Mod-2 Lec-7 Pile Foundations-2

Mod-2 Lec-7 Pile Foundations-2


Good afternoon, last lecture we started with
various aspects of Pile Foundations, first I
told you, that what exactly do you mean by pile foundations and why it is required. That,
when the above soil is strata, is not good or poor in nature that it is unable to take
the load, which is coming from the super structure.
Then, we need to extend the foundation to the larger stratum and those types of foundations,
they are called deep foundations and pile foundation is one of that types.
And then, we discussed that what are the various types of pile foundation depending on
the load transferring mechanism, piles can be classified as point bearing pile, skin
friction, then compaction pile, tension pile, etcetera. Then, depending on the material
of construction of pile, these can be classified
as timber piles, then we had steel piles and then we started with concrete piles.
In concrete piles, there were two types of piles, one was Bored Cast in-situ and another
was Driven Pre Cast concrete pile. So, we saw that; what are the main features of these
pile in the last class. Today, let us try to see that what are the various advantages
and disadvantages of these two types of concrete
piles, how one can be superior to another or
what exactly is the disadvantage or advantage of one particular type to the another one. .. First starting with advantages of driven cast
in-situ piles, once again I would like to recapitulate; that what are these words mean
by driven is that you are driving it by any mechanical mean by hammering or by vibration.
Cast in-situ, it is getting casted at the site itself, they can be driven to a predetermined
set; pile lengths are easily adjustable, since they are being casted at site only.
So, let us say at the 11th audio you change the length of the pile, then that can be taken
care of and an enlarged base can be formed which will help in generating much higher
end bearing capacity in a granular bearing stratum. Let us say that, you are driving
a pile in a granular bearing stratum and then towards
the end of the pile, that is where it is tip will be resting. There you can provide an
enlarged base, which will result into larger bearing capacity, this aspect also we discussed
in the last class. This case, reinforce is not needed to take
care of handling or driving stresses piles can be
driven with a closed end. So, as to exclude the effect of ground water, let us say that
while driving the pile you come across somewhere ground water table. So, if the pile is
driven with closed end, there you will not be having any effect of this presence of ground
water table. .. Some of the disadvantages of driven cast in-situ
piles, that heave of adjoining ground surface which could not affect the services
of structures or nearby. That is let us say a
very sophisticated structure is there, in nearby location that does not I mean you have
to protect that from any kind of vibration. So,
in that case while driving the pile, there will
be lot of vibration, which will take place and the heave also can occur, so that is one
of the disadvantage of driven cast in-situ piles.
Lifting of previously driven piles, where the penetration of the pile toes in to the
bearing stratum has not been sufficient, so that is
again one of the disadvantages. Possible damage to uncased piles by necking or waisting
due to lateral forces, when the concrete is green let us say that concreting is going
on and the concrete is green. In that particular case and in case of uncased pile, when after
concreting, you are not taking out the casing out.
So, you are taking out the casing and that type of pile is uncased pile, so possible
damage can be there. .. Then concrete cannot be inspected after completion,
concrete may be weakened, if artesian flow pipes up, the shaft of pile,
when the tube is withdrawn, again this is when
the concrete is weakened; obviously, the pile bearing capacity will decrease. So, that is
not desirable, so it becomes one of the disadvantages of driven cast in-situ piles. It
cannot be driven where headroom is limited, then limitation in length because of lifting
force required to withdraw the casing. So, let us say in case of uncased pile, you
have to take out the casing, so in case of higher
length of the pile, the lifting up of the casing becomes more and more difficult, in
case of driven cast in-situ pile. .. The maximum pile length, which you can go
for, in case of driven cast in-situ pile is of
the order of 25 to 30 meter and allowable load which any driven cast in-situ pile can
take depending on the soil condition can be of
the order of 1500 kilo Newton. . Now, some of the advantages of bored cast
in-situ piles, what do we mean by bored word here, that you have to first create a void
by boring or some excavation measure. And then, you have to fill that with concrete
which will form as bored cast in-situ pile, cast insitu, it is getting casted at the site
only. Very little displacement or no risk of ground
heave, as you know that this falls under the category of non displacement piles. .So, in this case and there is no mechanical
hammering, which is taking place while driving this kind of pile. So, very little
displacement is there and there is no risk of
ground heave at all, length can be readily varied, soil can be inspected and checked
with soil investigation data. See, when you will
be creating the void or excavation, so whatever is the soil, that you are taking
it out, you know at which particular depth, you
are taking that soil out by examining that particular soil.
