Bentonite use Procudur in Cast in Situ Pile

FEDERATION OF PILING SPECIALISTS
BENTONITE SUPPORT FLUIDS
IN
CIVIL ENGINEERING
January 2006
www.fps.org.uk
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
CONTENTS
Page
Acknowledgements 1
Preface 2
1 The use of bentonite support fluids in civil engineering 3
2 The production and properties of bentonite powder 3
3 Bentonite specifications 5
4 Bentonite slurry properties 6
4.1 The effect of slurry properties on required functions 7
4.1.1 Excavation support 7
4.1.2 Retention within the excavation 8
4.1.3 Suspension of solids 8
4.1.4 Displacement by concrete 8
4.1.5 Cleaning 9
4.1.6 Pumping 9
5 Preparation of bentonite slurry 9
6 Cleaning bentonite slurry 10
7 Re-use of bentonite slurry 11
8 Disposal of bentonite slurry 12
9 Testing 13
9.1 Density 14
9.1.1 Test procedure 14
9.1.2 Calculation of slurry density for mix proportions 15
9.1.3 Grain specific gravity of the bentonite powder 15
9.2 Sand content 16
9.2.1 Test procedure 16
9.3 Rheological measurements 17
9.3.1 Test procedure for the electrically driven viscometer 17
9.3.2 Test procedure for the hand cranked viscometer 18
9.3.3 Checking viscometers 19
9.3.4 Calculation of results 19
9.4 Flow cones 20
9.4.1 The Marsh funnel 20
9.5 The Shearometer 21
9.6 Ph 22
9.6.1 Test procedure 22
9.6.2 Typical pH values 23
9.7 Filtrate loss 23
9.7.1 Test procedure 24
9.7.2 Test results 25
9.8 Bleeding 25
9.9 Moisture content 25
9.10 Water compatibility testing 26
10 Bibliography 27
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
ACKNOWLEDGEMENTS
The Federation of Piling Specialists acknowledges the contributions made in the
preparation of this document by the following members of the Working Group:
D. J. Ball Steetley Bentonite and Absorbents Limited
M. T. Hutchinson Kvaerner Technology Limited
S. A. Jefferis Golder Associates (UK) Limited
P. G. Shotton Kvaerner Cementation Foundations Limited
L. Stansfield (Chairman) Bachy Soletanche Limited
A. J. Wills Kvaerner Technology Limited
and also the assistance provided by Dr. M. Stocker, Chairman of the Technical Working
Group of the European Federation of Foundation Contractors and Chairman of CEN/TC
288: Special Geotechnical Works.
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
PREFACE
Bentonite is the name used for a range of clays that can swell and gel when dispersed in
water. The name “bentonite” originates from the discovery of this type of clay near Fort
Benton, USA, in the 19th Century. This was a natural sodium bentonite, and has been
mined extensively for many years in Wyoming and Dakota for oil well drilling
applications.
Bentonite is now used extensively throughout the world in civil engineering, but the cost
of transporting original “Wyoming” bentonite from the USA has led to the use of
alternatives from other sources. A large proportion of bentonite now used is therefore
from other parts of the world.
It is important to recognise that the properties of bentonites from different sources vary,
and to take these variations into account when deciding on the suitability of a particular
bentonite for a specific purpose.
The purpose of this document is to provide information that will enable a decision to be
made as to whether or not a particular bentonite will produce a satisfactory support fluid,
and to give guidance on the preparation, use, re-use and disposal of the bentonite slurry,
and also on methods of testing.
Since the first edition of this guide the use of polymer support fluids as an alternative to
bentonite has become relatively common. Polymer support fluids are fundamentally
different to bentonite support fluids and their scope is too great to be covered in this guide.
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
3
1 THE USE OF BENTONITE SUPPORT FLUIDS IN CIVIL ENGINEERING
Bentonite support fluids are widely used in civil engineering.
One of their main uses is to support the sides of panel excavations for diaphragm walls. In
this application, the bentonite must be capable of forming a barrier or “filter cake” on the
sides of the excavations to prevent loss of fluid into the ground and provide a surface layer
against which the pressure of the fluid can act in order to resist external pressures from the
soil and groundwater.
Bentonite support fluids are also widely used in the construction of large diameter bored
piles. This application is similar to that for diaphragm wall construction except for the
shape of the excavation.
Small diameter boreholes for site investigation work or other purposes are often bored
through unstable strata using bentonite support fluid as an alternative to temporary casings.
Another widely used application is in the construction of cut-off walls below ground to
form barriers to groundwater or to surround areas of contaminated land where leachates
must be contained. In this application, cement and/or other materials are added to the
bentonite to form a slurry which remains fluid for several hours before setting to form a
relatively soft barrier wall. A thin flexible membrane is sometimes inserted into the slurry
in the excavation before it sets. This application is not treated in this document.
Bentonite support fluids are also used in some cases to support the excavation face in front
of tunnelling machines and to transport the cuttings to desanding equipment where they are
removed before the fluid is re-circulated for further use.
The properties of bentonites from different sources vary, and it is important to understand
that a property which may be required for one application may not be required for another.
For example, gel strength is important if material has to be kept in suspension while the
fluid is at rest, but may not be important if the fluid is agitated continuously in a
circulatory system. Differences in the properties of available bentonites should therefore
be considered before deciding which bentonite to use for a particular application.
Polymer support fluids, as an alternative to bentonite, have grown in usage for bored piling
operations in recent years. They behave and have different properties to to bentonite
support fluids and their scope is too great to be covered in this guide. Engineers may wish
to consider their use, but should always seek the necessary specialist advice on their usage
and application.
2 THE PRODUCTION AND PROPERTIES OF BENTONITE POWDER
Commercial bentonites are hydrated alumino silicates, and comprise predominantly the
mineral montmorillonite. The name “montmorillonite” is derived from the discovery of
this type of clay near Montmorillon in France.
© Federation of Piling Specialists – January 2006 (2nd edition)
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4
Bentonite occurs as a clay ore containing up to 50 % moisture. Commercially viable
deposits consist of accessible clay seams, low in accessory minerals, which can be cleanly
worked to minimise unwanted inclusions such as sand. The characteristics of the clay
vary, and selection is based on factors such as yield and gelling ability.
There are three common types of bentonite, namely:
• Natural sodium bentonite
• Natural calcium bentonite
• Sodium-activated bentonite
All bentonites have a capacity to exchange cations which is much greater than that of other
clays such as china clay, ball clays and attapulgite.
Natural sodium bentonite is characterised by very high swelling ability, high liquid limit
and low filter loss. This bentonite was used as the standard by which all other bentonites
were measured for many years. The predominant exchangeable cation in natural sodium
bentonite is the sodium cation but there may also be significant amounts of other cations
present.
Natural calcium bentonite, where calcium is the predominant exchangeable cation, is
mined world-wide. It has much lower swelling ability and liquid limit, and much higher
filter or fluid loss than natural sodium bentonite.
Sodium-activated bentonite is produced by the addition of soluble sodium carbonate to
calcium bentonite. This effects a base exchange on the surfaces of the clay particles,
replacing calcium ions with those of sodium. The result is a bentonite exhibiting many of
the typical characteristics of a natural sodium bentonite.
