Geotechnical Engineering

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A Comparative Study and Computer Coding of Limit Equilibrium-Based Slope Stability Analysis Techniques
(Addis Ababa University, 2003-07) Amare, Yohannes; Worku, Asrat (PhD)
General slope stability concepts and the parameters and major considerations associated with slope stability problems are discussed. General features and theories of some of the available methods of analysis such as limit equilibrium methods, limit analysis methods, finite element method, etc. are reviewed. Special emphasis is made to the limit equilibrium slope stability analysis techniques based on the method of slices. The underlying principles, common features and assumptions employed by these methods are elaborated. The various commonly used methods of slope stability analysis based on the limit equilibrium principles are then thoroughly assessed, compared and contrasted. These common limit equilibrium based methods of analysis, including Bishop’s simplified method, Janbu’s simplified method, Bishop’s rigorous method, Spencer’s/Morgenstern and Price’s method, Janbu’s rigorous method, are formulated in a unified manner. A computer program is then written for these selected methods, using the Visual Basic language, on the basis of this unified formulation. The computer program has the capacity of analyzing slopes with general-shaped slip surfaces and general loading conditions. It is also capable of incorporating earthquake loading based on the pseudo-static analysis. Finally, example problems are solved to illustrate the computer program developed and to reflect on the discussions and conclusions in the literature review section. The results obtained for the example problems, using the computer program developed and the different methods of analyses are in close agreement with those obtained by the author’s of the original methods. Key Words: slope stability analysis; limit equilibrium; method of slices; factor of safety
Examining the Swelling Pressure of Addis Ababa Expansive Soil
(Addis Ababa University, 2003-07) Teklu, Daniel; Haile, Messele (PhD)
So many damages have been reported as a result of expansive soil. Such damages occur when the pressure exerted by the soil is greater than the foundation pressure. Consequently assessing the swelling pressure is an important step in designing foundation on expansive soil. Several researchers have developed an empirical relation for the estimation of swelling pressure. The empirical expressions relate the swelling parameters to the geotechnical parameters that are determined by the identification tests. But the expressions should either be reformulated or checked for our local condition. Therefore in this paper: the various equations already developed are tested for the validity to our condition and new equations are developed that relates swelling pressure with the index and physical properties. The new formulas are developed, by taking one or more of the four parameters (liquid limit, moisture content, dry density and plastic index) in different combinations. The equations are developed by taking 14 samples and using v SPSS 9.0 for windows software. The equations are then tested for three control samples. The most important equations are proposed, and conclusions and recommendations are made.
Investigation of Influence of Compaction onThe Stabilityof Earthfill Dams of Tropical Soils
(Addis Ababa University, 2003-11) Hunde, Sintayehu; Boled, Girma(PhD)
Dam construction is one of the major civil engineering activities, which enhance development. If the construction of such infrastructures can be achieved within sound technical and affordable financial capacity, then it is possible to realize the dream for development and well-to-do life. One of the merits of earth fill dams is that they can be constructed from locally available natural material (soil) within sound technical and affordable financial capacity. For soils of a given locality to be used safely and economically, appropriate methods of determining the geotechnical properties and shear strength parameters have to be investigated and developed. In addition to this, the appropriate methods of compacting locally available tropical soils have to be developed. Accordingly, in this paper the influence of compaction on the stability of small earth fill dams of local tropical soils has been investigated. In addition to this, some peculiar geotechnical and geo-chemical characteristics of local tropical soils that have been used in construction of earth fill dams have been studied. Accordingly, the fill material of the cofferdam of Gilgel Gibe Hydroelectric Project has been obtained to be true laterite, while the core material of Dire Dam and the red clay soil of Addis Ababa (Semen Gebeya Area) have been obtained to be lateritic. The soil samples were compacted at optimum moisture content, at drier of the optimum moisture content and at wetter of the optimum moisture contents. Then, triaxial tests were done on the soil samples to investigate the variation in shear strength properties of these soils, which ultimately affect the stability of the earth fill dams. Compaction moisture has been obtained to influence the stability of earth fill dams. Samples compacted on the drier side of optimum moistures registered high shear strengths. But these strengths were observed to be lost upon further increasing the compaction moisture and also upon sample saturation. Variation of permeability of compacted soils with respect to compaction moistures and also the effect of saturation settlements were investigated. Accordingly, Permeability has been obtained to decrease towards the optimum moisture content. On the drier and wetter sides of the optimum, higher values of coefficients of permeability have been obtained. vi The problem of saturation settlement has been observed to occur when the soil is compacted on the drier side of the optimum moisture.
