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FACULTY OF ENGINEERING AND TECHNOLOGY
DEPARTMENT OF CIVIL ENGINEERING
COURSE: CEM 410
MSc. (TRANSPORTATION) CIVIL ENGINEERING DISSERTATION
TITLE: Effect of Salinity on Alkali Earth Metals and Zeolites Stabiliser.
NAME: WILLIAM A.T. MUTEPFA
ID NO:200608088
SUPERVISOR: Dr. J. Egwurube
i
Dedicated to my family
ii
Laboratory Evaluation of the Effect of Cement Concentration, Water Salinity and the Roadcem Additive on Kalahari Soil Strength
ABSTRACT Botswana is experiencing a rapid growth in road infrastructure, increasing from 10km at
independence to a total of close to 9000 km in 2008. This growth has increased demand for
suitable road construction materials. Coupled to this challenge is the fact that close to seventy
five (75) per cent of the country is covered in Kalahari Sands which are in most cases saline, and
do not meet road construction specifications (Botswana Roads Department Guideline No.6,
2001). Due to Botswanas arid climate surface water is scarce in many parts of the country
especially the western side. Water for road construction is sourced from local boreholes or
imported from elsewhere being hauled over long distances. The borehole water within the
vicinity of most construction sites has very often been found out to be highly saline with total
dissolved solids (TDS) exceeding the maximum road specification limit of 2000 mg/l (Botswana
Road Design Manual, 1982). The importation costs of suitable material for infrastructure
development increases construction costs. To this end alternative design approach, methodology
and alternative materials need to be investigated for future usage.
An investigation will be made on the effect of saline water (TDS 33296 as collected) on alkali
earth metals and zeolites stabilizer. This complex chemical compound is manufactured by
Powercem Southern Africa under the trade name Roadcem. Roadcem compound is reputed to
have been successfully utilized in many parts of the world in cement stabilization of several
types of problematic soils. One of the documented reports is that the additive can be used
successfully with saline materials.
The aim of this laboratory investigation is to identify the effect of material salinity on the
performance of Roadcem in improving the Unconfined Compressive Strength (UCS) of Kalahari
Sands. Kalahari Sand of G7 classification and saline water samples were collected from
Tsabong. Soil samples were prepared and stabilized with tap water and cement only for the
control samples. Roadcem additive was added for further testing of 7 day UCS cured samples.
The results achieved reflect an enhanced UCS strength for specimens with Roadcem additive but
even more so in saline water mixtures. Unconfined compressive strength ranging from 13% up to
iii
57.9% was achieved on samples when comparing the neat sample and that to which Roadcem
compound was added.
The results achieved show a strong agreement with the manufacturers claim as per the literature
provided. These tests were conducted as a preliminary investigation to verify two claims,
whether Roadcem improves the cement stabilized soil strength and that strength is still achieved
with saline materials. The total dissolved solids content of the test water was 33 296 with a
chloride and sulphate content of 13 995.7 mg/l and 8 704.4 mg/l respectively. This was in
contrary to expected knowledge that chlorides and sulphates are detrimental to cement hydration
and strength gain.
Based on the positive results of this testing it is proposed future work to be carried out to
establish the long term effects of Roadcem on saline material stabilization. Further work will be
able to capture if there are any long term effects of chlorides and sulphates on cement stabilized
materials.
iv
ACKNOWLEGMENT
I am grateful to Almighty God, Jesus Christ, for giving me the patience to complete this work. In
addition I express my deepest and hearty thanks and great indebtedness and gratitude to my
supervisor Dr. J. Egwurube, Civil Engineering Department, University of Botswana for his kind
supervision, valuable courses during my developing study, guidance, valuable advice, reviewing
the manuscript, and support during, my study program.
Special mention is also made of Dr. M. Dithinde, Civil Engineering Department, University of
Botswana for his review, technical input and recommendations made to develop the final
presentation.
I also extend my gratitude to Powercem Technology for their product, Roadcem, without which
this research would not have been made possible. All the literature and technical advice during
the experimentation have been very fruitful in achieving the objectives of this research.
I am deeply grateful to Mr. Kowa of Botswana Roads Department for their invaluable support,
research material and advice given in respect of pursuing this investigation.
I extend my special and heartily thanks and gratitude to my work supervisor and employer Mr.
V. Ponoesele of Lesedi Consulting Engineers (Pty) Ltd for granting me the time and opportunity
to pursue my studies during demanding working periods.
v
DISCLAIMER The opinions, findings and conclusions expressed in this report are those of the author and not
necessarily those of the University of Botswana. This is a product of the authors efforts and
investigation and where cited material is utilized acknowledgements are made.
vi
Table of Contents Abstract .......................................................................................................................................... ii
Acknowledgements ...................................................................................................................... iv
Disclaimer .......................................................................................................................................v
Table of Contents ......................................................................................................................... vi
List of Figures ............................................................................................................................... ix
List of Tables ..................................................................................................................................x
Chapter 1 ........................................................................................................................................1
1.0 General Introduction ..............................................................................................................1
1.1 Problem Statement .................................................................................................................8
1.2 Objectives of The Study .........................................................................................................9
1.3 Scope of The Study ................................................................................................................9
1.4 Importance of the Study .......................................................................................................11
1.5 Study Environment ...............................................................................................................11
1.6 Plan of The Study .................................................................................................................12
Chapter 2 ......................................................................................................................................14
2.0 Background Review of Literature .....................................................................................14
2.1 Kalahari Sands ......................................................................................................................14
2.2 Mechanism of Salt Damage .................................................................................................16
2.3 Incidences of Salt damage on Pavements in Botswana ........................................................18
2.3.1 Sua Pan Airfield ............................................................................................................18
2.3.2 Nata Maun Road .........................................................................................................18
2.3.3 Sekoma - Kang Road .....................................................................................................19
2.3.4 Tsabong Middlepits Road...........................................................................................19
2.4 Current Practice to Limit Salt Damage to Bituminous Surfaced Pavements ......................20
2.5 Historical Review of Chemical Stabilisation in Road Construction ....................................21
2.6 Review of Some Chemical Stabilisers .................................................................................24
2.6.1 Calcium or Magnesium Chlorides .................................................................................24
2.6.2 Clay Additives ...............................................................................................................24
vii
2.6.3 Enzymes ........................................................................................................................25
2.6.4 Lignosulphates ...............................................................................................................25
2.6.5 Synthetic Polymer Emulsions........................................................................................25
2.6.6 Tall Oil Emulsions From Paper Mills ...........................................................................25
2.6.7 Sulphonated Petroleum Products ...................................................................................25
2.6.8 Conaid Stabilisation.......................................................................................................26
2.6.9 Fly Ash Stabilisation .....................................................................................................26
2.6.10 Other Documented Tests on Kalahari Sands ...............................................................28
2.7 Traditional Stabilisers ..........................................................................................................28
2.7.1 Lime Stabilisation ..........................................................................................................28
2.7.2 Cement and Lime Stabilisation in Botswana .................................................................29
2.7.2.1 Jwaneng Kanye Road ....................................................................................29
2.7.2.2 Other Cement Stabilised Roads in Botswana ...................................................32
2.7.3 Properties of Cement .....................................................................................................32
2.7.4 Portland Cement and Roadcem Compound ..................................................................34
2.8 Roadcem Compound in Road Construction .........................................................................36
2.9 Laboratory Evaluation by Others .........................................................................................36
2.9.1 CSIR Transportek Initial Tests ......................................................................................37
2.9.2 Demonstration Projects .................................................................................................40
2.9.2.1 Rusternberg Koster Road ..................................................................................40
2.9.2.2 Danie Theron Road in Fochville .......................................................................41
3.0 Methodology ........................................................................................................................42