Most bentonites used in civil engineering to produce support fluids are sodium-activated.
Natural sodium bentonite is rarely used because of its high cost. Natural calcium bentonite
is usually not suitable for this purpose.
Processing methods used in the production of sodium-activated bentonite depend on the
deposit and its geographical location. They could include several of the following:
• Selective mining.
• Field laying and rotavation. This is a process used in hot, dry climates where the
bentonite is spread out and rotavated with the addition of sodium carbonate.
• Crushing to 50 mm maximum size at the production plant.
• Extrusion. In wet climates, where field laying is not possible, a blend of raw
bentonite and sodium carbonate is extruded to promote activation.
• Drying by rotary louvre drier to optimum moisture contents in the range 11 to
22 % of dry weight.
• Milling to a particle size that promotes good powder flow but does not diminish
clay performance. Generally, powders with 95 % of particles less than 150
microns (dry sieve basis) are used.
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
5
Fig. 1(a) Fig. 1(b)
The rheological characteristics of bentonites (i.e. their behaviour as bentonite slurries when
mixed with water) influence their suitability for use in civil engineering applications.
Natural sodium bentonite and sodium-activated bentonite, when dispersed in water under
conditions of high shear mixing, break down into minute plate-like particles, negatively
charged on their surfaces and positively charged along their edges. Typically, if 3% or
more bentonite powder is dispersed in water, a viscous slurry is formed which is thick
when allowed to stand but thin when agitated. This phenomenon is known as thixotropy,
and results from the orientation of the plate-like particles within the slurry. When the
slurry is allowed to stand, the plate-like particles become orientated as shown in Figure
1(a). Electrical bonding forces between the particles form an interlocking structure which
causes the slurry to gel. When the gel is agitated, the electrical bonds are broken and the
slurry becomes fluid, with the particles orientated in random fashion as shown in Figure
1(b).
3 BENTONITE SPECIFICATIONS
Bentonite powder is normally satisfactory for use in support fluids in civil engineering if it
complies with one of the following specifications:
• API Specification 13A, Fifteenth Edition, May 1, 1993, Section 6 (OCMA grade
bentonite)
• The Engineering Equipment and Materials Users Association (EEMUA)
Publication No. 163 entitled “Drilling Fluid Materials”, last reprinted in 1988.
The API Specification and the EEMUA Specification differ slightly in some respects. The
main differences in the specifications are in the requirements for the rheological properties
and filtrate loss of the slurry. The rheological properties of the slurry at different rates of
shear are determined using a direct reading viscometer. Filtrate loss is determined using a
filter press. Test methods are described in detail in Section 8.
Tests to determine the properties of a bentonite slurry in accordance with the API
Specification are carried out on a 6.4% suspension of bentonite in deionized water, aged
for up to 16 hours. This specification requires a minimum viscometer dial reading of 30 at
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
6
600 rpm, and a maximum Yield Point/Plastic Viscosity Ratio of 6. Expressed another
way, this equates to a minimum dial reading of 30 at 600 rpm, and a maximum dial reading
of 0.875 times the 600 rpm reading at 300 rpm. The filtrate volume must not exceed 16 ml
in 30 minutes.
The EEMUA Specification expresses the rheological properties in terms of Yield (not to
be confused with Yield Point). This specification requires the yield of a bentonite/distilled
water slurry, aged for 24 hours and having an apparent viscosity of 15 cP (centipoise), to
be not less than 16 m3/tonne. An Apparent Viscosity of 15 cP equates to a 600 rpm
reading of 30, and a Yield of 16 m3/tonne equates to a 6.4% bentonite suspension, which is
the same as that used in the API Specification. Thus, the requirement of the EEMUA
Specification can be re-written as follows:
• A 6.4% suspension of bentonite in distilled water, aged for 24 hours, should have a
minimum viscometer dial reading of 30 at 600 rpm.
This is the same as the API Specification except that the API Specification only requires
the bentonite suspension to be aged for up to 16 hours. The EEMUA Specification does
not specify a maximum Yield Point/Plastic Viscosity Ratio therefore does not require a
viscometer dial reading to be taken at 300 rpm. The filtrate volume is measured on a 7.5%
suspension, aged for 24 hours, and should not exceed 15 ml in 30 minutes.
The maximum moisture content of the bentonite powder is specified as 13% in the API
Specification and 15% in the EEMUA Specification. This difference will not affect the
performance of the bentonite, therefore the requirement of the API Specification could, if
necessary, be relaxed to 15% to accommodate some bentonites in common use.
Both Specifications require the residue greater than 75 microns (US standard sieve No.
200) not to exceed 2.5% by weight. The EEMUA Specification has the additional
requirement that the amount of bentonite passing through a dry 100 mesh (150 micron)
screen shall be at least 98% by weight.
Where applicable, testing procedures should be carried out in accordance with the latest
edition of API Publication RP13B “API Recommended Practice - Standard Procedure for
Testing Drilling Fluids”.
4 BENTONITE SLURRY PROPERTIES
Bentonite slurries of the type normally used to support excavations, can vary widely in
their physical and chemical properties. They must, however, perform the following
functions:
a) Support the excavation by exerting hydrostatic pressure on its walls
b) Remain in the excavation, and not flow to any great extent into the soil
c) Suspend detritus and prevent sludgy layers building up at the base of the excavation
© Federation of Piling Specialists – January 2006 (2nd edition)
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In addition, they must allow:
d) Clean displacement by concrete, with no significant interference with the bond between
reinforcement and set concrete
e) Cleaning to remove suspended detritus, by passing through screens and hydrocyclones,
in preparation for re-use
f) Easy pumping
In general, items a) to c) require thick, dense slurries, while items d) to f) need very fluid
slurries. There are therefore conflicting requirements which must be resolved before an
acceptable specification for slurry properties can be drawn up.
In the following paragraphs, consideration is given to the effect of slurry properties on
each function. This will assist in the establishment of limits for most of the slurry
properties and in defining a slurry which is acceptable for each function and also for the
overall excavation process.
The primary aim of any bentonite slurry specification must be to ensure that the slurry is
capable of fulfilling functions a) to d) without deleterious effects on the finished pile, wall
or other form of construction. In addition, for economic and environmental reasons, the
maximum re-use and minimum disposal of used slurry are required. The specification for
the slurry properties should be as wide as possible, consistent with achieving satisfactory
results. In most cases, for any given function, maximum and minimum values can be
chosen which then enable limits to be derived as the basis for the specification.
4.1 The effect of slurry properties on required functions
4.1.1 Excavation support
In order to exert stabilising pressure on permeable walls of an excavation, bentonite slurry
must form a seal on or near the surface of the soil. This avoids loss of slurry into the soil,
with consequent increase in pore pressure and reduction in shear strength, and enables the
slurry to exert its maximum stabilising effect.
