Geotechnical Engineering

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    Comparative Assessment of Excavation Supporting Systems in Addis Ababa
    (Addis Ababa University, 2024-01) Kidist Afewerk; Henok Fikre (PhD)
    Addis Ababa, Ethiopia's growing capital city, has witnessed rapid urban development, necessitating the construction of towering structures. As the need for deep excavation arises, Engineers face the challenge of maintaining stability while digging into the earth. This necessitates using temporary earth retaining structures known as excavation supports or shoring to prevent soil collapse and ensure precise excavation. Various methods are available for excavation support, including soldier beams and lagging, sheet piling, bored pile walls, soil nailing walls, and slurry (diaphragm walls). In Addis Ababa, specialized firms dedicated to foundations construct excavation support systems. The first foundation specialist company in the country, BAUER MIDROC Foundation Specialist Plc., was established on June 12, 1998. Addis Ababa has varying soil types, but the protection system used is uniform, causing specific problems such as costly temporary shoring systems and time-consuming installation. Local companies compete by compromising safety, leading to the collapse of excavation support systems with major damage, including loss of human life, additionally, machinery has a short lifespan due to difficult strata placement. The study aims to access different shoring methods in Addis Ababa and compare conventional contiguous pile walls with soil-nailing walls for various sites and soil conditions that are practical, reliable, appropriate, and adaptable solutions for local firms. To achieve this, first gather information about three different sites, including soil strength, excavation depths, surcharge load, and available space for the supporting system structure. Numerical methods such as finite element analysis and limit equilibrium analysis will be used to create models of the deep excavation stabilizing methods for each site and method. After that compare two methods of construction costs and construction period based on the obtained designs. In conclusion, this research aims to provide valuable insights into the most effective and suitable excavation supporting systems for specific site conditions and soil types in Addis Ababa. The particular approach employed in this study, along with considering various factors, ensures that the findings are reliable and applicable to the context at hand. The results of this study can be valuable for contractors and Engineers in selecting the appropriate excavation supporting system in Addis Ababa.
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    Correlating CBR values with basic soil parameters (by using Neuroxl Predictor)
    (Addis Ababa University, 2024-01) Airmeyas Aychew; Henok Fikre (PhD)
    The California bearing ratio (CBR) is an essential design parameter for soils and an indirect measure of soil strength. This is broadly used for the design of sub-grade, sub-base and base course materials for road, railway, and airfield projects. It is also used as a direct relationship to determine the response of the base or sub-base soil. This research studies the relationship between CBR values and other parameters of soil properties, although the samples include coarse and fine-grained soils, using advanced neural network programs to help us obtain an accurate predicted value. Most of the previous studies were conducted on fine-grained soil types and also used conventional multiple linear regression analysis methods. To satisfy the objective of this study, one hundred and ninety-eight soil sample test results were collected. I participated as a material engineer in the testing and reporting process. Laboratory testing was performed in accordance with AASHTO standard test methods. Modified compaction, soak three-point modified CBR, wet sieve analysis, and Atterberg limit tests were performed on a total of one hundred and ninety-eight soil samples. Statistical analyses were performed to validate the new model using 30 percent of the total sample size. Two types of analysis programs—Microsoft Excel software (ANOVA) for multiple non-linear regression relationships and the advanced NeuroXL predictive neural network program—were used to predict CBR values. Independent soil property parameters were liquid limit, plastic limit, plastic index, amount of particle size less than 0.075 mm, amount of particle size less than 0.425 mm, amount of particle size less than 2.00 mm, amount of particle size less than 4.75 mm, optimum moisture content, and maximum dry density. This study provided two alternative models. The first alternative model included compaction test parameters (OMC and MDD), particle size distribution parameters (4.75 mm PP, 2 mm PP, 0.425 mm PP, and 0.075 mm PP), and plasticity parameters (LL, PL, and PI). They were taken as independent parameters. The second alternative model excludes the compaction test parameters (OMC and MDD) as independent parameters when compared to the alternative one. This study used two alternative analysis techniques: the first group of analysis techniques developed model equations for each classified data set (sub-grade, sub-base, and sub-base), and the second technique developed model equations for the unclassified data set group. The predicted CBR values of both the NeuroXL prediction and multiple nonlinear ANOVA regression models were compared with the actual CBR values, which confirmed that there was an acceptable difference between the actual and predicted CBR values between both analysis methods.
