## 4 Field Experiments for Evaluating Vibration Isolation Performance
### Experimental Program  
This research encompasses a series of full-scale field experiments aimed at evaluating the vibration isolation performance of the BCCB Trench, a wave barrier fabricated from  
Figure 20: The filling material: polystyrene foam (Styrofoam). low-impedance materials, under impact loading. Considering the alluvial soil conditions and groundwater level of the test site, the trench depth was determined to be 1.5 m. This required the inner surfaces of the open trench to be supported. However, the impedance of the support material must be chosen so as not to alter the transmission behavior of ground waves when they encounter an open trench. To achieve isolation performance similar to that of an open trench without altering the refraction mechanism of ground waves, the BCCB Trench, made from low-impedance materials, was placed into the dug trench. Consequently, when the body waves generated by the sledgehammer hitting the impact plate encounter the BCCB Trench, their behavior will be similar to their interaction with an open trench. Additionally, since the BCCB Trench completely surrounds the interior of the open trench, it will prevent lateral stability issues that could arise from an open trench in the test site, which features soft soil deposits lacking self-supporting properties. Experimental measurements at the test site were conducted in four phases, with vibrations recorded at the same measurement points for all conditions to compare isolation performances. The layout of the accelerometers and the position of the BCCB Trench are illustrated in Figure 21.  
In the first phase of the field experiments, the free surface movements caused by the impact load were recorded with accelerometers (No Trench). In the second phase, to compare the isolation performance of the BCCB Trench with the wave absorption performance of an open trench, a trench of appropriate dimensions matching the barrier's geometry was excavated, and the waves generated during the sledgehammer impact on the impact plate were recorded via accelerometers (Empty Trench). Subsequently, the BCCB Trench, which was fabricated in a box shape with dimensions of 3 m in length, 1.5 m in height, and 0.6 m in width, was placed into the dug trench, and the body waves generated by the sledgehammer impacts were recorded (BCCB Trench). The process of trench excavation and the placement of the wave barrier into the trench, suitable to the dimensions of the BCCB Trench, is given in Figure 22. In the final stage of the experimental measurements, to investigate the effect of low-impedance waste materials on the vibration isolation performance of the wave barrier, the BCCB Trench was filled with waste polystyrene foam (Styrofoam) having a very low density of 0.01-0.03 t/m\({}^{3}\), and the vibration produced by the impact load was recorded (Styrofoam-filled BCCB Trench). Figure 21: Layout of measuring points from a schematic view of the monitoring field test area with BCCB Trench. In this study, vibration field tests were conducted on four different cases to investigate the vibration isolation performance of a box-shaped wave barrier (BCCB) made of balsa wood and corrugated cardboard, which surrounds the open trench on all sides: No Trench, Empty Trench, BCCB Trench, Styrofoam-filled BCCB Trench (Figure 23). These vibrations are surface waves generated by the energy released from the impact of a sledgehammer on the steel plate surface located on the ground. The impact-induced vibrations produced by the sledgehammer were recorded in the East-West (EW), North-South (NS), and Up-Down (UD) directions by accelerometers placed on the free-field surface. The EW, NS, and UD directions represent the directions parallel, perpendicular, and vertical to the dynamic vibration source, respectively. The need for sensors capable of detecting weak vibrations at very low noise levels to accurately measure impact-induced vibrations is addressed by the DAC-3 HDG series of 3-axis high-precision accelerometers (Figure 18).