Geotechnical Evaluation of the Influence of Saltwater Intrusion on the Shear Strength Properties of Coastal Soils: A Case Study of Ikot Abasi L.G.A.

Introduction/Objective Saltwater intrusion can significantly alter the geotechnical properties of coastal soils, with implications for infrastructure stability and environmental management. This study investigated the effects of salinity on the shear strength parameters of coastal soils in Ikot Abasi Local Government Area, Nigeria. Methods Laboratory tests, including triaxial, direct shear, Atterberg limits, and compaction tests, were conducted on soil samples subjected to different salinity levels. Samples were collected from four locations within the coastal zone and designated as Points A, B, C, and D for identification. Results Grain-size analysis classified soils from Points A, B, and C as clayey sand (SC), while Point D was classified as well-graded sand with silt (SW-SM) according to the Unified Soil Classification System (USCS). Accordingly, Points A–C exhibited cohesive behavior, whereas Point D was predominantly non-cohesive. Saltwater intrusion was simulated using NaCl solutions prepared with distilled water. Increasing salinity reduced the liquid limit of Points A–C by 23.2–32.0% and the plastic limit by 7.02–14.6%. Similarly, OMC decreased while MDD increased with increasing salt concentration. Cohesion increased progressively with salinity, from 34% to 48% at Point A, 42% to 58% at Point B, and 17.3% to 26.0% at Point C, whereas the corresponding variations in the angle of internal friction (ϕ) were nonlinear. At Point D, ϕ decreased progressively from 31.34° to 24.86° with increasing salinity. Discussion Increasing salinity reduced the liquid limit and the plasticity index of soils from Points A, B, and C, whereas Point D remained non-plastic. The reduction in PI indicates a narrower plasticity range and reduced susceptibility to shrink–swell behavior. The observed decrease in OMC and increase in MDD with increased salinity may be associated with salinity-induced changes in clay-particle interaction and soil fabric. Triaxial tests further showed that cohesion increased with salinity for the cohesive soils at Points A, B, and C, although the corresponding variations in friction angle (ϕ) were nonlinear. In contrast, Point D exhibited a progressive decline in ϕ, with a reduction of approximately 6.48° between distilled water and 15% saline water. The failure-envelope intercepts (−0.10 to +0.04 kPa) were close to zero, indicating negligible cohesion and confirming that shear strength at Point D was governed mainly by friction. Conclusion The results of this study demonstrate that increasing salinity was associated with the changes observed in the shear strength parameters and mechanical behavior of coastal soils. These findings highlight the need to account for salinity-induced changes in geotechnical design and assessment of coastal infrastructure.