And by comparing with the soil exploration data, it is gives you to check, whether your
soil exploration data is proper or not it just confirms that information Piles can be
installed in very great lengths and very large diameter, this is one of the main advantage
of bored cast in-situ piles. . End enlargements of up to two to three shaft
diameters are possible in clay or soft rocks; see in the earlier case, it was for driven
cast in-situ piles, it was the enlarged base was
able to form in granular deposit. However, in case of bored cast in-situ pile, this end
enlargement can be possible in clays or soft rocks. It can be installed without much noise
and vibration and with limited headroom. .. Now, some of the disadvantages of bored cast
in-situ piles, that boring methods may loosen sandy or gravelly soil, see if the
soil is of claye in nature then this bored cast in
situ piles are quite advantageous. But, in case of sandy or gravelly soil in the process
of boring or excavation or creating the void,
there can be loosening of the soil strata, which
is not at all desirable as far as construction is concerned.
Concreting under water will pose problem, because it is Cast in-situ, you are creating
a void and then, you are filling that with concrete.
So, in case if the water is present somewhere, along the length of the pile, that
can pose the problem, then concrete cannot be subsequently inspected. Once, you have
created the whole of let us say 30 meter depth, then you have to go on pouring the
concrete and then go on compacting it. But, we really cannot inspect that to what
extent, it has got compacted, whether we have achieved that desired degree of compaction
of concrete or not, so that becomes one of the disadvantage of this kind of piles. Now,
these are susceptible to waisting or necking in squeezing ground, enlarged base bases cannot
be formed in case of granular soil. In case of clay soils yes you can do that, but
in case of granular soils it is not possible. .. Then every time we are talking of enlarged
base or say providing little more cross section as compared to the pile shaft. So,
these kind of piles they are usually called as
under reamed piles, let us see that what exactly are the few features of these kind of pile,
an under reamed pile is a special type of bored pile, which is provided with a bulb
or pedestal at the end.
So, enlarged base, it forms a kind of bulb; that we call as under reamed piles, it is
constructed by making a hole in the ground by means of hand operated auger. So, what
are the steps how it is being carried out, how it is being constructed that we will be
discussing one by one in the subsequent slides, an under reamer is then lowered in the
cleaned hole. So, first a hole is created a bore hole is
created, then you can insert an auger into that and
by hand operation, you can increase the diameter at the end. And then, an under reamer
is lowered in the bore hole which has been cleaned, the under reamer is pressed down
and rotated. .. Then, under pressure the blades open up, the
blades of under reamer, they opens up and due to rotary action the soil is cut and falls
in the bucket. So, once the bore hole is created, you insert the under reamer and then
with pressure you try to rotate it. So, as soon as it, there is a pressure on the under
reamer, it is blades get opened and with the rotary action. In the clay part or in the
soil part, they are below at the end of the pile,
what happens is the soil is getting cut and then it falls into the bucket associated with
it. When the bucket is full, under reamer is pulled
out and cleaned. So, the soil is being cut it gets accumulated there in the bucket associated
with under reamer, it is then pulled out and then it is cleaned and then this process
is being repeated again. So, the under reamer is lowered again and the process is repeated.
After the enlarged end is formed, the reinforcement cage is lowered and the concreting
is done in this case. .. So, you see here with the help of figure,
if I explain you, so first this hole is prepared that
is called bore hole, that is boring guide with the help of this, you insert it the auger,
see it is of this kind of shape. Now, what happens
is with the auger, you just put the rotary action and then you take out the soil. So,
this is the process of preparing this bore hole,
once this bore hole is prepared, you insert the under reamer.
