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..SilOLIR gft SSULtr}ERV EgE} -
FR{}i)* Str* F SR&€Lit,4*" -
By
R.K. DHruaNE
Pap*r Nc. 5SS
l.
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A
6.
7.
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,
CONTENTS
i ntroduction
iicour Around Bridge Pier
Ii cour 3p gyg1r,.,i e.*,,
ilrese iit Practice for Determination of Maxirnum
Scour Around Piers
iled Material Characteristics of Bculdery Bed and
inter Rslated Factor for Scour
ljcour Oi:servations and Interpretation of Data -
Case Studies
Ilerivation of Farmu!a to Determine Scour Depth in
Bouldery Bed
f{ecorcmendations anci Conci usion
5C*
{ 1'" r}
in?
5{i.4
<'!r-,|
J-IL'
52i
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ABSTRACT
Bouldery beci basically consists of bed material of differeni size and shape ancl oi
ncn uniform stratification. Scour around bridge piers is gavernri by the bed rnateriai
around its vicinity and flolv conditions. Extreme care need to be exerciseri to estabiisii
maxirnum sccur depth to prevent undermining ciuring servic* life ci the bridge.
Estirnation of scour fbr bridge pier iocaied irr bouldery bed has been a chailenge fgr
bridge engineers. Consiruction of foundations has many times led to a time and cosi
overrun. as there is no empiricallralional f,ormula availabie to establish the same. Effbrts
havc becn made to cclh'ci the data of briciges aiready ccnstructed in bouidery bed anci
thi:ir be iraviour has been t-rbserved. Based on data coilection and analysis, an empirical
rclation i:as becn deve lcped to co-relate the scour rvith velocity in the cross section of
the rivci' and an atten:pt to develop fcrmuia in this type of strata has been made in
lhis papcr"
o lVriitcn commenis trn this Paper are ii^rvited and will be received up to 3 ist December
2{t*4
s llcputy {icr,eral lvia-::ager {P) Zaranj, Care F{Q DGBR, Seema Sadak Bharvan,
Delhi Cantt-110 CIC
2. 500 Dnttttrrx ox
1. INTRODUCTION
Stability of bridge foundation is dependent on scouring of river bed near
its vicinity. Extreme care should be exercised to establish foundations at
sufficient depths to prevent its undermining. Economy of bridge foundations
depend upon the degree of certainty with u'hich the scour is assessed. Estimation
of scour and foundation depth in rivers having bouldery beds pose a real challenge
to the bridge engineers as no method is presently available for prediction of
scour in such cases. Such uncertainty often leads to not only uneconomical bridge
foundation designs but also to expensive river training works and counter-
lneasures. In the absence of a rational formula, the present tendency is to apply
Lacey' s formula as applicable for alluvial bed with ajudicious choice ofvalues
of silt factor and unit discharge. The result so obtained is compared with past
experience before fixing up a value. This is not a very satisfactory solution
especially for situations where bridges are to be built on new alignments. To
work out the scour depth, consideration has to be given to general scour, local
scour, lateral channel, migration and degradation. Proper method for soil
investigation of bouldery strata is also an important factor for scour assessment.
Bouldery bed strata generally encountered is shown in Photo 1.
ij,*;
iir::i
i;i:i
t-!Bi
,*k il d :..t
:,ry! :-:r
YAtl7
V.;'.+
v*+*p*E
Photo l. Bouldery river bed
Border Roads Organisation has collected data on this aspect during last 9
years. Efforts have been made to derive a reliable and practicable formula for
estimation of maximum likely scour based on observations on number of bridges
in bouldery bed.
2. SCOUIT AROUND BRIDGE PIER
Scour is the erosive action of water in excavation and carrying away
3. Scoua n iici;roEnv.Sep -.,PRorosg.n Fonuula 501
materials from the channei ,bed. An.obstruction.s,uch-as a,bridge pier causes
interference in the flow'cf s**am,. rvhich'changes the JIow pattern at obstruction.