Assesment of Damage of Buildings Constructed in Expansive Soil Areas of Addis Ababa
(Addis Ababa University, 2004-10) Sisay, Afewerk; Haile, Messele(DhD)
The amount of damages caused by expansive soil is tremendously high and alarming. These damages are mainly dominated in light weight-engineering structures founded in expansive soils. These structures include lightweight buildings, roads, runways and buried utilities. The problem associated with expansive soils is not yet properly solved. It is important to assess buildings constructed in expansive soil areas in order to know the cause of failure. This helps in providing solutions for newly constructed buildings and to get remedial measures for those already damaged. Therefore, in this study an attempt was made to assess the damage that occurred on buildings that are constructed on expansive soil areas by taking ninety six randomly selected houses in the city of Addis Ababa. The houses are located in Bole, Olympia, Nifasilk, Lafto, Old Airport, Mekanisa, Gergi and Bole bulbula localities of the city. The study showed that 64 % of the houses suffered heavy damage, 8 % of the houses were slightly damaged and 28% of the houses showed no damage. Analysis was made to understand the cause, extent and type of damage that was observed in the houses. The damages that are observed are mainly caused by poor design, construction and non-controlled drainage. Detailed investigation was made for the Ethiopian Airlines maintenance hangar building which has shown pronounced failure in both the sub structure and the super structure Finally, conclusion and recommendations are made.
Investigation in to Shear Strength Characteristics of Expansive Soil of Ethiopia
(Addis Ababa University, 2004-11) Zewdie, Ayenew; Haile, Messele (PhD)
Considerable research work has been done on the shear strength properties of most soils. However, very little information is available on the shear strength characteristics of expansive clays. This is mainly due to the fact that many investigators have concentrated most of their efforts towards finding solutions to the swelling behavior associated with these clays. However, the strength behavior of expansive clays need not be overlooked. Factors such as swelling and shrinkage, for example, influence the soil strength significantly and their effects are difficult to account for in most laboratory shear tests. Furthermore a thorough knowledge of shear strength is required to estimate the bearing capacity of expansive clay soils and also to evaluate the stability of dams, road and railway embankments built in these soils areas. Therefore, in this paper, Unconsolidated Undrained triaxial shear strength tests were performed on unsaturated soil and on some saturated soil samples collected from different locations of Addis Ababa, unsaturated soil sample of Gambela and unsaturated soil sample of five major roads (i.e. Addis-Modjo, Addis-Ambo, Addis-Debreberhane, Addis-Weliso and Addis-Ghoastion roads). The purpose of the study was to know the shear strength properties of expansive soil. Range of values of shear strength parameters (cohesion and angle of internal friction) were obtained based on unsaturated soil samples. According to the outcome of the research, the shear strength of expansive soil ranges from 30-150Kpa in cohesion and 3-25 degree in angle of internal friction in UU test on unsaturated soil. For saturated soil sample in UU test the cohesion ranges from 55- 94Kpa. There is a decrease in strength in the saturated samples, which shows that the degree of saturation and the suction pressure can have major influence on the shear strength of expansive soil.
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.
Reinforced Earth Retaining Wall Design Using Reinforcement Steel With Gabion Facing
(Addis Ababa University, 2006-10) Tadesse, Tilahun; Teferra, Alemayehu ( Profasser)
This paper presents reinforced earth model retaining wall made of reinforcement steel and gabion facing. Four model reinforced earth retaining wall were designed and constructed at selected site in Technology Faculty (North) campus. Rigid walls were provided at the sides and rear face to simulate the plain strain conditions prevailing in retaining walls. A gabion was used as facing to keep the backfill soil from flowing. Reinforcement steel was used as reinforcement with vertical spacing of 40cm and horizontal spacing of 36cm. Sand with different relative densities was used as backfill and a surcharge for each model wall. The surcharge was applied over the model wall step by step and deformation of the reinforcement was measured under different surcharge load until certain group of reinforcement observed to fully mobilize their frictional resistance. To identify the level of loading at which frictional resistance was fully mobilized, a graph of applied surcharge load versus wall face displacement was plotted and carefully observed the level of loading at which frictional resistance was fully mobilized. This level of loading was taken as ultimate load that the particular reinforcement steel can carry. Based on the observed ultimate load, the coefficient of friction between the backfill soil and reinforcement was determined. It is this coefficient of friction which will be used for the design of actual retaining wall.
