Local Scour Around Differently Shaped Bridge Piers. An Experimental Investigation

Contents

Abstract

Acknowledgment

Chapter 1 INTRODUCTION
1.1 Background
1.2 Scouring Phenomenon around a Bridge Piers
1.3 Local Scour Importance
1.4 Problem statement
1.5 Aim and Scope
1.6 Research questions
1.7 Objectives
1.8 Previous work
1.9 Thesis organization

Chapter 2 LITERATURE REVIEW
2.1 The Scouring Process
2.2 Scour as a Failure Mode
2.3 Parameters Affecting Scour
2.4 Pier Influences
2.4.1 Pier size
2.4.2 Blockage Ratio, D/b
2.4.3 Pier Configuration
2.5 Time Development of Scour
2.6 Other Parameters
2.7 Scour Depth Estimation Experimental Methods
2.8 About SURFER 13.0

Chapter 3 RESEARCH METHODOLOGY
3.1 Sieve Analysis
3.2 Channel Preparation
3.3 Pier Model Preparation
3.4 Similitude Analysis
3.5 Geometric Similarities
3.6 Experimental Procedure

Chapter 4 RESULTS AND DISCUSSION
4.1 Experimental Analysis
4.1.1 Shape of Pier VS Scour around Pier
4.1.2 Size of Pier VS Scour around Pier
4.1.3 Duration of Experiment VS Scour around Pier.
4.1.4 Discharge in Channel VS Scour around Pier

Chapter 5 CONCLUSION AND RECOMMENDATIONS
5.1 Conclusions
5.2 Recommendations

References

Appendices

Abstract

Local scour is a complex phenomenon involving three-dimensional flow, typically developed around piers founded in movable bed rivers. Local scour can lead to partial failure or to collapse of bridge piers because of the high flood velocity. The cost of large bridges, with common and special complex piers, justifies carrying out an accurate prediction of scour depth, for both economic and safety reasons, which in turn leads to the interest of hydraulic engineers in predicting the equilibrium scour depth. The objective of the present study is to conduct laboratory model testing to investigate and compare scour hole depth of the diamond, square and elliptical scaled model of bridge piers in a cohesion less bedding material under clear water and to propose the most efficient cross section of the bridge pier, having less Local scour around piers under steady clear-water condition was studied experimentally for a variety of configuration, including different sizes and shapes of piers. Total number of forty experiments were performed in the flume using diamond, square and elliptical shape. The scouring around the square shape was found maximum and at elliptical shape was the least. The main reason was obstruction in flow area that was more in case of the square and less in case of the elliptical shape. However, the diamond shaped has the intermediate scouring under same condition.

Acknowledgment

First of all I am grateful to the ALMIGHTY ALLAH for the good health and wellbeing that were necessary to complete this book.

I wish to express my sincere thanks to our Supervisor Dr. Mujahid Khan and chairman of civil engineering department Dr. Qaisar Ali, for providing us all the necessary facilities for the research.

I pay my deep sense of gratitude to Dr. Mujahid Khan of Department of Civil Engineering, University of Engineering and Technology, Peshawar to encourage us to the highest peak and to provide us an opportunity to prepare the project. I am immensely obliged to my friends for their elevating inspiration, encouraging guidance and kind supervision in the completion of my project.

I take this opportunity to express gratitude to all of the Department faculty members for their help and support.

Last, but not the least, my parents are also an important inspiration for me. So with due regards, I express my gratitude to them.

Chapter 1

INTRODUCTION

Bridge pier scouring is an important issue in the safety evaluation of bridges (Huber 1991). It has been reported that most bridge failures were related to the scour of foundation material. The process of the local scour around bridge piers is fundamentally complex because it depends on many variables (e.g. the flow, pier and bed material characteristics). Accurate prediction of scour depth around bridge piers is essential for their safe and economical design. Numerous studies have investigated flow mechanisms and predictions of scour depth around bridge piers, and some have studied the determination of scour depth under uniform steady flow conditions. As already stated, many parameters influence the scour process and, although many studies have been carried out, a general theory has not been achieved because of the complex structure of the problem.

