108 Ground improvement by deep vibratory methods
for material backfilling and at the same time ensuring column continuity
for deeper columns in much softer soils (c u = 10 kN/m 2 ). With this dry
displacement bottom feed method the backfill is discharged directly at the
point of the vibrator through a pipe that is fitted to its outside and which is
fed by a backfill container with an airlock situated on top of the extension
tubes. The system is best used with dedicated cranes for the depth of less
than 20 m (these are called vibrocats and develop considerable pull-down
forces to compensate the increased friction force developing as a result of the
increased depth vibrator cross section) (Section 4.2). For greater depths standard crawler cranes can be used. A continuous column is formed by pulling
the vibrator in steps of 0.5–1.0 m within the ground, in this way allowing
the backfill to flow into the bore hole, helped by a moderate flow of compressed air. By repenetrating until resistance is met, the coarse material is
compacted and laterally displaced into the soil. This procedure is repeated
leaving behind a well-compacted stone column of about 0.5–0.8 m in
diameter up to working grade level. The method is to a certain degree selfcompensating since in softer soils column diameters tend to be larger than
in stiffer soils. Figure 4.1b shows a typical plant while forming vibro stone
columns with the dry bottom feed method.
It is evident that the wet system primarily replaces native soil by coarse
backfill with relatively little lateral displacement; the dry system is, in contrast, in the first place a displacement method. Nevertheless both methods
are today generally named vibro replacement stone column methods, but
we will see later that displacement of the surrounding soil can add extra
strength to columns of otherwise equal dimensions.
The grading of backfill stone for the vibro stone columns differs slightly
between the different methods of construction.
• For the replacement wet method rounded or subangular stone or gravel
30–60 mm in size and comparatively uniformly graded is used which
passes easily through the annulus around the machine. It is always
found, on excavation of a stone column, that the voids between the
stones, which are in close contact with each other, are always found to
be filled with the coarser particles (sand and coarse silt) from the native
soil ensuring a well compacted stone column. The finer parts of the
native soil are transported to the ground surface with the flushing water.
• For the bottom feed method, finer gravel or crushed stone of
10–40 mm is necessary to pass through the delivery system including
the feed pipe to the vibrator point. Well-graded sand can also be used
should no coarser backfill be available, but this will forfeit a considerable amount of column strength in comparison to the coarser stone
backfill (Section 4.3).
The material of the stone backfill should be environmentally acceptable
and can be taken either from natural gravel deposits, crushed natural stone,
for material backfilling and at the same time ensuring column continuity
for deeper columns in much softer soils (c u = 10 kN/m 2 ). With this dry
displacement bottom feed method the backfill is discharged directly at the
point of the vibrator through a pipe that is fitted to its outside and which is
fed by a backfill container with an airlock situated on top of the extension
tubes. The system is best used with dedicated cranes for the depth of less
than 20 m (these are called vibrocats and develop considerable pull-down
forces to compensate the increased friction force developing as a result of the
increased depth vibrator cross section) (Section 4.2). For greater depths standard crawler cranes can be used. A continuous column is formed by pulling
the vibrator in steps of 0.5–1.0 m within the ground, in this way allowing
the backfill to flow into the bore hole, helped by a moderate flow of compressed air. By repenetrating until resistance is met, the coarse material is
compacted and laterally displaced into the soil. This procedure is repeated
leaving behind a well-compacted stone column of about 0.5–0.8 m in
diameter up to working grade level. The method is to a certain degree selfcompensating since in softer soils column diameters tend to be larger than
in stiffer soils. Figure 4.1b shows a typical plant while forming vibro stone
columns with the dry bottom feed method.
It is evident that the wet system primarily replaces native soil by coarse
backfill with relatively little lateral displacement; the dry system is, in contrast, in the first place a displacement method. Nevertheless both methods
are today generally named vibro replacement stone column methods, but
we will see later that displacement of the surrounding soil can add extra
strength to columns of otherwise equal dimensions.
The grading of backfill stone for the vibro stone columns differs slightly
between the different methods of construction.
• For the replacement wet method rounded or subangular stone or gravel
30–60 mm in size and comparatively uniformly graded is used which
passes easily through the annulus around the machine. It is always
found, on excavation of a stone column, that the voids between the
stones, which are in close contact with each other, are always found to
be filled with the coarser particles (sand and coarse silt) from the native
soil ensuring a well compacted stone column. The finer parts of the
native soil are transported to the ground surface with the flushing water.
• For the bottom feed method, finer gravel or crushed stone of
10–40 mm is necessary to pass through the delivery system including
the feed pipe to the vibrator point. Well-graded sand can also be used
should no coarser backfill be available, but this will forfeit a considerable amount of column strength in comparison to the coarser stone
backfill (Section 4.3).
The material of the stone backfill should be environmentally acceptable
and can be taken either from natural gravel deposits, crushed natural stone,