Retrofitting for well foundation of a Railway Bridge subjected to scour

K.V.RAMA MEHER , I.R.S.E

Abstract: The Retrofitting technique to be adopted for strengthening the well foundation of

a Railway Bridge subjected to scour will be discussed in this paper. Two proposals of

retrofitting will be discussed ; the first one being Putting Piles around the well foundation,

which is based on the premise that the bearing pressure beneath the well would be very high

when subjected to MBG loading standard(1987) compared to BGML loading standard(1926).

There is no much difference in vertical loads between MBG loading standard and BGML

loading standard for Span of 18.3m but very large increase in moments. It is found that this

large increase in moments is not getting reflected as increase in bearing pressure, when the

grip length of the well is 8 meters or more. It is also found that there is no means by which

the full load on the well can be relieved and transferred to piles.

The proposal therefore was revised and a second proposal i.e. a shallow foundation is

suggested around the well to partly transfer the vertical loads; transfer all the moments from

the well to the shallow foundation for the depth of Shallow foundation . The shallow

foundation would also protect the well against Scour in the top 2.5 to 3 metres.

Introduction:

Bridge Parameters: The bridge in study is having 13 nos. of 18.3 m spans. The

superstructure is of steel girders without any sign of corrosion. The substructure (piers) is of

stone masonry and is in very good condition.The dimensions of the pier are approximately2m

width and 5m length at bottom with triangular cut waters . Height of the pier is

approximately 6m and the piers are having through bed blocks. The foundation is on single

circular stone well foundation of 3m diameter .The well is having a stone well cap of 1.2m

depth. The dimensions of the well cap are same as that of the pier.

Soil Parameters: The soil investigation report revealed that the soil is clayey in nature upto

30m with the top 10m having intermittent layers of sandy strata.

Site Conditions: The bridge is having only occasional water flows and the flow is confined

mainly to the central four spans. The central four piers are having scour of about 2 m and

this 2 m scour was filled up with boulders.

Depth of the Well: The bridge has been functioning well for the past 100 years. However

the depth of well is not known. For the stability of the well the equilibrium of moments is to

be satisfied. Indirectly the resisting moments can be equated with the applied moments to

calculate the minimum grip length (factor of safety one). However for a new well this factor

of safety is taken as 1.4.

Table 1

Loading

Standard

Applied

Moment(t.m)

Resisting

Moment (t.m)

Factor of Safety Grip Length

(m)

BGML (1927) 251.6 379.1 1 4.66

MBG (1987) 413.67 608.64 1 6

Assuming a physical scour of 2m the depth of well would be minimum 6m to 8m.

The well depth of near by bridge with similar spans and having single 3m dia circular

well foundations is 13m. However the nature of soil at this bridge is entirely different from the

bridge under study.

From the above it was concluded that well depth may be minimum 8m.

Strengthening mechanism proposed for well foundation of the bridge: It was decided

to provide 6 piles all around the stone masonry piers and connect this piles to a Pile Cap

which in turn is connected to well cap by Dowel bars (Fig 1). The substructure would be

jacketed by approximately 300mm thickness of RCC and at the bottom 1.7m a haunch would

be provided. The principle in adopting proposed pile arrangement was that if the well is

checked for MBG loading (1987) the bearing pressure beneath the well would be very high

compared to the nature of Soil.

.

Bearing Pressure Beneath the Well: The variation of Bearing Pressure with various grip

lengths of the well are as follows:

Table 2

Grip

Length

Loading

Standard

Vertical

Load (t)

Moment

(t.m)

P1

(t/sqm)

P2

(t/sqm)

Pmax.(t/

sqm)

Pmin.(t/

sqm)

10 MBG 423.40 620.9 53.17 3.80 56.97 49.36

10 BGML 420.52 377.60 55.23 2.31 57.89 53.27

8 MBG 392.30 543.28 48.42 6.30 54.73 42.12

8 BGML 393.40 330.4 50.97 3.83 50.97 47.17

4.66 MBG 340.36 413.67 40.44 20.49 60.94 19.95

4.66 BGML 341.48 251.58 43.24 12.46 55.7 30.77

Jacketing 300 mm thick

Piles 1000mm dia.

EXG. WELL 3000mm DIA

FIG 1

The factor P1 reflects the bearing pressure due to vertical load. The factor P2 reflects the

bearing pressure due to moment. At Grip length of 8m the P2 reduces very much. The

maximum bearing pressure is a sum of P1 and P2. It should be noted that there is not

much difference in vertical loads between MBG (1987)and BGML(1926) loading standards.

A comparison of the max bearing pressure with the theoretical bearing pressures and the

available factor of safety are as follows:

Table 3

Depth of

well (m)

Loading Std. Pmax (t/sqm) Theroretical

bearing

capacity

(t/sqm)

Factor of

safety

10 MBG 56.97 105.80 1.86

10 BGML 57.89 105.80 1.83

8 MBG 54.73 102.20 1.87

8 BGML 50.97 102.20 2.01

4.66 MBG 60.94 96.20 1.58

4.66 BGML 55.7 96.20 1.73

The bearing capacity of soil has been worked out by using skempton’s formula .The soil report

has given the cohesion value as 4.50t/sq.m. However the value of SPT is between 15 to 30 and in

most of the cases it is greater than 30.There exists correlation between N value and unconfined

compressive strength of cohesive soil.Assuming unconfined compressive strength as 30.0t/sq.m

the C value works out to be 15.0t/sq.m from SPT-tests.Therfore an average C value of

9.75t/sq.m has been adopted. The value of N may be more reliable than the cohesion value given

in the soil report as the tests are conducted on disturbed Samples.

