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8/12/2019 ARMA-10-165_Relationship Between Physical, Chemical, And Mineralogical Properties and Cohesion of Questa Roc
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ARMA 10-165
Relationship between Physical, Chemical, and Mineralogical
Properties and Cohesion of Questa Rock Pile Materials
Boakye, K.
Geotechnical Engineer, Knight Pisold and Co. 1580 Lincoln St., Suite 1000, Denver, CO, USA & Department
Mineral Engineering, New Mexico Tech, Socorro, NM, USA
Fakhimi , A.Professor, Department of Mineral Engineering, New Mexico Tech, Socorro, NM USA and Department of Civil
Engineering, University of Tarbiat Moderres, Tehran, Iran
McLemore , V. T.Senior Economic Geologist, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NMUSA
Copyright 2010 ARMA, American Rock Mechanics Association
This paper was prepared for presentation at the 44thUS Rock Mechanics Symposium and 5
thU.S.-Canada Rock Mechanics Symposium, held in
Salt Lake City, UT June 2730, 2010.
This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical reviewof the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the writtenconsent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not becopied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.
ABSTRACT:A modified in-situ direct shear test apparatus consisting of 30 cm and 60 cm metal shear boxes was designed anused to determine cohesion and internal friction angle of the Questa Rock Piles and natural analog materials. The main differenc
between the in-situ shear box and the conventional laboratory one is that this in-situ shear box is constructed of a single box thaconfines the prepared soil block. The lower half of the soil block is made of the earth material underneath the shear plane that is
semi-infinite domain. Tests were performed on the materials close to the surface of the rock piles and natural analog material
using normal stresses between 10 to 100 kPa to simulate the overburden stresses. Results indicate that cohesion shows a sligh
negative correlation with water content and a slight positive correlation with matric suction. The mineralogy and chemistry of the
rock-pile and analog materials have little or no correlation with cohesion, which suggests that no single mineral or chemicaelement affects cohesion within the rock-pile and analog materials. The evidence of cohesion in the Questa rock piles is due to th
presence of clay pockets within the rock piles, jarosite, gypsum, Fe-oxide cementing minerals, and soluble efflorescent salts
matric suction and interlocking of grains.
1. INTRODUCTION
Mine rock piles deposited at their angle of repose by
crest end-dumping have intrinsic stability at the time of
placement. The stability conditions can change with time
as a result of time-dependent changes in the strengthalong potential failure surfaces and the forces such as
pore water pressure acting on these potential failure
surfaces [1]. The heterogeneity of these rock piles and
their coarse nature makes it difficult to determine theirshear strength in-situ and in laboratory experiments.
Shear strength is variable in the rock piles as a result of
variations in grading, compaction density, rock type and
mineralogy, stress, and weathering characteristics. The
nature of rock piles requires large testing equipment totest representative samples containing large fragments.
So far, only a limited number of in-situ shear tests have
been conducted on rock piles worldwide. Using
laboratory test methods to determine the shear strength
parameters of rock piles is more traditional compared to
in-situ testing because the laboratory tests are less
expensive and easier to perform. Even with very
sophisticated techniques for simulating in-situconditions, sample disturbance is difficult to eliminate,
which cause variations in laboratory results as compared
to in-situ testing results. With the best sampling
technique, it is practically impossible to prevent sample
disturbance when collecting for laboratory shear tests,
especially in rock piles that contain large boulders and
rock fragments. The exact amount of disturbance that a
sample undergoes is difficult to quantify. Nevertheless,
most studies of the shear strength of rock piles involve
the use of conventional laboratory analysis performed on
disturbed samples. These laboratory tests are considered
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standard engineering practice for design purposes, but
they do not always take into account the existence of
cohesion within the rock pile. In fact cohesion is usually
considered to be zero for laboratory direct shear testing.
Yet cohesion can affect the overall stability of rock piles.
For example, previous studies have identified the
influence of microstructures such as cementation
increasing the shear strength [2, 3, and 4].
In order to evaluate the effect of cohesion on the slopestability of rock piles and to allow larger particles to be
included in the tests with no or little sample disturbance,
a modified in-situ shear testing apparatus was developed
and implemented. The in-situ shear tests performed in
this project are similar to the methods used by Fakhimi
et al. [5]. They used in-situ shear tests on soil material in
a tunnel in Tehran where the reaction of the normal force
was transferred to the tunnel roof. Subsequent sections
of this paper give oversight of the design, methodology,
and results for the modified in-situ shear tests performed
during this research work.
In addition to in-situ shear testing, laboratory shear tests
were conducted on the disturbed dry samples. The
laboratory friction angle results were used to obtain the
cohesion values from in situ shear tests.
This paper also presents an investigation and conclusion
on the effect of physical parameters, mineralogy, and
chemistry on the cohesion measured using the modified
in-situ direct shear test device. The investigation
involves correlating the results of index parameters,
mineralogy, and chemistry of the rock pile and analogs
material with the cohesion values.
2. BACKGROUND
2.1. Questa MineThe Questa molybdenum mine is located in a region
with a long history of mining 5.6 km (3.5 miles) east of
the village of Questa in Taos County, north central New
Mexico (Figure 1). The mine is on the south-facing slope
of the north side of the Red River valley between an
east-west trending ridgeline of the Sangre de Cristo
Mountains and State Highway 38 adjacent to the Red
River at elevation 2,438 m (8,000 ft) [6]. Mining startedin 1914 when molybdenum was first discovered in the
area. The mine encompasses three main tributary
valleys; from east to west they are Capulin Canyon,
Goathill Gulch, and Sulphur Gulch [7].
