DETERMINING THE EFFECT OF PARTICLE SIZE DISTRIBUTION ON SHEAR BEHAVIOR OF SOIL

CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

Soil behavior is a fundamental factor in geotechnical engineering, influencing the design and stability of infrastructure, including buildings, roads, and dams. Among the soil properties critical to engineering applications, shear strength is paramount as it determines a soil's resistance to deformation and failure under stress. Shear strength is profoundly affected by particle size distribution (PSD), a key parameter describing the range and proportion of particle sizes within a soil mass (Gee & Or, 2002). PSD directly impacts the soil's porosity, permeability, and mechanical stability, making it a crucial focus in the study of soil behavior.

Particle size distribution affects the contact points and frictional resistance between soil grains. Soils with a broader PSD often demonstrate higher shear strength due to better particle interlocking (Morgan, 1999). Conversely, uniformly graded soils, where particles are of similar size, may exhibit reduced shear strength as fewer inter-particle contacts develop. Gu et al. (2017) emphasized that PSD significantly influences the stiffness of granular soils, particularly at small strain levels. Their discrete element modeling highlighted the critical role of fine and coarse particle ratios in determining mechanical properties.

The unique geotechnical characteristics of Nigerian soils add another layer of complexity to understanding the influence of PSD. Nigeria's diverse geological zones encompass a wide variety of soil types, ranging from laterites in the tropical regions to sandy soils in arid areas. These soils often exhibit distinctive PSD profiles that are shaped by weathering, erosion, and sediment deposition processes (Raei et al., 2015). For instance, lateritic soils, abundant in iron and aluminum oxides, often feature a bimodal PSD that influences their compaction and shear strength characteristics. This makes studying the relationship between PSD and shear behavior particularly relevant for engineering applications in Nigeria.

The influence of PSD extends to critical engineering applications such as slope stability, foundation design, and earthworks. For example, particle size has been shown to affect the interface shear behavior between soil and reinforcement materials. Vangla and Gali (2016) investigated the role of sand particle size in geotextile applications, concluding that finer sands exhibited higher interface shear resistance due to greater contact area and friction. Similarly, Wang et al. (2016) explored the effects of particle size on coarse soil-geogrid interfaces, finding that larger particles tended to reduce post-cyclic shear strength, underscoring the need for careful consideration of PSD in soil-structure interactions.

In addition to engineering implications, PSD plays a crucial role in soil erosion and environmental sustainability. Raei et al. (2015) compared the initial motion of soil aggregates with sand particles of varying sizes, demonstrating that larger aggregates are more resistant to erosion forces. This finding is particularly pertinent to regions like Nigeria, where soil erosion is a prevalent environmental challenge. Understanding how PSD influences soil stability can inform erosion control measures and sustainable land management practices.

The dynamic interaction between PSD and shear behavior is further influenced by factors such as moisture content, particle shape, and loading conditions. Jiang et al. (2016) highlighted the impact of shear speed on granular materials, showing that higher shear speeds can exacerbate frictional instability, especially in soils with poorly graded PSD. This underscores the complexity of soil behavior under varying loading scenarios, which are common in real-world applications.

While extensive research has been conducted globally on PSD and soil mechanics, there is a paucity of studies specifically addressing Nigerian soils. This research gap is significant, considering the critical role of soil in Nigeria’s rapidly growing infrastructure sector. Stefanow and Dudziński (2021) emphasized that state-of-the-art methods for determining soil shear strength must be adapted to regional contexts, considering local soil characteristics and engineering needs. This study aims to bridge this gap by investigating the effect of PSD on the shear behavior of Nigerian soils, providing insights that can enhance geotechnical design and construction practices.

1.2 Statement of the Research Problem

The relationship between particle size distribution and soil shear behavior is well-established in the field of geotechnical engineering. However, the application of these principles to the specific context of Nigerian soils remains underexplored. This knowledge gap poses challenges for engineers and researchers attempting to predict and optimize soil performance for infrastructure development in Nigeria's diverse geotechnical landscape. Despite the availability of sophisticated soil analysis techniques, such as those outlined by Loveland and Whalley (2000), their use in analyzing Nigerian soils has been limited, often resulting in suboptimal design and increased project costs.

One of the primary challenges lies in the variability of Nigerian soils, which are shaped by diverse climatic, geological, and anthropogenic factors. For instance, lateritic soils, prevalent in southern Nigeria, differ significantly in composition and PSD from the sandy soils of the northern regions. The lack of a comprehensive understanding of how these variations in PSD influence shear behavior undermines the reliability of geotechnical designs, particularly for critical structures such as dams, bridges, and retaining walls (Gu et al., 2017).

