Scour Project

8/10/2019 Scour Project

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EXPERIMENTAL STUDY TO OBSERVE SCOUR

AROUND A BRIDGE PIER

MAJOR PROJECT

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE

AWARD OF THE DEGREE OF

B.TECH

IN

CIVIL ENGINEERING

BYA GROUP OF FINAL YEAR STUDENTS

UNDER THE GUIDANCE OF

DR. N.K.TIWARI

DEPARTMENT OF CIVIL ENGINEERING

NATIONAL INSTITUTE OF TECHNOLOGY

KURUKSHETRA-136119

MAY 2013


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CONTENTS

i. CERTIFICATE

ii. ACKNOWLEDGEMENT

ii. ABSTRACT

CHAPTERS

1.INTRODUCTION

Areas affected by Scour.Causes of Scouring.

2.REVIEW AND LITERATURE

Factors affecting scouring

Effect of Velocity of approach.Effect of depth of flow.

Effect of sediment size.

Effect of sediment grading.

Effect of pier shape.

Effect of pier size.

Theoretical Scouring.

Mechanism of Scouring.

3.EXPERIMENTAL STUDIES

4.CONCLUSION


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CERTIFICATE

Certified that the major project entitled EXPERIMENTAL STUDY TO OBSERVESCOUR AROUND A BRIDGE PIER which is being submitted in partial fulfilment

of the requirement of the degree of bachelor of technology in civil engineering

of National Institute of Technology, Kurukshetra is a record of the work carried

out by a group of 12 students:

1. Ajay Dev 109102

2. Abhimanyu Rana 109136

3. Anshul Sheokand 109383

4. Himanshu Grover 109509

5. Varun Bhagi 109512

6. Puneet Mehta 109513

7. Shubham Singhal 109536

8. Anurag Malik 109711

9. Akhilesh Dahiya 109712

10.Balvindra Singh 109729

11.Rahul Sharma 10973412.Arushi Jain 109825

DR. N.K. TIWARI

ASTT. PROFESSOR

CIVIL ENGG. DEPTT.

NIT KURUKSHETRA


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Acknowledgement

We express our sincere gratitude and indebtedness to our respected

guide Dr. N.K. Tiwari, Astt. Professor, Civil Engineering Department,NIT Kurukshetra for providing constant inspiration, co-operation and

encouragement throughout the study.

We are also thankful to the staff of fluid mechanics lab and Mr. Kewal

Singh of soil mechanics lab of NIT Kurukshetra for their co-operation

and help during the experiment.


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ABSTRACT

The most common cause of bridge failures is floods with the scouring of

bridge foundations being the most common cause of flood damage to

bridges.

The present study attempts to study the scour around a bridge pier

which is seated in soil containing a particular percentage of clay, and

produce an equation to determine the relationship between the velocity

of flow and the scour depth. As clay has adhesive and cohesive

properties, it is quite logical to expect clay to produce greater forces ofattraction between the particles of silt and decrease the extent of

scouring around a bridge pier.


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1. INTRODUCTION

Bridge scour is the removal ofsediment such assand androcks from

aroundbridge abutments orpiers. Scour, caused by swiftly movingwater, can scoop out scour holes, compromising the integrity of a

structure.

Bridge scour is one of the three main causes ofbridge failure (the others

being collision and overloading). It has been estimated that 60% of all

bridge failures result from scour and other hydraulic-related causes.

Areas affected by Scour:

Water normally flows faster around piers and abutments making themsusceptible to local scour. At bridge openings, contraction scour canoccur when water accelerates as it flows through an opening that isnarrower than the channel upstream from the bridge. Degradation scouroccurs both upstream and downstream from a bridge over large areas.Over long periods of time, this can result in lowering of the stream bed.

Causes:

Stream channel instability resulting in river erosion and changing angles-of-attack can contribute to bridge scour. Debris can also have asubstantial impact on bridge scour in several ways. A build-up ofmaterial can reduce the size of the waterway under a bridgecausing contraction scourin the channel. A build-up of debris on theabutment can increase the obstruction area and increase local scour.Debris can deflect the water flow, changing the angle of attack,increasing local scour. Debris might also shift the entire channel aroundthe bridge causing increased water flow and scour in another location.

During flooding, although the foundations of a bridge might not sufferdamage, the fill behind abutments may scour. This type of damagetypically occurs with single-span bridges with vertical wall abutments.

