 I am here to discuss with you on topic raft foundation. What are the learning outcomes of this session? At the end of the session, students will be able to explain necessity of raft foundation, its types and suitability. What is raft foundation dear students? Rraft foundation also known as mat foundation is actually a thick concrete slab resting on a large area of soil reinforced with steel, supporting columns or walls and transfer loads from the structure to the soil. So raft is nothing but a slab acting as a foundation, it is also called a mat foundation. Generally raft foundation is spread over the entire area of the structure it is supporting and raft foundation is generally used to support structures like residential or commercial buildings where soil condition is poor, storage tanks, silos, steel equipment etc. So raft foundation is used when bearing capacity of soil is low and also structure is heavily loaded. So here you can see a photograph of raft foundation, complete slab is provided. So beam and slab network is used to transfer load coming from the super structure through columns to the soil below. So let us see working principle of raft foundation. To get a better idea about when to use raft foundation it is important to understand how raft foundation works. So raft foundation transmits the total load from the building to the entire ground core area. Stress distribution mechanism of raft foundation is very simple, total weight of the structure and cell weight of the mat is calculated and is divided by the total area of the foundation it is covering to calculate the stress on the soil. As in the case of raft foundation the contact area of the foundation with soil is much more than any other type of foundation. So the load is distributed over the larger area and thus the stress on soil is lesser and the possibility of shear failure of soil is reduced. So what happens larger area of this particular foundation helps in distributing the load over a larger area. So intensity of pressure on soil below will be less, intensity will be less and shear failure of soil will also be reduced due to this type of foundation. Now when to choose this raft foundation? Remember dear friends raft foundation is preferred when soil has a low bearing capacity then load of the structure has to be distributed over a larger area. Again individual or any other foundation area would approximately cover 50 percent of the total ground area beneath the structure. If you are providing individual footing and total footing area of all these columns say is more than 50 percent of the total ground area beneath the structure then there is no fun in providing individual footings you can better go for mat foundation or raft foundation. The columns or walls are placed so closely individual footings would overlap in those cases. So mat foundation is the best alternative for these types of conditions. And also when stress on soil is to be reduced or needs to be reduced because soil is of low bearing capacity. There is a possibility of differential settlement in case individual footing is used. When soil state are unpredictable and contain pockets of compressible soil, when basement is to be constructed and any other type of footing cannot be used advantageously. So in all these cases you can go for raft foundation remember. Now let us see types of raft foundation. There are different types of raft foundation. Now few of them are flat plate mat foundation, plate thickened under the column, two-way beam and slab raft, plate raft with pedestals, piled raft, rigid frame mat or cellular raft foundation. So let us see one after another these in little detail. So flat plate mat is the simplest form of raft foundation. This type of mat is used when the columns and walls are uniformly spaced at small intervals and the subjected loads are also relatively small. Reinforcement is placed in both directions and more reinforcement is required at the column locations and load bearing walls. Here we can see a photograph of this particular type. So here only a slab is provided, flat plate raft foundation it is called. There are no beams provided, these slabs are not thickened anywhere. Some thickness is provided. So such type of raft foundation is called flat plate raft foundation. You can see plan and section also and these are used for low loads. What are the limitations of this? The thickness of this type of raft foundation is generally restricted within 300 mm remember within 300 mm for economic reason. A thicker slab would not be economical. So up to 300 mm thickness you can go for this type of foundation. Now next plate plate thickened under the column. So next type is the plates are or this raft slab is thickened under the columns. You can see in the photograph here. Below the column the thickness of slab is increased so that the slab can be rigidified and the load on this particular foundations can be more, this can take more load. Now next type is two-way beam and slab raft. So in this particular type of raft foundation what is done? Beams are cast monolithically with the slab connecting columns and walls. Here you can see two-way beam and slab raft. So load from column comes to the beams and beams transfer the load to the slab below. So this is two-way beam and slab raft type of foundation. Now next is plate raft with pedestals. So in this particular type of foundation what is done? The function of this mat is same as that of flat plate thickened under columns and in this mat pedestals, in this mat foundation pedestals are provided at the base of the columns. So here you can see pedestals are provided at the base of the column over slab. That means thickness of column is increased when it comes to in contact with slab. So here you will find that load from slab is distributed, load from column is distributed over a larger area when it gets transferred to slab. So that punching shear failure is prevented or intensity of pressure is minimized at the points of junction. Piled raft, this type of raft foundation is supported on piles. A piled raft is used when the soil at a shallow depth is highly compressible and the water table is high. Piles under raft help in reducing settlement and provides resistance against buoyancy. So we will see a photograph, here you can see. So piles are used and over these piles a raft is provided, remember. So pile cap here acts as a raft, pile cap here acts as a raft. So it is called piled raft foundation and when this particular type of foundation is used when the soil at a shallow depth is highly compressible and also water table is high and this type of foundation will reduce settlement and provide resistance against buoyancy because water is there in this particular area below the foundation. Resid frame mat is the next type. In this type of raft the foundation walls act as a dip beam. Resid frame mat is referred when columns carry extremely heavy loads and the connecting beams exceeds 90 centimeter depth, remember. So here you can see this is a rigid frame mat. So depth of these particular walls are increased, voids are provided and depths are increased. So two slabs are there parallel to each other at certain height. So what will happen? When you increase height moment of resistance will be increased and to observe economy or make the structure economical, voids are provided in between. So cost of material will also be less and moment of resistance will be much more because depth is more. These beams which are in between two voids those may act as dip beams also remember depending upon the span. So let us stop while here dear students. Here are two questions for you answer them neatly. The first question is the thickness of flat mat type of raft foundation is generally restricted within dash dash dash mm for economic reason and rigid frame mat is referred when columns carry extremely heavy loads and the connecting beam exceeds dash dash centimeter depth. Come on write down your answers neatly. Here are the answers first question B 300 mm and second question C 90 centimeter. So these are the correct answers hope you must have written them correctly. These are the references for today's session. Thank you.