Types of Shallow Foundations

Shallow Foundation - When the range of the lower most section of the foundation crosses the depth of the lower most section of the foundation from the top of the soil that is the ground level then it is termed as a Shallow Foundation.

Shallow foundations are developed where layer of soil at shallow depth (equal to 1.5m) can support the structural loads. The depth of shallow foundations is usually below its width.

Shallow foundation is perfect for different types of light and tolerably loaded structures like 5 storied residential buildings and other general type of works, but the soil should not be of substandard quality with collapsible nature and low bearing capacity.

Shallow foundation can be further categorized based on the site condition and type of loading and pattern.

Given below, various types of shallow foundation:

a) Strip footing
b) Spread or isolated footing
c) Combined footing Strap or cantilever footing
d) Mat or raft Foundation.

1. Strip Footing: A strip footing is set for a load-bearing wall. A strip footing is also set for a row of columns which are intensely positioned that their spread footings overlay or almost contact each other. In this situation, it is suggested to use a strip footing to minimize the cost rather than using a number of spread footings in a single line. A strip footing is also called continuous footing.

2. Spread or Isolated Footing: A spread footing alias isolated or pad footing is set to provide support to the specific column. A spread footing may be circular, square or rectangular slab that contains unchanging thickness. Occasionally, it is stepped or haunched to disperse the load along a broad area.

3. Combined Footing: A combined footing is based on two columns. It is applicable if the two columns are very adjacent to each other so that their separate footings may overlay. A combined footing is also used if the property line is very near to one column that a spread footing should be irregularly loaded when retained completely inside the property line. If it is integrated with that of an interior column, the load is equally circulated. A combined footing is rectangular or trapezoidal in plan.

4. Strap or Cantilever footing: A strap (or cantilever) footing contains two isolated footings linked through a structural strap or a lever. The strap attaches the two footings in a manner that they perform as a single unit. The strap is designed like an inflexible beam. The individual footings are designed in such a way that their combined line of action moves through the sequent of the total load. A strap footing is more cost-effective as compared to a combined footing if the tolerable soil pressure is comparatively high and the distance among the columns is huge.

Ref.: theconstructor.org

5. Mat or Raft Foundations: A mat or raft foundation refers to a big slab that is based on a number of columns and walls under the whole structure or a substantial part of the structure. A mat is necessary if the tolerable soil pressure is low or where the columns and walls are very adjacent so that individual footings may overlay or almost meet with each other.

Mat foundations are used in minimizing the differential settlements on dissimilar soils or where there exist a big difference in the loads on separate columns.