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The search for petroleum is eminent in offshore environment of Southern Nigeria, while information on challenges and requirements of seabed soils at depth is limited. Available geotechnical results are inadequately quantified and documented. This study evaluates the physical state of the seabed soils for the purpose of ensuring the useful life of offshore structures. Nineteen (19) submerged soils were sampled and tested in the laboratory and in the field to obtain the required chemical and geotechnical results for analysis. Clayey silty sand and organic matter forms the succession of processes at the seabed depth. The moisture content was above 40% in most of the samples tested while the hydrogen ion concentration range is 5 ≤ pH ≤ 7. The chloride content was found to be above 100 mg/l, and sulfate concentration was below 200mg/l for all the samples. The cohesion of the soils was from 0 to 35 kN/m2 which is a characteristic of normally consolidated clays with shearing resistance angles range of 0–32°. Relatively high shear vane torque occurred at depths 26.00–26.60 m and 19.00 m–19.60 m and 21.00 m–23.00 m. The coefficient of consolidation ranged from 3.52 to 21.63 m2/yr with generally low compressibility. The moisture and chloride contents could be more attributable to soil corrosivity than other chemical variables. The results indicate that reasonable allowable loads with minimal total settlements are achievable for effective foundation; however, more geotechnical data collection is required to enable reliable and high-quality validation of the seabed foundation soil characteristics. [ABSTRACT FROM AUTHOR]

Nine seismic refraction profiles were conducted and processed to study the near-surface sediments in the new urban area of Diriyah. The 2D geoseismic models illustrate two layers: a surface layer of soft sediments and weathered to hard limestone bedrock. Moreover, microtremor measurements were performed at 38 sites for 40 min using three-component seismographs and processed to assess the peak spectral amplitude and the corresponding fundamental resonance frequency. The seismic vulnerability index at each measurement site was estimated. These results correlate well with the geotechnical borehole data. The north-western zone is highly vulnerable due to the great thickness of the soft sediments. [ABSTRACT FROM AUTHOR]