Thesis 1

UNIVERSITY GHENT

UNIVERSITEIT GENT

INTERUNIVERSITY PROGRAMME MASTER OF SCIENCE IN PHYSICAL LAND RESOURCES Universiteit Gent Vrije Universiteit Brussel Belgium

Stabilization of soils from Cameroon for construction purposes

September 2006

Promotor: Prof. J. Wastiels Advisor: Dr. Mazen Y. M. Alshaaer

Master dissertation in partial fulfilment of the requirements for the Degree of Master of Science in Physical Land Resources by: Ndofor Akongnui Fai

i

Abstract Local soils are the most common, easily available and cheap construction materials used for simple structures in most parts of Cameroon. Due to their poor durability, severe limitations have been unraveled while using these soils. Various techniques have been used over the years to improve on the durability of these soils. The possibilities of using the mineral polymerization (MIP) technique for the stabilization of kaolinitic soils with the aim of making them more suitable for construction purposes was investigated using four soil samples ( FET, FBM, FNB, and FNK) from Cameroon. This technique is based on the micro structural transformation of some clay minerals into solid and stable materials having hydroxysodalite, feldspatiod or zeolite characteristics under the action of alkaline reactants, at atmospheric pressure and low (quasi environmental) temperatures. Granulometric analysis, plasticity and loss on ignition tests were used to characterize the soil samples. They all had the required amount of kaolinite; averagely 40%. Adding 12% sodium hydroxide at optimum water and sand content to sample FNK, compressive strengths which meet the requirements for construction materials precursors were obtained. These strengths were 25MPa and 10MPa under dried and immersed conditions respectively. Sample FBM also gave acceptable compressive strengths by adding 8% sodium hydroxide at optimum water and sand content to the sample which were 12MPa and 7MPa under dried and immersed conditions respectively. Despite all efforts, we were unable to obtain acceptable compressive strengths for the FET and FNB samples probably due to the presence in the samples of other kinds of clays or colloidal materials which are undesirable for the technique. With a soil sample that meets the requirements for this technique, optimizing the quantity of various constituents results in very good mechanical and physical characteristics of the resulting construction materials. The smaller the grain sizes used, the more the properties of the materials are improved as there will be a larger surface area for reactions to occur. Ideal curing temperatures lie between 75°C and 85°C as higher temperatures only increase cost and pollution with no considerable improvement in the mechanical and physical properties of the materials. Ndofor Akongnui Fai

PHYLARES 2006

ii The mineral polymerization technique can be considered as a potential technique for the improvement of some soils in Cameroon for it is cheaper, environmentally friendlier than traditional methods and produces strong and durable construction materials.

Ndofor Akongnui Fai

PHYLARES 2006

iii

Acknowledgements The scientific journey that has produced this thesis has seen the contribution of seasoned scientists, institutions, family and friends. Without them, I wonder whether this adventure of mine would have yielded any fruits. I am most grateful and will be eternally indebted to my thesis promoter Prof. Dr. Ir. Jan Wastiels and advisor Dr. Mazen Alshaaer. Their patience and invaluable guidance has been the main driving force behind this work. Words cannot express what I have learnt from them within and outside this research. I will also like to express my gratitude to the academic and administrative staff of the Physical Land Resources Programme in the University of Ghent and the Vrije Universitiet Brussels under the leaderships of Prof. Dr. E. Van Ranst and Prof. Dr. F. De Smedt respectively. Their guidance and assistance has been of much help and value to this work. Special thanks go to Dr. Uphie Melo Chinje the director of the Local materials promotion authority (MIPROMALO) Cameroon who assisted me immensely in sample collection. By granting me access to the library and laboratories of her institution under the supervision of her assistants, the fieldwork and collection of samples for this work saw the light of day. The mind searching discussions and exchanges we had on the subject were also enriching. Thank you once more madam. I acknowledge the warmth of my course mates throughout the two years. My cluster friends Lambive, Messiga, Uzoma, Prabin, Lee and Nawal have been exceptional “we remain united”. Hearty thanks go to my mum and dad Helen Nkeh and William Nkeh respectively for their support and love through out my educational ladder. I will always remain indebted to them reason why l dedicate this work to them. I am equally grateful to my sisters and brother, Relindis, Jacqueline, Loveline, Mary, Therese and Julius. I wish to thank all my friends and most especially, Eric Andangfung who inspired me into the field of engineering geology. I am particularly thankful to Ursula for her last minute support. Her support, understanding, and advice helped me to courageously surmount all difficulties provoked by my long absence from home. Finally I would like to thank all the good people at the Vlaamse Interuniversitaire Raad (VLIR) and the Flemish Government for financing my studies.

