Construction of Raft Foundation in Deep Sandy Beds for Major Bridges Across Perennial Rivers

8/3/2019 Construction of Raft Foundation in Deep Sandy Beds for Major Bridges Across Perennial Rivers

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Paper No.506

CONSTRUCTION OF RAFT FOUNDATION IN DEEPSANDY BEDS FOR MAJOR BRIDGES ACROSS

PERENNIAL RIVERS+

By

MS. ASHWINI UGALMUGLE* & DR. A.G. NAMJOSHI**

CONTENTS

Page

1. Introduction ... ... 422

2. Bridge Across River Bawanthadi ... ... 422

3. Bridge Across River Kanhan ... ... 439

SYNOPSIS

The choice of shallow foundation is conventionally restricted to minor

bridges (less than 60 m length) where the stream bed has non-erodable strata

or where the design depth of scour in erodable strata is not much and there is no

perennial flow/standing water in the river bed. In the present case, shallow

foundations have been adopted for construction of bridges across major rivers viz.

Bawanthadi & Kanhan having a linear waterway of 400 m & 180 m respectively.This paper describes in detail two different techniques adopted to tackle the

construction of cut off walls for raft foundations where heavy dewatering was

required.

In the first case of submersible bridge across river Bawanthadi, the site is

located very near to the confluence with river Wainganga and heavy dewatering was

contemplated. In order to overcome the difficulty, the cut-off walls were cast above

low water level and lowered down to the designed founding level below the bed.

An innovative method of lowering down a unit of 22 m long pre-cast cut off wallsusing temporary fabricated steel frame stiffeners is described here.

In the second case of high level bridge across river Kanhan, there was

standing water of around 3.0 m depth in the river bed extending almost over 80

per cent width of the river. Pre-Trenching method was adopted using Bentonite

clay for construction of cast-in-situ cut off walls.

+ Written comments on this Paper are invited and will be received upto 31st

December, 2004.

* Senio r Desi gn Engi neer,

** Consu lt ing Engineer ,Dr. Namjoshi & Associates, Nagpur.}


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MS. UGALMUGLE & DR. NAMJOSHION422

1. INTRODUCTION

It is a conventional practice to rest the foundations of major

bridges anchored firmly into sound rock. Streams where foundable strata

is not available at a reasonable depth, always pose a problem to select

appropriate type of foundations. The bridge engineers generally do not

like to deviate from guide lines given in the IRC codes 1,2,3,4 which are

general in nature covering various conditions all over the country. The

bridge engineer, however, has to exercise his insight to adopt unique

solution, to suit a particular site conditions. Such decisions a build upconfidence level, if they are supported by sound performance record of

precedents.

Over the years, Maharashtra state has developed raft foundations

as available alternative for deep foundations such as wells and piles in

locations where hard foundable strata are deep seated and dry weather

flow is such that construction of cut off walls with manual open excavation

is feasible. Recognizing the indisputable merits of raft foundation, designand construction, engineers of the department have been constantly

innovating to achieve economy and furtherance of constructional ease.

Construction of cut off walls upto desired depth is possible by

excavating an open trench in case of cohesive soils which can be vertically

cut. Dewatering is one of the major difficulties faced in the construction

of raft foundations6

specially in case of sandy soils and structures near

irrigated areas because recharging of the ground makes dewatering difficult.

Various innovative methods7 have been devised and put into practice in

the field to overcome such difficulties. A number of major bridges have

also been economically9 constructed successfully over raft foundations7

in the past with a sound performance record.

The Paper discusses construction of cut off walls for raft foundations

for two major bridges viz. submersible bridge across river Bawanthadi andhigh level bridge across river Kanhan where innovative methods were

developed during execution.

2. BRIDGE ACROSS RIVER BAWANTHADI

2.1. Brief History

Tumsar Belaghat A State Highway (S.H. No.271) connects Tumsarin Maharashtra with Belaghat in Madhya Pradesh. It had an unbridged

crossing across river Bawanthadi near Bapera. The State Highway at


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CONSTRUCTIONOF RAFT FOUNDATIONIN DEEP SANDY BEDSFOR MAJOR BRIDGESACROSS PERENNIAL RIVERS

423

present is a fair weather road and prior to the construction the bridge

temporary crossing was having constructed across this river during dryseason every year. The traffic, however, used to remain closed during

monsoon period (June to October).

Since heavy traffic is not likely to develop over this route in the

near future, a submersible bridge with a single lane carriageway was

constructed from economic considerations. The formation level of the

bridge has been fixed at such a level, that the traffic may get interrupted

only during high floods with a frequency and duration remaining withinthe permissible norms.

Therefore, a continuous unit of multiple cell RCC box construction

which satisfies all the requirements of a submersible bridge which causes

minimum obstruction to flood waters was adopted.

This single lane bridge being quite long (i.e. 381 m) cause

considerable hindrance to two way traffic. Therefore waiting places of

two lane capacity were provided at two locations (Plate 1).

This bridge length is made up of 15 independent units of RCC Box

Cells each having a length of 21.65 m. Each unit consists of 5 continuous

box cells with opening 4.0 x 3.7 each (Plate 1). The gap between two

independent (continuous) units is bridged by providing a simply supported

solid slab having length of 4.0 m (Plate 2). The simply supported solidslab superstructures rest over the brackets projecting out from the vertical

walls at ends of the box cell units. The foundation for each 5 cell unit

is provided with reinforced cement concrete (RCC) continuous raft slab

of length 22.55 m x 4.60 m (22.55 m x 7.80 m for 2 lane enlarged portion

provided at two intermediate locations). The salient features of the bridge

are given in Annexure-I.

The raft slab is confined by detached plain cement concrete (PCC)cut off walls (300 mm x 2300 mm) on upstream (u/s) and downstream

(d/s) sides extending for full length of the bridge from bank to bank.

