Mod-01 Lec-01 Basics of Soil Mechanics I


Today we are going to just briefly look at
the basics as we discussed yesterday the 5 modules in this subject we will spend some
time on the 1st module So the 1st module we are going to have brief introduction of various
types of foundations for offshore structures as well to some extent coastal structures
as you can see I have just briefly trying to introduce so you can see the difference
between coastal and offshore so that you can appreciate what difficulty we will face during
the design of you know deep foundations And then we will go on to a bit on construction
sequence which will give you an idea how we can actually differentiate between shallow
and deep foundation Then we will look at some of the codes and then the method of design
that is the most critical one to complete So I will try to do this within next few hours
probably within 2 weeks And followed by we will introduce Uhh you know brief idea about
how these soil formations take place over the period of time and the types of soil formations
so that you can appreciate as the material for engineering how you can quantify the characteristics
for strength So that is the idea behind you know basically
you know the different types formations and then we will take forward with engineering
properties how do we arrive at them either we do field testing or testing at the laboratory
and how do we bring the samples to the lab So basically the soil investigation part is
quite important in the whole process of project development because that is fair you know
most of the information is coming from and if the information is incorrect it could lead
to a wrong direction in the design So the soil investigation is a primary part
in fact that is the 1st activity you will carry out when you are thinking of development
of any project basically to go forward which type of structure you would like to place
on this place For example yesterday we were talking about gravity structure versus the
friction platform Depending on the type of soil soil layers how far soft soil or whether
the strong soil is on the surface this will completely change what type of structure you
can actually plan for the site So that is why the 1st activity that you will be planning
is to find out what type of pile soil investigation is integral part of the design process We will just quickly look at why we need a
foundation I think we spent a lot of time yesterday discussing about load transfer from
structures superstructure to the ground So you can see the evolution of various types
of foundations that require in terms of transfer of load So if you look at the single-storey
building versus multistorey building versus coastal structure where you have several activities
I think some of you might have seen Pearl Harbor if you visited either Chennai Coast
or other coast you could see structures on the coastline which makes them interface between
interface between land and sea so that you can bring in ships you can bring in floating
systems to a stationary position to transfer cargo men and material So basically you can see a porting harbour
structure is constructed near coast just because we need an interface because the interface
in nature is not very good For example if you want to bring a boat near the beach maybe
yes possible but if it is a big ship it will get grounded so you cannot move So that is
why you need to create an interface with sufficient water depth wherein you can go there So that
is the idea of the porting harbours jetties So you can see that variety of ideas can be
Uhh looked at For example you can also do a pile foundation for the small building but
you need to think about the necessity do we need it you know the load is quite small for
a two-storey building I do not think you will have a huge load the brick wall and floor
load So you do not need a pile foundation whereas
if it is a multistorey building and the loads are concentrated multifold increase in the
loads will cause excessive deformation to the soil just below maybe a soft soil or maybe
strong soil will make the difference whether we need a pile foundation or shallow footing
In some cases if the soil is good you can still go for a very good footing type of foundation
but then you need to look at the horizontal stability whether you are able to provide
that with such type of foundations In many cases you know basically if it is a high-rise
building you will go for pile formation to get both vertical equilibrium and the horizontal
equilibrium So if you look at the right side what we are
trying to do is basically shift ideas you look at the offshore platform you can think
of going for shallow foundation yes if the soil is very good but then the stability for
horizontal and overturning needs to be established which we will talk about in detail So you
can see here the type of construction that we are looking at is completely different
If you look at this building versus you could have this building as big as and offshore
platform like we have shown some pictures earlier But then you can see here the loading
is different there is no wave loads whereas when you come to offshore platforms you got
a large amount of horizontal loads compared to onshore structures So the design shift is required So that is
why the foundation what we are looking at for any type of structure is integral part
of the structure itself you cannot actually separate them So you need to understand the
behaviour of structures as well the foundation so that we could design all depends on what
is the subsurface conditions I will just put some typical profile you see here soft soil
medium soil and then bearing stratum and the hard-rock Typically this will bea kind of
idea that you will encounter in many places but not necessary that this kind of sequence
you will see some places suddenly hard-rock in between or a very steep soil and then followed
by or underlined by other medium characteristic materialů But So in such type of places you will have difficulties
in installation which we will talk about a later time So the need of foundation is very
important what we want to try and transfer the load without causing problems to the structure
So what are the problems for example we take a simple single-storey brick walled building
