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134 Barry’s Advanced Construction of Buildings
Long cone Flower pot cone Multiple point
cone
Traditional
weight
2.5
Consolidates strata
closer to the surface
10–20 tonnes
Energy does not
penetrate the ground as
much as for cone weights
Typical weight (mass)
7–11 tonnes
Used for densifying
deep layers of strata
Figure 3.55 Typical cone type tampers (adapted from http://www.roger-bullivant.co.uk).
objective of densifying deep layers. Subsequent tamping is then undertaken using multipointed
tampers (Photograph 3.13). The multi-point tamper improves ground consolidation
at shallower depths (Figure 3.55 and Figure 3.56).
Vibro compaction
Vibro compaction (also called vibro displacement or vibro replacement) uses large vibrating
mandrels (vibrating shafts or rods) to penetrate, displace and compact the soil (Figure
3.57 and Photograph 3.14). When the mandrel is removed from the ground, the subsequent
void is filled with stone. The mandrel is then forced back through the stone, further displacing
and compacting the ground and stone. The method that produces stone columns in
the ground compacts the surrounding strata, enhancing the ground-bearing capacity
and limiting settlement. Typical applications include support of foundations, slabs, hard
standings, pavements, tanks or embankments. Soft soils, man-made and other strata can
be reinforced to achieve improved specification requirements, while slopes can be treated
to limit the risk of slip failure. The allowable ground-bearing capacities for low- to mediumrise
buildings and industrial developments are in the region of 100–200 kN/m 2 . Beneath
tanks, on embankments or slopes of 100 kN/m 2 can be achieved. Ground conditions may
allow heavier loads to be supported.
The high-powered mandrel penetrates the ground, in a vertical plane to the designed
depth. When the mandrel is extracted, the resulting bore is filled with suitable aggregate
or stabilised solid; these are compacted in layers of 200–600 mm increments. The shape of
the mandrel (poker) tip is designed to ensure high compaction of the stone or stabilised
soil column and surrounding strata. The taper on the mandrel causes increased densification
of the strata.
Vibro displacement and compaction can be used in granular and cohesive soils. In
granular soils, the ground-bearing capacity is improved by the introduction of columns of
compacted stone or stabilised soil, and the compaction and densification of the granular
soil that surrounds each column.