Report of Investigations 9651

Material Properties Affecting the Stability

of a 50-Year-Old Rock Dump in an Active

Mine

D.R. Tesarik and R.W. McKibbin

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

December 1999
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

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

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

Site description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Field-determined material properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Hydraulic conductivity predicted from particle-size distribution ........................................... 6

Downhole water content measurements .............................................................. 8

Laboratory-determined material properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Particle size analysis using image processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Weather station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

ILLUSTRATIONS

1. Rock dump failure caused by water runoff ....................................................... 2

2. Rock dump with 15-m-high vertical benches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Drillhole and infiltrometer sites ............................................................... 4

4. Frame for in situ density tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5. Density frame covered with natural latex rubber sheeting ........................................... 5

6. Density pit lined with polyethylene sheeting and filled with water .................................... 5

7. Infiltrometer .............................................................................. 6

8. Water content versus depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9. Change in water content versus depth since September 16, 1994 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

10. Particle-size distribution curve for site 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

11. Particle-size distribution curve for site 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

12. Particle-size distribution curve for site 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

13. Particle-size distribution curve for site 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

14. Particle-size distribution curve for site 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

15. Particle-size distribution curve for site 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

16. Particle-size distribution curve for site 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

17. Particle-size distribution curve for site 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

18. Particle-size distribution curve for site 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

19. Particle-size distribution curve for site 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

20. Particle-size distribution curve for site 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

21. Particle-size distribution curve for site 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

22. Particle-size distribution curves generated from mechanical and digital image processing sorts . . . . . . . . . . . . . . 18

23. Maximum wind velocity and azimuth versus time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

24. Minimum and maximum temperature versus time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

25. Precipitation versus time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

TABLES

1. Investigations of rock dump stability ........................................................... 3

2. Density, water content, and void ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Results of infiltration tests ................................................................... 6

4. Hydraulic conductivity predicted from particle-size distributions, entire sample .......................... 7

5. Hydraulic conductivity predicted from particle-size distributions, sand fractions ......................... 7

6. Correlation coefficients for calculated hydraulic conductivity and measured infiltration rates ............... 8

7. Degree of saturation at 0.3 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8. Soil classification data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter mm millimeter

deg degree m/s meter per second

g gram m/s2 meter per second squared

kg kilogram m2

/s square meter per second

kg/m3 kilogram per cubic meter pct percent

km kilometer V volt

L liter ° degree

m meter °C degree Celcius

min minute
MATERIAL PROPERTIES AFFECTING THE STABILITY OF A 

50-YEAR-OLD ROCK DUMP IN AN ACTIVE MINE

By D.R. Tesarik1 and R.W. McKibbin2

ABSTRACT

Material properties affecting slope stability were measured in a large 50-year-old, partially consolidated rock

dump located in an active open-pit mine. Field tests included single-ring infiltration and density. In addition,

a nuclear depth-moisture gauge was used to measure water content in six stainless-steel-cased drillholes on the

crest and an upper bench of the rock dump. Precipitation, evaporation, wind speed and direction, and

temperature data were collected at a weather station installed on the dump’s crest. Laboratory tests included

particle-size distribution, specific gravity, Atterberg limits, and water content. By measuring material properties

of a rock dump presumed to be stable, the safety of miners working on or at the toe of old rock dumps

constructed of similar material and located in a similar climate can be assessed.

1

Mechanical engineer, Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, WA. 2

Mining engineer.
2

Figure 1.—Rock dump failure caused by water runoff.

INTRODUCTION

Mine waste dumps are generally located as close to an active

mining site as possible to limit transportation costs. Therefore,

expansion of open-pit mines or the discovery of economically

minable resources near old rock dumps may require partial

excavation of a dump, as occurred in this mine. 

Because haulage roads are sometimes constructed below

dumps, slope stability is of prime importance for the safety of

equipment operators. Factors leading to the instability of

dumps, particularly old ones, include unusually large

precipitation events, gravity sorting of the material during

construction, increased unit weight resulting from soil wetting,

increase in slope angle caused by excavation or erosion, and loss

of material strength caused by weathering. These factors can

lead to dump failures that range from inconsequential slumps to

catastrophic slides traveling great distances (Dunn and others,

1980) (figure 1). 

The slope stability problem can be compounded if the

material in the old dump has become weaker with time or if new

material has been added. Between 1990 and 1996, MSHA

databanks show that 136 haulage vehicles overturned while

dumping because material at the top of the dump collapsed

(Fesak and others, 1996). Slope failure accounted for more

accidents than all other subcategories in the category of “surface

powered haulage.” 

Knowledge of slope stability in this case study was made

even more important because the number of miners working

beneath the old dump slopes was greater than if the dump had

been located away from active mining. Although slope stability

studies may have been conducted prior to dump construction, it

was possible that the dump may not have had the same material

properties as it had 50 years ago.

