Novel vibratory compaction quality indices developed from particle migration and distribution characteristics for unbound aggregate materials

Unbound aggregate materials (UAM) with large air voids are increasingly used to construct pavement base and subbase layers as part of the initiative to create sponge cities and improve drainage performance. However, the motion of particles and the spatial distribution of kinetic energy during the particle rearrangement process induced by vibratory loading remain unclear. This study presents the results of laboratory vibratory plate compaction tests conducted on UAM specimens under various combinations of vibratory parameters and different levels of Gravel to Sand ratio (G/S). SmartRock (SR) sensors were placed within the specimens to monitor particle motion in real-time while kinematic energy and its spatial distribution were analyzed from the acceleration time-history signals collected by the SR sensors. Based on high-precision industrial X-ray computed tomography (XCT) and 3D reconstruction technology, this study analyzed the motion and migration rules of the UAM particles. A new compaction index was proposed based on particle motion and kinematic energy to evaluate the compaction quality of the specimens.The findings of this study reveal that vibratory compaction can be divided into two distinct stages. During the first stage, coarse particles primarily move vertically while energy dissipation occurs mainly through compression of air voids but does not form a dense skeleton structure. In the second stage, coarse particles translate horizontally while rotating vertically, resulting in a tendency of the particles to lie flat with their long axes oriented horizontally. During this stage, most of the energy is used to fill air voids, leading to the formation of a densely packed skeleton structure. Kinematic energy indices and particle movement in the middle of the specimen can be used to evaluate the compaction stage and quality. Additionally, the lateral particle motion in the specimen transitions from continuous ascent to gradual descent to almost no kinematic energy, indicating a relatively dense state of compaction. Reducing the void ratio and increasing the contact area between particles in these size ranges of 4.75-9.5 mm and 2.36-4.75 mm can significantly increase the compaction density and improve the stability and deformation resistance of the subgrade bed.The results of this study provide valuable insights into the mechanisms of vibratory compaction and can be used to optimize compaction methods and improve pavement performance.