Method variations and related processes 191
ground using a special vibratory hammer. After excavating, coarse backfill is placed in small quantities that are compacted utilizing an internal
mandrill compactor. In stiffer soils the necessary hole, generally 0.75 m in
diameter, can be drilled out using standard augers. Material backfill and
compaction is then performed in the same way, employing a special compactor (White et al., 2002).
It is obvious that the design approaches that are valid for vibro replacement
stone columns can also be applied to all types of columns that are installed
by full or partial displacement of the surrounding soil and by subsequent
insertion and compaction of granular material. With similar load-carrying
mechanisms, the key design parameter is the strength of the compacted coarse
backfill which is controlled by the compaction energy employed. The adequate
choice of the friction angle φ c and the area replacement factor are decisive for
a realistic prediction of bearing capacity and deformation of these foundation
methods.
All types of columns that consist of materials whose higher strength is
controlled by hardening substances such as cement or other types of hydrating binder act as piles that develop their bearing capacity from skin friction
and end bearing.
5.2 VIBRO CONCRETE COLUMNS FOR
FOUNDATIONS IN VERY SOFT SOILS
5.2.1 Process description
We have seen in Chapter 4 that vibro stone columns do require sufficient containing pressure from the soil which they are to improve in order to provide
necessary load carrying capacity without undue settlement. The threshold
expressed by the minimum undrained shear strength is generally c u = 5 kPa
for these soils. When in these borderline soils (see Table 4.3), particularly in
the presence of natural or artificial organic material, a vibro replacement
foundation would reach its limit or should not be carried out, either in view
of the loads a column would have to carry or, especially, in view of the resulting deformations, the column backfill material can be replaced by concrete.
When cured, this vibro concrete column, as it is called, has a stiffness which
is more than 25 times larger than that of the surrounding soil.
The vibro concrete column is constructed in a similar way and using
similar equipment as with the stone column method. When dry concrete is
added in the same way as with the bottom feed method (see Section 4.1),
a so-called premix VCC (PVCC) is built. The special premixed dry coarse
concrete behaves in the same way as normal source backfill material allowing its displacement into the surrounding soil by the vibrator movement
and vibrations. Column diameters range generally between 50 and 80 cm
allowing a working load of about 900 kN to be used for PVCCs.
ground using a special vibratory hammer. After excavating, coarse backfill is placed in small quantities that are compacted utilizing an internal
mandrill compactor. In stiffer soils the necessary hole, generally 0.75 m in
diameter, can be drilled out using standard augers. Material backfill and
compaction is then performed in the same way, employing a special compactor (White et al., 2002).
It is obvious that the design approaches that are valid for vibro replacement
stone columns can also be applied to all types of columns that are installed
by full or partial displacement of the surrounding soil and by subsequent
insertion and compaction of granular material. With similar load-carrying
mechanisms, the key design parameter is the strength of the compacted coarse
backfill which is controlled by the compaction energy employed. The adequate
choice of the friction angle φ c and the area replacement factor are decisive for
a realistic prediction of bearing capacity and deformation of these foundation
methods.
All types of columns that consist of materials whose higher strength is
controlled by hardening substances such as cement or other types of hydrating binder act as piles that develop their bearing capacity from skin friction
and end bearing.
5.2 VIBRO CONCRETE COLUMNS FOR
FOUNDATIONS IN VERY SOFT SOILS
5.2.1 Process description
We have seen in Chapter 4 that vibro stone columns do require sufficient containing pressure from the soil which they are to improve in order to provide
necessary load carrying capacity without undue settlement. The threshold
expressed by the minimum undrained shear strength is generally c u = 5 kPa
for these soils. When in these borderline soils (see Table 4.3), particularly in
the presence of natural or artificial organic material, a vibro replacement
foundation would reach its limit or should not be carried out, either in view
of the loads a column would have to carry or, especially, in view of the resulting deformations, the column backfill material can be replaced by concrete.
When cured, this vibro concrete column, as it is called, has a stiffness which
is more than 25 times larger than that of the surrounding soil.
The vibro concrete column is constructed in a similar way and using
similar equipment as with the stone column method. When dry concrete is
added in the same way as with the bottom feed method (see Section 4.1),
a so-called premix VCC (PVCC) is built. The special premixed dry coarse
concrete behaves in the same way as normal source backfill material allowing its displacement into the surrounding soil by the vibrator movement
and vibrations. Column diameters range generally between 50 and 80 cm
allowing a working load of about 900 kN to be used for PVCCs.