Examining Atterberg Limits for Expansive Soils
(Addis Ababa University, 2006-12) Kassahun, Habtamu; Tadesse, Samuel (PhD)
Expansive soils have been responsible for many structural damages that result in great financial losses in many parts of the world including Ethiopia. Proper understanding of the properties of the soil helps to understand the actual causes of failure of structure. The main objective of this research work is to examine Atterberg limits by giving emphasis on the range at which a reduction of water content will not cause a decrease in the volume of the soil mass, i.e. shrinkage limit. In addition to this an attempt has been made to define the maximum moisture content corresponding to the maximum swelling and the effect of initial moisture content on the swell pressure of expansive soils. To achieve the research objective, disturbed and undisturbed samples of both Expansive and red clay soils were collected from Addis Ababa. The red clay soil was used as a methodology control of Atterberg limits .The following laboratory and field test were conducted to attain the purpose of this work Free swell and linear shrinkage Atterberg limits Swelling measurement with and without seating load Artificial flooding of test pit and moisture content Measuring of volume at various stages of desiccation ( Drying process) Re wetting of the completely dry pat Swell pressure measurement by varying initial moisture content Laboratory test results on both clay samples show that the volume change of expansive soil is more significant than that of red clay soil. The volume change on expansive soil is mainly due to the swell property of the soil and this volume change is sufficient enough to develop distress on structures. The investigation of swell measurement and moisture content of undisturbed soil samples showed that the moisture content after a complete swelling is found with in the range of liquid x limit and plastic limit. In addition to this high moisture content soils experience less uplift, but the pressure required to maintain a constant volume is not altered.
Indepth Investigation of Relationship Between Index Property and Swelling Characteristic of Expansive Soil in Bahir Dar
(Addis Ababa University, 2007-02) Negussie, Dagmawe; Teferra, Alemayehu ( Profasser)
In this study, the relationship between Index Property and Swelling characteristic of Expansive soil of Bahir Dar is examined. The test results showed that the Specific Gravity ranges from 2.55 to 2.81. The Index Property Test showed that Plasticity Index ranges from 44.5 to 76.42%. The clay content of the soil is from 55.4 to 87%. Free Swell tests conducted on the collected samples showed a range from 78 to 215%. According to the Unified Soil Classification System. The soil is categorized as Fat or Organic Clay with a potential of expansion. AASHTO Classification System also shows that the soil is Plastic Clay with high volume change capacity. The dry density of the soil, which samples were taken from, ranges between 1.04 and 1.39 gm/cc. Another test conducted using one-dimensional consolidometer showed that the Swelling Pressure of the area ranges between 75.95 and 547.52 kpa. Relationship observed between Index Property and swelling characteristic of the area showed that with the increment of clay content in the soil the Plasticity Index, the Cation Exchange Capacity and the w max 24 is increased. The increment of the Plasticity Index is in turn reflected in increasing the swelling pressure of the soil. Dry Density of the soil also increases the swelling pressure. On the other hand Natural Moisture content affects the swelling pressure negatively. A single Index Property is not a reliable means of predicting the swelling pressure of the soil while a better estimation is observed when different Index Properties are involved in prediction the swelling pressure The relation of (w max 24/w)*gd or (w max 24/w) with swelling pressure showed a better indicative value than the PI for the study area. -------------------------------- 92
Stabilization of Light Grey and Red Clay Subgrade Soil Using Sa-44/Ls-40 Chemical and Lime
(Addis Ababa University, 2008-01) Argu, Yohannes; Teferra, Alemayehu ( Profasser)
This study is undertaken on a light grey (expansive) and a red clay soil samples taken from within Addis Ababa. The change in engineering properties of these soils treated with SA- 44/LS-40 chemical alone, lime alone and combinations of both at different application rates are investigated. The investigation of the modifications in engineering properties of the stabilized soils are made using laboratory tests that include soaked CBR and CBR swell tests, swelling pressure test, Atterberg limit tests, percent swell test and free swell tests. The modifications in engineering properties were evaluated after curing the treated sample for fourteen days. From the study, it is observed that the application of lime alone and SA-44/LS-40 chemical with lime shows improvements of varying degree on the engineering properties of both the light grey and the red clay soils. However, no specific mix ratio showed the maximum improvement in all the engineering properties of each soil. This suggests that before deciding for large-scale applications, the desired improvement level of the engineering properties of a particular soil should be specified first, and tests be conducted for different mix ratios to arrive at the optimum one.
