculated as 122 GPa

(17,700,000 psi), and Poisson’s ratio was calculated as 0.31. The

correlation coefficient, r2

, was 0.98.

Stress tensors were fit in a drift coordinate system, as shown in

Figure 6. The borehole azimuth was 2°,and borehole inclination

was 4° above horizontal. 

The stress tensor was fit with the program STRESsOUT and

checked using the code STRESS91. Table 3 lists stress

components calculated in the two coordinate systems. Table 4 lists

principal stresses and directions. The correlation coefficient, r2

,

was 1.00 using all strain changes in the fit. Standard errors of the

stress components ranged from 0.8% to 3.7% of the highest

compressive principal stress. The high correlation coefficient and

low standard errors indicate a good quality measurement and

linear, homogeneous material.

6. STRESS CHANGE MEASUREMENT

6.1 Procedure

The instrument was prepared and installed according to standard

installation procedures (Mindata [1976]); however, standard

installation equipment could not be used because of the small size

of the borehole. Instead, 19-mm (0.75-in) polyvinyl chloride

(PVC) pipe sections were assembled and marked at the collar end

so that the instrument could be properly oriented in the borehole.

The PVC pipe was left in place and wedged at the collar. Mindata

Pre-Formulated Epoxy Glue Pack HIC—0220 was used to bond

the gauge to the wall of the borehole.

The cable wires were connected to a Campbell Scientific

CR10X datalogger via a multiplexer. Monitoring of the gauges

began immediately (July 30, 1998) after they were installed and the

wires connected to the datalogger. Monitoring continued until

early morning of Aug. 7, 1998, when the gauge was destroyed by

blasting.

Table 1.—Initial and final strain measurements and change in strain resulting from overcoring HICell, microstrain

Measurement description Gauge number

1 2 3 4 5 6 7 8 9 10 11 12

Initial readings ..................... 1 7 8 5 7 10 -2 7 3 6 7 7

Final readings . . . . . . . . . . . . . . . . . . . . . 60 228 155 253 283 498 75 130 91 138 330 98

Strain changes . . . . . . . . . . . . . . . . . . . . 59 221 147 248 276 488 77 123 88 132 323 91

Table 2.—Strain measured at 0 MPa (0 psi) and -10.3 MPa (-1500 psi) during biaxial test on overcore, microstrain

Measurement description Gauge number

1 2 3 4 5 6 7 8 9 10 11 12

0 MPa . . . . . . . . . . . . . . . . . . . . . . 1 -4 -6 -4 -6 -5 -7 -2 -7 -3 -2 0

-10.3 MPa . . . . . . . . . . . . . . . . . . . 67 -204 -82 -71 -89 -217 55 -208 -87 -85 -201 -204

Strain changes . . . . . . . . . . . . . . . 66 -200 -76 -67 -83 -212 62 -206 -80 -82 -199 -204
Figure 7 .—Time versus measured HICell strains covering

period of drift advance Figure 6 .—Drift coordinate system shown in plan view with

respect to mine coordinate system and true north

Table 3.—Stress components determined by least squares fit to overcoring strains

Stress component Mine coordinate

system

Drift coordinate

system

MPa psi MPa psi

Normal north-south . . . . . . . . . . . . . . . -22.76 -3301 -18.34 -2660

Normal east-west . . . . . . . . . . . . . . . . -18.46 -2677 -22.88 -3318

Normal vertical . . . . . . . . . . . . . . . . . . -12.12 -1758 -12.12 -1758

Shear north-south/east-west . . . . . . . . -0.717 -104 -0.062 -9

Shear east-west/vertical . . . . . . . . . . . 0.438 64 1.572 228

Shear vertical/north-south . . . . . . . . . . -1.673 -243 0.722 105

Table 4.—Principal stresses calculated from fitted stress components

Stress component Principal stress Mine coordinate system Drift coordinate system

MPa psi

Azimuth,

deg

Inclination,

deg

Azimuth, deg Inclination,

deg

PS1 . . . . . . . . . . . . -11.82 -1714 152.3 79.9 52.3 79.9

PS2 . . . . . . . . . . . . -18.41 -2670 277.1 5.8 177.1 5.8

PS3 . . . . . . . . . . . . -23.11 -3351 8.0 8.3 268.0 8.3

6.2 Strain Results

Figure 7 shows measured strains collected during the monitoring

period. Measurements made during the first 20 hr of monitoring

show response to the exothermic reaction of the glue curing.