1.1 Background

Scour is the terminology used to describe the lowering of channel bed by erosion below a natural level tending to expose or undermine foundations that would otherwise remain buried. Because the openings of a bridge and specially the distance between the piers are less than the full width of the river, the water accelerates as it approaches and passes through them. Consequently, the velocity is higher than it would otherwise be, and this can cause scouring and undermining of the foundations of the bridge. Scour is a serious problem in bridge piles. Increases in river and stream flows that result in scouring are the principal cause of bridge failures (Hamill 1999). The ability to protect bridge piers from scouring is critical to bridge safety. Excessive scour can cause high maintenance costs or even bridge collapse. Bridge collapse results in costly repairs, disruption of traffic, and possible death of passengers travelling on the bridge when the collapse occurs (Barkdoll et al. 2006).

The mechanism has the potential to threaten the structural integrity of bridges and hydraulic structures, causing failure when the foundation of the structure is undermined (Masjedi et al. 2010). Lagasse and Richardson (2001) stated that in the United States, general and local scour are the major cause of hydraulic factors such as stream instability, long term streambed degradation, while general scour, local scour, and lateral migration were responsible for 60 % of all U.S. highway bridge failures. It has been long established that the basic mechanism causing local scour at bridge piles is the down flow at the upstream face of the piles and subsequent formation of vortices at the base (Muzzammil et al. 2004).

There are many parameters that affect the flow pattern and the process of scour around bridge piers. These include the size, cohesion, and grading of the bed material, depth of flow, size and shape of the bridge pier, flow velocity, and geometry of the bed. Other factors that influence scour that are the result of significant flood events include floating debris and accumulation and buildup of debris.

Usually, scour may occur during floods, and it can make bridges collapse (Melville and Coleman, 2000). The floods in Pakistan began in late July 2010, resulting from heavy monsoon rains in the Khyber Pakhtunkhwa, Sindh, Punjab and, Baluchistan regions of Pakistan, which affected the Indus River basin. Approximately one-fifth of Pakistan's total land area was affected by floods. Rivers Swat and Kabul experienced record floods in excessive of 400,000 cusecs crossing previous historic recorded flows of 1929 (250,000 cusecs) that caused inundation of Charsadda, Nowshera and adjoining areas. Exceptionally high floods were also recorded in Panjkora River, Budni and other nullahs, as well as flash floods in D.I. Khan hill torrents. Devastation was so massive that 278 bridges were damaged or washed away, besides severe damages to bridges; Irrigation Infrastructure damages included Munda, Amandrah and Kuram Garhi Headworks in Khyber Pakhtunkhwa (Hashmi et al. 2012).

1.2 Scouring Phenomenon around a Bridge Piers

Recent scour related bridge catastrophes throughout the world have received great attention. Scour is local lowering of streambed elevation that takes place around structures that are constructed in flowing water. It means the lowering of the river bed level by water erosions such that there is a tendency to expose the foundations of a bridge. It is the result of the erosive action of flowing water, excavating and carrying away material from the bed and banks of streams and from around the piers and abutments of bridges (Richardson et al. 2001). Such scour around pier can result in structural collapse and loss of life and property. The amount of this reduction below an assumed natured level is termed scour depth.

If an obstruction is placed in a stream, the flow pattern in the vicinity of that obstruction will be modified. Since the transport capacity is a function of the flow characteristics, the transport capacity pattern will also be modified. In any area where the transport capacity is not equal to the rate at which material is supplied, scour or deposition must occur. The bed configuration will then change until a balance is again achieved between capacity and supply. Scour around bridge piers and an abutment is a particular case of this general problem. The special characteristic of this case is the functional relationship of the rate of supply to the flow conditions in the unobstructed stream. The capacity in the vicinity of the pier or abutment is related to these same flow conditions and to the geometry of the obstruction.

Scour is usually divided into two categories. Aggradation and degradation. Aggradation is general and progressive buildup of the longitudinal profile of a channel bed due to sediment deposition. It involves the deposition of material eroded from the channel or watershed upstream of the bridge. Degradation is a general and progressive (long-term) lowering of the channel bed due to erosion, over a relatively long channel length. It involves the lowering or scouring of the streambed due to a deficit in sediment supply from upstream.