Channel Stabilty Analysis -the Case of Borkena River (Ethiopia)
(Addis Ababa University, 2008-03) Mengesha, Fantaw; Channe, Bayou
Rivers dynamically respond to changes in the discharge and sediment supply of catchments. A change of land use and other natural factors in catchments result in changes on sediment supply and discharge. And this process of change of river geomorphology seeking for its equilibrium may affect people and property especially in urban areas located adjacent to rivers. The Kombolcha town, situated on flat land plains in the middle course of the Borkena River is facing a danger of flooding and destruction of properties due to the instability of the Borkena and Berberie River reaches crossing the town. To prevent the loss of lands and properties resulting from the changes in the characteristics of those river reaches, stabilization measures must be employed. For proper design and implementation of any stabilization measures, analysis on stability of these channel reaches must be carried out. So this thesis deals with analysis of the stability of the channel reaches by evaluating their capacity and stability in response to discharge and sediments that are supplied from the catchments. Field investigation and surveying to collect data for channel cross sections, bed & bank material, and the vegetation covers is carried out. Gradations of bed and bank materials are determined in laboratory using sieve analysis. Design discharges of different return periods and quasi unsteady flow data are analyzed from measured maximum daily instantaneous discharge data. Using such flow and existing field data as input, HEC-RAS 4.0 beta version is used for simulation of flow and mobile bed sediment transport Based on the model results which are analyzed using the Meyer Peter Muller and Yang’s sediment transport formula, deposition is observed in almost all of the stations of the Borkena River. And there is deposition at some of the down stream stations in the tributary reach and degradation in the rest. It is also analyzed that the material on the banks is less resistance to the minimum shear induced by the flow, besides much of the stations in the tributary reach are geotechnically unstable. Over flooding still is the problem on upper and lower reaches of Borkena which is resulted from the complex processes of aggradational problems. And stabilization measures are proposed and should be implemented with further engineering, economical and hydraulic analysis for effective and sustainable stabilization of the rivers.
Detail Investigation on Index Properties of Lateritic Soils: The Case of Nedjo-Mendi-Assosa
(Addis Ababa University, 2008-03) Dibisa, Jiregna; Haile, Messele(PhD)
The classification properties and composition of lateritic soils, developed on Tertiary basalt, granite and metamorphic rocks like gneiss and Shists in Nedjo, Mendi and Assosa areas have been investigated. X-ray diffraction analyses showed that the soil comprised minerals of Kaolinite and Smectite group such as Kaolinite, Quartz, Dickite, Nacrite, Muscovite, Vermiculite, Geigerite, Zektzerite, Tridymite and Gibbsite. Chemical analyses showed there was a concentration of sesquioxide minerals but no amorphous material present in the soil. Comparison between moisture content at air-dry temperature (50oC with 30% relative humidity) and that of oven drying at 105oC indicates that there is no structural water. For the lateritic soil of Nedjo, Mendi and Assosa, unlike the oxidation of the free secondary clay minerals, the fact that there is no pronounced structural water found and/or amorphous clay minerals showed that, dehydration does not significantly affect the index and/or engineering properties of lateritic soils of this area. In all the cases there is no significant difference observed between the Atterberg limits obtained from test on a specimen mixed for 5min and that of 25min, this indicates that the cementation of the clay particles is too high to be disaggregated with in 25min further mixing and can also be related to the degree of laterization. Analysis of specific gravity of the soils shows that, increase in temperature from its in situ condition to an air-dried equivalent of 50oC oven temperature reduces specific gravity by an average of 4%. viii Analyses of the grain size distribution and other physical properties indicated that the soil was not homogeneous within the laterite zone. Atterberg Limit test results for the soil falls below the Casagrande A-line. There were significant differences between the values of plasticity index determined for as received condition compared with air dried samples. High values for both the liquid and plastic limits may be due to the water retention capacity of porous microaggregations in the soil. This can be correlated with the degree of laterisation, which depends on the ambient climate, topography and vegetation of the area.
