For optimisation of foundation depth of bridges

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TECHNICAL PAPER
SILT FACTOR FOR SCOUR CALCULATION AROUND BRIDGE FOUNDATION
R. K. Dhiman, VSM
1. INTRODUCTION
Foundation level for bridges are finalized based on the
hydraulic parameters and the nature of bed material
underneath. The subsoil is rated in term of silt factor,
which is a numerical value. It indicates the type of bed
material from clay to heavy sand. Gravel and Soil Mixed
with Boulder (SMB) falls beyond this range. Foundation
level is fixed below the scour level after considering
the grip length. The scour depth is determined by using
IRC-78 which incorporates the silt factor or on the basis
of results of model study wherever carried out. In bridges
the hydraulic parameters such as the discharge, velocity
need to be estimated accurately as it has direct bearing on
the depth of scour. This has financial bearing and affects
overall completion of bridge. Stress has been laid in this
paper to highlight the importance of scour and silt factor
used for calculation of scour.
2. SCOUR AND OVERVIEW
The design and construction of foundation of bridges is
linked with the realistic assessment of scour depth, both
global and local. The foundations are generally designed
to withstand the loads and moments transmitted by other
components of the bridge. They are also designed to have a
minimum grip length below the deepest scour level, which
is usually calculated based on various parameters. The best
way of assessing the depth of scour in a river is to observe
Abstract
Foundation of river bridges on alluvial soil is decided based on hydraulic data and subsoil strata. The subsoil strata is
represented by a numerical value called silt factor. This factor plays vital role as the foundation level depends upon soil
strata underneath, which is examined based on the bore log data. Bridges foundations are very costly due to various reasons
depending upon location. There is need to optimize the depth of foundation to a pragmatic level, which can be constructed
without undue delay as per construction practices. Any variation in foundation level at later date plays crucial role in the
overall cost of the bridges and affect the completion time of the project. It is highlighted that pre construction investigation
should be given more attention to avoid any variation in construction programme. Data analyzed based on investigation
need to be reviewed in term of construction trend in the area. The importance of silt factor has been discussed in this paper.
An accurate estimation of the same helps in completing the foundation in time without time and cost overrun. Border Roads
Organisation based on its experience recommend that silt factor upto ‘8’ can be used based on soil strata for calculation of
scour depth for bridge foundation in gravely/bouldery beds.
1
Chief Engineer, Border Road Organisation, Project Brahmank, E-mail: rkdhiman1964@gmail.com
the same during the highest flood period. Unfortunately
with the methods available it has not been possible to
approach the intended pier location during high floods and
observe deepest scour. Thus the Design Engineer generally
relies on the use of formulae for calculation of scour depth.
While the various available formulae have been known to
give reasonable results in respect of sandy strata, the results
havebeenerraticinothercases.Variousformulaehavebeen
originally evolved based on the study and observations
of particular type of strata, soil classification and water
flow regime. Over the years there has been an increasing
tendency to apply the same formulae for other types of
harder strata’s including conglomerates, large boulders
and soft rock. This has resulted in skewing of results and
arriving at 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 of
bridges are required to have their foundations taken deeper
than necessary depth due to the above referred approach.
Because of this, the time overruns in many cases have been
more than double with corresponding cost overruns. In a
number of well foundations, steinings have been damaged
due to extensive blasting during well 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.