3.1 Field Sampling..................................................................................................................42
3.2 Test Procedures ................................................................................................................45
3.2.1 Material Classification Tests .....................................................................................45
3.2.2 Soil Index Properties .................................................................................................45
3.2.3 Compaction Characteristics ......................................................................................45
3.2.4 Strength Tests............................................................................................................46
3.2.5 Chemical Tests ..........................................................................................................49
viii
4.0 Results ..................................................................................................................................50
4.1 Material Characterisation .................................................................................................50
4.2 Water Quality ...................................................................................................................50
4.3 Compaction Characteristics ..............................................................................................51
4.4 Chemical Tests .................................................................................................................52
4.4.1 Ph Test Results ..........................................................................................................52
4.4.2 Total Dissolved Solids Results .................................................................................53
4.5 Strength Characteristics ....................................................................................................53
4.5.1 Alkali Earth Metals and Zeolites Stabiliser ..............................................................56
5.0 Discussion of Results ...........................................................................................................58
5.1 UCS Results......................................................................................................................58
5.2 Chemical Test Results ......................................................................................................59
5.3 Recommendations for Future Work .................................................................................61
5.0 Conclusion ............................................................................................................................62
Bibliography ...............................................................................................................................63
Appendices
Appendix 1 Gravel Classification
Appendix 2 Compaction Characteristics
Appendix 3 Chemical Test Results for Water
Appendix 4 UCS Test Results
Appendix 5 Chemical Tests Results for Soil Samples
Appendix 6 Powercem Mixing Protocol
Appendix 7 Additional Photos
ix
LIST OF FIGURES Figure 1: Groundwater Total Dissolved Solids ...........................................................................4
Figure 2: Ecozones Map of Botswana ..........................................................................................5
Figure 3: Map of the Study Area ................................................................................................12
Figure 4: Decision Flow Chart as per Jones and Ventura .......................................................36
Figure 5: Sampling Positions ......................................................................................................42
Figure 6: Kalahari Sand Being Loaded for The Subgrade Layers by the Contractor ........43
Figure 7: Sampling Borehole Water for Road Construction ..................................................44
Figure 8: Sieve Analysis of the Sand Sample Using the Mechanical Shaker ........................46
Figure 9: Proctor Mould and Sample Being Weighed, Determination of OMC/MDD .........47
Figure 10: Soaked Specimens Prior to Compressive Strength Determination ......................48
Figure 11: Specimen Failure Due to Applied Loading ............................................................48
Figure 12: Graph of UCS against Cement Content .................................................................55
Figure 13: Maximum Dry Density Against Cement Content .................................................56
x
LIST OF TABLES Table 1: Areas Covered by Saline Soils .......................................................................................2
Table 2: Common Water Soluble Salts ......................................................................................16
Table 3: Typical Composition of Ordinary Portland Cement .................................................33
Table 4: UCS Results from CSIR Transportek ........................................................................38
Table 5: Material Properties .......................................................................................................50
Table 6: Results of Chemical Analysis of Borehole Water Samples ......................................51
Table 7: Compaction Characteristics of the Specimens ..........................................................52
Table 8: Results for Cured 7 Day Unconfined Compressive Strength ..................................54
xi
1
Effect of Salinity on Alkali Earth Metals and Zeolites Stabiliser.
Chapter 1
1.0 General Introduction
The prevalence of saline soils worldwide has led to deformation of structures founded on
them including numerous engineering failures. In the USSR, the estimated area occupied by
saline soils is in the region of 3. 5 million square kilometers. (Petrukhin, 1993). The extent of
the problem has been extensively reported in numerous investigations and studies and has
been evidenced by several pavement failures in India, Africa, the Middle East, Australia,
USA, Europe and USSR, particularly in regions where saline soils are prevalent.
The symposium on Engineering Characteristics Arid Soils, 1994 has a collection of detailed
studies and limitations on the current knowledge of arid soils. The effort has collated most of
the known structural failures linked to saline substructures. In particular aspects such as
classification, identification, chemical and engineering behaviour have been dealt with in
great detail from soils as diverse as clay strata at Bassilica to saline loess soils of inland
China. Petrukhin has also distinguished that saline soils is a broad term encompassing a wide
variety of soils which differ in their granular composition. Saline soils vary from detrital soils
to clays. Though the soils are grouped together as saline soils, they have very little in
common with respect to their physical and engineering properties and drainage
characteristics. Sepage in detrital soils is characterised by turbulent flow leading to piping,
whilst in sands, sandy loams and loams Darcys law is applicable. Clays do not have seepage
properties. The extent of the prevalence of saline soils is summarised in Table 1 below as
extracted from the Food and Agricultural Organisation of the United Nations Soil Map of the
World. Although this map was produced from a pedological point of view it gives an estimate
of the prevalence of saline soils. It is intresting to note the differences in areas as derived at
by Petrukhin for Russia and the Soil Map areas for Asia. However both estimates still
underscore the full extent of the global problem of soil salinity. In this report the main focus
of the study will be based on inland saline soils and borehole water. The materials used in the
enquiry were collected from Tsabong village in the south western part of Botswana and
assesed on their responses to Roadcem stabiliser additive and cement stabilisation.
2
Region Area (106 ha)
Africa 69.5
Near and Middle East 53.1
Asia and Far East 19.5
Latin America 59.4
Australia 84.7
North America 16.0
Europe 20.7
Total Area 322.9
Table 1: Areas Covered In Saline Soils as Per Food and Agricultural Organisation of
the United Nations Soil Map Of The World; adopted from FAO, March 2009.
The prevalence of saline soils has been linked to semi arid and desert regions by several
scholars (Obika, Woodbridge, Freer-Hewish, Newill, 1994; Petrukhin, 1993; Naifeng, 1994;
Warren, 1994; Ministry of Works and Transport Botswana Roads Department, 2000 and
2001), amongst others. To date a code of good practice has been developed to circumvert the
risks assosciated with pavement construction with saline materials. In Botswana this has
culminated in the development of Guideline No. 6 (Ministry of Works and Transport
Botswana Roads Department, 2001). Most of the recommendations in this guideline are
based on the early work by Obika et tal (1994). Additional work is still required to identify
alternative construction methods that can incorporate the saline material in pavement
structures without adversely affecting the performance of the pavement in service.
Pavement failure due to high salinity of construction materials on many local road
construction projects is reported. The scale of the problem in Botswana was been established
through observation of pavement failures in the past. Most notable amongst these failures
include Sua Pan Airfield (Bennet, 1991), Nata-Gweta road, Orapa-Mopipi-Rakops road,
Selibe-Phikwe runway, the trans-Kgalagadi road and several other road sections (Ministry of
Works and Transport Botswana Roads Department, 2001). A failure on these pavements is
characterized by surface blistering of the bituminous surfacing. The ensuing moisture ingress
3
on the exposed wearing course led to pothole formation and loss of pavement riding quality
and further deterioration through loss of compacted bases. Ongoing construction projects that
are adversely affected include Tsabong Bokspit Road (Botswana Roads Department
Progress report No. 6, 2008). The construction is behind schedule due to high water salinity
amongst some of the factors contributing to construction delays. On this construction project,
it is reported that construction water is imported from South Africa. Local saline water is
diluted with the imported water to buffer the salinity. This renders the water within the
acceptable limits of total dissolved solids (TDS) for usage in road construction.
Rapid expansion of the road network in Botswana has contributed to the growth of demand
for suitable construction materials. The network has expanded from 10 km of surfaced
flexible pavement in the central business district at independence in 1966, to a total of 8916
km of both paved and unpaved inter-district roads in 2008. ( Mokgethi, 2007).Other statistics
have estimated the road network to be 10km in 1966 and 18 000km in 1992 (Madzikigwa,
2007). The difference in lengths probably arise from the exclusion of unpaved District
Council Roads by Mokgethi and the inclusion of the same in Madzikigwas measurements.
Nevertheless this rapid growth has generated an insatiable demand for suitable road
construction materials especially for the base and subbase materials. Associated with this
growth are environmental concerns, material depletion, material scarcity, increased haulage
and subsequent construction costs. These factors are obstacles to infrastructure development
(Motswagole and Monametsi, 1996). This means that alternative road construction materials,
methods and design have to be identified and implemented accordingly to cope with the
rising demand for specified road materials (Gourley & Greening, 1999).
Botswana has significant deposits of Kalahari Sands. They are mainly located to the western
part of the country and these pose serious engineering problems in road construction.
