1

Scour countermeasures at bridge Scour countermeasures at bridge piers and abutmentspiers and abutments

A. H. Cardoso

2

Introduction

Main features of the flow field at bridge piers and abutments

Countermeasures for local scour at bridge piers Introduction Design of armouring countermeasures Notes on flow altering devices Experimental study on the effectiveness of slots, bed-sills and

combinations of slot plus bed-sill

Riprap mattresses as a countermeasure against scour at bridge abutments.

Contents of the presentationContents of the presentation

3

• Is scouring a real problem?

in the USA: – 383 bridges have been destroyed or damaged between1964 and 1972;

– 73 bridges were destroyed in Pennsylvania; Virginia; West Virginia in 1985;

– 17 bridges were destroyed in NY and in N Eng. in 1987;– ………………

in Portugal: Penacova; Alva; Gafanha; … Entre-os-Rios, 2001 (56 casualties).

IntroductionIntroduction

4

5

• What are the open issues? Available methods (scour depth prediction; design of protection

solutions) are often not satisfactory; this is particularly true for abutments.

• Why? The flow field is highly 3-dimensional at piers and abutments; The sediment transport phenomenon is complex in the scour

hole.

• Other aspects? Local phenomena may be superimposed to river-bed

degradation as well as to contraction scouring (due to the increase of U).

6

Main features of flow field around bridge piers and Main features of flow field around bridge piers and abutmentsabutments

The presence of obstacles implies the flow stagnation close to the walls pressure increase (kinetic energy potential energy).

Pressure increases are bigger at the free surface than close to the bottom.

h

y

u

Descending

flow

y

h

p = (u2)/2

bow wave

7

The local change of the pressure field originates:

bow wave; descending flow (which triggers the scouring process); flow separation.

stagnation point

main vortex

bow wave

descending flow

8

The combined action of the deflected descending flow and the separated flow creates:

the horseshoe vortex (in the case of piers); the main vortex (in the case of abutments).

Separation also occurs at the lateral walls of obstacles, inducing wake vortices (rotating at successively alternate senses).

For bridge piers:

descending flow

scour hole

horseshoe vortex

bow wavepier

approaching flow

wake vortices

9

The horseshoe vortex (or the main vortex) carries the bed material downstream as bed load.

Wake vortices pick up sand particles from the bottom; they transportthe particles downstream in suspension.

secondary vortex

main vortex

The flow structure around abutments is very similar to the flow structure around piers.

10

Types of countermeasures: Armouring countermeasures: act as barriers withstanding the

elevated shear stresses that occur around bridge piers or abutments; riprap mattresses; gabions and Reno mattresses; artificial riprap; cable-tied blocks; concrete-filled bags and mats.

Flow altering devices (for piers): act to reduce the strength of the main features of the flow field around piers (horseshow vortex; downflow; wake vortices). sacrificial piles; collars; flow deflecting vanes; permeable sheet piles; suction applied to pier; slot in pier.

IntroductionIntroductionIntroduction

Countermeasures for local scour at bridge piersCountermeasures for local scour at bridge piers

11

In practice, two different sediment transport conditions may be observed clear-water flow ( < c or U < Uc); live-bed flow ( > c or U > Uc).

Under clear-water flow, failure mechanisms of armouring countermeasures are: shear failure; winnowing failure; edge failure.

Under live-bed, failures mechanisms are as under clear-water plus: bed-form undermining; degradation failure.

12

Occurrence domains of failure mechanisms (excluding degradation).

0.35 for piers; variable for abutments

Riprap ≡ blocks of any kind

u*/u*c blocks

u*/u*c sand

13

shear failure – armour blocks do not withstand the local hydrodynamic forces and are entrained by the flow.

winnowing failure – the finer underlying bed material is eroded through the voids between the blocks under the action of turbulence and seepage flows.

remediation – sufficiently heavy blocks.

remediation – sufficiently thick mattress; underlying filter.

14

edge failure – armour blocks fall into the scour hole that develops in the periphery of the armour mattress.

remediation – sufficiently wide mattresses.

15

Bed-form undermining – armour blocks are undermined and settle with the migration past the pier/abutment of the trough of large dunes.

remediation – place the mattress sufficiently below the original sand bed.

