Geoscientists and structural engineers all around the world are concerned about the earth's subsurface inhomogeneity, which can lead to building foundation failure. Knowledge of the probable causes of the frequent failure of building foundations in Nigeria, due to the movement of the earth’s subsurface that gives rise to cracks or soil differential settlements is becoming of great concern to geoscientists. This will help to differentiate between a continuous movement, which is often more likely to be a problem, and those of single events, which may not require repair depending on the extent of the damage. When addressing the geological and geophysical causes of a building's failure, it is critical to have a thorough understanding of the types and patterns of foundation-based fissures and how to assess them. The majority of home settling cracks are caused by variances in the construction materials' expansion and compression coefficients, relative changes in the shapes and sizes of saturated soils, or the dynamic earth [1]. The direction of foundation movement is usually observed from swelling and cracks on the blocks. This reveals the risk of a possible vertical collapse or horizontal dislocation. Therefore, the development of new towns as well as university layout sites generally requires a detailed evaluation of the subsurface of the proposed site. Foundation studies of a new site are required to give subsurface and aerial information that aids civil engineers, builders, and town planners in designing and arranging civil engineering constructions' foundations [2].

Geohazard characterization surveys help civiliengineers and builders recognize and efficiently handle underground geological threats, includingimineisubsidence, vacuums, sinkholes, Soilimovement, landslides, underwater fooding, dam leakage, faults and industrial hazards. Geological hazard characterization requires pre-construction mapping of faults and fracture zones to ensure the site’s stability and competence. In addition, the ongoing failures of infrastructure systems suchias bridges, roads and the collapse of buildings in Nigeria have reached an alarming pace that both the federal and state governments are concerned and have ordered their urban planning department to draw up regulatory codes for any buildings above a foor [1]. The causes of this failure can be traced to several factors including poor/insufficient construction materials and incompetent structural soils [3].

The Federal University Gusau has just moved to its permanent site which is still at its developmental stage in Kotorkoshi Tsafe Local Government Area of Zamfara State, Nigeria. The site is situated on the crystalline basement complex with variable overburden thickness. It is expected that with increasing population, developments which may include the erection of high-rise buildings may have to be embarked upon. With the increasing demand for site development in the study region and the unfortunate experience of building collapse, it is vital to conduct site investigations to uncover any potential subsurface issues. As a result, geophysical studies are critical for determining the physical attributes of the subsurface in terms of soil type, soil competency, bedrock depth, and lithological succession. Site engineers, for reasons of cost and other considerations; such as assumptions in structural design, sometimes fail to incorporate pre-construction investigations in their job schedule.

One of the major geophysical techniques used for environmental and engineering site delineation, and routinely applied for studies of structural failures, is Electrical Resistivity Tomography (ERT) [4-5]. Previous research has shown that the ERT method can be used to identify bedrock structures cavities or sinkholes geotechnical site investigation and bridge foundation studies [6-11].

This research aims to demonstrate the ERT technique's application in solving foundation problems. This investigation was carried out to explore the suitability of the subsurface for building foundations, by identifying weak zones that may be fractured, faulted, or prone to subsidence which could pose danger to existing structures and competent areas that can support massive engineering structures to determine the depth and extent to which the bedrock has been weathered.

Location and Geology of The Study Area

The study area is located near Kotorkoshi town is located within the north-western Nigeria crystalline Basement Complex (Figure 1) and lies between latitude 6°45'3.11"N to 6°47'49.76"N and longitude 12°07'39.57"E to 12°07'49.65"E. camp comprises both older and younger metasediments and coarse to fine-grained older granites consisting essentially of quartz, feldspar and biotite [12]. While the coarse granites weather into water-bearing sandy residue, the syenitic rock types with the predominance the unstable minerals such as feldspars, decompose into plastic or soft clays and other argillites which behave only as aquitards or aquicludes. Generally, only a small amount of water can be obtained in the freshly unweathered bedrock below the weathered layers [13].

Figure 1: The simplified geological map of Zamfara shows the study area.

The 2D ERT was carried out using the Wenner array. The choice of direction of the profiles was, on the other hand for the continuous direction of the cavity to be mapped and to be able to get the lateral extent of these voids. A total area of 210 m by 20 m was surveyed with Abem tetrameter SAS 4000 and ES 464 electrode selector employed. The data was collected at 5 m electrode spacing and 5 parallel profiles laid at 5 m intervals. The profiles were laid with the origin at the West and the end at the East of the study area, covering a lateral length of 210 m.

The collected resistivity data was processed and inverted using the RES2DINV software developed by [14]. The inversion technique used was the standard least-square smoothness constraint. In general, the computer generates a 2D model by dividing the subsurface into rectangular blocks and iteratively adjusting the resistivity of the blocks to reduce the discrepancy between the observed and computed apparent resistivity values [14].

