Scouring Around The Bridge Pier-A State of Art

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International Journal of Recent Advances in Engineering & Technology (IJRAET)

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ISSN (Online): 2347 - 2812, Volume-5, Issue -3, 2017 6

Scouring Around The Bridge Pier-A State of Art

1Deepika Bhulla, ²Rajendra Magar

1(Water Resources Engineering) YadavRaoTasgaonkar College of Engineering & Technology

2AIKTC, Panvel Abstract –Local Scour can be defined as taking away

sediments around bridge pier and abutments due to movement of water. Scouring accounts of about 60% of bridge failure. Civil Engineers are presently facing with the problem related to the old structure, their monitoring, and maintenance. For the safe and economic life of the structure, method to reduce the effect of scouring is essential. Various efforts are made to reduce the scour depth around the bridge pier. This paper explains the phenomenon of scour around bridge pier, various factors affecting it, problems encountered during measurement of scour depth, methods for prediction of scouring, comparison for accurate result and also measures that should be taken to prevent scouring

Keywords: Scouring, Scour Protection, bridge pier, sediments

I. INTRODUCTION

In the case of bridge, evaluation of scour depth is important as it helps in predicting the depth of foundation. But the main concern for this estimation is 1) negligence or inadequate knowledge of scour phenomenon, 2) data required about the pattern of flow and sub-soil/material condition, 3) Overloading of the structure, during operation period. In 1969, Shenet. Al defined scour as the abrupt decrease in bed elevation near a pier due to erosion of bed materials by the local flow structure induced by the pier. Later, in 1980, Hopkins et al. defined scour as erosive action of water in excavating and carrying away materials from the channel bed. Bridge Scour is the taking away of sediments such as sand and rocks around bridge abutments or pier. Scour holes are caused because of speedy movement of water. This movement of water leads into erosion and thereby reduces the carrying capacity of bridges and pier. Scour holes can be classified into following three types: 1) Local Scour:

caused by the presence of pier 2) Contraction Scour:

caused by decrease in the width of the river 3) Abutment Scour: formed near the bridge abutments. This paper deals with Scour around bridge pier.

Huber¹acknowledged that in USA around 500 bridges failed since 1950 where majority of the failures were because of the scouring of foundation material. Also in 1993, the upper Mississippi flooding caused failure of 23 bridges. Chang in 1973 and Murillo in 198, stated that 46 bridges failed between the year 1961 to 1976

because of scour. Procedure for Paper Submission

II. FACTORS AFFECTING SCOUR

Following parameters mainly results into bridge scouring :

A. Characteristics of Stream flow

Stream flow characteristics such as velocity and flow depth significantly increases scour depth (Peggy A.

Johnson1, Student Member, ASCE, 1991). (Jain and Modi, 1986; Strautmannet. Al, 1987) stated velocity and possibly flow depth have a direct effect on the scour depth and erosion rate. (Peggy A. Johnson1, Student Member, ASCE, 1991) future explained that as the velocity and flow depth on upstream increase, it results into increase in the rate of erosion and scour depth on the downstream. They also stated along with velocity and scour depth, the direction of flow with respect to alignment of pier should be well studied because as the angle of incidence increases, exposed width increases which results into increase in scour depth. (Jain and Modi, 1986) confirmed if the angle of flow velocity vector changes during a flood then it may result into large increase in scour.

B. Characteristics of Bed Material

(Peggy A. Johnson1, Student Member, ASCE, 1991)commented, characteristics of bed material as the size; especially the gradation of sediment also influence bridge scour. They studied the behavior of sediments for slow moving flow, during which they concluded that scouring is less for a non-uniform size sediments than for uniform size. This is mainly because in non-uniform size material, small size sediments flow whereas the large size sediments tends to remain forming a protective cover to the bed, resulting to reduce or stop scouring action.

