influence of cemented mill tailing as backfill material in open stopes
DESCRIPTION
Filling of the mine voids has multiple reasons such as, a simple method of tailingsdisposal, or as a void filler, in a few cases it is followed as an economic method forsupporting the weak wall rocks, and lastly, for creating a working floor in a fewstoping operations. Based on the specific purpose of backfilling, the composition ofbackfill material has been varied. Of the various reasons of backfilling, the role ofbackfilling in providing a passive support to the weak sidewalls of a stope is gainingimportance in both cut-and-fill as well as open stoping operations. In the former, fill isintroduced periodically during the progressive extension of the stope while in the latercase, fill placement in a particular stope is delayed until production from it iscomplete. In both cases, the function and duty of the fill mass can be prescribedquantitatively. It is necessary to design the backfill to meet prescribed operationalfunctions and safety requirements. In order that the backfill material serves thepurposes of offering passive support to the sidewalls, cement is added as a binder todevelop strength to the backfill.TRANSCRIPT
Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 1
INTRODCUTION
1.1 General Filling of the mine voids has multiple reasons such as, a simple method of tailings
disposal, or as a void filler, in a few cases it is followed as an economic method for
supporting the weak wall rocks, and lastly, for creating a working floor in a few
stoping operations. Based on the specific purpose of backfilling, the composition of
backfill material has been varied. Of the various reasons of backfilling, the role of
backfilling in providing a passive support to the weak sidewalls of a stope is gaining
importance in both cut-and-fill as well as open stoping operations. In the former, fill is
introduced periodically during the progressive extension of the stope while in the later
case, fill placement in a particular stope is delayed until production from it is
complete. In both cases, the function and duty of the fill mass can be prescribed
quantitatively. It is necessary to design the backfill to meet prescribed operational
functions and safety requirements. In order that the backfill material serves the
purposes of offering passive support to the sidewalls, cement is added as a binder to
develop strength to the backfill.
Cemented backfill became popular when it is taken as a means to support the weak
wall rock. However, the high price of Portland cement has thrown open the challenge
of economic viability. The consequence is that the researchers have tried to look for
binder alternatives which have eventually resulted in the application of high density
slurry and paste backfill materials that have improved backfill mechanical strength
response, reduced cement consumption and water disposal.
Compressive strength and permeability are the two factors affecting fill stability. The
compressive strength of fill required in mining operation varies over a wide range
depending on its application. and the variation in the strength is dependent on the
percentage of binding material.
Permeability is an extremely important fill property, which affects the stope
dewatering capability, and it should be taken into account in slurry fill design. There is
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
no designated provision for percolation rate, but the universally accepted rate in the
slurry hydraulic fill is about 2.5 to 3 x 10-3 cm/s.
The backfilling method used is often dependent on the mining system adopted and can
be classified in to two general groups; these are cyclic and delayed filling methods.
With delayed backfill, the fill must be capable of existing as a free standing wall after
being exposed during pillar recovery. From the practical point of view the stability is
more governed by the dimension of the stope in most of the cases.
Cemented fill of various solid compositions has been employed as an artificial support
with considerable popularity in the recent years. The most widely used cemented fill
consists of classified tailings, or mixtures of rock, sand and cement. Results of
practical studies show that cement content in a fill and its slurry density are essential
factors affecting fill stability and the economy of backfilling.
With accumulation of large amount of carbonaceous waste from the thermal power
plants and mill tailings from the mineral beneficiation plants of metal ferrous mines,
the environmental concern regarding the disposal of mine waste has grown in recent
years and this trend is unlikely to end. Thus the backfill technology has to address
three basic issues, namely the property of the backfill material which enhances the
stability of the ground, the utility of mine waste as a backfill material to control the
environmental pressure and lastly it has to take into account the pressures created by
the conservation of mineral resource.
The present study focus is laid on the characterization of cemented mill tailings and
attempts were made to increase the mechanical properties of the cemented mill tailings
with out adding on to the cost of production. The basic objective of increasing the
strength of the mill tailings is to study its influence on the stability of the wall rocks,
which are under severe side pressure.
1.2 Overview Backfilling of mine voids is one of the two popular techniques adopted for controlling
local, stope wall behaviour and also mine near-filed displacements. Brady and Brown
(1985) outline three mechanisms that demonstrate the support potential of mine
backfill. The three mechanisms, illustrated in fig.1.1, represent fil1 performance as
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
superficial, local and global support elements in the mine structure. As a superficial
support element, fil1 provides kinematics constraint on surface blocks in distressed
rock. This helps prevent large-scale wall movements and collapse of openings. When
placed tight to the back, fill can provide back support as well. At a local scale. The
passive resistance of the fill may be mobilized by pseudo-continuous and rigid-body
displacements of the wall rock induced by adjacent mining. Lastly, fil1 may act as a
global support element and reduce stresses in the rock by carrying part of the load. In
order to provide such support the fil1 must either be very stiff or undergo large strains
in confined compression. The latter might occur in narrow-vein mining where the
hanging wall to foot wall closure strain is high.
Fig.1.1.Modes of support of backfill: (a) kinematic constraint on surface blocks in
de-stressed rock; (b) support forces mobilized locally in fractured and jointed rock; (c) global support due to compression of fill by wall closure (Brady and Brown, 1985).
The disposal of mill tailings in underground not only reduces the environmental
impact but provides the base of engineering material which can be used to improve
both the ground conditions and also acts as a working platform in a few stoping
operations. According to Barrett (1978) the purpose of the backfill is not to transmit
the rock stresses, but to reduce the relaxation of the rock mass so the rock itself will
retain a load carrying capacity and will improve load shedding to crown pillars and
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
abutments. This leads to less deterioration in ground conditions in the mine, improving
operations and safety.
When used as a support medium in open stope mining. The fill must be able to remain
stable as a freestanding wall during adjacent mining. It may also be required to remain
stable during undercutting. Depending on the mining sequence and ore body geometry,
the fill may undergo various combinations of monotonic and cyclic loading prior to
and during exposure.
Adding low percentages of ordinary Portland cement (OPC) of between 3% and 6% by
weight, permits the development of cohesive strength and the ability for the backfill
mass to be self supporting when exposed in vertical faces by adjacent pillar mining.
The self supporting nature of the backfill permits higher recovery of pillar ore, which
in turn improves the utilization of the mining reserve and the economics of the mining
operation. Increased ore recovery results in longer mine lives.
In some mining methods the backfill forms a working platform for people and
equipment and therefore must be capable of supporting the traffic. Generally, cement
is not required in this application.
The placement of backfill underground directly reduces the quantity to be disposed on
surface. This has direct operating and capital cost benefits and reductions in future
rehabilitation costs.
In India, large amounts of tailings and waste rock produced annually through mining
activity. The tailings consist of a coarse fraction and a fine fraction and can be either
reactive (acid generating) or non-reactive. Most mining operations utilize the coarse
fraction for backfilling while the fines have been traditionally disposed of at the
surface in tailings ponds. The development of paste backfilling technology allows an
underground mining operation to use a portion of the fines for backfill. reduction in
tailings pond accumulation and possible elimination of impoundment for reactive
tailings in ponds and on land; reduction in costs associated with constructing and
reclaiming tailings ponds during mine start up and final close down; removal of
constraints placed upon fine grinding as an effective means of mineral value liberation,
i.e. gold, uranium and metal bearing ores.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
One practical method to modify the physical and chemical properties of the fine mill
tailings so that they can be utilized in an appropriate manner for underground
applications is by using the superplasticizer (Alsayed, 1998).
In short it can be said that backfill is one of the most important processes used
extensively in underground mining operations around the world for many purposes,
and they are:
• To provide ground support;
• To create a fresh working surface and roadways for machinery;
• To permit maximum ore recovery;
• To safe and selection extraction of ore deposits without loss of ore and
encountering dilution problems; and
• To dispose of mining waste materials.
1.2.1 Cemented Mill Tailing as Backfill Considerations There are two main types of Cemented Mill Tailing as Backfill: Hydraulic Slurry Fill,
and Paste Backfill. Each has certain advantages and disadvantages and their use is
dependent on certain aspects of the present mining method and surrounding ground
conditions. An important parameter for determining the suitability of hydraulic and
paste backfill is its strength. An adequate uniaxial compressive strength for a backfill
in a typical mine is 0.7–2 MPa (100–300 Psi), and common strength specification is 1
MPa after 28 days (Andersion, &Pigdon 2005). The paste backfill must be designed so
that it will reach its target compressive strength values at 28 days of curing age and
beyond. This can be done by choosing optimal mixtures for tailings. In backfill the
cement strength can be reduced when rock walls are sufficiently close together to help
support the backfill by shearing stresses at the wall backfill contact (Mitchel and
Smith, 1982).
An adequate modulus for a backfill in a typical mine will vary widely depending on
the surrounding ground conditions, mining method, and mining depth. Resistance of
the fill to converge should be sufficient to absorb the strain energies expected as
mining progresses.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Hydraulic fills are slurry fills having a pulp density in the range of 60–75% solids
weight for weight. Viles and Davis (1989) state that as much as 30% of the total initial
fill volume is lost by dewatering. Hydraulic fills consist of classified coarse tailings
along with a binder. The fine tailings are usually excluded from the fill because their
removal improves flow characteristics provides better fill consolidation and
subsequent water drainage, the high water content allows the slurry to be transported
by gravity or pumping at relatively high placement rates through boreholes and
pipelines. Level preparation and clean-up can be very time-consuming with this type
of fill. The high binder dosage needed to create a hydraulic fill with good strength
properties can be expensive.
Paste fill, on the other hand, has high solids content, usually with a pulp density in the
range of 75–85% solids weight for weight. Paste backfill is cheap as comparison to
rock fill or hydraulic fill (Hassani & Archibald, 1998). This type of filling usually
contains fine material. According to Slatter (2006) as the concentration of fine particle
(below 20μm) increases, viscous stresses also increases, and paste become non-
Newtonian in nature. And it flow just like laminar. This viscous character is a dynamic
property of paste. When the paste is stationary, the attractive forces between particles
or agglomerates form a three- dimensional structure, which extends to wall of the pipe.