An important consideration in designing the pier is to predict themaximum depth of scour hole so that the foundation of the structure canbe sited deep enough to avoid the possibility of undermining.
http://en.wikipedia.org/wiki/Sedimenthttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Rockshttp://en.wikipedia.org/wiki/Bridgehttp://en.wikipedia.org/wiki/Abutmenthttp://en.wikipedia.org/wiki/Pier_(architecture)http://en.wikipedia.org/wiki/Bridge_failurehttp://en.wikipedia.org/wiki/Bridge_failurehttp://en.wikipedia.org/wiki/Pier_(architecture)http://en.wikipedia.org/wiki/Abutmenthttp://en.wikipedia.org/wiki/Bridgehttp://en.wikipedia.org/wiki/Rockshttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Sediment


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2. REVIEW AND LITERATURE

FACTORS AFFECTING THE SCOUR DEPTH

a)Effect of velocity of approach:Undisturbed approach flow velocity U, definitely influences the local

scour depth around bridge piers. As approach velocity increases, there

is a linear increase in the scour depth till clear water flow condition

exists. Maximum scour depth is attained at the critical velocity. Chabert

and Engeldinger (1956) and Laursen (1963) inferred that local live bed

scour depth is 10% less than the clear water scour depth irrespective of

the approach velocity of flow. However recent studies have shown that

when the approach velocity exceeds the threshold velocity. The scour

depth first decreases and then increases again (Melville, 1988).

b)Effect of depth of flow:

For shallow depth of flow local scour depth increases with increases in

depth of flow. But further increase in depth of flow for a deep flow, the

scour depth becomes independent of depth of flow. Many researchers

such as Laursen (1963), Breusers (1977), Ettema (1980) and Chiew

(1982) have observed the same trend in their experiments. Due to

obstruction caused by pier, two rollers having opposite direction of

rotation are created. One is surface roller created around pier near the

water surface and the other is horse shoe vortex roller created around

pier near alluvial bed of the channel. As per Melville (1988);

In principle, as so long (two rollers) do not interfere with each other, the

local scour depth is independent of floor depth.However following the regime theory, some researchers such as

Laursen (1963) etc. suggested that depth of local scour increases.


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c)Effect of sediment size:

Relatively recent studies have shown that sediment size has a definite

influence on the local scour, (Nicollet, 1971; Ettema, 1980) Ettema

(1980) inferred from the laboratory data in clear water, non ripple

forming sediments that local scour is independent of sediment size so

long as size of obstruction is greater than or equal to 50 times the size of

the sediments (D/d5050). Breuseres et al. (1977) argue that effect ofgrain size, d50is limited to a single particle size sediment.

d)Effect of sediment grading:

Ettema (1980) and Grade (1989) observed that increase in the standard

deviation of the particle size distribution of sediments; causing formation

of armour layer at the base of scour whole decreases both, rate of scour

and the equilibrium scour depth.

e)Effect of pier shape:

Most of the researchers agree on the influence of pier shape on local

scour depth. It is accepted that blunter the pier facing, the area facingthe flow increases, thereby reversing the flow direction and creating

more turbulence than a streamlined shape. Melville suggested a shape

factor to account for the effect of pier shape.

f)Effect of pier size:

Interferences made by the researchers regarding the influences of pier

size on the equilibrium scour depth are not quite debatable. It is directly

concluded that larger is the size of pier, more is the equilibrium scour

depth. For all other factors being constant the scour depth varies as D

where D is width or dia of pier & Toch(1956) design curve corresponds

to =0.7 (Hsm=1.35D0.7

h0.3), Larras(1963) suggests =0.75

(Hsm=1.05D0.75

).


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Theoretical Scouring:

On the basis of laboratory and some field data, a number of equations

have been developed. In India, Lacey-Inglis method to estimate scour

depth DSEis related to Laceys depth as:

DL= 0.47(Q/f)1/3

DSE= 2DL

Here Q is designed flood discharge in cm3/sec, f is Laceys Silt Factor, aconstant of proportionality was obtained from the analysis of scour data

on 17 bridges in indo- gangetic plains and its value varied from 1.76 to

2.59 with an average of 2.09 in its basic form with slight variation lacey.

Inglis method is recommended for D SE by Indian railways and Indian

road congress . This method is purely empirical in nature and gives

combined scour caused due to flow modifications by introduction of pier,

flow construction due to guide bunds and flow concentration due to non

uniform distribution of flow.

