A

Seminar report

On

GEOTECHNICAL CHARACTERISTICS OF STABILIZED AGED

BIOSOLIDS”

Submitted In partial fulfillment for the award of the degree of

BACHELOR OF TECHNOLOGY

IN CIVIL ENGINEERING

From Rajasthan Technical University

Batch: 2014-18

SUBMITTED BY: SUBMITTED TO:

ANJALI JAIN (14EJCCE007) Mr. TEEKEM CHOUDHARY

ANKIT GUPTA (14EJCCE008)
ANKIT SAHARIA (14EJCCE009)
 Department of Civil Engineering
Jaipur Engineering College & Research Centre
Jaipur (Raj.)
Candidate’s Declaration

I hereby declare that the work, which is being presented in the Seminar report entitled “GEOTECHNICAL
CHARACTERISTICS OF STABILIZED AGED BIOSOLIDS ”in partial fulfillment for the award of Degree of
“Bachelor of Technology” in Department of Civil Engineering with Specialization in Civil Engineering
and submitted to the Department of Civil Engineering, Jaipur Engineering College & Research
Centre, Rajasthan Technical University is a record of my own investigations carried under the Guidance
of Mr. Teekam choudhary, Department of Civil Engineering, Jaipur Engineering College & Research
Centre.

I have not submitted the matter presented in this report anywhere for the award of any other Degree.

(Name and Signature of Candidate)

Civil Engineering

Enrolment No.:

Jaipur Engineering College & Research Centre, Jaipur

Counter Signed by

Name(s) of Supervisor(s)

.....................................
 .....................................
CERTIFICATE

This is to certify that ANJALI JAIN of VII Semester, B.Tech (Civil Engineering) 2017-18, has presented
a seminar report titled GEOTECHNICAL CHARACTERISTICS OF STABILIZED AGED
BIOSOLIDS in partial fulfilment for the award of the degree of Bachelor of Technology under
Rajasthan Technical University, Kota.

Date:

Mr. Tikkam choudhary Mr. Kamlesh Kr. Choudhary

Project Co-ordinator H.O.D. Civil Dept.

 ACKNOWLEDGMENET

I take this opportunity to express my gratitude to all those people who have been directly and
indirectly with me during the competition of this project.

I pay thank to Mr. Tikkam choudhary who has given guidance and a light to me during this
seminar report. His versatile knowledge about “GEOTECHNICAL CHARACTERISTICS
OF STABILIZED AGED BIOSOLIDS” has eased me in the critical times during the span of
this minor project.

I acknowledge here out debt to those who contributed significantly to one or more steps. I take full
responsibility for any remaining sins of omission and commission.

ANJALI JAIN (14EJCCE007)

ANKIT GUPTA (14EJCCE008)
ANKIT SAHARIA (14EJCCE009)

1
 ABSTRACT

Stockpiles of air-dried sludge from wastewater treatment plants, known as biosolids, are constantly
increasing worldwide. This subsequently adds to the urgency of stabilising biosolids in an effort
to convert them into a suitable construction material for use in engineered fills. This paper reports
the result of a laboratory testing programme on biosolids stabilised with two types of waste
materials, namely, bauxsol and fly ash. The biosolids collected from the Western Treatment Plant
in Melbourne were stabilised in different ratios and tested to determine their compaction, strength,
permeability and deformation characteristics. Both static and dynamic compaction methods were
used in the preparation of the test specimens. For several years, land application has been the
preferred method of biosolids disposal.

The nutrient-rich nature of biosolids makes them an excellent soil amendment and consequently,
their use in agriculture as fertilizers has been wide spread. However, in recent years, an increasing
awareness of the disease-causing potential of biosolids has given rise to misgivings about this
method of disposal. Occurrences of ailments such as ear, eye, nose, throat and lung infections;
gastrointestinal problems; skin rashes; nausea; asthma; dizziness; coughing; and in some cases,
even death have been attributed to exposure to land-applied biosolids. The fact that the availability
of large tracts of land for land applying biosolids is rapidly decreasing has only compounded the
concern over land application.

Results suggest that the hydraulic conductivity values of biosolids consistently decrease with
increasing amounts of additives, which is an indication of decreasing void ratio of material. In
addition, increasing the amount of additives was found to lead to a subsequent decline in the
compressibility of the stabilised biosolids. The strength of the biosolids samples increased with the
addition of bauxsol and fly ash, with the highest shear strength found to be achieved with around
10% fly ash mixture. The application of stabilised biosolids in road embankments is discussed as
a sustainable solution in geotechnical engineering applications.

Biosolids are nutrient-rich organic matter produced during processing of treatment of

wastewater at a treatment facility. It is estimated that approximately 8.2 million tons of biosolids
will be produced in the United States by the year 2010. Because of their volume and potential
contamination by pathogens and heavy metals, biosolids disposal presents a major challenge to
facility owners and operators. Current disposal practices include land application, landfilling,
surface disposal, incineration, and composting.

2
 CONTENTS

Certificate i

Acknowledgement ii

Abstract iii

Contents iv-v

List of Figures vi

List of Tables vii

Chapter 1 Introduction 01-06

1.1 Background
1.2 Introduction
1.3 Previous studies
1.4 Materials

Chapter 2 Literature review 07-09

Chapter 3 Experimental Setup 10-19

3.1 CBR test procedure

3.2 Atterberg’s Limit

3.3 Triaxial shear test

3.4 Index properties

3.5 Compaction, CBR & swell properties

3.6 Hydraulic conductivity

3
Chapter 4 Discussions 20-27

Chapter5 Conclusion 28

Chapter 6 References 29

4
Chapter 1 INTRODUCTION

1.1 Background
Biosolids are nutrient-rich organic matter produced during the processing of wastewater at a
wastewater treatment plant. In addition to significant amounts of carbon, hydrogen, oxygen, sulfur,
and nitrogen, biosolids also contain phosphorus, potassium, and essential micronutrients such as
zinc and iron. Because of their high content of phosphorus, nitrogen and other nutrients, biosolids
make an excellent soil amendment and have been widely used as fertilizers.

The wastewaters that are generated by domestic, commercial and industrial sources are collected
by sewers and brought for treatment to wastewater treatment plants. Most industrial sources
provide some sort of pretreatment for their wastewaters in order to reduce the levels of such
contaminants as metals and chlorinated hydrocarbons before discharging them into the municipal
sewer system. Over the past 20 years, improvement in pretreatment technology and pollution
prevention programs has considerably improved the quality of wastewaters that are discharged by
these sources into the sewers (Walker,1998).

At the treatment plant, wastewater is given primary, secondary and, in some occasions, tertiary
treatment prior to discharge into the environment. The nature and amount of biosolids is influenced
by the quantity and quality of wastewater entering the treatment plant, the treatment given to the
wastewater and also, and the treatment given to the biosolids. Generally, the greater the treatment
given to the wastewater, the larger the quantity of biosolids produced. Also, since contaminants in
the wastewater tend to get concentrated in the biosolids. If an industrial pretreatment process uses
chemical precipitation using such chemicals as ferric chloride, alum, lime or polymers, these
chemicals can ultimately wind up in the biosolids (EPA, 1999).
Thus, the nature and the subsequent treatment of the biosolids is influenced by the quality of the
incoming wastewater. The marked improvements in biosolids quality resulting from pretreatment
and pollution prevention programs, for example, can encourage POTWs to process their solids
further. When biosolids achieve the low levels of pollutants that make the widest distribution of
biosolids products possible, processes such as composting become more attractive.