The shear stress required to rupture this structure and initiate flow, is called the yield
stress. Below this stress the material behaves like an elastic solid. As shear stresses and
shear rates increases, the agglomerates gradually reorientate and disintegrate, resulting
in a decrease in the viscosity of the backfill material. This process is known as shear
thinning. At very high shear stresses and shear rates, the reorientation and
disintegration process reaches equilibrium, and the viscosity becomes constant (Slater
2006).
Paste fills have gained popularity in the past few years due to several operational and
environmental benefits due to following achievement.
• The water solid ratio for the paste fill is low, producing greater strength gain
per unit volume of cement added to consolidate the fill. So strengths
approaching rock fill can be achieved, while using less cement than hydraulic
fills;
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
• Facilitates a rapid mining sequence because strength is achieved earlier
compared with hydraulic fill;
• Allows the use of waste rock and slag as well as the fine fraction of tailings,
thereby reducing surface tailings impoundment requirements; so reduced the
environmental costs of the mine;
• A uniform graded backfill is capable of generating greater compressive
strength due to fewer voids;
• Paste fill has higher stiffness than hydraulic sand fill because of reduced
porosity;
• Decant water from the fill is virtually eliminated, reducing costs and
problems associated with barricade set-up/level clean-up and wear on mine
dewatering pumps;
• The present bore-hold delivery systems of slurry fills can be used;
• In saturated condition within the paste backfill the ingress of oxygen, so
limiting the potential for generation of acid mine drainage.
Due to these advantages the use of paste technology has been accepted worldwide in
the modern mining industry of today.
1.2.2 Geology of Hutti Gold Mines The Hutti Gold Mine is situated in the Archaean Hutti–Maski Greenstone Belt
(HMGB) in the south Indian Dharwar Craton (Figure1.2). The hook-shaped HMGB
consists of a heterogeneous assemblage of volcano sedimentary units, which are
metamorphosed to the greenschist–amphibolite facies transition. The western
boundary is tectonically juxtaposed to gneisses; the northern, eastern and southern
contacts are intrusive. Two generations of granitoids can be distinguished, namely the
Yellagatti and Kavital Granites. The stratigraphy of the schist belt remains unresolved
because of the lack of any definite younging direction criteria and radiometric dating.
A granitoid pebble within a conglomerate forming the base of the HMGB has yielded
a SHRIMP weighted mean 207Pb/206Pb zircon age of 2576F12 Ma, which is
interpreted as the maximum age of the belt.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Fig.1.2. Schematic geological map of the Hutti– Maski Greenstone Belt (Jochen Kolb 2004). Inset shows location of the study area in South India.
1.3 Need of the present Investigation Several mines in India are using hydraulic backfill, cemented rock filling etc., but until
now there in case where paste backfilling is being used. Paste backfilling is far better,
cheap than hydraulic backfill. The published research work in India has been on the
preparation and transport of hydraulic backfil1 rather than improving its cured
geotechnical properties. However in the last two decades a considerable extent of work
has been reported to have been carried out in many other countries on the aspects of
improving the strength and flowability of concrete by adding superplasticizer (Alsayed
98, Yoshioka 2002, Nkinamubanzi 2004, Khatib 1999, and Radocea 1992). However
the effect of superplasticizer on strength and flowability in cemented paste backfill
was not attempted. In the absence of any detailed laboratory work on paste fill strength
and deformation behaviour, in the present project work, the initial design of cemented
backfill technology for The Hutti Gold Mines Ltd. has laid its focus on improving the
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
strength of the backfill material, with different proportions of cement as well as the
addition of superplasticizer.
1.4. Objectives In order to develop an understanding of the strength and deformation behavior of
Cemented mill tailing as backfill and to optimize its use at the Hutti Gold Mine. The
specific objectives of this thesis are to:-
• Review the pertinent physical and mechanical properties of paste backfill
along with relevant aspects of soi1 mechanics and published results of
laboratory testing Paste backfill;
• Laboratory testing is identify a cost – effective backfill mixture which will
fulfill the desired strength and deformation behavior of Hutti gold mine
Cemented mill tailing backfill as a function of binder content and cure time
in uniaxial, direct shear test. So that the mix characteristics will be adjusted
in such a way that when underground opening is filled with this mixture,
the filled structure will safely withstand strata loading, and will limit
underground and surface movements; and
• To develop an understanding of the performance of cemented paste backfill
when exposed to superplasticizer.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 2
LITERATURE REVIEW
The literature review is focused on the published results of laboratory testing of
backfill material. It is structured around specific geotechnical properties and includes
discussion of relevant aspects of established soil behavior. A vast amount of literature
exists on laboratory testing of conventional hydraulic fill. Since paste fil1 is a
relatively new technology, most of the existing literature focuses on preparation and
transport rather than geotechnical characteristics. The following aspects of paste
backfill behaviour will be critically reviewed:
I. Specific gravity;
II. Particle size distributions;
III. Porosity & void ratio;
IV. Permeability;
V. Unconfined compressive strength;
VI. Shear strength.
VII. Superplasticizer
2.1 Specific Gravity The specific gravity of a fill is the ratio the weight in air of a given volume of fill
particles to the weight in air of an equal volume of distilled water. The specific gravity
of a fill is often used in relating a weight of fill to its volume. By the help of specific
gravity we can find the unit weight of fill. And unit weight is very useful for finding in
nearly all pressure, settlement, and stability problem in backfilling.
2.2 Particle Size Distributions The size distribution has the largest effect on fill porosity and delivery. Tailings have
been the most widely used materials in backfill, and as such their properties have
become of main interest. It is the fine fraction of the tailings that makes a fill into a
paste. A paste fill contains at least 15 % by weight of particles less than 20µm (Slatter
2006; Amaratunga 1997). It provides the water retention properties needed to prevent
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
water bleeding, prevent One of the most important characteristics of any fill material is
a well-graded particle size segregation of larger particles in the fill, and act as a
lubricant during pumping (Amaratunga 1997).
A backfill should have a suitable particles grading for any of its coarse aggregates.
Viles and Davis (1989) state that suitable backfill materials have similar features to
those specified for concrete aggregates. If coarse aggregates (≤ 25μm) are incorporated
into the mix, it must have a well graded distribution to help reduce the void ratio in the
fill. Particles size distribution curve can be described by the coefficient of curvature
(Cc) which should be between 1 and 3 for a well graded distribution (Das 2002).
2.3 Porosity & Void Ratio Porosity is defined as the amount of air space in a soil that is available for fluid flow
(Meegoda and Gunasekera 1992). The porosity, η, defined as the ratio of void volume
to total volume in a fill mass.
While, the void ratio, e, is defined as the ratio of void volume to solid volume in a soil
mass,
So, ( )(%), 100( )volumeof airporosity nvolumeof soil
= × 2.1
(1 )en
e=
+ 2.2
Porosity decreases markedly with pouring pulp density within the practical hydraulic
range of 65- 75% pulp density. At low pulp densities a porous cement gel is formed
(Mitchel and Wong, 1982) which in turn, produces higher backfill porosity. The
porosity of a fine tailing decreases with addition of cement
Many author present the results of an investigation on the influence of porosity (n) on
the uniaxial strength σn of various engineering materials. Fig. 2.1 shows the results of
test work by Li. Li; and Aubertin. M (2003) has determined the effect of each of these
factors On UCS. Higher UCS from lower porosities due to greater particles
interlocking and the fact more cement is available per unit volume of backfill.
Porosity is generally easy to measure with an acceptable level of accuracy, so it is a
practical property to use to characterize the influence of material internal structure. It
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
should also be kept in mind, however, that porosity, taken as an average scalar
parameter, provides only part of the information.
Fig.2.1. The effect of porosity on UCS of cemented sand (Li. Li. et al. 2003).
2.4. Permeability Permeability is the measure of the ability for pore fluids to pass through the fill
material. Permeability is the key to successful drainage. Upon placement it is desirable
to have a fill drain as quickly as possible so that the fill material can settle and reach its
operating density. This process of drainage is dependent upon the permeability of the
fill. Thus preparation of the backfill is often required to remove the fine particles.
Paradoxically the best result strengths are obtained at high binder levels where the
binder is as fine as possible, therefore reducing permeability (Viles and Davis 1989).
Viles and Davis (1989) has given one solution to these problems is to eliminate the
need drainage by locking up the water within the backfill.
The importance of permeability can be seen in the fact that hydraulic mine backfill
must be dewatered from a moisture content of 30-40% (by weight) to around 20%
before being able to perform its ground support role (Viles and Davis 1989).The
processes by which dewatering occurs are decanting, exudation, and percolation, all of
which are dependent upon the permeability of the fill.
It is common practice to specify fill permeability in terms of percolation rate.
From Darcy’s Law the coefficient of permeability can be defined as (Aysen 2002)
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
hALQKΔ××
= 2.3
Where K= the coefficient of permeability (m/sec)
Q= the volume rate of flow through the fill (m3/sec)
L= length of fill (m)
A= the cross-sectional area of the fill (m2)
Δ h = the head differential across the fill (m)
The percolation rate is the same as the coefficient of permeability except that the
hydraulic gradient hLΔ is equal to unity. Therefore, the percolation rate of any fills
material can be written as:
Kp=Q/A 2.4
Where Kp = the percolation rate of fill (m/sec)
Since the percolation rate is dependent on the fill material, the grain size distribution
and the porosity affect the drainage behavior of the fill. As for the fluid flowing
through the fill, in this case water, the viscosity and degree of saturation of the fill also
influence its dewatering capabilities.
The permeability coefficient varies considerably with grain size, Plasticity, and void
ratio demonstrate that Hazen's formula is best for predicting the average permeability
of sand tailings(Rankine 2002)
K=C d102 2.5
K= permeability constant, cm/sec
Where d10 = grain size in millimeters for which 10% of the particles pass by weight.
C=constant (≈1)
For a given size distribution, permeability will decrease with void ratio. A 20%
decrease in void ratio can cause a 50% decrease in permeability. The addition of
cement to tailings also decreases the permeability of tailings with finer materials
experiencing a greater percentage decrease (Mitchell and Smith 1982). The effect of
cementing reactions is to reduce the porosity of the fill and block drainage paths.