Measurements taken during the next 95 hr show a relatively smooth

curve with clear trends and occasional spikes. After hour 128, all

gauges showed similar noise that masked the immediate strain

responses resulting from advancing the drift. It was assumed that

the noise made the same contribution to every measurement in any

one scan. Thus, for any gauge,

mi

 = I  e (1)

where mi

 = measurement of gauge i,

i = strain contribution, 

ande = noise contribution. 

Here, e is the same for each gauge, but changes with time.

On the basis of the data shown in Figure 7, most of the

response of gauge 7 appeared to be noise. Therefore, if gauge 7

measurements are subtracted from measurements from the other

gauges, a preliminary estimate of strain history is obtained (Figure

8). Strain increases imperceptibly during the first advance, but

increases abruptly during the second and third face advances.

Background noise was approximately the same for every gauge.

Figure 8 shows that equation 1 is a reasonable model. The jumps

provide preliminary estimates of strains resulting from each blast.

6.3 Analysis of Strain Results

The measurements shown in Figure 8 contain errors if gauge 7

strains are nonzero. To eliminate that error, Nichols & Johnson

(1998) used two statistical fitting techniques to separate structured

noise from strain signals. These methods were singular value

decomposition (SVD) and principal component analysis (PCA). 
Figure 8.—Time versus measured HICell strains less gauge 7

measurements

Figure 10.—HICell strains over time calculated with PCA to

extract external signal

Figure 9.—HICell strains over time calculated with SVD to

extract external signal

Figures 9 and 10 show the history of strains computed by SVD

and PCA, respectively. The increases in strain immediately after

the first blast are nearly imperceptible, especially in the SVD

computation, whereas the increase in strain after the second blast

is more noticeable. The third increase is considerably larger than

the changes shown following the first two blasts. The PCA curves

are smoother than the SVD curves, which indicates that PCA may

be a better fit method to use with these data.

Table 5 lists the strain jumps associated with each blast as

calculated by SVD and PCA. As stated, there appear to be less

noise in the PCA-fitted strains than in the SVD-fitted strains.

However, when comparing SVD-fitted strains with PCA-fitted

strains, the correlation coefficients were 0.99 for both blasts 2 and

3. This shows that strains computed using both SVD and PCA are

not significantly different from each other.

6.4 Stress Change Calculations

Stress changes were fit by using the least squares method along

with the computer program STRESsOUT (Larson, 1992) and

checked with the program STRESS91 (Mindata, Ltd., 1990). In

this report, positive strain change indicates a decrease in

compressive strain, and positive stress change indicates a decrease

in compressive stress. Elastic material properties used in the

calculations were determined from the aforementioned biaxial

compression test.

Stress change components were fit to strain changes determined

by SVD and PCA for all strain gauges. The correlation

coefficients, r2

, for all fits of SVD- and PCA-determined stress

changes were 0.90 and 0.91. The standard errors of measurement

ranged from 10% to 40% and from 8% to 31% of the highest

principal stress change calculated from SVD- and PCA-determined

strains, respectively. These ranges are not unusual for stress

measurements. The correlation coefficients were high considering

that noise had to be filtered from the measurements. This suggests

that the measurements were of good quality.

6.5 Analysis of Stress Change

6.5.1. Elastic Continuum Model

A three-dimensional, finite-element continuum model was

constructed for the code UTAH3 to assess stresses around a

homogeneous, linear-elastic medium. The drift coordinate system

was selected for constructing the model because it was easy to use.

The mesh was 20.1 m (66 ft) parallel to the drift, 18.3 m (60 ft)

parallel to the face, and 18.3 m (60 ft) in the vertical direction.

Element size was 0.3 m (1 ft) along any edge. The model

contained 237,600 elements, each of which was assigned the in situ

stress state as an initial condition. Elastic modulus was 55.2 GPa

(8 million psi), which had been measured previously using a

Goodman jack. Poisson’s ratio was 0.3. Far-field displacements

were fixed. Excavation of the initial face perturbed the stress state

only slightly (less than or equal to 1.3% of the largest compressive

principal stress) at the approximate location of the HICell. 