Researchers have investigated various aspects of mine rock

dump stability (table 1), yet until recently, a broad-based

research program did not exist. To address this problem, the

British Columbia Mine Dump Committee (BCMDC), comprised

of representatives from the Canadian mineral industry; the

Canada Centre for Mineral and Energy Technology

(CANMET); the British Columbia Ministry of Environment,

Lands, and Parks; and the Ministry of Energy, Mines, and

Petroleum Resources (MEMPR), was formed in

1990. Under the auspices of this committee, many topics were

addressed, including rock dump monitoring, failure runout, rock

dump design, evaluation of failures, and instability mechanisms.

In the western United States, the U. S. Forest Service (USFS)

conducted workshops with mining industry representatives,

university researchers, and personnel from other government

agencies to identify rock dump research needs and report on

recent research accomplishments. From surveys conducted at

these meetings, an evaluation of ongoing USFS research,

information obtained from USFS permits for new rock dumps,

and concerns expressed by mining industry engineers, Spokane

Research Laboratory (SRL) researchers determined that the

stability of old rock dumps in the western United States should

be investigated. The objective of this work was to obtain

information on the physical characteristics of rock dumps over

50 years old and arrange this information so that it could be used

by mine operators and land managers to compare the

characteristics of their own dumps. Such a comparison might

allow them to assess the stability of aging rock dumps and make

better decisions about designing new rock dumps based on

specific characteristics, such as rock type, climate, or waste rock

disposal methods.

SITE DESCRIPTION

This research study was conducted at a large, partially

consolidated rock dump constructed of overburden from an

open-pit mine. This dump, constructed prior to 1950, is

composed primarily of highly fractured quartzite waste rock

discarded from railroad trains into a mountain valley. The

angle of repose is approximately 37°. Railroad tracks are not

present on 1901 U.S. Geological Survey (USGS) maps of the

area, but are indicated on a 1950 map. Between 1978 and 1983,

the pit was expanded, and the dump was excavated up to 122 m

horizontally (figure 2). During this expansion, a series of
3

Figure 2.—Rock dump with 15-m-high vertical benches.

Table 1.—Investigations of rock dump stability

 Topic Investigator Date

Design of overburden piles . . . . . . . . . . . Piteau Assoc. (BCMDC) 1991

Rock dump monitoring . . . . . . . . . . . . . . Klohn Leonoff (BCMDC) 1991

HBT AGRA 1992

Failure runout . . . . . . . . . . . . . . . . . . . . Golder Assoc. (BCMDC) 1992, 1994, 1995

Rock dump design . . . . . . . . . . . . . . . . . Piteau Assoc. (BCMDC) 1991

Evaluation of failures . . . . . . . . . . . . . . . Broughton (BCMDC) 1992

Instability mechanisms . . . . . . . . . . . . . . Dawson and others (BCMDC) 1992

Liquefaction flowslides . . . . . . . . . . . . . . CANMET 1994

Dawson and others (BCMDC) 1998

Rock durability . . . . . . . . . . . . . . . . . . . . Olivier 1976, 1979

Vandre 1993

Welsh 1988

Vallejo and Robinson 1992

Particle-size distribution . . . . . . . . . . . . . Mariachi and others 1972

Kemeny 1993

Vukovic and Soro 1992

Material strength ................... Leps 1970

Mariachi and others 1972

Williams and Walker 1985

Hydrology . . . . . . . . . . . . . . . . . . . . . . . . Leps 1973

benches with near-vertical, 15-m-high highwalls were

constructed. These benches provided access to the interior part

of the dump for physical property measurements that otherwise

would not have been possible to obtain. Because of natural

cementation and consolidation over time, minimal sloughing

was observed at the toe of the dump highwalls. 

Measurements of in situ density, infiltration rate, and water

content were taken at the rock dump, and measurements of

particle-size distribution, Atterberg limits, and specific gravity

were obtained from rock samples from the dump. A weather

station was installed on the crest of the dump to collect

information on wind velocity and direction, temperature, and

evaporation; however, precipitation data were not compiled

because of an equipment design error. Instead, precipitation

data were acquired from the weather station at the mine office

approximately 3 km away and 445 m lower in elevation.

FIELD-DETERMINED MATERIAL PROPERTIES

Twelve pits designed to measure in situ density were

excavated on benches with elevations ranging from 2,176 to

2,362 m (figure 3). These pits were located near the crest, the

middle, and the toe of the rock dump to account for variations

in the physical properties of the waste material resulting from

gravity sorting as it was discarded. Procedures described in

American Society for Testing and Materials (ASTM) standard

D 5030-89, “Density of Soil and Rock in Place by the Water

Replacement Method in a Test Pit,” were used. Based on

guidelines presented in the ASTM standard for an estimated

maximum particle diameter of 15 cm (figure 4), researchers

from the Spokane Research Laboratory (SRL) constructed a

91-cm-diam aluminum test frame. An attempt was made to line

the density pits with 0.25-mm-thick natural latex rubber

sheeting. Although the latex sheeting conformed to hole

geometry quite well, angular rocks ripped the material (figures

5 and 6), and 4-mil polyethylene sheeting was used instead. All

material from the density pits was sealed in 19-L plastic buckets

for transport to the soils laboratory at SRL.

