Engineering Geological Properties of Transported

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General Introduction

Tropical soils in many ways have unique characteristics that can mainly be attributed to the compositions and micro-structures of material developed under humid soil-forming conditions. Most of these soils contain abundant iron and aluminium oxides due to the rapid breakdown of feldspars and ferromagnesian minerals, the removal of silica and bases and the concentration of iron and aluminium oxides or sesquioxides. Due to the high iron content and oxidation processes, these soils are more often than not red in colour. The chemical alteration of primary minerals under widely varying degrees of weathering intensity will produce clay minerals that vary in both their chemical and physical properties. The primary weathering processes are characterized by the rapid breakdown of feldspars and ferromagnesian minerals, the removal of silica and bases and the concentration of iron and aluminium oxides.
Different constituent elements are released into solution according to their solubility and resistance to weathering and are transported downward through the soil profile through percolating soil moisture (Scheffer and Schachtsabel, 1998). The overall decrease in percentage bases and silica content leads to the relative accumulation of weathering-resistant and stable iron and aluminium oxides and hydroxides such as goethite (FeOOH), hematite (Fe2O3) and gibbsite (Al(OH)3) (Hintermaier-Erhard and Zech, 1997). The distinctive red colour of many tropical soils can be attributed to the development of hematite in the material. Tropical red soils in this study refer to all non-lateritic tropical soils other than tropical “black” soils and which in reality may vary in colour (between shades of red, orange, yellow and brown), microstructure, mineralogy and grain size distribution. Due to the fact that soil formation in the tropics is a highly variable and often complex process, the classification of soils formed under such conditions as distinct and well-defined soil types appears to have been avoided in the past.
Much of the difficulty would also appear to stem from the inapplicability of ‘conventional’ soil mechanical concepts, developed almost exclusively from work on temperate soils, when applied directly to certain soils from tropical zones (Northmore et al., 1992). Tropical red soils are further also frequently recorded and loosely referred to in the literature as either ‘laterites’, ‘laterite soils’, ‘latosols’ or ‘lateritic soils’. This approach has led to the grouping of materials of largely varying characteristics and engineering behaviour under a single, all-encompassing term, resulting in the categorization of materials that may have significantly different chemical, geological and/or geotechnical properties as a single soil type, namely “tropical red soils”. Often no distinction is made between reporting the origin of the material to which the results pertain and so distinguishing between the results of residual and transported tropical red soils. Where specified however, the majority of available test results pertain to residual soils. Much has been written about the material characteristics and engineering behaviour of tropical soils and the sensitivity of these to the influence of test methods on the subsequent test results. The challenges associated with conventional index and strength tests for the identification and engineering classification of tropical soils as discussed in the literature exist primarily because of the dependency of such tests on the sample preparation process and the influence of the subsequent variation of the natural soil structure which occurs during preparation.
The usefulness of conventional index and strength tests for the identification and engineering classification of tropical soils has as such been questioned in the past, primarily because of the dependence of such tests on the sample preparation process and the variation of the natural soil structure which occurs during preparation. However, it has been shown that the unique engineering properties of tropical soils may be very useful for identifying and ‘grouping’ soils which are likely to possess distinctive or similar physical characteristics for specific or local application (Wesley, 1988). Conventional engineering classification tests are largely concerned with the determination of the index properties of soils. Index property tests were developed for temperate soils and are generally sufficient for these purposes. The properties can be easily assessed, such as their particle size distribution, plasticity limits or density through the test procedures as defined in the British and ASTM standards. These index tests are carried out on disturbed or remoulded soil samples and are thus independent of the in-situ characteristics of the soil (Northmore et al., 1992). It can therefore be argued that, by regarding the particle size and consistency limits as the basis of soil classification, the influence of mineralogy, chemistry and origin of a soil on its mechanical behaviour is adequately measured by these simple index tests (Schofield and Wroth, 1968). This cannot necessarily be assumed to be the case for tropical red soils. The inherent mineralogical and chemical composition and structure (in the case of residual soils) of the material has a pronounced influence on the engineering behaviour and measurement of the index and mechanical properties of tropical red soils.
Because of the mineralogical composition of tropical red soils, even partial air-drying at ambient laboratory temperatures may change the structure and physical properties of these soils. Some of these changes cannot be reversed when the soil is re-mixed with water. These structural changes are reflected in sometimes drastic changes in the index properties derived from plasticity, shrinkage and particle size tests or specific gravity. Conventional laboratory testing procedures, as defined in the British and ASTM standards, are therefore not necessarily applicable to tropical red soils without some modification or change in focus (Northmore et al., 1992) to take into account the unique mineralogical and chemical composition and structure. A detailed literature review was completed of the general characteristics and geotechnical properties of tropical red soils in an effort to evaluate and review the unique variations in the physical and chemical characteristics and subsequent engineering behaviour of tropical red soils compared to soils from more temperate climatic regions. An in-depth geotechnical investigation of a number of tropical red soil horizons of colluvial origin and identified in a selected area in the tropics is presented in the form of a case study, with the results and evaluations from the study compared to the findings and conclusions summarised from existing literature on tropical red soils.
The initial part of this thesis (Chapters 3 and 4) is a review of the researched physical and chemical characteristics and established engineering behaviour of a number of residual tropical red soils in the literature, as well as providing an overview of the selected study site and method of investigation. Chapters 5 to 8 quantify and evaluate the established characteristics of the tropical red soils covering the study area. The identification and evaluation process of the engineering characteristics of the material and any variations to standard test procedures, making provision for the differences in material properties of tropical soils, are discussed in detail. The variation in soil suction pressures with a change in natural moisture content is investigated and the results obtained using two different methods of determination compared, as well as providing conclusions on the findings and results of the research completed in the study area against those detailed in the existing literature.

