Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 251

The second step is to determine the depth of soil over which the stresses transmitted by
the block foundation are significant. This is usually taken as the depth at which the vertical
stress resulting from the net pressure at foundation level has decreased to 20% of the net
overburden pressure at that level (Figure 5.14). A deeper level should be considered for soft
highly compressible alluvial clays and peats.

0

0

0.1

1

2

3

5



Values of L/B

sz/q

z/B

1.5

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

0.2 0.4 0.6 0.8 1.0

Figure 5.13Calculation of mean vertical stress ( (^) z) at depth zbeneath rectangular area abon
surface loaded at uniform pressure q.