The seal can be formed by three different mechanisms:
• Surface filtration
• Deep filtration
• Rheological blocking
Surface filtration occurs when a filter cake is formed by the bridging of hydrated bentonite
particles at the entrance to the pores in the soil, with negligible penetration of the bentonite
into the soil. During and after its formation, water percolates through the filter cake from
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
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the slurry into the soil. Water lost in this way is referred to as fluid loss, and reduces in
relation to the square root of time.
Deep filtration occurs when slurry penetrates into the soil, slowly clogging the pores and
building up a filter cake within them. In this case, the seal may penetrate into the soil
about 40 to 50 mm.
In both surface filtration and deep filtration, the concentration of bentonite in the filter
cake is greater than in the slurry (typically 15% for a slurry containing 5% bentonite).
Rheological blocking occurs when slurry flows into the soil until it is restrained by its
shear strength. In this case the slurry may flow several metres into the soil.
Of these three mechanisms, surface filtration is much to be preferred, since the seal is
formed very rapidly with little or no penetration of bentonite into the soil.
4.1.2 Retention within the excavation
Retention of bentonite slurry in excavations in clay, silt or sand should not present any
problems provided the bentonite slurry has a minimum Marsh funnel viscosity of about 32
seconds (946 ml test volume). Excavations in gravel may require a Marsh funnel viscosity
of 40 to 50 seconds to limit the filtration depth into the soil. A Marsh funnel viscosity in
excess of 50 seconds will make desanding operations more difficult, and may inhibit
complete displacement of the bentonite slurry by concrete in excavations containing
complex steel reinforcement.
It may not be possible to retain bentonite slurry in very open ground containing cobbles
and boulders unless special measures are taken. These may include the addition of sand to
the bentonite to assist the blocking mechanism, or the use of bentonite-cement slurry or
weak concrete to seal off strata where losses occur.
4.1.3 Suspension of solids
While excavating under bentonite, fine soil particles will accumulate in the slurry. If this
material is to be kept in suspension, for example to prevent the formation of a layer of
sediment at the base of a pile bore, the bentonite slurry should have a high viscosity under
quiescent conditions. A measure of this can be obtained from the 10 minute gel strength
which can be determined when testing the rheological properties of the slurry, or other
testing method for gel strength.
4.1.4 Displacement by concrete
The bentonite slurry should have a low viscosity and contain the minimum possible
amount of suspended soil particles if it is to be displaced by concrete placed through a
tremie pipe or by pumping. It is therefore normal practice to use desanding equipment
and, if necessary, desilting equipment to remove soil particles from the slurry, or to replace
the
© Federation of Piling Specialists – January 2006 (2nd edition)
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9
contaminated slurry with fresh or reconditioned slurry before the concrete is placed.
Sometimes there can be a build-up of fine silt and clay particles in the slurry which cannot
be removed by conventional desanding or desilting equipment. A practical upper limit has
therefore to be set on the density of the slurry, after which it is considered to be unsuitable
for re-use.
4.1.5 Cleaning
Suspended soil particles can be removed from a slurry more easily if the slurry has a low
viscosity. Desanding becomes increasingly difficult as the viscosity of the slurry
increases, and also as the amount of suspended solids increases.
4.1.6 Pumping
Pumping bentonite slurry over distances of several hundred metres can become difficult
and inefficient if the viscosity of the slurry is too high or the slurry contains a large amount
of suspended solids. The slurry should have low viscosity to minimise the energy required
for pumping but should have sufficient gel strength to prevent soil particles from settling
out in the delivery lines if pumping is interrupted.
5 PREPARATION OF BENTONITE SLURRY
When a bentonite slurry is being prepared, the objective is to achieve maximum hydration
of the bentonite. Potable quality fresh water from a mains supply should be used in the
mixing process to achieve the best results. If there is any doubt about the quality of the
water, a chemical analysis should be carried out to determine its suitability or the need for
chemical treatment before use.
Salt water should not be used in the preparation of bentonite slurry because there is no
simple chemical treatment available to remove the sodium chloride.
The presence of calcium or magnesium in fresh water will inhibit dispersion of the
bentonite powder, but it is a relatively simple matter to treat these chemically before the
water is used. Calcium can be removed by soda ash (sodium carbonate) which precipitates
out the calcium as calcium carbonate. Care must be taken not to over-treat the water, since
this will provide an excess of carbonate ions which will hinder hydration. Magnesium can
be treated with caustic soda (sodium hydroxide) which can also be used to provide some
alkalinity to assist in dispersing the bentonite when it is mixed.
Bentonite slurry can be prepared either in batches or in a continuous process, depending on
the type of equipment used. The bentonite powder must be added to the mixing water
gradually in order to ensure that all the particles are wetted and do not clump into partially
hydrated balls. Typically, the bentonite powder is added through a simple venturi hopper
or directly into a high shear mixer. The mixing equipment must generate sufficient shear
to ensure that all the individual bentonite particles are dispersed in the mixing water. The
quantity of bentonite powder to be added to the mixing water depends on the quality of the
© Federation of Piling Specialists – January 2006 (2nd edition)
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bentonite and the required viscosity of the slurry. For most applications, concentrations
between 4% and 6% by weight are typical.
Following dispersion in the mixing water, the bentonite particles absorb water and swell.
The initial properties of the slurry will depend on the efficiency of the mixing process.
Thus, the greater the agitation and the longer the mixing time, the higher will be the initial
viscosity and gel strength. After mixing, the slurry is normally stored in tanks or lagoons
where it is kept agitated by circulating through pumps or by injecting compressed air while
further hydration takes place. Hydration is time dependent and will continue for many
days, but the rheological properties of the slurry will approach limiting values within a few
days after mixing. The slurry is normally stored for at least 12 hours before being used,
but it can be used immediately after mixing if necessary, provided tests show that its
properties are satisfactory.
Satisfactory results should be obtained if the properties of the fresh bentonite slurry
comply with the “fresh” column in Table 1.
TABLE 1 : CHARACTERISTICS FOR BENTONITE SUSPENSIONS
Stages
Property Units Fresh Ready
for re-use
Before
concreting
Test equipment
Density g/ml < 1.10 < 1.25 < 1.15 Mud balance
Marsh viscosity (946 ml) sec 32 to 50 32 to 60 32 to 50 Marsh funnel
Fluid loss (30 min) ml < 30 < 50 n.a. Filter press
pH 7 to 11 7 to 12 n.a. pH meter
Sand content % n.a. n.a. < 4 Sand content set
n.a. : not applicable
In order to keep sand particles in suspension, it is necessary for the bentonite slurry to have
sufficient gel strength. The gel strength can be checked by using a rotational viscometer or
other suitable equipment.
6 CLEANING BENTONITE SLURRY
Upon completion of an excavation, the slurry will contain soil particles held in suspension
which may include clay, silt, sand and fine to medium gravel. If concrete is to be placed
through a tremie pipe into the excavation, a sample of the slurry should be taken from the
bottom of the excavation and checked for compliance with the values in the “before
concreting” column in Table 1.