Cross cut-off walls of same size are provided at the beginning and end

of each 5 cell continuous unit so as to confine the RCC raft slab foundation

for each unit independently. The portion below the simply supported

span between two cross cut-off walls is provided with 600 mm thick PCC

flooring.

The clear width of roadway is 4.25 m for 13 units whereas 7.50 m

wide 2 units were provided at waiting places (Plate 3). 550 mm wide


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discontinuous road kerbs are provided on either side of the carriageway

along full length of the bridge [Photo 1]. The width of kerb has beenkept as 550 mm with removable type post and pipe railing on either side

for the safety of pedestrians. The riding spans on either side of the two

lane units (provided as waiting/overtaking zones) are enlarged from

4.2 m to 7.5 m wide carriageway to facilitate smooth transition. Thus the

total length of the bridge is built as under :

Photo 1. Two lane unit waiting place

Unit A (Single Lane) : 13 x 21.65 = 281.45 m

Unit B (Two Lane) : 2 x 21.65 = 43.30 m

Riding spans (Single Lane) : 10 x 4.0 = 40.00 m

Riding spans ( Flared ) : 4 x 4.0 = 16.00 m

Total Length = 380.75 m

2.2. Protection Works

The raft foundation & cut off walls are further protected by providing

1100 mm thick launching apron of rubbles. The velocity at bed level

during peak floods does not exceed 2.50 m/sec, however, weight of each

stone not less than 40 kg has been used for apron. The launching apronextends for a width of 6.75 m on u/s side and 9.0 m on d/s side. PCC toe

walls are provided along full length of the apron on both sides.


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425

2.3. Problem Encountered

The bed material predominantly consists of pure sand with almost

no silt/clay content & could not stand a vertical cut necessary to allow

excavation of an open trench. The work was, however, commenced with

a conventional method of excavating a wide open trench. A block of

28.0 m x 10.0 m was excavated in the sandy bed. The pit could hardly

be excavated to a depth of 1.50 m below the bed level & it was observed

that further digging is not possibly due to following impediments :

(a) Collapse of sides due to heavy sand blows during excavation

& dewatering, inspite of shoring & strutting.

(b) The water level remained constant even after employing 200

HP water pumps for dewatering.

Further work was hampered due to occurrence of nonseasonal

floods & the excavated pit got filled up to the original bed level. It was

therefore considered necessary to explore the possibility of casting the

cut off walls at bed level & thereafter lowering it down to the desired

level below the bed, instead of conventional method of casting in situ in

open trench.

2.4. Construction Technique

The above arrangement (Fig. 1) provided confinement to RCC raft

foundation (for each unit of 5 celled box) at foundation level by an

independent frame of cut off walls of 2.30 m depth (22.65 m length x

4.60 m width) all along the periphery.

It was, therefore, proposed to cast the PCC frame of cut off wall for

each unit separately & lower it down to the desired position by scooping

out the sand below the cut off & inside the frame simultaneously so

that it could gradually sink uniformly by its own weight.

2.5. General Features & Design Considerations for Raft Foundation

The detached cut off walls are designed to perform the following

main functions in service condition :

(a) To confine the bed material below the raft foundation.

(b) As a diaphragm that prevents the free flow of bed material/

particles underneath raft foundation during floods.(c) To extend the path of exit gradient below the bridge

foundation & its protection works.


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Fig.

1.


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427

In short, under service condition of the bridge proper, the detached

cut off walls are not designed to bear any stresses. It can therefore beconsidered as a non structural bridge component so far as transfer of

forces to the foundation stratum under various critical combinations of

loads are concerned . In the present case the cut-off walls are constructed

in PCC M-15 grade. However as a conventional practice, skin reinforcement

@ 5 kg/sqm is provided.

The stability of the cut off wall frame was checked for the loading

conditions, which it might have experiences during sinking process. Theinside dimensions of the rectangular frame under discussion was 22.55 x

4.60 m with a height of 2.30 m and the thickness of wall was 300 mm.

The frame of cut off wall was proposed to be lowered down to the desired

position by removing the sand inside the box & below the cut off wall.

The frame was designed to withsand following forces during sinking

process :

(a) The differential lateral pressure of saturated sand from outside

for a height of 2.30 m inducing bending stresses in the horizontal

plane.

(b) Occurrence of non uniform settlement along the periphery of

the frame resulting in bending stresses in the vertical plane

due to self weight of the frame. The height of cut off wall is

2.30 m and its thickness is 300 mm. The long walls are

connected by cross cut-off walls at the ends only. It was,therefore, necessary to restrict the span length to maximum

4.0 m, so as to safely bear the lateral thrust. The longitudinal

walls, were, therefore stiffened from inside at five intermediate

locations by providing removable vertical steel brackets thus

dividing it into six short bays of length not exceeding 4.0 m

each. The brackets on the opposite walls were braced by

removable horizontal steel strut fixed at 0.75 m height from

bottom.

After carrying out detailed analysis of forces occurring during the

lowering down operation, additional reinforcement wherever required was

provided in the cut off wall frame at appropriate locations. The reinforcement

for the cut off walls was designed for the stresses induced under critical

conditions of the construction phase. The stress in concrete & steel has

been allowed to exceed upto 33 per cent in the construction phase aspermissible.


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2.6. Construction Details

2.6.1. Sinking of single lane main unit: The bed was prepared,

levelled & timber logs were placed at one meter interval along the periphery

of the unit. A 500 mm wide strip in brick on edge, was constructed all

round the periphery so as to rest the base of 300 mm cut off wall. The

shuttering was erected for a full height of 2.30 m & reinforcement cage

was tied in position. The anchor bolts required for fixing the steel bracket

were placed in a 20 mm dia & 300 mm long PVC pipe casings so as to

facilitate easy removal. These bolts were placed in exact position &tightened with the centering plates.

