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EVALUATION OF LIQUEFACTION POTENTIALS OF SOILS AT SOME SITE IN UYO METROPOLIS
1.1 Background Study Of Overview:
Increasing losses due to environmental hazards in the past two decades have led to a general shift in policy from mere post event response to emphasis on loss reduction measure like mitigation, preparedness and recovery programme. Earthquake induced liquefaction and related ground failures have caused substantial casualties and major property losses in various parts of the world and as such the need for this research.
Liquefaction is a phenomenon in which saturated or partially saturated cohesion less soils are temporarily transformed into liquid state, most commonly as a result of earthquake induced ground shaking. Liquefaction occurs as a result of the build up of excess pore water pressure during shaking. When the pore water pressure exceeds the grain to grain (effective) contact with each other and the soil essentially behaves as a liquid. Pore- water pressure in aliquefied soil may become great as to result in small geysers from which water is ejected, leaving sediment any features commonly termed SAND BOILS
1.2 Statement ofthe Research Problem:
Although Nigeria is not located within the major seismic zones of the world, over the years, several minor earthquakes have been experienced in some parts of the country. Earthquake is a global phenomenon experienced in most regions of the world. It is classified as one of the most devastating natural disasters that pose threat and has the capability to impact negatively on human lives. Seismic activities in Nigeria and other countries located along the mobile belt of Africa between West African craton and Congo craton are believed to be a seismic (without earthquake or other seismic activities). However, Nigeria began experiencing series of earthquakes of low local magnitudes (between 4 and 5) from the early 1930s, and this led to in depth studies into the seismology of the country (Tsalha et al. 2015). Some studies have reported that earthquakes in Nigeria are triggered by intraplate tremors unlike what occurs in other regions of Africa (Eze et al. 2012). This tremor was attributed to regional stresses created as a result of the country’s position between the West Africa craton and Congo craton, and zones of weakness resulting from magmatic intrusions and other tectonic activities in the sediments (Eze et al. 2012; Afegbua et al. 2011). Most of the early earth quakes in Nigeria were not well documented because of the absence of seismology equipment at those times. Table 6 presents a list of historical earthquakes and tremors felt in Nigeria and the regions with documented scientific measures; and also other earthquake events which were not documented scientifically. One of the most recent earthquake events in Nigeria is the quake of 2009 which was felt in Abeokuta, Ago-Iwoye, Ajambata, Ajegunle, Imeko, Ijebu-Ode, Ilaro and Ibadan towns of south western Nigeria. The earthquake which had a local magnitude of 4.4 was reported to be triggered by a ruptured fault within the upper crust (Akpan et al. 2014). In 2016, series of earth tremors and quakes were felt in Saki, Oyo state in south western Nigeria, also in communities in Bayelsa and Rivers State in Southern Nigeria, and Jabal area of Kaduna state in north western Nigeria (Vanguard.com 2017). The most recent occurred in September 2018 in the countries capital city, Abuja (Premium Times.com 2018). The causes of these events are presently uncertain and studies on the events are still ongoing, a clear indicator that Nigeria may not be free of earth tremors in the near future. The review on the seismicity of Africa has shown that Africa is not only prone to seismic activities but the frequency of the occurrences has been on the rise at the turn of the century due to changing tectonic activities and increased human interaction with the earth surface. It is thus imperative for proactive measures to be instituted to avert any potential adverse consequences of seismic activities that may arise in future. Mitigation strategies for structural vibration African record Reports from the previous section have shown that Africa is prone to seismic activities of varying magnitudes especially the regions around the EARS. These seismic activities have caused serious damage to properties, reduced the quality of living and even led to instabilities in economies of some of the affected nations. The intensity of seismic events is often measured on the Modified Mercalli Intensity (MMI) scale which provides information on the observed effects of the event (extent of vibration) on people (and other living things), objects and buildings/infrastructures around the region of occurrence (Midzi et al. 2013). The information on the scale maybe subjective to accounts of residents in the area where the event took place, but it gives a guide on the extent of damage and shock per event. Studies have shown that the intensity of earthquakes in Africa vary between I and VIII, signifying that earthquakes around the African continent range from minimal or no effect to slight damages on well-designed buildings. Although the damages have not been very severe, it is necessary to devise earthquake mitigating methods, since the continent is known to undergo intra-plate earthquakes which increase the frequency of earthquake occurrences on the surface of the earth. Most earthquake deaths are reported to be related to partial or complete collapse of civil structures (Kenny 2009). These deaths are linked to the response of the structural and non-structural parts of building to ground vibrations during earthquakes. Arya, (2000) divided earthquake protection measures for buildings into three major parts which are; based on architectural design. However, studies around the world have been channelled towards the advancement of structural designs which would minimize the adverse effects of earthquakes on buildings which could eventually reduce the casualties associated with such seismic events. Effective methodologies for building design against earthquake should consider the dynamic response of the building to forces generated by earthquakes. This can be achieved by selection of the preferred response mode, selecting zones where inelastic deformations are acceptable and suppressing the development of undesirable response modes which could lead to building collapse (Constantinou and Symans 1993). The inherent strength of a building is one major parameter that affects its response to seismic events. Different buildings have different responses to seismic excitations. Adobe buildings and other non-engineered building are low cost buildings which are very popular in developing nations (countries in Africa). These buildings which are made from materials which have low compressive strength and stiffness are mostly patronized because of their low cost and simplicity of construction, and excellent thermal and acoustic properties (De Risi et al. 2011). Arya (2000) surmised that most non-engineering buildings lack good permanence under seismic action, and are therefore prone to partial or full collapse when subjected to earthquakes of relatively high.
The occurrences of an earth tremor in Nigeria were first reported in 1984, thensubsequently in 1990, 1994, and 2000. They had body wave magnitudes ranging from 4.3 to 4.5, local magnitudes of 3.7 to 3.9. The most frequent earthquakes were observed in the western part of Nigeria, near Warri, Lagos, Ibadan and Akure.Experts have warned that there is an impending earthquake looming in South-South and some other states in Nigeria before 2020.
The Nigeria Association of Water-Well Drilling Rig Owners and Practitioners (AWDROP) has called on the Federal Government to take measures that could reduce the effects of earthquakes as 6 states exist at the risk of possible earthquake. AWDROP, at a press conference by its National President, Mr Michael Ale, lamented that lack of Underground Water Abstraction Regulation in Nigeria could easilyinduce earthquake and sea water intrusion “hence the need for government to ensure strict compliance in the implementation of code practice in bore hole drilling in the country.
Ale said his association has been informed of the salt water intrusion affecting under - ground water usage and subsidence along the land ocean boundaries in shoreline areas