Key_design_parameters in Foundation Design

Technical Instruction No. 1 Key Design Parameters for Rail Bridges During the course of interaction with various Railways, I have come across some design parameters adopted for construction of important bridges like one over Ganga at Mungheyr, Kosi and Brahmaputra at Boghibeel, which are not at all in consonance with Railway’s own codes. Indian Railways have been pioneers in design and construction of important bridges and have the unique distinction of bridging all the important rivers of the country for over 150 years. The design of well foundation is so well known and construction methodology is so well established that it should have been possible to design and construct them with much more confidence.

They have proved

themselves and therefore I am totally surprised to find various changes being introduced, which are being adopted in supercession of provisions in IRS Code of Practices. These so called modern practices have to be established but there is apparently no case to totally alter the old ones which are proven ones. I have also noted that costly pile foundations have been constructed where well foundations would have been more suitable besides being more cost effective. Example is Sone bridge on East Central Railway, where all the three existing bridges are already on well foundations. In view of above, following may be adopted henceforth: (1)

Founding level of wells below HFL:

(a)

Under normal conditions: Normal depth of Scour(D) below HFL should be maximum of: •

Using

Lacey’s

(Cumecs)

formula

for

Design

discharge

Q

{DL=0.47 (Q/f)⅓}

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f is silt factor

= 1.76 √m (m being diameter of bed material in mm over

scourable depth). The value is generally taken as 1.00 which is in itself quite conservative. •

For design discharge intensity in cumecs due to constriction of waterway on account of pier width, as per provisions of IRS Substructure Code {DL=1.34(q2⁄ƒ)⅓}}

•

Increase in depth of scour for design of foundation due to local scour around nose of piers = 2DL. This, however, needs to be checked from observed scour around piers as per hydraulic model study.

Scour depth

reported by model study need not be doubled as in case of calculations done for normal scour. •

Grip length = one third of 2DL. However adequacy of grip length should be checked for stability of well pressure including safe bearing capacity of soil with all vertical and horizontal loads as applicable under normal conditions.

(b)

Under Seismic conditions: Procedure same as above under normal conditions, but with design parameters like discharge, intensity of discharge, HFL etc. should be for seismic conditions as per provisions of IRS Substructure Code.

Adequacy of grip length under this

condition shall be checked with values of loads and moments for seismic forces as per dynamic analysis carried out by approved methods like one done by IIT/Kanpur or Roorkee etc. (2)

Thickness of Well Steining:

2

Thickness of well steining is always designed in consideration of sinking effort required to sink the well without taking recourse to use of kentledge or dewatering. The

sinking

effort

available

may

be

calculated

by

simple

calculation based on following, taking due account of buyoncy. f= Axw P

H1 + (w-δ) X H2 + (w-δ) H3 w H3 w

Where f = Average Sinking Effort in t/m2 A = Cross sectional area of well steining in (m2) w = Unit weight of plain concrete in t/m3 δ = Unit weight of water in 1 t/m3 P = Perimeter of well

in (m) H1

Values of H1, H2, H3 are as shown in the figure

___________

___________

H1 = height of well above water H2 = height of well below water level and upto bed level

H2 xxxxxxxxxxxxxx

H3 = depth of well below bed level, where skin friction applies.

xxxxxxxxxx

_____ H3

In limiting conditions, H1 = 0, H2 < of H3, hence H2/H3 is neglected. Hence f = A x w { w - δ} P w Taking weight of concrete as 2.3 t/m3 f = 1.3 x weight of steining per meter length of well (w) 2.3 Perimeter (P) This is nearly taken as 4 x w 7 P The skin friction of soil varies at different level and is dependent upon type of soil also.

This can be calculated by using following

formula:-

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F = ½ ka. (Z – 2C ka) tan (2φ) 3 Where F = Skin friction in t/m2 Ka

=

Active earth pressure coefficient

φ

=

Angle of shearing resistance of soil (degrees)

C

=

Half of unconfined compressive strength.

Z

=

Depth of foundation below Scour level (m)

γ =

Density of soil in t/m3

This is calculated below LWL. But empirical values are also safely used with fair degree of confidence. Stiff and soft

=

0.73 to 2.93 t/m2

Clay

=

4.88 to 19.53 t/m2

Very soft clay

=

1.23 to 3.42 t/m2

Dense sand

=

3.42 to 6.84 t/m2

Dense gravel

=

4.88 to 9.76 t/m2

For alluvium deposits, minimum sinking effort required is of the order of 5t/m2. Thus using the Formula available, sinking effort can be verified from (f = 4/7 W/P). 3.

Design of Steining: The normal Railway practice is to provide plain cement concrete.

The reinforcement provided in such cases is very nominal in the form of bond rods and lateral ties.

Bond reinforcement of about 0.12% of

sectional area and ties of about 0.04% of the volume per unit length is found to be adequate and should be adopted.

Check against tensile

stresses in steining causing cracking should be made using following formula both for seismic and non-seismic conditions. 4

Soil Pressure = 2 q (B-Sin B Sin2x – sin B cos2x) π F= M - P Z A F = tensile stress in t/m2 M = Moment in t – m A = Cross Section area in sq.m Z = Sectional modules of well in m3 q = Density of soil = 1.5 t/m3 P = Total lateral pressure in t/m2 The above was used in checking stresses in Mokamah bridge over River Ganga. Details in Technical Paper No.336 ‘Ganga Bridge at Mokamah’ by Shri H.K.L.Sethi. Check for Bearing Capacity: Most of deep foundations are on sandy beds at foundation level. The allowable bearing capacity can be calculated by q = 5.4 N2B + 16 (100 + N2 ) D q = Allowable soil pressure in kg/sq.m. N = SPT value. B = Smaller dimension of well cross section in metre. D = Depth of foundation level below scour level in metre. For calculating Bending moment both active and passive soil pressures around the well should be considered. A factor of safety usually of ‘3’ is taken. 4.