C. Characteristics of Pier

(Peggy A. Johnson1, Student Member, ASCE, 1991) mentioned that bridge pier characteristics such as size, shape and length affect scouring. (Breusers et al. 1977, Richardson 1988) explained that the length of the pier has a negligible effect unless the pier is not aligned with the flow. Greater the angle of attack of flow over the pier, greater will be effective pier width and so would be

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scour depth. (Richardson, 1988) also said that accumulation of ice and debris or any accumulated material around footing may also increase effective width of pier causing deepening of scour hole. Whereas (Jones, 1989) commented that an exposed footing may turn out to be a dominant feature of scouring process.(Peggy A. Johnson1, Student Member, ASCE, 1991) also commented that scour depth will vary to some extend depending on shape of bridge pier. (Chang, 1988) experienced scouring is maximum in case of rectangular pier, 90% for circular pier whereas it is least, about 80% for sharp-nosed pier. The shape factor is neglected since other effects such as the angle of the attacking flow to the pier, overshadow the effect of the nose shape.

D. Change in Depth of Flow

(Melville and Sutherland2) confirmed that when (depth of flow/pier width) ratio is greater than 2.6, scour depth is irrespective of the depth of flow but when the ratio is less than 2.6,the scour depth depends on the depth of flow.

III. PROBLEMS ENCOUNTER TO ESTIMATE SCOUR DEPTH

(Peggy A. Johnson1, Student Member, ASCE, 1991) summarized following problem faced during estimating scour-depth:

1) Risk Sensitivity:

Current models do not offer sensitivity of risk towards the bridge failure for various parameters that results into bridge scour. Neither it is clear as what model should be used to estimate scour depths for design, data required nor define what should be tolerable error for the safe design.

2) Investigation and Monitoring

An engineer neither has any methods for determining the impact of future storm on scour nor to predict probability that the bridge will fail or survive.

3) Prediction

Models that are used for prediction gives single value for scour depth but the equilibrium depth is a function of the flow and its magnitude and time. Also existing models are time invariant and they cannot assess the cumulative effects of scouring over the life of the bridge or some other duration.

4) Lack of guidance

No proper guidance is available for making decision related to design and maintenance of pier for avoiding failure

5) Current Available Model

Present available models /equations consider only flow characteristics such as flow velocity and depth for the design return period but doesn’t contribute to cumulative

effects caused by a random series of storm with different magnitudes and frequencies.

6) Data Collection

Difficulties in collecting data and the complex nature of local scour, researchers turned to the collection of data from laboratory experiments. Hence, model fit for laboratory measurements were worked on but were difficulty to verify with field measurements.

7) Other difficulties:

Predicting magnitude and frequencies of future storm is easy, storm intensities varies from the area it passes through whereas engineers has no method for determining the scour depth considering the said case.

Also there is no method of measuring the safety of the bridge considering a proper definition of failure.

IV. RECENT EQUATIONS FOR PREDICTING SCOUR DEPTH

Discussed below are Lacey-Inglis equations commonly used in India and few recently developed approaches.

Lacey-Inglis Equation

Lacey analyzed the data from stable irrigation canals flowing through loose non-cohesive sandy material in Indo-gangetic plain.

Formula:

DLQ = 0.47 (Q/f)1/3 And

P = 4.75Q1/2 Where,

DLQ = depth (hydraulic width) Q = discharge in m3/s

Considering lacey’s Silt factor f = 1.76(d)1/2

Where,

f = Lacey’s silt factor related to medium size bed material

P = Perimeter (or width) Laursen –Toch Equation

The equation proposed by Laursen and Toch12 for prediction of dse is

dse /D= 1.35 (b/D)0.70 Where,

D = Average depth of flow b = sediment size

Kothyari –Garde –RangaRaju’s Method

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Kothyari conducted laboratory experiment proposed equations for determining clear water scour depth dsc and equilibrium scour depth dse for steady flow.

Formula

a) Clear Water Scour:

Where

Uc= average critical velocity, given by

b) Scour Under sediment transporting flow

Where,

= difference in specific weight of sediment = mass density of water

Melville and Sutherland’s Equation

Melville and Sutherland2 proposed a method to estimate bridge-scour based on the laboratory analysis. They assumed largest possible scour depth around bridge pier Formula:

dse = 2.4 b Pier Modification

Chabert and Engeldingers4, Shen and Schnieder35and Jones et al36 recommended that cassions or wells should be provided three times the diameter of pier

Scour Reduction using collar

Mubeen Beg1, Salman Beg2, 2013 mentioned that installing a collar around bridge pier reduce the direct impact of the downward flow on the streambed. This not only reduces the maximum scour depth but also lowers the rate of scouring. Hence the risk of failure of the bridge during flood is low.