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    Prediction of Soil Water Characteristic Curve Based on GSD and PI for Red Clay and Expansive Soils Found in Addis Ababa
    (Addis Ababa University, 2013-09) Nuru Ismail; Hadush Seged (PhD)
    Many researchers have done researches on unsaturated soil mechanics worldwide to bring the unsaturated soil mechanics in to practices. Among those researches some of them focused on simplifying the complex and expensive testing mechanism by developing models which can be used to predict the unsaturated soil parameters from basic soil prosperities such as Grain Size Distribution (GSD) and Plasticity Index (PI) along with the saturated shear strength parameters. Soil water characteristic curve (SWCC) was proved to have good relationship with the unsaturated soil parameters. In this study an attempt was made to check whether those developed models would work for expansive and red clay soils found in Addis Ababa. Using the model developed by Fredlund and Xing (1994) along with the correlation equation given by Perera (2005), family of SWCC for soils found in Addis Ababa has been plotted. It was observed that the family of SWCC plotted was out of the widely referenced family of curves developed by Zapata (2000) to some extent. Even though the family of curves seems to be out of the range provided by Zapata (2000), the model gave good prediction for the unsaturated shear strength parameter Φb. The percentage error between the measured values of unsaturated shear strength parameter Φb in previous researches of Habtom (2010) and Getaneh (2010) with predicted unsaturated shear strength parameter Φb in this study was compared. For red clay samples the percentage error lie in the range 2.07 to 6.37% for suction ranges of 36.67 to 52.9 kPa and at lower suctions of 16.75 and 21.5 kPa the percentage error ranges from 11.42 to 25.39% . For expansive soil samples for suction of 23.1 and 32.9 kPa, the percentage error is in the range from 3.86 to 6.13%. For suction values of 14.5 and 52 kPa the range of error is about 9.45 to 13.40% and for suctions of 38.8 and 48.6 kPa the error lies between 18.94 and 38.84%.
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    Developing Correlation Between Index properties and California Bearing Ratio, CBR of Soils: A Case Study on Guliso Cheliya Dilla Kondala Begi Road Project, Lot 2
    (Addis Ababa University, 2023-07) Gudata Mokonon; Henok Fikre (PhD)
    Soil properties are specific to a region and alter greatly from place to place based on climatic and geographic conditions. Geotechnical engineers often attempt to develop empirical methods specific to particular regions and soil types. However, empirical equations are more consistent for soil types where the correlation is developed. Therefore, it is extremely important to develop a correlation between CBR and subsoil index properties specific to different soil types. This project comprises developing the relationship between CBR values and soil index properties specific to the Guliso Cheliya Dilla Kandalama Begi road project. When finding the correlations, both simple and multiple regression analyzes were considered. Accordingly, one hundred and ten secondary data sets were collected from Best Consulting engineers, consulting firm, to attain the intended correlations. Using SPSS, the CBR correlation is constructed as a function of Atterberg limits, percentage grain size, and moisture-density relationship parameters. After evaluating developed correlation, it was found that CBR values and soil index properties are well correlated and this is advantageous for the preliminary detection of the geotechnical characteristics of the soil in the investigation area. Finally, a reliable correlation is derived from correlation work with a coefficient of determination of 0.691 using multiple linear regression analysis.