The concrete cut off wall was, thereafter cast in layers for full

height of 2.30 m in one operation so as to ensure adequate self weight

and to facilitate ease in sinking. [A top layer of 200 mm was left to be

cast initially so as to leave scope for achieving a perfect level after final

sinking] (Photo 2). The vertical shuttering plates were removed. Thereafter

steel brackets were tightened to the concrete cut-off walls at predetermined

locations. After that horizontal steel strut was braced at 0.75 m from

bottom to the brackets on opposite walls (Fig. 1). It was ensured to place

timber wedges at junction of bracket & steel strut on both sides so as

to allow easy removal of brackets after final sinking (Photo 3). The

sinking operation started after the concrete of cut off walls attained

adequate strength. The following arrangement was made for sinking

operation.

Photo 2. Cut-off wall unit cast at bed level


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429

Photo 3. Stiffening arrangement of RCC cut-off walls

Vertical steel bracket with horizontal strut

In all 28 labourers, 4 in each intermediate bays and 6 in end bays

were working inside the box. They first scooped out the sand beneath

the cut off wall uniformly all along the periphery & removed the temporary

timber supports placed beneath it simultaneously. The operation of removal

of sand within the box & all along the periphery was carried out gradually

& uniformly so as to avoid differential sinking. It was also observed that

the sinking of cut-off wall continued to be uniform all along the periphery.

When the whole unit of cut off wall was sunk upto the bottom level ofapron, the sand outside the cut off wall on u/s & d/s side was removed

upto that level so as to reduce intensity of the differential lateral pressure.

Thereafter, the sinking process was continued till the cut off wall reached

the desired level.

For removal of sand within the box, 3 nos. of Winch with Grab

(Latur cranes) of 350 kg capacity (65 HP) were used. The removed sand

was deposited at a distance of 10 m on d/s side i.e. beyond the toe ofapron on d/s side. In all 60 HP electrically operated water pumps were

required to work continuously for the sinking operation.


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The cut-off wall unit was seated to the desired level. Sand was

refilled in the box upto the top of horizontal steel strut (leaving theportion around the strut) so as to reduce the earth pressure on the cut-

off wall from outside. Thereafter the wooden wedges were removed &

the strut was relieved of axial thrust. The whole steel frame work i.e.

horizontal struts & vertical brackets were removed one by one and side

by side and sand filled in the box up to the bottom level of proposed raft

(Photo 4).

Photo 4. Cut-off wall unit sunk in final position

The following material & labour were required for casting & sinking

of one main unit (single lane) of cut off wall.

Casting Operation

PCC M15 - 39.50 cum

Cement bags required - 240 nos

Working hours required - 10

Concrete mixer used - 1 no

Labours (in 2 shifts) - 80 nos


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431

For removal of sand and sinking in all 60 HP water pumps were

required to operate continuously.

Period Required

Preparation of base platform, erection - 5 days

of centering, shuttering & tying of

reinforcement cage etc.

Casting & curing of cutoff wall - 7 daysSinking operation - 5 days

2.6.2. Intermediate unit: The cut off walls required for bridging of

the gap between the two adjacent main units (under simply supported

span) overlapped both units by 300 mm from outside (Photo 5), (Fig. 2a).

The cut-off walls were cast for the required length for full height on

u/s & d/s side separately. These two cut off walls were braced with a

single horizontal steel strut connected with a vertical steel bracket atboth ends for sinking purpose. The cut-off walls were cast leaving a gap

of 50 mm on u/s and d/s side so as to avoid friction with the main unit

Photo 5. Overlapping cut-off wall between two units


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while sinking. Both the cut-off walls were sunk to the desired level by

adopting the same procedure as that followed for the main unit. It was,however, observed that the self weight of the intermediate unit (being

small in length) was insufficient to overcome side friction & hence

counterweight had to be provided during final sinking.

The following labour & material were required for casting & sinking

of the intermediate units of cut off walls.

Casting Operation

PCC M-15 - 6 cum

No of bags required - 37 bags

For removal of sand and sinking in all 40 HP water pumps were

required to operate continuously.

Period Required



reinforcement cage etc.

Casting & Curing of cut-off wall - 7 days

Sinking operation - 3 days

2.6.3. Method for ensuring continuity of cut-off walls: In order toensure an efficient functioning of the cut off wall to prevent free flow of

bed material underneath the raft foundation, its functional continuity had

to be maintained. If construction joints are inevitable it should be ensured

that no gap is left in-between or is protected with additional lap joint

so that sand should not flow through the gap from u/s to d/s during

floods.

The horizontal surface steel in the longitudinal walls of the mainunit was extended for one lap length beyond the outer face of cross cut

off wall. After casting of concrete in cut off wall, the projecting

reinforcement bars were bent upward & plastered with lean cement

mortar flushed with the outer face of the cross cut off wall to ensure

obstruction to free sinking. Similarly 8 mm dia bars in U shape were

projected for a lap length in the transverse direction (Fig. 2b) in the cut

off wall of intermediate unit. The longitudinal extended bars from main

unit cut off were straightened (after scraping the lean plaster) and were

tied with 8 mm for `U bar projecting from cut off of intermediate unit.


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433

Fig. 2b. Details showing continuity of cut off wall

Fig. 2a.

300

f i ll e t 150 x 1 50

up to 1 .3 m from

bot tom

M A I N U N I T M A I N U N I T

I N T E R M E D I A T E U N I T

A

300300

300

2 5 0 0

30 0

Su r fa c e r e i n f

ex tended f rom

ma i n u n it i n te rmed ia te un i t

Su r fa c e r e i n fex tended f rom

M A I N U N I T

I N T E R M E D I A T E U N I T

cast in s i tu @ al l the four

6 0 0 x 3 5 0 x 2 3 0 0 m m t o b eConc re te b lock o f s i ze

corners o f i n te rmed ia te un i t

after f ina l s inking

DETAILS OF A

37 5 45 0 75

30 0

35 0

30 0 60 0

90 0

30 0

Thereafter this overlapping piece of cut-off wall was cast in-situ making

the lap joint fool-proof & thus maintaining a physical continuity throughoutthe length of the bridge foundation.