Kumar et.al. (1999) derived a formuls for the color around bridge pier,

Formula:

(dsp-dsc)/dsp = 0.057 x (B/b)1.612 x (H/Y0)0.837 Where,

dsp = depth of scour on pier without a collar dsc = depth of scour on pier with a collar B = diameter of collor

b = diameter of circular pier

H = elevation difference between water surface and collar surface

Y0 = depth of water above bed elevation A pier fitted with a collar is shown below.

Fig. Schematic Illustration of a pier fitted with a collar Riprap Protection

Using non-movable stones to protect the river bed and banks prone to erosion is an old

V. REDUCTION AND PROTECTION OF BRIDGE SCOUR

Scouring around bridge pier due to flooding is the man cause of bridge failure (Richardson and Davis, 1995;

Johnson and Dock, 1996; Lagasse et.al. 1995; Melville and Hadfield, 1999) Bridge failure results in extreme repairs, loss of accessibility and human loss (Chiew, 1995). To avoid these loss, a large depth of foundation is required but it is result to costly structure. Therefore, for the safe and economical design, scour around the bridge pier is required.

Following methods are adopted in the field with condifence:

practice. The thickness of the riprap (T) at the scour hole is given by

Formula

U12/gT=6(ds85/da15) Where,

U1 = twice the velocity in the river

ds85 = is such a size of river bed material that is 85% of the material is finer than this size.da15= is such a size of armour layer that is 15% material finer than this size Worman stated ds85/da15 should be less or equal to 0.01. Using above equation, twenty bridges were provided riprap protection in Sweden and the results observed had no significant scour. Moreover, it was noted that using the above design criteria, no filter is needed underneath the armour layer. (R J Garde and U C Kothyari, 1998)

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VI. CONCLUSION

Civil Engineers face problems during design as how much deep should the pier be than estimated scour depth.

During Monitoring, when relative scour depth is discovered, again engineer has to decide whether the structure is safe or not, whether protection action is required or not.

Various Laboratory experiments were conducted and with the help of which significant amount of data was collected. Using this data, modeling turned into dimensional analysis and empirical equation. Bu these empirical equations did not account of the varying flow characteristics.

The methods suggested by Melville and Sutherland2and Kothyariet al34are more logical but for predicting scour in gravel-bed rivers and should be used. But no proper field data is available to comment on the relative accuracy for estimating these methods.

Flood is time-dependent random process whereas scour is non-linear time-varying process therefore Time depends model should be developed which would help in estimating the probability of bridge failure over its life span.

REFERENCES

[1] Peggy A. Johnson^1,Student Member, ASCE.,

“Advancing Bridge-Pier Scour Engineering” J.

Prof. Issues Eng. Pract., 1991, 117(1):48-55.

[2] Garde R. J & Kothyari U. C. “Scour Around Bridge Pier”. PINSA 64, A, No.4. (1998)

[3] Mubeen Beg¹, Salman Beg². “Scour Reduction around Bridge Pier: A review” , International Journal Of Engineering Inventions, e-ISSN:

2278-7461, p-ISSN:2319-6491, Volume 2, Issue 7 (May2013), PP: 07-15

[4] Linda P. Warren, Scour at Bridges: Stream Stability and Scour Assessment at Bridges in Massachusetts, U.S.GeologivalSurvet, 2011 [5] Mark N. Landers, Bridge Scour Sata

Management. Published in Hydraulic Engineering: Saving a Threatened Resources-In Search of Solutions: Proceedings of the Hydraulics Engineering Sessions at Water Forum’92. Baltimore, Maryland, August 2-6, 1992. Pblishes by American Scoiety of Civil Engineers.

[6] Johnson P A(1991). “Advancing Bridge Pier- Scpour Engineering”. Journal of Professional Issues in Engineering Educaton and Prfactice, Bol.117 No.1.

[7] Chang. H.H. “Fluvial Processes in river engineering.”. John Wilet& Sons, New York, N.Y.,(1998)

[8] Chiew, Y.M. (1984), “Local Scour at bridge pier”. Report No.335, School of Engineering, University of Auckland, Auckland, New Zealand.

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