Dolgu barajların tasarımında temel ilkeler ve İ.T.Ü. göleti

Abstract

Bu çalışmada, ilk olarak baraj tipi seçimini etkileyen faktörler ele alınmıştır. Baraj tasarımının temel ilkelerinden ve barajın toptan göçmesinden, onarım gerektirmeyen basit hasarlara kadar geniş bir aralıkta değişen göçme ve hasarların nedenleri ve bunları önlemek için alınması gereken tedbirlerden söz edilmiştir. Î.T.Ü Göleti ortalama 30 m yüksekliğinde homojen dolgu bir baraj olarak projelendirilmiştir. Baraj gövdesini oluşturan yan geçirimli malzeme ve temel kayasının mühendislik özelliklerini belirlemek için laboratuvar ve arazi deneyleri yapılmış ve deney sonuçlan ilgili analizlerde kullanılmıştır. Barajların deformasyon analizleri ve analizlerde sonlu elemanlar metodunun kullanılması tezin ikinci hedef konusudur. Bu bölümde İ.T.Ü Göleti, SIGMA/W yazılımı ile modellenerek, elestik-plastik zemin modeli ile gerilme-deformasyon analizi yapılmıştır. Barajın tam kesitte inşaasını takiben yapacağı deformasyonlar, herbir inşaat aşamasındaki yerdeğiştirmeler, yatay ve düşey gerilmeler ile kayma dayanımının aşılması ile ortaya çıkan plastikleşme bölgeleri belirlenmiştir. SIGMA/W yazılımı ile modellenen Î.T.Ü Göletinin, SLOPE/W yazılımına transferi ile, önce barajın inşaat sonu durumu için memba ve mansap şevlerinin stabiliteleri incelenmiş, ardından SEEP/W yazılımı ile hesaplanan boşluk suyu basınçlan aynı programa aktanlarak, işletme durumunda mansap şevi stabilitesi, ani boşalma durumunda ise memba şevi stabilitesi analizleri yapılmıştır.

As with most civil engineering structures, the design of an earth dam is based both on precedent and on analytical studies. The personal experience and preferences of the designer, however, play a large role in earth dams than in the design of most other structures. At a given site it is possible to build a variety of dams that would be both safe and economical, and there are many examples where competent engineers have proposed widely different designs for the same reservoir. In addition, the characteristics of the particular site have a greater influence on the design of an earth dam than they do on many other structures. The quantities, types and location of the soil available for the construction of the embankment may dominate the entire design, although in most cases the soils must be considered in conjunction with conditions such as the nature of the foundation, the climate of the region, and size and shape of the valley. Besides these conditions imposed by nature, the design may be affected by the length of time available for construction or the intended function of the reservoir. The fundamental steps in designing an earth dam are: a) A through exploration of the foundation and abutments, and an evaluation of the quantities and characteristics of all the embankment construction materials available within a reasonable distance of the site. b) A study of all conditions which may influence the design. c) The selection of possible trial design. d) An analysis of the safety of the trial design. e) The modification of the designs in order to meet minimum stability requirements. f) The final selection of the design which seems to offer the best combination of economy, safety, and convenience in construction. In the second chapter, we have attempted to provide a summary of the most instructive experiences with failures and damages, in dams. The magnitude of damages to earth dams ranges from complete catastrophic failure, resulting in large property damage and loss of life, to relatively minor deterioration that may or may not necessitate remedial work. The worst type of complete failure occurs when the reservoir water suddenly break through the vn embankment and surges downstream in one devastating flood wave. Lesser damages fall into several categories, some of which can lead to complete failure if left unattended and some of which require only maintenance work even under the most extreme conditions. The most common cause of complete catastrophic failure has been that of waters flowing over the tops of earth dams during great river floods when the spillway capacities were inadequate. Failures due to overtopping are not deficiencies in the design of the earth dam itself but rather the result of inadequate hydraulic design. The two other principal causes of catastrophic failure are piping (the progressive erosion of leaks that develop under or through the dam) and earth slides in the downstream portion of the embankment or foundation. Upstream slides in the embankment have not often threatened to cause complete failure of the dam since they usually happen after the reservoir has dropped below a dangerous level. Embankment or foundation slides occurring during construction never threaten a catastrophic failure unless water is retained in the reservoir while the dam is being built. Neither do slope and crest erosion by waves, wind, and rain lead to danger of complete failure except in very special circumstances. The designer must make very conceivable effort to eliminate any possibility of unsatisfactory performance that could lead to a catastrophic failure. Design details that are provided against damages that would not lead to catastrophic failure, on the other hand, fall within the sphere of decision where a calculated risk may properly be taken. Because of the uncertainties in some aspects of earth dam design, the engineer is often faced with a situation where he can make large savings in the cost of the structure by risking certain damages. The following section is deals with the deformation analysis of the I.T.U Dam. In the deformation analysis of the dam, SIGMA/W a finite element program was used. An important purpose of the finite element analysis of embankment dams is to determine internal stress distribution. This is necessary to find out if there is a developing which could cause cracks. The analyses are also used to calculate displacements. Although displacements are at least in the case of clay core dams ultimately of less interest than the stresses they are important because they can relatively easily be measured. This makes it possible to calibrate an analysis against measurements taken at an early stage, or on a trial fill, so that the accuracy of the assumed soil stiffness can be improved. This in turn will lead to a more accurate stress