If the sand content is greater than 4%, slurry should be removed from the bottom of the
excavation by means of a pump or air-lift, while clean slurry is pumped into the top of the
excavation to maintain the required level. The slurry from the excavation can be pumped
directly to a desander and then returned to the excavation in a closed circuit, or can be
pumped to a lagoon for subsequent desanding and replaced by clean slurry from storage
© Federation of Piling Specialists – January 2006 (2nd edition)
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tanks or a separate storage lagoon. Up to 6% sand and a maximum density of 1.20 g/ml
may be permitted in special cases such as unreinforced walls, but it is preferable for the
sand content to be reduced as far as possible before the concrete is placed in order to
reduce the risk of blockages in the pipes when the slurry is being pumped back to the
mixing station during the concreting operation.
Several proprietary makes of desander are available on the market, all employing similar
principles. The bentonite suspension is first passed over a coarse vibrating screen to
remove large solids, typically greater than 5 mm. It is then pumped to a main
hydrocyclone (typically 250 - 450 mm dia.) where suspended solids, down to fine sandsize,
are concentrated and discharged as the underflow on to a vibrating dewatering screen.
This screen effectively dries the sand, which is then discharged on to a spoil heap. Most
desanders have a facility to recover the fluid, which passes through the dewatering screen,
and pump it to a secondary, smaller hydrocyclone which concentrates the coarse silt
particles in the fluid and discharges them back on to the sand bed on the dewatering
screen. The size of the hydrocyclone dictates both its flow capacity and the size of solids it
will remove. The smaller the hydrocyclone, the lower its flow capacity, but the smaller the
particle size it can remove.
The bentonite slurry which overflows from the hydrocyclones contains a much lower
concentration of suspended solids than the feed material, and may be sufficiently clean to
be re-used after passing through the hydrocyclones only once. However, as the weight of
suspended solids in the feed material increases and the viscosity of the fluid increases, the
ability of the hydrocyclones to clean the slurry reduces, therefore it may be necessary for
the slurry to circulate two or three times through the desander before it is sufficiently clean
to be re-used. The viscosity of the slurry can also be reduced by the use of suitable
admixtures.
After desanding, the slurry may still contain silt and clay-size soil particles which will
increase its density. During the excavation process, heavy slurry is lost and replaced by
lighter slurry, which may result in the density of the slurry increasing to a certain level
above which no further increase occurs. Should the build-up of fine soil particles continue
and become a serious problem, there are two possible solutions: either dispose of the slurry
or use a desilter or centrifuge to remove silt-size particles. This latter option may be
relatively expensive, but may still be preferable to disposal of large quantities of slurry. It
is not practical to remove clay-size particles therefore, should these continue to build up,
increasing the density of the slurry to more than the allowable limit, disposal is the only
practical solution.
7 RE-USE OF BENTONITE SLURRY
Bentonite slurry can be re-used repeatedly provided its properties are carefully monitored
and kept under control.
Whatever system of excavation is used, loss of slurry will occur. Some will be excavated
with the soil, some will permeate into the strata, and some will become too contaminated
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for re-use and have to be taken off site. Also, some slurry may be left in the excavation if
it is not filled with concrete to ground level. The slurry which is lost is replaced by fresh
slurry which is blended with the used slurry to top up the system. Satisfactory results
should be obtained if the properties of blended fresh and used slurry comply with the
“ready for re-use” column in Table 1.
Bentonite powder may have to be added to the slurry or admixtures may have to be
introduced to adjust its properties. These may include sodium bicarbonate or soda ash to
control the pH, organic thinners or polyphosphates to reduce viscosity, and sodium
carboxymethylcellulose (CMC) to reduce fluid loss.
The pH of the slurry will increase if it becomes contaminated by cement, and will reduce if
contaminated by acids or acidic groundwater. In both cases, there will be an increase in
viscosity accompanied by an increase in fluid loss, therefore the pH should be adjusted to
its original value before any other tests are carried out. This can be achieved by the
addition of sodium bicarbonate if the pH has to be reduced, or soda ash if it has to be
increased.
After adjustment of the pH, the next step is to check the density, Marsh viscosity and fluid
loss.
If the density rises above the acceptable limit due to the inclusion of clay and silt-size
particles, and cannot be reduced by the equipment available on site, the slurry must be
taken off site for disposal.
The Marsh viscosity will increase if the slurry contains an accumulation of clay and siltsize
particles, and will increase still further if contamination causes flocculation to occur.
In their dispersed state in the fluid, the individual clay particles are held apart by water
cushions but, when contamination occurs, the water cushions shrink and the particles move
closer together, causing flocculation. The flocs form a highly permeable filter cake
accompanied by high fluid loss which may result in partial or complete collapse of an
excavation. Flocculation can often be corrected by the addition of organic thinners or
polyphosphates, but it may be necessary to analyse a sample of the filtrate water to identify
the contaminant if the problem persists.
It is important to carry out regular filtrate tests on the slurry to check the fluid loss,
because this can increase with continued use of the slurry, even though other properties
may remain within acceptable limits.
8 DISPOSAL OF BENTONITE SLURRY
Under current UK waste regulations bentonite is classified as a non-hazardous waste.
Usually, the cheapest acceptable method of disposal of bentonite slurry is to place it in an
approved landfill tip, with transportation by a licenced carrier. However the availability
of approved tips is limited, and many tip operators will only accept limited daily quantities
(generally related to how much dry solid waste they are handling). Additionally, in wet
weather, some tips will not accept bentonite slurry for disposal.
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Alternative methods which are available but, to date, not cost effective are:
(i) Continuous belt filtration units which produce a clay product with a solids content of
approx. 65%.
(ii) Decanting centrifuge units, producing a similar product to the above.
(iii) Flocculation of the bentonite, followed by the addition of cement to form a clay-like
product, generally with a solids content of approximately 25%.
The purpose of these forms of treatment is to allow the products to be disposed of as solid
waste.
Waste disposal regulations have been the subject of significant changes in recent years and
users of this guide should always ensure that any transportation or disposal is in
compliance with the latest regulations.
9 TESTING
Much of the early technology for civil engineering slurries was developed from oil well
drilling practice, and many of the test procedures have also been borrowed from the oil
industry. Rogers (1988) provides a useful account of oil well drilling fluids and test
procedures.
The following paragraphs set out a range of procedures that can be used for testing
bentonite excavation slurries. Some of these tests are more appropriate to the research
laboratory than to a construction site therefore, in selecting parameters to be measured on
site it is important to consider the following questions:
• Is the test parameter relevant to the site situation?
• Does the test procedure produce repeatable results so that unacceptable materials
can be easily identified?
• Is the test equipment robust and suitable for site use?
• Can the test be performed reasonably rapidly?
It is not suggested that all the tests detailed below are appropriate for use on all sites.
Important parameters which may need to be tested include:
• Some measure of rheology to ensure that the slurry is appropriately fluid.
• The density of the slurry in the excavation prior to concreting to ensure
satisfactory displacement by the concrete.
• The sand content, if the slurry is to be cleaned and re-used (slurries with high
densities but low sand contents may be little improved by conventional cleaning
plants).
• The pH of the fresh bentonite slurry as a quality control measure (the result
should be consistent for a particular source/type of bentonite but may vary
between sources).
• The pH of the slurry in use to check for cement or other contamination.
