Draft for comments

GOVERNMENT OF INDIA MINISTRY OF RAILWAYS

GUIDELINES ON

WELL FOUNDATION FOR BRIDGES OVER INDIAN RAILWAYS

(REPORT NO. BS- )

DECEMBER 2005

ISSUED BY

RESEARCH DESIGNS AND STANDARDS ORGANISATION LUCKNOW - 226011

1

F O R E W O R D

20 years have passed since ‘Manual on the Design and Construction of Well

and Pile Foundations” was published in 1985. Since then lots of advances have

occurred in well design and construction. These developments have been captured

in recent A&C slips and editions of IRS Substructure & Foundation Code, IRC:78,

IS:456, and IRS Concrete Bridge Code. Very recently in 2005, Member

Engineering has also issued two technical instructions 1 & 2 for design and

construction of well foundation. All these developments, including learnings from

experiences of renowned ex-civil engineers like Vijay Singh, L. Singh and H.K.L.

Sethi have been captured in this draft guideline. We hope to get further suggestions

and comments from Zonal Railways for making it better.

I congratulate Mr. A.K. Gupta, Director/B&S/CB-II and his team consisting

of Mr. P.P. Singh, ADE/CB-II and Mr. Binay Kumar, SE/Design for coming out

with this draft.

( Lalloo Singh ) ED/B&S

2

C O N T E N T Sl.No. Description Page No.

1.0 Introduction 4

2.0 Comparison with Pile foundation 4

3.0 Types of Wells and its Suitability 6

3.1 Circular Well 6

3.2 Double D Well 6

3.3 Double Octagonal well 7

3.4 Rectangular Well 7

3.5 Twin Circular Well 7

3.6 Wells with Multiple Dredge Holes 8

4.0 Well Foundations in Existing Important Railway Bridges 8

4.1 Ganga Bridge at Mokameh 8

4.2 Jogigopha bridge near Jogigopha on River Brahmaputra 9

5.0 Design parameters 9

5.1 Founding Level of Wells Below HFL 10

5.2 Design of Well Steinning 12

5.3 Design of Well Curb 15

6.0 Material to be used 16

6.1 Concrete 16

6.2 Under Water Concreting 18

6.3 Steel 20

7.0 Well Sinking and Sinking Effort 21

7.1 Precautions during Well Sinking 21

8.0 Well Design and Soil Parameters 24

8.1 Preliminary Design 24

8.2 Design and Analysis of Well Foundation by Substructures Code

25

8.2.1 Design 25

8.2.2 Settlement of Well Foundation 31

References 37

Appendix-A (ME’s Technical Instruction No.2) 38

3

The guidelines on well foundation for Railway Bridges over

Indian Railways 1.0 Introduction:

Well foundations had their origin in India and have been used for hundreds

of years for providing deep foundations below the spring water level for

important buildings and structures. The technique of sinking masonry wells

for drinking water is very ancient and even today small drinking water wells

are constructed all over the country using the same methods as were

prevalent centuries ago. Well foundations were used for the first time for

important irrigation structures on the Ganga canal including solani aqueduct

at Roorkee (India), which were constructed in the middle of the nineteenth

century. With the advent of Railways in India, construction of a large

number of bridges across major rivers became necessary and it was

recognized very soon that much bigger and deeper well foundations were

required for their piers and abutments.

2.0 Comparison with Pile Foundation

i) Well foundations provide a solid and massive foundation for heavy loads as

against a cluster of piles which are slender and weak individually and are

liable to get damaged when hit by floating trees or boulders rolling on the

river bed in case of bridge piers.

ii) Wells have a large cross sectional area and the bearing capacity of soil for

this area is much greater than that of the same soil at the same depth for

bearing piles of small cross-section.

iii) Well foundations can be provided upto any depth if only open sinking is

involved and upto a depth of 33.5m if pneumatic sinking is required to be

done. Pile foundations are generally economical upto a depth of 18m and in

some cases for depths upto 27m.

iv) Piles can not be driven through soil having boulders. Logs of wood which

are very often found buried even at great depths also obstruct a pile. It is

possible to sink a well after over coming these obstructions.

4

v) The size of well foundations cannot be reduced indefinitely as the dredge

hole must be enough to enable a grab to work and the steining must have the

thickness necessary to provide the required sinking effort. It is, therefore,

not economical to use well foundations for very small loads and pile

foundations are more suitable for them.

vi) Wells are hollow at the center and most of the material is at the periphery.

This provides a large section modulus with the minimum cross-sectional

area. They can resist large horizontal forces and can also take vertical loads

even when the unsupported length is large. The section modulus of

individual piles in a cluster is small and cannot carry large horizontal force

or vertical loads when the unsupported length is considerable as in case of

bridge piers and abutments in scourable riverbeds.

vii) The bearing capacity of a pile is generally uncertain. In most cases, it is not

possible to determine the exact strata through which each individual pile has

passed. It can not be said with confidence in the case of bearing piles if they

have gone and rested on the strata taken into account while designing them

or if they are resting only on an isolated boulder.

