The Florida Institute of Phosphate Research was created in ...fipr.state.fl.us/wp-content/uploads/2014/12/02-030-061Final.pdfThe Florida Institute of Phosphate Research was created

The Florida Institute of Phosphate Research was created in 1978 by the Florida Legislature(Chapter 878.101, Florida Statutes) and empowered to conduct research supportive to theresponsible development of the state’s phosphate resources. The Institute has targeted areasof research responsibility. These are: reclamation alternatives in mining and processing,including wetlands reclamation, phosphogypsum storage areas and phosphatic clay contain-ment areas; methods for more efficient, economical and environmentally balanced phosphaterecovery and processing; disposal and utilization of phosphatic clay; and environmentaleffects involving the health and welfare of the people, including those effects related to radia-tion and water consumption.

FIPR is located in Polk County, in the heart of the central Florida phosphate district. TheInstitute seeks to serve as an information center on phosphate-related topics and welcomesinformation requests made in person, by mail, or by telephone.

Executive DirectorRichard F. McFarlin

Research Directors

G. Michael Lloyd Jr.Gordon D. NifongDavid J. RobertsonHassan El-ShallRobert S. Akins

Research Staff

-Chemical Processing-Environmental Services-Reclamation-Beneficiation-Mining

Florida Institute of Phosphate Research1855 West Main StreetBartow, Florida 33830(863) 534-7160

RECLAMATION OF PHOSPHATIC CLAY WASTE PONDS BY CAPPING

VOLUME 2: CENTRIFUGAL MODELING OF THE CONSOLIDATION BEHAVIOR OFPHOSPHATIC CLAY MIXED WITH LIME OR GYPSUM

Research Project: FIPR 82-02-030

Prepared by:

Department of Civil Engineering IMC, Bartow, FloridaUniversity of Florida Agrico, Mulberry, FloridaGainesville, Florida 32611 Mobil, Nichols, Florida

Principal Investigators:

T. E. SelfridgeF. C. TownsendD. Bloomquist

Prepared for

FLORIDA INSTITUTE OF PHOSPHATE RESEARCH1855 West Main StreetBartow, Florida 33830

FIPR Project Manager: Dr. Henry L. Barwood

September, 1986

DISCLAIMER

The contents of this report are reproduced herein as receivedfrom the contractor.

The opinions, findings and conclusions expressed herein are notnecessarily those of the Florida Institute of Phosphate Research,nor does mention of company names or products constitute endorse-ment by the Florida Institute of Phosphate Research.

ii

ACKNOWLEDGMENTS

The support and sponsorship provided by the Florida Institute of

Phosphate Research for this investigation is acknowledged. The

cooperation and assistance for this project is acknowledged for:

Dr. J. E. Lawver, Inc.

Mr. Steve I. Olson, Agrico

Mr. H. H. Miller, Agrico

Mr. A. J. Propp, Mobil

This report reflects the MS thesis of Mr. Thomas E. Selfridge,

Department of Civil Engineering, University of Florida, Gainesville,

Florida, 32611. Dr. J. L. Davidson served as a committee member of this

thesis.

V

viii

ix

Reclamation of Phosphatic Clay Waste Ponds by Capping

Volume 2: Centrifugal Modeling of the Consolidation Behaviorof Phosophatic Clay Mixed With Lime or Gypsum

Research Project: FIPR 82-02-030

ABSTRACT

The phosphate mining industry is actively seeking methods to

improve the consolidation characteristics of waste clays created during

the beneficiation process in hopes of reclaiming the large amounts of

land currently used for waste clay impoundment.

This research investigates the use of lime and gypsum as additives

which might enhance the consolidation of the waste clays. The research

was divided into two stages. In the first stage, experimental tech-

niques were used to determine the appropriate quantities of lime and

gypsum to be added to the waste clays and assess the strength gains due

to these additives. The second stage involved centrifuge modeling of

the consolidation of the waste clays with the determined quantities of

lime and gypsum added.

The results of this investigation reveal that the pH test pro-

vides a rapid method for determining the minimum lime content

required for strength producing clay-lime reactions. Lime percen-

tages of approximately 12% were required to achieve the pH percentage

for the clays tested. Miniature vane shear tests showed 28-day

strength gains of 3 to 5 times that of untreated clay when 12% lime

was added while relatively small strength increases occurred for

lower lime contents. Unfortunately, the high concentrations of lime

required, and the relatively low 28-day strengths (14 to 25 psf)

x

question the feasibility of lime treatment from the standpoints of

economics or surface strength to support equipment. However, sufficient

strength to support a sand cap might be feasible, but caution is

advised. In the case of gypsum, the pH test was unsuitable for

determining the target gypsum content. Gypsum/clay ratios as high as

12:1 produced practically no strength improvement.

Centrifugal model tests revealed that addition of lime hindered the

consolidation magnitude of the clays. Apparently the lime strengthens

the soil skeleton sufficiently that little self-weight consolidation

occurs. Instead, the consolidation behavior of lime treated clay is

akin to that of sand. In the case of gypsum addition, the consolidation

magnitude was enhanced. Consolidation occurred more quickly, and final

effective clay solids contents approached those of untreated clay. The

behavior of clay/gypsum mixtures is similar to that for sand/clay mixes.

xi

CHAPTER I: INTRODUCTION

Florida supplies nearly one-third of the world's phosphate

requirements. This phosphate is the source of phosphorous in inorganic

fertilizer and represents a vital non-renewable source. The nature of

the phosphate deposits and the mineralogy of the ore or matrix combined

with the present state of the art in phosphate beneficiation give rise

to several problems related to safety and environmental acceptance.

Perhaps the most pressing problem faced by the phosphate industry is the

adequate disposal of the clay wastes generated during the beneficiation

process. The problem , simply stated, is that the volume of waste

products exceeds the mined-out volume. Historically, waste clays have

been impounded in earthen retention structures with 60% of the clays

being retained below the ground surface. The resulting retention

structures are often as large as 800 acres and have earthen dams as

high as 60 feet. There is growing public and legislative pressure

stemming from the following concerns:

1. The fear of a dam failure which could pollute the ground watersupply.

2. The rising cost of land and the fact that waste clay pondshistorically have required 10 to 30 years for reclamation.

3. The demand for potable water tied up by the mining process.

The last waste clay retention dam failure occurred in 1971. This

failure brought about the development of clay settling area regulations

under Chapter 17-9 of the Florida Administrative Code. Since then,

1

there has been increasing pressure to reduce the turnaround time between

the mining and reclamation of the land.

BENEFICIATION

Mining of the Florida phosphate deposits today is accomplished by

large electric draglines using buckets with capacities as high as 65

cubic yards. These draglines strip off the overburden and cast it

aside. The ore layer is then removed and placed in a sump where it is

slurried and pumped to the beneficiation plant at solids contents of 25%

to 40% for processing.

The washer section of the beneficiation plant separates the pebble

phosphate from the slurry in a series of screening, scrubbing, and

washing operations. The slurry that remains consists of sand, phosphate

particles, and clays. The clays are separated from the sand/phosphate

mixture by hydrocyclones and are pumped at a 3% solids content to the

large earthen retention structures. The sand is separated from the

phosphate ore by means of a flotation process. The resulting tailings

sand is pumped from the beneficiation plant at a solids content of

30%. This tailings sand is used for dam construction and land

reclamation; therefore, disposal is not a problem.

Each year a typical mine will process about 10,000,000 tons of

phosphate matrix and will produce about 2,800,000 dry tons of clay. The

increased volume of the clay at 3% to 30% solids requires that 55% to

70% of the mined land be used for waste clay settling areas.

MINERALOGY

The mineralogy of a clay greatly effects its behavior. Typical

phosphatic clays contain the following minerals: montmorillonite

2

(smectite), attapulgite (palygorskite), kaolinite, mica (illite),

quartz, appatite, and aluminum phosphates (mostly wavelite).

The basic problems associated with phosphatic clays involve the

water holding capacity and low bearing capacity of the clays. The water

holding capacity of the clay is a function of three factors: 1) water

in pore spaces, 2) water held on surfaces, and 3) water in

interlayers. The bearing capacity of a clay is related to the water

content.

