###### Abstract
The current research on the compaction characteristics of UPFCs has made some progress, but further studies are still needed to describe the differences in the compaction characteristics between the field and indoor compaction of UPFCs more accurately to achieve better simulations of field cores with laboratory specimens. One of the most used design methods for UPFCs is the Marshall compaction method (MCM), particularly in China. Many studies have proven that there is a poor correlation between field cores and MCM-designed asphalt mixtures with ordinary thickness [20; 21; 22]. Laboratory UPFC specimens molded via the MCM are 2.5 to 6 times thicker than the actual paving thickness of the UPFCs. The difference in thickness inevitably leads to different compaction characteristics of the UPFC, which have different effects on its volume parameters. If a MCM-designed UPFC cannot reflect the actual compaction condition, the performance evaluation of the UPFC will deviate significantly from that of the actual pavement. Therefore, it cannot contribute to accurate guidance for the application of thin-layer overlays [23; 24]. Currently, no studies have used field compaction tests to verify MCM-designed UPFC mixes. There may also be a poor relationship between the MCM-designed UPFC specimens and the field core samples. Considering that the paving thickness of a UPFC is only 1-2 cm, more research on the correlation assessment of the compaction characteristics between on-site compaction and the MCM is required. The development of nondestructive technologies, especially image analysis and X-ray CT, has enabled road workers to thoroughly study the internal structure inside asphalt mixes over the past two decades. The relative properties can be obtained involving the distribution, orientation, contact, and AV distribution of the aggregates using X-ray CT technology [25]. The AV distributions of the specimens with different gradations molded by Superpave gyratory compaction (SGC) were obtained using X-ray CT to analyze the compaction characteristics [26; 27]. In addition, relevant research on the meso-mechanical response of asphalt mixes during the compaction process has made significant progress owing to the continuous progress in the discrete element method (DEM). It has been demonstrated that the SGC test simulated via the DEM was consistent with the laboratory test [28]. Moreover, the DEM was used to explore the effects of force chain evolution and aggregates in the compaction process [29]. There is no doubt that research progress on the application of CT technology and DEM in asphalt mixtures provides a powerful means and experience for exploring the current compaction characteristics of UPFCs. Therefore, this study explores the field compaction mechanism of a UPFC designed using an MCM. Owing to the lack of field core samples, SGC and rolling-wheel compaction (RWC) methods were used to form laboratory specimens with the same thickness as the paving thinness of a UPFC, with the aim to simulate on-site compaction conditions. Accordingly, the compaction characteristics and mesoscopic force of the UPFC during the compaction process were studied. The research results are significant for improving the road performance of UPFCs.