Effect of consolidation on local scour around bridge pier in cohesive soil

Effect of consolidation on local scour around bridge pier in cohesive soil

1. Dr. Abdul Karim Barbhuiya

   Associate Professor Civil Engineering Department

   NIT Silchar, Assam, India

2. Tamasa Chakma

Assistant Professor

Civil Engineering Department NIT Agartala, Tripura, India

ABSTRACT

The scouring process is very complicated involving 3D modification of flow around piers. In this present study,

dominant role in engineering problems. Compared with non cohesive sediment, cohesive particles have a large specific surface area which is defined as the surface of

experiments were conducted in a flume, to study the

the particles per unit weight. The fine sediment

scour around bridge pier in cohesive soil. The test

particles are connected with each other as a result of

samples investigated in this study are mixtures of clay

strong influences of the electrochemical reaction on the

and silt with variable compositions and different

surface of the particles in water. The finer the sediment

consolidation times. Effects of consolidation in terms of

particles, the more important the electrochemical

dry density on equilibrium scour depths and on time variation of scour were studied. Further, the variation of scour depth on percentage of clay and silt is also investigated. Moreover, the geometry of the scour hole

effects are. Such effects are favourable for the sediment particles to become more stable against erosion. In the course of consolidation of fine sediment particles, the texture of deposits change progressively into a denser

at different clay and silt content and also with time

state under the action of its own weight or other

variation was studied. It is observed that the depth of

external forces, and the deposits acquire a stronger

scour holes decreases with the increase of dry density due to consolidation. Again, it is found that the depth of

cohesion. The resistance against erosion increases with consolidation time, with bulk density as an indicator to

scour hole also decreases with the increase of clay and

represent the erosion resistance of the consolidated

silt content.

KEY WORDS

sediment. Detailed studies on the mechanism of scour around bridge piers and abutments were made amongst others by Laursen and Toch, (1956), Nakagawa and

consolidation, local scour, cohesive soil, dry density,

Suzuki (1975),Raudkivi and Robert Ettema, (1983),

scour depth.

Melville and Sutherland (1988), Ettema (1990),

1. INTRODUCTION

   Kandasamy and Melville (1998), Sheppard and Miller (2006), Dunn (1959), Bohan (1970), Steven and Ruff (1982), Shuyou and Fang (1991), Sekine and Iizuka

   In engineering design and safety, the effects of alluvial

   (2000) Jennifer (2005), Dey and Barbhuiya (2004,

   river flow around a bridge pier is an important practical problem which results into scour. The main cause of concern in stability of bridges founded in river beds in

   2005), Montanari (2006), Sheppard and William Miller Jr (2006), Tan Guang-ming, Wang Jun, Shu Cai-wen, Lai Yong-hui(2006). Underestimation of the depth of

   lowering of river bed level caused by river flow around

   scour and its areal extent results in design of too

   bridge elements such as piers, abutments and spur dikes

   shallow a foundation which may consequently get

   and is termed as local scour. The erosion behaviour of

   exposed to the flow endangering the safety of the

   cohesive sediments around an obstruction plays a

   bridge. Overestimation of the scour depth results in

   uneconomical

   design.

   Therefore,

   knowledge of

   the

   anticipated maximum scour depth for design discharge is essential for a proper design of the foundation of the bridge piers, abutment etc. The aim of the present study

   is the

   effect

   of consolidation

   time

   and

   sediment

   composition

   of clay

   and

   silt

   on scour

   rate

   of the

   cohesive sediment around bridge piers.

2. EXPERIMENTAL PROCEDURE

SETUP

AND

1. Experimental setup: Experiments were conducted in a 16 m long, 0.9 m wide and 0.7 m deep horizontal flume. The test section was located 10.90 m from the upstream end of the flume. At the inlet section, there

   was a

   vertical

   steel

   screen

   covering

   the

   full

   cross-

   section

   for

   damping

   the

   flow

   disturbances

   through

   which water entered into the flume as shown in Fig.1. An adjustable tailgate was installed at the downstream end of the flume to control the flow depth. The choice of the flume and the location of the test section were made in such a way that (a) the width of the flume was wide enough to have three-dimensional flow and

   (b) the flow became fully developed before it reaches the test section. The flume was connected to the water supply system in the laboratory. A sediment trap was constructed in the downstream side, having a length of

   1.5 m to arrest the scoured sediments. A centrifugal high discharge pump was used to pump water into the flume with a discharge of 26.9 lit/sec .The discharge pipe of diameter 20.32 cm and control valve to regulate the amount of discharge into the flume. The pump is driven by a 15 horse power motor under 1440 rpm. It is

   a three-phase

   continuous

   type

   motor.