In the constructed mesh, the centroid of the element nearest the

HICell was 0.3 m (1 ft) from the north rib line and 1.4 m (4.5 ft)

above the plane of the floor. Distance to the face between the

model and the field location differed by as much as 0.44 m (1.45

ft). While this difference introduced some error between modeled

and measured stresses, the errors were small enough to allow a

useful comparison.

In general, the elastic model showed that as the face advanced,

the stress normal to the face decreased in compression, while

stresses parallel to the face increased in compression.
Figure 11 .—Measured normal stresses parallel and perpendicular to the face at HICell location versus distance to face. Measured stresses were calculated with the in situ measurement and

stress changes calculated with SVD- and PCA-determined strains

Figure 12.—Measured shear stresses in drift coordinate

system at HICell location versus distance to face. Measured

stresses were calculated with the in situ measurement and

stress changes calculated with SVD- and PCA-determined

strains.

Table 5.—Strain jumps calculated by SVD and PCA associated with drift advance 

Gauge no. Singular value decomposition analysis Principal component analysis

Blast 1 Blast 2 Blast 3 Blast 1 Blast 2 Blast 3

1 . . . . . . . . . . . . -0.6 -5.7 -24.4 -0.8 -5.1 -22.6

2 . . . . . . . . . . . . -1.8 -16.1 -69.6 -2.1 -13.1 -57.8

3 . . . . . . . . . . . . -0.6 -5.7 -24.6 -0.6 -3.7 -16.1

4 . . . . . . . . . . . . 1.4 12.4 53.4 2.2 13.8 60.6

5 . . . . . . . . . . . . -0.3 -2.5 -11.0 -0.2 -1.5 -6.8

6 . . . . . . . . . . . . 1.3 12.0 51.8 2.3 14.4 63.5

7 . . . . . . . . . . . . 0.6 5.6 24.1 0.9 5.4 23.6

8 . . . . . . . . . . . . 0.8 7.4 31.8 1.5 9.4 41.2

9 . . . . . . . . . . . . 0.1 1.0 4.1 0.2 1.2 5.3

10 . . . . . . . . . . . -0.9 -7.8 -33.6 -1.1 -7.0 -30.7

11 . . . . . . . . . . . 1.6 14.9 64.3 2.8 17.6 77.3

12 . . . . . . . . . . . 1.0 -9.0 -39.0 -1.1 -6.9 -30.4

6.5.2 Measured Stress Change

The results show that only small amounts of stress change resulted

from the first two face advances. A greater amount of stress change

occurred after the third face advance when the distance from the

face to the instrument was 2.3 m (7.5 ft), or about 70% of the drift

width of 3.3 m (10.8 ft) at that location. During the third face

advance, the largest principal stress change was tensile (reduction

in compressive stress). The orientation of this change was

approximately 30° counterclockwise (in plan view) from the

direction of face advance and inclined about 21° above horizontal,

looking ahead of the face. The intermediate principal stress change

was also tensile and near horizontal. Only the principal stress

change closest to vertical was compressive. The amount of stress

change calculated using SVD strains differed from those calculated

using PCA strains. However, signs and principal stress change

orientations were consistent and remained nearly the same during

all three face advances.

Because the HICell monitored during drift advance was only

2.6 m (8.5 ft) from the overcored HICell and the two instruments

were about two drift widths from the initial face (Figure 3), it was

assumed that the in situ stress measured was a good approximation

of the stress state at both HICell locations. Stress changes resulting

from face advance can be added to in situ stress to approximate the

progression of stress as the face approached the HICell.

Figures 11 and 12 show the amount of normal and shear stresses

in the drift coordinate system as calculated from SVD- and PCAdetermined strains versus distance from the face. Also shown in the

figures are stresses calculated with the elastic model. Measured

horizontal normal stress parallel to the face differs significantly

from that calculated with the elastic model. Moreover, as the face

approaches the instruments, the significance of the trend becomes

opposite that expressed in the elastic model. that is, stress relaxes

instead of being concentrated. Other normal stresses are roughly in

agreement with the elastic model. 

Vertical shear stresses differ from vertical stresses calculated

with the elastic model, and as the face approaches the instruments,

the trend becomes opposite in sign from that in the elastic model.

Horizontal shear stress tracks stress calculated by the model. 
Figure 13.—Deformation patterns and resulting deviations of stress from the elastic model. A, Plan view of end of drift showing

how a shear slip on joint set 1 can cause stress deviations; B, Vertical section through drift, HICell, and joints in set 1 showing how

an inelastic zone near the face can push stress trajectories further ahead of the face.