1.3 Local Scour Importance

Improving the understanding of the local scour phenomena is vital to the engineer responsible for the design of piers and piles foundations. The knowledge of the maximum possible scour around a bridge pier is of paramount importance in safe and economic design of bridge piers. Because complete protection against scour is too expensive, generally, the maximum scour has to be predicted to minimize the risk of failure. Although scour research has become quite extensive, scour related failures still occur, which can be attributed to a lack of knowledge with respect to the process of scour, design criteria, and lack of publicly available results from such research (Sumer and Fredsoe, 2002).

Because of its prevalence as a cause of bridge failure, scour is highly prioritized by modern bridge engineers. Several national and provincial bridge design codes (including the American Association of State Highway and Transportation Officials (AASHTO), Load and Resistance Factor Design (LRFD), Ontario Highway Bridge Design Code (OHBDC) and Canadian Highway Bridge Design Code (CHBDC), include provisions for hydraulic design. Such provisions include recommendations for design of bridge piers with respect to scouring, which state that this design is to be done based on one of several code specified “approved methods.” These methods refer to empirical equations, which have been derived using experimental and field data over the past half-century. These equations are used to calculate the depth under which foundations must be placed in order to avoid failure due to scour. However, these widely used equations have a tendency to over-predict this depth (referred to as equilibrium scour depth).

The variables which contribute to scour are many and varied, further complicating scour prediction. These factors include those relating to geomorphology of the channel itself, flow transport, bed sediment, and geometry of the bridge in question (Melville and Coleman, 2000); while the complexity of the scouring process and varied nature of such contributing parameters are undoubtedly partially responsible for such an inclination, a phenomenon known as the scale effect is also a principal factor to which over-estimation of scour can be attributed.

In hydraulic modeling, scale effects refer to an imbalance of force ratios between model (laboratory) and prototype (field). If perfect geometric, kinematics, and dynamic similarity between model and prototype are not maintained, then scale effects will occur; however, the magnitude of these effects and their negligibility is highly dependent on the nature of the model in question (Heller, 2011).

1.4 Problem statement

Scour is a major issue causing bridge failure and most of bridge failures in the world have proved it. The ability to protect bridge piers from scouring is critical to bridge safety. Excessive scour can cause high maintenance costs or even bridge collapse. Bridge collapse results in costly repairs, disruption of traffic, and possible death of passengers travelling on the bridge when the collapse occurs. The cost of large bridges, with common and special complex piles and piers, justifies carrying out an accurate prediction of scour depth, for both economic and safety reasons, which in turn leads to the interest of hydraulic engineers in predicting the equilibrium scour depth.

Keeping in view the importance of scouring in bridges, it is necessary to analyze possible pier scour reduction. Therefore, in this research the scour reduction measures are analyzed experimentally.

1.5 Aim and Scope

This research examines the scouring for different shaped and different size scaled model bridge piers in a uniform non-cohesive bedding material within a large flume. Through the various experiments conducted and analysis of the data, the reader will be able to assess scour and erosion potential.

1.6 Research questions

Some of questions which we have tried to answer in this research are:

- By changing the flow what will be effect on the scour hole depth of bridge Pier?
- Can scour hole depth depends on the shape of bridge Pier?
- What is the effect of size of bridge pier on scour hole depth?

1.7 Objectives

The main objectives of this research are the following:

- To reduce local scour around bridge pier by altering of flow technique
- To compare local scour around different shaped and sized piers.

1.8 Previous work

A series of bridge failures due to pier scour, as reported during floods, has rekindled interest in our understanding of the scouring process and for developing improved ways of protecting bridges against scour. As such, attention is being given to the scour design of new bridges and to the inspection, maintenance and management of existing bridge structures.