The small size of the clay particles and pore spaces greatly

contributes to the water holding capacity of the clay. Often as much as

70% of the clay particles are less than 2 microns in size with an

average particle size in the colloid range. The small size of the pore

spaces combined with the presence of fibrous minerals (usually

palygorskite), which interfere with orderly stacking of the clay

particles, make dewatering of the voids difficult. Additionally, the

small particle size means that electrostatic forces prevail over the

body forces due to the large specific surface, thus hindering particle

settlement.

CLAY DISPOSAL TECHNIQUES

Dewatering methods for waste clays have been sought for many

years. Intuitively, the problem may be approached from two directions;

either modify the beneficiation process to avoid producing the slurry or

alter the produced clay slurry. Of these two methods, the latter has

generated the most interest. Research has confirmed that the poor

settling characteristics of the clays are a function of the low self-

weight consolidation forces. A logical solution to the problem is the

3

addition of tailings sand to the clay to increase the unit weight of the

material. Another solution is to provide a sand cap surcharge to

increase the effective stresses within the underlying clay slurry. In

these applications, the bearing capacity of the clay becomes a major

consideration. The initial solids content of the clay is on the order

of 2% to 6% and has insufficient strength to entrain added sand or

support a surcharge cap. Consequently, the clay must be allowed to

settle to a solids content of approximately 15% before pumping to other

disposal sites for sand addition. The use of flocculents to increase

the sedimentation rate of the particles has been investigated and holds

much promise.

LIME OR GYPSUM TREATMENT OF CLAYS

Inasmuch as gypsum is a waste by-product produced in the manufac-

ture of phosphoric acid, the feasibility of simultaneous disposal

analogous to a sand/clay mix is attractive. However, recognizing the

low pH and activity of the waste clays requires an examination of the

compatability of these materials.

Lime treatment of clays was practiced by the Romans who added burnt

limestone and volcanic ash to form a weak mortar. Accordingly, lime

treatment of waste clays would be anticipated to increase strength and

thus bearing capacity. Since gypsum can be converted to lime, the

affect of lime treatment on the consolidation and strength properties of

phosphatic clays warrants investigation.

PURPOSE OF RESEARCH

The research presented in this report investigates the effects of

the addition of lime and gypsum to the waste clay in the same manner as

4

the tailings sand. Addition of either of these two compounds to the

clays would occur after the clays had reached a solids content in the

13% to 15% range. Lime was suggested for its history of increasing the

strength and reducing the plasticity of troublesome clays. The addition

of gypsum is being investigated strictly on an experimental basis and

because disposal of by-product gypsum created in the manufacture of

fertilizers is rapidly becoming a problem.

SCOPE OF WORK

The objectives of the research are twofold. The first objective is

to experimentally estimate suitable quantities of lime and gypsum for

addition to the waste clays. The second objective is to determine the

consolidation characteristics of the waste clays with the suggested lime

and gypsum additions using centrifuge modeling techniques. Centrifuge

modeling was chosen because it permits the very lengthy consolidation

process to be modeled in a matter of hours as opposed to years.

5

CHAPTER II: BACKGROUND INFORMATION

LIME TREATMENT OF SOILS

Lime stabilization of plastic soils has become a widely accepted

method of preparing clayey soils for use as roadbeds and foundations.

Specifically, lime stabilization serves to reduce plasticity and provide

moderate strength gain. This behavior is attributed to the following

phenomena:

1. Cation Exchange,

2. Flocculation and Agglomeration,

3. Pozzolanic Reactions, and

4. Carbonation.

Cation exchange is the replacement of the exchangeable cations

(such as sodium, hydrogen, and potassium), which previously occupied the

exchange sites of the soil, by the calcium cations derived from the

lime.

An increase in grain size is created by the flocculation and

agglomeration of particles. This phenomenon is caused by suppression of

a double water layer surrounding the clay particles due to an increased

electrolyte concentration provided by the calcium cations.

Pozzolanic reactions involve the formation of cementitious minerals

from the reactions between the silica and alumina present in the soil

minerals and the calcium from the lime. These new minerals are

primarily responsible for the time dependent strength gain exhibited by

7

the lime-soil mixture. The formation of these minerals is dependent on

the amount of lime (calcium) available for reaction.

Carbonation involves the reaction of lime and atmospheric carbon

dioxide to form the cementing agent calcium carbonate. This cementing

agent is relatively weak, therefore, carbonation has only a minor

influence on the strength increase exhibited by lime addition.

MECHANISMS OF LIME TREATMENT

The following mechanisms occur as lime is added to a clay. Ini-

tially, the material becomes friable and a silty texture is attained

upon curing. This behavior is attributed to cation exchange and com-

pression of the double water layers surrounding the clay particles.

These reactions are made possible by an increase in hydrogen-ion concen-

tration (pH) of the system and an increase in calcium-ion concentration,

both made possible by the addition of lime. The plasticity of the

treated clay is subsequently reduced. The lime content at which all

cation-exchange reactions have occurred is termed the "lime fixation

point" (Townsend, 1979). This point is characterized by the increase in

pH of the lime-soil mixture to that of a calcium saturated lime slurry.

This pH approaches 12.4 for unhydrated "quick" lime. Since this point

represents the maximum modification percentage, it is the optimum

percentage generally used for lime addition to soil. Strength gains

created by the addition of percentages of lime below this optimum are

generally insignificant and are probably a result of an increase in the

angle of internal friction caused by the aggregation of the clay parti-

cles. The addition of lime in excess of the optimum percentage provides

free calcium ions which are available for further pozzolanic reactions.

8

SUITABLE SOILS FOR TREATMENT

Lime stabilization is effective for soils with silica and alumina

components available to take part in the pozzolanic reactions. Clayey

soils, with their silica tetrahedral and alumina octahedral layers,

provide a source of these ions, therefore, clayey soils generally

respond very well to lime treatment.

It has been found that plasticity, organic content, pH, and sulfate

concentration are additional factors which effect the lime reactivity of

soils (Townsend, 1979). A Plasticity Index (PI) of 12 is accepted by

the Corps of Engineers as the lower limit of soils responsive to lime

treatment. It has been reported that soils with organic contents

greater than 1% generally do not respond well to lime treatment. It has

additionally been found that acid soils are less responsive than soils

with pH greater than 7. The presence of sulfates has been proven

detrimental to lime stabilization due to the formation of minerals which

disrupt the bonding and formation of cementitious compounds.

BY-PRODUCT GYPSUM

Source of Gypsum

Calcium sulfate (gypsum) is obtained as a by-product in the manu-

facture of phosphoric acid. Phosphoric acid is the intermediate product

from which all phosphate end products are made and is created by the

reaction between sulfuric acid and ground phosphate rock. Some 4 to 5

tons of gypsum are formed for each ton of phosphoric acid (Watson,

1971).

9

Disposal and Storage

After the gypsum is separated from the phosphoric acid through

vacuum filters, it is typically slurried and pumped to settling ponds or

to the top of huge mounds of gypsum called gypsum stacks. These gypsum

stacks commonly cover areas as large as 300 acres and are often more

than 40 feet in height. The gypsum is deposited at the top of the stack

and the water is allowed to drain to a cooling pond. The water at this

point is highly corrosive, containing from 1% to 2% phosphoric acid and

having a pH of 1.5 to 2.0. Typically, only a small amount of the water

in the cooling pond is from the gypsum stack with the remainder coming

from various sources throughout the plant. This water is allowed to

cool before returning to the plant for reuse.

Uses of By-Product Gypsum

Only very limited uses have been found for by-product gypsum in the

United States because of the relatively low cost of sulfur and the

availability of good quality natural gypsum. The by-product gypsum

contains varying amounts of phosphate and fluoride impurities which must

be considered in any possible usage. Additionally, the occurrence of

small quantities of radioactive matter affects the possible usage of

this gypsum in cements and wall board in the building industry.

By-product gypsum has been used in expanding and prestressing

cements for road construction in Japan with favorable results. The

resulting concrete is said to have a better surface finish with fewer

shrinkage cracks than can be obtained with ordinary concrete. Addition-

ally, ground gypsum has been used as a soil conditioner, particularly on

grassland, to make up for calcium and sulfur deficiencies in soil. Soil

10

alkalinity is reduced with the addition of gypsum and a flocculation of

soil particles occurs, thereby improving drainage (Gutt and Smith,

1973).