There are at least two distinct joint sets near the HICell (Figure

3). The first set strikes N30°W (in the mine coordinate system) and

dips 80° to the southwest. The second set strikes S80°E, or roughly

parallel to the drift, and dips 70° to the north. A third joint set

strikes S88°E and dips 23° to the south. However, joints of this set

did not enter the tested rock volume.

The driving mechanisms for the differences between measured

and calculated stresses cannot be determined without an extensive

study of the influence of these joints and the possible existence of

inelastic processes. Even so, it is useful to point out that deviations

are consistent with the deformation patterns shown in Figure 13. 

The first of these patterns is shear deformation along joint set

1, as indicated by relief of compressive stress parallel to the face

and an increase in compressive stress perpendicular to the face (see

Figure 13A). The increase in compressive stress would not be as

much as the increase in tensile stress because of the free face. This

deformation is not likely to be slip being driven by static stress.

The elastic model indicates a mobilized friction angle of only 14°

after the third face advance. Although the friction angle was

measured at this mine, 14° is considered too small to have allowed

slip. Any slip that did occur had to have been driven by blasting.

However, assuming that shear stiffness of the joints was smaller

than stiffness of the intact rock, shear softening could have allowed

enough shear deformation (not slip) to occur and cause stress

deviations.

The second deformation pattern was inferred from differences

between the measured vertical shear stresses and those calculated

with the elastic model. Gravity was the driving force, while some

inelastic vertical deformation near the face (Figure 13B) forced

vertical stress trajectories away from the face, causing increased

vertical shear stresses at the HICell.

7. CONCLUSIONS

In situ stress was successfully measured ahead of the eastern

footwall drift in the Stillwater Mine. The near-vertical principal

stress was not much different from expected overburden stress.

The principal stresses nearest the horizontal plane are closely

aligned with regional geologic structures. More measurements are

needed to determine a more complete picture of the stress field

throughout the mine.

In spite of the presence of an external signal in the data, the

SVD and PCA methods of removing noise produced reasonable

estimates of changes in strain resulting from face advance. Stress

changes derived from these strains calculated using the least

squares method provided reasonable estimates of the stress change

tensor at the point where the HICell had been installed.

Although the stress response to excavation appeared to be in

the elastic range, some components departed from those calculated

for an elastic, homogeneous medium. These departures involved

the direction, as well as magnitude, of the stress change.

Horizontal shear displacement across joint set 1 is consistent with

the difference in horizontal normal stress parallel to the face.

Inelastic vertical strain near the face, perhaps involving slip on

joints that insected the face, is suggested by the increased level of

shear stresses. Modeling studies are planned to investigate the

deformation and its potential impact on ground control safety.

8. ACKNOWLEDGMENTS

The authors express their thanks to Stillwater Mining Company for

its cooperation in this work, including permission to conduct

research tests at the site, instruments, installation tools, and use of

the datalogger. Radford Langston determined site location and
arranged transportation of equipment. Other mine personnel drilled

the borehole. We are also grateful to Dr. W. G. Pariseau,

University of Utah, and Doug Tesarik, Spokane Research

Laboratory, for their technical reviews of this paper, which resulted

in several improvements. Thanks are extended to Ken Strunk for

illustrations and to Priscilla Wopat for editing.

9. REFERENCES

Larson, Mark. 1992. STRESsOUT—A Data Reduction Program

for Inferring Stress State of Rock Having Isotropic Material

Properties: A User’s Manual, U.S. Bur. Mines Inform. Cir.

9302, 168 pp.

Nichols, T., and S. Johnson. 1998. Noise Reduction of Strain

Gauge Data Using SVD, Report SRC-01, 22 pp.

Mindata, Ltd., [1976] CSIRO Hollow Inclusion Stress Cell Field

Manual. Mindata, Ltd., Seaford, Australia, 73 pp.

Mindata, Ltd., STRESS91 Users Manual. 1990. Available from

Mindata Ltd., Seaford, Australia.

Worotnicki, and R. J. Walton. 1976. Triaxial Hollow Inclusion

Gauges for Determination of Rock Stress In Situ, In

Proceedings, ISRM Symposium on Investigation of Stress in

Rock—Advances in Shear Measurement, Sydney,

Supplement, pp. 1-8.