Ndofor Akongnui Fai

PHYLARES 2006

iv

Table of Content Abstract ...................................................................................................................................... i Acknowledgements................................................................................................................... iii Table of Content ....................................................................................................................... iv List of Figures........................................................................................................................... vi List of Tables .......................................................................................................................... viii Chapter-1 General Introduction ............................................................................................1 1.1 General Background ....................................................................................................1 1.2 The use of soils for construction materials in Cameroon ..............................................1 1.3 Problem Description ....................................................................................................2 1.4 Objectives....................................................................................................................3 Chapter-2 Literature Review ................................................................................................4 2.1 Improvement of Soils for Construction ........................................................................4 2.1.1 Mechanical Stabilization ......................................................................................4 2.1.2 Physical Techniques.............................................................................................5 2.1.3 Physico-chemical techniques – fired bricks ..........................................................6 2.1.4 Chemical Stabilization .........................................................................................7 2.2 Clay Minerals and other silicate minerals in soils.........................................................9 2.3 Chemistry of Inorganic Polymers...............................................................................15 2.4 Mineral Polymerization Technique ............................................................................16 Chapter-3 Area Description ................................................................................................22 3.1 Introduction ...............................................................................................................22 3.2 Collection of Samples................................................................................................22 3.3 The Yaoundé area deposits ........................................................................................24 3.3.1 Nkolbison sample ..............................................................................................24 3.3.2 Mvog Betsi sample ............................................................................................24 3.3.3 Simbock sample.................................................................................................25 3.3.4 Parent material for Yaoundé soils.......................................................................25 3.4 Bambili Sample .........................................................................................................25 Chapter-4 Characteristics of Soil Samples ..........................................................................27 4.1 Introduction ...............................................................................................................27 4.2 Moisture Content .......................................................................................................27 4.3 Grain Size Distribution ..............................................................................................28 4.4 Atterberg Limits ........................................................................................................31 4.5 Plasticity Index (PI) ...................................................................................................32 4.6 Loss on Ignition.........................................................................................................33 Chapter-5 Methodology......................................................................................................38 5.1 Materials....................................................................................................................38 5.2 Fabrication of Specimens...........................................................................................38 5.2.1 Mixing ...............................................................................................................39 5.2.2 Moulding ...........................................................................................................39 5.2.3 Curing................................................................................................................40 5.2.4 Post curing and Pre-test Treatments ...................................................................40 5.3 Measurements............................................................................................................40 5.4 Testing.......................................................................................................................41 5.4.1 Uniaxial Compression test..................................................................................41 Ndofor Akongnui Fai

