Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No.N-266)

PHYSICAL MODEL STUDY OF SCOURING EFFECTS ON RAFT FOUNDATION OF

BRIDGE PIERS

T.I. Eldho Professor, Department of Civil Engineering, IIT Bombay; Email: eldho@civil.iitb.ac.in

B.V.S.Viswanadham Professor, Department of Civil Engineering, IIT Bombay; Email: viswam@civil.iitb.ac.in

B.A.Vijayasree, Research Associate, Department of Civil Engineering, IIT Bombay; Email: vijayasree@iitb.ac.in

N. Siva Naga Venkat, PG Student, Department of Civil Engineering, IIT Bombay; Email:venkat.nsn@gmail.com

ABSTRACT: Construction of raft foundation is one of the economical and viable options for bridge piers in Maharashtra.

The objective of the present study to evaluate the scouring behavior of bridge pier constructed with raft foundation through

physical model tests. In the on-going series of experiments being conducted at IIT Bombay, a horizontal masonry flume

constructed at the Hydraulics Laboratory of IIT Bombay was used. The flume is 11m long, 2m wide and 1.3m deep, having

a PCC floor. At a distance of 6m from the upstream end of the flume, a Perspex window of length 3m is provided, which

facilitates the observation of scour and flow behavior. Using the Froude’s similarity, the model scale was chosen as 1:10.

The pier model is made of steel. The raft model is made of marine plywood sheets. The water from the sump is pumped

into an inlet tank and re-circulated. The experiments were conducted for a range of discharges and the results are presented.

It was observed that majority of scour occurs at the upstream end of the pier only. The provision of raft foundation reduces

the scour to a large extent especially along the sides of the pier.

INTRODUCTION

Scour at bridge piers may be defined as a localized

lowering in the bed elevation around the pier or loss of

support to the bridge pier. This lowering is caused by the

three-dimensional boundary layer separation at the pier,

resulting in erosion of bed material by the local flow

structure, which is characterized by a high level of

turbulence and vortices. One of the main concerns about the

stability of bridge foundations is the occurrence of scour

around piers. The foundation strata for the bridge structures

while executing bridge projects, varies from exposed hard

rock to pure sand for considerable depth. If good founding

stratum is not available in a shallow depth, the construction

of small bridges becomes uneconomical. Majority of the

rivers in Maharashtra have thick sandy beds, where raft

foundation is a feasible solution.

Chiew [1,2] showed that by placing a one-fourth diameter

wide slot near the water surface or the bed level, it is

possible to reduce the clear-water scour depth by as much

as 20%. A one-half-diameter-wide slot placed near the

water surface can reduce the clear-water scour depth by as

much as 30% [3]. When a slot placed near the bed is

combined with a collar, the study showed that the

combination is capable of eliminating scouring altogether.

As per the IRC: 89-1997, 78-2000 [4, 5] whenever adoption

of shallow foundations for bridges becomes economical, in

order to restrict the scour, floor protection has to be

provided. The floor protection will comprise of rigid raft

with cutoff walls and a flexible apron, to check scour,

washing away or disturbance by piping action. Maximum

velocity of the flow should be less than 2m/s and the

intensity of discharge should be limited to 3m3/m. The rigid

raft shall be provided under the bridge extending for a

distance of at least 3m on upstream side and 5m on

downstream side of the bridge pier. The rigid raft shall be

enclosed by cutoff walls with a minimum of 2m on the

upstream side and 2.5m on the downstream side. The rigid

raft shall be continued over the top width of cutoff walls.

Flexible apron of 1m thickness comprising of loose stone

boulders, weighing at least 40kg, shall be provided beyond

the cutoff walls for a minimum distance of 3m on the

upstream side and 6m on downstream side. Cement

concrete blocks, or wire crates filled with stones can also be

used in place of boulders.

Ugalmugale and Namjoshi [6] have discussed about the

design and construction of two major bridges (submersible)

across River Bawanthadi and high level bridge across river

Kanhan where innovative methods were developed during

execution.

Borde and Jangde [7] have concluded that raft foundation is

a most economical solution in the case of weak soils where

open foundation is not possible. Raft foundations can be

most effectively used on small streams and rivers. In rural

road development programmes where such structures are

required in large number, lot of economy can be achieved.

Span lengths up to 10m are suitable for adopting raft

foundations since longer spans would make foundation

uneconomical. In Maharashtra, the major rivers, like

Godavari and Wainganga have been bridged with raft

foundations. Raft foundations are equally good for

submersible bridges.