if the structure settles vertically just because the soil is too soft so what will happen Basic
idea is the structure will start showing cracks if there is a differential settlement If it is a uniform settlement nothing will
happen in fact simply it will not be showing any cause of concern except the building is
going underground So the next time when you want to go inside the building you will walk
downwards is not it which is not very good So excessive deformation vertically even though
it is uniform not preferred by because we are going to occupy the building so it is
going down But no guarantee that the deflection or displacement will be very uniform You will
see a small difference immediately you will see that the architectural finishes or even
structural elements will start showing cracks So one of the important criteria is displacement
based criteria to limit the deformation of the soil is within the limits so that the
structure does not show any kind of distress against the loading that is going to be survived
for during the design life So we will just look at one by one and basically that is the
need of the foundation the foundation of variety of kinds can be developed what is our idea
is to make sure that the functional requirements are not violated For example you design a
residential building and the purpose for which it is built has to be served If it is offshore platform you are building
it for drilling you are building it for production so basically we need to look at functional
requirements which could be it could be different for each type of category For example residential
building we do not want a crack of any kind because we will be afraid the population will
not be guaranteed enough safe survival Whereas you go to offshore platform or deflation of
a metre also can be accommodated because it is a different type of structure different
class of material used you will not find any crack then So that means there is no uniform
requirement of design requirement for shallow foundation pile foundation and same pile foundation
for different types of structures will have different types of requirements For example use of pile formation for buildings
you will restrict the horizontal displacement to 5 MM whereas if you use a pile formation
to offshore structures you will restrict nothing because we are not worried about horizontal
displacement what we are worried about is the vertical capacity and the horizontal displacement
will cause more stresses so you will have to provide sufficient material strength to
take care of the stresses So that is where you will find you could have multiple solutions
for the same location depending on what type of structure you are building For example building a factory building onshore
you know the factory building can have cranes moving cranes or your verti cranes they cannot
sustain even a smaller displacement difference here For example from left side to right side
less than 2 MM the design criteria is 2 MM because if you have a differential settlement
of the building by 5 MM the crane will get stuck it cannot move So such places you design
a foundation where it does not deflect more than 2 MM So you can see such type of design
changes So same place if you look at another different
type of building where Crane is not required you do not need to design so stringent requirements
You can change the design criteria so that is where the type of foundation changes from
one type to other depending on what is required for the design In this we are going to not
discuss building bridges and onshore structures what we are going to look at 3 categories
you know retaining walls is part of the development I think most of the places if you go to ports
and harbours we call it Quay Walls retaining walls just to retain the air from falling
down so that you can bring the ship closer to the coastline Or jetty structures basically Uhh structures
constructed slightly away from coast You will see some photographs later on you can see
that the ships can be berthed against structure slightly away because water depth near the
coast is very small Then you can transfer the cargo by other means either by constructing
a bridge or by pipelines Then the last one category is offshore platform wherein we use
it for island gas exploration activities Just briefly the purpose for the coastal structures
seawalls primarily for coastal protection I think you can go around many fishing villages
you will see that the erosion is happening if you travel to North Chennai you know so
these seawalls are the primary part of coastal protection for you know the coastal villages
coastal structures sometimes you may have refineries you may have power plants which
are located most of the time near the coast because of the availability of large volume
of seawater for cooling purposes that is why most of the power plant will be located near
the coastline In order to protect such type of facilities
against erosion you try to build either a seawall or breakwaters just to diverge the
energy to other side And then jetties and harbours which are primary part of the sea
to land interface you know you might see so many cargo berths all around the world You
can go along the east coast many are there Also commercial harbours which are meant for
you know several commercial purposes including tankers high tankers you have container berths
and then passenger berths sometimes you have A typical seawall you can see here on the
left side picture is just a concrete wall nothing else But what it does is it is doing
2 things one is retaining the soil on this side exposed to sea activities If this wall
is not there everyday some soil will be carried away and you will see the land is slightly
reducing every time and then And it does also retain the earth and form a stable ground
here Otherwise we will not be able to use this particular place and that is the idea
behind the construction of the wall Many places you will see of different kind
you see here this seawall is made of just only rocks just rocks dumped around the coastline
which gives you such type of Of course all depends on the durability and the sea conditions
