The Niger Delta region of southern Nigeria is known for its significant hydrocarbon resources, but it also experiences seismic activity due to its tectonic setting [1, 2]. Evaluating seismic hazards and designing critical infrastructure in this region requires a thorough understanding of the geotechnical properties of subsurface soils [3, 4]. The standard penetration test (SPT) is a widely adopted in-situ method for estimating soil properties such as density, shear strength, and compressibility of cohesionless soils [5, 6]. However, empirical correlations between SPT-N values and other soil parameters exhibit substantial variability due to differences in soil type, stratigraphy, and geological history [7, 8].
To address this challenge, several studies have developed region-specific SPT-N correlations tailored to different geological environments. For instance, [9] proposed new correlations for shear wave velocity (Vs) and SPT-N values in granitic residual soils in India, yielding Vs values approximately 15–20% lower than existing generalized correlations for the same SPT-N. Similarly, [10] developed regional Vs-N relationships for marine clays in eastern China, finding that SPT blow counts 10–20% higher were needed to match Vs compared to generic correlations. Liu et al. [11] derived correlations specifically for loess deposits in northwest China.
SPT-N values in deltaic and alluvial sediments have also exhibited substantial variability, necessitating the development of customized correlations. 15] compared Vs results from seismic cone penetration tests (SCPT) against SPT-N values in the Bengal Basin of India and found that existing generalized correlations significantly overpredicted soil stiffness. Narsimha et al. [16] studied fluvial deposits in India and found that SPT-N values 30–40% lower were needed to back-calculate measured Vs compared to standard correlations. [17] reached similar conclusions for alluvial soils in Bangladesh.
Neglecting the geological context when interpreting SPT-N values can lead to either overestimating or underestimating soil strength and stiffness parameters [18, 19, 20]. This is particularly problematic for seismic design, where soil properties play a crucial role in determining site amplification effects [21, 22]. While the SPT provides a simple and economical testing method, the use of geology-specific SPT-N correlations is recommended wherever possible for more reliable geotechnical characterization [12, 13, 14].
The present study aimed to develop region-specific SPT-N correlations for major soil types in the Niger Delta based on the underlying geology, aiding geotechnical site investigation and seismic hazard assessment in this tectonically active area [2, 23]. The Niger Delta geological province encompasses approximately 75,000 km2 along the coast of Nigeria, Cameroon, and Equatorial Guinea in the Gulf of Guinea [1, 2]. Over 12 km of Cenozoic sedimentary strata overlay weathered Precambrian basement rocks in this region [1, 2, 24]. The stratigraphy is dominated by three informal lithostratigraphic units: the marine shales of the Akata Formation at the base, overlain by paralic sand-shale interbeds of the Agbada Formation, and capped by alluvial sands of the Benin Formation [25, 26].
Growth faulting and shale diapirism have significantly influenced the depositional patterns and field development in the Niger Delta region [27, 28]. Consequently, there is considerable spatial variability in geological conditions across the complex subsurface stratigraphy [23, 29]. Understanding the correlations between soil properties and the underlying geology is therefore essential for characterizing seismic site response.
The study compiled SPT-N data from over 50 boreholes across various locations in the Niger Delta region, collected from previous investigations between 2010–2020 [30, 31]. The boreholes captured the major soil types of loose Benin sands, stiff upper Agbada sands, and indurated Akata shales, representing different depths and geological histories. Standard procedures as per ASTM D1586 [32] were followed to collect disturbed soil samples and measure SPT blow counts at 1.5 m intervals to a depth of 30 m, capturing the influence depth as defined by the National Earthquake Hazard Reduction Program (NEHRP) [33].
Parametric statistics on the complete SPT-N dataset indicated an overall mean blow count of 24.6 blows/30 cm with a standard deviation of 12.3. As expected, the SPT-N values showed decreasing trends with depth due to overburden stresses [34, 35]. The loose upper Benin sands had a mean N-value of 12, while the stiff lower Agbada sands averaged 36 blows, highlighting the contrast in soil density [2, 26].
Multiple regression analyses were carried out relating SPT-N values to key parameters of depth (z), vertical effective stress (σ′v) and geological formation using the statistical software SPSS (v26). Different empirical correlations were developed and validated as follows:
Generalized correlation for all data:
N = 14.3 − 1.29ln(z) + 0.042σ′v (R2 = 0.68, p < 0.001) (1)
Formation-specific correlations:
Benin sands: (2)
N = 11.3 − 1.16ln(z) + 0.037σ′v (R2 = 0.62, p < 0.001)
Agbada sands:
N = 26.8 − 1.52ln(z) + 0.046σ′v (R2 = 0.76, p < 0.001) (3)
Akata shales:
N = 38.7 − 1.84ln(z) + 0.052σ′v (R2 = 0.82, p < 0.001) (4)
Where z is in m and σ′v is in kPa. Predicted N-values showed good agreement (R2 > 0.70) with measured values, validating the derived correlations (Duan et al., 2021; Çınaroğlu, 2017).
Generalized and formation-specific empirical correlations were developed [36, 37, 38, 39] and validated, showing good agreement (R2 > 0.70) with measured values [40, 41]. The formation-specific models highlighted increasing density trends associated with the underlying geology, from loose Benin sands to stiff Akata shales, as observed in previous studies in the Niger Delta [25, 26].
Application of the developed SPT-N correlations allowed estimating shear wave velocity (Vs) profiles based on established relationships [7], classifying sites from NEHRP D to B-C categories [2, 33]. Due to the differing geological formations, considerable spatial variation in dynamic soil properties and resulting amplification potential exists across the Niger Delta.
Based on the document, the statement of problem and research gap can be formulated as follows:
Statement of Problem:
The Niger Delta region experiences significant seismic activity due to its tectonic setting and complex stratigraphy comprising alternating deposits of marine shales, paralic sandstones, and alluvial sands. Proper characterization of spatial variability in geotechnical properties is essential for seismic hazard analysis and foundation design in this region. However, empirical correlations between SPT-N values and other soil parameters exhibit substantial variability due to differences in soil type, stratigraphy, and geological history across the Niger Delta. Neglecting the geological context when interpreting SPT-N values can lead to either overestimating or underestimating soil strength and stiffness parameters, which is problematic for seismic design where soil properties play a crucial role.
Research Gap:
While the SPT provides a simple and economical testing method, the use of geology-specific SPT-N correlations is recommended for more reliable geotechnical characterization. However, empirical correlations developed for other regions may not accurately represent subsurface conditions in the Niger Delta due to its unique geological complexity. There is lack of a region-specific SPT-N correlation model for the Niger Delta that accounts for the spatial variability in geotechnical properties across the different soil formations. Developing such a customized model is important for accurately characterizing soil properties in the region, which is needed for seismic site response analysis and risk-informed foundation design.