144 Ground improvement by deep vibratory methods
the 25.4 mm sieve, and 10%–80% passing the 12.7 mm and up to 5% passing the 4.75 mm sieve. Should higher sand contents be required for economic or technical reasons, up to 30% passing the 4.75 mm sieve can be
used. Typically, for these projects a maximum distance of the stone columns, (generally in the range of 3 m) together with their required diameter
of about 800–900 mm would be specified. The sand between the columns
would have to be compacted to certain equivalent clean sand CPT tip resistance values; in general, normalized and corrected for overburden pressure
in accordance with the seismic risk evaluation (see Section 3.3.3 and the
case histories in Sections 3.6.5 and 4.7.8).
4.3.5 Recommendations
The various steps in designing a vibro replacement foundation will generally commence with an assumed and preliminary arrangement of the stone
columns, which will have to be verified by the assessment of bearing capacity and deformation under load. These assumptions primarily concern
attributable load per column (generally between 200 and 500 kN), distance between column axis (generally between 2 and 5 column diameters),
stone column diameter (ranging between 0.5 and 1.2 m), and column depth
(ranging normally between 2 and 25 m). The choice will have to take into
account the prevailing soil conditions, the available coarse backfill material
and the chosen depth vibrator, and the method of installation. In waterbearing soils of low plasticity (sandy silts to clayey silts), depth vibrators
with larger diameters will be used in general, and column construction will
be by either the wet or the bottom feed dry method, always creating columns in excess of 0.8 m. Conversely, in stiffer cohesive soils, smaller diameter, high-frequency depth vibrators will ensure penetration to the required
depth and will generally form columns with diameters below 0.8 m.
As we have seen when discussing the design principles, essential parameters controlling the performance of the stone column foundation are their
diameter and the distance between each other, expressed by the area replacement factor a c , together with the angle of internal friction φ c of the column
material. The gravel material which is normally used is generally coarsegrained alluvial deposits or crushed rock. It’s stress-dependent friction angle
usually decreases with increasing normal pressure, with φ c,max corresponding with σ c,min and vice versa. The friction angle of gravel is, as with other
granular material, also directly proportional to its density. Installation
methods generally guarantee that the column material is very well compacted by the vibratory motions of the depth vibrator with void ratios in
general close to minimum values as field measurements have shown (Herle
et al., 2007). Their shear parameter φ c can be taken from a σ–τ diagram,
which can be determined relatively easily by shear box tests in the laboratory. Maximum friction angles can be as high as 60° (generally measured
at moderate pressures of ~50 kN/m 2 ) with minimum values of about 50°
the 25.4 mm sieve, and 10%–80% passing the 12.7 mm and up to 5% passing the 4.75 mm sieve. Should higher sand contents be required for economic or technical reasons, up to 30% passing the 4.75 mm sieve can be
used. Typically, for these projects a maximum distance of the stone columns, (generally in the range of 3 m) together with their required diameter
of about 800–900 mm would be specified. The sand between the columns
would have to be compacted to certain equivalent clean sand CPT tip resistance values; in general, normalized and corrected for overburden pressure
in accordance with the seismic risk evaluation (see Section 3.3.3 and the
case histories in Sections 3.6.5 and 4.7.8).
4.3.5 Recommendations
The various steps in designing a vibro replacement foundation will generally commence with an assumed and preliminary arrangement of the stone
columns, which will have to be verified by the assessment of bearing capacity and deformation under load. These assumptions primarily concern
attributable load per column (generally between 200 and 500 kN), distance between column axis (generally between 2 and 5 column diameters),
stone column diameter (ranging between 0.5 and 1.2 m), and column depth
(ranging normally between 2 and 25 m). The choice will have to take into
account the prevailing soil conditions, the available coarse backfill material
and the chosen depth vibrator, and the method of installation. In waterbearing soils of low plasticity (sandy silts to clayey silts), depth vibrators
with larger diameters will be used in general, and column construction will
be by either the wet or the bottom feed dry method, always creating columns in excess of 0.8 m. Conversely, in stiffer cohesive soils, smaller diameter, high-frequency depth vibrators will ensure penetration to the required
depth and will generally form columns with diameters below 0.8 m.
As we have seen when discussing the design principles, essential parameters controlling the performance of the stone column foundation are their
diameter and the distance between each other, expressed by the area replacement factor a c , together with the angle of internal friction φ c of the column
material. The gravel material which is normally used is generally coarsegrained alluvial deposits or crushed rock. It’s stress-dependent friction angle
usually decreases with increasing normal pressure, with φ c,max corresponding with σ c,min and vice versa. The friction angle of gravel is, as with other
granular material, also directly proportional to its density. Installation
methods generally guarantee that the column material is very well compacted by the vibratory motions of the depth vibrator with void ratios in
general close to minimum values as field measurements have shown (Herle
et al., 2007). Their shear parameter φ c can be taken from a σ–τ diagram,
which can be determined relatively easily by shear box tests in the laboratory. Maximum friction angles can be as high as 60° (generally measured
at moderate pressures of ~50 kN/m 2 ) with minimum values of about 50°