During the period of open pit mining (1969-1981), a
tremendous amount of mine rock material was mined.
This material was placed in nine valley-fill rock piles
using end dumping methods.
Fig. 1 Questa rock piles and other mine features, including
location of in-situ test sites (red circles). Test site
identification numbers are listed in Table 3.
The piles, including Sugar Shack South, Middle and Old
Sulphur (or Sulphur Gulch South), were deposited along
the sides of the mountain ridges and within and along
narrow mountain drainages, ultimately forming largerock piles along State Highway 38. These rock piles also
are referred to as the Front Rock Piles or Roadside Rock
Piles and are on the west-facing slope of the mountain.
Capulin, Goathill North, and Goathill South rock piles
are on the west-facing mountain slope on the west side
of the open pit. On the east side of the pit, the Spring
Gulch and Blind Gulch/Sulphur Gulch North rock piles
are located. The rocks piles are characterized by heights
extending nearly vertical from the Red River at
elevations from 2,440 m (8005 ft) to 2,930m (9613 ft),
making them some of the highest mine rock piles in
North America [7]. Additionally, these rock piles haveshallow depths. This combination results in movement
of air and moisture through the piles affecting their long
term oxidation, acid mine drainage, and slope stability.
The rock piles have an average slope of 36 to 38.
The Questa climate is semi-arid with mild, dry summers
and cold, wet winters. The mine is located in an area of
high relief with a complex distribution pattern of
precipitation and net infiltration. As a result of the
difference in snow pack at different elevations, there is a
general trend of increasing net infiltration with
increasing elevation.
The geological history of the mine area is characterized
by hydrothermal alteration as explained in detail in
descriptions of the geology of the district [8, 9]. The
basement beneath the mine consists predominately of
Tertiary volcanic rocks, granitic and gabbroic intrusive
rocks, and Precambrian schists, quartzites, and
metamorphosed ocean floor volcanics. Outcrops of
andesite flows overlain by rhyolitic welded ash flow
tuffs (approximately 26 Ma) can be seen along the
ridgeline at the crest of the hydrothermal alteration scars
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2.3. Shear Strength of Mine Rock Piles and In-Situ Direct Shear Test
between the Red River and Cabresto Canyon to the north
[8]. These geological features are important since they
can affect the shear strength of overlying rock piles
when subjected to weathering. The rate of weathering is
controlled by precipitation and the mineralogy of the
material making up the rock piles and analogs. The
change in mineralogy and chemistry due to weathering
subsequently can contribute to the presence of cohesion
within the rock piles and analogs. The evidence of
cohesion in the Questa rock piles is due to the presenceof clay pockets within the rock piles, jarosite, gypsum,
Fe-oxide cementing minerals, and soluble efflorescent
salts, matric suction and interlocking of grains.
The strength of soil is mostly defined by its shear
strength. Shear strength of soils is the resistance of the
soil to failure under applied shear force. McCarthy [15]
refers to soil stability as being governed by its strength,
durability, permeability and volume changes, but
especially by its shear strength. The shear strength of a
soil can be expressed by the modified Mohr-Coulomb
failure criterion as follows [16]
bwaa uuuc tan)(tan( ) (1)
where u2.2. Description of the Questa Mine Analogs a is the net normal stress, ua is the pore-airpressure, c is the effective cohesion, u -uA project hypothesis was established during the study of
the rock piles that the alteration scar areas and debris
flows around the mine site could serve as mineralogical
and physical proxies or analogs to long-term weathering
of the rock piles. Weathering processes operating in the
natural analogs share many similarities to those in the
rock pile, although certain aspects of the physical andchemical systems are different. Alteration scars are
natural, colorful (red to yellow to orange to brown),
unstable landforms that are characterized by steep slopes(greater than 25 degrees), moderate to high pyrite
content (typically greater than 1 percent), little or no
vegetation, and extensively fractured bedrock [10]. The
Goathill debris flow is formed by sedimentation due to
transportation of landslide rock material within thealteration scar by water and gravity. These features were
formed thousands to millions of years ago and have been
exposed to weathering conditions similar to those
affecting the rock piles today and in the future. Thealteration scars and debris flows represent weathered
rocks that are similar to the materials in the rock piles
[11, 12, 13 and 14] because weathering process
operating in the internal analogs share many similarities
to those in the rock piles. The spectrum of isotopic ages
determined, thus far, indicate that weathering in the
alteration scars has been active for at least 4.5 million
years [13]. High altitude scars (e.g. the Questa Pit Scar
and upper Goat Hill Scar, ~4.5 million years old) are
older than lower elevation scars (e.g. Southwest Hansen,
~300,000 years old). Charcoal in a pond deposit near the
top of the Goat Hill debris flow produced a calibratedcarbon isotope age of 4917 years BP [15]. These
reported ages represent maximum ages. Therefore, the
alteration scars and associated debris flows represent
long-term weathered analogs (1000 years to 4.5 million
years) for the material in the rock piles. By
characterizing and establishing the geotechnical
parameters of these analogs, their mineralogy and
chemistry, future changes of geotechnical properties of
the rock piles could be predicted. Comparison of the
analogs to the rock piles is in Table 1.
a wis the matric
suction, bis the angle indicating the rate of increase in
shear strength relative to the matric suction, and is the
friction angle. Based on equation (1), the cohesion thatincludes the effect of matric suction is obtained as
follows:
bwa uucc tan)( (2)
Shear strength parameters are usually determined by
performing laboratory direct shear or triaxial tests on soil
samples. The rock-pile materials in the Questa rock piles
contain small to very large rock fragments. Therefore,
the conventional tests with a small shear box might not
provide the true shear strength of the rock pile material
because of scalping of the larger size fraction.