Kalahari sands are fine-grained soils varying in colour from white to greenish grey. The soils
are collapsible, have a poor structure and exhibit saline conditions. The saline nature of the
soils makes them undesirable for base course materials due to the deleterious effect salts have
in the bond formation between the road base and the bituminous compounds used in priming
and surfacing. Compounding this problem is the fact that Botswana is a semi arid country
(Botswana Meteorological Dept, 2003; Lancaster, 1978; Wright, 1978) with annual rainfall
varying between 250 to 650 mm falling mainly in the summer months of October to the
following April. Consequently there is very little surface runoff and the few water pools that
collect in depressions eventually dry off in the summer heat. Some of the water infiltrates into
4
the ground recharging the ground water tables whilst the rest is held by capillary action
withinin the sand voids. Most road construction is carried out using borehole water that is
saline and has a high total dissolved solids content that does not meet the Road Design
Manual specifications (Ministry of Works and Communications Roads Department, 1982).
Where there is need for cement stabilization of subbases and bases to improve material
properties saline water cannot be used.
Fig. 1, Groundwater Total Dissolved Solids Map
Fig. 1 of the Botswana hydro geological map above shows the distribution of the salinity
levels of borehole water. The salinity ranges from 0 mg/l in tap water to peak in excess of 50
000 mg/l of total dissolved solids (TDS). Incidentally, the areas to the west of Botswana have
400000 500000 600000 700000 800000 900000 1000000 1100000 1200000 1300000 14000007000000
7100000
7200000
7300000
7400000
7500000
7600000
7700000
7800000
7900000
8000000
Orapa
Kanye
Molepolole
Ghanzi
Gaborone
Lobatse
Mahalapye
SerowePalapye
Francistown
Kasane
Maun
Tsabong0
1000
2000
5000
50000
Natural Neigbour
5
a higher occurrence of high salinity compared to the eastern part of the country. The same
areas are also overlain with Kalahari Sands. This is in alignment with the soil distribution of
Botswana as shown in Figure 2 below.
Fig 2. Ecozones Map of Botswana Botswana
The evident problems include the unavailability of suitable water that meets construction
specifications and inadequate water for road construction in the upper pavement layers
(subbase and base). There is a lack of good quality gravel materials to the western part of
Botswana. Road construction challenges include increased energy consumption associated
with exporting unsuitable material and importing suitable materials, gravel and water, for the
road. The additional haulage distances traversed to achieve these objectives impact negatively
on the environment. Low quality gravels demand for increased sampling frequency to reduce
risks arising from material variability. The construction of haul roads also contributes
significantly to deterioration of natural environments. There is increased air and noise
pollution, disturbance of the natural habitats, indigenous flora and fauna, dust and noise
6
pollution, scarring of the natural landscape from construction of access roads leading to
increased runoff and soil erosion, and depletion of the finite gravel resource. These factors
have negatively affected the infrastructure development in the western part of Botswana. In
recent years, the Government of Botswana has invested in major road links like the Trans
Kgalagadi highway in efforts to link the country with Namibia and South Africa. In the
process, it is anticipated that greater accessibility will promote economic development in that
area. To offset the rising construction costs of providing good roads in areas overlain with
poor quality construction materials it is necessary to find solutions that are more amenable
that promote the use of in situ materials at reasonable costs.
The unsuitable high saline local groundwater within road construction localities leaves
developers with no viable alternatives other than to resort to the use of portable water for
construction of the upper road layers. This imparts additional construction constraints in the
sourcing and haulage of portable water. These additional construction efforts are setbacks to
the provision of affordable infrastructure through out the country. Alternative construction
techniques should provide a solution sooner than later. Rapid advances in the chemical and
petrochemical industry in the past fifty years have developed products, which may offer
potential solutions subject to comprehensive laboratory testing and trials. To this end, the
documented benefits of some of the commercially available products need to evaluation as a
means of identifying value engineering designs and eco friendly construction techniques. The
balance achieved in improving the material quality as per the specifications of traditional
design and construction methods and establishing durability through observed field trials.
Based on pavement failures in the past and recurrent problems, the natural progression is to
identify synergistic methods of road construction that facilitate the usage of natural in situ
materials in as much as possible to offset the economic, environmental and social costs.
Chemical stabilization is one of the alternative means of modifying and enhancing ordinary
road construction materials to suit desired specified purposes.
Chemical stabilisation involves the use of chemical compounds to alter the physical-chemical
properties of an engineering soil so that its properties may conform to the minimum expected
design specifications and predetermined performance criteria.
The most common stabilisers used in Botswana on a large scale are traditional stabilizers,
mainly cement and lime (Lionjanga, Toole and Greening, 1987). In recent years, the usage of
fly ash, a pozzolana, has been gaining appeal due to the availability of fly ash in large
7
quantities from thermal coal fire stations. Sahu (2001) has also established that fly ash has
potential for use in improving CBR of several problematic soils in Botswana. For Kalahari
Sands minimal improvement to CBR is observed for fly ash content of up to 25% and
beyond. At fly ash content of 28% and 32%, there was a trend towards an increasing CBR,
which peaked at a CBR of 25% at fly ash content of 32%. Other scholars worldwide have
achieved similar results, as fly ash has proved effective in soil strength improvement as
evidenced by increased CBR and unconfined compressive strength when used on different
problematic soils. (Kroeger, Bane and Chugh, 2005; V.K.Mathur, 2007; P. Eskioglou, 2008).
Substantial studies have also been carried out through field trials which have culminated in
the construction of fly ash stabilised roadworks in India, United States of America and
Europe. Based on Sahus results fly ash is is not recommended as a potential stabiliser for
Kalahari Sands. The large volumes of fly ash required to bring appreciable changes in CBR
require hauling large volumes of fly ash over long distances. The main source of fly as in
Botswana is the Morupule Power Station near Palapye. Fly ash stabilisation in Botswana may
be useful within the vicinity of Moruple.
A Netherland based company, Powercem Southern Africa founded in 1995, has an
operational branch in South Africa. The company has produced innovative products for use in
the construction industry. One of their products, Roadcem has been developed as an additive
for use in cement stabilized road construction materials. Concrecem, Immocem and Nuclicem
are some of their other products manufactured respectively for use in concrete construction,
waste and chemical management and nuclear waste management. Based on the marketing
literature provided by Powercem and investigative technical reports form Universities in
South Africa evidence shows that there is great potential for the product in treating a number
of problematic soils. Demonstration projects have also established the efficacy of the
compound. The oldest road to be constructed using Roadcem is in Freiburg (Germany) and is
over 13 years old. The pavement is still intact and has not yet experienced potholes, distress
in use, rutting and is virtually maintenance free. The oldest Roadcem stabilized road in South
Africa is well over three years and is still intact with no reported rutting, potholes or
maintenance required to date.
The manufacturers have indicated that Roadcem is a compound consisting of alkali earth
metals and synthetic zeolites and has received acclaim abroad for its universal application in
stabilizing most soil types. The main objective of this paper is to evaluate the potential of this
compound in the cement stabilization of Kalahari Sands with saline water. Tests by
8
Universities in South Africa have verified some of the product qualities (University of the
Witwatersrand, Johannesburg School Of Civil and Environmental Engineering, 2007;
Powercem Synopsis, 2007). This compound has been launched by the manufacturer as having
several beneficial properties that has to date been established as one of the leading stabilizing
and environmentally friendly compound of its kind on the market.
Laboratory tests are conducted to assess the material response in regards to 7 day unconfined
compressive strength (UCS) tests. It is anticipated that based on the outcomes of this initial
laboratory enquiry sufficient data will be gathered which may support the development of site
field trials of the product. Of particular interest are the findings identified by other scholars
on the subject of chemical stabilization in Botswana and the challenges faced. One area that
has been repeatedly identified is the lack of suitable construction water and the unavailability
of water for curing stabilized layers (Sahu, 2001). High salinity from borehole water has
detrimental effects on the hydration of cement and lime used in stabilized layers. Borehole
water in Botswana has unreasonably high levels of chlorides, which are detrimental to
cement. Roadcem additive has been documented to be effective when used with saline
materials or water containing chlorides. However, the extent of salinity tolerance or
resistance to which the product can be subjected has not been reported in sufficient detail to
make a value judgement on the full potential effectiveness of the product. (Powercem
Technologies, 2008). This claim, if verified through results, has very strong socio-economic
and environmental importance to countries in semi arid climates, particularly Botswana,
which experience problems with road construction using saline materials. This property will
be investigated further in this report to identify if there is peak salinity level at which the
effectiveness of Roadcem in stabilization is affected.
