An overview of deep soil improvement by vibratory methods 3
capacity calculations. The study of the effect of resonance developing during compaction in granular soils surrounding the vibrator and improved
mechanical and electronic controls of the compaction process have, only relatively recently, opened opportunities for significant advances in this field.
The introduction of coarse backfill during vibro compaction and the
resulting formation of a granular or stone column was a logical and almost
natural development when unexpectedly cohesive or noncompactable soils
were encountered. The composite of the stone column and surrounding soil
stimulated theoretical studies on settlements and shear resistance by applying standard soil mechanics principles.
When soils needing improvement of their characteristics contain layers
of organic material or are too soft to allow the safe formation of a stone
column, with a modified depth vibrator columns can be built in the ground
using dry or liquid concrete as backfill material, which, after curing of the
cementitious material, act like small diameter concrete piles, frequently with
enhanced soil characteristics below the toe of the pile and alongside its shaft.
Each of the systems, vibro compaction, vibro stone columns, or vibro
concrete columns, has its characteristics and method of execution, and even
machine types are different for the two systems of ground improvement, as
are design principles, field testing, and quality control.
When in motion, depth vibrators send out horizontal vibrations, and are
all excellent boring machines in loose sandy and soft cohesive soils.
The horizontal motion emanating from the depth vibrator being positioned in the ground is the distinctive characteristic that differentiates this
method from all the other methods, which utilize vertical vibrations. These
methods, which are in general less effective for the compaction of granular
soils and which cannot be used for the improvement of fine-grained cohesive soils, are generally not recognized as true vibro compaction methods
and hence are not discussed here in great detail.
Depth vibrators are normally suspended like a pendulum from a standard
or special crane for vertical penetration into the ground. The vibrator sinks
by the desired depth, sometimes assisted by water or air flushing, additional
weight when necessary, or even by downward thrust developed by special
cranes with vertical leaders. In granular soils, the surrounding sand is compacted in stages during withdrawal, and in cohesive soils imported backfill
is employed to form a stiffening column.
The choice of technique follows from the soil and groundwater conditions given in site investigation reports. As will be explained later, the grain
size distribution diagram of the soil to be improved is a valuable tool for
this choice. Sand and gravel with negligibly low plasticity and cohesion can
be compacted by the vibrations emanating from the depth vibrator (vibro
compaction), while with an increasing content of fines vibrations are dampened rendering the method ineffective. Experience shows that the limit of
vibro compaction is reached with a silt content of more than approximately
10%. Clay particles at even smaller percentages (1%–2%) cause a similar
capacity calculations. The study of the effect of resonance developing during compaction in granular soils surrounding the vibrator and improved
mechanical and electronic controls of the compaction process have, only relatively recently, opened opportunities for significant advances in this field.
The introduction of coarse backfill during vibro compaction and the
resulting formation of a granular or stone column was a logical and almost
natural development when unexpectedly cohesive or noncompactable soils
were encountered. The composite of the stone column and surrounding soil
stimulated theoretical studies on settlements and shear resistance by applying standard soil mechanics principles.
When soils needing improvement of their characteristics contain layers
of organic material or are too soft to allow the safe formation of a stone
column, with a modified depth vibrator columns can be built in the ground
using dry or liquid concrete as backfill material, which, after curing of the
cementitious material, act like small diameter concrete piles, frequently with
enhanced soil characteristics below the toe of the pile and alongside its shaft.
Each of the systems, vibro compaction, vibro stone columns, or vibro
concrete columns, has its characteristics and method of execution, and even
machine types are different for the two systems of ground improvement, as
are design principles, field testing, and quality control.
When in motion, depth vibrators send out horizontal vibrations, and are
all excellent boring machines in loose sandy and soft cohesive soils.
The horizontal motion emanating from the depth vibrator being positioned in the ground is the distinctive characteristic that differentiates this
method from all the other methods, which utilize vertical vibrations. These
methods, which are in general less effective for the compaction of granular
soils and which cannot be used for the improvement of fine-grained cohesive soils, are generally not recognized as true vibro compaction methods
and hence are not discussed here in great detail.
Depth vibrators are normally suspended like a pendulum from a standard
or special crane for vertical penetration into the ground. The vibrator sinks
by the desired depth, sometimes assisted by water or air flushing, additional
weight when necessary, or even by downward thrust developed by special
cranes with vertical leaders. In granular soils, the surrounding sand is compacted in stages during withdrawal, and in cohesive soils imported backfill
is employed to form a stiffening column.
The choice of technique follows from the soil and groundwater conditions given in site investigation reports. As will be explained later, the grain
size distribution diagram of the soil to be improved is a valuable tool for
this choice. Sand and gravel with negligibly low plasticity and cohesion can
be compacted by the vibrations emanating from the depth vibrator (vibro
compaction), while with an increasing content of fines vibrations are dampened rendering the method ineffective. Experience shows that the limit of
vibro compaction is reached with a silt content of more than approximately
10%. Clay particles at even smaller percentages (1%–2%) cause a similar