It is of this shape and then you apply the pressure from this point, what happens is,
it is blades get open up and if you rotate it from
this point, that is top. The soil at this particular level will get cut and it will
get deposited in a bucket, which is hum which is
with this particular assembly. Then, you simply take this out, empty the bucket put lower
down the under reamer again with the pressure. Again, the blades will get open and the soil
will again cut and then you can take out the soil, I mean you can repeat this process again
and then you can simply lower down the reinforcement cage in this particular manner
and then the concreting can be done. So, these are the various stages in the construction
of single under reamed pile. .. Now, these under reamed piles, usual size
of piles is of the order of 150 to 200 millimetre shaft diameter, usual length of
piles, they are 3 to 4 meters length diameter of
under reamed portion 2 to 3 times the shaft diameter. So, if a pile is having a diameter
of let us say 500 mm, then you can go for the
size of the bulb for 2 to 3 times the 500 mm,
but usually in case of under reamed pile the diameter of the pile shaft is of the order
of 150 to 200. So, let us say if it is 150, then
the diameter of under reamed portion can be 300 mm to 450 mm.
. .These are considered useful in case of expansive
soils, where the use of shallow spread footings is ruled out due to excessive shrinkage
and swelling behaviours of such soils. That expansive soil with the presence of water
or the increase in temperature, they frequently either shrink or get swells. So,
in that case the shallow footing does not serve
the purpose. So, therefore, the deep foundations, they are usually recommended for such
type of soils and in those type of soils, this under reamed piles they are proved proven
to be very useful.
When, the number of bulbs is increased from 1 to 2, the carrying capacity of pile
increases by about 50 percent. This is just a rough idea it is not a hard and fast rule
that, it will get increased by 50 percent, it will
all depend upon what is the type of the soil, which is surrounding the pile. And then, what
is the bulb diameter or what exactly is the spacing, so all these things, you have to
design, in case of under reamed piles. Let us see some of the experiences that I
had at one particular site, where this pile driving and construction of pile was going
on. As, I told you that in case of driven precast piles, what happen is, your piles
they are casted at some particular work shop. And then, they are being transported to the
site and with the help of driving rigs, they are
being installed in the ground. So, while driving the pile lot of hammering and vibrations,
they occur . So, you can see here it such a huge driving
rig, you see when it just hammers one particular blow to the pile, how much noise
that it will be creating and if there is some .sensitive structure nearby, then it can cause
problem to that. But, in case of barren lands, you can see here, that nearby, there are no
structures only some trees and some barren land, it is there.
So, that is why at this particular site, they could go for this driven piles and then when
we were dealing with the person, who was taking care of these construction of piles,
usually in case of this driven piles, they go for 550 mm diameter in India. So, this
becomes a limitation of driving, I mean in case of driven piles, however in case of bored
cast in-situ piles, you can have the diameter of the pile as large as up to the extent of
1 or 1.5 meters.
So, you see this is the assembly; it is a kind of driving rig and here there is one
hammer, it is just centre at that particular point,
where you have to drive the pile and just with
hammering the pile is getting driven. You see with the help of cables, you can see here,
here and then some of the cables are here, they are held vertical in position.
. Then, in that was the case of driven piles,
in case of bored piles, first you have to prepare
the bore hole and then let us say the soil you have to go for say 25 to 30 meter pile.
So, for that much depth of the pile, when you
prepare the bore hole you have to make sure that the wall of the bore hole they should
not collapse. So, for that you have to provide either some temporary or permanent measure
to stabilise the walls of bore hole. So, what happens either you insert a casing
and then to the concreting and remain left the
casing as it is in the position or another method can be that with casing, you pour the .concrete, when the concreting is over you
take out the casing. Out of these two, usually there is the third one, which is being used
in the construction procedure is that, you know
the bentonite soil. Pond of bentonite slurry is being created and with the help of one
pipe, this bentonite slurry is being filled into
the bore hole which is prepared for the concreting.
So, what happens is this bentonite slurry, it gets in contact with the wall of the bore
hole and then, it for the time being it strengthens
the wall of the bore hole. So, you can see here that, it is filled with this bentonite
slurry, you can see this is the circumference of
that bore hole, this part this much part is seen here in this particular figure. Then,
what is done is with the help of a pipe, this bentonite
slurry is getting filled into this and there is
another pipe, which takes out this particular slurry.
