Design of single piles - Piling Engineering - page 128

Figure 4.21 Variation of pile capacity with time.

d 2 , where c h is the coefficient
of consolidation in a horizontal plane, t is the time since installation of the pile, and
d is the diameter of the pile (Randolph et al ., 1979; Randolph and Wroth, 1979).
Figure 4.21 shows the variation of pile capacity with time, for three piles driven into
soft silty clay. The time scale has been normalized by the square of the pile diameter.
Other factors which will affect the rate of dissipation of excess pore pressures are
(a) the consolidation coefficient, c h , and (b) the extent of the zone of excess pore
pressures. The latter will be a function of the rigidity index, G

period which is governed by the dimensionless group c h t /

c u , of the soil (where
G is the shear modulus), and the sensitivity of the soil. Also shown on Figure 4.21 is a
theoretical curve of the change in excess pore pressure, normalized by the maximum
value, with time. The curve is taken from Randolph and Wroth (1979), assuming that
the excess pore pressures extend out to a distance of 10 pile radii from the axis of the
pile. A value of 10 m 2 /year has been taken for c h , which is reasonable for silty clays
under conditions of horizontal drainage. The measured variations of pile capacity
follow the theoretical curve reasonably well. When piles are driven in a group, the
magnitude of the excess pore pressures generated will be similar to that for a single
pile. However, the zone over which they extend will be considerably larger, and the
resulting consolidation process will be more protracted.
The dissipation time will depend on the magnitude and extent of the excess pore pres-
sure field generated around the pile, and will thus be much shorter for open-ended piles
than for closed-ended piles. Dissipation curves for open-ended piles of different wall

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