Laboratory investigation of the effect of flanged collars on the local scouring process around circular foundations in clear water conditions

Master thesis on hydraulic structures

Abstract:

Washing water around the existing structures in the flow path is one of the important issues that should be considered in the design of such structures. For this purpose, the investigation of this phenomenon around the circular bases in the condition of clear water was carried out in laboratory physical modeling. For this purpose, two types of collars with edges were used, including collars with a straight edge and collars with an oblique edge, and the height of the edges used in each of these collars is 0.5 cm, 1.0 cm, and 1.5 cm. Also, in this research, the effect of height on the efficiency of edged collars was investigated. For this purpose, collars were installed at levels of +5 cm, +2.5 cm, 0.0 cm, -1.5 cm, and -3 cm. In this research, it was observed that collars with edges at higher levels performed better than collars without edges at the same level, and collars with an inclined edge of 0.5 cm compared to other collars with edges had the best performance in most levels compared to collars without edges at the same level. All the experiments conducted in this study were conducted in the river research department of the Soil Conservation and Watershed Research Institute of Jihad Agriculture. 

Key words: flanged collars, clear water conditions, scouring water, circular foundations

Chapter 1

Generalities

Introduction

Scrubbing water around the foundations of bridges that are placed on rivers, according to the conditions of the rivers, are mostly in the state of uniform flow [1] (in normal conditions and normal river flow) or It occurs in non-uniform flow conditions [2] (in flood conditions or in places where the course of the river changes). So far, a lot of research has been done in the field of water washing process. Many of these researches are in the fields of identifying the mechanisms affecting the scouring process, determining the depth of scouring water in equilibrium [3], timing of scouring water, investigating various factors affecting scouring water such as the size and type of bed soil, flow regime (clear water [4] or active bed [5]), flow speed, size of bridge foundations, shape of foundations and so on. Done.

1-2. Necessity of research

Considering the potential of destruction of bridges due to water scouring and its financial and human costs, trying to understand the process of water scouring and methods to deal with it seems essential. Estimating the depth of scour at the base of a bridge less than the actual value leads to its destruction, while overestimating it also leads to higher costs for bridge construction [1].

In the United States, several hydraulic factors such as flow instability, sedimentation and sedimentation of the bed in the long term, general scour [6], local scour [7] and lateral movement of bed particles are known as the main factors for 60% of bridge structural failures in the US. is[19]. The tragic collapse of the U.S. 51 bridge over the Hatchee River resulted in the death of 8 people [19]. As a result of the destruction of the bridge located on the Ario Passagero River in California, which occurred during a large flood, 7 people were killed[19].

1-3. Objectives

The main objective of this research is to investigate the effect of edged collars in reducing the depth of localized scour around circular foundations and in flat water conditions. For this purpose, first the tests with the mentioned conditions are done for the circular foundation without collar, then in the second stage, the tests are done exactly for the circular foundation with rimless collar, and in the third stage, the tests will be done for the collars with edges. In each test, the depth of the scouring water will be measured to obtain the percent change of the maximum scouring water depth compared to the rimless collar. Also, tests are performed for collars installed at different heights compared to the bed and with different edge heights and at different angles to the collar in order to check the effect of these three parameters on the depth of washing water.

1-4. Type of research

The presented research is the master's thesis of the Department of Hydraulic Structures of Qom University, which is based on the experiments conducted in the laboratory of the Soil Conservation and Water Resilience Research Center of the Ministry of Agricultural Jihad located in Tehran and using a physical model.

1-5. Dissertation structure

This thesis consists of five chapters, in the first chapter, the general topic of local washing water is given. In the second chapter, the research done by other researchers in the field of using collars in reducing the depth of washing water is described, and in the third chapter, the method of conducting experiments and the equipment used in the experiments are explained.In the second chapter, the research done by other researchers in the field of using collars in reducing the depth of washing water is described, and in the third chapter, the method of conducting experiments and the equipment used in the experiments are explained. The presentation and analysis of the results are stated in the fourth chapter and the conclusions are drawn in the fifth chapter according to the results obtained. Second chapter

Research history

2-1. Introduction

Until now, many articles and books have been published about local scour that occurs in non-cohesive soils around the foundations of a bridge. In this chapter, an attempt is made to summarize the information about local scour in non-cohesive soils and how scour develops over time, as well as methods to reduce local scour in bridge foundations.

2-2. What is scouring water?

Brothers et al. (1977) have defined scouring water as follows: scouring water is a natural phenomenon that occurs due to the flow of water in rivers and streams. This phenomenon is the result of the erosion effect of water, which washes away materials and materials from the bed and banks of rivers, as well as around the foundations of bridges and their supports, and places them in other places of the river.

Krimisinov et al.

The amount of lowering of the riverbed level (compared to before the occurrence of scouring water) is called scouring depth. A wash water pit is called a cavity left after the movement of bed particles by the flow of water.

2-3. Types of scouring water

Generally, scouring water is divided into three types, including general scouring water, obstructing scouring water [8] and localized scouring water.

Krimisinov et al.

General washing water

This type of washing water causes a change in the height and level of the river bed due to human or natural causes, and also causes the longitudinal profile to decrease in the entire river. This type of washing water is caused by a change in the river regime. As it can be seen from diagram 1-2, general scour is divided into two types, long-term and short-term. Short-term scour occurs as a result of one or more floods in a short period of time, while long-term scour occurs over a longer period of time (usually around a few years) and includes the erosion of the river banks.

Regional scour

Unlike In general, local scouring occurs due to the presence of a bridge or any other sea or river structure. This type of wash water is divided into two types of blocked wash water and local wash water.

Obstructive wash water

This type of wash water occurs due to the narrowing of the flow (which may occur naturally or due to human activities). As a result of this narrowing, the space that the water flow can pass through decreases, and as a result, the average speed of the water flow increases. As a result of this increase in speed, the erosive force that enters the bed from the side of the water flow increases, as a result, the river bed is subjected to erosion and its level goes down. The scouring water around the bridge foundations that are placed on the river bed are a good example of blocking scouring water.

Local scouring water

This type of scouring water refers to the erosion of a part of the river bed that is located near the foundation of the bridge. Due to the impact of the water flow with the bridge foundation, the acceleration of the water flow increases and causes the formation of small vortices, due to the formation of these vortices, particles are washed away from around the foundations and localized washing water is created. The wash water in the vicinity of the bridge foundations is a good example of local wash water, which is clearly visible in Figure 1-2. This type of wash water is clearly visible. Since the main topic of this research is about local wash water, this type of wash water will be further investigated in the next parts. In the rest of this treatise, rinse water means local rinse water.

-4. Local washing water mechanisms

According to the conducted research, the main mechanisms that cause washing water to occur in bridge foundations include downward flow (which occurs on the upper side of the bridge foundation) and eddies [10] (which are formed in the bed).