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TECHNICAL PAPER
The substrata may consist of boulders, shingle, gravel etc.
either in loose form or cemented by a matrix, which may
be calcareous in nature. Such heterogeneous combination
of material with individual particle size upto two or three
metres does not easily lead itself to any logical assessment
or interpretation of scour using available tools. Substantial
reliance needs to be placed on observation of behavior of
structures built in the past coupled with reasoned judgement
of the decision makers in each individual case. Similar
situations may also arise in other parts. Conglomerate
strata are known to have been encountered in the plains
in various locations leading to dilemma in the matter of
proper assessment of scour. At the outfall end of the river,
the tidal effect needs to be considered. Here fine suspended
sediment deposits are common. The deposition process as
well as scour if any is also affected by changes in the density
of water due to salinity. If these aspects are not considered,
scour depth is generally assessed on conservative basis,
resulting in wasteful design. In such cases, the assessment
of scour needs entirely different perspective. The South
Indian peninsula is geologically more stable. The bed and
banks of the river are generally highly resistant to erosion.
The tendency for a gradation or degradation is insignificant
with such a diverse scenario considering the characteristics
of rivers flowing through the different parts of the country,
it is no wonder that diverse problems are being faced by
the Engineers.
3. SILT FACTOR
Silt factor plays an important role in determining the scour
depth and also the founding levels for the foundation of
the bridges. Due to lack of adequate bore hole data and
also various uncertainties associated therewith, bridge
engineers are confronted with a difficult job of choosing an
appropriate value of silt factor. This assumes importance
because the present code used for design of bridge
foundation guidelines caters for a maximum factor of upto
2.42, which is applicable for heavy sand only. (Table 1)
Table 1 Silt Factor of Various Sizes of River Bed Material
S.N. Type of bed material Mean Size of particle: dm in mm Silt factor – f = 1.76 x √dm
1 SILT Very fine 0.081 0.520
(less than 75µ) Fine 0.120 0.600
Medium 0.233 0.850
Standard 0.323 1.000
2 Sand Fine (75 to 425µ) 0.323 1.000
Medium (425U to 2.0 mm) 0.725 1.500
Coarse (2.0 m to 4.75 mm) 1.290 2.000
3 Gravel Fine (4.75 mm to 20 mm) 5.160 4.000
Coarse (20 m to 80 mm) 26.000 9.000
Sincethesiltfactorhasasignificantroletoplayinfinalizing
foundation depths, it suffices to say that identification of
correct silt factor poses a problem wherein the selection of
this important parameter is left to the judgment, discretion
and experience of the designer. For calculation of silt
factor in any type of soil, the soil strata is examined in
greater depth and values are calculated in Table 2 & 3. A
worked out example of silt factor is given below:-
Table-2 Sample-1
Dia. of sieve Weight retained gms Percentage retained Average size of
sieve
Col (3) x (4) (Mean diameter
Col-5 mtr 100
2.360 293 29.30 - -
1.180 313 31.30 1.7700 55.401 79.136/100=0.7913
0.600 172 17.20 0.8900 15.308
0.425 109 10.90 0.5125 5.586
0.300 39 3.90 0.3625 1.413
0.150 59 5.90 0.2250 1.327 K=1.76 m=1.5656
0.075 9 0.90 0.1125 0.101
Pan 6 0.60 - -
1000 100.00 - 79.136

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TECHNICAL PAPER
Table-3 Sample-2
1 2 3 4 5 6
2.360 49 4.90 - -
1.180 190 19.00 1.7700 33.630 78.424/100=0.7842
0.600 295 29.50 0.8900 26.255
0.425 253 25.30 0.5125 12.966
0.300 83 8.30 0.3625 3.008
0.150 104 10.40 0.2250 2.340 K=1.76 m=1.5587
0.075 20 2.00 0.1125 0.225
Pan 6 0.60 - -
1000 100.00 - 78.424
Silt factor = 1.5656 + 1.5587 / 2 =1.5622
The Sample calculation of mean diameter of silt is based
on mathematical expression of averaging.
3.1
Role of silt factor in estimation of scour
The scour depth is calculated based on following
formula.
Db =
Design discharge per meter width
Ksf =
Silt factor for representative sample of bed
material
Dsm =
Mean scour depth
This formula is applicable upto heavy sand only. For
material having heterogeneous stratification in the river
where material is comprises soil mixed with boulder, are
comparedwithactualobservationatsiteorfromexperience
on similar structure nearby and their performance. Model
study is also carried out for bridges on requirement and
scour depth is finalized accordingly. Trend of normal scour
calculation with a fixed discharge of 50 cubic mtr per sec
with a different value of silt factor using the IRC fourmula
has been shown in Table 4.
Table - 4
Silt Factor Dsm
0.50 22.91 Fine sand to fine gravel
0.60 21.56
0.85 19.19
1.00 18.18
1.25 16.24
1.50 15.88
1.75 15.09
2.00 14.43
2.25 13.87
2.42 13.54
3.00 12.60 Soil strata is gravely
4.00 11.45
5.00 10.635
9.00 8.74
13.00 7.73 Soil mixed with boulders
17.00 7.073
19.00 6.82
20.00 6.70
3.2
Foundation in Gravely / Bouldery Beds
Importance of silt factor for scour calculation can be
further represented in graphical from as indicated below
here it is seen that after a value of silt factor of 8 the value
of mean scour is not changing much and likely to almost
constants for a higher value of discharge.
Fig. 1 Silt Factor for Scour calculation
Most of bridges gravely/bouldery beds whether
already completed or under construction have faced
foundation problems especially in sinking of wells of
multi span bridges. Whenever problems of sinking of
well are faced, case is examined with reference to soil
strata actually encountered. The task of subsequent
review of foundation levels based on actual strata