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2.7. Two Lane Unit

The area covered by this unit is larger by about 70 per cent than

the single lane unit. In order to limit the dewatering efforts, the two lane

unit (Fig. 3a) of size (22.55 m x 7.90 m) was divided into two independent

units of internal size 10.00 m x 7.90 m leaving a gap of 2.50 m (similar to

that left between the single lane units). The intermediate cross cut off

wall at the continuous end was cast 1.0 m below the raft top level so as

to avoid propping action during service condition. The 1.0 m height was

built up by constructing a temporary precast CC block masonry upto theraft top level in order to have uniform loading all along the periphery.

The longitudinal walls were stiffened from inside at two intermediate

locations by providing the removable vertical steel brackets thus dividing

the same in three bays of length less than 4.0 m each. The brackets on

the opposite walls were braced by removable horizontal steel strut fixed

at 0.75 m height from bottom. The cross cut off walls were also stiffened

from inside at center with a vertical steel bracket. The steel bracket on

the opposite cross cut off walls were braced by longitudinal removablehorizontal steel strut which simultaneously stiffened the two struts provided

in transverse direction also. The same procedure as adopted for other

units for casting & sinking was adopted for these two units also. The

intermediate unit was cast & sunk to desired level. The temporary walls

of block masonary of 1.0 m height constructed over intermediate cross

cut off wall was removed & sand was refilled upto the bottom level of

proposed raft. These two parts of the units were finally joined to form

a single unit for two lane flared portion (Fig. 3b).

Fig. 3a.


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435

Fig. 3b. Arrangement of twin segements of cut

off walls below two lane units

After final

sinking of

o v e r l a p p i n g

cut off the

main cut off

wall is cast

insitu to main-tain continuity

(All dimensions in mm unless otherwise specified)

The following material & labour were required for casting & sinking

of one part of Two lane unit of cut off wall.

Casting Operation

PCC M15 - 23.16 cum

Cement bags required - 140 nos

Working hours required - 8

Concrete mixer used - 1 no

Labours (in 2 shifts) - 80 nos

For removal of sand and sinking in all 50 HP water pumps wererequired to operate continuously.

Period Required



reinforcement cage etc

Casting & Curing of cut-off wall - 7 daysSinking operation - 5 days


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2.8. Difficulties Encountered

2.8.1. The cut off units were sunk to an average depth of 3.50 m

below the LWL and the inside dimension was increased by 150 mm

(4600 mm to 4750 mm) so as to provide adequate margin for the likely

shift, if any in the lateral direction during sinking. This was done so as

to ensure uniform & symmetrical raft about the center line of the bridge

of required minimum dimension. No change in dimension of the main

units was warranted to accommodate shift in the longitudinal direction,

as the same could be accommodated by varying the length of theintermediate unit which was cast after final sinking of main units on

either side.

2.8.2. The cut off wall has to satisfy the following main functional

requirements

(a) The minimum depth below the raft top RL should be 2.30 m

(b) The cut off wall top should be in level with the raft top.

The cut off wall was cast in upto 2.10 m height only in the initial

stage before sinking. The surface reinforcement required, however, was

tied for the full height of 2.30 m. There was no difficulty in sinking it

to a little more depth so as to observe minimum depth of 2.30 m of cut

off below raft top RL at all the corners & intermediate locations. After

meeting with the requirement of minimum depth on final sinking, the

remaining cut off height which was left to be cast initially (around 200

mm), was concreted upto the desired level at top of the raft after making

up the difference in height, if any, at all the corners. This ensured to

achieve a perfect horizontal plane at the top of the raft level.

2.8.3. It is already mentioned in the Brief History that a temporary

crossing used to be constructed for restoring the traffic during the open

season. This temporary crossing used to be constructed by drivingtimber piles/stamps in the river bed & laying branches of trees within, so

that it could hold the filled up soil in flowing water for preparation of

temporary service road. These timber stumps used to get plunged further

into the sandy bed during monsoon. The alignment of the bridge overlapped

a part of the service road. These timber logs stuck at some places while

sinking the units. Such obstacles had to be removed after resorting to

excavation from outside & pulling it out with a rope. The sinking operation

all over the periphery was required to be slowed down or suspended tillsuch obstacles were removed so that uniform sinking could be possible.


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437

2.8.4. Initially the cut off wall frame required for two units of

22.55 m each was cast & finally sunk in position one after the other. Agap of 4.0 m was left for simply supported span. Thereafter the intermediate

cut off unit was cast. While removing the sand between the intermediate

cut off wall for sinking purpose it was observed that the sand filled in

the adjacent bays of units on either side also slipped in this unit through

sand blows. Due to this phenomena the level of sand in the adjacent bays

used to subside. It was therefore necessary to make up the level of sand

filled in the adjacent bays also after filling sand in the intermediate unit.

It is, therefore, advisable to lay the levelling coarse below RCC raft ofmain unit only after the sinking & sand refilling of adjacent intermediate

units on either side of the main unit are completed.

2.8.5. During construction of cut off walls near the banks it was

observed that at some places, rock outcrops were struck under the units

(Photo 6). It is a structural necessity that the cut of walls all along the

Photo 6. Rock struck near banks


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periphery need to rest over a strata of uniform SBC in order to avoid

differential settlement. This requirement was artificially generated byexcavating an open trench of dimension about 1 m. wide and half a meter

deep below the desired seating level of the cut off walls in the rock

outcrop (Fig. 4). The extra portion of the excavated trench (i.e. 0.50 m

below the seating level of cut off) was refilled with sand cushion.