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    Lateral Static and Dynamic Response of Single Piles in Non-Homogenous Soils using a Standalone Two-Parameter Foundation Model
    (Addis Ababa University, 2023-03) Mathewos Endeshaw; Asrat Worku (PhD)
    In the design and analysis of structures supported on piles, geotechnical engineers are required to formulate pile responses. Such a formulation involves the use of subgrade models that relate the interface forces to the displacement. The primary focus of this thesis is one such relatively new subgrade model, initially developed for shallow foundations, applied to laterally loaded piles. The model is a Kerr-equivalent two-parameter Pasternak-type continuum subgrade. Furthermore, as with most pile applications, the variable nature of the soil along the pile depth is also taken into consideration. The goal of this thesis is to investigate the applicability of the aforementioned model for predicting the static and dynamic inertial response of a single pile embedded in non-homogenous soils represented by a power function. To this end, the model in question is initially calibrated by setting equivalences between the responses predicted and results from finite element analysis. Expressions for a calibrating factor left open in the model are developed as functions of relevant parameters. The results from the calibrated model are compared with the results from the finite element method and found to be comparable. Additionally, it is demonstrated that the critical slenderness ratio and critical relative stiffness can be approximated by threshold values form expressions developed for the calibration factor. Using the calibrated subgrade model, the static response of the soil is also investigated. Expressions for the pile head flexibility influence factor and pile head stiffness terms are provided. The proposed expressions compare well with published results. In addition, utilizing the unique characteristic of the model being studied, in that the shear interaction is explicitly accounted for, its effects on static pile response are also studied. In general, neglecting the shear term results in overestimation of the pile head displacement. Similarly, the use of a calibrated lower order Winkler-type model that implicitly accounts for shear interaction is investigated which by in large provides higher displacement profiles. The calibrated model is also used in predicting the dynamic inertial response of a laterally loaded pile in non-homogeneous soil. An approximate energy method is used to arrive at the dynamic pile head stiffness and damping ratios. With the exception of fixed-head piles, it is observed that the dynamic pile head stiffness can reasonably be approximated by the static pile head stiffness for low ranges of excitation frequencies commonly encountered in seismic events. Curve fit expressions are also provided for the pile head damping ratio. The effect of shear interaction is also investigated for the dynamic inertial case. As the analysis is confined to long flexible piles, very little effect is observed with the exception of piles in rocking oscillation.
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    Relationship between Water Content Ratio and Undrained Shear Strength of Fine Grained Soil
    (Addis Ababa University, 2023-07) Tigist Gemechu; Samuel Tadesse (PhD)
    Undrained shear strength of clay soil is a very important feature in geotechnical engineering. Obtaining undisturbed samples and testing them is a tough and time- consuming procedure; any experiment aimed at obtaining correlations between shear strength and consistency limits would be extremely beneficial. In this thesis, efforts have been made to obtain valid correlations between undrained shear strength and Water content ratio (WCR) for saturated soil samples and for unsaturated soil samples suction values were obtained from previous proposed mode of SWCC then suction value and undrained shear strength were correlate. For this purpose, Red clay soil samples collected from different sites in Addis Ababa city at the depth of 3m were used. In addition to collected soil samples secondary data were used. And for each samples Atterberg limits, Specific gravity, Sieve and hydrometer analysis, Compaction tests were conducted beside that by remolding these samples Unconfined Compressive Strength (UCS) tests were conducted. The best fit line, correlation, regression analysis, and comparison were performed using EXCEL. The findings are expected to have a wide range of applications in construction, making it easier for designers to produce a solid, cost-effective, and dependable design.
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    Use of DCP and CBR Tests to Characterize Subgrade Shear Strength The Case of Volcanic Ash/Pyroclastic Deposit of Bulbula - Alage Section
    (Addis Ababa University, 2023-04) Yohannes Woldechirkos; Tensay Gebremedhin (PhD)
    The existing manuals adopted in Ethiopia for road construction and management purpose have made CBR and/or DCP tests mandatory in order to characterize strength of unbounded materials. Executing CBR and DCP tests is obligatory that, the data is available easily in every road stretch throughout the country. Contrary to this, the shear strength parameters used in slope stability analyses and bearing capacity computations are not available in sufficient number. For this reason, this research is initiated with the objective of obtaining values of shear strength parameters of pyroclastic/volcanic ash deposits using CBR and/or DCP test results on a pilot study area Bulbula – Alage road section. Different test results of Atterberg limit, sieve analysis, proctor, California bearing ratio, direct shear, dynamic cone penetration, field density, collapsibility and dispersity tests have been used. According to the test results, the plasticity of subgrade is low to none; the sand fraction is significant; the maximum dry density of the deposit is mainly below 1.5 g/cm3; the deposit has high CBR and low CBR swell values; DCP penetration rate decreases with depth in the majority of the stretch. The level of collapsibility potential is moderate with dispersity potential ranging from none to moderate. For this non-plastic to low plastic, low dry density, high CBR and moderate collapsible potential pyroclastic deposit a correlation is developed between CBR and/or DCP test results with that of angle of internal friction value obtained from direct shear test. The correlating equation has been developed using Excel and SPSS statistical analysis software.