Permeability is an important design parameter for slurry fill. A good permeability is
required to ensure that excess transport water drains from the fil1 as quickly as
possible. The general industry accepted standard for minimum permeability of slurry
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
fill is about 2.5 to 3 x 10-3 cm/s determined using either a constant head or falling head
permeability test.
2.5. Unconfined Compressive Strength The unconfined compressive strength (UCS) is regarded as the most useful measure
for comparing the relative strengths of paste backfills. There are several factors which
affect UCS including cement content, water-solids ratio, size distribution, and cement
type. Although the cement content contributes directly to the strength of the fill, the
final amount of cement is controlled more by economics than by design strength
parameters.
Laboratory work has also indicated that the grain-size distribution of a fill material is
important in optimizing its strength. Modification of a fill’s grain-size distribution is a
useful tool for improving a fill’s strength characteristics. The “desirable” size
distribution can be described as well-graded, containing course to fine particles
Thomas (1973) indicated that as the fines content of a fill material was increased, the
strength increased. Three cemented fills, prepared from deslimed tailings and
distinguished according to their grain-size distributions, were tested for strength.
Although comparable, the strengths of the medium and coarsely graded fills are less
than that of the well-graded fill that contains more fine particles.
2.5.1. Effect of Curing Conditions The normal curing procedure for paste backfill, outlined by Archibald (1997), is to let
the sample drain for 48 hours then set it aside in a sealed polythene bag. Many paste
fills with a measurable slump are prepared in an oversaturated condition. If this is the
case, the paste fil1 may produce some decant water or bleed water from the base of a
sample tube which is not completely sealed. If the amount of water is small (e.g. 1-2%
of total sample weight), the effect of letting the water bleed from the sample prior to
curing should not be significant. The conditions under which the sample is cured have
a much greater effect on sample strength. If the sample is allowed to dry out at all, the
UCS will increase since lower moisture content will result in better cement hydration
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
(Mitchell and Wong, 1982). This is not considered realistic for paste fil1 because it is
not free-draining and remains close to saturation for a much longer period of time.
The curing time, temperature, and humidity have also been shown to have an effect on
cemented fills. As with normal concrete, the strength of a cemented fill increases with
curing time. Perry (1990) tested specimens of differing water/cement ratios and found
that ultimate fill strength is achieved after long periods of up to 90 days. Elevated
curing temperatures also accelerate the gain in strength of cemented backfills provided
that full saturation is maintained and the temperature does not exceed a critical level.
Kejin Wang et al. (2004) observed that strength can be increased by increasing curing
temperature and found that for common backfills, a curing temperature of 50°C is a
safe limit. As opposed to time and temperature, the strength of cemented fill decreases
with humidity. Compressive strength increases more rapidly when dry cured rather
than wet cured (Mitchell and Wong, 82). Curing temperatures less than 15° C retard
reactions while higher temperatures accelerate reactions, excessively high curing
temperatures degrade humic materials in highly organic soils, detrimentally affecting
strength gain (Jesse Jacobson 2002)
Although these factors affect the strength of cemented fills, they may be impossible to
control due to the underground environment. Whatever curing temperature is used, the
specimens should be properly spaced and fans or pumps should be used to ensure that
all specimens cure at the same temperature (Jesse Jacobson 2002). It is important to
note that, excluding curing time, few mining environments will be able to provide the
ideal hot and dry environment that can help improve a cemented fill’s strength.
G. M.M. Ley, C.M. Steed, D. Bronkhorst and Robert Gustas (1998) have found in the
case of the garson mines that the laboratory material values (average UCS .65 MPa
with an average Young’s Modulus of 215 MP) for the paste fill varied significantly
from those obtained from insitu samples, with the insitu values being stronger and
(1.5MPa) stiffer (920MPa). Temperature monitoring carried out during fill placement
indicated high hydration temperatures which may account for the increased strength.
Although humidity control is not standardized, Jesse Jacobson (2002) recommends
several methods for controlling the humidity in the curing environment: curing
samples in sealed, airtight tubes; curing underwater; or placing samples inside an
insulating jacket found that providing the samples access to water while applying a
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
confining pressure during curing, which may imitate field conditions more accurately,
reduces strength.
Although free water in sample will readily evaporated at a relative humidity less than
45%, the cement gel water appears to resist evaporation and cured water content, under
low relative humidity, increases with increased cement content (Mitechell and Wong,
1982). Humid cured specimen, on the other hand, showed a general deceases in water
content with increases cement content, a result due to Hydration. Uniaxial strength, for
given cement content, was found to increase as the water content decreased.
2.5.2 Pulp Density Pulp density is a measure of the percentage of the solids by total weight in the slurry or
paste backfill. Pulp density is an important factor in fill strength development and
pumping characteristics due to water content paste fills have a pulp density which
range from 75 % to 88% while hydraulic fills are placed at pulp density of 55% to 75
%( Amaratunga, and Yaschyshyn, 1997). For strength purpose a high pulp density is
ideal, however, paste fill pumping and placement consideration limit the density.
Fig. 2.2 shows the results of test work by Benzaazoua, (2003) have a significant
influence on the mechanical performance of the paste fill in different ways depending
on their concentration in the pore water. Fig.2.2 clearly shows that decrease in strength
(UCS) occurred with an increase in the water content (or slump value). This effect is
clear at short-term (7 days of cure) as well as medium term curing times (28 and 56
days of cure), however it is less clear at the long-term curing time (120 days of cure)
possibly due to a certain internal weathering of the samples as seen for samples made
of tailings
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Fig.2.2. Effect of the water content (w%) on UCS (M. Benzaazoua 2003)
2.5.3 pH When a significant quantity of lime is added to a mill tailing, the pH of the mill tailing-
lime mixture is elevated to approximately 12.4, the pH of saturated limewater at 25° C.
This is a substantial pH increase for mill tailing. Mehta and Monteiro (1993) show that
the solubilities of silica and alumina are greatly decreased at reduce the pH levels,
which can lead to a decrease in pozzolanic reactions as well as decreased cation
exchange capacity. Mehta (1993) suggested that the high pH causes silica from the
clay minerals to dissolve and, in combination with Ca++, to form calcium silicate. This
reaction will continue as long as Ca (OH)2 exists in the mill tailing and there is
available silica. A very small amount of Ca (OH)2 was required to raise the pH to the
target value.
2.5.4. Mineralogical Effects The mineralogy of the mine tailings and cement plays an important role in the strength
development of cemented paste backfill. The mineralogical composition is determined
by X-ray diffraction analysis (XRD), which provides the crystalline mineral
assemblage of a sample.
It is common knowledge that sulphide mine tailings generate sulphuric acid in the
presence of water and oxygen, which may lead to possible chemical weathering and
other consequences. The presence of sulphide minerals within cemented composites as
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
well as the soluble sulphates has a deleterious effect on the strength of paste backfill
due to sulphate attack (Kesimal, & Ercikdi, 2004).
The effect of sulphate on the strength of cemented paste backfill depends on the
strength of sulphate concentration, the curing time, and the cemented composition and
content (Benzaazoua et al. 2003).
According to Archibald (1999) the deleterious nature of sulphide on cement
performance increases with increasing O2 concentration, specific surface area and
temperature. pH is also an important factor in sulfide oxidation. At low pH, bacterial
action is an important factor in oxidation. But this role is eliminated at pH levels above
approximately 4. A typical sulfide oxidation reaction for pyrite, described in the
following equation illustrates the reason for its harmful effect on cement hydration.
FeS2 +15/4 O2 +7/2O2→Fe(OH)3(s) +2SO42- +H+ 2.6
the strength loss experienced by cement, curing the presence of this reaction, is due to
the production of hydrogen ions causing an acid attack that dissolves the calcium
hydroxide found in hydrating cement and the precipitation of gypsum, ettringite and
monoaliminate sulphate by the reaction of aluminates in the cement the mineral
species have low molar densities compared to the cement components they replace and
thus, cause expansion in cements.
Mitchell and Wong (1982) note that large variations in strength should be expected
between cemented tailings sands of different mineral or chemical composition. They
performed tests on tailings with 2.5% sulphur content and found that it produced
strengths that were only 60% of the strength of non-sulphide tailings prepared with the
same cement content due to an internal sulphate attack, which results from the
chemical interactions of the sulphate ions with the Portland cement hydration products.
They indicate that the possibility of long-term strength loss in some cemented tailings
backfills should be considered in backfill design and mine operations.
Kesimal (2004) also find that effect of sulfide on UCS. It was pointed out that for high
sulphide tailings, neither the slag-based binders nor the fly-ash based it was pointed
out that for high sulphide tailings; neither the slag-based binders nor the fly-ash based
binders were effective, whereas the sulphate-resistant-based binder gave good long-
term strength. To control the deteriorating effects of sulphate attack, the amount of
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
calcium hydroxide and calcium aluminates hydrate must be kept to a
minimum(Hassani et al. 2001).
2.6. Shear Strength
2.6.1. The Mohr –Coulomb Failure Criterion The shear strength of backfill has traditionally been represented by the Mohr-Coulomb
failure criterion which States that shear strength increases with increasing normal
stress on a plane:
τ=c+ σ tanΦ 2.7
Where τ = shear stress on the failure plane.
c = cohesion intercept.
σ = normal stress on the failure plane, and
Φ = angle of shearing resistance.
Since shear stress cm only be resisted by the skeleton of solid particles according to
Terzaghi’s effective stress law. Shear strength is expressed as a function of effective
normal stress:
τ=c'+ (σ –u) tanΦ' 2.8
τ=c'+ σ' tanΦ' 2.9
Where u= pore pressure, and
c' and Φ' = shear strength parameters in terms of effective stress (drained shear
strength parameters).
2.6.2. Effective Shear Strength The determination of the internal friction angle (φ) and the effective cohesion (c) is
commonly accomplished by the direct shear test or the triaxial test. The direct shear
test is preferred because of its simplicity and lower cost. The advantages and
disadvantages of direct shear tests are given below.
Advantages of direct shear testing are as follows:
• The test is relatively inexpensive and quick to perform.
It requires less sophisticated equipment than other methods and it is easier to reduce
the data and interpret the results
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
• It has been found that soil parameters φ and c obtained by direct shear
testing are nearly as reliable as triaxial values. Typical values obtained
with the direct shear test are 1 to 2 degrees larger than values obtained
with the triaxial test (Bowles, 1979).