   The

   average

   velocity is found to be 13.4 cm/sec.

2. Sample Preparation: The test samples used in this experimental work are divided into 6 groups according to their compositions. The size distributions are shown

   in Fig.2.

   The

   median

   size

   d50

   determined

   from

   the

   particle size distribution curve for different sediments

   were

   found

   to be

   0.18mm,

   0.145mm,

   0.12mm,0.078mm,0.075mm and 0.048mm respectively and clay content ratio of 0%, 20%, 40%, 60%, 80% and

   100% respectively.

   Fig.1: Sketch of experimental setup

   Grain Size Distribution Curve

   In the first set of experiment, the 100% clay and silt sample was used and was placed in the flume at the area provided for placing of material. The clay and

   100

   silt

   sample

   was

   thoroughly

   mixed

   with

   water

   and

   90 compacted gently. Before leaving the bed to settle

   Cumulative percentage passing

   down completely, the obstruction was placed exactly at

   80 the centre of the bed and a smooth finishing was

   70 provided. Then the bed was left as it is to drain out

   60 water and to

   settle

   down

   completely.

   Four of such

   similar experiments were carried out to check the

   50

   40

   30

   20

   10

   0

   0.001

   0.01

   100% silt and clay 80% silt and clay 60% silt and clay 40% silt and clay 20% silt and clay 0% silt and clay

   0.1 1 10

   consolidation variation having different dry densities. In the second set of experiments an obstruction size was kept as constant but the percentage of clay and silt content was changed. In the second set the dry density was not taken into consideration. The depth of water and velocity of flow of water was kept as constant. While performing the experiments initial bed reading was taken before the start of the experiment. During the

   process of the experiment a scour depth reaing was

   Particle size in mm

   Fig. 2: Grain size distributions of samples

   taken as the function of time. Then with the help of a periscope the scour depth reading was taken at every interval of time such as 30min or 1 hr. Finally, the bed condition was observed and noted down throughout the flume. The geometry of the scour hole was taken with the help of a ruler or a scale. The final scour hole length

3. Experimental Procedure: Each group of the sample

was

noted

down.

The

readings

were

recorded

and

is mixed

with

water

and

left

for

consolidation.

The

analysis was carried out.

consolidation period for each sample are 1d, 5d, 10d, 15d, 20d, 30d, 50d and 60d. After that dry density of

3. RESULTS AND ANALYSIS

different samples were found out. Nine experiments

were carried out which has been divided into two parts. In the first set four tests were carried out with same 100% clay and silt content with an obstruction size of 75mm diameter with different dry densities as shown in

The following analysis are based on the experimental results.

1. Dry density during the course of consolidation: It

   Table 1. In the second set five tests were carried out

   has

   been

   observed

   that

   during

   the

   course of

   with

   0%,

   20%,

   40%,

   60%

   and

   80%

   clay

   and

   silt

   consolidation,

   deposits

   change

   progressively

   into a

   content with same obstruction size of 75mm diameter.

   Expt. No.	Silt and clay (%)	Sand (%)	Obstruction size, D(mm)	Dry density, g/cc
   1	100	0	75	1.2
   2	100	0	75	1.22
   3	100	0	75	1.578
   4	100	0	75	1.682

   Table 1: First set of test (with consolidation)

   denser state, and eventually they have a relatively high dry density with a strong cohesion. It is necessary to consider the variation of the dry density at different stages of consolidation and the dry density can be used to reflect the effect of consolidation on scour process. It is seen that in the early stage of consolidation, the dry density increases very fast. After several days the dry density reaches nearly steady values. A graph showing dry density versus consolidation of different clay and silt content is shown in Fig.3.

   formed when the flow impinges with the seabed; the resulting vortex system wraps around the cylinder and trails off downstream. The main scouring force is the primary vortex system, which develops in front of the cylinder. Flow pattern in the wake system are formed by the rolling up of the unstable shear layers generated at the surface of the obstruction and these are detached from both the sides of the obstruction at the separation line.

   Analysis of data on scour around an obstruction in cohesive sediments has revealed interesting aspect about the process of scour. In cohesive sediments, the geometry, location and extent of scour hole are found to be significantly different from those in cohesion less

   sediments.

