Fig. 6.17. In this figure the abscissa of the curve C1 represents the vertical stress C1z at different depths z below the center of a square area B x B which carries a surcharge q per unit area or a total surcharge load of B2q. This curve is obtained by the exact method explained under Sect. 6.6. The abscissa of the curve C2 represents the corresponding stresses due to a concentrated load Q = B2q acting at the center of the square area. The figure shows that the difference between the two curves becomes very small for values of z/B in excess of three. Hence in a computation of the vertical stress C1z at a depth z below an area, the area should be divided into convenient squares or rectangles such that the least width of any block is not greater than z/3.