1.1 Problem Statement
The problems experienced in road construction with saline materials included:
Suitable road construction materials, both water and gravel, that meet the Botswana Road Design Manual specifications are scarce.
Surface deterioration through blistering of surfacing seals leads to moisture ingress into the base and subgrades, rutting of surfaced areas. (Obika, Freer-Hewish and
Newill, 1992)
In some countries, cases of piping failure have been reported. This occurs during phase changes of solid salt crystals as they dissolve in water during periods of rain or
9
underground moisture movements. The resulting voids from the dissolved crystal lead
to piping failure and subsequent structural soil collapse. (Petrukhin, 1989)
Maintenance and rehabilitation of salt damaged pavements has proven to be costly in the past (Bennet and Netterberg, 2004).
It has also been reported that the hydration products from some salts produce expansive forces, which lead to upliftment or heaving of pavement surfaces.
The remedial measures and current construction practice developed to mitigate the problem of salt damage do not address the fundamental issue of in situ material usage.
The guidelines developed still limit the usage of insitu materials through setting the
minimum acceptable tolerances of salinity expected in any given constructed
pavement layer to reduce failure risks. Alternative and effective strategies include the
usage of vapour barriers between the subgrade and base course layers to prevent salt
migration to the pavement surface where it is most harmful. Observations have
identified that thin bituminous surfacing are more susceptible to salt damage. Thick
asphalt and bituminous pavements are not easily affected ( Thagesen and Robinson,
2004).
1.2 Objectives of the Study
The key objectives are to determine the following hypotheses :
Determine the effect of salinity in Roadcem stabilised Kalahari Sand. Determine the mechanism, if it exists, of how Roadcem overcomes
material salinity. Whether strength gain is sustained within high saline
conditions or salinity is eliminated through chemical reaction leading to
improvement in strength.
Determine the optimum Roadcem/cement/Kalahari Sand for stabilization.
1.3 Scope of the Study
Kalahari Sand and borehole water were collected from Tsabong village from some of the
Tsabong Bokspits road construction material sources. The investigation here will focus on the
laboratory study of 7 day cured unconfined compressive strength results of test samples of
cement stabilised Kalahari Sands and cement stabilized Kalahari Sands with Roadcem
compound. The cement concentration will be varied from 1.5% to a maximum of 9 %.
10
Ordinary tap water (0 total dissolved solids) and saline water (33296 total dissolved solids)
will be used in the mixtures to determine the effect if any of salinity on Roadcem compound.
All the samples will have their Ph, electrical conductivity and total dissolved solids measured
before and after crushing the unconfined compressive strength test samples.
The basis of assessment that will govern the procedure and outcomes of the results are drawn
from The Botswana Standard Specification for Roads and Bridges (1983), The Botswana
Road Designers Manual (Ministry of Works and Communications Roads Department, 1982) ,
Road Note 31 and Roads Department Gudeline Number 6 (Ministry of Works and Transport
Botswana Roads Department, 2001). These documents are used extensively in the design and
construction monitoring of roads and highways within Botswana. The Botswana Road Design
Manual requires a minimum unconfined compressive strength (UCS) of 0.75 Mpa for
material to qualify for use in sub-base layers. Materials designated for construction of base
layers are expected to have a minimum UCS of 1.5MPa. The Technical Recommendations
For Highways (Department of Transport South Africa, 1996) recommends the use of 2 3 %
cement stabilization as the ideal compositions for material modifications to reduce the risks
of carbonation and shrinkage cracking when drying. Thagesen (1996) has recommended that
soils with a Plasticity Index (PI) of less than 20% and a Coefficient of Uniformity (Cu) of at
least 5 would be suitable for cement stabilization. The success of the stabilization experiment
with Kalahari Sands will be weighed in against these criteria which are standard practice in
Botswana.
Results previously achieved by others on Kalahari Sand stabilization will also be reviewed as
a means of assessing the general trends observed to date on with the different stabilizers to
date. A brief analysis of results also achieved with Roadcem compound will also be looked at
to see the general expectations comparison of results with those achieved in this study.
Kalahari Sand was selected for this analysis because it occupies by area, a large geographical
land mass of Botswana and an equally vast area within southern Africa. Kalahari Sands are
problematic regarding their geotechnical properties and may be considered a representative
soil in Botswana.
11
1.4 Importance of the Study Based on the documented claims of Roadcem capabilities, it is necessary to establish its
efficacy with a problematic soil like Kalahari Sand which is prevalent across the country in
large quantites. Additionally the asserted ability of Roadcem compound to work effectively
with saline construction materials will be an additional bonus if established. Most of
Botswana water sources used in road construction include borehole water which is highly
saline. Establishement of these potential properties can reduce construction costs
tremendously enabling insitu material to be used on construction sites.
1.5 Study Environment Tsabong, 26o 1 12 S, 22o 24 20 E, is located 494 km to the south west of Gaborone, the
capital city of Botswana. Kalahari Sand samples were collected from Tsabong on the
construction site for the Tsabong Middlepits road located at coordinates X = 62138.0854, Y
= 2888714.2734, close to chainage 9.84 km along the Tsabong Middlepits Road. This was
selected as ideal because the road construction has suffered from lack of suitable water that
has low total dissolved content to meet the Botswana Road Design Manual (1982)
specification. At some stage, during construction, it has been reported that non saline water
had to be imported from neighbouring South Africa to dilute the water on site and make it
usable for road construction purposes. Two other road projects in the area, Tsabong
Bokspits and Kang - Hukuntsi, which are also currently under construction are currently
facing similar difficulties. The Kang Hukuntsi project is reportedly 8 months behind the
program due to unavailability of non saline water in the construction area.
Based on these facts, the study area forms an intresting case study in that it presents two
challenges for road construction. The unavailability of non saline water/construction water in
general and the prevalence of heavy Kalahari Sands which are a difficult road construction
material. The southern map of Botswana for the study area is shown in Fig 3 in relation to
the capital city, Gaborone.
12
Fig 3. Map of the Study Area 1.6 Plan of The Study This report is has been prepared with the objective of identifying the effect of salinity on
Roadcem material. For clarity the report is divided into five chapters. Chapter 1 introduces
the problem and extent of saline soils and a brief historical background to its nature together
with current developments. Chapter 2 discusses the detailed literature review and looks into
the background of chemical stabilization in general world wide and within Botswana. A
review of the problems of saline materials, their origins and impact on pavement structures is
discussed in further detail as well as a review of current construction and damage mitigation
practice in saline prone areas. This chapter also introduces Roadcem compound and the
stabilization mechanisms that are expected to aid in reducing material salinity. Chapter 3
focuses on the methodology and processes adopted for the experimental review. The
13
strengths and weaknesses of each method will be outlined and weighed against current road
design practice. Chapter 4 deals with the presentation, analyses and discussion of results.
Chapter 5 is the conclusion and recommendations for the way forward in view of the report
findings.
