In Situ Stress Measurements at the Stillwater Mine, Nye, Montana

Jeffrey Johnson and Thomas Brady

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

Mary MacLaughlin 

University of Montana at Butte, MT

Radford Langston

Stillwater Mining Co., Nye, MT

Hendrik Kirsten

Fontainebleau, South Africa

ABSTRACT

The magnitudes and directions of in situ stresses affect the stability of mine openings, as well as the type and amount

of ground support needed to maintain a safe working environment for miners. Using hollow inclusion stress cells,

researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health

obtained two in situ stress measurements from the face of two footwall lateral drifts at the Stillwater Mine near Nye,

MT. The first measurement, obtained in 1997, was collected under the valley beneath the Stillwater River. The second

measurement was obtained in 2002 under the main frontal massif of the Beartooth Mountains in the western sector of

the mine. Although few data are available, the major principal stress under the valley has been recorded as nearly

horizontal in a north-south direction perpendicular to the ore body, while beneath the mountains, stress runs east-west

parallel to the ore body. This paper documents the measurements, describes the in situ stress state, and discusses a

finite-difference analysis of the biaxial test. 

1. INTRODUCTION

Because ground falls continue to be a major

contributor to underground injuries and fatalities in

the mining industry, researchers at the Spokane

Research Laboratory of the National Institute of

Occupational Safety and Health and associates are

studying the conditions that control ground falls.

One condition that influences excavation stability is

in situ stress. To obtain measurements of in situ

stress, hollow inclusion cells (HI-Cells) developed

by Australia’s Council for Scientific Investigation

and Research Organization (CSIRO) were installed

at two locations at the Stillwater Mine.

The Stillwater Mine, the largest primary producer of palladium and platinum in the Western

hemisphere, is situated in the Stillwater igneous

complex in south-central Montana, USA. The Stillwater Complex is a 42-km- (26-mile-) long, layered,

stratiform, mafic-to-ultramafic igneous intrusion

approximately 2.7 billion years old that hosts a
Figure 1—Plan view of surface structures at the Stillwater Mine and location of stress

measurement sites. Dashed lines show J-M Reef and South Prairie fault projected

underground.

palladium- and platinum-rich layer known as the JM Reef. Currently, production is primarily accomplished using variations on horizontal cut-and-fill

mining, but longhole stoping practices are playing

an increasingly important role.

Originally deposited almost horizontally, the

complex was tilted to the north during the Eocene

Laramide Orogeny. This event resulted in the strike

of the J-M Reef being approximately 290° with a

dip from 45° to 70° north. The predominant orientation of the faults is parallel with strike. A large

reverse fault lies from 0 to 91 m (300 ft) stratigraphically above the hanging wall of the J-M Reef

and is known as the South Prairie fault. Two other

large thrust faults (the Lake and the Horseman)

have the same strike but opposing dip angles to the

ore zone. The Lake fault lies about 915 m (3,000 ft)

below the J-M Reef while the Horseman fault lies

approximately 1,040 m (3,400 ft) above it. Doleritic

or diabasic mafic dikes cut the Reef and typically

strike northwest and dip about 35° to 70° southeast.

Several strike-slip faults lie perpendicular to the

Reef and dip steeply east or west (Page and Zientek, 1985). Figure 1 is a plan view of the J-M Reef,

the faults, and the Stillwater River, as well as the

two in situ stress measurement sites. Figure 2 shows

the location of the two sites in vertical section. The

first overcore was done on September 11, 1997,

under the valley 515 m (1,700 ft) beneath the Stillwater River (Johnson et al., 1999). At the second

site, located under the plateau of the Beartooth

Mountains and west of the western wall of the Stillwater valley, two successive overcores were taken

on April 5 and 6, 2002, at a depth of approximately

850 m (2,800 ft) below the surface. 

2. IN SITU STRESS MODELS

Before the measurements are described, it is

useful to predict in situ stress using overburden

pressures and the Sheorey equation. 

Hoek et al. (1998) review the current models

used to estimate in situ stress. Hoek reports that

although there is significant amount of scatter in

vertical stress measurements taken around the

world, vertical stress can be estimated using overburden stress. Overburden stress is the product of

the depth and the unit weight of the rock. At the
Figure 2.—Vertical section of Stillwater Mine showing location of stress measurement sites.