Specific weight values ranged from 1,924 to 2,345 kg/m3

(table 2). The average value was 2,156 kg/m3, and the standard

deviation was 120 kg/m3

. Density calculations were also made
4

Figure 3.—Drillhole and infiltrometer sites.

for a control fraction consisting of minus 3.8-cm material. This

size of material was chosen because it was to be used for direct

shear tests in a 35.6-cm-diam cylindrical mold.

Density and water content were recorded at each site using

a Troxler 3400-B nuclear surface-moisture density gauge.1

 The

 1Mention of specific products or manufacturers does not imply endorsement

by the National Institute for Occupational Safety and Health.

density measurements were taken with a cesium-137 source

mounted 5 cm from the end of a source rod. The source rod was

inserted into a vertical drill hole punched into the soil with a

drill rod and hammered to depths from 5 to 31 cm. Water

content measurements were taken with an americium-241 source

rod near the center of the 37- by 23-cm gauge base. Detectors

for attenuated radiation from these nuclear sources are located

near the edge of the gauge base.
5

Figure 4.—Frame for in situ density tests.

 Figure 5.—Density frame covered with natural latex rubber

sheeting.

 Figure 6.—Density pit lined with polyethylene sheeting and filled

with water.

Average density readings from the gauge were approximately 117 kg/m3

, or 5 pct, lower than density readings obtained

using the water replacement method. A linear regression

analysis on these two sets of data, minus the readings from sites

2 and 3, yielded a correlation coefficient of 0.88 and the

following linear equation.

Y = 1.13X - 148.4, (1)

where Y = density, kg/m3

 using the water replacement 

method

and X = density, kg/m3

 using a nuclear surfacemoisture density gauge. 

Table 2.—Density, water content, and void ratio

Site

Specific weight, kg/m3 Water content determined by

oven method, pct

Water content

determined by 

nuclear density

 gauge, pct

Void

ratio

Entire sample Control fraction Entire

sample

 Control fraction

1 .......... 2045 1930 3.44 3.83 5.1 0.37

2 .......... 2058 1937 6.24 6.75 8.8 0.39

3 .......... 2243 1977 4.32 5.03 8.9 0.37

4 .......... 2184 NA 5.41 5.64 8.6 0.29

5 .......... 2184 2102 3.08 3.36 5.4 0.30

6 .......... 2316 2292 4.57 4.90 5.2 0.22

7 .......... 2167 2020 2.24 2.56 4.2 0.38

8 .......... 2345 2293 3.34 3.70 7.0 0.20

9 .......... 2054 1969 4.58 5.02 4.5 0.38

10 . . . . . . . . . 2153 2108 5.36 5.61 6.1 0.35

11 . . . . . . . . . 2203 2147 5.10 5.67 7.2 0.30

12 . . . . . . . . . 1924 1920 9.10 9.79 9.4 0.53

NA. Not applicable.
6

Figure 7.—Infiltrometer.

Sites 2 and 3 were not included in the analysis because they

appeared to be outliers. The correlation coefficient when these

sites were included was 0.19.

Infiltration tests were performed at the bottom of each

density pit (figure 7) except at site 12, where the test was

conducted approximately 10 m northwest of the pit. These tests

were conducted according to ASTM standard D 3385-88, except

that only one infiltrometer ring was used, and changes in water

volume were recorded at time intervals as small as

1 min. A mixture of powdered bentonite and water was packed

around the outside of the infiltrometer ring to prevent water

from seeping under the ring. Water volume flow into the ring

was measured using 3.8-L gradations on a plastic tube plumbed

into a 208-L steel barrel. Infiltration rates for sites 7 and 11

(table 3) could not be determined because water flow into the

rock dump was the same as unrestricted water flow through the

supply hose from the steel barrel. The minimum infiltration rate

at these two sites was 1.47E-3 m/s. 

Table 3.—Results of infiltration tests

Site Infiltration rate, m/s Test duration,

min

1 . . . . . . . . . . . . . 2.94E-4 275

2 . . . . . . . . . . . . . 4.98E-7 1005

3 . . . . . . . . . . . . . 6.55E-5 413

4 . . . . . . . . . . . . . 1.54E-5 234

5 . . . . . . . . . . . . . 2.19E-5 297

6 . . . . . . . . . . . . . 4.53E-6 252

7 ............. (1) NA

8 . . . . . . . . . . . . . 5.82E-5 107

9 . . . . . . . . . . . . . 1.17E-4 235

10 . . . . . . . . . . . . 2.12E-5 1403

11 . . . . . . . . . . . . (1) NA

12 . . . . . . . . . . . . 5.34E-6 1168

NA. Not applicable.

