Cracks on concrete and masonry walls could be bothersome for quality of life and for property claims. Diverse patterns of wall cracks, leaning, and differential settlement of three building structures  in  Zaria  area,  north  central  Nigeria were investigated using geophysical imaging technique. Seismic Refraction and Electrical Resistivity Imaging were integrally applied in the study. Hence the mechanisms of the failures, cracks’ identity and classifications, patterns and sizes, based on their cause traced in the survey were accentuated. The results of the integrated geophysical imaging were provided to resolve some ambiguous questions raised by indigenous geoscientists and engineers over the prevalent structural failures. 

TABLE OF CONTENTS

 TITLE PAGE

APPROVAL PAGE
DEDICATION
ACKNOWELDGEMENT
ABSTRCT
TABLE OF CONTENT

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE STUDY
• SIGNIFICANCE OF THE RESEARCH
• SCOPE OF THE STUDY
•  PROBLEM AND LIMITATIONS
• TYPES OF CRACKS
• OBJECTIVE OF THE STUDY

CHAPTER TWO               

LITERATURE REVIEW
2.0      LITERATURE REVIEW
2.1      OVERVIEW OF THE STUDY
2.2     OVERVIEW OF SEISMIC REFRACTION
2.3     OPERATION PRINCIPLE OF SEISMIC REFRACTION METHOD
2.4     APPLICATIONS OF SEISMIC REFRACTION

CHAPTER THREE

3.1      RESEARCH DESIGN AND METHODOLOGY

CHAPTER FOUR
RESULT ANALYSIS
4.1 RESULT AND DISCUSSION

CHAPTER FIVE

5.0      CONCLUSIONS, RECOMMENDATION AND REFERENCES

• CONCLUSIONS
• RECOMMENDATION

5.2     REFERENCES

 

 

CHAPTER ONE
1.0                                                        INTRODUCTION
Structural failure is said to have taken place when there are unacceptable differences between expected and observed performance of any structure.  In many parts of the world, lives have been lost and casualties have been recorded due to structural failures. Common structural failures in the world today include the failures of bridges, dams and the failure of buildings, which is the most prominent of all.  Records of such structural disasters are common in Western Canada, Colorado, Texas, Wyoming, India, Nigeria, Israel, South Africa, and to some extent South Australia, California,  Utah,  Nebraska  and  South  Dakota most of which were associated with swelling clays (Blyth and de Freitas, 1988).
causing the ground to respond with uneven and excessive movement.   Building  failures can be considered  to  have  occurred  in  a  component when  that  component  can  no  longer  be  relied upon to fulfill its principal functions. Limited deflection in  a  floor  which  causes  a  certain amount of cracking and distortion in partitions could reasonably be considered as defects, whereas excessive deflection resulting in serious damage to partitions, ceilings and floor finishes could be classified as utter failure. Investigators and reporters of structural failures are not only expected to identify trends leading to structural safety problems but are also expected to suggest topics for critical research leading logical solution against the trend (Chapman, 2000).
Foundation cracks are evaluated based on their size and extent. The British Royal Institute of Chartered  Surveyors  (RICS)  evaluated  cracks and tilting of buildings based  on the questions pointing to whether they are slight, isolated, moderate, severe, or very severe cracks. Other questions used for evaluation are based on whether there are multiple small cracks, leaning, shifting (creep and crawl) which can be serious. Generally, cracks associated with displacement of original structural or mechanical components are considered to be significant (Donald and Cohen,
1998).
Most   settling   cracks   on   building   walls   are basically caused by either the differences in expansion and compression coefficients of construction materials, relative changes in the shapes and  sizes  of  saturated  soils  or  the dynamic earth. The amount, type and direction of foundation movement are commonly noted from the bulging of brick or masonry block. These in turn   reveal   the   risk   of   vertical   collapse   or horizontal dislocation. The risk could be traced to the height of construction, material used for the building, site factor, earth loading or waterlog. Other    factors    include    the    seismic    action, atmospheric  extremes,  or  mere  physical accidents.  However, if cracks are old with no sign of continuing or recurrent movement, building inspectors accept monitoring rather than quickly recommending repairs (Tim, 2006).
The cause of rampant failure of building foundations due to subsurface movements giving rise to cracks or structural differential settlements is of great  concern  to  geoscientists.    There is need to distinguish between a continuing movement, which is often more likely to be a problem, and single eventual movements, which may not require repair depending on the extent of damage.
Adequate insight on the types, and patterns or foundation-based cracks and their evaluation is vital as one considers the geological and geophysical basis to buildings’ failures. Therefore this investigation is therefore aimed at reviewing diverse cracks patterns and failure mechanisms of some buildings in Zaria area, northwestern Nigeria.  Two  geophysical  imaging  techniques were applied in the investigation with the aid of state-of-the art geophysical equipment as an arm of an on-going multidisciplinary survey on buildings’ foundation failures in Zaria area.

