Ground loosening and subsurface cavities potentially cause ground cave-ins, even if they are deep in the ground. Loosened soil and cavities, for example, formed by shield tunnel excavation or breakage of underground pipes occur frequently. Recently, ground-penetrating radar method has been utilized to detect subsurface cavities, and studies such as dynamic wave surveys have been considered. However, these methods assume that cavities several meters deep can be detected by surface-based surveys, and do not target loosened soil directly above a deep tunnel. This contribution is a fundamental study aimed at detecting loosening depth in the ground, with the goal of measuring dynamic waves from the inside of a tunnel. To understand wave propagation and particle-scale response around loosened sandy soil, this study adopts the discrete element method (DEM) using cohesionless spherical particles. A series of DEM simulations are performed to understand how dynamic waves propagate or reflect around loosened sandy soil in comparison with dense ground without loosening.