Structures

Milan Zdravkovic

Raft Foundation

Raft foundation is the monolithic RCC slab of uniform thickness provided throughout the entire area below the superstructure, which supports walls and columns. Thus, it acts as a structural mat.

The compensated foundation is that foundation in which the weight of the building is approximately equal to the full weight of soil excavated for the foundation.

Types of Raft foundation

The raft foundation is generally divided on the basis of two categories. They are on the basis of support and on the basis of structural component.

A) On the basis of support:

1. Raft on soil: The raft foundation is directly erected over the top layer of excavated and compacted layer of soil.

2. On pile: In this, the raft foundation is constructed over the cap of the pile foundation. The soil which is very fragile on top and over which massive structure is needed to be built, at such conditions raft is built over the pile.

B) On the basis of structural component:

1. Flat plate: It is a single slab foundation of uniform thickness provided throughout the area covered by the structure. This kind of mat foundation is used when loads from the superstructure are comparatively low. Also, used when the spacing column is small.

2. Flat plate slab thickened under column: the slab thickness under the column base is provided more in order to reduce the negative bending moment and diagonal shear in the foundation.

3. Two-way beam and slab: in such mat foundation, beams are provided along the joining the base of the column in all directions, and above the beam, the slab of uniform thickness is provided as a mat.

Such a beam is generally called inverted beams, and the column is located at their intersection. Such foundation provides enough strength when column spacing is large and suitable when the column is not equally loaded eccentric loading over the structure.

4. The rigid frame (Box structure frame): In such a foundation, the box structure is provided along with the basement wall as stiffener forming a hollow compartment in the mat.

The boxes are cellular or like a rigid frame having slabs, basement shear walls. Such kind of mat foundation is used for high bending stress reduction. And also such a foundation reduces settlement and helps to control upthrust.

Advantages Of Raft Foundation

• The raft foundation is shallow so it doesn’t require more excavation.

• Raft foundation can provide the provision of a basement compartment in the building.

• Raft foundation is extremely advantageous to the soil which has less bearing capacity.

• Raft foundation has a larger base area so reduces the pressure intensity to ground strata.

• Raft foundations are economic in the sense that the slab of raft can be directly used without any other concrete work.

• Raft foundation reduces differential settlement.

Disadvantages Of Raft Foundation

• It is costlier than individual foundation

• The unnecessary area where there is no significant stress, there also the reinforcement is used.

• There is a probability of uplift of structure when the groundwater table rises making nearby soil with more pore water pressure.

• Waterproofing and proper management of water in the basement is difficult.

• Edge erosion can occur.