Seismic Site Classification and Correlation Between SPT N-Value and Shear Wave Velocity in The Niger Delta Region

This study investigates seismic site classification and the empirical correlations between SPT blow counts (N-values) and shear wave velocity (Vs) in the Niger Delta region of Nigeria. Accurate site classification is vital for evaluating seismic ground response and liquefaction potential in earthquake-prone areas like the Niger Delta. To overcome the challenges of costly and time-consuming direct geophysical measurements of shear wave velocity, the study utilizes the SPT as an in-situ test to indirectly estimate Vs through empirical correlations. The research involves subsurface exploration at two sites in Agudama Town, Bayelsa State. SPT borings were conducted to depths of 30m, collecting disturbed soil samples at 1.5m intervals and undisturbed samples from deeper cohesive layers. Laboratory testing was performed to classify the soils according to the Unified Soil Classification System and determine key engineering properties. Grain size analyses revealed a transition from sandy soils near the surface to silty clay with increasing depth, attributed to natural sorting processes in deltaic sediments. Consistency limits indicated low to intermediate plasticity soils in the upper layers, transitioning to high plasticity at greater depths. Downhole geophysical tests directly measured Vs profiles, which, combined with SPT blow counts, facilitated site classification based on the NEHRP and Eurocode 8 systems. Additionally, one-dimensional equivalent linear site response analysis estimated amplification factors across various strains and periods, predicting peak amplifications of 0.4 to 0.7 seconds that aligned with the subsurface conditions. Empirical correlations were established to relate SPT-N values to Vs, achieving R2 values exceeding 0.90. Predictive models for liquefaction potential factor of safety demonstrated good agreement with observed trends. These findings offer valuable insights into subsurface conditions and establish initial predictive tools for seismic hazard assessment in the soft deltaic soils of the Niger Delta region. Further studies are necessary to expand the database, refine correlations and classifications, and account for geological variability in this seismically active area. These investigations will enhance the understanding of the region's subsurface characteristics and improve the accuracy of seismic hazard assessments.