Paper No.506
“CONSTRUCTION OF RAFT FOUNDATION IN DEEP
SANDY BEDS FOR MAJOR BRIDGES ACROSS
PERENNIAL RIVERS”+
By
MS. ASHWINI U GALMUGLE* & 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 walls
using 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 31 st
December, 2004.
Senior Design Engineer,
Dr. Namjoshi & Associates, Nagpur.
Consulting Engineer,
}
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MS. UGALMUGLE & D R. NAMJOSHI
ON
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 up
confidence 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, design
and 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
6
of raft foundations 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 economically 9 constructed successfully over raft foundations 7
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 and
high 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 Tumsar
in Maharashtra with Belaghat in Madhya Pradesh. It had an unbridged
crossing across river Bawanthadi near Bapera. The State Highway at
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS 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 dry
season 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 within
the 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 solid
slab 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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MS. UGALMUGLE & D R. NAMJOSHI
ON
discontinuous road kerbs are provided on either side of the carriageway
along full length of the bridge [Photo 1]. The width of kerb has been
kept 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)
Unit B (Two Lane)
Riding spans (Single Lane)
Riding spans ( Flared )
Total Length
2.2.
:
:
:
:
13 x 21.65
2 x 21.65
10 x 4.0
4 x 4.0
=
=
=
=
281.45 m
43.30 m
40.00 m
16.00 m
————
= 380.75 m
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 apron
extends 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.
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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)
(b)
Collapse of sides due to heavy sand blows during excavation
& dewatering, inspite of shoring & strutting.
The water level remained constant even after employing 200
HP water pumps for dewatering.
Further work was hampered due to occurrence of non–seasonal
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)
(b)
(c)
To confine the bed material below the raft foundation.
As a diaphragm that prevents the free flow of bed material/
particles underneath raft foundation during floods.
To extend the path of ‘exit gradient’ below the bridge
foundation & its protection works.
MS. UGALMUGLE & D R. NAMJOSHI
ON
Fig. 1.
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CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 be
considered 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. The
inside 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 as
permissible.
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MS. UGALMUGLE & D R. NAMJOSHI
2.6.
ON
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
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 of
apron, 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 of
apron 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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MS. UGALMUGLE & D R. NAMJOSHI
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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 the
portion around the strut) so as to reduce the earth pressure on the cutoff 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 M–15
Cement bags required
Working hours required
Concrete mixer used
Labours (in 2 shifts)
-
39.50 cum
240 nos
10
1 no
80 nos
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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
of centering, shuttering & tying of
reinforcement cage etc.
-
5 days
Casting & curing of cut–off wall
Sinking operation
-
7 days
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 at
both 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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MS. UGALMUGLE & D R. NAMJOSHI
ON
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
No of bags required
-
6 cum
37 bags
For removal of sand and sinking in all 40 HP water pumps were
required to operate continuously.
Period Required
Preparation of base platform, erection
of centering, shuttering & tying of
reinforcement cage etc.
Casting & Curing of cut-off wall
Sinking operation
-
2 days
-
7 days
3 days
2.6.3. Method for ensuring continuity of cut-off walls: In order to
ensure 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 main
unit 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.
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
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433
Thereafter this overlapping piece of cut-off wall was cast in-situ making
the lap joint fool-proof & thus maintaining a physical continuity throughout
the length of the bridge foundation.
IN TE R ME D IAT E U N IT
3 00
A
3 00
fillet 150 x 1 50
up to 1.3 m fro m
bo ttom
M A IN U N IT
M A IN U N IT
25 00
3 00
3 00
Fig. 2a.
30 0
37 5
Su rfa ce r einf
ex tend ed fr om
45 0
75
Su rfa ce r einf
ex tend ed fr om
int erm e diate un it
30 0
m ain u nit
35 0 C onc re te b loc k of si ze
60 0 x 350 x 23 00 m m t o be
30 0
c as t in s itu @ all t he four
M A IN U N I T
30 0
60 0
90 0
c orne rs o f interm e diat e uni t
aft er fin al s ink ing
IN TE R ME D IAT E U N I T
D ETAILS OF A
Fig. 2b. Details showing continuity of cut off wall
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MS. UGALMUGLE & D R. NAMJOSHI
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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 the
raft 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 removable
horizontal 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.
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
435
After
final
sinking
of
overlapping
cut
off
the
main cut off
wall is cast
insitu to maintain continuity
(All dimensions in mm unless otherwise specified)
Fig. 3b. Arrangement of twin segements of cut
off walls below two lane units
The following material & labour were required for casting & sinking
of one part of Two lane unit of cut off wall.
Casting Operation
PCC M–15
Cement bags required
Working hours required
Concrete mixer used
Labours (in 2 shifts)
-
23.16 cum
140 nos
8
1 no
80 nos
For removal of sand and sinking in all 50 HP water pumps were
required to operate continuously.
Period Required
Preparation of base platform, erection
of centering, shuttering & tying of
reinforcement cage etc
Casting & Curing of cut-off wall
Sinking operation
-
5 days
-
7 days
5 days
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MS. UGALMUGLE & D R. NAMJOSHI
2.8.
ON
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 the
‘intermediate 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)
(b)
The minimum depth below the raft top RL should be 2.30 m
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 driving
timber 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 till
such obstacles were removed so that uniform sinking could be possible.