This resuits in deepening.the seour,hole around,the bridge,pier beyond the Ieve I
that rvoulC naturaily occur frcm,dqgradation and generalscour.: This is coutmoniy
termed as iocal scour:. The.florv,arourid:'the bridge pier is complex. As the
stream flow approach€s,the.pier,..adverse gradient'caused by the pier. drives a
portion of the approach flou'do*ryrwards,justahead of the pier,',A change in the
d"*n*urd flow velocity has a direct,effec-t on the rati,of scour and thus on the
depth of scour hole (Fi5,,1);
Il.f3Lil l*,
-',.'t'
-i.+'-*"* 5A:.Cf,tJ rs- l'lCtLE
ei. l r| ::lg *$"i rj'r € *t ('itF.4,'f ,€ x
:*r-:"tltt-l E F|r*
Fig. 1. Scour profile around
2.1. Local Scour -
Local scour is the local lorvering of the bed in the vicinity of hydraulic
struciur€s such as bridge pier, spur, guide bund, etc. Bridge pier locally distorts
the flow pattern by'increasing local,velocities or by inducing whirls, eddies and
r,'ortices etc. resultin-s in increased sediment transporting capacity of the stream.
The bed particles are lifted up and carried away with the current.: This process
continues till the norrnal {ransponting capacity of the channel is restored (Photos
7. &, 3). This stage of dynarnic equilibrium may also be achieved when the
armouring of the bed reaehes a limit. When,upper particles of,the bed can no
more be dislodged by the strearn action, further development of scour ceases.
The finally attained scour:depth is known as maximum or limiting scour. The
pllenomenon of local scour is very complex due to large variation in the field
conditions, besides numerous variables describing the flow, fluid and sediment
{:r.;{1F{
4. 542 DHtuex oN
r lran
.'.:ra. a
iii:iti;^Yi-f
. ::'i;F{$;;i;i1i;r:
::/j.;.4*:.4Ma
1'za,,: ';i *.:'::-:
Photo 3. Local scour around pier in bouldery bed
characteristics, the channel and the pier geometry etc., which have their own
cffect ol1 this phenomenon. The efforts made by designers and research scholars
in this regard are broadly classified as below :
Use of empirical formulae for estimation of scour depth.
Laboratory investigations by research scholars to gain insight into
the mechanism of scour around a bridge pier, the various parameters
entering into the problem and their effects and estimate scour depth
in terms of known variables such as depth of flow, velocity, grain
size, geometry of the pier and other variables.
i
.i
t.
(a)
(b)
Photo 2. Lacal scour around pier in sandy bed
5. Sqoun r.: Bor-'Loe*y Bnn _. Pnoposen F:onuula 503
(ci Prototype observations of scour with a view to investigate model
and place the conclusions from laboratory siudir:s.
3. SCOUR AN OVERVIEW ,.i:
The design and construction of foundations of bridges is linked to realistip
assessment of scour depth, both global and local. The foundations are genera|ly
designed to withstand the loads and moments transmitted by the other
components of the bridge. They are also designed to have a minimum grip
Iength below the deepest scour level, which is usually calculated based on various
parameters. The best way of assessing the depth of scour in :r river is to observe
the same during the highest flood period. Unfortunately with the methods
available in the country it has not been possible to approach the intended pier
location during high floods and observe the deepest scour. Thus the design
engineer generally relies on the use of empirical formulae for calculations of
scour depth. While the various available formulae have been known to give
reasonable results in respect of sandy strata, the results hirve been erratic in
other cases. lv{oreover the various formulae have been origirrally evolved based
on the study and observations of particular type of strata, soil classification and
water flow regime. Over the years there has been a incri:asing tendency to
apply the same formulae for other types of harder strata including conglomerates,
large boulders and soft rock. This has resulted in skewing of,results and totally
unrealistic scour value in extreme cases. While, fortunately in India, there has
not been many cases of failure of foundations due to scour, a large number cf
bridges are required to have their foundations taken deeper than necessary due
to the above referred approach. The consequences to this, the time overrun in
many cases have been more than double with corresponding cost overrun. In a
number of well foundations, steinings have been damaged due to blasting while
sinking, necessitating extensive repairs. In a few cases, the wells had to be
rejected because of extensive damages. The situation is acute while dealing
with conglomerate strata, particularly encountered in the rivers flowing through
the foothills of Himalayas. The substrata may consist of boulders, shingles,
gravels etc. either in loose form or cemented by a matrix, which may be
calcareous in nature. Such heterogeneous combination of materials with
individual particle size upto two or three meters does not easily lend itself
to any Iogical assessment or interpretation of scour using available tools
(Photo 4).