In case of wells sunk by dewatering or pneumatic sinking, it is possible to

visually examine the strata through which sinking is done in its natural state

and the material on which they are finally founded. Even when sinking is

done by dredging, the dredged material gives a fairly good idea of the strata

through which the well is sunk. Drilled piles and caisson piles also have this

advantage over the driven piles.

viii) Masonry in the steining wells is done under dry conditions and the quality of

masonry or concrete is much better than in case of cast in situ piles for

which concreting is done below the ground level and in many cases below

the water level, where it can not be inspected. Even in case of precast piles,

the concrete is subjected to a lot of hammering and damage to it cannot be

ruled out.

ix) In case of wells raising of the well steining and sinking are done in stages

and a decision about the foundation level can be taken as the work

progresses piles and the strata conditions become known . In case of precast

piles, a decision about the depth has to be taken in advance. If the bearing

5

capacity of the piles at the design depth is found to be less than the

calculated value after testing, it may become necessary to redesign the

foundation and the piles of short length already cast may have to be rejected

or additional number of piles may have to be provided in each cluster. On

the other hand if the strata is too hard, it may not be possible to sink them to

the design depth and the piles may have to be cut which is costly and

wasteful. This does not apply to cast in situ piles.

3.0 Well Types and Their Suitability

The followings are the different types of well in common use in Indian

Railways as well as roadways. The advantages and disadvantages of each

type have also been discussed as below:

3.1 Circular well

This type of well is used most commonly and the main points in its favour

are its strength. Simplicity in construction and ease in sinking. It requires

only one dredger for sinking and its weight per sq. metre of surface is the

highest due to which the sinking effort for this well is also high. The

distance of the cutting edge from the dredge hole is uniform all over and the

chances of tilting are the minimum for this type of well. The well is

generally adopted for piers of single track railway bridges and those of

bridges on narrow roads. When the piers are very long the size of circular

wells becomes unduly large, which makes them costly and disadvantageous

hydraulically also as they cause excessive obstruction to the flow of water.

Nine metres is generally considered as the maximum diameter of circular

wells. Allowing cantilever of one metre on either side the maximum length

of the pier resting on this type of well is about 11 metres.

3.2 Double D well

This type of well is most common for the piers and abutments of bridges

which are too long to be accommodated on circular well. The shape is

simple and it is easy to sink this type of well also. The dimensions of the

well are so determined that the length and the width of the dredge holes are

almost equal. It is also recommended by some engineers that the overall

6

length of the well should not be more than double the width. The

disadvantage of this type of well is that considerable bending moments are

caused in the steining due to the difference in the earth pressure from outside

and water pressure from inside which result in vertical cracks in the steining

particularly in the straight portions where join the partition wall.

3.3 Double Octagonal Well

These type of wells are free from the shortcoming of double D-well. Blind

corners are eliminated and bending stresses in the steining are also reduced

considerably. They, however, offer greater resistance against sinking on

account of the increased surface area. Masonry in steining is also more

difficult than in case of double D wells.

3.4 Rectangular Well

These type of foundations are generally adopted for bridge foundations

having shallow depths. They can be adopted very conveniently where the

bridge is designed for open foundations and a change of well foundations

becomes necessary during the course of construction on account of adverse

conditions such as excessive in flow of water and silt into the excavation.

3.5 Twin circular well

This type of foundation consists of two independent circular wells placed

very close to each other with a common well cap. It is necessary to sink

these wells simultaneously to ensure that the cutting edges are almost at the

same level all the time. The wells have a tendency to tilt towards each other

during the course of sinking on account of the fact that the sand between

them becomes loose and does not offer as much resistance against sinking as

on the other sides. If the depth of sinking is small say upto 6 or 7 metres, the

clear space between the two wells may be kept 0.6 to 1 m to avoid tilting.

For greater depth of sinking spacing of 2 to 3 meters may be necessary.

Since it is necessary to sink these wells simultaneously it is obligatory to

have two sets of equipment for well sinking and in this respect they do not

offer any advantage over double D or double octagonal wells. They are,

7

however, advantageous where the length of the pier is considerable and the

sizes of the double D or octagonal wells become unduly large to

accommodate the pier. If , however, the soil is weak, the larger size of

double D or double octagonal wells may be required to keep the bearing

pressure on the soil within limits. Twin circular wells are advantageous only

when the depths of sinking is small and the foundation material is soft rock

or kankar or some other soil capable of taking fairly high loads. Design of

well caps for the twin circular wells also requires special care. Allowance is

made for relative settlement of the two wells and this adds to its cost. The

possibility of development of cracks in the pier due to relative settlement can

not be ruled out inspite of the heavy design of the cap except where the

wells are founded on rock or other incompressible soils.

3.6 Wells with Multiple Dredge Holes

For piers and abutments of very large sizes, wells with multiple dredge holes

are used. Wells of this type are not common in India. Wells of this type

were, however, used for the towers of Howrah Bridge. The size of these

wells is 24.8m x 55m and there are 21 dredge holes in each of them, In the

United States wells of this type are more common. The overall dimension of

the largest well are 60.5m x 29.6m and they support the piers of San

Francisco okland bridge. Each well has 55 square dredge holes of 5.2m x

5.2m size.

4.0 Well Foundations in Existing Important Railway bridges

4.1 Ganga Bridge at Mokameh

Well for the foundation has been fixed to be at 53 ft. 6 inch x 32 ft. size

with semicircular ends. It has two dredge holes 14 ft. D shaped. They are

covered by a 6 ft. deep well raft. The well curbs of the main piers were

made of mild steel and are 14 ft 10 inch in height and weight about 125 ton

each. The well curb were designed in steel instead of reinforced concrete.

1 HKL Sethi 1963

8

The wells were designed in mass concrete with 9 ft. thick steining. The

wells were provided with cement concrete plugs both at the top and at the

bottom, the intermediate portion being filled up with sand. A grip equal to

one third of the maxm. scour was required, while according to Gales a grip

of 65 ft. should be provided. Keeping in view the enormous discharge and

the importance of this bridge, a grip of 65 ft. was provided. This gave a

depth of 126 + 65 = 191 ft. below the HFL. Two wells next to abutments

were sunk 10 ft. deeper to counter the extreme scour conditions, which may

be experienced around them.

