Beach ridges and barrier islands along the Nigerian coastline are a critical geomorphic unit which serve as

foundation for several significant engineering structures, yet their geotechnical behavior remains unevenly

characterized. This study synthesizes existing sedimentologic, geotechnical, and geophysical data to

characterize the engineering properties of Nigerian beach‐ridge and barrier–island systems, with emphasis on

the barrier–lagoon complex of Lagos, Brass, Bonny and Eket. The study identified hidden, shallow,

incompetent, and highly compressible plastic clay/peat layers beneath the predominant seemingly stable sand,

which can lead to significant consolidation settlement or building collapse. The near-surface sediments reveal

poorly graded sands or silty sands, with high natural moisture content, low plasticity, moderate to high

hydraulic conductivity, and decreasing relative density and relative compaction within the depth of

foundation of 100kN/m

resistance of the soil at 3.5m depth is not less than 10% of the structural load placed on the surface. The

combined evidence indicates that while Nigerian beach ridges and barrier islands often provide adequate near-

surface materials for light to moderate structures, careful site-specific characterization is essential to account

for underlying soft sediments, shoreline erosion, and associated geohazard risks.

Keywords: Geotechnical, Resistivity, Characterization, Beach Ridges, SPT(N), Relative density, Compaction,

Compressibility

Coastal beach ridges in Nigeria host and serve as foundations for storage tanks, pump stations, pipelines and

conductive and severely corrosive. In such cases, according to Abam & Ngah, (2014) resistivity surveys along

an onshore pipeline route are required to quantify the extent of corrosivity and guide cathodic protection (CP)

and groundbed design. Geo-electric characterization therefore provides baseline data to size CP systems, select

coatings, and place anode beds to avoid interference with existing systems. Consequently, understanding their

geological and geotechnical characteristics is crucial for the satisfactory performance of this infrastructure

during their service life.

According to Ishola et al., (2022) and Adamu et al., (2024) geotechnical and geo-electric characterization of

beach ridges provide insights into shallow subsurface conditions and potential construction challenges. The

integration of geotechnical and geophysical technics combines the complimentary perspectives of both

methods and therefore enhances the understanding of these structures, ensuring safer and more effective

with engineering practices to ensure that construction projects are designed to withstand environmental

changes and geological challenges. Okiotor (2023) on the other hand investigated the geological structure

beneath Okerenkoko Primary School, identifying four geoelectric layers, including unstable peat/clay/sandy

clay. This characterization is crucial for assessing site suitability for civil construction and understanding

subsurface conditions affecting structural integrity. Oyeyemi et al., (2020) on the other hand conducted subsoil

characterization using a combination of geotechnical and geophysical tests, revealing lithological units of loose

silty sand and compacted clayey sand, which are crucial for determining suitable foundation types for civil

construction, particularly pile foundations. Similarly, geotechnical techniques were used to characterize near-

subsurface geomaterials in Lagos, identifying lithologies like loose sand and clay/peat, which are crucial for

assessing foundation stability in civil construction projects (Oyeyemi et al 2016). Table (1) provides a summary

According to Otvos (2000), Taylor and Stone (1996) Beach ridges are relict semi-parallel, multiple wave- and

wind-built landforms that originated in the inter- and supratidal zones formed through sediment deposition and

represent ancient shorelines, providing a record of coastal evolution during the Quaternary period. Beach

been documented severally (Adegoke et al 2010, Obowu and Abam 2014, Dada et al., 2016). Studies have

shown that these ridges can reach significant thicknesses (Johnston et al., 2002), with depositional units

identified through Ground Penetrating Radar (GPR). Abam (2016) described the Beach Ridge as constituting

the shoreline interface with the Atlantic Ocean in the Niger delta as evident in Fig (1) and comprises well

sorted uniformly graded fine sand, deposited and densified by wave and tidal action.