That is once, this slurry is being has come in contact with the soil, which is lying at
the wall of this bore hole, once it get stiffened,
the wall of the bore hole get stabilised. So,
what happens is, then you can take out the remaining slurry, which is lying there in
this and then with the help of water you have to
clean the hole. Because, if this bentonite slurry remains in the bore hole, concrete
will be in contact with this bentonite not with
the soil, which is really lying over there that is not a desirable situation.
So, you see here a worker, he has put some weight, some weight is there with this cable
he is trying to measure that at what particular depth, the soil has been taken out. See you
have to go, let us say for 30 meter, so there should be some measure to ascertain,
whether you have reached to that particular 30 meter length or not. So, this is a kind
of you know check that they provide at the site. .. Then lowering of reinforcement cage in bore
hole, see if you are going for 30 meter long pile, then the length of the reinforcement
cage will be of that order and becomes like these are all steel bars and there you can
see here this circular ones, rounded ones, they
are ties, which tie these vertical main reinforcement of the pile. So, while lowering it
down you see at a stretch you really cannot prepare a reinforcement cage of 30 meter and
in one go you really cannot lower it down. So, what happens is that, first they prepare
a reinforcement cage of, let us say 10 meter, they lower down it to some depth, then they
hold it back, they weld the another reinforcement cage of 10 meter. Once, it is
getting welded, then they lower down this remaining part and so on, so in one go it
is not been lowered. So, you can see that with
the help of this rig, they just go on lowering this particular reinforcement cage. .. Then, as I told you that in one go you cannot
lower this reinforcement, so you see to get this reinforcement cage, in position what
is the kind of arrangement, that usually at site
they go for, it is like a bamboo this thing, slab kind of a thing. They just place it in
this bore hole and then you can see that they have
made, it rested with the help of this lateral reinforcement, what are vertical reinforcement
bar, we are main reinforcement. However, this one is lateral reinforcement
and these are lateral ties, which are either wired or welded to the main reinforcement.
So, this is how they place or keep this reinforcement in position, that is some of
the portion is inside the bore hole and some is
outside the bore hole, why it is done, that will be clear in this one. .. You see here, this is the bamboo that you
are seeing with the help of which the reinforcement has been kept in position and
you can see here that this particular labour is
welding here. So, you see, this was up till this one, there was one reinforcement cage;
some most of the portion has been lowered into the bore hole. However, with the help
of this rig, another reinforcement cage has been
lowered to the extent that, this particular worker is welding it.
So, once it is getting welded at all the location, what happens is, then you can proceed
this lowering down of this particular reinforcement cage. I hope that now, you can
appreciate, why it is necessary to held the reinforcement in position using this particular
measure. So, that the work which has to be done, above the bore hole can be conducted
in this particular manner. .. Then, you can see here, that particular reinforcement
cage is lying, see these are longitudinal reinforcement, these are the,
this is the particular rig, which is your lateral
reinforcement and these are your ties. To bind this vertical parts, in position, then
you can see there is a slab sort of cylindrical
kind of a thing with a hole in between, you can
see here in this one. See here, this is a hole and this kind of
thing is there you see this facilitates, when you
lower down this reinforcement cage in the bore hole, it should not get in contact with
the wall of the bore hole. So, it gives you it
works as a spacer plus being of this shape, it is
not fixed, so it can rotate, so it facilitates the lowering of the reinforcement cage in
the bore hole.
See all these things, you will not be getting in any of the books, it is just that the
experience, that we get from various sites. Because, I did not even IS code any standard
book, they do not give any specification, that you should provide such kind of this
while going for this lateral ties and all. But,
then to ease in construction to ease in work man
ship, all these measures are taken care of while construction is going on. .. You can have a look here, this is a completed
pile, it was of one meter diameter, when we were there at the site. So, we recommend
that the one meter diameter pile should be there. So, this is how it looks, with reinforcement
and all, ever all the concreting has been done and then from top also, all the
concreting has been done and it is quite clean. And, if you see, but if you see the subsequent
slide, then you will really get the feel that how exactly these pile foundations look like
above the ground. . So, you see here these are the groups of piles,
as I mentioned you that, you never go for one particular pile, it is always that group
of piles they are provided. So, you see this .much excavation has been taken place and
then the two views are there, these are all completed piles. And, if you check of some
of them concrete from the top of this particular the pile, what will happen that
reinforcement will get exposed of which is here.