Stillwater Mine, the specific gravity of the rock is

about 2.8, giving a density of about 2,800 kg/m3

(175 lb/ft3) and producing a vertical stress gradient

of about 0.03 MPa/m (1.2 psi/ft). An estimate of

vertical stress at a depth of 518 m (1,700 ft) under

the valley is 14.4 MPa (2,090 psi). Under the mountain, at a depth of 850 m (2,800 ft), overburden

vertical stress is 23.8 MPa (3,450 psi). 

Models for horizontal stress are usually expressed as a ratio of average horizontal to vertical stress,

denoted as “k.” Hoek et al. (1998) present the

Sheorey equation for computing k as—

k = 0.25 + 7E(0.001 + 1/z)

where E = elastic field modulus, GPa, 

and z = depth, m.

Johnson et al. (1999) report an elastic modulus

of the rock of 55 GPa (8 million psi). k then equals

1.3, giving an average horizontal stress under the

valley of 18.7 MPa (2,700 psi). The computed

Sheorey value under the mountain is 1.1, giving an

average horizontal stress of 26.2 MPa (3,800 psi).

3. TEST DATA 

Azimuth and dip, as well as position in a

coordinate system, are required to orient the

direction of the stresses. At the Stillwater Mine, a

mine grid coordinate system is used that takes

advantage of the general east-west trend of the ore

body by defining east and west as being parallel to

the ore body and equal to the Montana state plane

azimuth minus 20° (State of Montana, 2001). The

mine grid coordinate system is thus aligned with the

J-M Reef, allowing stresses to be presented

orthogonally to the reef. The mine grid azimuths for

the valley and mountain sites are 100° and 90°,

respectively, as measured as a positive angle east of

north. Dip is defined as a positive angle measured

downward from the horizontal. The borehole dip

angle at both sites was -5° to allow for water

drainage. 

The standard procedure for installing HI-Cells

was followed (Geokon, 1976). Overcoring strain

changes were determined by taking the difference

between initial and final measurements. The initial

strains are approximately zero. Picking final strains

is a bit of an art. The manual indicates that for a

normal overcore, the final strain readings should

become stable or flat and show little change in strain

with additional overcoring distance. Figure 3A, 3C,

and 3E show overcoring strains as a function of

distance for each HI-Cell. Generally, each HI-Cell

plot shows a peak strain followed by a fairly flat

portion that deceases toward the end of the

overcore. The reason for the decrease in strain near

the end of the overcore is not known, although each

HI-Cell was equipped with a 120-ohm precision

resistor that showed only a small variation (about

0.1 ohm) in resistivity change during overcoring,

indicating that temperature effects were not likely.

The final strains for the valley and the first

mountain overcore were obtained by averaging the

readings in the flat portion of the curve. The final

readings for the second mountain overcore were

taken as the peak strains. Table 1 lists the strain

changes from overcoring for the three cells at the

two sites.

The elastic properties of the overcore were

determined from a biaxial chamber test in which a

radial pressure of 6.9 MPa (1,000 psi) was applied

to the core, and the strains were recorded as listed in

table 2. The biaxial test is extremely important

because it uniquely determines the elastic properties

of the system, including core, epoxy, and cell, that

unlock in situ stress from overcoring strains. This

test should be performed immediately after each

overcore. Plots of strain-versus-pressure are shown

in Figure 3B, 3D, and 3F. Each figure shows linear,
Figure 3.—Overcoring (A, C, E) and biaxial (B, D, F) strains for the valley (top row) and mountain sites 1

(middle row) and 2 (bottom row). 

Table 1. Change in strain from overcoring HICell, microstrain.

ID G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12

Valley . 57 275 177 304 333 580 80 142 90 159 394 96

Mtn1 . . 741 1400 712 752 433 462 389 995 746 535 661 1190

Mtn2 . . 135 760 806 202 1080 1030 100 93 155 180 486 320

Table 2. Averaged change in strain from cyclic biaxial chamber loading to 6.90, MPa.

ID G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12

Valley . . 44 -131 -49 -45 -55 -140 40 -134 -55 -55 -132 -134

Mtn 1 . . 120 -428 -100 -106 -102 -385 79 -297 -132 -109 -355 -259

Mtn 2 . . 56 -177 -71 -54 -59 -211 61 -196 -69 -37 -192 -196
Figure 4.—Calculated strains from finite-difference analysis

using measured elastic rock moduli from the biaxial tests and

varying epoxy elastic moduli by three orders of magnitude.