Fig. 4. Sinking of cut off wall in rocky out crops


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439

If a continuous stretch of hard strata/rock is met with where the

cut off wall is designed to rest it is desirable to provide a clear verticalseparation joint in the cut off wall at the junction of the two different

strata. In order to protect this vertical joint an additional piece of cut off

wall (covering cut off wall) with an overlap of 750 mm on either side of

the vertical joint should be constructed from outside so as to arrest the

free flow of sand through the construction joint.

2.8.6. Similar rock outcrops were met with below the raft portion

within the cut off walls, where the rock outcrops were cut to a depth suchthat a uniform sand cushion of minimum 900 mm could be laid below the

proposed raft.

2.8.7. Rock was continuously met with on both the banks. The cut

off walls of the end units were therefore constructed by adopting

conventional method of excavating an open trench and casting in situ of

the PCC cut off walls.

2.8.8. The rubble apron was deleted in the portions where rock was

met with in the bed. However the apron and the toe wall was extended

horizontally for 2 m length inside the rock so as to overlap the rocky bed

and to ensure sufficient grip against local erosion (Fig. 5).

2,8.9. PCC solid abutments supporting open foundation were end

riding spans with constructed at both the ends where rock foundation

was met. The end units of box cell superstructure were converted into

intermediate units of box cell with simply supported riding spans at the

ends.

The actual construction of the bridge was commenced on 18 th Nov.

2003. The most difficult task was to construct cut off walls. The construction

of box cell superstructure units was, however, simultaneously completed

leaving a margin of atleast one unit of cutoff frame sunk in positionahead, in the direction it progressed. The bridge work at a cost of Rs.190

Lacs was completed on 15 th June 2003 @ Rs.50000 per m length

(Photo 7).

3. BRIDGE ACROSS RIVER KANHAN

3.1. Brief History

The bridge across river Kanhan is at 13 km on Parseoni -

Khaperkheda road (MDR) in Nagpur District. River Kanhan is a perennial


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Fig. 5. Details of rubble apron anchoring in rocky banks

Photo 7. Bapera Bawanthadi bridge work nearing completion


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441

river with catchment area of about 5221 sq km (2017 sq miles) upto the

bridge site. There is a minor irrigation project constructed on u/s sidein Madhya Pradesh across this river . Therefore there is a perennial flow

at the bridge site due to regeneration of the catchment. The low water

spread extends for about 80 per cent of the bed width at bridge site

having a maximum depth of about 3 m above the lowest bed level. Ferry

crossing was the only available mode to cross at this site for the local

population. The bridge provides an all weather connection from thermal

power station at Khaparkheda to Nagpur Jabalpur N.H. 7 by a shorter

route. Moreover the MSEB was also in a need of a high level crossingfor carrying the ash pipe line across river Kanhan. In order to serve both

the purposes simultaneously, it was, therefore, decided to construct a

high level bridge across this river.

A linear water way of 180 m was required from hydraulic

considerations. Foundable strata of sound rock was available at a depth

of about 35 m below the L.W.L in the main gorge as revealed from the

trial bores. The flood depth at HFL was around 11 m above the L.W.L.The total height of substructure including foundation worked out to 46

m. The proposal was therefore framed as per conventional practice of the

department. A proposal of 4 spans of 45 m each (Plate 4) with PSC box

superstructure & hollow R.C.C. piers resting over well foundations was

finalized and tenders were invited accordingly.

The bids received were ranging between 5.0 to 6.5 crores with a

similar arrangement as contemplated by the department. However one

alternative proposal with R.C.C. raft foundation and elaborate protection

work was offered at 4.27 crores. This alternative proposal being

unconventional one (RCC raft foundation for a major bridge having

perennial flow & standing water in the bed) took some time to arrive at

a considered decision after carrying out a detailed technical scrutiny &

examining construction viability. The contract was finally awarded in

November 1997.

3.2. Details of Bridge Proper

This proposal contemplated a two lane bridge having 15 simply

supported spans of 12 m center to center of solid slab construction

(Plate 5). A separate trough slab superstructure adjacent to main

carriageway was provided to carry MSEB ash pipe line. Both the

superstructures rested over neoprene bearings placed over RCC pedestalon R.C.C. cantilever pier caps. R.C.C. wall type piers with semicircular cut

and ease water were provided over R.C.C. raft foundations. The raft slab


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was provided with a haunched beam below each pier support. Abutment

on left bank and Piers P-1 to P-11 had R.C.C. raft foundations whileremaining 3 piers and abutment on right bank rested over rock with open

foundations. The foundable strata on right bank was available at a

reasonable depth.

3.3. Protection Work

3.40 m deep and 0.35 m wide PCC detached cut off walls with

nominal reinforcement were provided to confine the 900 mm thick sandfilling below the R.C.C. raft. The sand cushion was overtopped by 100 mm

thick P.C.C. leveling coarse. 700 mm thick rubble apron overtopped by 300

mm thick PCC paving cast in staggered bays of 2 m x 2 m were provided.

This protection work extended for a length of 15.70 m on d/s side & 11.75

m on u/s side beyond the cut off walls. The hydraulic features with

calculation for protection work are given in Annexure-II.

3.4. Design Features of Haunched Footing

The RCC raft slab for foundation is designed as a wide beam on

elastic foundation, on the same theory.

The coefficient of subgrade reaction is assumed as under:

(a) Sandy strata - 5600 T/m3

(b) Sandy clay - 2800 T/m3

The raft slab is provided with a haunched base below the RCC pier

for even distribution of concentrated load over a span of 12.0 m

(Photo 8).

This part of the pier footing being a rigid one is not deformable.

The load at bottom of pier is assumed to disperse uniformly through the

deep concrete footing at 45 degrees to the base of raft.

The value of the load transmitted at the end of haunches of the

footing in the form of shear force on both sides has been considered

for the design of raft slab of uniform thickness in between ends of two

haunched footings.