CENTRIFUGE MODELING OF CONSOLIDATION BEHAVIOR

Limitations of Physical Modeling

Physical modeling is often used in engineering in an attempt to

duplicate operating and field conditions. For the model to be an ideal

representation of the prototype, similar stresses must be applied.

While geometric and material properties are often easy to duplicate in

reduced scale models, the stresses which the prototype is subjected to

are much harder to duplicate, especially in models involving soil. Soil

stresses in corresponding points of the model and prototype are a func-

tion of the height of the overlying material and its unit weight when

both model and prototype are subject to the same gravitational field.

This is the major disadvantage of physically modeling soil behavior with

a reduced scale.

Centrifuge Modeling

To overcome the stress dissimilarity of soil models previously

alluded to, the unit weight of the material in the model must be

changed. This can occur either by using a material with a higher

specific gravity, or by increasing the gravitational force. Since, in

this case, it is desired to model both material and geometric proper-

ties, the logical solution is to increase the gravitational force. This

can be accomplished by subjecting the model to high centrifugal forces.

By increasing the acceleration of the model by a scaling factor and

reducing the dimensions of the model by the same scaling factor, equal

11

stress levels at equal points in both prototype and model will be

produced.

Since the settling of a clay is a time dependent function, it

becomes necessary to study the correlation between time in the model and

time in the prototype in centrifuge modeling. When modeling the settle-

ment behavior of a slurry, the different processes of sedimentation and

consolidation must be considered.

The initial settling of clay particles is referred to as sedimenta-

tion. Bromwell and Raden (1979) state that the majority of sedimenta-

tion ends at clay solids contents of 3% to 8%. Since it is desired to

model clay slurries with relatively greater solids contents (13% to

15%), it is assumed that sedimentation will not be a major considera-

tion.

As the sedimentation process ends, the consolidation process

begins. This process is characterized by a compression of solid

particles through the dissipation of pore water pressures within the

soil mass over a period of time.

For centrifugal modeling it has been shown (Bloomquist, 1982) that

the time scaling relationship is

T P = T m x n2

This means that the model time for consolidation is increased by the

square of the scaling factor. This is obviously a major advantage of

centrifuge testing. For a centrifuge test in which the model is

accelerated to 60 times the force of gravity (i.e. n = 60), a prototype

time of 1 year can be modeled in 2.4 hours.

12

CHAPTER III: TEST PROCEDURES AND EQUIPMENT

INTRODUCTION

This chapter describes the test procedures and equipment used for

this research. In reiteration, the primary objective of this research

was to determine the consolidation properties of phosphatic waste clays

treated with lime or gypsum using centrifugal modeling techniques.

However, prior to centrifugal model tests, the optimum quantities of

lime or gypsum to be added to the clays must be determined. Thus a

series of preliminary bench tests were required. An additional

consideration was the time scaling relationships for centrifugal

modeling. Specifically, the consolidation phenomena models as N2, while

the lime/clay chemical reactions model as N1. An accelerated curing

program investigated this aspect.

From these considerations, the testing program can be divided into

four phases; (a) pH testing for optimum lime or gypsum percentage,

(b) bench tests for strength, (c) accelerated curing tests for scaling

compatibility and (d) centrifugal modeling of consolidation behavior.

pH TEST

The pH test of Eades and Grim (1966) was used to determine the

quantity of lime to be added to the waste clay. Essentially, this test

was developed to determine the minimum lime percentage required to

provide sufficient lime for clay/lime pozzolanic reactions to occur.

Since the pH of saturated lime solution is 12.4, then the percentage of

13

lime added to a clay to produce a pH of 12.4 in a clay/lime solution is

the "pH percentage." For lack of better criteria, the same tests and

concepts were used to try and estimate suitable quantities of gypsum to

be added to the waste clay. However in this case, the pH for gypsum was

about 2.8. A flowchart of the pH testing procedures followed is shown

in Figure 1, while Appendix A provides detailed procedures.

BENCH TESTS

Although the pH test provides a good indication of target lime

percentages, it does not assure "reactivity," i.e., the clay may not

react favorably to produce strength gains. Hence, the pH tests were

followed by the bench testing phase of the research to assess strength-

gaining reactivity. The target lime and gypsum quantities estimated

from the pH test data, in addition to two lower percentage quantities,

were added to the waste clays at approximately 12% to 14% solids. The

mixtures were allowed to cure at room temperature in sealed containers

underwater for periods of 0, 7, 14, and 28 days. At the end of the

specified curing periods, shear strengths of the samples were determined

using a miniature vane shear apparatus. A flowchart of the bench

testing procedures followed is shown in Figure 2, while Appendix A

provides detailed procedures.

ACCELERATED CURING TESTS

Lime stabilization is a chemical process which cannot be adequately

modeled in the centrifuge, that is to say, the time-scaling exponent for

consolidation is 2.0, while for gravity-independent chemical reactions

it is 1.0. Thus consolidation in a centrifuge model would be over many

days before the chemical reactions had occurred. Accordingly, a method

14

15

Figure 2: Flowchart of Bench Testing Procedures

16

was devised to avoid the problem associated with the time required for

the clay-lime mix to achieve adequate strength. From plots of the lime

bench test data, it was estimated that the majority of the strength

gained through the lime addition had occurred by the 16th day of

curing. The solution was to spin the suggested lime clay mixture in the

centrifuge at 60 g's for the equivalent of 16 days and then remove the

samples and allow them to cure for 16 days. Following the 16 days of

curing, the samples would be placed in the centrifuge and allowed to

consolidate to completion. Recognizing that chemical reactions can be

accelerated by heat (Townsend, 1979) to reduce the 16-day time required

for curing, two accelerated curing tests were performed to determine the

time required to reach maximum strength with a curing temperature of 105

degrees Fahrenheit. The clay was mixed with the required amount of lime

and placed in mason jars. The jars were then placed in an oven set at

105 degrees and al lowed to cure for 1, 2, 3, 4, and 5 days. At the end

of each specified curing time, a jar was removed from the oven and

tested with the vane shear device. The number of days required to reach

full strength was determined from plots of shear strength versus curing

time. A flowchart of the accelerated curing testing procedures is

presented in Figure 3, while Appendix A provides detailed procedures.

When the lime mixtures were subsequently tested in the centrifuge, the

buckets were sealed to prevent drying and placed in the oven for the

required curing time after being spun for the initial 16-day equivalent

at 60 g's. Following the curing period, the samples were placed in the

centrifuge and were allowed to consolidate to completion at 60 g's.

17

Figure 3: Flowchart of Accelerated Curing Test Procedures

18

CENTRIFUGE MODELS

The actual centrifuge testing of the waste clay mixes is a simple

but time consuming process. The consolidation behavior is determined by

visual (photographic) in-flight inspection of the slurry/supernatant

interface height. Pictures were taken at geometric time increments

(e.g. 1, 2, 4, 8, 16, 32, 64, etc. minutes) to facilitate plotting on a

logarithmic time scale. When the interface heights stabilized, the

tests were ended. The centrifuge samples were then cored and vane shear

tested to provide solids content with depth and average strength

information. The operation of the UF Centrifuge is discussed in detail

in Volume 1 of this report series (Townsend, et al. 1986) and the

centrifuge operation procedures furnished in Appendix A of this report

are for completeness.

EQUIPMENT

The equipment utilized in this research is discussed in this

section and consists of two facets; (a) bench tests for optimum lime or

gypsum content and accelerated curing, and (b) centrifugal model tests.

ADDITIVE QUANTITY DETERMINATION

Figures 4 and 5 show photographs of the pH and bench testing

equipment, respectively. Figure 4 shows the equipment used to determine

lime and gypsum fixation points while Figure 5 shows the equipment used

to determine the strength gains with time of the mixtures.

pH Meter

To determine the lime and gypsum fixation points, over a pH range

of approximately 2.6 to 12.4, a digital pH meter was used. Appropriate

buffer solutions were used for calibration, depending upon the pH range

19

Figure 5: Bench Testing Equipment

20

of interest. However, it should be mentioned that in determining pH

fixation percentages, a great deal of sensitivity is not required and

inexpensive equipment can be used.