There is no difference in vertical loads between the two loading standards BGML (1926)

and MBG (1987) for span of 18.3m. However there is a large increase in the moments due

to increase in Horizontal Forces (Tractive Effort and Braking Forces).

If the well depth is 8.00m the bearing pressure for MBG loading is 54.73t/sq.m.If this value is

compared with the bearing pressure for BGML loading the difference is 3.76t/sq.m which is

marginal.Though there is large increase in moment by 212.89t-m the bearing pressure increase is

only 3.76t/sq.m. This is because, this moment is being opposed by passive resistance of soil

and creating bending stresses in the steining and not getting reflected as increase in bearing

pressure.

Failure of well foundation:-

The failure of foundation cannot be expected due to inadequate Bearing capacity but by scouring

action and leaching of mortar joints of well foundations. The increased moments due to MBG

loading would create more bending stresses in the steining especially in the well cap and steining

junction.In the event of failure of well foundation the well cap made of stone masonry would fail

in bending. The 6 nos. of piles driven around the well and connected to the well cap and pier by

dowel bars would then become futile. Some have opined that even in the event of failure of well

the dowel bars connected to the well cap and upto a height of 1.70m above the well cap would

give rise to a lot of friction and prevent the pier from collapsing .Friction would generate when

there is normal reaction perpendicular to the movement of pier.

Jacketing 300 mm thick

Piles 1000mm dia.

EXG. WELL failed

W

N

F

N

FIG 2

To prevent the pier from collapsing we should have W=F=µN. Where W is the dead load +live

load,F is the frictional force and N is the compressive reaction which is normal to the direction of

movement of pier. It can be easily understood from figure 2 that haunches with dowel bars

driven in to pier cannot offer a compressive reaction so as to prevent pier failure in the event of

well foundation failure.

There is no other mechanism by which the full load on the well can be relieved except to

strengthen the existing well foundation.

Revised Strengthening mechanism proposed for well foundation:

First Step: The physical scour at site in the central piers may be about 2 to 2.5m with an

allround width of 2m. An allround width of 1.50m to 2.0m with a depth of 2.50m to 3.0m

should be filled with reinforced cement concrete.It is not advisable to go to a Larger depth as

that may endanger the safety of the existing well . The reinforcement around the well would be

vertical and circular bars ; one as inner ring and other as outer ring.The spacing and size of inner

ring bars would be decided based on the moments developed in the existing stone steining. As

far as moments are concerned the new RCC ring should be treated as new well with existing old

stone well as core. The spacing and size of outer ring bars can be decided based on minimum

reinforcement considerations of shrinkage and temperature stresses. By doing this arrangement

we would create a shallow foundation around central well which is deep foundation. This

would reduce the bending stresses in the well steining and also protect the well foundation

against scour for top 2.50m to 3.0m which is most important from failure point of view.

Some have pointed out that since there is no dowel bars between shallow foundation and the

deep foundation, no vertical load transfer would take place . The well cap and the Pier are

approximately 2.0m x 5.0m( 5m inclusive of cut waters ) where as the well is circular with

3.0m Diameter (fig 3 ).The well cap is longer in the tranverse direction to traffic. Some load is

getting transferred to well from the pier through cantilever action in the well cap. This is 20% of

the total load (based on area calculations with triangular cut waters making angle of 30 deg ).It is

this vertical load which will be transferred to shallow foundation .The existing well would carry

80% of vertical load and also get relieved from the bending stresses due to moments . There has

been no increase in vertical Loads of MBG Lading Standard compared to BGML Standard and

therefore the well getting relieved from Bending Stresses is more important than Vertical Loads.

Second Step: A sheet pile should be driven around the piers atleast for the central piers to

protect pier against scour greater than 2.5m. The depth of sheet pile can be judiciously chosen

from practical considerations. If driving of sheet pile is not possible from practical

considerations like agencies not available to execute the work etc then flooring to be done to

prevent scour. This will reduce the hydraulic gradient and the effect of scour. The depth of

flooring can be decided on the consideration of hydraulic gradient to be achieved. The Shape of

flooring need not be in the conventional way, i.e., along the Length of the Bridge with Curtain

and Drop wall arrangements . Instead it can be designed in a circular manner around the well .

The radius of Flooring can again be decided based on Scour Considerations. Such an

arrangement (only dry stone circular pitching) has been provided to a Bridge with well

foundation which is functioning well from the past several years. The depth of this Circular

Concrete Flooring can be large near the well and tapering towards the outer radius.

Method of Execution of work : The work has to be carried out under Traffic Conditions

during running of Trains. The concreting for each well is to be done for a depth of 2.5m to 3m

with a width of 1.5m to 2m. The Consumption of Concrete can be around 75 Cum. Therefore it

is advisable to Plan a Traffic Block of about 4 hours for each Pier and do excavation and

reinforcement work in the first 2 and half hours. Since Large Concreting has to be done in a

5m

2m

3mFig 3

Short time it is advisable to use ready-mix Concrete by which work can be executed speedily.

The Traffic can be restored immediately after the completion of Concreting at normal Speed.

Conclusion: It is suggested that the revised method of retrofitting the well foundation be

carried out as discussed for transfer of vertical loads and Moments from the old well

foundation to the new shallow foundation and also for protection against Scour.

References:

a.) ARORA K.R, Soil Mechanics and Foundation Engineering.

b.) PUNMIA B. C , Soil Mechanics.