Soil resistance during vibratory driving in sand

Abstract

The mechanism that can explain pile shaft resistance during vibratory driving is discussed. One hypothesis is that horizontally oscillating stresses temporarily reduce the shaft resistance. To investigate this hypothesis, vibration measurements were carried out in medium-dense to dense sand during the vibratory driving. A vibrator with variable frequency was used to install a large compaction probe. The driving process and ground response were documented in detail. Geophones were installed on and below the ground surface. Horizontal ground vibrations were measured at three levels below the ground surface. The difference in vibration response of the ground during driving at a high frequency (27 Hz) and at the system resonance frequency (15 Hz) showed the effect of the vibrator operating frequency on penetration speed and emitted ground vibrations. During vibratory driving, horizontal stress pulses are emitted along the probe shaft, which can temporarily reduce static horizontal stresses acting against the probe shaft. This phenomenon can explain the temporary reduction of the shaft resistance and the efficiency of vibratory driving in granular soils. As a result of the horizontal stress pulses, the horizontal stresses are increased permanently. By using a monitoring and process control system, it is also possible to determine the system resonance frequency in the field, which is a critical parameter for vibratory driving resistance, emission of ground vibrations, and vibratory compaction of granular soils.