190 Ground improvement by deep vibratory methods
pile method, the Franki gravel pile method, the controlled modulus column
(CMC) method, the Geopier method, and the dynamic replacement method
(see also Table 1.1).
The sand compaction pile method has been extensively used in the Far
East for improving very soft marine clays, both in offshore and onshore
applications to depths of about 20 m. The process uses closed-end steel
pipes, with diameters between 0.5 and up to 1.6 m, that are vibrated by
vibratory hammers into the ground, and the process is largely automated
including sand backfilling. Area replacement ratios are generally between
0.3 and 0.5, and settlement improvement is based on experience, with an
empirical stress concentration factor n = 3 (Tanimoto, 1973). To reduce the
settlements of large diameter replacement sand columns a method has been
introduced in Germany whereby a geotextile liner encases the sand (Raithel
and Kempfert, 1999). Considerable vertical deformations activate the tensile
forces of the geotextile before, ultimately, horizontal deformations of the
column generate supporting stresses of the surrounding soil (Raithel, 1999).
In the Franki gravel pile method, which is similar to the well-known piling method, a steel pipe is driven, after a gravel plug was formed at its base,
to the required depth and is then gradually withdrawn while the coarse fill
material is rammed out by means of an internal drop weight.
From a multitude of similar small diameter piling methods being installed
for ground improvement purposes in soft soils at relatively close spacing,
the CMC method utilizes a special displacement auger that penetrates the
ground to the predesigned depth essentially without spoil. Grouting of the
pile shaft is carried out through the hollow auger stem. If necessary, this
process can be repeated to push the grout into the soil thereby increasing
contact with it to enhance skin friction at the pile perimeter. In contrast
to the vibro concrete columns, end bearing of these columns is generally
avoided and their design is based on the equal strain concept of soil and
columns whereby column strength can be selected to keep the ratio of soil
and column modulus within reasonable limits.
By contrast with the CMC method, which is used to improve soils at considerable depths, the dynamic replacement method is restricted to moderate
depth ranges of about 5–7 m. With this method, a large diameter gravel
column is formed in cohesive soils by hammering crushed stone into the
ground in a controlled way applying a similar procedure to the well-known
dynamic compaction method, but using moderate weights and drop heights.
The heavy impact of the drop weight is accompanied by very severe shearing
in soft cohesive soils generally without any compaction. To avoid unwanted
heave and to increase column depths pre-excavation may be necessary.
Generally, the method requires experience during its execution and close
supervision of the development of the imprints and their filling with coarse
backfill material (Varaksin, 1990; Luongo, 1992).
The Geopier method constructs impact stone columns in soft cohesive
soils by driving a steel tube of approximately 0.5 m diameter into the
pile method, the Franki gravel pile method, the controlled modulus column
(CMC) method, the Geopier method, and the dynamic replacement method
(see also Table 1.1).
The sand compaction pile method has been extensively used in the Far
East for improving very soft marine clays, both in offshore and onshore
applications to depths of about 20 m. The process uses closed-end steel
pipes, with diameters between 0.5 and up to 1.6 m, that are vibrated by
vibratory hammers into the ground, and the process is largely automated
including sand backfilling. Area replacement ratios are generally between
0.3 and 0.5, and settlement improvement is based on experience, with an
empirical stress concentration factor n = 3 (Tanimoto, 1973). To reduce the
settlements of large diameter replacement sand columns a method has been
introduced in Germany whereby a geotextile liner encases the sand (Raithel
and Kempfert, 1999). Considerable vertical deformations activate the tensile
forces of the geotextile before, ultimately, horizontal deformations of the
column generate supporting stresses of the surrounding soil (Raithel, 1999).
In the Franki gravel pile method, which is similar to the well-known piling method, a steel pipe is driven, after a gravel plug was formed at its base,
to the required depth and is then gradually withdrawn while the coarse fill
material is rammed out by means of an internal drop weight.
From a multitude of similar small diameter piling methods being installed
for ground improvement purposes in soft soils at relatively close spacing,
the CMC method utilizes a special displacement auger that penetrates the
ground to the predesigned depth essentially without spoil. Grouting of the
pile shaft is carried out through the hollow auger stem. If necessary, this
process can be repeated to push the grout into the soil thereby increasing
contact with it to enhance skin friction at the pile perimeter. In contrast
to the vibro concrete columns, end bearing of these columns is generally
avoided and their design is based on the equal strain concept of soil and
columns whereby column strength can be selected to keep the ratio of soil
and column modulus within reasonable limits.
By contrast with the CMC method, which is used to improve soils at considerable depths, the dynamic replacement method is restricted to moderate
depth ranges of about 5–7 m. With this method, a large diameter gravel
column is formed in cohesive soils by hammering crushed stone into the
ground in a controlled way applying a similar procedure to the well-known
dynamic compaction method, but using moderate weights and drop heights.
The heavy impact of the drop weight is accompanied by very severe shearing
in soft cohesive soils generally without any compaction. To avoid unwanted
heave and to increase column depths pre-excavation may be necessary.
Generally, the method requires experience during its execution and close
supervision of the development of the imprints and their filling with coarse
backfill material (Varaksin, 1990; Luongo, 1992).
The Geopier method constructs impact stone columns in soft cohesive
soils by driving a steel tube of approximately 0.5 m diameter into the