30 Ground improvement by deep vibratory methods
the compaction point generally into a settling pond. Sludge handling and
water treatment to allow its safe return into the environment can be time
consuming and expensive. However, this wet method is still being used
today whenever the local conditions permit.
A major breakthrough was achieved when, in 1972, a patent was registered for the bottom feed vibrator. It allows the stone backfill to be channeled via special piping in the extension tubes, at the outside of the vibrator,
and supported by compressed air to be released directly at the vibrator
point. The vibrocats were adapted for the new requirements and a special
material lock was subsequently developed that allowed the stone backfill to
be fed in a controlled way into the extension tubes now acting as material
containers. The advantages of the improved method were soon fully realized and were so convincing that the wet method for stone column installation was almost completely replaced by the new bottom feed method.
It was not only the sludge-handling problem that was responsible for this
development, it was also the new concept itself, which enabled for the first
time a safe—and even for critical users visible and verifiable—construction
of an uninterrupted stone column. Hitherto occasionally occurring discontinuities or even interruptions in stone columns, which had caused excess
settlements, were a key reservation that critics raised against the method.
Not only did clients and their consultants welcome the new bottom feed
method, but also the practitioners on-sites, because by the new method the
quality of stone columns was substantially increased without increasing the
accompanying controls.
After the main patent rights to the vibrator elapsed at the end of the
1960s, other specialist contractors, especially in Germany, started designing and constructing their own vibrators. Since hydraulic motors had gained
general acceptance in the design of construction equipment, it was not surprising that hydraulic drives were also used for these vibrators. Above all, it
was the aim of the designers to increase the stamina and endurance of the
vibrator for its rough usage on-sites. Nevertheless, design and development
targets for the improvement of vibrator performance were—and are still
today—not at all equal among the few companies specialised in the manufacture of vibrators. And it is interesting to see that very little has been published so far about their key characteristics. It is generally accepted today
that an optimal ground improvement can only be achieved by an optimal
choice of the compaction equipment.
2.6 DESIGN ASPECTS
Although densification of clean sands by vibro flotation is the oldest form of
deep vibratory compaction, no computational design method has yet been
developed. As the degree of density in given sand achieved by vibro compaction depends on the distance between compaction centers, the vibratory
the compaction point generally into a settling pond. Sludge handling and
water treatment to allow its safe return into the environment can be time
consuming and expensive. However, this wet method is still being used
today whenever the local conditions permit.
A major breakthrough was achieved when, in 1972, a patent was registered for the bottom feed vibrator. It allows the stone backfill to be channeled via special piping in the extension tubes, at the outside of the vibrator,
and supported by compressed air to be released directly at the vibrator
point. The vibrocats were adapted for the new requirements and a special
material lock was subsequently developed that allowed the stone backfill to
be fed in a controlled way into the extension tubes now acting as material
containers. The advantages of the improved method were soon fully realized and were so convincing that the wet method for stone column installation was almost completely replaced by the new bottom feed method.
It was not only the sludge-handling problem that was responsible for this
development, it was also the new concept itself, which enabled for the first
time a safe—and even for critical users visible and verifiable—construction
of an uninterrupted stone column. Hitherto occasionally occurring discontinuities or even interruptions in stone columns, which had caused excess
settlements, were a key reservation that critics raised against the method.
Not only did clients and their consultants welcome the new bottom feed
method, but also the practitioners on-sites, because by the new method the
quality of stone columns was substantially increased without increasing the
accompanying controls.
After the main patent rights to the vibrator elapsed at the end of the
1960s, other specialist contractors, especially in Germany, started designing and constructing their own vibrators. Since hydraulic motors had gained
general acceptance in the design of construction equipment, it was not surprising that hydraulic drives were also used for these vibrators. Above all, it
was the aim of the designers to increase the stamina and endurance of the
vibrator for its rough usage on-sites. Nevertheless, design and development
targets for the improvement of vibrator performance were—and are still
today—not at all equal among the few companies specialised in the manufacture of vibrators. And it is interesting to see that very little has been published so far about their key characteristics. It is generally accepted today
that an optimal ground improvement can only be achieved by an optimal
choice of the compaction equipment.
2.6 DESIGN ASPECTS
Although densification of clean sands by vibro flotation is the oldest form of
deep vibratory compaction, no computational design method has yet been
developed. As the degree of density in given sand achieved by vibro compaction depends on the distance between compaction centers, the vibratory