calculation. VIII A succession of finite element analyses are carried out with the loading for each one comprising the application of gravity to the new layer. Each successive analysis inherits stress and displacements from its predecessor and these are accumulated. The displacements, however, are accumulated in a special way. The finite element in layers not yet placed can either be omitted and added to the mesh as they come into existence, or they may all be included from the start and simply given a very low stiffness while they are in the " ghost " category. This later procedure has the advantage of simplicity, but the alternative of adding the element as they are needed involves less computing at the expense of added complication. The first stage in the analysis is to make the necessary geometric idealization, i.e., whether the analysis is to be two- or three-dimensional, whether or not the foundation is to be included in the mesh, and what type of elements should be used. A factor to be considered here is the construction sequence. îs the fill to be risen evenly so that layer boundaries can be assumed horizontal? or is part of the fill (such as upstream cofferdam) to be constructed ahead of the rest? The layer boundaries should be chosen to accommodate this, i.e., they should roughly correspond to the actual fill surface as the fill is raised. A finite element layer, although it cannot be less than one element thick, can contain more than one layer of finite element. Then the material properties must be chosen. This involves the selection of a material model and the major decision of selecting parameter values. Provision must also be made for the stiffness adjustment of the new layer. In general initial stresses must be prescribed. These must be self equilibrating, and if the foundation is included in the finite element model insitu stresses should be assigned to it. If the ground surface is level these can be prescribed directly as yh vertical and Koyh horizontal. Otherwise a preliminary excavation analysis from a level ground surface may be required to calculate a self equilibrating stress field. The initial stresses in the embankment itself will usually be set to zero- at least in term of total stresses. This is consistent with the concept of a new layer being initially weightless. The applied gravity imposes the stresses. This does not, however take into account any stresses locked into the material by the compaction plant. SIGMA/W is a finite element software product that can be used to conduct stress and deformation analyses of earth structures. The comprehensive formulation makes it possible to analyze both simple and highly complex problems. For examples, you can perform a simple linear elastic deformation analysis or a highly sophisticated nonlinear elastic plastic effective stress analysis. SIGMA/W is integrated with other geotechnical engineering products, such as SLOPE/W AND SEEP/W IX First goal of this study, is to find out the stress and strain behavior of the I.T.U Dam. Strain behavior of the dam is investigated in the cross section which is assumed to have the maximum deflection. As it was considered, first stage of the finite element analysis is the idealization of the engineering structure. This means that a geometry is established to have the finite element problem solved. The finite element meshes that was used in the analysis have seven hundred and twenty-six elements. Two different element types was used, one of them is four pointed quadrilateral, other one is three pointed triangular. The second stage of the finite element analysis is to determine the boundary conditions of the problem. First condition the increment at the vertical stresses is not affected by the body of the dam. Same stresses are assumed not to cause lateral deflections. I.T.U Dam is a homogen dam with 30 m. height. In order to calculate the deformations, four different layers is chosen and this is assumed to be adequate. Maximum vertical deflection for the first stage of loading is 3.68 centimeters, for the second stage of loading 9.82 centimeters, for third stage of loading 16.56 centimeters. Finally while fourth stage can be called dam crest with 18.73 centimeters. In the fifth section of this thesis, I.T.U Dam is modeled by using SLOPE/W Software and slope stability of the dam is analyzed. SLOPE/W is a software product that uses the limit equilibrium theory to solve for the factor of safety of earth and rock slopes. The comprehensive formulation of SLOPE/W makes it possible to select a variety of methods for computing the factor of safety, and to analyze both simple and complex geometric,stratigrafic, and loading conditions. In the stability analysis of embankments and slopes there are at present two basic lines of approach. The first is the limit equilibrium approach and the second is the stress-strain analysis. With the advance of the finite element technique, given the material properties and the cross section of the embankment or the slope, it is not difficult to analyze the section for deformation and safety by computing the stresses and strains in the structure. However, the use of such a sophisticated method requires very accurate input data. Otherwise, the results obtained from such analysis become as doubtful as the input data itself. Therefore, for a designer who has yet to satisfy himself about the soil properties obtained from laboratory test, and to finalize a cross section of a dam or and embankment, the use of the finite element method becomes uneconomical. On the other hand, simple methods such as those based on limit equilibrium principle, even though these cannot give the deformations, picture of the structures under stress, are able to produce comparable results as regard the safety of the structure. Furthermore, the strength parameters computed from analysis of slope failures agree reasonably well with laboratory test results, which gives more confidence in the analytical procedure of the limit equilibrium approach. Once a final design section is arrived at, it can always be analyzed with a finite element or similar sophisticated method to check the results obtained from the simple analysis. The basic idea behind the limit equilibrium approach is to assume a surface which is likely to fail and try to find a state of stress along the surface so that the