PHYLARES 2006

v 5.4.2 Loss of compressive strength .............................................................................42 5.5 Water Absorption ......................................................................................................42 5.6 Efflorescence and pH.................................................................................................42 5.7 Homogeneity of the mixture ......................................................................................43 5.8 Checking the effect of maximum grain size of soil samples .......................................43 5.9 Checking the effect of curing temperature..................................................................43 5.10 Improving specimen’s characteristics using sodium hydroxide ..................................43 Chapter-6 Observations, Results and Discussions ...............................................................44 6.1 Mvog Betsi sample, (FET).........................................................................................44 6.1.1 Fabrication of Specimens ...................................................................................44 6.1.2 Physical Characteristics of Specimens................................................................45 6.1.3 Compressive strength and Stability ....................................................................50 6.1.4 Efflorescence .....................................................................................................53 6.2 Bambili sample, (FBM) .............................................................................................53 6.2.1 Fabrication of specimens....................................................................................54 6.2.2 Physical Characteristics of Specimens................................................................54 6.2.3 Compressive strength and stability .....................................................................59 6.2.4 Efflorescence .....................................................................................................62 6.2.5 NaOH as stabilizing argent.................................................................................63 6.3 Simbock sample, (FNB).............................................................................................64 6.3.1 Fabrication of Specimens ...................................................................................64 6.3.2 Physical Characteristics of Specimens................................................................65 6.3.3 Compressive strength and stability .....................................................................70 6.3.4 Efflorescence .....................................................................................................73 6.4 Nkolbison sample, (FNK) ..........................................................................................73 6.4.1 Fabrication of Specimens ...................................................................................74 6.4.2 Physical Characteristics of Specimens................................................................75 6.4.3 Compressive strength and stability .....................................................................79 6.4.4 Efflorescence .....................................................................................................82 6.5 Changing the maximum grain size of soil samples .....................................................83 6.5.1 Physical Characteristics of Specimens................................................................83 6.5.2 Compressive strength and Stability ....................................................................85 6.6 Influence of curing temperature .................................................................................86 6.6.1 Influence of curing temperature on density.........................................................86 6.6.2 Compressive strength and stability .....................................................................87 Chapter-7 Conclusions and Recommendations ...................................................................89 7.1 Specifications for the use of soils in construction .......................................................89 7.2 Mvog Betsi sample, (FET).........................................................................................90 7.3 Bambili sample, (FBM) .............................................................................................90 7.4 Simbock sample, (FNB).............................................................................................91 7.5 Nkolbison sample, (FNK) ..........................................................................................91 7.6 Changing the maximum grain size of soil samples .....................................................92 7.7 Influence of curing temperature .................................................................................92 7.8 Sodium hydroxide as stabilizing argent......................................................................92 7.9 Recommendations .....................................................................................................93 References ................................................................................................................................94

Ndofor Akongnui Fai

PHYLARES 2006

vi

List of Figures Figure 1, Basic structural unit of silicates [21]...........................................................................10 Figure 2, Linking of (SiO4)-4 tetrahedra to form silicates............................................................10 Figure 3, An exploded view of the aluminum octahedral unit [21].............................................12 Figure 4, Structure of gibbsite [21]............................................................................................12 Figure 5, Structure of kaolinite..................................................................................................13 Figure 6: Sketch picture of kaolinite structure showing distances between atoms ......................15 Figure 7, General formulas of polysiloxanes (1) and polyphosphazenes (2)...............................16 Figure 8, Hydrosodalite unit cell, [(Si-O-Al-O), nNa, nH2O] (Source : S. Kowalak et al, (Modified 2000)).......................................................................................................................18 Figure 9, Schema showing reaction between Kaolinite and NaOH.............................................19 Figure 10, Map of Cameroon showing sampling locations.........................................................23 Figure 11, Grain size distribution curve and summary of other properties - FET........................29 Figure 12, Grain size distribution curve and summary of other properties - FBM ......................30 Figure 13, Grain size distribution curve and summary of other properties – FNB ......................30 Figure 14, Grain size distribution curve and summary of other properties – FNK ......................31 Figure 15, An illustration of boundaries between Atterberg’s limits ..........................................31 Figure 16, Incremental loss on ignition at various temperatures - FET.......................................34 Figure 17, Total loss in weight at various temperatures - FET ...................................................34 Figure 18, Incremental loss on ignition at various temperatures – FBM.....................................35 Figure 19, Total loss in weight at various temperatures – FBM .................................................35 Figure 20, Incremental loss on ignition at various temperatures – FNK .....................................36 Figure 21, Total loss in weight at various temperatures – FNK..................................................36 Figure 22, Incremental loss on ignition at various temperatures – FNB .....................................37 Figure 23, Total loss in weight at various temperatures - FNB...................................................37 Figure 24, Variation of density with % NaOH – FET ................................................................47 Figure 25, Variation of density with water content – FET..........................................................47 Figure 26, Variation of density with sand content - FET............................................................48 Figure 27, Variation of water absorption with NaOH content - FET ..........................................49 Figure 28, Variation of water absorption with water content - FET............................................49 Figure 29, Variation of water absorption with sand content – FET.............................................50 Figure 30, Variation of compressive strength with water content – FET ....................................51 Figure 31, Variation of compressive strength and NaOH content – FET ....................................52 Figure 32, Variation of compressive strength with sand content – FET......................................53 Figure 33, Variation of density with NaOH content – FBM.......................................................55 Figure 34, Variation of density with water content- FBM ..........................................................56 Figure 35, Variation of density with sand content – FBM..........................................................57 Figure 36, Variation of water absorption with NaOH content – FBM ........................................58 Figure 37, Variation of water absorption with water content - FBM ..........................................58 Figure 38, Variation of water absorption with sand content - FBM............................................59 Figure 39, Variation of compressive strength with NaOH content - FBM ..................................60 Figure 40, Variation of compressive strength with water content - FBM....................................61 Figure 41, Variation of compressive strength with sand content – FBM ....................................62 Figure 42, Stabilization potentials of NaOH - FBM...................................................................64 Figure 43, Variation of density with NaOH content – FNB........................................................66 Figure 44, Variation of density with water content - FNB..........................................................67 Ndofor Akongnui Fai

