PSI - Issue 82

Koji Uenishi et al. / Procedia Structural Integrity 82 (2026) 79–83 Uenishi et al. / Structural Integrity Procedia 00 (2026) 000 – 000

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Kato, 2024), by means of dynamic photoelasticity in conjunction with high-speed video cameras, we have experimentally shown dynamic stress transfer and associated fracture evolution in two-dimensional granular media made of penny-shaped birefringent polycarbonate particles, and for instance, we have identified two different failure patterns, i.e. complete collapse by unidirectional stress transfer and toppling due to broadly spreading multi dimensional waves (Uenishi and Goji, 2018). Here, our attention is paid to the experimental observations of mechanical stability of a three-dimensional granular medium, especially, the one with a cavity inside the granular medium. As observed e.g. at the subway construction site in Fukuoka in 2016 (Public Works Research Institute, 2017; Mitani, 2023) and at the sewer pipe breakage spot in Yashio in 2025, both in Japan, instability of cavities in a granular ground can lead to subsidence of the surface above and cause tremendous damage to the surface and underground structures in the surroundings. 2. Pressurized cavity in a three-dimensional granular medium For a preliminary study, we prepare a three-dimensional granular medium (dimension 300 mm  300 mm  height some 130 mm) that is made of glass beads (diameter about 1 mm) in a transparent tank, and constantly supply compressed air (approximately 1 MPa) through the outlet of a nozzle located at depth 0, 5 or 35 mm from the top surface of the granular medium. We align the nozzle with one of the walls of the cuboidal tank and supply compressed air downwards into the medium so that we can observe the cross-section of the cavity formation process. In order to trace the particle motion more clearly, we preset airsoft pellets at some positions (see Fig. 1). With a high-speed video camera (Photron FASTCAM Nova S), we trace dynamic motion of particles (glass beads and airsoft pellets) and formation of a pressurized cavity inside the medium. Figure 1 shows the case where the nozzle outlet is located at depth 35 mm from the top surface of the granular medium. The photographs, experimentally taken at a frame rate of 30,000 frames per second (fps), indicate that as soon as the compressed air is supplied a pressurized cavity emerges and expands, and the airsoft pellet just below the outlet, marked by a yellow circle, drops quickly due to the action of compressed air and gravity (see the photographs at time 10 and 20 ms (milliseconds)). By the time 40 ms, the pressurized cavity seems to reach a stable dynamic equilibrium state and the cavity has a teardrop form with a sharp top and a widened bottom. Once established, the pressurized cavity seems stationary and statically kept without altering the form so long as compressed air is supplied. However, surprisingly, instead of being suppressed and kept stationary, the particles that compose the wall of the cavity continuously move from the bottom upwards along the cavity wall due to the action of compressed air and then at the top point of the cavity they drop to the bottom by the effect of gravity (and again compressed air). We can recognize this particle motion by tracing the airsoft pellets marked by light blue, light green and red circles. Thus, in fact, the pressurized cavity in the granular medium is at stable dynamic equilibrium, and a steady flow of particles exists and the particles experience a cyclic motion to keep the form of the cavity. As soon as the supply of compressed air is stopped, the cavity disappears (see the photograph at time 500 ms). A closer look at the photographs and the videos obtained by the high-speed video camera indicates that, as shown in Fig. 2 taken at time 280 ms, the global cyclic upward/downward particle motion explained above is made of local rotational motion of each particle near the wall of the pressurized cavity. That is, each particle globally climbs the wall with the assistance of the local rotation such as illustrated in the figure. Also, inside the conical region with vertex at the nozzle outlet, particles are gradually moving downwards but at the same time they are supplied from outside. If the supply of particles from outside is decreased or stopped, the pressurized cavity may become mechanically unstable and subsidence or collapse of the granular medium may occur. 3. Conclusions We have newly identified a distinctive phenomenon related to dynamic equilibrium of a pressurized granular cavity and provided a qualitative explanation. However, quantitative evaluations that include the relation of particle friction with air pressure and gravity as well as the link between mechanical equilibrium and subsidence are definitely needed, either analytically (Tordesillas et al., 2021; Sano, 2024; Zeng et al. 2024) or by continuum (de Vet and de Bruyn, 2012) / discrete (Staron et al., 2005; Teufelsbauer et al., 2011; Uenishi and Sakurai, 2015; Debski and Klejment, 2021; Uenishi and Xi, 2022) numerical modeling.