Furthermore, intact, undisturbed samples are difficult to
collect that will be truly representative of the rock-pilematerial. Previous studies of the shear strength of the
Questa rock piles concentrated mainly on laboratorytesting of disturbed samples with 5, 10, and 30 cm (2, 4,
and 12 inches) shear boxes to determine internal friction
angle [17, 18, and 19]. In order to allow larger particles
to be included in the tests, a modified in-situ shear
testing was developed and implemented.
In-situ direct-shear tests to obtain more realistic field
data are not new in studying landslides, but have not
been widely employed to examine the gravitational
stability of rock piles. Brand et al. [20] described an in-situdirect-shear machine used on residual soils in Hong
Kong. Marsland [21] described a field test apparatus
involving a shear box and the application of a normal
load by a ballast tank. Endo and Tsurata [22] used an in-
situshear box to shear soil that was strengthened by tree
roots. Some other in-situshear tests have been reported
by [5, 24, and 25].
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Table 1. Comparison of the different weathering environments in the rock piles and analog sites in the Questa area. QSP=quartz-
sericite-pyrite. SP=poorly-graded sand, GP=poorly-graded gravel, SM=silty sand, SC=clayey sand, GW=well-graded gravel,
GC=clayey gravel, GP-GC=poorly-graded gravel with clay, GP-GM=poorly-graded gravel with silt, GW-GC=well-graded gravelwith clay, SW-SC=well-graded sand with clay, SP-SC=poorly-graded sand with clay.
Feature Rock Pile Alteration Scar Debris Flow
Rock types Andesite
Rhyolite
Aplite Porphyry Intrusion
Andesite
Rhyolite
Aplite Porphyry Intrusion
Andesite
Rhyolite
Aplite Porphyry Intrusion
Unified soilclassification
(USCS)
GP-GC, GC, GP-GM, GW,GW-GC, SP-SC, SC, SW-SC,
SM
GP-GC, GP GP, SP, GP-GC
% fines 0.2-46Mean 7.5 Std Dev. 6
No of Samples=89
0.6-20
Mean 5.2 Std Dev. 4
No of Samples=18
0.3-6Mean 1.8 Std Dev. 2 No of
Samples=12
Water content (%) 1-24
Mean 10 Std Dev. 4
No of Samples=390
1-20
Mean 9 Std Dev. 4
No of Samples=48
1-29
Mean 5 Std Dev. 4
No of Samples=36
Paste pH 1.6-9.9Mean 4.8 std dev 1.9
No of samples=1368
2.0-8.3Mean 4.3 std dev 1.6
No of samples=215
2.0-6.9Mean 4.5 std dev 1.3
No of samples=58
Pyrite content (%) Low to high
0-14%(mean 1.0%; std dev. 1.2%,
No of samples=1098)
Low to high
0-11%(mean 0.7%, std dev 1.8%,
No of samples=62)
Low to medium
0-0.2%(mean 0.03%, std dev 0.06%,
No of samples=22)
Dry density kg/m3 1400-2400
Mean 1800 Std Dev. 140
No of Samples=153
1500-2300
Mean 1900 Std Dev. 210
No of Samples=13
1300-2200
Mean 1900 Std Dev. 340 No
of Samples=10
Particle shape Angular to subangular to
subrounded
Subangular Subangular to subrounded
Plasticity Index (%) 0.2-20
Mean 10 Std Dev. 5No of Samples=134
5-25
Mean 12 Std Dev. 5No of Samples=30
3-14
Mean 7 Std Dev. 3No of Samples=18
Degree of chemical
cementation (visualobservation)
Low to moderate (sulfates, Iron
oxides)
Moderate to high
(sulfates, Iron oxides)
Moderate to high
(sulfates, Iron oxides)
Slake durability
index (%)
80.9-99.5
Mean 96.6 Std Dev. 3.1
No of Samples=120
64.5-98.5
Mean 89.2 Std Dev. 9.2
No of Sample=24
96.1-99.6
Mean 98.4 Std Dev. 0.9
No of Samples=18
Point load strength
index (MPa)
0.6-8.2
Mean 3.8 Std Dev. 1.7
No of Samples=59
1.7-3.8
Mean 2.8 Std Dev. 0.8
No of Samples=4
2.6-6
Mean 4 Std Dev. 1
No of Samples=12
Peak friction angle
(degrees), 2-inch
shear box (NMIMTdata)
35.3-49.3
Mean 42.2 Std Dev. 2.9 No of
Samples=99
33.4-54.3
Mean 40.7 Std Dev. 4.8 No of
Samples=22
39.2-50.1
Mean 44.3 Std Dev. 3.9 No
of Samples=12
Average cohesion
(kPa), in-situ shear
tests
0-25.9
Mean 9.6 Std dev 7.3
No of samples=20
12.1-23.9
Mean 18.1
No of samples=2
31.4-46.1
Mean 38.8
No of samples=2
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The in-situ shear tests performed in this project are
similar to the methods used by Fakhimi et al. [5]
conducted on soil material in a tunnel where the
reaction of the normal force was transferred to the
tunnel roof. However, the situation at the Questa mine
was more challenging because of difficulty in applying
the normal force to the shear block. At Questa, it was
decided to use the bucket attached to an excavator to
carry the reaction of the normal load.