1.2                              BACKGROUND OF THE STUDY
Foundation cracks on buildings occur as a result of differential movement on the building. In most cases, serious damages caused by cracks on building can be safely repaired when these differential movements stop.   The size, shape, pattern, and location of foundation cracks on a building, when correlated with other site and construction conditions, help to distinguish among probable causes foundation based failures (Tim,2002).
Before the present study, some geological and geophysical works have been carried out in Zaria area yet, none of them considered the structural failure of some buildings in the area. Table 1 shows the summary of the previous geophysical investigations in Zaria area.
Presently, there are ongoing environmental geophysical surveys in the area which have included the investigation of structural failure of buildings, origin of some valleys, dam site investigations, pollution studies, and classification of Zaria rocks.  
The present study is therefore the first to address the structural failures using integrated geophysical technique. Seismic refraction tomography and Electrical resistivity imaging techniques are simultaneously applied at three building site.

Year of Study	Geoscientist(s)	Subject of Study	Infere nces
1968	Ososami	vertical electrical sounding for ground water potential	Depth to the aquifer in the area varied from about 1m to about 30m. demarcation of the boundary between the weathered basement and the fresh basement
1978	Baimba	resistivity and seismic refraction studies for ground water exploration	basement complex generally forms poor source of ground water
1979	Onugba	detailed resistivity and seismic refraction survey of Kimberlite pipe area	correlation of seismic velocities of shallow structure with resistivity profiling results, has shown close agreement between the regions of anomalous
1985	Olufemi,	Resistivity survey	Siting of Borehole
1990	Ogah	applied the beam forming techniques to enhance low quality seismic refraction data at Kubanni drainage in the area	beam forming technique adequate for extracting useful information from low quality seismic refraction record
1990	Shemang	Resistivity survey	weathered and fractured basement constitute the main aquifer components
1991	Hassan, et al.	Geo-electrical investigation	Bedrock undulating

1.2                               SIGNIFICANCE OF THE STUDY
The study area lies within 70  35 17″E and 70 4117″E longitude, and 110 7 50″N and 110 11 22″N latitudes on the National grid of Nigeria and at an elevation of about 670 m above the mean sea level. It lies on a dissected portion of the Zaria– Kano plains. The plains are an extensive peneplane developed on the crystalline rocks of the   Nigerian   Basement   Complex.   Residual granite inselbergs, the largest of which is the Kufena Hill, provides the main relief in Zaria area. The area has a tropical continental climate with distinct wet and dry seasons.  Three of affected buildings were selected for study from the area. Table 2 shows the locations of the three buildings’ sites selected for study as measured with the aid of a geo-positioning system.

Site one’s building has been under maintenance with masonry patches which have not provided a permanent solution to the problem.   Although some of the cracks observed were not originated from the foundation but from the roof owing to poor masonry work. The most prominent crack is a major vertical cracks which divides the building at its centre in NE-SW direction (Figure 1).

1.3                                                 SCOPE OF THE PROJECT
Cracks can be broadly classified as either active or dormant. Active cracks show some change in direction, width or depth over a measured period of time while dormant cracks remain unchanged. If left unrepaired, both active and dormant cracks provide channels for moisture penetration, which can lead to future damage.