Substantial reliance needs to be placed on observations of behavior of
structures built in the past coupled with reasoned judgernent of the decision
makers in each individual case. Similar situations may also arise in other parts
of the country as well. Conglomerate strata are known to have been encountered
6. 504
iriiiiitli{x&;x
Photo 4. Lat'ge size of boulders encountered during tht
in a boulde:'Y bed
in the plains in various locations leading to dilemm'
assesslnent of scour'
1. PRESENr I',RAcrIcE ooto
ifJf$|tt-flt""
The theoretical method recon'lmended to er
based on Lacey's formulae IRC:78-2000 is repr
dsnt : 1.34 (Dbz lf)ttz
Where "dsm' is normal scour depth belor
Where Db: intensity of discharge in cum''
be the maximum of the following'
(i) l'he total discharge divided by the effective '
abutments 0r guide bunds;
(ii) J.lte value obtained taking into accoullt any co,
throughaportionofthewaterrvayassessedfron.
cross section of the river;
(iii) Actual observation, if anY'
f : Silt factor given by the expression
1.76tr m
where
.m, is the weighted mean diameter of the bed material in millime'
7. Rt
160.735
150.780
145.360
141.180
130.730
125.000
CN-
!. lJt-
Plctor1al Descrlptlon Strata descrlptlon
Strata comprising of
sand sllt mlxed wlth
60t boulders slze
varylng from 300 to
600 mm
Sandy soll mixed
r+1th 60t boulders
size varying from
600 mm to 900mm.
60t red clay mlx
Sandy soll mlxed
wlth boulders
varylng from 300 mm
to 1500 mm.
Compacted strata
comprlslng of red
clayey sandy soll -
401. Boulders upto
300 mm - 30t.
Boulders 300 to ?00
mm = J[$.
{)N - PNqf ATtc s I.NX,K
]I,A 505
1
,
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Irurther IRC:78-2000 recommends that if a river is fl;rstry in nature and the bed
does not lend itself readily to the scouring effect of floods, the method given
above for calculating maximum depth of scour shal.t be assessed from actual
observations. However for bouldefy bed there is no rational or empirical
formulae available for use.
5. BED MATERIAL CI{ARACTERISTICS OF BOULDERY BED AND INTER
RELATED FACTOR FOR SCOUR
The size of the channel material is important at low veiocity, the scour
depth is less for a bigger size boulder since it is difficult for slow moving water
to pick up and carry the large size boulder. This may eventually result in an
armoured bed rvithin the scour hoie, slowing or stopping of erosion process
until higher velocity scours the armored layer. Soil strata generally available in
bouldery bed is indicated in Fig. 2. Various interreleited important parameters
which affect the type and depth of foundation are type of strata, design discharge,
silt factor and soil parameters. The important aspect affecting the scour are as
under:
:
1
during foundation construction in bouldery
(Typical)
Fig. 2. Soil strata encountered bed
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5.1. Type of Str*t*
Er.odible/nonerCIdeble beds lead to adopting foundation based on scourl
non scour criteria and also an entirely .different philosophy irr planning and
designing oibridge foundat!ons. The aspects ofweathered/fissureci rocks further
adci to the uncertainty in branching out to the two approaches described above'
In a,Jdition, the type of strata encountered during the actual execution is
invariably at variance from the one catered for during the planning stage' Thus
owing ro partially/entirely clifferent strata encountered during execution stage
at times necessitates in redoing the entire sub soil investigation (SSI), and/or
also adopting an entirely diff'erent type of foundation on the other extreme besides
eausing exceptional dela.v in order to ensure safety of the structure. on a
moder;te site, it tantamounts to raising/lowering of the foundations, the effect
of ,,vhich however becomes quite substantial, incase there is a significant raising/
lo,rering of the founding levels.
5.2. Design Discharge
'l'he design discharge for the foundation design is done based on various
ernpirical/rational forrnul,ae er,'olved quite some time back. T'he applicability
of ihese formulae is best suited for rivers flowing in the plains. It is seen from
experience that these empirical relations are not entirely dependable/reliable in
esiimatio' of discharge for hilly regions of the Himalayas and Northeastern
Region. Invariably this leads to an enigma regarding this vital parameter leading
to a doubt regarding the safety of foundations at alater date.