1.2 Introduction

Waste disposal and waste recycling is one of the biggest challenges of our time. There have been
numerous attempts in the past two decades to use solid wastes in a range of applications including
substitutes for naturally occurring materials in the construction industry (Arulrajah et al., 2011).

5
Roadwork construction and development often impose heavy demand on virgin construction
materials such as aggregates and fill material. Reusing solid wastes (in their original form, blended
with natural aggregates or stabilised with additives if necessary) in roadwork applications can
significantly reduce the demand for virgin soils and aggregates (Ghataora et al., 2006). In addition,
recycling solid waste materials will reduce demand for landfill sites and carbon generation, which
alternatively will accelerate the move towards a more sustainable construction industry (Hoyos et
al., 2011). Using recycled material in roadwork applications as a substitute for naturally occurring
material requires technical feasibility assessments such as laboratory testing, field trials and
numerical modelling to ensure that these recycled solid wastes are accepted by road authorities,
consultants and contractors (Maghoolpilehrood et al., 2013).

With rapid increases in population, improved health standards and higher number of sewage
treatment facilities around the world, there is a sharp surge in the quantity of waste production in
waste water treatment facilities. Sludge, which is the solid–water mixture pumped from wastewater
treatment lagoons, is the main waste byproduct of these facilities. Sludge typically contains 2–15%
oven-dried solids and shows the characteristics of slurry. Sludge is treated using different
techniques varying from one treatment plant to another, including drying in drying pans, which
ultimately results in a soft highly organic material with 50–70% of solids (mass ratio) called
biosolids (Arulrajah et al., 2011). Issues such as surface deformation caused by differential
settlement, desiccation cracking and odour emission caused by gas leakage from containment
regions are some of the potential problems associated with improper biosolids placement (Kayser
et al., 2011). The state of Victoria in Australia currently stores more than 2 million tonnes of
biosolids in lagoons or in stockpiles, with an annual production rate of 66 700 tons added to current
stockpiles (DNRE, 2002; EPA Victoria, 2004). The combination of high organic contents, high
water contents, significant deformation potential and low strength makes biosolids a problematic
material for reuse in any form of construction, with its improper use potentially leading to severe
consequences (Kayseret al., 2011). Limited knowledge and research on the relevant engineering
characteristics of wastewater biosolids is one of the obstacles for reusing this waste material in
road work applications (Arulrajah et al., 2011). This is furthermore exacerbated considering that
the characteristics of biosolids vary around the world depending on the source of sewage, treatment
process and technique and age of the biosolids (Arulrajah et al., 2013; U.S. EPA, 1999).

Despite these variations, there are still similarities in biosolids’characteristics and behaviour
produced at different treatment plants. There is a certain similarity in mechanical behaviour of
biosolids to that of highly organic clays in that biosolids possess a high potential for excessive
deformation (Koenig et al., 1996) and also a low bearing capacity or shear strength (Arulrajah et
al., 2011; Kayser et al., 2011). It is recognised that organic matter affects the geomechanical
behaviour of soils by modification of some soil properties, which is the case for biosolids. Organic
matter, for instance, provides an aggregated structure to the soil by molecular complexation
involving metallic, organic and clay molecules, which in turn contributes to increasing its
compressibility (Tremblay et al., 2002). The enhancement of shear strength and deformation

6
characteristics of biosolids have been tried by either drying (Stone et al., 1998), chemical
stabilisation through mixing it with a range of additives such as lime, fly ash, cement and slag
(Kayser et al., 2011); physical stabilisation using coarse grained aggregate such as recycled
crushed glass (Disfani et al., 2009) or a combination of these techniques.

The focus of this paper is on the improvement of mechanical behaviour of biosolids for potential
use in road fills through a chemical stabilisation process using fly ash and bauxsol. Use of waste
material such as fly ash and bauxsol as binders in the stabilisation of biosolids is an attractive zero-
waste option. Fly ash and bauxsol are both relatively inexpensive industrial byproducts compared
with conventionally used cement and lime. This solution can promote a sustainable construction
industry through reduction of energy use and cutting down the greenhouse gas emissions (Tastan
et al., 2011).

An experimental study was conducted on biosolid samples collected from Western Treatment Plant
(WTP), located approximately 50 km west of Melbourne, Australia. Fly ash and bauxsol were
mixed with biosolids at two different ratios, and tests including compaction, California Bearing
Ratio (CBR), swell index, permeability, consolidation, creep and Consolidated Undrained (CU)
triaxial shear tests were carried out on stabilised and also untreated specimens. The geotechnical
characteristics of stabilised biosolids were then compared with those of similar studies and also
characteristics of highly organic clays and road authority requirements to ascertain the viability of
using biosolids as a construction material in road fills and embankments.

1.3 Previous studies

Construction of roadways on soft organic soils or using organic soils as fill material in roads is
potentially problematic because organic soils typically exhibit low shear strength and high
compressibility(Tastan et al., 2011). Chemical stabilisation of soils, which involves mixing with a
binder to increase its strength and stiffness through chemical reactions, is widely used for soft
inorganic soils (Tastan et al., 2011). In this approach, a cementing agent is added to organic soils
(peat; biosolids or sludge) to convert the highly compressible material into a mechanically
competent, stiff and compact end product that can be used as fill material in reclaimed lands,
landfill covers, or bearing material to support light structures (Lim et al.,2006). There are shortterm
and long-term effects in chemical stabilization of soils. In the short term, adding lime or fly ash
causes flocculation and agglomeration of the clay particles by ion exchanges at the surface of the
soil particles, which results in improved workability and provides an immediate reduction in swell,
shrinkage and plasticity (Lim et al., 2002). In the long term, chemical reactions are accomplished
over a period of time depending on the rate of chemical breakdown and hydration of the silicates
and aluminates, which can result in further amelioration and binds soil grains together by the
formation of cementitious materials (Lim et al., 2002). For cementation to occur and enhance over
this long-term period, sufficient sources of pozzolans, either from the soil itself or from the
chemical additive, is required (Lim et al., 2002; Tastan et al., 2011). Nevertheless, there are