In this particular case, these vertical bars are seen, they are in the process in this
particular pile and then in this particular pile it has not been chipped off. So, you
can see that that the concreting is done in soil cover
is also there little bit of soil layer is there.
. Now, how do you chip off the piles, see once
the concreting is done and we really go for m 30 grade of concrete. So, and if the compaction
has been done properly it becomes really difficult to chip off the concrete
from that particular pile. So, with the help of this
drilling machine, they just chip off that particular concrete, you must be wondering,
that first we completed the pile at 30 meter and
now we are chipping off this particular material from the top.
Why exactly, it is being done, so the answer to this particular question will be obvious
in the subsequent slides, it is not that only
pile will be there, you have to provide a connection in all the piles. Such that, the
load from the super structure will come to this
and will get distributed uniformly to all the piles and then to subsequent soil strata.
So, this reinforcement of the pile will go in
the in that particular structure, which will be
coming just on top of the pile. Let us say it is a kind of a raft that will
come or that will join all the piles, so we will be
seeing typical view of that particular thing. Also then, you will be appreciating, this .particular thing that, why exactly it is
necessary to expose off, some of the reinforcement part from top of the pile.
. So, you can have a look, that many workers
are doing their work, some are drilling here you see, that drilling part is going on some
are just washing off that particular area. On
that day, it was rainy day, so you can see some of the water is getting accumulated at
different places. So, if you see at if you have a look on this particular thing, particular
pile, what happens is that typical lateral tie is also exposed in this case and then
these vertical bars, they have got bended like this
in the different direction. So, likewise if you have an area, in that
one you particularly design that how you have to
provide these piles, what should be the spacing, what should be the arrangement,
whether they should be arranged in triangular fashion or rectangular fashion or some
other pattern. That you have to first decide upon and then only go for the construction,
but this is how the situation looks like at any construction site and this is how it looks
above the ground. .. Then, this is another view, you see here that
all the piles they have chipped off the concrete from the top and they have cleaned
it and this is how I mean all the piles, you have the large area, lot of many number of
piles are there. Some are being cleaned and some are there in the process of that chipping
off and then cleaning of the concrete. . As, I was telling you, that this is what is
this vertical bars, they which are visible here in
this particular picture, they are the main reinforcement of the pile. And, this is the
grid, which you are seeing this one, you can see
that this is the particular grid; that is for the
raft. .When you will be studying the shallow foundation,
then you must have studied this kind of foundation that isolated footing and then
combined footing and one particular type of shallow foundation is raft foundation. So,
when the raft foundation is provided with this
pile foundation, the combination is called pile raft foundation.
So, to have monolithic in construction, that monolithic word means, that the pile and the
raft, they will behave as one single unit, it is not that that the when the load will
be coming, the raft will behave separately and
the piles will be behaving separately know. They will behave as one unit to so to be in
monolithic in construction, to have monolithic construction, this thing is being done.
That the reinforcement of the pile is continued, till the raft is placed; that means that the
main reinforcement of the pile gets into the raft also, to have monolithic construction.
. Then, now you see first we saw that, what
the various types of pile are and then we had
some of the pictures; that I took from one particular construction site, where the pile
for pile construction was going on, that how it
is being constructed. Now, once it has constructed, we have to see that, what exactly
is the magnitude of the load, that a particular pile can take or what exactly is
the load, which are group of piles can take. So,
it is construction procedure is over, now the estimation of it is capacity will be there.
So, what exactly do we mean by that, how we can calculate that load carrying capacity
of the pile in compression, that we will be studying
in the subsequent slides. That in general, this is general thing, general requirements
for satisfactory behaviour of pile .foundations are adequate safety against shear
failure and permissible settlement. The load capacity of pile can be estimated by
several methods, which may be grouped into several categories.
So, in case of shallow foundation, when you were checking that ultimate bearing
capacity, as well as the settlement criteria, likewise in case of pile foundation also you
have to see to it, that pile is not failing against shear. As well as, it is not undergoing
excessive settlement; that means, one is from this safety point of view, another one is
from serviceability point of view, stability and serviceability.