free body, contained within the slip surface and the free ground surface, is in static equilibrium. This state stress, which is known as the mobilized stress is then compared with the available strength, i.e., the stress necessary to cause failure along the surface. The factor of safety, F, is then defined as that factor by which the available shear strength should be reduced so as to bring it into equilibrium with the mobilized stress. There are at present several methods of stability analysis in existence which apply the limit equilibrium principle. Most of these methods apply the technique of slices, Fellenius (1936), Bishop (1955), Janbu (1957), Morgenstern and Price (1965), Spencer (1967). İn these methods, the available strength is computed on the basis of the Mohr-Coulomb failure criterion. These methods mainly differ in the shape of the assumed slip surfaces and in the handling of the indeterminacy of the problem. There are three critical situation for earth dams. These are a) End of construction b) Steady seepage with maximum storage pool c) Sudden drawn down In an embankment composed partially or entirely of impervious soils placed at water contents higher than those corresponding to optimum water contents after complete consolidation under the imposed loading, pore pressure will be induced because the soil cannot consolidate readily during the construction period. Where this is indicated, applicable shear strengths are determined from UU test on specimens compacted to anticipated field placement water contents and densities. A condition of steady seepage from the maximum water storage level that can be maintained sufficiently long to produce a condition of steady seepage throughout an embankment may be critical for downstream slope stability. A flow net should be constructed to determine the phreatic surface and seepage forces when the assumption of a horizontal phreatic line in the impervious zone is overly conservative. Shear strength used in this case, should correspond to a strength envelope midway between the CU and CD test envelopes, except for large downstream zones consisting of cohesionless materials that may be analyzed by the infinite slope method using CD strength envelope. XI Embankments may become saturated by seepage during prolonged high reservoir stages. If subsequently the reservoir drawn down faster than pore water can escape, excess pore water pressures and unbalanced seepage forces result. Shear strength to be used in sudden draw down case shall be base on the minimum of the combined CU and CD envelopes. Second goal of this study is the analysis of the stabilities I.T.U Dam's slope. The hearth of the analysis is based on the critical downstream and upstream slopes stability analysis. At the beginning, end of construction analysis of the upstream and down stream slopes are carried out. In the next section, seepage analysis of I.T.U Dam has been done. SEEP/W Software has been used in the analysis. In the analysis of the stability of slopes in terms of effective stresses, the pore water-pressure distribution is of fundamental importance and its evaluation is one of the prime objectives in the early stages of any stability study. This may involve exsentive field measurements or modeling the seepage pattern. A seepage pattern will develop if the necessary overall hydraulic gradients are present. İn a slope, it is conceivable that such a flow pattern would not develop, even solely under the effects of rainfall, because water infiltrating at the head crest of a slope must have a higher potential energy than that infiltrating at the toe. Water- impounding embankments have an even more apparent hydraulic imbalance. Seepage or the flow of water through soil or rock, can be treated in two separate parts:the steady flow of water in a pattern that equilibrium with the external hydraulic constraints(steady seepage), and the equilibrium of non-equilibrium pore water pressures to the steady state. ( unsteady seepage, consolidation, swelling) In steady seepage, the flow pattern that results is dependent on the distribution of soil permeabilities in the slope, and the nature and extent of the hydraulic boundary conditions which control that seepage. Even so, identification and quantification of these factors in order to make an entirely satisfactory prediction of pore water pressures for slope analysis and design is very difficult indeed. The situation for unsteady seepage is even more complex: the generation of non-equilibrium pore water pressures being the result of a sometimes long, but always involved, stress history, and the equilibration process may be partly complete, or proceeding concurrently with the stress changes in a slope. For many problems, the prediction of field pore water pressures may not be economical or practical, and it may be preferable or expedient to attempt to measure the pore water pressures insitu and then leave the predictive element to likely future changes in those pressures.. XII The amount of water seeping through and under an earth dam, together with the distributions of the water pressure, can be estimated by using the of flow through porous media. The computed amount of seepage is useful in estimating the loss of water from the reservoir, the estimated distribution of pressure in the pore water is used primarily in the analysis of stability against shear failure and also occasionally to study hydraulic gradient at the point of seepage discharge which gives a rough idea of the piping potential. SEEP/W is a finite element software product that can be used to model the movement and pore water pressure distribution within porous materials such as soils and rocks. The comprehensive formulation makes it possible to analyze highly complex seepage problems. SEEP/W has application in the analysis and design of geotechnical, civil, hydrotechnical, and mining engineering facilities. At the end of the study, finite element seepage analysis with SEEP/W software program is done. By the help of these analysis, pore- water pressures which represents the reasonable steady state seepage conditions and sudden draw dawn circumstances are found. These pore-water pressure heads values used in the slope stability analysis. Use of the actual pore water pressure values, let the analysis to calculate more reasonable factor of safety values.