3. LARGE SCALE IN-SITU DIRECT SHEARTESTS
3.1. Design of ApparatusThe apparatus consists of a 30 cm or 60 cm square
metal shear box, a metal top plate, a fabricated roller
plate, normal and shear dial gages with wooden
supports, and two hydraulic jacks with cylinders
having a maximum oil pressure of 69 MPa (10,000 psi)
or a load capacity of 10 metric tons (Figure 2). A
conventional laboratory shear test apparatus typically
consist of upper and lower boxes that move relative to
each other. The shear plane is the boundary between
the two boxes. However, the in-situ direct shear box
designed for this project consists of only one box,
which confines the entire excavated rectangular soil
block. The shear plane in the in-situ test set up is the
boundary between the soil block and the unexcavated
material beneath the block that behaves like a semi-
infinite domain. This innovation allows for easier and
faster site preparation. Additionally, this technique can
accommodate a large shear displacement without any
reduction in the surface area of the shear plane during
the test. Further details about this box, its accessories,
and the procedures employed to obtain an undisturbed
rock pile block can be found in Fakhimi et al [26].
Fig. 2. Set-up of in-situ test using the bucket of an excavatorto support the hydraulic cylinder.
3.2. Test Location and Sample DescriptionIn-situ test locations were selected based on geologic
characteristics, personnel safety factors, and easy
accessibility for equipment. Test sites of varying
degrees of weathering (as determined using the Simple
Weathering Index (SWI), petrographic analysis and
other indications of weathering) and cohesion were
selected in the rock piles (Figure 1), Questa Pit
alteration scar (QPS in Figure 1), and Goat Hill debris
flow (MIN in Figure 1). The SWI is a simple,
descriptive weathering index classification tool
developed for the Questa material that consists of five
classes (Table 2) and is based on relative intensity ofboth physical and chemical weathering of the matrix,
modified in part from [27, 28 and 29]. Even though the
simple weathering index introduced in this study is not
a precise tool in evaluating the weathering intensity
(because of the overlapping hydrothermal alteration
and fine-grained nature of the soil matrix), it is
relatively simple and can be readily used in the field.
Blocks of material were excavated as described herein
to perform the in-situ shear tests. Samples were
collected along the shear plane for geological and
geotechnical characterization. The collected samples
consisted of a mixture of rock fragments ranging insize from boulders (0.5 m) to
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Table 2. Simple weathering index for rock pile material (including rock fragments and matrix) at the Questa mine [32].
SWI Name Description
1 Fresh Original gray and dark brown to dark gray colors of igneous rocks; unaltered pyrite (if present); calcite,
chlorite, and epidote common in some hydrothermally altered samples. Primary igneous textures preserved.
2 Least weathered Unaltered to slightly altered pyrite; gray and dark brown; very angular to angular rock fragments; presenceof chlorite, epidote and calcite, although these minerals not required. Primary igneous textures still partiallypreserved.
3 Moderately weathered Pyrite altered (tarnished and oxidized), light brown to dark orange to gray; more clay- and silt-size material;
presence of altered chlorite, epidote and calcite, but these minerals not required. Primary igneous textures
rarely preserved.
4 Weathered Pyrite very altered (tarnished, oxidized, and pitted); Fe hydroxides and oxides present; light brown to yellow
to orange; no calcite, chlorite, or epidote except possibly within center of rock fragments (but the absence ofthese minerals does not indicate this index), more clay-size material. Primary igneous textures obscured.
5 Highly weathered No pyrite remaining; Fe hydroxides and oxides, shades of yellow and red typical; more clay minerals; no
calcite, chlorite, or epidote (but the absence of these minerals does not indicate this index).
Table 3. Description of the lithology and texture of rock pile and analog material (including rock fragments within a soil matrix) at
the in-situ test locations. QSP (quartz-sericite-pyrite) or phyllic alteration is alteration assemblage defined by the predominance of
quartz, sericite and pyrite. Propylitic alteration consists of essential chlorite (producing the green color), epidote, albite, pyrite,quartz, carbonate minerals, and a variety of additional minerals depending upon host rock lithology, temperature, and composition
of the fluids [31]. Locations of sample sites are shown in Figure 1.
Test id Sample id SWI LithologyOriginal magmatic
texture
Hydrothermal
alteration and
intensity
Indications of Weathering
MID-VTM-
0002-1
(Middle Rockpile)
MID-VTM-0002
(MID2, Figure 1)4
100%
andesite, traceintrusion
textures visible,
moderate feldsparreplacement
QSP: 40%Iron oxide present, skeletal feldspar
crystals, rounded pyrite grains
MIN-AAF-0001-1
(Goat Hill debrisflow)
MIN-AAF-0001
(MIN, Figure 1)3
98% intrusive,
2% rhyolite
tuff
texture still visible
but slightly
overprinted by
hydrothermaltexture
QSP: 50% Iron oxide present
MIN-AAF-
0012-1
(Goat Hill debris
flow)
MIN-AAF-0013(MIN, Figure 1)
4
100%
andesite, trace
intrusion
texture visible,
moderate-heavy
feldspar replacement
QSP: 55%Iron oxide present, skeletal feldsparcrystals
QPS-AAF-0001-
3(Questa Pit scar)
QPS-AAF-0005
(QSP, Figure 1)4
100%
andesite, traceintrusion
texture visible,
limited feldsparreplacement
QSP: 25%
Propyllitic: 5%
Iron oxide present, authigenic gypsum
present, skeletal feldspar crystals
QPS-VTM-
0001-1 (Questa
Pit scar)
QPS-VTM-0001
(QSP, Figure 1)5 100% andesite
texture visible,
moderate-heavy
feldspar replacement
QSP: 55%
Propyllitic: 1%
Iron oxide present, authigenic gypsum
present
SPR-AAF-0001-1
(Blind Gulch
rock pile)
SPR-AAF-0001
(SPR1, Figure 1)2 100% andesite textures visible Propyllitic: 25%
Iron oxide present, authigenic gypsum
present
SPR-AAF-0001-
2
(Blind Gulch
rock pile)
SPR-AAF-0003
(SPR1, Figure 1)2 100% andesite
texture visible,
moderate feldspar
replacement
QSP: 45%
Propyllitic: 3%
Iron oxide present, authigenic gypsum
present, skeletal feldspar crystals
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Fig. 3. Backscattered electron microprobe image of least
weathered (SWI=2) large silicified andesite rock fragments
with quartz, jarosite, and goethite in hydrothermal-clay-rich
matrix (sample SSS-VTM-0600 from in situ test id SSS-VTM-0600-1, Table 3). There is minor cementation of the
rock and mineral fragments by pre-mining, hydrothermal
clay and gypsum. Pyrite grains (bright white cubes and
euhedral crystals) are relatively fresh.