7
significant issues related to chemical stabilisation of highly organic material such as biosolids and
peat.
This is because when an organic soil is stabilised with a cementing additive, organic matter may
interfere with the hydration process by coating additive grains, retarding or preventing the
hydration reaction (Kamon et al., 1989). However, whether or not all organic compounds are
harmful to cementing is still the subject of on-going research studies (Tremblay et al., 2002).
Previous studies suggest that organic materials may cause some interference in the solidification
process by affecting the cement setting of stabilisers (Malliou et al., 2007). In a study on
stabilization of organic soils with fly ash, Tastan et al. (2011) concluded that when cement, fly ash
(or any source of calcium ions (Ca2+)) is added to organic soils, following the hydration of lime,
released Ca2+ ions are likely to be exhausted by the organic matter (humic acids), which limits the
availability of Ca2+ ions for pozzolanic reactions (Tastan et al., 2011). In addition, organic soils
require greater quantities of stabiliser compared to clay soils partially because, in organic soils
such as peat, there are fewer solid particles to stabilise and as solid particles are those that provide
structure, a greater quantity of stabiliser is required (Axelsson et al., 2002). Organic substances
such as humus and humic acids may also cause the soil pH to drop, which alternatively affects the
reaction rate of the binders, resulting in a slower strength gain in mud and peat than in clay
(Axelsson et al., 2002). Saride et al. (2013) also noticed reduction in pH of stabilised organic clays
with the progress of curing. Tastan et al. (2011) acknowledged that soil organic content acts as a
detrimental agent for stabilisation with an increase in the organic content of soil resulting in an
exponential decline in strength of soil-fly ash mixtures (Tastan et al., 2011). Hampton and Edil
(1998) indicated that the organic matter in soils can also retain large amounts of water, which can
reduce the amount of available water for hydration reactions when a cementitious additive is
blended with soil (Tastan et al., 2011). Considering that organic soils are known to be more
difficult to stabilise chemically compared to inorganic soils (Malliou et al., 2007) and the fact that
little is known regarding the effectiveness of stabilising soft organic soils (especially waste
material such as biosolids) with fly ash (Tastan et al., 2011), hence there is a requirement for further
research in this area.

As stated previously, before they can be disposed, biosolids have to meet certain regulatory
requirements as regards their pathogen content, vector attraction potential and heavy metal content
in order to ensure that they are not detrimental to public or environmental health. Only high quality
biosolids can be land applied or composted. Also, the water content of biosolids can affect many
aspects of biosolids management such as the ease of handling and the transportation costs as well
as the magnitude of treatment required. The nature of biosolids also influences the decision
regarding treatment or disposal. Some biosolids treatment processes reduce the volume or mass of
the biosolids (such as biosolids digestion processes), while others increase biosolids mass (for
example, when lime is added to control pathogens). The two most common types of biosolids
treatment processes are stabilization and dewatering. Stabilization refers to a number of processes
that reduce pathogen levels, odor, and volatile solids content.

8
Biosolids must be stabilized to some extent before they can be used or disposed. Typical
methods of stabilization include alkali (lime) stabilization, anaerobic digestion (digestion of
organics by microorganisms in the absence of oxygen), aerobic digestion (digestion of
organics by microorganisms in the presence of oxygen), composting, and/or heat drying.

1.4 Materials
The laboratory tests were performed on specimens obtained from biosolids stockpiles at the WTP
in Victoria. Laboratory experiments were conducted on untreated biosolids along with biosolids
stabilised with bauxsol (3% and 5%) and fly ash (10% and 20%). These ratios were selected based
on previous studies on stabilization of sludge and soft soils using chemical additives (Asakura et
al.,2009; Lim et al., 2002; Nikraz et al., 2003; Suthagaran, 2010).All test specimens were cured
for a period of 24 hours, at room temperature of 20–25°C and relative humidity of 95–99%, prior
to the laboratory testing.

Biosolids
Sampling of the biosolids was carried out from the top of three existing biosolids stockpiles. It has
been recognised that the characteristics of biosolids in the stockpiles could vary naturally or could
vary during the life of stockpiling (Arulrajah et al., 2013). The biosolid samples were taken from
several locations at the top of the stockpiles and mixed to obtain representative biosolids samples
from each of the stockpiles. Particles size distribution results indicate that biosolids specimens
contain moderate gravel and clay fractions (3–4% gravel and clay content) with the majority of
particles in the sand and silt size range (40–50% silt and sand percentage). The biosolids showed
liquid limit (LL) of 100–110% and a plasticity index (PI) of 21–27% (Arulrajah et al., 2011). Loss
on ignition (LOI) was in the order of 35–38% and the average specific gravity (Gs) of the solids
was 1·86–1·88, which is significantly lower than that of the inorganic soil and aggregates
(Arulrajah et al., 2013).

According to the Unified Soil Classification System (USCS), the WTP biosolids can be classified
as organic silts of high plasticity (OH).

It is important to note that in a geotechnical engineering context, high organic content, high
deformation potential, low shear strength and a strain-rate dependency of shear strength are all
significant features of biosolids properties (Arulrajah et al., 2013; Kayser et al., 2011; Tastan et
al., 2011).

Bauxsol
Bauxsol is an inert stabilised byproduct of the aluminium industry and is a carefully modified
residue from alumina refineries, also known as red mud (Suthagaran, 2010). Bauxsol has been
used in the treatment of soils for agricultural and construction purposes (AustRoads, 1998).
9
Fly ash
Fly ash is predominantly silt size, light to dark grey pozzolanic powder that is a byproduct of coal-
fired power plants. The properties and reactivity of the ash vary depending on the coal properties
and also the combustion process (Axelsson et al., 2002). Consequently, it is recommended that
each batch must undergo technical and environmental quality assessment (Axelsson et al., 2002).
The significant strength increase of fly ash is a result of its high silica and aluminium content,
which generates pozzolanic reactions. As a result of these pozzolanic activities, minerals such as
ettringite are formed. The correlation between their formation and the increase in strength of
stabilised soil has been reported in several studies (Lim et al., 2002; Tastan et al., 2011; Yin, 2001).
The positive characteristic of fly ash has made it a useful additive not only for the solidification of
sludge and biosolids, but also in other geotechnical applications including soft-soil improvement
(Kayser et al., 2011).

The low-calcium fly ash (class F) used in this research was supplied from Gladstone power station
in Queensland, Australia. Table 1 presents the chemical composition and LOI of the fly ash
determined by X-ray fluorescence (XRF) (Nematollahi and Sanjayan, 2013). The use of fly ash in
highway construction provides significantshort- and long-term environmental benefits specifically
when fly ash is used in lieu of other materials, such as Portland cement, which helps with reducing
energy use, greenhouse gas emissions and conserves natural resources (U.S. EPA, 2005).

Chapter 2

LITERATURE REVIEW

2.1 Debra R. Reinhart, Manoj B. Chopra, Aparna Sreedharan, and Binoy

Koodhathinkal presented a paper on “ Co Disposal Of Sludge And Biosolids
Landfills”
According to the paper Disposal of biosolids presents a great challenge to facility owners. There
are a number of ways of disposing of biosolids. The most popular methods include land application
and landfilling. For a significant period of time, a large portion of biosolids has been disposed by
land application. The application of biosolids to land is regulated at the federal level under the

10
Clean Water Act (40 CFR part 503) and at the Florida state level under Chapter 6 1640, Florida
Administrative Code (FAC). These regulations establish pollutant limits and treatment
requirements for pathogen control. In recent times, there has been a growing uneasiness about the
health impacts associated with land application of biosolids. Several incidents of serious ailments
associated with land application of biosolids have raised concerns over this method of disposal.
This change in the mindset of the general public as well as facility owners has been heightened by
a substantial reduction in availability of land for biosolids disposal. Due to these reasons,
landfilling is being regarded with renewed interest as a method of disposal. Landfilling of biosolids
has several advantages. It generates additional revenue for the landfills. Biosolids provide a source
of moisture for landfills facilitating their operation as bioreactors. As a result, the addition of
biosolids to landfills speeds up the process of gas production. If facilities are capturing this gas for
energy generation this enhanced production translates into significant economic benefits. These
advantages make biosolids disposal in landfills an option worth exploring. However, landfilling of
biosolids has several challenges. Biosolids have been shown to create operational difficulties.
Mainly due their extremely wet nature, they create soft spots that are difficult to compact. Co-
disposal of biosolids with MSW may also create odor situations which will have to be addressed.
Also, the increased gas production might prove to be a disadvantage if gas is not captured and
odors are promoted.

from the Western Treatment Plant in Melbourne were stabilised in different ratios and tested to
determine their compaction, strength, permeability and deformation characteristics. Both static and
dynamic compaction methods were used in the preparation of the test specimens. Results suggest
that the hydraulic conductivity values of biosolids consistently decrease with increasing amounts
of additives, which is an indication of decreasing void ratio of material. In addition, increasing the
amount of additives was found to lead to a subsequent decline in the compressibility of the
stabilised biosolids. The strength of the biosolids samples increased with the addition of bauxsol
and fly ash, with the highest shear strength found to be achieved with around 10% fly ash mixture.
The application of stabilised biosolids in road embankments is discussed as a sustainable solution
in geotechnical engineering applications.