16

Degradation failure – armour blocks are undermined and settle due to the general erosion of the river bed.

remediation – construction of bed sills or check dams (one or more) immediately downstream of the bridge site.

17

To be reminded (above all):

1. Completely avoid scouring is practically impossible;

2. Armouring countermeasures are to be considered only temporarily effective;

a. At the edge of mattresses, scour holes typically develop, in any case: scour holes are attenuated and displaced from the pier/abutment;

b. Maintenance and monitoring are recommended;Remote monitoring is an issue.

c. Preferably, new bridges should be conceived to withstand scouring.

3. Flow altering devices may be useful to reduce scour at bridge piers

Research is required in this front.

18

Questions to be answered (assuming that degradation scour does not occur or is mitigated):

1. How big should blocks be to face shear failure ?;

2. How thick should the mattresses be to face winnowing ?;

3. How wide should mattresses be to face edge failure ?;

4. How deep should mattresses be placed to face bed-form undermining ?

Design of armouring countermeasuresDesign of armouring countermeasures

Main issuesMain issues

19

How big should blocks be to face shear failure ? There are many formulae available in the literature:

Isbash 1938 Inglish 1942 Blench 1957 Sousa-Pinto 1959 Maza and Sanchez 1964 Nicollet & Ramette 1971 Neil 1973 Bonasoundas 1973 Quazi & Peterson 1973 Posey 1974 Hjorth 1975 Breusers et al. 1977 Farraday & Charlton 1983 Worman 1989 Parola & Jones 1989 Breusers & Raudkivi 1991 Parola 1993, 1995 Austroads 1994 Richardson & Davis 1995 Chiew 1995 Fotherby 1995 Croad 1997 Lagasse et al. 1997 Lauchlan 1999 Fotherby & Ruff 1999 Choi et al. 2002 ……………………………..

Riprap mattressesRiprap mattresses

20

I have used the equation of Parola & Jones 1989, as recommended by Parker et al. 1998:

I also use those of Bonasoundas 1973, Quazi & Peterson 1973 and Breusers & Raudikiwi 1991, which lead to “central”predictions, for U 5 ms1.

gsKU

D fr 189,2

22

50

U – approach flow velocity;Kf – pier shape factor; s – density of the blocks.

25,1

5,2

0

50

185,0

s

Fh

D rr250 43,36 UUDr

5.1

3

0

50

1278,0

s

Fh

D rcr Frc – critical approach Froude number,defined with Uc = 2U;

h0 – approach flow depth.

21

Melville & Coleman 2000 suggest the use of the equation of Parola 1993 + 1995 and Lauchlan 1999.

Assuming that Dr50 is known, the riprap gradation curve can be given by (Parker et al. 1998):

» 100% finer than 1,50 Dr50

» 80% finer than 1,25 Dr50

» 50% finer than 1,00 Dr50

» 20% finer than 0,60 Dr50

How thick should the mattresses be to face winnowing ? For piers, settling can be observed for mattresses of up to

t = 12Dr50 !!!!!!!!!!! thick (Nanyang Technological University). 2Dr50 t 3Dr50, placed on geotechnical filter (Terzaghi

criteria; usually difficult to built) or on geo-textile filter (properly designed and dully attached to the pier).

22

How wide should the mattresses be to face edge failure ? According to Parker et al. 1998:

For cylindrical piers, adapt accordingly (3D B1 4D; B2 = 0.75B1).

B1

B2

filter

riprap

Flow

h1h2

D

B1 = 4D/cos;B2 = 3D/cos;h1 = 1.5D/cos;h2 = D/cos.

23

How deep should mattresses be placed to face bed-form undermining ? only the study of Lauchlan 1999 could be found in the

literature.

2.175.2

00

50 13.0 FrhYS

hD r

fr

Sf – safety factor (minimum of 1.1).

h0 – approach flow velocity

24

Preliminaries: Gabions and Reno mattresses should only be used in sandy

bottom rivers with small bed load discharge (to reduce abrasion effects);

They allow the reduction of block sizes (compared to riprap); There is not enough experience on the durability of this solution.