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• Filtrate loss of the slurry to check its ability to form a seal on or near the surface
of the soil in the excavation.
9.1 Density
For excavation slurries, the standard instrument for density measurement is the mud
balance. This is an instrument similar to a steelyard, except that the scale pan is replaced
by a cup. The instrument thus consists of a cup, rigidly fixed to a scale arm which has a
sliding rider and counterweight, and is supported on a fulcrum, as shown in Figure 2.
9.1.1 Test procedure
(i) Check that the instrument is thoroughly clean and dry, paying particular attention to
the inside of the cup.
(ii) Fill the cup with the slurry to be tested. Try to avoid trapping air bubbles. If
necessary, tap the cup a few times to release any bubbles.
(iii) Insert the lid with a firm twisting movement to ensure that it is properly seated and
that no air is trapped under it. Make sure that some excess slurry has been squeezed
out of the central vent hole in the lid. If none has, remove the lid, top up with slurry
and repeat the seating process.
(iv) With a finger over the vent hole carefully dry the outside of the cup. Check that the
vent hole is still full of slurry. If not, top up and repeat.
(v) Seat the instrument on the fulcrum and adjust the rider until the beam is in balance,
as shown by the level bubble.
(vi) Read off the slurry specific gravity against the calibration mark on the rider.
To familiarise oneself with the instrument, several tests can be done on a single batch of
clean slurry. The results should agree to 0.01. With contaminated slurries, it may be
difficult to get good repeatability, as the spoil may tend to settle. Such slurries should be
well stirred before a sample is taken for testing.
Fig. 2 Mud balance
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The calibration of the instrument should be checked regularly. The procedure is simply to
test a sample of clean water. This should show a density in the range 0.995 to 1.005 g/ml.
If the reading is outside this range, set the rider to a density of 1.000 and adjust the
counterweight until the beam is in balance. The counterweight is at the far end of the
beam from the cup, and usually consists of a small recess containing lead shot, closed by a
screw plug.
As the balance was developed for the oil industry, the range of the instrument is rather
wider than is necessary for civil engineering work (typically 0.72 to 2.88). Construction
slurries usually have specific gravities in the range 1.0 to 1.4. The smallest division of the
scale is 0.01 but, with care, the instrument can be read to 0.005 though the repeatability
between readings is seldom better than 0.01. A resolution of +/- 0.005 represents a range
of solids contents of the order of +/- 8 kg of bentonite per cubic metre of slurry (see
Section 8.1.2, Equation 2) and thus will not allow the bentonite content of a fresh slurry to
be estimated to an accuracy better than about +/- 20%. The mud balance can therefore
only be used to identify gross errors in batching, and is not suitable for accurate quality
control work.
It should be noted that the instrument has three scales in addition to the specific gravity
scale. These are: pounds per cubic foot, pounds per U.S. gallon (0.833 of an Imperial
gallon) and pounds per square inch per 1000 foot depth (a one thousand foot column of
water exerts a pressure of 433 psi). In general none of these other scales is useful for civil
engineering work, and only the specific gravity scale should be used.
9.1.2 Calculation of slurry density for mix proportions
The density of a slurry, ρs is related to the concentration, C of bentonite by weight of mix
water as follows:
ρs = ρw (1 + C) / (C/Gp + 1) (1)
where ρw is the specific gravity of the mixing water (assumed to be 1.0) and Gp is the grain
specific gravity of the bentonite powder used to prepare the slurry (see Section 9.1.3)
If the concentration, Cs is expressed as kilograms of bentonite per cubic metre of final
slurry, then the formula becomes:
ρs = ρw + Cs (1 - 1/Gp) (2)
9.1.3 Grain specific gravity of the bentonite powder
If the density of a bentonite slurry is to be calculated from the clay content, it is necessary
to know the grain specific gravity of the clay. Typically, this may be in the range 2.5 to
2.8 for the oven dry powder but bentonites, as supplied, will contain some moisture
therefore the effective grain specific gravity of the powder will be less than that of the
oven dry powder.
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If the grain specific gravity of the oven dry powder is Gs and that of the bentonite, as
supplied, is Gp then:
Gp = (1 + mc) / (1/Gs + mc) (3)
Where mc is the moisture content of the bentonite powder by oven dry weight. If Gs = 2.7
and mc = 10%, then Gp = 2.34 and thus it can be seen that the effect of moisture content on
the grain specific gravity is significant.
It should be noted that the above formula has been derived from a simple rule of mixtures
and assumes that the moisture in the bentonite has the same density as free water. In
practice this may not be strictly true for water sorbed on to the particle surfaces.
9.2 Sand content
During excavation with a bentonite slurry, the density will increase due to suspension of
spoil. The density of a contaminated slurry provides a measure of the total amount of spoil
in the slurry but no information as to whether this is sand, silt or clay
The sand content set is designed to measure the bulk volume of sand (strictly material
coarser than 200 mesh U.S., 0.075 mm, 75 microns) in a given volume of slurry. The
apparatus consists of a tapered graduated tube, a small 200 mesh U.S. sieve and a funnel,
as shown in Figure 3.
9.2.1 Test Procedure
(i) Carefully fill the graduated measuring tube with
slurry to the “mud to here” line.
(ii) Add sufficient water to fill the tube to the “water
to here” line. The exact amount of water is not
important. Cover the mouth of the tube and
shake vigorously.
(iii) Pour the mixture on to the screen a little at a
time. After each addition, wash the bulk of the
fines through before adding more slurry (if the
whole batch of slurry is poured on to the screen
at once, it may block it and make subsequent
washing difficult). Wash any remaining material
out of the tube and on to the screen. Wash the
residue on the screen until free of all bentonite.
(iv) Fit the funnel over the screen, invert it, and put
the tip of the funnel into the tube. Wash the residue back into the tube.
Fig. 3 Sand content set
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(v) Record the volume of residue in the tube.
The result of the test is quoted as the sand content (percent bulk volume of sand by volume
of slurry).
9.3 Rheological measurements
The most common instrument used for
measuring actual rheological parameters (rather
than ranking slurries, for example, by flow time
from a funnel) is the Fann viscometer
(sometimes referred to as a rheometer), as shown
schematically in Figure 4. Two general versions
of the instrument are available: electrically
driven and hand cranked. All versions of the
instrument can be operated at 600 and 300 rpm,
and have a handwheel so that the bob can be
slowly rotated for gel strength measurements.
Some electrically driven versions also have
additional fixed speeds of 200, 100, 6 and 3 rpm,
or variable speed motors. For all versions of the
instrument, there is a central bob, connected to a
torque measuring system, and an outer rotating
sleeve. The gap between bob and sleeve is only
0.59 mm, therefore it is necessary to screen spoil
contaminated slurries before testing. The full
test procedure is given in API Publication
RP 13B “API Recommended Practice - Standard Procedure for Testing Drilling Fluids”.
