Well foundation_Introduction in soil mechanics

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UCE607_FOUNDATION ENGINEERING BY RAJESH PATHAK

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Well foundation is the most commonly adopted
foundation for major bridges in India. Since then many
major bridges across wide rivers have been founded on
wells.
Well foundation is preferable to pile foundation when
foundation has to resist large lateral forces and strata
consists of gravels and boulders.
 The construction principles of well foundation are similar
to the conventional wells sunk for underground water.
 Type of Deep Foundation
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 Well foundations have been used in India for
centuries.
 The famous Taj Mahal at Agra stands on well
foundation.
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 Well foundations were used for the first time for
important lrrigation structures on the Ganga Canal at
Roorkee, constructed in the middle of 19th century.
 In 1937, Largest size of caisson 29.6m x 60.1m on San
Francisco Oakland bridge, California (USA) with sinking
depth of 73.8m was completed.
 Largest well used for main towers of Howrah bridge
having a size of 24.8m x 53.5m with maximum depth of
sinking as 31.4m below ground level.
 On Rajendra Pul across the Ganga at Mokameh double
D wells were used. The Largest size was 16.3m x 9.7m
and 50.3m deep.
 On Mahanadi Bridge Pneumatic sinking was done for
the first time up to a depth of 32m.
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 Open foundations are economical at shallow depth
under dry conditions
 When the soil is not stable and runs in to the excavation
it becomes necessary to provide shoring and under such
circumstances it is economical to provide well
foundations even under dry condition for depth, more
than 7.5m
 For foundation of piers and abutments of bridges, it is
generally economical to provide well foundations for
depth greater than 6m below spring water level.
 For well foundations concreting is done above the
ground level in dry conditions and the quality is better.
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 A large number of pumps are required for open
foundations under spring water level.
 In built up area, a lot of danger is caused to the adjoining
property by slips and subsidence of ground lf open
excavations are made. This danger is obviated if well
foundations are used and precautions are taken to
prevent blows.
 The advantage of open foundations are that they are
simple to design and construct and are open for
inspection all the time.
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 Well foundations provide a solid and massive foundation for
heavy loads as against a cluster of piles which are slender and
weak individually and are liable to get damaged when hit by
floating trees, ice, boulders etc.
 Wells have a large cross sectional area and the high bearing
capacity of soil.
 Piles can not be driven through soil having boulders, logs of
wood found burled at greater depth, whereas it is possible to
sink a well.
 Well are hollow and most of the material is at periphery. This
provides a large section modulus with minimum cross sectional
area. They can resist large horizontal forces and vertical loads
even when the unsupported length is large in scourable river
bed. This is not possible with piles.
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 The bearing capacity of a pile is generally uncertain. They
may be resting on desired strata or on a boulder. On other
hand in case of wells, it is possible to visually examine the
strata through which sinking is done and material on which
it is finally resting.
 Concreting in the steining of wells is done under dry
conditions and the quality of concrete is much better than in
case of cast-in-situ piles.
 In case of wells raising of the well steining and sinking are
done in stages and a decision about the foundation level
can be taken as the work progress and the strata condition
become known.
 Well not suited for smaller loads. It is because the size of
well foundation cannot be reduced indefinitely as the
dredge hole must be big enough to enable a grab to work
and the steining must have the thickness necessary to
provide the required sinking effort.
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 Well foundation is a member of the caisson family.
Caisson is a box-like structure usually round or
rectangular which is sunk from the surface of
either land or water to some desired depth.
 Caisson have been used mainly to support bridge
piers and abutments as well as building columns
subject to large horizontal loads apart from the
vertical loads.
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PNEUMATIC
CAISSON
OPEN CAISSON
DROP CAISSON
BOX CAISSON
(OPEN BOX)
CAISSON
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 Box caissons are generally used in an ocean beds and
carried little vertical load. It is also called a floating
caisson.
 The caisson is pre cast at the shore with the bottom
which is generally provided with opening which are
plugged, and toed to the required position by tugs and
then plugs are removed to permit the caisson to reach
the bottom of the ocean bed. The ocean bed is pre-
levelled by divers.
 After the caisson rests in its final position, concrete
blocks are normally dumped in to the hollow portion to
increase its lateral stability. Box caissons rests on the
surface of the soil and no attempt is made during the
construction process to penetrate the surface of soil.
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 Caissons which are deliberately sunk into the soil are
called drop caissons. They are used to transfer heavy
vertical loads to safe bearing strata.
 Caissons used as foundations for bridge piers are
constructed partly at the bank with a fake bottom and
towed to the position by tugs and the false bottom is
removed to lower the caisson on to the river bed.
 When caissons are used for bridge piers, their design is
exactly the same as that of well foundation. Therefore
well foundation is a type of drop caisson.