So, the settlement should be within permissible limit as and along with that the pile
should be safe against shear failure. There are various methods from where, this pile
load carrying capacity in compression can be estimated
and usually they are grouped in four categories.
. And they are that you can find out the load
capacity of pile in compression by static load
formulae. There are various formulae for different types of soils, then by conducting pile
load test in the field, let us say that you constructed a pile and there itself you tested
and find it out, that what exactly is the load
that it is taking. Then, pile driving formula depending on what type of hammer that you
are using, what type of motor that you are using, depending on it is efficiency and all
other things. You can estimate this load carrying capacity
of the pile in compression, using this pile driving formulae and then there are some correlations,
which have been developed over .many years using the penetration data. I
hope that, you have studied that standard penetration test, while you studied soil exploration
chapter and from there, once you get the data; that can be correlated to the load
carrying capacity of the pile in compression. So, these are the mainly four categories from
where you can estimate the load carrying capacity of the pile.
. So, let us start with static pile load formulae,
that how we can find out the capacity of the pile using these formulae, when a compressive
load Q is applied at the top of the pile, the
pile will tend to move vertically downward related to the surrounding soil. Obviously,
when you apply the load to any kind of a structure, it will try to displace, so in the case
of pile, also let us say that you are applying a load Q from the top, which is compressive
in nature, the pile will is have the tendency to move with respect to the surrounding soil.
And then, this will result in shear stress to develop between the soil and surface of
the shaft, there will be relative movement in
pile shaft and the soil strata, which is surrounding that pile shaft, which will develop
the shear stress. So, as a result the applied load is distributed as friction load, along
a certain length of pile measured from the top.
So, as soon as the shear stress is developed, whatever is that is applied load, this Q which
is compressive in nature. It will get distributed as the frictional load along the shaft
length. .. But, earlier it will be to some particular
length, now what will happen, that when the load
at the top will go on increasing, the length of generation of this particular friction
load will go on increasing along the pile shaft.
So, you see here, as at the top is increased, the
friction load distribution will extend more and more towards the tip of the pile. Till
at a certain load level, the entire length of the
pile is involved in generating the frictional resistance.
You remember when we when we were discussing about that end bearing pile and skin
friction resistance of the pile there also we saw this exactly the same kind of mechanism.
What happens is that from the top, when the load is being applied, it will be getting
applied slowly, you can just imagine a process of construction which will be taking
place. Let us say first the dead load of any you
cannot just create, let us say if you are going for
7 storey or 8 storey building, you really cannot construct 8 storey building at one
go, you will be going in stages. Let us say first
you completed first storey, then second, then third
and then subsequently, the load will be increasing. So, the load is not applied suddenly, it
is a gradual one. So, as you go on increasing the load on the
pile from the top, the length along which the
friction load is developed, will go on increasing and this length will go on moving
towards the tip of the pile. One situation will come; whole length of pile or the pile
shaft will be creating or causing that whole friction
load. In that case, that particular resistance .is called ultimate skin friction resistance
of pile and we represent that by Q subscript f,
that is Q f. You can see here, this is this is ultimate
skin friction resistance of the pile, where the
friction load is getting mobilised, all along the length of the pile shaft, it is only,
when the load at the top of the pile exceeds Q
f. . That the load in excess of Q f, begins to
be transferred to soil at the base of the pile, so
you see the load is gradually increasing and from there a situation will come, when this
friction load will get mobilised, along the total length of the pile shaft. And that
particular stage, we are calling as ultimate skin friction resistance Q f, so at that
particular situation or condition, your Q which is applied from the top will become
equal to Q f.
Let us say that this Q f you attain after the completion of construction of the second
storey, but you have to go till eight storey building. So, what will happen that, when
you will start the construction of beyond this
second storey or the construction of third storey,
this Q value will go on increasing. The moment it increases the value of Q f, that is the
ultimate skin friction resistance, this will get transferred to the base of the pile.
This load is known as point load and this will go on increasing till the soil at the
base of pile, fails by punching shear failure. It
is not that to the infinite extent you can go on
increasing the load and pile can take that particular load, it has it is capacity. So,
the .frictional capacity you can estimate using
that Q f, whatever is your Q f; that will be the
frictional resistance or the capacity of the pile.
