The shallow foundation shown in Fig. 1.1 has a width B and a length L.
The depth of embedment below the ground surface is equal to Df Theoretically, when B/L is equal to zero (that is, L = ∞), a plane strain case will exist in the soil mass supporting the foundation. For most practical cases when B/L <=1/5 to 1/6, the plane strain theories will yield fairly good results. Terzaghi  defined a shallow foundation as one in which the depth, Df , is less than or equal to the width B (Df /B " 1). However, research studies conducted since then have shown that, for shallow foundations, Df /B can be as large as 3 to 4.
FIGURE 1.1 Individual footing