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    Reliability of Probabilistic Finite Element Method Over Deterministic and Traditional Probabilistic Method in Slope Stability Analysis
    (Addis Ababa University, 2022-04) Eleyas, Wolde; Tensay, Gebremedhin (PhD)
    Slope Stability is one of the crucial topics in geotechnical engineering to be investigated well because oftentimes slope exists naturally or formed artificially during construction of various civil engineering structures which needs to be at equilibrium. For that matter, there are various methods of analysis. For the last couple of decades, it is observed hard for Engineers to use Probabilistic Slope Stability Analysis due to many reasons. But these days, technological advancements which led to having more rigorous software packages made it be possible to use in the analysis and its merit. The aim of this thesis is to show the use of more reliable method of slope stability analysis for hypothetical back slopes in Addis Ababa where red clay soil is dominant and that is achieved by comparing methods based on a factor of safety, probability of failure, and reliability index. Limit Equilibrium Method (LEM) and Numerical Method are used for this purpose. Both deterministic and probabilistic analyses are carried on by modeling seven slope geometries with different input parameters from Addis Ababa ring road III project and hypothetical slopes using a two-dimensional Slope Stability analysis software called SlideV6.0. A Global Minimum type of analysis are carried on by using the Slide software. For finite element analysis and probabilistic finite element analysis, RS2 software from Rocscience is used. Uncertainty is accounted for in a better way in probabilistic analysis by defining parameters as a random variable in link with other features. It is shown by this study that, Probabilistic Finite Element Analysis (PFEA) seeks out the most critical slip surface than that of the finite element method. In addition, even if the factor of safety found from the deterministic analysis is greater than one still a probability of failure is observed. Moreover, it is attested that the factor of safety is not the only measure for slope stability, but other measures exist while using probabilistic analysis.
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    Bored Soldier Pile Analysis and Design Practice in Addis Ababa (Case of 4B+G+6 Podium and 15 floors Assem Building)
    (Addis Ababa University, 2022-03) Roza, Tsegaye; Tezera, Firew (PhD); Leamlak, Minwuyelet (Mr.) Co-Advisor
    The purpose of this research is to show the benefit of the Finite Element Method over the Conventional Analysis Method for deep excavation design. This research is conducted on the Assem Building excavation, which has a 4B+G+6 podium and 15 stories and is located in Addis Ababa, Ethiopia, and is surrounded by busy roads and buildings. Following a comprehensive assessment of the literature on the analysis and design of excavation support walls, specifically anchored soldier pile walls, FEA and conventional analytic methods are used to model the excavation of the Assem Building. In the conventional analysis approach, the Assem Building's excavation support walls were analyzed and designed using the DeepEX software program and the GEO5 "Sheeting Check" software tool. Similarly, the FEA method uses the Plaxis 2D software program. Similar model sizes and soil and structural element parameters are used throughout all software programs. The following result was achieved by analyzing the Assem Building excavation. According to the FEA and optimized FEA results, the bending moments along the pile length are 40 % and 36 % lower, respectively, than those found using conventional analysis methods. Similarly, shear force calculated using FEA and optimized FEA are 41 % and 44 % lower, respectively, than those calculated using conventional analysis. The conventional method makes considerable assumptions about the solutions, but the FEA method makes no such assumptions and treats the problem as it is. As a result, the FEM result is lower than that of the conventional method. Because of the lower results, the FEA approach saves 9% to 11 % of the cost of the conventional analysis method. The optimized FEA also results in a cost reduction of 14 to 15%.
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    Estimation of Soil Shear Strength Parameters from Index Properties Using ANN the Case of Addis Ababa
    (Addis Ababa University, 2022-05) Hanna, Yoseph; Tensay, Gebremedhin (PhD)
    Shear strength of a soil is perhaps the most important of its engineering properties, as stability analyses in the field of geotechnical engineering are dependent on it. This research work seeks to develop models for predicting the shear strength parameters (cohesion and angle of friction) of soils in Addis Ababa city using artificial neural network modeling technique; with a view to reducing time, effort and cost usually incurred in determining these shear strength parameters in the laboratory for future planning, design and construction projects in the study area. An attempt has been made to develop separate neural network models for c and ϕ from the index properties of soil consisting of Sand % (SP), Fines % (FP), Liquid limit (LL), Plasticity Index (PI), water content (ω), and Bulk density (BD) as input parameters. A multi-layer perceptron network with feed forward back propagation is used to model varying the number of hidden layers. For this purpose, 284 soil test result data was used. The geotechnical soil properties were determined in accordance with ASTM Standards. Direct shear box method was used to determine soil cohesion and soil internal friction angle. The developed models were found to be quite satisfactory in predicting shear strength parameters with correlation coefficients of about 0.98 and 0.92 for cohesion and angle of internal friction, respectively during the testing phase. The models are validated by primary soil test data and compared with some existing correlation methods. The result showed that the artificial neural network method gave better fit and accuracy than the selected empirical formulae in the prediction of shear strength parameters.
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    Assessment of Damages Caused by Expansive Soil on Roads Constructed in Addis Ababa
    (Addis Ababa University, 2022) Desalegn, Workie; Henok, Fikre (PhD)
    Roads are one of the basic infrastructures and serve as means of a communication system for any country. The investment budgeted and the cost for original construction and repair of roads of our country is relatively high. Most of the time road lines constructed in our country failed early before functioning the design life. This is because of various problems arising from different causes. Among these roads which are failing before serving its design life is the Addis Ababa Roads. The main reason is that large areas of these roads are constructed on problematic expansive soils. This is due to improper construction and operation of the road. It is well known that Addis Ababa is a populated and investment area as it is the capital city of Ethiopia. As a result, the roads in Addis Ababa serve or carry the highest daily traffic in the country. These roads were constructed and opened to traffic in different years accordingly. The design pavement life was estimated at 15 and above years, however; pavement structure failures have been observed since construction time, within the first few years after opening to traffic. From the end of construction to date, the Addis Ababa roads are repeatedly under periodic maintenance. Considerable resources were spent during this period on periodic maintenance, but without solving the basic problem. This maintenance is not working as each maintained section is failing within a short period, hence, it needs to investigate the root causes of the failure to come up with an appropriate solution. This paper tries to present some issues of geotechnical cases for roads built on expansive soils. Some case study sites, representatives knew problem-areas in Addis Ababa were carefully selected for geotechnical site investigation. The field exploration consisted of detailed observation and measurement of damaged roads. These road damages have been identified and characterized based on the theoretical aspect of soil property and its effect on construction. This work was also have been used any secondary data concerning the study area as a baseline and it was helped as an input. Field investigations which include visual condition survey, DCP test, Benkelman Deflection measurement, and other methods were conducted to investigate causes of failure. Finally, based on the investigation, all causes of road failure are identified. Based on the identified damage types and their causes, maintenance and rehabilitation mechanism or techniques are recommended and suggested.
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    Numerical Modeling of Embankment Dams in Liquefiable Soils During Earthquakes
    (Addis Ababa University, 2022-04) Getu, Debebe; Tensay, Gebremedhin (PhD)
    Ethiopia has built several embankment dams up to the present day for different purposes. Some dams are situated in the rift valley, which is highly susceptible to high earthquake natural disasters. Dynamic loads induced by earthquakes are often major factors in the design of earth dams founded on liquefiable soil. Earthquake-induced stresses are major factors in determining the angles of the dam slopes and significantly influence the selection of materials, the zoning of the dam, and the construction method. These have taken a great attention to dams constructed from loose or medium dense saturated cohesionless soils and subjected to strong ground shaking. This project aims to simulate the effect of earthquake shaking on the stability of the embankment dam on liquefiable soil using the finite element method-based state-of-the-art software package, Geo-Slope software. The numerical simulation of embankment dams on liquefiable soil has been carried out by taking many factors into account. Among these factors, in this project numerical simulation has been made to see the effect of dam geometry, foundation material property, and liquefiable soil foundation thickness on the stability of the dam during dynamic loading. The evaluation also has been made to determine the response of the embankment dam on liquefiable soil under different earthquake ground shaking conditions. This includes assessing the safety margins against the risks of failure due to extreme scenarios, particularly soil liquefaction of the structure‟s foundation. A finding of this study shows that all those aforementioned factors have a significant impact on the stability of the liquefiable foundation of the embankment dam under dynamic loading. As a result an embankment dam having with 1 to 3 slope ratios, 5 meter foundation thickness with dense sand foundation material can withstand 0.37g peak ground acceleration safely with respect to slope stability, deformation and liquefaction.
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    Bio-Enzyme Stabilization of Red Clay Soil
    (Addis Ababa University, 2021-04) Dagnachew, Seifu; Mesele, Haile (PhD)
    Enzyme based liquid soil stabilizers have emerged as an alternative soil stabilization technique and are being introduced to the road construction industry, with an objective to improve the geotechnical properties of soils and ultimately reduce the overall cost of the projects. However, evidence to support the effectiveness of these products as stabilizer is based on manufacturer’s claims in their marketing brochures and manuals. Thus there is a need for an independent assessment of these products. Accordingly, this study is performed to evaluate the effect of TerraZyme which is an Enzyme based liquid stabilizer in improving the engineering properties of a red clay soil. Soil samples were collected from five different location in the northern part of Addis Ababa, and different laboratory tests were conducted on the soil samples with and without the application of TerraZyme. These tests include Particle Size Distribution, Plasticity Index, CBR and Unconfined Compressive Strength tests. The result obtained shows a slight increment in the UCS and CBR tests of the soil but the improvement is very small to warrant a large scale application of TerraZyme for red clay soil. Therefore, the type of soils in which the TerraZyme application can produce an optimal result should be investigated before applying TerraZyme on a large scale projects.
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    Use of Artificial Neural Network to Predict Compaction Characteristics of Soil from Soil Index Properties (Case of Addis Ababa)
    (Addis Ababa University, 2021-10) Hana, Adugna; Tensay, Gebremedhin (PhD)
    Soil compaction is the most commonly practiced mechanical method used to improve soil behavior and has a significant impact on earthwork structures. However, determining the compaction characteristics of soil in the laboratory requires considerable time and effort. For the purpose of attempting the problem of spending too much time and effort, ANN models that can predict the values of compaction parameters namely Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) from soil index properties are developed using Artificial Neural Network (ANN). A total of 300 secondary data divided as coarse and fine grained soils were used to develop the ANN models. All data were soil laboratory test result records of different soil samples taken from Addis Ababa, Ethiopia. Percent of grain size distribution was used as input parameter for the coarse grained soils while plasticity index along with the percent fine were used as input variables for the fine grained soils. The two variables MDD and OMC were the desired outputs from the models. Supervised learning with a backpropagation algorithm was implemented to train the models using MATLAB R2020a. The models were validated using 15 primary data and the models’ performance was evaluated using statistical values. The fifteen soil samples were collected from different locations in Addis Ababa. Three soil laboratory tests namely grain size analysis, Atterberg limits, and modified compaction tests were done on each sample. The outputs of the developed models were compared with the actual experimental soil laboratory test results and showed a good accuracy with a determination coefficient value of R=0.92 and R=0.81 for both maximum dry density and optimum moisture content respectively. Equations were derived for the ANN models. The models were also compared with existing regression equations developed using Addis Ababa soil type. From the research, it was concluded that the developed ANN models can be applied to predict the value of compaction parameters from soil index properties for both coarse and fine grained soils of Addis Ababa.
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    A Discrete Element Model for the Numerical Simulation of Granular Geo-Materials
    (Addis Ababa University, 2021-11) Agegnehu, Gizaw; Tensaye, Gebremedhin (PhD)
    This work presents computer simulation tools that use a coupled discrete- nite element methods for the modeling of granular materials. The simulation provides a new insight (perspective) in understanding the behavior of granular media at a microscopic level.It introduces and implements prevailing methods of computer sim- ulation to examine the internal dynamics of granular material that are typically not amenable to direct observations in the laboratory and eld investigations. The sim- ulation techniques are implemented using an object-oriented programming approach via C++ and python in the open-source Linux package YADE. The fundamental formulation of DEM ,FEM and the coupling scheme of discrete- nite element methods is presented. Along with Discrete Element soil sample and packing generation algorithms are discussed. Among the proposed packing algo- rithms, Multi-layer under compaction packing process is adopted since it mimics the natural layer-by-layer depositing process of granular materials . Accordingly, an algorithm is presented and coded in the platform (YADE). Then the generated DE soil sample is used to simulate a series of direct shear tests. The microscopic view of Direct Shear Test (DST) is studied on the shear behavior of dense and loose sand. The output results of the numerical simulation is validated with laboratory DST. The deformation pattern, stress-strain relationship, and change are analyzed.The DEM assemblies with porosity =0.65 (dense sand) shows strain softening and volumetric dilation behavior. While strain hardening and volumetric contraction is observed for the assemblies with porosity =0.8 (loose sand) which is the typical shear behavior of Dense and loose sand in laboratory DST.Furthermore, the deformation pattern observed in the simulation is localized within the shear zone of the sample which is comparable to observations from the laboratory Direct Shear Test. when the shear stress arrives at a steady-state after large shear displacement, the soils inside the shear band reach a critical state while the entire sample doesn't. The critical properties of the sand in DST should be investigated from the shear band instead of the entire sample. This property is di cult to capture in laboratory DST however the simulations allow us to resolve the limitation.Finally, a technical manual for the development process of computer simulation tools to provide an e cient approach for solving coupled discrete- nite geotechnical problems is presented.