• It is good for measuring residual strength values.
Disadvantages or limitations of the direct shear test (Holtz and Kovacs, 2003) are as
follows:
• Shearing stress is not uniformly distributed across the sample. Initial
failure occurs at the corners and ends of the box, and propagates
towards the center.
• The test forces failure to occur along a fixed zone or plane.
• There is an uncontrolled rotation of principal planes and stresses that
occurs between the start of the test and failure.
• Pore water pressures for fine-grained soils are neither controlled nor
monitored.
2.7 Superplasticizer
2.7.1 Backfill Strength Enhancement by Superplasticizer
The properties of backfill material are governed by its strength and flow behavior,
which can achieved by the dispersion of backfill material. It is widely known that
better strength and fluidity is achieved by the addition of superplasticizer (Chandra,
2002.).
A superplasticizer is one of type of admixtures called water reducers (Stuart.1980) that
are used to reduction in water requirement of concrete. Water reduction results in
undesirable effect on setting, bleeding, segregation and hardening characteristics.
Superplasticizer is chemically different from normal water-reducers, and is capable of
reducing water contents by about 30%. The presence of superplasticizers (SP) in a
concrete mixture is quite advantageous, in that they assist in the effective dispersion of
cement particles due to the electrostatic repulsion (Termkhajornkit 2004) and yield
stress value decreased but the plasticity does not decrease significantly, paste will
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
obtain good flow ability without ingredient segregation. So it improves the workability
of concrete (V.S. Ramachandran, 1979 and P. Termkhajornki, 2004). The water to
cement ratio is reduced when SP is added to cement paste, which leads to reduced
permeability, increased strength and producing durable concrete
Due to these excellent properties superplasticizer can use in cemented paste backfill.
So basic advantages of superplasticizers in Cemented Paste Backfill (CPB) are -
(1) High workability of CPB, resulting in easy placement without reduction in cement
content and strength; (2) high strength CPB with normal workability but lower water
content (fig. 2.3); and (3) a binder mix with less cement but normal strength and
workability (4) CPB of good workability has advantages in that it permits easy and
quick placement. (5) It increases the flow characteristics and becomes self leveling.
.
Fig. 2.3. Effect of Super plasticizer on Strength (Stuart, 1980)
Sokalan HP 80 is an advanced superplasticizer that consists of a polycarboxylate. It has
the following advantages.
• It creates strongly plasticizing effect on cement for high strength CPB;
• Longer working time; and
• Low consumption compared to conventional plasticizers.
Khatib (1999) states the inclusion of superplasticizer in cement paste leads to a
reduction in the total pore volume and to a refinement of the pore structures. The
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
dominant pore size is unaffected and the threshold diameter is reduced in the presence
of SP. The initial curing regime has a substantial influence on porosity and pore
structure. Less pore volume and finer pore structure are obtained when cement paste is
subjected to initial moist curing as compared with initially dry curing (Khatib, Mangat
1999). Flow velocity of cement paste increases with the increased concentration
(Yoshioka 2002).
2.7.2 Rheological Test The transportation of cemented mill tailings in the form of paste through pipelines is
one of the main stages of paste backfill operations. One of the data-sets used for
pipeline design purposes are those correlating the yield stress of fluid material changes
with changes in friction loss and the diameter of pipes (Li and Moerman, 2002).
Furthermore, the yield stress-water content curves are usually used in the design of
pumping energy requirements for the transportation of paste backfills through
pipelines (Van Dijk, 2001); (Sofra and Boger, 2000). Therefore, the yield stress
measurement is of great importance to understanding the pipeline transportation
principles of paste backfill.
The yield stress of viscoplastic fluid material is defined as a critical shear stress at
where it begins to flow as viscous material with finite viscosity. Within the context of
rheology, paste backfill is categorized as a viscoplastic fluid; a non-Newtonian,
homogenous mixture that may show time-independent or time-dependent behaviour
(Cooke, 2001); (Brackebusch, 2002); (Li and Moerman, 2002). In either case, the yield
stress depends on the characterization of the mixture (Nguyen and Boger, 1992).
2.7.3. Apparatus The yield stress of the specimens can find by using the vane method in controlled
shear rate (CSR) and controlled shear stress (CSS) modes, and slump tests by cone or
cylinder (Clayton 2003).
In the slump test methods, standard conical and cylindrical slump moulds can use.
Clayton (2003), Saak (2004) has find that the cylindrical model is an excellent
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
technique for slump test because its value is fully agreement with the experimental
yield stress data obtained using the vane method.
According to Clayton (2003) the cylindrical slump test has many advantages over the
cone slump test:
• Cylindrical test value is fully agreement with the experimental yield stress data
obtained using the vane method.
• The cylinder model is mathematically simpler which is important for operators
who may not have a strong background in mathematics.
• Due to the more complex cone geometry, the cone is more difficult to fill,
during experiment; full extraction of material is also very difficult. Due to this
cause leading to the likely presence of air bubbles which can adversely affect
the results.
• Cylinder slump measurements can be completed with a section of pipe or even
a beer can, whereas cone measurement must be completed with a cone
manufactured to certain specifications.
2.7.4. Theory The cylinder model is generalized for any-sized cylinder. The cylinder theory
developed by Pashias (1996) is also presented to enable easy comparison with the
generalized cone theory. Schematic diagrams for the cylinder are presented in fig. 7.
The schematics display the important variables and the stress distributions involved in
slumping.
Fig.2.4. Schematic diagram of the cylinder slump test, showing initial and final stress distributions (developed from Pashias et al., 1996). The dimensionless yield stress of backfills can be calculated from slump height, s,
using the following equations (Pashias et al.1996):
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
'0
1 h'2yτ = 2.10
)]2ln(1[21s yy τ′−τ′−=′ , 2.11
Where τ'y is the dimensionless yield stress
' yy gh
ττ
ρ= 2.12
s' is the dimensionless slump
' ssH
= 2.13
h0' is dimensionless height of undeformed region,
' 00
hhH
= 2.14
h1' is dimensionless height of deformed region.
' 11
hhH
= 2.15
and H is the height of cylinder.
The actual yield stress can then be estimated by multiplying τ'y with ρgH.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 3
LABORATORY INVESTIGATION
3.1. Experimental Method
3.1.1 Specific Gravity The specific gravity of backfill material was carried out in 25 ml bottles (also known
as pycnometer bottle). The density bottle with fill material, half full with water, were
placed in warm water bath for 30-45 minutes to remove entrapped air. This was latter
placed in a desecrator where a Crompton Parkinson vacuum pump, of 0.25 KW, 2.3
amp, rpm 1425 was used for about an hour to completely remove any remaining air
within the sample.
3.1.2. Particle Size Analysis
Particle Size Analysis is performed to evaluate the particle size by sieving procedure
most appropriate to the samples from Hutti gold mines. Generally, the distribution of
particle sizes larger than 75 μm is determined by sieving.
Following sieves was used for particle size analysis is numbers: 4, 16, 25, 40, 60, 70,
100, 140, 200, and 270
3.1.3 Porosity 50 gram mill tailing with different cement % taken in different measuring cylinder
and volume was measured. Then transfer the content of different cylinders in other
cylinders which are contains 50 ml water. Now leaved the composition in cylinder for
settle 1 day and then record the combined volume of the MT composition and water.
And measured the porosity by formula 2.1
( )(%), 100( )volumeof airporosity nvolumeof soil
= ×
3.1.4. Permeability Using the constant and falling head permeability tests the coefficient of permeability
was determined. Initially water flows through the sample until flow (Q) and the
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
hydraulic head loss (∆h) has reached a steady state. The flow rate and head loss are
then measured and the coefficient of permeability calculated by equation 2.3:-
hALQKΔ××
=
Where, ‘Q’ is the flow rate (q/t), ∆h is the constant head loss and, ‘A’ and ‘L’ are the
cross sectional area and length of the sample respectively.
3.1.5 pH
Air-dried soil samples were adjusted to 100% water content by adding distilled water,
and pH values were measured using a calibrated pH probe.
3.2. Uniaxial Compression
3.2.1 Sample Preparation The purpose of the uniaxial compression tests was to obtain unconfined compressive
strengths (UCS) and moduli as a function of binder content and cure time. Al1 test
samples were cast in the laboratory. The different cement contents were sampled for
each type of test: 3%, 6%, 10% and 20% by dry weight (Cement: mill tailing). In all,
60 samples were cured on laboratory for 3, 7, 14, 21 and 28 days in summer season.
Other samples were cured on laboratory for 28 days for different pulp density in winter
season. Again 24 samples were cured on laboratory for 28 and 90 days for different
composition of superplasticizer in winter season.
The samples of 54 × 110 mm diameter by length were cast in the Department in
wooden molds (Fig.3.1). After allowing them to set for 48 hours, al1 of the sample
were removed from the wooden molds and were waxed at both ends to prevent
moisture loss due to evaporation and possible oxidation of the samples. The samples
were cure for 3, 7, 14, 21 and 28 days
Fig.3.1. Sample preparation in the wood mold
3.2.2 Uniaxial Compression Test
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Both ends of the samples initially were done parallel by polish. Before the testing
samples length, diameter and weight were measured. The sample was placed in the
testing frame its stroke control rate was 0.315mm/min and brought into contact with
the load cell by adjusting the hydraulic ram (fig.3.2). When the sample was failed load
and deformation was noted. UCS was calculated with the Secant value of Young’s
modulus at 50% peak stress (Unal 2002).
Fig.3.2 Uniaxial Compression test machine
3.3. Direct Shear Test under Consolidated Drained Conditions
The direct shear box of 50 × 50 × 25 mm was used for conducting shear strength
tests. All the direct shear tests were conducted in the laboratory using manual operated
equipment. Prior to testing, all samples were air-dried. All of these samples were
tested under five different values of normal stress from 19.6 to 76.6 KPa and at a
shearing rate of 0.25mm/min.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 4
LABORATORY TEST RESULTS
Several tests were performed on cemented Hutti Gold mill tailings to evaluate the
basic material properties that influence the strength and deformation behaviour of
paste fill.
