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136 Barry’s Advanced Construction of Buildings
(a) A grid is marked out and the
vibrating mandrel (poker) is
inserted to the required depth
(b) As the mandrel drives into
the ground, the soil is displaced
(surrounding granular soil
is compacted)
Rigs weighs 14–55
tonnes
(c) Having reached the engineered
depth, the mandrel is withdrawn and
hardcore is placed up to the first
level. The hardcore is built up in
layers of 0.3–0.6 m. The mandrel
is inserted into the hardcore; it
penetrates and compacts each layer
before the next load of hardcore is placed
(d) By compacting in layers and
reintroducing the cone mandrel, a
dense stone column is constructed
Figure 3.57 Vibro displacement – typical sequence.
Cohesive soils are not compacted in the same way that granular soils are. In clays the
stone columns help to share and distribute the loads. The columns of stone carry the loads
down the pile and distribute them through the strata; however, they are not end bearing.
Where the density and consistency of the ground varies, the installation of stone columns
helps to stabilise the ground, enhances loadbearing performance and makes the conditions
more uniform, thus limiting differential settlement. The benefits of vibro compaction
include: