A semi-analytical method of dynamic analysis, capable of predicting both the magnitude and pattern of earthquake induced deformations, is presented. The analysis is based on a modulus reduction approach which uses a reduced modulus to simulate the softening induced in soils during cyclic loading. The effects of the inertia forces developed during dynamic loading on the induced deformations are also included through an appropriate selection of the reduced modulus. The reduced modulus is utilized in a static stress-strain analysis to predict the magnitude and pattern of the deformations induced during earthquake loading. The appropriate modulus reduction is determined from laboratory tests on undisturbed soil samples. Three methods of computing a suitable post-cyclic modulus were investigated but only the cyclic strain approach, in which the modulus is determined from cyclic loading tests that duplicate the field stress conditions, yields reductions of sufficient magnitude to provide realistic estimates of earthquake induced deformations. The modulus reduction analysis was used to predict the deformations occurring during dynamic loading of a model tailings slope in a laboratory shaking table test and of the Upper San Fernando Dam during the earthquake of February, 1971. These studies showed that the modulus reduction analysis is capable of reproducing the dynamically induced deformations and that reductions in the modulus of up to 1000 times may be required. Unfortunately, limitations of the testing equipment and inadequacies in the available data required that the appropriate modulus reductions could not be determined entirely through laboratory and field investigations. Some assumptions were necessary in selecting the reduced modulus values used in the analyses. Although these case studies were, hence, unable to provide full verification of the proposed method, they do demonstrate the reliability and simplicity of the analysis as a method of assessing the performance of soil structures during earthquake loading. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

Ph.D. (Geography) / Please refer to full text to view abstract

In the classical design method for gravity dams, the designs are conducted in the linear elastic isotropic material domain. For many decades the so-called ‘classical method’ (or conventional method) was used to design gravity dams. This method is based on the Bernoulli shallow beam theory. Despite much criticism expressed by academics regarding the basis of the theory, dam design engineers are still using the classical method to design gravity dams. Currently, in most dam building countries the various codes of practice are standardised and based on this method, and engineers have confidence in these codes. This state of affairs will probably continue until structural engineers come up with a viable alternative for designing gravity dams more precisely. The perception of increased risk is always a critical aspect to overcome when introducing an alternative design method, especially when the established, well-known methodology has proved to be safe. However, when so-called ‘back analyses’ are performed on existing dams, it is not so straightforward to assess the safety margin of these structures. Material properties and their yielding or failure characteristics are now becoming important in evaluating these structures accurately in the non-linear domain. With the growing popularity of roller compacted concrete as a dam building material, the attractiveness of gravity dams has also increased and the author is of the opinion that the finite element method could be utilised more efficiently to optimise gravity dams. But, as with any new or alternative design method, the parameters and means of evaluation should be developed to establish a workable and reliable technique. The objective of this dissertation is to develop a practical methodology for the non-linear analysis of gravity dams by means of the non-linear finite element method. A further aspect of this dissertation is the inclusion of a broad guideline on the application of the latest dam design standards used in South Africa for both the classical and finite element methods. In order to gain a better understanding of the basic design criteria, a literature survey was conducted on the evolution of dams and the various theories developed in the past to design and optimise gravity dams. The literature survey included the examination of gravity dam safety criteria and some available statistics on dam failures. The International Committee on Large Dams (ICOLD) has interesting statistics on dam failures and their causes. A few typical dam failures are presented to illustrate what can go wrong. During this literature research, a thorough study was done on the non-linear material theory, with special reference to the Mohr-Coulomb and Drucker Prager material models. The findings of the study are used to illustrate how the non-linear material models are incorporated into the finite element method and in what manner the different material parameters have an influence on the accuracy of the results. As already mentioned, currently the classical method is still a recognised design standard and for this reason a summary is presented of the South African Department of Water Affairs and Forestry’s practice for designing gravity dams. This includes the latest concepts on load combinations and factors of safety for these load conditions. This summary of current practice is used as a stepping stone for the proposed load combinations that could be used for the finite element method as these are not always compatible. However, this dissertation does not deal with the full spectrum of load combinations and the scope is limited to hydrostatic loads. Although the finite element method is a very powerful structural engineering tool, it has some serious potential deficiencies when used for dam design. The most serious problem concerns the singularities and mesh density, which develop high stress peaks at the heel of the dam wall. This problem is illustrated and some practical finite element examples are given. Some solutions for addressing this problem are also presented. It is concluded that an effective method for overcoming the singularity problem is to use the non-linear material yielding model. In order to calibrate the non-linear Drucker Prager model, several finite element benchmarks were conducted, based on work done by other researchers in the fracture mechanics field. Although the theory of the Drucker Prager model is not based on fracture mechanics principles, this model simulates the failure of the concrete material very well. To demonstrate this, various benchmarks were conducted, such as a pure tension specimen, a beam in pure bending, a beam combined with bending and shear, the flow models of Chen (1982), a model of a gravity dam and, finally, a full-size gravity dam. The next step in the study was to calibrate the Drucker Prager model with the concrete material properties used in existing dams of different construction methods and ages. The material strength of the concrete was statistically analysed and the average strength was calculated. The important ratio of tensile strength to compression strength (ƒt/ƒc) was also examined and the findings are presented. This ratio is important to get accurate results from the Drucker Prager model. The normal input parameters for the Drucker Prager model are the internal friction angle of the material (φ) and the cohesion (c). Scrutiny of the work done by Chen (1982) helped to find a useful solution to obtaining the parameters for the non-linear finite element method without determining the ö and c values, but by using the material tensile and compression strengths instead. The formulation is demonstrated in the chapter on theory. To illustrate the usefulness of the non-linear yielding model a few case studies were conducted. A hypothetical triangular gravity dam structure was analysed because it was widely used in other literature studies and a useful comparison could be made. Then, a case study of an 80-year-old concrete gravity dam was performed. The uniqueness of this dam lies in the fact that it was designed before the theory of underdrainage was used in South Africa and the dam has a characteristic shape due to its relatively steep downstream slopes compared with today’s standards. A study of material strength sensitivity was also done on this dam to evaluate its stability under severe load conditions. The last case study presented is on a recently designed 75-m-high roller compacted concrete gravity dam, optimised primarily by the classical method. The non-linear Drucker Prager yield model was used to evaluate this structure, with the actual material strengths taken from the laboratory design mix results. Although the finite element method was used during the design stage of this dam, it was used mainly to check the results of the classical method. The finite element method was also used to do studies on this dam where the classical method could not be used, such as studies of temperature and earthquake load conditions (not included in this research). The factor of safety against sliding was also determined using the results obtained from the finite element method and compared with the results obtained from the classical method. This case study gives an approximate comparison between the classical method and the finite element method. Finally, a methodology is proposed for analysing a gravity dam. Procedural steps are given to describe the methodology. With regard to the future, the advantage of the non-linear finite element method is that it can easily be extended to contemporary 3-D analysis, still using the same concept. Many dams can only be accurately evaluated by a full 3-D analysis. There is a modern tendency to design gravity dams in 3-D as well so as to evaluate their stability against sliding in the longitudinal direction. The non-linear 3-D finite element method is also used for arch dams, for which very few alternative numerical analysis methods are available. Moreover, the non-linear finite element method can be extended to earth and rock-fill embankments. / Dissertation (MEng)--University of Pretoria, 2009. / Civil Engineering / unrestricted

In general, all water retaining structures are subjected to seepage through or under their embankments. If the seepage is not controlled, structural failure is certain. The object of this project is to present a review of the seepage problems and stability considerations involved in the analysis of earth dams. It contains a review of the seepage characteristics of soils and their effects on earth embankments. The problem resulting from seepage and different methods for controlling seepage are fully discussed. Because many uncertainties remain, the stability of the embankment must be determined. Four different methods are presented for analysis of earth embankments to determine their factor of safety against stability failure. / Master of Engineering

No description available.

Inflatable dams are flexible, cylindrical structures anchored to a foundation. They are used for a variety of purposes, e.g. diverting water for irrigation or groundwater recharging, impounding water for recreational purposes, and raising the height of existing dams or spillways. The vibration behavior of such dams is analyzed. Single-anchor inflatable dams with fins are considered. First, a static analysis is performed which yields the equilibrium shapes of the dam, both in the presence and absence of water. Then, a dynamic analysis is undertaken which analyzes the small vibrations of the inflatable dam about the equilibrium configuration, both in the presence of water (hydrostatic water as well as parallel flowing water) and absence of water. The dam is modeled as an elastic shell. It is assumed to be air-inflated and resting on a rigid foundation. The cross-sectional perimeter, material thickness, modulus of elasticity, and Poisson's ratio are given. The analysis is performed for different values of internal pressure and external water heads. Initially, the dam is assumed to lie flat. The internal pressure is then increased slowly until it reaches the desired value. Then the external water is applied and the equilibrium configuration is obtained. Small vibrations about this configuration are considered. The water is assumed to be inviscid and incompressible, and potential theory is used. The infinite-frequency limit is assumed on the free surface. A boundary element technique is utilized to determine the behavior of the water, and the finite element program ABAQUS is used to analyze the structural behavior. Both the cases of fluid at rest and flowing parallel to the dam are considered. The vibration frequencies and mode shapes are computed. The effect of the internal pressure of the dam is investigated, and the results are compared to those for the dam in the absence of external water. / Master of Science

Conventional design of cellular cofferdams is largely based upon semi-empirical concepts derived from classical earth pressure theories. Recent studies have suggested that most existing design methods are excessively conservative. In part, this derives from an inability to fully describe the true, relatively complex soil-structure interaction process which occurs in the flexible cellular cofferdam. In addition to the apparent conservatism in the conventional theories, they provide no means to predict movements of cofferdams. This deficiency has become more important with time, since performance and safety monitoring systems are largely oriented towards measuring deformations, and it is not possible to define reasonable levels of movements before construction. Further, finite element analysis techniques are being developed which are designed to predict cofferdam deformations, but the means for verifying the predictions are not definitive because of the lack of a broad data base on cofferdam performance. This investigation is directed towards providing a data base through the documentation and comparison of five case histories where instrumentation was used to monitor the behavior. Primary consideration was given to the response of the cofferdams under differential loading. In the course of the investigation, the response of each cofferdam was documented and dissected in terms of the behavior at each stage of the differential loading. A strong correlation is found to exist between the lateral cell deflection and the level of differential loading, with the exact nature of the correlation depending on certain key parameters including cell foundation, cell fill material, cell width to height ratio, presence of a stabilizing berm, and nature of loading. It is found for conservatively designed cells that the response falls into a predictable pattern which can be characterized in terms of non-dimensionalized parameters for both normal and more severe levels of loading. In addition to deformations, information is provided on interlock tensions where available. The results are believed to provide a baseline for cofferdam behavior that can be useful in gaging the expected behavior of other cofferdams and for verifying the accuracy of new forms of predictive tools such as the finite element method. / M.S.

This work deals with the linear two-dimensional free vibrations of an inflated cylindrical membrane. An air-inflated membrane is considered first. Vibration frequencies and modes are determined for various cases. The lowest mode shape is anti-symmetric. In the rest of the work, the membrane is inflated with water. In some cases there is a reservoir of water on one side of the membrane. The membrane equation of motion is solved using a finite difference method, and the hydrodynamic pressures on the membrane, caused by the motion of the internal and external water domains, are treated by the boundary element method. The effects of the membrane parameters, internal and external water head, and density of the membrane on the lowest four frequencies are illustrated. For the membrane without the outside water, the first two natural frequencies agree well with experimental values. The existence of the upstream head has a significant influence on the frequencies, and the mode shapes are shown to be tilted toward the downstream side of the membrane. / Ph. D.

Renewable energy sources are getting much more important to reduce the increasing threat coming from greenhouse gases. Hydropower is the most important source of renewable energy. However, development of a hydropower project is a challenging engineering process. Several computer programs have been developed to make initial estimations on hydropower schemes. A computer program named RETScreen Small Hydro Project Model has been developed with the objective to make complete pre-feasibility studies including costing and financial analysis. Two case studies, which have been under construction in Turkey, will be used to check the accuracy of software in Turkish practice. Then in light of the results, RETScreen software will be used to make a pre-feasibility report on an existing multipurpose dam in Turkey. Electricity can be generated at existing dams which requires minor civil works. Porsuk Dam which is a 36 year old dam used for domestic, industrial and irrigation water supply will be evaluated for energy generation by constructing a penstock, powerhouse and installing electromechanical equipment.

Thesis (M.Eng.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 101-108.