5.3. Silt Factor:
(a) Silt factor plays a significant role in finalising the scour depth and
also the founding levels for the bridge structures' Due to lack of
adequate borehole data and also various uncertainties associated,
the bridge engineers are confronted with a difficult job of choosing
an appropriate value of silt factor. This has importance because
IRC:78-2000 caters for a maximum silt factor of upto 2.42
(applicable for heavy sand) only. Though IS 7784(Pt-l) gives a
values beyond the range of 2.42, there is in discreet jumps of
4.75,g,12,15 &24 rather than a continuous spectra. However, in
the absence/identification of correct silt factor for boulderly bed,
there is a problem in the selection of this important parameter as
the same is left to the judgement, discretion and experience of the
designer. Also results obtained by the above formulae for bouldery
bed are erratic and impracticable.
9. Scoun rN Bouloenv Beo - Pnoposeo FoRlrrul,q s07
(b) Ttt understand the affect of silt factor, a special study was carried
out where the value of discharge was fixed (50 Cumees/rn) and
silt factor was changed frorn 0.5 to 20. The results are given in
Table 1.
Tanr,B 1. Nonnal.t Scoun Drprs FoR A Dtscs'rRcs or 50 Cuuecs/nr
It can be seen that small change in the value of silt factor has much
variation in normal scour depth and accordingly depth of foundation will change
substantially. In case there are number of foundations, the result will have
more impact in all over all cost. Accordingly based on these observations result
have been plotted in graphical fornrat in Fig 3.
From the graph it can be seen that normal scour depth does not have
nruch variation beyond silt factor value 8 for a particular value of discharge
per meter Iength
Sl No Silt Factor dsm
0.50 22.91
2 0.60 21.56
3 0.85 19.19
4 1.00 i 8.18
1.25 16.24
6t' 1.50 15.88
B 2.00 14.43
I
i
0
1
;
2.42 13.54
3.00 12.60
4.00 11.45
15 13.00 7.73
16 17.00 7.473
t7 I 9.00 6.82
t 8 20.00 6.74
10. 508 DrxuaN ox
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t.t
}utq fi|;t
AEJ i
Itlt'* r.r.xa !
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b
a
a
I
**.Fl
!ilt* lLEltll *
Fig 3. Mean scour depth vs. silt factor - a graphical representation
There is problem of sinking of well foundations in bouldary bed due to
construction difficulties and in case deeper depths are planned the same will
lead to cost and time overrun. Review of such cases has been done based on
soil strata actually encountered. The application of silt Lacey,s factor theory
has been critically examined for its application to bouldary bei and it has been
found that the theory can not be applied in present shape to bouldary bed.
However foundation level in bouldary bed can be finaiised based on past
experience in such strata and also on the basis of results of model study.
5.4. Factors affecting Scour Depth
In addition to the various unknown parameters and uncertainties mentioned
above, the problem assumes a bigger dimension because of lack of appropriate
formulae for finalising the scour depths. Scour pattern in various natural channel
in case of different soil starta are shown in Fig. 4.
(a) dsm <d Normal scour depth is less than depth of water
(b) dsm =d Normal scour depth is equal than depth of water
(c) dsm >d Normal scour depth is greater than depth of water
(d) dsm : Normal scour depth
d: depth of water in channel
various reasons connected with it are enumerated as under :
(a) The type of strata/bed ie erodiblity or nonerodibility has a direct
impact on deciding whether to finalise a foundation based on scour/
non scour criteria, and also to decide whether an open foundation
would suffice or a deep foundation would have to be catered for. It
11. Scoun N BouI-psnv Bep - Pnorosep Fon*ruu 509
I
g:Qil," lv{ I }C E E 1AttT}l ,ft S l+ Ltt i= r+s { rrt4,tl r+,*. *rn}
€ Cisl;' FrI:lg,€t tt 'f4i, 7*I. Sf,JLJLt}era s
$. Ll- tir 1,.itJl E-' , *fi I l-
Fig. 4. Typical scour pattern
would be in order to state that this significant difference in adopting
different philosophies and depth of two types of foundations have
direct impact on the cost of the structure.
(b) Scour depth calculations significantly depend on the design
discharge to be adopted for foundation design. Since the formulae
for design discharge are by themselves not applicable, the reliability
of scour depth calculations is thus questionable.
(c) The applicability of these formulae for rivers in hilly region with
bouldery beds is debatable and quite often disputed since these have
an origin from alluvial/quasi alluvial streams. This aspect has also
been highlighted in clause 7A3.2.5IRC 78: 2000.
(d) IRC 78:2000 also clearly states that these formulae are not
applicable to rivers with flashy nature.
(e) A close scrutiny of the formulae indicates that the uncertainties
associated with the selection of strata, design discharge and silt
factor as pointed out above gets further substantiated for various
anomalies in the adoption of these formulae for various reasons.