Mason jars with sealable lids were used to contain the clay/lime or

clay/gypsum mixtures. The wide mouth of these jars allowed easy mixing

and testing with the miniature laboratory vane shear device. Mixing of

the slurries was easily accomplished using a variable speed drill with a

paint mixing attachment.

Vane Shear Device

A laboratory vane shear device was used to determine the shear

strength of the slurry mixtures at various curing times. The device

consists of a four-bladed vane which is inserted into the slurry and

rotated. The resistance to rotation, or torque, is measured. The shear

strength may subsequently be calculated from the torque required to fail

the sample.

UNIVERSITY OF FLORIDA CENTRIFUGE

Description of Centrifuge

The centrifuge used to model the consolidation behavior of the

mixtures is a Rucker Model 57-2380 shown schematically in Figure 6 and

discussed in detail elsewhere (Bloomquist, 1982; McClimans, 1984;

Townsend, et al., 1986). Power is supplied by a 2-horsepower, 3-pole,

208-volt electric motor. Two rotating arms, 180 degrees apart, each

support a platform capable of holding two samples. The distance from

the axis of rotation of the centrifuge to the centroid of each platform

is 34.5 inches. Modifications to the centrifuge arms allow a capacity

of approximately 80 pounds to an acceleration of 100 g's. A metal

21

Figure 6: Schematic Drawing of University of Florida Centrifuge

housing encloses the entire assembly. Access to the arms and platforms

is provided through a double door on the side of the centrifuge, a

hinged top panel, and a plexiglass covered viewport located on the top

of the centrifuge.

Electronic Monitoring Devices

Three devices are used to monitor and control the speed of the

centrifuge during operation: the accelerometer, the digital tachometer,

and the acceleration divergence limiter.

An Entran Miniature Damped Accelerometer (No. EGA-125f-250D)

monitors the acceleration of the samples during the tests. The output

voltage is converted to g units and displayed on the LED panel of a

Doric Series 420 Digital Transducer Indicator after passing through a

slip ring on the centrifuge spindle. The accelerometer is mounted on

one of the sample bucket housings at a distance of 37.5 inches from the

center of rotation of the centrifuge. This distance differs from the

center of gravity of the sample necessitating the calculation of an

operational offset acceleration.

For redundancy, a digital bench tachometer (Power Instruments,

Model 1723) is used to monitor the model acceleration. The tachometer

measures the speed (RPM) of the centrifuge by means of a photo-electric

pickoff monitoring the spindle. The rotational speed of the model is

displayed on the LED panel of the tachometer. Knowing the rotational

speed of the centrifuge and the radial distance to the model point of

interest, the correct acceleration of the model may be calculated.

An Acceleration Divergence Limiter (ADL) was developed by the

Digital Design Facility of the University of Florida's College of

23

Engineering. The ADL provides a means of controlling the acceleration

of the centrifuge for safety purposes and to allow overnight testing to

occur without the presence of an operator. The device allows the

selection of an appropriate acceleration level and operating window.

When the acceleration of the centrifuge increases beyond or falls below

the specified operating window, the centrifuge automatically shuts

off. This device therefore protects against damage due to overheating

of bearings, unauthorized adjustment of the speed control, and possible

breakage of the platform or sample housings during the test.

Specimen Container Assembly

For proper correlation between stresses in the prototype and model,

centrifuge modeling requires proper inflight orientation of the sample

to be modeled. The consolidation process occurs vertically in the

field, however, the increased centrifugal forces which, make

model/prototype correlation possible, occur horizontally. The sample

must therefore be oriented horizontally in flight. This orientation is

made possible by the use of platforms which allow the models to pivot

freely. This design allows the acceleration vector to pass through the

bottom of the sample at all times. The components of the mounting

assembly include the aluminum bucket housings, the aluminum buckets, the

pivot bolts, and the plexiglass containers. The components of the

mounting assembly include the plexiglass containers, aluminum buckets,

pivot bolts, and aluminum bucket housings.

Since movement of the slurry/supernatant level must be monitored

during the test, transparent plexiglass containers, 5.5-in. ID by 6 in.

high, were used to carry the samples. Transparent metric scales with

24

millimeter divisions were attached to the outside of the container for

accurate measurement of interface heights.

Since high stresses are developed in the centrifuge, the plexiglass

containers alone would not be strong enough and some means of support is

required. Accordingly, an aluminum bucket was fabricated from 6 9/16-

in. OD by 1/4-in. thick pipe with 1/4-in. plate heliarc welded to the

bottom. A 1 in. by 4-in. cut-out in the bucket permits viewing of the

plexiglass slurry container. Two 3/4-in. holes drilled through the top

of the bucket allow mounting of the bucket to the container housing by a

pivot bolt.

A 3/4-in. high-strength steel bolt is used to mount the buckets to

the housing. The bolts pass through the bucket and allow them to rotate

upward into a horizontal position upon centrifugal loading. The bolts

are slightly offset to allow the buckets to overswing and rest against a

restraining bracket mounted on the bucket housing. This design insures

that the resultant acceleration vector is always perpendicular to the

model's interface (Bloomquist, Davidson, and Townsend, 1984).

Two aluminum bucket housings provide a support system for the

buckets to be mounted to the centrifuge arm, and allow simultaneous

modeling of four samples.

Data Acquisition

Data acquisition is a photographic record using a stroboscope and a

Polaroid camera. The major components of the system include the strobe,

the flash delay, the camera, and the camera mount.

25

A Stroboslave Model 1539-A manufactured by General Radio Company is

used to provide high intensity light flashes of short duration. The

strobe is connected with one of two photoelectric pickoffs mounted on

top of the centrifuge. An external trigger allows just one flash to

occur, thus eliminating multiple exposures.

A model 1531-P2 Flash Delay allows the rotating samples to be

oriented directly underneath the camera as a flash occurs. This flash

delay allows the proper orientation of the samples with the flashing

light source and camera and permits minor adjustments to compensate for

fluctuations in centrifuge speed.

A Polaroid camera (Model 600 SE) was chosen because of its ability

to take close-up, instant photographs. The model features an

interchangeable lens but a close-up lens set was added to enable

accurate readings of the metric scale on the side of the plexiglass

buckets. High-speed film (ASA 3000) is used and provides excellent

photographs as illustrated in Figure 7.

Camera Mount. Since the centrifuge vibrates during operation, the

camera is mounted to a platform anchored to the ceiling joists directly

over the view port. A tube extends down from the platform and allows

the camera to be lowered into place over the opened viewport for

pictures to be taken. An adjustable mechanism allows the camera to be

raised and lowered vertically along the tube or three inches

horizontally.

26

Figure 7: Example of Photographic Monitoring

CHAPTER IV: PRESENTATION AND DISCUSSION OF RESULTS

INTRODUCTION

The testing performed on the phosphatic waste clays may be divided

into four phases:

1. The pH testing phase, where stabilization concentrations oflime and gypsum to be added to the waste clays were determined,

2. The bench testing phase, where strengths of the proposed claymixtures were determined after certain curing periods,

3. The accelerated curing test phase, where lime/clay mixtureswere subjected to increased temperature to decrease the curingtime required for adequate strength gain of centrifuge samples,and

4. The centrifuge testing phase, where the consolidation charac-teristics of phosphatic waste clays mixed with lime and gypsumwere observed.

pH TESTS

The pH test developed by Eades and Grim (1966) was used to

determine the percentages of lime and gypsum required for stabilization

of the waste clays. A total of six waste clays from different mines in

Florida were tested, including I.M.C.- Kingsford, Occidental-Suwannee

River, Agrico-Ft. Green, Amax-Big Four, Brewster-Haynsworth, and CF

Mining-Hardee. Table 1 presents the results of the pH testing and shows

the percent lime or gypsum (by weight of clay solids) required to

achieve a pH value of approximately 12.4 or 2.8, respectively. The pH

of the untreated clays ranged from 7.2 to 7.8.

29

Lime Addition

Relatively large quantities of lime were required to increase the

pH of the waste clays to the target pH. In general, six percent lime is

accepted as being the cut-off point for cost effective stabilization.

