Engineering Behaviour of Bentonite-Modified Coastal Plain Sands from Southeastern Nigeria: Integrated Interpretation of Plasticity, Compaction and Microstructural Evolution
Coastal Plain Sands are widely distributed across southeastern Nigeria and are extensively used as foundation, embankment, and pavement construction materials. However, their predominantly sandy composition and low natural fines often result in poor plasticity, limited cohesion, and engineering behaviour that may require improvement for demanding geotechnical applications. This study investigates the influence of bentonite modification on the plasticity and compaction characteristics of Coastal Plain Sands obtained from Akanu, Mgboko Itungwa Ward, Obingwa Local Government Area, Abia State, Nigeria. Three representative soil samples (S1, S2, and S3) were blended with bentonite at varying proportions ranging from 0% to 50% by dry weight. Laboratory testing included particle-size distribution analysis, Atterberg limits, and Standard Proctor compaction tests in accordance with relevant ASTM standards. The natural soils consisted predominantly of sand-sized particles (95.76–97.93%) with low fines contents (2.07–4.24%) and were classified as poorly graded sand (SP) under the Unified Soil Classification System. Bentonite addition produced systematic increases in liquid limit, plasticity index, maximum dry unit weight, and optimum moisture content, reflecting progressive modification of the soil fabric. The results demonstrate that bentonite initially improves particle packing through intergranular void filling and subsequently promotes clay coating of sand grains and physicochemical bonding, resulting in the evolution of a cohesive granular–clay composite. An integrated conceptual model is proposed to explain the observed relationships between particle-size distribution, plasticity, and compaction behaviour through progressive microstructural evolution. The findings demonstrate that bentonite functions simultaneously as a physical void-filling material and a physicochemical modifier, providing a mechanistic basis for optimizing the engineering performance of Coastal Plain Sands in applications such as compacted liners, hydraulic barriers, pavement subgrades, embankments, and engineered fills.
Keywords: Bentonite modification; Coastal Plain Sands; Compaction characteristics; Plasticity; Microstructural evolution; Geotechnical engineering