(0 Structures designed from scour considerations on the basis of above
formulae had led to unnecessary time and cost over run during
execution.
12. -)iti
/:;.:ri-rN.
Drirptnti oN
In addition to above, tirere are other factors such as whether the flow is
ciear or it carries sediments, depth of flow, angle of inclination of pier, cpening
ratio, etc. which have significant.'effect on scour depth.
6. SC{}UR OBSERVATIONS AND INTERPRETATION OF
DATA - CASE STUDIES
6. i . There are number of bridges in bouldery beds in our country.
Observations of' Ranga - I bridge is being discussed. Ranga-l river is basically
a tributary of the Brahmaputra river. Bridge is located on Kimin-Ziro road in
Arunachal Pradesh. Bridge was completed during 1968 on bouldery strata.
The salient features of the bridge are as under (Fig. 5):
(a) Length
(b) Deck Level
(c) HFL
(d) LwL
(e) LBL
(f) Foundation Level
(e) Type of foundation
(h) Dia of well
(i) Soil strata
[i) Year cf construction
90 m (2 x45 m)
RL 267.53
RL 254.00
RL 250.00
RL 249.115
RL 233.853
Well
9.14 m
Soil mixed
with Boulders
1968
r, '.. ,''.'' 1:l'. ':j':': :l:::'111:.ii:l]
Fig. 5. Gencral arrangernent ranga-I
13. Scor-rn rN Boulueny Brn - Pnoposeo Fonuula
Well founclation construction planning was done as per codal provision.
Based on the construction difficulties faced, the foundation level was reviewed
and well was plugged at RL 233.853 m in bouldery packed strata, where as
designed foundat ion level was 229.5 m. During construction, the advantige of
changed properties of the soil strata was considered. Soil starta of bouldery bed
are less erodible Bridge was completed and opened for traffic during Feb
1968. During June (1968)there was heavy flood with increased discharge about
1.6 times the designed discharge, which changed the pattern of flow around the
pier and was observed as oblique. This enhanced discharge caused heavy scour
around the pier foundation and reoriented the flow on right channel only.
Observations of scour reveal that there was increased scour from 1979 onwards
(Table 2 refers). Rehabilitation measures were taken to arrest the scour. For
this purpose, plain concrete block of 2.Ax2.Ax2.0 m size were laid around the
foundation as per practice in vogue. There have been instances when most of
the concrete blocks laid around the pier were either washed away or dislocated
due to water force. Scour remained within the designed limit.
Tanr,r 2. Ossrnvro Scoun Anourn Prnn
Year Observed Scour
From Lowest
Bed Level (m)
Designed Scour
From Lowest Bed
Level (m)
Remarks
1979 1.0
4.95
Protection of pier foundation
was done on different
occasion with PCC blocks to
arrest further scour
l98l 1.5
t982 1.5
I 983 2.0
1984 1.0
I 985 1.5
l 986 t.25
t987 1.5
I 988 t.9
t999 2.1
2000 2.2
To control the scour damage during pastyears PCC block were cast during
the years 1981, 1984 and 1985. Few of these blocks got dislocated and later on
all the blocks were interconnected using 20 mm steel wire rope (SWR). Under
pining qf the blocks was also done. In fact, more damage was noticed due to
oblique flow hitting the pier at an angle and on left bank there was very less
flow (Photo 5). There is sharp turn on down steam (d/s) leading to sudden
change in flow pattern. Detailed examination of the problem done during 2000
5ll
14. 512 Dgit*x ox
keeping in view the past scour and rehabilitation measures taken so far revealed
fc!lowing:
(b) All the pCC btocks
order to avoid the
eddies.
with the bed level in
avoid formation of
(a) FIow in the channel is oblique and maximurn flow is passing through
the right side span.
need to be made matching
obstruction of flow and to
it has been observed that inspite of erratic flow pattern in the river specially
on u/s the scour had been within limit and the bridge has behaved well so far.
However, it has been observed thar in case of bourdJry t.J;;; ; arge size of
particles in the river bed, more force of water is required to lift una'"urry-ti"
material to d/s' Scour hole was observed near the pier and more stress was
given to have streamlined/smooth flow under the entire length of the bridge
(Photos 6 u7 and Fig.6).
