METHODS OF GROUND IMPROVEMENT - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 223

shown in Fig 13.15. The value of U at the end of consolidation at S S & is 0.994,

when t = T hd , and at 1/4 th of this period U = 0.764.

stop vacuum
pressure

S

(v)

1.0

(u)

0.764

( )

U= S/S &

(') S &

S

t

¼T T

Figure 13.15

A vacuum pressure is simply additive and, therefore, its value causing a
reduction of the consolidation period by a factor 4 can be obtained by
proportionality, according to

v/ ( v+ ) = u t= ¼ T / = (1 0.764) = 0.236 or v = 0.31

In this analysis the vacuum pressure is assumed uniformly distributed in the clay
layer, which is not usually the case. In the pipes and/or drains the vacuum pressure
is significantly lower, and there a practical lowest value is about 70% of the
atmospheric pressure ( ~ 100 kPa), which limits the extent of the vacuum method. In
this case, with v max 40 kPa, the maximum load which can be dealt with for a

reduction of 4 times is

130 kPa, or a height of 130/18 = 7.2 m. This
shows that vacuum consolidation can be effective.

= 40/0.31

application 13.2
Calculate the required spacing of vertical drains in a soft soil deposit to achieve
a reduction of excess pore pressures to 30% of an applied surface load within 5
months. Data: prefabricated drains, width 150 mm, thickness of 6 mm.
Consolidation coefficient of soft soil 2x10 -8 m 2 /sec.

application 13.3
Calculate the time required to obtain a degree of consolidation of 90% using
vertical drains in a soft soil deposit to which a surface fill is applied, using the
following data: prefabricated drains, width 150 mm, thickness 6 mm, spacing = 2
m on a rectangular grid. Consolidation coefficient of soft soil 2x10 -8 m 2 /sec. Is the
period you find acceptable in practice? Determine the dependency between this
period and the drain spacing. From this, give the drain spacing needed to fulfil the
specification within 1 year.