4.2 Jogighopa Bridge near Jogighopa on river Brahmaputra2

The conventional double –D shaped wells with outer plan dimensions of

17.00 x 11.00m were provided at all locations except abutments and two

adjacent piers. Abutments are supported on 6.0m diameter twin circular

wells with common well cap. Double D wells as well as circular wells were

provided with 2.5m thick diaphragms in longitudinal direction. The

thickness of steining, in double D wells is 2.80m with 2.5m thick diaphragm

and that in circular wells is 3.5m at base and 3.0m at the top:

Foundation Design parameters:

Design discharge – 90400 cumecs.

Design discharge intensity – 90 cumecs/m max.

Max. velocity of flow – 5.0m/sec.

Design founding level – RL (-) 36.75m

Buoyancy. Well resting on sand – 100%

Well resting on rock – 75%

Max. tilt – 1 in 80

Maxm shift - D/40

5.0 Design Parameters3

2 L. Singh 1998

3 ME’s Technical Note No.1

9

5.1 Founding Level of Wells Below HFL:

Rivers with scourable beds increase their cross sections when they are in

flood by the rise in the water level and also by scouring their beds thus

increasing the depth of flow. Rivers in regime, which flow through

incoherent alluvium and are free to adjust their width of flow and their depth

with equal ease, acquire an elliptical cross section in the straight reaches

with the highest flood level as their major axis.

a) Under Normal Conditions

The depth of well foundations is decided with respect to the maximum scour

and stability. The depth of foundation should not be less than 1.33 times the

deepest scour below HFL, and it should be so selected that it provides

necessary stability with respect to overturning and sliding. The method of

determining foundation depth is explained in following paras:

Normal depth of scour (D) below HFL should be maximum of:

• Using Lacey’s formula for Design discharge Q (Cumecs) {DL =0.473

(Q/f)1/3}

Where f is the silt factor for representative sample of bed material obtained

from scour zone, and value of f for different bed material is given in para

4.6.5 of IRS Substructure Code (applicable for medium sand) = 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.

Extra allowance should however, be made when the bank or a portion of the

river bed are non scourable.

• Before applying Lacey’s equation for scour depth, the width of the

channel should be measured and checked with the width calculated

by the following equation given by the Lacey:

L = 4.85√Q

10

Where L = Linear watering in metres, Q = maximum flood discharge in

m3/sec.

If actual width is lesser than the width given by Lacey’s equation on account

of restraint on the river due to non-scourable banks or if it is proposed to

construct a bridge having a lesser water way than that given by Lacey’s

equation, the scour depth worked out by Lacey’s equation should be

increased by multiplying with factor ,61.0

⎟⎟⎠

⎞⎜⎜⎝

⎛

cww where wc is the constricted

water way and w is the Lacey’s regime width. Alternatively; 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/f)1/3}

where q is the discharge intensity in cubic metre per second per meter width

and f is the silt factor

• When the bridge piers are placed in the flow due to obstructions

caused by them, the scour increases around them. The multiplying

factor is given in para 4.6.6 of IRS Substructure Code. 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 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

11

moments for seismic forces as per dynamic analysis carried out by approved

methods like one done by IIT/ Kanpur or Roorkee etc.

c) Low water level

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 Brahmaputra 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.

d) Check for bearing capacity

Most of deep foundations are on sandy beds at foundation level. The

allowable bearing capacity can be calculated by

q = 5.4N2B + 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.

5.2 Design of Well Steining:

a) 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

12

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.

Soil Pressure = π

)cos Bsin - (2 22 xxSinBSinBq −

AP -

ZMF =

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.

b) Thickness of Well Steining

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 buoyancy .

f = }w

wHHX

ww

HH

PAxw )()(

3

2

3

1 δδ −+

−+

⎩⎨⎧

Where,

13

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)

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

H3 = depth of well below bed level,

where skin friction applies.

In limiting conditions, H1 = 0, H2 < of H3, hence H2/ H3 is neglected.

Hence f = { }⎩⎨⎧ −

ww

PAxw δ

Taking weight of concrete as 2.3 t/m3

(P)perimeter (w) welloflength meter per steining ofweight

3.23.1 xf =

This is nearly taken as Pwx

74

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:

32tan)2.(/ 2

1 Φ−= CkaZkaF

Where,

F = Skin friction in t/m2.

H1

H2

H3

14

Ka = Active pressure coefficient

Φ = Angle of shearing resistance of soil (degrees)

C = Half of unconfined compressive strength.

Z = Depth of foundation below Scour level (m)

r = 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 alluvial 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).

c) Grade of Concrete

Concrete steining for the well is traditionally and conventionally treated as

MCC/ Plain concrete only and never as RCC. This is not withstanding the

fact that reinforcements are provided in the concrete but they are meant for

temperature, shrinkage, and bond. This has been the practice in Indian

Railways for ages and has stood well. The concrete is generally not richer

than M-15 (1:2:4)

5.3 Design of 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.

15

There is no known methodology for the design. More commonly 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, know 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.

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.

Well curb should have an offset (7.5sm in Jogigopha bridge) all around the

well steining. This is for the purpose of reducing skin friction during

sinking operation by keeping the soil close to the steining in disturbed

condition. Cutting edge inner angle was 24o upto 2m height in Jogigopha

bridge.

Well curb should be placed on a platform/ Island built on river bed. In

Jogigopha bridge, for example, island was created upto depth of 5m by

driving two rows of 6” sal ballies by 1 ton monkey at 0.8m c/c upto a depth

of 4m below river bed. Bamboo mattings were tied with the two rows and

the space between them were filled with sand bags, to build a 25m dia

islands by filling with sand by crane/ dredger. For wells in deeper locations

Caissons were fabricated and launched in the river. These were of mild steel

plate shells with angle struts, with 215 T of steel in 15 m height of Caisson.