Fundamentally, for both types of instrument, four measurements can be made:
(i) The dial reading at a rotational speed of 600 rpm (the 600 rpm reading)
(ii) The dial reading at a rotational speed of 300 rpm (the 300 rpm reading)
(iii) The 10 second gel strength, obtained by slowly rotating the gel strength knob until
the gel breaks after the slurry has been agitated at 600 rpm and then left to rest for
10 seconds
(iv) The 10 minute gel strength, determined as for the 10 second gel strength but after a
rest period of 10 minutes
9.3.1 Test procedure for the electrically driven viscometer
The test procedure for the electrically driven instrument is as follows:
(i) Fill the mud cup with slurry and place it on the platform of the instrument. Raise the
platform so that the surface of the slurry is level with the engraved line on the rotor.
Fig. 4 Schematic diagram of Fann viscometer
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(ii) Set the instrument to rotate at 600 rpm. If any clicking or grinding sounds are heard,
it means that the sample contains coarse material which is being ground between the
rotor and the bob. The instrument must be stopped immediately and cleaned, and the
slurry sieved on a 52 mesh B.S. sieve (i.e. approx. 0.3 mm or finer) before re-testing.
Most slurries taken direct from the excavation will need to be screened before
testing.
(iii) Leave the instrument running at 600 rpm until a steady reading is obtained. Record
this reading as the 600 rpm reading.
(iv) Shift the speed to 300 rpm and again record the steady reading.
(v) To obtain the ten second gel strength, set the instrument to 600 rpm, run it for ten
seconds, stop it and allow the slurry to stand undisturbed for 10 seconds. Then
slowly and steadily rotate the larger handwheel on top of the instrument so as to
produce a positive dial reading. Record the highest dial reading before the gel
breaks. It may be necessary to turn the handwheel through quite a large angle before
the gel breaks so, before starting, position the hand so that the necessary rotation can
be achieved without undue difficulty. As this test is quick, it should be done several
times until a repeatable value is obtained.
(vi) The ten minute gel strength is obtained in the same way as the ten second gel
strength except that the slurry must be left undisturbed for ten minutes. Take great
care when taking this reading so as to avoid having to repeat it.
9.3.2 Test procedure for the hand cranked viscometer
The test procedure for the hand cranked viscometer is very similar to that for the
electrically driven version. The speed is controlled by a gear shift lever. When this is
turned fully clockwise, the 300 rpm speed is selected. 600 rpm is obtained by moving the
lever anticlockwise one indent. When turned fully anticlockwise, a high stirring speed is
obtained (the electrically driven instrument does not have this facility).
The manufacturer’s recommended operating procedure is as follows:
To obtain the 300 and 600 rpm reading:
(i) Place a recently agitated sample in a suitable container and lower the instrument
head until the rotor sleeve is immersed exactly to the scribed line. To hold the
instrument in this position, tighten the lock screw on the left leg of the instrument.
With the gear shift lever at the high speed setting, rotate the crank for about 15
seconds, move the lever to the 600 rpm position and continue cranking.
(ii) Wait for the dial reading to come to a steady value (the time required depends on
sample characteristics). This is the high speed reading (600 rpm). Turn the gear
shift
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lever fully clockwise, crank and wait for the dial reading to come to a steady value.
This is the low speed reading (300 rpm).
To obtain the gel strength:
(i) Stir a sample at the highest speed for about 15 seconds.
(ii) Allow the desired rest time (10 seconds or 10 minutes) and then turn the gel strength
knob on the hub of the speed change lever clockwise, slowly and steadily. The
maximum deflection of the dial before the gel breaks is the gel strength in lb/100 ft2.
N.B. When the gel strength knob is turned, the crank also turns. This may prevent
sufficient rotation of the knob unless the crank is initially well clear of the knob. To
ensure that this is so, it is best to always finish cranking with the crank at the “10 o’clock”
position. Also, as in Section 9.3.1 (v), position the hand before taking a reading so that it
can be obtained without undue difficulty.
9.3.3 Checking viscometers
The instruments should be checked at regular intervals to ensure that the speeds are
correct. This is best done with a stroboscope. If a 60 cycles/sec lighting supply is
available, then the drilled holes in the top of the rotor can be used. If only a 50 cycles/sec
light is available, the strobe effect can be obtained by fitting a ring of paper, marked with
10 equally spaced dots, over the rotor. The dots should appear stationary (or very slowly
moving) at both 600 rpm and 300 rpm when there is a sample in the cup (this slows the
rotation very slightly). If the speeds are not correct, the instrument must be recalibrated.
Caution: if the cover of the instrument is taken off (in particular, if the gel strength knob is
removed), it is very likely that the speed calibration will be disturbed.
9.3.4 Calculation of results
The Fann viscometer is designed so that, for the test sample:
(i) Apparent viscosity in centipoise (cP) = 600 reading/2
(ii) Plastic viscosity in cP = 600 reading - 300 reading
(iii) Gel strength (10 sec or 10 min) in lb/100 ft2 = dial reading. (lb/100 ft2 is a unit used
in the oil industry. To convert to N/m2 multiply the dial reading by 0.48.)
For completeness note that:
(i) Yield value in lb/100 ft2 = (2 x 300 reading - 600 reading)
(ii) The yield value is the extrapolation of the line passing through the 600 and 300
points on to the shear stress axis of a plot of shear stress against rotational speed. In
contrast, gel strength is the stress necessary to cause flow in a slurry which is at rest.
For an ideal Bingham fluid, the yield value and gel strength would be equal.
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Bentonite slurries are not ideal Bingham fluids, and they also show thixotropy.
As a result, the yield value will not be equal to the gel strength (either 10 second
or 10 minute). The yield value is seldom used in civil engineering.
For thick slurries, there may be doubt as to whether the slurry has fully penetrated the
narrow annular gap between rotor and bob of the instrument (the gap is only 0.59 mm).
Results for such slurries should be viewed with caution.
9.4 Flow cones
The Fann viscometer is an expensive instrument and must be used with care by a trained
operator if reliable results are to be obtained. In the laboratory, the detailed information
that can be obtained from it can be invaluable in the investigation of different slurry
systems, treatment chemicals, etc. However, there is often a need for a simple test which
can be used for compliance testing at mixers, the trench side, etc. In general some form of
flow cone is used. For excavation slurries the most common cone is the Marsh funnel.
However, there is a wide variety of different cones, and it is important that the type of cone
is specified when reporting results. The following data should be specified:
(i) The outlet diameter of the funnel spigot
(ii) The volume of slurry to fill the cone
(iii) The test volume to be discharged (this may not be the full volume of the cone)
(iv) The flow time for water
Test procedures are similar for all cones, therefore
only the procedure for the Marsh funnel is
detailed.
9.4.1 The Marsh funnel
The Marsh funnel, as shown in Figure 5, is the
simplest instrument for routine checking of slurry
viscosity. The test procedure is as follows:
(i) Clean and dry the funnel.
(ii) Hold the funnel upright with a finger over
the outlet spigot.
(iii) Pour a freshly stirred sample of the slurry
through the screen to fill the funnel to the
underside of the screen (a volume of 1.5
litres).
Fig. 5 Marsh funnel
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(iv) Immediately the funnel is full, keeping the funnel upright, remove the finger and
allow the slurry to flow into a graduated receiver. Record the time for the flow of
one US quart (946 ml). The volume discharged should also be quoted.
