AN INTEGRATED SURVEY FOR SITE INVESTIGATION AT THE ABANDONED SENATE AUDITORIUM OF UNIVERSITY OF NIGERIA NSUKKA BEHIND ECO BANK AND UBA

ABSTRACT An integrated investigation involving electrical resistivity method, Induced polarization method, and triaxial tests were used to investigate an abandoned senate auditorium in university of Nigeria, Nsukka, southeastern Nigeria. This was with a view to determining or imaging the subsurface structure as they may cause instability and led to multiple cracks of the building. The building in the study area is intensively affected by severe cracks. The electrical resistivity and induced polarization method were carried out using Abem Terameter SAS 1000. The latitude and longitude data of each electrode position were collected and keyed into Google Earth software to imagine the exact spatial positions. In the study area the survey pattern was designed in a way that survey lines were carried out in a form of trapezium following the shape of the building fig.10, each of the profile lines was executed using a Wenner array method with a minimum electrode spread (a) of 10m and maximum of 30m and extended about 160m respectively.  The acquired data were processed and interpreted to elucidate the subsurface geologic structure of the study area. The interpretation of the data acquired led to the delineation of resistivity anomaly. The resistivity anomaly that is typically of a vertical structure (seemed to be a fault that filled up with low resistivity materials) was delineated along traverse one (fig.11) and three (fig.15), and it is surrounded with high background of high resistivity, showing fault zone. In profile line ERT2, it shows an inclined anomalous zone of relatively low resistivity fig.13. Profile line four is opposite of profile line two picked a similar low resistivity anomaly at the same spatial location. Fig.17. Data acquired in field were plotted using suffer 9 software to determine the 3D view of raw apparent resistivity, fig.12, 14, 16 and 18.  The traixial tests conducted on the soil samples collected at the same locations of profile lines revealed that there is a failure in the soil mass in which the building is erected.

CHAPTER ONE

1. INTRODUCTION

1.1     BACKGROUND

It is a well known fact that subsurface investigations involving the use of geophysical techniques are important for assessing the suitability of an area for the construction of buildings, bridges and dams among others. Collapsing of buildings and foundation problems have been in existence for long, it is common knowledge that most of these buildings have been constructed without prior geophysical investigations to determine the nature of the subsurface structure.

Paramount to the construction of any building is the suitability of the geology of the site. Most buildings are constructed on soil that has inadequate bearing capacity to support the weight of the structure. Subsurface geological features such as fractures, fault, nearness of the depth to bedrock, nearness of the water table to the surface are among the inconveniences that pose constraint to building constructions especially to their foundations.

The use of the geophysical method as an effective tool for gaining knowledge into the subsurface structure, in particular, for identifying anomalies and defining the complexity of the subsurface geology is fast gaining grounds (Soupois et al., 2007, Lapenna et al, 2005).

In recent times, much attention is being paid to the electrical resistivity imaging (ERI) method (Griffiths and Baker, 1993; Loke and Barker,1996, Schmutz et al., 2000), which provides a high spatial resolution with a relatively fast field data acquisition time and is low in cost.  Geophysical methods are implemented in a wide range of applications ranging from ground investigations in building constructions to the inspection of dams and dikes (Soupios et al., 2007),

 aiming at the exploration of geological structures and the determination of the physical parameters of the rock formations.

In the case of building construction, geophysics can be applied for exploration purposes to provide useful information regarding the early detection of potentially dangerous subsurface conditions. The procedure for obtaining subsurface information is divided into two broad categories: indirect and direct methods. Indirect methods include aerial photography, topographic map interpretation and the study of existing geological reports, maps, and soil surveys. Direct methods comprise of the following modules: (a) geologic field reconnaissance, including the examination of in-situ materials, man-made structures, groundwater level and exploration of shafts, (b) application of modern geophysical techniques for mapping subsurface structures, (c) borings, test pits, trenches and shafts from which representative disturbed and/or undisturbed samples of the in-situ materials may be obtained and analysed and (d) simple geotechnical field tests, such as the standard penetration test (SPT), which can be correlated with other engineering parameters (Soupois et al., 2007). Some of the geophysical methods that can be applied in the direct method for mapping subsurface structures include: ground penetrating radar (GPR), seismic, electromagnetic terrain conductivity, electrical resistivity tomography (ERT), 2D and 3D electrical resistivity imaging (ERI). The 2D and 3D electrical resistivity imaging can be carried out using the electrical resistivity method with the necessary software for processing of the acquired field data.

This geophysical investigation that has been carried out at the Senate Auditorium of the University of Nigeria, Nsukka for determining the subsurface geological condition of the site is therefore needed a detailed subsurface investigations to delineat the cause of cracks on the building; the presence of fractures, faults, voids and clay, which are important in characterizing the suitability of the area for building purposes. The research was carried out using the Abem Terrameter (SAS 1000) system based on the Wenner electrode configuration. As many as 4 profile lines were investigated at the site using the ABEM terremeter for the data acquisition. The data inversion method proposed by Loke and Baker (1996) was carried out using the RES2DINV and RES3DINV software. The 2D and 3D resistivity models generated were interpreted to determine the subsurface geologic conditions.

