Types of Shallow Foundations & Its Use

Shallow foundations are constructed where hard soil layer is at shallow depth (up to 1.5 m). According to Terzaghi`s, the foundation is shallow if its depth is less than or equal to its width.

   

                                             

Types of Shallow Foundations:-

               

Spread Foundation :-

Spread footing foundation is basically a pad used to ‘‘spread out’’ loads from walls or columns over a sufficiently large area of foundation soil. Spread footing required to support a wall is known as a continuous, wall, or strip footing, while that required to support a column is known as an individual or an isolated footing.

Strip Footing (Continuous Footing) :-

             A strip footing is provided for a load-bearing wall. A strip footing is also provided for a  row of columns which are so closely spaced that their spread footings are overlap or nearly touch each other. In such case, it is more economical to provide a strip footing than to provide a no. of isolated footings in one line.               

Isolated Footing (Individual Column Footing) :-

              Isolated footing is also known as pad footing or individual column footing. An isolated footing may be square, circular, or rectangular in shape in plan, depending upon factors such as the plan shape of the column and constraints of space. Isolated footing has two small categories namely 

a.  Isolated Footing with Uniform Thickness or depth

b.  Isolated Pad Footing

                                
                                                         

                                         Common Arrangements of Strip Footing and Isolated Footing

Strap Footing :-

A ‘strap footing’ comprises two or more footings connected by a beam called ‘strap’. This is also called a ‘cantilever footing’ or ‘pump-handle foundation’. The strap beam is so connected that the whole assembly behave like a single unit. The individual footings are so designed that their combined line of action passes through the resultant of total load.

This may be required when the footing of an exterior column cannot extend into an adjoining private property. A strap footing is economical than a combined footing when the allowable soil pressure is relatively high and the distance between the columns is large. Common arrangements of strap footing are as shown in fig.

             

                  

                                                  Common Arrangements of Strap Footing

Combined Footing :-

A combined footing is supports two columns. It is provided when two columns are so close to each other that their individual footings may overlap. It is also provided when the property line is so close that a spread footing would be eccentrically loaded for kept entirely within the property line. Combined footing may be rectangular or trapezoidal in shape. Common arrangements of combined footing are as shown in fig.                  

      

                    

Common Arrangements of Combined Footing

Mat or Raft Foundations :-

A mat or raft foundation is a large slab supporting a no. of columns and walls under the entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is a large variation in the loads on individual columns. Common arrangements of mat footing are as shown in fig.

             

                        

                         Common Arrangements of Raft or Mat Footing (Flat Slab Type)