Analysis and Parametric Study of Piled Raft Foundation Using Finite Element Based Software
(Addis Ababa University, 2009-05) Abate, Simeneh; Teferra, Alemayehu (Professor)
The design of group piles depends on either the group or single pile capacity of piles. In conventional design method of such foundations, the stiffness of the pile cap is barely taken into account. Such design becomes too conservative if the pile cap is in contact with the ground. Because the pile cap contributes in transferring load to the ground and distributing load over the piles. The design method that considers the contribution of the pile cap and interaction between the different elements of group piles is called piled raft foundation. The concept of piled raft foundation leads to economical design. In this paper, analysis and parametric study of piled raft foundation has been conducted. The study is performed using powerful finite element based software, PLAXIS. A number of parameters were selected from the elements of the piled raft system. According to their effect on the response of piled raft system, some are taken to be constant while others are varied. Among the varied parameters, raft thickness, pile length, pile spacing and pile number were considered. Other possibilities were also investigated in search of an optimum placement of piles. The analysis results from the PLAXIS software have shown a close prediction to that of in-situ measurements and other numerical methods. From the parametric study, concentrating piles around the center for uniformly loaded rectangular piled raft foundation reduce the differential settlement.
Application of Gis And Remote Sensing for Flood Hazard and Risk Analysis: The Case Of Boyo Catchment
(Addis Ababa University, 2009-06) Gobena, Destaye; Suryabhagavan, K.V (PhD)
Flood is a natural phenomenon that will remain a major hazard as long as people live and work in flood prone areas. It occurs when the volume of water in a river or stream exceeds the capacity of the channel. It also takes place along lake when higher than normal water levels inundate low-lying areas. The present study was carried out to identify and delineate flood hazard and risk zones in the study area. In order to do this, it was found important to understand the catchment characteristics, and hence flood hazard assessment was done to the whole Lake Boyo catchment area. An integrated Remote Sensing and GIS approach was found to be very helpful to delineate flood hazard and risk zones in the study area. Factors that were found to be significant in triggering flood hazard in the study area in decreasing order of importance were: drainage, elevation, geomorphology, land use land cover, rainfall and slop. These factors were weighted in hierarchical order using the MCE approach to produce flood hazard map of the catchment. From the flood hazard map, the areas of flood hazard levels were calculated. Since all the areas of the very high and more than 90% of high flood hazard levels are found in Shashogo Woreda, flood risk analysis was done for the Woreda using the two elements at risk, viz., land use and population density of the Woreda. Major findings of the study revealed that, PAs in the down stream part of the catchment: Doesha, Mololicho, Shemo, Biramora, Musagesa, Urbecha and Kemetcho Borara PAs were subjected to very high flood hazard and risk and the different land uses in these areas are within high to very high flood hazard and flood risk level.
A Study on the Design and Advantage of Conical Type Shell Foundation Using Analytical and Fem
(Addis Ababa University, 2009-07) Taye, Endalkachew; Seged, Hadush (PhD)
This thesis introduces shell foundations in general and conical type shell foundation in particular as an alternative to the conventional plain foundation. The different types of shells that may be employed in foundations are introduced with their geometry and applications under different situations. Shell foundations can be used as combined or isolated footing. The simplest form of shell appropriate for isolated footing is conical shells. Conical shells are employed as an alternative to plain circular footings. The design of conical shell foundation is based on membrane theory in order to determine membrane stresses, and ultimate strength theory to obtain the ultimate load which enables to compute the load factors involved in the design. A design example is provided and comparison is made with plain circular footing. The result clearly indicates that conical shells save more material than do so plain circular footings. A finite element analysis is carried out using the finite element software PLAXIS for different soil properties and footing size. The analysis was performed for conical shells and plain circular footing in order to compare the results. The finite element analysis results prove that conical shells have high load carrying capacity than circular footing under same soil properties. Shell foundations need smaller quantities of material than the conventional plain foundations. In a country like Ethiopia where material to labor cost ratio is high, shell foundations may be used as an economic alternative.