Brass

Figure 1: Geological map of the Niger Delta (Reijers, 2011)

October) and calmer conditions during the dry season (Dada et al., 2016). Steeper, coarser-grained beaches,

like those around Lagos, foster plunging breakers, while gentle, low-gradient beaches as in Bonny tend to have

spilling breakers. Coastal engineering projects like port construction can interfere with longshore sand

transport and significantly alter the coastline. Sea-level rise and increased wave run-up exacerbate coastal

erosion and flooding events in settlements along the Nigerian coastline. It follows therefore that while beach

ridges offer valuable geological insights, their dynamic nature and susceptibility to erosion and climate change

pose challenges for long-term civil engineering projects.

microprocessor and classifying the layers based on their corrosivity (Table 2). The investigation utilized the

Wenner electrode configuration of the Vertical Electrical Sounding (VES) technique, which is well-suited for

shallow subsurface profiling and is particularly effective in evaluating near-surface soil corrosivity along linear

infrastructure such as pipelines.

Table 2: A correlation of soil resistivity with soil corrosivity (Safeway, 2018)

In this study, twenty-eight (28) shallow electrical resistivity measurements were acquired along a 14.0 km

segment of the Beach ridge. Since the investigation was concerned with depths of engineering significance, a

total electrode spread of 30 meters was adopted, providing an effective depth of investigation for pipelines that

are buried between 1.5m and 5m. In general, the measured apparent resistivity value corresponds to the average

resistivity of the soil at a depth roughly equivalent to 30% of the total current electrode separation. For instance,

if the outer (current) electrodes are spaced 20 meters apart, the estimated investigation depth is approximately

6.0 meters.

Additionally, deep resistivity sounding using the Schlumberger array configuration was conducted at strategic

well sites to investigate subsurface resistivity profiles down to depths above 60 meters. These measurements

The Angle of Internal Friction (Deg) was derived using Dalai & Patra (2019) =8.103*N

Cu = q

σ

Total consolidation settlement, p

= H Log

Po

C

Po

Where σ

Po

= effective overburden pressure at the center of the layer before any excavation or load application.

These relationships were reliably used to estimate settlement.

and 3). Table 3 is also sample of summary interpreted of geo-electric layers, depth, thicknesses and corrosivity

rating for soils at all sounding locations. A sample ground model of the VES output for deep groundbed is

presented in Fig (4) showing the significant occurrence of silty clay soil to serve a possible groundbed horizon

for the optimum cathodic protection of an oil and gas asset. Similarly, a linear surface topography of the profile

alignment was captured (Fig.5) so that the ultimate geo-electric ground model or tomograph can be corrected

for elevation to aid the decision of depth of pipeline placement. Figure 6 is the result of the generated geo-

resistivity section across the survey profile alignment for a pipeline case study within the project area,

synthesized with elevation corrected to reflect the natural topography. The horizons of low resistivity coincide

of shallow occurrences of clay and groundwater table and swampy zones surrounding creeks.

Figure 4: Geoelectric Section and ground bed potential for V1 VES survey point

(Fig. 7). In addition, were geo-rectified and transformed into a 3-D Fence diagram (Fig.8) to provide a 3-D

perspective of the sub-surface geology of the beach ridge. The soil profiles revealed a 3-layer soil structure

comprising top thin organic silty clay underlain by silty-sand and fine sand which is then followed by medium

dense to dense. This sandy units have occasional clay laminations as intercalations which occur in BH2, BH3,

BH5 and BH6. Apart from the top clay, the other occurrences of clay within the depth explored are non-

persistent and could nonetheless have profound effect on foundation settlement. Groundwater level was

encountered at shallow depth, generally between 0 and 2.0m below ground level and subject to tidal fluctuation

with a small phase difference. Fresh groundwater on the beach ridge exists as a mound (Abam 2016).

Consequently, its extent and depth are a function of the size of the ridge. Since, its shallow, groundwater is

highly vulnerable to contamination within the environment of the beach ridge.