Dey and Sen [8] have done extensive studies to determine

the scour depth in boulder-bed Rivers under high stream

907

T.I. Eldho, B.V.S. Viswanadham, B.A. Vijayasree & N. Siva Naga Venkat

velocities. If the pier is fitted with a circular collar of

diameter three times the pier diameter at the river bed level,

the reduction in scour depth is 100%.

Studies on physically scaled-down models of the pier

resting on a raft foundation subjected to different flow

conditions in controlled channel flow condition in a

laboratory may reveal useful information, which can be

used to finalize optimum parameters for the design.

Literature review showed that not many model studies have

been conducted in Indian conditions to understand the

scouring behavior of pier with raft foundations. With an

objective of getting better understanding, a physical model

study has been undertaken at Department of Civil

Engineering, IIT Bombay, sponsored by the Ministry of

Road Transport and Highway. Preliminary experiments

were conducted to understand the scouring phenomena of

simple bridge piers were conducted [9]. In continuation of

this project, experiments were conducted on a physical

model of pier with raft foundation and some of the results

are presented in this paper.

EXPERIMENTAL DETAILS

The present study is focused on physical model studies of

the scouring behavior at a bridge pier with raft foundation.

The experiments were conducted in a horizontal masonry

flume constructed at the Hydraulics Laboratory of IIT

Bombay. The flume is 11m long, 2m wide and 1.3m deep,

having a PCC floor. At a distance of 6m from the upstream

end of the flume, a Perspex window of length 3m is

provided which facilitates the observation of scour and flow

behavior. Using the Froude’s similarity [10,11], model

scale chosen is 1:10. The pier model is made of steel. The

raft model is made of marine plywood sheets of 12 mm

thickness. The water from the sump is pumped into an inlet

tank and re-circulated. In the study, a bridge with two spans

is considered so that one central pier and two side piers are

provided with a raft foundation which extends to the entire

width of the flume and was placed in the test section. The

water from the sump was pumped into an inlet tank.

Adequate stilling arrangements were made to dissipate the

energy of falling water. From a big tank, water enters into

the channel. A bell mouth arrangement was provided. A

wire mesh was provided and pebbles were laid at the entry

point to enable smooth entry of water in to the flume.

Figure 1 shows the plan view of the test section. Figure 2

shows the schematic diagram of the longitudinal cross

section of the flume at the test section.

The bed material used was River sand (d50= 0.5mm,

specific gravity = 2.93 with 98.1 % sand content).The grain

size distribution is shown in Figure 3. The sand used in the

present study can be classified as SW according to Unified

Soil Classification system [12].

The pier and raft model were placed at the centre of the test

section. Sand was filled to a height of 300 mm and leveled.

The initial bed levels were recorded using 2D Bed Profiler

[13].Water was allowed into the channel with the discharge

u/s d/s

Flow of water

Sand Bed Sand Bed

Raft ModelPier

Model

1m

1m

1m

2m0.09m

Fig.1 Plan view of the test section.

1m

Cut Off Walls

u/sd/sFlow of water

Sand Bed

Pier Model

Raft Model

1m

0.3m

Fig.2 Longitudinal Section of the flume at the test section

Fig. 3 Grain size distribution of the river sand used as bed

material.

being introduced gradually without disturbing the sand bed.

The discharge was measured using Ultrasonic Flow Meter.

The velocity was measured with a Programmable

Electromagnetic Liquid Velocity Meter [14].

The velocity of flow, in the model for the maximum

possible discharge, in the model was 0.923m/s, with a flow

depth of 210mm. The corresponding values in the prototype

are 2.92m/s and 2.1m respectively. The experiments were

conducted for a period of 8 hours to attain regime

908

Physical Model Study of Scouring Effects on Raft Foundation of Bridge Piers

conditions. Figure 4 shows the vortex formation during the

flow at the upstream end of the pier.

Flow Direction

Fig. 4 Horse Shoe Vortex formation at the u/s end of pier.

After 8 hours, the flow was allowed to stop and the water

was drained out. The bed levels after the formation of the

scour were also recorded using the 2D bed profiler. The

data from the bed profiler were analyzed and scour contours

were plotted using Surfer software package.

RESULTS AND DISCUSSION

The experiments were conducted with simple piers and

piers with raft foundation, for flood discharge conditions. In

both cases, the maximum scour was observed at the

upstream end of the pier. But provision of raft foundation

has helped in reducing the scour depth by approximately

17%, for the same value of discharge, depth and flow

velocity. This endorses the fact that provision of raft

foundation reduces the damaging power of the horse shoe

vortex and thereby the scour.

