Browsing by Author "Tadesse, Samuel(PhD)"

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    Basic Engineering Properties of Lateritic Soils Found in Nejo – Mendi Road Construction Area, Welega
    (Addis Ababa University, 2005-11) Zelalem, Abebaw; Tadesse, Samuel(PhD)
    Lateritic soils are highly weathered and altered residual soils formed by the in-situ weathering And decomposition of rocks in the tropical and sub-tropical regions with hot, humid climatic Conditions. Their formation also consists of leaching out of free silica and bases and Accumulation of oxides of iron, aluminum or both. This process is called laterization. Laterites Are rich in sesquioxides (iron oxides, aluminum oxides or both) and low silicates but may Contain appreciable amounts of kaolinite. Due to the presence of iron oxides lateritic soils are Red in colour. Laterites are grouped under group of soils with a strong mineralogical influence derived from Clay minerals only found in residual soils. Soil engineering properties under this group are Highly influenced by the presence of sesquioxides. Sesquioxides appear to act as cementing Agents which bind the other mineral constituents into clusters or aggregations. With sufficient Concretion of sesquioxides, the hard concretionary materials are formed. This concretionary Bond is broken down during test manipulation for some lateritic soils. In this thesis work, some peculiar geotechnical and geo-chemical characteristics have been Investigated on soils sampled from Nejo- Mendi road construction area. Accordingly, the soil Samples under investigation have been obtained to be true laterites, in short referred to as Laterites. Moisture content determination using oven temperature of l05°C and oven Temperature of 50°C with maximum relative humidity 30% were also carried out on the soil Samples to investigate presence of loosely bound water of hydration. It was obtained that the Soil samples did not contain loosely bound molecular water in a significant amount. The soil specimens were tested at different sample preparation conditions prior to testing. The Engineering properties investigated in the course of the research work include index tests, Compaction and California Bearing Ration (CBR) tests at oven temperature of l05°C dried, air Dried (oven temperature of 50°C with maximum relative humidity 30%) and as received Conditions on the soil samples. Accordingly, pre-treatment variation has not significantly Change index, compaction and CBR test results. Viii Atterberg limits were investigated for different testing procedures on the soil samples to see Effect of test manipulation on concretionary bond. The liquid limit tests were carried out on Soil specimens mixed for 5 minutes and 30 minutes durations. It was observed from the test Results that the mixing durations has significant effect on the values of liquid limits. Accordingly, the soil samples have been sensitive to te
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    Computation of Soil Compressibility Using Tangent Modulus Approach
    (Addis Ababa University, 2003-07) Sissay, Dessalegn; Tadesse, Samuel(PhD)
    Keywords: tangent modulus, resistance, modulus number and strain There are several mechanisms causing settlement of a structure. Different structures have varying potential to withstand the settlement that takes place Thus, its determination has to be given due consideration by engineers as it affects these structures from functioning properly if not limited to a permissible value. Soils consist of solid particles and void space. Gas or water or combination of both occupies the void space. When loaded the water and air gets squeezed out resulting in settlement. Determination of settlement and the rate at which the expulsion takes place is determined from the theory of consolidation. The classical approach, which is widely used in our country and other parts of the world, has limitations. These limitations have contributed for the development of the tangent modulus approach. The tangent modulus approach provides better way of determining settlement for various types of soil ranging from hard to very soft soil. In the method the soil property, which is determined from laboratory soil deformation tests, are used. In the approach a compression modulus also called the constrained modulus, the tangent to a linear plot of stress versus strain curves determined from laboratory tests play a significant role. It is emphasized on this property of the soil, as it is a measure of resistance of the soil against deformation due to change in loading condition The compression modulus, measures of the resistance of a media or an isolated part of it against a forced change of equilibrium condition. It is based on the resistance concept, which is very well developed in other field of engineering, except that its application for settlement analysis in soil mechanics is new. 1 1. Introduction 1.1 General Construction scheme is advancing day to day. This has made possible construction of high rising buildings, heavy structures and highways. Due to this heavy loads are induced on to the soil underneath. Besides the use of undesirable construction site is increasing. The above factors results in compressive strains causing settlement of a structure. The resulting settlement has to be limited to a minimum to allow the structure function properly and maintain the required aesthetic value. Further cost of maintenance has to be limited to a minimum. For estimating the settlement that occurs in cohesive sols the classical approach is widely used in most parts of the world including Ethiopia. This approach assumes the compressibility of the soil remains constant. Further it was believed that the initial pore water pressure describes the consolidation characteristics of thick clay. These and other assumptions limit the validity of the approach. Thus, another method of settlement evaluation which amends at least the above mentioned drawbacks have to be adopted. This paper introduces another method as compared to classical method for the determination of settlement, which is known as the tangent modulus approach. 1.2 Background The probable settlement any structure undergoes on different types of soils is predicted by compressibility study. Thus, based on the compressibility characteristics the method which relies on the stress- strain analysis of soil for determining this property is dealt with. 1.3 Objective of the Present Work The objective of the research work is to: i) Introduce a recent and more practical way of determining settlement and compare the applicability with the existing and more popular classical approach. ii) To
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    Detemination of Parameters for The Tangent Modulus Approach
    (Addis Ababa University, 2004-11) Nerea, Tesfay; Tadesse, Samuel(PhD)