Fig. 4. Backscattered electron microprobe image of
weathered (SWI=4), hydrothermally-altered rhyolite rockfragments cemented by jarosite, iron oxide, and
hydrothermal-clay minerals (sample QPS-AAF-0005 from in
situ test id QPS-AAF-0001-3, Table 3). Relict pyrite (point22) has been oxidized to iron oxides.
3.3. Testing ProgramIn-situ tests were performed on both rock piles andtheir natural weathering analogs. The in-situ tests were
performed close to the rock-pile surfaces; the depth of
the shear planes were within 1 to 4 meters from the
surface. A total of 52 in-situ shear tests were conducted
[30]. The applied normal stress for the in-situ tests
ranges from 15 to 70 kPa. For low normal stresses,
dead weight was used while the high normal loads
were applied through a hydraulic cylinder (Figure 2).
The reaction of the hydraulic cylinder was transferred
to the bucket of an excavator through a roller plate to
prevent any induced shear resistance. This range of
normal stresses was lower than the overburden
pressure to prevent consolidation of samples and loss
of material cementation due to large vertical
deformation. One dial gauge was used to measure
shear displacement, while two dial gauges attached to
the lateral sides of the top platen were used to measure
normal displacement of the rock pile block. The shear
load is gradually increased. The hydraulic jack loads
and dial gauges were read after each 0.51 mm (20/1000inch) of shear displacement. The average shear
displacement rate was approximately 0.025 mm/s.
Each in-situ shear test was normally continued for a
shear displacement of 7.5 cm. Each test takes
approximately 3 hours to excavate and set up and
approximately 1 hour to run. Measurements of matric
suction and soil temperature were taken at the shear
plane following the appropriate standard operating
procedures. Representative samples were selected for
Atterberg limits, specific gravity, and disturbed
laboratory direct-shear tests, plus moisture content,
particle size, mineralogical, chemical, and petrographicanalyses.
After each in-situ shear test, the shear plane wasinspected for the maximum particle size. Particle size
analyses were performed in the laboratory on the
samples that were collected from the in-situ sites. The
samples were classified based on the Unified Soil
Classification System (USCS).
Laboratory direct shear tests on the air-dried samples
were conducted using a 2-inch shear box. The tests
were performed on rock-pile materials collected fromthe in-situ direct shear test shear planes at each in-situ
direct shear test location. This was to make sure that
the conventional laboratory direct shear tests were
performed on the same materials as tested in-situ. The
collected rock-pile materials passed through the sieve
#6 was used for the laboratory direct shear tests. Each
specimen was compacted to the field dry density
before testing. The normal stress varied between 19 to
110 kPa for the laboratory tests. The laboratory direct
shear tests were performed in accordance with ASTM
[33].
4. TESTS RESULTS
Figure 5 shows the grain size distribution curves for
the materials tested using the in-situ direct shear box.
Particle size analyses were performed in the laboratory
on the rock-pile samples that were collected from the
in-situ tests locations. Atterberg limit tests were
performed on the rock-pile materials to determine the
plasticity of the materials tested. Sieve analysis on the
rock-pile material indicates that these materials consist
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of 32% to 80% gravel, 16% to 67% sand, and 1% to
15% fines. These values correspond relatively well
with those of [24, 25, and 26]. The amount of fine
material corresponding to different rock piles and
natural analogs is shown in Figure 5. Notice that due to
the fact that a small amount of the material is made of
fine particles, the shear resistance is mostly controlled
by the sand and gravel particles. The plasticity indexes
range from 19 to 40 for liquid limit, 13 to 32 for plastic
limit, and 0.2 to 19 for plasticity index. The plasticityindices indicate that the material at Questa mine has
low plasticity. The rock-pile materials at the locations
of in-situ shear tests were classified as GP-GM to SP-
SC based on the Unified Soil Classification System
(USCS). The rock-pile materials are not saturated. The
water content in these materials range from 1 to 29%
(See Table 1). At any location of the in-situ shear test,
the density of the material was measured using the
sand replacement technique. The dry density in these
materials range from 1300 to 2400 kg/m3 (See Table
1).
Particle Size Distribution
0
10
20
30
40
50
60
70
80
90
100
0.0010.010.11101001000Grain Size, mm
PercentPassingbyWeight
COBBLES GRAVEL SANDSILT CLAY
BOULDERS
Coars Fin Coarse M ed ium F in e
Hydrometer3/3 41.5 1 103/4 16 30 40 5 60 200100
U.S. Standard Sieve Numbers
2 6
SPRING GULCH ROCKPILES
SUGAR SHACK WEST ROCK PILES
SUGAR SHACK SOUTH ROCK PILES
MIDDLE ROCK PILE
ALTERATION SCAR
DEBRIS FLOW
Fig. 5. This graph shows the range of grain size distribution
for samples from the in-situ tested sites.
