RAFT FOUNDATIONSubmitted By :-

Rahul AsodariyaDarshan BavadiyaUrmila DhameliyaNikhilesh DhadukVatsal GadhiyaDarshan SavsaiyaRidham Vekariya

Raft is shown in the part plan A & section B.

• The whole of the ground floor is covered with concrete reinforced with a layer of steel fabric ,lapped joint 150mm near the bottom surface, an alternative and better arrangement is shown in D.

• Two layers of fabric heaving 40mm cover of concrete top and bottom are provided.

• The reinforced concrete beam foundation under the wall is incorporated with the floor by extending the mild steel stirrups – enlarged detail at E.

• The lean concrete blinding shown provides a clean bed of structural concrete alternatively ,a layer of graval may be used for this purpose.

How Do Raft Foundations Work?• A raft foundation spreads the weight of the building over the whole ground floor area of

that building. The raft is laid on a hard-core, or scalping bed and usually thickened at the edges, especially in very poor ground. Rafts are most suitable when the ground is of good load bearing capacity and little work is required to get a solid foundation.

• Raft Foundations are built is this following steps:• The soil removed down to correct depth• The foundation bed is then compacted by ramming• Lay reinforcement on spacers over the foundation bed• Pour the concrete over the reinforcement• The foundation may stiffened by ribs or beams built in during construction which will add

extra strength and rigidity.

Types of raft foundation1. Raft support on the soil,2. Raft support on pile, and3. Buoyancy raft.

On the base of structural system adopted for the structure of the raft , these can be classified as :

1. Plain slab raft which are flat concrete slab having uniform thickness throughout. This can be with pedestals or without pedestals .

2. Beam and slab raft which can be design with down stand beam or up stand beam 3. Cellular raft or framed raft with foundation slab , wall , columns and on the floor slabs

acting together to give a very rigid structure.

• Raft of uniform depth is most popular due to its simplicity of design and construction.• This type is most suitable where the column loads are moderate and the column

speacing fairly small and uniform.• Pedestals are utilize to distribute the load on a bigger area in case of heavy column

loads.

• Slab and beam raft is use as foundation for heavy buildings where stiffness is the principal requirement to avoid excessive distortion of the super structure as a result of variation in the load distribution over the raft or the compressibility of the supporting soil.

• These rafts however have many obvious difficulties.

• If the beam are deep, ribs placed below the basement floor or raft ,the bottom of excavation becomes badly cut up with trenches, impeding the bearing value of the soil because of its disturbance.

• Waterproofing in case of basement become more complicated and involved.

• If the beam are projecting up , usefulness of the basement is distrait unless the entire foundation is lower and the gap filled up or an upper slab is provided supported on these inverted beams to form the ground floor of the structure.

• Buoyancy Foundation• Buoyancy raft are necessarily to be provided with a basement so that the weight of soil

removed balances to a large extent, the impose load.

• Cellular raft consisting of foundation slab, wall, column and ground floor can be designed, but it creates considerable amount of uncertainties, difficulty of construction and quite often even in such cases, raft is design as a slab of uniform thickness.

• Raft, as a slab of uniform thickness, as a additional advantage of providing better water proofing treatment ease of reinforcement fabrication and laying of concrete.

Need of Raft Foundation•Whenever buildings loads are so heavy or the allowable pressure on the soil so small that individual footing would cover more then floor area.

•Whenever soil contains compressible lenses or the soil is sufficiently erratic and it is difficult to define the assess the extent of each of the week pockets or the cavity and , thus , estimate the overall and differential settlement.

•When structures are equipment to be supported are very sensitive to differential settlement. •Where structure naturally lend themselves for the use of raft foundation such as silos,chimneys,water tower, etc.

•Floating foundation cases where in soil is having where poor bearing capacity and the weight of superstructure is proposed to be balanced by the weight of the soil removed. •Buildings where basements are to be provided or pits located below ground water table.•Buildings where individually foundation is provided will be subjected to large widely varying bending moments which may result in differential rotation and differential settlement of individual footings causing distress in the building.

When Raft Foundations Are Used?• A raft foundation is usually preferred under a number of circumstances:• it is used for large loads, which is why they are so common in commercial building

which tend to be much larger, and therefore heavier, than domestic homes• The soil has a low bearing capacity so the weight of the building needs to be

spread out over a large area to create a stable foundation• The ratio of individual footings to total floor space is high. Typically if the footings

would cover over half of the construction area then raft foundation would be used• If the walls of the building are so close that it would cause the individual footings to

overlap, then raft foundations should be used

Advantages and Disadvantages of Raft Foundations• Raft foundations tend to be cheaper and quicker to use than traditional

footings. There are a number of reasons why this is the case:• The foundation and floor slab is combined, which saves time and materials• Less excavation is required• Other reasons that make raft foundations preferable to footings are due to

their engineering benefits. They are ideal for poor ground condition where normal footings would not cope well as they cannot spread the load as effectively.

• Related to this is that raft foundations can reduce differential settlement, where settlement occurs at different rates across the ground surface of the building, which reduces cracking and other more serious problems.

• The main disadvantage is that they can prone to edge erosion if they are not treated properly. They are not effective is the load of the building is going to be focused on a single point, although this is rare in domestic construction, so this isn't generally of concern.

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