The sum of shear force at both ends of the haunches together isconsidered as a single concentrated load over the raft slab of uniform

thickness.


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443

Photo 8. Reinf. details of haunched footing

below pier

The span between two such consecutive concentrated loads, over

the raft slab (in the form of continuous beam of uniform thickness) is

considered as distance between two consecutive ends of haunches ignoring

the width of footing at the base of pier 5.

3.5. Construction Technique for Casting of Cut off Wall

Bentonite clay technique is commonly used for construction of

diaphragm walls to enclose the area to be built up for building or some

other structure. This is predominantly used in coastal areas/reclamation

are as where heavy dewatering is involved and open excavation is not

possible where or the soil cannot stand a vertical cut. This technique

however is not adopted for the bridge works where there exists flowing

water in the channel.

As already explained, there was flowing/standing water in the river

bed and the bottom level of the cut off wall was designed to rest at about


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6.0 m depth below the LWL. The water spread extended for almost about

80 per cent width of the river. This situation naturally warranted heavy

dewatering before laying raft foundation and the cut off walls. In order

to reduce the dewatering efforts to the barest minimum, it was necessary

to go in for such a technique which would allow the operation of open

trenching as well as concreting of cut off wall under water. Hence the

bentonite clay technique was employed here.

The excavator was slightly modified to suit the specific purpose by

extending the length of stick by 1.2 m and reducing the width of bucketto 490 mm from that of its standard size, was procured for construction

of cut off walls. The width of cut off wall was increased to 500 mm to suit

the operatable minimum size of bucket though 350 mm width of cut off

wall was adequate from the design considerations.

The cut off wall was to be constructed for a total length of 315 m

i.e length on both sides and that required at ends. This operation was

however tackled by taking a panel of about 30 m at a time. A workingearthen platform for a width of about 15 m was constructed in the river

bed with its top at about 0.50 m above the low water level so as to cover

the area required for construction of bridge foundation as well as the

width of path required for operating the machinery and equipment.

The following equipments were required for the above work.

(i) Excavator (Photo 9)(ii) Crane RB 19 (For concreting & lifting of

tremie set & compaction)

(iii) Guide rail (To guide the bucket) (Photo 10)

(iv) Miller mixing pump (To mix Bentonite slurry)

(v) Two Bentonite slurry tanks each capacity (Photo 11)

of 8000 litres (For recycling purpose)

(vi) 2 sets of tremie pipe with hopper (For concreting) (Photo 12)

(vii) Tremie spanner (For fixing of tremie pipe)

(viii) Concrete mixer.

(xi) Generator 45 HP (For lighting & welding purpose)

(x) Chain (To measure depth/sounding)

The alignment of the cut off wall and its width was precisely marked

on the ground. The guide rails were embedded in the platform to form a

defined width of the cut off wall trench (Photo 10). The trench wasexcavated to a depth of about 0.50 m and it was filled with bentonite

slurry. The slurry was allowed to penetrate and saturate the adjacent soil


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445

Photo 9. Trench excavator

Photo 10. The guide rails aligned


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Photo 11. Bentonite slurry tank

Photo 12. Tremie pipe fitted with hopper


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447

for about 6 hours. This process stabilized the adjacent soil so as to

facilitate further excavation. Thereafter excavation was resumed and the

soil removed was simultaneously replaced by an equal amount of bentonite

slurry. It is essential to maintain the level of bentonite slurry up to the

level of guide rail and the rate of refilling should match with the rate of

removal of soil. This process is continued till the trench was excavated

upto RL at the bottom of cut off wall.

A panel of length about 3-4 m was earmarked by placing two stop

end pipes of diameter little less than that of the trench (Photo 13) forconvenience during concreting. Once the panel was ready a reinforcement

cage was lowered into the panel (Photo 14) with the help of a crane.

Thereafter the tremie pipes were lowered and a hopper was tightened in

position at top (Photo 15). Generally one tremie pipe can conveniently

feed a length of about 2 m. If the length of panel is larger, additional

tremie pipe is to be lowered down.

Photo 13. Stop end pipes placed to form a panel


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Photo 14. Lowering down of reinforced cage with crane

Photo 15. Well point method with electrically operated pumps


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449

A tremie plug was provided at the base of hopper and concrete was

filled up to the brim of the hopper. The plug was thereafter released with

the help of a sling attached to the crane. The concrete just gushes down

the tremie pipe as soon as the plug is removed and uniformly spreads

over the bottom of the cut off wall. The concrete thus smoothly spreads

over the panel bottom gradually replacing the bentonite slurry. A base

layer of about 300 mm depth was thus carefully laid.

The hopper was fitted with a hook attached to the sling of the

crane. When the concrete passes through the tremie pipe it is raised andlowered so as to induce vibration and compaction of the concrete laid.

In this process care is taken that the bottom of the tremie pipe always

remains submerged in the concrete mass laid in the cut off wall so that

there is no chance of bentonite slurry getting entrapped in the concrete

mass. Concreting was thus continued till the panel was filled upto the

desired height. The final level was checked by lowering a chain.

The process of excavation and concreting of the cut off wall wascontinued in a phased manner till the full length of cut off wall was cast.

3.6. Construction of Raft Slab and Substructure

The construction of toe wall on u/s side together with the apron

was tackled first. This reduced the dewatering effort required for the

construction of the raft and the d/s apron. For construction of the above,

the dewatering was done by adopting conventional method of wellpoint. A circular concrete caisson of 5.50 m internal diameter was sunk

to a depth of about 5 m below the LWL and a number of electrically

operated water pumps were used to keep the water level below the raft

bottom till concrete was set (Photo 15). The RCC wall type piers

together with cantilever caps were constructed thereafter.

3.7. Construction of Counterfort Abutment

Abutment on right bank was resting over open foundation and was

designed and constructed as per usual practice. The left side abutment

was located well inside the bank which was composed of sandy clay.