The main purpose for performing the in-situ shear tests
was to measure the rock-pile cohesion to investigate
the intensity of cementation between particles.Therefore, a few shear tests at different locations in
some rock piles were conducted with identical low
normal stresses. In order to obtain the cohesion from a
single in-situ shear test, the normal stress and the peak
shear stress of an in-situ shear test were used together
with the friction angle from the laboratory shear tests.Substitution of these normal stress, shear stress and
friction angle in the Mohr-Coulomb shear strength
equation results in the cohesion value. The laboratory
friction angles were obtained by conducting direct
shear tests on dry specimens compacted to the in-situ
dry density using low normal stresses in the range of
20 to 110 kPa as described herein. Typical shear stress-
shear displacement curves and the corresponding
Mohr-Coulomb failure envelope for sample SSS-AAF-
0001-1 are shown in Figures 6a and 6b, respectively.
Detailed laboratory and in situ shear tests plots are
reported in [30]. For the intent of this paper, the
estimated cohesion values will be correlated with
physical, mineralogy and chemistry of the rock pile
material to see what parameter controls the cohesion
existing within the rock piles.
SSS-AAF-0001-1
0
50
100
150
200
0 2 4 6 8 10 1
Shear displacement (mm)
Shearstress(kPa)
2
Normal stress =101kPa
Normal stress = 71kPaNormal stress = 52kPa
Normal stress = 22kPa
(a)
y = 1.15 x + 29.75
R2= 0.9757
0
50
100
150
200
0 50 100
Normal stress (kPa)
Shearstress(kPa)
150
(b)Fig. 6. Laboratory direct shear test results for sample SSS-
AAF-0001-1, (a) shear stress vs. shear displacement, (b)
Mohr-Coulomb failure envelope.
Based on the results of direct shear tests on rock-pile
material with an oversize particle [34], only the results
of in situ tests where the maximum particle size was
less than 1/5 the width of the shear box were
considered valid and used. Using this criterion, only 24
in-situ shear test results remained for further analysis;
the remaining 28 tests were not acceptable (Table 4).
The cohesion of the Questa material range between 0
to 46.1 kPa.
Table 5 and 6 show detail results of mineralogy and
chemistry of in-situ samples collected from the shear
plane of each individual test, respectively. More detailsabout these tests can be found in [30].
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Table 4. Selected geotechnical parameters of rock pile and analog material, and the descriptive statistics of field cohesion values.
Cohesion
(kPa)
Rock piles and Analogs Test id
Matric
Suction
(kPa)
Fine (%)PI
(%)USCS SWI
Field
Cohesion
(kPa)
No.
of
Tests Mean STD
Sugar Shack South Rock Pile SSS-VTM-0600-1 1 6.9 8.7 GP-GC 2 1.9
Spring Gulch Rock Pile SPR-AAF-0001-1 10 1.3 1.4 GP 2 8.1
Spring Gulch Rock Pile SPR-AAF-0001-2 9 0.6 9.5 GP 2 12.8
3 7.6 5.5
Sugar Shack South Rock Pile SSS-AAF-0001-1 1 1.8 7.3 GP 3 6.7
Sugar Shack South Rock Pile SSS-AAF-0005-1 9 1.4 4.7 SP 3 17.2
Sugar Shack South Rock Pile SSS-AAF-0009-1 0 2.0 10.5 GP 3 2.0
Sugar Shack West Rock Pile SSW-AAF-0004-1 n/a 13.6 14.2 GP-GC 3 8.7
Sugar Shack West Rock Pile SSW-VTM-0026-1 13 0.7 7.1 SP 3 0.3
Sugar Shack West Rock Pile SSW-VTM-0030-1 3 0.7 7.1 GP 3 12.2
Debris Flow MIN-AAF-0001-1 25 3.6 3.6 GP 3 31.4
7 11.2 10.6
Middle Rock Pile MID-VTM-0002-1 1 1.0 1.9 GP 4 0.5
Debris Flow MIN-AAF-0012-1 31 0.7 8.9 GP 4 46.1
Sugar Shack West Rock Pile SSW-AAF-0005-1 5 2.9 8.2 GP 4 25.9
Sugar Shack West Rock Pile SSW-AAF-0007-1 9 0.6 7.2 SP 4 13.2
Sugar Shack West Rock Pile SSW-VTM-0600-1 n/a 0.2 7.7 GP 4 19.3
Sugar Shack West Rock Pile SSW-VTM-0600-2 n/a 1.5 6.2 GP 4 13.6
Sugar Shack West Rock Pile SSW-VTM-0600-3 n/a 13.6 9.2 GC 4 2.2
Spring Gulch Rock Pile SPR-VTM-0012-1 2 8.4 2.5GP-GM
4 12.7
Spring Gulch Rock Pile SPR-VTM-0012-2 0 6.7 6.2 GP-GC 4 4.0
Spring Gulch Rock Pile SPR-VTM-0012-3 0 7.9 4.3 GP-GC 4 0.0
Spring Gulch Rock Pile SPR-VTM-0019-1 5 10.0 7.3 SP-SC 4 14.5
Spring Gulch Rock Pile SPR-VTM-0019-2 2 8.5 6.9 GP-GC 4 16.8
Questa Pit Alteration Scar QPS-AAF-0001-3 0 6.8 16.4 GP-GC 4 23.9
13 14.8 12.6
Questa Pit Alteration Scar QPS-VTM-0001-1 11 4.2 5.3 GP 5 12.1 1
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Table 5. Selected mineralogical compositions (in percent) of rock pile and analog material
