The correct answer is: C. Half the column spacing.
A raft foundation is a type of foundation that distributes the load of a structure over a large area. This type of foundation is often used in areas with soft soil, as it can help to prevent the structure from sinking or settling.
The size of the soft area that can be bridged safely by a raft foundation depends on a number of factors, including the type of soil, the depth of the soft area, and the load of the structure. However, in general, the soft area should be no larger than half the column spacing. This is because the raft foundation will only be able to distribute the load of the structure if the soft area is not too large.
If the soft area is larger than half the column spacing, the raft foundation may not be able to distribute the load of the structure evenly. This could lead to the structure sinking or settling. In some cases, it may be possible to use a different type of foundation, such as a pile foundation, to support a structure on soft soil.
Here is a brief explanation of each option:
- Option A: The column spacing is the distance between the columns of a structure. If the soft area is larger than the column spacing, the raft foundation will not be able to bridge the soft area safely.
- Option B: One-third the column spacing is too small to safely bridge a soft area. The soft area should be at least half the column spacing in order to be safely bridged by a raft foundation.
- Option C: Half the column spacing is the minimum size of the soft area that can be safely bridged by a raft foundation.
- Option D: Three-fourth the column spacing is too large to safely bridge a soft area. The soft area should be at least half the column spacing in order to be safely bridged by a raft foundation.