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TECHNICAL PAPER
S.No. Name of
Bridge
Ksf
V
(m/sec)
Type of
founda-tion
Span
(m)
Strata available on
ground
Photographs Remarks
1. Pasighat (AP) 24 8 Well 762 Large size of
boulders available at
foundation level and
in river beds.
Pneumatic sinking
was done at this
bridge.
2. Siku (AP) 1.25 5.572 Well 480 Large size boulders
and khadir width
is about 1.2 Kms.
Bridge is located at
one side of channel
as two river joins on
U/stream.
3. Sissri (AP) 9.88 6.18 Well 135 Bridge is located on
bouldery beds. Both
side high bank.
4. Bakcha-chu
(S)
7.22 7.50 Well 120 Large size boulders
in beds. Both bank
has high vegetation.
5. Toong Bridge
(S)
1.26 21.93 Well 130 River bed is bouldery
and straight reach.
6. Sibokor-ang
(AP)
10.40 3.34 Open 110 River straight
channel and has
tendency to outflank.
Table-5 Silt Factor Taken and Actual Strata on Ground
encountered (review of silt factor) need reprocessing
of case. Data of silt factor actually used for few
bridges with gravely to bouldery soil strata in Border
Road Organisation is given in Table 5.
1.
Scour
has
been
recorded
less
than
15%
of
the
designed
value
in
all
the
bridges.
(Srl
no
1
to
6).
2.
This
is
due
to
less
erosion
of
bed
due
to
large
sized
of
material
laying
in
river
bed

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TECHNICAL PAPER
7. B h a g i r a t h
(UA)
1.50 9.87 Open 110 Bouldery bed river
8. Dalai (AP) 9 8.83 Well 130 Soil strata at
foundation level is
compacted very near
to rock.
9. Lohit (AP) 5.05 9.00 Well 410 Pneumatic sinking
was used due to
difficulty in sinking.
Rock encountered at
foundation level.
10. Diffo (AP) 7.72 5.78 Well 426 Bouldery beds and
channel straight and
flat.
11. S h i m o n g
(AP)
8.00 3.40 Well 140 Compacted bouldery
strata
12. Sime Korang
(AP)
2.00 8.79 Well 140 Bouldery beds
and bank are loose
soil mixed with
boulders.
13. Dundi (H.P) 6.792 6.20 Open 160 Large size of
boulders. Bridge
located at
downstream of
confluence point.
1.
Scour
has
been
recorded
less
than
15%
of
the
designed
value
in
all
the
bridges.
(Srl
no
7
to
13).
2.
This
is
due
to
less
erosion
of
bed
due
to
large
sized
of
material
laying
in
river
bed

