ARMA-10-165_Relationship Between Physical, Chemical, And Mineralogical Properties and Cohesion of Questa Rock Pile Materials

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


Andesite

Rhyolite


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


3-14


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


No of Sample=24

96.1-99.6


No of Samples=18

Point load strength

index (MPa)

0.6-8.2


No of Samples=59

1.7-3.8


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%


present

SPR-AAF-0001-1

(Blind Gulch

rock pile)

SPR-AAF-0001

(SPR1, Figure 1)2 100% andesite textures visible Propyllitic: 25%


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%


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].


10/18

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-0019-1 5 10.0 7.3 SP-SC 4 14.5


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


11/18

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


12/18


13/18

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.


14/18

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.


15/18

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.


16/18

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.


17/18

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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Documents

ARMA-10-165_Relationship Between Physical, Chemical, And Mineralogical Properties and Cohesion of Questa Rock Pile Materials