Design and efficiency of a composite vibration isolating screen in soil

This paper reports on the design, the installation, and the efficiency of a vibration isolating screen that has been installed in the soil near a test track of the Brussels public transport company in 1995. The dynamic soil characteristics of the site have been determined by means of a seismic cross-hole test, a seismic refraction test, and a Spectral Analysis of Surface Waves test. The screen is designed to isolate frequencies above 20 Hz, which requires a screen depth of 8 m to effectively reflect the incident surface waves. The vibration isolating screen is conceived as a sandwich panel, using extruded polystyrene as core material and concrete side panels, which act as a ballast material to prevent uplift of the screen during installation. The efficiency of the screen is measured in situ by means of hammer impacts on a foundation installed in front of the screen. The isolation performance is further studied by means of a coupled finite element- boundary element model, comparing the performance of the screen to the ideal case of an open trench, and a polystyrene screen without concrete side panels. The analysis shows that the polystyrene screen has a lower performance than an open trench, whereas the concrete side panels do not significantly modify the effectiveness of the vibration isolating screen. It is demonstrated that the stiffness contrast between the soil and the in-fill material is the key factor determining the performance of the in-fill material. The isolation performance could therefore have been improved by using a material with a lower Young's modulus. A further study also shows that a higher performance could have been achieved using a concrete isolating screen. (C) 2012 Elsevier Ltd. All rights reserved.