How big should blocks be to face shear failure ? How thick should the mattresses be ?

Blocks should be able to withstand a velocity 2U Ucr 3.5U(after Hancu 1971; after Ciew 1995); U – approach flow velocity; Ucr – design velocity.

Gabions and Reno mattressesGabions and Reno mattresses

25

Gabions and Reno mattresses without bitumen (Agostini et al.)

Gabions and Reno mattresses with bitumen (Agostini et al.)

26

How wide should mattresses be to face edge failure ? How deep should the mattresses be placed to face bed-form

undermining ? Use the same criteria as for riprap; Place the mattresses on adequate filter.

Artificial riprapArtificial riprap

What is this ?

+ ……

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Apart from in Japan, there are very few examples of the use of artificial riprap as a bridge pier countermeasure.

Artificial riprap is an alternative to riprap where it does not exist with proper dimensions or it is very costly.

Design criteria are the same as for riprap with the exception of block sizing (in the case of toskanes and dollos). Ruff and Fotherby 1995 suggested the equation:

where: De = equivalent spherical diameter of toskane/dollo; Dp = projected pier width; Ut = 1.5ClCsChU (Cl = coefficient of pier location; Cs = coefficient

of pier shape; Ch = coefficient for the level of the top surface of the toskane layer).

0

5.0

00 1255.0

ghUFr

hD

sFr

hD tpe

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CableCable--tied blockstied blocks

System typically consisting of concrete blocks or slabs interconnected with steel cables. The blocks may be unstable by themselves but the mat is capable of withstanding large forces.

The solution has already been used in the USA.

Weight of blocks per square meter:

Height of blocks:

2

1200 U

ssW

1gWh

sb void fraction in

the mat

29

ConcreteConcrete--filled bags and matsfilled bags and mats

Concrete or grout-filled bags are sacks that are filled with concrete and stacked to form an armour layer. Typically, the mat is strengthened with steel cables.

The solution applies in sandy rivers only and in absolutely exceptional circumstances, due to the lack of angularity of blocks.

Each “pillow” should be D50 = 1.2Dr50 (Parker et al. 1998). For the rest (thickness, area to be covered, depth of installation,

filter), the recommendations for riprap will probably apply.

30

Notes on flow altering devicesNotes on flow altering devices

Piles placed upstream of the bridge pier for the purpose of protecting it from scour, by deflecting the high-velocity flow and creating a wake region behind them.

Effectiveness of sacrificial piles depends on number of piles; their protrusion from the bed, geometrical arrangement, approach flow angle, , flow intensity, U/Uc.

The results presented hereafter were obtained by Hadfield 1997.

Sacrificial pilesSacrificial piles

31

The best configurations (see below), extending up to 40% of the flow depth, reduce scour up to the percentages indicated in the table (in parenthesis for rectangular pier).

U/Uc

Few field applications of sacrificial piles are know; Sacrificial piles are recommended only when flow remains aligned and

the flow intensity (sediment transport rate) is relatively small.

32

CollarsCollars Collars are thin horizontal plates

attached to the piers assumed to shield the sediment bed from downflow and horseshoe vortex.

Effectiveness of collars depends on collar dimension (diameter); position of the collar relative to the bed

For cylindrical piers, Dcollar = 2Dp, under clear water, the collar effectiveness is 50% when the collar is 0.2h0 below the bed. It reduces to 20% when it is flush with the bed (Chiew 1992).

For cylindrical piers, Dcollar = 3Dp, under clear water, the effectiveness increases to 50% when it is flush with the bed (May et al. 2002).

Collars have only been tested for clear-water; they should not be considered for use under live-bed.

33

Flow deflecting vanesFlow deflecting vanes

These vanes are similar to Iowa vanes. They form arrays of vertical plates installed upstream of the pier.

The most efficient, seem to interact with the sediment bed rather than with the approach flow.

According to studies carried out at the University of Auckland, their best configuration is as follows:

Effectiveness can be as much as 50% for relative submergence of 5/6 (protrusion of h0/6), plate lengths of 1.5D and alignment angle of 30º, under live-bedconditions.

Promising countermeasure; deserves further investigation.