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    Developing Correlation between Index Properties and California Bearing Ratio, CBR of soils: A Case Study on Gedo Menabegna Road Upgrading Project; on the Way from Gedo to Fincha
    (Addis Ababa University, 2021-12) Abyinur, Solomon; Henok, Fikre (PhD); Zetseat, Gashaye (Mr.) Co-Advisor
    California Bearing Ratio (CBR) is a very common test to determine the thickness of overlying pavement layers in pavement design and to assess potential strength of subgrade, sub base and base course material. Engneers are continually searching for simplified tests with the view to increase their knowledge of soils via using a simple and rapid soil tests. These simplified tests which are indicative of the engineering properties of soils are called index properties. In road construction, civil engineers constantly come upon difficulties in obtaining representative CBR value for pavement design. Laboratory CBR test requires relatively large effort to conduct the test and it is time consuming. The alternate method could be to correlate CBR with simpler test results such as soil index properties. These tests are much economical and rapid than CBR test. This project gives an overview to obtain a correlation between CBR values with soil index properties for Gedo-Manabegna road upgrading project. Primarily, in order to address the intended objectives of the study, basic theories and descriptions of CBR test in general and in relation to index property of subgrade soil is reviewed. Subsequently, previous works of different researchers with regard to prediction of CBR value from basic soil index properties were assessed. In order to have satisfactory data for utilizing the correlations, conducted laboratory tests are collected from Gedo-Manabegna road upgrading project to get records of test results of CBR values along with the associated soil indices particularly the grain size analysis, Atterberg limits, moisture-density relationships. Then, discussions on sample collection and summary of laboratory test results are presented. Statistical regression analyses of test results carried out, correlations are developed and analyzed to fit the test results with a reliable accuracy with a higher regression coefficient that approaches to unity. Finally, a generalized conclusion and recommendation is made.
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    Parametric Study on the Influence of Tunneling on Adjacent Deep Foundation
    (Addis Ababa University, 2021-09) Kalkidan, Gashw; Henok, Fikre (PhD)
    Numerous high rise buildings are supported by deep foundations in major cities or urban areas. Due to the rapid increasing demand for infrastructures construction in congested urban areas, tunnels are often preferred for under ground transportation. The construction of tunnels are close to some existing piles foundations will affect pile capacity and stability due to soil deformation caused by vertical and horizontal movement, which changes the axial load distribution along the piles. In this study, the effect of a 6m open face advancing tunnel on a three by three group pile in c- ϕ soil has been investigated using three dimensional finite element analysis software (PLAXIS 3D). Hardening soil (HS) constitutive model and isotropic elastic model for piles, pile cap, tunnel lining, and tunnel boring machine (TBM) were used for analysis. A parametric study has been carried out to investigate the behavior of piles through varying ratios of tunnel depth (H) to tunnel diameter (D) (namely H/D= 2.0, 2.5, and 3.0). From the analysis result, the factor of safety of the pile was reduced from 3.0 to1.62 due to tunnel advancement 2m near to the pile tip, large pile settlement was observed when the tunnel face below the pile tip(H/D=3.0). And also the load carrying capacity of the pile at the base significantly decreases. In addition, skin friction of the pile was significantly decreased when the tunnel approaches to the shaft of the pile.
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    Site Response Analysis of Selected Sites of Adama City
    (Addis Ababa University, 2021-09) Temesgen, Abiy; Asrat, Worku (PhD); Atalay, Ayele (Prof.)