4.1 Specific Gravity Specific gravity has found 2.67
4.2. Mineralogy and Chemical Composition A qualitative assessment of tailings mineralogy using X-Ray diffraction (XRD)
indicated the mine tailings consists mainly of quartz (SiO2), followed by Albite
(NaAlSi3O8), Calcium Peroxide (CaO2), Cordierite(Mg2Al4Si5O18), Potassium sulfate
oxide(K2S2O5), sulfur (S7)and Sodium Manganese Silicate (Na2Mn6Si7O21) as shown
in the Fig. 4.1. The relative proportions of the minerals are based on peaks from an X-
ray diffraction analysis of the Hutti gold mine tailing. Chemical composition had
determined by scanning electron microscope method given in Table 4.1.
The mineralogy analysis shows that sulphide minerals are not present. And its pH
value is 7.89. So strength will be not more affected due to bacterial action in oxidation.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
P o s i tio n [°2 The ta ]
2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0
C o unts
0
1 0 0
4 0 0
9 0 0
1 6 0 0
Mg2
Al4
Si5
O18
Ca
Mn
Si;
Ca
Mn2
As2
K2
S2
O5;
Mg2
Al4
Si5
O18
; S7
Si O
2; M
g2 A
l4 S
i5 O
18N
a A
l Si3
O8;
Mg2
Al4
Si5
O18
; S7
K2
S2
O5;
Na
Al S
i3 O
8; C
a M
n S
i; C
a M
n2 A
s2; S
7N
a A
l Si3
O8;
Ca
Mn
Si;
Ca
Mn2
As2
; S7
Si O
2; N
a A
l Si3
O8;
Mg2
Al4
Si5
O18
; S7
Na
Al S
i3 O
8; C
a M
n2 A
s2; S
7K
2 S
2 O
5; N
a A
l Si3
O8;
Mg2
Al4
Si5
O18
; S7
K2
S2
O5;
Mg2
Al4
Si5
O18
; S7
K2
S2
O5;
Na
Al S
i3 O
8; C
a M
n S
i; S
7
K2
S2
O5;
Ca
Mn
Si;
S7
K2
S2
O5;
Na
Al S
i3 O
8; C
a M
n2 A
s2; S
7K
2 S
2 O
5; S
i O2;
Na
Al S
i3 O
8; M
g2 A
l4 S
i5 O
18; S
7
K2
S2
O5;
Si O
2; N
a A
l Si3
O8;
Ca
Mn
Si;
Mg2
Al4
Si5
O18
; S7
K2
S2
O5;
Si O
2; M
g2 A
l4 S
i5 O
18; S
7
K2
S2
O5;
Si O
2; N
a A
l Si3
O8;
Mg2
Al4
Si5
O18
; Ca
Mn2
As2
; S7
K2
S2
O5;
Na
Al S
i3 O
8; M
g2 A
l4 S
i5 O
18; C
a M
n2 A
s2; S
7
K2
S2
O5;
Na
Al S
i3 O
8; S
7K
2 S
2 O
5; S
i O2;
Na
Al S
i3 O
8; C
a M
n S
i; C
a M
n2 A
s2; S
7
K2
S2
O5;
Si O
2; C
a M
n S
i
K2
S2
O5;
Si O
2; C
a M
n S
i
Si O
2; C
a M
n S
i; C
a M
n2 A
s2
Si O
2; C
a M
n S
i
K2
S2
O5;
Si O
2; C
a M
n S
i
K2
S2
O5;
Si O
2; C
a M
n2 A
s2S
i O2;
Ca
Mn
Si;
Ca
Mn2
As2
Si O
2; C
a M
n S
i; C
a M
n2 A
s2
Si O
2
Si O
2
Si O
2
TA IL IN G b .rd
Fig.4.1 X-ray diffractogram of hutti gold mill tailing
Table 4.1 Chemical Composition of Hutti gold mill tailing (determined by Scanning Electron Microscope method)
Chemical component % by weight Element % by weight Na2O 5.88 Na 6.72 MgO 6.23 Mg 6.20 Al2O3 9.83 Al 8.97 SiO2 42.31 Si 35.55 SO3 4.89 S 3.79 K2O 1.1 K 1.67 CaO 9.08 Ca 11.76 TiO2 1.46 Ti 1.58 MnO2 .34 Mn .47 Fe2O3 17.43 Fe 21.29 NiO .1 Ni .13 ZnO 1.35 Zn 1.89
4.3. Particle Size Distribution The particle size distribution of the tailings was measured using a sieve analysis (table
4.2). Fig. 4.2 shows the size distribution obtained by sieve analysis method compared
with a size distribution obtained by sieve analysis:
4.3.1 Sieve Analysis Mass of dry mill tailing taken was 500 gram
Mass loss during sieve analysis is given by the following equation
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
1( )% W WMass lossW
100−= × 4.1
(500 498.6)% 100 0.28%500
Mass loss −= × =
Table.4.2. Sieve Analysis result
Sieve
no.
Opening
size
(mm)
Mass of
soil retained
on each sieve
Wn
% of mass
soil retained
on each sieve
Rn
Cumulative percent
retained ∑Rn
Percent
finer,
100 -∑Rn
4 4.75 0 0 0 100
16 1.18 1.6 0.32 0.32 99.68
25 0.71 1 0.2 0.52 99.48
40 0.425 2.1 0.42 0.94 99.06
60 0.25 8.7 1.74 2.68 97.32
70 0.212 21 4.2 6.88 93.12
100 0.15 80 16 22.88 77.12
140 0.106 266.3 53.26 76.14 23.86
200 0.075 58.1 11.62 87.76 12.24
270 0.053 59.1 11.82 99.58 0.42
Pan
0.7
0.14 99.72 0.28
∑=498.6
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
0.1
1
10
100
0.01 0.1 1 10
Grain Size D(mm)
% P
assi
ng
Fig.4.2 Plot of Percent Finer vs. Grain Size
The following properties were obtained from the particle size distribution:-
Effective diameter D10 = .071mm
D30 = .125mm
D50 =.14 mm
D60 =.15 mm
60u
10
C = DUniformity CoefficientD
4.1
So Cu=2.1126
230
c60 10
C =( )
DCoefficient of gradationD D×
4.2
Cc= 1.4671
Percent of silt= 12.22
Percent of fine sand =86.82
Percent of medium sand = .96
4.3.2 Effective Grain Size, D10
The effective grain size of a soil, D10, refers to the diameter of soil particles for which
10 % of particles are finer. D10 is an important value in regulating flow through fills
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
and can significantly influence the permeability of fills. The higher the D10 value, with
the coarser the soil and the better is the drainage characteristics.
The effective grain size is 0.071mm, which is very small signifying that permeability
of fill material will be less. Hazen (1930) related the permeability to the effective grain
size of a soil using equation 2.5
K=Cd102
K= permeability constant, cm/sec
Where d10 = grain size in millimeters for which 10% of the particles pass by weight.
C=constant (≈1)
4.3.3 Coefficient of Uniformity The coefficient of uniformity (Cu) is defined as the ratio D60/D10.
The uniformity coefficient provides a measure of uniformity. Aysen (2002) suggests
that a uniformity coefficient of less then four indicates uniform grading of soil grains.
Whereas, values greater then four indicate a wider assortment of grain sizes. In
general, the uniformity coefficient of the fills was less than four, indicating that the
hydraulic fills were not well graded.
4.3.4 Coefficient of Curvature
The coefficient of curvature (Cc) is defined as the ratio230
60 10( )D
D D×.
Where D60, D30, and D10 are refers of fill particles for which corresponding to 60, 30,
and 10%of particles are finer on the cumulative particle size distribution curve.
For sand, a coefficient of curvature between one and three with the uniformity
coefficient greater then 6, indicates a well-graded soil. Here from the laboratory testing
we have obtained from the laboratory testing Uniformity coefficient Cu = 2.11267,
Coefficient of gradation Cc=1.4671. So this result is also giving information that fill
material is not well graded soil
4.4. Permeability
Falling head permeability tests result is given in Table 4.3.
From table the coefficient of permeability is 4.08×10-3cm/sec which is very less for
hydraulic backfill. Because for hydraulic backfill Coefficient of permeability should be
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
more so that after placement of backfill in open stope in mine, water drainage will be
good. But by this sample, paste backfill will be good because for paste backfill
permeability should minimum so that water will be not drainage.
Table 4.3. Determination of permeability by falling head method
Sample
no.
Length
of water
L (cm)
Height
of soil
∆h (cm)
Area of
tube
A
(sq cm)
Time
t
min
Water
quantity
q (ml)
qL
∆h At
K
(cm/min)
1 11 29 15.21 185 1712 18832 81576.43 0.23
2 11 48 15.21 60 820 9020 43791.26 0.20
3 14.5 44.5 15.21 122 1105 16022.5 82549.57 0.19
4 14.5 24.5 15.21 100 580 8410 37252.99 0.22
5 19.5 37.5 15.21 115 655 12772.5 65572.86 0.19
6 24 42 15.21 65 465 11160 41510.47 0.27
7 28 31 15.21 1020 4010 112280 480791.6 0.23
8 28 21 15.21 145 538 15064 46300.14 0.32
9 32.5 26.5 15.21 262 1055 34287.5 105570.4 0.32
4.5. pH At first determined the weight into a cup. After that added pure water to the sample to
bring the solution to a weight to weight ratio of 1:1. Again stir vigorously for 5
seconds and let stand for 10 minutes .now placed electrodes in the slurry, swirl
carefully and read the pH on pH meter.
Weight of beaker (W1) =102.9gm
Weight of beaker+ mill tailing (W2) = 172.6gm
Weight of beaker +mill tailing +water (W3) = 242.3gm
Result =7.89.
It means mill tailing nature is basic so it is also indicates that in mill tailing sulfate
concentration is negligible.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
4.6. Unconfined Compression
4.6.1. Compressive Strength and Young’s Modulus The unconfined Uniaxial Compressive Strength (UCS) is calculated as the mean value
of the maximum stresses obtained during the testing of three samples of the same mill
tailing and cement mixture. Table 4.4 summarizes the strengths and moduli obtained
from uniaxial compression testing of fully sealed 54 mm diameter samples for
different cement % and curing period. The secant values of Young’s Modulus are
calculated at the point corresponding to 50% of the compressive strength value (Unal
2000). Different Uniaxial Compressive Strength (KPa) and Young Moduli (MPa) were
obtained for different cement proportions and the curing time.