34

Permeable sheet pilesPermeable sheet piles

Permeable sheet piles are based on the principle of permeable dikes in rivers and snow fences, supposedly inducing deposition.

The best configuration seems to be as follows: Two 3D long panels forming an arrow pointing upstream; placed at an

angle of 90º; apex at a distance of 4D; Panels made of 3D x 0.25D slabs, covering 50% of the area; Top of the panels protruding 1D above the bed and forming a 10º upward

angle downstream; two triangular panels on the first and third slabs extending 3D

downstream.

Effectiveness 45% for cylindrical piers; 30% for rectangular piers, under live bed conditions. (Parker et al. 1998).

35

The technique involves the removal of fluid from the surface of the pier by internal suction.

Suction applied to the pierSuction applied to the pier

The only known study (Rooney & Machemehl 1997) claims that it is possible to eliminate scouring.

This technique seems quite unrealistic due to the need of driving the pump at field installations and the potential clogging of the holes.

36

Experimental study on the effectiveness of slots, bed-sillsand combinations of slot plus bed-sill

Carmelo GRIMALDI, 2005

37

Literature reviewOn slots• Effectiveness depends on: slot length, ls;

slot width, ws; sinking depth, zs; skew angle, .

• For zs 0, Tanaka and Yano (1967) + Chiew (1992) suggest slot effectiveness of 15% 30%, depending on zs, ls and ws. Performance increases as zs → 0.

• According to Kumar et al. (1999), the best effectiveness ( 30%) is achieved when ls > h0, for zs 0.

• For ls h0, Heidarpour (2002) has shown that the lower effectiveness is achieved when the slot is placed near the water surface.

• No field applications are known.

On bed-sills• Bed-sills are regularly used to mitigate bed degradation (5th failure

mechanism).

38

Experimental study

• 15 tests on the effectiveness of isolated slots. • ls = h0; ws = 0.2Dp.• Variables: sinking depth, zs; pier diameter, Dp.

• 15 tests on the effectiveness of bed-sills. Variables: distance between the pier and the sill, Ls; pier diameter, Dp.

• 3 tests on the effectiveness of combined bed-sill + slot.

Reported experiments

39

Experimental SetExperimental Set--upup

Tests were carried out at UBI and LNEC

Flume dimensions: 1 recess box:Length = 12.70 m Length = 2.50 mWidth = 0.8 m Width = 0.8 mDepth = 0.7 m Depth = 0.35 m

Flume dimensions:Length = 40.7 mWidth = 2 mDepth = 1.0 m

2 recess boxes:Length = 5 mWidth = 2 mDepth = 0.35 m

40

Two sands:

2.651.440,70Sand #2 (LNEC)

2.651.461.28Sand #1 (UBI)

sr

D

D50(mm)Material

Measuring equipment:• Point gauges installed on

rolling bridges

scour depthsfree-surface levels

• Leica Reflectorless Total Station TCR307 Topographical surveys

Design of the tests:• UDp/ ≥ 7000 (Franzetti et al., 1994);• h0/Dp ≥ 2 (Laursen and Toch, 1956; Breusers et al., 1977)• B/Dp ≥ 10 (Laursen and Toch, 1956)• Dp/D50 ≥ 50 (Ettema, 1980; Chiew, 1984; Breusers and Raudkivi, 1991)

• Cylindrical piers with rectangular slot, aligned with the flow direction;

• Uniform flow at the condition of beginning of sediment motion (U Uc).

41

Results on isolated slotsResults on isolated slots

100d

dd%r0se

se0sede

100A

AA%r0e

e0eAe

100V

VV%r0e

e0eVe

• zs/h0 = 1/3 provided satisfactory scour reductions in all cases (Best performances of 30% of dse).

• The slot acts from the beginning of the tests, sucking downflow and weakening horseshoe vortex.

