Investigation Of Causes Of Geological Activity That Resulted In Offices Floor Crack Using Seismic Refraction Method

Project and Seminar Material for Civil and Environmental Engineering

Investigation Of Causes Of Geological Activity That Resulted In Offices Floor Crack Using Seismic Refraction Method


Abstract


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.


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 StudyGeoscientist(s)Subject of Study

Infere

nces

1968Ososamivertical electrical sounding for ground water potentialDepth 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
1978Baimbaresistivity and seismic refraction studies for ground water explorationbasement complex generally forms poor source of ground water
1979Onugbadetailed 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
1985Olufemi,Resistivity surveySiting of Borehole
1990Ogahapplied 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
1990ShemangResistivity surveyweathered and fractured basement constitute
the main aquifer components
1991Hassan, et al.Geo-electrical investigationBedrock 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.


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