It is necessary to record the volume discharged, as the specification for the instrument also
allows a discharge volume of 1000 ml.
The funnel may be checked by measuring the flow time for water. For clean water at 21oC
(70oF), the times should be as follows:
25.5 to 26.5 seconds for 946 ml
27.5 to 28.5 seconds for 1000 ml
No adjustment of the funnel is possible, and if readings outside the above ranges are
obtained, it must be assumed that the funnel (or the stopwatch) is damaged or that the
funnel has not been properly cleaned. Solids can build up around the discharge orifice and
constrict the flow. Clogging of the discharge orifice may be particularly severe if the
funnel has been used previously for polymer based slurries. In this case, it may be
necessary to immerse the cone in a chemical polymer breaking agent (e.g. bleach).
The Marsh funnel is suitable for testing most bentonite slurries.
9.5 The Shearometer
The Shearometer is an instrument that is no longer in common use for the testing of
excavation slurries. The instrument is designed to measure the gel strength of slurries but
gives results which tend to be markedly lower than those from the Fann viscometer. When
reading older publications quoting slurry gel strength results, it may be necessary to
consider whether the Shearometer or Fann viscometer was used.
The instrument consists of a duraluminum tube, 3.5 inches long, 1.4 inches internal
diameter weighing 5 grams, and a stainless steel cup, mounted within which is a vertical
scale graduated in lb/100 ft2. To carry out a test, the cup is filled to the zero line of the
scale with a freshly agitated sample of the slurry. The duraluminum tube is then wetted
and the excess water wiped off. The tube is placed over the scale, lowered to the slurry
surface and released at the appropriate test time (ten seconds or ten minutes). After
allowing the tube to sink for one minute, the scale should be read directly opposite the top
of the tube. If the tube sinks completely, the time it takes to sink should be recorded.
The lowest scale division of the instrument is 3 lb/100 ft2 (1.4 N/m2). Some specifications
use this figure for the lower limit of gel strength - i.e. it would seem that the slurry is
deemed acceptable if it gives a reading on the scale of the instrument. As the instrument
was designed to test bentonite slurries, this is not unreasonable.
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9.6 pH
pH is a measure of acidity or alkalinity. pH 7 is neutral, below 7 is acid and above 7 is
alkaline. pH may be measured with a glass electrode and a matched millivolt meter or
with pH papers. With an electrode, it should be possible to measure the pH of pure
solutions to a repeatability better than 0.05 pH unit. It is necessary to calibrate the
electrode with a buffer solution prior to use and, preferably, to check its operation using a
second buffer solution.
By selecting narrow range pH papers it is, in theory, possible to measure pH to 0.1 unit.
However, there can often be doubts about the indicated colour. When testing suspensions,
to avoid masking the colour with deposited solids, apply the suspension to one side only of
the paper and read the colour from the other.
9.6.1 Test procedure
A pH meter is a delicate instrument and must be treated with great care. The basic
components of the instrument are an electrode ( an electrical cell) and a millivolt meter.
The electrode has a very fragile glass bulb as one element of the cell; the other element is
normally a porous plug or wick connecting with a fluid inside the electrode.
Before use, the electrode should be checked to see that it is filled with liquid. A bottle of
filling solution is provided with the instrument, and this (and only this) should be used to
top up the electrode when necessary. The electrode should be stored in the electrode
filling solution or distilled water when not in use. The manufacturer’s instructions should
be checked for precise details.
The instrument must be calibrated before use and, for this, a buffer solution is required.
This is supplied either as a liquid or as a small sachet of powder that must be dissolved in a
specified volume of distilled water (generally 100 ml). The calibration procedure is to
pour some of the buffer solution into a small beaker and dip the electrode into it. The
beaker should then be rotated gently so as to swirl the solution around the electrode. The
meter should settle down to a steady reading within about a minute. Once the reading is
steady, the meter should be adjusted so that it shows the pH of the buffer solution. If the
meter has an adjustment for sample temperature, the temperature of the buffer solution
should be measured prior to calibration and the control set to this temperature.
After calibration, the electrode should be washed in a stream of distilled water and gently
dried with a tissue. The used buffer solution should not be returned to the stock bottle as
there is always a risk of contamination. However, it should not be discarded immediately,
as it can be used to make occasional rough checks on the calibration during testing, but it
should be discarded at the end of the day and a new sample used for the following day’s
work.
Once calibrated, the controls on the pH meter must not be touched. In theory the
calibration should hold good for some time but, in practice, when testing slurries, it can
drift significantly and the response of the meter may become very sluggish. The reason for
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this seems to be that the electrodes have a rather short useful life. To prolong the life of an
electrode, do not leave it standing in slurry for longer than necessary. Always wash off the
slurry at the end of a test, and keep the number of tests to a minimum.
The actual test procedure for a slurry is as follows:
(i) Set the temperature control to the slurry temperature (if appropriate).
(ii) Dip the electrode into a beaker of the slurry.
(iii) Gently stir the slurry with the electrode until a steady recording is obtained.
(iv) Record the pH to the nearest 0.1 of a pH unit.
It should be noted that, if the electrode is left stationary in the slurry, a slightly different
reading from that obtained as in (iii) may be indicated. This will not be the correct
reading.
9.6.2 Typical pH values
Most fresh slurries made with bentonite which has been converted from the calcium form
to the sodium form by the addition of sodium carbonate, will have a pH in the range 9.5 to
10.5. Used slurries, unless contaminated by cement, often have a slightly lower pH than
their fresh counterparts. Processes which will contribute to this pH reduction include ion
exchange of the sodium ions in the slurry with ions present on the natural clays in the
ground, and reaction of any free sodium carbonate in the slurry with atmospheric carbon
dioxide.
Natural sodium bentonites, such as Wyoming bentonite, can be of more nearly neutral pH.
Cement contaminated slurries may have very high pH values of the order of 11.5 to 12.5.
pH may be used as a quality control parameter for the bentonite as delivered to site. For
this, the pH of a slurry of fixed concentration (typically 5%) should be measured, though
the variation of pH with concentration will be quite modest.
9.7 Filtrate loss
Filtrate loss (sometimes known as fluid loss), bleed, settlement and syneresis all represent
segregation processes which may suggest slurry instability. Segregation of solid and liquid
phases is a common theme and thus there may be common causes at the micro structural
level. Slurries which show high values for any one parameter may show high values for
the others.
The standard apparatus used for filtrate loss measurement is the American Petroleum
Institute standard filter press, as shown in Figure 6. The instrument consists of a 3 inch
diameter cell with a detachable base in which a filter paper, supported on a wire mesh, can
be fitted. In the test, the volume of filtrate collected from a slurry sample, subjected to a
pressure of 100 psi for 30 minutes, is measured.
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9.7.1 Test procedure
The test procedure for the standard Baroid instrument is as follows (the procedure for
assembling other versions of the instrument may be slightly different):
(i) Assemble the dry parts in the following order: base cap, rubber gasket, screen, a
sheet of filter paper, rubber gasket, and cell. Secure the cell to the base cap.