    The local soil deposits found in Adama area have the potential of seismic site amplification. Site response analysis of the study area is mainly motivated by the presence of thick local soil deposits in a seismic prone area. Compressional (P-wave), horizontally polarized shear (SH-wave) and surface waves was acquired at four sites in the study area. Multichannel Analysis of Surface Wave (MASW) and Micrometer Analysis Measurements (MAM) were used to evaluate the shear waves from the surface waves and refraction time term inversion, to evaluate shear waves from body waves. The Poisson's ratio and anisotropy of the site have been calculated using the P-wave, SV-wave and SH-wave results. Using the results of the average shear wave velocity for a depth of 30 m (VS30), the sites were characterized as site class C according to EN 1998:2015 and site class D according to National Earthquake Hazards Reduction Program (NEHRP). The geotechnical data collected from different private and government organizations have also been used to characterize additional five sites and to generate dynamic properties such as the modulus reduction and material damping curves. The input ground motions from the Pacific Earthquake Engineering Research Center (PEER) database were selected using several criteria such as magnitude, site conditions, and target spectrum. These are then scaled to the site PGA value of 0.15g as established based on Global Seismic Hazard Assessment Program (GSHAP). One-dimensional site response analyses have been conducted by adapting equivalent linear analysis (EQL), nonlinear analysis with Modified Kondner-Zelasko (MKZ) and General Quadratic/Hyperbaric (GQ/H) model at nine sites in the study area. The results have been presented in terms of peak ground acceleration (PGA) profiles, displacement profiles, strain profiles, and response spectra. The area shows an amplification between 1.30 and 3.73, 1.15 and 2.00, 1.25 and 2.36 in PGA value for EQL, MKZ and GQ/H models. The results indicate that the current code design spectra (EN 1998:2015) for the site class C underestimates the shaking intensity of the sites.
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    Static and Dynamic Lateral Response of Single Piles Using Kerr-Equivalent Pasternak Subgrade Model
    (Addis Ababa University, 2021-09) Abey, Lulseged; Asrat, Worku (PhD)
    The most widely used model to perform piles analysis under static and dynamic lateral loads consists of modeling the pile as beam elements and representing the soil as a group of unconnected, concentrated springs perpendicular to the pile (Discrete Winkler Model). The absence of interaction between the individual springs in the Discrete Winkler Model produces an unrealistic response. This study aims to perform an analytical and numerical study of the static and dynamic response of the pile-soil system under lateral loads (considering homogeneous soil profile) using a two-parameter subgrade model. In order to develop a rational method that includes shear interaction between the concentrated springs, a two-parameter model proposed by Worku (2014) was used. A simplified expression for the adopted model parameters was obtained by performing a series of numerical and parametric investigation. The proposed model was calibrated using outputs from PLAXIS 3D, and from the result, it was observed that the proposed model could predict pile-head stiffness within a maximum error of 3% when compared to FE output. A new approach for determining the critical pile length is also introduced. Using this new approach, critical pile lengths for different boundary conditions are introduced. Based on the variation of pile-head stiffness, a new classification, other than "short" and "long" piles, was introduced, namely "transitional" piles. Simplified expressions for pile head stiffness for piles in a "transitional" state are provided. The dynamic stiffness (or impedance) and the kinematic response are investigated. Equivalent spring and dashpot coefficients at the pile-head are computed to provide a simplified procedure to replace the entire soil-pile system with a spring and a dashpot at the top that will produce the same effect on the overlying structure. Equivalent spring coefficients at the top of the pile are obtained from the beam on elastic foundation techniques. Equivalent damping coefficients are obtained following a simplified approach that uses the concept of conservation of energy. Results are compared with those given by a dynamic finite element (DFE) analysis over a range of frequencies for piles in a homogeneous soil layer, and from the result, it is evident that the model is in good agreement with the DFE results.
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    Correlating Liquidity Index with Vane-shear Strength of Clays in Addis Ababa
    (Addis Ababa University, 2017-10) Jara, Mengistu; Asrat, Worku (PhD)
    Investigation of soil properties is one of the main tasks of Geotechnical Engineer. Soil testing methods don’t require the same amount of time and effort; even some of the apparatus are scarce in developing countries. This usually leads to unreliable and impractical test results. That is why it is important to develop empirical equations that fit the local area. In this thesis, attempts have been made to obtain valid correlations between undrained shear strength and liquidity index. For this purpose, 6 disturbed samples were collected from 5 sites (from depth of 1m to 3m) and for each sample Atterberg limit and standard compaction tests were conducted. Beside that by remolding these samples 12 vane shear tests for each of the samples in total 72 vane shear tests were conducted. From these tests undrained shear strength and liquidity index are determined. For the analysis, SPSS and EXCEL softwares have been used for the best fit line, correlation, regression analysis and comparative study. Equations were developed for each soil sample with a very strong correlation of coefficient of determination (R2) values ranging from 0.927 up to 0.991. From the result it has been shown that the use of the commonly assumed 100-fold factor increase in strength from the liquid limit to plastic limit over predicts the measured data of the soil strength. In this study the ratio factor fold in strength is between 30 up to 63. The results are expected to have wide application in the construction sector for soft clays.