PHYLARES 2006

vii Figure 45, Variation of density with sand content - FNB ...........................................................68 Figure 46, Variation of water absorption with NaOH - content ..................................................69 Figure 47, Variation of water absorption with water content - FNB ...........................................69 Figure 48, Variation of water absorption with sand content – FNB............................................70 Figure 49, Variation of compressive strength with NaOH content – FNB ..................................71 Figure 50, Variation of compressive strength with water content – FNB....................................72 Figure 51, Variation of compressive strength with sand content – FNB .....................................72 Figure 52, Variation of density with NaOH content – FNK .......................................................75 Figure 53, Variation of density with water content – FNK.........................................................76 Figure 54, Variation of density with sand content - FNK ...........................................................77 Figure 55, Variation of water absorption with NaOH content - FNK .........................................78 Figure 56, Variation of water absorption with water content - FNK...........................................78 Figure 57, Variation of water absorption with sand content – FNK............................................79 Figure 58, Variation of compressive strength with NaOH content – FNK..................................80 Figure 59, Variation of compressive strength with water content - FNK ....................................81 Figure 60, Variation of compressive strength with sand content – FNK.....................................82 Figure 61, Variation of density with grain size – FBM ..............................................................84 Figure 62, Variation of water absorption with grain size – FBM................................................85 Figure 63, Variation of compressive strength with grain size – FBM.........................................86 Figure 64, Variation of density with curing temperature - FBM.................................................87 Figure 65, Variation of compressive strength with curing temperature – FBM...........................88