The width of raft below piers was reduced to 8.5 m. The abutment,

however, had to be provided for the full width i.e 11.25 m to retain the

approach earth. The total length of raft provided below abutment was

17.35 m in order to restrict the pressure within the safe being capacity(SBC) of the soil. This raft, stiffened by counterforts, was projected in


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the front by about 45 per cent of the total length so as to restrict the

difference between maximum and minimum pressure over foundation by

not more than 1t/sqm under all conditions. The raft below abutment was

separated out from the main raft below piers by providing a cross cut

off wall.

3.8. Treatment Near Junction of Open & Raft Foundation

The raft foundation below P1 to P11 was terminated at the end

of haunch base below P11 by providing a cross cut off wall. ThoughP-12 was resting over open foundations in order to simulate smooth

transition of the hydraulic conditions from raft foundation to open

foundation the following measures were taken.

(a) The cut off walls on u/s and d/s side were extended in the

longitudinal direction further upto mid span between P-12 &

P-13 though these were anchored in rock.

(b) The vent portion around P-12 was provided with 300 mm thicknominally reinforced PCC M-20 grade paving resting over

250 mm thick PCC M-10 base & 900 mm rubble filling.

(c) This paved vent portion was confined by providing cut off

walls all around which were anchored into rock.

(d) The end piers (P-10 & P-11) over RCC raft foundation were

provided with 1500 mm thick rubble filling below the 900 mm

thick sand cushion as an additional precaution.(e) The rubble apron and the toe wall was extended horizontally

for 2 m length inside the rock so as to overlap the rocky bed

and to ensure sufficient grip against local erosion (Fig. 8).

3.9. Construction of Solid Slab Superstructure

The height of pier was about 13 m above the LWL and also due to

standing water in the bed, suspended truss type centering was used for

supporting the superstructure. The width of the pier cap was slightly

increased so as to rest the ends of trusses. The superstructure was cast

after placing the neoprene bearing pads over the RCC pedestals.

ACKNOWLEDGEMENTS

The Authors express their thanks to Shri D.G. Marathe, Chief Engineerand Shri S.K. Mukharjee, Superintending Engineer, Designs for their

encouragement and able guidance during the construction of bridge


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451

across river Kanhan. A word of appreciation is also due to M/s. Khare

& Tarkunde Infrastucture Pvt. Ltd., Nagpur for their relentless efforts in

successful completion of the bridge.

The Authors wish to thank Shri S.K. Veermani, Superintending

Engineer, P.W. Circle, Nagpur for his valuable views and suggestions

during execution of bridge across river Bawanthadi. Thanks are due to

Shri D.H. Fulekar, Executive Engineer, PW Division, Bhandara and

Shri B.K. Chilkalwar, Sub-divisional Engineer for their help in efficient

execution of the above work. Similarly the authors express their gratitudeto M/s. Chaphekar & Co., Nagpur for their enterprising approach in

adopting the technique and successfully executing this major bridge

under difficult conditions.

REFERENCES

1 . I R C: 5 1 9 98 S t an d ar d s p ec i fi c at i on s a n d c o de of pr a ct i ce fo r r o ad

bridges : Section I General features of design (Seventh Revision)

2 . I R C: 6 2 0 00 S t an d ar d s p ec i fi c at i on s a n d c o de of pr a ct i ce fo r r o ad

bridges : Section II Loads and stresses (Fourth Revision)

3 . I R C: 7 8 20 0 0 S ta n da rd s pe c if i ca t io ns a nd c od e of p ra c ti c e f o r r o ad

bridges : Section VII Foundation and substructure (Second Revision)

4. IRC: 89 1997 Guidelines for design & construction of river training and

control works for road bridges. (First Revision)

5. Beams on elastic foundations Hetenyi (1946) ANN ARBOR. The University

of Michigan Press.

6 . Namjoshi , A.G. , Dewater ing Problem in construct ion of Raft Foundat ion

bridge across Morna River on Borgaon Hatrum Road in Akola district,

Journal of the Indian Roads Congress, Vol. 40, Part 3, 1979.

7. Namjoshi, A.G. & Dr. Kulkarni, S.S., Shallow caissons design, construction

and sinking technique and review of some field techniques adopted for construction

of cut off walls, Journal of the Indian Roads Congress, Vol. 532, Sept.

1992.

8. Namjoshi , A.G. , Anticipated Scour depth in non al luvial /clayey beds

International Seminar on Bridge Substructure and foundations, Delhi Conference

Documentation, Vol. 2, Jan. 1992.

9. Namjoshi, A.G. & Dr. Kulkarni S.S, Economic design of raft foundation

bridges. Journal of International Seminar on Bridge Substructure and Foundations,

Delhi Conference Documentation, Vol. 1, Jan. 1992.


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Annexure-I

Bridge Across River Bawanthadi

Salient Features

1) Catchment Area - 2380 sq. km (920 sq. miles)

2) Design Discharge - 5999 cumecs

3) Maximum Mean Velocity - 2.77 m/sec

4) Maximum Flood Level - 263.800 m5) E. O. H. F. L - 264.800 m

6) Bridge Formation RL - 261.325 m

7) Lowest Bed Level - 257.630 m

8) Length of Bridge - 381 m

9) Nature of Bed - Medium sand upto 6 m depth varying

from 6 to 10 m depth below bed level

in the main gorge. Silt Factor - 1.80

10) Type of Bridge - Submersible11) Foundation - M25 RCC raft slab with detached

cut off wall.

12) Cut Off Wall - PCC cut off wall 2.30 m deep.

13) Protection Work - 900 mm thick rubble apron over

topped by 150 mm thick PCC paving

6.75 m on u/s side & 9.0 m on d/s

side.