Figures 5 and 6 show the scour contours with simple pier

and pier with raft foundation. For the prototype, the

maximum scour depth without raft foundation was 0.93m,

whereas the maximum scour depth with raft foundation

measured to 0.77 m. For the model, Figures 7 and 8 shows

the photographs of the scour hole, with simple pier and pier

with raft foundation. Minimal scour was observed at the

downstream end of the pier.

Further experiments were conducted for a range of

discharges and the results were analyzed. It was observed

that majority of scour occurs at the upstream end of the

pier. The provision of raft foundation reduces the scour to a

large extent especially along the sides of the pier.

0.00 200.00 400.00 600.00 800.00 1000.00 1200.00 1400.00 1600.00 1800.00 2000.00

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

1800.00

2000.00

-100.00

-90.00

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-10.00

0.00

20.00

Pier

Flow Direction

Fig. 5 Scour pattern with simple pier (scour in mm)

0.00 500.00 1000.00 1500.00 2000.00 2500.00 3000.00

0.00

500.00

1000.00

1500.00

2000.00

-80.00

-60.00

-40.00

-20.00

0.00

Edge of raft

foundation

Flow Direction

Pier

Fig.6 Scour Pattern for pier with raft foundation (scour in

mm)

Flow

Direction

Fig. 7 Photograph showing scour pattern without any raft

foundation

909

T.I. Eldho, B.V.S. Viswanadham, B.A. Vijayasree & N. Siva Naga Venkat

Flow

DirectionRaft

Fig. 8 Photograph showing the scour pattern with pier on

raft foundation.

CONCLUSIONS

In this study, a 1:10 physical model has been constructed to

test the scouring pattern around bridge pier with raft

foundation and various parameters affecting it. The

experiments were conducted for different depths of flow

and discharges. The flow behavior, scour pattern and

behavior of the horse shoe vortex were observed. It was

observed that maximum scour takes place on the upstream

side of the pier, though provision of raft foundation has

reduced the scour depth by almost 17%, than the simple

pier case. In the next phase, the effect of extending the

width of raft foundation to the upstream and downstream

side of the pier would be pursued.

ACKNOWLEDGEMENTS

The Authors are thankful to Ministry of Road Transport and

Highways (MORTH) and Mr. Arun Kumar Sharma,

Chief Engineer for co-coordinating a project entitled

“Hydraulic Model Investigations for Design of Raft

Foundations for Bridges (B-40)”. Thanks are also due to

Mr. P.L. Bongirwar for his continuous support and

encouragement during the course of the study. Finally, the

staff at Hydraulics laboratory, Department of Civil

Engineering, IIT Bombay needs a special mention for their

involvement in executing model flume tests and in the

development of experimental setup.

REFERENCES

1. Chiew, Y.M. (1992), Scour protection around bridge

piers, Journal of Hydraulic Engineering, ASCE,

118(9), 1260-1269.

2. Chiew, Y.M. (1995), Mechanics of Riprap Failure at

Bridge Piers, Journal of Hydraulic Engineering,

ASCE, 121(9), 635-643.

3. Chiew, Y.M., and Lim, F.H. (2000), Failure Behavior

of Riprap Layer at Bridge Piers under Live-Bed

Conditions, Journal of Hydraulic Engineering, ASCE,

126(1), 43-55.

4. IRC: 78-2000, Standard Specifications and Code of

Practice for Road Bridge, The Indian Roads Congress,

New Delhi, India.

5. IRC: 89-1997, Guidelines for Design and Construction

of River Training and Control Works for Road Bridges, The Indian Roads Congress, New Delhi,

India.

6. Ugalmugle, A and Namjoshi, A.G. (2004), Construction

of Raft Foundation in Deep Sandy Beds for Major

Bridges Across Perennial Rivers, Journal of the Indian

Roads Congress, Paper no.506, pp.421-459.

7. Borge, V.B and Jangde, K.S. (2005), Innovative Bridge

Foundations in Weak Soils-Experiment and Practice in

Maharashtra, Paper no.518, Journal of Indian Roads Congress, 455-484.

8. Dey, S., and Sen, D. (2005), Determination of Scour

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Stream Velocities, Research and Development scheme

No. B-33, Final Report. Indian Institute of Technology,

Kharagpur, India.

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B.A. (2010), Physical model study of scouring effects

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McGraw-Hill Publishing Company Limited, New

Delhi.

12. Head, K.H. (2006), Manual of Soil Laboratory

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13. HR Wallingford (2010), 2D bed profiler,

www.hrwallingford.co.uk (last visited on July 12,

2011).

14. P-EMS (1998), Manual, WL | Delft Hydraulics, pp 2-

1.

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