Ndofor Akongnui Fai

PHYLARES 2006

viii

List of Tables Table 1, Main types of silicates .................................................................................................11 Table 2, Geographical location of sampling sites.......................................................................24 Table 3, Hygroscopic moisture content of samples ....................................................................28 Table 4, Atterberg limits and Plasticity indices for the various samples .....................................33 Table 5, Composition of each series of specimens - FET ...........................................................44 Table 6, Variation of density with NaOH content - FET ............................................................46 Table 7, Variation of density with water content - FET .............................................................46 Table 8, Variation of density with sand content - FET ...............................................................46 Table 9, Variation of water absorption with NaOH content - FET .............................................48 Table 10, Variation of water absorption with water content - FET .............................................49 Table 11, Variation of water absorption with sand content - FET ..............................................50 Table 12, Variation of compressive strength with water content - FET ......................................51 Table 13, Variation of compressive strength with NaOH content - FET.....................................52 Table 14, Variation of compressive strength with sand content - FET........................................52 Table 15, Composition of each series of specimens - FBM........................................................54 Table 16, Variation in density with NaOH content - FBM .........................................................55 Table 17, Variation of density with water content - FBM ..........................................................56 Table 18, Variation of density with sand content - FBM............................................................56 Table 19, Variation of water absorption with NaOH content - FBM ..........................................57 Table 20, Variation of water absorption with water content - FBM............................................58 Table 21, Variation of water absorption with sand content - FBM .............................................59 Table 22, Variation of Compressive strength with NaOH content - FBM ..................................60 Table 23, Variation of compressive strength with water content - FBM.....................................61 Table 24, Variation of compressive strength with sand content - FBM ......................................61 Table 25, Effects of NaOH on compressive strength - FBM ......................................................63 Table 26, Composition of each series of specimens - FNB ........................................................64 Table 27, Variation of density with NaOH content - FNB .........................................................66 Table 28, Variation of density with water content - FNB ...........................................................66 Table 29, Variation of density with sand content - FNB ............................................................67 Table 30, Variation of water absorption with NaOH content - FNB...........................................68 Table 31, Variation of water absorption with water content - FNB ............................................69 Table 32, Variation of water absorption with sand content - FNB..............................................70 Table 33, Variation of compressive strength with NaOH content - FNB ....................................71 Table 34, Variation of compressive strength with water content - FNB .....................................71 Table 35, Variation of compressive strength with sand content - FNB .......................................72 Table 36, Composition for each series of specimens - FNK.......................................................74 Table 37, Variation of density with NaOH content - FNK .........................................................75 Table 38, Variation of density with water content - FNK...........................................................76 Table 39, Variation of density with sand content - FNK ............................................................77 Table 40, Variation of water absorption with NaOH content - FNK...........................................77 Table 41, Variation of water absorption with water content - FNK ............................................78 Table 42, Variation of water absorption with sand content - FNK..............................................79 Table 43, Variation of compressive strength with NaOH content - FNK....................................80 Table 44, Variation of compressive strength with water content - FNK .....................................81 Table 45, Variation of compressive strength with sand content - FNK.......................................81 Ndofor Akongnui Fai

PHYLARES 2006

ix Table 46, Variation of density with grain size - FBM ................................................................83 Table 47, Variation of water absorption with grain size - FBM..................................................84 Table 48, Variation of compressive strength with grain size - FBM...........................................85 Table 49, Variation of density with curing temperature - FBM ..................................................87 Table 50, Variation of compressive strength with curing temperature - FBM ............................87

Ndofor Akongnui Fai

PHYLARES 2006

1

Chapter-1

General Introduction

1.1 General Background Cameroon is highly under-developed, with poverty, poor healthcare, malnutrition, poor road infrastructure, and poor housing conditions being very prominent. The use of cheap and high quality local natural resources for construction purposes is therefore very vital for its sustainable development. With very low purchasing powers, very few Cameroonians can afford for conventional building materials which entail a lot of money to buy and transport them to construction sites. However, Cameroon is potentially provided as far as natural construction materials are concern. Their advantages are increasing from the scientific, economical and environmental points of view. Qualitative and quantitative data on these materials are not yet sufficient for their valorization. Knowledge of the benefits and usefulness of natural resources, how people used them in the past and how they are using them now for construction is very vital in order to boost development in Cameroon. To gain this knowledge, an assessment of the geotechnical properties of materials and various methods that have been used or maybe used to improve upon these properties is necessary.

1.2 The use of soils for construction materials in Cameroon One of the demands of rising populations and rising standards of living is the increasing use of resources for construction. As a result, some construction resources such as soils, which rarely possess the characteristics of volume stability, strength and durability required in construction, have to be used. In order to use these materials and come out with good results, there is the need for their improvement.