 However well foundation, unlike caisson, is constructed
generally at its final location.
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 The dredge hole should be large enough to permit
dredging.
 The steining thickness should be sufficient to transmit
the load and also provide the necessary weight for
sinking and adequate strength during sinking and
service.
 It should accommodate the base of structure and not
cause undue obstruction to flow of water.
 The overall size should be sufficient to transmit the loads
to the soil.
 It should allow for the permissible tilt and shift of the well.
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.
Common Shapes of Well Foundation
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 Strength, simplicity in construction,
ease in sinking, minimum steining
thickness due to minimum flexural
stresses in steining.
 It requires only one dredger for
slinking
 Its weight per sq. meter of surface
area is the highest due to which
sinking effort is high
 The distance of the cutting edge
from the dredge hole is uniform
and the chances of tilting are
minimum.
9m is considered as the
maximum dia. Allowing
cantilever of 1m on either
side, the maximum Length
of pier is about 11 meters.
CIRCULAR
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 Greater Lateral stability. Simple
shape. Easy to sink but greater
effort than circular well.
 Desired to have overall Length of
the well not more than double the
width.
 Dimensions are so determined
that the Length and the width of
the dredge holes are almost
equal.
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 Considerable bending moments are caused in the
steining due to the difference in the earth pressure from
outside and water pressure from inside, resulting in
vertical cracks in the steining particularly in the straight
portion where they join the partition wall.
 Four square corners at either end of the partition wall
are far away from the center of the dredge hole and
these blind corners offer considerable resistance to
sinking specially in clay.
 Steining is relatively thick. More prone to tilt and shift
due to unsymmetrical shape and possible unequal
dredging.
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 Consist of two independent
circular wells placed very
close to each other with a
common well cap.
 If the depth of sinking is small
say up to 7m, the clear space
between the two wells may
be kept 0.6 to 1m to avoid
tilting.
 For greater depth of sinking
spacing of 2 to 3m may be
necessary
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 Useful where the length of the pier is considerable and
the size of the double D or octagonal wells become
unduly large to accommodate the pier.
 Twin circular wells are advantageous only when the
depth of sinking is small and the foundation material
capable of taking fairly high Loads.
 It is necessary to sink these wells simultaneously to
ensure that the cutting edge are almost at the same level
all the time.
 Tendency to tilt towards each other during sinking due to
the fact that sand between them become Loose.
 Simultaneous sinking is a must and thus requires two
sets of equipments for sinking.
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 Design of well caps for the twin circular wells also
requires special care. Allowance is made for relative
settlement of the two wells and this adds to its cost.
 The possibility of development of cracks in the pier due
to relative settlement cannot be ruled out inspite of the
heavy design of the cap except where the wells are
founded on rock on other incompressible soil.
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 Free from short
comings of double
D well
 Blind corners are
eliminated
 Bending stresses
in the steining are
also reduced
considerably . Greater resistance against
sinking on account of increased
surface area
Concreting in steining is also
more difficult than in case of double
D wells.
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 For bridge foundations having
shallow depths.
 Where the bridge is designed
for open foundations and a
change to well foundation,
become necessary during the
course of construction on
account of adverse condition
such as excessive in-flow of
water and silt into the
excavation.
 Rectangular well can be
designed exactly of the same
size as of open foundation.
Generally adopted for depth up
to 7.5m. For bigger foundations
having shallow depth double
rectangular well, are adopted. Bending stresses in the steining are the
maximum.
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 Used for piers and
abutment of very Large
sizes
 Not common in India
 Used in Howrah bridge
with 21 dredge holes for
main tower
 It is common in USA.
San Francisco Oakland
bridge, having 55
square dredge holes of
56.2m x 5.2m for each
well depth of well
foundations.
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 Cutting Edge: The cutting edge is provided at the
bottom of well below the curb to cut through the soil
during sinking. This is generally made of steel and
welded to angle iron to suit the outer dimensions of well
steining.
 Well Curb: The curb of the well transfers all the
superimposed load to the soil through the cutting edge.
The material used for curbs is reinforced concrete.
 Steining: This is subjected to different types of Stresses.
At the sinking Stage, it is subjected to water and earth
pressure. At the dredging stage, inside surface is
subjected to water pressure while outside surface to the
earth pressure.
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The thickness of the steining should be fixed with
following considerations:
It should be possible to sink the wells without
excessive kentledge.
The well do not get damaged during sinking.
It should be possible to rectify tilts and shifts without
damaging the well.
Stresses at various stages should be under
permissible limits.
Resistance against sudden drop/sand blow condition.
If Sinking is done through clay, the steining should be
made as thick as possible.