Numerous experimental and numerical studies have been carried out by researchers in an attempt to quantify the equilibrium depth of scour in various types of soil material. Moreover, while a lot of work has been done to develop equations for predicting the depth of scour, researchers have also worked extensively to understand the mechanism of scour. Raudkivi and Ettema (1983), Ahmed and Rajaratnam (1998), Chiew and Melville (1987) and Breusers et al. (1977), among others, are some of the researchers that have worked on pier scour. Local scour around bridge piers was studied by Shen and Schneider (1969) while Breusers et al. (1977) gave a “state of the art” review on local scour around circular piers. Posey (1974) provided guidance on how bridge piers in erodible material can be protected from under-scour by means of an inverted filter extending out a distance of 1.5 to 2.5 pier diameters in all directions from the face of the pier Numerous experimental and numerical studies have been carried out by researchers in an attempt to quantify the equilibrium depth of scour in various types of soil material. Moreover, while a lot of work has been done to develop equations for predicting the depth of scour, researchers have also worked extensively to understand the mechanism of scour. Raudkivi and Ettema (1983), Ahmed and Rajaratnam (1998), Chiew and Melville (1987) and Breusers et al. (1977), among others, are some of the researchers that have worked on pier scour. Local scour around bridge piers was studied by Shen and Schneider (1969) while Breusers et al. (1977) gave a “state of the art” review on local scour around circular piers. Posey (1974) provided guidance on how bridge piers in erodible material can be protected from under-scour by means of an inverted filter extending out a distance of 1.5 to 2.5 pier diameters in all directions from the face of the pier. Moreover, the researches carried out on scouring is briefly discussed in chapter 2.

1.9 Thesis organization

We organized our thesis in the following format;

- Chapter1: Introduction to scouring phenomenon
- Chapter2: Literature Review
- Chapter3: Research Methodology
- Chapter4: Results and Discussions
- Chapter5: Conclusions and Recommendations.

Chapter 2

2. LITERATURE REVIEW

The literature review consists of an overview of the scouring process and its mechanisms as a cause of bridge failure, a description of the parameters affecting scour and their influence on equilibrium scour depth (determined through prior experimentation), an examination of bridge pier scour depth estimation through experimentation.

2.1 The Scouring Process

Flow around a bridge pier is a class of junction flow, or “flow [which envelops] at the junction of a structural form and a base plane” (Ettema et al., 2011). The flow field around a pier consists of a horseshoe vortex system, wake or lee vortices, trailing vortices, or a combination of any of these (Chiew, 1984). This flow field is three- dimensional and unsteady due to the ongoing interactions between these turbulent flow structures (Ettema et al., 2011), which are illustrated in Figure 1-1.

The magnitude of approach flow velocity decreases in the vertical direction, such that maximum velocity occurs at the water surface and velocity at the channel bed is zero due to the no-slip condition. When the flow encounters the upstream face of the pier, velocity of flow abruptly becomes zero, and velocity at the sides of the pier increases. This causes pressure to decrease around the pier in the downstream direction (Figliola and Beasley, 2011). It is at this point (the pier sides) that scouring action will be initiated. Scour then increases in the upstream direction until the upstream face of the pier is reached, creating a partial “ring” of scoured material (Guo, 2012).

A downward pressure gradient will also form at the pier face, causing increased flow velocity in a downward motion (Dey et al., 1995). The down flow, once initiated, induces scouring action upstream of the pier (Chiew, 1984). The down flow will then curl up and around itself and the pier, initiating formation of the horseshoe vortex, which is named for its plan-view shape (Melville and Coleman, 2000). The aforementioned ring from the scoured sides of the pier then “traps” the still-forming horseshoe vortex, allowing rapid removal of sediment to commence and continue until equilibrium is reached (Guo, 2012).

For clear-water scour, the equilibrium scour state is defined by the point in time at which the velocity of flow circulation in the scour hole is no longer capable of removing bed material from the hole (Chiew, 1984), or when the shear stress caused by the horseshoe vortex is equal to the critical shear stress of the bed material at the bottom of the scour hole (Deng and Cai, 2010). The corresponding equilibrium scour depth represents, for live-bed scour, the scour depth at the point in the scour process at which the rate of sediment transport into the scour hole is equal to the rate of sediment transport out of the scour hole (Chiew, 1984). In practice, equilibrium scour depth (dse) can require many hours or even weeks to develop under clear- water conditions, and even when dse is eventually reached, some removal and deposition of sediment may still occur in the vicinity of the scour hole; however, this continued scouring action is typically not significant enough to affect the “overall scour form” (Ettema et al., 2011).