--17.4--185.582660120C1--25.2--167.988931/6120C2

--19.2--144.78388090B1---0.0471.569179.08876-90B0

68.171.930.10.0150.441125.289201/390B3

--21.7--140.290571/690B2

---0.0791.666224.68765-120C072.377.025.60.0130.361133.18932190B4

65.465.127.90.0090.34387.572352/375A573.165.034.50.0070.34479.543221/275A465.467.931.00.0090.31583.869401/375A3

57.054.524.00.0340.758170.688081120C460.857.121.10.0310.715177.187961/3120C3

76.973.830.20.0060.25784.77670175A6

61.560.120.20.0100.39296.986701/675A2

34.648.49.90.0170.507109.47705075A1---0.0260.982121.45760-75A0

rVe(-)

rAe(-)

rde(-)

Ve(m3)

Ae(m2)

dse(mm)

T(min)

zs/h(-)

b(mm)Test

42

Results on isolated bedResults on isolated bed--sillssills

• Bed-sills placed close downstream the pier reduce scouring by cutting the lower part of the wake vortices (inside the scour hole).

• Bed sills do not act immediately at the beginningof the experiments.

• The smaller the distancebetween the pier and the bed-sill, the larger the effectiveness is.

• Best effectiveness of 25%of dse.

72.281.325.40.0220.311167.688390120C172.281.819.60.0220.303180.585750.5120C2

70.286.225.40.0140.217133.68901090B1

---0.0471.569179.08876-90B0

68.184.217.10.0150.248148.48877190B378.788.524.50.0100.181135.186950.590B2

---0.0791.666224.68765-120C0

59.679.315.00.0190.325152.18822290B4

65.481.112.70.0090.186106.05770275A476.983.314.80.0060.164103.45780175A3

40.561.012.10.0470.650197.487102120C4

59.575.318.00.0320.411184.287561120C3

80.887.214.50.0050.126103.858400.575A284.683.125.80.0040.16690.16016075A1

---0.0260.982121.45760-75A0

rVe(-)

rAe(-)

rde(-)

Ve(m3)

Ae(m2)

dse(mm)

T(min)

L/b(-)

b(mm)Test

43

Results on combined bedResults on combined bed--sill+slotsill+slot

1/3

1/3

1/3

zs/h(-)

120

90

75

b(mm)

49.091.30B5

41.5131.40C5

45.166.60A7

rde(-)

dse(mm)

L/b(-)Test

• The best configuration of a given isolated countermeasure was chosen to be combined with the other best one.

• Effectiveness increased to become of the order of 40% to 50%.

44

Riprap mattresses as a countermeasure against scour at Riprap mattresses as a countermeasure against scour at bridge abutmentsbridge abutments

Cristina FAEL, 2007

Tests on clear-water, corresponding to the most usual flow conditionsin flood plains, where abutments tend to be more frequent.

Tests for vertical-wall abutments, supposed the most unfavourable.

45

Riprap stone sizes (to face shear failure)• General formula for vertical-wall abutments (after Pagan-Ortiz (1991);

Atayee et al. (1993), Austroad (1994); Richardson & Davis (1995):

where Dr50 = median riprap stone diameter; h0 = flow depth; s = specific gravity of blocks; Fr = Froude number; C, n = coefficients.

Mattress thickness (to face winnowing)• With a suitably graded filter or filter cloth placed underneath

according to NZ Ministry of Works and Development, according Lagasse et al. 1997

nr FrsC

hD

10

50

Literature reviewLiterature review

502 rDt 100505.1 rr DtorDt

46

Layout of riprap mattresses (to face edge failure)

• According to Richardson & Davies 1995

• According to Eve & Melville 2000

• According to Melville & Coleman 2000

02hw

LBB

hD

hw r

0

50

082.15.0

sese dVdHw 5.1/

47

Experimental Set-up

• Tests were carried out at UBI.

Recess box:Length = 3 mWidth = 4 mDepth = 0.6 m

Flume dimensions:Length = 30 mWidth = 4 mDepth = 1.0 m

48

• Vertical-wall abutments were placed on the bottom of the recess, at its mid cross-section, protruding at right angle from the glass wall.

Abutment dimensions:

Length =

Width = 0.14 m

0.30 m0.51 m0.72 m0.93 m1.13 m

L/d2.50...9.42

• The recess was practically filled with natural quartz sand; riprap stoneswere placed around the abutments, on top or embedded in the quartz sand.