Typical sketches of well curbs have been shown in Appendix A, as included

in ME’s Technical Instruction No.2.

6.0 Material to be used:

6.1 Concrete

In specifying a particular grade of concrete, the following information

should be included:

a) Type of mix, that is, design mix concrete as nominal mix concrete.

b) Grade designation

16

c) Type of cement

d) Maximum nominal size of aggregate.

e) Minimum cement content (for design mix concrete)

f) Maximum water cement ratio.

g) Workability

h) Mix proportion (for nominal mix concrete)

i) Exposure conditions – As guided by table No. 4 & 5 of IS-456:2000.

j) Maximum temperature of concrete at the time of placing.

k) Method of placing and

l) Degree of supervision.

The protection of the steel in concrete against corrosion depends upon an

adequate thickness of good quality of concrete. The free water cement ratio

is an important factor in governing the durability of concrete and should

always be the lowest value. Cement content not including fly ash and

ground granulated blast furnace slag in excess of 450 kg/m3 should not be

used unless special consideration has been given in design to the increased

risk of cracking due to drying shrinkage in thin sections as to early thermal

cracking and to the increased risk of damage due to alkali silica reactions.

(Clause N0. 5.2.1 of IRS CBC) When the designer wishes to have an

estimate of the tensile strength from compressive strength, the following

expression may be used.

fcr = ckf7.0

where,

fcr is the flexural strength in N/mm2; and

fck is the characteristic compressive strength of concrete in N/mm2.

(Clause No. 14.2.2 of IS-456:2000) Under water concrete should have a

very high degree of workability and confirm to IS:9103. The water cement

ratio shall not exceed 0.6 and may need to be smaller, depending on the

grade of concrete or the type of chemical attack. For aggregates of 40 mm

17

maximum particle size, the cement content shall be at least 350 kg/m3 of

concrete.

(Clause No. 708.3.1 IRC 78:2000) In case of plain concrete wells, the

concrete mix for the steining shall not normally be leaner than M-15. In

case of marine or other similar conditions of adverse exposure, the concrete

in the steining shall not be less than leaner than M-20 with cement not less

than 310 kg/m3 of concrete and the water cement ratio not more than 0.45.

(Clause No. 708.7.3 IRC 78:2000) The well curb shall invariably be in

reinforced concrete of mix not leaner than M-25.

(Clause No. 708.8.2 IRC 78:2000) The mix used in bottom plug shall have

a minimum cement content of 330 kg/m3 and a slump of about 150mm to

permit easy flow of concrete through tremie to fill up all cavities. Concrete

shall be laid in one continuous operation till dredge hole is filled to required

height. For under water concreting the concrete shall be placed gently by

tremie boxes under still water condition and the cement contents of mix be

increased by 10 percent.

(Clause No. 708.8.3 IRC 78:2000) In case grouted concrete, e.g. concrete is

used, the grout mix shall not be laner than 1:2 and it shall be ensured by

suitable means, such as, controlling the rate of pumping that the grout fills

up all inter stices upto to the top of the plug.

(Clause No. 708.8.4 IRC 78:2000) If any dewatering is required it shall be

carried out after 7 days have elapsed after bottom plugging.

(Clause No. 708.10.1 IRC 78:2000) A 300mm thick plug of M-15 cement

concrete shall be provided over the filling.

6.2 Under water concreting: (Clause N0. 14.2 & 14.2.4 of IS-456:2000)

When it is necessary to deposit concrete under water, the method,

equipment, materials and proportions of the mix to be used shall be

submitted to and approved by the engineer-in-charge before the work

started.

18

Concrete cast under water should not fall freely through the water.

Otherwise it may be leached and become segregated. Concrete shall be

deposited continuously until it is brought to the required height. While

depositing, the top surface shall be kept as nearly level as possible and the

formation of seams avoided. The method to be used for depositing concrete

under water shall be one of the following-

i) Tremie- The concrete is placed through vertical pipes the lower end

of which is always inserted sufficiently deep into the concrete which

has been placed previously but has not set. The concrete emerging

from the pipe pushes the material that has already been placed to the

side and upwards and thus does not come into direct contact with

water.

When concrete is to be deposited under water by means of tremie,

the top section of the tremie shall be a hopper large enough to hold

one entire batch of the mix or the entire contents the transporting

bucket, if any. The tremie pipe shall be not less than 200mm in

diameter and shall be large enough to allow a free flow of concrete

and strong enough to withstand the external pressure of the water in

which it is suspended, even if a partial vacuum develops inside the

pipe. Preferably, flanged steel pipe of adequate strength for the job

should be used. A separate lifting device shall be provided for each

tremie pipe with its hopper at the upper end. Unless the lower end of

the pipe is equipped with an approved automatic check valve, the

upper end of the pipe shall be plugged with a wedding of the gunny

sacking or other approved material before delivering the concrete to

the tremie pipe through the hopper, so that when the concrete is

forced down from the hopper to the pipe. It will force the plug (and

along with it any water in the pipe) down the pipe and out of the

bottom end, thus establishing a continuous stream of concrete. It

will be necessary to raise slowly the tremie in order to cause a

uniform flow of the concrete but the tremie shall not be emptied so

that water enters the pipe. At all times after the placing of concrete

19

is started and until all the concrete is placed, the lower end of the

teremie pipe shall be below the top surface of the plastic concrete.