Lime percentages of from 12 to 14 were required to bring the waste clays

up to the target pH, which ranged from 12.2 to 12.4 depending on the

degree of hydration of the lime used. Typical liquid limits (LL) of the

phosphatic waste clays are well over 200. Typical plasticity indices

(PI) of the clays range from 140 to 180. Some restrictions which have

been placed on the LL and PI of clayey soils suitable for effective lime

stabilization are a liquid limit not less than 40 and a plasticity index

not less than 20 (Townsend and Donaghe, 1976). According to these

criteria, lime stabilization of phosphatic waste clays should be

30

successful. A possible reason that such large quantities of lime are

required to achieve this pH percentage could be the organic content of

the clays. In general, clays with organic contents greater than 1% do

not respond well to lime stabilization. A typical organic content of

phosphatic waste clays is on the order of 2% (Bloomquist, 1982). With

these facts in mind, it was decided to proceed to the bench testing

phase using the experimental target lime percentage and two lower lime

percentages to test the validity of the pH testing phase.

Gypsum Addition

The addition of gypsum as a stabilizing agent is being investigated

on an experimental basis. While gypsum has been used as a soil

conditioner, documentation of its use as a soil additive for stabili-

zation purposes is nonexistent.

For lack of better criteria, the pH test of Eades and Grimm, which

was developed to determine lime stabilization concentrations was used to

determine the quantity of gypsum required for waste clay

stabilization. Gypsum concentrations of from 700% to 1200% were

required to bring the clay to the target pH, which ranged from 2.8 to

2.85, using Eades and Grimm criteria. Obviously, the stabilization

processes of lime and gypsum are different and require different tests,

assuming that gypsum indeed acts as a soil stabilizer.

It was decided to use the same approach adopted for lime in the

gypsum bench testing phase. The target pH additive concentration would

be tested as well as two lower percentages.

31

BENCH TESTS

Introduction

The bench testing technique outlined in the previous chapter was

used to determine 28-day strengths of the phosphatic waste clay mixed

with lime and gypsum. In typical lime stabilization laboratory

investigations, the soil is mixed with the required lime percentage at

the appropriate water content and then compacted in a Harvard miniature

or Proctor mold. The resulting sample is generally cured for a

specified time and then tested in unconfined compression. The nature of

phosphatic waste clays, however, requires a different testing

procedure. Compaction of the slurry cannot occur in the field, and the

slurry form of the clay prevents the use of the unconfined compression

device. It was therefore necessary to employ the miniature vane shear

device to provide a measure of the undrained shear strength of the cured

phosphatic clay mixtures. The samples were cured at room temperature

(20-25 degrees Celsius).

The six phosphatic waste clays which were tested in the pH testing

phase were also tested in this phase of the research. Table 2 presents

a summary of the 28-day strengths of the various clay mixtures.

Shear Strength versus Curing Time Curves

In addition to the 28-day strengths, the undrained shear strength

was measured at 7 and 14 days as well as immediately after mixing with

lime or gypsum. When the shear strengths are plotted against the curing

time, the plots give an indication of the curing behavior, i.e. chemical

reactions of the particular mix.

32

Table 2.

Summary of 28-Day Strengths from Bench Tests

MINE % SOLIDS ADDITIVE % ADDED 28-DAY

STRENGTH (PSF)

I.M.C. KINGSFORD 13.6

OCCIDENTAL SUWANNEE RIVER

AGRICO-FT. GREEN 13.3

AMAX-BIG FOUR 12.7

BREWSTER HAYNSWORTH

CF MI NING-HARDEE

12.5

12.2

13.1

LIME LIME LIME

GYPSUM GYPSUM GYPSUM LIME LIME LIME

GYPSUM GYPSUM GYPSUM

LIME LIME LIME

GYPSUM GYPSUM GYPSUM LIME LIME LIME

GYPSUM GYPSUM GYPSUM

LIME LIME LIME

GYPSUM GYPSUM GYPSUM

LIME LIME LIME

GYPSUM GYPSUM GYPSUM

i 12

100 300 700

: 14

300 500 700

5 7

12 400 800

1200 4 8

12 400 800

1200 4

1: 400 800

1200 4 8

12 400 800

1200

;*i 25:2 4.5 4.0 4.7 4.5 5.4

15.8 3.5 6.2 8.4 4.1 4.6

14.3 4.8 6.5

10.2 3.7 4.5

21.1, 4.4 7.5 8.9 3.3

1::; 4.0 7.5

i-i 8:l

17.7 5.5 7.9

10.6

33

Figures 8 through 13 present the curves for each clay type that

lime was added to. In general, the addition of lime below the target

value produced very little strength gain in any of the samples. The

addition of the target concentration, however, produced strength gains

as high as 5 times the initial strength of the slurry with the average

strength gain being approximately 3 times the initial strength. The

strength gains with time observed for the target pH percentages indicate

that chemical reactions are occurring with the clay/lime mixture. They

also verify the validity of the pH test for rapidly determining the lime

contents required for stabilizing reactions.

In all of the samples that lime was added to, there was a

noticeable reduction in plasticity during the initial mixing. The time

required for the maximum strength gain varied widely between the samples

as seen in the figures. Samples from Agrico, Occidental, and Brewster

reached a plateau in strength gain between 16 and 20 days. In contrast,

samples from I.M.C., CF Mining, and Big Four exhibited strength gains

throughout the 28-day period. These samples also exhibited the highest

strength gains of the lime treated clays, indicating that they react

more favorably to lime stabilization.

Figures 14 through 19 present the curing curves for the samples to

which gypsum was added. Strength gains were minimal in comparison with

the lime samples, even when gypsum-clay ratios of 12:1 were tested. The

highest strength gains were from approximately 5 psf to 10 psf. While

this represents a doubling of strength over the 28-day period, the gain

of 5 psf is negligible in terms of improving the bearing capacity of the

clay. Furthermore, the addition of gypsum to the clay slurry initially

34

Figure 8: Shear Strength vs Curing Time, I.M.C. Lime Samples

Figure 9: Shear Strength vs Curing Time, Occidental Lime Samples

Figure 10: Shear Strength vs Curing Time, Agrico Lime Samples

Figure 13: Shear Strength vs Curing Time, CF Mining Lime Samples

Figure 14: Shear Strength vs Curing Time, I.M.C. Gypsum Samples

Figure 15: Shear Strength vs Curing Time, Occidental Gypsum Samples

Figure 16: Shear Strength vs Curing Time, Agrico Gypsum Samples

Figure 17: Shear Strength vs Curing Time, Big Four Gypsum Samples

Figure 18: Shear Strength vs Curing Time, Brewster Gypsum Samples

)

Figure 19: Shear Strength vs Curing Time, CF Mining Gypsum Samples

tended to decrease the viscosity of the mixture, making it flow more

readily and reducing the strength to below that of an untreated clay

slurry of the same clay solids content. This phenomenon was observed

even when adding a 12:1 gypsum to clay ratio. It is difficult to

discern whether these minor strength gains are chemical or mechanical

reactions or a combination. The time-related strength gains are

indicative of chemical reactions and were observed for all gypsum

mixtures. Although quite small, in all of the gypsum samples, the

majority of the strength gains occurred within 12 to 16 days.

ACCELERATED CURING TESTS

It was found that the majority of the strength gain in the lime

samples occurred within 16 to 20 days. This is a time dependent, i.e.,

chemical, process that cannot be modeled or accelerated in the

centrifuge. In order to examine centrifugally the consolidation

behavior of these clay/lime mixtures, a method of circumventing this

problem was devised in which the lime-clay mixtures would be accelerated

in the centrifuge for the equivalent of the 16 days required for

strength gain, removed from the centrifuge, allowed to cure for the 16-

day period, and then re-accelerated to completion in the centrifuge.

The major problem with this method was the 16-day period required for

curing the samples. In order to reduce the curing time and to quicken

the total testing process, accelerated curing tests were performed as

outlined in the previous chapter. A curing temperature of 105 degrees

Fahrenheit was used and samples from Agrico-Ft. Green and Occidental-

Suwannee River mines were tested. Lime concentrations of 12% were added

to the samples since it was proven that the addition of lime in lower

47

percentages produces little or no strength gain. Figures 20 and 21

present the results from these curing tests. From the figures, it is

clear that the majority of the strength gain occurs after 2 days of oven

curing. The centrifuge buckets were therefore allowed to cure for 2

days at 105 degrees Fahrenheit after initial spinning. It is curious to

note that the strength gains for the bench test specimens during

accelerated curing were as much as 100% higher than the specimens that

were cured at room temperature. This increased strength could be due to

a change in the pozzolanic reaction products brought about by the

increased temperature (Townsend and Donaghe, 1976).