Because of sudden sinking in clay, chances of
horizontal, cracks are more.
Dewatering of well can be done once the well has
sunk to a sufficient depth in clayey soil.
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 Bottom Plug
After final grounding of the well to the required foundation
level a concrete plug is provided. The bottom plug transfers
the entire load, viz., weight of steining, super imposed load,
the weight of saturated sand filled in the well, to the ground.
The bottom plug functions as an inverted dome being
supported along the periphery of the steining. As it is not
feasible to provide reinforcement at the bottom it is generally
made thick and a rich concrete mix (M20) is used.
 Sand Filling
The bottom plug concrete is cured and after curing, the well is
filled with sand in a saturated condition. Sand filling, although
is not mandatory, provides stability to the bottom of the well,
helps in the elimination of tensile forces at the base and also
cancels the hoop stresses induced in the steining.
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 Top Plug
A top plug is provided after the filling is completed. This
enables to transfer the load of pier and the superimposed
load to the well steining. The thickness of top plug is generally
kept greater than 50 of the smaller dimension of the dredge
hole. If a sand filling is provided, no reinforcement is needed
and only PCC mix of 1 : 2 : 4 is enough.
 Well Cap
As the shapes of pier and well are different, the well cap
forms an interim transition layer to accommodate the pier. The
well cap is so designed such that the base of pier is provided
with minimum all round offset. Further the centre of the cap is
made to coincide with that of the pier and not of the well.
Such positioning nullifies the effect of minor shifts which might
have occurred during well sinking.
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Design discharge (as per substructure code)
Water surface slope on upstream and down stream
Low water level and high flood level
Silt factor
Cross section of the river
Plain table survey of the river
The deepest existing scour in the vicinity of the river
Strata chart i.e. bore Log data
Availability of construction materials
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At Least one bore hole must be available at each well
foundation location prior to commencement of work.
The depth of borehole below anticipated foundling
level = one and a half times the outer dia/least
dimension.
If well has to rest on a rocky strata, additional borings
may be required prior to commencement of work to
ascertain the actual profile and quality of rocky strata.
In case bore hole data shows steeply dipping rock,
chiseling may have to be resorted to so as to obtain
the proper seating of foundation. For this purpose
pneumatic sinking may be required.
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Vertical Loads:-
(i) Self weight of well
(ii) Buoyancy
(iii) Dead Load of super structure, substructure
(iv) Live Load, and
(v) Kentledge during sinking operation
Horizontal Loads:-
(i) Braking and tractive effort of moving vehicles
(ii) Forces on account of resistance of bearing
(iii) Forces on account of water current or waves
(iv) Centrifugal force( bridge is situated on a curve)
(v) Wind forces or seismic forces
(vi) Earth pressure
(vii) Other horizontal and uplift forces due to
provision of transmission Line tower (broken
wire condition) etc.
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 Size of Well Foundation
 The minimum width/dia of well is dependent on the
plan dimension of pier / abutment / superstructure.
 The minimum dia /width of well is 4m (which is
maximum width of shallow foundation).
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Depth is decided with respect to the
following consideration
 Scour, where applicable
 Stability
Total depth = Depth (stability) + Scour depth
Depth of foundations should not be Less than 1.33
times the deepest scour below H.F.L. and it should
provide necessary stability with respect to
overturning and sliding.
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As well foundations are constructed in river beds they
have to be taken to a safe depth well below the
anticipated scour level. Scour around piers depends on
several factors like flood discharge, angle of attack of
flow, flow obstruction etc. It is generally assumed that
the flow at the high stage of the river is straight. The
most common is Lacey’s formula which is given as :
where d = scour depth (m)
Q = design discharge (m3 / s)
f = silt factor
= 1.76 (Dm)1/2
Dm = mean diameter of soil in river bed (mm) = d50
3
/
1
473
.
0 








f
Q
d
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Nature of the river Depth of scour
In a straight reach 1.25d
At the moderate bend conditions
e.g. along apron of guide bund
1.5d
At a severe bend 1.75d
At a right angle bend or at nose of piers 2.0d
In severe swirls
e.g. against mole head of a guide bund
2.5 to 2.75d
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Following steps are required to be followed
Layout
Making of island if required
Fabrication of cutting edge
Well curb, construction and pitching
Construction of steining
Well slinking
Plugging, sand filling and castling of well cap
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 Accurately lay out the
centre line of the bridge
and the locations of the
piers and abutments with
great degree of accuracy.
 It should always be cross
checked by at least two
independent surveys before
starting of work
 Permanent theodolite
stations with the base line
on the bank will be
established to mark
reference points.
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Well Curb May be precast or cast in situ
The outer face of the curb shall be vertical
The form work on outer face & inner face of the curb may
be removed within 24 hours & 72 hours respectively
All concreting in well curb should be done in one
continuous operation.