14) Substructure/Superstructure - RCC M25 Box Cell15) Kerbs & Railing - 550 mm wide discontinuous kerbs with

GI Pipe & Post removable type railing.

(i) Calculations for mean scour depth

5999Unit Discharge q = = 17.885 Cumecs/m

(380.75 45.32)

Unit discharge to be increased by 22 per cent as per IRC:781983

Cl.No. 703.1

Hence designed unit discharge is 21.82 Cumecs/m

Dsm = 1.34 x [21.822/1.80]1/3

= 8.57 m

C R F D S B M B


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453

(ii) Depth of cut off required

L.B.L. = 257.630 m

Mean Scour RL = 263.8008.57 = 255.230 m

Cut off depth required = 257.330 255.230 = 2.10 m

Provide minimum depth of cut off = 2.30 m

(iii) Design of floor protection work to RCC raft foundation

Max Scour Depth = 1.27 x 8.57 = 10.884 mMax Scour RL = 263.80 10.884 = 252.916 m

Top RL of raft : 257.330 m

Designed max. scour depth below the raft top

(257.330 - 252.916) = 4.414 m

U/S apron length = 1.5 x 4.414 = 6.621 m say 6.75 m

D/S apron length = 2.0 x 4.414 = 8.828 m say 9.0 m

(iv) Size of stone for apron

(Designed velocity at HFL - 263.800 is 2.77 m/sec)

Velocity at raft top

= [(257.33 - .252.916)/(263.80 252.916] x 2 x 2.772

= 2.49 m/sec

Size of stone for apron

d = (2.49/4.893)2

= 0.259m

w = 2650 x (4/3) x (22/7) x (0.1294)3

= 24 kg

Minimum weight of stone shall be 40 kg as per IRC:891997 cl.No. 5.3.7.2

(v) Thickness of apron

t = 0.06 x Q1/3

= 0.06 x 5999 1/3

= 1.09 m

Provide 900 mm rubble apron overtopped by 150 mm PCC paving

or 1100 mm rubble apron.

M U & D N454


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Annexure-II

Bridge Across River Kanhan

1) Catchment Area - 5221 sq. km (2017 sq. miles)

2) Design Discharge - 8872 cumecs

3) High Flood Level - 102.500 m

4) Maximum Mean Velocity - 4.95 m/sec

5) E. O. H. F. L - 104.050 m6) Maximum Mean Velocity - 5.17 m/sec

7) Bridge Formation RL - 105.325 m

8) Low Water Level - 91.400 m

9) Silt Factor - 1.70

10) Length of Bridge - 180 m

11) Nature of Bed - Coarse sand

12) Type of Bridge - High Level

13) Foundation - RCC raft slab M-25 with haunch below

piers & with detached cut off wall.

14) Cut Off Wall - PCC cut off wall 3.40 m deep.

15) Protection Work - 700 mm thick rubble apron over topped

by 300 mm thick PCC paving 11.75 m

on u/s s ide & 15.70 m ond/s side.

16) Substructure - RCC M-20 Wall Type Pier with cantilever

Pier cap.

17) Superstructure - RCC M-25 Solid Slab Superstructure

over Neoprene bearing pads.

18) Kerbs & Railing - 375 mm wide continuous kerbs with

RCC Parapet

(i) Calculations for mean scour depth

8872

Unit Discharge q = = 51.88 Cumecs/m

(171)

CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES 455


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455

Unit discharge to be increased by 19 per cent as per IRC:78-1983

Cl.No. 703.1

Hence designed unit discharge is 61.74 Cumecs/m

Dsm = 1.34 x [61.742/1.70]1/3

= 17.54 m

(ii) Depth of cut off required

Normal scour level - (102.5017.54) = 84.96 m

Local scour level (1.27 Dsm)

for Abutment foundation and

design of floor protection works

for raft foundation. (102.501.27 x 17.54) = 80.22 m

Lowest bed level 88.35 m

Top R.L. of raft to be kept 300 mm below lowest bed level.R.L. 88.35-0.30 = 88.05 m

Cut off wall bottom to be kept 300 mm below normal scour level

R.L.84.96-0.30 = 84.66 m proposed bottom R.L. = 84.65 m

Max. depth of detached cut off wall

88.05-84.65 = 3.40 m

(iii) Design of floor protection work to RCC raft foundation

Designed max. scour depth below the raft top

88.05-80.22 = 7.83 m

Length of floor protection from nose of pier on u/s side

7.83 X 1.5 = 11.75 m

Proposed length - 11.75 m

Length of floor protection on d/s side from the nose of pier.

7.83 X 2.00 = 15.66 m

Proposed length - 15.70 m

UGALMUGLE & DR NAMJOSHI ON CONSTRUCTION OF RAFT FOUNDATION IN DEEP456


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(iv) Size of stone for apron

(Designed velocity at EOHFL - 104.050 is 5.17 m/sec)Velocity at raft top

= [(88.05 - 80.22)/(104.05 - 80.22)] x 2 x 5.172

= 4.19 m/sec

Size of stone for apron

d = (4.19/4.893)2 = 0.7332 m

w = 2650 x (4/3) x (22/7) x (0.3666)3

= 547 kg Say 550 kg

(v) Thickness of apron

t = 0.06 x Q1/3 = 0.06 x 88721/3 = 1.240 m

As Per IRC:89-19974 Cl. 5.3.5.2. The thickness computed from above

formulae shall be subject to an upper limit of 1.0 m. Provide 700 mm rubble

apron overtopped by 300 mm PCC paving in bays of 2 m x 2 m in

staggered position.

UGALMUGLE & DR. NAMJOSHION CONSTRUCTIONOF RAFT FOUNDATIONIN DEEPSANDY BEDSFOR MAJOR BRIDGES ACROSS PERENNIAL RIVERS

456

Construction of Raft Foundation in Deep Sandy Beds for Major Bridges Across Perennial Rivers

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