Question

For a proposed building, raft foundation, isolated footings and combined footings are being considered. These foundations are to be listed in the decreasing order of preference in terms of performance. Which one of the following is the correct order of listing?
1. Raft foundation - Combined footings - Isolated footings
2. Isolated footings - Raft foundations - Combined footings
3. Combined footings - Raft foundations - Isolated footings
4. Combined footings - Isolated footings - Raft foundations

Answer 1

Correct Answer - Option 1 : Raft foundation - Combined footings - Isolated footings

Explanation:

Isolated footing:

(i) For ordinary structures located on relatively firm soil, it is usually sufficient to provide individual footing for every column. Such a footing is called an isolated footing.

(ii) Isolated footings are generally square or rectangular in plan but other shapes can also be adopted in special circumstances.

Combined footings:

(i) Many a times it is inconvenient to provide separate isolated footings for columns (or walls) due to insufficient areas available in plan.

(ii) This usually occurs when two or more columns (or walls) are located close to each other or if they are relatively heavily loaded or rest on soil with low safe bearing capacity, resulting in overlap of areas if isolated footings are provided. In such cases, it is preferred to provide a single combined footing for the column.

Mat/Raft footings:

(i) These are special types of combined footings which support a large number of columns. This footing is provided when the column loads are heavy and the soil below is poor.

(ii) Mat footing minimizes the chances of differential settlements and transfer the column load to a very large area.

So, from above three concepts it is concluded that mat foundation is superior than combined and isolated foundation and combined foundation is superior than isolated foundation.