Chapte
2.0 Bac
2.1 Kal
Kalahar
75% of
Thomas
2000).
deposits
deposits
from 10
Caylor,
are form
(2002)
general
They ex
reddish
Departm
the soil
10 on th
Based
with ca
Kalahar
requirem
Commu
state, as
and loo
Sahu a
have lit
mm has
and Lan
indicate
wetting
er 2
ckground -
lahari Sand
ri sands are
f the countr
s and Shaw
Simmers (
s from the
s making th
00 200 m
Parsons, F
med throug
have also
ly scarce du
xhibit colou
. According
ment, 2000)
ls. They are
he 0.075mm
on Botswa
alcrete, Kal
ri Sands h
ment accor
unications,
s with all sa
se their bea
and Piyo (2
ttle or no str
s been reco
nge as quo
ed that Kala
g and rapid
Review of
ds
e arid aeolia
ry and 2.5 m
w, 1991; Mi
(1987) also
Late Cret
hem silcrete
as docume
Frost, and S
h river eros
identified
ue to free d
ur variation
g to Guidel
), the colour
e generally n
m sieve. (Di
ana Roads D
ahari Sands
ave been i
rding to B
1982) and
ands in the
aring capaci
001) have
ructure and
orded by th
ted in (Wa
ahari Sands
d drying o
Literature
an, soils loc
million squ
inistry of W
o adds that
acious to r
es. Aeolian s
ented by Ty
Shugart (20
sion and as
that Kalah
drainage of
ns from pur
line No. 1 (
r of the san
non plastic
erks, 1992)
Department
s can be us
identified t
Botswana
Dierks (19
unbound st
ty when we
established
exhibit col
he Namibian
ng, DOdor
have a crus
of the soil
e.
ated to the
are kilomet
Works and
they are
recent. Th
soils are wi
son and Cri
002). Fluvia
s lake depo
hari Sands
the sands a
re white to
(Ministry o
nd is also ref
on the 0.42
t unpublish
sed as subg
to have hig
Road Des
992). The m
tate, is that
etted.
d that Kalah
llapse chara
n Departme
ricoa, Ring
st impregna
l and the
west of the
ters of Sout
Transport B
fluvial and
he sands m
ind transpor
imp (1998)
al and lacus
sits. Simme
lie in a re
and low ann
grayish, lig
of Works an
flective of t
25 sieve but
hed records,
grade or su
gh CBRs
ign Manua
main proble
they are su
hari sands a
acteristics. S
ent of Trans
groseb, Coet
ated with sa
upward ca
Botswana
thern Africa
Botswana, R
d lacustrine
may be calc
rted soils an
quoted in
strine sands
ers (1987) a
egion wher
nual rainfal
ght brown
nd Transpor
the engineer
t can have p
, at the righ
ubbase laye
which mee
al (Ministr
em with the
usceptible to
are fine gra
Subgrade co
sport (Dierk
tzeec and M
alts. The cru
apillary mo
and occupy
a. (Simmer
Roads Dep
sedimenta
cified with
nd vary in th
Scholesa, D
s on the oth
and Schole
re surface w
ll (Simmers
to dark bro
rt Botswan
ring charac
plasticities
ht mix pro
ers. On the
et the base
ry of Wor
e soils in th
o moisture
ained, cohes
ollapse of up
ks, 1992). B
Macko; 200
ust is forme
ovement o
14
y at least
rs, 1987;
artment,
ary sand
calcrete
hickness
Dowty,
her hand
sa et tal
water is
s, 1987).
own and
a Roads
terics of
of PI >
portions
eir own,
e course
rks and
heir neat
changes
sionless,
p to 700
Belknap
07) have
d by the
f saline
15
underground water on evaporation. (Dierks, 1992; Woodbridge, Obika, Freer-Hewish &
Newill, 1994; Sahu and Piyo, 2001; Thagesen, Robinson, 2004; Thomas & Dougill, 2006).
Wanga et tal have also observed that the soils are acidic in nature. The salt impregnated soils
are a source of salinity in road construction material. The other contributory source arises
from low annual rainfall, high evaporation rates and the fine grained structure of the sand
particles which impede infiltration rates leading to poor groundwater recharge (Simmers,
1987; Dierks, 1992). This leads to concentration of salts within the groundwater rendering it
unsuitable for construction of upper pavement layers.
Dierks (1992) has also indicated that densification of the Kalahari subgrades is necessary if
the upper pavement layers are to maintain the design integrity under traffic loading. Dierks
has observed that the experience of the Namibian Roads Department, has been that achieving
densities of 100% without collapsing the subgrades was sufficient to improve bearing
strength. This was achieved through impact or vibratory rollers and the trial sections along
main road 61 and the Gobabis Hospital has withstood 15 years of construction without
collapse of the subgrade Kalahari Sands. On the other hand, the Botswana Roads Design
Manual (1982) recommends the following treatment to collapsible soils in order to achieve
stable subgrades :
Depths of 0 to 500mm compacted to 90% MOD AASHTO Depths of 500mm to 1000mm compacted to 85% MOD AASHTO.
The compaction method is normally established on sites through trial runs.
Road construction through areas overlain by Kalahari Sands is normally collapsed to give
sufficient subgrade support for the pavement layers as reported by Thagesen and Robinson
(2004). Thagesen et tal have also concluded that Kalahari Sands cannot be effectively
stabilized with cement due to their single sized particle grading. Instead they recommend
foamed bitumen as a potential stabilizing agent.
Based on this experience of others it can be identified that the problems assosciated with
Kalahari Sand include:
Fine grained soils with low permeability Have a collapsible structure Have saline properties which may render them unusable as road bases in surfaced
roads
16
The fine grained structure renders the sands unsuitable for cement stabilization. The soils are susceptible to moisture changes even when compacted resulting in loss
of bearing capacity when used for pavement subgrades.
2.2 Mechanism of Salt Damage on Pavements
Substantial research work has been carried out in Botswana regarding the effects of salt
damage to bituminous surfaced pavements. Four sources of salts in pavements have been
identified, from the soil, construction water and salts derived form underground water
movements (Obika, Freer-Hewish and Newill; 1992). Other sources of salts may be external
as was the case with the pavement failure at Selibe-Phikwe runway. Maswikiti and Obika
(2000) have identified that mine waste used to construct the runway was the source of salts
which caused blistering to the pavement. The deterioration has since halted.
Obika, Freer-Hewish and Newill (1992) have suggested the following mechanisms described
briefly herewith as culprits in pavement surfacing damage. Pavement surfacing damage has
been attributed mainly to water soluble salts. The most common soluble soil salts have been
tabulated in Table 2 below as per the findings by (Obika, Woodbridge, Freer-Hewish and
Newill, 1994) and the National Park Service U.S. Department of the Interior (1998).
Soluble Salts Insoluble Salts Chlorides Carbonates Nitrates Sulphides Sulphates Phosphates Gypsum (Calcium Sulphates) Table 2: Common Water Soluble Salts The salts are described as moving in solution through capillary action to the ground surface
where they are deposited as the water evaporates during the day at high temperatures. The
deposited salts exert large pressures on thin bituminous surfacing, of thicknesses < 50 mm.
The type of failure exhibited by the bituminous surfacing will involve either or all in
combination of blistering, doming, fluffing or powdering and disintergration of the surface.
The exposed pavement is then susceptible to further failure through moisture ingress from
rainwater or kneading by traffic action leading to potholes, pavement rutting and general loss
of service through prolonged rut formation and pothole growth. This failure mechanism has
been assosciated with climatic zones in arid and semi arid regions of the world where
evaporation exceeds precipitation. (Obika, Freer-Hewish and Newill, 1992; Dierks, 1992;
Woodbridge, Obika, Freer-Hewish and Newill, 1994; Woodbridge, Obika, Freer-Hewish and
17
Newill, 1995; Ministry of Works and Transport Botswana Roads Department, 2001; Biggs
and Mahony, 2004; Bennet and Netterberg, 2004; Thagesen and Robinson, 2004). These arid
climatic zones are found all across the world in Australia, Africa, The Middle East, North and
South America and Asia (Obika et tal, 1992; Petrukhin, 1993; Fookes and Parry, 1994).