Fhoto 5. Damage to concrete blocks
Photo Bridge
i
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a
prer bank
15. Scoun N BouLoenv Beo - Pnoposeo Fonuula
Photo 7. Bridgc pier and left bank
BE/-.) /.1v' /2/.iM ,<EN{3.4'-.r
513
FY.'
trlflt csRaarv€ ln€a3ur<ss
-g A tt c;/r.- .'- t:5F,rC)G E
3Ot-l L.l)t::f< L'f Bl=.IJ
Le- r*-r :r r E) b,
ABrJ-r M E iJ-T
't.- R1<:'lfi +5tFF:
-"-' r- t E- ,.. L- {> cA'r' t {2
^J
f r-, rrr e x F rt LL
----1 cr r s<7 u r- E r:-r+s
Fig. 6. Plan showing bed of Ranga-I bridge
16. 5t4 DmueN ow
6.2. scour observations for Bridges under construction
In addition to above, there are number of bridges on bouldery beds under
construction, where regular record of soil strata is being compared with the
sub soil investigalion report during actual execution oi work. The scour
observations have also been made in these bridges. For pasighat Bridge, model
study was also carried out before start of work and on facing OitficultGs during
construction of bridge, review of foundation level was done in consultation
with Central Water Research Station, Pune (CWRS) and IIT Roorkee. and
final foundation levels were approved for implementation on ground based on
such reviewed due to large size of boulders encountered during the process of
well foundation sinking.
Revised Scour Level
a. Max. Design Flood Discharge
b. Design HFL
c. Model study carried out by Up Irrigation
Research Institute (maximum scour level)
d. Designed Maximum Scour level
e. Final level after review
41543 Cumsec
163.50 m
134.00 m
I 18.00 m
125.00 m
After the greater review of the soil strata and comparison of data, the
foundation depth was reduced from 50 m to 25 m. Bridge was started in iqSt
and still under construction due to over delay in sinking of well foundation
(Figs. 7 & 8 and Photos 8,9 & t0).
i'
.-'t-:--
( <-;..:.>)
'---{ -.--
i:::.t. $i.!:!... _ t:*,. dt..f:r_
Fig. 7. Bridge pier model prepared for bridge before start of construction
during model study
,
17. Scoun nq BouloBny Beo .*.pnoposeo FoRNt,L,q
!.gg-Er{g,:-
;:::f#;.ji;;t:
Fig' 8' X section of River siang showing position of river b,ed profile before
and after flash flood occurred on ll June 2A00
Photo 8. Scour actually recorded during the construction of bridges
5i5
r69
161
t6t
t6,
t6t
t59
t5t
Itl
rtt
r5I
t{t
t17
til
lct ,
Photo 9. Bridge pier in bouldery bed _ twin wells
18. 516 Dsrv..qx cN
The monitoring of the scour pattern reveals the following in bouldery
bed:
(a) Scour depth in bouldery bed should be constantly monitored with
proper records after construction of bridge to ensure the
serviceability and soundness of the bridge for designed life.
(b) Reasons for damages if any due to excessive scour should be
investigated and remedial measures be taken on priority.
{c) Documentation of the scour data in bouldery bed can save lot of
time and money in finalisation and construction of bridges.
(d) Scour record observations at each bridge site should be taken
judiciously as it will be helpful to optimisation of cost of many
bridges in future as due to present use of formulae there is sizable
and cost overrun for completion of foundation;
Similar problems have already been faced by bridges in bouldery beds.
After due consideration of Pros and cons of the scour in bouldery bed, a detailed
discussion was cione by the Core Croup in the depaltment and bridges were
selected for data collection. The guidelines for collection of data was issued to
the site staff. The data for every bridge was collected as per (Table 3). Data
collection was done with a systematic approach to correlate maximum scour
with other important parameters. In fact the data of scour observed and existing
hydraulic data was critically examined keeping in view the difficulties faced to
finalise the foundation level in absence of bouldery bed formula. There are
number of bridges constructed on bouldery bed in the country, where the
foundation levels for these were finalised on the basis of existing formula/IRc
Codes and have behaved reasonably well barring few where excessive scour
Photo 10. 480 m (60x8) long bridge under construction in bouldery bed
19. 517
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iras been reported. Since this practice irad been fcllowecl in various bridges irr
the couutry, an effort r.tas made tcl collect ihe design elata of these bridges as
these act as a live rnodeis'for observations and have fuced numbr:r of flash
f-loods lvithout any damage and abnormal scour around the piers. Based on
the, data of bridges, collected resuits were examined with refere nce to tlre
velocity and maximum scour below the lowest bed level and subsequent to
various reviews of foundation levels in each case. It is further menl.ioned that
r.vhenever the velocity is calculated based on the Manning formula, it takes into
consideration all the properties ofthe cross-section i.e. area, bed slope, rugosity
coefflcient and wetted perimeter but ignore shape of pier. Based on above
concepts certain assumptions are made for oirr data collection and analysis
apprt-rach.