Low Water Level:

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Depth of foundation is always measured below LWL.

It is

customary to place the bottom of the well cap at LWL. This is done in order to enable inspection of the well cap. Low water level is determined from gauge levels of the river for as large period as possible particularly from consideration of as long working period as possible. From the available charts, LWL adopted should give ideally 150/180 days for working. Of course in river like Bramhaputra this is not available where maximum time available is 130/140 days. Thus LWL is not necessarily the lowest gauge level.

This is also

important so that the well cap can be cast without use of coffer dams etc. 5.

Well Curb: Most important element of well curb is the cutting edge. This is

designed from consideration of following. •

It should be able to cut through hard strata.

•

It should be able to stand on a single point in case of a sloping rock/large boulder, tree trunk etc. without getting damaged.

•

It should be able to withstand additional forces caused by occasional blasting.

There is no known methodology for the design. More common is to use a design which has proved itself for various important Railway bridges under very difficult conditions. For a typical circular and Double ‘D’ well for large well foundations, known design is available as per the enclosed sketch. Double D type is more prone to tilt and shift due to unsymmetrical shape and possible unequal dredging. Thus it is essential that the well is heavy in deep foundation.

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Only part of the well curb should be armoured, may be 1 to 1.5 metres level from the cutting edge level, as shown in the sketch. On above considerations, as mentioned, for bridge over River Ganga, Mungher, the maximum depth of foundation below HFL should not be more than 59.10 metres.

The thickness of steining could be

between 2.8/3.0 metres, which will give the sinking effort of over 5t/m3. Steining concrete could be of M20 (200 kg/cm2) to be treated as plain concrete although ordinary M15 (1:2:4) concrete has served very well in the past. The well curb is usually of M25 (250 kg/cm2) 6.

Pier Shaft: The pier is designed as column subjected to vertical forces and

moments for both seismic and non-seismic conditions. 7.

Calculation of lateral earth pressure for soils with cohesion: It is seen that in many case of back fill of soil having c and φ, only

φ is considered and active earth pressure coefficient for Rankien’s forumula is calculated accordingly. This is totally incorrect. In such cases, the earth pressure may be calculated using Bell’s equation obtained from Mohr’s failure stress circle. Principle shear stresses σ1 and σ2 will be: σ1 = σ2 tan2 (45 + φ/2) + 2 c tan (45 + φ/2) σ3 = σ1 tan2 (45 - φ/2) + 2 c tan (45 - φ/2) Using Coulumb’s and Rankien’s k factors to calculate Earth pressures at depth Z. Pa = γ z ka – 2 c √ka

where Z = 2c . γ.√ka

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Resultant R and its location y can be calculated by either neglecting tension zone or altering pressure diagram for overall depth of soil. (i)

R

= Pa (H-Z)/2 at y = (H-Z)/3 above base Or

(ii)

R

= Pa H/2 at y = H/3 above base

Where ka = coefficient of active earth pressure for Rankien = 1 – sin φ 1 + sin φ φ = Angle of Shearing Resistance in degrees. γ

=

c =

Density of soil. Cohesion

of

soil generally obtained from

unconfined comprehensive test. By neglecting tension crack(Z), the lateral pressure obtained is generally higher and is considered more conservative. 8.

Use of concrete blocks Vs. crated boulders for launching apron as well as for side slopes of guide bunds and approach embankments: The subject has been well debated. But general experience, which

has served successfully for over 100 years, is to use crated boulders in alluvium deposits for launching aprons. The size of crate is determined on the basis of water velocity to prevent it being swept and lifted away. The basic principle of the launching apron is best described as the one of a carpet which takes the shape of the scoured bed. The bottom of the apron is normally placed at LWL.

It is not possible to lay the apron

below knee deep water because of difficult manual working.

In such

cases, it is best to fill the area, if necessary by using suction dredgers. Because of these considerations, concrete blocks are not suitable in such cases, unless the strata is bouldery or non erodible. In such 8

cases

also,

it

is

customary to

secure

these

blocks by

proper

methods(chains anchored in the shore). They are very costly and may be impossible to execute, if quantities are large both from the point of view of casting and handling of concrete blocks.

Crated boulders launch

smoothly and being flexible, easily take the shape of the scoured beds and are not lost as in case of concrete blocks. For pitching guide bunds, slopes also, boulders in the form of grids are used.

For upstream and Down stream side in guide bunds and

upstream of approach bank, suitable inverted filters are used with provision of suitable weep holes above the ponding levels.

The usual

designs are available and have been found to be very successful. Approach banks must however be provided with proper longitudinal and cross drains so that side slope erosions can be prevented. The aforesaid directions may be brought to the notice of all concerned and suitable action must be taken, wherever needed. Ganga Bridge at Patna & Mungher and one at Kosi must particularly be checked and corrective action taken accordingly.

( R.R.Jaruhar ) Member Engineering 06.06.2005.

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