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

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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

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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