Obika, Freer-Hewish and Newill (1992) have described in detail the mechanisms involved in
crystal formation and the high expansive pressures exerted due to crystal growth. One key
observation has been that at humidities of less than 76% evaporation of NaCl solutions takes
place. NaCl crystals are said to be precipitated in places where the mean relative humidity is
less than 76%. At night when the temperatures drop sodium chloride (NaCl) crystals aborb
water and go into solution. During the day as the humidity drops recrystallisation occurs. The
process creates high pressures which disturb road surfacing.The crystals formed are whisker
shaped for sodium chloride (Obika et tal, 1992; Ministry of Works and Transport Botswana
Roads Department, 2001). Obika et tal (1992) have observed that alum and CaSO3 crystals
growing between two plates of glass exert sufficient pressures to lift a kilogram mass through
several tenths of millimeters. When pressures of this magnitude are exerted on thin
bituminous surfacing surface upliftment (blistering) and cracking results. Correns (1949), as
quoted in Obika et tal (1992) has established that crystal growth is also dependant on phase
boundary relationships. An additive or inherent property of the bitumen or soil is therefore
said to likely to increase the phase boundary tensions of the materials and either increase or
decrease the chances of crystal growth. Additives can therefore diminish the conditions
conducive for crystal growth. The laboratory findings of Obika et tal (1992) have established
that warm arid and semi arid regions are more prone to salt attack on thin bituminous
surfacing of les than 50 mm thickness. It has also been established that primes consisting of
penetration grade bitumen with more volatile solvents are more susceptible to salt damage as
compared to primes made from emulsions. Bituminous primes probably act as a continous
thick skin which is pushed up by crystalline pressures. Emulsions however lose their water
content and volatiles through evaporation and form a coating around gravel particles. Crystal
growth is likely to take place adjacent the coated gravel particles and may go unnoticed.
Further salt damage has been established to be more likely to occur within the period
immediately after road construction is complete. Obika et tal (1992) have recommend that
tests have to be carried out to assess if there is potential salt damage in particular material
sources. Despite the negative evidence of salts on surfaced roads, it has also been established
that salts are beneficial to gravel roads. This arises from the fact that the salts precipitated at
18
the surface are hygroscopic and absorb atmospheric moistures. This helps to entrap dust
particles and helps reduce lifting of dust under traffic. This property has been utilized
worldwide in the stabilization of unsurfaced gravel roads with salts like CaCl2 and brine
water .
2.3 Incidences of Salt Damage on Pavements in Botswana 2.3.1 Sua Pan Airfield The construction of the Sua Pan airfield, in 1988, was carried out without any controls
applied to the salt content for the pavement construction materials (Woodbridge, Obika,
Freer-Hewish and Newill; 1994). Due to scarcity of portable water, which was 40 kilometres
away, brine with total dissolved solids of 15% was used in the compaction of the subgrades.
The pavement was constructed with insitu Kalahari Sand, calcified sand for the subgrades
and calcrete for the subbase and base. The surfacing was Cape Seal. Within a month of
completion of construction the surface erupted with cracks and blisters originating on the
outer untrafficked pavement and was characterized by occurring in strips and turning bays.
Comprehensive testing followed in 1989 which linked the failure on the Cape Seal surface to
salinity (Obika et tal, 1994; Ministry of Works and Transport Botswana Roads Department,
2001; Bennet and Netterberg; 2004). Expensive remedial measures were tried unsuccessfully
until the failed sections had to be reconstructed and on some sections concrete pavement
slabs were later used (Ministry of Works and Transport Botswana Roads Department, 2001).
2.3.2 Maun Nata Road Total Dissoloved Salts (TDS) ranging from .1% to 7 % were observed on the Maun Nata road
where it crosses the northern extension of the Makgadikgadi Pans. Design trials revealed
damage to bituminous cutback (MC30) and emulsion (KR60) primes. Bituminous prime
coats were identified to be more susceptible to salt damage. Blistering and powdering damage
occurred within 48 hours to several days of construction. Single and double surfacing seals
also experienced damage where the TDS exceeded .4%. Plastic sheeting, 0.25mm thick, laid
across the full road width, to depth 450 mm below base level was used in areas of high
salinity to prevent salt migration and this was identified to be very effective in limiting the
salt damage. Contrary to expectations, placing a thick bituminous layer placed in a similar
position was unsuccessful in the same regard. In other places correct timing between
19
construction completion of road bases, prime application and surfacing seals was observed to
limit the risks (Ministry of Works and Transport Botswana Roads Department, 2001).
2.3.3 Sekoma Kang Road Salt water was used in the compaction of earthworks. The salt migrated towards the surface
and caused blistering, powdering and fluffing of the single seal surfacing on the carriageway
and road shoulders. Some of the damage occurred twelve months after construction.
Remedial measures carried out included removal of damaged seals and resealing and
reconstruction of damaged shoulder sections. Isolated spots were cut out and filled with
emulsion based premixes. The salt content is still high and timely reseals have been
scheduled to prevent future damage (Ministry of Works and Transport Botswana Roads
Department, 2001).
2.3.4 Tsabong Middlepits Road
The construction of the Tsabong Middlepits road has faced serious construction constraints
to the extent that the project has lagged five months behind schedule, and possibly increasing,
from construction completion. One of the contributory factors was the unavailability of water
for construction. The water from Tsabong village is highly saline. At one stage of
construction water had to be imported from South Africa for mixing with the saline water in
efforts to reduce the salinity levels. (Ministry of Works and Transport Botswana Roads
Department, 2008) Samples collected for the purposes of this research had Total Dissolved
Solids (TDS) which ranged from 0 for ordinary tap water to TDS of 33 296. This is consistent
with the groundwater TDS distribution map shown earlier in Figure 1. Tsabong is shown with
an underground water TDS that varies between 5000 and 50000. This high water salinity is a
major constraint for the future development of infrastructure in the village.
Other pavements which have also experienced surfaced pavement damage due to high
salinity in the past include:
Selibe-Phikwe runway which was attributed to pyrite salts originating from the mine waste
which was used to construct the pavement.
Sekoma - Kang road (Trans-Kalahari road)
Sekoma - Makopong
Kang - Hukuntsi
20
Tsabong - Makopong road
Orapa - Mopipi road
Rakops-Motopi
Maun Runway
Evidence from the preceding pavement failures shows that:
high salinity materials (water and gravel) were used in the pavement construction suitable construction water is scarce there is a scarcity of suitable road construction gravels no prior risk assessment was carried out to determine potential damage from salinity
All the above mentioned cases eventually cost the client considerably in terms of
maintenance and rehabilitation of the salt damaged pavement surfaces. This agrees with
literature reviewed earlier.
2.4 Current Practice to Limit Salt Damage to Bituminous Surfaced Pavements Based on the works of Obika, Freer-Hewish and Newill (1992), Woodbridge, Obika, Freer-
Hewish and Newill (1993,1994 & 1995), Ministry of Works and Transport Botswana Roads
Department (2001) and the Overseas Transport Research Laboratory the following techniques
have been used in Botswana successfully to mitigate the effects of salt damage to bituminous
surfaced pavements.
Control of road construction materials through specification of the upper salt contents of gravels for subbase and base and water for construction. The Botswana Roads
Design Manual (Ministry of Works and Transport Botswana Roads Department,
1982) has set limits of 0.05% for NaCl and 0.2% for SO3.
Design control by specifying thicker bituminous surfacing or asphalt seals, although this would be at greater construction cost.
Placing of vapour proof membrane between the upper subgrades and the subbase to prevent upward migration of concentrated soluble salts to the surface.
Careful construction practice by ensuring that in salt prone areas, the road bases are primed immediately after compaction has been completed. Further a thicker prime
will ensure an effective barrier preventing evaporation. Surfacing should immediately
follow within a week to safeguard the pavement from any further salt attack from the
subgrades.
21
Remedial treatments of mild salt damage include rolling or brooming of surfaces to break down and counter crystal formation. Trafficking has also been identified to
assist in counteracting the salt crystal pressures that form at the pavement surface.
Avoidance of areas with poor subgrade soils by realigning the road to prevent engaging in costly engineering countermeasures on new projects (Paige-Green, 2008).
Reconstruction has been implemented as the last alternative where any or all of the above recommendations have not yielded a positive result.
The techniques established to date are based on numerous field trials and observations. The
only disadvantage is they do not resolve the challenge of utilization of substandard materials.
The promise of Roadcem use has appeal in that it will incorporate material that would
otherwise be discarded through application of the specification and render them suitable for
the road construction. Infrastructure developers would be interested in utilization of the
economic design mixes that would yield beneficial construction and maintenance costs
without adversely affecting the environment. Road construction involves massive energy
consumption assosciated pollution more so in regions with problematic materials. A means to
reduce costs assosciated with road construction will provide huge dividends to developers.