1.4                                           PROBLEMS AND LIMITATION
The preceding models assume planar boundary interfaces. Conformable sequences of sedimentary rock may form planar boundaries. However, erosion and uplift easily produce irregular boundary contacts. More sophisticated algorithms can process refraction surveys where irregular interfaces might be expected. Profile length and source energy limit the depth penetration of the refraction method. Typically, a profile can only detect features at a depth of one-fifth survey length. Thus, refraction imaging of the Moho would require profile lengths of over one hundred kilometers; an unreasonable experiment. Larger sources could be utilized for greater depth detection, but certain sources (e.g. explosives) may cause
problems in urban areas. Refraction depends on layers to increase in velocity with depth. In the hidden slow layer senario, a buried layer is overlain by a faster layer. No critical refraction will occur along the boundary interface. Thus, refraction will not detect the slow layer. All is not lost since reflection seismology could detect the slower layer.

Seismograms require careful analysis to pick first arrival times for layers. If a thin layer produces first arrivals which cannot easily be identified on a seismogram, the layer may never be identified. Thus, another layer may be misinterpreted as incorporating the hidden layer. As a result, layer thicknesses may increase.

1.5                                       TYPES OF CRACKS

The severity of a crack can be characterized in terms of its direction, width, and depth; cracks may be longitudinal, transverse, vertical, diagonal or random. Different risks for cracking exist for cured versus uncured concrete, and for reinforced concrete. Breakages occur through thermal, chemical or mechanical processes causing shrinkage, expansion or flexural stress. Below is a list of types of concrete cracks, and some of their possible causes:

• Plastic-shrinkage cracking: Cracks that run to the mid-depth of the concrete, are distributed across the surface unevenly, and are usually short in length. Most often occurs while concrete is curing, due to the surface of the concrete drying too rapidly relative to the concrete below.
• Crazing/Map cracking/Checking: A web of fine, shallow cracks across the surface of the concrete.
• Also occur during curing due to the surface of concrete drying faster than the interior concrete, but the surface drying occurs at a lesser depth.
• Because this type of cracking is limited to the surface, it does not usually pose serious structural problems.
• Hairline cracking: Very thin but deep cracks.
• Due to settlement of the concrete while it is curing.
• Due to their depth, these cracks can allow for more serious cracking once the concrete is hardened.
• Pop-Outs: Conical depressions in the concrete surface
• Occurs when a piece of aggregate near the concrete surface is particularly absorbent, causing it to expand and pop out of the surface of the concrete.
• Scaling: Small pock marks in the concrete surface, exposing aggregate underneath.
• Once cured, if concrete does have an adequate finish to prevent water penetration, water that seeps into the concrete will expand when it freezes, pushing off pieces of the concrete surface.
• Scaling can also be caused by delamination, which occurs when too much water (due to insufficient curing) or air (due to insufficient vibrating) remains in the concrete when it is finished. The water and air rise to the top and form pockets below the surface. These pockets may form blisters or which may break open to create scaling. 
• Spalling: Surface depressions that are larger and deeper than scaling, often linear when following the length of a rebar.
• Also caused by pressure from under the surface of the concrete.
• Most often occur due to improperly constructed joints or the corrosion of rebar in the concrete
• Corrosion creates pressure as rust forms, which can push away large chunks of concrete, and expose the corroded metal below.
• Spalling that exposes corroded metal can be particularly problematic because the corrosion is likely to accelerate due to exposure to air and water.
• D-Cracking: Cracks that runs roughly parallel or stem from a concrete joint and are deeper than surface cracks. 
• Due to moisture infiltration at the joint.
• Offset cracking: Cracks where the concrete on one side of the crack is lower than the concrete on the other side.
• Due to uneven surfaces below the concrete, such as subgrade settlement or pressure from objects such as tree roots, previously-placed concrete, or rebar. 
• Diagonal corner cracking: Cracks that run from one joint to its perpendicular joint at the corner of a slab
• The corners of concrete slabs can be prone to curling (due to differences in temperature at different depths in the curing concrete) or warping (due to differences in moisture evaporation at different depths in the curing concrete). The dryer or colder level of concrete will shrink more and create cracks as the concrete dries.
• Because the warped or curled-up corners often have some empty space below them, they are also prone to cracking after curing due to weight overload causing the corner to snap downward into the empty space. 

Different type of crack is shown as below:

 

Figure 1: Some Views of Cracks on Site 1 Building

 

1.6                                               OBJECTIVE OF THE STUDY
The aims of the project are to establish geoscientific on the ground support for a sustainable development and management of save building constructions, lifelines, infrastructure and also natural resources. Seismic reflection method is used to achieve the goal of the present study.

 

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