## 1 Introduction
If the entire image area is divided into numerous small interrogation windows, the displacement field can be obtained by calculating the displacement in each interrogation window [17]. DIC technology has been widely used in geotechnical engineering due to the advantage of allowing high precision and non-contact full-field strain measurement [18; 19; 20]. Using the DIC technique, [PERSON] et al. [21] determined that an increase in the roughness of the shaft results in an increase in the average unit of shaft resistance and in displacements and strains in the soil next to the pile shaft. [PERSON] et al. [22] determined the distributions  
Figure 1: The process of creating a drill-expanded concrete pile and a photo of a real pile: (**a**) drilling the hole; (**b**) expanding the hole; (**c**) expanding completed; (**d**) pouring concrete; (**e**) photo of a real drill-expanded concrete pile. of displacement during pile installations with different jacking depths, frictions, and pile shoes. By using an indoor half-face pile model test and DIC technology, [PERSON] [23] was able to determine the law of soil displacement development around the pile during static pressure piling. [PERSON] et al. [24] used the digital image correlation technique to generate the displacement and strain fields of sand and determined that reversing the loading direction substantially reduced the units of shaft resistance, which was attributed to a drop in the radial strain exhibited by the soil elements surrounding the model pile shaft. [PERSON] et al. [25] applied digital image correlation throughout the test to better examine the performance of screw-shaft piles. As previous work demonstrates, studying the deformation characteristics of the soil around a pile is important to the ongoing study of the interaction between piles and soil. In the current research, most of the soil used is transparent soil, and most of the research objects are traditional piles. The soil displacement characteristics around the drill-expanded concrete piles are relatively complex, and there are currently no systematic studies on this topic utilizing digital image correlation technology. The introduction of digital image correlation technology into the study of soil displacement fields around expanded bodies can provide valuable data to help reveal the mechanisms of pile-soil interactions. This study involved tests of small-scale half-face piles in undisturbed soil and sand. The dynamic development process of the soil displacement field around the pile under vertical load was recorded by the VIC-3D non-contact full-field strain measurement system. The soil displacement field information was analyzed using digital image correlation (DIC) technology. The characteristics of the soil around the pile under load were described as they evolved during pile development. The model test was verified by numerical simulation, and the results of the numerical simulation are relatively consistent with those of the physical test. The novelty of this paper is to use the DIC technology to observe the horizontal and vertical displacements of the soil around the expanded body visually, and then the distribution characteristics of the soil around the pile were analyzed, which could provide a new method and idea to study the soil displacement characteristics around the pile and the mechanism of interaction between soil and pile.