Mechanism of Scouring

A bridge pier is spur like obstruction which causes flow acceleration and

separation at the upstream face of the pier. As the flow moves pass the

obstacle creating a vortex trial that moves downstream in a direction

approximately perpendicular to the structure. This results in scouring of

the bed around the structure locally, Lim(1998). Once a scour hole is

formed, the scouring mechanism is dominated by the vortex system and

an associate downfall cause by the stagnation pressure gradient, which

developed ahead of the structure. The downfall acts like a vertical jet

impinging and eroding sediment from the bed. The vortex system and

the down flow, along with cone around the trip of the structure. Melville

(1992) reported that out of 108 bridges failures recorded in Newzealand

between the years,1960-1984,29 attributed to fail scour.


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According to Liu et al (1957) at an pier the approach flow is considered

to consists of an upper hand lower layer which separates into an upflow

and downfall on hitting the pier.

The flow from a surface roller, while the down flow rolls up to form thebottom vortex called the principle vortex . On approaching the pier upper

layer tends to divide ,part of the flow accelerates along the upstream

corner of the pier and the remaining flow slowly circulates in the near

segment pool .


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3. EXPERIMENTAL STUDIES

Properties of Soil Used in the Experimental

Studies

Sieve size inmicron

Mass ofparticles

retained

Mass ofparticles finer

Percentagefiner

4750 0 1000 100

2360 20 980 98

1180 15 965 96.5

600 20 945 94.5

300 545 400 40

150 317.5 82.5 8.25

75 57.5 25 2.5

Retained on

pan

25 1000

Total 1000


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10098

96.594.5

40

8.25

2.50

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

%

Retained

SIZE(mm)

Sieve Analysis


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SEDIMENTS SOURCE AND PROPERTIES

The sediment was collected from Samani village, Karnal from a lake bed

to be used in the flume having the following properties.

Type of sediment = Fine sand

Medium size of sediments, d50 = 0.45 mm

D84 = 0.51mm

D60 = 0.42mm

D10 = 0.18mm

D30 = 0.28mm

Co-efficient of uniformity = d60/d10= 2.33 < 3

Co-efficient of Curvature = (d30)2/d10.d60 =1.034


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Properties of Clay Used:

35.5

36

36.5

37

37.5

38

38.5

39

39.5

40

40.5

3.688 3.583 3.258 2.944

Watercontent

(W%)

No. of blows (N)

Flow Curve


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S.No. Weight of

empty

container(gm)

W1

Wt. of

container +

wet soil(gm)

W2

Wt. of

container

+dry soil(gm)

W3

Water content

(%)

(W2 -W3)*100/(W3W1)

No. of

blows

1. 26.79 52.63 45.64 37.08 40

2. 25.36 43.78 38.72 37.87 36

3. 26.26 57.52 48.76 38.9 26

4. 27.23 55.88 47.76 40.14 19

Liquid Limit (L.L.) = 39%

Ip = 0.73( L.L. -20) = 14%

Plastic Limit = L.L. - Ip = 25%

Specific Gravity = 1.658

So, clay is Medium plastic clay.


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Experimental Setup

Flume: A flumeis man-madechannel for water, in the form of anopen inclinedgravity chute whose walls are raised above the

surrounding terrain, in contrast to atrench orditch.Flumes

leadwater from adiversion dam orweir to their desired location.

The experiment was conducted in a 12m long, 40cm wide and 60cm

deep tilting bed flume in a fluid mechanics laboratory of NIT kurukshetra.

There were 2 panels of glass for visual observation and rest of panels

were made of steel plates on both sides. It was supported on a steel

truss with a jack for the adjustment of bed slope. The water pumped

from a sump channel with the help of 15 hp centrifugal pump which is

discharging into a stilling tank upstream of the flume. The pump was

drawing water from the sump 2.75 x 1.75 m in a plane and 1.75 m deep

and delivering water at the upstream channel. Channel bed was levelled.

Further, a tailgate was provided at the flume to control the flow of water

after passing through the flume goes to a rectangular tank. The outlet

channel from the concrete tank passes the flow over the sharp crested

weir of height 40 cm provided in a 60 cm wide channel to measure the

discharge downstream before entering to the sump channel. The pier is

a placed at a distance of 3 m diameter and is fitted in a prepared bed of

soil length 40 cm consisting of 30% clay and 70% sand by weight the

detail of experimental setup is given in the diagram.
http://en.wikipedia.org/wiki/Watercoursehttp://en.wikipedia.org/wiki/Chute_(gravity)http://en.wikipedia.org/wiki/Trenchhttp://en.wikipedia.org/wiki/Ditchhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Diversion_damhttp://en.wikipedia.org/wiki/Weirhttp://en.wikipedia.org/wiki/Weirhttp://en.wikipedia.org/wiki/Diversion_damhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Ditchhttp://en.wikipedia.org/wiki/Trenchhttp://en.wikipedia.org/wiki/Chute_(gravity)http://en.wikipedia.org/wiki/Watercourse