In situ sample In situ test idCohesion
(kPa)Quartz
K-
spar/orthoclasePlagioclase Illite Chlorite Smectite Kaolinte
MID-VTM-0002 MID-VTM-0002-1 0.45 48 17 2 20 1 2 1
MIN-AAF-0001 MIN-AAF-0001-1 31.39 46 13 3 23 8 2
MIN-AAF-0013 MIN-AAF-0012-1 46.11 50 19 4 13 1 10
QPS-AAF-0005 QPS-AAF-0001-3 23.90 30 10 16 12 0 21 2
QPS-VTM-0001 QPS-VTM-0001-1 12.1 33 12 16 25 3 3 1
SPR-AAF-0001 SPR-AAF-0001-1 8.10 24 16 28 2 7 12 1
SPR-AAF-0003 SPR-AAF-0001-2 12.77 22 18 27 2 5 16 1
SPR-VTM-0012 SPR-VTM-0012-1 12.68 56 11 0.8 26 2 2
SPR-VTM-0014 SPR-VTM-0012-2 3.96
SPR-VTM-0017 SPR-VTM-0012-3 0.00 49 18 24 5 2
SPR-VTM-0019 SPR-VTM-0019-1 14.51 52 21 20 3 2
SPR-VTM-0021 SPR-VTM-0019-2 16.80 51 22 20 4 2
SSS-AAF-0001 SSS-AAF-0001-1 6.73 28 18 7 15 3 19 3
SSS-AAF-0005 SSS-AAF-0005-1 17.20 35 4 5 17 1 29 1
SSS-AAF-0009 SSS-AAF-0009-1 2.01 47 15 1 23 7 1
SSS-VTM-0600 SSS-VTM-0600-1 1.87 36 17 13 18 2 1 7
SSW-AAF-1009 SSW-AAF-0004-1 8.65 30 16 16 5 2 2
SSW-AAF-0005 SSW-AAF-0005-1 25.86 33 11 18 25 0.4 3 1
SSW-AAF-0007 SSW-AAF-0007-1 13.16 29 26 8 2 1 21 4
SSW-VTM-0600 SSW-VTM-0600-1 19.29 56 3 1 17 1 14
SSW-VTM-0001 SSW-VTM-0600-2 13.64 44 29 2 2 16
SSW-VTM-0004 SSW-VTM-0600-3 2.23 49 10 25 6 1
SSW-VTM-0027 SSW-VTM-0026-1 0.32 30 16 8 23 6 2 2
SSW-VTM-0030 SSW-VTM-0030-1 12.18 31 8 13 23 5 2 1
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5. DISCUSION OF RESULTS
The main purpose for performing in-situ direct shear
tests was to measure the rock-pile cohesion in order to
investigate the intensity of cementation between
particles. To understand the effect of physical
properties of the rock piles and analogs on the
measured cohesion, correlation between cohesion of
rock piles and analogs materials with index parameters
were investigated. The index parameters investigated
are water content, dry density, liquid limit, plastic
limit, plasticity index, percent gravel, percent sand,
percent fine and matric suction.
The influence of plasticity index on the cohesionwas investigated and is shown in Figure 7 which
suggests no significant correlation between cohesionand plasticity index.
Figure 8 shows a plot of cohesion versus dry density.There is no correlation between dry density and
cohesion. This indicates that the cohesion measured in
the field is not a result of gravitational compaction of
the material alone. In general, gravitational compaction
of materials can have some influence on the cohesionbut other controlling factors areinvolved as well. Notealso that the in-situ tests were conducted at shallow
depths within the rock piles where the compaction
effects are minimal.
0
5
10
15
20
25
30
35
40
45
50
0.0 5.0 10.0 15.0 20.0
Plasticity Index (%)
Cohesion(kPa)
Fig. 7. Correlation between cohesion and plasticity index.
0
5
10
15
20
25
30
35
40
45
50
1000 1500 2000 2500 3000
Dry Density (kg/cm3)
Cohesion(kPa)
Fig. 8. Correlation of cohesion with dry density.
0
5
10
15
20
25
30
35
40
45
50
0.00 5.00 10.00 15.00
% Fines
Cohesion(kPa
)
Fig. 9. Correlation of cohesion with % fines.
The influence of percent fines on the cohesion wasinvestigated and is shown in Figure 9, which suggests
little correlation between cohesion and percent fines.
The lower cohesion values tend to correlate with
higher %fines, but some samples with lower cohesion
also have low %fines.
To see the correlation between the measured cohesion
and matric suction, a plot of the two parameters was
generated (Figure 10). Figure 10 shows a weak positive
correlation between cohesion and matric suction. This
can indicate that the measured cohesion is only partly
due to the existing negative pore water pressure withinthe rock pile and analog samples. Figure 11 shows a
plot of cohesion vs. water content. This plot shows a
slight negative correlation between cohesion and water
content that is consistent with the plot in Figure 10.
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0
5
10
15
20
25
30
35
40
45
50
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Matric Suction (kPa)
Cohesion(kPa)
Fig. 10. Correlation of cohesion with matric suction.
0
5
10
15
20
25
30
35
40
45
50
4.0 6.0 8.0 10.0 12.0 14.0 16.0
Water Content (%)
Cohesion(kPa)
Fig.11. Correlation of cohesion with water content.