here if the sea condition is very high then this kind of rock dumping will not work you
may require construction of this kind You can see this is one of the jetties which
was built several years back You can see there are 2 jetties around and there is a connectivity
between the jetty and the this is actually an island in so you can see the oil transfer
so this is the place where the ships will come and they will transfer the oil from the
ship to the shore And this oil goes to the refinery which is located the slightly away
So this water depth is 25 meters not small so you can see why the jetty is constructed
slightly away I just to make sure the ships have sufficient depth to come there Of course you could do one thing you can construct
the jetty very close to the coastline by doing this removal of the soil by dredging which
is another activity which you can plant This is the jetty that is located off Karaloor
which we designed in 2007 and 8 basically 2 kilometres away from the ocean which is
the purpose of doing such thing is basically to avoid dredging Because if you keep the
jetty very close everyday every year you have to do the maintenance dredging which is very
expensive So you just put the jetty on the waterside only thing is the transfer of cargo
by pipeline from jetty to land there is no bridge So you could see the purpose and the structures
are designed in such a way that this can survive for open sea conditions cyclonic conditions
in fact they jetty has survived 2 cyclones within last 2 years Every year we get a cyclone
exactly crossing at this location Multipurpose Harbor I think this is something that you
will see around you know many major ports and harbours in our country If you go to Chennai
port this is basically the Ennoor port You can see the breakwater and this breakwater
is protecting the inner area from external sea conditions so that the berths located
inside is exposed to lower sea state condition so that ships and vessels coming inside can
transfer cargo without much problem So comparison to this you see this is an open sea condition
you cannot use this jetty all the time whenever there is a high sea condition you cannot bring
the ship closer Whereas when you are inside you could do most of the time except maybe
a severe cyclone So that the difference can see between all-weather and open sea ports All of them involve one thing important the
retaining structures For example if you look at this you have to retain the this soil in
this particular area so that the soil is not getting eroded and basically you will construct
some kind of wall that is why we need to learn about what is that it is basically retaining
wall or sometimes we call it seawall You see this jetty is also located outside
the coastline but then protected by a big breakwater on the seaside so that the incoming
waves will be diverted and protecting the jetty within that area So this is another
type of idea where you can have design of a breakwater I think some of you might be
already having some idea breakwater is nothing but barrier wall You can actually construct
in RC Wall in some places we do actually instead of random we construct a wall using RC but
then the customer itself is quite difficult because it is in water So you need to think about how we can do a
construction of a seawall in the middle of the ocean you have to think about it it is
not feasible So you need to 1st fill up the soil then construct the wall then remove the
soil so many times people do that But the easiest way is to do a rock dumping of certain
height certain width so that it will be stable against the sea waves coming from outside
So this side is exposed to see waves and the jetty is well protected All of them involve
one important aspect is the soil mechanics because this is going to be a huge structure
which is going to be resting under seabed Over a period of time if it is going to settle
we just need to make sure that the calculations are made for the soil mechanics How do we
build pile foundation in coastal structures Most of the times you know you will see 99
percent will use concrete piles I think you will see the difference in cost very high
If you look at the still pile versus concrete pile most of them will be circular in shape
very rarely we use square or rectangular cross-sections for onshore structures Sometimes people do use but for coastal and
offshore applications we never use I think we learned about that last semester regarding
the circular sections providing efficient form of several kind including strength and
hydrodynamic efficiency so that is why we use circular shape So these circular shapes
helps also in construction and you can see here circular shape means you can use the
circular hollow section pipes readily available as pipes So you can use it So in construction
of circular solid RC pile the sequence you can see in the picture when you actually make
a hole in the seabed or in the side it has to be stable Like if you go to villages or even in cities
open excavation foundations you might have seen just do an excavation and make sure that
the soil does not fall down we may actually do we side shuttering in some places some
places if the ground is very good you simply make an open excavation and just make the
foundation because it is only shallow depth But here we are talking about several metres
so when you excavate inside something like this what will happen the soil will collapse So what we need is the shuttering so while
we just drive a steel pipe of smaller thickness up to a depth where the soil is stable after
which even if you do actually excavation soil is going to be almost not going to fall down
So the method of construction of concrete pile is very simple drive a liner excavate
the soil by several means you have either boring or chisel and bale many many methods
are there so you just remove the soil somehow and stabilise the soil by means of a denser
fluid pump inside That means the pressure created by a fluid inside is higher than the
outside so that your the soil without the liner is going to be sustaining without falling
down So normally the denser fluid is nothing but
bentonite slurry it is very similar to cementers material but with the specific viscosity and
density you pump inside continuously And basically will sustain this soil will not fall down
