Question

A raft foundation subjects its supporting soil to a uniform pressure of 300 kPa. The dimensions of the raft are 6.1m by 15.25m. Determine the vertical stress due to the raft at a depth of 4.58m below it at the central points of the long edges.

Answer 1

Influence factors for the vertical stress beneath the corner of a rectangular foundation (after

Fadum, 1948).

i) at the center of the raft

z= 4.58 m

m= $\frac{B}{z}= \frac{3.05}{4.58}= 0.67$

n=$\frac{L}{z} = \frac{7.63}{4.58}$ = 1.67 obtained from the graph.

$l_\alpha = 0. 16$

$4l_\alpha = 0.64$

$\sigma _z= 4l_\alpha P = 0.64 \times 300 = 192 kPa$

ii) at the central point of long edges.

$\Delta\sigma_z= \frac{PBL}{(B+Z)(L+Z)}$

$\Delta\sigma_z = \frac{300\times 15.25 \times 6.1}{(6.1 +4.58)(15.25+4.58)} = 131.65$

$\Delta\sigma_z = 132 kPa$