Table 3. Elastic properties calculated from each biaxial test.

ID Young’s modulus Poisson’s ratio

Valley . . . . . . . . . . . . . . . . 124.8 GPa (18.1 Mpsi) 0.31

Mountain 1 . . . . . . . . . . . . . 48.3 GPa (7.0 Mpsi) 0.29

Mountain 2 . . . . . . . . . . . . . 86.2 GPa (12.5 Mpsi) 0.30

isotropic behavior. The program, STRESS91 (RST

Instruments, 1991), was used to calculate material

properties from the averaged circumferential and

axial strains at 6.9 MPa (1,000 psi) for each overcore (table 3). Although each biaxial test was linear

and repeatable, an extremely large variation can be

seen in the elastic modulus of the overcores. As a

check on the elastic modulus of the rock, a standard

compression specimen was prepared from the EX

core from the mountain site and mounted with strain

gages and tested. The results of the compression test

gave an elastic modulus of 96.6 GPa (14.0 Mpsi). 

Because the J-M Reef is composed of similar

rock, it was very surprising to see such a wide

variation in elastic moduli. Researchers considered

whether the epoxy might have been the source of

such variation and investigated this possibility with

a numerical model. Thus, as an independent check

of the elastic parameters calculated from the biaxial

test strains using the STRESS91 program, a finitedifference analysis was done using Itasca’s FLAC

software (Itasca, 2002). The model was a simple

two-dimensional axisymmetric geometry of the biaxial test consisting of an EX hole, HI-Cell, epoxy,

and rock. The elastic properties of the HI-Cell are a

Young’s modulus of 2.6 GPa (377,000 psi) and a

Poisson’s ratio of 0.4 (RST Instruments, 2000). The

Poisson’s ratio of the rock and the epoxy is 0.3. The

Young’s modulus of the rock (Erock) was varied

between the values of 125 GPa (18.1 million psi)

from valley biaxial test, 96.6 GPa (14 million psi)

from an unconfined EX specimen compression test,

86.2 GPa (12.5 million psi) from the second mountain biaxial test, and 50 GPa (145,000 psi) from the

first mountain biaxial test. The shear moduli of the

epoxy (Gepoxy) were varied between 1 MPa and 1

GPa (145 and 145,000 psi) (Biehl, 2000). Young's

modulus of the epoxy was calculated by FLAC from

the shear moduli.

The results of the analyses are shown in Figure

4. The data points connected by lines depict the

strains calculated using FLAC for the different

values of Erock and Gepoxy chosen for the models. The

separate data points on the right side of the graph

are the actual measured axial and circumferential

strains recorded from the biaxial tests in the field.

The results showed very little sensitivity to Gepoxy

and Eepoxy over three orders of magnitude and generally good agreement between the model results and

the measured data.
Figure 5.—Plan view of mountain stress site showing location of overcores and structural

geology.

4. RESULTS

With the known stress orientation, elastic

properties calculated from biaxial chamber tests,

and strain changes obtained from overcoring, the in

situ stress field can be calculated. It is interesting

that such a large difference in the magnitude of

Young’s modulus between the two overcores under

the mountain did not produce a large difference in

calculated in situ stress. Table 4 lists the mine grid

components of stress from each overcore. All the in

situ stresses reported in this paper used all 12 strain

gages of the HI-Cell to compute the standard error,

which is defined as the standard deviation of the

computed stress about the regression line.

The two mountain overcores do not show reproducibility. Since the overcore measurements were

taken, the footwall drift was extended an additional

12 m (40 ft), allowing detailed mapping of the faults

and joints in the drift (Figure 5). According to the

map, the overcores were close to a 5- to 8-cm- (2- to

3-in) wide, gouge-filled fault. The higher shear

stress and odd overcoring plot of the second overcore in the mountains appears to be affected by this

fault, but the effect has a limited range of influence

since the overcores were separated by only 0.6 m (2

ft).

A useful way to examine the two stress sites is

to normalize the stress. This is a common practice

since most in situ stress theories express horizontal

stress in terms of vertical stress. Table 5 lists the

mine grid stress ratios. If the second overcore at the

mountain site is excluded because of the influence

of the fault, then from the valley to the mountains,

the east-west horizontal stress ratio is nearly constant at 1.5 times vertical stress, but varies from 1.9

to 0.8 for the north-south stress. 

