The effect of non-uniformity of the foundation on the dynamic response of weighted concrete dams

Dissertation for Master's Degree<>

Trend: Hydraulic Structures

Abstract:

One of the assumptions made in order to simplify the dynamic analysis of existing weight concrete dams is the uniformity of the foundation in terms of physical parameters such as modulus of elasticity and damping. This is despite the fact that, in practice, different geological conditions may prevail in different areas of the dam's foundation area, and the geological and mechanical reports of the dam's foundation rock are drawn in a non-uniform manner. In such conditions, generally, either in a conservative state, the parameters of the weakest area are entered as uniform foundation parameters in the dynamic analysis of the dam, or under certain conditions, the parameters obtained from the weighted averaging of the areas are defined as uniform foundation parameters as inputs to the model. The seismic behavior of a weighted concrete dam with a height of 110 meters under the effect of non-uniformity of the foundation has been investigated. For this purpose, a finite element model of dam, lake and foundation system has been created using Abaqus software. With the help of this model, the response of the dam has been calculated by considering the interaction of the dam, foundation and lake under the effect of an earthquake with a maximum acceleration of 0.8 g. By comparing the analysis results in non-uniform conditions with the analysis results in uniform conditions, the effect of this non-uniformity has been investigated. The comparison of stress values ??and displacement values ??in the crest of the dam shows that the assumption of non-uniformity can have a significant effect on the stress values ??applied to the dam. Also, the results show that in non-uniform foundations, the pattern of stress distribution in the dam body is generally different from the uniform case.     

Key words: weighted concrete dam, linear dynamic analysis, finite element method, non-uniformity of foundation, Abaqus software

Chapter 1

Introduction

1-1- General

Today, the construction and construction of dams in order to collect and store river water for drinking, irrigation, electricity generation, flood prevention, fishing and It is inevitable. However, dams are huge structures that carry potential risks for the downstream community and their failure can be catastrophic. In fact, the issue of safety of dams has received much attention in recent decades due to social and economic issues. This has attracted special attention due to the increasing age of the built dams and the appearance of failures in the dams that were built in the beginning and middle of the 20th century. There is no accurate information on the number of dams that have failed throughout history. However, there have been several reports of damage to dams during earthquakes. Of course, the risk of dam failure does not depend only on the height of the dam, and the volume of water stored behind the dam and the shape of the valley are also effective in its failure.

         During the many years that mankind has attempted to build dams, it has always adopted assumptions according to the available information as well as the available facilities, and has developed models to define the behavior of dams, construction materials, and applied forces, which have lost their credibility due to the progress made. In addition to that, with the development of advanced software, the refinement of material behavior models based on laboratory results, the study of the responses of existing dams during earthquakes, the more accurate understanding of the nature of applied earthquakes, and the increase in the speed of computers, it is possible to conduct more detailed three-dimensional nonlinear dynamic analyzes and studies on the behavior of dams.

One of the assumptions made to simplify the dynamic analyzes of existing dams is the uniformity of the foundation. In terms of physical parameters such as modulus of elasticity and damping. This is despite the fact that, in practice, different geological conditions may prevail in different areas of the dam's foundation area, and the geological and mechanical reports of the dam's foundation rock are drawn in a non-uniform manner. In such conditions, generally, either in a conservative state, the parameters of the weakest area are entered as uniform foundation parameters in the dynamic analysis of the dam, or under certain conditions, the parameters resulting from the weighted averaging of the areas are defined as uniform foundation parameters as model inputs.. Therefore, by using the software provided for finite element modeling, it is possible to check to what extent the assumption of non-uniformity of the foundation compared to the uniform state affects the seismic response of weighted concrete dams.

In this research, the seismic behavior of a weighted concrete dam at a height of 110 meters has been investigated under the effect of non-uniformity of the foundation. The weighted concrete dam investigated in this research has a height of 110 meters. The upstream and downstream slopes of this dam are 0.1 to 0.1 and 0.85 to 1 (horizontal), respectively. For this purpose, a finite element model of dam, lake and foundation system has been created using Abaqus software. With the help of this model, the response of the dam has been calculated by considering the interaction of the dam, foundation and lake under the effect of an earthquake with a maximum acceleration of 0.8 g. In order to investigate the impact of non-uniformity of the dam foundation on the obtained response, the behavior of this dam under different conditions and considering different physical parameters for the foundation has been investigated. In all the analyses, the behavior of the dam and foundation materials has been considered in the linear range.

The results of the analyzes show that in non-uniform foundations, the pattern of stress distribution in the dam body is generally different from the uniform state. In addition, the assumption of non-uniformity of the foundation will have a significant effect on the stresses generated in the dam body. This is despite the fact that in the non-uniform state, the displacements do not increase significantly compared to the uniform state. 1-2- Safety in dams Operation along with safety control are two inseparable and continuous processes during the life cycle of dams. With the construction and start of operation of a superstructure such as a dam, potentially dangerous conditions can be created for the downstream community, and the failure of the dam is an abnormal phenomenon that is associated with flooding downstream and can cause significant financial and human losses. The scope of these damages is very wide both in terms of time and in terms of location and even reduces the national reputation of a country. According to the above mentioned materials, the issue of safety in dams is very important, especially in our country where many dam construction points are located in places with high seismicity. In fact, due to the high cost of dam construction, not paying enough attention to the safety of dams can also lead to the loss of the national capital of the country.

(Images are available in the main file)

(Formulas are available in the main file)

1-3- Types of forces acting on weighted concrete dams

The forces in the discussion of stability and stress analysis can be divided into two categories: static and Dynamically segmented. Static loads include weight force, hydrostatic force, underpressure force, thermal loads, sediment and ice pressure, etc. and the forces caused by earthquakes and wind are dynamic.

3-1- Static loads

1-3-1-1- Load due to weight

This force is the most important load in weighted concrete dams and plays the role of stability and resistance against other types of forces.

To calculate this force, the product of the specific weight of the material in the volume is used.

(Formulas are available in the main file)

It should be noted that because in stability calculations, the width perpendicular to the monolith plane (the desired block for which stability calculations are made) is assumed to be equal to one, instead of calculating the volume, the cross-sectional area is used. In the calculation, the cross-sectional area of ??the holes in the body of the dam, such as galleries, is ignored, unless it is determined in special cases (such as short dams) according to engineering judgment that ignoring them has a significant impact on the calculations. Also, when the weight of the dam accessories such as valves, bridges, etc. If they cannot be ignored, they should be added to the dead load of the body, and if there is an embankment on the body of the dam, its effect should be added to the dead load.

Another important point about roller concrete dams is that the specific weight of roller concrete depends on the density and density of its aggregates. Due to the reduction of air bubbles and a small volume of water in the mixture, roller concrete is slightly denser compared to ordinary concrete with similar aggregates.