(1) Infiltration rate exceeded 1.47E-3 m/s.

HYDRAULIC CONDUCTIVITY PREDICTED FROM PARTICLE-SIZE DISTRIBUTION

The 10 most often applied empirical formulas for determining hydraulic conductivity from particle-size distribution are

reduced to the following generalized formula by Vukovic and

Soro (1992).

(2)

where K = hydraulic conductivity, m/s,

g = acceleration due to gravity at 9.81 m/s2

< = kinematic coefficient of viscosity, m2/s,

C = dimensionless parameter,

N(n) = empirical function of porosity,

and de = effective grain diameter, m.

Specific values for C and de, expressions for N(n), and valid

particle-size ranges for each of the 10 formulas represented by

equation 2 are supplied by Vukovic and Soro (1992). Although

these formulas were derived from experiments performed with

sand-sized particles, researchers attempted to apply them to the

entire range of particle-size distributions found at the 12 study

sites (table 4), as well as the particle-size distributions for

material passing U.S. Standard No. 4 mesh (4.75 mm) (table 5).

A linear regression analysis was used to compare these

results with infiltration rates (table 6). For these sets of data, the

highest correlation coefficients were calculated using the entire

particle-size distribution curve and formulas reported by Hazen,

Slichter, Terzaghi, Beyer, and Sauerbrei (Vukovic and Soro

1992).
7

Table 4.—Hydraulic conductivity predicted from particle-size distributions, entire sample, meters per second

 (adapted from Vokovic and Soro 1992)

Author Site 1 Site 2 Site 3 Site 4 Site 5 Site 6

Hazen . . . . . . . . . . . . 0.220E-3 0.229E-5 0.407E-4 0.130E-5 0.649E-4 0.146E-4

Slichter . . . . . . . . . . . 0.525E-4 0.522E-6 0.773E-5 0.248E-6 0.126E-4 0.339E-5

Terzaghi . . . . . . . . . . 0.863E-4 0.841E-6 0.968E-5 0.296E-6 0.176E-4 0.282E-5

Beyer . . . . . . . . . . . . 0.744E-4 NA1 0.206E-4 0.321E-6 0.390E-4 0.292E-4

Sauerbrei . . . . . . . . . 0.420E-2 0.758E-5 0.266E-3 0.304E-5 0.310E-4 0.824E-5

Kruegerr . . . . . . . . . . 0.377E-5 0.691E-6 0.219E-5 0.551E-6 0.540E-4 0.143E-4

Kozeny . . . . . . . . . . . 0.726E-6 0.517E-6 0.137E-5 0.922E-7 0.458E-4 0.644E-5

Zunker . . . . . . . . . . . 0.843E-6 0.404E-6 0.124E-5 0.132E-6 0.365E-4 0.693E-5

Zamarinu . . . . . . . . . 0.215E-5 0.586E-6 0.154E-5 0.266E-6 0.440E-4 0.796E-5

USBR . . . . . . . . . . . . 0.271E-1 0.168E-4 0.733E-2 0.167E-4 0.102E-3 0.718E-4

Site 7 Site 8 Site 9 Site 10 Site 11 Site 12

Hazen . . . . . . . . . . . . 0.391E-3 0.771E-5 0.151E-3 0.184E-6 0.143E-5 0.422E-5

Slichter . . . . . . . . . . . 0.825E-4 0.182E-5 0.352E-4 0.384E-7 0.282E-6 0.118E-5

Terzaghi . . . . . . . . . . 0.127E-3 0.149E-5 0.573E-4 0.589E-7 0.403E-6 0.201E-5

Beyer . . . . . . . . . . . . 0.178E-3 0.649E-5 0.441E-4 NA1 NA1 0.119E-5

Sauerbrei . . . . . . . . . 0.768E-2 0.203E-4 0.599E-2 0.957E-5 0.265E-4 0.225E-5

Kruegerr . . . . . . . . . . 0.105E-5 0.423E-6 0.717E-5 0.217E-6 0.750E-6 0.382E-5

Kozeny . . . . . . . . . . . 0.161E-6 0.312E-7 0.476E-5 0.108E-6 0.652E-6 0.349E-5

Zunker . . . . . . . . . . . 0.209E-6 0.590E-7 0.385E-5 0.989E-7 0.511E-6 0.243E-5

Zaamarinu . . . . . . . . 0.538E-6 0.136E-6 0.591E-5 0.158E-6 0.623E-6 0.355E-5

USBR . . . . . . . . . . . . 0.903E-1 0.112E-2 0.671E-1 0.555E-4 0.282E-3 0.105E-5 1

Beyer's equation produced a negative number. 