   Observation

   revealed

   that

   scouring in

   Fig.3: Dry density versus consolidation time.

   sediments having silt and clay content between 5% to

   30% commenced from the sides of the obstruction then

   propagates to

   the

   upstream

   along

   the

   sides

   of the

2. Scouring process and monitoring of deepest

   obstruction and met the nose of the obstruction. The time for scour to happen is very fast and also reached

   scour: From the experimental observations it is found

   equilibrium very fast. The scour depth then increased

   that initiation of scour occurred in either from wake or

   rapidly creating the deepest scour hole at the nose of

   at the sides of the obstruction. The different bed

   the obstruction. For sediments with silt and clay

   materials were consolidated for different time period. In 100% clay sample, scouring occurs at a fast rate which was consolidated for one day as compared with

   the ones which were consolidated for more days. The scouring which occurred at the sides later turned into

   content ranging from 60% to 90% a similar behavior was observed but the deepest scour is found to occur at the sides and nose of the obstruction. For sediments with higher silt and clay content, i.e. more than 50% scour initially developed at the obstruction sides, and

   the entire area where the pier was fixed. Maximum

   then propagate towards the nose of the obstruction. The

   scouring occurred at the upstream portion and the shape

   scour depth decreases with the increased in clay content

   of the scour hole was approximately circular. With

   and the time variation also increased respectively. It

   lower clay content scouring started very quickly,

   was observed from the experiment that at smaller

   whereas it took much longer time with highly cohesive

   obstruction size the scour depth also is less and at the

   material. The maximum scour depth was found at

   larger obstruction size the scour depth is more, the

   upstream of the obstruction in all experiments.

   shape of the scour hole is cone shape. The horseshoe

   However, in soil with silt and clay content at or above 30%, the scour depth on both sides and in the wake of

   vortex is more prominent when the obstruction size is large. Whether the obstruction size is small or large the

   the obstruction is found to be about the same as the

   maximum scour depth was found to be at the front

   maximum scour depth. The scoured sand in the soil

   (nose) of the obstruction. The depth of the water flow is

   mixtures deposited downstream and formed a bar,

   kept at 80mm so that it does not influence scouring of

   while the clay particles were in suspension during the scouring process and were carried out by the flowing water.

   the bed. In another case of the experiment when the clay content is changed it was observed that the sand particles is coated with clay all around their surface, so

   The initiation of scour is mainly due to the

   when the silt and clay content is between 5% to 30% in

   accelerated flow and the formation of flow induced

   the mixture the particles tends to dislodged and

   vortices in the vicinity of an obstruction causing

   detached from each other and get deposited at the

   erosion of sediments particles. The approach flow

   downstream. The time required for scouring is

   velocity goes to zero at the upstream face of the pier.

   minimum but scour hole is maximum and reached

   Since the flow velocity decreases from a maximum at

   equilibrium stage very early. But when the silt and clay

   the free surface to zero at the bed, the stagnation

   content is between 60% to 90% the particle from a

   pressure decreases with distance from the water surface

   bulk of mass together which is very difficult to detach

   and this pressure difference

   drives

   the

   flow

   them

   due

   to the

   physico

   chemical

   properties of

   downstream. A recalculating eddy (primary vortex) is

   cohesive

   soils.

   The

   time

   required

   for

   scouring is

   maximum as

   the

   obstruction

   sizes

   increases

   and

   the

   from the sides of the pier and then propagates to the

   clay content decreases.

   At the beginning f the test for time variation

   upstream along the sides of the pier and met the nose of the pier. In this case, scouring occurs very fast and

   versus silt and clay content present in the mixture is

   reached

   equilibrium

   condition.

   The

   scour

   depth

   also measured. We need to be more careful and more increases rapidly by creating the deepest scour hole at

   precaution

   should

   be taken

   so that

   unnecessary

   and

   the nose of the pier. For sediments with clay and silt

   undesirable scour hole may develop at the start of the experiments. The motor is allowed to turn on and off

   content ranging from 60% to 80% a similar behavior was seen but the maximum scouring was seen at the

   for

   several

   interval

   of time

   and

   very slowly.

   A full

   sides and the nose of the pier. Here the scour depth

   discharge

   is allowed

   when

   the

   bed

   has

   attained

   decreases with the increase in clay and silt content and

   stability,

   then the reading is

   started taken with very

   takes longer time to reach the equilibrium. The shape of

   small time interval.

