Improvement of fine-grained and cohesive soils 107
the bore from collapsing as a result of the suction developing when the vibrator is withdrawn, as it is generally in a tight fit in the ground. The column is
built by tipping the backfill into the bore hole and by lowering the machine
back into it in lifts of about half a meter. By the action of the depth vibrator,
the coarse backfill is compacted and displaced laterally and downward forming in this way a tightly compacted stone column of about 0.5–0.6 m diameter
which is well interlocked with the surrounding sheared soil. To safeguard
column verticality and to assist penetration into stiff soil, custom-built cranes
with leaders were developed, replacing the standard cranes as base machines.
As this method has its downsides with deeper penetration, it being relatively slow and requiring well-supervised workmanship, a special depth
vibrator was developed avoiding the need to remove it from the ground
Gravel
Wet
Dry
(a)
(b)
W
a t e r
Figure 4.1 Stone columns built (a) by the wet vibro replacement method and (b) by the
dry displacement bottom feed method.