CENTRIFUGE TESTS

Introduction

The final phase of the research involved the actual centrifuge

modeling of the consolidation behavior of phosphatic waste clays mixed

with lime and gypsum. A total of four clays from different mines were

tested; specifically, Agrico-Ft. Green, Occidental-Suwannee River, Amax-

Big Four, and Brewster-Haynsworth. A total of 16 tests were per-

formed. Four tests each were performed on untreated and lime treated

clays, while eight tests were performed on gypsum treated clays.

Untreated clay was tested to provide a baseline against which the

consolidation characteristics of lime and gypsum treated clays may be

compared. Twelve percent lime was added to the lime treated clay

samples as predicted by the pH and bench testing phases. Gypsum to clay

ratios of 8:1 and 2:1 were tested. It was decided to use these lower

quantities of gypsum rather than the 12:1 ratio determined in the pH

testing phase because it was felt that adding such large quantities of

48

Figure 20: Accelerated Curing Test Results, Occidental Samples

material to the clay would be economically infeasible. Initially, the

8:1 ratio was tested but subsequently discarded as sand clay mix tests

showed this ratio was not beneficial (McClimans, 1984). Then it was

decided to test a 2:1 ratio in an attempt to lower the final interface

height and enhance consolidation of the clay. Table 3 provides a brief

summary of the centrifuge tests performed. All tests were performed at

60 g's and had initial interface heights of approximately 8 centi-

meters. Additionally, all samples had initial clay solids contents of

approximately 13%.

Consolidation Characteristics of Treated Clays

The objectives of this modeling were to determine the effects of

the addition of lime and gypsum on consolidation behavior of the phos-

phatic waste clays mentioned above. Figures 22 through 25 present plots

of average effective clay solids content versus elapsed model time on a

log scale. The average effective clay solids content was determined

from the photographs of the interface heights and the following equation

derived by McClimans (1984):

51

53

n

54

55

56

Effect of Lime Addition on Consolidation Behavior

Figures 22 through 25 show that the addition of lime did nothing to

enhance the consolidation behavior of the phosphatic waste clays. The

addition of lime in fact hindered the consolidation when compared to the

untreated samples. This behavior is probably explained by the phenomena

of flocculation and agglomeration which occurs in the lime stabilization

process. An increase in grain size is produced by the suppression of

the double water layer. While increased grain size intuitively means

better drainage, it also means a stronger soil skeleton which resists

consolidation. An analogy to testing a lime-clay mix is placing a

sample of sand in the centrifuge and attempting to consolidate it. At a

certain point, the soil skeleton will resist further movement. It is

believed that this phenomenon as well as a drastic reduction in

plasticity on addition of the lime to the clay caused the observed

consolidation behavior.

Effect of Gypsum Addition on Consolidation Behavior

Figures 22 through 25 also show that the addition of gypsum in the

2:1 ratio greatly improved the consolidation behavior of the four clays

tested. The final interface height was reached an average of two times

faster in the gypsum treated samples. Additionally, the figures show

that similar intermediate clay solids contents were reached up to six

times faster in the gypsum treated waste clays. A possible explanation

for this behavior is the hypothesis that gypsum might act as a

flocculent in phosphatic waste clays. Unlike lime, which also produces

a flocculation of particles, a strengthening of the soil skeleton

probably does not occur. This was proven in the bench testing phase of

57

the research as negligible strength gain occurred even at 12:1 gypsum to

clay ratios. A flocculation of the particles could produce the enhanced

consolidation behavior observed in the gypsum samples.

More than likely, however, the gypsum particles serve to increase

self-weight and enhance drainage in much the same way as the tailings

sand in sand/clay mixes. Similar consolidation behavior is evident in

work performed by McClimans (1984) reported in Volume 1 (Townsend, et

al., 1986) in which sand/clay mixes were tested in the centrifuge.

Unlike sand, however, the gypsum seems to be very soluble in water and

drastically reduces the viscosity of the clay when mixed with it. In

order to fully understand the reaction between the gypsum and the waste

clay, further research should be conducted to prove or disprove the

theory that gypsum acts as a flocculent when mixed with phosphatic waste

clays. It is clear, however, that gypsum aids the consolidation process

of phosphatic waste clays.

Final Sample Analysis. After the completion of testing in the

centrifuge, each sample was removed and cored as outlined in the

previous chapter. Additionally, two vane shear tests were performed on

each sample to determine the final average shear strength. Table 3

shown on page 52 also presents a summary of the final analysis data. It

is interesting to note that the highest strength gain occurs in the

gypsum treated samples. The gypsum treated samples consolidated to

final model heights which were roughly equivalent to the final heights

of the untreated samples; however, the final strengths of the gypsum

treated samples ranged from 1.5 to 1.9 times higher than the untreated

samples, with shear strengths in the 30 psf range. This strength gain

58

is due to the increase in total solids content created by the addition

of gypsum since both the untreated and gypsum treated samples

consolidated to roughly the same final clay solids content.

The addition of lime to the waste clay obviously hindered the

consolidation process. Final clay solids contents of only 60% to 80% of

the final solids contents achieved in the untreated clay centrifuge

tests were attained. Final strengths were highly variable and

significant gains were noted only in the Agrico and Occidental waste

clays. The final strengths of the lime treated samples were generally

less than the untreated samples, however, this is understandable because

of the lower clay solids content achieved by the lime treated samples.

Figures 26 through 29 present plots of the final total solids

content with depth of each of the untreated centrifuge samples. The

untreated samples showed consistent profiles of increasing solids

content with depth. These solids contents were relatively variable from

top to bottom, differing by as much as 15%.

Figures 30 through 33 present similar plots of the lime treated

soils. The trend showed in these figures is a similar increasing solids

content with depth. The magnitude of the overall increase with depth is

much less than the untreated samples, differing by only 4% or less.

This perhaps indicates an overall strengthening or stiffening of the

clay from top to bottom, when the lime is added.

Figures 34 through 37 show plots of the final total solids content

profiles of the gypsum treated samples. Again, the observed trend of

increasing solids content with depth is present. Like the untreated

samples, there is a relatively high difference between the solids

60

61

63

contents at the top and bottom of the gypsum treated samples. This

could be indicative of migration of the gypsum particles to the bottom

of the containers during testing especially in the Amax-Big Four sample

which has a solids content variation of 36% from top to bottom. Because

of the degree of consolidation achieved by the gypsum treated samples,

and by work done by McClimans, 1984, (Townsend, et al., 1986) in which

it was shown that significant migration of sand particles through the

clay during centrifuge testing does not occur, it is believed that the

gypsum to clay ratio with depth stayed relatively intact. Since the

cored sections were not washed through a sieve to determine the

gypsum/clay ratio profile, there is no sure way of proving or disproving

this belief.

64

Figure 30: Solids Content With Depth, Agrico Lime Treated Clay Centrifuge Samples

Figure 31: Solids Content With Depth, Occidental Lime Treated Clay Centrifuge Samples

Figure 32: Solids Content With Depth, Big Four Lime Treated Clay Centrifuge Samples

Figure 34: Solids Content With Depth, Agrico Gypsum Treated Clay Centrifuge Samples

Figure 35: Solids Content With Depth, Occidental Gypsum Treated Clay Centrifuge Samples

Figure 36: Solids Content With Depth, Big Four Gypsum Treated Clay Centrifuge Samples

Figure 37: Solids Content With Depth, Brewster Gypsum Treated Clay Centrifuge Samples

CHAPTER V: CONCLUSIONS

LIME ADDITION TO WASTE CLAYS

The pH testing phase revealed that the pH test provides a rapid

assessment of minimum lime contents required for strength gains.