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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. A
gap 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 of
‘main 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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MS. UGALMUGLE & D R. NAMJOSHI
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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 by
excavating 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
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 vertical
separation 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 such
that 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 position
ahead, 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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MS. UGALMUGLE & D R. NAMJOSHI
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Fig. 5. Details of rubble apron anchoring in rocky banks
Photo 7. Bapera Bawanthadi bridge work nearing completion
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 side
in 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 crossing
for 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 pedestal
on 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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MS. UGALMUGLE & D R. NAMJOSHI
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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 while
remaining 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 sand
filling 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)
(b)
Sandy strata - 5600 T/m3
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 is
considered as a single concentrated load over the raft slab of uniform
thickness.
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 pier5.
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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MS. UGALMUGLE & D R. NAMJOSHI
ON
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 bucket
to 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 working
earthen 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)
(ii)
Excavator
Crane – RB – 19 (For concreting & lifting of
tremie set & compaction)
(iii) Guide rail (To guide the bucket)
(iv) Miller mixing pump (To mix Bentonite slurry)
(v) Two Bentonite slurry tanks each capacity
of 8000 litres (For recycling purpose)
(vi) 2 sets of tremie pipe with hopper (For concreting)
(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)
(Photo 9)
(Photo 10)
(Photo 11)
(Photo 12)
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 was
excavated 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
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
Photo 9. Trench excavator
Photo 10. The guide rails aligned
445
446
MS. UGALMUGLE & D R. NAMJOSHI
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Photo 11. Bentonite slurry tank
Photo 12. Tremie pipe fitted with hopper
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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) for
convenience 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
448
MS. UGALMUGLE & D R. NAMJOSHI
ON
Photo 14. Lowering down of reinforced cage with crane
Photo 15. Well point method with electrically operated pumps
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 and
lowered 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 was
continued 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 well
point”. 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
450
MS. UGALMUGLE & D R. NAMJOSHI
ON
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 P–1 to P–11 was terminated at the end
of haunch base below P–11 by providing a cross cut off wall. Though
P-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 thick
nominally 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 Engineer
and Shri S.K. Mukharjee, Superintending Engineer, Designs for their
encouragement and able guidance during the construction of bridge
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
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 gratitude
to M/s. Chaphekar & Co., Nagpur for their enterprising approach in
adopting the technique and successfully executing this major bridge
under difficult conditions.
REFERENCES
1.
IRC:5–1998 “Standard specifications and code of practice for road
bridges : Section – I General features of design” (Seventh Revision)
2.
IRC:6–2000 “Standard specifications and code of practice for road
bridges : Section – II Loads and stresses” (Fourth Revision)
3.
IRC:78–2000 “Standard specifications and code of practice for road
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., “Dewatering Problem in construction of Raft Foundation
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. 53–2, Sept.
1992.
8.
Namjoshi, A.G., “Anticipated Scour depth in non – alluvial/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.
452
MS. UGALMUGLE & D R. NAMJOSHI
ON
Annexure-I
Bridge Across River Bawanthadi
Salient Features
1)
2)
3)
4)
5)
6)
7)
8)
9)
Catchment Area
Design Discharge
Maximum Mean Velocity
Maximum Flood Level
E. O. H. F. L
Bridge Formation RL
Lowest Bed Level
Length of Bridge
Nature of Bed
-
2380 sq. km (920 sq. miles)
5999 cumecs
2.77 m/sec
263.800 m
264.800 m
261.325 m
257.630 m
381 m
Medium sand upto 6 m depth varying
from 6 to 10 m depth below bed level
in the main gorge. Silt Factor - 1.80
Submersible
M–25 RCC raft slab with detached
cut off wall.
PCC cut off wall 2.30 m deep.
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.
RCC M–25 Box Cell
550 mm wide discontinuous kerbs with
GI Pipe & Post removable type railing.
10)
11)
Type of Bridge
Foundation
-
12)
13)
Cut Off Wall
Protection Work
-
14)
15)
Substructure/Superstructure Kerbs & Railing
-
(i)
Calculations for mean scour depth
5999
Unit Discharge q =
= 17.885 Cumecs/m
(380.75 – 45.32)
Unit discharge to be increased by 22 per cent as per IRC:78–1983
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
CONSTRUCTION OF RAFT FOUNDATION IN DEEP SANDY BEDS FOR MAJOR BRIDGES
ACROSS PERENNIAL RIVERS
(ii)
453
Depth of cut off required
L.B.L. = 257.630 m
Mean Scour RL = 263.800–8.57 = 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 m
Max 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 2.77 2
= 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:89–1997 cl.
No. 5.3.7.2
(v)
Thickness of apron
t = 0.06 x Q 1/3
= 0.06 x 5999
= 1.09 m
1/3
Provide 900 mm rubble apron overtopped by 150 mm PCC paving
or 1100 mm rubble apron.
454
MS. UGALMUGLE & D R. NAMJOSHI
ON
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 m
6)
Maximum Mean Velocity
- 5.17 m/sec
7)
Bridge Formation RL
- 105.325 m
8)
Low Water Level
- 91.400 m
9)
Silt Factor
10) Length of Bridge
- 1.70
- 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 side & 15.70 m on
d/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
ACROSS PERENNIAL RIVERS
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
(ii)
=
=
1.34 x [61.742/1.70]1/3
17.54 m
Depth of cut off required
Normal scour level - (102.50–17.54) = 84.96 m
Local scour level (1.27 Dsm)
for Abutment foundation and
design of floor protection works
for raft foundation. (102.50–1.27 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
456
(iv)
UGALMUGLE & DR. NAMJOSHI ON CONSTRUCTION OF RAFT FOUNDATION IN D EEP
SANDY BEDS FOR MAJOR BRIDGES ACROSS P ERENNIAL RIVERS
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
(v)
Say 550 kg
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.