Fig. 4.5.1 and 4.5.2 show the relationship between UCS and cement %, Young’s
modulus and cement % respectively for different curing period. These figures clearly
show the compressive strength and Young’s modulus of the fill increases with binder
content and curing time as expected. Compressive strength and stiffness are relatively
low for 3% cement in mill tailing, with a notable increase starting to occur for some
mixes with a binder dosage higher than 6%. All samples gradually gained strength and
elastic modulus up to 28 days of curing. These results agree with reports by Belem
(2000). UCS increases nonlinearly with the cement dosage for the all cement mill
tailing composition. Modulus values also follow the same trend, increasing nonlinearly
with binder dosage for all cement mill tailing composition. The rate of increase in UCS
and modulus values is higher in the initial 21 days compared to its increment after 21
days.
High strength values were obtained in the samples containing high amount of cement.
It can also be seen from Fig. 4.5.1 that binders have high strength gain in the
corresponding sample at curing 28 days. Therefore, it can be concluded that longer
curing period also plays an important role for increasing the strength and moduli of
paste backfill. In the present studies it has been noticed that the strength increment is
not as high with curing period as compared to the previous reported works of Lamos
and Clark, 1989. This is may be due to change of temperature in environment.
Fig. 4.5.3 shows the stress–strain curve for different cement composition and for
different curing period.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Table 4.4. UCS and Young’s Modulus of Hutti Gold mines paste fill.
UCS(KPa) Curing Time (days) MT:C
3 7 14 21 28
97: 3 173.44 184.6 277.7 299.1 288.6
94: 6 635.25 819.48 929.4 1278.0 1288.2
90:10 1702.8 1863.0 1990.7 2068.8 2075.6
Pulp density
80 %
80:20 5159.3 5727.9 5793.8 5718.0 5867.0
Curing Time (days) Young’s Modulus(MPa)
3 7 14 21 28
97: 3
26.6
27.0 27.3 28.4 91.8
94:06 159.20 187.3 193.7 233.5 237.0
90:10 277.7 331.9 345.1 346.9 467.8
Pulp density 80 %
80:20 654.6 694 705.4 765.1
811.1
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
3% 6% 10% 20%
Cement %
UC
S (M
Pa) for 3 days
for 7 daysfor 14 daysfor 21 daysfor 28 days
Fig. 4.5.1 Effect of Cement and Curing time on Uniaxial Compressive Strength
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
0
100
200
300
400
500
600
700
800
900
3% 6% 10% 20%
Cement %
Youn
g's M
odul
us (M
Pa)
for 3 days for 7 daysfor 14 daysfor 21 daysfor 28 days
Fig. 4.5.2 Effect of Cement and curing time on Young’s Modulus
0
50
100
150
200
250
300
350
0 0.5 1 1.5 2
Strain (%)
UC
S(K
Pa)
curing 3 days
curing 7 days
curing 14 days
curing 21 days
curing 28 days
a
0
200
400
600
800
1000
1200
1400
1600
0 0.5 1 1.5
Strain (%)
Stre
ss (K
Pa)
curing 3 days
curing 7 days
curing 14 days
curing 21 days
curing 28 days
b
0
500
1000
1500
2000
2500
0 0.5 1 1.5
Strain (%)
UC
S (K
Pa)
curing 3 days
curing 7 days
curing 14 days
curing 21 days
curing 28 days
c
0
1000
2000
3000
4000
5000
6000
7000
0 0.5 1 1.5 2
Strain (%)
UC
S (K
Pa)
curing 3 days
curing 7 days
curing 14 days
curing 21 days
cuirng 28 days
d
Fig. 4.5.3 Typical stress- strain curves for paste backfill specimen for different curing period containing (a) 3% cement (b) 6% cement (c) 10% cement (d) 20 % cement.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
4.6.2 Effect of Pulp Density on UCS Table 4.5 summarizes the strengths and moduli obtained from uniaxial compression
testing of fully sealed 54 mm diameter samples for different pulp density at 28 days
curing period.
Figure 4.5.4 shows the relationship between UCS and different pulp densities for 28
days curing period at 6% cement. Figure 4.5.5 shows the relationship between moduli
and pulp density for 28 days curing period and 6% cement dry weight composition.
When pulp density 83.3% is used then result of UCS of these sample after 28 days
curing in laboratory was 566 KPa, which is 135% more compared with UCS of sample
with pulp density 66.7%. Similarly, Young’s Modulus of pulp density 83.3% sample is
86% more as compared to that of the sample with 66% pulp density (Table 4. 5 and Fig.
4.5.4 and 4.5.5). It can therefore be inferred that the compressive strength and Young’s
moduli of the backfill samples are related to the pulp density. It is also noticed from Fig
4.5.4 that there is no difference in strength values for the samples with pulp densities
between 83.33% and 80 %. This may be due to the required amount of water to react
with cement and develop bonds between tailing materials. The strength of the backfill
decreases as the pulp density decreases mainly because of the subsequent increase in
overall porosity caused by the water-filled voids. On drying these samples air voids are
created which are likely to decrease the strength of samples. On the contrary, the lower
the pulp density ratio the stronger due to greater cement particle interlocking with mill
tailing and less air voids creation.
Fig. 4.5.6 also shows the stress –strain curve at different pulp density for 6% cement.
Table 4.5 Effect of pulp density on UCS and Young’s Modulus.
Pulp density (%)
Composition
(MT:C:W)
UCS
(KPa)
Young’s Modulus
(MPa)
83.3 94:6:20 566.3 157.1
80.0 94:6:25 503.3 127.9
76.9 94:6:30 369.4 128.5
74.0 94:6:35 269.4 121.0
71.4 94:6:40 242.9 86.9
66.7 94:6:50 240.8 84.3
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
566.3503.3
369.4
269.4 242.9 240.8
0
100
200
300
400
500
600
83.3 80 77 74 71.4 66.7
Pulp density (%)
UCS
(KPa
)
Fig.4.5.4 Effect of Pulp density on uniaxial compressive strength
157.1
127.9 128.5121
86.9 84.3
0
20
40
60
80
100
120
140
160
180
83.3 80 77 74 71.4 66.7
Pulp density (%)
Youn
g's
Mod
ulus
(MPa
)
Fig.4.5.5 Effect of pulp density on Young’s Modulus
0
100
200
300
400
500
600
700
0 0.5 1 1.5
Strain (%)
Stre
ss (K
Pa)
PD 83.3 %PD 80%PD 77 %PD 74 %PD 71.4%PD 66.7 %
Fig. 4.5.6 Typical Stress- Strain curves at different pulp density for 6% cement
(dry weight %)
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
4.6.3 Effect of Porosity on Uniaxial Compressive Strength Porosity has decreased with addition of cement in mill tailing due to fineness of cement.
So when we mix cement in mill tailing then void ratio of mill tailing decreased. So
higher uniaxial compressive strength has found in lower porosity due to greater particle
interlocking and the presence of more cement is available per unit volume of backfill.
The effect of cement addition is given in table 4.6. And effect of porosity on uniaxial
compressive strength is given in fig. 4.5.7.
Table 4.6: Effect of Cement addition on Porosity and UCS
Cement content in
Mill tailing (%)
Porosity (n %) UCS(KPa)
3 60 173.44
6 59.18367 635.25
10 58.69565 1702.8
20 55.5556 5159.3
173.44635.25
1702.8
5159.3
0
1000
2000
3000
4000
5000
6000
55 56 57 58 59 60 61
Porosity (n%)
UCS
(KPa)
Fig. 4.5.7 Effect of Porosity on UCS for (after curing 3 days)
4.7. Shear Strength Parameters
Normal stresses required for testing were estimated by dividing the applied
load by the area of the shear box. Peak shear strength was determined from plots of
shear stress versus shear strain. Internal friction angle was obtained using a linear best-
fit line from the plot of peak shear strength versus normal stress. The residual friction
angle was obtained using a similar best-fit line. Fig. 4.6.1 shows the variation of shear
stress with shear strain. Table 4.7 and fig 4.6.2 show the shear strength with normal
stresses which gives internal friction angle 300 and cohesion. 17.4 KPa.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7 8 9 10
Shear Strain (%)
Shea
r Str
ess
(KPa
)
normal stress 19.6 Kpanormal stress 35.28 KPanormal stress 47 KPanormal stress 58.8 KPanormal stress 70.56 KPa
Fig. 4.6.1 shear strain vs. shear stress curve from direct shear test
Table 4.7. Variation of shear strength with normal strength
Normal Stress (KPa)
Shear Strength (KPa)
19.6 31.85
35.28 35.53
47.00 40.15
58.80 50.63
70.56 60.57
y = 0.5697x + 17.4R2 = 0.928
0
10
20
30
40
50
60
70
0 20 40 60
Normal stress (KPa)
Shear S
tren
th (K
Pa)
80
Fig.4.6.2 Normal stress vs. Shear strength from direct shear test
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 5
BACKFILL STRENGTH ENHANCEMENT
5.1. Introduction The evolution of backfill technology is closely related to the advancement of modern
mining methods and the development of new backfill technology continues to evolve.
Backfill operations employ large quantities of cement and experience large costs
associated both with backfilling practice and cement use. Opportunities exist for
reducing backfill energy costs and enhancing the application of alternative cementing
agents in backfill. One of the techniques is the application of superplasticizer
[Stuart.1980] which is equally effective both in terms of enhancing the strength as well
as in the cost. This project paper presents the results of only two design stages
associated with the superplasticizer: Strength development and flow performance. The
advancement of this technology has reached a stage that should promote industrial
implementation within the Indian mining industry. A number of Indian mining
operations exhibit potential for plant optimization through possible binder source
replacement, and might thus realize associated reductions in backfill and energy costs.
Utilization of superplasticizer in mine backfill would create a new market for
manufacturing of this, so that its cost will be reduced.
5.2. Effect of Super Plasticizer on Compressive Strength and Young’s Modulus For the cemented paste backfill with superplasticizer specimens, the same procedure
was performed, but differed by adding the superplasticizers after 5 minutes and mixing
the superplasticizer with paste backfill for extra 2 minutes in mixing bucket.