With rapid increases in population, improved health standards and higher number of sewage
treatment facilities around the world, there is a sharp surge in the quantity of waste production in

1
.2 Arul Arulrajah, Farshid Maghool presented a paper on “ Geotechnical
Characteristics Of Stabilised Aged Biosolids”-
According to the paper Stockpiles of air-dried sludge from wastewater treatment plants, known as
biosolids, are constantly increasing worldwide. This subsequently adds to the urgency of
stabilising biosolids in an effort to convert them into a suitable construction material for use in
engineered fills. This paper reports the result of a laboratory testing programme on biosolids
stabilised with two types of waste materials, namely, bauxsol and fly ash. The biosolids collected
11
waste water treatment facilities. Sludge, which is the solid–water mixture pumped from wastewater
treatment lagoons, is the main waste byproduct of these facilities. Sludge typically contains 2–15%
oven-dried solids and shows the characteristics of slurry. Sludge is treated using different
techniques varying from one treatment plant to another, including drying in drying pans, which
ultimately results in a soft highly organic material with 50–70% of solids (mass ratio) called
biosolids (Arulrajah et al., 2011). Issues such as surface deformation caused by differential
settlement, desiccation cracking and odour emission caused by gas leakage from containment
regions are some of the potential problems associated with improper biosolids placement (Kayser
et al., 2011). The state of Victoria in Australia currently stores more than 2 million tonnes of
biosolids in lagoons or in stockpiles, with an annual production rate of 66 700 tons added to current
stockpiles (DNRE, 2002; EPA Victoria, 2004).

2.3 Great Lakes Soil And Environment Consultants presented a paper on

“Geotechnical Characteristics of Biosolids”-
According to the paper The Metropolitan Water Reclamation District of Greater Chicago (District)
generates approximately 200,000 dry tons of biosolids annually. Most of this biosolids is generated
at the District's Calumet and Stickney water reclamation plants (WRP). Following anaerobic
digestion of sewage sludge, biosolids are produced by taking the digestedsludge through two main
processing trains: by centrifugation (high solids processing train-HSPT) and by gravity thickening
(low solids processing train-LSPT). Except for some of the centrifuge cake biosolids (25 percent
solids) which are immediately applied to farmland, after generation, most of the biosolids are
stored in lagoons for greaIer than 18 mouths (aged) or less than 18 months (under aged), then dried
to approximately 65 percent solids before final utilization. The biosolids are used in a variety of
beneficial reuse projects such as final cover at municipal solid waste landfills, construction of golf
courses, parks, and athletic fields, and for reclamation of brownfields. In these projects, biosolids
are utilized as a soil substitute or at relatively high application rates (usually greater thm. 25 percent
of soil volume) as a soil amendment. Also, the biosolids are utilized as n fertilizer amendment to
farmland.

Information on the geotechnical characteristics of biosolids is essential to adequately evaluate the

suitability of biosolids for various applications, especially in civil engineering projects. For
example, if biosolids are to be used as a fill material, pmperties such moisture-density relationships
and shear strength need to be determined. If biosolids are considered as subbase or base course
underneath pavements, geateclmical properties such as Illinois bearing ratio (IBR) and immediate
bearing talue (XBlr) will be relevant. Index geotechnical properties such as grain size distribution
md Atterberg Limits are needed to compare biosolids to natural soils

12
Chapter 3

Experimental Analysis

The geotechnical characteristics of the stabilised biosolids are presented and compared with the
values of the untreated biosolids in this section. The various tests procedure that has to performed
on biosolids are as follows-

CBR(California bearing ratio) Test

There are two types of methods in compacting soil specimen in the CBR moulds

i. Static Compaction method. ii.

. Dynamic Compaction method.

Take representative sample of soil weighing approximately 6kg and mix thoroughly at OMC. •
Record the empty weight of the mould with base plate, with extension collar removed (m1).
Replace the extension collar of the mould.

• Insert a spacer disc over the base plate and place a coarse filter paper on the top of the spacer
disc.

• Place the mould on a solid base such as a concrete floor or plinth and compact the wet soil in to
the mould in five layers of approximately equal mass each layer being given 56 blows with
4.90kg hammer equally distributed and dropped from a height of 450 mm above the soil.

• The amount of soil used shall be sufficient to fill the mould, leaving not more than about 6mm
to be struck off when the extension collar is removed.

• Remove the extension collar and carefully level the compacted soil to the top of the mould by
means of a straight edge.

• Remove the spacer disc by inverting the mould and weigh the mould with compacted soil (m2).

• Place a filter paper between the base plate and the inverted mould. Replace the extension collar
of the mould. Prepare two more specimens in the same procedure as described above.

13
• In both the cases of compaction, if the sample is to be soaked, take representative samples of the
material at the beginning of compaction and another sample of remaining material after
compaction for the determination of moisture content.

• Each sample shall weigh not less than 100g for fine-grained soils and not less than 500 for
granular soils.

• Place the adjustable stem and perforated plate on the compacted soil specimen in the mould.
Place the weights to produce a surcharge equal to the weight of base material and pavement to
the nearest 2.5kg on the perforated plate.

• Immerse the whole mould and weights in a tank of water allowing free access of water to the top
and bottom of specimen for 96 hours.

Hence by following the whole procedure the CBR value can be find and the strength of sand can
be find by using calibration chart.

Atterberg’s Limits- Liquid

Limit:
(1) Take roughly 3/4 of the soil and place it into the porcelain dish. Assume that the soil was
previously passed though a No. 40 sieve, air-dried, and then pulverized. Thoroughly mix the soil
with a small amount of distilled water until it appears as a smooth uniform paste. Cover the dish
with cellophane to prevent moisture from escaping.

(2) Weigh four of the empty moisture cans with their lids, and record the respective weights
and can numbers on the data sheet.

(3) Adjust the liquid limit apparatus by checking the height of drop of the cup. The point on
the cup that comes in contact with the base should rise to a height of 10 mm. The block on the end
of the grooving tool is 10 mm high and should be used as a gage. Practice using the cup and
determine the correct rate to rotate the crank so that the cup drops approximately two times per
second.

(4) Place a portion of the previously mixed soil into the cup of the liquid limit apparatus at the
point where the cup rests on the base. Squeeze the soil down to eliminate air pockets and spread it
into the cup to a depth of about 10 mm at its deepest point. The soil pat should form an
approximately horizontal surface.