TABLE OF CONTENTS :

• 1. General Introduction
• 2. Overall Aim and Specific Objectives
• 3. A Review of Tropical Red Soils
  • 3.1 Soil Formation in the Tropics
  • 3.2 Identification and Classification of Tropical Red Soils
    • 3.2.1 Pedological Maps and Soil Classification Systems
    • 3.2.2 Geological Maps
    • 3.2.3 Application
  • 3.3 Tropical Red Soils and Laterites
  • 3.4 Description of the Tropical Weathering Profile
    • 3.4.1 Morphological Properties
    • 3.4.2 Classification by Secondary Mineral Content
    • 3.4.3 Classification by Saturation Moisture Content
  • 3.5 Engineering Characteristics of Residual Tropical Red Soils
    • 3.5.1 Index Properties
    • 3.5.2 Strength and Consolidation Characteristics
    • 3.5.3 Compaction Characteristics
    • 3.5.4 Permeability
• 4. Investigation of Engineering Geological Properties of Transported
  • Tropical Red Soil
  • 4.1 Case Study Area
    • 4.1.1 Introduction
    • 4.1.2 Topography
    • 4.1.3 Climate
    • 4.1.4 Geology
  • 4.1.4.1 Regional Geology
  • 4.1.4.2 Local Geology and Stratigraphy
  • 4.2 Methodology
  • 4.2.1 Profiling and Sampling
    • 4.2.2 Geotechnical Drilling
    • 4.2.3 Laboratory Testing
    • 4.2.3.1 Chemical and XRD/XRF Analyses
    • 4.2.3.2 Index Properties
  • i) Particle Size Distribution
  • ii) Particle Density
  • iii) Bulk density
  • iv) Void ratio
  • v) Linear Shrinkage
  • vi) Natural Moisture Content
  • vii) Atterberg Limits
  • 4.2.3.3 Mechanical Properties
  • i) Tri-axial Testing
  • ii) Consolidation
  • iii) Proctor Compaction
  • iv) Permeability
  • v) Dispersivity
  • vi) Swelling Pressure
  • 4.2.3.4 Unsaturated Soil Behaviour
  • i) Filter Paper Method
  • ii) Pressure Plate Extractors
• 5. Chemical and Mineralogical Properties
  • 5.1 XRF Results
  • 5.1.1 XRF Results – Metallurgical Characterization of Transported Ore
  • 5.1.2 XRF Results – Geotechnical Characterization
  • 5.2 XRD Results
  • 5.2.1 XRD Results – Metallurgical Characterization
  • 5.2.2 XRD Results – Geotechnical Characterization
  • 5.3 Chemical Properties
  • 5.3.1 Cation Exchange Capacity (CEC)
  • 5.3.2 pH
  • 5.3.3 Conductivity
  • 5.3.4 Dispersivity
  • 5.4 Discussion
• 6. Index Properties
  • 6.1 Particle Size Distribution
  • 6.2 Atterberg Limits
  • 6.3 Natural Moisture Content
  • 6.4 Particle Density
  • 6.5 Discussion
  • 6.5.1 Particle Size Distribution
  • 6.5.2 Atterberg Limits
  • 6.5.3 Natural Moisture Content
  • 6.5.4 Particle Density
• 7. Mechanical Properties
  • 7.1 Shear Strength
  • 7.2 Compaction
  • 7.3 Consolidation
  • 7.4 Swell Pressure
  • 7.5 Permeability
  • 7.6 Discussion
  • 7.6.1 Shear Strength
  • 7.6.2 Compaction Characteristics
  • 7.6.3 Consolidation
  • 7.6.4 Permeability
• 8. Suction
  • 8.1 Evaluation of Soil Suction – Filter Paper Method
  • 8.2 Evaluation of Soil Suction – Pressure Plate Extractor Apparatus
  • 8.3 Discussion
• 9. Conclusions and Recommendations
  • 9.1 Classification and Nomenclature of Tropical Soils
  • 9.2 Sampling and Sample Disturbance
  • 9.3 Effect of Dispersing Agent on Particle Size Distribution
  • 9.4 Sensitivity to Change in Natural Moisture Content and Drying of Test Specimens
  • 9.5 Index Properties
  • 9.6 Mechanical Properties and Permeability
  • 9.7 Suction
  • 9.8 Recommendations and Concluding Remarks
• 10. References

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