PRE-FOUNDATION STUDIES USING VERTICAL ELECTRICAL SOUNDING AND SOIL SAMPLE ANALYSIS

Subsurface rock properties investigation to categories foundation competent layer for the proposed engineered structure capable of housing offices, lecture halls


INTRODUCTION
Developed countries use GPHY and GTECT site investigation as a benchmark before beginning the design phase of engineering projects.According to (Oyeniran and Falae, 2018), this routine procedure aims to reduce construction failure by determining the geological conditions and the ability of the underlying soil formations to withstand the load capacity of the structure.Such practices are not prioritized in the developing countries, such as Nigeria, consequently leading to structural defects and a string of building collapses (Oyedele, 2011).A foundation is important parts of engineering structures that support the weight of the structure and transfers it to the soil underneath it.However, when the subsurface soil materials are geologically deformed and/or lack requisite GTECT properties, construction problems may arise with an outcome of structural defect (Soupois 2007, Oyedele et al., 2011, Adeoti et al., 2016, Olayanju et al., 2017).Investigations of vital parameters (subsurface soil qualities and geologic conditions) to be considered before designing an engineering structure have been demonstrated by (Bremmer, 1999, Omoyoloye, et al., 2008, Arora, 2008, Nwankwoala and Warmate, 2014).Natural phenomena, which include natural activity like earthquakes, tremors, and faulting are one of the reasons for engineering structural failures/defects in addition to poor pre-investigation (Oyedele, 2009, Aghamelu, 2011, Khatri, 2011, Olorode, et al. 2012, Cardarelli, 2018).Earthquakes and tremor are significantly manifested in an area where faulting is enormous.In such an area, GPHY investigation can be used to delineate fault and fracture system to facilitate pre design of engineering structure.Detecting the existence of geological structure such as fracture system and its spatial continuity in the subsurface is however, a major drawback for GTECT investigation.These constraints require the combination of GPHY and GTECT to totally exploit the subsurface conditions.
Engineering GPHY deals with the unraveling of engineering performance of earth materials (soil and rock) as related to foundations of roads, railway lines, buildings, tunnels, and power plants using appropriate GPHY prospecting techniques.Foundation investigation methods such as boring, drilling, pitting and trenching are very costly, invasive, and timeconsuming unlike engineering GPHY method which provides less laborious and cost-effective alternative with accurate results without disturbance of the earth materials (Olorunfemi, et. al, 2002;Akintorinwa and Adesoji, 2009;Akintorinwa and Adeusi, 2009;Ofomola et al., 2009).Frequently used GPHY methods in engineering GPHY survey include Electromagnetic (EM), Electrical and Seismic Refraction (Reynolds, 2011;Rungroj, 2015;Bharti, et al., 2016;Fajana, et al., 2016;Das, et al., 2017;Pazzi, et al., 2018;Bharti, et al., 2019;Singh, et al., 2019;Guptal et al., 2020).These methods exploit the science of natural phenomenon of the earth to assess the physical properties of the subsurface (Olaleye, et al., 2020), by revealing depth to bedrock, the presence of geologic structures, and the competency of subsurface (Guptal, et al., 2018;Srivastava, 2020).On the other hand, the GTECH investigation involves subsurface GTECH evaluation such as Natural Moisture Content, Particle Size Analysis, Atterberg Limits comprising liquid limit (LL) and plastic limit (PL).While Linear Shrinkage, Compaction Test, and Unconfined Compression are further GTECH metrics that aid the determination of the soil's competency (Bharti et al., 2016).
Pre-foundation studies were conducted in the Study Area using both GPHY and GTECH technique as a result of expansion drive of an Institution management.The proposed structure is to host offices, laboratories, and lecture rooms.The research's findings will establish (1) the capacity of subsoil materials to support the foundation of the structures (2) the potential depth at which the foundation could be positioned (3) prevent economic loss that could accompany future structural failure and, (4) the need for pre-foundation studies as a precautionary measure to prevent widespread building collapse in the nation.

GEOLOGICAL SETTING
Geology of the research area has been described in detail by (Rahman, 1989).Biotite granite and gneiss migmatite are the primary geological features in the region.