At the valley site, the measured vertical stress of

14.7 MPa (2,130 psi) is 2% higher than the

calculated overburden stress of 14.4 MPa (2,090

psi). Under the mountains, the average measured

vertical stress of 25.3 MPa (3,700 psi) is 6% higher

than the calculated overburden stress of 23.8 Pa

(3,450 psi). The ratio of measured average horizontal stress at site 1 gives a value of k of 1.7. The

computed k, assuming an elastic modulus of the

rock of 55 GPa (8 million psi), gives a k value of

1.3, which is about 25% less than the measured

average horizontal stress ratios. At the second site,

the computed k value is 1.1, and the average

measured ratio 1.2.

The principal in situ stresses were also

calculated by the program, and the results are shown

in Table 6.

Orientation of the major principal stress under

the valley is almost due north-south, or perpendicular to the ore body, the intermediate principal stress

is east-west, and the minor principal stress is ver-
Table 4. Mine grid in situ stress and standard error in parenthesis, MPa.

ID Sns Sew Svert Sns/ew Sew/vert Svert/ns

Valley . . . . 28.0 (0.3) 21.4 (0.8) 14.7 (0.3) 1.1 (0.3) -0.8 (0.3) 2.3 (0.2)

Mountain 1 . 20.4 (2.4) 38.5 (6.2) 28.4 (2.4) 2.1 (2.5) -2.7 (2.8) 0.7 (1.4)

Mountain 2 . 35.4 (0.8) 26.8 (2.1) 22.2 (0.8) -11.4 (0.8) -5.2 (0.9) 8.7 (0.5)

Table 5. Stress ratios of mine grid components of stress.

Location East-west:north-south East-west:vertical North-south:vertical

Valley . . . . . . . . . . . 0.8 1.5 1.9

Mountain 1 . . . . . . . . 1.9 1.4 0.7

Mountain 2 . . . . . . . . 0.8 1.0 1.3

Table 6. Principal stress from each overcore, MPa.

ID S1 (azimuth, dip) S2 (azimuth, dip) S3 (azimuth, dip)

Valley . . . . . . . . . . 28.5 (8, 9) 21.4 (276, 9) 14.2 (323, -78)

Mountain 1 . . . . . . . 39.4 (264, 14) 27.9 (235, -75) 20.0 (353, -7)

Mountain 2 . . . . . . . 24.2 (146, -14) 15.2 (109, 73) 10.9 (234, 10)

tical. At the mountain site, the first overcore shows

the orientation of the major principal stress as eastwest or parallel to the ore body. However, both

measurements under the mountain show that intermediate principal stress is ±15° from vertical. The

dip angle of the ore body is about 60° under the

valley and about 45° under the mountains and does

not appear to influence the direction of the principal

stresses.

5. CONCLUSIONS

The orientation of the principal stresses seems to

be controlled by the general east-west structure of

the mine. Vertical stress is well approximated by the

overburden stress gradient of 0.03 MPa/m (1.2

psi/ft). The east-west horizontal stress parallel to the

strike of the ore body appears to be a constant value

about 1.5 times vertical stress. The north-south horizontal stress perpendicular to the ore body appears

to be a variable value about 1.9 times vertical stress

under the valley and 0.7 under the mountain.

It is recommended that in situ stress

measurements be continued in the Stillwater Mine

to establish a database on the distribution of stresses

in the mine. Future stress measurements should be

made to determine the effect of depth and topography on north-south stress.

Large variations in the elastic modulus obtained

from the biaxial tests do not seem to affect the

magnitudes of in situ stresses. A finite-difference

model of a biaxial test showed little sensitivity to

the elastic modulus of the epoxy and consequently

indicated that the HI-Cell is a relatively robust

instrument for determining in situ stress. 

ACKNOWLEDGMENTS

Special thanks goes to the Stillwater Mining

Company for allowing access to the mine. Svante

Andersson tested his newly developed “termite”

portable core drill. Spokane Research Laboratory

personnel Paul Pierce and Mike Jones performed the

drilling, Dale Avery conducted geologic mapping,

and Mark Larsen wrote the FLAC fish functions to

calculate strains. Montana Tech students, Kat Clapp,

Kathy Miller, and Russell Sheets, helped with

overcoring and biaxial testing.

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