1.2     OBJECTIVES

The objectives of the study

1. To determine the cause of cracks on the wall
2. To determine the subsurface geology of the study area
3. To determine the bearing capacity of soil  of the study area

1.3     GEOLOGY OF STUDY AREA

The study area is located in the Anambra basin of south-eastern Nigeria. It consists of three major geologic formations; the Mamu, Ajali and Nsukka formations, respectively. The Mamu formation, previously known as lower coal measures (Reyment, 1965), consists of fine-medium grained, white to grey sandstones, shaly sandstones, sandy shales, grey mudstones, shales and coal seams. The thickness is about 450m and it conformably underlies the Ajali formation. The Ajali formation, also known as false bedded sandstone, consist of thick friable, poorly sorted sandstones, typically white in colour but sometimes iron-stained. The thickness averages 300 m and is often overlain by considerable thickness of red earth, which consists of red, earthy sands, formed by the weathering and ferruginisation of the formation. The Nsukka formation, previously known as the upper coal measures (Reyment, 1965), lies conformably on the Ajali sandstone. The lithology is very similar to that of Manu formation and consists of an alternating succession of sandstone, dark shale and sandy shale, with thin coal seams at various horizons. Eroded remnants of this formation constitute outliers and its thickness averages 250 m.

The study area is located at Ajali formation, fig. 1and 2.

AGE	FORMATION
Paleocene-Danian	Nsukka formation
Maastrichtian	Ajali formation
Campanian-Maastrichtian	Mamu formation *                                                               Nkporo formation *                                                                                       Agata shale *                                                                                                        Enugu shale*                                                                                                 Owerri sandstone*
Coniatian-Santonian	Agwu formation
Turonian	Eze-agu formation*                                                                                Amasiri sandstone*
Cenomanian	Odukpani formation
Albian	Abakaliki shale                                                                    Unnamed formation
Pre cambrian	Basement complex

Table.1, STRATIGRAPHY OF ANAMBRA BASIN

      *= Lateral equivalents,   #=Asu-River group (Reyment, 1965)

Fig.1, geologic map of Anambra basin

1.4    LOCATION

The study area is located at University of Nigeria, Nsuka, Enugu state, Nigeria. It lies within latitude 6⁰51” N -6⁰53’’N and Longitude 7⁰24’’ E -7⁰26’’

fig.2, the geologic map of University of Nigeria, Nsukka

Fig.3, map of nsukka

Fig. 4, Geologic map of Nigeria

1.5          FAULT

It is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the earth’s crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as sub-duction zone or transform faults.

1.5.1                FAULT LINE

It is the surface trace of a fault, the line of intersection between the fault plane and the earth’s surface. Since faults do not usually consists of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.

1.5.2 MECHNISM OF FAULT

Because of friction and the rigidity of the rock, the rocks cannot glide or flow past each other. Rather, stress builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is dissipated by the release of strain, which is focused into a plain along which relative motion is accommodated.

1.5.3       TYPES OF FAULT

1.  Normal fault: In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and result in extension.

2.  Reverse fault: In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compress ional forces and results in shortening.

3.  Strike-dip fault: In a strike-dip fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, the fault is called left-lateral. If it is right, is called right-lateral.

4.  Transform fault: Is a type of strike-dip fault wherein the relative horizontal slip is accommodation the movement between two ocean ridges or tectonic boundaries.

1.6     SHEAR STRENGHT

Shear strength of a soil is its maximum resistance to shear stresses just before the failure. However, the shear stresses develop when the soil is subjected to direct compression. In the field soils are seldom subjected to tension, as it causes opening of the cracks and fissures. These cracks are not only undesirable, but are also detrimental to the stability of the soil masses. Thus, the shear failure of the shear mass occurs when the shear stresses induced due to the applied compressive loads exceed the shear strength of soil. It is noted that the failure in soil occurs by relative movement of the particles not by breaking of particles.

Shear strength is the principal engineering property which controls the stability of the soil mass under loads. It governs the bearing capacity of soil, the stability of slopes in soil, the earth pressure against retaining structures and the many other problems.

1.7  MOHR’S CIRCLE

It is a graphical method for the determination of stresses on a plain inclined to the principal plains. The graphical construction is known as Mohr’s Circle. In the method, an origin O is selected and the normal stresses are plotted along horizontal axis, and the shear stresses on the vertical axis. As the compressive stresses are taken positive in soil engineering, these are plotted towards the right of the origin that is along positive x-axis.

The soil is a particulate material. The shear failure occurs in soil by slippage of particles due to shears stresses. The failure is essentially by shear, but shear      stresses at failure depend upon the normal stresses on the potential failure plane. The soil fails when the shear stress on the failure plane at failure is a unique function of the normal stress, acting on that plane. The Mohr circle is connected with shear at failure plane at failure. Failures of materials occur when the Mohr circle of the stresses touches the Mohr envelope fig.5. Any Mohr circle which does not cross failure envelope and lies below the envelope represents a (non-failure) stable condition. The Mohr circle cannot cross the Mohr envelope, as the failure would have already occurred as soon as the Mohr circle touched the envelope.

Fig.5. Failure Envelope

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