Using the finite-difference approach, the apparent resistivity values were computed and compared to the measured data by the program. The modeled resistivity values are changed during iteration until the model's computed apparent resistivity values match the real data. The iteration is stopped when the inversion process converges (i.e., when the RMS error is below acceptable bounds (typically less than 5%) or when the difference between RMS errors for consecutive iterations is infinitesimally small). Before data inversion, the apparent resistivity data set was reviewed for poor datum points, as suggested by Loke and such points were eliminated [15].

Figure 2: Google Earth Map of Federal University Gusau Showing the Profile Position in the study area.

The data were used to plot the 2D pseudo-sections for each of the profiles. The profiles show the inversion result with an absolute error (RMS) between 4.2% and 10.5%, thus indicating that a good fit between the measured and calculated apparent resistivity data has been achieved.

Boreholes are often used to correlate results obtained from geophysical surveys as these surveys are indirect. This is often done by correlating apparent resistivity obtained in an area with the lithological information in the area. Boreholes which provide lithologic information, are a necessary and reliable source of primary data and Electrical Resistivity Imaging (ERT) interpretations provide secondary information. Although borehole data provide a good sample for a six-inch diameter vertical cylindrical volume, it can be a poor representation of the several square meters surrounding the borehole. The 2D inversion results of the survey were correlated with the borehole log taken within the study area (Figure 3).

Figure 3: Driller’s log with data from this study.

Figure 4-8 shows the results of the profiles, profile one (figure 4) is situated close to a borehole, as such, its log was used as a guide in interpreting the inversion model of the profile. It was also used as a measure of interpreting the remaining pseudo-sections because lithological successions hardly change over small areas.

Figure 4: Results of 2D inversion of Wenner array along with profile 1.

Figure 5: Results of 2D inversion of Wenner array along profile 2.

Figure 6: Results of 2D inversion of Wenner array along profile 3.

Figure 7: Results of 2D inversion of Wenner array along profile 4.

Figure 8: Results of 2D inversion of Wenner array along profile 5.

This investigation revealed a minimum of three and a maximum of four geo-electric sections in all profiles (figure 4-8). The corresponding lithological layers were correlated with borehole log and standard resistivity values used as control.

The overburden is the first layer, which is primarily made up of laterite and lateritic clay. The laterite has a resistivity range of 26 m to 394 m. The resistivity value of the lateritic clay ranges from 24 ?m – 108 ?m, with a thickness of the overburden ranging from 0-6 m. The weathered basement is the second lithological layer. Its length is between 8 and 13 meters, and its resistivity is between 400 and 735 meters. The aquifer in the basement complex where this study area is located includes a weathered basement. Its geotechnical impact on foundations and buildings is that it usually contains clay components, which shrink during periods of dryness, causing uneven support beneath a structure. The fractured basement is the third. This is only believed to be limited to a section of the surveyed region near the middle (figure 8). At an average depth of 15 m meters, it has a resistivity value ranging from 201 to 735 meters. It is also an aquifer component, and its impact on foundations and structures is comparable to that of a weathered basement. The fresh basement or bedrock is the fourth lithological layer. It has a depth range of 22 meters to infinity. It has a resistivity range of 745 m to 1206 m.

Because competent bedrock has high resistivity values, geotechnical engineers look for it when creating long-lasting structures [15]. The fresh basement, as suggested by the inquiry, is the competent layer of this study due to its high resistivity value. The low resistivity lateritic clay deposit should be avoided in the overburden. This is due to the clay material's poor geotechnical qualities, shear strength, and high compressibility, which make it susceptible to differential settlement and potential flow under load.

Geophysical and geotechnical investigations have been carried out at Federal University Gusau, Nigeria. Five 2D resistivity profiles were carried out to identify the depth to competent layer for the foundation of engineering structures a priori to differential settlement of structures in this environment. The result of the 2D tomography indicates that the subsurface consists of three distinct layers; overburden (lateritic and lateritic clay), weathered and fresh basement, using the resistivity range, and borehole log of the study area as a control. It was found that in all profiles, no faults or fractures were present to pose danger to the existing structures. The result also revealed that the building foundation should be at the depth of the fresh bedrock because of its competence and the lack of clay materials. The 2D resistivity investigation revealed important details about the lateral and vertical variation of the layer capable of supporting engineering structures. Buildings in this location should have deep foundations that are pilled to the competent layer. The Electrical resistivity tomography method appears to be a valuable geophysical tool for engineering structure studies. It is however recommended that geotechnical studies be carried out in the geotechnical incompetent zones.

The first author gratefully acknowledges the Department of Physics, Ahmadu Bello University (ABU), Zaria Nigeria and the Federal University Gusua, for the platform it created for this research to be carried out.

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