Table 5.—Hydraulic conductivity predicted from particle-size distributions, sand fraction, meters per second 

(adapted from Vokovic and Soro 1992)

Author Site 1 Site 2 Site 3 Site 4 Site 5 Site 6

Hazen . . . . . . . . . . . . . 0.873E-5 0.234E-7 0.914E-7 0.789E-7 0.123E-4 0.406E-5

Slichter . . . . . . . . . . . . 0.208E-5 0.531E-8 0.174E-7 0.151E-7 0.239E-5 0.946E-6

Terzaghi . . . . . . . . . . . 0.342E-5 0.856E-8 0.218E-7 0.179E-7 0.334E-5 0.786E-6

Beyer . . . . . . . . . . . . . 0.653E-5 0.121E-7 0.877E-7 0.857E-7 0.227E-4 0.216E-4

Sauerbrei . . . . . . . . . . 0.483E-5 0.331E-7 0.196E-6 0.138E-6 0.815E-5 0.251E-5

Kruegerr . . . . . . . . . . . 0.229E-6 0.981E-7 0.163E-6 0.147E-6 0.122E-4 0.358E-5

Kozeny . . . . . . . . . . . . 0.449E-7 0.733E-7 0.101E-6 0.245E-7 0.103E-4 0.161E-5

Zunker . . . . . . . . . . . . 0.519E-7 0.574E-7 0.918E-7 0.351E-7 0.820E-5 0.173E-5

Zamarinu . . . . . . . . . . 0.132E-6 0.832E-7 0.114E-6 0.709E-7 0.989E-5 0.199E-5

USBR . . . . . . . . . . . . . 0.490E-5 0.370E-7 0.610E-6 0.496E-6 0.237E-4 0.161E-4

Site 7 Site 8 Site 9 Site 10 Site 11 Site 12

Hazen . . . . . . . . . . . . . 0.375E-8 0.285E-7 0.310E-6 0.641E-8 0.127E-7 0.243E-5

Slichter . . . . . . . . . . . . 0.791E-9 0.675E-8 0.721E-7 0.134E-8 0.250E-8 0.676E-6

Terzaghi . . . . . . . . . . . 0.122E-8 0.552E-8 0.117E-6 0.206E-8 0.357E-8 0.115E-5

Beyer . . . . . . . . . . . . . NA1 0.522E-7 0.188E-6 NA1 0.390E-8 0.167E-5

Sauerbrei . . . . . . . . . . 0.926E-6 0.434E-6 0.945E-6 0.554E-8 0.758E-8 0.151E-5

Kruegerr . . . . . . . . . . . 0.473E-7 0.446E-7 0.328E-6 0.406E-7 0.850E-7 0.163E-5

Kozeny . . . . . . . . . . . . 0.735E-8 0.329E-8 0.220E-6 0.205E-7 0.739E-7 0.150E-5

Zunker . . . . . . . . . . . . 0.953E-8 0.623E-8 0.177E-6 0.187E-7 0.579E-7 0.104E-5

Zamarinu . . . . . . . . . . 0.244E-7 0.144E-7 0.272E-6 0.297E-7 0.706E-7 0.151E-5

USBR . . . . . . . . . . . . . 0.176E-5 0.241E-5 0.103E-5 0.622E-8 0.919E-8 0.612E-6 1

Beyer's equation produced a negative number.
8

Table 6.—Correlation coefficients for calculated hydraulic conductivity and

measured infiltration rates (adapted from Vokovic and Soro 1992)

Author Entire curve Curve from minus No. 4

mesh

Hazen . . . . . . . . . . . . . . . . . 0.92 0.35

Slichter . . . . . . . . . . . . . . . . 0.93 0.42

Terzaghi . . . . . . . . . . . . . . . 0.93 0.51

Beyer . . . . . . . . . . . . . . . . . 0.81 0.12

Sauerbrei . . . . . . . . . . . . . . 0.75 0.29

Kruegerr . . . . . . . . . . . . . . . 0.14 0.23

Kozeny . . . . . . . . . . . . . . . . 0.17 0.21

Zunker . . . . . . . . . . . . . . . . 0.17 0.22

Zamarinu . . . . . . . . . . . . . . 0.14 0.22

USBR . . . . . . . . . . . . . . . . . 0.58 0.10

DOWNHOLE WATER CONTENT MEASUREMENTS

Measurements of the water content in rock dumps were taken

using a Troxler 4300 nuclear depth-moisture gauge. This gauge

was calibrated for use in stainless steel casings according to the

procedures recommended by the manufacturer. That is, the

gauge was inserted into a section of stainless steel pipe sealed

at one end, and then readings were taken in a barrel of water. 

