Improvement of fine-grained and cohesive soils 109
or from recycled concrete or brick debris. The material itself should be of
sufficient hardness and strength to resist the strong abrasive forces resulting
from the vibratory action of the depth vibrator. It should not contain any
organic and other deleterious material. It should be chemically inert and
should resist—in cases of aggressive groundwater conditions—any potential ion attack during the lifetime of the foundations.
Regardless of the method, whether suspension from crawler cranes, or
guided by leaders, vibrators are always applied in the vertical orientation.
The vibrator forms a vertical hole largely by displacement, sometimes
enlarged by water flushing, ultimately hanging as a pendulum in the bore
hole. While crane-hung vibrators receive no guidance when penetrating
and during column construction, the special rigs (Section 4.2) are generally
equipped with vertical leaders providing excellent guidance to the vibrator in all working phases. Varying soil strengths during penetration, and
obstructions, may cause the vibrator to depart from its vertical position. It
is very important that these deviations are detected and corrected in good
time in order to ensure the verticality of the stiffening columns.
Vibro stone columns improve the foundation ground because they are
stiffer and of higher strength than the soil which they have replaced. The
reinforcing effect depends primarily on the column diameter and the distance between adjacent columns. They are generally installed with center
spacing of 1.5–3.5 m, in rows below strip footings and in triangular or
square patterns below single footings and widespread loaded areas. As with
vibro compaction, the upper 0.3–0.5 m of a stone column are generally less
well compacted as a result of vibrator shape and missing lateral support
from the surrounding soil. Therefore, before concreting work begins, the
foundation area is excavated to the required level, about 0.3 m of wellgraded coarse gravel is placed on top of the treated area and the whole
surface prepared in this way is well compacted with a standard surface
compactor.
Production rates depend, in the main, on the stiffness of the ground
needing improvement but also on the stone volume consumed for the stone
columns, on the characteristics of the vibrator, and on the efficiency of
the working crew. Rates generally range between 150 and 450 m of stone
columns for a 10-h shift per vibrator unit.
4.2 SPECIAL EQUIPMENT
For the construction of vibro replacement stone columns employing the
wet method, essentially the same equipment is needed as for the vibro
compaction process and as described in Sections 3.1 and 3.2. When working in cohesive soils, the water management and sludge handling on-site
requires special attention. Silt and clay particles in suspension cannot
be completely or easily removed from the water before it is released into
or from recycled concrete or brick debris. The material itself should be of
sufficient hardness and strength to resist the strong abrasive forces resulting
from the vibratory action of the depth vibrator. It should not contain any
organic and other deleterious material. It should be chemically inert and
should resist—in cases of aggressive groundwater conditions—any potential ion attack during the lifetime of the foundations.
Regardless of the method, whether suspension from crawler cranes, or
guided by leaders, vibrators are always applied in the vertical orientation.
The vibrator forms a vertical hole largely by displacement, sometimes
enlarged by water flushing, ultimately hanging as a pendulum in the bore
hole. While crane-hung vibrators receive no guidance when penetrating
and during column construction, the special rigs (Section 4.2) are generally
equipped with vertical leaders providing excellent guidance to the vibrator in all working phases. Varying soil strengths during penetration, and
obstructions, may cause the vibrator to depart from its vertical position. It
is very important that these deviations are detected and corrected in good
time in order to ensure the verticality of the stiffening columns.
Vibro stone columns improve the foundation ground because they are
stiffer and of higher strength than the soil which they have replaced. The
reinforcing effect depends primarily on the column diameter and the distance between adjacent columns. They are generally installed with center
spacing of 1.5–3.5 m, in rows below strip footings and in triangular or
square patterns below single footings and widespread loaded areas. As with
vibro compaction, the upper 0.3–0.5 m of a stone column are generally less
well compacted as a result of vibrator shape and missing lateral support
from the surrounding soil. Therefore, before concreting work begins, the
foundation area is excavated to the required level, about 0.3 m of wellgraded coarse gravel is placed on top of the treated area and the whole
surface prepared in this way is well compacted with a standard surface
compactor.
Production rates depend, in the main, on the stiffness of the ground
needing improvement but also on the stone volume consumed for the stone
columns, on the characteristics of the vibrator, and on the efficiency of
the working crew. Rates generally range between 150 and 450 m of stone
columns for a 10-h shift per vibrator unit.
4.2 SPECIAL EQUIPMENT
For the construction of vibro replacement stone columns employing the
wet method, essentially the same equipment is needed as for the vibro
compaction process and as described in Sections 3.1 and 3.2. When working in cohesive soils, the water management and sludge handling on-site
requires special attention. Silt and clay particles in suspension cannot
be completely or easily removed from the water before it is released into