    The estimation of settlement often receive less attention than they deserve (or even not at all), with the result that excessive settlements cause far more problem than do bearing capacity failures. Settlement analysis is therefore one of the two most important types of analysis made by soil engineer for designing safe and economical structures. This paper introduces different method for the determination of settlement, which is known as the tangent modulus approach, this approach deals with the use of a coherent definition of compression modulus leading to a unified procedure of practical settlement calculation for different types of soils ranging from rocks to very soft soil. In this approach a compression modulus, the tangent to stress Vs strain curves obtained from laboratory tests play a significant role. It is a measure of the resistance of soil against deformation due to application of loading. The compression modulus describes the resistance of the media or an isolated part of it against a forced change of equilibrium conditions. The resistance concept is very well developed in other field of engineering except its application for settlement analysis in soil mechanics is new. The procedure is therefore believed to give a much clearer mechanical understanding of what volumetric soil compression actually is. Moreover, the use of classical concepts have lead to a simpler, and more straight forward calculation procedure, where the practical advantage are most apparent when applied to layered soils. Even though the classical approach for determining the consolidation settlement suggested by Karl Terzagi has contributed more for the development of settlement computations, it has its limitations in that it assumes the value of the compression modulus to be constant, which is true only for elastic material. Further more, it was believed it is the initial pore water pressure that describes the consolidation characteristics of thick clay layer. But it is shown here in that a new theory for stress distribution, compatible with the actual soil properties is included; moreover, it is shown that the calculation of the time rate of consolidation of clay has to be based on strain distribution, instead of pore pressure distribution, to avoid fundamental misunderstandings. It is therefore the application of this tangent modulus approach by introducing and fixing parameters for Addis Ababa clay soils that this paper introduces.
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    Preparation of Ethiopian Standard Sand for The Purpose of Construction and Testing
    (Addis Ababa University, 2005-09) Berhanu, Belayneh; Tadesse, Samuel(PhD)
    Sand is a naturally produced material resulting from the mechanical and chemical breakdown of rocks. Sand is the most common and loosely packed mineral on the earth's surface. Sand particles mostly range from 0.02 mm to 2.00 mm in diameter. Silica sand is the term used to describe sand that has a very high percentage of silicon dioxide (SiO2). In the field of civil engineering, silica sand is used for several purposes like, in construction, in industries for the production of different construction materials. Sand, which is graded according to a specified standard, is used for testing of Portland cement and concrete, in field density determination using sand replacement method, and in water and sewage treatment plants as a filter media. Due to lack of standard silica sand in Ethiopia, the country is losing high foreign currency by importing standard silica sand from abroad. The purpose of this research is the identification of the different sources of sand in different parts of the country for the purpose of standard sand production. To achieve the research objective, sand samples for laboratory investigation and field test were collected from North Showa (Jema river valley) and Dire Dawa town. Visual inspection, index property tests, test for impurities and chemical contents were conducted on both samples. Sand replacement and mortar test were done to check the applicability of the local samples 9 in field density determination and in testing of Portland cement respectively. Grain size analysis and impurity test results revealed that both local sand samples cannot be used as standard sand as they are collected directly from their natural places. Thus, inorder to be used as standard sands both samples must be washed, oven dried, sieved and separated to various size fractions and mix each fraction according to the grading requirement. In all of the test results, the compressive and flexural strength of mortar cubes made with local samples gave lower values compared to that of imported sand. However, a relatively uniform trend was observed both in the compressive and flexural test results between the local samples and the imported one. The field density test by sand replacement method showed that, both local sand samples gave the same result with the imported Ottawa standard sand in almost all test locations irrespective of the silica content. Laboratory and field tests conducted on both local sand samples showed that there is a high possibility that both local sand samples can replace the imported standard sand.
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    A Study of Undrained Behavior of Saturated Sand and The Concept of Steady State
    (Addis Ababa University, 2005-01) Enawgaw, Abraham; Tadesse, Samuel(PhD)
    It has been very common to see collapse of structures, slopes, and foundations due to flow of foundation soils without observing any structural failure during earthquake. This can also occur frequently and cause severe damages in this country following the occurrence of earthquake, particularly in the Rift Valley region where there is a considerable deposit of saturated sand and silty sand. This problem has been studied in various countries and attributed to Liquefaction Phenomenon. The phenomenon occurs in saturated sand, which either completely loses its strength and flow as a liquid (Flow Liquefaction, F.L) or shows a progressive softening (Cyclic Mobility, C.M.) under cyclic loading. Since liquefaction of sand is entirely related to its undrained behavior, the undrained behavior of saturated sand under cyclic or equivalent monotonic loading has been studied in the laboratory by different researchers, simulating field conditions properly for the better understanding of liquefaction phenomenon. The undrained behavior of saturated sand is useful to predict the occurrence of the probable type of liquefaction (C.M. or F.L) and to provide suitable solutions for liquefaction problem. The objective of this research is therefore to study the undrained behavior of saturated sand in relation to liquefaction phenomenon based on intensive literature survey. This is best explained by identifying factors affecting the behavior and studying their effects observing the resulting stress-strain and p-q diagrams. The concept of Liquefaction and steady state are then discussed based on the undrained behavior of sand. Finally, the application of Steady State Line (SSL) on the assessment of liquefaction potential and in the identification of partial liquefaction, limited liquefaction, and complete liquefaction is discussed.