This will cause to the concrete to build up from below instead of

flowing out over the surface, and thus avoid the formation of laitance

layers. If the change in the tremie is lost while depositing, the tremie

shall be raised above the concrete surface and unless sealed by a

check valve, it will be replugged at the top end, as at the beginning,

before refilling for depositing concrete.

ii) Direct placement with pumps – As in the case of tremie method, the

vertical end piece of the pipe line is always inserted sufficiently deep

into the previously cast concrete and should not move to the side

during pumping.

iii) Drop bottom bucket – The top of the bucket shall be covered with a

canvas flap. The bottom doors shall open freely downward and

outward when tripped. The bucket shall be filled completely and

lowered slowly to avoid backwash. The bottom door shall not be

opened until the bucket rest on the surface upon which the concrete

is to be deposited and when discharged, shall be withdrawn slowly

until well above the concrete.

6.3 Steel: (Clause No. 708.3.4 IRC 78:2000) For plain concrete wells, vertical

reinforcements (whether mild steel or deformed bars) in the steining shall

not be less than 0.12 per cent of gross sectional area of the actual thickness

provided. This shall be equally distributed on both faces of steining. The

vertical reinforcements shall be tied up with hoop steel not less than 0.04

percent of the volume per unit length of the steining.

(Clause No. 708.3.5 IRC 78:2000) In case where the well steining is

designed as a reinforced concrete element, it shall be considered as a column

section subjected to combined axial load and bending. However, the amount

of vertical reinforcement provided in the steining shall not be less than 0.2

percent (for either mild steel as deformed bars) of the actual gross section

area of the steining, on the inner face, a minimum of 0.06 percent of gross

20

area steel shall be provided. The transverse reinforcement in the steining

shall be provided in accordance with the provisions for a column but in no

case shall be less than 0.04% of the volume per unit length of the steining.

(Clause No. 708.6.1 IRC 78:2000) The mild steel cutting edge shall be

strong enough and not less than 40 kg/m to facilitate sinking of the well

through the types of strata expected to be encountered without suffering any

damage. It shall be properly anchored to the well curb. For sinking through

rock cutting edge should be suitably designed.

(Clause No. 708.7.3 IRC 78:2000) The well curb shall invariably be in

R.C. of mix not leaner than M-25 with minimum reinforcement of 72 kg/m3

excluding bond rods. The steel shall be suitably arranged to prevent

spreading and splitting of the curb during sinking and in service.

(Clause No. 708.7.4 IRC 78:2000) In case blasting is anticipated, the inner

faces of the well curb shall be protected with steel plates of thickness not

less than 10mm upto the top of well curb.

7.0 Well Sinking and Sinking Effort

7.1 Precautions During Well Sinking

The following precautions must be taken during sinking of the wells.

i) When the wells to be sunk close to each other and the distances

between them is not greater than the diameter of the wells, they

should be sunk alternately i.e. one sunk ½ the dia in advance of the

other as the wells tend to draw towards each other in case they are

sunk simultaneously. Similarly when two parallel rows of wells

have to be sunk with centers of each at about 1m apart one row

should be sunk before the other or they can be started on different

ends or from the center towards two ends. The purpose of this is to

disturb the least possible area of the soil in the vicinity of well at one

time. It is also advisable to sink the alternate wells in a row in

preference to sinking them one after the other.

21

ii) In sinking of wells joined together, for example, dumb bell shaped

wells, the excavation in both the dredge holes should be carried out

simultaneously and equally to facilitate even sinking.

iii) The sinking of number of wells commenced in one season should be

such that they can be sunk to atleast 66% of their depth before the

seasonal flood. Wells not reaching this stage before rains should be

brought to the notice of the engineers in charge for his advise about

protective measures to be taken e.g. provision of temporary cap.

Sand drilling etc.

iv) All precautions should be taken against possible damage to the

foundations of structures in the vicinity of the wells prior to

commencement of dredging of the material from inside of the well.

v) During sinking, there is tendency to dump all the dredged material

close to the well and only on one side. This causes appreciable

difference of pressure on the sides of the well which tends to lean

towards the side where material has not been dumped. It is therefore

to be ensured that the dredged material is never allowed to

accumulate near the well . Firstly, it should be dumped at the time of

dredging as far away from the well as possible and then it should be

kept being removed simultaneously. The water running out of the

excavated material should not be allowed to flow close to the well

steining. A temporary drain should be made to take away this water.

vi) In sinking a pair of wells through sandy strata there is a tendency for

the two wells to draw closely to each other. These wells may,

therefore get considerably titled. To prevent this timber pieces may

be introduced in between the steining of the two wells.

vii) Generally in case of abutment wells, there is high bank on one side

of the well. The well curb is usually cast by digging up a pit slightly

bigger than the dimensions of the well. This results in surcharge on

one side of the well, which tends to lean away from the bank. In

such cases, it is worth while to spend a little more money in digging

22

the original pit of sufficiently larger sizes leaving about 6.8 metres

clear distance round the well and by not permitting steeper than 1:1

slopes for the walls of the pit.

viii) Sometimes, in case of well situated in the river bed, the river stream

flow along one edge to the coffer dam made for the sinking of the

well. Generally, the dredged material is disposed off on that side

where derrick etc. are situated i.e. the edge close to the bank. This

causes adverse effect and the well tends to tilt towards the side on

which the river current is flowing. Arrangements have therefore to

be made for dumping the dredged material on the river current side.

ix) The sinking operations should be carried on with great caution

whenever cutting edge approaches the junction of different types of

strata. To control this boting chart should be consulted regularly.

x) When the well curb approaches a hard strata which dips at a

considerable angle the well may have a tendency to lean when being

sunk. This tendency should be prevented by supporting the well at

two or three places on its steining high the ground. A well should

also be secured against such possibility where the soil is fluid or semi

fluid in nature.

b) Sinking of wells:

The wells as far as possible be sunk true and vertical. Sinking should not be

started till the steining has been cured far at least 48 hours. A complete

record of sinking operations including tilt and shifts, kentledge, dewatering,

blasting etc. done during sinking shall be maintained.

c) Tilt and Shifts4:

As far as possible well shall be sunk without any tilt and shift. A tilt of 1 in

---------------------------------------------------------------------------------------------- 4 Manual on the design and construction well and pile foundation-1985 Cl. No. 15

23

100 and shift of D/40 subject to a minimum of 150mm shall be taken into

account in the design of well foundation (D is the width or diameter of well).