14
(5) Use the grooving tool carefully cut a clean straight groove down the center of the cup. The
tool should remain perpendicular to the surface of the cup as groove is being made. Use extreme
care to prevent sliding the soil relative to the surface of the cup.

(6) Make sure that the base of the apparatus below the cup and the underside of the cup is clean
of soil. Turn the crank of the apparatus at a rate of approximately two drops per second and count
the number of drops, N, it takes to make the two halves of the soil pat come into contact at the
bottom of the groove along a distance of 13 mm (1/2 in.). If the number of drops exceeds 50, then
go directly to step eight and do not record the number of drops, otherwise, record the number of
drops on the data sheet.

(7) Take a sample, using the spatula, from edge to edge of the soil pat. The sample should
include the soil on both sides of where the groove came into contact. Place the soil into a moisture
can cover it. Immediately weigh the moisture can containing the soil, record its mass, remove the
lid, and place the can into the oven. Leave the moisture can in the oven for at least 16 hours. Place
the soil remaining in the cup into the porcelain dish. Clean and dry the cup on the apparatus and
the grooving tool.

(8) Remix the entire soil specimen in the porcelain dish. Add a small amount of distilled water
to increase the water content so that the number of drops required to close the groove decrease.

(9) Repeat steps six, seven, and eight for at least two additional trials producing successively
lower numbers of drops to close the groove. One of the trials shall be for a closure requiring 25 to
35 drops, one for closure between 20 and 30 drops, and one trial for a closure requiring 15 to 25
drops. Determine the water content from each trial by using the same method used in the first
laboratory. Remember to use the same balance for all weighing.

Plastic Limit:
(1) Weigh the remaining empty moisture cans with their lids, and record the respective weights
and can numbers on the data sheet.

(2) Take the remaining 1/4 of the original soil sample and add distilled water until the soil is
at a consistency where it can be rolled without sticking to the hands.

(3) Form the soil into an ellipsoidal mass. Roll the mass between the palm or the fingers and
the glass plate. Use sufficient pressure to roll the mass into a thread of uniform diameter by using
about 90 strokes per minute. (A stroke is one complete motion of the hand forward and back to the
starting position.) The thread shall be deformed so that its diameter reaches 3.2 mm (1/8 in.), taking
no more than two minutes.

15
(4) When the diameter of the thread reaches the correct diameter, break the thread into several
pieces. Knead and reform the pieces into ellipsoidal masses and re-roll them. Continue this
alternate rolling, gathering together, kneading and re-rolling until the thread crumbles under the
pressure required for rolling and can no longer be rolled into a 3.2 mm diameter thread.

(5) Gather the portions of the crumbled thread together and place the soil into a moisture can,
then cover it. If the can does not contain at least 6 grams of soil, add soil to the can from the next
trial (See Step 6). Immediately weigh the moisture can containing the soil, record its mass, remove
the lid, and place the can into the oven. Leave the moisture can in the oven for at least 16 hours.

(6) Repeat steps three, four, and five at least two more times. Determine the water content from
each trial by using the same method used in the first laboratory. Remember to use the same balance
for all weighing.

Triaxial Shear Test-

(1) Position the specimen in the chamber and assemble the triaxial chamber.

(2) Bring the axial load piston into contact with the specimen cap several times to permit proper
seating and alignment of the piston with the cap.

2.1 During this procedure, take care not to apply a deviator stress to the specimen
exceeding
0.5% of the estimated compressive strength.

2.2 If the weight of the piston is sufficient to apply a deviator stress to the specimen
exceeding 0.5% of the estimated compressive strength: the piston should be locked in
place above the specimen cap after checking the seating and alignment; and left locked
until application of the chamber pressure.

(3) Place the chamber in position in the axial loading device.

(4) Carefully align the axial loading device, the axial load-measuring device, and the triaxial

chamber to prevent the application of a lateral force to the piston during testing.

(5) Attach the pressure-maintaining and measurement device.

(6) Fill the chamber with the confining fluid to a predetermined level.

(7) Adjust the pressure-maintaining and measurement device to the desired chamber pressure and
apply pressure to the chamber fluid.

7.1 If the axial load-measuring device is located outside the triaxial chamber, the chamber

16
will produce an upward force on the piston that will react against the axial loading device. In this
case, start the test with piston slightly above the specimen cap, and before the piston comes
in contact with the specimen cap, measure and record the initial piston friction and upward
thrust of the piston produced by the chamber pressure. Later correct the measured

axial load, or adjust the axial load-measuring device to compensate for the friction and
thrust.

7.2 If the axial load-measuring device is located inside the chamber, it will not be necessary
to correct or compensate for the uplift force acting on the axial loading device or for
piston friction.

7.3 In either case, record the initial reading on the deformation indicator when the piston

contacts the specimen cap.

(8) Approximately 10 minutes after the application of chamber pressure begin to apply the axial
load to produce axial strain at a rate of approximately:

ieve maximum deviator stress at approximately 3–

6% strain.

8.1 At these rates, the elapsed time to reach maximum deviator stress will be approximately
15– 20 minutes.

(9) Record load and deformation values at approximately 0.1, 0.2, 0.3, 0.4, and 0.5% strain, and
at increments of about 0.5% strain, then to 3%; and thereafter at every 1%, except that the
load and deformation may be recorded at 2% increments of strain for strains greater than 10%.
Take sufficient readings to define the stress-strain curve; hence, more frequent readings may
be required in the early stages of the test and as failure is approached.

(10) Continue the loading to 15% strain except:

17
Index properties
Liquid limit (LL), plastic limit (PL) and plasticity index data of untreated and stabilised biosolids
are summarised in Table 1 Untreated biosolids studied in this research show a much higher LL
value compared to Markey peat studied by Tastan et al. (2011) and Klang peat studied by Wong et
al. (2013).
s. no. Chemical Component weight %
1 Al2O3 27.0
2 SiO2 48.8
3 Fe2O3 10.2
4 CaO 6.2
5 MgO 1.4
6 TiO2 1.3
7 P2O5 1.2
8 K2O 0.85
9 Na2O 0.37
10 SO3 0.22
11 BaO 0.19
12 SrO 0.16
13 MnO 0.15
14 LOI 1.7

Table 1: Chemical composition of Gladstone flyash

For all the stabilised blends, it is evident that there is steady and significant decline in the LL and
PL values with the increase in the percentage of bauxsol (25–40% decline in LL with addition of
3–5% bauxsol) and fly ash (10–20% decline in LL with addition of 10–20% fly ash). The plasticity
index (PI), however, decreases only slightly with the increase in the percentage of the admixtures.

Compaction, CBR and swell properties

Optimum water content (wopt) and maximum dry unit weight (gd,max) values obtained through
standard compaction tests (Standards Australia, 2003) are summarised in Table 2. Untreated
biosolids reported in Table 2 have a maximum dry density slightly lower than the dry density of
Markey peat and Klang peat studied by Tastan et al. (2011) and Wong et al. (2013), respectively.
This can be attributed to higher particle density of peat compared to particle density of biosolids
reported by Arulrajah et al. (2011). The optimum water content of Markey and Klang are found to
be very close to the value of 53% reported here for untreated biosolids.