Six 6.1-m-long holes were drilled vertically into the rock

dump, two on the 2362-m elevation and four on the 2310-m

elevation, using schedule 40, 5-cm nominal diameter, stainless

steel, flush-coupled drill rods and three-wing carbide insert bits

with pin threads. A short stainless steel adapter sub was

fabricated for the bits. The bit, sub, and rod had nearly identical

outside diameters to minimize the annulus between the drill

string and the borehole wall. A small annulus was desirable

because the drill string was left in the hole to serve as a hole

casing for subsequent water content readings. The holes were

advanced using only enough water to suppress dust and disturb

as little of the rock dump material as possible. The annulus was

filled with screened drill cuttings and then capped with a cement

and sand mixture. Water content readings were taken 

approximately 10 weeks after the holes were capped to allow the

drill water to equilibrate. Both the calibration and water content

measurements were performed by a contractor with a Utah state

license to use nuclear devices. 

Downhole water content readings were recorded at 0.3-m

intervals approximately every week for 4 weeks after the

casings were installed and then after significant precipitation

events (figure 8). The readings ranged from 2.6 pct at a depth

of 0.3 m at site 5 on September 16 and 23, 1994, to 11.5 pct at

a depth of 3.7 m at site 6 on June 5, 1995. Changes in water

content ranged from 0 to 4.2 pct (figure 9). The largest increase

occurred at site 1 at a depth of 2.4 m on June 5, 1995, following

a period of rainfall (May 22 to June 4) when rainfall totaled

8.4 cm. In general, changes in water content decreased with

depth up to 1.5 m below the dump surface and remained

relatively constant from 1.5 to 5.8 m. 

The maximum degree of saturation was 81.6 pct at a depth of

0.3 m. This amount was recorded at site 4 on February 7, 1995

(table 7). Values for depths greater than 0.3 m could not be

calculated because void ratios were not available. 
9

 Figure 8.—Water content versus depth. A, September 16 through December 6, 1994; B, February 7 through June 30,

1995.
10

 Figure 9.—Change in water content versus depth since September 16, 1994. A, September 16 through December 6,

1994; B, February 7 through June 30, 1995.
11

Table 7.—Degree of saturation at 0.3 m, percent

Site Sep.

16

Sep. 23 Sep. 30 Oct. 10 Oct. 21 Dec. 6 Feb. 7 May 17 June 5 June 30

1 . . . . . . . . 33.6 33.6 55.5 48.9 53.3 48.9 58.4 58.5 58.5 58.5

2 . . . . . . . . 45.6 44.3 56.7 56.7 62.2 55.3 62.9 61.8 61.8 61.8

3 . . . . . . . . 45.3 44.6 72.3 60.6 66.5 59.2 74.5 67.9 67.9 67.9

4 . . . . . . . . 45.4 40.8 72.3 60.2 67.7 63.0 81.6 72.2 72.2 72.2

5 . . . . . . . . 23.6 23.6 36.3 29.9 35.3 38.1 44.4 57.5 57.5 57.5

6 . . . . . . . . 35.8 35.8 50.5 59.2 76.4 66.6 71.5 79.0 79.0 79.0

LABORATORY-DETERMINED MATERIAL PROPERTIES

Water content of the rock dump material was determined by

using the oven drying method described in ASTM standard D

2216-90. All material collected at each site was weighed before

and after it was dried in an oven. Water contents ranged from

2.2 to 9.1 pct, with an average of 4.7 pct and a standard

deviation of 1.8 pct (table 2). The average value obtained with

the nuclear surface-moisture density gauge was 6.7. Except at

site 9, water contents obtained from the density gauge were

larger than contents obtained by drying the material. The

correlation coefficient for the data collected using these two

methods was 0.67.

After drying, all material was screened over nested meshes

with opening sizes of 76.20, 38.10, 19.05, 9.53, and 4.75 mm

(U.S. Standard No. 4) to determine particle-size distribution

(ASTM standard C 136-93). The minus 4.75-mm fraction was

split repeatedly using a Jones splitter until a sample weighing

approximately 0.5 kg was obtained. Particle-size distribution in

this material was determined using the procedures specified in

ASTM standard D 422. That is, approximately 300 g of the

minus 4.75-mm fraction was washed over a 0.075-mm opening

mesh (U.S. Standard No. 200). The material remaining on the

200 mesh was dried in an oven and screened through a nested

set of sieves with openings of 2.00 (No. 10), 1.18 (No. 16), 0.60

(No. 30), 0.30 (No. 50), 0.15 (No. 100), and 0.075 (No. 200)

mm. At 11 sites, over 90 pct of the material passed the

76.2-mm mesh (figures 10 through 21). The diameter of the

largest rock fragment was 25 cm, but the average diameter of

material retained on the 76.2-mm mesh was only 15 cm.