As described, there are other turbulent structures in the flow field surrounding the pier, which will affect scour. Wake vortices occur as a result of flow around a pier and the “surface roller” flow structure forms at the air-water interface (Figure 1.1). The behavior of wake vortices mimics that of a tornado, removing sediment from the channel bed in an upward motion. The volume of sediment transported by wake vortices is smaller than the volume of sediment transported by the horseshoe vortex system. Trailing vortices are only induced in the case of a pier that is entirely submerged in the flow (Chiew, 1984), and extend from the top of the pier in a downstream direction (Breusers et al., 1977).

Once equilibrium has been attained, the scour hole is generally of an inverted- frustum shape; physically, the upstream slope of the hole tends to be close to the angle of repose of the sand in which it has formed (Ettema et al., 2011).

2.2 Scour as a Failure Mode

A structure (in this case, bridge) can be in danger of failure if one of its structural components (here, a pier or abutment) fails; pier and abutment foundations are therefore crucial in bridge stability, since failure of foundation is highly likely to result in the failure of the column it is supporting. It is necessary to recognize the ways in which a pier can fail such that the span it is supporting also fails or collapses. If pier failure is considered primary failure, the pier foundation or foundation material has failed, and the pier will experience downward motion. The linkage or connection to the span (and therefore, supporting action of the pier) then no longer exists, and the span is therefore susceptible to failure and likely to collapse. If the pier failure is considered secondary, the failure has resulted from motion of the pier in a vertical, lateral, or rotational direction. For example, lateral and vertical movement of the pier can occur because of seismic forces, and lateral and rotational pier motion occurs as a result of debris, ice, and marine traffic colliding with the pier. Vertical and rotational pier movement can occur due to scouring around the pier foundation and soil-bearing failure when scour reaches the foundation support (Lebeau and Wadia-Fascetti, 2007). In general, “piers fail as scour develops” (Ettema et al., 2011).

If equilibrium scour depth is not reached until after the pier or abutment foundations have been exposed, or in extreme cases, undermined, then failure of the foundation is likely to occur, resulting in failure of the pier and subsequent failure of the bridge itself. Pier structure (or pier type) will also affect the way in which a pier fails. Behaviour of piers with footings will differ from behaviour of piles during development of scour (Ettema et al., 2011). Fig 2.1.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2. 1 Topi bridge has been exposed making it dangerous for traffic (Dawn News)

2.3 Parameters Affecting Scour

Experimentation has contributed to determination of the effect of each of these variables on scour depth and geometry, particularly in clear-water scour. Clear-water scour experimentation was more common than live-bed until the 1980s, when a sudden influx of results demonstrated that scour depth in live-bed conditions could exceed scour in clear-water conditions (Melville and Sutherland, 1988).

Prediction of equilibrium scour depth can be done using experimentally-derived empirical equations or computational methods. Temporal scour depth (time development of scour) can also be predicted using either of these methods.

Most variables which affect scour depth and geometry can be categorized into one of five groups, which are generally interdependent (Chiew, 1984):

- Fluid properties (density; kinematic viscosity; and temperature, which is not a primary concern in the lab but rather in the field, where it cannot be controlled)
- Time, as scour is a temporal process, is also related to the type of scour under consideration (live-bed equilibrium is typically achieved within a shorter time than in clear-water conditions); in the case of increased scour induced by flooding after a storm of some magnitude, the length of time of flooding or storm is pertinent
- Flow properties (water depth; energy slope; shear stress in uniform flow; angle of attack; mean flow velocity; and critical velocity of bed material), pier characteristics (pier diameter; shape; surface condition; pier orientation; and debris accumulation)
- Sediment characteristics (sediment density; median sediment size or diameter (d50); uniformity of particle size distribution; cohesiveness; shape factor; angle of repose; and fall velocity)

Richardson et al., (1990) stated that pier shape will also alter scour geometry and depth; a more streamlined pier will induce a weaker horseshoe vortex system, lessening intensity of scouring action. The scour depth of a square-nosed pier can be 1.2 times higher than the scour depth of a sharp-nosed pier, and 1.1 times the depth of scour for a cylindrical or otherwise blunt-nosed pier.

