8 Ground improvement by deep vibratory methods
surface vibrators generally working with vertical oscillations. Compaction
was achieved by simultaneously adding water while casing and vibrator
was slowly withdrawn at a rate of 8 min./m. The compaction effect was visible at the surface in a crater developing around the casing, into which the
excavation material and additional imported sand was backfilled as necessary. Compaction centers were arranged in a triangular grid at a distance of
2.16 m from each other, representing 4.05 m 2 per probe. Sand consumption
was 5.5 m 3 for each compaction point, an astounding amount, exceeding
all expectations. Degebo confirmed the excellent compaction and raised
the allowable bearing capacity from originally 2.5 to 4.5 kg/cm 2 (250–
450 kPa). The unexpectedly favorable results, which were also confirmed
by seismic tests, could not obscure grave problems identified during execution. Not only was the method very time consuming, but the vibrator itself,
still just a prototype, could only be kept operational at considerable repair
expenses. This was the reason that only a relatively minor portion of the
foundation work could be performed by the novel vibro flotation method in
spite of the convincing test results. The major part of the gigantic foundation totaling some 22,000 compaction centers was performed employing
the Franki method compacting crushed gravel and sand backfill (Ahrens,
1941).
Even during the execution of the trial compaction, mechanical
improvements were made to the vibrator, extending its operational time.
However, it was Rappert, then Keller’s project site manager, who had the
decisive idea that led to the breakthrough with this method. To complete
Figure 2.2 Vibro flotation method used at the Congress Hall at Nuremberg. (Courtesy
of Keller Group plc, London, UK.)
Fun
surface vibrators generally working with vertical oscillations. Compaction
was achieved by simultaneously adding water while casing and vibrator
was slowly withdrawn at a rate of 8 min./m. The compaction effect was visible at the surface in a crater developing around the casing, into which the
excavation material and additional imported sand was backfilled as necessary. Compaction centers were arranged in a triangular grid at a distance of
2.16 m from each other, representing 4.05 m 2 per probe. Sand consumption
was 5.5 m 3 for each compaction point, an astounding amount, exceeding
all expectations. Degebo confirmed the excellent compaction and raised
the allowable bearing capacity from originally 2.5 to 4.5 kg/cm 2 (250–
450 kPa). The unexpectedly favorable results, which were also confirmed
by seismic tests, could not obscure grave problems identified during execution. Not only was the method very time consuming, but the vibrator itself,
still just a prototype, could only be kept operational at considerable repair
expenses. This was the reason that only a relatively minor portion of the
foundation work could be performed by the novel vibro flotation method in
spite of the convincing test results. The major part of the gigantic foundation totaling some 22,000 compaction centers was performed employing
the Franki method compacting crushed gravel and sand backfill (Ahrens,
1941).
Even during the execution of the trial compaction, mechanical
improvements were made to the vibrator, extending its operational time.
However, it was Rappert, then Keller’s project site manager, who had the
decisive idea that led to the breakthrough with this method. To complete
Figure 2.2 Vibro flotation method used at the Congress Hall at Nuremberg. (Courtesy
of Keller Group plc, London, UK.)
Fun