A history of vibratory deep compaction 17
phase due to its astounding flexibility and adaptability in solving different
construction problems. Whenever the density of naturally deposited or filled
sand was insufficient for foundation purposes, the depth vibrator was a useful
tool to remedy the situation. Eventually, the outdated gantries were replaced
by crawler cranes which, together with other modern equipment, reduced site
installation costs and increased production rates considerably.
Besides the densification effect of the depth vibrator as described earlier,
its potential to temporarily liquefy fully saturated sand below the water table
was soon realized and further developed for practical use. Prefabricated concrete piles and steel tubes as well as sheet piles could be easily lowered into
the liquefied zone surrounding the depth vibrator. The process was further
supported by strong water flush emanating close to the point of the vibrator.
This was of significance as top vibrators and vibratory hammers did not then
exist. Many such jobs were successfully executed during the first decades
after the war including one spectacular case: a complete lighthouse, Tegeler
Plate, was sunk 40 km northwest of Bremerhaven 18 m deep into the sandy
bottom of the North Sea by means of six depth vibrators attached to its
foundation shaft consisting of a 30 m long, 2.5 m diameter heavy steel pipe.
In 1953, when the planning of a new dry dock at Emden commenced, the
designers were considering a special proposal to use the ability of the depth
vibrator to sink larger bodies into the ground to install anchors which were
necessary for the uplift protection of the dock. Experiments were carried out
to establish the optimal size of the anchor blocks together with the number
of depth vibrators needed to lower the blocks to the predetermined depth.
For the dry dock, 500 cross-shaped concrete anchor blocks were sunk 14 m
into the underlying sand layer, simultaneously using two or three vibrators
for each block. The 46 mm diameter corrosion-protected steel cables, which
were securely fastened in the anchor blocks, extended into the slab of the
dry dock from where they were posttensioned with 1 MN. As the sand
below the dry dock was also well-compacted, when the vibrators were withdrawn, the system of anchors and dense sand provided an excellent solution
for the foundation of the dry dock which was more economical than a conventional gravity structure.
In 1956, at Braunschweig, similar difficulties were experienced during
vibro compaction works for an engine shed for the German Railway as at
the above-mentioned project in Norway in 1942. Here, the subsoil again
consisted of very fine, very silty water saturated sand that was totally liquefied by the vibratory motions, transforming the ground surrounding the
vibrator into a viscous liquid. A measurable compaction effect, if at all,
was only achieved after a very long time. The vibro flotation method had
reached its limit of application. The solution that was finally adopted consisted of lowering the vibrator into the ground without the use of flushing water, withdrawing it completely from the ground after it had reached
its final depth and filling the temporarily stable hole left behind by the
vibrator with coarse material. By repeated repenetration and backfilling,
phase due to its astounding flexibility and adaptability in solving different
construction problems. Whenever the density of naturally deposited or filled
sand was insufficient for foundation purposes, the depth vibrator was a useful
tool to remedy the situation. Eventually, the outdated gantries were replaced
by crawler cranes which, together with other modern equipment, reduced site
installation costs and increased production rates considerably.
Besides the densification effect of the depth vibrator as described earlier,
its potential to temporarily liquefy fully saturated sand below the water table
was soon realized and further developed for practical use. Prefabricated concrete piles and steel tubes as well as sheet piles could be easily lowered into
the liquefied zone surrounding the depth vibrator. The process was further
supported by strong water flush emanating close to the point of the vibrator.
This was of significance as top vibrators and vibratory hammers did not then
exist. Many such jobs were successfully executed during the first decades
after the war including one spectacular case: a complete lighthouse, Tegeler
Plate, was sunk 40 km northwest of Bremerhaven 18 m deep into the sandy
bottom of the North Sea by means of six depth vibrators attached to its
foundation shaft consisting of a 30 m long, 2.5 m diameter heavy steel pipe.
In 1953, when the planning of a new dry dock at Emden commenced, the
designers were considering a special proposal to use the ability of the depth
vibrator to sink larger bodies into the ground to install anchors which were
necessary for the uplift protection of the dock. Experiments were carried out
to establish the optimal size of the anchor blocks together with the number
of depth vibrators needed to lower the blocks to the predetermined depth.
For the dry dock, 500 cross-shaped concrete anchor blocks were sunk 14 m
into the underlying sand layer, simultaneously using two or three vibrators
for each block. The 46 mm diameter corrosion-protected steel cables, which
were securely fastened in the anchor blocks, extended into the slab of the
dry dock from where they were posttensioned with 1 MN. As the sand
below the dry dock was also well-compacted, when the vibrators were withdrawn, the system of anchors and dense sand provided an excellent solution
for the foundation of the dry dock which was more economical than a conventional gravity structure.
In 1956, at Braunschweig, similar difficulties were experienced during
vibro compaction works for an engine shed for the German Railway as at
the above-mentioned project in Norway in 1942. Here, the subsoil again
consisted of very fine, very silty water saturated sand that was totally liquefied by the vibratory motions, transforming the ground surrounding the
vibrator into a viscous liquid. A measurable compaction effect, if at all,
was only achieved after a very long time. The vibro flotation method had
reached its limit of application. The solution that was finally adopted consisted of lowering the vibrator into the ground without the use of flushing water, withdrawing it completely from the ground after it had reached
its final depth and filling the temporarily stable hole left behind by the
vibrator with coarse material. By repeated repenetration and backfilling,