Method variations and related processes 195
on the column is remarkably lower than its inner strength, a case where in
addition the column module is very much higher than the module of the surrounding soil (E c ≫ E s ). In a two-step calculation, the settlement of a group
of columns can be estimated including any additional deformation below the
column tow, stemming from, what is generally called, the punching effect.
The displacement effect of the bottom feed vibrator during PVCC and
VCC installation and its beneficial influence on the soil properties has been
investigated by Sondermann and Kirsch (2009). They report of closer bonding between column surface and soil and enhanced skin friction leading to
higher bearing capacities of the columns (see also Section 4.3.5).
The effective transfer of the building loads into the column heads
requires a load distribution and transfer layer of sufficient strength, which
can be achieved either by a concrete slab capable of carrying the single
column loads, or by a granular cushion of appropriate thickness and friction value of its material to allow an arching effect to develop according
to Figure 4.12. To reduce the thickness of such a load distribution layer,
a geotextile of sufficient strength may be placed across the column heads
(Kempfert, 1995; Sondermann and Jebe, 1996; Topolnicki, 1996).
Preparation of the column heads for the following construction work
requires special attention and care. When cutting cured VCCs and PVCCs
to foundation level, cracking of the columns must be avoided and the cutting tools have to be selected accordingly. It is therefore recommended to
achieve this by using handwork or only very light excavators for cutting of
the columns which have not yet been cured. After careful removal of the
debris, the excavations should be backfilled again for column head protection to ground level with sand or other appropriate material.
5.2.4 Quality control and testing
Since vibro concrete columns are generally quasi by definition executed for
the improvement of very soft soils (although these soils are generally only
bypassed and not improved in their characteristics), an adequate quality
assurance program is indispensible (see also DGGT, in press). It includes all
the elements which are deemed necessary for vibro replacement stone columns according to Section 4.4. In addition the verticality of the columns is
very important and must be carefully controlled, measured, and reported for
each vibro concrete column. In this regard, a competent, horizontal working
platform is required for a stable positioning of the heavy vibrocat machine.
When fluid ready-mix concrete is used, pumping pressure and rate must be
carefully coordinated with the rate of extracting the vibrator from the ground
and with the hollow volume it theoretically leaves behind, and which needs
to be refilled without delay by the concrete. Only by this way a continuous
vibro concrete column can be constructed with a stable minimum diameter.
Vibro concrete columns displace the soil according to the volume of the
construction apparatus. This results in a certain amount of heave at ground
on the column is remarkably lower than its inner strength, a case where in
addition the column module is very much higher than the module of the surrounding soil (E c ≫ E s ). In a two-step calculation, the settlement of a group
of columns can be estimated including any additional deformation below the
column tow, stemming from, what is generally called, the punching effect.
The displacement effect of the bottom feed vibrator during PVCC and
VCC installation and its beneficial influence on the soil properties has been
investigated by Sondermann and Kirsch (2009). They report of closer bonding between column surface and soil and enhanced skin friction leading to
higher bearing capacities of the columns (see also Section 4.3.5).
The effective transfer of the building loads into the column heads
requires a load distribution and transfer layer of sufficient strength, which
can be achieved either by a concrete slab capable of carrying the single
column loads, or by a granular cushion of appropriate thickness and friction value of its material to allow an arching effect to develop according
to Figure 4.12. To reduce the thickness of such a load distribution layer,
a geotextile of sufficient strength may be placed across the column heads
(Kempfert, 1995; Sondermann and Jebe, 1996; Topolnicki, 1996).
Preparation of the column heads for the following construction work
requires special attention and care. When cutting cured VCCs and PVCCs
to foundation level, cracking of the columns must be avoided and the cutting tools have to be selected accordingly. It is therefore recommended to
achieve this by using handwork or only very light excavators for cutting of
the columns which have not yet been cured. After careful removal of the
debris, the excavations should be backfilled again for column head protection to ground level with sand or other appropriate material.
5.2.4 Quality control and testing
Since vibro concrete columns are generally quasi by definition executed for
the improvement of very soft soils (although these soils are generally only
bypassed and not improved in their characteristics), an adequate quality
assurance program is indispensible (see also DGGT, in press). It includes all
the elements which are deemed necessary for vibro replacement stone columns according to Section 4.4. In addition the verticality of the columns is
very important and must be carefully controlled, measured, and reported for
each vibro concrete column. In this regard, a competent, horizontal working
platform is required for a stable positioning of the heavy vibrocat machine.
When fluid ready-mix concrete is used, pumping pressure and rate must be
carefully coordinated with the rate of extracting the vibrator from the ground
and with the hollow volume it theoretically leaves behind, and which needs
to be refilled without delay by the concrete. Only by this way a continuous
vibro concrete column can be constructed with a stable minimum diameter.
Vibro concrete columns displace the soil according to the volume of the
construction apparatus. This results in a certain amount of heave at ground