If greater tilts and shifts occur their effects on bearing pressure on soil

steining stress, change in span etc. should be examined individually.

d) Sinking of well by resorting to blasting –

Blasting may be employed with prior approval of competent authority to

help sinking of well for breaking obstacles, such as boulders or far leveling

the rock layer for square seating of wells, blasting may be resorted to only

when other methods are found ineffective.

8.0 Well Design and Soil Parameters

8.1 Preliminary Design

a) Shape and size of the well

i) The outer sides of the wells should be preferably be vertical. In

special cases small offset may be allowed.

ii) The horizontal cross section should satisfy the following

requirements:

• The dredge holes should be large enough to permit dredging.

• The steining thickness should be sufficient to transmit the load and

also provide necessary weight for sinking and adequate strength

against forces acting on the steining both during sinking and

services.

• It should accommodate the base of the sub-structure and not cause

under obstruction to the flow of water.

• The overall size should be sufficient to transmit the loads to the soils and

• It should allow for the permissible tilt and shift of the well.

iii) When a group or groups of wells are sunk, the minimum spacing

between them should not be less than 1m.

b) Forces Acting on the Well

The following forces which act on the well should be first calculated:

i) Dead load of the bridge.

24

ii) Self weight of the wells

iii) Live load

iv) Longitudinal forces

v) Temperature forces

vi) Water forces

vii) Wind load

viii) Seismic force

ix) Buoyancy effect

x) Earth pressure

xi) Skin friction

8.2 Design and Analysis Of Well Foundation By Sub-Structures Code:

8.2.1 Design

The design of well foundations shall be carried out for either of the

following two situations:

i) Wells surrounded by non-cohesive soils, below maximum scour

level and resting on non-cohesive soils;

ii) Wells surrounded by cohesive soils or mixed strata below maximum

scour level and resting on any strata viz. Cohesive soil, non–cohesive

soil or rock.

a) Wells resting on non-cohesive soils

For wells resting on non-cohesive soils like sand and surrounded by the

same soil below a maximum scour level, the design of foundations shall be

checked by both Elastic Theory and Ultimate Soil Resistance Methods as

given below which are based on IRC:45-1972 ‘Recommendations for

Estimating the Resistance of Soil below the maximum scour level in the

design of Well Foundation of Bridges.’ Elastic Theory Method gives the

soil pressure at the side and the base under design load, but to determine the

actual factor of safety against failure, the ultimate soil resistance is

computed.

25

The provisions given below shall not apply if the depth of embedment is less

than 0.5 times the width of foundation in the direction of lateral forces.

The resistance of soil surrounding the well foundation shall be checked :

i) for calculation of base pressures by the elastic theory with the use of

subgrade moduli ; and

ii) by computing the ultimate soil resistance with appropriate factor of

safety.

i) Elastic Theory

Step 1: Determine the values of W, H and M under combination of normal

loads without wind and seismic loads assuming the minimum grip length

below maximum scour level,

Where,

W = total downward load acting at the base of well, including the self

weight of well.

H = external horizontal force acting on the well at scour level.

M = total applied external moment about the base of well, including

those due to tilts and shifts.

Step 2 : Compute IB and IV and I

Where,

I = IB + mIv (1+2µ’ α)

IB = moment of inertia of base about the axis normal to direction of

horizontal forces passing through its C.G.

Iv = moment of inertia of the projected area in elevation of the soil mass

offering resistance = 12

3LD

where,

26

L = projected width of the soil mass offering resistance multiplied by

appropriate value of shape factor.

Note: The value of shape factor for circular wells shall be taken as 0.9. For

square or rectangular wells where the resultant horizontal force acts parallel

to a principal axis, the shape factor shall be unity & where the forces are

inclined to the principal axis, a suitable shape factor shall be based on

experimental results :

D = depth of well below scour level

m= KH / K : Ratio of horizontal to vertical coefficient of subgrade

reaction at base. In the absence of values for KH and K determined by field

tests m shall generally be assumed as unity.

µ’ = Coefficient of friction between sides and the soil = tan δ, where δ is the

angle of wall friction between well and soil.

α = DB

2 for rectangular well

= D

diameter.π

for circular well.

Step 3 : Ensure the following :

H > r

M (1+ µ µ’) - µ W

and H < M/r (1- µ µ’) + µ W

where,

r = (D/2) (I / m Iv )

µ = coefficient of friction between the base and the soil. It shall be taken as

tan φ

φ = angle of internal friction of soil.

Step 4 : Check the elastic state

27

mM/I not greater than γ (Kp – KA)

If mM/ I is > γ (Kp – KA), find out the grip required by putting the limiting

value mM/ I = γ (Kp – KA)

Where,

γ = density of the soil (submerged density to be taken when under water or

below water table)

Kp & KA = passive and active pressure coefficients to be calculated using

Coulomb’s theory, assuming ‘δ’ the angle of wall friction between well and

soil equal to 2/3 φ but limited to a value of 22-1/20.