(ii) Fill the cell with the sample to be tested to within 6 mm of the top. Set the unit in
place in the filter press frame.
(iii) Check the top cap to make sure the gasket is in place. Place the top cap on the cell
and secure the unit in place with the T-screw.
(iv) Place a dry graduated cylinder under the filtrate tube.
(v) With the regulator T-screw at its maximum outward position (closed position), close
the safety bleed valve. Apply 100 psi pressure to the filter cell by rapidly screwing
the regulator T-screw inwards. The pressure should be applied in 30 seconds or less.
A pressure in the range 95-105 psi is acceptable. Timing of the test should start at
the time of pressure application.
Fig. 6 Standard filter press
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(vi) At the end of 30 minutes, record the volume of filtrate. Return the regulator T-screw
to its maximum outward position. Open the safety bleed valve to release the cell
pressure.
9.7.2 Test results
The volume of filtrate normally shows a linear relationship to the square root of time for
which the sample is under pressure. That is:
V = m t0.5 (4)
where V is the volume of filtrate collected in time t, and m is a constant.
Thus, the filtrate volume at 7.5 minutes will be half that at 30 minutes. Some
specifications may allow the test to be terminated after 7.5 minutes and the filtrate loss
reported as twice the 7.5 minute value.
The test result is independent of the volume of slurry used in the test, provided that all the
water is not expelled from the slurry within the test period If this occurs, gas will start to
discharge from the filtrate tube. For some versions of the test cell, the pressure is provided
by a small, disposable, carbon dioxide bulb. If such bulbs are used, the test cell should be
filled with as much slurry as possible so as to minimise the amount of gas required.
Caution: If carbon dioxide (“Sparklet”) bulbs are used to pressurise the cell, they must be
stored away from direct sunlight or sources of heat.
9.8 Bleeding
Bleeding may be defined as the separation of water from the solids in a slurry, principally
due to gravitational settlement of the solids. In bentonite slurries, bleeding will be
effectively a self weight consolidation process which will continue until the slurry has
sufficient strength to support its own weight, though there may be secondary effects (e.g.
syneresis) which lead to continued separation of water over a very long time scale.
Bleeding of bentonite slurries should be small, once the slurry has been hydrated for about
24 hours. Severe bleeding in hydrated slurries normally suggests an incompatibility of the
bentonite with the mix water. Normally, it should not be necessary to test or specify bleed
for excavation slurries, though problems may occur if slurries are continually used and
cleaned without regular topping up with fresh slurry.
9.9 Moisture content
As indicated in Section 9.1.1, density is a poor indicator of the bentonite content of fresh
slurries. The density of such slurries is close to that of water, therefore quite precise
measurements are necessary. Measurement of the moisture content of a fresh slurry can
give much better resolution, though the test will take longer (typically at least 24 hours if a
conventional oven is used).
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Generally the test procedure should follow that given in BS 1377: Part 2: 1990, except that
fluid slurry should replace the soil. The quantity of slurry should be selected so that a
reasonable weight of solids will be left after drying, which will depend on the resolution of
the balance used for weighing. Typically, at least 100 grams of slurry should be dried.
It should be remembered that the bentonite used to prepare the slurry will include some
moisture, therefore the moisture content of the bentonite powder used to prepare the slurry
and that of the slurry should both be measured.
If the moisture content of the bentonite powder is mc and that of the slurry is ms (this may
be of the order of 2000%), then the concentration, C of bentonite powder at moisture
content mc in the slurry (in kg/kg of water) is as follows:
C = (1 + mc) / (ms - mc) (5)
It should be noted that, in most of the literature on bentonites, the moisture content is given
as a percentage of the moist weight of the powder. In soil mechanics, moisture contents
are calculated by oven dry weight. Thus, if mm is the moisture content calculated as a
percentage of the moist weight, the moisture content as a percentage of oven dry weight,
mc is:
mc = mm / (100 - mm) (6)
9.10 Water compatibility testing
In general, potable water from a mains supply has been found suitable for the preparation of
bentonite slurries. In contrast, water from untreated sources such as lakes and streams may
not be acceptable.
Water containing significant quantities of dissolved salts can inhibit proper dispersion of the
bentonite. The levels at which dissolved ions inhibit dispersion vary with the type and source
of the bentonite which is generally more sensitive to cations than anions. Magnesium is often
the most sensitive ion and may begin to inhibit dispersion at levels greater than about 50
mg/litre. Calcium may inhibit dispersion at levels greater than about 250 mg/litre.
If chemical analyses are to be carried out on the water it is suggested that the following ions
are determined:
• Cations: sodium, calcium, magnesium and potassium
• Anions: chloride, sulphate and bicarbonate
pH and electrical conductivity should also be measured.
On the basis of the anion and cation analyses, an anion/cation balance should be undertaken. If
there is a significant imbalance, or other ions are known or expected to be present, a fuller
analysis may be necessary.
© Federation of Piling Specialists – January 2006 (2nd edition)
(first published April 2000)
27
It is difficult to predict whether a water is suitable from chemistry alone. Therefore, if there is
any doubt about the water, and particularly if non-mains water is to be used, hydration trials
should be undertaken. These should take the form of a full investigation of bleeding, rheology
and filtrate loss, etc. carried out on bentonite slurries prepared with both distilled water and the
test water. A simplified procedure could be based on bleed alone, as this can be a sensitive
parameter. A possible procedure could be as follows:
• A slurry consisting of the bentonite and the proposed mix water for the site should
be prepared.
• If there is any significant bleed at 24 hours and at 48 hours (after re-mixing), the
mix water should be considered unsuitable for use, and an alternative source of
water sought.
10 Bibliography
Boyes, R.G.H., Structural and Cut-off Diaphragm Walls, Applied Science Publishers,
1975.
Federation of Piling Specialists, modifications (1977) to Specification for Cast-in-Place
Concrete Diaphragm Walling (published 1973) and Specification for Cast-in-Place Piles
Formed Under Bentonite Suspension (published 1975).
Hajnal, I., Marton, J. and Regele, Z., Construction of Diaphragm Walls, Wiley, 1984.
Hodgson, F.T., Design and control of bentonite/clay suspensions and concrete in
diaphragm wall construction, in A Review of Diaphragm Walls, Institution of Civil
Engineers, 1977, pp 79-85
Hutchinson, M.T., Daw, G.P., Shotton, P.G. and James, A.N., The properties of bentonite
slurries used in diaphragm walling and their control, Conference on diaphragm walls and
anchorages, Institution of Civil Engineers, 1975, p33-40
Jefferis, S.A., Effects of mixing on bentonite slurries and grouts, ASCE specialty
conference on grouting in Geotechnical Engineering, New Orleans, 1982 pp 62-76.
Jefferis, S.A., Slurries and Grouts, in the Construction Materials Reference Book, Doran,
D.K., Editor, Butterworths, 1992.
Rogers, W.F., Composition and Properties of Oil Well Drilling Fluids, 5th Edition, Gulf
Publishing, 1988.
Xanthakos, P.P., Slurry Walls, McGraw Hill, 1979.