18
Table 2 suggests that the addition of admixtures increases the dry unit weight in the order of 5%,
while there is a noticeable reduction (up to 25%) in the optimum water content. Table 2 also
presents soaked CBR values and swell percentages for all blends. CBR tests were performed on
specimens compacted at optimum water content using standard Proctor compaction effort and
soaked for 96 h with a 4·5 kg surcharge (Standards Australia, 1998; VicRoads, 1998). It is evident
that the CBR value increases significantly (roughly to four times higher than untreated biosolids
for blends with 20% fly ash) with an increase in the percentage of admixture, while the swell index
only decreases for blends stabilised with fly ash.

Hydraulic conductivity
Hydraulic conductivity tests were conducted on untreated and stabilised biosolids with bauxsol
and fly ash by following ASTM D5084 standard test method using a flexible wall permeameter
(ASTM, 2003). Constant head tests were conducted on specimens measuring 50 mm dia. × 100
mm height. Test specimens were mixed at optimum water content and after 24 h of curing were
compacted under a static pressure of 400 kPa in five layers to reach a minimum of 95% maximum
dry density. The static pressure was increased from the bottom layer to the top layer of the
specimen to reach constant density within the specimen. This method proved to produce
consistency among the samples. In addition, all test specimensachieved a saturation ratio (B value)
of at least 0·96 by applying incremental back pressures before starting the permeability test. The
confining pressure level during the tests was kept at 500 kPa with a constant head of 407·5 kPa
acting on the bottom and a pressure of 410 kPa acting on the top of the specimens. Measurements
were taken for three consecutive days using a data logger up to the point that stable and consistent
values of permeability were obtained for specimens. The hydraulic conductivity values are
presented in Table 2. A consistent decline in hydraulic conductivity of biosolids is observed with
the increasing percentages of bauxsol and fly ash, with lowest hydraulic permeability recorded for
20% fly ash with a permeability coefficient only 40% of untreated biosolids.

One-dimensional primary and secondary consolidation

One-dimensional oedometer tests were carried out on test specimens prepared using standard
compaction in a 63·5 mm dia. by 20 mm height oedometer ring. The loading sequence of 31-
62124-248-496-992 kPa and loading duration of 7 d were applied for each loading stage, which
was then followed by unloading. This testing programme provided results for both conventional
consolidation tests (compression index and recompression index) reported in Table 3 and
consolidation creep test results summarised in Table 4.

19
 Blend LL % PL % wopt: % gd,max: Soaked Swell % Hydraulic
kN/m3 CBR % Conductivity
Untreated 100-110 21-27 53 8.3 1 0.48 1·49 × 10–7
Biosolids
3% 76 17 45 8.8 2.3 0.77 1·17 × 10–7
Bauxsol
5% 59 15 40 8.7 3.4 0.76 1·08 × 10–7
Bauxsol
10% 90 21 46 8.5 4.6 0.33 0·86 × 10–7
Flyash
20% 83 16 42 8.7 4.7 0.24 0·63 × 10–7
Flyash

Table 2: Basic geotechnical properties of blends

Blend Compression Index(Cc) Recompression Index(Cr)

Untreated Biosolids 0.56-0.64 0.038-0.043
3% Bauxsol 0.47-0.50 0.029-0.032
5% Bauxsol 0.39-0.41 0.019-0.032
10% Flyash 0.47-0.51 0.034-0.041
20% Flyash 0.37-0.41 0.021-0.301

Table 3: 1-D Oedometer test results

Table 3 suggests that while adding bauxsol and fly ash has a dominant impact on reducing the
compression and recompression indices of blends. Increasing the percentage of additive results in
further reduction of compression and recompression indices as outlined in Table 3. Both
compacted untreated and stabilised biosolids have compression indices far below values reported
for peats (Cc > 4) and more similar to those of Bangkok clay (Cc = 0·4) and Boston blue clay (Cc
= 0·3–0·5) (Holtz et al., 2011).Comparing values of secondary compression reported in Table 4
with values of secondary compression for untreated biosolids reported earlier by Arulrajah et al.
(2011) suggests that adding bauxsol and fly ash reduces the potential for longterm settlement for
biosolids.

20
CU triaxial shear test
The shear strength properties of the blends were measured by a set of CU triaxial shear tests on 50
mm dia. × 100 mm height specimens following the ASTM D4767 method (ATSM, 2011).

For specimen preparation, the same method of static compaction under 400 kPa pressure level
(similar to method used for hydraulic conductivity tests) was followed. Test specimens were sealed
and cured in room temperature for 24 h prior to start of tests. All test specimens achieved a
saturation ratio (B value) of at least 0·96 in the saturation phase, which was then followed by a 24
h consolidation phase. This 24 h consolidation period was considered to be the end of the primary
consolidation stage and was carried out under an effective confining pressure of 430 kPa. This
pressure is higher than the static pressure of 400 kPa used during sample. preparation and
compaction, which ensures test specimens are under normally consolidation conditions. The
consolidation phase was followed by a shear phase with a deviator strain rate of 0·1%/ min under
the same confining pressure level. The CU triaxial test parameters (before and after test) and results
of both consolidation and shear phases are presented in Table 5.

Blend Applied Vertical Stress

31 kPa 62 kPa 124 kPa 248 kPa 496 kPa 992 kPa
3% Bauxsol 0.0019 0.0034 0.0058 0.0100 0.0162 0.0219

5% Bauxsol 0.0009 0.0025 0.0049 0.0073 0.0149 0.0211

10% Flyash 0.0017 0.0036 0.0061 0.0110 0.0170 0.0189

20% Flyash 0.0013 0.0024 0.0045 0.0079 0.0134 0.0177

Table 4: Coefficient of secondary compression (Ca) of blends

Biosolids 3% Bauxsol 5% Bauxsol 10% Flyash 20% Flyash

Water content% 96 95 96 96 95
Compaction ratio 95 100 96 100 99
%
Void ratio before 1.45 2.09 2.3 2.07 2.05
test
Void ratio after 0.99 1.02 1.26 0.95 0.93
consolidation
stage

21
 Coeff. Of 4·8 × 10–2 4·3 × 10–2 3·7 × 10–2 4·0 × 10–2 3·5 × 10–2
consolidation(Cv):
m2/year
Undrained shear 138 124 120 195 194
strength: kPa
Strain at failure: 7.4 7.9 5.8 7.3 6.7
%

Table 5: CU triaxial shear test results for biosolids blends

The results of the Atterberg limit tests presented in Table 2 suggest that the LL and PI of biosolids
decreases with the addition of both bauxsol and fly ash. The addition of bauxsol is seen to lead to
a sharp decline in LL with up to 40% decline in LL values for 5% bauxsol ratio in the blend. The
decline in LL and PL values with increasing stabiliser ratio can be attributed to a decline in
effective surface area of fine particles as a result of agglomeration and flocculation of these
particles. Similar results have been observed stabilising peat and organic clays with additives such
as cement and fly ash.