T'his data was examined along with photographic records of the bridges.
Tirere is no substantial scour near the pier in case of bridges indicated in the
photographic records (Photos 1l to 15).
Photo 11. Scour Jammu-Srinagar
Photo 12. Bridge build during 1973 on bouldery bed - No scour
observed due to largc size of bed matcrial
21. Scoun N BouI-peny Beo . PRoposno Fonuula
pier Dalai
519
Photo 13. Scour observation around bridge
Photo 15. Lateral migration has becn observed in Ranga-Il trridge
22. 520 Dnritiax or
6.3. Present Practice for Boulderv Red Scour
It is rvell knarvn fact tirat soii scour depth in boulder.v ired does not foliow
regime conditions as in the case of sandy beds. Norv keeping in view the size
of bed material and bank conditions, the scour around each prier can be different
in case of longer span on a same cross-section of river having defined flow
pattern for years together. Accordingly, based on the obsr:rvations, different
foundations are placed at different levels in same bridge cc,nsidering the cross
section features. Necessary revierv of the final scour level should be made based
on the construction problems, toughness of strata and revision of soil parameters
subsequent to re-examination of the same. However, the sound engineering
practice should be the main aim of reconsideration for review of level if any,
Less scourable strata has been considered as engineering friendly and revision
be carried out keeping in view the safety requirement. Bouldery bed generally
consists of soil strata predominantly consisting of boulder size of 300 mm
above and remaining rnaterial is soil-sand-gravel-boulder-nratrix. This material
is embedded in disorderly manner in natural bed available for siting the bridge.
The site data is investigated Lrefore actual construction in the form of bore log
data. Aiso the sfrata actually encountered is generally cr:mpared to known
engineering properties of the material for finalising the scour level.
6.4. Assumptions for Bouldery Bed Scour
. Data of existing bridges in bloudery bed has been cotrlected. In case of a
few bridges, there has been lateral migration of bed instead of vertical cutting
indicating that there is less scour around piers. In fact this is due to armoured
bed around the pier. Also the results of model studies of various bridge piers
were also analysed. Based on the detailed analysis of data collected, the
following assumptions are made.
(a) A soil stratum in the river bed is predominantly boulders of size
300 mm dia and above.
(b) Scour observation is a site specific for each bri,Cge which depends
otr number of factors, Iike bed material, discharge and direction of
flolv.
Scour depth is a function of area of cross section (A) wetted
perimeter (P), bed slope (S), rugosity cofficient (N) which in turn
depend upon the total discharge per meter length of the cross-
section. Infact when velocity is calculated by Mannings formula, it
takes into account all the properties of the particular cross section
of river.
(c)
23. Scoun rN Bour-ornv Bpp - Pnopospo Fonuule 521
(d) Flow pattern in bouldery bed takes a sudden turn and change bed
slope. There is frequent gain and dissipation of energy. Scour depth
at a particular section is a continuous function of time.
Flow pattern in bouldery bed is turbulent unsteady and non-uniform.
Aggradations at particular cross-section while siting the bridge be
examined and same shall be taken care.
(g) Scour depth is to be measured beiow the lowest bed level in a cross-
section. ,
7. DERIVATION OF FORMIJLA TO DETERMINE SCOUR DEPTH IN
BOUI,DERY BED
Scour pattern of bridges and data collection thereon has been shown in
1Figs. 5 to 8, Photos 5 to l3 and proforma developed in B.R.O in Table 3). It
has been observed that whenever the existing formulae are used for bouldery
bed, the value of mean depth of scour (dsm) has been observed less than the
water depth. This indicates that there is no scour observed which is not possible
on ground (Figs. 9 and 10). Keeping in view this aspect, a thoughtful exercise
rvas made to check the value of actual date scour from the lowest bed level in
case of all the bridges constructed in bouldery bed for which was available
(r-4).
To derive the formula, following steps were tbllowed:
(al Data was collected as per performa given in the Table 3
(b) Data was further modified and represented as per Table 4.