It should transmit the super imposed load to the bottom
plug without getting overstressed.
It should offer minimum resistance to slinking.
The slope to the vertical of the inner faces of the curb shall
be not more that 30 degree. In sandy strata lt may be up to
45 degree.
An offset on the outside (about 50mm) may be provided to
ease slinking.
The curb is usually of M-25 grade
In case blasting is anticipated the inner face of the curbs
shall be protected by steel plates or any other means to
sufficient heights
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RCC well curb should be allowed to set for at Least one
week before sinking is started.
The chances of tilting increases considerably lf the well ls
made top heavy by raising the steining too high in the first
stance.
The best course is to sink the well curb alone after allowing
it setting time without raising the steining over it.
The steining may then be raised by about 1.5m at a time
and sinking done after allowing at Least 24 hrs of settling
time once the well has acquired a grip of about 6m in sand,
the steining can be raised about 3m at a time.
Steining shall be built of cement concrete not weaker than
M-150.
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For wells in cohesionless soils, as per IS:3955
Qa = 5.4 N2B + 16(100+N2) D
Where Qa = allowable bearing pressure in kg/m2
N= corrected value of SPT
B = smaller dimensions of well section, m
D = depth of foundation below scour level, in m
For wells in cohesive soils, consolidation and shear strength
should be obtained from tests on undisturbed samples. The
allowable bearing pressure is then determined from these
parameters
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 Crabs or Dredgers
 Hoist/Crans
 Straight Chisels, Fan chisels
 Under Cutting Chisels
 Pumps
 Air and Water Jet
 Explosive and Detonators
 Diving Equipments
 Concrete Equipments
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Erect cutting edge
Erect inside (conical) shuttering
Fix Reinforcement for the curb
Erect outside shuttering of curb
Concrete the curb and ground it
Remove the curb shuttering
Fix reinforcement in steining
Erect steining for one lift
Concrete the steining
Dredge inside the well and well to sink in stages till final
level.
The well should be sunk by excavating material uniformly
Use of water jetting, explosives and divers may also be
adopted
Normally dewatering should not be done.
Kentledge may be used for rectifying the tilts
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Placing of curb:
If the bed of the river is dry, the bed is excavated
up to about 150 mm. Firstly, cutting edge is laid.
The curb is then laid there and after it has
sufficiently hardened (in about 5 days) and
steining is built over it.
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 The removal of the material by dredging the wells is
usually accommodated by clamshell buckets.
 When the caisson approaches very hard strata, special
rock teeth should be added to the grab or very often
heavy chiseling resorted to for breaking such strata.
 In those cases where due to the depth of water and
peculiarities of soil, sinking by the use of grabs, etc.
cannot be done, pneumatic sinking or in some cases
sinking by divers with the use of steel helmet and
compressed air is resorted to.
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The rate of sinking of a well is a function of the rate at
which the bed material is removed from the well bottom.
The following rate of sinking (cm/day) has been
achieved:
 Medium sized well through sandy strata 60 co 90 cm/day
 Medium sized well through clayey strata 40 to 50 cm/day
 Large sized well through sandy strata 50 to 60 cm/day
 Large sized well through clayey strata 30 to 40 cm/day
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 A well sinks by its own weight when the soil below its cutting
edge is removed. If the side friction is so great as to retard
sinking under its own weight, additional weight or kentledge
will have to be added.
 When addition of kentledge does not make the well sink, it is
best to suspend the work for some time and allow the water in
the well to its normal level, then lower the water level by
about 3 m to 6 m by pumping.
 The differential head causes increased flow in the well. This
flow reduces surface friction and helps the well in sinking. In
such cases the well sinks rapidly.
 The sinking process may be hastened by temporarily
reducing the skin friction. The outer surface of the steining is
finished smooth. The surface may also be coated with coal tar
or bentonite solution.
 Alternatively air or water jets may be used which is the
effective method of reducing the skin friction.
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IRC: 78-2000 values
 Tilt of 1 in 80
 Shift of 150mm
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 Non-uniform bearing capacity
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 Method of sinking: Material should be removed
from all sides equally otherwise the well may
experience tilt.
 Sudden sinking due to blasting may also cause
tilting of well.
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 Eccentric grabbing
 Eccentric loading
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 Water jets are used on the outer face of the
higher side. The jets will reduce friction on the
higher side which will be helpful in rectifying tilt.
The method, if used alone, is not very effective
but is helpful if used with other methods.
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 Temporary obstacles like wooden sleepers or
hooking below the cutting edge of the well on the
lower side to avoid further tilt of the well could be
useful.
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Well foundation_Introduction in soil mechanics

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