and then you replace this area with concrete So what will happen concrete is denser than
this fluid this fluid will come out and before that you lower the reinforcement cage and
pump in the concrete the concrete will displace the bentonite slurry out and leave it for
several days which will make the concrete cured So the construction of marine concrete piles
is obviously a long-term process it will take probably 2 days 3 days the whole process of
driving a liner excavating the soil pumping bentonite slurry and putting the reinforcement
cage and filling concrete by tremie and then curing it So you could see it is not a very
simple process so if you have several piles it may take a longer time So this method why
we need to understand why I was just talking about this Now you have created an interface between
the superstructure which is going to sit on top of this basically here on the soil and
the interface needs to be understand because the interface partly having a steel casing
and partly the concrete is directly expose You see here at the bottom concrete is directly
in touch with the soil Now the load is applied to the top we need to understand how the load
transfer going to happen from the structure to the pile pile is also a structure in fact
it is a it can be a concrete structure it can be steel structure Then from that part to the soil somehow we
need to transfer and that is where we need to understand so that the design can be easily
performed So that is why we need to know how it is constructed I think I have explained
that procedure in this line from so if you are asked to write you can read this procedure
it is exactly what I have explained there How do we construct a retaining wall for coastal
structures Very similar except that it is not circular shape it is in rectangular but
along the length of the wall only difference is the liner may not be used Because driving a liner of such kind is not
feasible because it is longer in length So what we normally do is we try to do this construction
without the liner but of course only up to that certain depth maybe we can put a guide
wall which will be either steel plate or it can be concrete wall but only about 1 metre
half a meter just to avoid collapsing of topsoil But the remainder has to be self-sustaining
using bentonite slurry So the construction is not feasible when you have water on right
side is not it because it is exposed So what we normally do is we fill up soil
here on this side and make a slope stable flow and construct the wall after the construction
of the wall to remove this soil by dredging or by other means So you can see the difficulty
if it is a pile you do not need to worry because the liner is providing interface between the
water and inside whereas here we have a problem of construction So in here of course one one
thing is very clear you cannot get any strength from this soil on the left side because the
right side there is no soil So you will be able to transfer the load only
the part of the wall below the dredged level which is basically this Most of the olden
day berth is constructed using this type of idea If you go to Chennai port or many of
the British time ports you know Uhh basically construction using either the step of ideas
of making concrete walls or sometimes gravity wall You will see from this type of picture something
like this Many of the olden days ports constructed during British period is simply aligning and
just keeping big big concrete blocks or even rocks so that they are stable but of course
this is feasible only when you have very good ground below the dredged level Otherwise what
will happen it will keep sinking See the other idea is a retaining wall needs to be taking
sustained loading from the activities of the portal harbour So that is why we need to have sufficient
strength in the wall itself as the structural element and then sufficient strength in the
soil to take the horizontal load because you will see that the surcharge is going to be
very large The portal harbour activity you will have cranes you will have transfers so
you will have stacking of material so you will have a huge surcharge where in this is
a simple soil mechanics to transfer the load horizontally and sustain reasonable deflection
What is that reasonable deflection we need to verify Whether it is a 1 metre or 5 MM
that will govern the design of this wall and the penetration of theů Imagine if this wall is not penetrating so
much it is just half a metre what will happen the whole wall will be drifting horizontally
So we need to have sufficient embedment of the foundation into the ground Or you could
also come up with an idea of sustaining lesser deflection by means of holding this wall backwards
But one of the important thing is the failure plane You know when you have such a system
you have to have the retaining wall to hold this anchor it should be away If this is whole
thing within this failure block the whole thing will that is the major important activity
that you need to look at when you are designing anchor wall you need to make sure that anchor
itself is away and the failure plane that not cross each other The failure plane of the retaining wall and
the anchor wall or anchor block have to be separated so that they areů Otherwise what
will happen the stresses on the soil from this and this will overlap there will be double
stresses the soil will fail terribly And then sometimes we do this kind of idea most of
the recent designs we have done for ports Chennai port or the others we will be using
relieving kind of platform where you see here the loads from the harbour activities is directly
transferred to a pile not going as lateral process to the front wall So you can see here how we have manipulated
that the load is taken directly by this and going down to the soil here so that the surcharge
pressure horizontal pressure on the vertical wall is limited only to the soil behind it
So the surcharge is taken directly to the so this is some idea where if the soil conditions
are very bad for example the retaining wall here are the soil is very soft you will not
be able to design In such cases of course slightly expensive but then at least we are
