An appraisal of the problems related to the application of Casagrande Plasticity Chart and Unified Soil Classification System in classifying lateritic soils in Nigeria

Abstract Lateritic soil is the predominant soil type in Nigeria, making its accurate classification crucial for understanding its engineering properties in foundation and construction applications. Various classification systems for residual soils exist, based on pedological, geochemical, or engineering criteria. However, the Unified Soil Classification System (USCS) is widely used globally for classifying lateritic soil. It has been noted that the field behavior of many lateritic soils classified under the USCS often does not correspond with their positions on the Casagrande Chart. This review paper examines the classification challenges associated with lateritic soils in Nigeria using both the Casagrande Plasticity Chart and the USCS, focusing on the soil properties that contribute to these difficulties. Existing literature detailing the consistency limits, plasticity characteristics, and grain size distributions of lateritic soils derived from various parent rocks was analyzed to construct a Casagrande Plasticity (CP) classification chart specific to Nigerian lateritic soils. The analysis indicates that most lateritic soils fall above the A-line, in regions typically designated for clay soils, leading to their classification as either low plasticity clay (CL) or high plasticity clay (CH). However, these classifications often do not reflect the primary compositional characteristics of the soils. This discrepancy is likely due to the limitations of the classification tests used for lateritic soils. Factors such as coating and cementing agents, degree of pre-drying, chemical treatment, and mechanical instability can affect the test results. Therefore, it is essential to exercise caution when using the Casagrande chart for classifying lateritic soils, considering the unique attributes of residual soils, including their in-situ structures and properties influenced by weathering effects. Recognizing the limitations and understanding the weaknesses of current classification systems can facilitate the development of more appropriate methods and better evaluation techniques for these critical construction materials.