A Sedigraph 2000 particle-size analyzer was used to

determine the particle-size distribution of the minus No. 140

mesh (0.106-mm) material. Approximately 25 g of soil was

mixed with a solution of 99.9-pct water and 0.1-pct dispersing

agent for analysis.

Following procedures described in ASTM standard D 4318-

84, the liquid and plastic limits of the soil were determined.

These results, along with the coefficients of uniformity and

concavity, plastic index, Unified soil classification, and a

specific gravity of minus No. 140 mesh (0.106-mm) material,

are shown in table 8.

Table 8.—Soil classification data

 Site 1

Cu

2

Cc Liquid limit,

 pct

Plastic limit,

pct

Plastic index,

 pct

Flow index,

 pct

Unified soil

classification

 SG3

1 ........ 107 9.9 21.0 18.3 2.7 -7.1 GP-GC 2.71

2 ........ 700 2.3 24.0 16.0 8.0 -10.7 GC 2.69

3 ........ 167 15.0 23.2 19.4 3.8 -10.6 GP-GC 2.71

4 ........ 335 1.2 20.2 17.9 2.3 -9.2 SC 2.66

5 ........ 105 0.1 NA NA NA NA GP 2.71

6 . . . . . . . . 95 0.2 18.9 NA NA -2.1 GP 2.70

7 ........ 100 11.1 22.0 17.4 4.6 -8.3 GP-GC 2.73

8 ........ 230 14.4 22.2 16.2 6.0 -5.2 GP-GC 2.73

9 ........ 177 24.2 25.3 20.8 4.5 -7.3 GP-GC 2.70

10 . . . . . . . 1030 11.7 23.9 18.2 5.7 -7.6 GC 2.70

11 . . . . . . . 955 15.1 25.5 18.3 7.2 -5.4 GC 2.76

12 . . . . . . . 135 0.2 30.9 25.5 5.4 -9.1 SM 2.73 1

Coefficient of uniformity. 2Coefficient of concavity. 3Specific gravity. NA. Not applicable.
12

Figure 10.—Particle-size distribution curve for site 1.

Figure 11.—Particle-size distribution curve for site 2.
13

Figure 12.—Particle-size distribution curve for site 3.

Figure 13.—Particle-size distribution curve for site 4.
14

Figure 14.—Particle-size distribution curve for site 5.

Figure 15.—Particle-size distribution curve for site 6.
15

Figure 16.—Particle-size distribution curve for site 7.

Figure 17.—Particle-size distribution curve for site 8.
16

Figure 18.—Particle-size distribution curve for site 9.

Figure 19.—Particle-size distribution curve for site 10.
17

Figure 20.—Particle-size distribution curve for site 11.

Figure 21.—Particle-size distribution curve for site 12.
18

 Figure 22.—Particle-size distribution curves produced from mechanical and digital image

processing sorts.

PARTICLE-SIZE ANALYSIS USING IMAGE PROCESSING

Because mechanical sorting of large samples can be time

consuming, a digital image processing program (Kemeny, 1993)

was used to produce particle-size distribution curves from site

7 material. This program incorporates algorithms into the

National Institute of Health’s program “Image.” It delineates

individual particles in the images, applies statistical procedures

that account for overlapping particles and two-dimensional

aspects of the images, and produces a single particle-size

distribution curve from multiple images of the same physical

sample.

Two samples from site 7 were used to compare particle-size

distribution curves obtained from mechanical and digital image

processing. One sample contained 23 kg of minus 38-mm

material, and the other contained only the material from the first

sample retained on a 4.75-mm mesh. The material was spread

out on a flat surface and videotaped with a high-resolution

camera. Selected images from this tape were then imported to

a computer disk for processing.

Two different types of light sources were used to illuminate

the minus 38-mm sample: overhead fluorescent lighting and a

blue artificial light that compensated for daylight film. For each

type of light source, five images taken with different zoom lens

settings on the camera were processed by the computer program.

Although the shape of the distribution curves produced by the

computer program followed the trend of the curve from the

mechanical sort, both methods indicated a higher percentage of

coarse material (figure 22). This discrepancy most likely

occurred because the fines were too
19

Figure 23.—Maximum wind velocity and azimuth versus time.

small to be delineated and “blackened” in the image (Kemeny,

1993). Adjustments in the particle-size distribution curve can

be made by estimating the percentage of the black area of the

image that represents fines instead of shadows or objects placed

in the image for scale. Manually tracing individual particles on

the digital image using a computer mouse increased the

accuracy of the curve produced from photo images, but the

percentage of

coarse particles predicted by the computer

program was still too high. Similar results were obtained when

the material used in these tests was screened with a 4.75-mm

mesh and video-imaged under natural light. The imageprocessing program has been improved recently, and computergenerated particle-size distributions were compared to the

results of on-site mechanical sorting (Girdner and others, 1996).