2.5 Historical Review of Chemical Stabilisation in Road Construction.
Chemical stabilization involves the addition of chemical additives or stabilizers to improve
soil strength, or its characteristics to render the soil more suitable for engineering purposes.
The process is inclusive of chemicals added to reduce dust on unsealead roads. Chemical
stabilization is here defined as soil modification through chemical additives for the purpose
of improvement of the soils engineering properties. Ismaeil (2006) reports that chemical
stabilizers enhance interparticle bonds by formation of gels in the void spaces which improve
cohesion and adhesion between particles. The concept of strength gain through gel formation
has also been reported much earlier by The US Army Corps (1997), and Dithinde (1999).
Most of the products on the market are either binders, compaction aids or dust palliatives
(Department of International Development, 2000). Jones and Emery (2003) have documented
that chemical stabilisation is one method that has been in use over the past 50 years and is yet
to gain popular usage. Pinard (1998) had raised similar observations earlier, wherein he stated
that usage of chemical stabilizers is not yet as widespread and little benefit was being derived
due to lack of effectiveness of most products. However, other scholars document the usage of
22
this technology to as far back as ancient Rome and China. Jones et tal (2003) may have
referenced their findings regarding chemical stabilization in Africa. The most common
stabilizers in use on the continent are cement and lime. Evidence also indicates that large
tracts of roads in modern day China, Russia, United States of America, Europe, United
Kingdom, Asia and Australia have been constructed using various types of chemical
stabilizers in the past. There is plenty of visible evidence today of chemical stabilization as
seen in the remnants of some of the roads and walkways which are now historical
monuments in Italy, Greece, South America, and other parts of the ancient world. Cement,
lime, pozzolanas and volcanic ash are some of the early chemical stabilizers that were used
(Ismaiel, 2006). The need for stabilization centers on one engineering principle, all structures
are founded on the soil or they are made of soil. If the soil does not have adequate strength or
impermeability to water it has to be modified to suit the design capabilities.
The siting of civil engineering structures relies heavily on soil. In situations where the soil
does not have adequate bearing capacity to withstand the structural loads alternative
expensive foundation designs will have to be considered. Soil with low engineering strength
parameters has given rise to soil stabilization to improve soil bearing capacity. Where
unstable conditions existed the engineer was faced with several options (CAT Stabilisation
Guideline, 2006):
relocate the structure to more stable ground or alternative route redesign the structure to enable good load distribution that can be sustained by the soil
bearing capacity
cut to spoil unsuitable material and import good material ground improvement to enable the soil to sustain imposed structural loads. another more costly method involved raising the structure very high above the
deleterious material
These solutions have withstood the verity of testing through time, albeit some of them at
considerable cost to the developer. Each solution has been applied uniquely to a particular
problematic site based on economy, durability and structural integrity. Rapid infrastructure
development globally has also led to diminishing land resources on which to relocate new
structures on. Exportation of unsuitable and importation of suitable materials may generate
prohibitive costs at construction stage. In Botswana with a land mass of at least 75% covered
by Kalahari Sands this solution will not work. Construction costs arise from the high energy
23
consumption and depletion of suitable materials due to increasing demand. It is the solutions
that offer competitive economic, environmental, mitigate social impacts and long lasting
engineering solutions that will survive obsoletion. Ground improvement is gaining
significance in that it offers opportunities to utilize otherwise weak in situ material.
The demand for alternative sustainable methods and materials to enhance road construction
materials has been accelerated by the rapid depletion of good construction soils and the
scarcity of suitable soils. This has resulted in many commercial products appearing on the
market. Despite the proliferation of commercial chemical stabilizers, they are still limited in
their performance according to soil characteristics. Furthermore certain stabilizers are
designed to target only specific problematic soil properties. The quality and quantity of the
chemical ingredients also affects the effectiveness of most compounds. (Netterberg and
Paige-Greene, 1984; Wilmot, 1994; Ismaiel, 2006). According to Wilmots work on
traditional stabilisers, there is an ideal mixture workability which can be reached before
resitance to compaction begins. This has been related to laboratory density loss (LDL) period
which relates a mix performance to the laboratory test for maximum dry density. Ultimately
this determines the working time in the field between wet mixing and compaction. However
Wilmort indicates that more work is required to establish LDL values and correlating them to
site working times. A full understanding of some of these mechanisms can aid in improving
the correct field usage of chemical stabilisers and their long term performance.
Chemical stabilizers vary in their properties and mechanisms in which they effect soil
strength gain. An understanding of this classification and the properties of these chemical
groups can assist in selecting potential stabilizers for a particular soil type.
According to TRL, The Sulphonated Petroleum Products Toolkits 1 chemical stabilisers have
been classified into the following key categories:
Traditional stabilisers e.g. cement and lime, which are binders Calcium or magnesium chlorides used as dust palliatives Clay additives e.g. bentonite to increase plasticity Enzymes e.g. earthzyme which work by consuming clays Lignosulfates from paper mills used as dust palliatives Synthetic Polymer Emulsions e.g. soil cement which glue soil particles together Tall oil emulsions from paper mills
24
Sulphonated petroleum products (SPP) which are surface active agents and are compaction aids.
Through the course of this investigation it will be possible to classify the category into which
Roadcem falls in. Roadcem is likely to fall in the category of traditional stabilizers or SPP
products as determined by its chemical reactivity. It is also intresting to note that some of the
laboratory and field trials of Roadcem has distinguished it to be a unique soil cement additive
that has the following capabilities :
Can be used with saline water and gravels Can be used with all types of problematic soils in immobilizing peats, expansive
clays, sands with appreciable strength gains.
Prevents carbonation of cement stabilized soils Can be used to neutralize chemically contaminated soils reducing their hazards Provides a durable and stronger material Enables a thinner pavement layer to be used for the same loading capacity of a
conventionally designed pavement layer.
2.6.0 Review of Some Chemical Stabilisers
2.6.1 Calcium or Magnesium Chlorides
Calcium and magnesium chlorides are used as dust palliatives in gravel road construction. It
has been observed that in dry arid climates, the application of the chlorides assists in
absorbing atmospheric moisture which coats dust particles. The heavier dust particles are not
easily lifted by wind or under traffic action. Dust palliatives add little or no engineering
strength improvement to the soil structure. Consequently they are not ideal for subgrade
strength improvement.
2.6.2 Clay Additives
Clay additives like bentonite or sodium montmorillonite have been used to improve the
binding soil characteristics in otherwise non cohesive granular material. The fine sized clay
fraction acts as a cementetious material that binds the different soil particles together through
interpaticle bond formation. Furthermore when optimum moisture is added the clay fraction
acts as a lubricant that reduces the compactive effort required. This method of stabilization is
more akin to mechanical blending than chemical stabilization. The exact proportions of clay
25
and non cohesive granular material determine the final properties of the blended material and
these mixes need careful design to produce the required blend properties. This method is
unlikely to be suitable for Kalahari Sands which are fine grained soils. A combination of the
two materials types is not likely to produce a good matrix with sufficient aggregate interlock.
The end product is highly likely to produce a soil with very poor shear resistance
characteristics.
2.6.3 Enzymes
Some of the available commercial enzymes include Earthzyme, EMC-squared and
Permazyme. The enzymes are believed to react with the air to form compounds that digest
clay particles and release inert material in the process. These enzymes are only suitable for
soils with clay minerals.
2.6.4 Lignosulphates
Lignosulphates are byproducts from the sulphite paper manufacturing process. The products
are mainly dust palliatives and are not chemical stabilisers.
2.6.5 Synthetic Polymer Emulsions
This range of products is manufactured mainly for the paint industry and react with soil by
direct bonding or gluing of the soil particles together. Examples include Soil Sement which
usually have acryllic or acetate polymers/coplymers as their base materials.