The results of all the UCS tests due to variation of superplasticizer are summarized in
table 5.1 and fig. 5.1 and 5.2 for the different % of composition after 28 and 90 days of
curing. A typical stress- strain curve is shown in the fig.5.3 and 5.4 for the different %
of composition after 28 and 90 days of curing.
The unconfined compressive strength was calculated as the arithmetic mean of the
maximum stresses obtained during the testing of three samples of the same paste
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
mixture. Secant values of Young’s modulus are calculated at the point corresponding to
50% of the compressive strength value
Table 5.1. Effect of SP on Compressive strength and Young’s Modulus for 28 and 90 days curing
for 28 days curing
Composition UCS(KPa)Young's Modulus(MPa)
94:6:.2 654.26 214.2 96:4:.2 545.91 165.36 94:6:0 313.64 124.84 97:3:.3 130.16 150.90
for 90 days curing 94:6:.2 938.9 234.0 96:4:.2 892.1 198.6 94:6:0 586.1 170.2 97:3:.3 331.87 97.3
654.26545.91
313.64
130.16
938.9 892.1
586.1
331.87
0100200300400500600700800900
1000
94:6:.2 96:4:.2 94:6:.0 97:3:.3
Composition (MT:C:SP)
UCS
(KP
a)
for 28 daysfor 90 days
Fig.5.1 Effect of Superplasticizer on UCS for different curing days
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
214.2
165.36
124.84150.9
234
198.6170.2
97.3
0
50
100
150
200
250
94:6:.2 96:4:.2 94:6:.0 97:3:.3
Composition (MT:C:SP)
You
ng's
Mod
ulus
(MP
a)
for 28 daysfor 90 days
Fig.5.2 Effect of superplasticizer on Young’s Modulus for different curing days
0100200300400
500600700800
0 0.2 0.4 0.6 0.8 1
Strain (%)
Stre
ss (K
Pa)
.2% SP mixed withMT:C (94:6) .2 %SP mixed withMT:C (96:4) no SP mixed withMT:C (96:4) .3% SP mixed withMT:C (97:3)
Fig.5.3 Typical Stress- Strain curve after 28 days of curing for the different % of composition
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
0
200
400
600
800
1000
1200
0 0.2 0.4 0.6 0.8 1 1.2Strain (%)
Stre
ss (K
Pa)
.2% SP mixed withMT:C (94:6) .2% SP mixed withMT:C (96:4) no SP mixed withMT:C (96:4) .3% SP mixed withMT:C (97:3)
Fig. 5.4 Typical Stress- Strain curve after 90 days of curing for the different % of composition
Fig.5.1 and 5.2 show the variation in the UCS and Young’s Moduli with the variation of
composition of paste backfill with superplasticizer for 28 and 90 days curing. Fig. 5.1
shows the maximum compressive strength 654.26KPa (just double) of the composition
MT:C:SP containing 94:6:.2 ratios respectively as compression to compressive strength
313.64 KPa of composition MT: C: SP containing 94:6:0 ratios (control binder)
respectively for 28 days curing. Compressive strength of another binder in which MT:
C: SP containing 96:4:.2 ratios are also 74% more strength as compression to that
control binder. But effect of superplasticizer is not good in binder which contains MT:
C: SP containing 97:3:.3 ratios. Compressive strength 130.16 KPa of this binder is less
than half value of compressive strength of control binder. Fig. 5.2 shows Young’s
modulus 214.19 MPa of binder which contains MT: C: SP containing 94:6:.2 ratios is
also 70 % more than that of the Young’s modulus of control binder. This type of
increment in compressive strength and stiffness has happened due to renders a lower
porosity hardened material and increased the rate of cement hydration in well dispersed
cement so that between cement –mill tailing better particle packing and denser
structure upon hardening in pastes contains admixture superplasticizer.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Fig. 5.1 and 5.2 clearly show the variation of curing time on its strength and moduli.
Increment on strength due to curing varies from 50- 100% for different composition.
This has happened may be due to long term hydration between cement and mill tailing.
Fig. 5.3 and 5.4 produced not cleared stress-strain relationship for 28 and 90 days
curing.
Superplasticizer affects the unconfined compressive strength with curing. The cement
paste backfill mixture MT: C: SP containing 94:6:.2 developed the highest unconfined
compressive strength over a 90 days curing period and showed the maximum stiffness
development as compared to with other those of paste backfill specimens without
admixture.
But the cement paste backfill mixture MT: C: SP containing 96:4:.2 also developed the
required unconfined compressive strength over a 90 days curing period and showed
the maximum stiffness development as compared to with other those of paste backfill
specimens without admixture. So for economical purpose this composition is also best.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 6
RHEOLOGICAL TEST
6.1. Laboratory Preparations The materials used for characterizing the rheological properties were non-plastic mine
tailings from a Hutti gold mine.
The two types of homogenous solid-liquid mixtures characterized in this study were
cemented paste backfill with superplasticizer and other without superplasticizer
cemented paste backfill, in both case pulp densities was used 77%. For the cemented
paste backfill without superplasticizer specimens were prepared by adding 6% cement
and 23% tap water to mill tailing, the water was mixed for 5 minutes in the mixing
bucket. For the cemented paste backfill with superplasticizer specimens, the same
procedure was performed, but differed by adding the superplasticizer after 5 minutes
and mixing the superplasticizer with paste backfill for extra 2 minutes.
6.2. Experimental Procedure Cylindrical mould was used for determination of slump value. There is no required
standard for the cylinder test. Cylinder was made by PVS with the length 115 mm and
diameter 102 mm. The both side of the cylinder was opened so that slumped material
is 100% consistent during lifting. And filling with sample time used 1 strong smooth
steal plate on top of cylinder. The cylinder was filled with sample, and the cylinder
lifted slowly and evenly. The change in height between the cylinder and deformed
material was measured (fig.6.1). The midpoint of the slumped material was taken as
the representative height. Heights were measured with a scale. Density and
concentration were measured at the time of testing. Average value was finding of three
tests for each. Cylindrical slump test (a) with superplasticizer (b) without
superplasticizer
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Fig. 6.1 Slump test of backfill with Superplasticizer
6.3 Calculation Length of PVC Cylinder = 115 mm
Diameter of PVC Cylinder = 102 mm.
So volume of Cylinder = 9.4× 10-4 m3
Weight of material used in cylinder =2 kg
So slurry density of material = 2127.66 kg/m3
For without superplasticizer Height of undeformed region = (115-20)/2 = 47.5
Dimensionless height of undeformed region, ' 00
hhH
=
h0' =47.5/115 = .413
The dimensionless yield stress of backfills, '0
1 h '2yτ =
τ'y =.413/2 = .206
Yield stress of backfills τy = τ'y× ρgH
τy= .206×2127×9.8×.115 = 493 Pa
Dimensionless slump )]2ln(1[21s yy τ′−τ′−=′
s' = .2226
So slump height = .2226× 115 = 25.6 mm
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
For with Superplasticizer
Height of undeformed region = (115-50)/2 = 32.5
Dimensionless height of undeformed region, ' 00
hhH
=
h0' =32.5/115 = .282
The dimensionless yield stress of backfills, '0
1 h '2yτ =
τ'y =.282/2 = .141
Yield stress of backfills τy = τ'y× ρgH
τy= .141×2127×9.8×.115 = 338 Pa
Difference of yield stress between both = 155 Pa
Dimensionless slump )]2ln(1[21s yy τ′−τ′−=′
s' = .36
So slump height s = .36×115 = 41.51 mm
Difference of yield stress between both = 155 Pa
Difference of slump height between both =41.51-25.6 =15.91 mm
6.4 Result
Yield stress of backfills without superplasticizer = 493 Pa
Slump height = 25.6 mm
Yield stress of backfills with superplasticizer =338 Pa
Slump height = 41.51 mm
The results of the slump tests performed with 6% (dry weight %) cements with .2%
superplasticizer and without superplasticizer (fig. 6.2). It has seen that there is very
much difference (155 Pa) on the yield value and also much difference between slump
height(15.91mm) between both while water 23% in solution present in both condition.
So here fluidity increased with superplasticizer.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
493
338
0
100
200
300
400
500
600
without SP with SP(.2%)
Yie
ld S
tres
s (P
a)
Fig.6.2 Effect of Superplasticizer on Yield Stress
6.5 Setting Time
6.5.1 Procedure Setting time was determined by Vicat needle test (penetration test). Specimen For the
vicat needle test were cylindrical cup 70 mm in diameter and 40 mm high. After being
filling with paste (pulp density 80%). The standard test method, the Vicat needle test,
was used to determine the initial and final setting times hydraulic cement. The initial
setting is the determined for the needle to reach a penetration depth 5mm in standard
Vicant apparatus. The final setting takes place when the needle does not visibly
penetrate into the paste i.e., the specimen has a solid structure.
6.5.2 Result Result is given in table 6.1.
Table 6.1.Vicat needle test result
Setting time(min) Binder
content
Additive
Water(%) of total weight
Initial Final
94:6 MT:C none 23 65 125
94:6 MT:C SP.2% of dry
weight 23% of total weight 45 120
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Table 6.1 shows the initial and final times of setting for paste in which one is without
superplasticizer and other is with superplasticizer. The data indicates that initial time
setting of paste with superplasticizer is less as compared to without superplasticizer
paste. And final time setting of both pastes is about same while superplasticizer paste
is wet as comparison to without paste. So we can say for same slump value time
setting will be reduced in superplasticizer paste than without superplasticizer paste.
6.6. Flowability Test For flow behavior test, one galvanized iron sheet 120 cm length was used at inclination
200 degree as shown in figures 6.3-6.7.
Figures 6.3-6.7 show the flow characteristics of backfill material. The result of the test
performed with 4 different compositions. In first experiment for flow test, .2 %
superplasticizer was used in MT: C contains 94:6 ratios binder. In 2nd experiment no
superplasticizer was used for same combination. In 3rd experiment, .2%
superplasticizer was used in MT:C contains 96:4 ratios respectively. In 4th experiment,
.3% superplasticizer was used in MT: C contains 97:3 ratios respectively. It is seen
that there is significant difference on the fluidity of different composition. At .2%
superplasticizer in Mill Tailing –Cement (94:6 ratios) binder, the fluidity increased
compared to the other composition. And in other composition some part of paste has
flowed and some part has not flowed. Higher the fluidity in first case was observed
due to electrostatic repulsion between particles, causing dispersion (Nkinamubanzi and
Aitcin 2004). In 3rd and 4th experiment an insufficient amount of cement may be
available to react with main hydration (i.e. calcium silicate hydrates or C-S-H) to
produce effective dispersion at later stage. Fine particle is also important role played
with superplasticizer for fluidity purpose.