(c) Value of designed scour taken for all the bridges and recently
measured scour were recalculated with reference to lowest bed
level. This has been done to check the scour from lowest bed
level. As the result normally obtained by existing formula are not
representative of actual, because scour generaliy occurs in bcluldery
bed. This is due to limitation of existing formula in vogue which is
also being applied for bouldery beds.
(d) Value of scour obtained from design data and actually observed
since construction of bridge was also compared.
Based on the inference, it has been observed that even after construction
of bridge , value of scour observed in bouldery bed is less than maximum
velocity value from its lowest bed level.
{e)
(0
24. 522
Dstuarq oN
= NIean depth ofscour
drn, ( D (Impractical)
K.1: Silt factor
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Irig 9. Cross section of bouldery river and general observation of scour
,loxltrATt.
GE NER./. L ST[)T M
STA'I TS
E, NT5 MOTION
Fig. 10. Concept of equilibriunr scour
In view of para (a) to (d), it has been observed that value of scour from
the lou,est bed can be safely assumed as equivalent to velocity value measured
lrom the lowest bed level as after this value there is a tendency to attain the
positioit of equilibrium scour in a river. There may be situation where cross
section of the river is irregular and one side is deeper and other side is at higher
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26. 524 DurnreN ox
level. To be on safer side, the lowest bed level be taken as ref,erence for
determ in ing scour value.
It may be pointed out that in USA aiso the reference point for ealaculation
of scour fbr foundation is lowest bed lr:vel. However. keeping in view our
country's system to keep the reference point as HFL only, the sarne can be
measured from HFL also.
It has been observed
bridges from the lowest bed
that the value of scour observed so far in all these
levels are wlthin or less than the numeric value of
Dmax:
V:
velocity of flow taken for calculation of design discharge for these bridges.
Actual scour depth (D) is proportional to Veiocity (V)
Dmax =KV
maximum scour depth from lowest bed level
maximum calculated velocity on the basis of "Manning
formula"
K depends upon shape of the piern bed material and bed slope.
For circular pier K:1.2 and for rectangular pier K: 1.3
Value of 'li' has been initially taken while considering the worse case of
scour observed as in case of Ranga -l bridge in Arunachal Pradesh (lndia). It
is mentioned that all the bridges considered for study are having circular well
only.
Tlie re sults obtained with this formula are more reasonable to result based
on the existing practice. Now while planning bridge foundations in bouldery
bed if the soil strata are predominantly bouldery, this formula can be adopted
and accordingly maximum value of scour below the lowest bed level can be
calculated. The proposed formula has been found fit to be used for such major
bridges in bouldery bed where large size of boulders are available. The other
bridges of this type in Border Roads are L,ohit bridge, Lai bridge, Tiding bridge,
Iphipani bridge and Siku bridge. All these bridges are Multi span arrangement
and on well foundations. It is also mentioned that validity of this concept has
also been checked with reference to all the bridges made in past on bouldery
bed in the organization.
27. Scoun rN Bor;l-oenv Beo - PRoposeo FoRturula
8. RECOMMENDATIONS AND CONCLUSION
s25
(a) Scour determination in bouldery bed need optimization as the present
folmulae have certain limitation for their applicability to bouldery beds; When
bridges are planned on river having large size of material it requires special
investigations for assessment of scour as same will have financial implications
and likely cost over run if optimum value is not finalized.
(b) Soil strata anticipated during the subsoil investigations and on actual
construction are at variance, it is desirable that special efforts are made for
collection of data at pre-construction stage to avoid variation later on. Any way
tougher strata if encountered subsequently should be considered as engineering
friendly and advantage be taken to review scour depth accordingly.
(c) It is understood that MORT &H has given bouldery bed scour project
to IIT Khargpur, Border Roads should also be associated with this R&D
scheme.
(d) Scour depth in bouldery bed can be better understood with regular
observations on prototype structures. Studies carried out subsequent to data
collection and existing bridges considered as live models, it has emerged that
results are quite satisfactory and further study on this are undertaken to ensure
wide applicability of this formula.
(e) Since the formula suggested in this paper has been developed based
on actual data of bridges in bouldery bed, the same should be applied on
requirement.
ACKNOWLEDGEMENTS
Author expresses sincere thanks to The Director General Border Roads
and Additional Director General Border Roads fbr their guidance for preparation
of this Paper.
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t
a
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t,.)t i .
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t J.
16.
17.
