Dissertation for M.SC degree
String: Water Engineering
Abstract:
Dams are the largest man-made structures. Their design often stems from the unique nature and consequences of failure, which is certainly greater than the collapse of other types of structures. The stability of dam body and foundation structures can be analyzed and checked in various ways. Nowadays, there are various methods to check the dynamic stability of earthen dams. Among these methods, limit equilibrium methods, limit analysis methods (upper and lower limits in plasticity theory) and finite element methods can be mentioned. The most important use of numerical analysis in the design stage is to evaluate the amount of permeable water pressure at the end of the dam construction, check the hydraulic failure in the clay core, check the arching phenomenon, predict the amount of settlement to determine the volume of required materials and determine the confidence factor to determine the stability of the dam body. In this research, using flac 7.0 software, the influence of different materials as cores in dams and the stability of upstream and downstream gables of the dam in two cases of vertical core and inclined core have been investigated and the results obtained from them have been investigated. The behavioral model used in this research is the Moore-Columb model, which is a complete elastic-plastic model. In the aforementioned behavioral model, the rupture envelope is obtained by the famous Moore-Columb criterion, which is a function of shear rupture, taking into account the tensile rupture criterion. According to the obtained results, the amount of confidence factor versus stability depends on the type of behavior assumed for the core and shell materials, the location of the core (inclined or upright core), and Poisson's ratio of the core materials. Keywords: earthen dams, stability, confidence factor, effective stress, flac software
Chapter One
1- Introduction and purpose:
1-1 Introduction
According to the basic need of the vast country of Iran to contain and store surface water to supply water for drinking purposes, water needed for agriculture, industry, power generation, as well as flood control, river flooding, and on the other hand, the first step in the development and exploitation of the country's water resources is the construction of high dams. Statistics show that about 300 dams have been built in our country and more than 200 embankment type dams are under construction, which reveals the importance of these types of dams for various reasons. Therefore, it seems necessary to control the stability of these types of dams according to the material of the core. Dams are important infrastructure structures of a country, because it takes a lot of money and time to build a dam, unlike a normal building. The central core in earthen and gravel dams is responsible for the main task of sealing and maintaining water behind the dam, so choosing materials with high permeability in the dam core is inevitable. The integrity of the dam structure must be maintained during its operation or possible events that occur during operation. For this purpose, the corresponding range of stress and loading conditions is investigated. In all foreseeable events, the stability of the dam must be ensured by the stresses placed at acceptable levels and the integrity of the dam core. In this research, the effect of different materials as the core in dams and, accordingly, the evaluation of the problem of hydraulic cracking and the stability of the upstream and downstream gables of the dam in two cases: vertical core and inclined core, and the results obtained from them are analyzed.
1-2 The purpose of this research:
Since Conventional reliability coefficients are still important in limit equilibrium stability analysis, and other considerations for the accuracy of the analysis of an earthen dam, including the ability to change location, body stresses, and its resistance to the effects of leakage, must also be evaluated. Finite element method and other advanced techniques for the analysis of dams have made it possible to study the behavior of the embankment under various conditions. A large amount of data obtained from multiple analyzes has created a facility for drawing the predicted curves of dam performance. Also, the effective phenomena in the internal instability of earthen dams under the influence of impermeable core materials should be evaluated and compared, and these factors can be investigated with the finite element technique.. These phenomena can have a different behavior in dams with vertical and inclined core shapes, phenomena that start from the differential settlements of the core and shell due to the difference in their compressibility and turn into the phenomenon of arching of the core on the shell and finally can cause hydraulic cracks inside the core of earthen dams or at the interface between the core and the shell. Therefore, due to the fact that dams with earthen and inclined cores exhibit different behavior against all these factors, a detailed and scientific investigation of these conditions is carried out. In general, despite all the things that are presented in the third chapter as the advantages and disadvantages of vertical and inclined cores, a coherent and coordinated theory has not been presented regarding the decisive superiority of any of the two methods, and in this case there is a difference of opinion between design engineers and researchers. In fact, in addition to the above issues, it is necessary to consider various factors such as special geological and geotechnical conditions of the place, climatic conditions, existing operational issues and facilities, quality and quantity of loan materials, topographical conditions of the construction site, etc. In any case, the comparison between the advantages and disadvantages resulting from the inclined core or ridge will be evaluated based on the relative shear strength of the core materials and the upstream and downstream shells. In this research, by controlling the reliability coefficients for the slopes under different loading conditions and analyzing the stress conditions in the dam body, the phenomenon of arching, which is one of the factors of hydraulic failure, is investigated, and this important issue is controlled in a case dam with two different core shapes. In order to achieve the desired goals, this thesis has been prepared in five chapters, which chapters include the following contents:
The first chapter includes the introduction and also the purpose of this research.
In the second chapter, an explanation is given about the technical literature and the background of the research related to the subject.
In the third chapter, the factors that can lead to the loss of stability in soil cores with different geometric shapes are discussed.
In the fourth chapter, the introduction of the device needed in this research and its capabilities, which are used for the analysis of earthen dams, and the working method and method of dam modeling for dam stability analysis are explained.
In the fifth chapter, the discussion and conclusions of the work are stated.
Chapter Two:
2- Technical literature and research background
2-1 Introduction:
Dams usually Based on the materials used in the body and their cross-sectional shape, they are classified such as concrete dams, earthen dams and dams with other materials, each of which has different types. Earthen dams are much older than concrete dams. It is obvious that some earthen dams were built in the cradle of ancient cultures such as eastern countries around 3000 years ago. According to the standard booklet prepared by the International Committee of Large Dams ICOLD, in which 63 important countries of the world are participating, dams with a height of more than 15 meters are called high dams. Out of about 14,000 high dams in the world that have been registered so far, more than 70% of them are earthen dams. The recent report on the construction of high dams also shows that out of about 1000 high dams built in the last 2 years, only about 20% of them are concrete dams and the remaining 80% are earth dams. Soil or a combination of soil and rock are called embankment dams. The types of embankment dams are as follows depending on the type of materials used in their bodies:
2-2-1 Homogeneous^1 or non-homogeneous^2 earthen dams
Earth dams can be placed on different types of stone or alluvial foundations, while concrete dams must be built on hard and healthy stone foundations, which is one of the main advantages and distinguishing features of earthen dams. is, their types are shown in Figure 1-2.