The rheological behaviour of two paste backfills characterized in this study was yield-
pseudoplastic. The superplasticizer controls not only the rheological behaviour of paste
backfill, but also their yield stress. Yield stress measurements in slump test method
show reliable results for superplasticizer as comparison to non superplasticizer paste
backfills. So Based on the results of this research, we can conclude that the use of
superplasticizer in backfill material will be economical because this will not increase
the strength but also aids in the rheological characteristics of paste backfill material.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Fig.6.3 Flow characteristics of mill tailing when .2 % superplasticizer mixed with MT: Cement (94: 6)
Fig.6.4 Flow characteristics of mill tailing When no superplasticizer used in
MT: Cement (94: 6)
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
Fig.6.5 Flow characteristics of mill tailing when .2 % superplasticizer mixed with MT: Cement (96: 4)
Fig.6.6 Flow characteristics of mill tailing when .3 % Superplasticizer mixed with MT: Cement (97: 3)
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
CHAPTER 7
CONCLUSIONS
An effort is made in the present study to generate value on physical, chemical,
mechanical and rheological characteristics of Hutti gold mill tailing samples.
Mechanical and rheological characteristics of mill tailings were found with different
percentage of cement, water and superplasticizer. Ninety different cemented backfill
materials were tested to determine the influence of their composition on the
unconfined compressive strength in laboratory.
7.1 Conclusions The studies lead to the following conclusions which may be of significance for the
further research and for the practice of mine backfill using cemented paste backfill
method.
• Predominant oxides found in the Hutti mill tailing samples are SiO2, Fe2O3, Al2O3,
CaO, Na2O, MgO, SO3, and TiO2. The sums of these oxides were above 90%. The
presence of CaO at 9 % in the mill tailing samples indicates the strong pozzolanic
characteristic of mill tailings.
• Generally mill tailings contain sulfate concentration. pH was found to be at 7.89 in
this sample and by SEM very less sulfur concentration has been observed. So it is
also good for strength purpose. Because sulfate concentration decreased strength
after 90 days. Due to this cause dilution problem will be decreased.
• Particle size distribution show that the Percent of fine sand is 86.82%, for paste
backfill purpose minimum15% of below size 20μm mill tailing will be required
will be required.
• Coefficient of permeability of mill tailing is 4.08×10-3cm/sec, which is very less
and after cement, addition its value again will be decreased. So it is not good for
drainage in hydraulic backfill purpose without any flocculent. This is good for
paste backfill purpose. After addition of cement and fine particle permeability will
be decreased.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
• One hundred twelve different cemented backfill materials were tested to determine
the influence of their composition on the unconfined compressive strength in
laboratory. 60 samples were tested with different percentage of cement at 80%
pulp density. 18 samples were tested with different pulp density. And 24 samples
were tested with different percentage of binder with superplasticizer.
• The material composition strongly influenced the strength of cemented backfill.
Pulp density is a critical determining factor in the strength of cemented backfill.
Increase in its value significantly increased the backfill strength.
• Increased cement content increased the backfill strength.
• Superplasticizer also play good impact for increment on its strength with cement,
• Slump heights obtained from the slump cylinder experiments is observed to
increase and decrease yield stress with mixing of superplasticizer in binder. So its
flowability increased with mixing of superplasticizer. Setting time has also not
increased with superplasticizer. Due to these good properties of superplasticizer, it
may play a vital role in paste backfill.
So the use of paste backfill in place of hydraulic backfills will be correct choice for
backfill operation. This shall not only enhance the performance of the backfill as a
ground support system but also likely to reduce the dilution of muck, and thus may
result in the full recovery of ore.
7.2 Scope for further Studies
• Numerical modeling is required for determination of accurate strength
requirement of backfill for stability in stope.
• Transportation of paste backfill could not be studied. Transportation rate and
quantity of discharge from surface to stope should be conducted both
numerically and experimentally to model and verify long term characteristics.
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
APPENDIX
BACKFILL DESIGN
The aim of backfill design is to develop a procedure for calculating the fill strength
requirement for a different height, strike length and width of fill exposure.
Various authors have published backfill design techniques and these are briefly
reviewed in the following section. The techniques discussed are:
1. Freestanding Vertical Faces
2. Vertical Slopes
3. Limit Equilibrium Wedge,
4. Arching.
A1. Free Standing Vertical Face Mitchell et al (1982) note that the largest shear stresses in backfill are caused by self-
weight. The cemented backfill can be designed as a freestanding vertical face. A
freestanding wall where the unconfined compressive strength required at any depth in
the fill is given as
σz≥ ρg z A.1
Where
σz =unconfined compressive strength (kPa)
ρg =bulk density of backfill (KN/m3)
z =depth of backfill from the surface (m)
A2. Vertical Slopes
Mitchell et al (1982) discuss the vertical slope method where φ= 0 for a constant
strength fill
=2zgHρσ A.2
Where: H = overall height of exposed fill (m)
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
A3. Limit equilibrium wedge by Mitchell
Fig.A.1. Confined block mechanism (Mitchell et al. 1982)
Mitchell et al. (1982) modeled the failure of a single exposure 3-D fill mass as shown
in Fig.A.1. A shear plane for sliding failure was defined within the ‘block’. By
assuming that there exists some shear resistance between the fill and stope walls due to
the fill cohesion, the design uniaxial compressive strength required to maintain
stability with safety factor of F can be evaluated by using following relationship
(Mitchell et al. 1982):
A.3
Where γ is the Fill bulk unit weight (kN/m3), α is the angle of failure plane from
horizontal (= 45 + /2), is the friction angle of fill (Degree), c is the cement bond
strength ( cohesive strength) of fill (kPa), l is the strike length of block (m), w is the
width of block (m), h is the height of the block (m), h* is the height of block from top
to the centroid of the triangular section of the sliding wedge (m), F is the Factor of
safety.
In the long term, the compressive strength of the fill material is mainly due to binding
agents and strength contributed by friction can be neglected (i.e. = 0). For a
frictionless material, cohesion is considered as half of the uniaxial compressive
strength (UCS) (i.e. c = UCS/2). Therefore, Eq. (A.3) can be modified as:
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
A.4
The stability of a free standing backfill can also be determined when height of the
stope to be much greater than width. The required strength (UCS) of frictionless fill is
given by;
A.5
A4. Extended Arching - Marston theory by Pirapakaran (2006)
The analytical solution presented arching theory by Marston (1930), Terzaghi (1943)
and Aubertin et al. (2003) are for a 2-dimensional stope where the fill is subjected to
plane strain loading. In reality, mine stopes are rarely 2-dimensional, and therefore it is
quite useful to extend these theories to three dimensions.
An attempt is made here to extend the above theories and develop expressions for
vertical and horizontal stresses within a 3-dimensional mine fill stope
A schematic diagram of a 3-dimensional stope is shown in Fig. A.2.(a) With the
dimensions. Figure A.2 (b) shows the free body diagram of the forces acting on an
infinitesimal horizontal layer within a vertical stope, where h is the backfill height, w
the stope width, dh the thickness of the layer element, W is the weight of the backfill
above the layer element. dC is the lateral compressive force, dS is the shearing force at
the fill- rock interface and V and V+dV are vertical forces at the position h and h+dh
respectively.
Weight of the element
A.6
Compressive forces acting on the vertical faces
A.7
The shear force S is defined as:
A.8
Where δ is the friction angle between the backfill and the wall it cannot be greater than
(may be assumed between 1/3 to 2/3)
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
For the equilibrium of the small element,
A.9
Here, it is assumed σv uniformly distributed over
Fig. A.2. Schematic diagram of a 3-dimensional stope (a) and the free body diagram With forces (b) the entire width w (Pirapakaran 2006)
A.10
Earth pressure coefficient from soil mechanics theories is defined as;
A.11
The following relationships can be found out from equations (A.6)–(A.11);
A.12
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
A.13
By integrating Eq. (A.13), vertical and horizontal stresses which are acting within the
stope can be found out as follows;
A.14
A.15
So for the strength requirement purpose I assume the following condition
1. The fill material is non cohesive i.e. c=0,
2. The friction angle between the backfill and wall δ = 1/3 ,
3. Rock’s stiffness is stiffer than backfill material so that rock would be in rest
condition i.e. K=Ko =1-sin
4. Factor of Safety =1.5
1 exp 2 tan2 tandesign
w l l wUCS Kh FSK l w lwγ δ
δ⎡ ⎤⎧ + ⎫⎛ ⎞ ⎛ ⎞= − −⎜ ⎟ ⎜ ⎟⎨⎢ ⎥+⎝ ⎠ ⎝ ⎠⎩ ⎭⎣ ⎦
×⎬ A.16
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Influence of Cemented Mill Tailing as Backfill Material in Open Stopes
0
200400
600800
1000
12001400
1600
0 15 30 45 60 75 90 105
Height (m)
UCSr
eq (K
Pa)
l=60 m & w =5 ml=60 m & w=10 ml=60 m & w=20 ml =80 m & w =20 ml= 100m & w=10 m
Fig.A.3. UCSreq vs. height of fill for different width and length in backfill
Fig.A.3 shows strength requirement in backfill material increases with height, width
and strike length of fill. Here figure shows Impact of width, height is more than length
on strength.
From fig. A.3 we can say that uniaxial strength for narrow mining width 5
m, height 45 m and length 60 m (e.g. Hutti gold Mines) will be required approximate
515 KPa for stability of backfill material in stopes. We know that temperature also
play important role in backfill strength. Strength increases with temperature up to
control condition.. So we can say that in Hutti gold mines strength of backfill will be
increased than these laboratory values. Because 3% cement was given 288 KPa after
28 days curing in paste backfill composition and UCS of 6% cement was given 1288
KPa which is more than required value. So for safety and economical purpose we can
use cement in mill tailing between 3 to 6% in paste backfill.
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