Percentages of lime were approximately 12% for the clays tested. The

bench testing phase of the research showed that relatively high strength

gains were achieved in waste clay samples where lime concentrations of

12% were added and relatively little improvement was obtained at lower

lime contents. Unfortunately, the magnitude of the 28-day strengths

could prove inadequate in bearing capacity considerations for supporting

equipment. However, the concept of using lime to strengthen the upper

few feet of a waste pond to provide a working platform for supporting a

sand cap may be feasible, but caution is advised. Given the relatively

high percentage of lime required for stabilization of the waste clays

and the magnitude of the strength gain resulting from the addition of

lime, lime stabilization is probably unsuitable for natural phosphatic

waste clays.

The addition of lime to the waste clays tended to hinder the

consolidation process. This is probably due to the decrease in

plasticity of the clay occurring as the lime is added and the curing

process proceeds. An overall strengthening of the soil skeleton occurs,

although final strengths after consolidation are weaker than untreated

clay after consolidation due to relatively lower clay solids contents.

73

The addition of lime to the clay seems to liken the consolidation

behavior to thatof sand which resists consolidation after

grain contact is made.

GYPSUM ADDITION TO WASTE CLAYS

grain to

The pH testing phase of the research showed that relatively large

quantities of gypsum were required to stabilize the waste clays when

using Eades and Grimm criteria. Since little time-dependent strength

increases indicative of chemical reactions occurred for clay/gypsum

mixes, this testing method is unsuitable for gypsum, which reacts

differently than lime with the clay.

The bench testing phase of the research showed that initial mixing

of gypsum with the waste clay dramatically decreased the viscosity of

the clay. This phenomenon is probably a key to the behavior observed in

the centrifuge testing phase of the research and should be investigated

further. It is assumed that a flocculation of particles occurs after a

certain period of time and results in an increased effective grain size

without the strength gain observed in lime stabilization. After

approximately 12 days of room temperature curing, maximum strengths were

achieved. These strengths were relatively low and probably insufficient

for bearing capacity considerations, even when 12:1 gypsum to clay

solids ratios were added.

The addition of gypsum to the waste clays had a marked effect on

the consolidation behavior of the clays. When compared to untreated

waste clays subjected to the same centrifugal acceleration, consoli-

dation to similar intermediate clay solids contents occurred much faster

and final solids contents were reached an average of 2 times faster when

74

a 2:1 gypsum to clay solids ratio was tested. It is believed that this

behavior is caused by a combination of the flocculation of clay and

gypsum particles and an increased self-weight and enhanced drainage of

the mixture similar to mixing sand with the waste clay. To fully

understand the reactions or mechanisms that occur, further research

should be conducted. Different gypsum to clay ratios should be tested

to determine an optimum additive concentration for consolidation

enhancement. Additionally, the environmental impact of adding gypsum to

the clays should be studied. After the gypsum centrifuge tests were

completed, a measure of the supernatant pH indicated a highly acidic

solution with pH values ranging from 2.5 to 2.9. This could have a

detrimental effect on the groundwater supply if unchecked seepage

occurs.

75

REFERENCES

Bloomquist, D.G. 1982. Centrifuge Modeling of Large StrainConsolidation Phenomena in Phosphatic Clay Retention Ponds, Ph.D.Dissertation, University of Florida.

Bloomquist, D.G., J.L. Davidson and F.C. Townsend. 1984. PlatformOrientation and Start-up Time During Centrifuge Testing.Engineering and Industrial Experiment Station, University ofFlorida. Research Report 242 W18.

Bromwell, L.G. and D.J. Raden. 1979. Disposal of Phosphate MiningWastes (Current Geotechnical Practice in Mine Waste Disposal).ASCE Geotechnical Division Special Publication.

Eades, J.L. and R.E. Grim. 1966. A Quick Test to Determine LimeRequirements for Lime Stabilization. National Academy of Sciences,National Research Council, Highway Research Board, Washington,D.C. Highway Research Record No. 3.

Gutt, W. and M.A. Smith. 1973. Utilization of By-Product CalciumSulphate. Chemistry and Industry. 63-65.

McClimans, S.A. 1984. Centrifugal Model Evaluation of theConsolidation Behavior of Sand/Phosphatic Clay Mixes. ReportSubmitted in Partial Fulfillment of Requirements for the Degree ofMaster of Engineering, University of Florida.

Townsend, F.C. 1979. Use of Lime in Levee Restoration. U.S. ArmyEngineer Waterways Experiment Station, CE, Vicksburg, Miss.Technical Report GL-79-12.

Townsend, F.C. and R.T. Donaghe. 1976. Investigation of AcceleratedCuring of Soil-Lime and Lime-Flyash-Aggregate Mixtures. U.S. ArmyEngineer Waterways Experiment Station, CE, Vicksburg, Miss.Technical Report S-76-6.

Townsend, F.C., S.A. McClimans, D. Bloomquist, and M.C. McVay. 1986.Reclamation of Phosphatic Waste Clay Ponds by Capping, Vol. 1:Centrifugal Model Evaluation of Reclamation Schemes for PhosphaticWaste Clay Ponds. Florida Institute of Phosphate Research, Bartow,Florida. FIPR Research Report 82-02-030.

Watson, G.V. 1971. The Disposal of Gypsum from Phosphoric AcidPlants. Chemistry and Industry. 78-84.

77

APPENDIX A

INTRODUCTION

This appendix details via step by step and flowcharts the

procedures follow for:

a. pH Tests

b. Bench Tests

c. Accelerated Lime/Clay Tests

and d. Centrifuge Testing

pH TESTING PROCEDURES (Figure A-1)

1. Standardize the pH meter with a buffer of known pH.

2. Prepare lime and gypsum slurries to determine target pH values in

clay mixes. This step calibrates divergences in pH from 12.4 or 2.8

due to pH meter, local water, and quality of lime or gypsum being

used in the test program.

a. Weigh approximately 40 grams of lime and gypsum.

b. Place lime and gypsum in two plastic 200-ml bottles.

c. Add water to bottles to 200-ml mark.

d. Mix thoroughly by shaking bottles every 10 minutes for an hour.

e. Measure the pH of the slurries with the pH meter. This is the

target pH.

3. Determine bulk solids content of clay to be tested.

a. Thoroughly mix the clay to be tested.

b. Weigh three tare to nearest .01 gram.

A-1

STANDARDIZE' pH METER

1 PREPARE LIME &

GYPSUM SLURRIES

L MEASURE TARGET pH OF SLURRIES 4

I

. PLACE CLAY IN

PLASTIC BOTTLES

I c ADD WATER I TO 200 ml MARK

i SHAKE BOTTLES EVERY 1 10 MINUTES FOR 1 HOUR1

I 1

1 MEASURE pH OF MIXTURES 1

1 DETERMINE TARGET PERCENTAGES 1 I +

[PROCEED TO BENCH TESTS 1

Figure A-l: Flowchart of pH Testing Procedures

A-2

c. Place clay in tare and weigh.

d. Place tare cans in oven set at 110 degrees Fahrenheit for 24

hours.

e. Remove tare cans from oven and weigh.

f. Determine solids content from S% = W solids/W total.

4. Prepare clay slurry mixtures with lime and gypsum.

a. Weigh six clay samples and place in plastic bottles.

b. Add lime percentages of 4, 6, 8, 10, 12, and 14 (% of clay

solids), or gypsum to clay solids ratios of 4:1, 6:1, 8:1, 10:l,

12:1, and 14:1.

c. Add water to 200-ml mark.

d. Shake bottles every ten minutes for an hour to mix thoroughly.

5. Measure and record the pH of the mixtures. The percentage of lime

or ratio of gypsum at which the target pH is reached is the

percentage required for stabilization.

6. Repeat procedure for another clay.

BENCH TEST PROCEDURES (Figure A-2)

1. Thoroughly mix bulk sample of waste clay and obtain solids content

as previously outlined. If solids content is between 12% and 14%

continue, otherwise add water and remix.

2. Weigh clay samples into mason jars using tare function on electric

balance. Eighteen jars are required for 1 set of lime or gypsum

bench tests.

3. Calculate weight of solids in jar based on determined solids

content.