Another issue is the lack of standardization in soil testing practices across Nigeria. While global studies, such as those by Wang et al. (2016), have emphasized the importance of controlled laboratory conditions for assessing soil properties, such rigor is often lacking in local contexts. This can lead to discrepancies in test results, making it difficult to establish consistent correlations between PSD and shear strength. Furthermore, environmental factors such as erosion and land degradation, which are widespread in Nigeria, exacerbate the variability in soil properties, adding to the complexity of predicting shear behavior (Raei et al., 2015).

The limited integration of regional data into geotechnical modeling tools further compounds the problem. Many existing models are based on studies conducted in temperate regions, which may not accurately reflect the behavior of tropical soils like those in Nigeria. This disconnect highlights the need for localized research to provide data-driven insights and practical recommendations for the engineering community.

In light of these challenges, this study seeks to address the critical gap in understanding the effect of particle size distribution on the shear behavior of Nigerian soils. By employing advanced soil analysis techniques and focusing on regional soil types, the research aims to contribute to the development of more reliable and efficient geotechnical practices tailored to Nigeria's unique conditions.

1.3 Objectives of the Study

The primary objective of this study is to investigate the effect of particle size distribution (PSD) on the shear behavior of Farin Gida soil in Kaduna State, Nigeria. The specific objectives are:

1. To analyze the particle size distribution of Farin Gida soil using standard laboratory methods.

2. To evaluate the shear strength parameters of Farin Gida soil under varying PSD conditions.

3. To establish the relationship between PSD and shear behavior for Farin Gida soil.

4. To compare the findings with existing studies on similar soils in Nigeria and other regions.

5. To provide recommendations for geotechnical design and construction practices in areas with similar soil characteristics.

1.4 Research Questions

This study seeks to answer the following research questions:

1. What is the particle size distribution profile of Farin Gida soil in Kaduna State?

2. How do variations in PSD influence the shear strength parameters of Farin Gida soil?

3. What is the nature of the relationship between PSD and the shear behavior of this soil?

4. How do the results for Farin Gida soil compare to findings from other studies on Nigerian soils?

5. What geotechnical recommendations can be derived from the study's findings?

1.5 Significance of the Study

This study holds significant implications for both academic research and practical applications:

Academic Contribution
The research addresses a critical gap in understanding the geotechnical properties of Farin Gida soil, a representative soil type in Kaduna State. It builds on the limited body of work on Nigerian soils, providing data that can inform future studies and refine existing models of soil behavior under varying PSD conditions.

Practical Relevance
For geotechnical engineers, the study provides valuable insights into the relationship between PSD and shear behavior, which are crucial for designing stable and cost-effective structures in Kaduna State and similar regions. Specifically, the findings can guide decisions on foundation design, slope stability, and earthworks, minimizing the risk of structural failure due to soil instability.

Policy and Environmental Implications
The study’s results can also inform land use planning and soil conservation strategies, particularly in areas prone to erosion. By understanding how PSD impacts shear strength, policymakers can develop targeted interventions to mitigate land degradation in Kaduna State.

1.6 Scope and Limitations of the Study

Scope
This study focuses exclusively on the Farin Gida soil in Kaduna State, examining its PSD and shear behavior through laboratory experiments. The research emphasizes geotechnical applications, particularly in the context of construction and infrastructure development.

Limitations

The study is confined to the Farin Gida soil, and its findings may not be directly applicable to other soil types in Nigeria.

Laboratory conditions may not fully replicate field conditions, which could introduce variability in results when applied to real-world scenarios.

The research does not account for the long-term effects of environmental changes, such as climate variations and erosion, on PSD and shear behavior.

1.7 Definition of Key Terms

Particle Size Distribution (PSD): A measurement of the range and proportion of different particle sizes in a soil sample, typically expressed as a cumulative percentage.

Shear Strength: The resistance of soil to deformation or failure under applied shear stress, influenced by factors such as cohesion and friction.

Farin Gida Soil: A specific soil type found in Farin Gida, Kaduna State, characterized by its unique geotechnical properties.

Geotechnical Engineering: A branch of civil engineering concerned with the behavior of earth materials and their application in construction.

Soil Stability: The ability of soil to resist deformation and maintain structural integrity under stress.

Lateritic Soil: A type of soil rich in iron and aluminum oxides, commonly found in tropical regions, including parts of Nigeria.