49

2.651.351.170,860.64Sand #2

2.651.1818.6615.6913.43Riprap #3

2.651.4410.917.485.28Riprap #2

2.651.485.793.592.65Riprap #1

2.651.461.871.280.87Sand #1

sr

D

D84.1(mm)

D50(mm)

D15.9(mm)Material

• 2 types of sand + 3 types of riprap stones were used in the tests

Measuring equipment• Point gauge installed on

a rolling bridge

scour depthfree-surface level

• Video camera installedinside the abutment

visualization of riprap instability

50

t = 3Dr50

Dr50 = 3.59 mm; Dr50 = 7.48 mm; Dr50 = 15.69 mm

on a filter

• tests started with a low flow velocity.• the velocity was successively increased while the flow depth was

kept constant until riprap stones began to move close to the abutment.

1st set of experiments (15 tests on riprap stone size)

51

• Evaluation of existing contributions

0.00

0.05

0.10

0.15

0.1 0.2 0.3 0.4 0.5 0.6F r

D r50

d

Austroads (1994) Present study

L/d=6 L /d =9.3 L/d=4.1

L/d=7.8 L/d=2.3

0.00

0.05

0.10

0.15

0.1 0.2 0.3 0.4 0.5 0.6F r

D r50

d

Pagán-Ortiz 1991 Atayee et al. 1993 Richardson e Davis 1995 Present study

Fr of the approach flow Fr of the contracted cross-section

• None of the equations adequately fits the experimental data.• Dr50 depends both on Fr and L/h0 (L/d).

d ≡ h0

52

• Critical value, Ic, of the approach flow intensity, Ic = (U/Uc)s, below which scour does not show up, for a given value of L/h0:

• Ic decreases with L/h0.

• It seems that Ic increases with h0/Dr50.

• An envelope curve was established that ensures the stability of riprap stones.

0.0

0.5

1.0

0 5 10 15 20 25 30

Lh

I c Sand Riprap #1Riprap #2 Riprap #3

Fael et al . 2006

Hager and Oliveto 2002

Envelope curve h ≡ h0

41

0521

hLI c

53

2nd set of experiments (42 tests on the minimum mattress thickness to face winnowing)

• Velocities were kept equal to 90% of those inducing scour at the abutments, as defined in the first set of tests.

t = 1Dr50; 2Dr50 …. 20Dr50

Dr50 = 7.48 mm; Dr50 = 15.69 mm

sand #1 and sand #2

without filter

54

• riprap mattresses should preferably lay on a filter;

riprap #3; sand #1

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

Lh

h se

h N = 1N = 2N = 3

riprap #3; sand #2

0.0

0.2

0.4

0.6

0.8

0 5 10 15 20N

h se

h

L/h = 9.42L/h = 7.75

• Increase of layer thickness decreases scour depth;

• t = 3Dr50 seems to be enough to stop scour if riprap acts as granular filter.

• Scour is negligible for N > 6 when it does not act as filter, but it is still present for N = 20.

h ≡ h0

55

3rd set of experiments (14 tests on the minimum plan dimensions of mats so as to avoid edge failure)

2.46 L/h 9.42

t = 2Dr50 = 31 mmriprap #3

on filter clothU Uc of sand

w = ?

56

• Riprap mattresses reduce the equilibrium depth of the associated scour hole. Reduction is not very significant, particularly, for short abutments.

0

2

4

6

8

10

0 5 10 15 20 25 30 35Ld

dsed

unprotected bedprotected bed

dy

abutment

wdx

Flow direction

wu

wd

w

w

20

25

30

35

40

2 4 6 8 10 12Ld

a

a =30.6º-2.98º

a =30.6º

a =30.6º+2.9º

• is practically constantirrespective of L/h0.

d ≡ h0

57

Variation of w/h0 with L/h0

• w increases with L/h0. The influence of h0/Dr50 cannot be addressed since it was kept 120/15.69 7.65.

• Richardson & Davis’s (1995) equation leads to safe predictions of w.

• A better predictor of w is suggested:

0.0

1.0

2.0

3.0

0 2 4 6 8 10 12

Lh

wh

Richardson and Davis 1995

Failure

Non-failure

w u

ww

ww

abutm ent

w d

h ≡ h0

53

0

0

2

hLh

w

58

END