A research approach that integrated geophysical and geotechnical methods for subsoil evaluation using for prefoundation study was carried out in this research paper. The study involved eighteen Vertical Electrical Sounding (VES) using Schlumberger configuration and geotechnical investigation. Three major geoelectric layers were delineated; the topsoil, weathered layer and basement bedrock. The layer resistivity and thickness ranges are 137 – 1559 ohm-m; 189 – 441 ohm-m and 1253- ∞ ohm-m and 0.3 – 2.3 m and 0.9 – 5.4 m for the three layers respectively. The estimated depths to basement bedrock vary from 0.5 – 5.9 m, for most parts of the property, the overburden thickness are generally less than 2.0 m. There are no indications of the major geologic structure such as faults. The percentage passing 0.075mm ranges from 30 – 32%, the plastic index ranges from 16 – 20% and the linear shrinkage ranges from 7 – 8%. The subsoil on or within which civil structures can be founded in the area are composed of mainly clayey sand and laterite with resistivity values generally greater than 250 ohm-m and all the geotechnical parameters fall within specification for subsoil foundation materials. It can be concluded therefore that the subsoil within the study area are generally competent and buried metallic structures within the area are at extremely low risk of being corroded.

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This paper establishes empirical relationships between subsoil electrical resistivity and geotechnical parameters in a typical basement complex terrain of Nigeria. Two pits and two trenches were dug to a maximum depth of 3.0 m along and upstream of a proposed dam axis. Core cuttings (undisturbed soil samples) were collected at the base of the pits/trenches and the coefficients of Compaction, Compressibility, and Permeability were determined. Down-the-hole micro-resistivity measurements using Wenner and Pole-Dipole (Half-Wenner) arrays were made at 0.10 m interval from top to bottom of the pits/trenches with inter-electrode spacing of 0.10 m. On a linear-log plot, both coefficients of Consolidation and Compressibility increase with increase in subsoil resistivity values. The established empirical equations gave correlation coefficients that vary from 0.70 to 0.96. Beyond a threshold permeability value of around 4 x 10-5 mm/s, subsoil permeability increases with decrease in soil resistivity while below the threshold value, soil permeability tend to increase with increase in resistivity. The study demonstrates that engineering geotechnical parameters can be estimated from resistivity measurements provided the relevant empirical equations had been established for the area of interest.

Geophysical investigation was carried out along segments of the Ilesa-Akure highway located in the Precambrian Basement Complex of Southwestern Nigeria with a view to establishing the cause(s) of the road pavement failure. Four failed segments and two stable segments serving as control were studied. The magnetic and the Very Low Frequency Electromagnetic (VLF-EM) measurements were taken at intervals of 5 m along traverses established parallel to road pavement. The Vertical Electrical Sounding (VES) involving the Schlumberger array and 2-D imaging using the dipole-dipole configuration were adopted for the resistivity survey. The magnetic and VLF-EM results revealed that the control stable segments are founded on a near homogeneous substratum devoid of major geological features while anomalies typical of linear features suspected to be fault/fractured zones and lithological boundaries were identified within the failed localities. The geoelectric sections generally identified four geologic layers comprising the topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. The subsoils on which the road pavement was founded at control stable segments have moderate to high resistivity values (>200 Ω-m) while the subsoils on which the failed segments are founded are of relatively low resistivity values that are generally less than 200 Ω-m. Network of suspected linear (geological) structures such as fractures and faults were identified from the 2-D resistivity structure across the failed segments. The causes of highway pavement failure in the studied highway may have been clayey topsoil/sub-grade soils with characteristically low resistivity values of < 200 Ω–m and near-surface linear features such as faults, fractures and lithologcal contacts beneath the highway pavements.

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Geophysical and Hydro-Chemical Investigation of the area around a Waste Dump Site

Geophysical and geotechnical studies were conducted at a proposed Switch station facility for Telecommunication at a site in the south-eastern part of Nigeria. The aim of the study is to evaluate the sub-soil conditions and electrical properties of the soil which may have effect on the proposed mast and switch facilities system. The geophysical investigation involved the Vertical Electrical Sounding (VES) technique using the Schlumberger configuration and a geotechnical investigation. A total of sixteen (16) VES stations were occupied within the study site. The geotechnical study involved Boreholes drilling as well as Cone Penetration Tests (CPT). A total of six (6) CPTs and three (3) Boreholes drilling were utilised within the study area. This was done to provide controls on the geophysical interpretation. Four subsurface layers were delineated within the study area which include: the topsoil (mixture of sand, silt and clay), coarse sand, clayey sand and sand. This correlated with the sub-soil investigation. The study area is underlain by a stratum of medium stiff to stiff lateritic clayey/silty sand to the depth of about 20 m as explored by the Borehole. The choice of foundation construction for the proposed structure must take care of the settlement characteristics of the clayey material. The subsurface layers up to a depth of 5 m is of moderate to high resistivity values (> 180 ohm-m) and it may not serve as a good electric earthing material, therefore there is a need to improve the subsurface conductivity of this layer most especially within the area where the electrode for the earthling system will be buried.

The velocity-deviation log, generated by combining the sonic log with the neutron-porosity log, provides a tool for characterizing the predominant pore types and fractures in clastic drilled holes in the Niger delta area. Five geophysical wireline logs comprising lithology and porosity logs were acquired for the study. The velocity-deviation is calculated by converting neutron porosity-log data at a reasonable (VTA) depth interval to a synthetic velocity using time-average equation while the sonic porosity-log data (Vsonic) is converted to a sonic velocity by taking the reciprocal of the interval transit time (t) at the same (Vsonic) depth intervals of interest. The difference between the sonic velocity and the neutron log–derived (VTA) synthetic velocity is utilized in generating the velocity-deviation log. The resulting velocity-deviation, which could be ‘negative’ ‘zero’ or ‘positive’ log is the result of the variability of velocity at equal porosity. It gives down-hole information, which is interpreted with respect to the predominant primary poretypes and fractures. Positive velocity deviation (i.e zones where formation velocity is higher than thesynthetic velocity) marks zones where frame-forming poretypes are dominant. Zero deviation (approximate VTA) equality of Vsonic and shows intervals where the rock lacks rigid frames such as in clastic formation with high inter-particle poretype. Negative-deviation (low formation velocity) marks zones of cavernous borehole wall, fracture, or high content of free gas. The study revealed that the penetrated clastic formations largely exhibit zero deviation diagnostic of interparticle poretype structure. In addition, it shows the applicability of the velocity-deviation concept in the identification of permeable zones.

As part of effort to examine the geological factors responsible for highway failure in basementcomplex terrain, geophysical survey involving Schlumberger Vertical Electrical Sounding (VES) and dipole-dipole electrical resistivity, magnetic and Very Low Frequency Electromagnetic (VLF-EM) methodswas carried out along Ilesha – Owena Highway. This was with the view to detailing the subsurface geoelectric sequence, mapping the subsurface structural features within the sub-grade soil and delineating the bedrock relief as a means of establishing the cause(s) of the highway pavement failure. The magnetic profile, inverted VLF-EM model, dipole-dipole and geoelectric sections along the stable segment are diagnostic of generally resistive and homogeneous subsurface devoid of any geological feature. Along the failed segments low resistivity clay enriched, water absorbing substratum and linear features suspected to be faults, fracture zones, joints and buried stream channels were delineated. The clayey sub-grade soil below the highway pavement and identified suspected geological features are the major geologic factors responsible for the highway failure. Other factors include poor drainage and excessive cut into near-surface low resistivity water absorbing clay enriched substratum.

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