According to Manfred Stocker, the A38 project is the first time the vibratory tube system with bottom gate and leader-operated top vibrator has been used on such a large scale.
Its big advantage is the centrifugal force of the top vibrator of between 2,000kN and 3,000kN, which is about 10 times greater than the centrifugal force of a bottom-feed vibrator.
And in contrast to the bottom-feed vibrator, which produces horizontal vibrations, the top vibrator produces high frequency vertical vibrations in the axis of the vibratory tube, which, Stocker maintains is more effective for the production process.
The bottom gate system has a large diameter material transfer tube, Stocker says.
'This ensures the integrity of the stone columns particularly in soft to very soft soils or soils with a low residual strength which tend to liquefy under the influence of energy transfer.
'It has therefore been possible to extend the area of application of vibrodisplacement stone columns to soils beyond the usual limitations.
'A further advantage of the new installation technique is the possibility of using filter-resistant widely-graded sandy gravels as fill material. This means the columns will continue to function long term as drainage elements as the migration of fine particles into the columns is limited.'