Coduto [35] indicates that cementation by cementing
agents, electrostatic and electromagnetic attraction
hold soil particles together, and adhesion that occurs in
overconsolidated clays are the prime indicators of
existing cohesion. Cementing agents that exists within
Questa rock piles and analogs are gypsum, jarosite,
iron oxides, and pre-existing clay minerals. These
existing clays are hydrothermal clays and are not
weathered clays [36]. To understand the effect the
mineralogy and chemistry on cohesion, several plots of
correlations between cohesion and mineralogy and
chemistry were generated.
Figure 12 shows little correlation between cohesion
and percent gypsum. Low cohesion values correspond
with low gypsum values but not all low cohesion
values correspond with low gypsum values which
support the observation that cohesion existing within
the rock piles and analogs is not controlled by only one
mineral. Figure 13 show no correlation between sulfate
and cohesion.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 0.5 1 1.5 2 2.5 3 3.5
Gypsum
Cohesion(
kPa)
4
Fig. 12. Correlation of cohesion with percent gypsum. Note
that some samples with low amounts of gypsum have highcohesion, but not all.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 0.5 1.0 1.5 2.0
SO4
Cohesion(kPa)
Fig. 13. Correlation of cohesion with SO4 (in percent).
Figure 14 shows little correlation between cohesion
and percent Authigenic gypsum, which also supports
the observation that cohesion existing within the rock
piles and analogs is not controlled by only one mineral.
Authigenic gypsum is the gypsum formed after the
placement of the rock piles due to oxidation of the
pyrite minerals.
Figure 15 shows little correlation between cohesionand percent pyrite, which supports the observation that
the cohesion existing within the rock piles and analogs
is not only a factor of oxidation of pyrite to form
cementing agents within the rock piles and analogs.
Figure 16 show no correlation between sulfur and
cohesion which support the observation made between
cohesion and pyrite.
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0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Auth gypsum
Cohesion(kPa)
Fig. 14. Correlation of cohesion with percent Authigenic
gypsum (in percent).
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 0.5 1 1.5 2 2
Pyrite
Cohesion(kPa)
.5
Fig. 15. Correlation of cohesion with pyrite (in percent).Note that some samples with low pyrite have low cohesion.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
S
Cohesion(kPa)
Fig. 16. Correlation of cohesion with S (in percent). Note
that some samples with low S have low cohesion.
Figure 17 shows no correlation between cohesion and
percent calcite, which supports the observation made
earlier related to the correlation between cohesion and
pyrite oxidation since these two mineral are reciprocal
of each other. Figure 18 shows no correlation between
cohesion and carbon.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0
Calcite
Cohesion(kPa)
.7
Fig. 17. Correlation of cohesion with calcite (in percent).
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 0.1 0.1 0.2 0.2 0
C
C
ohesion(kPa)
Fig. 18. Correlation of cohesion with C (in percent).
Figure 19-22 shows little to no correlation between
cohesion and individual clay minerals, which supports
the fact that different combination of minerals and
other factors within the rock piles and analogs accountsfor the existence of cohesion.
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0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 2 4 6 8 10 12 14 16 1
Kaolinte
Cohesion(kPa)
8
Fig. 19. Correlation of cohesion with Kaolinte (in percent).
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 1 2 3 4 5 6 7
Chlorite
Cohesion(kPa)
8
Fig. 20. Correlation of cohesion with Chlorite (in percent).
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 5 10 15 20 25 30
Illite
Cohesion(kPa)
Fig. 21. Correlation of cohesion with Illite (in percent).
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0 1 2 3 4 5 6 7
Smectite
Cohesion(kPa)
8
Fig. 22. Correlation of cohesion with Smectite (in percent).
Note that samples with high smectite have low cohesion.
6. CONCLUSION
Laboratory and in-situ direct shear tests were
conducted on the Questa rock-pile materials toinvestigate the effect of physical, chemical, and
mineralogical properties on the shear strength of these
materials. To classify the rock-pile material based on
the weathering intensity, a simple weathering index
was used that was defined by color, mineralogy, and
texture of the material. A series of geotechnical tests
were conducted on samples with different weathering
intensities from four of the Questa rock piles and from
weathering analogs of the rock piles (alteration scar
and debris flows on the Questa mine site). It should be
noted that all in-situ tests were performed at or near the
surfaces of the rock piles, and the conclusions maderegarding the effect of mineralogy and chemistry on
cohesion are valid only for the shallow surface portion
of the rock piles and not the interior. The synthesis of
these analyses lead to the following conclusions:
The index properties studied in this paper havelittle to no correlation with cohesion. Cohesion
shows a slight negative correlation with water
content and a slight positive correlation with
matric suction. The lower cohesion values tend
to correlate with higher %fines, but some
samples with lower cohesion also have low%fines.
The mineralogy and chemistry have little or nocorrelation with cohesion, which shows that no
single mineral or chemical element affects
cohesion within the rock piles and analogs;
combination of all the physical, chemical, andmineralogical factors are responsible for the
observed cementation within the Questa rock
piles.
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ACKNOWLEDGEMENTS
This project was funded by Chevron Mining Inc.
(formerly Molycorp Inc.) and the New Mexico Bureau
of Geology and Mineral Resources (NMBGMR), a
division of New Mexico Institute of Mining and
Technology. The design and construction of the in-situ
direct shear test device and the direct shear testing was
done at the New Mexico Institute of Mining and
Technology in Socorro, New Mexico and at the Questamine. We also thank the professional staff and students
of the large multi-disciplinary Questa Rock Pile
Stability Project field team for their assistance. We also
thank the entire group of Chevron Mining Inc.
employees who assisted in the successful completion
of the in-situ direst shear testing program.
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