In case the Q exceeds the value of Q f in that particular case, this remaining load
that is Q minus Q f load will gets transferred to
the base of the pile. And you can go on increasing the value of the load Q, till the
soil at the base of the pile fails by punching shear failure. So, whatever is the resistance
of the soil at the base of the pile, that much
only extra load, you can put as point load. The load in bearing at this stage is called
as ultimate point load, which is represented by
Q p u, p is for point, u is ultimate and that is ultimate point load. So, you got familiar
with two terms; that is ultimate skin friction resistance and now this ultimate point load.
. Then, the maximum load which the pile can
support through combined resistance of skin friction and point bearing is known as ultimate
load capacity, which we represent usually by Q u of the pile. So, whatever is the distribution
or how it is getting transferred, whatever the load which is coming on the pile;
that is compressive load from super structure.
First, it will get transferred to the pile shaft in the form of frictional resistance
and once that particular stage is over, where Q becomes
equal to Q f and if Q goes on increasing beyond that, then that particular increase
will get transferred to the point bearing. And
sum of these two, that is ultimate skin friction resistance and ultimate point load, gives
you the maximum load which a pile can support through combined resistance of skin .friction. And point bearing, which is called
as ultimate load capacity or Q u, it is notated by Q u.
So, you see here, this Q u will become equal to Q p u plus Q f, first one is ultimate point
load, second is ultimate skin friction. If this Q p u is quite high than as compared
to this Q f, the pile may be called point bearing
pile; that means, the soil surrounding the pile
shaft is such that. That it is not able to mobilise, that much of the friction resistance
as compared to the ultimate point load.
So, mainly the pile resists the load through point bearing that is this Q p u is quite
higher as compared to Q f; in that case the pile
is called as point bearing pile. Now, there can be
another situation that if, this Q f is quite high as compared to Q p u, then this pile
is called frictional pile.
That the resistance, through friction becomes, so much that the load which is coming
from the super structure is not getting transferred to the tip of the pile or the base of the
pile. In that case, the function of the pile or the load transfer mechanism usually is
by this friction only, so in that case the pile is
called the frictional pile. . The relative proportion of loads carried by
point load and skin friction they depend on the shear strength and elasticity of the soil,
so you see depending on the type of the soil, which is lying surrounding the pile shaft.
Let us say, if able to mobilise good friction resistance along the pile shaft; obviously,
the frictional resistance will be more and then
the pile will be acting as friction pile. .And in case, the soil is not able to provide,
that much frictional resistance or in case the
soil is lying below the pile is resting on the good soil strata or may be rock. Then,
in that case the soil surrounding the pile shaft will
not be able to mobilise, that much of the frictional resistance as compared to the end
bearing resistance and so it will be acting as
point bearing pile. And then, the vertical movement of pile required
to mobilize full skin friction resistance is much smaller than that required to mobilize
full point bearing resistance. This is a very important the aspect which is that vertical
movement of pile required to mobilize full friction resistance is much smaller than that
required to mobilize full point bearing resistance.
So, you can see first as soon as the load is coming on the pile first the friction is
getting mobilized along the length; obviously, when
the load is coming, the pile is getting displaced from that the shear stresses are
getting developed and so they are resulting into
frictional resistance. So, by the time your Q will become equal to Q f; that is at ultimate
skin friction resistance, the deflection of the pile is not that much.
So, to mobilize the full point bearing resistance of the pile, you need large vertical
movement of the pile. . In clays and for driven piles in sand, the
vertical movement required to fully mobilize the
base resistance is about 10 to 20 percent of the base diameter. So, we really have to
keep a check that, what exactly should be the vertical
movement, such that the total skin .friction is being mobilised. Along the length
of the pile shaft and then beyond that the point bearing is getting mobilized.
So, in case of clays and for driven piles in sand, this to mobilise the base resistance
fully, the vertical movement is of the order of 10
to 20 percent of the base diameter. So, let us
say, if you are the diameter of at the base is one meter then around 01 meter to 0.2 meter
is the vertical movement; that will be required to fully mobilize the base resistance.