22
Chapter 4 Discussions

The dry unit weight of the biosolids (treated or untreated) is only about 50% of what is reported
for natural aggregates (for example, given in Terzaghi et al., 1996) but only slightly less than values
reported for peat (Tastan et al., 2011; Wong et al., 2013). Results of standard compaction tests
presented in Table 2 indicate that with the addition of 3% bauxsol and 10% fly ash there is a 15%
reduction in optimum water content of blends and 2–5% increase in maximum dry unit weight.
Agglomeration of fine particles (resulting in consequent drop in available surface area) and also
filling the pore space between biosolids particles by stabilisers, decrease the volume available for
uptake of water and contributes to a denser packing of the mixture. Thus, the optimum water
content reduction of the blends continues with increasing the percentage of stabilisers. This trend
is likely to hold beyond the maximum percentage of admixtures tested herein, but the behaviour
may change or reverse at higher fly ash or bauxsol content. Higher contents of the additives were
not tested because of practical and economic considerations associated with using stabilisers
beyond a certain level. Adding pozzolanic additives such as cement and fly ash beyond a certain
level can cause development of shrinkage cracks or microcracks due to fast rate of pozzolanic
reaction. This phenomenon is well recognised when General Portland (GP) cement is used as an
additive for chemical stabilisation of soils (Paul and Gnanendran, 2012). Results of compaction
tests reported by Lim et al. (2002) in a similar study indicate that the maximum dry density of
biosolids increases with increasing percentage of additives such as lime and fly ash, while optimum
water content decreases.

The results of the Atterberg limit tests presented in Table 2 suggest that the LL and PI of biosolids
decreases with the addition of both bauxsol and fly ash. The addition of bauxsol is seen to lead to
a sharp decline in LL with up to 40% decline in LL values for 5% bauxsol ratio in the blend. The
decline in LL and PL values with increasing stabiliser ratio can be attributed to a decline in
effective surface area of fine particles as a result of agglomeration and flocculation of these
particles. Similar results have been observed stabilising peat and organic clays with additives such
as cement and fly ash.

The untreated biosolids indicated a soaked CBR of about 1·0, indicating their poor load bearing
capacity (Arulrajah et al., 2011). However, there is a significant increase (e.g. 370% rise with
addition of 20% fly ash) in CBR values of the blends with the addition of both bauxsol and fly ash.
This effect is more pronounced with the first increment of fly ash added to the biosolids. Contrary
to the findings of this research, Lim et al. (2002) reported that the addition of lime and fly ash to
sewage sludge did not result in a significant increase in CBR values of blends, and concluded that
an alternative method needs to be sought, especially for the use of construction materials, due to

23
its low CBR (Lim et al., 2002). Tastan et al. (2011) indicated that the increase in strength of organic
soils obtained by fly ash stabilisation is attributable to chemical reactions and the reduction in
water content obtained by adding dry solids, but the significance of the reactions depends on the
type of fly ash and the soil (Tastan et al., 2011). One explanation for the different CBR results
reported here compared with those of Lim et al. (2002) is that some fly ashes contain lime and
pozzolans, such as aluminium oxide (Al2O3) and silicone dioxide (SiO2) (similar to Gladstone fly
ash used in this study; refer to Table 1), which result in self-cementing ability of fly ash (Tastan et
al., 2011), whereas in other fly ashes, the self-cementing characteristic is quite low.

The other explanation is the difference between the biosolids studied here compared with the
sewage material studied by Lim et al. (2002), especially the difference in percentage and nature of
organic content. Axelsson et al. (2002) reported that in soils with high organic contents, the
quantity of chemical stabiliser needs to exceed a threshold to ensure that humic acids present in
the organic content of the soil are neutralised (Axelsson et al., 2002).

Hydraulic conductivity of all blends (Table 2) is found to range between 0·63 and 1·49 (×10−7
m/s), which is considered very low according to the hydraulic conductivity classification
introduced by Terzaghi et al. (1996). The test results indicate the efficacy of adding fly ash to
biosolids with hydraulic conductivity reductions in the order of 200–250%. While variability in a
parameter such as hydraulic conductivity in soils can be very large (Barr, 2001), comparing these
results with the ones reported by Maghoolpilehrood et al. (2013) indicates that fly ash has the
greatest efficacy in reducing the hydraulic conductivity of biosolids compared with cement, lime
and bauxsol. In a similar study, Asakura et al. (2009) reported that the hydraulic conductivity of
sludge can be improved by adding slag and also construction and demolition waste materials to
sludge. Agglomeration of fine particles, a denser packing of particles and voids filled up with
chemical stabilisers are the possible reasons for decline in hydraulic permeability of chemically
stabilised biosolids.

Figure 1 presents the variation of coefficient of secondary compression with the applied vertical
pressure from consolidation creep tests on stabilised blends. Comparing values reported in Table
4 to coefficients of secondary compression of untreated biosolids reported previously by
Suthagaran (2010) (under slightly different stress levels) confirms that adding bauxsol and fly ash
reduces the potential for long-term settlement of biosolids, thus making the blends suitable for

24
roadworks and embankments. For all the stress levels, the 20% fly ash blend shows the lowest
coefficient of secondary compression within the range of tested blends.

Similar studies using long-term one-dimensional oedometer tests of wastewater biosolids indicate
that the value of secondary consolidation of untreated biosolids is higher than biosolids treated
with added cement and lime (VicRoads, 2007). Chu et al. (2005) and Lim et al. (2004) proposed
the option of using cement-treated sewage sludge as a fill material for land reclamation activities
in Singapore and determined its satisfactory settlement behavior compared with untreated material
The triaxial tests consisted of a 24-h consolidation phase after the saturation. Figure 2 shows the
change in volume against the square root of time obtained from the consolidation stage of triaxial
tests. Values of t90 extracted from Figure 2 were used to calculate the coefficient of consolidation
(CV) values reported in Table 5. These values suggest that the coefficient of consolidation of
untreated biosolids decreases continuously with the addition of the bauxsol and fly ash additives;
however, the decrease is not substantial and only in the order of 10–25% as presented in Figure 3.
Figure 3 indicates that adding 5% bauxsol and 20% fly ash leads to a higher effect in decreasing
the consolidation settlement of biosolids. This is in agreement with results of long-term one-
dimensional consolidation tests (creep) presented in Figure 1, which also suggests that the samples
prepared with 5% bauxsol and 20% fly ash have the lowest creep potential.

Figure 4 shows both the deviator stress and changes in sample pore pressure against the vertical
strain during the shearing phase of triaxial tests. Specimen conditions such as water content,
compaction ratio, initial void ratio, and void ratio after consolidation are all reported in Table 5 as
they have a proven impact on shear strength test results. Since static compaction under a constant
stress level was used for all specimens (treated and untreated), the blends with bauxsol and fly ash
did not reach the same void ratio as untreated biosolids. This is simply because more stress
(compaction energy) is required to compact the stabilised biosolids as compared with the energy
required to compact highly compressible untreated biosolids.

