The additional value of a sonic drill in a new compaction method

Abstract

Cities are growing and more land is reclaimed. Reclamation is the process of creating new land from ocean, riverbeds or lake beds. New land is created by hydraulic placement of sand. The hydraulically placed fills are characterized by low densities and show a high variety in packing throughout the field. The relative density of the fills depend on placement method and whether the placement takes place aqueous or subaqueous. After the placement Vibro-densification techniques are used to densify the soil. Conventional vibro-densification techniques have difficulties penetrating hard soil layers to reach the loosely packed layers. This makes that land reclamation is not always financially feasible. The Sonic drilling technique might be used to improve the penetration in hard layers. Reducing the process time make the use of land reclamation more accessible. However, the sonic drill technology is not well understood at the moment. There are no reliable guidelines that make it able to estimate the driveability of a certain profile in to the ground with the help of sonic vibrations. This research focusses on better understanding of the sonic drill technology and the contribution of a sonic drill to a new compaction technique.

On a reclaimed land site in Amsterdam an innovative method for installing elements in dense and hard sand layers was tested. The method focuses on sonic drilling/installation with vibrations up to 180Hz to reduce soil resistance whilst pushing the elements into the ground. This method has shown to generate less disturbances to the surroundings whilst achieving a higher penetration rate in comparison to conventional installation methods.
The sonic vibrator generates pressure waves at the top of the element. The elasticity together with the inertial properties of the element allow the waves to propagate through the element. The pressure waves travel through the element and become partially transmitted to the surrounding soil. At the sides the mainly shear waves occur while at the tip both pressure and shear waves are present. During the wave propagation energy is lost due to scattering, radial damping and proper damping.
The sonic drill technology uses high frequency vibrations to strongly reduce soil resistance. The resistance of the element against penetration is reduced by liquefaction, inertia effects and a temporary reduction of porosity of the soil.
The observations from the tests form the basis for the establishment of model which simulates vibratory pile driving. The model is able to describe the installation process for different type of elements. The model might be used in the future to predict the penetration time using only the CPT data.
It can be concluded from the tests that the sonic vibration method is an effective solution for installing elements in a short amount of time in challenging ground conditions and with minimal vibration and settlement disturbance.
Finally, in terms of further study, it must be noted that several major simplifications and assumptions underlie the model, which may be improved upon with more rigorous constitutive modelling and numerical analysis.