Electrical Resistivity Survey
GPHY method engaged in the studied area was electrical resistivity GPHY survey with the aid of R-50 resistivity meter.With the use of a Schlumberger array and electrode spacing (AB/2) of up to 65 m, 19 VES data were collected in order to map the distribution of the subsurface apparent resistivity (A_ RESIST).To determine the type of depth sounding curves, observed A_RESIST (Ω m) values were plotted against electrode spacing AB/2 (m).A qualitative assessment involving visual evaluation of the sounding curves was conducted in an effort to gain first-hand knowledge of the subsurface structure of the research area.In order to establish geoelectric parameters for each location within the study region, the sounding curves were additionally subjected to curve matching using conventional electrical resistivity master and auxiliary curves.Geoelectric parameters are dependable clue of soil competence classification (Olorunfemi et al., 2004).The result of the iteration was then presented as geoelectric sections and maps.These were subsequently used to quantitatively evaluate the resistivity and thickness of the subsurface layers (Olorunfemi et al., 2004) (Table 1).

Geotechnical Investigation
Five (5) soil samples were collected at a depth approximately 1m in the study area (Figure 1).The samples were adequately tagged and taken to the laboratory for the following geotechnical test; natural moisture content, particle size analysis, Atterberg limits test (Liquid Limit, Plastic Limit), linear shrinkage, unconfined compression, and compaction test.These tests were carried out in accordance with global best practices utilizing (BSI, 1990) as a reference point.Presentations of the geotechnical data include graphs, charts, curves, and tables.

Electrical Resistivity
Four (4) sounding curve types: A, KH, H, and HKA; identified from the geoelectric curves of the study area; and Table 2 shows the result of the geoelectric parameters.A maximum of five (5) subsurface layers with reliable indications of soil competency were identified based on the geoelectric parameters classifications (Table 2).The delineated subsurface layers include topsoil, weathered layer, partly weathered/fractured bedrock, faulted basement, and fresh basement (bedrock).Generally, topsoil is an incompetent soil layer for foundation emplacement because it is expected to be dug out, therefore, emphasis are laid on the subsequent layers beneath the topsoil for soil competent investigations.Depending on the resistivity range of the weathered layer, some are considered competent.From Table 1, clay have flow propensity under stress, render the soil material incompetent as they cause differential displacement on building walls (Sheriff, 1991).On the other hand, sand, clayey sand, and crystalline rocks (bedrock) are competent subsurface materials due to their ability to hold on to stress (Sheriff, 1991;Olorunfemi et al., 2002).Due to the significant depth of occurrence, VES analysis at the third layer mostly indicates compacted soil at depth of about 2 to 5 m.However, the third layer of VES 5and 10, and the fourth layer of VES 14 (Table 2) have a resistivity classification of loose soil, an indication of a fractured/faulted filled with clayey sand or sandy clay materials, therefore rendering the VES locations unsuitable for the deployment of foundations.Overburden layer is a term used to describe the weathered, topsoil and clay incompetent layers.(Sheriff, 1991), therefore can cause collapse.Around VES 10, the geologic structure was observed (Figure 2a), making this region of the research area completely unsuitable for a foundation.Bedrock resistivity varies from 447 to1607 Ωm and occurs as shallow as 3.8 m around VES 9 (Figure 2a).This resistivity range is an indication of a competent layer for foundation placement due to the stress holding capacity of the material as described (Sheriff, 1991;Olorunfemi, et al., 2002) (Table 1).Also, Figure 2b revealed weathered later resistivity values ranging from 36 -80 Ωm, which indicates clay material, to the depth of 3 m, except VES 18 where the overburden thickness is up to 17 m.The thick overburden around VES 18 is likely due to bedrock depression, which must be taken into consideration during foundation designs.
Figure 3a revealed an overburden layer (consisting the topsoil and weathered clay soil) of depth of about 5 m with a resistivity values implying loose and incompetent soil, as presented on Table 1.Bedrock resistivity values are between 891 and 1958 Ωm, occurring at a depth between 3 and 5 m (Figure 3a).This layer is considered competent for foundation placement.Figure 3b revealed clay weathered layer of resistivity between 43 and 119 Ωm.However, the cumulative thickness of the overburden layer to the top of the competent soil is between 5 and 7 m (Figure 3b).Thick overburden, up to 13 m, around VES 18 may be as a result of subsurface/basement depression, which must be taken into consideration while designing the foundation type.In order to provide a general impression of the near-surface soil features of the research area, isoresistivity (Figure 4) and isopach (Figure 5) maps describes the spatial distribution of the overburden layer (that is, topsoil and incompetent weathered layer) in terms of resistivity and thickness respectively.Figure 4 demonstrated that the overburden layer is made up of clay and sandy clay formations based on the overall resistivity values.This renders the layer incompetent to build engineering structures.Figure 5 revealed that this incompetent overburden layer is thin towards the south, central and north-eastern Part of the study area with thickness between 2 and 5 m.Presence of fault are observed towards the western and northern part of the study area (Figure 5; Table 2).The existence of this geologic structure may be the cause of the high thickness of the overburden layer (up to 12 m) at this area (Figure 5).