25
26
27
Shear strength test results from CU triaxial tests reported in Table 5 suggest that not all the
stabilised biosolids have higher values of undrained shear strength in comparison with untreated
biosolids. The mixture of biosolids with bauxsol produced lower shear strength materials than
untreated biosolids. In contrast to triaxial results, CBR values of bauxsol blends reported in Table
2 were found to be higher than untreated biosolids. This can be explained by different compaction
techniques used in the preparation of triaxial specimens (static compaction resulting in higher void
ratio for stabilised specimens) as compared with dynamic compaction used for preparation of CBR
specimens. At the same time, the long curing period of 4 d used for CBR specimens certainly
provides more time for pozzolanic activities to progress and form bonds. The latter also explains
the much higher strength gain of fly ashtreated specimens in CBR tests compared with triaxial
tests. Further research including scanning electron microscopy (SEM) and X-ray diffraction
(XRD) analysis can provide essential information to explain the behaviour of treated specimens
with different curing age. In similar studies on stabilisation of organic soils, Kayser et al. (2011)
and Lim et al. (2002) stated that for increasing the strength of organic soils, chemical additives that
can initiate the formation of either ettringite (3CaO-Al2O3-3CaSO432H2O) or calcium carbonate
(CaCO3) are required because these minerals are active in increasing strength. They also stated
that the larger the amount of available quicklime or aluminum and silicon, the more ettringite or
CaCO3 is likely to be formed, leading to higher shear strength for organic soils.

Among all the specimens, biosolids treated with fly ash have the highest undrained shear strength.
Moreover, the experimental results indicate that adding more than 10% fly ash has a limited or
even negative impact on the shear strength of specimens. This behaviour is similar to that reported
for adding cement to biosolids, in which adding cement in proportions of more than 3% was found
to reduce the strength of specimens (Maghoolpilehrood et al., 2013) due to formation of shrinkage
micro-cracks. Prabakar et al. (2004) also reported that shear strength of stabilised soils increases
nonlinearly with the increase in fly ash content of blends. Remarkably, Lim et al. (2002) also
reported that adding fly ash to sewage sludge has a significant effect on shear strength, but adding
fly ash in large amounts results in a negative effect. Tastan et al. (2011) have also reported that
shear strength of stabilised organic soils with fly ash do not increase linearly with fly ash content.

With increasing fly ash content in the blends, the rate of increase in shear strength drops and
consequently a recommendation was made that the benefits of adding more fly ash diminish as the
fly ash content increases (Tastan et al., 2011). Tastan et al. (2011) also claimed that the significant

28
characteristics of fly ash contributing to the increase in shear strength of blends are calcium oxide
(CaO) content and CaO/SiO2 ratio. In the absence of CaO in fly ash, fly-ash mixtures perform
poorly and are not considered suitable for biosolids stabilisation (Kayser et al., 2011). In this
situation, the addition of lime has been recommended in order to initiate the pozzolanic reactions
(formation of elements such as ettringite and calcium carbonate), which cause increase in shear
strength of biosolids (Kayser et al., 2011; Lim et al., 2002). XRD has supported the existence of
ettringite, and SEM has shown the presence of needle-shaped crystals, and the increase in strength
is attributed to the interlocking of these crystals (Lim et al., 2006; Malliou et al., 2007).

Figure 4 suggests that while untreated and stabilised biosolids specimens have different peak
undrained shear strength, their behaviour during the shearing is very similar. Leroueil and Vaughan
(1990) explain that the effect of structure in behaviour of soils and rocks is as important as porosity
(void ratio) and stress history. Soil structure can arise from different causes such as cementing
agents such as fly ash in this study or the deposition of organic matters in case of untreated
biosolids. Since the stress history of all triaxial specimens reported in this paper are the same, it is
believed that the combination of void ratio, presence of organic content and cementing agents all
together play in shear strength curves of blend.

Stabilised biosolids in road embankment fills

Results from the laboratory characterisation were compared with local roadwork specifications for
embankment fills. According to VicRoads (2007), there are three types of road formation
construction material for road and bridge works. VicRoadsʼ requirement for type B fill materials
for road embankments is a CBR value of 2–5% (VicRoads, 2007). Table 2 indicates that the CBR
value of biosolids stabilised with 3%, 5% bauxsol and 10% and 20% fly ash satisfies this
VicRoadsʼ specification for type B fill materials.

As bauxsol did not have a positive effect on shear strength obtained in triaxial tests and the addition
of fly ash in ratios greater than 10% did not significantly increase the shear strength of blends,
biosolids stabilised with 10% fly ash is found to be the optimum additive in this study to meet
VicRoadsʼ requirements for embankment fill material.

Overburden pressure and the traffic load on the layer of biosolids used in road embankment and
the consequent long-term settlement can eventually cause premature structural distress in the form
of cracking, rutting and differential settlement (Sobhan et al., 2012).

This makes settlement control a critical aspect of using stabilised biosolids in road embankments.
While road requirements and specifications may vary from district to district (and indeed from
country to country), the results presented here suggest that the blends of biosolids could potentially
be used in roadworks based on material performance measures.

29
Chapter 5 CONCLUSION

Biosolids samples from the WTP in Melbourne, Australia, were tested to determine their
geotechnical characteristics in both untreated and stabilised conditions. Different proportions of
bauxsol and fly ash were mixed with biosolids to improve its consolidation and strength
behaviours. The experimental laboratory results indicate that the maximum dry density of biosolids
increases with increasing proportions of bauxsol and fly ash while subsequently decreasing the
optimum moisture content of the stabilised biosolids compared with the untreated biosolids. The
stabilisation of biosolids with additives significantly increases the CBR value of the biosolids
specimens to the extent where they meet local road authorities’ requirement for type B of fill
material for road embankments. Reduction in the coefficient of consolidation and secondary
compression values were noticed when additives were added to biosolids. In CU triaxial tests, the
shear strength of untreated biosolids was significantly improved by adding fly ash up to 10%
content, while adding bauxsol was found to have a negative effect on the shear strength.

XRD and SEM assessments are required to further study the role of formation of ettringite and
calcium carbonate on shear strength behaviour of stabilised biosolids using fly ash and bauxsol. It
should be taken into account that biosolids mineralogy and reaction conditions (such as
temperature, moisture and curing time) can significantly affect the geotechnical characteristics of
stabilised blends (Lim et al., 2002). The exact effect of these conditions on behaviour of stabilised
biosolids needs further investigation. High pH has also been widely used as an indicator of
biochemical stabilisation because a pH above 11 leads to the immobilisation of heavy metals as
well as the destruction of pathogens and lowering of microbial activity (Kayser et al., 2011). A
decline in pH can lead to a surge in biodegradation of organic matter, and a subsequent decrease
in strength. Because the organic content of biosolids is very high, limiting biodegradation is
important, and therefore maintaining a pH above 11 is necessary (Kayser et al., 2011). Further
study is required to look into the pH values of stabilised biosolids and their relationship with the
strength characteristics of blends.

30
Chapter 6 References

1. Arulrajah A, Disfani MM, Suthagaran V and Bo MW (2013)

Laboratory evaluation of the geotechnical characteristics of wastewater biosolids in road
embankments. Journal of Materials in Civil Engineering 25(11): 1682–1691.

2. Arulrajah A, Disfani MM, Suthagaran V and Imteaz M (2011)

Select chemical and engineering properties of wastewater biosolids. Waste Management
31(12): 2522–2526.

3. Asakura H, Endo K, Yamada M, Inoue Y and Ono Y (2009)

Improvement of permeability of waste sludge by mixing with slag or construction and
demolition waste. Waste Management 29(6): 1877–1884.

4. ASTM. 2011. Standard test method for consolidated undrained triaxial compression test
for cohesive soils. American Society for Testing and Material, ASTM D4767-11, West
Conshohocken, PA, USA.

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