Civil engineering structures such as buildings of different sizes, bridges, dams, and other structures are situated on land formations which are mostly best for them because the soil formation below them has enough shear strength to hold their weight. The load of the building is transferred to the soil formation by a unit called “A Foundation”. In engineering, a foundation is the element of a structure that connects it to the ground, transferring loads from the structure to the ground. Foundations are generally considered either shallow or deep (wikipedia.com, 2020). mm. The foundation can be damaged or weakened due to a series of conditions like substandard building materials, the fracturing of the rock or formation, the existence of large void spaces in the subsurface, the occurrence of water-saturated zones, the occurrence of sinkholes, etc. Therefore, from a geological perspective (not related to civil engineering causes), it is necessary to carry out Engineering Geophysical investigations, which will delineate these subsurface features and occurrences that can result in the failure of the Foundation that can cause loss of lives and properties. However, In Nigeria for the past few decades, there has been a large number of collapsed buildings and failed structures due to inadequate pre-foundation investigation before construction, poor government policies, and a general lack of awareness of the importance of geophysical and geotechnical investigations; which would give specifications for depth of foundation depending on the geology condition of the subsurface of the area such as faults, joints; which could cause failure in the integrity of the said structure and may lead to serious loss of lives and properties. As a consequence of this environmental problem, several geophysical investigations have been conducted in many parts of Nigeria to forestall unforeseen and future building collapse. Most of these works used electrical geophysical techniques, which represent one of the oldest and cheapest geophysical techniques for investigating subsurface rocks, properties, and structural anomalies such as fractures, faults, voids, sinkholes, cavities, etc. Which could lead to failure in the integrity of the structure. The VLF-EM (very low-frequency electromagnetic) method is traditionally used to study the electrical properties of the earth’s crust. This method is relatively cheap and the source is always available, although its strength (source) differs throughout the day and year. Geophysical techniques involving VLF-EM surveys have been used to assess variations in the overburden composition and bedrock lithology, i.e. to characterize the overburden materials and delineate the concealed basement structures within the basement rock. This method is also used to complement the data obtained from the Electrical survey to delineate shallow seated faults, fractures, joints, cavities, etc. Which could source a hazard to the building causing it to collapse. Constant Geophysical and soil integrity test is important to ensure that soil formation and subsurface lithology can withhold the load from Civil Engineering structures in other to avoid disastrous events or to foresee and take immediate action to counter such events. Because Ilorin is of Basement complex geology (little overburden thickness) and the University of Ilorin falls under the same category, it is very common that pre-foundation study is neglected and there could be the occurrence of small fractures or faults which can endanger Structures built upon weak zones. This study tries to delineate possible weak zones which may pose danger shortly,
Description of the Study Area
The study area falls within latitude 80 11ʹ 59ʺ, 80 11ʹ 68ʺ E and longitude 40 09ʹ 11ʺ and 40 17ʹ 18ʺN on the map (Fig. 1a and 1b). The study profiles encompass three buildings located at the College of Health Sciences, University of Ilorin, Kwara State Nigeria. Ilorin and its environs are in the transition zone between a semi-arid northern climate and a sub-humid southern climate, it has two distinct seasons: the wet season, which begins in late March and ends in mid-October (wettest month in September) and the cold and dry season, which occurs in December and January due to the influence of Harmattan (driest in December and coldest in January). The rainfall is moderate, with an annual average of about 1,264 mm, with the wettest months being June and August/September, and a low humidity level of about 50%. The average annual temperature in this study region ranges between 27.2 and 31 degrees Celsius, with a mean annual minimum of 22 to 23 degrees Celsius. With an average monthly temperature of about 28°C, the warmest months are January and March (Olasehinde, 1984). The climate of Ilorin supports the growing of tall grass vegetation dotted with medium-sized trees (Fig. 2).
Geology Vegetation and Drainage of Study Area
The study areas fall within the University of Ilorin campus which falls within Geological Sheet 223 Ilorin southeast in the basement complex of Nigeria. The basement complex of Nigeria is considered to be Precambrian to lower Paleozoic in age (Oyawoye, 1964; 1972; Rahaman, 1976; Annor et al., 1987). The campus falls within the eastern part of Ilorin. The study zone is a semi-arid region of Nigeria with vegetation mainly of the guinea savannah type with shrubs and undergrowth. Rugged troughs and crests due to erosions characterize the topography of the area. The main river within the campus is River Oyun which flows from a southeast-northwest direction. Ilorin Township is underlain by the Precambrian to Cambrian basement complex rocks represented basically by migmatite-gneisses, granitic gneisses, and metasediments such as quartzites. The western part of the university is made up of gneiss rocks with strike foliation in NW-SE direction and dipping 10° − 45° westerly. In the eastern part, granite rock suites emplaced in the NE-SW direction dip 70°- 90° easterly. The area appears to have been clinically folded into a south-plunging antiform with a steep eastern limb. Several joints trending 120° – 160° dominated the rocks in the area as presented by Olasehinde. The study area lies within the crystalline basement complex of Nigeria consisting of high-grade metamorphic rocks in the form of gneiss migmatites and Granite suites found occasionally.
Ilorin is dominated by Guinea Savannah, which is the largest vegetation belt in Nigeria. Ilorin's vegetation is dominated by derived savannah, which can be found in the east and west of the city, and is known for its dry lowland vegetation Oyebanji (1993). This region has characteristic tall grasses (these grasses are green during the wet or rainy season and turn brown during the dry season), and a few scattered deciduous trees. The tree has thick Barks and long tap roots. Trees found in this region also form an umbrella-shaped canopy. The major rivers are Asa, Agba, Alalubosa, Okun, Osere and Aluko. Some of these rivers drain into river Niger or river Asa (Oyegun, 1986). The drainage system of Ilorin is dendritic in pattern due to its characteristics. The most important river is the Asa River which flows in south northern direction. Asa River occupies a fairly wide valley and goes a long way to divide Ilorin into two parts namely the Eastern and the western part. Ilorin is primarily drained by the Asa River, which runs through a fairly wide valley and flows south to north (Ajibade and Ojelola, 2004). The drainage system is depicted in the Fig. 3 below. Ilorin elevations vary from 273m to 333 m on the western side, and a Hill (Sobi Hill) rises to around 394m above ground level, while it varies from 273m to 364m on the eastern side (Ajibade and Ojelola, 2004). The lowest point is along the Asa and Oyun river valleys, while the highest point is Sobi Hill.