Geotechnical Investigation
Summarized GTECH analysis is presented in  4).A moderate PI value of about 20% (Table 3) is moderately good for engineering material (Jegede, 2000), and values below it are considered good whereas those with a value higher than that are deemed incompetent.In general, PI of the soils samples 1 and 4 within the area were lower than the 12% maximum suggested by (FMWH, 1997), the soil possess good engineering material as competency of the soil is defined by the lower PI, which is consistent with geophysical results of these areas (soils sample 1 and 4) which showed that the area is dominated by sand and clayey sand.The Linear Shrinkage (LS) value of the tested soils ranges between 6 and 11 % (Table 3).Brink et al (1992) put forward that soils with LS lower than 8% are apparently inert and are fairly good foundation materials.
Considering that the average value of the LS is 8.7% within the study area, the soils may swell and fall away during dry and wet seasons, which should not be taken for granted in the course foundation design.
Assessment on the Shear Strength (SS) revealed Unconfined Compression (UC) strength ranges between 120 and 230 KN/m 2 (Table 3) whereas the Undrained Shear Strength (UDSS) ranges from 60 -115 KN/m 2 .The elevated values of UC strength signify a substantial percentage of clay within the samples.Comparing the SS result with the 103KN/m 2 minimum acceptable standard of FMWH, 1997, the subsoil within the study area possesses reasonably high strength property.The intention of the test is to get hold of compressive strength for the soils that have sufficient cohesion to allow testing in the unconfined state.Soil samples such as soft clays, dry and crumbly soils, silts and/or sandy samples generally display higher UDSS (ASTM, 1996).
Grain size distribution and grading curves for all the samples (Table 3) revealed moderately elevated percentage of finer soil particles, at Percentage Passing 0.075 mm, ranging from 35 to 50%.The soils samples from the study area to a large extent graded well.Generally, the tested soils have a Percentage Passing 0.075 mm with an average of 41%.FMWH (2010) advocated 35% maximum rating of foundation formation (Table 3).These ranges of values reveal that the overburden layer is majorly characterized by clay and clayey sand materials, such materials will be liable to swelling in the event of a rise in water table.This result agrees with the initial geophysical results of overburden layer composition mainly of clay material.
Towards ascertaining desirable load-bearing properties (density) of the soil within the study area, compaction test from Maximum Dry Density (MDD) reveals density between 1635 and 1940 Kg/m 3 ; and Optimum Moisture Content (OMC) between 11 and 21.9 % (Table 3).At a MDD of 1940 Kg/m 3 , OMC is as low as 12.5%.These values demonstrate that the soils react steadily to compaction, Jegede, 1999; suggested high MDD and low OMC soil material for a foundation purpose.