3. Variation of scour depth with dry density: The scour process in cohesive, fine grained soil is different from that in non cohesive, coarse grained soils. During

   the scour hole is conical with clay content of 60% to 80% and the shape is circular with clay content of 5 to 40%. When the clay content is changed it is seen that the sand particles is coated with clay all around their surfaces, but when the clay content is in between 0% to

   the

   course

   of consolidation,

   deposits

   change

   40% the particles tends to get dislodged and detached

   progressively into a denser state, and eventually they

   from each other and gets deposited at the downstream.

   have a relatively high dry density with a strong

   Here, time required for scouring is minimum but the

   cohesion. Thus, it is necessary to consider the variation of the dry density at different stages of consolidation, and the dry density can be used to reflect the effect of consolidation on scour process. It can be seen that in

   scour hole is maximum and reached equilibrium stage very early. In case of higher clay contents particles are very difficult to detach. Variation of scour depth with clay and silt content is shown in Fig.5 and variation of

   the early stage of consolidation, the dry density

   dry density with different clay and silt content is shown

   increases very fast. After several days, the dry density reaches nearly steady values. The closer the dry density is to its steady value, the more difficult the deposits are scoured. Fig.4 shows a variation of dry density with scour depth.

   in Fig.6.

   Fig.4:

   Variation of dry density with scour

   depth.

   Fig.5: Variation of scour depth with clay and silt content.

4. Variation

   of scour

   depth

   with

   clay

   and

   silt

   content: In cohesive sediments, the geometry, location and extent of scour hole are found to be significantly

   different

   from

   those in

   cohesionless

   sediments.

   Observations have shown that scouring in sediments having silt and clay content between 0% to 40% started

   Fig 6: Variation of dry density with various clay and silt content.

   Fig 7: Variation of scour depth with elapsed time for 100% clay and silt content.

5. Time variation of scour depth around pier: Time variation of scour depth around piers is an important

   vision.

   In cohesive

   soils

   scour

   phenomenon is

   very

   much

   different

   from

   that

   in non-cohesive

   soils.

   Scouring process is much slower in cohesive soils and

   mainly

   dependent

   on the

   soil

   properties so

   the

   equations available for non cohesive sediments for time

   variation

   cannot be

   applied

   to cohesive

   sediments.

   Therefore, the cohesive materials needs to consider the

   time

   variation and soil properties. Time

   variation of

   scour depths are shown in Fig.7 to 12. From the figures it is seen that, the rate of scour for sediment mixture with less silt and clay content is more in the initial

   period

   and

   decreases

   with

   time

   and

   reaches

   the

   dynamic equilibrium condition within 20 hours. But

   Fig 8: Variation of clay and silt content with

   with more clay and silt content the rate of scour is very less in the beginning and increases with time and again decreases and reach to equilibrium. The time to reach the equilibrium is very large particularly for sediment

   time to reach equilibrium stage for 0%, 20%, 40%, 60%, 80% clay and silt content.

   with

   clay

   and

   silt

   content

   of about

   80%.

   For

   bed

   sediments with 60% to 80% clay and silt content it is

   found

   that

   the

   scour

   hole

   sometimes

   decreases

   with

   time which is due to the deposition of material from the slopes due to shear failure.

   Fig 9: for (20% clay and silt)

6. Geometry of the scour hole: From the experimental results it is seen that the scour volume increases with increase of sediment size and pier size. The scour hole

around

the

pier

is produced

due to

the

increase of

velocity of the flow around the pier and formation of vortex. The system of vortices producing local scour are the horseshoe and wake vortices, which combine to produce a scour hole with its maximum depth around the pier. From the experiments of 100% clay and silt

content

having

different

dry

densities

(the

contour

Fig10: for 100% clay and silt (without consolidation)

Fig 11: for (0% clay and silt)

diagram are shown in Fig. 13 & 14) it is seen that the scour hole length decreases with the increase in dry density and from the experiments of different clay and silt content without consolidation it is seen that with the increase of clay and silt content steepness of the slope increases and vice versa. With the increase of clay and silt content the scour at the base decreases.

Fig 13: Contour diagram of scour hole with 100% silt and clay having dry

Fig 12: for (80% clay and silt)

density

1. g/cc.

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      1. SUMMARY

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1. CONCLUSIONS

engineering,Vol.132,No.7,July1.ASCE,ISSN 0733-9429. [14]Tan Guang Ming (2010), experimental study of scour

1. Due to the gravitational action or other external

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   sediment,

   Journal of

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   hydrodynamics, 22(1):51-57.

   difficult to be scoured.

   [15]A.K.Barbhuiya, S. Talukdar (2010), scour and 3D

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