The improvement of these materials is termed stabilization. Soil

stabilization can be described as the modification of soils to meet specific engineering requirements. Soil stabilization is commonly used to describe any physical, chemical or biological method or combination of such methods used to improve certain properties of a natural soil for intended purpose. The use of conventional construction materials requires a high level of technical know how. In Cameroon, well trained technicians who can easily apply these methods are rare and expensive to pay for by an average citizen. Cheap and easy to use naturally

Ndofor Akongnui Fai

PHYLARES 2006

2 occurring local materials which require simple and cheap techniques to improve upon their properties will therefore be of great necessity and advantage. It is rather unfortunate that in Cameroon a greater majority of the population uses basic earth materials without any modification on their properties for construction. Most buildings built with only basic earth materials suffer relatively rapid deterioration due to low dry strengths which decrease to zero under wet conditions and also have high porosities and water absorption. They also show the development of shrinkage cracks under dry conditions, swelling under wet conditions and a generally high susceptibility to damage due to periodic wetting and drying.

1.3 Problem Description Modern construction materials are accompanied by a lot of energy consumption and environmental degradation. The high cost of energy and transportation has made the prices of cement and its related products beyond the rich of most Cameroonians. The first stage of all industrialization, and increasing standards of living, requires concrete for building infrastructures. The manufacture of traditional Portland cement requires calcining calcium carbonate. This yields calcium oxide and carbon dioxide gas. The emission of carbon dioxide is therefore, becoming a growing concern; it is an important contributor to the green house effect. As regards development statistics, the worldwide level of cement production is expected to be 3.5 billion tons by the year 2015. This would put the share in the global pollution (all human activities combined) at 18% [1]. Most specifications used in tropical countries were developed to meet the needs of the temperate climatic conditions of Europe and North America and do not seem to recognize the special characteristics of tropical soils.

In Cameroon, much still has to be done to adapt these

specifications to local realities, especially as the country has a varied surface geology. Most of the specifications in use are those borrowed from France. Modifications to existing specifications, taking into account the peculiarities of local soils are indeed necessary if they are to be used effectively. An elaborate study of the properties of the natural construction materials and ways of improving on their properties in all parts of Cameroon so as to come out with specifications that will reflect local climatic and environmental conditions is of great necessity. Ndofor Akongnui Fai

PHYLARES 2006

3

This study is going to evaluate the potentials of the Mineral polymerization (MIP) Technique which has been well studied in the Mechanics of materials and construction laboratory of the Vrije Universiteit Brussel at solving one of these problems. This technique of soil stabilization is not well known in Cameroon though soils suitable for the technique are likely quite abundant given the local climatic conditions and geology. With this technique, stone-like materials are produced from kaolinitic soils at atmospheric pressures and low temperatures. This requires less equipment and is less expensive to produce and also more environmentally friendly since rapid deforestation due to utilization of wood for energy and the emission of large quantities of carbon dioxide to the atmosphere will be greatly reduced. The products of this technique can be used as structural materials with attractive properties in construction and other applications.

1.4 Objectives Based on the problems outlined above, four soil samples were collected from Cameroon, whose quality and suitability for construction purposes will be evaluated using the Mineral Polymerization (MIP) technique. We aim at producing high quality and durable construction materials using an environmentally friendly and easy to apply technique at low cost. Secondly, different relationships between materials compositions and manufacturing processes, physical and mechanical properties and durability of the materials produced will be assessed.

Ndofor Akongnui Fai

PHYLARES 2006

4

Chapter-2

Literature Review

2.1 Improvement of Soils for Construction The most common soils which are used for construction in Cameroon are lateritic soils. In regions where it is available and cheap, wood is commonly used but its greatest short coming is its low durability since it is hardly seasoned and treated to fight against insect attacks and humidity. Lateritic soils are the products of the intensive weathering that occur as a result of the tropical and sub-tropical climatic conditions in this country. These soils are generally rich in secondary oxides and sesquioxides of iron and/or aluminum. They are nearly devoid of bases and primary silicates but may contain large amounts of secondary quartz and kaolinite. [2] The most common and simplest process for the manufacturing of bricks from lateritic soils in most countries in Africa consists in taking these soils and drying them in open-air [3]. Given the nature of the type of soils found in this region, it is rather difficult to manufacture bricks by the traditional process of firing at high temperature around 900°C to 1100°C. Bricks manufactured simply by mixing the soils with water and drying in open-air have not been able to give good results in terms of dimensional stability, strength, stiffness, permeability and durability. With these problems encountered, there is thus a need to look for means of improving these soils. Efforts have been made of recent towards this and results have shown that blocks and bricks made from lateritic soils can be improved to produce masonry units with strengths high enough to meet building standards [4]. 2.1.1