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TECHNICAL PAPER
There are bridges as indicated in above table the value
of silt factor is not matching with the grade of material.
Hence deeper depth was planned which was not possible
to achieve. This need better examination for all bridges
under planning.
As per IRC formula given in code not applicable to
bouldery/gravely soil but same has been extended to soil
mixed with boulders.
There are certain important points which require
attention of bridge engineer for better planning of bridge
foundation.
Correct finalization of silt factor at initial stage will
a.
be helpful to optimize cost of the bridge. The will
also be helpful to adhere to original time schedule
as the there is no likely mismatch of strata.
Fig: 2 Comparison between scour hole in sandy and
bouldery beds
Cost of sinking of foundation can be adhered if there
b.
is no variation in soil parameter including silt factor.
Foundation depth is required to be finalized as
c.
per the construction technology available in the
country.
Infact there is need to take a stock of situation
d.
about the construction methodology, in such a way
that the proposal finalized should be executable on
the ground.
The completion of the particular project and related
e.
difficulties encountered be examined with reference
to silt factor, as this is the only one major factors
affecting the design scour. There are other factors
affecting scour around bridge pier viz. Whether
the flow is clear water flow or carries sediments,
change in depth of flow, shape of pier nose, angle of
inclination of pier, opening ratio, bed characteristics,
stratification and effect of flow parameters.
Based on the above information and experience
f.
‘BRO’ firmly believe that silt factor value upto
‘8’ can be considered with present formula as per
IRC:78 for calculation of scour in gravely / boulder
beds. This has also been explained in paper No.508
published in IRC Journal in 2004 as “Bouldery Bed
Scour-Proposed formula”.
4. RECOMMENDATIONS
There are number of bridges where completion got delayed
and in a few cases bridge could not be completed to due
difficulties in construction of foundation. This may be due
to incorrect assessment silt factor and design of foundation
or other construction difficulties. Few locations are as
under (Fig 3 4)
Fig: 3 4 Bridge foundation could not be completed at proposed location of bridges in JK due to non
matching of strata
Fig.4
Fig.-3

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TECHNICAL PAPER
Various steps to be followed to arrive at correct value of
silt factor are as follows:-
Keep a drilling record of the entire bore log and
a.
assess the value of silt factor upto foundation level
at initial stage.
During construction of foundation better picture
b.
of soil strata can be seen and accordingly the silt
factor can be further reviewed if it does not match
with the calculation as finalized initially during
sub soil investigation state. Maximum value upto
‘8’ can be considered for finalisation of scour
depth in gravely/bouldery beds. This has been
applied in no of Bridges constructed by BRO as
shown in Table 6.
Table 6.
Value of different silt factor value can be calculated
c.
for 2 to 3 of bore log details and average value can
be adopted.
Wherever that silt factor value is not assessable,
d.
the soil strata actually encountered during
execution is reviewed and practical aspect is kept
in view and final value is arrived at.
5. CONCLUSION
Silt factor plays an important role in finalization of
foundation levels. In case of difficulties faced in finalizing
level of foundation based on silt factor, model study can
also be reviewed if carried initially otherwise, experienced
gained at previous bridges can be dove-tailed for future
bridges for finalizing their foundation level. Efforts should
be made to assess correct value of silt factor to optimize
the depth of foundation and there will be no time and cost
overrun. However in case of gravely/bouldery soil the
value of silt factor upto ‘8’ can be considered as done in
Border Roads Organisation in various bridges constructed
in hilly areas.
REFERENCES
Dhiman RK “Effective Construction Management
i.
for Bridges” Dec-1996 International Association of
Bridges and Structural Engineers (IABSE).
Dhiman RK “Pneumatic Sinking–ACase Study” Indian
ii.
Highways Feb 1996 Indian Roads Congress (IRC).
Dhiman RK “Caisson launching A–Case Study–
iii.
1006” Civil Engineering and Construction Review
(CECR).
DhimanRK–EssenceofSoilFactorBridgeFoundation
iv.
– IGS Conference Baroda – 1997.
Dhiman RK “Construction Problem of Bridges in Hilly
v.
Region–A Review–1997” InternationalAssociation of
Bridges and Structural Engineers (IABSE)
Dhiman RK “DIMWE Bridge Foundation–A Case
vi.
Study” 4th
International Seminar on Bridge and
Aquetunnel - 1998
DhimanRK“WellFoundationConstructioninBouldery
vii.
Bed–A Case Study–1999” International Association of
Bridges and Structural Engineers (IABSE).
Dhiman RK
viii. “Bouldery bed Scour –Proposed Formula”
IRC Journal 65 Vol-3 Paper No. 508