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Model setup and Experimental procedure

The pier models are used in experiments in circular pier of a diameter 4cm and length 100 cm. A uniform flow of a certain known velocity is set

in the flume. Care is taken to ensure the height of water flow is atleast

2.15 times the diameter of the pier and also there is no disturbance in

the soil bed profile .The flow goes on for 3 hours and then the maximum

scour depth around the pier is noted down by using a bent point gauge

so that the scour depth can be taken more accurately and efficiently.

The height of water over the rectangular sharp crested weir installed at

the end of the flume is also noted down .And the discharge of water is

calculated in the flume using the following formulae:

1. Rehbocks formula is usedto calculate the coefficient of dischargeof the sharp crested weir

Cd= 0.611 + 0.08 Hw/ P

Where, Hw= head over weir

P = height of weir

2. Discharge is calculated using the following equation

Q = 2/3 Cd 2 g X L X Hw3/2

Where, Q = discharge

L= Width of channel where weir is provided

Velocity of flow V = Q/(H X W)


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FLOW CONDITIONS

Uniform Flow ConditionsStudies on sediment bed were conducted under conditions of uniformflow. Turbulence is diminished using honeycombing at the head of thejet.

Velocity of Flow

After a few initial trails by regulating tail control gate & regulating valveand varying the velocities varying from 10cm/sec to 21cm/sec, a criticalvelocity equal to 19.32cm/sec was established just below the incipientmotion of the sediment where the particle just started moving under theinfluence of flow without formation of bed features. This is the velocityjust below the incipient motion of the sediment and which wasmaintained throughout for most of the experiment. Discharge wasmeasured with the help of an already calibrated orifice meter fitted in thedelivery outlet

Depth of Flow

In order to minimize the effect of flow depth on the flume was kept equalto or greater than 2.5 x diameter of the pier. Scour depth becomesalmost independent of flow depth.

Duration of Test Run

Duration of test run was taken as 3 hours.


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Scour Measurements

Initially the sediment bed mixed with 30% of clay and labelled acrossand along the flow direction. The pier model was fixed with the glasspanel section of the flume. The critical velocity of the Ucwas determinedto be 19.32cm/sec .

The flow was started and water was allowed to accumulate in thechannel by closing the gate. Then water head maintained to avoid themovement of particles with water by adjusting the gate. Then theparticular required head is maintained in the manometer for the

experiment. This is done by varying the discharge in the jet by the helpof valve. Then the required water depth of upstream of the pier hasmaintained by varying the tailgate opening. After this, labelling in thechannel around the pier model was done. This instant markedcommencement of the test run.

Reading for the scour depth was taken at the upstream of the outer edgeof the pier and also at the locations where the scour depth could bemaximum. Scour depth measurement were taken with the help of

improvised Z shaped point gauge . After 3 hours stopping the flow thestatic scour profile of the sediment was recorded by taking scour depthof the pier. critical velocity was calculated by taking the observations ofthe sediments at the various discharge .Scour depth for various velocities and flow depths were observed .

Criteria For Analysis

The maximum scour depth at a point the periphery of the pier observedis used for the particular experiment at the end of experiment was takenas the criteria for the analysis of the study.


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OBSERVATIONS

EXPERIMENTAL DATA

S. No. Hw

(cm)

Cd Q(cm3/s) H

(cm)

Velocity

of flow

(cm/s)

Time

(hours)

Scour

depth (cm)

1. 5.0 0.621 12300 22.20 14.0266

1 0.800

2 1.175

3 1.325

2. 5.0 0.621 12300 18.30 17.0159

1 0.950

2 1.550

3 1.900

3. 5.0 0.621 12300 15.60 19.9610

1 1.125

2 1.800

3 2.250


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As it can be observed, the results point closely to the equation

SD = 0.1558V-0.8613

Where SD= Scour Depth (cm)

V= Velocity of Flow (cm/s)

0

0.5

1

1.5

2

2.5

14.0266 17.0159 19.961

Scourdepth(cm)

Velocity(cm/s)


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CONCLUSIONS

1.Local scouring phenomenon was studied for a

circular pier resting in a prepared bed consisting

of 30% clay & 70% sand.

2.Relationship was established between velocity of

flow and scour depth

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Scour Project