able to find a solution Just give you an idea of variety of kinds
some cases we have done this kind of idea where the pile is in front wall is at the
back again depending upon the situation you have to design it Anchor block versus the
gravity type gravity type is almost similar only thing is the stability is obtained by
its own weight provided if the soil and ground conditions permit because otherwise it will
not work out A typical picture of a recent project which
we were doing in Qatar you can see here the existing ground conditions are like this so
simply prepare the ground dump the concrete blocks of prefabricated with variety of sizes
with interlocking You can see here interlocking shear interlocking and then just fill up the
remainder of the portion by layered engineered fill and prepare the ground So you can make
this idea what we are doing here if you make this one up here you need to drift this area So by doing this you avoided completely any
removal of the seabed soil by dredging which is quite good And also you have created a
new space which is basically not occupying the existing land space So reclamation is
together So most of the reclamation is like this you create a wall and fill up the gap
between the land and the wall itself Some other type of constructions where you
have jetties built away from the coast it is nowhere near So you can see you can use
steel pile simply driven into the ground and construct the deck or you can construct a
concrete wall concrete pipe and then construct a deck The only difference is every material
has to be transported from coastline to this particular place no access so that makes slightly
difficult for construction but otherwise the procedure is exactly same So you see here in this one there is no concrete
involved you take a circular hollow section drive it and achieve sufficient capacity from
the pile depth embedded into the ground Now you see here this this and is open this is
not closed after not having clear idea the pipe is open-ended that is why sometimes we
call it open-ended paid pile Because if you close it what will happen you will not be
able to drive the amount of resistance that will develop during driving will be very large
that you will not be able to drive So that is why we drive it open-ended but
alternatively you can also make a hole in the ground and put this pile and do a concrete
all-around Sometimes we do this whenever you encounter a very hard rock for example but
still we want to go for still pile so you can actually drill hole in the ground and
simply place the pipe and do a concrete ground but very expensive that is why many times
we do not prefer to use this So so far I think you have got a clear idea
of the class of structure that is being constructed on coastal areas or maybe on land Now we will
quickly look at offshore structures basically for hydrocarbon exploration I think most of
you are very familiar So the purpose of the offshore platform is to do drilling and production
of oil and gas You might see this picture in the earlier lectures variety of class of
structures wherein we require some kind of foundation Either it is a fixed still pile
foundation here or other classes of foundation to hold back the floating structures it could
be section anchors or it could be gravity type anchor So you cannot leave away from the foundation
design for any offshore type of project A typical jacket where the pile foundations
are focused at the 4 corners So you can see here the purpose why we do this instead of
distributing the number of foundations to everywhere This will be effective because
all the loads are decoupled at the extreme points of the structures especially when you
have a rectangular portal frame and you have a horizontal load and you have a vertical
load I think when we were looking at design course if you apply the horizontal load it
will be decoupled to the maximum at the extreme points So if you put a pile formation at the middle
it will be of not much use it can only carry the local vertical load coming from the deck
structure So if you locate them further apart 2 things will happen the decoupling distance
becomes larger the magnitude of the load arising from the horizontal load will be reduced I
think we did a simple problem if you apply a horizontal load the decoupling will be moment
divided by the distance between the 2 foundation locations So that is why we keep the foundations as
much as to the corners so that the decoupled forces on the foundation will be minimal Also
you do not want to have too many piles in offshore systems because as you know very
well many more you can actually instead of having 12 only here you can have 20 of them
But then as we discussed yesterday minimising risk is the most important in any offshore
projects because you want to reduce the time that you spend offshore The more the time that you stay the higher
the risk with respect to many aspects is with respect to exposure to see conditions risk
with respect to accidents risk with respect to supply so you can see the lesser the time
you spend is better So that is why minimise the number of foundations that you require
to construct drive or install Many times we keep 2 corners some cases you may actually
have foundation here and here all depends on the design configurations It is a typical
platform you can see here is an 8 legged with 12 numbers of corner piles You could have many number of solutions to
this this is not the finished idea that every 8 legged jacket will have only 12 number of
piles you may have 4 piles you may have 8 piles depending on the magnitude of loads
that is coming at this location and the type of size that you actually have for the site The other types of Uhh foundations normally
used for FPSO section casings or mooring lines You can simply replace this one with a gravity
type big concrete block Many times we do this depending on the magnitude of the load What
it has to survive is the drag load coming from the floating systems and has to sustain
with a minimum displacement So that several times we use fluke anchors If you see ships
standing outside the harbour area they have the anchors which are simply a big size weight
with a specific shape so that it can go into the sub ground And when the load is applied it will be resisting