There are other scour-influencing factors which are difficult to quantify; for example, inter- particle behavior in any given sediment will affect scour depth and development. Similarly, the propensity of sediment to develop bed formations (planar beds, ripples, dunes and anti- dunes) under certain flow conditions will also alter the magnitude of scour (Richardson et al., 1990).

Several researchers investigated scour at bridge piers. Among them, for example, are the following:

- (Gampathi, 2010) concluded that scour depth increases with the increase of flow angle relative to the pier axis.
- (Ismael, Gunal and Hussein,2009 ) Carried out experimental work to examine the effect of bridge pier position in reducing the scour. The percentage reduction in scour hole depth for the normal and opposite piers as compared to the circular pier was 40% and 54.5% respectively.
- (Dahe and Kharode, 2015) Employed HEC-RAS model to estimate the scour depth for bridge piers. It is found that the computed scour depth around sharp nose shaped pier has better value whereas the square shaped pier has maximum value of scour depth. Circular pier, rounded nose, and pier with group of cylinders have identical values of scour depth.
- (Samuel and Aziz, 2002) Used 1-D model to compute local scour for Aswan Bridge at a flow of 270 Mm3/day. The results show that the local scour at the smallest piers is 5.75 m. for round nose and 6.32 m for square nose and rise to 12.57 m at the biggest piers for round nose and 13.82 for square nose.
- (Ismail, 2009) Used 1-D model for Aswan Bridge at a flow of 270 and 350 Mm3/day. The results show that the local scour at the biggest piers is 13.66 and 14.47 m respectively.
- (El-Sersawy, 2004) Implemented 2-D model to compute local scour for Aswan Bridge for two main piers of five piers. It was found that the total scour at Aswan bridge were 11.82, 12.15, and 12.81 m at discharges of 270, 300 and 350 Mm3/day, respectively.
- (Salam and Aziz, 2003) Used 1-D model for El-Minia Bridge at a flow of 181 and 350 Mm3/day and the angle of attack is 30°. The results show that the local scour at all piers is 4.9 and 5.6 m respectively.
- (Ismail, 2009) Used 1-D model for El-Minia Bridge at a flow of 181 Mm3/day and the angle of attack is normal and 30°. The results show that the local scour at all piers is 1.81 and 4.53 m respectively.

2.4 Pier Influences

2.4.1 Pier size

Pier size is a governing parameter with one of the greatest influences on scour depth and geometry. Frequency of vortex shedding and the amount of vorticity in the wake of a pier are directly related to the projected width of a pier, demonstrating how influential D is on the surrounding flow field. Because of this, non-dimensional quantities are typically compared with dse normalized with pier diameter (i-e. and etc. are plotted with to isolate the effects of these variables without influence from pier diameter alone (Ettema et al., 2006). If all test parameters are held constant and pier diameter D is increased, the frequency of vortex shedding will decrease, causing a subsequent decrease in ; similarly, as pier diameter is increased will increase and will decrease demonstrating the relationship between relative sediment size and frequency of vortex shedding (Ettema et al.,2006).

2.4.2 Blockage Ratio, D/b

The effects of blockage of a flow channel have been extensively investigated at the University of Windsor by Hodi (2009), D’Alessandro (2013), and Tejada (2014). It has been previously stated in literature that the blockage effects are negligible if blockage ratio is held below ten percent (Chiew, 1984). However, previous experimentation had been shown to employ testing conditions for which blockage ratio exceeded this recommended value; since data from these experiments would have been used for development of empirical equations for pier design, use of this equation might not have been judicious.

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