WEATHER STATION

An automated weather station was erected on the 2362-m

elevation of the rock dump to record air temperature, wind

velocity and direction, evaporation rate, and precipitation. This

information was collected to help interpret soil water content

recorded in the six holes, as well as to provide a baseline for

rock slope stability studies.

An air temperature probe with a solar radiation shield; an R.

M. Young Co. vane anemometer; a Campbell Scientific

measurement and control module with a data storage unit; a

12-V power supply; and a solar panel were mounted on a 3-mhigh tower. The control module, data storage module, and

power supply were enclosed in a weatherproof container.

The collection of data started on July 14, 1994. Anemometer data indicate that maximum wind velocities are lower

in August and September than during the rest of the year.

Average maximum wind velocities of 9.8, 26.5, and 10.0 m/s

were recorded between July 14 and October 9, 1994; October

10, 1994, and July 26, 1995; and July 27 and September 10,

1995, respectively. Gusts of 61.6 m/s were recorded on March

10 and 21, 1995, and gusts of 73.2 m/s were recorded on June

5, 1995. Maximum wind velocity directions generally had

azimuths of 150° to 300o

, indicating that the wind rarely blew

from the northeast (figure 23). Temperature data were consistent with seasonal temperature cycles (figure 24), with a

maximum recorded on August 5, 1994 (29 °C), and a minimum

on December 31, 1994 (-19 °C).

A raised platform was constructed adjacent to the tower for

a Novalynx evaporation pan and all-season precipitation gauge.

Data from these two instruments were also collected by the

Campbell Scientific datalogger on the tower. Data collection

started on August 30, 1994; however, a design error in the

precipitation gauge resulted in erroneous values. A replacement

sensor was installed in late July 1995, but observed rainfall was

not always recorded, resulting in insufficient data for

calculating evaporation. Readings from the rain gauge installed

near the mine office at an elevation of 1917 m indicated that

significant precipitation events occurred in the period between

March through July 1995 (figure 25). 
20

Figure 24.—Minimum and maximum temperature versus time.

Figure 25.—Precipitation versus time.
21

SUMMARY AND CONCLUSIONS

Material properties of a rock dump constructed of mine

overburden at least 50 years ago were measured using field and

laboratory techniques. Field measurements included in situ

density, surface water content, single-ring infiltrometer, and

downhole water content. Instruments installed at a weather

station near the dump's crest collected wind, temperature,

precipitation, and evaporation data. The rationale for conducting this work was to correlate the performance of an old

rock dump with its material properties. This information could

be used to evaluate the stability of other old rock dumps.

Soil water content measurements were taken with a Campbell

Pacific nuclear depth-moisture gauge in 6.1-m-deep, vertical,

stainless-steel-cased holes. These measurements indicated that

water content decreased with depth up to 1.5 m below the

dump’s surface and remained relatively constant from 1.5 to 5.8

m. Despite numerous daily precipitation events in which

rainfall exceeded 15 mm, the maximum degree of saturation at

a depth of 0.3 m in the rock dump was 82 pct. The condition of

a partially saturated surface, a relatively constant water content

in the dump’s interior, and the self-cementing

nature of the dump material may be the reasons why the dump

has remained stable for over 50 years. 

Researchers determined that a nuclear surface-moisture

density gauge could be used to measure surface density

provided that the gauge was calibrated with measurements

obtained by the water replacement method. However, the

average water content of the dump material measured by the

gauge exceeded values obtained from oven drying by 40 pct,

and correlation coefficient of data obtained from these two

measurement techniques was only 0.67.

This work was intended to identify those physical properties

and climatic conditions common to old, stable rock dumps.

Similar investigations at failed rock dumps could identify

differences in material properties between stable and unstable

dumps that are critical to slope stability. With this knowledge,

safer working practices could be developed. Future research

could also include direct shear tests to determine material

strength, slope stability analyses to calculate factors of safety,

and investigations of the physical processes associated with

natural cementation observed at this site. 

ACKNOWLEDGMENTS

The authors wish to express their appreciation to the mine

management and engineering staff involved in this project for

suggestions in the experimental design, access to the research

area, construction and maintenance of the weather station,

contracting with the drilling company to drill and case the

access holes for water content measurements, and providing the

water truck for the infiltration and in situ density tests. Bill M.

Stewart and Ronald R. Backer, mining engineers at the Spokane

Research Laboratory, provided assistance with the field tests.

Terrance J. McMahon, mining engineer, SRL, retired, assisted

with the laboratory tests; Bruce C. Vandre, geotechnical

engineer, USFS, retired, helped to identify research opportunities; and John M. Kemeny, associate professor, Dept. of

Mining and Geological Engineering, University of Arizona,

provided the digital image processing software and assistance in

its use. 

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DHHS (NIOSH) Publication No. 2000-110.

December 1999

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