Step 5 : Calculate

2

1

σσ } =

IMB

APW

2

'

±− μ

where,

σ1 & σ2 = max. and min. base pressure respectively.

A = area of the base of well.

B = width of the base of well in the direction of forces and moments.

P = M/r

P = horizontal soil reaction.

Step 6 : Check σ2 not smaller than 0 i.e. no tension

σ1 not greater than allowable bearing capacity of soil.

Step 7 : If any of the conditions in Steps 3, 4 and 6 or all do not satisfy,

redesign the well accordingly.

Step 8 : Repeat the same steps for combination with wind and with seismic

case separately.

28

ii) Ultimate resistance method

Step 1: Check that W/A not greater than σu/2

W = total downward load acting at the base of well, including the self

weight of well, enhanced

by a suitable load factor given vide Step 5.

A = area of the base of well

σu = ultimate bearing capacity of the soil below the base of well.

Step 2 : Calculate the base resisting moment Mb at the plane of rotation and

side resisting moment Ms by the following formulae :

Mb = QWB tan φ

B = width in case of square and rectangular wells parallel to direction of

forces and diameter for circular wells.

Q = a constant as given in Table 1 below for square or rectangular base. A

shape factor of 0.6 is to be multiplied for wells with circular base.

φ = angle of internal fricture of soil.

TABLE -1

D/B 0.5 1.0 1.5 2.0 2.5

Q 0.41 0.45 0.5 0.56 0.64

NOTE: The values of Q for intermediate D/B values in the above range may

be linearly interpolated.

Ms = 0.10 γ D3 ( KP – KA) L

Where,

Ms = Side resisting moment

29

γ = density of soil (submerged density to be taken for soils under water or

below water table)

L = projected width of the soil mass offering resistance. In case of circular

wells. It shall be 0.9 diameter to account for the shape.

D = depths of grip below max. scour level.

KP , KA = passive and active pressure coefficient to be calculated using

coulomb’s Theory assuming “δ” angle of wall friction between well and soil

equal to 2/3 φ but limited to a value of 22-1/2°.

Step 3 : Calculate the resisting moment due to friction at front and back

faces (Mf) about the plane of rotation by following formulae :

(i) For rectangular well

Mf = 0.18 γ ( KP – KA) L.B.D2 Sin δ

(ii) for circular well

Mf = 0.11γ ( KP – KA) B2.D2 Sin δ

Step 4: The total resistance moment Mt about the plane of rotation shall be

Mt = 0.7 (Mb + Ms + Mf)

Step 5 : Check Mt not less than M

Where,

M = Total applied external moment about the plane of rotation, viz, located

at 0.2D above the base, taking appropriate load factors as per combinations

given below :

1.1 D . . . . (1)

1.1 D – B +1.4 (Wc +EP + W of S) . . . . (2)

1.1 D +1.6 L . . . . (3)

1.1 D – B + 1.4 (L + Wc +EP ) . . . (4)

30

1.1 D – B + 1.25 (L + Wc +EP + W or S) ….. (5)

Where,

D = Dead load.

L = Live load including tractive/braking etc.

B = Buoyancy

Wc = Water current force

Ep = Earth pressure

W = Wind force

S = Seismic force

Note : Moment due to shift and tilt of wells and piers and direct loads, if

any, shall also be considered about the plane of rotation.

Step 6 : If the conditions in steps 1 and 5 are not satisfied, redesign the well.

Note : Notation, symbols given in the clause 3.0 of Bridge Substructure &

Foundation Code, Revised in 1985 are not applicable for the above

Appendix-V.

b) Wells resting on cohesive soils

For wells founded in clayey strata and surrounded by clay below max. scour

level, the passive earth pressure shall be worked out by C & φ parameters of

the soil as obtained from UU (unconsolidated undrained) test and for

stability against overturning, only 50% of the passive earth pressure will be

assumed to be Mobilized (Refer para 6.9.3).

In wells through clayey strata, the skin friction will not be available during

the whole life of the structure, hence support from skin friction should not be

relied upon.

8.2.2 Settlement of Well Foundation:

i) The settlement of well foundation may be the result of one or more of

the following cases:

• Static loading,

• Deterioration of the foundation structure;

31

• Mining subsidence; and

• Vibration subsidence due to underground erosion and other causes.

ii) Catastrophic settlement may occur if the static load is excessive.

When the static load is not excessive, the resulting settlement may be

due to the following :

• Elastic compression of the foundation structure;

• Slip of the foundation structure relative to the soil;

• Elastic deformation or immediate settlement of the surrounding soil

and soil below the foundation structure ;

• Primary consolidation settlement of the surrounding soil;

• Primary consolidation settlement of the soil below the foundation

structure.

• Creep of the foundation structure under the constant axial load; and

• Secondary compression of the surrounding soil and soil below the

foundation structure.

iii) If a structure settles uniformly, it will not theoretically suffer

damage, irrespective of the amount of settlement. In practice,

settlement is generally non-uniform. Such non-uniform settlements

induce secondary stresses in the structure. Depending upon the

permissible extent of these secondary stresses, the settlements have

to be limited. Alternatively, if the estimated settlements exceed the

allowable limits, the foundation dimensions or the design shall be

suitably modified.

iv) The following assumptions are made in settlement analysis :

• The total stresses induced in the soil by the construction of the

structure are not changed by the settlement;

• Induced stresses on soil layers due to imposed loads can be

estimated, and

• The load transmitted by the structure to the foundation is static and

vertical.