2.6.6 Tall Oil Emulsions from paper mills
These are by products of sulphate paper making processes particularly when pulping the
Douglas Fir or Southern Pine. They act as dust suppressants by coating light gravel particles
so they are not lifeted by traffic action.
2.6.7 Sulphonated Petroleum Products
Sulphonated petroleum products (SPPs) are surface active agents. Active surface agents
reduce the surface tension of water. They have a sulphonic head and a hydrophobic tail. They
are marketed as compaction aids or stabilisers. (Greening and Paige-Greene, 2003; CSIR &
TRL, nd) In principle these compounds have been found to be useful in the stabilisation or
compaction of clayey soils. Intial tests are required to determine the clay component within
26
the soil since it is this which influences the SPP effectiveness. In Botswana, an evaluation of
SPP was done using Conaid by Abadjieva (1997).
2.6.8 Conaid Stabilisation An experimental study on the effect of Conaid stabiliser on black cotton clay soils and
calcrete was carried out by Abdjieva(1997). Conaid is available in liquid form and the
application rate recommended by the manufacturer is 0.01 to 0.03 litre/m2 per 15 cm
thickness. It has been documented that Conaid can be used to stabilise soils with Plasticity
Index greater than 11 and clay content of 15% or more. Conaid works by coating the weak
interparticle bonds between the clay particles and prevent water absorbtion. This reduces
volumetric changes in the clay and aids in the compaction process (Abadjieva, 1997).
Moderate reductions in plasticity and moderate increase in CBR were observed for the black
cotton clay and calcrete samples immediately after the mixing. However with extended
curing periods CBR values were observed to double as compared to the untreated sample
CBR values. Laboratory results achieved on black cotton soils saw an increase of CBR to the
order of at least 67%. But practically this achieved by an improvement of CBR from 3 to 5
for black cotton soils. A trial road section was visually observed over a three month period.
The treated section resulted in higher DCPs compared to the untreated section. The trial
section was less dusty in dry weather and was less slippery or muddy during wet weather.
Based on these findings and the stabilisation mechanisms observed sulphonated petroleum
products are unlikely to improve the engineering strength of Kalahari Sands. Kalahari Sands
have no plasticity and therefore do not provide the base required for the chemical reactions
required to initiate setting by SPP reactivity.
2.6.9 Fly Ash Stabilisation
Fly Ash is a by product from the coal burning process for thermal power generation. In
Botswana this is produced at Morupule Colliery. Usage of fly ash has been gaining ground in
areas such as flowable fills, road bases, earth embankments and is also blended with cement
and other stabilizers used in road construction. Flowable fills are fills comprised of fly ash
only which is mixed thoroughly with water and placed as a structural layer. Final strength is
gained through drying of the fly ash layer. Sahu and Piyo (2001) have carried out a
laboratory investigation with six different soil types, namely Kalahari Sand; calcrete; silty
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sand, silts of intermediate and low plasticity and black cotton clay soil, all sourced from
different parts of Botswana.
Fly ash has pozzolanic properties which are enhanced by the presence of high free lime and
low unburnt carbon content. Sahu and Piyos (2001) investigation involved variation of fly
ash composition at 4, 8, 16 and 24% by weight of soil mixed accordingly. Classification,
compaction and CBR characteristics were carried out on all the samples. Sahus et tal (2001)
findings were that the maximum dry density (MDD) decreased with increasing fly ash
content except for the black cotton clay which increased in by up to 16% of fly ash
concetration and then decreased at 24% concetration. The optimum moisture content (OMC)
was unaffected by the fly ash content. For all the soil types tested Kalahari Sand required a
high fly ash content for appreciable results to be obtained. At fly ash contents of 28% and
32% the CBR increased beyond 24%. Thagessen (2004) has indicated that it is expected that
fine grained soils would require higher dosages of cement for effective stabilization to take
place. This reflects a lineal strength relationship with increasing stabilizer content. Similar
relationships were observed by El-Rawi and Al-Samadi (1995 in working with lime and
cement stabilization of a Jordanian soil.
Sahu and Piyo (2001) also identified that fly ash stabilization efficacy varied with the soil
type, and was observed to be less effective with Kalahari Sands and Black Cotton Clays. Fly
ash are therefore not very ideal for Kalahari Sand stabilization. Per unit mass a large volume
of fly ash would be required, at least 32 % according to Sahu et tal (2001), to bring about
marginal increases of CBR. The large distance between Morupule colliery, where fly ash is
produced as a by product, and the Kgalagadi District is prohibitive for any economic haulage
of the product. Ideally infrastructure within the vicinity of the colliery should be developed
with increased usage of fly ash where this is viable. Basak, Bhattacharya and Paira ( 2004 )
have also established that fly ash usage in emabankment construction is more economical
within the vicinity of the thermal fire station. This is applicable to any structural layer for
which fly ash usage is intended as this minimizes hauling distance from supply to demand
side.
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2.6.10 Other Documented Tests on Kalahari Sands
In other studies Kalahari Sands have been stabilised with Ecobond. The compound is
recommended for stabilization of silty sand and gravelly soils. Ecobond is a polymer that is
used in aqueous solution. With water only hardening is achieved rapidly through an
exorthermic reation, developing from a gel to a hard rock. When mixed with soil the part of
the heat is absorbed by soil particles and this slows down the reactions improving workability
in the field. yielding low results as compared to the requirements of the Botswana Road
Design Manuals minimum Enginerring parameters (Joas, Kgengwane, Mmeso; 2001). The
tests focused on unconfined compressive strength (which yielded low results) and shear
strength parameters and unsoaked CBR. It was identified that the stabilization of Kalahari
Sand with Ecobond may render Kalahari Sands suitability for usage as a subbase material
only (Joas et tal, 2001). The Kalahari Sands had been classified as G7 material suitable for
use in subgrades. The strength improvement to render the Kalahari Sands suitable for subbase
quality is a valuable achievement. Further investigation in regards to durability and
environmental stability will be required to establish the efficacy and effectiveness of the
Ecobond stabiliser. If theses criteria are satisfied as per the specifications, only then can field
trials be recommended.
2.7 Traditional Stabilisers
2.7.1 Lime Stabilisation
Cement and lime stabilization are the oldest known stabilizers. They have been used
extensively worldwide and throughout Africa in various road construction projects with great
success. Both have been used individually or in combination in cement-lime stabilization.
Lime has been used in clay soil stabilization as slaked lime or hydrated lime [either CaO or
Ca(OH)2] on many road construction projects throughout the world. (Ismaiel, 2006). Three
mechanisms involved in clay soil improvement have been identified : hydration, flocculation
and cementation. The first two mechanisms are believed to be short term reactions whilst the
cementation is understood to take place over a longer period of time. Ismaels study (2006)
identified that the strength gain in cement stabilised soil was greater than that observed on
soils stabilised with lime. However lime stabilised soils are not adversely affected by
construction delays as compared to cement treated soils. The cementation reaction in lime
stabilised soils results in long term strength gain and arises from a long term pozzolanic
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reaction (Little, 1999). On the other hand, the hydration reactions of cement take place within
2 to 3 hours. Failure to place and compact the material within this period will result in
breaking down of the cementitious bonds resulting in weak strenghts being achieved. Porr
construction methodology such as poor control of the compaction and hydration moisture will
lead to weakened cementious reactions.
Lime is the prefferred treatment for clayey soils because it improves soil structure through
flocculation and reduces the plasticity through Ca++ ions bonding with clay minerals leading
to a reduction in water affinity (Little, 1999). The National Lime Assosciation (2004) also
gives recommendations for soils with plasticity >10 and with 25 % or more passing passing
sieve number 200 (75mm) to be ideal for lime stabilisation. This being attributed in part to
the physical and mineralogical composition of the materials. Cement is a very fine material
and may not spread as evenly throughout the clay sized soil fraction to enable coating of the
particles to take place and encourage hydration and uniform gel formation. Whereas lime will
flocculate the clay particles improving the soil structure in the process.
Despite the reported success of cement and lime stabilization post construction failures have
also been observed worldwide. The failures are mainly due to carbonation of stabilized layers
and cracking of bases and result in weakened strength and failure to from bonds within the
cemen