CONCLUSION
An integrated GPHY and GTECH study were performed with the intention of understanding the subsurface soil properties prior to construction of an engineering structure.In achieving the research purpose, nineteen (19) VES stations, for GPHY sounding; and five (5) locations, for GTECH soil sample analysis, were occupied.The GPHY results revealed that the topsoil and weathered layer is characterized with clay material, with average thickness of about 5 m, which was referred to as overburden layer to the competent sandy layer suitable for engineering foundation.2-D Geo-electric sections from GPHY sounding also unearthed the undulating nature of the subsurface topography with depth to competent layer between 5 and 12 m.GTECH analysis show that the soils within the study area is generally characterized by low NMC between 4.20 to 8.20 % and Percentage Passing of 0.075 mm sieve greater than 35% in most areas (an indication of the clay nature of the soil).Consistency Limits of the soils revealed LL of 40% maximum and average PI of 20%.Although, at these Consistency Limits values, the soil is expected to experience moderate swelling, however, the values are within the average values suggested by (FMWH, 2010) for sub-grade materials.The general Linear Shrinkage of the soils has an average of 8 %, which implies expansiveness of the soil.
Overall GPHY and GTECH results show that the weathered layer is clayey in nature and is not good foundation material.The clay material will need to be excavated and the undulating nature of the depth to competent sandy layer should be considered while designing the foundation type.This research revealed the importance of GPHY and GTECH methods of investigation, as both methods complemented each other limitations well.GTECH analysis provided insitu and quantitative subsurface soil properties while GPHY analysis provided spatial distribution of subsurface parameters as well as the delineation of geologic structure, which could serve as threat to the engineering structure.This research will not only ensure a proper design and planning of the proposed structure but will also showcase the significant of GPHY and GTECH investigation as a yardstick to minimize structural failure.

ACKNOWLEDGMENT
The author will like to express appreciation to the anonymous reviewers whose honest comments on the original manuscript have greatly improved the quality and presentation of the paper.

Figure 1 :
Figure 1: Study area map showing VES stations and soil sample locations.

Figures 2
Figures 2 and 3 show associated VES positions along the strike (SW-NE) and dip (NW-SE) directions, respectively, for a quantitative 2D subsurface geologic model of the research region, which was created from the results of Table2in order to completely comprehend the subsurface geology of the area.The weathered layer on (Figure2a) has a resistivity range of 37 to 125 m, showing clay soil material down to a depth of approximately 7 m on VES 8. Clay soil materials are incompetent as they cause differential dislodgment on engineering structures(Sheriff, 1991), therefore can cause collapse.Around VES 10, the geologic structure was observed (Figure2a), making this region of the research area completely unsuitable for a foundation.Bedrock resistivity varies from 447 to1607 Ωm and occurs as shallow as 3.8 m around VES 9 (Figure2a).This resistivity range is an indication of a competent layer for foundation placement due to the stress holding capacity of the material as described(Sheriff, 1991;Olorunfemi, et al., 2002) (Table1).Also, Figure2brevealed weathered later resistivity values ranging from 36 -80 Ωm, which indicates clay

Table 2 :
Geoelectric Parameters of Interpreted VES Note: ρ stand for resistivity; h stand for thickness.

Table 3 .
The soil analysis reveals 4.20 to 8.20 % of Natural Moisture Content (NMC) which is considered very low.Considering discrepancies in NMC of soil due to the amount of rainfall, depth at which samples is collected as well as textural value of the soil; Jegede, 2000 recommended that such soil showing NMC value of 4.20 to 8.20 % underlain the area under study will not pose severe threat to the propose structure provided the strength/intensity of rain is reasonable for the period of rainy seasons.For Consistency Limit (CL) test (Table3); moderate Liquid Limit (LL) ranging from 25 to 40 %, moderate Plastic Limit (PL) between 18 and 30 %, and 8 and 22 % Plastic Index (PI) were recorded.Soils having high values of LL and PL are not recommended for foundation material.The intermediate LL and PL values in the study area point toward the clayey character of the soil/formation.However, FMWH, 2010 recommends standard values of 40%, 12% for LL, PI respectively for sub grade materials.The maximum recorded PI of the soil samples is 21.7% at the Northern part of the study area.This result confirms the clayey nature of the soil material as interpreted from geophysical survey (Figure