32

In the present state of knowledge, the settlement computations at best

estimate the most probable magnitude of settlement.

v) It is presumed that the load on the foundation will be limited to a safe

bearing capacity and, therefore, catastrophic settlements are not

expected. Settlement due to deterioration of foundations, mining and

other causes cannot, in the present state of knowledge, be estimated.

Such methods are not also available for computation of settlement

due to the slip of foundation structure with reference to the

surrounding soils and, therefore, not covered.

Wells Founded In Cohesionless Soil :

For wells constructed in cohesionless soils, the settlement due to dead load

of sub-structure will take place by the time the construction is completed and

the necessary adjustment in the final level can be made before erection of the

girder. In such cases, settlement shall be evaluated only for the dead load of

the super-structure.

Wells Founded In Cohesive Soil :

When wells are founded in cohesive soil, the total settlement will be

computed as per the provisions of clause 6.4. The settlements in clay occur

over a long period and time rate of settlement will be computed as per the

provisions of clause 6.4.2.3 of Substructure Code.

• Determination of bearing capacity

Bearing capacity for foundations in cohesive strata will be

determined in the similar manner as determined in case of

foundations in non-cohesive soils (para 6.3.1).

• Estimation Of Immediate And Primary Consolidation Settlements For computation of immediate settlement and primary consolidation

settlement, procedures provided in IS:8009 Part I and Part II –“Code

of Practice for Calculation of Settlement of Foundations”, shall be

followed.

33

• Estimation of secondary consolidation settlement may be computed as under: The Secondary consolidation settlement may be computed as under:

(a) If the load increment is more than (pc-po)

[i.e. p > (pc-po)], then

Ps = 01 e

Cc

+E log 10

o

c

pp

(b) If the load increment is smaller than

pc-po [ i.e. p < (pc-po)], the corresponding equation will be :

Ps = ( )

o

oc

ppp

LogEe

C Δ++ 10

0

.1

Where, Ps = Secondary settlement cc = Compression index

eo = Initial void ratio Pc= Pre-consolidation pressure Po = Initial effective pressure E = Thickness of clay layer

p = Pressure increment

• Time Rate of Settlement

The Time Rate of Settlement will be computed in accordance with

the provisions of IS:8009 (Pt.I) based on Terzaghi's One

Dimensional Consolidation Theory. In practice, the consolidation

settlements take place much faster than those predicted from

Terzaghi’s Consolidation Theory.

Following reasons partly explain the faster rates :

34

i) Three dimensional consolidation i.e. lateral release of excess pore pressure;

ii) Release of hydrostatic pressure outside the footing area; and iii) Horizontal permeabilities are usually much higher than the vertical.

Therefore, the rate of settlement should be corrected by factor of three to

five times faster. Actual rates of settlements in the area for similar cases

will be of great value for the accuracy of prediction for rate of settlement.

Note: 1. Settlement will be computed for the probable/actual sequence of loading

and correction for construction period will be allowed as per the provisions

of IS:8009 (Pt.I), clause 10.2, Appendix D.

2. While computing pressure increment below abutments, due care will be

taken to include the pressure increment due to earth fill behind abutment

also with the help of appropriate monograms (IS:8009-Pt.I, clause 8.3,

Appendix B).

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 Rankine’s formula 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.

Principal shear stresses σ1 and σ 2 will be:

σ1 = σ2 tan 2 (45 + Φ/2) + 2c tan (45 + Φ/2)

σ3 = σ1 tan 2 (45 + Φ/2) + 2c tan (45 + Φ/2)

Using Coulumb’s and Rankine’s k factors to calculate Earth pressures at

depth Z.

Pa = r z ka – 2 c√k where ka

2c τ

=Z

35

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 = PaH/2 at y = H/3 above base.

Where ka = coefficient of active earth pressure for Rankine = φφ

sin 1sin - 1+

Φ = Angle of shearing resistance in degrees.

R = Density of soil

C = 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.

36

References:

1. R.R. Jaruhar, Member Engineering- Technical Instruction No. 1 & 2 on Key

Design Parameters for Rail Bridges dated 06.06.2005.

2. Vijay Singh, BE (Civil), IRSE, Chief Engineer, India Railway – Wells and

Caissons, Nem Chand & Bros, Roorkee (U.P.), 1981.

3. L. Singh, Chief Engineer, North Central Railway, Allahabad- Salient Design

feature of Jogighopa Bridge, National Seminar on Bridge Engineering in

North East, Maligaon, Guwahati, 29th- 31st Oct., 1998.

4. H.K.L. Sethi, CE, M.I.E., IRSE (Retd.), Ganga Bridge at Mokameh,

Research Designs and Standards Organisation, Ministry of Railway,

Lucknow.

5. IRC:78-2000, Standard specifications and Code of practice for road bridges.

Section-VII, Foundation and Substructure (Second Revision), The India

Road Congress, Jamnagar House, Shahjahan Road, New Delhi- 110 011.

6. IS-456:2000- Plain and Reinforces Concrete Code of Practice (Fourth

Revision) Bureau of Indian Standards, Manak Bhawan, 9 Bahadur Shah

Zafar Marg, New Delhi- 110 002.

7. Concrete Bridge Code- IRS Code of Practice Plain, Reinforced and

Prestressed Concrete for General Bridge Construction, RDSO, Lucknow-

226 011.

8. IRS Code of Practice for the design of Sub-structures and Foundations of

Bridges, RDSO, Lucknow- 226011.

9. Technical paper No. 335, River training and control for bridges.

10. Manual on the design and construction of well and pile foundation (1985),

RDSO, Lucknow.

-------

37

Appendix-A

38

39

40

41

42