Site characterization for engineering purposes using geophysical and geotechnical techniques

The study area is located at Akole Community in Oke-Ata, Abeokuta North, Ogun State, Southwestern Nigeria, which lies between latitudes 7°8′16.9″ N and 7°8′24″ N and longitudes 3°17′9.2″ E and 3°17′13.4″ E. The ground in the study area lies at an elevation between 62 m and 78 m above sea level (Figure 1).

Figure 1

The climate is warm and tropical due to the rain-bearing ocean wind of the south-western monsoon and the northwest wind that arises from the Sahara desert. The rainy season of the study area starts around April and ends in October, with rainfall of nearly 1,238 mm per annum, while the dry season starts in November and ends in March. The area is located in a hummocky terrain with a well-pronounced undulating topography and prominent hills, characterized by a moderate slope varying in altitude. The study area falls within the Precambrian Basement rocks of Southwestern Nigeria, with six major lithologic units, namely quartzite, banded-gneiss, biotite-schist, quartz-biotite schist and pegmatite [13].

The method used for the geophysical survey was the vertical electrical sounding (VES) using the Schlumberger electrode array. The Schlumberger array focusses on the vertical variation of sub-surface layers. The Schlumberger configuration of an electrode is quite sensitive to vertical sub-surface resistivity below the centre of the array and it is less sensitive to horizontal changes in the sub-surface [14]. Data from a total of four VESs were acquired in the study area, and each of the potential differences and currents measured at each point were recorded. The apparent resistivity (ρ_a) was computed from measurements of voltage (ΔV) and current (i) using Equation 1. (1) ρa=π[(s2-a2)/4]aΔVi {\rho _a} = {{\pi \left[ {\left({{s^2} - {a^2}} \right)/4} \right]} \over a}{{\Delta V} \over i} where ρ_a is the apparent resistivity obtained, s is the distance between the potential electrodes, a is the distance between the current electrodes, ΔV is the potential difference measured and i is the current measured.

Using WinResist software, the apparent resistivity values obtained were plotted against the electrode spacing to acquire the VES curves.

The soil moisture content (SMC), which is the water between the pores of the soil, was determined using the gravimetric method expressed by Equation 2 [15].

The Atterberg limit test verifies the liquid (LL) and plastic (PL) limits of a fine-grained soil. The LL refers to the moisture content at which the soil begins to behave as a liquid material and begins to flow, while the PL is defined as the moisture content at which soil begins to behave as a plastic material. The LL and PL were determined using the Casagrande method, as described in American Society for Testing and Materials (ASTM) Standard D4318. The difference between the LL and the PL gives the plasticity index (PI). The compaction limit test describes the relationship between the moisture content and the dry density of a soil for a specified compactive effort (amount of energy that is applied to the soil). The compaction properties were determined using standard methods (ASTM D698 and ASTM D1557), the standard and modified efforts of 6,000 kN-m/m³ and 27,000 kN-m/m³, respectively, were chosen for the determination of the moisture–density relationship. The California bearing ratio (CBR) expresses the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the rate of 1.25 mm/min to that required for the corresponding penetration of a standard material [16]. The CBR was determined following the procedure of ASTM D1883.

The specific gravity (SG) of the soil was determined using a water pycnometer-based standard test (ASTM D854-00) expressed by Equation 3. (3) SG=((w2-w1)(w2-w1)-(w-w4)), SG = \left({{{({w_2} - {w_1})} \over {({w_2} - {w_1}) - (w - {w_4})}}} \right), where w₁ = empty weight of pycnometer, w₂ = weight of pycnometer + oven-dried soil, w₃ = weight of pycnometer + oven-dried soil + water, and w₄ = weight of pycnometer + water.

The grain size analysis estimates the percentage of sand that was passed or retained by an individual sieve. A soil sample of 500 g was sieved to appropriate sieve sizes of 475, 236, 118, 600, 300, 150, 75 μm and weighed. The percentages of particles passing and particles retained, as well as the quantity passing, were calculated using Equations 4, 5 and 6, respectively. (4) P=MrTm*100, P = {{Mr} \over {Tm}}*100, (5) R=100-P, R = 100 - P, (6) QP=Tm-Mr, {Q_P} = {T_m} - {M_r}, where T_m = total mass of the soils, R = percentage retained, P = percentage passing, M_r = mass retained and Q_P = quantity passing.

The PI of the soil samples (Table 2) revealed that soil samples SP2, SP3, SP4, SP6, SP7, SP8, SP9 and SP10 fall between PIs of 1% and 10%, which implies low plasticity, consisting of sand or silt with traces of clay, while soil samples SP1 and SP5 have PIs between 10% and 20%, depicting medium plasticity and composed of clayey loam soil. All the soil samples SP1–SP10 fall within the limits of the specifications, except SP1 and SP5, for which the LL and the PI exceeded the stipulated values of 35% and 12%, respectively, contrary to the Federal Ministry of Works and Housing (FMW&H) specification requirement in Clauses 6201 and 6252. The PI of soil samples SP1 and SP5 is above the standard limit and, therefore, they are considered to be highly plastic, which may pose a threat to the structure and consequently lead to structural failure.

Table 2 shows the SG values obtained for the different soil samples. All the samples fall between the ranges of specification, varying from 2.5 to 2.75, excluding samples SP1 and SP5, which fall below the limit recommended by FMW&H [18]. In a previous paper [19], it was noted that the SG of soil grains is a key attribute in the assessment of aggregate parameters for construction purposes. The higher the SG of the soil towards the upper limit of the soil standard, the better is the soil for construction purposes.

All the samples (Table 2) have moderate moisture content, except for samples SP1 and SP5, which have very high moisture content. Samples SP1 and SP5 are considered to be poor for engineering purpose because of their high content of moisture, and this implies that they have high ability to retain water without releasingit.

The results for the compaction limit test for each sample shown in Table 2 illustrates that the MDD for the soil samples ranges from 1,250 kg/m³ to 3,500 kg/m³, and the OMC ranges from 10.21% to 47.56%. All the samples, except SP1 and SP5, fall within the specifications of the FMW&H (1997), which recommends the MDD to be >1,680 kg/m³ and the OMC to be <18%. The density of the soil mass affects the strength of the soil, which implies that SP1 and SP5 have lower values compared to the standard values. The strength of a soil increases as its dry density increases; the potential for the soil to take on water at later times is decreased by higher densities.

The overall CBR values for the soaked (CBR_s) and unsoaked (CBR_u) samples, as shown in Table 3, fall within the specified limits for all the soil samples analysed, except for SP1 and SP5. The FMW&H specification states that the minimum strength of the material should not be <80% for CBR (for unsoaked samples), while the minimum strength of the material should not be <10% after at least 48 h of soaking (for soaked samples). The CBR_s values ranged from 3.05% to 29.76%, while CBR_u ranged from 60.25% to 98.95%. The values for CBR_u and CBR_s are 60.25% and 5.62% for SP1 and 70.45% and 3.95% for SP5, respectively. This implies that the soil is clayey lateritic type of soil, which does not support heavy structures. In addition, moisture influx would be highly detrimental to the structures constructed at those locations.

All the soil samples SP1–SP10 (Table 4) fall within the limit of specifications for sieve analysis since the percentage by weight of 15.18% passing the No. 200 sieve does not exceed the stipulated value of 35%, as required by the FMWH (1997) in Clause 6201.