against dragging so we call it drag anchors sometimes But they are temporary because you
want to remove them next time when you want to relocate the ship you simply have to pull
out and remove them Whereas these FPSOs are going to be permanent for a longer period
of time so you want to have some kind of better system So you can also drive a pile instead
of this kind of large diameter many many options you can drive a pile very similar to platform
or you can have a large cup like what you see here it is an inverted cup simply put
down to the ground We will talk about this section casing and
the concept by which it is achieving its capacity in I think in several classes later Then we
have Uhh TLP you can see here TLP the load transfer is slightly different you know part
of the load is taken by the buoyancy because it is the floating system But then you also
have tethers or so-called vertical mooring lines you can call them And holding them in
vertical position from the movement So you need to have sufficient anchor capacity so
that does not come out And basically of gravity type pile type you can design either way But the gravity loads are taken by the buoyancy
horizontal loads will get transmitted to these tethers but not fully because they are flexible
so you will be allowing the system to move horizontally so part of the load will go as
the tether moves but most of the loads are resisted by buoyancy in vertical condition
So TLPs also do have pile formation of gravity formation depending on the magnitude of loads
and design Like the FPSOs talk about the FPSO with the
Turret so you can clearly will require a good-sized foundation it could be anchors or it could
be pile foundations Jack up I think last time we discussed the jack up whenever design class
quite substantially And you could see here the computation of bearing capacity for the
spud sitting on the seabed It is an important aspect in the drilling time because everytime
you are going to relocate this jack up So everytime you have to do the foundation design
make sure that it is able to sustain the loads during the duration of one year or so for
drilling purposes And basically everytime when you relocate
the jack up will be a new project Unlike jacket is installed one-time and one design whereas
the jack up you have to do this as time you type of soil new type of environment new type
of load conditions I think this picture you might have seen soil
plays a major role in the stability itself and you must make sure that it does not also
have instability against rotation I think we discussed about this problem in detail
Basically the levelness of the ground evenness of the layers so a lot of study needs to be
carried out before you can establish that the jack up can be lowered at this particular
site So now come down to final 3 foundations that
we will go through in depth in this course is basically the pile foundation is primary
part of this course and then we will also look at a shallow footing very similar to
shallow footing is the mat foundation which is used for temporary purposes to keep the
jacket in equilibrium position without shrinking without drifting because you are going to
bring the jacket and place it on a particular site and make sure that the jacket does not
sink That means we need to have a temporary foundation prior to piling and basically just
call the mud mat And the 3rd class of foundation basically
for gravity type of platforms we have a large base with ballast it could be solid ballast
or it could be liquid by ballast with and without shear keys to obtain horizontal resistance
from passive side pressure So these are the 3 types of foundation we will be looking at
in detail Of course most part of the lecture we will be focusing on pile foundation the
last few sessions we will be talking about design of mud mats and gravity type of foundations The loading I think you will be familiar predominantly
gravity loads for gravity type of platform but for the class of structures that we are
looking at for jacket type of structures the larger wave loads or the environmental loads
from wave current and wind makes the design slightly different from onshore Of course
you will also be designing for Seismic process which is derived from your gravity loading
Earth pressure may not be an issue for offshore structure except for coastal structures Mooring and berthing loads also magnitude
wise will be very smaller for offshore structures compared to the coastal structures There the
jetties or port and Harbor structures will have a larger because you bring in a big ship
whereas for offshore structures we do not bring to bigger ships there we may have smaller
boats for supply We have this picture basically last time you
know the load transfer the spacing between piles the reason why we widen the base so
the magnitude of reaction reduces is the best principle that he wanted to adapt so that
you achieve actually the material does not change the configuration makes efficient form
of load transfer you know that is the idea behind because you have the same diameter
of the pipe but then you have made wide at the base Of course you can do that here but
in this particular aspect we want to keep the template for driving in accurate position Similar idea and the configuration of the
jacket is going to change the amount of loading that is going to the soil so the types of
load that we have on these structures Gravity foundations very similar principle only thing
is there is no depth you know more than 10 metres most of the gravity foundations will
not be able to sink further So what we are looking at is sinking the gravity type of
foundation to a stable ground where subsequent time bound displacement will not happen You
know you need to find such type of layer and also you have reasonable depth of skirt which
will provide you for horizontal stability Otherwise you will have to rely purely on
the frictional resistance between the soil and the foundation So the principle is very
simple the stability is achieved by its own weight and most of the gravity type of platforms
are built where you have a very good ground below 5 metre below or slightly low higher

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