Mechanical Stabilization

Mechanical stabilization is a very commonly used technique in Cameroon especially in road construction, where lateritic soils are used for sub-base and base course. This involves decreasing of the soil voids by mechanical means so as to increase the density and strength, and to achieve a decrease in compressibility, permeability and porosity [5]. Laboratory analysis on the soils is carried out in order to determine optimum conditions before this method is applied. The parameters mostly analyzed are the grain size range and optimum moisture content that will

Ndofor Akongnui Fai

PHYLARES 2006

5 give high density values after compaction. These laboratory analyses already constitute a limitation to average Cameroonians. Even with a proper application of this method, long-term problems still occur. The improved physical contact between the soil grains leads to an increase in strength and a reduction in porosity, which in turn leads to some reduction in water absorption and migration. Even though initial strength may be high, long-term stability cannot be assured. The improvement in physical strength alone is not enough to ensure that the ingress of water is significantly reduced on a longterm or permanent basis [5]. This short coming has been observed in many roads in Cameroon which gave good results at the time they were constructed. 2.1.2

Physical Techniques

Additional fines or aggregates maybe blended into a material to adjust its granulometric composition before compacting. This will result in a uniform, well graded, dense soil-aggregate mixture after compaction [6]. Various means of defining the assortment of sizes required to achieve the maximum density have been devised, of which the most common is to regard the material as a selection of spheres of decreasing size such that [7]: p=P

d D

n

…………………. (1)

Where p is the proportion of spheres smaller than d in diameter, P is the proportion of spheres smaller than D in diameter and n is the grading coefficient (D>d). For maximum density, n ranges from 0.33 to 0.5 This is a very cheap and easy to apply method, e.g. the use of sand in stabilizing fine grained soils rich in clays will lead to a decrease in their degree of plasticity and swelling. Using this method, lateritic soils in Kumasi Ghana were blended with alluvial gravel and this resulted in a more remarkable improvement in the particle size distribution with the resulting mixture producing strength gain and markedly enhanced material quality [8]. Four day soaked CBR values of at least 80% could be achieved in the laboratory for some of the blended specimens compared to about 46% for the unblended specimens. These blending techniques are very popular in Cameroon especially for road construction. A typical example where this was applied Ndofor Akongnui Fai

PHYLARES 2006

6 is the Yaoundé-Nsimalen-Mbalmayo road which was constructed in 2003 with the base course made of lateritic soils (75%) blended with crushed rocks (25%) of diameter 0-25mm. Based on the specifications in use, very good results were obtained (Razel Cameroun,2003). Soil reinforcement by incorporating components such as fibers, electrical or electro-osmosis treatment and the use of sand drains are other physical techniques which can be used to improve upon the properties of soils [5]. These methods require a lot of technical know how and money and are not commonly used in Cameroon. 2.1.3

Physico-chemical techniques – fired bricks

These techniques use a combination of both physical and chemical methods. A typical example is firing of bricks. When bricks are fired, the actual reactions which enable a suitable product to be formed are chemical while the heating itself is a physical process. Some people refer to this as thermal stabilization. Though sun-dried bricks are the most popular, heat stabilized bricks are also widely used in Cameroon. The technique of making fired bricks requires a lot of experience and knowledge. The problem of cost of energy and controlling baking temperatures has lead to a lot of research. This motivated Mbumbia et al [9] who worked on lateritic soils from Cameroon and found out that it is possible to produce strong bricks at lower temperatures without additives, simply by crushing the raw materials to obtain medium to fine particles. However the means to crush is a limitation to the average Cameroonian. Chinje and Monget [10] also tried to solve this problem of cost of energy. They constructed a traditional down-draught wood fired kiln for firing tests of clay bricks through which they discovered that massive dry wood gave better results than wet wood which is mostly used by local producers. They also discovered that adding saw dust of