Strength and Ductility Demand A Case Study of Ethiopian Design Practice to Seismic Loads
(Addis Ababa University, 2009-09) Ayele, Eyasu; Taye, Shifferaw (PhD)
This thesis is conducted to investigate the relationship between strength and ductility demand. The base shear and the story displacement ductility were used as parameters to establish the relationship and a five story and ten story reinforced concrete shear frames were used to this end. Eighteen shear frames with different combinations of ductility class and sub-soil class, according to the Ethiopian Building Code Standard EBCS 8, 1995, were considered. Each model was subjected to one linear static with EBCS 8, 1995 base shear distribution, two linear response spectrum EBCS 8, 1995 and EC 8, 2004, two non-linear static and one non-linear time-history analyses cases using the computer program SAP 2000 Ver.12. A range of base shear values were applied on each model to study the post-yield displacement ductility demands imposed on it. From the analyses results, different curves showing the relationship between the reduced (%) base shear yield strength and ductility demand were produced. To this effect, the non-linear static or the pushover analysis method was used as a tool. Two types of non-linear properties were considered for all the models; the first, material non-linearity following the recommendations by Federal Emergency Management Agency FEMA 356 Pre-standard and Commentary for Seismic Rehabilitation of Buildings and Applied Technology Council ATC 40 Seismic Evaluation and Retrofit of Concrete Buildings and the second Geometric non-linearity by implementing the iterative P-D analysis procedure included in SAP 2000 Ver.12. The influence of undermining the design base shear on ductility demand and the validity of permitted reductions by EBCS8, 1995 of design base shear through a behavior factor for each ductility class for the five and ten story shear frames was assessed by making use of theses curves. Finally the implementation of the relationship between these two important seismic design parameters in the current seismic design practice of Ethiopia was investigated by conducting a case study on an existing building located in Debre Birhan. Key words: Base shear yield strength; Ductility demand; Pushover analysis
Software Development for Consolidation Analysis Under Various Loading Conditions Using Finite Difference Method
(Addis Ababa University, 2010-05) Redie, Tesfahun; Seged, Hadush (PhD)
Consolidation analysis of soils can be undertaken using empirical and numerical methods. The problems related to empirical analysis of consolidation are the involvement of the determination for time factor using empirical equation obtained from curve fitting method. The empirical correlation of the time factor and the load application do not represent the actual situation where soil properties are assumed to be spatially and temporally invariant and thus, average property values are used. In reality, however, these soil parameters usually vary from point to point (heterogeneous) and these parameters may vary in time while a consolidation process is occurring due to an external influence such as surface pressure. Moreover, it is difficult to model and make consolidation analysis for variable loading conditions. In order to solve this problem, numerical analysis using Finite Difference Method is proposed. The finite difference method uses the original governing consolidation equation and provides the solution of a continuous problem by representing it in terms of a discrete set of elements such that there is a number of points in depth and time at which one calculates the field variables, in this case the excess pore water pressure, consolidation settlement and average degree of consolidation. Thus, this thesis involves development of user friendly application software for consolidation analysis using finite difference method. In the hope of achieving this goal, a computer program named ConsolAnalysis has been developed. The analysis capabilities of the software comprises of consolidation analysis for homogeneous and stratified soils subjected to constant loading, variable loading, and abrupt change of loading with constant and variable coefficient of consolidation.
