Measurement Of Radiation Impedance Using A Portable Vibrator To Evaluate Mechanical Properties Of The Subsurface

In civil engineering and environmental applications, it is important to evaluate the mechanical properties of<br>soils and rocks. Reijnders et al., (1999) introduced a method to measure the radiation impedance of the very shallow<br>subsurface using a portable vibrator. The method is based on the determination of the frequency-dependent integral<br>elastic response of the soils below the vibrator baseplate using reference signals measured at the source.<br>Radiation impedance is defined as the ratio of the stress acting on a particle and the resulting particle<br>velocity. A vibratory source equipped with accelerometers on baseplate and reaction mass allows the determination<br>of the force that acts on the surface as well as the baseplate velocity. If one makes the assumption that the velocity of<br>the baseplate is in phase with the velocity of the material directly beneath it, both the stress and the velocity, and thus<br>the radiation impedance, can be determined from measurements of the accelerations at the source.<br>Several field experiments were conducted to evaluate this method and are introduced in this paper. One of<br>these experiments was performed to evaluate the variation of radiation impedance over an area of re-filled and<br>compacted ground. High impedance was observed for the re-filled area. Obviously, this variation of radiation<br>impedance reveals the effect of compaction. This observation indicates that measurement of radiation impedance can<br>be applied to evaluate the mechanical properties of very shallow subsurface materials.