This usually implies that, when piles are designed to carry a working load equal to
onethird or half of the ultimate load the chances are that, the shaft resistance is
fully mobilised at this working load. So, usually
what happens is that piles are designed to carry a working load equal to one-third or
half of the ultimate load. See ultimate load is
being calculated using this static pile load formula or any other measure.
Then, it is half or one-third and then the design structural design of the pile is carried
out using this particular half or one-third of
this ultimate load. So, as you can see that, the
base to fully mobilize the base resistance, you required 10 to 20 percent vertical
movement of the base diameter. So, at this particular position, there are chances that
the shaft resistance is fully mobilized at that
that particular working load it may happen that
the fully mobilisation of point load may not take place.
. It has been observed that, one case is that,
when the ultimate skin friction resistance is
mobilized, only a fraction of ultimate point load is mobilized, so first is that the skin
friction is getting mobilized. So, whenever a load is coming from the super structure
to a .pile, first it is getting mobilized, along
the pile shaft to a particular length, as you go on
increasing this value of Q. The length over which this will get mobilized
or the length along which the frictional load will get mobilized will go on increasing
and a stage will come, where you will achieve ultimate skin friction resistance.
So, when the ultimate skin friction resistance is
mobilized, what does this means, that at this particular point the applied load becomes
equal to the ultimate skin friction resistance. In this particular case, only a fraction of
the ultimate point load is mobilised, that is very
obvious and when you will increase the load which is coming from the super structure,
beyond this particular stage, then this ultimate point load will go on increasing. And
then, the second one is, when the ultimate point load is mobilized, the skin friction
resistance has decreased to a lower value, than it is peak.
See, what is happening in this procedure is that, as you are increasing the value of Q,
which is coming from the super structure. A condition is occurring, that a particular
condition is occurring, that this load Q is becoming equal to ultimate skin friction
resistance, there will be a point, when this point bearing resistance will be just equal
to 0. When you will go on increasing the load, what
will happen, the resistance which has been developed at the tip of the pile will
go on transferring to the point bearing resistance.
And in that process at any particular point of time, what will happen is that, the ultimate
skin friction resistance value will be always less than it is peak value. So, what is there
is the second point, which we need to keep in
mind that when the ultimate point load is mobilized, the skin friction resistance will
be reduced to the lower value from it is peak value.
So, today we saw that, what are the various advantages and disadvantages of concrete
piles; that is two types of concrete piles, that we discussed one was driven cast in-situ
and another was bored cast in-situ piles. Then, I shared some of my experiences with
you, as far as the construction of the pile is concerned.
We saw that, how driving rig drives the pile, how high is that particular rig, then what
how this reinforcement cage is being lowered, what are the various techniques, that is .being adopted at as far as the construction
of the pile is concerned. How the piles can be
constructed monolithically to the super structure or other parts of the foundation.
And then, after the construction, it was the point of concern; that how we can find out
or estimate the load capacity of the pile, under
the compressive loads. So, we saw that there are four categories, that we can find out
or four ways; that we can find out this load capacity of the pile in compression.
They were static load formulae, then pile load test and then dynamic load formulae and
the fourth one was correlation with penetration data. Then, we started the detail of static
pile load formulae and in that one, we saw that; what exactly is the load transfer
mechanism from the super structure to the pile.
We saw that when a compressive load from the super structure comes to a pile, first the
skin friction is mobilized, a condition is reached. When this friction becomes equal
to the applied load and when this load is further
applied, then it gets transferred as point bearing resistance. And then, depending on
which whichever is the more, whether the ultimate skin friction resistance is more
or the ultimate point bearing is more, the pile is
named as skin friction pile or end bearing pile.
We will see the mathematical details of static pile load formulae, how we can utilise
them as far as estimating the pile load capacity in compression is concerned. And then,
we will proceed that, how we can estimate the pile load capacity using in-situ tests
and all other things in the subsequent classes.
Thank you. .

10 thoughts on “Mod-2 Lec-7 Pile Foundations-2

  1. Chipping is done to remove top portion of concrete which got mixed with bentonite solution and soil. I am not sure it is done to maintain continuity with pile cap as it can be achieved without chipping and just by leaving reinforcement

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