Performance of Unconventional Soil Stabilizers in Stabilization of Substandard Materials for Road Subgrade & Subbase
(Addis Ababa University, 2010-06) Ashuro, Tesfahun; Haile, Mesele (PhD)
Road connectivity is one of the key components for development, as it promotes access to economic and social services, generating increased income and employment. However, the construction of such economically vital sector is the most expensive of public works project undertaken by the society. Traditional pavement design and construction practices require high quality materials to fulfil minimum construction standards. In many of road projects it is very difficult to get standard materials with in economical haul distances. There is an increasing effort around the world towards introducing innovative and unconventional road construction approaches that aim at reducing costs of construction by enabling use of marginal materials found with in the road route. One of proven technologies in connection to this effort is stabilization of soil. Stabilization can be derived from thermal, electrical, mechanical or chemical means. Chemical stabilizers can generally be categorized in to two broad categories: conventional and unconventional stabilizers. Traditional stabilizers such as cement lime, fly ash, and bituminous products have been intensely researched, and their fundamental stabilization mechanisms have been identified. Unconventional soil stabilization additives consist of a variety of chemical agents that are diverse in their composition and in the way they interact with the soil. Unfortunately, relatively little is known about their interaction with geotechnical materials or their fundamental stabilization mechanisms due to proprietary nature of commercial stabilization additives, their exact chemical compositions are not disclosed. In this research two unconventional stabilizers were used for investigation of their effect on the engineering properties of soils that are deemed to be marginal for road subbase and subgrade. The two stabilizers used were: PURE CRETE and Anyway Natural Soil Stabilizer (ANSS). ANSS Natural Soil Stabilizer is claimed to be inorganic, hydration activated powderbased stabilizer that reacts with the soil particles to create layers that are interconnected through a complex inter-particle framework. The manufacturer claims PURE CRETE to be a complex non-bacterial concentrated multi-enzymatic formulation that alters the properties of earth materials, providing one of the most cost effective methods to stabilize roads and seal ponds and landfills. The stabilizers were combined with a total of five different soils with classifications according to AASHTO as A-7-5, A-7-6, A-2-7, A-2-4, and A-2-6. ANSS stabilization induced the most improvement on engineering properties of all soils tested at manufacturer recommended dosages and PURE CRETE failed to show any improvement at manufacturer recommended application rate. However, at very high application rates modest effects were noticed. It is recommended based on the results of this research that ANSS can be considered as a viable option for stabilization of subgrade soils after conducting durability tests. It will particularly help use of marginal materials where materials that met standard specifications are far away.
Shear Modulus and Damping Ratio of Dry Koka Sand Using Cyclic Simple Shear Test
(Addis Ababa University, 2010-06) Girmachew, Yimer; Alemayehu, Teferra (Prof.)
The nature and distribution of earthquake damage is strongly influenced by the response of soils to cyclic loading, and the behavior of soils subjected to dynamic loading is influenced by dynamic soil properties. The determination of these dynamic soil properties is invaluable in the response analysis of soils to cyclic loading. Of these dynamic soil properties, shear modulus and damping ratio are the most important; and this paper is concerned with the determination of the shear modulus and damping ratio of sand. In this research the shear modulus and damping ratio of Koka sand is determined using the cyclic simple shear test. The results have shown that the obtained normalized shear modulus values are located lower than those suggested in literature, especially at lower strains. For higher strains the obtained normalized shear modulus values agree with those suggested in literature. The obtained damping values are almost within the range suggested in literature.
Unsaturated Shear Strength Characteristics and Stress Strain Behavior of Expansive Soils of Addis Ababa
(Addis Ababa University, 2010-11) Habtom, Gebre; Hadush, Seged (PhD)
Considerable research work has been done on the shear strength properties of most soils. However, very little information is available on the shear strength characteristics of expansive clays. This work studies the shear strength characteristics of an expansive unsaturated soil found in Addis Ababa. Matric suction is one of the stress state variables that control the shear strength of unsaturated soils. Previous studies on the expansive soils are done using the conventional triaxial machine, which is unable to measure matric suction of the soil. The main aim of this study is to determine the unsaturated shear strength properties and the stress strain behavior of expansive soil found in two locations (Bole and CMC) in Addis Ababa. The shear strength behaviour of unsaturated soil is studied in this work using the consolidated undrained modified triaxial compression equipment with measurements of matric suction during the shearing stage. In this work, modified consolidated undrained tests were conducted on 12 undisturbed potentialy expansive soil samples collected from Bole and CMC area at 2.5m depth. Six of the tests were on completely saturated samples and the other six on unsaturated samples. Range of values of shear strength parameters (c, ∅’, and ∅b) were obtained from the consolidated undrained test depending on the applied matric suction. According to the outcome of the research, the deviator stress of soils from Bole area ranges from 90.65 kpa to 130.08 kpa for effective consolidation of 250 kpa for matric suction range of 0 Kpa to 75 Kpa. Similarly the soils from CMC area ranges from 54.03 to 93.41 kpa in deviator stress for effective consolidation of 150 kpa and for matric suction range of 0Kpa to 75 Kpa. The result clearly shows that the shear strength of expansive soils tested for unsaturated case is higher than that of saturated soil and the shear strength increases when matric suction increases.

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