A-3

ADD SUGGESTED PERCENTAGES OF LIME AND GYPSUM TO JARS

1 MIX CLAY AND ADDITIVE IN

EACH JAR FOR 2 MINUTES

1 LABEL SAMPLE JARS WITH SUGGESTED INFORMATION

1 ALLOW JARS TO CURE FOR

PRESCRIBED PERIODS

1 DETERMINE SHEAR STRENGTH

OF SAMPLES AT END OF CURING PERIODS

I . PLOT SHEAR STRENGTH

VERSUS CURING TIME

1 , REPEAT pti & BENCH TESTS

FOR ANOTHER CLAY

Figure A-2: Flowchart of Bench Testing Procedures

A-4

4. Add target pH percentage of lime or ratio of gypsum to 6 jars based

on weight of solids, again using the tare function.

5. Add two lower percentages or ratios to remaining 12 jars.

6. Thoroughly mix each sample for approximately 2 minutes with drill

mixer.

7. Label each sample with the following information:

a. where the clay is from

b. the additive used (lime or gypsum)

c. the percentage or ratio added

d. the curing time (7, 14, or 28 days)

e. the test number (two tests for each curing time)

8. Immediately test the vane shear strength of the 7-day samples for a

measure of the initial shear strength of the mixes.

a. Place the jar on the raised platform of the device.

b. Lower the vane down into the clay mixture by turning the top

crank.

C. When vane is inserted to a predetermined depth, turn crank on

the side of the device to rotate the vane and obtain the shear

strength.

9. Remix the 7-day samples immediately after testing O-day vane shear

strengths.

10. Allow jars to cure for prescribed periods, testing each as

previously outlined on the required dates.

11. When all tests are complete, plot shear strength versus curing time.

12. Repeat procedure for another clay.

A-5

ACCELERATED CURING TEST PROCEDURES (FOR LIME ONLY) (Figure A-3)

1. Mix bulk sample of waste clay and determine batch solids content as

previously outlined.

2. Weigh samples of clay into 5 mason jars using tare function.

3. Determine weight of solids in jar from solids content.

4. Add target pH percentage of lime to clay and mix thoroughly with

drill mixer.

5. Place jars in 105 degree Fahrenheit oven.

6. Remove one jar each day for 5 days and

as previously outlined.

7. Plot shear strength versus curing time

time required for adequate strength gain.

CENTRIFUGE TESTING PROCEDURES (Figures A-4

1. Adjust centrifuge for testing.

test the vane shear strength

for five samples to determine

and A-5)

a. Calculate offset acceleration by the following equation:

Ao = (Ra/Rs) x At

where: Ao = offset acceleration

Ra = radius to accelerometer = 37.5 inches

Rs = radius to the center of gravity of the sample

At = test acceleration

b. Place 8 centimeters of water in the four plexiglass containers.

c. Place the plexiglass containers in the aluminum buckets and

secure the buckets to the bucket housings with the pivot bolts.

A-6

I DETERMINE BULK SOLIDS CONTENT OF CLAY I

IADD 12% LIME ~0 CLAYI

Figure A-3: Flowchart of Accelerated Curing Test Procedures

A-7

CALCULATE OFFSET ACCELEFUWON

I PLACE 8 cm OF WATER

IN PLEXIGLASS CONTAINERS

I PLACE CONTAINERS

IN CENTRIFUGE

4 a REMOVE DEBRIS FROM

CENTRIFUGE INTERIOR

1 1 TURN ON POWER/

I +

ACCELERATE WATER SAIYPLES TO OFFSET ACCELERATION

I +

ADJUST OVERSWING ADJUSTMENT BOLTS AS OUTLINED L

I +

[ADJUST ADL AS OUTLINED] I 1

Figure A-4: Flowchart of Centrifuge Preparation Procedures

A-8

/MIX WASTE CLAY

I

/CHECK on LEVEL1

[DETERMINE % SOLIDS], 1

I

ICLEAR INTERIOR

I #IX CLAY WITH LIME

OR GYPSUM 1 SECURE ACCESS DOORS)

I I

ADD CLAY TO PLEXIGLASS f tUtIN POWER ON]

CONTAINERS 1

I

4 ICOMPRESSED AIR ONI

c PLACE CONTAINERS IN I

ALUMINUM BUCKETS

I

[START CENTRIFUGE 1

'PLACE BUCKETS I

INCREASE SPEED TO IN HOUSING OFFSET ACCELERATION

I TAKE PHOTOGRAPHS

,

AT PRESCRIBED TIMES

c 1 END TEST WHEN

INTERFACE STABILIZES I

CLEAN CAMERA CLOSE-UP LENS

I 'INSERT FILM

PACKET

I LOWER CAMERA

TO TEST POSITION

1 ----{STOP CENTRIF=]p

* 1 REMOVE CONTAINERS]

' DETERMINE INTERMEDIATE 1

PERFORM VANE ~ CLAY SOLIDS CONTENT SHEAR TESTS

1

L

I PLOT VERSUS ELAPSED

MODEL TIME 1 CORE SAMPLES 1

I

Figure A-5: Flowchart of Centrifuge Operations

A-9

d. Remove tools and loose debris from inside of centrifuge and

secure access door.

e. Turn on the main power at the circuit breaker.

f. Turn on the compressed air to cool the main spindle housing.

g. Adjust the overswing adjustment bolts in the restraining

brackets across each bucket housing. This is an iterative

procedure in which the water samples are spun-up to the required

offset acceleration. The heights of the water interfaces are

observed and the centrifuge is stopped. Adjustments are made to

the bolts if the interface deviates from the known 8

centimeters. If the water level is too high, the bucket has

under-rotated and the screw should be raised. If the water

level is too low, the bucket has over-rotated and the screw

should be lowered. Once the proper interface level is obtained,

for the given accelerationno further adjustment

level.

h. Adjust Acceleration

is necessary

Divergence Limiter (ADL). This is another

iterative procedure in which the acceleration level and

operating window are adjusted. After each adjustment, the

acceleration is increased and decreased from the given offset

acceleration to determine where the centrifuge shuts off (the

operating window) when the device is turned on. Once the

desired window is obtained no further adjustments are necessary

for the particular offset acceleration. A window of plus or

minus 5 g's was used for this testing.

A-10

2. Prepare samples.

a. Thoroughly mix clay samples from four different sites in four 10

gallon buckets. (Note: solids content of clays at this time

must be greater than desired test solids content.)

b. Determine the solids content of the clays as previously

outlined.

c. Place approximately 1000 grams of clay in a container, weighing

to the nearest .01 gram (WC).

d. Knowing batch solids content (%Sb) and desired test solids

content (%Sd), determine the quantity of water to be added (Ww)

by the equation:

Ww = (WC x %Sb/%Sd) - Wc.

e. Determine quantity of lime or gypsum to be added (%A) by the

following equation:

%A = (Wc + Ww) x %Sd x (%T/100).

where %T = 12 for lime or 200 for gypsum

f. Make the required additions to the clay and m

egg beater.

ix thoroughly with

g. Add clay to plexiglass container to initial model height of 8

centimeters. Actual initial model height should be determined

from the first centrifuge photographs.

h. Repeat procedures for three other clay samples.

A-11

3. Prepare the centrifuge for testing.

a. Check oil level in main spindle housing under centrifuge. Add

oil if necessary.

b. Place plexiglass containers in aluminum buckets.

c. Place aluminum buckets in bucket housings, securing with pivot

bolts.

d. Inspect inside of centrifuge. Remove all tools and loose

debris.

e. Secure access door by inserting locking rod.

f. Start centrifuge and increase acceleration to offset value.

g. Lower camera into position over viewport.

h. Begin taking photographs one minute after the offset

acceleration is reached.

i. Take following photographs in geometrically increasing time

increments to allow logarithmic plots with equally spaced data

(e.g., 1, 2, 4, 8, 16, 32, etc.)

j. If centrifuge is left unattended while operating, ADL should be

engaged. The switch is set such that the indicator light is

off.

k. Test is completed when two sequential photographs of the same

sample indicate no movement of the interface.

4. Analyze centrifuge samples.

a. Remove the plexiglass buckets from the centrifuge and decant the

supernatant.

A-12

b. Obtain the average shear strength of the sample with the vane

shear apparatus, testing the sample in two different locations

(see Figure A-6).

c. Obtain the final solids content with depth of the sample by

making a coring with the modified syringe. The coring should be

sectioned into approximately 1 centimeter thick samples. The

solids content of the samples should be determined as previously

outlined and plotted versus average depth.

A-13